United States ; I ( i
^Environmental Protection
Agency ; / , :
Office^of
Policy, Plannng
and Evaluation (2122)
EPA-230-R-96-006
'November 1:995
vvEPA
Inventory 6f U.S. Greenhouse Gas
Emissions and Sinks: 1990-1994
i«»l«;*:
-• ;-,'.-", ^SK^j^l^i
-------
Recycled/Recyclable • Printed with Vegetable Based Inks on Recycled Paper (20% Postconsumer)
-------
Additonal Readings
Callaway, M., Smith, J., and S. Keefe. 1994. The Economic Effects of Climate Change for U.S.
Forests. Report of RCG/Hagler, Bailly. (to obtain the report contact NCEPI at 513-489-8190)
Hohenstein, William G., and Lynn Wright. 1994. Biomass Energy Production in the United
States: An Overview. Biomass and Bioenergy, Vol. 6, No. 3. Pp. 161-173. (to obtain a copy of
this report write to the Office o'f the Economy and the Environment U.S. EPA 401 M St. SW
(MC: 2122) Washington D.C. 20460)
IPCC/OECD/IEA/UNEP, 1995. IPCC guidelines for National Greenhouse Gas Inventories,
'Vol. 1-3; Intergovernmental Panel on Climate Change, Organization for Economic Co-Operation
and Development, International Energy Agency, United Nations Environment Program:
Brucknell, UK.
Michaels G., O'Neal K., Humphrey, J., Bell, K., Camacho, R., Funk, R. 1995. Ecological
'Impacts From Climate Change: An Economic Analysis of Fresh Water Fishing. U.S. EPA.
(to obtain a copy of this report contact NCEPI at 513-489-8190)
Sathaye, J., Makundi, W., and K. Andrasko. 1995. A Comprehensive Mitigation Assessment
Process (COMAP)for the Evaluation of Forestry Mitigation Options. Biomass and Bioenergy.
In press, (to obtain a copy of this report write to the Office of the Economy and the Environment
U.S. EPA 401 M St. SW (MC: 2122) Washington D.C. 20460)
Titus, J.G., Narayanan, V.K., 1995. The Probability of Sea-Level Rise. U.S. EPA. (to obtain a
copy of this report contact NCEPI at 513-489-8190) - .
U.S. EPA. 1995. Anticipatory Planning for Sea-Level Rise Along the Coast of Maine. U.S.
EPA. (to obtain a copy of this report contact NCEPI at 513-489-8190)
U.S. EPA. 1994. Inventory of U.S Greenhouse Gas Emissions and Sinks: 1990-1993. U.S.
EPA. EPA 230-R-94-014, Washington, DC. (to obtain a copy of this report contact NCEPI at
513-489-8190)
U.S. EPA. 1993. Opportunities to Reduce Anthropogenic Methane. Emissions in the United
States. U.S. EPA. (to obtain a copy of this report contact NCEPI at 513-489-8190)
U.S. EPA. 1995. States Workbook; Methodologies for Estimating Greenhouse Gas Emissions,
Second Edition. EPA-230-B-95-001 Office of Policy Planning and Evaluation.
-------
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
APR 8 1996
OFFICE OF
POLICY, PLANNING AND EVALUATION
Dear Colleague,
I am pleased to announce the release of the Inventory of Greenhouse Gas Emissions and
Sinks: 1990-1994. The emissions estimates contained in this report, along with future updates,
will be used to monitor and track the progress of the U.S. in meeting the U.S. commitment to
return greenhouse gas emissions to 1990 levels by 2000. Decision 3/CP.l under the Framework
Convention on Climate Change (FCCC) states that Annex I Parties should submit "National
inventory data on emissions by sources and removals by sinks ... should be provided annually on
15 April." In accordance with this decision, Inventory of Greenhouse Gas Emissions and Sinks:
1990-1994 was prepared and is the second official U.S. submission to the The Framework
Convention on Climate Change (FCCC). This report complies with the reporting guidelines
established by the scientific and technical organizations that have been recommended to the
Conference of Parties and is consistent with the reports from all Parties to the FCCC.
We greatly appreciate the efforts of the Energy Information Administration, The
Department of Agriculture, and other EPA Offices for their strong cooperation and contributions
to this document.
To obtain additional copies of this document, please FAX your requests to the National
Center for Environmental Publications and Information (NCEPI) at (513) 489-8695. For other
relevant EPA publications please refer to the list on the back of this page. If you have any
questions or comments please call Wiley Harbour at (202) 260-6972.
Sincerely,
David Gardiner
Assistant Administrator
Roeycled/Reeyclabla «Printed with Vegetable Oil Based Inks on 100% Recycled Paper (40% Postconsumer)
-------
Inventory of U.S. Greenhouse
Emissions and Sinks:
1990-1994
U.S. Environmental Protection Agency
Office of Policy, Planning and Evaluation
Washington, D.C., U.S.A.
November 1995
-------
This document has undergone U.S. Environmental Protection Agency internal review, interagency review, and
public review. Mention of trade names or commercial products does not constitute endorsement or recom-
mendation for use.
-------
Acknowledgements
The U.S. Environmental Protection Agency, Office of Policy, Planning and Evaluation would like to thank
those who contributed to the writing of this report, in particular, Craig Ebert, Barbara Braatz, Alexei Sankovski,
Doug Keinath, Colin Polsky, Cathleen Kelly, Melissa Lavinson, Susan Barvenik, Sonali Shah, Shali Bogavelli,
Mary DePasquale, Michael Gibbs, Jonathan Woodbury, Dana Slevin, Paul Jun, and Vikram Bakshi of ICF
Consulting Group working as consultants to the U.S. EPA for the project. Other Agencies and EPA Offices con-
tributed greatly to data collection and review, including: EPA's Office of Programs, Office of Air Quality
Planning and Standards, and the Ak and Energy Engineering Research Laboratory; the Energy Information
Administration of the Department of Energy; and the Department of Agriculture.
-------
-------
Table of Contents
Executive Summary
Introduction
Part I. Energy
Emissions from Fossil Fuel Consumption
Carbon Dioxide Emissions from Fossil Fuel Consumption
Other Greenhouse Gas Emissions from Stationary Fossil Fuel Combustion
Other Greenhouse Gas Emissions from Mobile Combustion
Fossil Fuel Production, Transport, Storage, and Distribution
Emissions from Coal Mining
Emissions from Natural Gas Production, Processing, Transport, and Distribution
Emissions from Production, Refining, Transportation, and Storage of Petroleum
Emissions from Biomass and Biomass-Based Fuel Consumption
Emissions from Wood Consumption
Emissions from Ethanol Consumption
Part II. Industrial Processes
Carbon Dioxide Emissions
Cement Production
Lime Manufacture
Limestone Use
Soda Ash Manufacture and Consumption
Carbon Dioxide Manufacture
Aluminum Production
Nitrous Oxide Emissions
Adipic Acid Production
Nitric Acid Production
ES-I
I
9
10
10
18
21
24
24
26
28
32
32
33
35
36
36
38
40
41
42
43
44
44
45
Table of Contents B i
-------
Other Emissions 45
Emissions of Halogenated Compounds 46
Emissions of Criteria Pollutants: NOX, NMVOCs and CO 51
Emissions of NF3 51
Part III. Emissions from Solvent Use 53
»
Part IV. Emissions from Agriculture 55
Methane Emissions from Enteric Fermentation in Domestic Livestock 57
Methane Emissions from Cattle 58
Methane Emissions from Other Domestic Animals 60
Methane Emissions from Livestock Manure 60
Methodology 61
Methane Emissions Estimates from Livestock Manure 62
Methane Emissions from Rice Cultivation 62
Methodology 63
Methane Emissions from Rice Cultivation 64
Nitrous Oxide Emissions from Agricultural Soil Management 66
Methodology 67
Nitrous Oxide Emissions from Agricultural Soils 67
Emissions from Field Burning of Agricultural Wastes 69
Methodology 70
Emissions from Field Burning 70
PartV. Emissions from Land-Use Change and Forestry 75
Part VI. Emissions from Waste 79
Landfills 79
Wastewater 82
Waste Combustion 83
References
Annex A Method of Estimating Emissions of CO2 from Fossil Energy Consumption
Annex B Emissions from Mobile Combustion
Annex C Emissions of HFCs, PFCs and SF6
Annex D Estimation of Methane Emissions from Enteric Fermentation in Cattle and from Animal
Manure Management
Annex E Methane Emissions from Landfills
Annex F Sulfur Dioxide: Effect on Radiative Forcing and Sources of Emissions
Annex G D?CC Reporting Tables
ii • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
List of Boxes,
Tables, and Figures
Boxes
Executive Summary
Box ES-1 The Global Warming Potential (GWP) Concept
Box ES-2 Emissions of CFCs and Related Compounds
Box ES-3 Sulfur Dioxide: Effects on Radiative Forcing and Sources of Emissions
Introduction
Boxl
Box 2
Parti
Box 1-1
PartlV
Box IV-1
Greenhouse Gases and Other Photochemically Important Gases
The Global Warming Potential (GWP) Concept
About Energy Data and Estimating Carbon Emissions
Estimating Nitrous Oxide Emissions Using the DNDC Model
Tables
ES-2
ES-15
ES-17
2
3
15
68
Executive Summary
Table ES-1
Table ES-2
Table ES-3
Table ES-4
Table ES-5
Table ES-6
Introduction
Table 1
Parti
Table 1-1
Table 1-2
Table 1-3
Recent Trends in U.S. Greenhouse Gas Emissions: 1990-1994
Sources of CO2 Emissions: 1994
Sources of CH4 Emissions: 1994
Sources of N2O Emissions: 1994
Emissions of HFCs, PFCs, and SF6: 1994
Emissions of CO, NOX, and NMVOCs: 1994
Recent Trends in U.S. Greenhouse Gas Emissions: 1990-1994
U.S. CO2 Emissions from Energy Consumption by End-Use Sector
and Fuel Type: 1990-1994
Key Assumptions for Estimating Carbon Dioxide Emissions
U.S. Greenhouse Gas Emissions from Stationary Combustion: 1990-1994
ES-3
ES-5
ES-10
ES-13
ES-14
ES-1 6
4
11
17
18
List of Boxes, Tables, and Figures H Hi
-------
Table 1-4
Table 1-5
Table 1-6
Table 1-7
Table 1-8
Table 1-9
Table 1-10
Table 1-11
Table 1-12
Table 1-13
Table 1-14
Table 1-15
PartH
Table H-l
Table H-2
Table H-3
Table H-4
Table H-5
Table H-6
Table E-7
PartlE
Table IH-1
Table ffl-2
PartIV
Table IV-1
Table IV-2
Table IV-3
Table IV-4
Table IV-5
Table IV-6
Table IV-7
Table IV-8
Table IV-9
Table IV-10
Table IV-1 1
U.S. Greenhouse Gas Emissions from Stationary Combustion by Sector
and Fuel Source: 1994
Ratio of CH4 to NMVOCs Released During Combustion
U.S. Greenhouse Gas Emissions from Mobile Combustion: 1990-1994
U.S. Greenhouse Gas Emissions from Mobile Combustion by Vehicle Type: 1994
Coal Mine Methane Emissions Estimates
Methane Emissions from the U.S. Natural Gas Industry: 1990-1994
Methane Emissions from the Production and Refining
of Petroleum Liquids: 1990-1994
CO2 Emissions from Flaring of Natural Gas
NOx, NMVOCs and CO Emissions from Oil and Gas Activities: 1990-1994
CO2 Emissions from Wood Consumption by Sector: 1990-1994
Residential and Industrial Biomass Combustion: 1990-1994
U.S. CO2 Emissions from Ethanol Consumption: 1990-1994
Recent Trends in U.S. Greenhouse Gas Emissions
from Industrial Sources: 1990-1994
CO2 Emissions from U.S. Cement Production: 1990-1994
CO2 Emissions from U.S. Lime Production: 1990-1994
CO2 Emissions from U.S. Limestone Consumption: 1990-1994
Emissions of HFCs and PFCs: 1994
Emissions of ODSs: 1994
U.S. Emissions of NOX, CO, and NMVOCs
from Industrial Processes: 1990-1994
Emissions of NMVOCs , NOX and CO from Solvent Use: 1990-1994
U.S. Emissions of NMVOCs, NOX, and CO by Category: 1994
Recent Trends in U.S. Greenhouse Gas Emissions
from Agricultural Sources: 1990-1994
Methane Emissions from Animals: 1990-1994
U.S. Animal Populations: 1990-1994
Methane Emissions from Manure Management: 1990-1994
Area Harvested and Flooding Season Length for Rice-Producing States
CH4 Emissions from Rice Cultivation in the U.S.: 1990-1994
Fertilizer Consumption and N2O Emissions in the U.S.: 1990-1994
Key Assumptions for Estimating Emissions from Crop Waste Burning
Using Annual Activity Data
Annual Trace Gas Emissions from Field Burning: 1990-1994
Based on Annual Activity Data
Key Assumptions for Estimating Emissions from Crop Waste Burning
Using Three-year Averages of Activity Data
Average Annual Trace Gas Emissions from Field Burning: 1990-1994
Based on Three-year Averages of Activity Data
20
21
22
23
25
28
29
31
32
33
33
34
36
37
39
40
47
50
52
54
54
56
59
60
62
65
65
69
71
72
73-
74
iv • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
PartV
Table V-l
Table V-2
Part VI
Table VI-1
Table VI-2
Table VI-3
Table VI-4
Table VI-5
U.S. Carbon Storage Estimates
Carbon Fluxes from U.S. Forests in 1990-1992
Recent Trends in U.S. Greenhouse Gas Emissions from
Waste Sources: 1990-1994
U.S. Methane Emissions from Landfills: 1990-1994
U.S. Methane Emissions from Wastewater
U.S. NMVOC, CO, and NOX Emissions from
Waste Incineration: 1990-1994
U.S. NMVOC, CO, and NOX Emissions from
Waste Incineration by Source: 1994
Figures
Executive Summary
Figure ES-1 Total U.S. Emissions by Source: 1994
Figure ES-2 Recent Trends in U.S. Greenhouse Gas Emissions
Figure ES-3 Total U.S. Greenhouse Gas Emissions by Gas: 1994
Figure ES-4 Primary Sources of Energy in the U.S.: 1994
Figure ES-5 Carbon Dioxide Emissions from Fossil Fuel Combustion
Figure ES-6 Sources of CH4 Emissions: 1994
Figure ES-7 CH4 Emissions from Agriculture by Source: 1994
Figure ES-8 Sources of N2O Emissions: 1994
Parti
Figure 1-1
Figure 1-2
Figure 1-3
Primary Sources of Energy in the U.S.: 1994
Change in Acquisition Price of Crude Oil
Carbon Dioxide Emissions from Fossil Fuel Combustion
by End-Use Sector: 1994
Figure 1-4 Total Vehicle Miles Traveled by Major Vehicle Type: 1978-1994
Figure 1-5 Carbon Dioxide Emissions from Fossil Fuel Combustion
by Sector and Fuel Type: 1994
Part IV
Figure IV-1 Total U.S. Emissions by Source: 1994
Figure IV-2 U.S. Methane Emissions By Source: 1994
Figure IV-3 U.S. Nitrous Oxide Emissions By Source: 1994
77
78
80
81
83
84
84
ES-1
ES-4
ES-4
ES-5
ES-6
ES-10
ES-10
ES-13
10
12
12
13
14
55
56
56
List of Boxes, Tables, and Figures
-------
-------
Executive
Summary
This document provides information on greenhouse gas sources and sinks, and estimates of emissions and
removals for the United States for 1990-1994, as well as the methods used to calculate these estimates
and the uncertainties associated with them. The emission estimates presented here were calculated using the
JPCC Guidelines for National Greenhouse Gas Inventories (IPCC/OECD/IEA, 1995) to ensure that the green-
house gas emission inventories prepared by the United States to meet its commitments under the Framework
Convention on Climate Change are consistent and comparable across sectors and between nations. In order
to fully comply with the IPCC Guidelines, the United States has provided a copy of the IPCC reporting tables
in Annex G of this report. These tables include the data used to calculate emission estimates using the IPCC
Guidelines. The United States has followed these guidelines, except where more detailed data or methodolo-
gies were available for major U.S. sources of emissions. In such cases, the United States expanded on the IPCC
guidelines to provide a more comprehensive and accurate account of U.S. emissions. These instances have been
documented, and explanations have been provided for diverging from the IPCC Guidelines
(IPCC/OECD/IEA, 1995).
Figure ES-
The Greenhouse Gases and
Photochemical!/ Important
Gases
Naturally occurring greenhouse gases
include water vapor, carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), and
ozone (O3). Chlorofluorocarbons (CFCs) (a
family of human-made compounds), its substi-
tute hydrofluorocarbons (HFCs), and other
compounds such as perfluorinated carbons
(PFCs), are also greenhouse gases. In addition,
other photochemically important gases — such
as carbon monoxide (CO), oxides of nitrogen
(NOX), and nonmethane volatile organic com-
pounds (NMVOCs) — are not greenhouse
gases, but contribute indirectly to the green-
house effect (see Box ES-1 for explanation).
Executive Summary • ES-1
-------
These are commonly referred to as "tropospheric
ozone precursors" because they influence the rate at
which ozone and other gases are created and
destroyed in the atmosphere. For convenience, all
gases discussed in this summary are generically
referred to as "greenhouse gases" (unless otherwise
noted), although the reader should keep these distinc-
tions in mind. In addition, emissions of sulfur dioxide
(SO2) are reported. Sulfur gases, primarily sulfur
dioxide, are believed to contribute negatively to the
greenhouse effect.
Recent Trends of U.S.
Greenhouse Gas Emissions
Although CO2, CH4 and N2O occur naturally in
the atmosphere, their recent atmospheric buildup
appears to be largely the result of anthropogenic
activities. This growth has altered the composition of
the Earth's atmosphere, and may affect future global
climate. Since 1800, atmospheric concentrations of
CO2 have increased by more than 25 percent, CH4
concentrations have more than doubled, and N2O
concentrations have risen approximately 8 percent
(IPCC, 1992). From the 1950s until the mid-1980s,
the use of CFCs increased by nearly 10 percent per
year. Now that CFCs are being phased out under the
Montreal Protocol on Substances that Deplete the
Ozone Layer (Montreal Protocol), the use of CFC
substitutes is expected to grow significantly.
The current U.S. greenhouse gas inventory for
1990-94 is summarized in Table ES-1 and Figures ES-
1, ES-2, and ES-3. For 1994, total U.S. emissions
were 1,666 MMTCE. To be consistent with the
IPCC-recommended guidelines, this estimate
excludes emissions of 23 MMTCE from international
transport. Changes in CO2 emissions from fossil fuel
consumption had the greatest impact on U.S. emis-
Box ES-!
The Global Warming Potential (GWP) Concept
As mentioned, gases can contribute to the greenhouse
effect both directly and indirectly. Direct effects occur when
the gas itself is a greenhouse gas. Indirect radiative forcing
occurs when chemical transformation of the original gas pro-
duces a gas or gases that are greenhouse gases, or when a gas
influences the atmospheric lifetimes of other gases. The con-
cept of Global Warming Potential (GWP) has been developed
to allow scientists and policy makers to compare the ability
of each greenhouse gas to trap heat in the atmosphere rela-
tive to another gas. The GWP of a greenhouse gas is the
ratio of global warming, or radiative forcing (both direct and
indirect), from one kilogram of a greenhouse gas to one kilo-
gram of carbon dioxide over a period of time. While any
time period can be selected, the 100-year GWPs recom-
mended by the IPCC are used in this report. Carbon diox-
ide was chosen as the "reference" gas to be consistent with
IPCC guidelines. Carbon comprises 12/44 of carbon dioxide
by weight In order to convert emissions reported in million
metric tonnes of a gas to MMTCE, the following equation is
used:
MMTCE = (MMT of gas) (GWP of gas) (12/44),
where
MMTCE = million metric tonnes, carbon-equivalent,
MMT = million metric tonnes, full molecular weight,
GWP = global warming potential, and
(12/44) = carbon to carbon dioxide molecular weight ratio.
GWPs are not provided for the photochemically impor-
tant gases CO, NOX, NMVOCs, and SO2 because there is no
agreed-upon method to estimate their contribution to cli-
mate change. These gases only affect radiative forcing indi-
rectly.
Gas
Carbon dioxide
Methane*
Nitrous oxide
HFC-23
HFC-125
HFC-l34a
HFC-IS2a
PFCs**
SF*
GWP1
(100 Years)
I
24.5
320
12,100
3,200
1,300
140
9,400
24,900
* The methane GWP includes the direct effect and those
indirect effects due to the production of tropospheric ozone
and stratospheric water vapor. The indirect effect due to the
production ofC02 is not included.
** This figure is an average GWP for the two PFCs, CF4 and
C2F6. The GWP for CF4 is 6,300 and the GWP for C2F6 is
12,500.
• IPCC, 1994
ES-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table ES-1
/RecfeiftTrendsiMiJ^
| Gas/Source
Emissions Emissions
*: *£ t, . *~ff(Full Molecular Weight) (Direct and Indirect Effects;
cL -*" * '^Jf^" ff ' "«*••* -*<• * -% Carbon-Equivalent)
rt ,s4' «?^,-*fo %S5?"&%--;'?,"1» * .^^^^(Million Metric Tonnes)
,, ?I Jfr^^**,t||1,^ j|92 j943' 1*9^4 " 1990 1991 1992 1993 1994
''Greenhouse Gases
Carbon Dioxide (CO2)
- Fossil Fuel Combustion
Other
Total
Forests (sink)
Net Total .'.'
I Methane (CH4)
Landfills
^Agriculture
l Mining
l and Gas Systems
4,899 4,839
62 6*1
4;96I 4,901
(458) (458)
4,503 4,443
4,914
62
4,976
(458)
4,518
5,020
64
5,084
NA
NA
0.2
O.I
O.I
0.4
*
0.2
"'"O.I"'"'
O.I
0.4
*
0.2
6.1
O.I
0.4
*
0.2
0^1
0.1
0.4
*
0.2
0.1
O.I
0.5
*
fe.;"."'. .Total'" "...
Citrous Oxide (N2°)
prAgriculture
pL Fossil Fuel Consumption
fe Industrial Processes
|F Total
FHFCs and PFCs
Photochemically Important
tGases
|r NOX 20.6 20.4 20.6
t NMVOC 18.7 18.3 18.2
t CO 83.4 82.7 81.6
I- '
21.0
18.2
81.3
U.S. Emissions
Net, Including Sinks
5,098
~ 63
5,161
NA
NA
9.9
8.4
4.4
3.2
0.9
27. 1
10.1
8.5
4.3
3.3
1.0
2t-3
9.9
8.8
4.1
3.3
1.0
27.2
10.0
8.8
3.7
3.2
0.9
* 26.7
10.2
9.2
4.3
3.3
0.9
28.0
21.2
18.6
83.1
1,336
17
1,353
(125)
1,228
1,320
17
1,336
(125)
1,211
1,340
17
1,357
(125)
1,232
1,369
18
1,387
NA
NA
1,390
17
1,408
NA
NA
66
56
29
22
6
181
67
57
28
22
7
1 82
66
59
27
22
7
182
67
59
24
22
6
179
68
61
29
22
6
188
16
12
8
37
18.8
6.4
17
12
9
37
19.3
6.5
17
12
8
37
21.1
6.7
17
12
9
38
19.8
6.8
19
12
9
41
23.5
7.0
1,595 jl,582 1,604 l,63a 1,666 :
1,470 1,457 1,479 NA NA |
:* As this category contains multiple gases, an aggregate full molecular weight sum is not calculated.
Total of this gas does not exceed 0.01 million metric tonnes.
= not available
Note: Totals may not equal the sum of the individual source categories due to independent rounding.
sions from 1990 to 1994. While these emissions of
CO2 in 1991 were approximately 1.2 percent lower
than 1990 emission levels in the U.S., in 1992 they
were about 1.5 percent over 1991 levels, thus return-
ing emissions to slightly over 1990 levels. By 1993
CO2 emissions from fossil fuel combustion were
approximately 2.5 percent greater than 1990, with
emissions in 1994 about 4 percent higher than 1990.
This trend is largely attributable to changes in total
energy consumption resulting from the economic
slowdown in the U.S. during the early 1990s and the
subsequent recovery, as can be clearly seen in Figure
ES-2.
Methane, N2O, and HFCs and PFCs represent a
much smaller portion of total emissions than CO2. In
most cases, emissions of these gases remained rela-
tively constant from 1990 to 1994. However,
methane emissions from coal mining declined signifi-
cantly in 1993, largely due to decreases in coal pro-
duction as a result of labor unrest in 1993. As coal
Executive Summary • ES-3
-------
Figure ES-2
SF6
All MFCs & PFCs
N20
CH4
C02
Sinks ore not included in these graphs.
production has risen since the end of the strikes, emis-
sions have increased commensurately. Also, emissions
of HFCs and PFCs have fluctuated significantly in the
1990s, initially declining in response to lower CFC
production. The use of these chemicals has begun to
increase, however; as replacements for CFCs and
other ozone-depleting compounds being phased out
under the terms of the Montreal Protocol and Clean
Air Act Amendments.
Figure ES-3 illustrates the relative contribution of
the primary greenhouse gases to total U.S. emissions
in 1994. Due largely to fossil fuel consumption, CO2
emissions accounted for the largest share of U.S.
emissions on a carbon equivalent basis — almost 85
percent. These emissions were partially offset by the
sequestration that occurred on forested lands.
Methane accounted for 11 percent of total emissions,
including contributions from landfills and agricul-
tural activities, among others.
The other gases contributed less to emissions, with
N2O emissions comprising about 2 percent of total
U.S. emissions, HFCs accounting for just over one per-
cent, PFCs about 0.2 percent, and SF6 about 0.4 per-
cent. Any gases covered under the Montreal Protocol
are not included because their use is being phased out,
and the IPCC Guidelines (IPCC/OECD/ffiA, 1995)
recommend excluding gases covered by the Montreal
Protocol.
The following sections present the anthropogenic
sources of greenhouse gas emissions, briefly discuss
the emission pathways, summarize the emission esti-
mates, and explain the relative importance of emis-
sions from each source category.
Figure ES-3
N2O HFC/PFC/ Net
SF* Emissions
* Sinks are not included here.
Fjp
ES-4 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Carbon Dioxide Emissions
The global carbon cycle is made up of large car-
bon flows and reservoirs. Hundreds of billions of tons
of carbon in the form of carbon dioxide (CO2) are
absorbed by oceans, trees, soil, and vegetative cover
and are emitted to the atmosphere annually through
natural processes. When in equilibrium, carbon flows
between the various reservoirs roughly balance each
other. Since the Industrial Revolution, however,
atmospheric concentrations of carbon dioxide have
risen more than 25 percent, principally because of the
combustion of fossil fuels (IPCC, 1992). While the
combustion of fossil fuels accounts for 99 percent of
total U.S. carbon dioxide emissions, carbon dioxide
emissions also result directly from industrial
processes. Changes in land use and forestry activities
both emit carbon dioxide (e.g., as a result of forest
clearing) and can act as a sink for carbon dioxide (e.g.,
as a result of improved forest management activities).
Table ES-2 summarizes U.S. emissions of carbon
dioxide for 1994, while the remainder of this section
presents detailed information on the various anthro-
pogenic sources and sinks of carbon dioxide in the
United States.
Energy
Approximately 88 percent of U.S. energy is pro-
duced through the combustion of fossil fuels. The
remaining 12 percent comes from renewable or other
energy sources such as hydropower, biomass, and
nuclear energy (see Figure ES-4). As they burn, fossil
fuels emit carbon dioxide due to oxidation of the car-
bon contained in the fuel. The amount of carbon in
fossil fuels varies significantly by fuel type. For exam-
ple, coal contains the highest amount of carbon per
unit of energy, while petroleum has about 20 percent
less carbon than coal, and natural gas has about 45
percent less.
FossiV Fuel Consumption
In 1994, the United States emitted a total of
1,390 MMTCE of carbon dioxide from fossil fuel
combustion. (Bunker fuels, or fuels used in interna-
tional transport, accounted for an additional 23
Table ES-2
CO2 Emissions CO2 Emissions ;
" (Molecular Basis) (Carbon-Equivalent)'
(Million Metric Tonnes) (
^Sources ,
issil Fuel Consumption ~;
.esidential 1,001 273 !
798 218
1,709 466
1,553 424
36 10
5,098 1,390
5.0 1.4
Commercial
Industrial
^-^Transportation
«==._ j^ r
A.J.S. Territories
"Tlbtal
Production
jJ Processing
^Cement Production
Hume Production
ilirtxestone Consumption
i.daAsTi Production
Consumption
35.5
12.7
4.6
4.0
9.7
3.5
1.2
The totals provided here do not reflect emissions from bunker
Tfuelsjj_sed in international transport activities. The INC 9th Session
nsjruaed countnes to report these emissions separately, and not to
ode them in national totals US. emissions from banker fuels
'ere approximately 23 MMTCE in 1994
Figure ES-4
i
Nuclear,
Renewabl
Other ,
11.7% '
Natural Gas
24.8%
Approximately 88 percent of U.S. energy is produced
through the combustion of fossil fuels.
MMTCE.) The energy-related activities producing
these emissions included heating in residential and
commercial buildings, the generation of electricity,
Executive Summary H ES-5
-------
steam production for industrial processes, and gaso-
line consumption in automobiles and other vehicles.
Petroleum products across all sectors of the economy
accounted for about 42 percent of total U.S. energy-
related carbon dioxide emissions; coal, 36 percent;
and natural gas, 22 percent.
Industrial Sector. The industrial sector accounts
for 34 percent of U.S. carbon dioxide emissions from
fossil fuel consumption, making it the largest end-use
source of carbon dioxide emissions (see Figure ES-5).
About two-thirds of these emissions result from the
direct consumption of fossil fuels in order to meet
industrial demand for steam and process heat. The
remaining one-third of industrial energy needs are
met by electricity for such uses as motors, electric fur-
naces and ovens, and lighting.
The industrial sector is also the largest user of
nonenergy applications of fossil fuels, which often
store carbon. Fossil fuels used for producing fertiliz-
ers, plastics, asphalt, or lubricants can store carbon in
products for very long periods. Asphalt used in road
construction, for example, stores carbon indefinitely.
Similarly, the fossil fuels used in the manufacture of
materials like plastics also store carbon, releasing this
carbon only if the product is incinerated.
Transportation Sector. The transportation sector
is also a major source of carbon dioxide, accounting
for just over 30 percent of U.S. emissions. Virtually
all of the energy consumed in this sector comes from
petroleum-based products. Nearly two-thirds of the
emissions are the result of gasoline consumption in
automobiles and other vehicles, with other uses,
including diesel fuel for the trucking industry and jet
fuel for aircraft, accounting for the remainder.
Residential and Commercial Sectors. The resi-
dential and commercial sectors account for about 20
and 16 percent, respectively, of carbon dioxide emis-
sions from fuel consumption. Both sectors rely heav-
ily on electricity for meeting energy needs, with about
two-thirds to three-quarters of their emissions attrib-
utable to electricity consumption. End-use applica-
tions include lighting, heating, cooling, and operating
appliances. The remaining emissions are largely due
to the consumption of natural gas and oil, primarily
for meeting heating and cooking needs.
Figure ES-5
..by End-Use Sector
500 --——
by Sector and Fuel Type
500
Commercial Residential Industrial Transportation
Commercial Residential Industrial Transportation Utilities
Energy Sectors
oil • Coal T Gas
End-Use Sectors
Emissions from Direct g| Emissions
Fuel Consumption ™* from Electricity
In this graph, emissions generated by electric utilities are allocated to
each end-use sector according to each sector's share of electricity
consumption.
However, when the emissions attributable to electric utilities are
pulled out of the various end-use sectors, as depicted in this graph
U.S. reliance on electricity is evident.
ES-6 II Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Electric Utilities. The U.S. relies on electricity to
meet a significant portion of its energy requirements.
In fact, as the largest consumers of fossil fuels, electric
utilities are collectively the largest producers of U.S.
carbon dioxide emissions (see Figure ES-5). Electric
utilities generate electricity for uses such as lighting,
heating, electric motors, and air conditioning. Some
of this electricity is generated with the lowest carbon
dioxide-emitting energy technologies, particularly
nonfossil options, such as nuclear energy,
hydropower, or geothermal energy. However, electric
utilities rely on coal for 55 percent of their total
energy requirements and account for about 86 per-
cent of all coal consumed in the United States.
Fuel Production and Processing
Carbon dioxide is produced via flaring activi-
ties at natural gas systems and oil wells. Typically,
the methane that is trapped in a natural gas system
or oil well is flared to relieve the pressure building
in the system or to dispose of small quantities of gas
that are not commercially marketable. As a result,
the carbon contained in the methane becomes oxi-
dized and forms carbon dioxide. In 1994, the
amount of carbon dioxide from the flared gas was
just over 1 MMTCE, or about 0.1 percent of total
U.S. carbon dioxide emissions.
Biomass and Biomass-Based Fuel Consumption
Biomass fuel is used primarily by the industrial
sector in the form of fuelwood and wood waste.
Biomass-based fuel use, such as ethanol from corn or
woody crops, occurs mainly in the transportation sec-
tor. Ethanol and ethanol blends, such as gasohol, are
typically used to fuel public transport vehicles, such
as buses or centrally fueled fleet vehicles.
Biomass, ethanol, and ethanol-blend fuels do
release carbon dioxide. However, in the long run, the
carbon dioxide they emit does not increase total
atmospheric carbon dioxide because the biomass
resources are consumed on a sustainable basis. For
example, fuelwood burned one year but regrown the
next only recycles carbon, rather than creating a net
increase in total atmospheric carbon.
Carbon dioxide emissions from biomass con-
sumption in 1994 were approximately 49 MMTCE,
with the industrial sector accounting for 75 percent
of the emissions and the residential sector 23 per-
cent, the rest being made up of commercial and elec-
tric utility consumption. Carbon dioxide emissions
from ethanol use in the United States have been
increasing in recent years due to a number of factors,
including the extension of Federal tax exemptions
for ethanol production, the Clean Air Act
Amendments mandating the reduction of mobile
source emissions, and the Energy Policy Act of 1992
which established incentives to increase the use of
alternative fuels and alternative-fueled vehicles. In
1994, total U.S. carbon dioxide emissions from
ethanol were 1.85 MMTCE.
Industrial Processes
Emissions are often produced as a by-product of
various nonenergy-related activities. For example, in
the industrial sector raw materials are chemically
transformed from one state to another. This transfor-
mation often releases such greenhouse gases as car-
bon dioxide. The production processes that emit
carbon dioxide include cement production, lime pro-
duction, limestone consumption (e.g., in iron and
steel production), soda ash production and use, and
carbon dioxide manufacture. Total carbon dioxide
emissions from these sources were approximately 16
MMTCE in 1994, accounting for about 1 percent of
total U.S. carbon dioxide emissions.
Cement Production (9.6 MMTCE)
Carbon dioxide is produced primarily during
the production of clinker, an intermediate product
from which finished portland and masonry cement
are made. Specifically, carbon dioxide is created
when calcium carbonate (CaCO3) is heated in a
cement kiln to form lime and carbon dioxide. This
lime combines with other materials to produce
clinker, while the carbon dioxide is released into the
atmosphere. Since 1990, carbon dioxide emissions
from cement production have increased about 8.4
percent, from 8.9 MMTCE in 1990 to 9.6 MMTCE
in 1994.
Executive Summary • ES-7
-------
Lime Production (3.5 MMTCE)
Lime is used in steel making, construction, pulp
and paper manufacturing, and water and sewage
treatment. It is manufactured by heating limestone
(mostly calcium carbonate) in a kiln, creating calcium
oxide (quicklime) and carbon dioxide, which is nor-
mally emitted to the atmosphere. Since 1990, carbon
dioxide emissions from lime production have
increased by approximately 7 .percent, from 3.3
MMTCE in 1990 to 3.5 MMTCE in 1994.
L/mestone Consumption (1.2 MMTCE)
Limestone is a basic raw material used by a wide
variety of industries, including the construction, agri-
culture, chemical, and metallurgical industries. For
example, limestone can be used as a purifier in refin-
ing metals, such as iron. In this case, limestone heated
in a blast furnace reacts with impurities in the iron
ore and fuels, generating carbon dioxide as a by-
product. It is also used in flue gas desulfurization sys-
tems to remove sulfur dioxide from the exhaust gases.
Since 1990, carbon dioxide emissions from limestone
consumption have declined by about 10 percent,
from 1.38 MMTCE in 1990 to 1.24 MMTCE in
1994.
Soda Ash Production and Consumption (I.I MMTCE)
Commercial soda ash (sodium carbonate) is used
in many consumer products, such as glass, soap and
detergents, paper, textiles, and food. During the man-
ufacturing of these products, natural sources of
sodium carbonate are heated and transformed into a
crude soda ash, in which carbon dioxide is generated
as a by-product. In addition, carbon dioxide is
released when the soda ash is consumed. Of the two
states that produce natural soda ash, only Wyoming
has net emissions of carbon dioxide, because produc-
ers in California recover the carbon dioxide and use it
in other stages of production. U.S. carbon dioxide
emissions from soda ash production were approxi-
mately 0.4 MMTCE in 1994, while U.S. soda ash con-
sumption generated about 0.7 MMTCE. Since 1990,
carbon dioxide emissions from soda ash manufacture
and consumption have declined slightly, from 1.13
MMTCE in 1990 to 1.10 MMTCE in 1994.
Carbon Dioxide Manufacture (0.4 MMTCE)
Carbon dioxide is used in many segments of the
economy, including food processing, beverage manu-
facturing, chemical processing, crude oil products.,
and a host of industrial and miscellaneous applica-
tions. For the most part, carbon dioxide used in these
applications will eventually be released into the
atmosphere. Since 1990, carbon dioxide emissions
from carbon dioxide manufacture have increased
slightly, from 0.33 MMTCE in 1990 to 0.37
MMTCE in 1994.
Forests and Land Use Change
When humans use and alter the biosphere
through changes in land use and forest-management
activities, they alter the natural balance of trace gas
emissions and uptake. These activities include clear-
ing an area of forest to create cropland or pasture,
restocking a logged forest, draining a wetland, or
allowing a pasture to revert to a grassland or forest..
Forests, which cover about 737 million acres of U.S.
land (Powell, et al., 1993), are a potentially important
terrestrial sink for carbon dioxide. Because approxi-
mately half the dry weight of wood is carbon, as trees
add mass to trunks, limbs, and roots, carbon is stored
in relatively long-lived trees instead of being released
to the atmosphere. Soils and vegetative cover also
provide a potential carbon sink.
Carbon fluxes can also be attributed to biomass
that is harvested and used in wood products or dis-
posed in landfills. The potential carbon flux associ-
ated with these biomass pools, however, is
significantly smaller than the carbon flux associated
with forests. Therefore, the majority of this discus-
sion focuses on the carbon flux associated with land-
use change and forest management activities.
In the United States, improved forest-manage-
ment practices and the regeneration of previously
cleared forest area have actually resulted in a net
uptake (sequestration) of carbon on U.S. lands. This
carbon uptake is an ongoing result of land-use
changes in previous decades. For example, because of
improved agricultural productivity and the wide-
spread use of tractors, the rate of clearing forest land
for crop cultivation and pasture slowed greatly in the
ES-8 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
late 19th century, and by 1920 this practice had all
but ceased. As farming expanded in the Midwest and
West, large areas of previously cultivated land in the
East were brought out of crop production, primarily
between 1920 and 1950, and were allowed to revert
to forest land or were actively reforested. The regen-
eration of forest land greatly increases carbon storage
in both standing biomass and soils and the impacts of
these land-use changes continue to affect forest car-
bon fluxes in the East. In addition to land-use
changes in the early part of this century, forest carbon
fluxes in the East are affected by a trend toward man-
aged growth on private land in recent decades, result-
ing in a near doubling of the biomass density in
eastern forests since the early 1950s. More recently,
the 1970s and 1980s saw a resurgence of federally
sponsored tree-planting programs (e.g., the Forestry
Incentive Program) and soil conservation programs
(e.g., the Conservation Reserve Program), which have
focused on reforesting previously harvested lands,
improving timber-management activities, combating
soil erosion, and converting marginal cropland to
forests.
The net carbon dioxide flux in 1990, 1991 and
1992 due to these activities is estimated to have been
an uptake (sequestration) of 125 MMTCE per year.
This carbon uptake represents an offset of about 9
percent of the average annual carbon dioxide emis-
sions from energy-related activities during this
period. Emission estimates are not yet available for
1993 and 1994 because the last national forest inven-
tory was completed in 1992.
There are several major sources of uncertainty
associated with the estimates of the total net carbon
flux from U.S. forests. These sources are briefly
described below:
D The forest surveys used to compile these estimates
are based on a statistical sampling instead of actual
measurements. The surveys are based on a statisti-
cal sample designed to represent a wide variety of
growth conditions present over large territories.
The actual values of carbon stored in forests,
therefore, are represented by average values that
are subject to sampling and estimation errors.
The impacts of forest management activities on
soil carbon are quite uncertain. Forest soils and
forest floors contain over 60 percent of the total
U.S. forest carbon. However, because of uncertain-
ties associated with soil and forest floor carbon
fluxes, these components are not included in the
U.S. estimate at this time.
The current estimate does not include forest land
in Alaska and Hawaii or reserved timber land.
However, forests in these states are believed to be
in equilibrium, so their inclusion would not signif-
icantly affect the flux estimates presented here.
Forest management activities may also result in
fluxes of other greenhouse and photochemically
important gases. Dry soils are an important sink
for CH4, a source of N2O, both a sink and a
source for CO, and vegetation is a source of sev-
eral NMHCs (nonmethane hydrocarbons, a subset
of NMVOCs). However, the effects of forestry
activities on these gases are highly uncertain, and
are therefore not included in the U.S. inventory at
this time.
Estimates from wood products pools and landfills
are based on limited data and subject to significant
uncertainties. Research continues on the potential
magnitude of these sources.
Methane Emissions
Atmospheric methane (CH4) is second only to
carbon dioxide as an anthropogenic source of green-
house gas emissions. Methane's overall contribution
to global warming is large because it is 24.5 times
more effective at trapping heat in the atmosphere
than carbon dioxide over a 100-year time horizon,
when the direct as well as most indirect effects are
considered (IPCC, 1994). Furthermore, methane's
concentration in the atmosphere has more than dou-
bled over the last two centuries. Scientists have con-
cluded that these atmospheric increases are largely
due to increasing emissions from anthropogenic
sources, such as landfills, agricultural activities, fossil
fuel combustion, coal mining, the production and
processing of natural gas and oil, and wastewater
treatment (see Table ES-3 and Figure ES-6).
Executive Summary H ES-9
-------
Landfills
Landfills are the largest single anthropogenic
source of methane emissions in the United States.
There are an estimated 6,000 methane-emitting land-
fills in the United States, with 1,300 of the largest
landfills accounting for about half of the emissions.
In an environment where the oxygen content is
low or nonexistent, organic materials, such as yard
waste, household waste, food waste, and paper, are
decomposed by bacteria to produce methane, carbon
dioxide, and stabilized organic materials (materials
that cannot be decomposed further). Methane emis-
sions from landfills are affected by such factors as
Table ES-3
Source
CH4
(Molecular Basis)
Landfills
Agriculture
Coal Mining
Oil and Natural
Gas Systems
Fossil Fuel Combustion
: Wastewater Treatment
CH4
(Carbon-Equivalent;
GWP=24.S) I
(Million Metric Tonnes) '}
10.2 68.2 ^
9.2 61.5 ]
4.3* 28.9* 1
3.3* 22.1*
0.9*
0.2
6.0*
.0
* Pre/iminory estimate
Figure ES-6
waste composition, moisture, and landfill size.
Methane emissions from U.S. landfills in 1994
were 68.2 MMTCE, or about 36 percent of total U.S.
methane emissions. Emissions from U.S. municipal
solid waste landfills, which received approximately
67 percent of the total solid waste generated in the
United States, accounted for about 90 to 95 percent
of total landfill emissions, while industrial landfills
accounted for the remaining 5 to 10 percent.
Currently, about 15 percent of the methane emitted is
recovered for use as an energy source.
Agriculture
The agricultural sector accounted for approxi-
mately 33 percent of total U.S. methane emissions in
1994, with enteric fermentation in domestic livestock
and manure management together accounting for the
majority (see Figure ES-7). Other agricultural activi-
ties contributing directly to methane emissions
include rice cultivation and field burning of agricul-
tural crop wastes. Several other agricultural activities,
such as irrigation and tillage practices, may con-
tribute to methane emissions, but emissions from
these sources are uncertain and believed to be small;
therefore, the United States has not included them in
the current inventory. Details on the emission path-
ways included in the inventory are presented below.
Figure ES-7
Fossil Fuel
Consumption
3.3%
Agricultural
Waste
Burning
1.3%
Agriculture
32.7%
Enteric Fermentation
Wastewater *
0.6%
Landfills and agriculture are the largest sources of atmos-
pheric methane in the United States.
ES-IO • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Enteric Fermentation in Domestic Livestock (40,2 MMTCE)
In 1994, enteric fermentation was the source of
about 21 percent of total U.S. methane emissions,
and about 65 percent of methane emissions from the
agricultural sector. During animal digestion, methane
is produced through enteric fermentation, a process
in which microbes that reside in animal digestive sys-
tems break down the feed consumed by the animal.
Ruminants, which include cattle, buffalo, sheep, and
goats, have the highest methane emissions among all
animal types because they have a rumen, or large
"fore-stomach," in which a significant amount of
methane-producing fermentation occurs. Non-
ruminant domestic animals, such as pigs and horses,
have much lower methane emissions than ruminants
because much less methane-producing fermentation
takes place in their digestive systems. The amount of
methane produced and excreted by an individual ani-
mal also depends upon the amount and type of feed
it consumes.
Manure Management (17.0 MMTCE)
The decomposition of organic animal waste in an
anaerobic environment produces methane. The most
important factor affecting the amount of methane
produced is how the manure is managed, since cer-
tain types of storage and treatment systems promote
an oxygen-free environment. In particular, liquid sys-
tems (e.g., lagoons, ponds, tanks, or pits) tend to pro-
duce a significant quantity of methane. However,
when manure is handled as a solid or when it is
deposited on pastures and rangelands, it tends to
decompose aerobically and produce little or no
methane. Higher temperatures and moist climate
conditions also promote methane production.
Emissions from manure management were about
9 percent of total U.S. methane emissions in 1994,
and about 28 percent of methane emissions from the
agricultural sector. Liquid-based manure manage-
ment systems accounted for over 80 percent of total
emissions from animal wastes.
R/ce Cultivation (3.4 MMTCE)
Most of the world's rice, and all of the rice in the
United States, is grown on flooded fields. When fields
are flooded, anaerobic conditions in the soils develop,
and methane is produced through anaerobic decom-
position of soil organic matter. Methane is released
primarily through the rice plants, which act as con-
duits from the soil to the atmosphere.
Rice cultivation is a very small source of methane
in the United States. In 1994, methane emissions
from this source were less than 2 percent of total U.S.
methane emissions, and about 5.6 percent of U.S.
methane emissions from agricultural sources.
Field Burning of Agricultural Wastes (0.8 MMTCE)
Large quantities of agricultural crop wastes are
produced from farming systems. Disposal systems
for these wastes include plowing them back into the
field; composting, landfilling, or burning them in
, the field; using them as a biomass fuel; or selling
them in supplemental feed markets. Burning crop
residues releases a number of greenhouse gases,
including carbon dioxide, methane, carbon monox-
ide, nitrous oxide, and oxides of nitrogen. Crop
residue burning is not considered to be a net source
of carbon dioxide emissions because the carbon
dioxide released during burning is reabsorbed by
crop regrowth during the next growing season.
However, burning is a net source of emissions for
the other gases. Because this practice is not common
in the United States, it was responsible for only
about 0.4 percent of total U.S. methane emissions in
1994, and 1.3 percent of emissions from the agri-
cultural sector.
Coal Mining
Coal mining and post-mining activities, such as
coal processing, transportation, and consumption,
are the third largest source of methane emissions in
the United States. Estimates of methane emissions
from coal mining for 1994 were 28.9 MMTCE,
which accounted for about 15 percent of total U.S.
methane emissions.
Produced millions of years ago during the forma-
tion of coal, methane is trapped within coal seams
and surrounding rock strata. When coal is mined,
methane is released into the atmosphere. The amount
of methane released from a coal mine depends pri-
Executive Summary H ES-11
-------
marily upon the depth and type of coal, with deeper
mines generally emitting more methane (U.S. EPA,
1993a). Methane from surface mines is emitted
directly to the atmosphere as the rock strata overly-
ing the coal seam are removed.
Methane is hazardous in underground mines
because it is explosive at concentrations of 5 to 15
percent in air. Therefore, all underground mines are
required to remove methane by circulating large
quantities of air through the mine and venting this
air into the atmosphere. At some mines, more
advanced methane-recovery systems may be used to
supplement the ventilation systems and ensure mine
safety. The practice of using the recovered methane
as an energy source has been increasing in recent
years.
Oil and Natural Gas
Production and Processing
Methane is also the major component of natural
gas. Any leakage or emission during the production,
processing, transmission, and distribution of natural
gas emits methane directly to the atmosphere.
Because natural gas is often found in conjunction
with oil, leakage during the production of commer-
cial quantities of gas from oil wells is also a source of
emissions. Emissions vary greatly from facility to
facility and are largely a function of operation and
maintenance procedures and equipment condition.
Fugitive emissions can occur at all stages of extrac-
tion, processing, and distribution. In 1994, emissions
from the U.S. natural gas system were estimated to be
20.3 MMTCE, accounting for approximately 11 per-
cent of total U.S. methane emissions.
Methane is also released as a result of oil pro-
duction and processing activities, such as crude oil
production, crude oil refining, transportation, and
storage, when commercial gas production is not
warranted due to the small quantities present.
Emissions from these activities are generally
released as a result of system leaks, disruptions, or
routine maintenance. For 1994, methane emissions
from oil production and processing facilities were
1.8 MMTCE, accounting for about 1 percent of
total U.S. methane emissions.
Other Sources
Methane is also produced from several other
sources in the United States, including energy-related
combustion activities, wastewater treatment, indus-
trial processes, and changes in land use. The sources
included in the U.S. inventory are fossil fuel combus-
tion and wastewater treatment. In 1994, 6.1
MMTCE of methane were emitted from fossil fuel
combustion, which accounted for about 3.3 percent
of total U.S. methane emissions. Approximately 1.1
MMTCE, or less than 1 percent of total U.S. methane
emissions, were emitted due to wastewater treatment.
Additional anthropogenic sources of methane in the
United States, such as land use changes and ammo-
nia, coke, iron, and steel production, are not included
because little information on methane emissions from
these sources is currently available.
Nitrous Oxide Emissions
Nitrous oxide (N2O) is a chemically and radia-
tively active greenhouse gas that is produced natu-
rally from a wide variety of biological sources in soil
and water. While actual emissions of nitrous oxide
are much smaller than carbon dioxide emissions,
nitrous oxide is approximately 320 times more pow-
erful than carbon dioxide at trapping heat in the
atmosphere over a 100-year time horizon.
Over the past two centuries, human activities
have raised atmospheric concentrations of nitrous
oxide by approximately 8 percent. The main anthro-
pogenic activities producing nitrous oxide are soil
management and fertilizer use for agriculture, fossil
fuel combustion, adipic acid production, nitric acid
production, and agricultural waste burning. The rel-
ative share of each of these activities to total U.S.
nitrous oxide emissions is shown in Figure ES-8, and
U.S. nitrous oxide emissions by source category for
1994 are provided in Table ES-4.
Agricultural Soil Management
and Fertilizer Use
The primary sources of anthropogenic nitrous
oxide emissions in the United States are fertilizer use
and soil management activities. Synthetic nitrogen
ES-12 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table ES-4
Figure ES-8
N2O N2O
'^(Molecular Basis) (Carbon-Equivalent,
^ * GWP=*320) **
v, - „ , " i
f , f(Millioji Metric Tonnes)
0.21 18.4
Fossil Fuel
Consumption
30.7%
_
gncultural Soil
anagement and
Fertilizer Use
014
0.06*
004
0.005
ossil Fuel Consumption
!5ipic Acid Production
itncAqd Production
[cultural Waste
Burning
Agricultural Soils
45.4%
Agricultural
Waste Burning
.1%
fertilizers and organic fertilizers add nitrogen to soils,
and thereby increase emissions of nitrous oxide.
Nitrous oxide emissions in 1994 due to consumption
of synthetic and organic fertilizers were 18.4
MMTCE, or approximately 45 percent of total U.S.
nitrous oxide emissions.
Other agricultural soil management practices,
such as irrigation, tillage practices, or the fallowing of
land, can also affect nitrous oxide fluxes to and from
the soil. There is much uncertainty about the direc-
tion and magnitude of the effects of these other prac-
tices. Only emissions from fertilizer use and field
burning of agricultural wastes are included in the U.S.
inventory.
Fossil Fuel Combustion
Nitrous oxide is a product of the reaction that
occurs between nitrogen and oxygen during fossil fuel
combustion. Both mobile and stationary sources emit
nitrous oxide. Emissions from mobile sources are
more significant and are better understood than those
from stationary sources. The amount of nitrous oxide
emitted varies, depending upon fuel, technology type,
and pollution control device. Emissions also vary with
the size and vintage of the combustion technology, as
well as maintenance and operation practices.
For example, catalytic converters installed to
reduce air pollution resulting from motor vehicles
have been proven to promote the formation of
nitrous oxide. As catalytic converter-equipped vehi-
cles have increased in the U.S. motor vehicle fleet,
emissions of nitrous oxide from this source have also
increased (EIA, 1994d). Mobile emissions totaled 9.3
MMTCE in 1994 (23 percent of total nitrous oxide
emissions), with road transport accounting for
approximately 95 percent of these nitrous oxide
emissions. Nitrous oxide emissions from stationary
sources were 3.2 MMTCE in 1994.
AdipicAcid Production
Nitrous oxide is emitted as a by-product of the
production of adipic acid. Ninety percent of all adipic
acid produced in the United States is used to produce
nylon 6,6. It is also used to produce some low-tem-
perature lubricants, and to provide foods with a
"tangy" flavor. In 1994, U.S. adipic acid production
generated 5.4 MMTCE of nitrous oxide, or 13 per-
cent of total U.S. nitrous oxide emissions.
Nitric Acid Production
Production of nitric acid is another industrial
source of nitrous oxide emissions. Nitric acid is a raw
material used primarily to make synthetic commercial
fertilizer, and is also a major component in the pro-
duction of adipic acid and explosives. Virtually all of
the nitric acid that is manufactured commercially in
the United States is obtained by the oxidation of
ammonia. During this process, nitrous oxide is
formed and emitted to the atmosphere. Nitrous oxide
Executive Summary • ES-13
-------
emissions from this source were about 3.8 MMTCE
in 1994, accounting for about 9 percent of total U.S.
nitrous oxide emissions.
Other Sources of Nitrous Oxide
Other activities that emit nitrous oxide include
the burning of agricultural crop residues and changes
in land use. Emissions from agricultural crop residue
burning are extremely small relative to overall U.S.
nitrous oxide emissions. Nitrous oxide emissions in
1994 from this source were approximately 0.4
MMTCE, or about 1 percent of total U.S. nitrous
oxide emissions.
Forestry activities may also result in fluxes of
nitrous oxide, since dry soils are a source of nitrous
oxide emissions. However, the effects of forestry
activities on fluxes of these gases are highly uncertain;
therefore, they are not included in the inventory at
this time. Similarly, the U.S. inventory does not
account for several land-use changes because of
uncertainties in their effects on trace gas fluxes, as
well as poorly quantified land-use change statistics.
These land-use changes include loss and reclamation
of freshwater wetland areas, conversion of grasslands
to pasture and cropland, and conversion of managed
lands to grasslands.
Table ES-5
Compound
HFCs
HFC-23
HFC-125
HFC-l34a
HFC-IS2a
HFC-227
PFCs
SF.
Molecular
Basis
GWP
(Million Metric Tonnes)
0.00418 12,100
0.00113 3,200
0.01041 1,300
0.00153 140
0.00089 3,300
0.00200 6,300
0.00020 12,500
0.00103 24,900
Source: Abseck(l 995;
In 1994, the use of substitutes for ODS was minimal. Thus, emissions of HFCs were quite small,
and were largely tfie result of by-product emissions from the production of HCFC-22. PFC emis-
sions were the result of aluminum smelting activities.
HFC, RFC, and SF6 Emissions
Emissions of hydrofluorocarbon (HFC) and per-
fluorocarbon (PFC) chemicals occur for three rea-
sons. First, these chemicals were introduced as
alternatives to the ozone-depleting substances (ODS)
under phaseout by the Montreal Protocol and Clean
Air Act Amendments of 1990. Second, some of the
HFCs and PFCs are emitted as by-products of indus-
trial reactions. Third, some manufacturing proce-
dures employ these chemicals intentionally.
As substitutes for ODSs, HFCs and PFCs do not
dkectly harm the stratospheric ozone layer, but they
are powerful greenhouse gases. In many cases, HFCs
and PFCs absorb much more radiation than equiva-
lent amounts of carbon dioxide. For this reason, their
emissions are addressed by the Framework
Convention on Climate Change (FCCC). An example
of an ODS substitute with a high global warming
potential (GWP) is HFC-134a, with a GWP of 1,300
over a 100 year time horizon. Emissions of HFC-
134a reached 3.7 MMTCE in 1994. Other HFCs
included in the Inventory are HFC-125, HFC-152a,
and HFC-227; their emissions are listed in Table ES-
5. From 1990 to 1994, the use of CFC substitutes has
grown primarily due to HFC-134a use in automobile
air conditioners. Emissions of HFCs
and PFCs as ODS substitutes are
expected to rise.
Emissions of HFCs and PFCs
also occur as by-products of indus-
trial reactions. HFC-23 is produced
and emitted as a by-product of
HCFC-22 production; 1994 HFC-
23 emissions were estimated to be
13.8 MMTCE. The PFCs, CF4 and
C2F6, were emitted as by-products
of aluminum smelting; 1994 CF4
and C2F6 emissions reached 3.4
MMTCE and 0.7 MMTCE, respec-
tively.
Sulfur hexafluoride (SF6) use
occurs primarily in electrical trans-
mission and distribution systems
where it serves as a dielectric and
Carbon
Equivalent
13.80
0.99
3.69
0.06
'(XSO
3.43
0.68
6.96
ES-I4 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Box ES-2
Emissions of CFCs and Related Compounds
Chlorofluorocarbons (CFCs) and other halogenated com-
pounds were first emitted into the atmosphere this cen-
tury. This family of man-made compounds includes
Chlorofluorocarbons, halons, methyl chloroform, carbon
tetrachloride, methyl bromide, and hydrochlorofluorocar-
bons (HCFCs). These substances are used in a variety of
industrial applications, including foam production and
refrigeration, air conditioning, solvent cleaning, steriliza-
tion, fire extinguishing, paints, coatings, other chemical
intermediates, and miscellaneous uses (e.g., aerosols, pro-
pellants and other products).
Because these compounds have been shown to
deplete stratospheric ozone, they are typically referred to
as ozone depleting substances, or ODSs. In addition, they
are important greenhouse gases because they block
infrared radiation that would otherwise escape into space
(IPCC, 1990).
Recognizing the harmful effects of these compounds
on the atmosphere, in 1987 many governments signed the
Montreal Protocol on Substances that Deplete the Ozone
Layer to limit the production and consumption of a num-
ber of CFCs and other halogenated compounds. As of
August 1995, 149 countries have signed the Montreal
Protocol. The United States furthered its commitment to
phase out these substances by signing and ratifying the
Copenhagen Amendments to the Montreal Protocol in
1992. Under these amendments, the United States com-
mitted to eliminating the production of all halons by
January 1, 1994, and all CFCs by January 1, 1996.
The IPCC Guidelines do not include reporting emissions
of CFCs and related compounds because their use is
being phased out by the Montreal Protocol. The United
States believes that no inventory is complete without
these emissions; therefore, emission estimates for several
Class 1 and Class II ODSs are provided in the table below.
Compounds are classified as "Class I" or "Class II" sub-
stances based on their ozone-depletion potential, and
must adhere to a distinct set of phase out requirements
under the Montreal Protocol. Class I compounds are the
primary ODSs in use today; Class II compounds include
partially halogenated chlorine compounds (known as
HCFCs), some of which were developed as interim
replacements for CFCs. Because these HCFC com-
pounds are only partially halogenated, their hydrogen-car-
bon bonds are more vulnerable to oxidation in the
troposphere, and therefore pose only about one-tenth to
one-hundredth the threat to stratospheric ozone com-
pared to CFCs. Also, it should be noted that the effects
of these compounds on radiative forcing are not provided
here. Although CFCs and related compounds have very
large direct GWPs, their indirect effects are believed to be
negative and, therefore, could significantly reduce the mag-
nitude of their direct effects (IPCC, 1992). Given the
uncertainties surrounding the net effect of these gases,
they are reported here on a full molecular weight basis
only.
U.S. Emissions of Ozone-Depleting Substances and Relate'd Compounds: 1994
Compound Emissions
(Million Metric Tonnes;
Molecular Weight)
Class I ODSs
CFC-II 0.037
CFC-12 0.059
CFC-II3 0.017
CFC-114 0.005
CFC-II 5 0.003
Carbon Tetrachloride 0.016
Methyl Chloroform 0.078
Halon-1211 0.001
Halon-1301 0.002
Compound
Class II ODSs
HCFC-22
HCFC-123
HCFC-124
HCFC-141 b
HCFC-l42b
Source: Abseck (1995)
Emissions
(Million Metric Tonnes;
Molecular Weight)
0.105
0.002
0.002
0.016
0.010
insulator in circuit breakers, gas-insulated substa-
tions, and related equipment. Emissions occur from
this use due to older, leaky equipment, improper
maintenance, or intentional venting of the gas. The
metals industries also employ SF6 in degassing and
magnesium protection. For this latter use, SF6 pro-
tects molten metal from catastrophic oxidation, a
process which emits most or all of the chemical.
Overall emissions will likely grow if the need for
magnesium in alloys increases as expected. In 1994,
emissions of SF6 reached 7.0 MMTCE.
Chlorofluorocarbons (CFCs) and other halocar-
bons, which were emitted into the atmosphere for the
first time this century, have been shown to deplete
Executive Summary • ES-15
-------
stratospheric ozone, and thus are typically referred to
as ozone-depleting substances, or ODSs. Emission
estimates for several ODSs are provided in Box ES-2.
The growing semiconductor industry emits such
greenhouse gases as CF4, C2F6, NF3, SF6, C3F8, and
HFC-23 due to use in plasma etching and chemical
cleaning applications. Emissions of these gases in the
semiconductor industry are expected to grow.
Criteria Pollutant Emissions
Carbon monoxide (CO), nitrogen oxides (NOX),
nonmethane volatile organic compounds (NMVOCs),
and sulfur dioxide (SO2) are commonly referred to in
the United States as "criteria pollutants".1 Carbon
monoxide is created when carbon-containing fuels
are burned incompletely; oxides of nitrogen, NO and
NO2, are created from lightning, biomass fires, fossil-
fuel combustion, and in the stratosphere from nitrous
oxide (N2O); NMVOCs include compounds such as
propane, butane, and ethane, and are emitted pri-
marily from transportation and industrial processes,
as well as biomass burning, and nonindustrial con-
sumption of organic solvents (U.S. EPA, 1990b); SO2
can result from the combustion of fossil fuels, indus-
trial processing (particularly in the metals industry),
waste incineration, and biomass burning (U.S. EPA,
Table ES-6
Source
Fossil Fuel Combustion
3ndustr!al Processes
Fossil Fuel Production,
0.35
.09
Distribution, and Storage
', ,\.i
iaste
Agricultural Waste Burning
t j, ,._*,
^Solvent Use
1993b).
Because of their contribution to the formation of
urban smog, criteria pollutants are regulated under
the 1970 Clean Air Act and successive amendments.
These gases also have an impact on global climate,
although their impact is limited because their radia-
tive effects are indirect (i.e., they do not directly act as
greenhouse gases, but react with other chemical com-
pounds in the atmosphere). It should be noted, how-
ever, that SO2 emitted into the atmosphere affects the
Earth's radiative budget negatively; therefore, it is dis-
cussed separately from the other criteria pollutants
(see Box ES-3).
The most important of the indirect effects of the
criteria pollutants — CO, NOX and NMVOCs — is
their role as precursors of tropospheric ozone. In this
role, they contribute to ozone formation and alter the
atmospheric lifetimes of other greenhouse gases. For
example, CO interacts with the hydroxyl radical
(OH) — the major atmospheric sink for CH4 — to
form CO2. Therefore, increased atmospheric concen-
trations of CO limit the number of OH compounds
available to destroy CH.4, thus increasing its atmos-
pheric lifetime.
These criteria pollutants are generated through a
variety of anthropogenic activities, including fossil
fuel combustion, solid waste incineration, oil and gas
production and processing, indus-
trial processes and solvent use, and
agricultural crop waste burning.
Table ES-6 summarizes U.S. emis-
sions from these sources for 1994.
The United States has annually
published estimates of criteria pol-
lutants since 1970. Table ES-6
clearly shows that fuel consump-
tion accounted for the majority of
emissions of these gases. In fact,
motor vehicles that burn fossil fuels
comprise the single largest source of
CO emissions in the United States,
contributing nearly 90 percent of
,0=57,
1 The term criteria pollutant refers to those compounds for which attainment criteria have been established under the Clean Air Act
Amendments of 1970. NO, NOX, NMVOCs, and SO2 all have air quality standards for which air quality criteria have been issued.
ES-16 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
all U.S. CO emissions in 1994. Motor vehicles also
emit about half of total U.S. NOX and NMVOC
emissions. Industrial processes, such as the manufac-
ture of chemical and allied products, metals process-
ing, and industrial uses of solvents, are also major
sources of CO, NOX and NMVOCs.
Box ES-3
Sulfur- Dioxide: Effect on Radiative Forcing and Sources of Emissions
Sulfur dioxide (SO2) emitted into the atmosphere
through natural and anthropogenic processes affects the
Earth's radiative budget through photochemical transfor-
mation into sulfate particles that I) scatter sunlight back to
space, thereby reducing the radiation reaching the Earth's
surface; 2) possibly increase the number of cloud conden-
sation nuclei, thereby potentially altering the physical char-
acteristics of clouds, and 3) affect atmospheric chemical
composition, e.g., stratospheric ozone, by providing sur-
faces for heterogeneous chemical processes. As a result
of these activities, the effect of sulfur dioxide on radiative
forcing may be negative (IPCC, 1992). Therefore, since its
effects are uncertain and potentially opposite from the
other criteria pollutants, emissions of SO2 have been pre-
sented separately.
The major source of SO2
emissions in the U.S. is the burn-
ing of sulfur containing fuels,
mainly coal. Metal smelting and
other industrial processes also
1 JJKS^JT^S - ' •--••_
release significant quantities of IjFossil Fuel Combustion
SO2. As a result, the largest con-
tributor to overall U.S. emissions
. ..... . .. .. ... ..
Industrial Processes
of SO2 are electric utilities,
accounting for about 70 percent.
Coal combustion accounted for
approximately 96 percent of SO2
emissions from electric utilities.
The second largest source is
industrial fuel combustion, which
produced about 14 percent of
1994 SO2 emissions.
pSoivent Use .
-Waste Incineration
P^:'' '" " '•"" ' """"'' ""-" "'-
fJpssjL'Fuel Production,
B;,Qistribuitio,nyan
-------
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ES-18 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Introduction
^^he Earth naturally absorbs radiation from the sun, primarily at the surface, and reradiates this energy to
I space. A portion of this reradiated energy is absorbed or "trapped" by gases hi the atmosphere. This
"trapped" energy warms the Earth's surface and atmosphere, creating what is known as the "natural green-
house effect." Without the natural heat-trapping properties of these atmospheric gases, the Earth's temperature
would average about 55°F lower than today.
Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous
oxide (N2O), and ozone (C^).1 Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), a family
of human-made compounds, their substitutes hydrofluorocarbons (HFCs), and other compounds such as per-
fluorocarbons (PFCs), are also greenhouse gases. In addition, there are other photochemically important gases
such as carbon monoxide (CO), oxides of nitrogen (NOX), and nonmethane volatile organic compounds
(NMVOCs) that, although not greenhouse gases, contribute indirectly to the greenhouse effect. These are com-
monly referred to as tropospheric ozone precursors because they influence the rate at which ozone and other
gases are created and destroyed in the atmosphere. Box 1 contains a brief description of these gases, their
sources, and their roles in the atmosphere.2 In addition, emissions of sulfur dioxide (SO2) are provided in Annex
F of this report. Sulfur gases, primarily sulfur dioxide, are believed to contribute negatively to the greenhouse
effect. Therefore, the U.S. has discussed these emissions separately.
Although CO2, CH4 and N2O occur naturally in the atmosphere, their recent atmospheric buildup appears
to be largely the result of anthropogenic activities. This buildup has altered the composition of the Earth's
atmosphere, and possibly will affect future global climate. Since 1800, atmospheric concentrations of CO2 have
increased more than 25 percent, CH4 concentrations have more than doubled, and N2O concentrations have
risen approximately 8 percent (IPCC, 1992). And, from the 1950s until the mid-1980s, when international con-
cern over CFCs grew, the use of these gases increased nearly 10 percent per year. The consumption of CFCs is
declining quickly, however, as these gases are phased out under the Montreal Protocol. Use of CFC substitutes,
in contrast, is expected to grow significantly.
1 Ozone exists in the stratosphere and troposphere. In the stratosphere (about 20-50 km above the Earth's surface), ozone provides a
protective layer shielding the Earth from ultraviolet radiation and subsequent harmful health effects on humans and the environment. In
the troposphere (from the Earth's surface to about 10 km above), ozone is a chemical oxidant and major component of photochemical
smog. Most ozone is found in the stratosphere, with some transport occurring to the troposphere (through the tropopause, i.e., the
transition zone separating the stratosphere and the troposphere) (IPGC, 1992).
2 For convenience, all gases discussed in this inventory are generically referred to as "greenhouse gases," although the reader should keep
in mind the distinction between actual greenhouse gases and other photochemically important trace gases.
Introduction • I
-------
Box I
Greenhouse Gases arid; Other Phptochemically Important Gases
The Greenhouse Gases
Carbon Dioxide (CO2). The combustion of liquid,
solid, and gaseous fossil fuels is the major anthropogenic
source of carbon dioxide emissions. Some other non-energy
production processes (e.g., cement production) also emit
notable quantities of carbon dioxide. Carbon dioxide emis-
sions are also a product of forest clearing and biomass burn-
ing. Atmospheric concentrations of CO2 have been
Increasing at a rate of approximately 0.5 percent per year
(IPCC, 1992), although recent measurements suggest that
this rate of growth may be moderating (Kerr, 1994).
In nature, CO2 is cycled between various atmospheric,
oceanic, land biotic, and marine biotic reservoirs. The largest
fluxes occur between the atmosphere and terrestrial biota,
and between the atmosphere and surface water of the
oceans. While there is a small net addition of CO2 to the
atmosphere (;.e., a net source of COJ from equatorial
regions, oceanic and terrestrial biota in the Northern
Hemisphere, and to a lesser extent in the Southern
Hemisphere, act as a net sink of CO2 (/.e., remove more CO2
from the atmosphere than they release) (IPCC, 1992).
Methane (CH4). Methane is produced through anaero-
bic decomposition of organic matter in biological systems.
Agricultural processes such as wetland rice cultivation,
enteric fermentation in animals, and the decomposition of
animal wastes emit CH4, as does the decomposition of
municipal solid wastes. Methane is also emitted during the
production and distribution of natural gas and oil, and is
released as a by-product of coal production and incomplete
fuel combustion. The atmospheric concentration of CH4,
which has been shown to be increasing at a rate of about 0.6
percent per year, may be stabilizing (Steele, et al., 1992).
The major sink for CH4 is its interaction with the
hydroxyl radical (OH) in the troposphere. This interaction
results in the chemical destruction of the CH4 compound, as
the hydrogen molecules in CH4 combine with the oxygen in
OH to form water vapor (H2O) and CH3. After a number of
other chemical interactions, the remaining CH3 turns into
CO which itself reacts with OH to produce carbon dioxide
(COJ and hydrogen (H).
Halocarbons. Halocarbons covered by the Montreal
Protocol are human-made compounds that include chloroflu-
orocarbons (CFCs), halons, methyl chloroform, carbon tetra-
chloride, methyl bromide, and hydrochlorofluorocarbons
(HCFCs). All of these compounds not only enhance the
greenhouse effect, but also contribute to stratospheric ozone
depletion. Under the Montreal Protocol, which controls the
production and consumption of these chemicals, the U.S. will
phase out the production and use of CFCs, HCFCs, and
other ozone-depleting substances by December 31, 1995.
Perfluorinated carbons (PFCs) and hydrofluorocarbons
(HFCs), a family of CFC and HCFC replacements not cov-
ered under the Montreal Protocol, are also powerful green-
house gases.
Nitrous Oxide (N2O). Anthropogenic sources of
N2O emissions include soil cultivation practices, especially
the use of commercial and organic fertilizers, fossil fuel com-
bustion, adipic (nylon) and nitric acid production, and biomass
burning.
Ozone (O3). Ozone is both produced and destroyed in
the atmosphere through natural processes. Approximately
90 percent resides in the stratosphere, where it controls the
absorption of solar ultraviolet radiation; the remaining 10
percent is found in the troposphere and could play a signifi-
cant greenhouse role. Though O3 is not emitted directly by
human activity, anthropogenic emissions of several gases
influence its concentration in the stratosphere and tropos-
phere. Chlorine and bromine-containing chemicals, such as
CFCs, deplete stratospheric O3. However, as previously
stated, under the Montreal Protocol, the U.S. will phase out
the production and use of CFCs and other ozone-depleting
substances by December 31,1995.
Increased emissions of carbon monoxide (CO), non-
methane volatile organic compounds (NMVOCs), and oxides
of nitrogen (NOX) have contributed to the increased pro-
duction of tropospheric ozone (otherwise known as urban
smog). Emissions of these gases, known as criteria pollutants,
are regulated under the Clean Air Act of 1970 and subse-
quent amendments.
Other Photochernically Important Gases
Carbon Monoxide (CO). Carbon monoxide is created
when carbon-containing fuels are burned incompletely. Carbon
monoxide elevates concentrations of CH4 and tropospheric
ozone through chemical reactions with atmospheric con-
stituents (e.g.,the hydroxyl radical) that would otherwise assist
in destroying CH4 and O3. It eventually oxidizes to CO2.
Oxides of Nitrogen (NO*). Oxides of nitrogen, NO
and NO2, are created from lightning, biomass burning (both
natural and anthropogenic fires), fossil fuel combustion, and in
the stratosphere from N2O. They play an important role in
climate change processes due to their contribution to the
formation of ozone.
Nonmethane Volatile Organic Compounds
(NMVOCs). Nonmethane VOCs include compounds such
as propane, butane, and ethane. These compounds partici-
pate along with NOX in the formation of ground-level ozone
and other photochemical oxidants. Nonmethane VOCs are
emitted primarily from transportation and industrial
processes, as well as biomass burning and non-industrial con-
sumption of organic solvents (U.S. EPA, I990b).
2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
The Inventory Process
Central to any study of climate change is the
development of an emissions inventory that identifies
and quantifies a country's primary sources and sinks
of greenhouse gases. Developing and participating in
the inventory process is important for two reasons:
(1) it provides a basis for the ongoing development of
a comprehensive and detailed methodology for esti-
mating sources and sinks of greenhouse gases, and (2)
it provides a common and consistent mechanism
through which all signatory countries to the United
Nations' Framework Convention on Climate Change
(FCCC) can estimate emissions and compare the rel-
ative contribution of different emission sources and
greenhouse gases to climate change. Moreover, sys-
tematically and consistently estimating emissions at
the national and international levels is a prerequisite
for evaluating the cost-effectiveness and feasibility of
pursuing possible mitigation strategies and imple-
menting emission reduction technologies.
This report presents estimates by the United
States government of U.S. greenhouse gas emissions
and sinks for 1990-1994. A summary of these esti-
mates is provided in Table 1 by gas and source cate-
gory. The remainder of this document discusses the
methods and data used to calculate these emission
estimates. The emission estimates in Table 1 are pre-
sented on both a full molecular basis and on a car-
bon-equivalent basis in order to show the relative
contribution of each gas to total radiative forcing (see
Box 2 for an explanation of how the relative contri-
bution of each gas was calculated).
The U.S. views this submission as an opportunity
to fulfill its commitment under Article 4-1 of the
FCCC, which came into force on March 21, 1994,
following ratification. As decided at the Ninth
Session of the International Negotiating Committee
Box 2
The Global Warming Potential (GWP)'Concept
As mentioned, gases can contribute to the greenhouse
effect both directly and indirectly. Direct effects occur when
the gas itself is a greenhouse gas. Indirect radiative forcing
occurs when chemical transformation of the original gas pro-
duces a gas or gases that are greenhouse gases, or when a gas
influences the atmospheric lifetimes of other gases. The
concept of Global Warming Potential (GWP) has been devel-
oped to allow scientists and policy makers to compare the
ability of each greenhouse gas to trap heat in the atmosphere
relative to another gas. The GWP of a greenhouse gas is the
ratio of global warming, or radiative forcing (both direct and
indirect), from one kilogram of a greenhouse gas to one kilo-
gram of carbon dioxide over a period of time. While any
time period can be selected, the 100-year GWPs recom-
mended by the IPCC are used in this report. Carbon diox-
ide was chosen as the "reference" gas to be consistent with
IPCC guidelines. Carbon comprises 12/44 of carbon dioxide
by weight. In order to convert emissions reported in million
metric tonnes of a gas to MMTCE, the following equation is
used:
MMTCE = (MMT of gas) (GWP of gas) (12/44),
where
MMTCE = million metric tonnes, carbon-equivalent,
MMT = million metric tonnes, full molecular weight,
GWP = global warming potential, and
(12/44) = carbon to carbon dioxide molecular weight ratio.
GWPs are not provided for the photochemically impor-
tant gases CO, NOX, NMVOCs, and SO2 because there is no
agreed-upon method to estimate their contribution to cli-
mate change. These gases only affect radiative forcing indir-
ectly.
Gas
Carbon dioxide
Methane* .
Nitrous oxide
HFC-23
HFC-125
HFC-l34a
HFC-l52a
PFCs**
GWP'
(100 Years)
I
24.5
320
12,100
3,200
1,300
140
9,400
24,900
* The methane GWP includes the direct effect and those
indirect effects due to the production oftropospheric ozone
and stratospheric water vapor. The indirect effect due to the
production ofC02 is not included.
** This figure is an average GWP for the two PFCs, CF4 and
C2F6. The GWP for CF4 is 6,300 and the GWP for C2F6 is
12,500.
IPCC, 1994
Introduction
-------
Table I
, ........
. Recnt Trends '
"Gas/Source
I
Emissions
(Full Molecular Weight)
......
Emissions
(Direct and Indirect Effects;
fcori^Equivalent)
(Million Metric Tonngs)
.Greenhouse Gases
Carbon Dioxide (CO2)
^ Fossil Fuel Combustion
t Other
£ Total
: Forests (sink)
1 Net Total
_i..
f Methane (CH4)
t Landfills
• Agriculture
L Coal Mining
_; Oil and Gas Systems
-, Other
Total
Nitrous Oxide (N2O)
- Agriculture
i i .Fossil Fuel Consumption
i Industrial Processes
|";,,, rlbtal
[ HFCs and PFCs
|SF6
"
4,899
62
4,961
(458)
4.503
9.9
8.4-
4.4
3.2
0.9
27-l
0.2
O.I
O.I
0.4
*
+
4",839
61
4,90™
(458)
4,443
—
10.1
8.5
4.3
3.3
1.0
27.3
0.2
O.I
O.I
0.4
*
+
L
4,914 5,020
62^ 6f
4.97?"1 "5,084 "
(458)
4,518
NA
NA
-ni iiHw*
5,098
HgilVlii^
5,161
NA
NA
1,336
17
1,353
(125)
1.228
•3T**&acss*,
It320
1,336
(125)
1211
^SSJSBKSh,
L340
1,357
(125)
1.232
1^369
•4L™
1 387
NA
NA
j" ^ T1^- "^T 'sfM^*'^""^ *^rf i^s^'Si* '"tSi.^K^1 ttt
9.9
8.8
4.1
3.3
1.0
27.2
0.2
O.I
O.I
0.4
*
+
10.0
8.8
3.7
3.2
0.9
26.7
1
D.2
O.I
O.I
0.4
^4 —,^
#
+
10.2
9.2
4.3
3.3
0.9
28.0
0.2
O.I
O.I
0.5
*
+
66
56
29
22
6
181
'"•
~ - -|g«
12
8
18.8
6.4"
Up Ji SjiUfi flir SH4.& mif
67
57
28
22
*«7
uu.
17
12
9
37
19.3
6.5
66
59
27
22
•mZt-
|§2,
, HJ m^
17
12
67
59
24
22
6
J79
,.=4*--
17
12
^_J^ , *9
21. 1 ^
6.7
^^ n "i^SII
19.8
6.8
'S'Srfeuf ^
"".V
390
r?
,408
NA
NA
68
61
29
22
^,6.
188
19
12
9
.3',
23.5
7.0
W f f,
[ Photochemically Important
ilGases
Eiri^Tnj « I I I II inn i i i
t NMVOC
P CO
it
20.6
18.7
83.4"
20.4
18.3
"82.7
20.6
ia"2 ""
8L6""
. .
21.0
18.2
81.3
21.2
18.6
83.1
™ffi™M|*"*°''m imeroi*™n, i«i-s
-
_
,Mi bd- «KJUb nSldMWAAM
^t-ft^Vi/fm -nivwkL,
_
d!IW&tUfUW tUWWnJ
sL jjjwtea
-
_
WBlJihiirttllf^lllfe.
—
an«?sL ij^iimj^,^1
h "&
~
__
*.«
31
.. an
j-1
i
j»j
«ufr
U.S."erpisst6hs".i'!tl!
Net, Including Sinks
1,595 1,582 1,604 1,630 1,666
1,470 1,457 K479 NA NA
Jf-As this category contains multiple gases, an aggregate full molecular weight sum is not calculated.
Total of this gas does not exceed 0.01 million metric tonnes.
J^iA = not available _^ ^ _ . -,
•: Totals may not equal the sum of the individual source categories-due to independent rounding.
(INC), emission estimates are to be estimated and
presented in accordance with the IPCC Guidelines
for National Greenhouse Gas Inventories
(IPCC/OECD/IEA, 1995)3 to ensure that the emission
inventories submitted to the FCCC are consistent and
comparable across sectors and between nations. The
information provided in this inventory is presented in
accordance with the IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC/OECD/IEA,
1995), unless otherwise noted.
Methodology and Data
Emissions of greenhouse gases from various
J Discussions of inventory methods can also be found in Estimation of Greenhouse Gas Emissions and Sinks: Final Report from the
OECD Experts Meeting, 18-21 February 1991 (August 1991). That report documents baseline inventory methodologies for a variety of
source categories, which have subsequendy been further refined based on recommendations provided at an EPCC-sponsored experts
workshop held in Geneva, Switzerland in December 1991 and at an OECD/Netherlands-sponsored workshop in Amersfoort,
Netherlands in February 1993. The proceedings from these meetings, the Final Report (OECD, 1991), as well as several other
international meetings, form die basis for the current IPCC Guidelines.
4 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
sources are estimated using methodologies that are
consistent with Volumes 1-3 of IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC/OECD/
IEA, 1995). To the extent possible, the present U.S.
inventory relies directly on published activity and
emission factor data.4 Inventory emission estimates
from energy consumption and production activities
are based primarily on the latest official information
from the Energy Information Administration of the
Department of Energy (DOE/EIA). Emission esti-
mates for NOX, CO, and NMVOCs are based
directly on available U.S. Environmental Protection
Agency (U.S. EPA) emissions data. These estimates
are supplemented by calculations using the best avail-
able activity data from other agencies. Complete doc-
umentation of emission estimations can be found in
the sources referenced throughout the text. In these
supplementary calculations, attempts were made to
adhere as closely as possible to IPCC methods. In
many cases, the IPCC default methodologies have
been followed. However, for emission sources consid-
ered to be major sources in the U.S., the IPCC default
methodologies were expanded and more comprehen-
sive methods used. These instances, including energy
consumption, forest sinks, and some CH4 sources are
documented in the text, along with the reasons for
diverging from the IPCC default methodologies.5
The majority of U.S. CH4 emission estimates pre-
sented in this inventory are based on methods devel-
oped in the U.S. EPA report, Anthropogenic Methane
Emissions in the United States Estimates for 1990:
Report to Congress (U.S. EPA, 1993a). That U.S. EPA
report provided 1990 U.S. CH4 emissions for a vari-
ety of domestic sources, including natural gas sys-
tems, coal mining, landfills, domesticated livestock,
manure management, rice cultivation, fuel combus-
tion, and production and refining of petroleum liq-
uids. The methodologies used to arrive at the
emissions estimates in U.S. EPA (1993a) are concep-
tually similar to IPCC methodologies. Where the
methodologies differ, information is provided in the
text and/or appendices to ensure that the estimates
presented are reproducible.
Emission estimates for NOX, CO, and NMVOCs
were taken directly, except where noted, from the
U.S. EPA report, Draft National Air Pollutant
Emission Trends 1900-1994 (U.S. EPA, 1995b),
which is an annual U.S. EPA publication that pro-
vides the latest estimates of regional and national
emissions for criteria pollutants.6 Emissions of these
pollutants are estimated by the U.S. EPA based on
.statistical information about each source category,
emission factor, and control efficiency. While the U.S.
EPA's estimation methodologies are conceptually sim-
ilar to the IPCC-recommended methodologies, the
large number of sources EPA used in developing the
estimates makes it difficult to reproduce the informa-
tion from EPA (1995b) in this inventory document. In
these instances, the sources containing the detailed
documentation of the methods used are referenced
for the interested reader.
Organization of the Inventory
In accordance with the IPCC guidelines for report-
ing contained in the IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC/OECD/IEA,
1995), this inventory is organized into six parts.
These six parts correspond to the six major source
categories below. In addition, annexes provide addi-
tional data on calculations which are not included in
the main text. (Note: while the list below follows the
IPCC's list of recommended source categories, emis-
sion sources that are not applicable to the U.S. are not
included).
I. Part I covers emissions from all energy activities,
including:
A. Fuel Combustion Activities:
1. Industry
2. Transportation
4 Depending on the emission source category, activity data can include fuel consumption or deliveries, vehicle- miles traveled, raw
material processed, etc.; emission factors are factors that relate the quantity of emissions to the activity.
5 In order to fully comply with the IPCC Guidelines, the United States has provided a copy of the IPCC reporting tables in Annex G.
6 Criteria pollutants include carbon monoxide (CO), lead (Pb), nitrogen oxides (NOX), particulate matter less than ten microns (PM-10),
sulfur oxides (SOX), total particulate matter (TP), and nonmethane volatile organic compounds (NMVOCs).
Introduction
-------
3. Residential
4. Commercial/Institutional
5. Electric Utilities
B. Fuel Production, Transmission, Storage, and
Distribution:
1. Coal mining
2. Crude oil and natural gas
C. Biomass for Energy
II. Part II covers emissions from other industrial
production processes
HI. Part HI covers emissions from solvent use
IV. Part IV covers emissions from agriculture,
including:
A. Enteric Fermentation (in domestic animals)
B. Manure Management (for domestic animals)
C. Rice Cultivation
D. Agricultural Soils
E. Agricultural Crop Waste Burning
V. Part V covers emissions resulting from land-use
change and forestry.
VI. Part VI covers emissions from wastes and waste-
treatment processes, including:
A. Landfills
B. Wastewater Treatment
C. Waste Combustion
Uncertainty and Limitations
of Emissions Estimates
While the current U.S. emissions inventory pro-
vides a solid foundation for the development of a
more detailed and comprehensive national inventory,
it has several strengths and weaknesses. First of all,
this report by itself does not provide a complete pic-
ture of past or future emissions in the U.S.; it only
provides an inventory of U.S. emissions for the years
1990-1994. However, the U.S. believes that common
and consistent inventories taken over a period of time
can and will contribute to understanding future emis-
sion trends. The U.S. plans to update this compre-
hensive inventory of greenhouse gas emissions and
sinks on an annual basis and to use the information
gained to track progress of commitments made under
the U.S. Climate Change Action Plan. This report
represents the second inventory compiled by the U.S
for meeting its commitments under the FCCC. The
methodologies used to estimate emissions will be
periodically updated as methods and information
improve, and as further guidance is received from the
n>cc.
Secondly, there are uncertainties associated with
the emissions estimates. Some of the current esti-
mates, such as those for CO2 emissions from energy-
related activities and cement processing, are
considered accurate. For other categories of emis-
sions, however, a lack of data or an incomplete
understanding of how emissions are generated limit
the scope or accuracy of the inventory. For certain
categories, emission estimates are given as a specific
range to reflect the associated uncertainty. Where
applicable, specific factors affecting the accuracy of
the estimates are also discussed in detail.
Finally, while the IPCC methodologies provided
in the three volume IPCC/OECD/IEA report, IPCC
Guidelines for National Greenhouse Gas Inventories.,
represent baseline methodologies for a variety of
source categories, many of these methodologies are
still being refined. The current U.S. inventory uses the
IPCC methodologies where possible, and supple-
ments with other available methodologies and data
where needed. The U.S. realizes that not only are the
methodologies still evolving, but that additional
efforts are necessary to improve methodologies and
data collection procedures. Specific areas requiring
further research include:
B Completing estimates for various source cate-
/
goriest Quantitative estimates of some of the
sources and sinks of greenhouse gas emissions are
not available at this time. In particular, emissions
from some land-use activities and industrial
processes are not included in the inventory either
because data are incomplete or because method-
ologies do not exist for estimating emissions from
these source categories.
• Understanding the relationship between emissions
and sources. This is a crucial step in completing
and refining existing methodologies and in devel-
oping methodologies for emission source cate-
6 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
gories where none currently exist. For example, a
great deal of uncertainty exists in how nitrous
oxide emissions are produced from energy-related
activities and fertilizer consumption. As a conse-
quence, the quality of emission factors and activity
data for these categories are particularly weak.
Improving the accuracy of emission factors. A sub-
stantial amount of research is underway that could
improve the accuracy of emission factors used to
calculate emissions from a variety of sources. For
example, the accuracy of current emission factors
used to estimate emissions from surface coal min-
ing is limited by a lack of available data. Emission
factors for CH4 from landfills are also currently
undergoing revision. To more accurately assess
CH4 emissions from landfills, researchers are
working to determine the relationship between
moisture, climate, and waste composition and
CH4 generation rates. Emission factors used to
estimate greenhouse gas emissions from biomass
burning and land use are also being revised.
Providing appropriate activity data. Although
methodologies exist for estimating emissions for
some source categories, problems arise in obtain-
ing data that are compatible with methodology
requirements. For example, the ability to estimate
emissions from oil and gas systems is constrained
by a lack of information on compressor type,
amount of leakage, and emission control technol-
ogy. In the agricultural sector, estimating emissions
of animal wastes using the IPCC methodology is
arduous because of the complexity of the data
required. Obtaining information on animal
weights, waste management systems, and feeding
practices by animal type is difficult. Efforts need to
be made to collect activity data appropriate for use
in the IPCC methodologies.
The uncertainties and limitations associated with
calculating greenhouse gas emissions are both qualita-
tive and quantitative. Emissions calculated for the U.S.
inventory reflect current best estimates; in some cases,
however, estimates are based on approximate method-
ologies and incomplete data. Efforts need to be made
to improve existing methodologies and data collection
activities, so that methodologies and data are consis-
tent with one another and so that they allow both the
U.S. and other countries to estimate emissions with
greater ease, certainty, and consistency.
Introduction
-------
This page left blank intentionally.
8 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Part li
Energy
Energy-related activities are the most significant source of U.S. anthropogenic greenhouse gas emissions,
accounting for roughly 88 percent of total U.S. emissions annually on a carbon equivalent basis. This
includes almost 99 percent of carbon dioxide (CO2) emissions and just under one third of the nation's methane
(CH4) and nitrous oxide (N2O) emissions. Energy-related CO2 emissions comprise close to 85 percent of total
national emissions on a carbon equivalent basis, while the non-CO2 emissions represent a much smaller por-
tion of total national emissions (less than 5 percent collectively).
Emissions from fossil fuel combustion comprise the vast majority of energy-related emissions, with CO2
being the main gas emitted. Due to the relative importance of combustion related CO2, these emissions are con-
sidered separately from other emissions. Fossil fuel combustion also emits CH4 and N2O, as well as criteria pol-
lutants such as nitrogen oxides (NOX), carbon monoxide (CO), and non-methane volatile organic compounds
(NMVOCs).
Energy-related activities other than fuel combustion, such as the production, transmission, storage, and dis-
tribution of fossil fuels, also emit greenhouse gases. These emissions consist primarily of CH4 from natural gas
systems, oil production and refining, and coal mining. Smaller quantities of CO2, CO, NMVOCs, and NOX are
also emitted.
ndustry Agri- Wastes Total
culture
Emissions from Energy • 9
-------
The combustion of biomass and biomass-based fuels also emits greenhouse gases, although CO2 emissions
from these activities are not included in the U.S. total because biomass resources in the U.S. are used on a sus-
tainable basis. That is, the carbon released when biomass is consumed is recycled as U.S. forests regenerate,
causing no additional CO2 to be added to the atmosphere. The net impact of land-use activities on the forest
sector carbon cycle are accounted for in Part V of this document.
Overall, the energy sector was driven by a strengthening U.S. economy following an economic slowdown
in 1991. This resulted in a general increase in the production and consumption of fossil fuels since 1990, with
an associated increase in greenhouse gas emissions. Overall, emissions due to energy-related activities have
increased nearly 4 percent from 1990 to 1994, rising from 1,408 MMTCE in 1990 to 1,461 MMTCE in 1994.
This largely defines the trend in total U.S. anthropogenic greenhouse gas emissions, since energy-related emis-
sions represent the vast majority of total emissions (about 88 percent). Discussion of specific energy sector
trends is presented below.
EMISSIONS FROM FOSSIL FUEL CONSUMPTION
Carbon Dioxide Emissions
from Fossil Fuel Consumption
Background and Overall Emissions
The majority of energy in the United States,
approximately 88 percent, is produced through the
combustion of fossil fuels such as coal, natural gas,
and petroleum (see Figure 1-1). "The remaining 12 per-
cent comes from other sources such as nuclear energy,
hydropower, and biomass fuels.
After 1990, during which carbon dioxide (CO2)
emissions from fossil fuel combustion were 1,336
MMTCE, there was a Figure I-1
slight decline of emissions
in 1991, followed by an
increase to 1,390 MMTCE
in 1994. These trends are
directly attributable to
increased economic activ-
ity since the economic
downturn in the early
1990's. About 60 percent
of the increase since 1990
has come from increased
natural gas consumption,
30 percent from coal, and
10 percent from oil. Over
tiuo thirds of the increase
Approximately 88% of U.S. energy is produced
through the combustion of fossil fuels.
from 1993 to 1994 was due to oil products, of which
two thirds can be attributed to growth in the trans-
portation sector (see Table 1-1).
As fossil fuels are combusted, the carbon stored
in the fuels is emitted as CO2 and smaller amounts of
other gases, including CO, CH4, and NMVOCs.
These other gases are emitted as a by-product of
incomplete fuel combustion. The amount of carbon
in the fuel varies significantly by fuel type. For exam-
ple, coal contains the highest amount of carbon per
unit of useful energy. Petroleum has about 80 percent
of the carbon per unit of energy as compared to coal,
and natural gas has only
about 55 percent.
Petroleum supplies the
largest share of U.S.
energy needs, accounting
for just over 40 percent of
total energy consumption
on an annual basis (see
Figure 1-1). Natural gas
and coal follow in order
of importance, account-
ing for an average of 25
and 23 percent of total
consumption, respec-
tively. Most petroleum is
consumed in the trans-
10 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
portation sector, while the vast majority of coal is
used by electric utilities, and natural gas is consumed
largely in the industrial and residential sectors.
Following just over a one percent decline in
1991, U.S. emissions of CO2 from energy increased
approximately two percent annually through 1994.
The major factor behind this trend was the growing
domestic economy, combined with relatively low
energy prices and extreme weather conditions (EIA,
1994a).
Coal production fell in 1993 due primarily to a
United Mine Workers strike, but consumption was
largely unaffected as stocks were drawn down (EIA,
1994b). In fact, energy-related combustion of coal
was up 3 percent in 1993 and leveled off in 1994, due
Table I- 1
almost exclusively to increased use by electric utilities.
Oil combustion increased by about 1 and 3 per-
cent in 1993 and 1994, respectively. This was spurred
largely by U.S. economic growth and lower petro-
leum prices (EIA, 1994a), which are shown in Figure
1-2. A major portion of the increase was in the trans-
portation sector, which accounted for almost two-
thirds of the growth in petroleum demand during this
period.
Natural gas combustion rose by about 3 percent
and 2 percent, respectively, in 1993 and 1994, driven
largely by heating uses in the residential and commer-
cial sectors due to exceptionally cold winters. This con-
sumption increase was accompanied by an increase in
gas prices, production, imports, and storage as the nat-
oromercial
Coal
Natural J3as
Petroleum
dustrial
Coal
Natural Gas
Petroleum
ransportation
*T~ ^"l
Coal
Natural Gas
Petroleum
1LU.S. Territories
Coal
j^latural Gas
* etroleum
Sectors
Coal
Natural Gas
Petroleum
_|5irce Based on energy consumption estimates from EIA (I995f, 199Sb, I994a, and I995d), and carbon content coefficients from EIA (1995a) and
IIPCC (1PCCIOECD/IEA, 1995,Vols 1-3) For complete references see Annex A
fflote: Totals may not equal the sum of components due to independent rounding.
Emissions from Energy • 11
-------
Figure 1-2
Figure 1-3
1978 1980 1982 1984 1986 1988 1990 1992 1994
Year
Changes based on real 1987 dollars.
Although not shown in this chart, U.S. territo
an 1% of emissions.
ural gas industry responded to pressures placed on it
by intense seasonal demands and adjustment to indus-
try restructuring under Federal Energy Regulatory
Commission (FERC) Order 636, which increased the
complexity of natural gas service and heightened com-
petition among gas providers (EIA, 1994h).
Sectoral Contributions
The four end-use sectors that contribute to CO2
emissions from fossil fuel combustion include:
• industrial;
• transportation;
• residential; and
• commercial/institutional.
Electric utilities also emit CO2, although these
emissions are produced as they consume fossil fuel to
generate electricity which is ultimately consumed by
the four end-use sectors. For the discussion below,
utility emissions have been distributed to the end-use
sectors based on electricity consumption in those sec-
tors. Emissions from utilities are addressed separately
after the end-use sectors have been discussed.
Industrial Sector
From 1990 through 1994, the industrial sector
accounted for just over one-third of U.S. CO2 emis-
sions from fossil fuel consumption (see Figure 1-3).
On average, nearly two-thirds of these emissions
resulted from the direct consumption of fossil fuels in
order to meet industrial demand for steam and
process heat. The remaining one-third of industrial
energy needs was met by electricity for uses such as
motors, electric furnaces and ovens, and lighting.
Coal consumption by industry has remained rel-
atively constant since 1992, with a slight increase in
coal consumption for general industrial use offset by
a drop in coal consumption at coke plants (EIA,
1994b). In comparison, industrial use of natural gas
was up in 1993 due to overall sector growth, and lev-
eled out in 1994 as the result of localized economic
slowdowns in the Northeast and California (EIA,
1994h). The opposite trend occurred with petroleum
consumption, which declined in 1993 and subse-
quently increased in 1994. This trend was largely dri-
ven by decreased 1993 demand for distillate fuel oil,
LPG, and still gas, which rebounded in 1994.
The industrial sector is also the largest user of
non-energy applications of fossil fuels. Fossil fuels
used for producing fertilizers, plastics, asphalt, or
lubricants can store carbon in products for very long
periods. Asphalt used in road construction, for exam-
ple, stores carbon indefinitely. Similarly, fossil fuels
used in the manufacture of materials like plastics also
store carbon, releasing this carbon only if the product
is incinerated. Industrial non-fuel use rose about 22
percent between 1990 and 1994 (3,910 TBtu in 1990
12 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
to 4,788 TBtu in 1994).1 This increase, however, has
a negligible effect on total U.S. emissions since non-
fuel use comprises less than 7 percent of fossil fuel
consumption and the annual change in non-fuel use is
less than 0.3 percent of total consumption.
Transportation Sector
The transportation sector is also a major source
of CO2, accounting for slightly over 30 percent of U.S.
combustion-related emissions on an annual basis.
Virtually all of the energy consumed in this sector
comes from petroleum-based products, with nearly
two-thirds of the emissions resulting from gasoline
consumption in automobiles and other vehicles. Other
uses, including diesel fuel for the trucking industry and
jet fuel for aircraft, account for the remainder.
Following the overall trend in U.S. energy con-
sumption, fossil fuel combustion for transportation
has grown steadily after declining in 1991, with asso-
ciated increases in CO2 emissions (410 MMTCE in
1990 to 424 MMTCE in 1994). From 1992 to 1994,
petroleum consumption increased about 4 percent.
This increase was caused by the rising consumption
of motor gasoline, distillate fuel, and jet fuel, as travel
increased. This was slightly offset by decreases in the
consumption of residual fuel and liquefied petroleum
gases, as well as a slight decline in the energy inten-
sity of the U.S. passenger vehicle fleet. Declining
petroleum prices during these years (see Figure 1-2),
combined with a stronger economy, was largely
responsible for transportation growth, causing an
overall increase in vehicle miles traveled by on-road
vehicles, as shown in Figure 1-4 (EIA, 1994a).
Residential and Commercial Sectors
From 1990 to 1994, the residential and commer-
cial sectors have, on average, accounted for about 19
and 16 percent, respectively, of CO2 emissions from
fossil fuel consumption. Unlike in other major U.S.
sectors, residential and commercial emissions did not
decline in 1991, but rather grew slowly during the
entire period from 1990 to 1994. Both sectors are
heavily reliant on electricity for meeting energy needs,
with about two-thirds of their emissions attributable
Figure 1-4
to electricity consumption for lighting, heating, cool-
ing, and operating appliances. The remaining emis-
sions are largely due to the dkect consumption of
natural gas and petroleum products, primarily for
heating and cooking needs.
Coal consumption is a small component of
energy use in the residential and commercial sectors,
but has increased slightly from 1990 to 1994
(accounting for about 1 percent of total residential
consumption and 2 to 3 percent of total commercial
consumption). Residential and commercial natural
gas consumption increased slightly during both 1993
and 1994. This slow upward trend was primarily due
to sectoral growth, a shift toward natural gas heat in
new-home starts and old-home conversions, and
abnormally cold winter months in these two years
(EIA, 1994h). The majority of the annual increases
are due to colder than normal weather patterns,
which occurred in the eastern U.S. for the winters of
1993 and 1994. Specifically, the cold spell of March
1993 caused a combined 5 percent increase in resi-
dential/commercial consumption over 1992 levels,
and the cold spell of January 1994 resulted in 18 per-
cent higher residential deliveries and 30 percent
higher commercial deliveries compared to those in
January 1993. Oil consumption and related emis-
Throughout this document, TBtu stands for trillion Btus, or 1012 Btus.
Emissions from Energy SB 13
-------
sions in these sectors remained relatively constant
during this period, with only a slight increase in resi-
dential consumption and a slight decrease in com-
mercial consumption. This static trend is largely
caused by the offsetting factors of sectoral growth
and a shift toward natural gas heating.
Electric Utilities
The U.S. relies on electricity to meet a significant
portion of its energy requirements. In fact, as one of
the largest consumers of fossil fuel in the U.S. (aver-
aging 28 percent of total fossil fuel consumption on
an energy basis), electric utilities are collectively the
largest producers of U.S. CO2 emissions (see Figure I-
5). These emissions are produced as electricity is gen-
erated for such uses as lighting, heating, electric
motors, and air conditioning in the industrial, resi-
dential, and commercial/institutional end-use sectors.
Since electric utilities consume such a substantial por-
tion of U.S. fuel to generate this electricity, the type of
fuel they use has a significant effect on the total
amount of CO2 emitted. For example, some of this
electricity is generated with the lowest CO2-emitting
energy technologies, particularly non-fossil options
such as nuclear energy, hydropower, or geothermal
energy. However, electric utilities rely on coal for over
half of their total energy requirements and account
for about 87 percent of all coal consumed in the U.S.
Consequently, changes in electricity demand have a
significant impact on coal consumption and associ-
ated CO2 emissions.
Due almost exclusively to an increase in utility
consumption, coal-related emissions increased
approximately 3 percent in 1993 over 1992 levels.
This increase occurred despite a large drop in 1993
coal production due to a United Mine Workers strike,
resulting in a substantial depletion of utility stock-
piles (EIA, 1994b). There are three reasons for the
consumption increase (EIA, 1994b). First, there was
nearly a 4 percent increase in electricity demand
caused by a hot 1993 summer following a relatively
cool summer in 1992. Second, there was general
growth in the U.S. economy accompanied by reduced
imports of Canadian electricity. Third, compared to
earlier year?, coal constituted a slightly larger share of
Figure 1-5
Although not shown in this chart, U.S. terrfto
less than 1% of emissions.
the electricity mix, primarily due to a reduction in
nuclear power generation caused by plant outages in
the Midwest and Southeast. A smaller increase in util-
ity coal consumption occurred in 1994 as the U.S.
economy continued to strengthen. This gain was par-
tially offset by more moderate summer temperatures.
Utility consumption of natural gas declined in
1993 due to rising gas prices relative to coal and oil
and a displacement of marginal production by hydro-
electric generation following record levels of precipi-
tation in the Midwest and West (EIA, 1994h and EIA,
1994i). Utility natural gas use increased slightly in
1994, as the natural gas industry stabilized following
a series of cold winters and industry restructuring.
Also, gas prices fell, making gas-based electricity pro-
duction more economical.
Petroleum constitutes a relatively small portion
of utility fossil fuel consumption (approximately 5 to
6 percent), mostly occurring in the eastern United
States. Utility petroleum consumption increased
slightly in 1993 and leveled off in 1994. This trend
was due largely to a 1993 summer heat wave in the
eastern U.S., combined with relatively low petroleum
prices compared to natural gas through the first half
of 1993. This was followed in 1994 by declining gas
prices relative to petroleum, which encouraged gas
consumption at the expense of petroleum use.
14 H Inventory of U.S. Greenhouse'Gas Emissions and Sinks: 1990-1994
-------
Methodology Used to Estimate Emissions
The methodology used by the U.S. for estimating
CO2 emissions from fossil fuel combustion is concep-
tually similar to the approach recommended by the
IPCC for countries that intend to develop detailed,
sectoral-based emission estimates (IPCC/OECD/EIA,
1995; Vol. 3). A detailed description of the U.S.
methodology is presented in Annex A, and is charac-
terized by the following five steps:
1. Determine fuel consumption by fuel type and sec-
tor. Fuel consumption data were obtained directly
from the Energy Information Administration
(EIA) of the U.S. Department of Energy (DOE),
which is responsible for the collection of all U.S.
energy data. By aggregating consumption data by
sector (e.g., commercial, industrial, etc.), primary
fuel type (e.g., coal, oil, gas), and secondary fuel
category (e.g., gasoline, distillate fuel, etc.), E3A
estimates total U.S. energy consumption for a par-
ticular year.2 A discussion of the data sources and
comparison of different methodological
approaches can be found in Box 1-1.
Box I-1
About Energy Data and Estimating Carbon Emissions
When fuels are burned, the carbon contained within them combines with atmospheric oxygen to form CO2. In
theory, if the carbon content of the fuel and the combusted quantity is known, the resulting volume of CO2 can be esti-
mated with a high degree of certainty. Therefore, energy-related CO2 emissions can be estimated with a fairly high
degree of precision using available energy data.
I. Data Availability
In the U.S., the organization responsible for reporting
and maintaining annual energy statistics is the Energy
Information Administration (EIA), an agency of the U.S.
Department of Energy (U.S. DOE). EIA reports con-
sumption statistics for the SO U.S. states (e.g., the State
Energy Data Report) and U.S. territories as well as inter-
national statistics. EIA is also responsible for reporting
U.S. data to the IEA and U.N.
2. Data format
For consistency of reporting, the IPCC has recom-
mended that national inventories report energy data
(and emissions from energy) using the International
Energy Agency (IEA) reporting convention and/or IEA
data.
Data in the IEA format are presented "top down" —
that is, energy consumption for fuel types and categories
are estimated from energy production data (accounting
for imports, exports, stock changes, and losses). The
resulting quantities are referred to as "apparent con-
sumption."
The data collected in the U.S. by EIA are more of the
"bottom up" nature, i.e., they are collected through EIA
surveys at the point of delivery or use and aggregated to
determine national totals. In other words, the EIA data
reflect the reported consumption quantities of fuel cat-
egories and types.
For reporting to IEA, EIA converts the data for the
50 states into IEA fuel categories and units, calculates
"apparent consumption," and adjusts for production,
imports, exports, and stock changes in U.S. territories
and islands.The "converted" data are then submitted to
the IEA, along with the conversion factors used and
other relevant information.
Both of the above approaches have advantages and
disadvantages. For example, while the "top down"
approach more accurately captures fuel flow (and there-
fore the carbon flow) in most countries, the "bottom
up" approach allows for more detailed information by
end-use sectors and fuel types.
3. Estimating Carbon Emissions
Theoretically, both approaches should yield similar
carbon emissions results. In reality, most countries' esti-
mates will vary depending on the method used to esti-
mate consumption totals, the definition and
interpretation of data sources, the carbon coefficients
used, and the assumptions regarding both the quantity of
carbon stored in products and combustion efficiency.
Both approaches are believed to produce highly accu-
rate results in the U.S. For example, carbon emissions
estimates for 1990 from the "bottom up" approach total
1,336 MMTCE, while the "top down" approach results in
carbon emissions of 1,320 MMTCE.
2 Fuel consumption by U.S. territories (Le., American Samoa, Guam, Puerto Rico, U.S. Virgin Islands, Wake Island, and other U.S.
Pacific Islands) is included in this report and contributed about 10 MMTCE of emissions in 1993 and 1994.
Emissions from Energy 8 15
-------
2. Determine the total carbon content of fuels con-
sumed. Total carbon is estimated by multiplying the
amount of fuel consumed by the amount of carbon
in each fuel. This total carbon estimate defines the
maximum amount of carbon that could potentially
be released to the atmosphere if all of the carbon
were converted to CO2. The carbon emission coef-
ficients used by the U.S. are presented in Table 1-2.
3. Subtract the amount of carbon stored in products,
Non-fuel uses of fossil fuels can result in storage of
some or all of the carbon contained in the fuel for
some period of time, depending on the end-use. For
example, asphalt made from petroleum can
sequester up to 100 percent of the carbon for
extended periods of time, while other products,
such as lubricants or plastics, lose or emit some car-
bon when they are used and/or are burned as waste
after utilization. The amount of carbon sequestered
or stored in non-energy uses of fossil fuels was
based on the best available data on the end uses
and ultimate fate of the various energy products.
These non-energy uses occur in the industrial and
transportation sectors. Carbon sequestered by
these uses was estimated to be about 66 MMTCE
in 1990, which rose to 79 MMTCE in 1994.
4. Adjust for carbon that does not oxidize during
combustion. Because combustion processes are not
100 percent efficient, some of the carbon contained
in fuels is not emitted to the atmosphere. Rather, it
remains behind as soot or other byproducts of inef-
ficient combustion. The estimated amount of car-
bon not oxidized due to inefficiencies during the
combustion process range from 1 percent for oil
and coal to 0.5 percent for natural gas (see Table I-
2 for the assumptions used by the U.S.).
5. Subtract emissions from international bunker
fuels. According to the IPCC guidelines
{EPCC/OECD/IEA, 1995) emissions from interna-
tional transport activities, or bunker fuels, should
not be included in national totals. Therefore, since
EIA consumption statistics include these bunker
fuels (primarily residual oil) as part of consump-
tion by the transportation sector, emissions from.
this source are calculated separately and sub-
tracted from the transportation sector. The calcu-
lations for emissions from bunker fuel follow the
same procedures used for emissions due to con-
sumption of all fossil fuels (i.e., estimation of con-
sumption, determination of carbon content, and.
adjustment for the fraction of carbon not oxi-
dized). Emissions from bunkers resulted in emis-
sions of over 22 MMTCE in 1990, which rose to
a peak of 25 MMTCE in 1992 and then declined
to approximately 23 MMTCE in 1994.3
Uncertainty in the Carbon
Dioxide Emission Estimates
Uncertainties exist for all of the emission esti-
mates provided in this report. For estimates of CO2
from energy consumption, in principle the amount of
CO2 emitted is directly related to the amount of fuel
consumed, the fraction of the fuel that is oxidized,
and the carbon content of the fuel. Therefore, a care-
ful accounting of fossil fuel consumption by fuel type,
carbon content of fossil fuels consumed, and con-
sumption of products with long-term carbon storage
would yield an accurate estimate of CO2 emissions.
There are uncertainties, however, over the levels
of detail, data sources, carbon content of fuels and
products, and combustion efficiency. For example,
given the same primary fuel type (e.g., coal), the
amount of carbon contained in the fuel per unit of
useful energy can vary. Non-energy uses of the fuel
can create situations where the carbon is not emitted
to the atmosphere (e.g., plastics, asphalt, etc.) or is
emitted at a much delayed rate. The proportions of
fuels used in these non-fuel production processes and
their carbon content can also vary. Additionally, inef-
ficiencies in the combustion process, which can result
in ash or soot remaining unoxidized for long periods,
can vary. These factors all contribute to the uncer-
tainty in the CO2 estimates. For the U.S., however,
these uncertainties are believed to be relatively small.4
' 1994 estimates of emissions from bunker fuel consumption by the U.S. and its territories are preliminary estimates based on 1992 and
1993 data. Final estimates will be available in late 1995 or early 1996, when international data from EIA are published.
4 U.S. C(>2 emission estimates from fossil fuel consumption are considered accurate within one or two percent. See, for example,
Marland and Pippin, 1990 or EIA, 1993a.
16 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table 1-2
Key Assumptions for Estimating Catrbipn Dio>cid6; Enjiissipns^
'---• "-""---Content Coefficient Fraction Fraction j
,...*. .. i L. "-"^-^^^^^^-i-fei^^^j^J,^^.'^ is^i,.^ ^tf-jfe^V^^ •-' ,y "-->•; _•• '-- HT-.-I-, •-«
jBtu5) Oxidized Sequestered :;
TICI^^
SmmerciaTCoai" "~'"~ ' ~~" "" '"; ': " "r^™™"*"*"* -^«a-»E-»-;:**^ ^i^«™-.---- :.— -••.•.-•---. .,,-..-^ .-..-,.„, ,, -1
Vss-^«^»a}SM!5«s«^;.-?ea^^^ ,-.»~vi«MiW>«-.:.-.v-~.'^ »••«•*-• '.- <.,.-.*..<.-., ---.
0.99 0.75
iS«W"u^-3^^^*Mi&I^WEe-*^n'
[a]
^gfe^sScgSftKa -*>>;*|gis:3sssfl!>$i ,?.¥; -,§
C4S1-li^11S^^
1
->-*
0:99
&99
ndustrial Other Coal
.pKB Imports
transportation Cfoal
,_.T...,^ __y.. ^.. ^j^B|^f™
tix!«€iS?Ji^fiiss^
•Jatural Gas 14.47 0.995 f.00[d] •
B»:»'»jsoi»^-::'.:;;'»;ss»^^»«s«ww!^ -,.;,;,'•, .,y|
ijsffi8f%}Sj3I!I8s«84^^ ,*iM?'''.-r:"« ;:5p,'-•:.-•./.' *~,::'*'''• '.-'41
"10.62 0.99 i.OO j
f^^^^S^^:^^^^^^^^^1^?-.^:^;-^^^^ ^^,^~^'¥^-^^,^-,Jv-«",trv-.
19.95
^p^wfc?^^rf^^:<^^
L^J
..... ,._ . _.....,. «•& *...*.. ^..- - |^y2- -
*'5'vv-*r«i!?j.»ftS:.»jr,r^-W*4-^
!$ff-t'J;:"^:^:f^-l\-^?'£^-2&^ SJiif'-j;* LT.i-;--n';J?,V:i*i=('-"ri',i-v'tf-=S-,
?0 24
•""H «s k s Cf-* " ^r"'
tt. 1, ^° gun Vrti -IT *- -W "* •«-
21.49
• ."" "^ i* * t * ^ »^ ~ * V HI
ents' "^ *'* i§87
^ u^- r ^-2t" N
ieMs ^"^ * "* !9.4l
*""* K " 20.31
1^.14
pp**,*« -««»|^5 ' - ~ ~~
«n L-^JWAS igfUra^ «fr ^tH^-fffa^ t -ge^t ts^-wffljr ^ SW SiJMiJ-Bf *1
1824
Icks* **"r-Jt* "*" |9.3^""
.^»«* «t,^*^,8r ,_^*
**" " " I7.SI
eiafvajBiar' * vtttpfr^^or •* »« , ^ —
19.86
" "*" *"**" 20.2*1 '
~^? * t"^F J- ^*" fe-»*= ?s
19.81
^ *s, *e.*«ai.. a* < !>• ^
1981
.^.-t ?'.'-.} sfa,.'<.. '-T'1". ' -•-' -\- L"---.T= ',j,,\, •..',„-., . - ., •
-^.-ft^---.--; • ;--"
•^"^^^— ---/.-;--
0.99
-,1^ -"-TiB-^-j.'ifcS ",'.'a,, * »"v "»-)-:, riiW-r'.-v'-Tii-*'--. V- ---,'-'1,1 .-I-,
* "';w^9? -'•'--•-" ••-- '.---•
Q 99
0.99
0.99
0.9'9
0.99
* f*
0.99 " "" *
0.99
0.99
0.9$
0.99
H
0.99
*0.99_
_^099
-t _^
w 0.99
0.99
0.99
• - . -
..:, M:
\^ ,.,-- ,..,' . , .,, . .-\ ,,-.,;-.,.,,, .
0.80
" 0.50
-
M
.
.
.
[b]
[cj
[c]
_
0.80
0.00
.
0.00
_
[b]
I.OO
.- .--- 'v*
'. '- .'" -i!
•'-.- -•. -)i
«-,." ,„.„, , g
'-••.- -' - " ^
J
4
,'.
f
1
«
-1
f
5»
1
•«
*
e
1
S
e
€
tf
lotor Gasoline
-._ L T »
esiduai Fuel
er Petroleum
, »«(•« ^ 4;,,,
'AvGas Blend Components
"irudeOil
MoGas^Blend Components
llMisc^ Products
^Naphtha (<40I deg F)
?Otfier OiTt>4of degjf
fPentanes|'tus\^
t Petrochemical Feedstocks
^4 •*>*." •*»
* Petroleum Coke
2Ni$H*Wi~ -
Special Naphtha
'TJnfinf he3*Oils '
'Waxes
KOther Wax & Misc.
^,. " *, ^ *^"^\ *:•*-„, -,-e ^ -.'/ - 1
ources Carbon Coefficients from 0A (I995a) Stored Carbon from Marland and"Pippen (1990) and Rypmsh (1994) Combustion efficiency for
oal from Bechtel (1993) and for oil and gas from IPCC (IPCC/OECDI1EA, 1995,Vol 2) '
jjgtes tJC = Not Calculated "^ ' " ** "7
^J "= T/iese"coefl!aents vary annually due to fluctuations in fuel quality See Annex A for more information.
= Non-fueTuse values of distillate fuel, miscellaneous products, residual Jue\, ana1 waxes are reported in aggregate in the "Other Mixes & Misc "
"g°'?' -..r*". J.'T"*^"" "^-s^TJUI'r ^ ~ „«"" !"„ ^~^*«^ , *
= ZNon-)Set use values of^ap^ff'^01 *yegtf*and;Other ^^>40I aeg^Fjare reported /jToggre^ate in tfte "Petrochemical Feedstocks"category
' = ft /s assumed th'at 100 percent of the carbon in natural gas used as a chemical feedstock is sequestered (There are actually two major non-
fuses for natural gas I for ammonia production in nitrogenous fertilizer manufacture, and 2 as a chemical feedstock. The carbon in natural gas
I for ammonia production oxidizes quickly)
5 One QBtu is one quadrillion Btu, or 101S Btu. This unit is commonly referred to as a "Quad."
Emissions from Energy H 17
-------
Other Greenhouse Gas
Emissions from Stationary
Fossil Fuel Combustion
Stationary combustion encompasses all fuel com-
bustion activities except transportation (i.e., mobile
combustion). Other than carbon dioxide (CO^,
which was addressed in the previous section, gases
from stationary combustion include the greenhouse
gases methane (CH^ and nitrous oxide (N2O) and
the photochetnically important gases such as nitrogen
oxides (NOx), carbon monoxide (CO), and non-
methane volatile organic compounds (NMVOCs),
which are all products of incomplete combustion.
The amount of emissions varies depending upon fuel,
technology type, and pollution control equipment.
Emissions also vary with the size and vintage of the
combustion technology as well as maintenance and
operational practices.
Stationary combustion is a significant source of
NOxand CO emissions. In 1994, emissions ofNOx
from stationary combustion represented 50 percent
of national NOX emissions, while CO and NMVOC
emissions from stationary combustion contributed 5
and 4 percent, respectively, to the national totals for
the same year. Emissions of these criteria pollutants
have declined from much higher levels in the past due
to a combination of technological advances and more
stringent emissions requirements. From 1990 to
Table 1-3
a :
I
I I9go
f Year'
(Thousand Metric Tonnes)
10,414 ' 834 4394 "'':'" '708 '' 35'"'
11991 10,319 887 4,857 760 35 I
| 1992 10,358 926 5,079 802 35 ^
| 1993 10,607 815 4,493 691 36 *
1 1994 10,636 804 4,431 678 36 "
{• - -ii|iinn n ,,,.,, •,,- , , ; „!,;,;,„;- -^
* Sources: I.Criteria pollutant emissions estimates from U.S. EPA (I995b)
| 2. CH4 emission estimates based on NMVOC emissions from
! US. EPA (19956; and emission ratios from US. EPA (1993a).
I 3. N^O emissfons are based on IPCC emission factors for j
p uncontrolled fossil fuel and wood fiiel combustion "*'
t (/PCC/OHCD//EA, / 995J, and US. foss/7 fuel and wood fuel con- :
|; sumption data (EM i995f,l994a;and 19944 " "~
1994, emissions of NOX increased by about 2 per-
cent, while emissions of CO and NMVOCs showed
a slight decline (see Table I-3).6
Stationary combustion is also a small source of
CH4 and N2O. CH4 emissions from stationary com-
bustion in 1994 accounted for about 2 percent of
total U.S. CH4 emissions, while N2O emissions from
stationary combustion accounted for about 8 percent
of all N2O emissions. From 1990 levels, CH4 emis-
sions rose by almost 8 percent in 1991 and 6 percent
in 1992, but dropped by more than 14 percent in
1993 and 2 percent in 1994. Despite slightly more
than a 1 percent drop in 1991, N2O emissions rose
from the 1990 level of 35.2 thousand tonnes to 36.1
thousand tonnes in 1994 (an increase of nearly 3 per-
cent).
Background and Overall Emissions
Nitrous oxide and NOX emissions from station-
ary source combustion are closely related to air-fuel
mixes and combustion temperatures, as well as pol-
lution control equipment. Carbon monoxide emis-
sions from stationary combustion are generally a
function of the efficiency of combustion and emission
controls, and are highest when there is less oxygen in
the air-fuel mixtures than necessary for complete
combustion. This is likely to occur during combus-
tion stopping and starting, or switching of fuels (for
example, the switching of coal grades at a coal-burn-
ing utility plant). Methane and NMVOC emissions
from stationary combustion are believed to be a func-
tion of the CH4 content of the fuel and post-combus-
tion controls.
Methane emission estimates from stationary
sources are highly uncertain, primarily due to major
uncertainties in emissions from wood combustion
(e.g.,, fireplaces and wood stoves). The largest source
of N2O emissions comes from utility coal combus-
tion, accounting for almost 38 percent of total N2O
emissions from stationary combustion over the
period 1990 to 1994. It is important to note, how-
ever, that both of these gases are currently not regu-
lated in the U.S., and therefore, their emission
' Tables in this document are generally reported in million metric tonnes or thousand metric tonnes, depending on the relative
magnitude of the emissions being presented. When comparing information across tables, please note the units.
18 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
processes /are not as well understood as emission
processes for some criteria pollutants. The estimates
of CH4 and N2O emissions presented here are based
on broad indicators of emissions (i.e., aggregate emis-
sions ratios of CH4 emitted to total NMVOCs and
rate per amount of fuel used, respectively), rather
than specific emission processes (i.e., by combustion
technology and type of emission control).
Greenhouse gas emissions from energy-related
stationary combustion activities have been grouped
into four sectors:
• industrial;
• commercial/institutional;
• residential; and
• electric utilities.
The major source categories included in this sec-
tion are similar to those used by U.S. EPA (1995b):
coal, fuel oil, natural gas, wood, other fuels (includ-
ing bagasse, LPG, coke, coke oven gas, and others),
and stationary internal combustion (which includes
emissions from internal combustion engines that are
not used in transportation). A summary of the emis-
sions from stationary combustion sources in 1994 is
provided in Table 1-4.
The slight decline in CO and NMVOC emissions
from 1990 to 1994 can largely be attributed to resi-
dential wood combustion, which is the most signifi-
cant source of these pollutants in the energy sector
(74 percent of CO and 77 percent of NMVOCs). As
fossil fuel prices have decreased in the last several
years, residential wood consumption for home heat-
ing has declined (EPA, 1995b). Overall, NOX emis-
sions from energy have increased largely due to an
increase in emissions from electric utilities, which
constitute over 66 percent of stationary NOX emis-
sions. However, utility emissions have increased more
slowly than utility fuel consumption, due in large part
to emission control systems in plants burning fossil
fuel.
Like NMVOCs, emissions of CH4 decreased
over the period 1990 to 1994, due largely to the
decline in residential wood use. Nitrous oxide emis-
sions, on the other hand, increased slightly over this
period as fossil fuel combustion increased (72,300
TBtu in 1990 to 76,200 TBtu in 1994). Like NOX,
the emission increase was due in large part to electric
utility consumption, which grew at a faster rate than
overall energy consumption.
Methodology Used to Estimate Emissions
Emissions estimates for NOX, CO, and
NMVOCs in this section were taken directly from the
U.S. EPA's Draft National Air Pollutant Emissions
Trends: 1900-1994 (U.S. EPA, 1995b). U.S. EPA
(1995b) estimates emissions of NOX, NMVOCs, arid
CO by sector and fuel source using a "bottornAip"
estimating procedure, i.e., the emissions were calcu-
lated either for individual sources (e.g/, industrial
boilers) or for many sources combined, using basic
activity data (such as fuel consumption or deliveries,
etc.) as indicators of emissions. The national activity
data used to calculate the individual source categories
were obtained from many different sources.
Depending on the source category, these basic activ-
ity data may include fuel consumption or deliveries of
fuel, tons of refuse burned, raw material processed,
/
etc. Activity data are used in conjunction with emis-
sion factors, which relate the quantity of emissions to
the activity. The basic "bottom-up" calculation pro-
cedure for most source categories presented in U.S.
EPA (1995b) is represented by the following equa-
tion:
Ep,s=AsxEFp,sx(l-CpjS/IOO)
where
E = emissions
p = pollutant
s = source category
A = activity level
EF = emissions factor
C = percent control efficiency
Emission factors are generally available from the
U.S. EPA's Compilation of Air Pollutant Emission
Factors, AP-42 (U.S. EPA, 1995a), often referred to
as AP-42 emission factors. The EPA currently derives
the overall emission control efficiency of a source cat-
egory from a variety of sources, including published
reports, the 1985 NAPAP (National Acid
Precipitation and Assessment Program) emissions
inventory, and other EPA data bases. The U.S.
Emissions from Energy
19
-------
Table 1-4
,.! " . . _, _ - '.ii1, I.*! ' ,iii.,L_ • ' ' ; .• IP !t« IP" . :fmm;''-.'7TS"- -. swa*? • •tvswi.rs-. . . M — ' •
U.S. Greenhouse Gas' EnriislsiOns from Stationary GombtllStion i
- .. •''.'• '- • ^ifaVSedtor ^:I:F|l;.Spu|||/r994«VV %"£;: lifc,: ,..- ,
1 i L
Sector/Fuel Source
n in i » mi in in nn ii mi i in il i n i mini nil in ill nil ill ii n mill in in in in in nil u
Electric Utilities ~ — -L- —
£ Wood
4. Other Fuels*
* Internal Combustion
Tbt4l 1 1 |ii . tl ) '
.Industrial , Mm M —
|- Coa|
- Fuel Oil
Natural gas
" Wood
f Other Fuels*
1 Internal Combustion
Total :.; ill: ' !;• ill ; 1. ' |
NOX
III nn 1 111 ii lii 1 iini i
*,Jj|I.L-™
NA,
NA
50
1 7,07p i
288^
1,482
NA"
5I4U
uig&rini
— is
NMVOCs
" *tl( (Thousand
,._„.. 26 n JTO-
, ,,^^^
"n » "w m HrifiS. nn "T*
6
55
NA
"33^"" "**
iV
it frzs'.,: -
^co"""""1"
Metric Tonnes)
""•48"" "
NA
II
KlEEflHHIl
45
248
" T?* " 1~1
H&iiHHfflft
NA
NA
HBHBm
_ 3
NA
"NA
""T
i
IfflONHII
, ^g,
i
^lt&5nL-1"ilLl^P»!%fs'V'" fe*11*!
. „ W —-.
_ *. i
NA I
NA
nan
\ - 1
""y !
i
*
7
NA *
'Commercial/Institutional
~ Coal
Fuel Oil I
Natural gas
Wood
Other Fuels*
Internal Combustion
-W11!^™^
93
NA;
10
U\ " 1 F III 1
1
**-"*"
4
IP> w*«
6
11
NA
4
^w
NA
46
tf * 1-^
,NA
47
™ PV ^ ^n
NA
+
WRK^
NA
*- l» _»
, N4
+
^ hi T-
NA
lk
NA
NA
tj
s
^ f
t
f,
I
in
I
1
•
Fuel Oil<=
Natural gasc
Wood
Other Fuels*
Internal Combustion
Total ' iJMIi !':
N/S
NA
--°t -
" NA""
)! ''I' !i:'''3:7()!
NA
NA
NA
» W-.-.--,
ii" ' :;633
" "NA"""
NA
3j272
3,405
NA
NA
620
,. ,MK
•l'*2i.'
\
l
2
NA
r. ^NA
iMfflfflHffiiiiiiiigJSii
i
i
f
i
i
~^ " '• , • „ '" ,»,„ '.. , '..li"',:,;",!"^*1 ."'"1"'' ~X "i1 ^V'11*1"11''1^11^^^ "".t'-"-"' ii'",' '"-„>! "-•"
jpjl. _U,_ a_ Mi^illli*l|l!|i*"itliiiiiiiiiil*|iipiiiiiiiiiPi»iiiiJ|iiiiir»l life"A^ •".: ;i^:iw,*^yiiii,n,iiL .lai »,;,& ^..r, »i,i!';"1«?
STATIONARY COMBUSTION TOTAL ! )p,63§
804 4,431
^Sources; NOX. NM VOCs, and CO data are from U.S.EPA (1995b) (original data in short 'ton^
data using the midpoint of CH4 to NMVOC ratios from ~OJS.'&K~(f99'3a) (&ee"tex^.~" '
'factors. (£IA199Sf;l 994a; ..... IJ9J^aj^J^OECD§^, ..... 19951 ....... . ..... , _'„„, ..... ,, I ™ ..... „ ,,,,, ......... .. ...... ,„, ........... ...... . ........ . ...... ..... „„,. , ....... , ________ ...... „ .............. , , ........... ......... . ..... ..... . ..... ......... . ,,„ , .............. -
Noces; /. Technically, of the goses /isted in th/s table, only CH4 and N20 are greenhouse gases. See Box I in the Introduction for further explanation.
'- 2. Components may not sum to totals due to independent rounding.
, ....... , ________ ...... „ .............. , , ........... ......... . ..... ..... . ..... ......... . ,:,„ , .............. - .......... •
_ " ff. When referring to criteria pollutants and CHA other fuefe includK^G,wasteoilJcol^ oven gas, colw, and wood. For these pollutants wood
BIBB'has been disaggregated in the residential sector. For N^O, wood has been disaggregated in all sectors. (US. EPA, I995b) "
•pnaueiir -^•n»iniiiiijmmiinmiiii»ffli H»atMi»»H»mi^»iiiHiHiiiiiii»iiiiii|i||n»nnH|inn!ii!in»mn ipin^B^i||i|iiiiiii||iii|p!iiipip|iii||ipiMnHpp^p&ihi||ii| hiiniiniiiipigifii^!! hipii juiiiiiapaipniMiiiiiifiitfj^ iii^nuL' i.nJ^m^, iia^is.*;;- **& jFtyyyM.."- n. "-.JM .d M:.,^
... flS fe1"6 includes an additional 32 kt ofCH4 emissions from natural gas stationary sources, which was not distributed among the sectors
ft) the rest of the t
-------
reported NMVOC emissions for each activity from
U.S. EPA (1995b), and emission ratios of CH4 to
NMVOCs (U.S. EPA, 1993a) for these activities. The
emission ratios used are provided in Table 1-5. The
estimates for emissions from natural gas consumption
came from U.S. EPA (1993a).
The estimates of CH4 emissions from stationary
sources, other than gas-fired sources, are subject to
considerable uncertainty due to the lack of accurate
data regarding the technology type and the pollution
control equipment in each of the other source cate-
gories (e.g., coal, fuel oil, and especially wood).7 As a
result, estimates are based on broad estimates of the
percentage of CH4 emissions relative to NMVOC
emissions — a methodology that results in very impre-
cise estimates. The estimates for gas-fired stationary
combustion are more precise due to the greater level
of disaggregation by sector and technology.
Nitrous oxide emissions were estimated using
IPCC-recommended emission factors and U.S. fossil
fuel and wood fuel consumption data. Estimates were
obtained by multiplying the appropriate emission fac-
tors (by sectors and fuel types) by the appropriate
U.S. energy data. The emission factors used were: 4.3
g N2O/GJ of energy input for wood in all sectors; 0.1
g N2O/GJ for gas use and 0.6 g N2O/GJ for oil use in
all sectors; and for coal use, 0.8 g N2O/GJ for the util-
ity sector and 1.4 g N2O/GJ for the industrial, com-
mercial, and residential sectors.8
Table 1-5
•"-. Ratio^ of.CH4
Released Durin
^Activity
(Source Category)
Ratio of CH4 to NMVOCs
(Low-High)
;uel Oil Combustion, „„,„,. ^pjr 0.05 to^O.IO
FWood Combustion (Industrial Use) 0.2
~ Wood Combustion (Residential Use) 2
tOther O.I
^Source U.S. EPA (1 993a); except for "Other", where the upper end of
"yhe fuel oil category was used as an approximation.
Uncertainty in the Emission Estimates
Estimating emissions other than CO2 from sta-
tionary combustion can be time consuming and com-
plex. Moreover, the amount of gases emitted from
these activities are not thought to be major contribu-
tors to climate change. The uncertainties associated
with the emission estimates of these gases, especially
CH4 and N2O estimates, are also much higher than
the uncertainty associated with estimates of CO2
from fossil fuel combustion. Uncertainties in the CH4
estimates are due primarily to the fact that they are
based on simple ratios of CH4 to emitted NMVOCs
and are derived from a limited number of emissions
tests. Uncertainties in the N2O estimates are due to
the fact that emissions were estimated based on a lim-
ited set of emission factors. For the other gases, the
uncertainties are partly due to assumptions concern-
ing combustion technology types, age of equipment,
and the emission factors used.
Other Greenhouse Gas
Emissions from Mobile Combustion
: Emissions ratios from wood-fired equipment are based on U.S.
(/995b). For industrial wood combustion, the rrtean CH4 to ]
^NMVOC ratio is based on wood cgmbustion in. fepi/ers.. ,Jfar residential ^
Siiqod conjfaustiqn, the mean ratio is based on available emission fac- ^
rtqrs for residential wood stoves.
Mobile sources emit greenhouse gases other than
CO2, including methane (CH4) and nitrous oxide
(N2O), and photo chemically important gases, includ-
ing carbon monoxide (CO), nitrogen oxides (NOx),
and non-methane volatile organic compounds
(NMVOCs). Emissions of these trace gases are pro-
duced by the incomplete combustion of the fossil
fuels used to power vehicles.
Fossil fueled motor vehicles comprise the single
largest source of CO emissions in the United States as
well as being a significant contributor of NOX and
NMVOC emissions (see Table 1-6). In 1994, CO
emissions from mobile sources contributed about 84
percent of all U.S. anthropogenic CO emissions and
46 and 42 percent ofNOx and NMVOC emissions,
respectively. Mobile emissions are also a small source
ofCH4 and N2O in the U.S. Road transport accounts
for the majority of mobile source emissions.
For the period 1990 to 1994, emissions of the cri-
teria pollutants as a whole declined through 1992,
Methane emissions from gas-fired sources were extensively researched and documented in U.S. EPA (1993a).
GJ = Gigajoule = one billion joules. One joule = 0.9478 Btu.
Emissions from Energy
21
-------
after which there was an increase through 1994, The
increase was caused by a drop in gasoline prices com-
bined with a strengthening U.S. economy. These fac-
tors pushed the vehicle miles traveled (VMT) of road
sources up, resulting in increased fuel consumption and
higher pollutant emissions. Some of the increased
activity was offset by increasing energy efficiency of
highway vehicles and an increasing portion of the vehi-
cle population meeting established emissions stan-
dards.
Methane and N2O emissions have increased
slightly over the period 1990 to 1994. As with crite-
ria pollutants, this was driven largely by high eco-
nomic growth and low oil prices. On-road vehicles
accounted for nearly all of the increase.
Background and Overall Emissions
As in combustion in stationary sources, N2O and
NOx emissions are closely related to air-fuel mixes
and combustion temperatures, as well as pollution
control equipment. Carbon monoxide emissions from
mobile combustion are a function of the efficiency of
combustion and post-combustion emission controls.
Carbon monoxide emissions are highest when air-fuel
mixtures have less oxygen than required for complete
combustion. This occurs especially in idle, low speed
and cold start conditions. Methane and NMVOC
emissions from motor vehicles are a function of the
content of motor fuel, the amount of hydrocar-
Table 1-6
,Mobile Comustion: 99p-l;9M;
••Year
£1990
~ 1991
^1992
i 1993
I 1994
NOX NMVOCs
CO CH4 N,O
i ,, .......... i „ „ ............. , ,f ...... , IN;,,!: , , HI',,, n,i,,r, ,,'•" ........ n, ,IL ........... nil rt I
(Thousand Metric Tonnes)
9,371 8,141 70,308 248 98
9^26 7,821 69,559 250 100 \
?,369 ........................... 7^46? ....................... 67,820 ...... \ ........................ 256 ...... 102 ;
9,521 7,535 68,469 262 104*
9,636 7,753 69,607 267 106
^Sources:NOX, NMVOCs, and CO emissions data are from U.S. EPA
(/9956J;CH| and N20 emissions were calculated with data provided
' by the US. EPA and E/A (U.S. EPA (/ 995b;, Brezinsfci, et al. (/ 992J;
Cofbon (!994);Nizlch (1994 and I995);EIA (!994e);DOT (1994)
and FM (1994 and 1995)).
bons passing unburnt through the engine, and any
post-combustion control of hydrocarbon emissions,
such as catalytic converters.
Emissions from mobile sources are estimated by
major transport activity, (i.e., road, air, rail, and ships),
where several major fuel types, including gasoline,
diesel fuel, jet fuel, aviation fuel, natural gas, liquefied
petroleum gas (LPG), and residual fuel oil are consid-
ered. Road transport accounts for the majority of
mobile source fuel consumption, and hence, the major-
ity of mobile source emissions. Table 1-7 summarizes
the preliminary 1994 emissions from mobile sources
by transport activity, vehicle type, and fuel type.9
Since 1990, emissions of CO and NMVOCs have
decreased by 1 and 5 percent, while emissions of
NOX have increased by almost 3 percent. All criteria
pollutants experienced a slight drop in emissions dur-
ing 1991 and 1992, after which emissions have
increased steadily. For NOX this increase surpassed
1990 levels, due to increased vehicle miles traveled
(VMT) for gasoline vehicles, as well as increased fuel
consumption for non-road vehicles for which NOX
control measures are not yet in effect.
Methane and N2O emissions increased from
1990 to 1994, rising from 1.67 and 8.58 MMTCE in
1990 to 1.79 and 9.29 MMTCE in 1994. The change
was dominated by gasoline on-road vehicles, where
CH4 and N2O emissions increased by 7 and 8 percent
respectively, or 0.12 MMTCE for CH4 and 0.71
MMTCE for N2O, respectively.
Methodology used to Estimate Emissions
, NMVOCs, and CO
Emissions estimates for NOX, NMVOCs, and
CO (U.S. criteria pollutants) in this section were
taken directly from U.S. EPA (1995b), except for
emissions from bunker fuels (fuels delivered to
marine vessels, including warships and fishing ves-
sels, and aircraft for international transport), which
were calculated based on U.S. EPA data. The U.S.
EPA provided emission estimates for eight categories
' Annex B contains a description of the methodology and data sources used for these estimates. Estimates of CO2 emissions from mobile
combustion are provided as part of the transportation sector in the section titled "Carbon Dioxide Emissions from Fossil Fuel
Consumption." These CO2 estimates are not provided at the level of detail indicated in Table 1-7, because fuel consumption data for
each of these categories, which would be needed to complete calculations, are not readily available.
22 B Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
of highway vehicles,10 aircraft,11 and seven categories
of off-highway vehicles.12
CH4 and N20
Emission estimates of CH4 and N2O from mobile
sources historically have not been calculated by the
U.S. Emission estimates for these gases were calculated
using the IPCC-recommended methodologies and
emission factors (EPCC/OECD/IEA, 1995). Activity
data were derived from U.S. EPA (1995b), Brezinski, et
al. (1992); Carlson (1994); Nizich (1994 and 1995);
EIA (1994e); DOT (1994) and FAA (1994 and 1995).
Table 1-7
Uncertainty in the Emission Estimates
Estimating emissions from mobile combustion,
as with stationary combustion, can be time consum-
ing and complex. Also, the estimates can vary signif-
icantly due to many factors, including fuel type,
technology type, extent of emission control equip-
ment, equipment age, and operating and maintenance
practices. However, compared to stationary sources,
more detailed data are available on activity levels and
emission factors by vehicle type.
.Source Category
Gasoline Highway Vehicles
-"- Passenger Cars
p;, Light-Duty Trucks
^Heavy-Duty Vehicles
Motorcycles
NOX
3,390
1,299
302
II
NMVOCs CO CH4a
(Thousand Metric Tonnes)
f Diesel Highway Vehicles0
Passenger Cars
" Light-Duty Trucks
Heavy-Duty Vehicles
msJEJjBp^^istiujSk '^n
Other Mobile Sources
fe Aircraft
^ Locomotives
£• Vessels/Boats
^:_ Farm Equipment
h; Construction Eq
Pf Other Off-Highway
N2Oa
3,524
1,510
357
33
35,481
13,734
4,757
174
126
78
26
4
74
21
1
+
36
7
1,784
13
'3
271 '
31 + +
6 + +
1,218 16 6
u
Equipment
way
139
859
189
241
980
399
192
" '""'" 39 -
444
54
iss
1,159
-sr-rBjwiriMMfcs^!*
964
112
1,197
250
958
10,723
'^TKldTV^HttlPw^wrt
5
2
3
5
1
b
.-^;----lir"HB*l
+ •
.... . . {
2 '•
1
+ :
b
ft^n j - rmr*i~9
BHP(--.7»~
W^iijn^
I Sources: NOX, NMVOCs, and CO emissions data are from US. EPA (i995b);CH4 and N20 emissions were calculated with data provided by the U.S. EPA
fland E/A (US. EPA (I995b), Brezinski, et al. (1992);Carlson (!994);Nizich (1994 and I995);EIA (!994e);DOT (1994) and FM (1994 and 1995)).
|cNotes: /. totals may not equal the sum of components due to independent rounding.
jj: 2. "+" Denotes negligible (i.e., <0.5 Thousand MT).
a. Average of high and low estimates reported for diesel vehicles.
Ip fa. for CH4 and N20,"0fter Of ^^
P'il c. ^rnates carry an error rangeof ± 50 percent, of which these numbers are the midpoints.
10 These categories include: gasoline powered automobiles, diesel powered automobiles, light duty gasoline trucks less than 6,000
pounds in weight, light duty gasoline trucks between 6,000 and 8,500 pounds in weight, light duty diesel trucks, heavy duty gasoline
trucks and buses, heavy duty diesel trucks and buses, and motorcycles.
11 Currently, emissions factors are not available for aircraft flying above 3000 feet. These emissions may be significant and do affect
atmospheric chemistry, but sufficient information to calculate these emissions is unavailable at this time.
12 These categories include: gasoline and diesel farm tractors, other gasoline and diesel farm machinery, gasoline and diesel construction
equipment, snowmobiles, small gasoline utility engines, and heavy duty gasoline and diesel general utility engines.
Emissions from Energy
23
-------
FOSSIL FUEL PRODUCTION,TRANSPORT,
STORAGE, AND DISTRIBUTION
Emissions from Coal Mining
The most significant emissions from coal mining
are methane (CH^). Emissions from coal mining are
currently the third largest source of methane emis-
sions in the U.S., behind landfills and domestic live-
stock, accounting for approximately 15 percent of
national methane emissions. Estimates of methane
emissions from coal mining for 1994 were about 28.9
MMTCE.
The amount of methane released during coal
mining is primarily a function of coal rank and depth,
although other factors, such as moisture, also affect
the amount of methane released. In most under-
ground coal mines, methane is removed by circulat-
ing large quantities of air through the mine and
venting this air (typically containing a concentration
ofl percent methane or less) into the atmosphere. In
some very gassy underground mines, however, more
advanced methane recovery systems are used to sup-
plement the ventilation systems and ensure mine
safety. In surface mines, methane is emitted directly to
the atmosphere as the strata overlying the coal seam
are removed. In addition to emissions from under-
ground and surface mining, a portion of the methane
emitted from coal mining comes from post-mining
activities such as coal
processing, transporta-
tion, and consumption.
Background and
Overall Emissions
The process of coal
formation, commonly
called coalification, inher-
ently generates methane
and other by-products.
The degree of coalifica-
tion (defined by the rank,
i.e., quality of the coal)
determines the quantity
of methane generated.
Once generated, the amount of methane stored in
coal is controlled by the pressure and temperature of
the coal seam and other, less well-defined characteris-
tics of the coal. The methane will remain stored in the
coal until the pressure on the coal is reduced. This can
occur through erosion of the overlying strata or
through coal mining. Once the methane has been
released from the coal seam, it flows through the coal
toward a pressure sink (such as a coal mine) and
methane is released into the atmosphere (U.S. EPA,
1990a).
Methane emissions from coal mining in 1994
ranged from 3,300,000 to 5,300,000 metric tonnes of
methane (22.3 to 35.5 MMTCE), with a central esti-
mate of approximately 4,300,000 metric tonnes
(28.9 MMTCE). This is a decline of about 2 percent
since 1990 (see Table 1-8).
From 1990 through 1992, methane emissions
from coal mining decreased by about 1 MMTCE per
year (from 29.4 MMTCE in 1990 to 28.4 in 1991 to
27.4 in 1992). The decrease from 1990 to 1991 was
caused by lower coal production levels in 1991, par-
ticularly for coal produced from underground mines.
In 1992, total annual coal production for both under-
ground and surface mines was very similar to 1991
production. However, coal production from under-
ground mines in the
Central - Appalachian
Basin decreased, while
production from other,
less gassy basins
increased. An additional
factor contributing to the
reduced emissions levels
was that a large methane
recovery and utilization
project, involving four
extremely gassy Virginia
mines, started during the
second half of 1992.
While emissions de-
clined gradually between
24 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table 1-8
-.. ... -.
Goal Mine Metharie Emissit|Esii
;I990
|I99I
P992
1993
Q994
Range Point Estimate
* (Million Metric Tonnes)
3.4 - 5.4 4.40
3.3 - 5.2 " 4.25
3.Y-5.0 4.10
3.0-4.3 3.65
3.3 - 5.3a 4.33
1994 estimates are preliminary as detailed production data and emis-
•gons from ventilation and degasification systems ore not yet available.
^ ^ __ _ _j&
1990 and 1992, there was a substantial decrease
between 1992 and 1993 — from 27.4 MMTCE in
1992 to 24.4 MMTCE in 1993. Two primary factors
account for this decrease. First, a lengthy strike by the
United Mine Workers of America against many large
underground mines resulted in substantially lower
coal production levels from underground mines
(underground production decreased from 406 million
tons in 1992 to 351 million tons in 1993). Second,
the new methane recovery and utilization project in
Virginia, which started producing in the second half
of 1992, was in full operation throughout 1993. This
one project alone accounted for emissions reductions
of 1.2 MMTCE. During the time period between
1990 through 1993, the total quantity of methane
liberated13 per ton of coal mined remained fakly con-
stant.
Methane emissions increased from 24.4
MMTCE in 1993 to 28.9 MMTCE in 1994. The
1994 estimate is preliminary as coal production data
by basin and type and methane emissions estimates
from underground ventilation systems are not yet
available for that year. The increase in emissions is
due to the increase in coal production from about
945 million short tons in 1993 to about 1,031 million
short tons in 1994. In particular, production from
gassy underground mines increased in 1994 because
the strike against many large underground mines in
the eastern U.S. was over by the start of 1994.
Methodology Used to Estimate Emissions
For 1990 and 1993, these estimates were based
on detailed analysis of coal mine methane emissions
from surface and underground mines and post-min-
ing activities. Emissions were estimated for each
major coal mining source, including both ventilation
and degasification systems at underground mines,
surface mines, and post mining operations. Detailed
emissions data for underground mines were not avail-
able for 1991 and 1992. For 1991, 1992, and 1994,
the 1990 emissions were adjusted to account for dif-
ferences in coal production between 1990 and these
subsequent years. Coal production levels for all
basins for surface and underground mines were com-
pared to coal production levels for 1990.
Detailed emissions data for 1994 are not yet
available. Accordingly, for 1994, preliminary emis-
sions estimates were based on 1990 emissions factors,
adjusted for the increase in total coal production and
the increase in the amount of methane recovered and
used for pipeline sales. 1990 emissions factors, rather
than 1993 emissions factors, were used to estimate
1994 emissions due to the impact of the strike on
1993 emissions estimates.
For 1990 and 1993, the following data were used
to estimate emissions from underground mines:
• methane emissions from ventilation systems for all
underground mines with methane emissions
greater than 0.1 million cubic feet per day (mea-
sured by the Mine Safety and Health
Administration and reported by the U.S. Bureau of
Mines (Bureau of Mines, 1995i);
B estimated ventilation emissions from mines for
which measurements were not available;
B reported emissions from degasification systems;
• estimated emissions from degasification systems
from mines for which reported values were not
available; and
B reported and estimated methane recovered from
degasification systems that was sold to pipelines,
rather than emitted to the atmosphere.
For all years, emissions from surface mines were
13 Total methane liberated is the total quantity of methane released from the coal seams, which includes both methane emitted and
methane recovered and used for energy purposes.
Emissions from Energy & 25
-------
estimated using reported in-situ methane contents for
the surface coals mined in each U.S. coal basin and by
assuming that total emissions were from 1 to 3 times
the in-situ content of the coal. Post-mining emissions
from both underground mines and surface mines
were estimated to be between 25 to 40 percent of the
in-situ methane content of the coal mined in each
basin. A more detailed discussion of emissions from
each stage of the coal extraction process can be found
in U.S. EPA (1993a).
Further research is being conducted into emis-
sions from surface mines, post-mining emissions, and
emissions from abandoned mines. This research will
potentially make it possible to provide improved esti-
mates of emissions from these sources in the future. A
more detailed description of this recent research may
be found in Piccot, et al (1995).
Uncertainty in the Emission Estimates
The key uncertainties in these estimates arise
from emissions for which measurements are not
available. The most significant source of uncertainty
stems from emissions from degasification systems at
underground mines. While the EPA has developed a
list of mines known or believed to have degasification
systems in place, there is still some uncertainty
regarding which mines have degasification systems.
Furthermore, the quantity of methane that is emitted
from these systems has not been reported and is not
known for most mines. Accordingly, emissions from
degasification systems must be estimated. For mines
with unknown degasification emissions, it was
assumed that mines generally emit between 35 to 65
percent of their total emissions from degasification
systems. To the extent that the degasification strategy
varies by mine or coal basin, emissions could be over-
or underestimated.
Due to a combination of instrument error and
sampling and aggregation errors, measured estimates
of emissions from ventilation systems may have an
uncertainty range of ±20 percent. Estimates for sur-
face mining are less certain as these emissions are not
measured by the Mine Safety and Health
Administration. Surface mining emissions are esti-
mated to range from 1 to 3 (and possibly as much as
5) times the amount of methane contained in the coal.
As mentioned previously, recent research by EPA
should help to clarify these emissions in the future.
Emissions from Natural
Gas Production, Processing,
Transport, and Distribution
The production, processing, transport, and dis-
tribution of natural gas produces methane emissions.
These emissions from U.S. natural gas systems
account for about 11 percent of total U.S. methane
emissions. Between 1990 and 1994, methane emis-
sions from natural gas systems have remained rela-
tively constant at approximately 3 million tonnes (20
MMTCE). Although no emissions trend is discern-
able using the estimation method applied here, emis-
sions reductions of nearly 0.1 million tonnes have
been reported under the Natural Gas STAR program
for 1993 and 1994. Future emissions estimates will
need to develop an approach that considers the
impact of this program.
In addition to not reflecting the impacts of the
Natural Gas STAR program, the emissions estimates
remain very uncertain because the basis for estimat-
ing emissions remains extremely weak. Currently,
work is ongoing to improve the accuracy of the esti-
mates, which may change the emissions estimates
substantially as new information becomes available
and new methods are developed.
Background and Overall Emissions
Emissions from the U.S. natural gas systems are
generally process related. Normal operations of sys-
tems, routine maintenance, and system upsets all con-
tribute to methane emissions. Emissions from normal
operations include: emissions from the exhaust of
engines and turbines that use natural gas as fuel,
bleed and discharge emissions from pneumatic
devices, and fugitive emissions from system compo-
nents. Routine maintenance emissions are released
from pipelines, equipment, and wells during repair
and maintenance activities. System upset emissions
originate from pressure surge relief systems and acci-
dents. These sources of emissions exist throughout
26 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
the natural gas system, in different quantities and
degrees in different stages and activities.
There are a variety of activities that exist and
take place throughout the U.S. natural gas system.
This complex system encompasses hundreds of thou-
sands of wells, hundreds of processing facilities, hun-
dreds of thousands of miles of transmission pipelines,
and over a million miles of distribution pipeline. The
system can be divided into six stages, each with dif-
ferent factors affecting methane emissions, as follows:
• Field Production. In this initial stage, wells are
used to withdraw raw gas from underground for-
mations. Emissions arise from the wells them-
selves, treatment facilities, gathering pipelines, and
process units such as dehydrators and separators.
Fugitive emissions and emissions from pneumatic
devices account for the majority of total methane
emissions. Emissions from field production
accounted for roughly one-third of total emissions
from natural gas systems (U.S. EPA, 1993a).
• Processing Plants. At this stage, undesired con-
stituents in the raw gas are removed before the gas
is injected into the transmission system. Based on
estimates from model plant analyses, emissions
from the venting of glycol dehydrators account
for a large portion of emissions, with compressor
start/stops and fugitive emissions accounting for
most of the remaining discharge (U.S. EPA,
1993a). Processing plants contribute less than five
percent of the total emissions from natural gas
systems.
• Storage and Injection/Withdrawal Facilities.
Natural gas is injected and stored in underground
formations during periods of low demand, and
withdrawn, processed, and distributed during peri-
ods of high demand. From a 1990 analysis of five
plants (Tilkicioglu, 1990), the only significant
source of emissions originated from compressor
start/stops and routine maintenance. Less than one
percent of total emissions from natural gas systems
can be attributed to these facilities.
• Transmission Facilities. These are high pressure,
large diameter lines that transport gas long distances
from sources of supply to distribution centers or
large volume customers. In 1993, the transmission
system consisted of approximately 272,200 miles of
line. An additional 77,300 miles of field and gather-
ing line also fall into this segment. Throughout the
system, compressor stations pressurize the gas. The
majority of emissions were found to arise from rou-
tine maintenance, fugitive emissions, and pneumatic
devices. Methane emissions from the transmission
sector accounted for approximately one-third of
total emissions from natural gas systems.
• Distribution Systems. Distribution pipelines are
low pressure pipelines used to deliver gas to cus-
tomers. The distribution network consists of over
1.3 million miles of line (AGA, 1991). In U.S. EPA
(1993a), emissions from distribution were shown
to arise mostly from fugitive emissions from non-
plastic pipe and gate stations. The distribution sys-
tem accounts for roughly 10 percent of total
emissions from natural gas systems.
B Compressor Engines. These engines, which are
used throughout the entire industry, produce emis-
sions in their exhaust. Reciprocating engines
account for the majority of exhaust emissions,
with turbines contributing a small amount.
Compressor engines account for less than 15 per-
cent of total emissions from the gas system.
Taking into account the high level of uncertainty
in the calculation of emissions estimates, the small
fluctuations in the emissions estimates from 1990 to
1994 are negligible (see Table 1-9). Thus, during the
period 1990 to 1994, estimates show that methane
emissions from natural gas systems have remained
virtually unchanged.
Methodology Used to Estimate Emissions
The methodology used to estimate methane emis-
sions from the natural gas system, as described in the
EPA report, Anthropogenic Methane Emissions in the
United States: Estimates for 1990 (U.S. EPA, 1993a),
is as follows:
1. One or more model facilities were defined for
each stage of the natural gas system. These model
Emissions from Energy
27
-------
Table 1-9
1 1 .<* 1- •
Metha'he
LI • ' * r • •-
li.. ; _:„_;
^Segment
jl ;; ; : ; ;
1 Production
| Processing
•JjStorage
I Transmission
I Distribution
1 Compressor Engines
TOTA^-: ' I;!;-
I " 1994 data are preliminary.
^m's*'^iSl^l''S!^£1rS^ly(r
1990 1991
'""! ' i." " " '.'..'11+ i|, 'h'l'i',,' H, T" 'n." • ', , ,n -,_,,'i,|. , 1, "Jill ,„, »',!. "iiSL.ir * ' I' «il
LOS I..08
0.08 0.09
0.02 0.02
1.04 I.OS
0.33 0.34
0.42 0.41
2.S»7 ! i; 2J99
' Vi "•-"• ' " • -• "*
1992
:r,«.,ii, (Million Metric Tonnes)
1.08
0.09
0.02
1.06
0.34
0.40
i 2.99 i
"-I Tl ^
?Rt$»}
1993
1.07
0.02
1.01
0.42
2.9:4; -r i
,,|C *•* ,;>
19943
:'•
?isS3fc»Sisigl
1. 10
0.09
0.02 i
1.04
0.35 :
0.43
3.03 H !!:i|
vy-_ $
facilities were selected based on the extent to
which they were representative of the system.
2. Emissions types were identified for each model
facility based on detailed data describing the
facility and die processes that lead to emissions.
3. Emissions factors for each model facility were
estimated based on an appropriate measure of
the facility's size (e.g., throughput in ft3/year or
miles of pipeline).
4. Average emissions factors were estimated by
averaging the emissions factors estimated for
each of the model facilities in each stage of the
industry.
5. National emissions were estimated by multiply-
ing the average emissions factors for each stage
by the total applicable size of the national system,
such as billion cubic feet of throughput, number
of wells, or miles of pipeline.
6. Total industry emissions were determined by add-
ing the national segment emissions for each year.
Estimates of each emissions type within each seg-
ment of the industry were obtained by multiplying
emissions factors with their corresponding activity
levels. To estimate emissions for 1991 to 1993, the
emissions factors developed for 1990 in the EPA
report (U.S. EPA, 1993a) were multiplied with
updated activity factors for 1991 to 1993. National
aggregate activity factors were obtained from the
AGA publication Gas Facts (AGA, 1994), and the
Natural Gas Annual (EIA, 1994i). Since appropriate
1994 activity factors were unavailable, 1994 emis-
sions were estimated by increasing 1993 emissions
data based on the percent increase in national natural
gas production (approximately 2.7 percent).
Uncertainty in the Emissions Estimates
Due to the complexity and size of the U.S. nat-
ural gas system, activity levels are uncertain..
Similarly, extrapolating measurement data from a
small number of "model" facilities to determine aver-
age emissions factors for the whole industry also
becomes a large source of uncertainty. Recognizing
the weaknesses in the bases for the estimates pre-
sented above, the U.S. EPA and the Gas Research
Institute are conducting in-depth data collection and
analysis to improve the basis for making emissions
estimates (Kirchgessner, et al, 1995). The ongoing
work involves collecting field data to estimate the
number of components in each stage of the natural
gas system. These field data will then be extrapolated
to estimate national activity factors. Additionally,
detailed field measurements are being conducted to
improve the emissions factors for each component
type. The results of the ongoing work are anticipated
in the coming year, at which time the emissions esti-
mates will be updated.
Emissions from Production,
Refining, Transportation, and
Storage of Petroleum
The major gas emitted from the production and
refining of petroleum products is methane (CH4). The
28 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
activities that produce these emissions are production
field treatment and separation; routine maintenance
of production field equipment; crude oil storage;
refinery processes; crude oil tanker loading and
unloading; and venting and flaring. Together, these
activities account for approximately one percent of
total U.S. methane emissions. From 1990 to 1994,
these emissions remained relatively constant at
approximately 0.27 million tonnes per year (1.8
MMTCE). The emissions estimates remain very
uncertain as the basis for estimating emissions
remains weak due to the complexity of sources and
factors affecting emissions. Work is ongoing to
improve the accuracy of the estimates, which may
change substantially as new information becomes
available and sounder methods for characterizing the
system are developed.
The flaring of gas from oil wells is also a small
source of carbon dioxide (CO?), while the above
activities release small amounts of nitrogen oxides
(NOx), nonmethane volatile organic compounds
(NMVOCs), and carbon monoxide (CO). Each of
these sources, however, is a small portion of overall
emissions. Emissions of CO2, NOX, and CO from
petroleum production activities are all less than one
percent of national totals, while NMVOC emissions
are roughly three percent of national totals.
Table I-10
Methane: Background and Overall Emissions
Several activities during the production and refin-
ing of petroleum products produce methane emis-
sions. Tilkicioglu and Winters (1989) identified the
major emissions sources as:
• Production Field Emissions. Fugitive emissions
from oil wells and related production field treat-
ment and separation equipment are the primary
source of field production emissions. From 1990 to
1994, these emissions accounted for about 60 per-
cent of total emissions from petroleum production
and refining (see Table 1-10). Routine maintenance,
which includes the repair and maintenance of
valves, piping, and other equipment, accounted for
less than 1 percent of total emissions from petro-
leum production and refining.
• Crude Oil Storage Emissions. Crude oil storage
tanks emit methane during two general processes.
When the tank is in use, roof seals and joints
become a source for "breathing losses." When
tanks are emptied and filled, the methane in the
space above the liquid in the tank is often released;
these emissions are referred to as "working
losses." Also, piping and other equipment at stor-
age facilities produce fugitive emissions. Between
1990 and 1994, crude oil storage emissions
accounted for about 5 percent of total emissions
. ^Methane EmiisiqjnS. from"tj|eSlPr^dj£t|p|^|^ 990- 1 994 =
f K r
Emissions Source
f
Production
r Fugitive Emissions
r~ Routine Maintenance
[Refining
g^ Waste Gas Streams,
Storage
f-^Crude Oil Storage
-Transportation
CT Tankers _
";/;i99o'/
* 22.4
0.05
J0.3
~" 1.8
5-6
1^91
'
22.4
0.05
10.1
" 1.8
HBianvw^
1992
• (Thousand Metric Tonnes)
22.5
0.05
9.9
T.8"
•••«•••••••«••••••••«•
1993
22.1
0.05 "
lo.r
1 5
-'s.4 ; / ' ; ' ;
•§•*!•••••••••
1994
22.1
0.05
9.7
1.9
5.3
m^——smm
'- ^
i
A
' 1
1
jg
ma
9.9- 158.9
92.5 - 462
1614-620.9
10.0- 160.3
92.5 - 462
l02.S-62'2.3
ing and Flaring (V&F)
withV&F
9.9-157.6
92.5 - 462
1014-619.6
9.75-156.0
92.5 - 462
102:2-618.0
POINT ESTIMATE (W/V&F) 271
Emissions from Energy • 29
-------
from petroleum production and refining.
• Refining: Waste Gas Streams. Waste gas streams
from refineries are a source of methane emissions.
Based on Tilkicioglu and Winters (1989), which
extrapolated waste gas stream emissions to
national refinery capacity, emissions estimates from
this source accounted for approximately 25 percent
of total methane emissions from the production
and refining of petroleum.
• Transportation: Tanker Operations. The loading
and unloading of crude oil tankers releases
methane. From 1990 to 1994, emissions from
crude oil transportation on tankers accounted for
roughly 14 percent of total emissions from petro-
leum production and refining activities.
• Venting and Flaring. Gas produced during oil pro-
duction that cannot be contained or otherwise han-
dled is released into the atmosphere or flared.
Vented gas typically has a high methane content,
and flaring does not always destroy all the methane
in the gas. Venting and flaring can potentially
account for up to 90 percent of emissions from the
production and refining of petroleum, but there is
a wide range of potential estimates for this cate-
gory, which reflects the considerable uncertainty in
the estimate for this emissions source.
Methane: Methodology used to
Estimate Emissions
The methodology used for estimating emissions
from each source is described as follows:
• Production Field Emissions. Fugitive emissions
and routine maintenance emissions during produc-
tion are driven by the size of the production system,
i.e., the number of oil wells. Emissions estimates
are obtained by multiplying emissions factors
(emissions per well) with their corresponding activ-
ity level (number of wells). To estimate emissions
for 1990 to 1993, emissions factors developed for
1990, in the EPA report (U.S. EPA, 1993a) were
multiplied with updated activity levels for 1990 to
1993. The updated activity levels were obtained
from the Oil and Gas Journal ("Worldwide Look
at Reserves and Production", 1994). Since no
updated activity factors were available for 1994,
1993 values were used as preliminary estimates.
I Crude Oil Storage Emissions. There are significant
uncertainties in estimating crude oil storage tank
emissions because a good census of tank character-
istics that influence emissions is not available.
Tilkicioglu &c Winters (1989) estimated crude oil
storage emissions based on a model tank farm facil-
ity with fixed and floating roof tanks. Emissions
factors developed for the model facility were
applied to published crude oil storage data to esti-
mate total emissions. Crude oil storage data for
1990 to 1993 were obtained from the Energy
Information Administration (EIA, 1994k). Since no
updated activity factors were available for 1994,
1993 values were used as preliminary estimates.
I Refining: Waste Gas Streams. Tilkicioglu &
Winters (1989) estimated national methane emis-
sions from waste gas streams based on measure-
ments at 10 refineries. These data were
extrapolated to total U.S. refinery capacity to esti-
mate total emissions from waste gas streams for
1990. To estimate emissions for 1991 to 1994, the
1990 emissions estimates were scaled using
updated data on U.S. refinery capacity. These data
were obtained from the Energy Information
Administration (EIA, 1994k).
I Transportation: Tanker Operations. Methane
emissions from tanker operations are associated
with: (1) the loading and unloading of domesti-
cally-produced crude oil transported by tanker; and
(2) the unloading of foreign-produced crude trans-
ported by tanker. The quantity of domestic crude
oil transported by tanker was estimated as Alaskan
crude oil production less Alaskan refinery crude oil
utilization, plus 10 percent of non-Alaskan crude
oil production. Crude oil imports by tanker were
estimated as total imports less imports from
Canada. An emissions factor based on the methane
content of hydrocarbon vapors emitted from crude
oil was developed (Tilkicioglu & Winters, 1989).
The emissions factor was multiplied by updated
activity data to estimate total emissions for 1990 to
30 B Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
1994. Data on crude oil stocks, crude oil produc-
tion, utilization, and imports was obtained from
the Energy Information Administration (EIA,
1994k).
• Venting and Flaring. The basis for estimating emis-
sions from venting and flaring is weak. Also, the
data for estimating how much gas is vented and
how much is flared are very poor. Because of this
considerable uncertainty in estimating venting and
flaring emissions, the 1990 estimates were used in
the calculations of total estimates for the years
1991-1994. The range of emissions estimated in the
EPA report (U.S. EPA, 1993a) for 1990 is 92.5 to
462 million kilograms.
Methane: Uncertainty in the
Emissions Estimates
The fkst range in Table 1-10 accounts for uncer-
tainty in the emissions estimates excluding venting
and flaring. Following the uncertainty range adopted
in U.S. EPA (1993a), the range consists of one fourth
to four times the total estimate. The venting and flar-
ing estimates are those determined by Radian
(1992b) and Barns and Edmonds (1990). The addi-
tion of venting and flaring results in a combined
range of approximately 102 to 620 million kilograms
of methane per year.
As the wide range reveals, considerable uncer-
tainty is present in these emission estimates. Presently,
sufficient national emissions data do not exist.
Consequently, the emissions factors were determined
based on measurements at several model facilities
that may not encompass the range and diversity of
factors that affect emissions. However, ongoing
efforts to develop more precise assessments may sig-
nificantly improve the emissions estimates.
Carbon Dioxide: Emissions,
Methodology, and Uncertainty
Carbon dioxide emissions from oil and gas pro-
duction come from the natural gas that is flared at the
production site, which releases CC>2 as a by-product
of the combustion process. Barns and Edmonds
(1990) note that of total reported U.S. venting and
Table I-11
r=:
fc"
fe"?;;--
^,"~
p-
fa —
^r "~
|I Source:
y • 1 co2 Ep-i
if larihg 1
Year
1990
1991
1992 "--'-'-
1993
1994 '
£/A(/994()
'•'""V: C02' . :
(Million Metric Tonnes) '•.
6.5 }
7.4 ;
-•;'"";";: ;•"• 7.3 • ]
8.3
": , 5.0. .. '
flaring, approximately 20 percent is actually vented,
with the remaining 80 percent flared. The amount of
natural gas vented and flared was obtained from the
Natural Gas Monthly (EIA, 1994J) and used to esti-
mate the amount of CO2 resulting from the flared
gas. For 1994 these emissions were estimated to be
approximately 5 million metric tonnes (1.4
MMTCE), which was down by approximately 23
percent from 1990 (see Table 1-11).
The estimates were prepared using a conversion
factor of 525 grams of carbon per cubic meter of
flared gas, as determined by Marland and Rotty
(1984), and an assumed flaring efficiency of 100 per-
cent. The assumed uncertainty range is ±25 percent.
The 20 percent vented as methane is accounted for in
the above section on methane emissions from petro-
leum production, refining, transportation, and stor-
age activities.
Nitrogen Oxides, NMVOCs, and CO:
Emissions, Methodology, and Uncertainty
Criteria pollutant emissions from oil and gas pro-
duction, transportation, and storage constitute a rel-
atively small and stable portion of the overall U.S.
emissions of these gases for the 1990 to 1994 period
(see Table 1-12).
The U.S. EPA (1995b) provided emission esti-
mates for NOX, NMVOCs, and CO from petroleum
refining, petroleum product storage and transfer, and
petroleum marketing operations. Included are gaso-
line, crude oil and distillate fuel oil storage and trans-
fer operations, gasoline bulk terminal and bulk plants
operations, and retail gasoline service stations opera-
tions. Emission estimates were determined using
industry-published production data and applying
Emissions from Energy
31
-------
Table I-12
* NMVOCs, and G5"EMfslidnS'"firom""
Oil and das A^ivities: 1 ffi - 99^ *f 3
-Year
1990
1991
V! 11 HI II Illll Ill I
NO*
NMVOCS
co
average emission factors.
Due to the diverse nature of the various types of
emissions and the fact that some emissions occur peri-
L "" TLCT^°isjm!^?Hi5~'?nnesi. j odically or unexpectedly, precise measurements are
91
395
"I991
'source; b.S. EPA
7/9955,}."
583
88 575 374
«r i in ni ilwramini* tin TMnrnrtmt ***
87 579 372
.86 ;572p,
86 572 .. 354
not practical in many cases. As a result, the uncertain-
ties associated with the emission estimates in this sec-
tion vary, ranging anywhere from 25 to 50 percent.
EMISSIONS FROM BIOMASS AND BIOMASS-BASED FUEL CONSUMPTION
The combustion ofbiomass fuels (such as wood,
charcoal, and wood waste) and biomass-based fuels
(such as ethanol from corn or woody crops) produce
carbon dioxide. However, the carbon dioxide (CO^)
emitted from biomass consumption in the long run
does not increase total atmospheric carbon dioxide if
this consumption is done on a sustainable basis (i.e.,
annual emissions of CO 2 due to consumption ofbio-
mass are completely offset by the annual uptake of
CO2 from regrowing biomass). As a result, CC^
emissions from biomass combustion have been esti-
mated separately from fossil fuel-based emissions and
are not included in the U.S. totals. Net carbon flux in
the forest sector resulting from land-use activities and
forest management practices are accounted for in
Part V: Emissions from Land-Use Change and Forest
Managetnent.
Carbon dioxide emissions from stationary bio-
mass fuel consumption were estimated to be about 49
MMTCE in 1994. Emissions from this source have
increased 4 percent since 1990, primarily due to
increases in biomass fuel consumption in the indus-
trial sector in response to U.S. economic growth.
Carbon dioxide emissions from ethanol fuel con-
sumption were about 1.9 MMTCE, increasing about
19 percent from 1990 levels. This increase can be
attributed to rising consumption of ethanol due to
new legislation establishing incentives for ethanol fuel
use.
Emissions from
Wood Consumption
Background and Overall Emissions
In 1994, total CO2 emissions due to burning of
woody biomass within the electric utility, industrial,
residential and commercial sectors were about 49
MMTCE (181 million metric tonnes CO2) (See Table
1-13). As the largest consumer of biomass fuels, the
industrial sector was responsible for about 74 percent
of the CO2 emissions from biomass-based fuels. The
residential sector was the second largest emitter of
CO2, making up about 24 percent of total emissions
from biomass. The commercial and electric utility
sectors accounted for the remainder.
Between 1990 and 1994, total emissions of CO2
from biomass burning have increased about 4 percent
despite a 5 percent decrease in biomass fuel use in the
residential sector during this same time period (See
Table 1-14). This increase in total emissions is largely
due to a 7 percent rise in industrial biomass fuel con-
sumption between 1990 and 1994. Increases in
industrial biomass use are directly due to growth in
the U.S. economy. The 5 percent decline in biomass
fuel use in the residential sector is attributable to both
the rising cost of wood burning stoves and a falling
number of households relying on wood as a primary
heating source (Thompson, 1995). Consumption of
32 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table I-13
;::Emissions;iJp^
1992.
" (Million Metric Tonnes)
1993
1994
[f lectric Utility
^Industrial
Residential
[Commercial
•JMBf^^^H
"Sources. 1990-92 emissions estimates are based on biomass consumption estimates in trillion Mu from EJA (I994c). 1993-94 emissions estimates
'for the industriafcbmrnercial, anS"electnc utility sectors, "and the 1993 emissions estimate for the'residenfia/ sector are based on EIA unpublished bio-
Ifiass consumption estimates* See Table 1-14 for industrial and residential biomass consumption estimates for I990-1994.
:: /. Components may not sum to total because of rounding.
2. Consumption estimatesTn triffibn Btu were converted to sHorf tons based on an average energy content of 17.2 million Btu per short ton of
'"„ dry wood (ElA I994c).
3. Estimates carry an error range of ±25 percent
4. According to ElA^commeraal"wood energy useTs typically not reported because there are no accurate data sources to provide reliable esti-
mates (EIA, I994c). However, ElA's 1989 Nonresidential Building Energy Consumption Survey estimates that commercial sector use is about
20 to 40 trillion Btu. An average of 30 trillion Btu is used here.
" Data on residential wood consumption for 1994 were unavailabfe at the time this report was compiled. Emissions for 1993 have been used
as a proxy until 1994 data becomes available.
biomass fuels within the commercial and electric util-
ity sectors remained relatively stable and thus had lit-
tle impact on changes in overall CO2 emissions from
biomass combustion.
Methodology and Uncertainty
Emissions estimates were calculated based on the
methodology recommended by the IPCC. Emissions
were estimated by first converting U.S. consumption
data (in trillions of Btu) to tonnes of dry matter using
EIA assumptions.14 Once consumption data for each
sector were converted to tonnes of dry matter, the
carbon content of the dry fuel was estimated based on
IPCC default values of 45 to 50 percent carbon in dry
biomass. The amounts of carbon released from corn-
Table I-14
bustion were also estimated using IPCC-provided
default values of 87 percent combustion efficiency.
This is probably an underestimate of the efficiency of
wood combustion processes in the U.S. The IPCC
assumption has been used, however, since better data
are not yet available.
Emissions from
Ethanol Consumption
Background and Overall Emissions
Biomass-based fuel use in the U.S. consists
mainly of ethanol use in the transportation sector.
Ethanol is mostly produced from corn grown in the
: -£^' • - - i -. ,- ;-^:VV:^H^ ::v •:^u-:;->^;-s;;T^-^-:^^:i-1i:^;^;:
; Residential and Industrial Biomassi Cottibustion
-- •- - - :~ : ' ' -- . .- < vv : ...^.^ . ..^"^'i; - • '^;!;.-.''^- ^y "'."- \ --"-'• ;-.:/
i^l1^i9"94~^;!
':l ' .$•.-• ' i-;-\ K.;xff->'<-
^Sector '""*"" "*"" * *T$90 * "* 7991 * ***** *" " 19*92 " 1993 1994
pri,, * ,-;£•" ^vr^^wswar^j^ -^S8«i -» -i. ^?^ rn-.n.«n R*»^
g.^ ^^^f t •. f> \i^ffr st-^-Ksat ^-ii^-, » . (Inllion litus)
|Sustnal " A*- - *'^0^2- - *^28 ~ " "7,593 ~ 1,619 1,671
[Residential " ~~*~ "* "587 " "61 3 **" 645 550 550^
Sources 1990-1992 biomass consumption estimates from EIA (I994c) 1993-94 biomass consumption estimates are preliminary and were obtained
Fpwn an EIA database ^
^Estimate for 1994 residentia?biomos*s>consumption waslTOtlTvailabte at the time*thG report was compiled ^1993 biomass consumption data have
^Eeenjused as a proxy until 1 994 data become available
\
1
1*
1
-*
«
*•
^
J
14 Data for 1990-1992 are from the EIA report entitled Estimates of U.S. Biomass Energy Consumption 1992, published in May 1994.
Preliminary data for 1993-1994 were obtained from an EIA database.
Emissions from Energy • 33
-------
Table I-15
- . , , 1990
r - - - . -
^ Ethanol Consumption (trillion Btu) 82
LCO2 Emissions (million metric tonnes) 5.7
1991
65
4.5
1992
79
5.5
1993
88
6.1
1994
98
6.8
^Sources: 1990-1992 biomass consumption estimates from EIA (I994c). 1993-94 blomass consumption estimates are preliminary and were obtained
! from an E/A database.
Midwest, and used primarily in the Midwest and
South. Ethanol can be used directly, or mixed with
gasoline as a supplement or an octane enhancing
agent. The most common form is a 90 percent gaso-
line, 10 percent ethanol blend known as gasohol.
Ethanol and ethanol blends are used to fuel public
transport vehicles such as buses, or centrally-fueled
fleet vehicles. Ethanol and ethanol blends are believed
to burn "cleaner" than gasoline (lower in NOX and
other hydrocarbons), and are being tested in urban
areas with poor air quality. However, because ethanol
is a hydrocarbon fuel, its combustion releases CO2.
Emissions of CO2 in 1994 due to ethanol fuel
burning were estimated to be approximately 1.9
MMTCE (6.8 million metric tonnes of CO2) (See
Table 1-15). Between 1990 and 1991, emissions of
CO2 due to ethanol fuel consumption fell by about
21 percent. Since this decline, emissions from ethanol
have steadily increased through 1994. Between 1991
and 1992, CO2 emissions due to ethanol consump-
tion increased about 22 percent. Increases in CO2
emissions continued at an average annual rate of
about 11 percent between 1992 and 1994. Emissions
from ethanol consumption are not included in the
U.S. total since the corn from which the ethanol is
derived is produced on a sustainable basis.
Increases in CO2 emissions from ethanol con-
sumption between 1991 and 1994 can be attributed
to several factors. In 1990, the Budget Reconciliation
Act extended Federal tax exemptions for ethanol pro-
duction through the year 2000 and the Clean Air Act
Amendments mandated the reduction of mobile
source emissions. In 1992, the Energy Policy Act
established incentives to encourage an increase in the
use of alternative fuels and alternative-fueled vehicles.
Other factors also influencing ethanol consumption
include prices of corn, gasoline, and other alternative
fuels (EIA, 1994c).
Methodology
Emissions from ethanol were estimated based on
EIA (1994c). In 1994, the U.S. consumed an esti-
mated 98 trillion Btus of ethanol (1.3 billion gallons),
mostly in the transportation sector. Using an ethanol
carbon coefficient of 19 milligrams C/Btu (OTA,
1991), 1994 emissions of CO2 from the use of
ethanol were calculated to be about 6.8 million met-
ric tonnes (1.85 MMTCE).
34 B Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Part II:
Industrial Processes
Emissions are often produced as a by-product of various non-energy related activities. That is, these emis-
sions are produced directly from the process itself and are not a result of energy consumed during the
process. For example, in the industrial sector raw materials are chemically transformed from one state to
another. This transformation often results in the release of greenhouse gases such as carbon dioxide (CO2).
Other emissions result from use of greenhouse gases in manufacturing and subsequent emissions of the excess
gas. The production processes addressed in this section include cement production, lime production, limestone
use (e.g., for iron and steel making, flue gas desulfurization, and glass manufacturing), soda ash production and
use, CO2 manufacture, aluminum production, adipic acid production, nitric acid production, and HCFC-22 pro-
duction. Total CO2 emissions from industrial processes were approximately 58.1 million metric tonnes (15.9
MMTCE) in 1994. This accounts for 1.1 percent of total U.S. CO2 emissions. Nitrous oxide (N2O) emissions
from adipic acid and nitric acid production were about 105.8 thousand metric tonnes (9.2 MMTCE) in 1994,
or 22.8 percent of total U.S. N2O emissions. In the same year, emissions of hydrofluorocarbons (HFCs) and
perfluorocarbons (PFCs) combined for about 23.5 MMTCE, and emissions of sulfur hexafluoride (SF6) were
about 1 thousand metric tonnes (7.0 MMTCE). Table tt-1 contains a summary of non-energy related green-
house gas emissions from industrial processes in
the U.S.
Greenhouse gases are also emitted from a
number of industrial processes not covered in
this section. For example, ammonia production
is believed to be an industrial source of methane
(CH4) emissions, N2O, and non-methane volatile
organic compounds (NMVOCs). However,
emissions for these sources have not been esti-
mated at this time due to a lack of information
on the emission processes, the manufacturing
data, or both. As more information becomes
available, emission estimates will be calculated
and included in future greenhouse gas emission
inventories.
The emission estimates presented here gen-
erally follow the EPCC-recommended guide-
lines, although the only processes for which the
Industrial Processes • 35
-------
IPCC provides a specific methodology for estimating
emissions are cement, adipic acid, and nitric acid pro-
duction. The IPCC has not provided specific details
(e.g., default emission factors) to calculate emissions
from the other sources, but recommends a basic
approach that can be followed for each source cate-
gory. This involves multiplying production data for
each process by an emission factor per unit of pro-
duction. The methods used to estimate emissions in
this section generally follow this basic approach.
Most of the emission factors used below were derived
using calculations that assume precise, efficient chem-
ical reactions. As a result, uncertainties in the emis-
sion coefficients can be attributed to, among other
things, impurities contained in the raw materials or to
inefficiencies in the chemical reactions associated
with each production process. Additional sources of
uncertainty specific to an individual source category
are discussed in the appropriate section.
Table II-1
Carbon Dioxide Emissions
Cement Production
Carbon dioxide (COj) emitted during the cement
production process represents the most significant
non-energy source of industrial CO2 emissions.
Cement is produced in most states and is used in all
of them. Carbon dioxide is created when calcium car-
bonate (CaCOj) is heated in a cement kiln to form
lime (calcium oxide or CaO) and CO2. This lime
combines with other materials to produce clinker (an
intermediate product), while the CO2 is released into
the atmosphere. Clinker is then used to make port-
land and masonry cement. The production of
masonry cement requires additional lime and thus
results in additional CC>2 emissions. However, since
this lime is already accounted for in the lime manu-
facture section of this chapter, the resulting emissions
- : - ~•' ..., p '| |:. ij JfT*& -r«l! * •' ' '
RecentTreijds in U.S. G«j6n|iousC&l^^
•Gas/Source
Emissions
(Full Molecular Weight)
,.,,122 31.9" ill
L_ H.7 ,1M
"4.9 4,5
4.0 4.1
5 7.6 7.5
'1.4
""33.9"
,,-12.4
4.1
4.0
6.8
l.f
35.4™
4.6
4.0
6.1
0.3~
"1.9" "
L-4
I.I
_ 2.0
8.7*"
^SSsE
1.3
I.I
2.1
J994
0.4 _ 0.4 0.4
8.8 93 9.6
•3A ,«M^ 1!L
1.4 I.I |.2
I.I " I.I I.I
""2.0 """"").9"" "l.7
Nitric Acid Manufacture
BFCsandPFCs
HFC-23
HFC-Vis
I HFC-l34a
HFC-IS2a
rtFC-227
CF4
U.S. Industrial Emissions
±ure 0.04
cture 0 06
-
+
na
na
+
0.04
0.06
^ — -
+
na
na
+
0.04
0.05
— ^
+
na
na
t i
+
0.04
0.06
±—~~*
+
na
i n?
+
4^ I,
0.04
0.06
+
0.01
J ^MuaBitiiimittiMUK.iJIfc,, iLmuunl*
:-•
+
.. ^™ILr m. ,„. ,1.^ 4T „*-*«-
3.5
49
^^,^ ^ST^TSE
137
na
'6]3*l """
0.8
18.8
"* 6.4* "
3.5
51
'-"L^- "^-
141
na
0.0
«\'
0.9
19.3^
"6.5* ~
3.5
47
148
na
1.2
'0.04
"4.2*
0.8
•» » H* ^ 1»
6.7
3.6
5 1
, J3,!,.
na
^J).04
" ,^3.8
0.8
^J^
3.8
54
138
1.0
3.7,
0.06*
0.8,
3.4
0.7
23.5
7.0
, ^K
!T
£
i
J
48.5 I 49.0 50,9 50;.6 iSS.S
Total does not exceed 6.01 million metric tonnes.
sfofls from aluminum manufacture are not included in the industry totals to avoid doub/e-counting (see text).
|;Note: Totals presented may not equal the sum of the individual source categories due to rounding.
36 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
are not counted towards the cement manufacturing
emissions totals, although they are calculated here for
informational purposes.
In 1994, U.S. clinker production totaled 69.79
million metric tonnes, and U.S. masonry cement pro-
duction reached 3.28 million metric tonnes. As a result,
CO2 emissions from clinker production were esti-
mated to be 9.65 MMTCE, or 0.7 percent of total U.S.
CO2 emissions (Table II-2).1 Emissions from masonry
production were estimated to be 0.02 MMTCE.
Carbon dioxide is produced during the produc-
tion of clinker, an intermediate product from which
finished portland and masonry cement are made. The
quantity of the CO2 released during cement produc-
tion is directly proportional to the lime content of the
clinker. During cement production, calcium carbon-
ate (CaCO3) from limestone, chalk, or other calcium-
rich materials are heated in cement kilns to form lime
(CaO) and CO2:
CaCO3 -»• CaO + CO2
This process is known as calcination or calcining.
The lime is then combined with silica-containing
materials (provided to the kiln as clays or shales) to
form dicalcium or tricalcium silicates, two of the four
major compounds in cement clinker (Griffin, 1987).
Carbon dioxide emissions were estimated by
applying an emission factor, in tonnes of CO2
released per tonne of clinker produced, to the total
amount of clinker produced. The emission factor
used in this analysis is the product of the fraction of
lime used in the cement clinker and a constant reflect-
ing the mass of CO2 released per unit of lime. The
Table 11-2
Soda Ash
Manufacture
& Consumption
7%
Limestone
Use
emission factor was calculated as follows:
This analysis assumes an average lime fraction
for clinker of 64.6 percent, which yields an emission
factor of 0.507 tonnes of CO2 per tonne of clinker
produced.
Clinker production in the U.S. (including Puerto
Rico) was reported as 70 million metric tonnes in 1994
(Solomon, 1995). Combined with the emissions factor
derived above, CO2 emissions from cement manufac-
turing in 1994 were estimated to be 35.4 million met-
ric tonnes (9.65 MMTCE). Carbon dioxide emissions
from U.S. clinker production reached 32.6 million met-
ric tonnes in 1990, 31.9 million metric tonnes in 1991,
32.1 million metric tonnes in 1992, and 33.9 million
metric tonnes in 1993 (see Table II-2).
"Source , Cement Production CO2 Emissions
T * (Thousand Metric Tonnes)
g- . * ivvu 1991* l992 l99? 1993 * (990 1991 1992 1993 1994
[Clinker ' \ 64,355 62,918 63,415 6*6~,957 69,786 32,626 31,897 32,149 33,945 35,379
fHasonry=> 2,911 2,591 2,806 2,975 3,283 65 58 63 67 74
Source: Production data taken from the Bureau of Mines: 1990 (I992a); 1991 & 1992 (1994c); 1993 (1995a); 1994 (Solomon, 1995).
3' Emissions from masonry cement production are not counted towards the cement manufacturing emissions totals.
Please note that emissions in Table II-2 are expressed in thousand metric tonnes.
Industrial Processes • 37
-------
After falling by 8.3 percent from 1990 levels,
U.S. cement production has grown every year since
1991, growing by 5.0 percent in 1992, 10.6 percent
in 1993, and 7.3 percent in 1994. Demand outpaced
production in 1994, causing imports to grow by 67
percent (totaling 11.8 million metric tonnes), due in
part to shortages in some parts of the country.
Despite the magnitude of this growth, imports were
still shy of their all-time high (13.3 million metric
tonnes, established in 1987). Consumption also grew
in 1994, to its highest point of the decade (90.5 mil-
lion metric tonnes), representing an 11.6 percent
increase over the previous year. This consumption
increase is attributed in large part to greater con-
struction activity resulting from the economic recov-
ery and the 1991 transportation bill (Bureau of
Mines, 1995d).
Masonry cement requires additional lime over
and above the lime used in clinker production. In par-
ticular; non-plasticizer additives such as lime, slag,
and shale are added to the cement, increasing its
weight by 5 percent. Lime accounts for approxi-
mately 60 percent of this added weight. Thus, the
additional lime is equivalent to roughly 2.86 percent
of the starting amount of the product, since
(0.6* 0.05/1.05) =0.0286.
An emission factor for this added lime can then
be calculated by multiplying that percentage by the
molecular weight ratio of CO2 to CaO:
(
fraction of weight added \
/ +• fraction of weight added/ \ substance ) v>6-°8 g/mo/e CaO/
g/mofeCoA
(
0.05
a°6 x °'785
oo
= 0.0286 x 0.785
= 0.0224
Thus, 0.0224 tonnes of additional CO2 are emit-
ted for every tonne of masonry cement produced.
Masonry cement production in the U.S. was reported
to be 3.3 million metric tonnes in 1994 (Bureau of
Mines, 1995b). Combined with the emissions factor
derived above, this translates into 73.6 thousand met-
ric tonnes (0.02 MMTCE) of CO2 emitted. U.S.
masonry production reached 2.9 million metric
tonnes in 1990, 2.6 million metric tonnes in 1991,
2.8 million metric tonnes in 1992, and 3.0 million
metric tonnes in 1993.
The CO2 emissions from the additional lime
added during masonry cement production are
already accounted for in the section on CO2 emis-
sions from lime manufacture. Thus, these emissions
are estimated in this chapter for informational pur-
poses only, and are not included in the emission
totals.
The uncertainties contained in these estimates are
primarily due to uncertainties in the lime content of
clinker and in the amount of lime added to masonry
cement. For example, the lime content of clinker
varies from 64 to 66 percent. Also, some amount of
CO2 is reabsorbed when the cement is used for con-
struction. As cement reacts with water, alkaline sub-
stances such as calcium hydroxide are formed.
During the curing process, these compounds may
react with CO2 in the atmosphere to create calcium
carbonate. This reaction only occurs in roughly the
outer 0.2 inches of surface area. Since the amount of
CO2 reabsorbed is thought to be minimal, it is not
included here.
Lime Manufacture
Lime, or calcium oxide (CaO), is a manufactured
product with many chemical, industrial, and environ-
mental uses. Lime has historically ranked fifth in total
production of all chemicals in the United States. Its
major uses are in steel making, construction, pulp
and paper manufacturing, and water and sewage
treatment. Lime is manufactured by heating lime-
stone (mostly calcium carbonate — CaCO3) in a kiln,
creating calcium oxide (CaO) and carbon dioxide
(CO]). The CO2 is driven off as a gas and is normally
emitted to the atmosphere.
Lime production in the U.S. was estimated to be
17.4 million metric tonnes in 1994 (Miller, 1995).
This resulted in CO2 emissions of 3.5 MMTCE, or
0.25 percent of total U.S. CO2 emissions.
Lime is an important chemical with a variety of
industrial, chemical, and environmental applications
38 B Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Soda Ash
Manufacture
& Consumption
• • 7%
Limestone
Use
8%
in the U.S. Lime production involves three main
processes: stone preparation, calcination, and hydra-
tion. Carbon dioxide is generated during the calcina-
tion stage, when limestone (calcium carbonate or a
combination of calcium and magnesium carbonate)
or other calcium carbonate materials are roasted at
high temperatures. This process is usually performed
in either a rotary or vertical kiln, although there are a
few other designs. Carbon dioxide is produced as a
by-product of this process, just as CO2 is released
during clinker production (see previous section on
cement production). The CO2 is driven off as a gas
and normally exits the system with the stack gas. The
mass of CO2 released per unit of lime produced can
be calculated based on their molecular weights:
44.01 g/mole CO2 * 56.08 g/mole CaO = 0.785
Table 11-3
Lime production in the U.S. was 17,400 thou-
sand metric tonnes in 1994 (Miller, 1995). This
results in potential CO2 emissions of 13.66 million
metric tonnes. Some of the CO2 generated during the
production process, however, is recovered for use in
sugar refining and precipitated calcium carbonate
(PCC) production. Combined lime production by
these producers was 1.377 million metric tonnes in
1994, generating 1.081 million metric tonnes of CO2.
Approximately 80 percent of this CO2 is recovered
and not emitted, resulting in net CO2 emissions of
about 12.8 million metric tonnes (3.5 MMTCE) from
U.S. lime production in 1994.
Domestic production has increased every year
since 1991, when it declined by 1.0 percent from
1990 levels. Production grew by 3.4 percent in 1992,
3.5 percent in 1993, and 3.6 percent in 1994 (see
Table II-3). This growth is attributed in part to
growth in demand from environmental applications.
For example, in 1993, the Environmental Protection
Agency (EPA) completed regulations of the Clean Air
Act concerning sulfur dioxide (SO2) emissions caps
for electric utilities. The initial phase of this legisla-
tion has already resulted in greater lime demand; for
example, consumption for flue gas desulfurization
increased by 16 percent in 1993 (Bureau of Mines,
1994b).
The term "lime" is actually a general term that
includes various chemical and physical forms of this
commodity. Uncertainties in the emission estimate
can be attributed to slight differences in the chemical
composition of these products. For example,
although much care is taken to avoid contamination
f
'*"•"•_' Lime Production CO2 Emissions
_ s ' • ^CTnou?anc' Metric Tonnes)
I9?0 1991 1992 1993 1994 1990 1991 1992 1993 1994
.tential CO, from " "15,859" 15,694 ^16,227 16,800 17,400 12,445 12,317 12,737 13,188 13,659
L, * *• f ^ -^^ ^ + -
All Lime Producers t_w,_
•^cweredJlOjfromSugar155^1!* **?64* " \ffi5 "pro*" 1,377' 519 605 642 823 865
: PCC Manufacturers
Net Emissions
11,927
12,092 12,365 12,794
pources; Production fata taken from'the Bureau of Mines: 1990 & 91 (1992b), 1992 (79946). Data for 1993 & 1994, Miller (1995).
Industrial Processes • 39
-------
during the production process, lime typically contains
trace amounts of impurities such as iron oxide, alu-
mina, and silica. Due to differences in the limestone
used as a raw material, a rigid specification of lime
material is impossible. As a result, few plants manu-
facture lime with exactly the same properties.
A portion of the CO2 emitted during lime pro-
duction will actually be reabsorbed when the lime is
consumed. In most processes that use lime (e.g.,
water softening), CO2 reacts with the lime to create
calcium carbonate. This is not necessarily true about
lime consumption in the steel industry, however,
which is the largest consumer of lime. A detailed
accounting of lime use in the U.S. and further
research into the associated processes are required to
quantify the amount of CO2 that will be reabsorbed.
As more information becomes available, this emission
estimate will be adjusted accordingly.
Limestone Use
Limestone is a basic raw material used by a wide
variety of industries, including the construction, agri-
culture, chemical, and metallurgical industries. For
example, limestone can be used as a flux or purifier
in refining metals such as iron. In this case, limestone
heated in a blast furnace reacts with impurities in the
iron ore and fuels, generating carbon dioxide (CO^
as a by-product. Limestone is also used for glass man-
ufacturing and for sulfur dioxide (SO^ removal from
stack gases in utility and industrial plants.
Table 11-4
Soda Ash
Manufacture
& Consumption
Limestone -* 7%
Use
In 1994, approximately 3.9 million metric
tonnes of limestone and 0.7 million metric tonnes of
dolomite were used as flux stone in the chemical and
metallurgical industries, in flue gas desulfurizatiort
systems, and for glass manufacturing. This results in
total CO2 emissions of 1.2 MMTCE, or 0.09 percent
of total U.S. CO2 emissions (see Table II-4).
Limestone is widely distributed throughout the
world in deposits of varying sizes and degrees of
purity. Deposits of limestone occur in nearly every
state in the U.S., usually in tremendous amounts.
Great quantities of limestone are extracted for com-
mercial use. For example, limestone can be used as a
Limestone Production* CO2 Emissions
-•- • ='-- - ------=-------- (Thousand Metric Tonnes)
1990 1991 19?? J 993 _! ?94,_J 99Q I 99'.I 1992 1993 ! 994
Flux Stone
Limestone
Dolomite
Glass Making
SO2 Removal
1 TOT Si! ;; . ,.,,..41=
5,776
929
428
4,303
j7^!T!!
5,213
838
386
4,499
ijrirr
4,422
735
504
4,403
:::::! 'iT:;::1: ••::;:
3,63 1
632
622
4,307
111:,-';.;.;.;
3,984
694
683
4,991
'.. ii 1... i. ... . '
2,541
444
188
1,928
s.l Oil
2,294
400
170
2,003
14:867-3:
1,946
351
222
1,990
sS08::;:;;{
1,597
loi
274
1,895
1,068
•:.
1,753 1
331 :j
300 ;
2,1% ;
S^5801:1:^:H|
. Source: Production data taken from the Bureau of Mines:! 99 1 (I993a), 1 993 (I995c) ;
* A/though the U.S. Bureau of Mines reports production of total crushed stone annually, limestone and dolomite production are provided for odd-num-
bered years only. Limestone consumption fir 1992 was estimated by taking the average of the numbers reported in 1991 and 1993. Consumption fig- ":
iires for 1990 and 1994 were estimated by assuming that limestone and dolomite accounted for the same percentage of total crushed stone in those
" years as they did in 1991 and 1993. ------- ;
40 II Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
flux or purifier in metallurgical furnaces, as a sorbent
in flue gas desulfurization (FGD) systems in utility
and industrial plants, or as a raw material in glass
manufacturing. Limestone is heated during these
processes, generating CO2 as a by-product. Carbon
emissions can be calculated by multiplying the
amount of limestone consumed by the carbon content
of the limestone (which is approximately 12 percent
for calcite, 13 percent for dolomite).2
Using U.S. Bureau of Mines reports for 1990-93,
it was estimated that approximately 3,984 thousand
metric tonnes of limestone and 694 thousand metric
tonnes of dolomite were used as flux stone in the
chemical and metallurgical industries in 1994
(Bureau of Mines, 1995c and g).3 Additionally, 683
thousand metric tonnes of limestone were used for
glass manufacturing (Bureau of Mines, 1995c and g)
and 4,991 thousand metric tonnes of limestone were
used in FGD systems (EIA, 1994n, 1993b, 1992,
1991). Assuming that all of the carbon is released
into the atmosphere, these applications result in total
emissions of 1.2 MMTCE, or 4.6 million metric
tonnes of CO2 (see Table H-4).
Uncertainties in this estimate are due to variations
in the chemical composition of limestone. In addition
to calcite, limestone may contain smaller amounts of
magnesia, silica, and sulfur. The exact specifications
for limestone or dolomite used as flux stone vary with
the pyrometallurgical process, the kind of ore
processed, and the final use of the slag. Similarly, the
quality of the limestone used for glass manufacturing
will depend on the type of glass being manufactured.
Uncertainties also exist in the activity data. Much of
the limestone consumed in the U.S. is reported as
"other unspecified uses." Furthermore, some of the
limestone reported as "limestone" is actually dolomite
(which has a higher carbon content than limestone).
Soda Ash Manufacture and Consumption
• Commercial soda ash (sodium carbonate) is
used in many familiar consumer products such as
glass, soap and detergents, paper, textiles, and food.
Internationally, two types of soda ash are produced
— natural ahd synthetic; the U.S. produces only
natural soda ash. During the production process,
natural sources of sodium carbonate are heated and
transformed into a crude soda ash that requires fur-
ther refining. Carbon dioxide (CO?) is generated as
a by-product of this reaction, and is eventually
emitted into the atmosphere. In addition, CO2 is
released when soda ash is consumed.
Only two states produce natural soda ash:
Wyoming and California. Of these two states, only
Wyoming has net emissions of CO2. Because a dif-
ferent production process is used in California, those
soda ash producers never actually release the CO2
into the atmosphere. Instead, the CO2 is recovered
and used in other stages of production. U.S. CO2
emissions from soda ash production in 1994 were
Soda Ash Manufacture
& Consumption
7%
2 Limestone (CaCO3) and dolomite (CaMg(CO3)2) are collectively referred to as limestone by the industry, and intermediate varieties are
seldom distinguished.
3 Of the 723.5 million metric tonnes of limestone consumed in the U.S. in 1993,213.1 million metric tonnes, or 29.5 percent, were
reported as "unspecified uses," and only 2.6 million metric tonnes were reported as "flux stone." The Bureau of Mines recommends that
when analyzing the industry, however, the quantity reported as unspecified should be distributed among the various reported uses. For
example, limestone used as flux stone accounts for 0.5 percent of specified limestone uses. Assuming the same percentage of the
unspecified limestone was actually used as flux stone, total limestone used would be (0.005 x 213.1) + 2.6 = 3.666 million metric tonnes.
A similar calculation was applied for dolomite and glass manufacturing. 1994 consumption for SO2 removal was calculated as the average
of the 1992 and 1993 ratios of consumption for SO2 removal to consumption for the other three end-uses (flux stone, dolomite, and glass
manufacturing), multiplied by the 1994 total consumption for the flux stone, dolomite, and glass manufacturing end-uses.
Industrial Processes • 41
-------
approximately 0.39 MMTCE in 1994.
Soda ash consumption in the U.S. generated
about 0.71 MMTCE of CO2 in 1994. Annual soda
ash consumption in the U.S. decreased slightly in
1991 and 1992, and recovered in 1993 before
decreasing once again in 1994. Combined with pro-
duction, which accounted for 0.39 MMTCE, total
emissions from this source were about 1.1 MMTCE
in 1994, or about 0.08 percent of total U.S. CO2
emissions.
Soda ash (sodium carbonate, Na2CO3) is a white
crystalline solid that is readily soluble in water and is
strongly alkaline. Commercial soda ash is used as a
raw material in a variety of industrial processes. It is
used primarily as an alkali, either in glass manufac-
turing or simply as a material that reacts with and
neutralizes acids or acidic substances. About 75 per-
cent of world production is synthetic ash made from
sodium chloride; the remaining 25 percent is pro-
duced from natural sodium carbonate-bearing
deposits. The U.S. produces only natural soda ash.
During the production process, trona (the princi-
pal ore from which natural soda ash is made) is cal-
cined in a rotary kiln and chemically transformed
into a crude soda ash that requkes further processing.
Carbon dioxide and water are generated as a by-
product of the calcination process. CO2 emissions
from the calcination of trona can be estimated based
on the following chemical reaction:
2(Na3H(C03)2'2H2O) — f 3Na2CO3 + 5H2O + CO2
[trona] [soda ash]
Based on this formula, it takes approximately 10.27
metric tonnes of trona to generate 1 metric tonne of
CO2. Thus, the 14.6 million metric tonnes of trona
mined in 1994 for soda ash production (Bureau of
Mines, 1995f) resulted in CO2 emissions of approxi-
mately 1.4 million metric tonnes (0.39 MMTCE).
Changes in production from 1990 to 1994 may
be attributed in large part to European antidumping
actions against the U.S. industry. In late 1990, an
antidumping duty of 67.5 European Currency Units
(ECUs) was rescinded on U.S. imports, but another
investigation opened in mid-1993. Thus, the U.S.
share of the European market jumped from 1 percent
in 1990 to 11.3 percent in 1992, but then fell by
about 35 percent in 1993 and again by 33 percent in
1994 (Bureau of Mines, 1993b, 1994c, & 1995h).
Nevertheless, total U.S. soda ash exports hit an all-
time high of 3.23 million metric tonnes in 1994 due
to other favorable global economic trends, such as
the global price increase for caustic soda, a substitute
product (Bureau of Mines, 1995h).
An alternative method of natural soda ash pro-
duction uses sodium carbonate-bearing brines. To
extract the sodium carbonate, the complex brines are
first treated with CO2 in carbonation towers to con-
vert the sodium carbonate into sodium bicarbonate,
which will precipitate under these conditions. The
precipitated sodium bicarbonate is then calcined back
into sodium carbonate. Although CO2 is generated as
a by-product, the CO2 is recovered and recycled for
use in the carbonation stage and is never actually
released.
Glass manufacture represented about 50 percent
of domestic soda ash consumption, with smaller
amounts used for chemical manufacture, soap and
detergents, flue gas desulfurization, and other miscel-
laneous uses. In each of these applications, a mole of
carbon is released for every mole of soda ash used.
Thus, approximately 0.113 tonnes of carbon (or
0.415 tonnes of CO2) is released for every tonne of
soda ash consumed.
In 1994, U.S. consumption of soda ash was
reported as 6.26 million metric tonnes (Bureau of
Mines, 1995h). This generated about 2.6 million met-
ric tonnes (0.71 MMTCE) of CO2 for the year.
Between the years 1990 and 1994, emissions from
consumption were essentially static. However, the flat
glass and fiberglass sectors had significant growth in
the second half of 1994, primarily due to growth in
the automotive and construction industries (Bureau
of Mines, 1995h).
Carbon Dioxide Manufacture
Carbon dioxide (CO^ is used in many segments
of the economy, including food processing, beverage
manufacturing, chemical processing, crude oil prod-
ucts, and a host of industrial arid miscellaneous
42 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
applications. For the most part, CO2 used in these
applications will eventually be released into the
atmosphere.
Carbon dioxide emissions from this source were
about 1.3 million metric tonnes in 1994. This trans-
lates to approximately 0.4 MMTCE, or 0.03 percent
of total CO2 emissions. Carbon dioxide demand in
the merchant market is expected to expand 4.2 per-
cent annually through 1998 (Freedonia Group, 1994).
Carbon dioxide is used for a variety of applica-
tions, including food processing, chemical production,
carbonated beverages, and enhanced oil recovery
(EOR). Carbon dioxide used for EOR is injected into
the ground to increase reservoir pressure, and is there-
fore considered sequestered.4 For the most part, how-
ever, CO2 used in these applications will eventually
enter the atmosphere.
With the exception of a few natural wells, CO2
used in these applications is a by-product from the
production of other chemicals (e.g., ammonia), or
obtained by separation from crude oil or natural gas.
Depending on the raw materials that are used, the by-
product CO2 generated during these production
processes may already be accounted for in the CO2
Soda Ash
Manufacture
& Consumption
7%
Limestone
Use
8%
emission estimates from fossil fuel consumption
(either during combustion or from non-fuel use). For
example, ammonia is manufactured using natural gas
and naphtha as feedstocks. Carbon dioxide emissions
from this process are included in the portion of car-
bon for non-fuel use that is not sequestered (see Part
I), rather than in this section.
Carbon dioxide emissions were calculated by
estimating the fraction of manufactured CO2 that is
not accounted for in these other emission sources.
Carbon dioxide consumption for uses other than
EOR was about 4.6 million metric tonnes in 1994
(Freedonia Group, 1994). Carbon dioxide wells, nat-
ural gas wells, and fermentation account for approx-
imately 30 percent of total production capacity in the
U.S. Assuming that the remaining 70 percent is
accounted for in the CO2 emission estimates from
other categories (the most important being fossil fuel
consumption), CO2 emissions from industrial sources
were approximately 1.35 million metric tonnes in
1994, or 0.37 MMTCE. This is 12 percent higher
than CO2 emissions in 1990, which totaled 1.20 mil-
lion metric tonnes (0.33 MMTCE).
Aluminum Production
The production of aluminum results in emissions
of several greenhouse gases, including carbon dioxide
(CO^ and two perfluorocarbons (PFCs), CF4 and
C2F6. Carbon dioxide is emitted as carbon contained
in the anode and cathode of the electrolytic produc-
tion cell is oxidized during the reduction of alumina
to aluminum. Emissions ofCO2 from aluminum pro-
duction in the U.S. were about 6.1 million metric
tonnes (1.7 MMTCE) in 1994. However, the CO2
emissions from this source are already accounted for
in the non-fuel use portion of CO2 emissions from
fossil fuel consumption. Thus, to avoid double-count-
ing, CO2 emissions from aluminum production are
not included in the industrial processes emission
totals, although they are described here for, informa-
tional purposes.
4 It is unclear to what extent the CO2 used for EOR will be re-released. For example, the CO2 used for EOR is likely to show up at the
wellhead after a few years of injection (Hangebrauk, et al, 1992). This CO2, however, is typically recovered and reinjected into the well.
More research is required to determine the amount of CO2 that in fact escapes. For the purposes of this analysis, it is assumed that all of
the CO2 remains sequestered.
Industrial Processes B 43
-------
The aluminum production industry is also
thought to be the largest source of GF4 and C2F6
emissions. Emissions of these two PFCs occur during
the reduction of alumina in the primary smelting
process. As with emissions of CO& the carbon is pre-
sent in the anode and cathode material; the fluorine is
present in the molten cryolite in which the reduction
of alumina occurs. Total U.S. emissions of PFCs in
1994 were 3.4 MMTCE for CF4 and 0.7 MMTCE
for C2Ffr A detailed description of these emissions
may be found in the "Other Emissions" section of
this chapter.
Carbon dioxide is emitted during the aluminum
production process when alumina (aluminum oxide)
is reduced to aluminum. The reduction of the alu-
mina occurs through electrolysis in a molten bath of
natural or synthetic cryolite. The reduction cells con-
tain a carbon lining that serves as the cathode.
Carbon is also contained in the anode, which can be
a carbon mass of paste, coke briquettes, or prebaked
carbon blocks. During reduction, some of this carbon
is oxidized and released to the atmosphere as CO2.
U.S. primary aluminum production fell to a
seven-year low in 1994, continuing a decline since
1991. Production had increased by 2 percent in 1991
to 4,121 thousand metric tonnes, but then began to
drop: by 2 percent in 1992,9 percent in 1993, and ll
percent in 1994 (Bureau of Mines, 1995e). These
declines were due in part to a continued increase in
imports for consumption, primarily from the newly
independent states and the former Soviet Union. For
example, in 1994 these countries exported 60 percent
more ingot (metal cast for easy transformation) to the
U.S. than in 1993, pushing the total for aluminum
imports to a record high of just under 3.4 million
metric tonnes. However, the Bureau of Mines reports
that due to the temporary nature of this supply sur-
plus, a more normal global supply and demand equi-
librium should return beginning in 1995. Indeed,
through the first quarter of 1995, data akeady
pointed to lower demand, more stable prices, and
dramatically diminished world inventories (Bureau of
Mines, 1995e).
Approximately 1.5 to 2.2 tonnes of CO2 are
emitted for each tonne of aluminum produced
(Abrahamson, 1992). As a result, 1994 U.S. produc-
tion yielded CO2 emissions of approximately 6.1 mil-
lion metric tonnes (1.7 MMTCE). The CO2
emissions from this source are akeady accounted for
in the non-fuel use portion of CO2 emissions from
fossil fuel consumption, which was estimated in Part
I of the Inventory (i.e., the carbon contained in the
anode is considered a non-fuel use of petroleum
coke). Thus, to avoid double-counting, CO2 emis-
sions from aluminum production are not included in
the industrial processes emission totals. Aluminum
production is also thought to be the largest source of
two PFCs, CF4 and C2F6. Emissions of these two
potent greenhouse gases occur during the reduction
of alumina in the primary smelting process. Emission
estimates and a description of the estimation method-
ology are provided in the HFCs and PFCs section of
this chapter.
Nitrous Oxide Emissions
AdipicAcid Production
Adipic acid production has been identified as a
significant anthropogenic source of atmospheric
nitrous oxide (N2O). Adipic acid is a major compo-
nent used in nylon production, as well as production
of some low-temperature lubricants. It is also used to
Industrial N26 Emissions byjSource: I9J94
44 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
provide foods with a "tangy" flavor. Estimates of
1994 U.S. adipic add production were 815 thousand
metric tonnes (C & EN, 1995). Nitrous oxide emis-
sions from this source were 5.4 MMTCE for 1994, or
13.3 percent of total U.S. N2O emissions.
Adipic acid is a white crystalline solid used in the
manufacture of synthetic fibers, coatings, plastics,
urethane foams, elastomers, and synthetic lubricants.
Commercially, it is the most important of the
aliphatic dicarboxylic acids, which are used to manu-
facture polyesters. Ninety percent of all adipic acid
produced in the United States is used in the produc-
tion of nylon 6,6.
Adipic acid is produced through a two-stage
process during which N2O is generated in the second
phase. The second stage involves the oxidation of
ketone-alcohol with nitric acid. Nitrous oxide is gen-
erated as a by-product of this reaction and enters the
waste gas stream. In the U.S., this waste gas is treated
to remove NOX and other regulated pollutants (and,
in some cases, N2O as well) and is then released into
the atmosphere. There are currently four plants in the
U.S. that produce adipic acid. Since 1990, two of
these plants have employed emission control mea-
sures destroying about 98 percent of the N2O before
its release into the atmosphere (Radian, 1992a). By
1996, all adipic acid production plants will have N2O
emission controls in place as a result of a voluntary
agreement among producers.
Since emissions of N2O in the U.S. are not regu-
lated, very little emissions data exist. However, based
on the overall reaction stoichiometry for adipic acid,
it is estimated that approximately 0.3 kg of N2O is
generated for every kilogram of adipic acid produced
(Radian, 1992a). Estimates of 1994 U.S. adipic acid
production were 815 thousand metric tonnes (C 8c
EN, 1995). When combined with existing levels of
control, this yields N2O emissions from this source of
5.4 MMTCE for 1994.
Adipic acid production reached its highest level in
ten years in 1994, growing 6.5 percent from the pre- -
vious year. Production reached 735 thousand metric
tonnes in 1990, grew to 771 thousand metric tonnes
in 1991, dropped to 708 thousand metric tonnes in
1992, and rebounded to 765 thousand metric tonnes
in 1993 (C&EN, 1992,1993,1994,1995). However,
emissions should follow a significantly lower path by
1996, due to the imminent increase in pollution con-
trol measures mentioned above.
Because N2O emissions are controlled in some
adipic acid production facilities, the amount of N2O
that is actually released will depend on the level of
controls in place at a specific production plant. Thus,
in order to calculate accurate emission estimates, it is
necessary to have production data on a plant-specific
basis. In most cases, however, these data are confi-
dential. As a result, plant-specific production figures
were estimated by disaggregating total adipic acid
production using existing plant capacities. This cre-
ated a significant degree of uncertainty in the adipic
acid production data used to derive the emission esti-
mates. The most accurate N2O emissions estimates
would be derived from actual production figures, if
these data were reported by each plant.
Nitric Acid Production
The production of nitric acid (HNO3) produces
nitrous oxide (N2O) as a by-product via the oxida-
tion of ammonia. Nitric acid is a raw material used
primarily to make synthetic commercial fertilizer. It is
also a major component in the production of adipic
acid (a feedstock for nylon) and explosives. In 1994
this inorganic chemical ranked 13th in total produc-
i Industrial N2O Emissions by Source: l|994
Industrial Processes H 45
-------
tion of all chemicals in the United States.
Nitric acid plants are in operation in all regions
of the U.S., with a total annual operating capacity of
9.7 million metric tonnes in 1994 (SRI, 1994). Nitric
acid production reached about 8.0 million metric
tonnes in 1994. Based on an average emissions factor
of 5.5 kg N2O per metric tonne of nitric acid, N2O
emissions from this source were about 3.8 MMTCE,
accounting for about 9.5 percent of total U.S. N2O
emissions.
Nitric acid is an inorganic compound used pri-
marily as a feedstock for nitrate fertilizer production.
It is also a raw material used in the production of
adipic acid and explosives. Relatively small quantities
of nitric acid are employed for stainless steel pickling,
metal etching, rocket propellants, and nuclear-fuel
processing. Virtually all of the nitric acid produced in
the U.S. is manufactured by the catalytic oxidation of
ammonia (U.S. EPA, 1995a). During this reaction,
N2O is formed as a by-product and is released from
reactor vents into the atmosphere. While the waste
gas stream may be cleaned of other pollutants such as
nitrogen dioxide, there are currently no control mea-
sures aimed at eliminating N2O.
Nitric acid production in the U.S. was approxi-
mately 8.0 million metric tonnes in 1994 (C & EN,
1995). Off-gas measurements at one nitric acid pro-
duction facility have shown N2O emission rates to be
approximately 2-9 g N2O per kg of nitric acid pro-
duced (Reimer, et al., 1992). Using the midpoint of
this emission factor range, 1994 N2O emissions from
nitric acid production were about 44.0 thousand met-
ric tonnes (3.8 MMTCE). This represents a slight
increase over the prior years of the decade, as pro-
duction resulted in 39.9 thousand metric tonnes in
1990, 39.5 thousand metric tonnes in 1991, 40.1
thousand metric tonnes in 1992, and 41.2 thousand
metric tonnes in 1993.
These emission estimates are highly uncertain
because of insufficient information on manufacturing
processes and emission controls. Although no abate-
ment techniques are specifically directed at removing
N2O, existing control measures for other pollutants
will have some effect on the N2O contained in the gas
stream. While the emission coefficients used here do
account for these other abatement systems, there may
be some variation between different production facil-
ities depending on the existing level of pollution con-
trol at a given plant.
Other Emissions
Emissions of Halogenated Compounds
Hydrofluorocarbons (HFCs) and perfluorocar-
bons (PFCs) are used primarily as alternatives to the
ozone depleting substances (ODSs) being phased out
under the Montreal Protocol and Clean Air Act
Amendments of1990. ODSs, which include chloro-
fluorocarbons (CFCs) and hydrochlorofluorocarbons
(HCFCs,) are used in a variety of industrial applica-
tions, including refrigeration, solvent cleaning, foam
production, sterilization, and fire extinguishing.
Although the ODS replacements (i.e., HFCs and
PFCs) are not harmful to the stratospheric ozone
layer, they are powerful greenhouse gases. For exam-
ple, HFC-134a is 1,300 times more heat absorbent
than an equivalent amount of CO2 by weight in the
atmosphere.
In 1994, HFCs and PFCs were not used as
widely as more common commercial chemicals.
However, these gases were emitted from other indus-
trial production processes. For example, HFC-23 was
emitted as a by-product ofHCFC-22 production, and
CF4 and C2F6 (two PFCs) were released during alu-
minum smelting. Emissions of these gases totaled
approximately 23.5 MMTCE in 1994. The manufac-
ture and emissions of HFCs and PFCs are expected to
rise as their use as ODS replacements increases.
Sulfur hexafluoride (SF^ is a gas used in the elec-
trical and metals industries. In particular, it is pri-
marily used as insulation in high voltage electrical
equipment, as well as in aluminum degassing
processes and as a protective atmosphere for casting
of magnesium alloys. Emissions from the use of this
gas have increased by about 2 percent annually for
the period 1990 to 1994, when they totaled 1,030
metric tonnes (7.0 MMTCE).
46 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Hydrofluorocarbons (HFCs) and perfluorocar-
bons (PFCs) were introduced as alternatives to the
ozone depleting substances (ODSs) being phased out
under the Montreal Protocol and Clean Air Act
Amendments of 1990 (see discussion on ODSs
below). ODSs, which include chlorofluorocarbons
(CFCs), hydrochlorofluorocarbons (HCFCs), and
related compounds, are used in several major end use
sectors, including refrigeration, air conditioning, sol-
vent cleaning, foam production, sterilization, fire
extinguishing, paints, coatings, and other chemical
intermediates, and miscellaneous uses (e.g., aerosols,
propellants, and other products). Because HFCs and
PFCs are not harmful to the stratospheric ozone layer,
they are not controlled by the Montreal Protocol.
However, HFCs and PFCs are powerful greenhouse
gases, and therefore are covered under the
Framework Convention on Climate Change (FCCC).
For example, HFC-134a has an estimated direct
GWP of 1,300, which makes HFC-134a 1,300 times
more heat absorbent than an equivalent amount by
weight of carbon dioxide (CO2) in the atmosphere.
As a result, emission estimates for these gases have
been included in the U.S. inventory and are provided
in Table tt-5. Emissions for the entire period 1990 to
1994 may be found in Annex C.
Because the use of CFC and HCFC substitutes
Table 11-5
Etniss ons of HFCs and JPiGist
^Compound
Molecular GWP Carbon-
Basis Equivalent
w.,.-. (Million Metric Tonnes)
0.00418
0,00113
0.01041
0.00153
0.06089
0.00200
0.00020
12,100
3,200
1,300
140
3,300
6,300
12,500
13.80
0.99
3.69
0.06
0.80
3.43
0.68
Source' Abseck (f995).
was minimal in 1994, emissions of HFCs and PFCs
were largely the result of by-product emissions from
other production processes, and not the result of their
use as CFC alternatives. For example, HFC-23 is a
by-product emitted during HCFC-22 production,
and PFCs (CF4 and C2F6) are emitted during alu-
minum smelting. Emissions of HFCs and PFCs
should continue to rise, however, as their use as ODS
replacements increases.
Hydrofluorocarbons (HFCs). Emission estimates were
Emissions of HFCs; PFCs, and SF6
1990 1991 1992 1993 1994
-H- All RFC & HFC -fir SF6
Industrial Processes B 47
-------
developed using the Vintaging Framework Model
developed by EPA that estimates ODS emissions
based on:
• a vintaging framework that generates results using
information on the stock of equipment in each end
use, chemical use per piece of equipment, equip-
ment lifetimes, and emission rates from each piece
of equipment, and
• substitution scenarios that describe when chemi-
cals will replace ODSs as they are phased out
under the Copenhagen Amendments to the
Montreal Protocol. The scenarios are based on
estimated market penetration and the number of
years it may take to fully implement a substitute.
Because HFCs were not used widely as commer-
cial chemicals in 1994, emissions of these compounds
were relatively small, but are growing. Emissions of
HFC-134a were close to zero in 1990 but grew to
approximately 10,410 metric tonnes (3.7 MMTCE) in
1994. This was due to the introduction of HFC-134a
as a substitute for CFC-12 and other refrigerants being
phased out under the Montreal Protocol. Emissions of
HFC-152a (a component of the refrigerant blend R-
500) were estimated to be approximately 1,530 metric
tonnes (0.06 MMTCE). Hydrofluorocarbons continue
to be evaluated and introduced to the market as refrig-
erants, solvents, fire extinguishing agents, sterilizers,
and foam blowing agents.
HFC-23 is currently emitted as a by-product of
HCFC-22 production. Even after HCFC-22 is phased
out under the Montreal Protocol, production of
HCFC-22 as a polymer precursor will continue. By-
product emissions of HFC-23 are assumed to be 3
percent of HCFC-22 production. HCFC-22 produc-
tion was about 139 thousand metric tonnes in 1994,
resulting in 4.2 thousand metric tonnes of HFC-23
(13.8 MMTCE). This represents a 5.5 percent
increase over 1993 emissions, itself the low-point of
the five-year period. Emissions grew by 2.8 percent in
1991 and 4.8 percent in 1992 before dropping by
about 12 percent in 1993. HFC-125 and HFC-227
each came into production during 1994.
Perfluorocarbons (PFCs). The aluminum production
industry is thought to be the largest source of emis-
sions of two PFCs — CF4 and C2F6. Emissions of
these two potent greenhouse gases occur during the
reduction of alumina in the primary smelting process.
Aluminum is produced by the electrolytic reduction
of alumina (A12O3) in the Hall-Heroult reduction
process, whereby alumina is dissolved in molten cry-
olite (Na3AlF6), which acts as the electrolyte and is
the reaction medium. PFCs are formed during dis-
ruptions of the production process known as anode
effects, which are characterized by a sharp rise in
voltage across the production vessel. The PFCs can be
produced through two mechanisms: direct reaction of
fluorine with the carbon anode; and electrochemical
formation. In both cases the fluorine originates from
dissociation of the molten cryolite.
Because CF4 and C2F6 are inert, and therefore
pose no health or local environmental problems,
there has been little study of the processes by which
emissions occur and the important factors controlling
the magnitude of emissions. In general, however, the
magnitude of emissions for a given level of produc-
tion depends on the frequency and duration of the
anode effects during that production period. The
more frequent and long-lasting the anode effects, the
greater the emissions.
The methodology used to estimate emissions of
PFCs from aluminum production first calculates a per
unit production emissions factor as a function of sev-
eral important operating variables, including average
anode effect frequency and duration. Total annual
emissions are then calculated based on reported
annual production levels. The five components of the
per unit production emissions factor are:
• the amount of CF4 and C2F6 emitted during every
minute of an anode effect, per kAmp of current;
• the average duration of anode effects, expressed in
anode effect minutes per effect;
B the average frequency of anode effects, expressed
in anode effects per day;
• the current efficiency for aluminum smelting (no
units); and,
n the current required to produce a metric tonne of
aluminum, assuming 100 percent efficiency.
48 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Using currently available data for the U.S., this
methodology yields a range in the emissions factor of
0.3 to 0.9 kg CF4 per metric tonne of aluminum pro-
duced (Jacobs, 1994). The emissions factor for C2F6
is estimated to be an order of magnitude lower, and
therefore ranges from 0.03 to 0.09 kg C2F6 per met-
ric tonne of aluminum produced. Based on 1994 alu-
minum production of 3.299 million metric tonnes,
total U.S. emissions of PFCs in 1994 averaged about
2 thousand metric tonnes of CF4 (3.4 MMTCE) and
200 metric tonnes of C2F6 (0.7 MMTCE). U.S. alu-
minum production increased by 2 percent in 1991,
but then began to drop: by 2 percent in 1992, 9 per-
cent in 1993, and 11 percent in 1994 (Bureau of
Mines, 1995e).
Because there has been relatively little study of
emissions from this source, considerable uncertainty
remains in several of the values used in the estimates
presented here. In particular, the value for emissions
per anode effect minute per kAmp is based on a sin-
gle measurement study that may not be representative
of the industry as a whole (U.S. EPA, 1993b). For
example, this emissions factor may vary by smelter
technology type, among other factors. The average
duration of anode effects, according to preliminary
results of ongoing research, may in fact be consider-
ably shorter than the current values used. The aver-
age frequency of anode effects and the current
efficiency are well documented, although they may
change over time as operating efficiencies improve.
Because recent studies indicate that the values for the
important variables used in developing the emission
coefficient may actually be lower than previously
thought, the estimates presented here are likely to be
conservatively high. Annex C contains a more
detailed description of the calculations regarding
these gases and their emissions.
SF6. Sulfur hexafluoride (SF6) is a gas used in the
electrical and metals industries. In particular, it is
used as insulation in high voltage electrical equip-
ment, as well as in aluminum degassing processes and
as a protective atmosphere for the casting of magne-
sium alloys. Sulfur hexafluoride production in the
United States was estimated to be approximately 6.0
million pounds, or 2.7 thousand metric tonnes annu-
ally for the period 1990 to 1994. This translates to
emissions of about 1,000 metric tonnes of SF6 per
year. About 80 percent of SF6 use is attributed to the
electrical industry. When SF6 is sealed in such equip-
ment, it leaks at about 1 percent per year, so there is
a significant difference between production and emis-
sions. For SF6 used in the metals industry, most or all
of the chemical is emitted during use. Emissions from
production and leakage combined for an annual
increase of about 2 percent from 1990 to 1994,
reaching 1,030 metric tonnes in 1994 (7.0 MMTCE).
Annex C contains a more detailed description of the
calculations regarding this gas and its emissions.
Em/ss/ons of CFCs and Related Compounds.
Chlorofluorocarbons (CFCs) and other halocarbons,
which were emitted into the atmosphere for the first
time this century, are a family of man-made com-
pounds used in a variety of industrial applications,
including foam blowing, refrigeration, and solvent
cleaning. These compounds, which contain chlorine
and bromine, have been shown to deplete stratos-
pheric ozone, and thus are typically referred to as
ozone-depleting substances, or ODSs. In addition,
they are important greenhouse gases because they
block infrared radiation that would otherwise escape
into space (EIA, 1993a). Unlike other greenhouse
gases, however, these compounds do not occur natu-
rally in the atmosphere. ODSs include the following
substances: chlorofluorocarbons, halons, methyl
chloroform, carbon tetrachloride, methyl bromide,
and partially halogenated fluorocarbons (HCFCs).
Many governments, recognizing the harmful
effects of these compounds on the atmosphere, signed
the Montreal Protocol on Substances that Deplete the
Ozone Layer in 1987 to limit the production and con-
sumption of a number of CFCs and other halo-
genated compounds. The U.S. furthered its
commitment to phase-out these substances by signing
and ratifying the Copenhagen Amendments to the
Montreal Protocol in 1992. Under these amend-
ments, the U.S. committed to eliminating the produc-
tion of all halons by January 1,1994 and all CFCs by
January 1, 1996.
Industrial Processes • 49
-------
Under the Clean Air Act (CAA), which developed
the U.S. phaseout schedule for the Montreal Protocol,
ODSs were categorized based on their ozone deple-
tion potential. Compounds are classified as "Class I"
or "Class II" substances, and must adhere to a dis-
tinct set of phase-out requirements.
• Class I ODSs include fully halogenated CFCs,
halons, tetrachlorocarbon (commonly known as
carbon tetrachloride), and 1,1,1 trichloroethane
(a.k.a. methyl chloroform). Fully halogenated
compounds have no hydrogen atoms in their
makeup and are so called because they contain
chlorine, fluorine, or bromine atoms (elements
belonging to the halogen family). Of the elements
in the halogen family, chlorine and bromine are
thought to be ozone-depleting agents, while fluo-
rine is believed to be a potent greenhouse gas.
Halon compounds contain bromine atoms instead
of chlorine atoms, while methyl chloroform is
actually a partially halogenated compound (the
only one to be included in this Class). These com-
pounds are the primary ODSs in use today.
• Class II ODSs include hydrochlorofluorocarbons
(HCFCs), some of which were developed as
interim replacements for CFCs. Because these
HCFC compounds are only partially-halogenated,
their hydrogen-carbon bonds are more vulnerable
to oxidation in the troposphere, and therefore
pose only about one-tenth to one-hundredth the
threat to stratospheric ozone compared to CFCs.
Although HCFCs pose less of a threat to the
Earth's stratospheric ozone layer, they are still
powerful greenhouse gases, with GWPs several
orders of magnitude larger than CO2 (for exam-
ple, HCFC-22 has an estimated direct GWP of
1,700, which makes HCFC-22 1,700 times more
heat absorbent than an equivalent amount by
weight of CO2 in the atmosphere).
The production and use of Class I and Class II
substances in the U.S. are being phased out in accor-
dance with the Montreal Protocol and the 1990
Clean Air Act. Under these measures, the production
of Class I substances in the U.S. will cease by January
1996, while the production of Class n substances will
be gradually phased out between 2003 and 2030.
Another group of partially-halogenated compounds
that do not contain chlorine, known as HFCs, are
being developed as long-term replacements for Class
I and Class II substances.
Although the IPCC emission inventory guidelines
do not include reporting emissions of CFCs and
related compounds, the U.S. believes that no inven-
tory is complete without the inclusion of these emis-
sions; therefore, emission estimates for several Class I
and Class n ozone-depleting substances are provided
in Table II-6. It should be noted that the use of these
compounds is declining as the U.S. fulfills its obliga-
tions under the Montreal Protocol. Also, the effects of
these compounds on radiative forcing are not pro-
vided here. Although CFCs and related compounds
have very large direct GWPs, their indirect effects are
believed to be negative, possibly equal in magnitude
to their direct effects. Given the uncertainties sur-
rounding the net effect of these gases, they are
reported here on a full molecular basis only.
Emissions of ODSs were estimated by the U.S.
EPA using the Atmospheric and Health Effects
Framework (AHEF) model. The EPA model starts
Table 11-6
Emissionlaof QDSs:
ompound
Wlassl
ft^CFC-II
£L CFC-12
CFC-II3
CFC-J 14
fJCFC-115/
Carbon Tetrachloride
lyl Chloroform
fc.Halon-1211
Halon-1301
Emissions
(Million Metric Tonnes;
" Molecular Basis)
0.037
0.059
0.017
0.005
,
lass II
J-1CFC-22
HCF;C-L23
0.016
0.07&T
0.001
0.002
0.105
0.002
0.016
0.010
Source: Abseck(>/995).
50 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
with global production forecasts for each compound
and estimates U.S. consumption based on forecasted
regional shares. These data are further divided by
end-use.
With the exception of aerosols and solvents,
emissions from CFCs and related compounds are not
instantaneous, but instead occur gradually over time,
i.e., emissions in a given year are the result of both
CFC and related compound use in that year and their
use in previous years. Each end-use has a certain
release profile, which gives the percentage of the com-
pound that is released to the atmosphere each year
until all releases have occurred.
The emission estimates provided here account for
ODS use in both the current year and in previous
years. Uncertainties exist over the levels of produc-
tion, data sources, and emissions profiles that are
used by the model to estimate yearly emissions for
each compound.
Emissions of CFC-12, HCFC-22, and methyl
chloroform were three of the most prevalent ODS
emissions in 1994. An estimated 59 thousand metric
tonnes of CFC-12 were emitted into the atmosphere,
along with an estimated 105 thousand metric tonnes
of HCFC-22, and an estimated 78 thousand metric
tonnes of methyl chloroform.
Emissions of Criteria Pollutants:
NOX, NMVOCs and CO
In addition to the main greenhouse gases
addressed above, many industrial processes generate
emissions of criteria air pollutants. Total U.S. emis-
sions of nitrogen oxides (NOX), non-methane volatile
organic compounds (NMVOCs), and carbon monox-
ide (CO) from non-energy industrial processes from
1990-1994 are reported by detailed source category
in Table II-7. The emission estimates in this section
were taken directly from the U.S. EPA's Draft
National Air Pollutant Emissions Trends, 1900-1994
(U.S. EPA, 1995b). This EPA report provided emis-
sion estimates of these gases by sector, using a "top
down" estimating procedure: the emissions were cal-
culated either for individual sources or for many
sources combined, using basic activity data (e.g., the
amount of raw material processed) as an indicator of
emissions. National activity data were collected for
individual source categories from various agencies.
Depending on the source category, these basic activ-
ity data may include data on production, fuel deliver-
ies, raw material processed, etc.
Activity data are used in conjunction with emis-
sion factors, which together relate the quantity of
emissions to the activity. Emission factors are gener-
ally available from the U.S. EPA's Compilation of Air
Pollutant Emission Factors, AP-42 (U.S. EPA,
1995a). The EPA currently derives the overall emis-
sion control efficiency of a source category from a
variety of sources, including published reports, the
1985 NAPAP (National Acid Precipitation and
Assessment Program) emissions inventory, or other
EPA data bases.
Emissions of NF3
Nitrogen trifluoride (NF3) is a gas used in plasma
etching applications in the semiconductor industry. A
range of lifetimes are reported for NF3, from 50 to
740 years. These estimates vary due to different
destruction pathway assumptions, with the high end
estimate assuming photolysis and the low end esti-
mate considering ferrous ions present in water
droplets in the atmosphere. The global warming
potential (GWP) of NF3 relative to CO2 is estimated
for both lifetime extremes. The 100 year GWP of NF3
based on a 50 year lifetime is estimated to be 6,300;
for a lifetime of 740 years, the GWP is estimated to
be 13,100. Although the concentration of NF3 in the
atmosphere to date has not yet been determined,
experts have found that if all the NF3 produced in the
U.S. over the past 15 years were released into the
atmosphere, the temperature rise would be less than
IxlO"60 degrees Celsius (Maroulis, 1994). Because of
the uncertainties surrounding its contribution to the
greenhouse gas effect, NF3 is not included in this
inventory. However, as the understanding of this gas
increases, NF3 may be included in future inventories.
Industrial Processes 9 51
-------
Table 11-7
-T- , [f; I] '• », -f : -I,, ' ]«
U.S. Emissions of NOX> Cd, and NMl*y
• 1 • '- - i. : . i* t'i -•,-.„]. -JA
£ - j. „- „ i,ta. ^..i ji.),| i -yg. iffftf-ff-fffj^yf
i, ^ ~ -. T r,^^47
[Source
f ' " — :T— T— -1990
^Chemical & Allied Product Manufacturing 250
Metals Processing 73
,Other Industrial Processes ^_ t _ ^ 278
••Storage and Transport 2
TOTAL' ' : llji • ;. :|| | ' 1'603!
"Chemical & Allied Product Manufacturing 1,760
, , Metals Processing 1,887
i Other Industrial Processes 650
•Storage aridTransport 50
TOTAL'' !j!!!l ' "' !!! :' i | 1 4;i47;| 1
h-sni i'
t h
[Chemical & Allied Product Manufacturing 1,384
[Metals Processing 65
fOther Industrial Processes 364
I Storage and Transport 1.596
TOTAL':,, -.;•;•! .||l - „„ ' [| '.}| .; ], 3,409 1
-jfF -f if • , ••-' 1»;:.:. """i! -F-'llKf- ''--ymv.-: -1 i
^CsJfrdm-lSdUstnal'Proclisltes: 1 99. ^Thousand Metric Tonnes)
L^ ** Lu.J*»^-**u^Stx
1991 1992 1993
252 258 259
71 73 73
,„ ,,_^§t^__^ , 277, _ ^286
233
• - !! '594 " |6IO 111 ;. 62 li ' i
^ ^vn~® L ^f ^- S ^i^f^-^ ^ ^ ^ ^^flfc m^
, „..-,„„ ""CO
1,764 1,782 " J/813
1,807 1,854 1,897
644 652 t 664
.^ '""50" *"" "Yj""
' Ji' 4^264 : . ; 4,|338 ' . 1!!4,424; ' .1
h* , t -^-« , Wn «*— - v, *£ . -aTv« .
k^,_ »^ ^^* .NMVOCs
1,391 1,403 I{4I2
63 65 67
361 " 366 368
1,560 ' 1,583 " 1,594 "
: '3,375 ! , : 3,4 16 3,442 - I
»94
i^V-Wj" If,
1994
264
76
298
3
KBbl
1,858
1,965
681
53
4,557
v:
1,431
70
373
1,608
3,482
1
,|
1
II
, f
"1
1
l
i
1
i
1
Ml
fc° Totofe may not equal the sum of individual source categories due to independent rounding.
jj Source: US £PA/995a
, HJ SH.i'JL*' T,lii
<|
52 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Part
Emissions from Solvent Use
The use of solvents and other chemical products can result in emissions of various photochemically impor-
tant trace gases. Nonmethane VOCs (NMVOCs), commonly referred to as "hydrocarbons," are the pri-
mary gases emitted from most processes employing organic or petroleum-based solvents, along with small
amounts of carbon monoxide (CO) and oxides of nitrogen (NOX). While these gases are not greenhouse gases,
they are photochemically important gases, and so contribute indirectly to the greenhouse effect.
Emissions from solvent use in the U.S. consist mainly of NMVOCs, along with trace amounts of CO and
NOX. NMVOC emissions from solvent use increased nearly 6 percent from 1990 to 1994, while emissions of
NOX increased by 50 percent and CO emissions remained constant (Table EDH). Surface coatings accounted for
the majority of NMVOC emissions from solvent use (over 40 percent), while "non-industrial" uses accounted
for about 32 percent and dry cleaning for slightly over 3 percent of NMVOC emissions during the same period.
Overall, solvent use accounted for approximately 31 percent of total U.S. 1994 emissions of NMVOCs.
Although a comparatively minor source category
in the U.S., emissions from solvent use have been
U.S. NMVOC Emissions by Sourcb:' \994\
reported separately by the U.S. to be consistent with
the reporting guidelines recommended by the IPCC.
These guidelines identify solvent use as one of the
major source categories for which countries should
report emissions. In the U.S. emissions from solvents
are primarily the result of solvent evaporation,
whereby the lighter hydrocarbon molecules in the sol-
vents escape into the atmosphere. The evaporation
process varies depending on different solvent uses and
solvent types. The major categories of solvents use
include:
• Degreasing;
• Graphic arts;
• Surface coating;
• Other industrial uses of solvents (i.e., electron-
ics, etc.);
• Dry cleaning; and
• Non-industrial uses (i.e., uses of paint thinner,
etc.).
Emissions from Solvent Use • 53
-------
Table ffi-2 contains detailed 1994 emission esti-
mates from solvents by major source category.1
Estimates of emissions from solvents came from
U.S. EPA (1995b), which estimated emissions based
on a "bottom up" process. This process involves
aggregating solvent use data based on information
relating to solvent uses from different sectors such as
degreasing, graphic arts, etc. Emission factors for
Table III-1
each consumption category are then applied to the
data to estimate emissions. For example, emissions
from surface coatings are mostly due to solvent evap-
oration as the coatings solidify. By applying the
appropriate solvent emission factors to the type of
solvents used for surface coatings, an estimate of
emissions can be obtained.
Table 111-2
jflmissions of NMVO£?TOE3fttii P^lrM • -JFu.S. Emissions of pirf^i^Nbf 12%
| r : from jSolveS-'Use: ifcfifrtf: 111.*! : " 1 ' and ^O btgat^gorilg9iif Mil
(Thousand Metric Tonnes) •, '
Gas 1990 1991 1992 1993 1994 Source
* ^°" h. T- ^
^_ _____ _^_ _^ g^j _ 5,585 _ ,5,717,; ^Degreasing
r. NOX 2 2 3 3 35 ^Graphic Arts'
; CO 2 2 2 2 2,, ^Surface Coating
; Source US. EPA (I995b) •• ^Other Industrial
j[Non-lndustrial
^^^^^^^^^^^^^^^^^^^^^^^^^^^^H Total3
^-& *t^^*w ' ' =- '^ ^»»f^ ^^m "' " ^ -"\>i"«
^» .,-.„;,., ""' 1
(Thousand Metric Tonnes)
NMVOCs NOX CO
"7|2 " r +" **" ',
.359 "* + *"
2,516 2 1 '"
115 + + i
200 na na
1,824 na na *
,:"5;,727: !|t:--".-M.3.-; j- '-:' -''Zj. I'
f Source: U.S. EPA (1995b) "
rNbte:"+" Denotes"Jess than 453.$ metric tonnes (500 short tons). *
%- a The totals provided may not equal the sum of the individual
§ „ source categories presented due to independent rounding.
fLu_ ___ „ ___„ _ _^ „ , ,__jf
Please note that emissions in Tables DI-1 and m-2 are expressed in thousand metric tonnes.
54 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Part
Emissions from Agriculture
Agricultural activities contribute directly to emissions of greenhouse gases through a variety of different
processes. This part of the U.S. inventory presents emission estimates for five types of agricultural activi-
ties: management of domestic livestock, management of the manure of domestic livestock and poultry, cultiva-
tion of rice, fertilizer use, and field burning of agricultural crop wastes. Several other agricultural activities, such
as irrigation and tillage practices, may contribute to greenhouse gas emissions; however, due to uncertainty sur-
rounding the impact of these practices, emissions from these sources are not included in the inventory.1 Agri-
culture-related land-use change activities, such as conversion of grassland to cultivated land, are discussed in
part V of this inventory.
In 1994, agricultural activities were responsible for emissions of 80 MMTCE, or approximately 5 percent of
total U.S. GHG emissions (see Figure IV-1). Methane (CH4) is the most significant gas emitted by agricultural
activities, accounting for 61 MMTCE. Domestic livestock, manure management, rice cultivation, and field burn-
ing of agricultural crop wastes are all sources of CH4. Methane emissions from domestic livestock enteric fer-
mentation and manure management represent
about 21 percent and 9 percent of total CH4
emissions from all anthropogenic activities in the
U.S. (see Figure IV-2). Together, emissions from
these sources make up 93 percent of CH4 emis-
sions from agricultural activities. Of all domestic
animal types, beef and dairy cattle are by far the
largest emitters of CH4. Rice cultivation and agri-
cultural crop waste burning are minor sources of
CH4, comprising about 2 percent and 0.4 percent
of total CH4 emissions in the U.S., respectively.
These sources together account for about 5 per-
cent of U.S. CH4 emissions from agriculture.
Table IV-1 presents emissions estimates for
the agriculture sector between 1990 and 1994.
Between 1990 and 1994, CH4 emissions from
domestic livestock enteric fermentation and
manure management increased about 6 percent
Figure IV-1
1 Irrigation associated with rice cultivation is included in this inventory.
Emissions from Agriculture H 55
-------
Figure IV-2
Figure IV-3
U.SJMethane Emissions
By Source: 1994
Manure
Management |
9%
Agricultural
Waste Burning
1.1%
and 15 percent, respectively. During the same time
period, CH4 emissions from rice cultivation increased
about 21 percent, while emissions from agricultural
waste burning rose about 16 percent.
In addition to CH4, agricultural activities are a
source of nitrous oxide (N2O), carbon monoxide (CO)
Table IV-1
and nitrogen oxides (NOX). Fertilizer use on agricul-
tural soils is a major contributor to total N2O emis-
sions, responsible for about 45 percent of total U.S.
emissions (see Figure IV-3). Emissions of N2O from this
source increased about 14 percent between 1990 and
1994. Agricultural crop waste burning is a source of
Gas/Source
Emissions
(Full Molecular Weight)
Emissions
(Direct and Indjrect Effects;
^ _,7, ,.,~__=r, Carbon-Equivalent)
(Million MetricTonnes)
1 CH<
• Enteric Fermentation
: Cattle
Other
Manure Management
? Rice Cultivation
* Reid Burning
N20
- Soil Management
Field Burning
Field Burning
CO
j Reid Burning
| U.S. Emissions '| ' ;
1990
5.4
0.3
2.2
0.4
O.I
0.2
-t. ,,,.
O.I
2.2
.;!
I f Emissions of these gases do not exceed 0.01
1991
5.4
0.3
2.3
0.4
O.I
0.2
t
0.1
2.1
r!!'11!!!!!!!!!:11!!!!!!11!111!!!!!!
1992
5.5
0.3
0.5
0.2
. .t ::.;
O.I
2.5
i!« •-!« IfHiNiillsi
!:!!!'!! I!!!!:!::111!:?'1!!"!""!!: I1';1 hf!1:*!1!
1993
5.6
0.3
0.4
.tiEBM,^,,^™^,^
0.2
f
O.I
1.9
|iiT':"|ii; '! :.. i i, •:"'•..• ||::;
N'. 1 i'11 "•• '•'••'•• !•«
1994
5,8
0.3
2.5
J.5 ^
O.I
0.2
t
O.I
2.6
;'•,}: ", • "'.I
".'•j • •• • ;.
a I99J)
36,2
1.8
14.8
0.7
16.1
0.3
.••.'.iWl
!??l
MsS^i&SsftfciiiS
36.3
1.9
15.2
«sa^=Jafa^fm.
0.7
16.4
0.3
73.7
_I992_
^MiL
•^|;|-""-
~ws*o78w'
16.7
0.4
|||.;:75^|:,,
million metric tonnes.
- Note; Tbtofe presented in the summary tables in this chapter may not equal the sum of the
individual source
1993
_"!_ x~a'r* -•-', >",
iayrsElf .SBST^hS-iisE i. 1-
37.4^ _
1,8
16.0
3.0
0.6
16.6
0.3
75.6
1994
38.4
1.8
17.0
"*ol"
18.4
0.4
--,><-—
80.3i
-*i
s
••;-H
:.i
•1
categories due to rounding. '•
56 D Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
N2O, CO and NOX, in addition to CH4. However, agri-
cultural crop waste burning accounts for only about 1
percent or less of total U.S. emissions of each gas.
Methane Emissions from Enteric
Fermentation in Domestic Livestock
Methane is a natural by-product of animal diges-
tion. During digestion, methane is produced through
a process referred to as enteric fermentation in which
microbes that reside in animal digestive systems break
down feed consumed by the animal. Ruminants,
which include cattle, buffalo, sheep and goats, have
the highest methane emissions among all animal
types because of their unique digestive system.
Ruminants possess a rumen, or large "fore-stomach,"
in which a significant amount of methane-producing
fermentation occurs. Non-ruminant domestic ani-
mals, such as pigs and horses, have lower methane
emissions than ruminants because much less
methane-producing fermentation takes place in their
digestive systems. The amount of methane produced
and excreted by an individual animal depends upon
its digestive system (i.e., whether or not it possesses a
rumen), and the amount and type of feed it consumes.
Enteric fermentation in domestic livestock is a
major source of methane in the U.S. Methane emis-
Methane Emissions from; i
Agriculture by Source: 1994
Rice
Cultivation
Agriculture Waste
Burning
sions from enteric fermentation in the U.S. amounted
to 5.7 million metric tonnes (38.1 MMTCE) in 1990,
rising to 6.0 million metric tonnes (40.2 MMTCE) in
1994. Of all domestic livestock, cattle are by far the
largest source of methane. In 1994, cattle accounted
for 96 percent of total emissions from enteric fer-
mentation in domestic livestock. Of total cattle emis-
sions in 1994, beef cattle accounted for about 70
percent, while dairy cattle accounted for the rest.
Increases in methane emissions from enteric fer-
mentation in livestock are primarily due to increasing
beef cattle populations. Between 1990 and 1994, the
total beef cattle population increased by about 8 per-
cent. The population of dairy cattle, on the other
hand, decreased by about 2.8 percent between 1990
and 1994. Despite this decrease in dairy cattle popu-
lation, methane emissions from dairy cattle increased
by 0.7 percent because feed intake per cow increased
as milk production per cow increased.
Methane is produced during the normal digestive
processes of animals. During digestion, microbes res-
ident in the digestive system ferment feed consumed
by the animal. This microbial fermentation process,
referred to as enteric fermentation, produces methane
as a by-product, which is exhaled or eructed by the
animal. The amount of methane produced and
excreted by an individual animal depends primarily
upon the animal's digestive system and the amount
and type of feed it consumes.
Among animal types, the ruminant animals (i.e.,
cattle, buffalo, sheep, goats, and camels) are the
major emitters of methane because of their unique
digestive system. Ruminants possess a rumen, or
large "fore-stomach," in which microbial fermenta-
tion breaks down consumed feed into soluble prod-
ucts that can be utilized by the animal. The microbial
fermentation that occurs in the rumen enables rumi-
nants to digest coarse plant material that non-rumi-
nant animals cannot digest. Ruminant animals have
the highest methane emissions among all animal
types because a significant amount of methane-pro-
ducing fermentation occurs within the rumen.
Non-ruminant domestic animals, such as pigs,
horses, mules, rabbits,'and guinea pigs, also produce
Emissions from Agriculture B 57
-------
methane through enteric fermentation, although this
microbial fermentation occurs in the large intestine. The
non-ruminants have much lower methane emissions
than ruminants because much less methane-producing
fermentation takes place in their digestive systems.
In addition to the type of digestive system that an
animal possesses, its feed intake also affects the
amount of methane produced and excreted. In gen-
eral, the higher the feed intake, the higher the
methane emissions. Feed intake is positively related to
animal size, growth rate and production (i.e., milk
production, wool growth, pregnancy, or work).
Therefore, feed intake varies among animal types as
well as among different management practices for
individual animal types.
This section presents estimates of methane emis-
sions resulting from enteric fermentation in domestic
livestock. Only animals managed by humans for pro-
duction of animal products, including meat, milk, hides
and fiber, and draft power are included.2 Although
methane emissions from non-ruminants are signifi-
cantly less than those for ruminants, both animal types
are included in order to produce a complete inventory.
The emission estimates for all domestic livestock
were determined using the emission factors developed
in U.S. EPA (1993a). To derive emissions estimates,
emission factors were multiplied by the applicable
animal populations. The resulting emissions by ani-
mal type were summed over all animal types to esti-
mate total annual methane emissions for all domestic
livestock. Emission estimates for 1990 to 1994 were
derived using annual animal population statistics
from the U.S. Department of Agriculture (USDA)
National Agricultural Statistics Service (NASS).
The principal uncertainty in estimates of methane
emissions from livestock digestion results from the
large diversity of animal management practices found
in the U.S., all of which cannot be precisely charac-
terized and evaluated. Also, the methodological
assumptions used to derive emission factors are only
as accurate as the experimental data upon which they
are based. Nevertheless, significant scientific litera-
ture exists that describes the quantity of methane pro-
duced by individual ruminant animals, particularly
cattle. Also, cattle production systems in the U.S. are
well characterized compared to other livestock man-
agement systems in the U.S.
Methane Emissions from Cattle
With the availability of cattle management data, it
is possible to estimate methane emissions from cattle in
the U.S. using fairly detailed analyses of feeding prac-
tices and production characteristics. Also, due to their
large population, large size, and particular digestive
characteristics, cattle account for the majority of
methane emissions from livestock in the U.S. Thus, a
model can be constructed to determine estimates of
emissions from cattle. The estimates presented in Table
IV-2 are based on a detailed analysis that accounts for
regional differences in sizes, ages, feeding systems, and
management systems among cattle subgroups.
In order to derive emission factors representative
of the diverse types of cattle found in the U.S., U.S.
EPA (1993a) applied a mechanistic model of rumen
digestion and animal production (Baldwin, et al.,
1987) to 32 different diets and nine different cattle
types.3 The cattle types were defined to represent the
different sizes, ages, feeding systems and management
systems that are typically found in the U.S. (see Table
IV-2). Representative diets were defined for each cat-
egory of animal, reflecting the diverse feeds and for-
ages consumed by different types of cattle in different
regions of the U.S. Using the mechanistic model, an
emission factor was derived for each combination of
animal type and representative diet. Based upon the
extent to which each diet is used in each of five
regions of the U.S., regional average emission factors
for each of the nine cattle types were derived.4
2 Wild animals also produce methane emissions. The principal wild animals that contribute to U.S. emissions are ruminant animals such
as antelope, caribou, deer, elk, and moose. Termites have also been identified as a potentially important source of methane emissions and
are generally examined separately from other wild animals. These sources are not included in the U.S. inventory because they are not
considered anthropogenic.
3 The basic model of Baldwin, et al. (1987) was revised somewhat to allow for evaluations of a greater range of animal types and diets.
See U.S. EPA (1993a) for more detail.
4 Feed intake of bulls does not vary significantly by region, so only a national emissions factor was derived for this cattle type.
58 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table IV-2
For each cattle type, except dairy cows, emission
estimates for 1990 to 1994 were obtained using the
nationally weighted-average emission factors from
U.S. EPA (1993a) and national population data from
the USDA National Agricultural Statistics Service
(NASS),5 (USDA, 1995a-d, 1994a). The emission fac-
tors were multiplied by the applicable animal popu-
lations in each region, and the results were summed
over all cattle types to produce the total emissions
estimate for U.S. cattle. Dairy cow emission factors
from U.S. EPA (1993a) were modified to reflect
increasing milk production per cow. The following
factors should be considered when assessing these
emission estimates:
• Because all estimates except for dairy cows
were done nationally (rather than regionally),
regional shifts in these populations were not
considered.
• Dairy cow emission factors were developed
regionally, and reflect both
increasing milk production
per cow by region and the
shift in dairy cows away
from the North Central
region to the West (see
Annex D for detail). The
regional estimates were
summed to determine a
national emissions estimate.
• Emission factors for mature
dairy cattle were increased to
reflect the higher feed intakes
required to achieve the
increases in milk production
per cow.
• The mix of Weanling and
Yearling slaughters was kept
constant (see Annex D for
detail). Despite indications of
a shift toward more Weanling
slaughters, this change has not
been quantified.
Table IV-2 presents emissions estimates for each
animal category for the years 1990 to 1994.
Emissions from beef cattle increased by 8 percent,
from 3.95 million metric tonnes to 4.27 million met-
ric tonnes, reflecting increases in the beef cattle pop-
ulation (see Table IV-3). Emissions from dairy cattle
were relatively static, despite a declining population
as emissions per head increased due to higher milk
production per cow.
There are a variety of factors that make the emis-
sions estimates uncertain. First, animal population
and production statistics, particularly for range fed
cattle, are uncertain. Second, the diets analyzed using
the rumen digestion model are broad representations
of the types of feed consumed within each region, so
the full diversity of feeding strategies is not repre-
sented. And last, the rumen digestion model is itself
uncertain since it was validated using uncertain
experimental data. Together, these sources of uncer-
tainty result in an overall uncertainty of about 20 per-
Emissions
(Million Metric Tonnes)
Emissions Factor
(kg/head/yr)
ggw^.-.j,: '-rr T^rr v^. : 'KXv
Sfe--:;; - -I..' -•-•.-". -•.,:.;•.-.-•- .. ., Vi--".- -
ifiai'-y
"^',.'. '. ," '•'„ :.,.
H£ Replacements 6- 1 2
^s.*:>-^ ^V- -"-j, ,,--',-...'.i— ,- ., ',, .-.„„.,-;, .
p^- Replacements 12-24
EJgff^M'-P'-V ••"- :"':: *•:•*£:-•-:
fs&sf '"'""'"''"""•' ; ;" '"" '":
EtKows';.,' ;;.., ' , ' .:."".
11 Replacements 6-12
j|J|eplacements 12-24
ESjaughter-Weanlings
is-Slaughter- Yearlings
^is^,'^-.^,-^.-^^-
— •- -'"--'-'-
Bfsheep -
jpxfqats . ;/; . ,"".":." ...__".'-,'.».'
^fefeirWl-:" £".••-
itlP^^f^:;- j":* VS- JT' ^
U.SJ.Tptal
1.47
T.I 5
T" 0.08
0.24
3.95
2.18
'0.1 '1
6.33
o.ii
0.98
6.22,
0.27
6.69
0.01
0.09
6.68
5.70
1.46
1.14
6.08
6.24
3.98
2.20
0.12
-035
6.12
0.98
0.22
0.28
0.09
0.01
0.69
6.08
5.72
1.47
i.is
0.08
0.24
4.04
2.23
0.13
6.37
6.12
0.97
6.22
0.28
0.09
0.01
0.09
6.09
5[79
1.47
1.15
0.08
0.24
4.12
2.28
O.IJ
6.38
0.12
0.98
0,22
0.27
0.08
0.01
0.09
0.09
5.86
1.48
1.16
0.08
0.24
4.27
2.36
0.14
6.40
0.12
1.02
0.23
0.27
0.08
0.01
0.09
0.09
6.02
a ';
19.6
58.8 •
66.7 i
22.3
65.0 ':
23.1 ;
47.3 :
100.0 :
8.0 \
5.0
18.0
1.5
>,;- -cmm
iJ J-rnjssions from Hairy cows ore estimated using regional emissions factors. See Annex D.Tab/e D-/. j
5 USDA annual population data from 1990 through 1993 were revised. Due to these revisions, emissions estimates for 1990-through
1993 are also revised.
Emissions from Agriculture
59
-------
••miTnuifir
I CattleTypes
Dairy
g«f,-,;,;,,.,,,_.,,,,.,.,;;, ,
i Replacements1 6-12
^_l!,;i;;?:^,;:,,
ll^™^,.,.,,^^,^™™.,,,,^,,,,.,,,!, .^^u,,.
cent in the emission estimate (U.S. Table IV"3
EPA, 1993a).
Methane Emissions from
Other Domestic Animals
Methane emissions from other
animals (i.e., sheep, goats, pigs, and
horses) account for a very small frac-
tion of total methane emissions from
livestock in the U.S. Also, the variabil-
ity in emission factors for each of
these other animal types (e.g., vari-
ability by age, production system and
feeding practice within each animal
type) is much smaller than for cattle.
Therefore, emissions from each of
these other animal types are derived
using a more simple analysis that is
based on average emission factors
representative of entire populations of
each animal type.
Methane emissions from other domestic animals
were estimated by using emission factors from
Crutzen, et al. (1986), utilized in U.S. EPA (1993a)
and population data from NASS data sets (USDA,
1995h,m, and 1994c-d).6 These emission factors are
representative of typical animal sizes, feed intakes
and feed characteristics in developed countries. The
methodology employed in U.S. EPA (1993a) is the
same as the method recommended by the E?CC
(BPCC/OECD/IEA, 1995).
In 1994, total methane emissions from other
animals are estimated to be 270 thousand metric
tonnes CH4 (1.8 MMTCE). The uncertainty in this
estimate is probably greater than that for the cattle
emissions estimate because a less detailed analysis
was performed. However, since cattle account for
over 95 percent of the emissions from all domestic
livestock, the uncertainty in the cattle estimates dri-
ves the overall uncertainty for all livestock.
Therefore, the same uncertainty range that was
applied to cattle (20 percent) has been applied to
other animals (U.S. EPA, 1993a). This results in low
and high estimates of 200 and 350 thousand metric
1990
Population
' (thousand head)
1991 1992
1993
9,679"
_ ___«,„ „ _ 1A88
4,135 4,097 4,Tl6 "4,088
"l 0,007 "9,883 9,714
1994
9,614
4,072 ^
4,072
,32,677^,32,960 33,453 34,132 35,^325
Replacements 0-12 5,141 5,321 5,621 5,896 6,133
-Replacements 12-24 5,141 5,321 5,621 5,896 6,133
•Slaughter-Weanlings 'S,199 ' 5 J 60" S.'l50' " 5,198 " 5,408"
: Sbughter-Vearlirjgs " "" "20,794 "206392"'20,600 '^20,794 '' 2J ,*632
slis
53,807
5,215
56,535
T8"~:G " " :'" - i G ,, i IB ~~ ~ ™ »»ji™»!ari»ir'U8»'jn-"ir. -~ 7S~fSS^3 "H '"F!' ~TJ "nE^E TEW™^
Sheep 11,356 11,174 10,797
illWr"',: ,: , ,, ,a I in i • -"-nm,-n "-TIM liiin j .iiiin u* 4ii»"'iTiii',iai',Jiiifsn ii'iiiiii''.*tiii-njrTU'Ti,'i'5iiiL iun'Tini'i nn, »- «-
^ " " ""
Horses 5,215 5,215 5,215
58,553
10,201 9,742 i
5,215
56,919
5,215
60,628
Sa /99Sv '9?5& /995
-------
Methane Emissions from; j
Agriculture by Source: 19941
Rice
Cultivation
5.6%
Agriculture
Waste Burning
-.3%
2SZLJ
mated to have been 2.54 million metric tonnes (16.97
MMTCE). Between 1990 and 1994, methane emis-
sions from manure management increased about 15
percent. The largest increases occurred between 1991
and 1992, when the emissions level increased by 3.9
percent, and between 1993 and 1994, where there
was a 6.3 percent rise. Emissions for each category of
animal except "other" have increased annually.
Animals in the "other" category, representing those
that produce negligible amounts of methane from
manure, exhibited no change in their total methane
emissions from manure.
These increases in methane emissions reflect
changes in animal populations in the beef, swine and
poultry categories, and shifts in dairy and swine
manure management towards lagoon management
systems. Additionally, the increases reflect the regional
redistribution of dairies to the Southwest, as well as a
small increase in feed consumption by dairy cows.
Livestock manure is primarily composed of
organic material and water. When manure decom-
poses in an anaerobic environment (i.e., in the
absence of oxygen), the organic material is broken
down by methanogenic bacteria. Methane, carbon
dioxide and stabilized organic material are produced
as end products.
The principal factors that affect the amount of
methane produced during decomposition are the way
in which the manure is managed and the climatic
environment in which the manure decomposes.
Methane production will only occur under anaerobic
conditions. Therefore, when manure is stored or
treated in systems that promote an oxygen free envi-
ronment (e.g., as a liquid in lagoons, ponds, tanks, or
pits), the manure tends to produce a significant quan-
tity of methane. When manure is handled as a solid
(e.g., in stacks or pits) or when it is deposited on pas-
tures and rangelands, it tends to decompose aerobi-
cally and produce little or no methane. Air
temperature and moisture also affect the amount of
methane produced since they influence the growth of
the bacteria responsible for methane formation.
Methane production generally increases with rising
temperature. Also, for non-liquid based manure sys-
tems, moist conditions (which are a function of rain-
fall and humidity) favor methane production.
The composition of the manure also affects the
amount of methane produced. Manure composition
depends on the composition and digestibility of the
animal diet. The greater the energy content and
digestibility of the feed, the greater the amount of
methane that potentially could be produced by the
resulting manure. For example, feedlot cattle fed a
high energy grain diet produce manure with a high
methane-producing capacity. Range cattle feeding on
a low energy forage diet produce manure with only
half the methane-producing capacity of feedlot cattle
manure. However, as described above, in either case
the amount of methane that is actually produced and
emitted to the atmosphere depends largely on the way
that the manure is managed.
Methodology
Using annual livestock population data obtained
from the USDA National Agricultural Statistics
Service (NASS), the methods used in U.S. EPA
(1993a) were applied to derive methane emission esti-
mates for manure management in each state for the
year 1990. For 1991-1994, state methane emissions
for each animal subcategory were estimated by mul-
tiplying the corresponding 1990 emissions by the per-
Emissions from Agriculture H 61
-------
cent change in population between each respective
year and 1990. To incorporate shifts in manure man-
agement practices, changes in the methane conversion
factor (MCF) were estimated for seven states for
1992 and after. Additionally, to reflect changes in the
feed intake of dairy cows, a yearly volatile solids
change factor was applied to each state.
Methane Emissions Estimates from
Livestock Manure
From 1990 to 1994, methane emissions from
manure management increased from about 2.21 to
2.54 million metric tonnes of methane, or 15 percent
(see Table IV-4). A variety of factors contributed to
these changes in emission levels, including changes in
total animal populations, manure management sys-
tems, animal diets, and regional shifts in the dairy
industry:
Populations. The increases in emissions reflect a gen-
eral increase in animal populations in all categories,
except dairy cows, heifers and the other minor ani-
mals populations. These population increases are pri-
marily due to increases in demand.
Manure Management. Emission levels have
increased with the general shift in swine and dairy
management from dry storage to lagoon storage and
treatment, the latter method producing higher quan-
tities of methane. The increased use of anaerobic
lagoon manure management systems is primarily a
result of larger herd sizes that warrant the use of cost
effective, automated (liquid) manure management
systems. Increased concern over the effect of
Table IV-4
Methane Emissions from Manure ; |^
Management: 1990-1994 i ||
Dairy Cattle
Beef Cattle
Poultry
OTfier
mmmjfmmmmt
1990
0.75
0.26
0.95
0.26
0.06
1991
'a*'"'
0.20
0.99
0.27
0.06~ ~
Metric Jonnes)
1992 1993
0.79* '0.80 "
0.21 0.2l
1.04
0.28
0.06
1.03
0.28
0.06
""i fl
t
' 1.14*
0.06*
^ ™ M
improper manure management is also encouraging
such conversions.
Diet The decrease in dairy cow and heifer popula-
tions was accompanied by increases in emissions
from these animals. Increases in milk production per
cow result in increased feed intake, leading to
increased manure production per cow.
Regional Shifts. State emissions data have indicated a
shift in dairy operations to states where wastes were
more likely to produce methane. This is due largely to
the types of waste management systems in use in
these states; factors such as climate and rainfall are
secondary.
Methane Emissions
from Rice Cultivation
Most of the world's rice, and all rice in the U.S.,
is grown on flooded fields. When fields are flooded,
anaerobic conditions in the soil develop, and methane
is produced through anaerobic decomposition of soil
organic matter. Methane is released primarily through
the rice plants, which act as conduits from the soil to
the atmosphere.
Rice cultivation is a very small source of methane
in the U.S. Emissions from this source are estimated to
Methane Emissions ifirom
Agriculture by SpXirc^: 1994
Agriculture
Waste Burning
/ 1.3%
2.2 { 2.28 2.37 2.39 2.54
.Source; The, emissions data used above are demed from population
Fnlimfaers quoted in USDA (1995a-e, g-m, o) '
I™1"! * ' ' '•' ' •» ' •• "" .'....• »• -• ,.,--, '^,^jj4tf
62 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
have been approximately 0.9 - 6.0 MMTCE in 1994,
accounting for about 1 percent of U.S. methane emis-
sions from all sources and about 6 percent of U.S.
methane emissions from agricultural sources. Seven
states grow rice: Arkansas, California, Florida,
Louisiana, Mississippi, Missouri, and Texas. Emis-
sions from Arkansas accounted for over 35 percent of
total U.S. emissions from 1990 through 1994, pri-
marily because it has the largest area of rice fields har-
vested. Louisiana, which has a longer growing season,
has the second highest level of emissions, accounting
for more than 20 percent of the national total.
Between 1990 and 1994 methane emissions
from rice cultivation increased about 21 percent.
While emissions remained relatively constant
between 1990 and 1991, they increased about 13
percent between 1991 and 1992, due primarily to
the relatively large areas harvested for most states in
1992. Emissions decreased about 9 percent in 1993
as a result of reductions in the total area harvested
for that year. In 1994, emissions increased approxi-
mately 16 percent as total area harvested increased
again in each state.
Most of the world's rice is grown on flooded
fields. When fields are flooded, aerobic decomposi-
tion of organic material gradually depletes the oxygen
present in the soil and floodwater causing anaerobic
conditions in the soil to develop. Methane is pro-
duced through anaerobic decomposition of soil
organic matter by methanogenic bacteria. However,
not all of the methane that is produced is released
into the atmosphere. As much as 60 to 90 percent of
the produced methane is oxidized by aerobic
methanotrophic bacteria in the soil (Holzapfel-
Pschorn, et al., 1985; Sass, et al., 1990). Some of the
methane is also leached away as dissolved methane in
floodwater that percolates from the field. The
remaining non-oxidized methane is transported from
the submerged soil to the atmosphere primarily by
diffusive transport through the rice plants. Some
methane also escapes from the soil via diffusion and
bubbling through the floodwaters.
The water management system under which rice
is grown is one of the most important factors affect-
ing methane emissions. Upland rice fields are not
flooded, and therefore are not believed to produce
methane. In deepwater rice fields (i.e., fields with
flooding depths greater than 1 meter), lower stems
and roots of the rice plants are dead, and thus effec-
tively block the primary CH4 transport pathway to
the atmosphere. Therefore, while deepwater rice
growing areas are believed to emit methane, the
quantities released are likely to be significantly less
than the quantities released from areas with more
shallow flooding depths. Also, some flooded fields
are drained periodically during the growing season,
either intentionally or accidentally. If water is drained
and soils are allowed to dry sufficiently, methane
emissions decrease or stop entirely. This is due to soil
aeration, which not only causes existing soil methane
to oxidize but also inhibits further methane produc-
tion in soils.
Other factors that influence methane emissions
from flooded rice fields include soil temperature, soil
type, fertilization practices, cultivar selection, and
other cultivation practices (e.g., tillage, seeding and
weeding practices). Many studies have found, for
example, that methane emissions increase as soil tem-
perature increases. Several studies have indicated that
some types of nitrogen fertilizer inhibit methane gen-
eration, while organic fertilizers enhance methane
emissions. However, while it is generally acknowl-
edged that these factors influence methane emissions,
the extent of the influence of these factors individu-
ally or in combination has not been well quantified.
Methodology
Estimates of methane emissions from rice culti-
vation in the U.S. are based on the IPCC methodol-
ogy (IPCC/OECD/IEA, 1995), using emission factors
that slightly vary from recommended values. The
IPCC Guidelines suggest that the "growing" season
be used to calculate emissions assuming that emission
factors are based on measurements over the whole
growing season rather than just the flooding season.
Applying this assumption to the U.S., however, would
result in an overestimate of emissions because the
emission factors developed for the U.S. are based on
measurements over the flooding rather than the
Emissions from Agriculture H 63
-------
growing season. Therefore, the method used here is
based on the number of days of flooding during the
growing season and a daily emission factor, which is
multiplied by the harvested area. Agricultural statisti-
cians in each of the seven states in the U.S. that pro-
duce rice were contacted to determine water
management practices and flooding season lengths in
each state, and all reported that U.S. rice growing
areas are continually flooded and that none are either
upland or deepwater. Because flooding season lengths
varied considerably among states, the IPCC method
was applied to each of the seven states separately to
calculate total emissions.
Daily methane emission factors were taken from
results of field studies performed in California
(Cicerone, et al., 1983), Texas (Sass, et al., 1990,
1991a, 1991b, 1992) and Louisiana (Lindau, et al,
1991; Lindau and Bollich, 1993). Based on the max-
imal and minimal estimates of the emission rates
measured in these studies, a range of 0.1065 to
0.5639 g/m2/day was applied to the harvested areas
and flooding season lengths in each state.7 Since
these measurements were taken in rice growing
areas, they are representative of soil temperatures,
and water and fertilizer management practices typi-
cal of the U.S.
The climatic conditions of southwest Louisiana,
Texas and Florida allow for a second, or ratoon, rice
crop. This second rice crop is produced from re-
growth on the stubble after the first crop has been
harvested. The emission estimates presented here
account for this additional harvested area. Acreage
for a second cropping cycle, or ratoon cropping, was
estimated to account for about 30 percent of the pri-
mary crop in Louisiana, 40 percent in Texas (Lindau
and Bollich, 1993) and 50 percent in Florida
(Schudeman, 1995).
Rice fields for the second crop typically remain
flooded for a shorter period of time than for the first
crop. Recent studies indicate, however, that the
methane emission rate of the second crop may be sig-
nificantly higher than that of the first crop. The rice
straw produced during the first harvest has been
shown to dramatically increase methane emissions
during the ratoon cropping season (Lindau and
Bollich, 1993). It is not clear to what extent the
shorter season length and higher emission rates offset
each other. As scientific understanding improves,
these emission estimates can be adjusted to better
reflect these variables.
Since the number of days that the rice fields
remain permanently flooded varies considerably with
planting system and cultivar type, a range for the
flooding season length was adopted for each state.
The harvested areas and flooding season lengths for
each state are presented in Table IV-5. Arkansas and
Louisiana have the largest harvested areas, account-
ing for approximately 40 and 20 percent of the U.S.
total, respectively. The flooding season lasts the
longest in California (138 days), Louisiana (105 days)
and Florida (105 days).
Methane Emissions from Rice Cultivation
Table IV-68 presents annual emission estimates
based on the actual area harvested in each state
between 1990 and 1994. Emissions for the U.S. from
1990 to 1994 increased 21 percent, from about 112-
744 thousand metric tonnes (0.75-4.97 MMTCE) to
131-900 thousand metric tonnes (0.88-6.02
MMTCE), largely due to increases in rice production
during this time period. Emissions from Arkansas
account for over 35 percent of total emissions, pri-
marily because it has the largest rice area harvested.
Louisiana, because of its relatively large rice area and
long growing season, has the second highest level of
emissions, accounting for over 20 percent of the
national total.
For comparison, Table IV-6 also presents
national emission estimates based on three year
averages of the area harvested for each state — a
7 Two measurements from these studies were excluded when determining the emission coefficient range. A low seasonal average flux of
0.0595 g/m2/day in Sass, et al. (1990) was excluded because this site experienced a mid-season accidental drainage of floodwater, after
which methane emissions declined substantially and did not recover for about two weeks. Also, the high seasonal average flux of 2.041
g/m2/day in Lindau and Bollich (1993) was excluded since this emission rate is unusually high, compared to other flux measurements in
the U.S., as well as in Europe and Asia (see IPCC/OECD/IEA, 1995).
8 Please note that emissions in Table IV-6 are expressed in thousand metric tonnes.
64 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
methodology recommended by the IPCC to avoid
unrepresentative results due to fluctuations in eco-
nomic or climatic conditions. The U.S. believes that
annual data should be used, particularly as year-by-
year estimates are developed as they are in this doc-
ument.
Table IV-5
..v.---1 -.: -",' : • •. -rf^r^v:;"M>;<.-fc^^ • • : x
V:V.-.. . Area^Harvested and^lb^ii^ " _ < , J
State
ZU
r
^w
[Arkansas
California
Florida3
p' primary
iM'atopn
^Louisiana3
ppriniary
jT^ratoon
Mississippi
^Missouri
pexas3
^primary
Igjatoori
Tota\b
Area Harvested
•tr
1989
461,352
165,925
5,585
2,792
"196,277
58,883
95,103
3 1,971
I36>87
54,715 _"
1.209,389
Flooding Season Length
..-<•* "(ha)
1990
* » t
485,633
159,854
4,978
" 2,489
"220,558
66,168
101,174
32,376
"
* 142,857
^57,143
1,273,229
1991
509,915
'141,643
8.S80
4,290
s
206,394
61,918
89,033
37,232
1 §8,8 10""
"55,524'"
I,253i339
1992
558,478
" 159,450*
8,944
4,472
250,911
75,273
111,291'"
45,326
142,048
"56,819"
1,413,011
1993
497,774
" 176,851
8,449
4,225
"214,488
64,346
'99,150
37,637
120,599
48,240
1,271,759
1994
574,666
196,277
8,902
4,451
250,911
75,273
126,669
50,182
143,262
57,305
1,487,897
(days> j
low
75
123
90
90
75
80
60
high
100
153
120
120
82
100
80
1
I
1
• •;
. "1
J
1
. " ^
IS
1-
-Source: Area harvested data taken fromjett and Bequet, l995;Lindau and Bollich, I993;and Schudeman, 1995.
These states have a second, or -, iSS|?': KT^:'K'!gff'----'--^'f;^ • ,'- - - ;
:Iy|ti^n;-il^h'^U^I*l%f6Sf99,4;|^ .. -.
... - ,- .'! ?'-: - • :.->v.,.-. „;,*;•; •-'•f-!:^--~"':^^rfi^f--.^^'Jt^.-'\-'--j\:Jt\-'!, ""» " i-;-.,'-5*M.V--.,. . .-•- >J1"
m . -.*.•' ,
"••^^•••B
""^ -i, &
*"-*•-«- » * Annual Emissions
^ : , , (using annual data on harvested areas)3
• (Thousand Metric Tonnes)
|^ - - 1990
e
Arkansas
California
Florida
ff^^r'i. m g, s,.
f . .
Louisiana
Mississippi
^Missouri
^xas
Total
fc'"
fe^»*
Jow
,38.8 '
- «0^9~
, ,0.7 „
27/4 „
8.1
' 2.8
12.8
II 1.5
4*-«
^ high
7" 27|8 "'
" Y4|2~^
1, SL^.,
\y$s>
440""
' "il.6 "
86.4
743.7
*-"•*"•- ,
1991
low
4Q.7
*l8.6 *
T.,^, ?,
„!•?«
"^25.7.
7.1
13".2
12.4
108.9
.,.
- ^
high
287.5
*7212
*_"JJ_
* I8L6 "^
" ln".2*
"Ifto"
87.6
J749.$
, •* •*
T /
1992
low
44.6
* *2&.*9
T,y
31.2
8.9
3.9"
12.7
123.5
Annual
high
314.9
-H ^
137.6
Jf ,
'220J
'' 51.5^
" ' 25.6
89.7
849.0
Emissioins
1993
low
39.8
23.2
rr '-2/.,
26.8
7.9
""3.2
10.8
1 1 2|.8
^1 t\a.h
high
280.7
152.6
8.6
188.6
45.8
21.2
76.2
773.8
•1
.
•994 ]
low
45.9
25.7
1.3
31.2
10.1
" 4.3
12.9
131.4
high
324.1
169.3
9.0
220.7
58.6
28.3
90.5
900,.5
1
j
j
*
^
1
si
S
** n~, ' fc t»-«. *± (usinS three-year averaging method)3-11
109.0 748.7 114.7 788.3 I 15.1 790.9 122.6 841.1 122.1 837.1
urceiBosed on: Ocerpne, efai.,"l9s|Jsasst etjl., >990, / 9/IaJ 99 ib~ 1992;t/ndau,'et al., / 99 /; and Undau and Bollich, 1993.
factor:0.1565^0.5639 (s CH4im2lday)
sion estimates for 1994 are based on two-year averages for harvested area |i.e., / 993 and / 99-ty.
;:„_: J
Emissions from Agriculture H 65
-------
Nitrous Oxide Emissions from
Agricultural Soil Management
Various agricultural soil management practices
contribute to greenhouse gas emissions. The use of
synthetic and organic fertilizers adds nitrogen to soils,
thereby increasing natural emissions of nitrous oxide
(N2O). Other agricultural soil management practices
such as irrigation, tillage practices, or the fallowing of
land can also affect trace gas fluxes to and from the
soil since soils are both a source and a sink for carbon
dioxide (COy) and carbon monoxide (CO), a sink for
methane (CHj) and a source of nitrous oxide (NOx).
However, there is much uncertainty about the direc-
tion and magnitude of the effects of cropping and
other soil management practices on GHG fluxes to
and from soils. Due to these uncertainties, only N2O
emissions due to fertilizer consumption are included
in the U.S. Inventory at this time.
Fertilizer use is a significant source ofN2O in the
U.S. Consumption of organic and synthetic fertilizers
(both multi-nutrient and nitrogen) increased about 3
percent between 1990 and 1992. While fertilizer use
declined slightly between 1992 and 1993, heavy
Nitrous Oxide Emissions from;
Agriculture by Souride: 1994
Agriculture
Waste Burning
2.3%
Agricultural Soil
Management -j
: 97.7%
flooding in the North Central region in 1993 led to
anil percent increase in fertilizer application in 1994
to replace depleted nitrogen in affected cropland
soils. Total increases in fertilizer consumption
between 1990 and 1994 caused emissions ofN2O to
expand from about 16 MMTCE to about 18.4
MMTCE. Fertilizer emissions in 1994 represent
approximately 45 percent of total U.S. N2O emis-
sions, and about 98 percent ofN2O emissions from
all agricultural sources.
In 1994, the North Central region was responsi-
ble for approximately 55 percent of the total amount
of fertilizer consumed in the U.S., while the South
Central region consumed about 20 percent. Between
1990 and 1994, fertilizer consumption within each
region, as a percentage of total U.S. fertilizer con-
sumption, remained constant.
Nitrous oxide is produced naturally in soils
through the microbial processes of denitrification and
nitrification.5 A number of anthropogenic activities
add nitrogen to soils, thereby increasing the amount
of nitrogen available for nitrification and denitrifica-
tion, and ultimately the amount of N2O emitted.
These activities include cropping practices, such as
application of fertilizers, irrigation and tillage, acid
deposition, and cultivation of nitrogen-fixing crops.
This section focuses the discussion on emissions of
N2O due to. fertilizer use (organic and synthetic
nitrogen and multi-nutrient fertilizers). Other factors
impacting N2O emissions are also discussed such as
tillage and irrigation practices, local climate, rainfall,
and soil properties. Emissions due to atmospheric
deposition and nitrogen-fixing crops are not included
for two reasons: these emission sources are highly
uncertain, and activity data are not readily available.
Research has shown that a number of factors
affect nitrification and denitrification rates in soils,
including: water content, which regulates oxygen
supply; temperature, an important factor in microbial
activity; nitrogen concentration, in particular nitrate
and ammonium concentration; available organic car-
' Denitrification is the process by which nitrates or nitrites are reduced by bacteria, which results in the escape of nitrogen into the air.
Nitrification is the process by which bacteria and other microorganisms oxidize ammonium salts to nitrites, and further oxidize nitrites
to nitrates.
66 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
bon for microbial activity; and soil pH. These condi-
tions vary greatly by soil type, crop type, manage-
ment regime, and fertilizer application. Moreover, the
interaction of these conditions and their combined
effect on the processes leading to nitrous oxide emis-
sions are not fully understood.
Scientific knowledge regarding N2O production
and emissions from fertilized soils is limited. Significant
uncertainties exist regarding the agricultural practices,
soil properties, climatic conditions, and biogenic
processes that determine how much fertilizer nitrogen
various crops absorb, how much remains in soils after
fertilizer application, and in what ways the remaining
nitrogen either evolves into N2O or into gaseous nitro-
gen and other nitrogen compounds.
A major difficulty in estimating the magnitude of
N2O from soil has been the relative lack of emissions
measurement data across a suitably wide variety of
controlled conditions, making it difficult to develop
statistically valid estimates of emission factors.
Several attempts have been made to develop emission
factors for the purpose of developing national emis-
sions inventories. However, the accuracy of these
emission factors has been questioned. For example,
while some studies indicate that N2O emission rates
are higher for ammonium-based fertilizers than for
nitrate, other studies show no particular trend in
N2O emissions related to fertilizer types (see Eichner,
1990; and Bouwman, 1990 for reviews of the litera-
ture). Therefore, it is possible that fertilizer type is not
the most important factor in determining emissions.
One study suggests that N2O emissions from the
nitrification of fertilizers may be more closely related
to soil properties than to the type of fertilizer applied
(Byrnes, et al, 1990). Other factors, such as tillage
and irrigation practices, local climate and crop type
impact the production of N2O in soils. By linking
these factors through modeling, several studies esti-
mate N2O emissions from both crop and pasture
lands (Li, et al., 1992a, 1994, 1995) (See Box IV-1).
Agricultural practices not only affect atmos-
pheric fluxes of N2O, but also impact oxidation and
uptake of CH4 in soils. In addition, various land use
changes affect fluxes of both N2O and CH4. A study
assessing the impact of land use and management
changes on soil as a CH4 sink in temperate forest and
grassland ecosystems indicates that intensive land
cover changes and increased use of nitrogen fertilizers
reduced soil uptake of CH4 about 30 percent over the
past 150 years (Ojima, et al, 1993). Another study
provides evidence that cultivation of former grass-
lands between the spring and late fall of 1990
decreased soil uptake of CH4 and increased N2O pro-
duction (Mosier, et al., 1991).
Due to the uncertainty surrounding the variety of
possible emission factors, the IPCC recommends that
countries estimate emissions from soil based on fertil-
izer use only, assuming 1 percent of nitrogen applied
as fertilizer is released into the atmosphere
(IPCC/OECD/IEA, 1995).
Methodology
Nitrous oxide emissions from fertilizer use have
been estimated using the IPCC methodology,
although with a slightly higher emission coefficient.
The emission coefficient used (1.17 percent) is based
on research done by the USDA (CAST, 1992). The
amount of fertilizer consumed (synthetic nitrogen,
multiple-nutrient and organic fertilizer, measured in
mass units of nitrogen) was multiplied by this emis-
sion coefficient. Fertilizer data for the U.S. were
obtained from the Tennessee Valley Authority's
(TVA) National Fertilizer and Environmental
Research Center (TVA, 1994).10 Nitrous oxide emis-
sions from fertilizer use were calculated as follows:
N2O Emissions =
Fertilizer Consumption (tonnes N)
xO.OII7x4'//28
Nitrous Oxide Emissions
from Agricultural Soils
Between 1990 and 1994, total fertilizer consump-
tion (multiple nutrient, synthetic nitrogen and
organic) increased approximately 14 percent, largely
reflecting the increase in fertilizer use after flooding in
1993. Fertilizer use increased at a 2 percent average
10 Fertilizer consumption data may be underestimated since they do not include organic fertilizers that do not enter the commercial market.
Emissions from Agriculture
67
-------
Box IV-1
^Estimating Nitrous Oxide Emissions Using thej DNDC Model;
To more fully understand and quantify sources of nitrous
oxide within soils and to estimate emissions from agricultural
lands, the Denitrification-Decompositfon (DNDC) model was
developed. This model links the decomposition and denitrifica-
tion processes and uses data on soil properties, climate and agri-
cultural practices to simulate processes that impact nitrous oxide
production in soils. These processes include soil heat flux and
moisture flows, decomposition of soil residues, denitrification
rates, plant growth, and nitrogen uptake (Li, et a/, I992a and
1994).
The DNDC model estimates emissions of nitrous oxide from
both crop and pasture lands resulting from different soil proper-
ties, climate, crop type, fertilizer applications, and tillage and irri-
gation practices (Li, et a/, 1992a). Nitrous oxide emissions from
a variety of agricultural lands simulated by the DNDC model
have proven to be consistent with actual field measurements of
emissions (Li,eta/, 1995,1994,1992b).
In light of uncertainty surrounding the validity of the IPCC
methodology for determining nitrous oxide emissions from soils,
emissions estimates based on DNDC model criteria may offer
new and important insights into nitrous oxide emissions from
agricultural lands. Using the DNDC model, estimates of nitrous
oxide emissions in 1990 due to fertilizer consumption ranged
between 110 thousand metric tonnes (9.6 MMTCE) and 126
thousand metric tonnes (II MMTCE). While this estimate is
lower than the 1990 emissions estimate based on the IPCC
methodology (185 thousand metric tonnes, or 16 MMTCE), once
differences in fertilizer consumption data used to calculate the
two emissions estimates are reconciled, the IPCC and DNDC
emissions estimates are more consistent3
Like fertilizer consumption, cropping practices such as tillage
and irrigation, local climate, crop type and soil properties are
important factors affecting production of nitrous oxide in soils.
By simulating the effects of factors other than fertilizer con-
sumption through the DNDC model, emissions of nitrous oxide
from both pasture and croplands were estimated to range
between 1,226 and 1,807 thousand metric tonnes (107 and 158
MMTCE) in 1990. According to DNDC model estimates, nitrous
oxide emissions from cropland make up about 60 percent of
total nitrous oxide emissions from agricultural lands, while emis-
sions from pasture land account for the remaining 40 percent (Li,
eta/., 1995).
The DNDC model expands on the IPCC methodology esti-
mates of nitrous oxide emissions by assessing both crop and pas-
ture lands as sources of nitrous oxide and by examining
emissions factors beyond fertilizer consumption. However, by
incorporating a wide variety of emissions factors to estimate
nitrous oxide emissions from soil, the DNDC model measures
nitrous oxide emissions that may be unrelated to different land
uses and farming practices. As soil is a natural source of nitrous
oxide, the DNDC model may provide an overestimation of
nitrous oxide emissions due to anthropogenic activities.
* The DNDC model estimate of 1990 nitrous oxide emissions due to fertilizer use is based on consumption of 8,100 thousand metric tonnes of fertilizer
(data from TVA, 1989). The IPCC emissions estimate is based on consumption of 10,048 thousand metric tonnes of fertilizer, a more recent estimate of
US. fertilizer consumption in 1990 (TVA 1994). To make the emissions estimates more compatible, the IPCC method estimate was re-calculated using
the same fertilizer consumption data as the DNDC model estimate. This changes the IPCC estimate for 1990 to 149 thousand metric tonnes, a figure
closer to the DNDC model estimate.
annual rate between 1990 and 1992. Due to severe
flooding of cropland in the North Central region and
low total acreage harvested in 1993, fertilizer con-
sumption decreased 0.5 percent and cropland yield
declined about 20 percent. In response to low crop-
land productivity in 1993, total acreage planted in
1994 increased about 8 percent (USDA, 1995f;
Dowdy, 1995). Fertilizer consumption increased
about 11 percent in 1994 (TVA, 1994) due to both the
increase in acres planted and efforts to restore nitro-
gen to cropland soil depleted by the heavy rainfall and
flooding in 1993 (Taylor, 1995).
Regional fertilizer use, as a percent of total U.S.
fertilizer consumption, has remained fairly constant
between 1990 and 1994 for all regions with the
exception of the North Central region between
1993 and 1994. From 1990 to 1992, the North
Central region made up about 55 percent of total
fertilizer consumption in the U.S. While fertilizer use
within this region began to decline in 1993, it again
stabilized at 55 percent of total U.S. fertilizer con-
sumption in 1994, following cropland flooding in
1993. The South Central, West, South East, and
North East regions remained constant from 1990 to
1994, making up approximately 20 percent, 15 per-
cent, 10 percent and 5 percent of total U.S. fertilizer
consumption, respectively (TVA, 1993; TVA,
1994).11
11 Regional percentages of total U.S. fertilizer consumption are approximations. Therefore, the sum of percentages for each region will
not equal 100. North East, South East, North Central, South Central, and West are defined as the following: North East: Maine, New
Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, and West
Virginia. South East: Virginia, North Carolina, South Carolina, Georgia, and Florida. North Central: Ohio, Indiana, Illinois, Minnesota,
Wisconsin, Iowa, Missouri, North Dakota, South Dakota, Nebraska, and Kansas. South Central: Kentucky, Tennessee, Alabama,
Mississippi, Arkansas, Louisiana, Oklahoma, and Texas. West: Montana, Idaho, Wyoming, Colorado, New Mexico, Arizona, Utah,
Nevada, California, Washington, and Oregon. For 1993 and 1994, fertilizer consumption data on Georgia as part of the South East
region are unavailable.
68 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Based on annual fertilizer consumption statistics,
emissions of N2O from agricultural soils have been
estimated from 1990 through 1994 (see Table W-7)12.
In 1994, emissions of N2O were estimated to be
about 211 thousand metric tonnes (18.4 MMTCE).
After increasing at an average rate of about 2 percent
between 1990 and 1992, N2O emissions declined
about 0.5 percent from 1992 to 1993. Due to the dra-
matic increase in fertilizer use following cropland
flooding in 1993, emissions of N2O from agricultural
lands increased approximately 11 percent in 1994.
Because agricultural activities fluctuate from year to
year due to economic, climatic and other variables,
the IPCC recommends that emissions are estimated
based on three year averages of fertilizer consumption
data. While the U.S. believes that annual data should
be used to calculate emissions, estimates based on
three year averages are also presented in Table IV-7.
Estimates using the IPCC methodology are
highly uncertain due to the large degree of uncer-
tainty associated with the emission factor. A survey of
the current scientific literature on field N2O flux pro-
vides a rather broad range for the emission coefficient
— greater than 0.001 and less than 0.1 (CAST, 1992).
Also, the emission coefficient used (1.17 percent) is
probably too low for organic fertilizers and, as men-
tioned earlier, organic fertilizer consumption may be
underestimated since the statistics only include fertil-
izers that enter the commercial market. Uncertainty is
also introduced due to the variable nitrogen content
of organic fertilizers. Nitrogen content varies by type
of organic fertilizer as well as within individual types,
and average values are used to estimate total organic
fertilizer nitrogen consumed.
Table IV-7
fcL- " . 1990
I Fertilizer Use (IO3 t N) f 0048.1
tbl2OEmissions(l03tN2O) 184.7
li-Year Average4 of Fertilizer Use(l03 t N) 9965.7
fts-'rear Average3 of N2O Emissions (IO3 t N2O) 183.2
Emissions from Field Burning
of Agricultural Wastes
In some parts of the U.S., agricultural crop wastes
are burned in the field to clear remaining straw and
stubble after harvest and to prepare the field for the
next cropping cycle. When crop residues are burned, a
number of greenhouse gases are released, including
carbon dioxide (CO^, methane (CH4), carbon
monoxide (CO), nitrous oxide (N2O), and oxides of
nitrogen (NOy). However, crop residue burning is not
thought to be a net source of CO2 because the CO2
released during burning is reabsorbed by crop
regrowth during the next growing season.
Field burning of crop residues is not a common
method of agricultural waste disposal in the U.S., so
emissions from this source are minor. Annual emis-
sions from this source over the period 1990-1994
averaged approximately 107 thousand metric tonnes
of CH4 (0.7 MMTCE), 2,247 thousand metric tonnes
of CO, 4 thousand metric tonnes of N2O (0.4
MMTCE), and 98 thousand metric tonnes of NOX.
These estimates are highly uncertain because data on
the amounts of residues burned each year are not
available. The average annual emission estimates for
field burning of crop residues over the 1990-1994
time period represent less than 1 percent of total U.S.
emissions ofCH4 and NOX, around 3 percent of total
U.S. CO emissions, and less than 1 percent of total
U.S. emissions of N2O. Cereal crops (e.g., wheat,
corn and sorghum) account for about 75 percent of
the CH4 and CO released and 50 percent of the N2O
and NOX released.
Emissions decreased on average about 4 percent
"L
1991
10239.4
188.3
10223.9
188.0
1992
10384.1
190.9
10319.6
189.7
1993
10335.2
190.0
10729.6
197.3
1994
11469.5
210.9
10902.3
200.4
§ Notes: Fertilizer consumption data obtained from TVA, 1994.
ID" Emission estimates for (994 are based on two-year averages for harvested area (i.e., (993 and /994J.
12 Please note that emissions in Table IV-7 are expressed in thousand metric tonnes.
Emissions from Agriculture
69
-------
between 1990 and 1991, increased about 18 percent
between 1991 and 1992, decreased about 21 percent
between 1992 and 1993, and then increased by
approximately 34 percent between 1993 and 1994.
These fluctuations in emissions estimates reflect
annual fluctuations in the amount of crops produced.
Large quantities of agricultural crop wastes are
produced from farming systems. There are a variety
of ways to dispose of these wastes. For example, agri-
cultural residues can be plowed back into the field,
composted, landfilled, or burned in the field.
Alternatively, they can be collected and used as a bio-
mass fuel or sold in supplemental feed markets. This
section addresses field burning of agricultural crop
wastes. Field burning of crop wastes is not thought to
be a net source of CO2 because the carbon released to
the atmosphere during burning is reabsorbed during
the next growing season. Crop residue burning is,
however, a net source of CHLt, CO, N2O, and NOX,
which are released during combustion. In addition,
field burning may result in enhanced emissions of
N2O and NOX many days after burning (Anderson,
et al., 1988; Levine, et al, 1988), although this
process is highly uncertain and will not be accounted
for in this section.
Methodology
The methodology for estimating greenhouse gas
emissions from field burning of agricultural wastes is
based on the amount of carbon burned, emission
ratios of CH4 and CO to CO2 measured in the smoke
of biomass fires, and emission ratios of N2O and
NOX to the nitrogen content of the fuel. The method-
ology is the same as the IPCC methodology
(IPCC/OECD/IEA, 1995).
The first step in estimating emissions from agri-
cultural waste burning is to estimate the amounts of
carbon and nitrogen released during burning:
Carbon Released =
Annual Crop Production x Residue/Crop Product
Ratio x Fraction of Residues Burned in situ x Dry
Matter content of the Residue x Burning Efficiency x
Carbon Content of the Residue x Combustion
Efficiency13
Nitrogen Released =
Annual Crop Production x Residue/Crop Product
Ratio x Fraction of Residues Burned in situ x
Dry Matter Content of the Residue x Burning
Efficiency x Nitrogen Content of the Residue x
Combustion Efficiency13
Estimates of the amounts of crop residues burned
in situ, or in the field, are not readily available.
Therefore, the default value of 10 percent, recom-
mended by the IPCC for developed countries, was
used. However, this default value, based on Crutzen
and Andreae (1990), may be an overestimate for the
U.S. because open burning is banned in many states.
Emissions of carbon as CH4 and CO are calcu-
lated by multiplying the amount of carbon released by
the appropriate emission ratio (i.e., CH4/C or CO/C).
Similarly, N2O and NOX emissions are calculated by
multiplying the amount of nitrogen released by the
appropriate emission ratio (i.e., N2O/N or NOX/N).
The specific values used in this inventory and the
results are presented in Tables IV-8 and IV-9.M
Emissions from Field Burning
Estimates of emissions based on each year's activ-
ity data have been calculated for the years 1990
through 1994. Field burning of agricultural wastes
was estimated to release an average of approximately
107 thousand metric tonnes CH4 (0.7 MMTCE),
2,247 thousand metric tonnes CO, 4 thousand metric
tonnes N2O (0.4 MMTCE), and 98 thousand metric
tonnes NOX annually from 1990 through 1994.
Cereal crops account for about 75 percent of the car-
bon released and 50 percent of the nitrogen released.
Emissions from field burning of agricultural
13 Burning Efficiency is defined as the fraction of dry biomass exposed to burning that actually burns. Combustion Efficiency is defined
as the fraction of carbon in the fire that is oxidized completely to CO2. In the methodology recommended by the EPCC, the "burning
efficiency" is assumed to be contained in the "fraction of residues burned" factor. However, the number used here to estimate the
"fraction of residues burned" does not account for the fraction of exposed residue that does not burn. Therefore, a "burning efficiency
factor" is added to the calculations.
14 Please note that emissions in Tables IV-8 and IV-9 are expressed in Thousand Metric Tonnes.
70 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table 1V-8
Crop Type
t -
if. :.
1990
Annual Production
(Thousand Metric Tonnes)
1991 1992 1993
res/crop dry fraction fraction
ratio3 matter carbon nitrogen
1994
f
§--
b
£•-
f
t"
j:
;tr
{?-
£
f1
'ni'~
jj...
S"-
"ereals
7 Wheat
- Barley
Cornd
Oats
Rye
Milletb
Sorghum
Pulse
Soya
Beans
Peas
Lentils
74,473
9,192
201,534
5,189
258
7,080
180
14,563
52,416
1,469
•55
40
53,918
10,110
189,886
3,534
248
7,142
180
14,856
54,065
1,532
217
76
67,135
9,908
240,719
4,271
291
8,149
180
22,227
59,612
1,026
115
71
65,220
8,666
160,954
3,001
263
7,081
180
13,569
50,919
994
149
91
63,157
8,162
256,629
3,336
283
8,971
180
16,638
69,626 ;
1,324 :
102
84 ;
.3
.2
.0
.3
.6
.4
.4
.4
L\
LI
.5
LI
85.0
85.0
78.0
90.4
90.0
85.0
88.5
88.0
86.7
85.4
90.2
86.7
0.4853
0.4567
0.4709
0.4853
0.4853
0.4144
0.4853
0.4853
0.45
0.45
0.45
0.45
0.003
0.004
0.0081
0.007
0.007
0.0067
0.007*
0.0085*
0.023*
0.023*
0.023*
0.023*
24,959
56
1.634
18,239
574
25,525
25,585
56
2,235
18^943
511
27,444
26,438
56
1,943
19,294
548
27,545
23,812
46
1,539
19,445
504
28,214
29,037
55
1,934
20,835
596
28,863
0.3
0.8
1.0
0.4
0.4
0.8
90.0
90.0
90.1
86.7
86.7
90.0
0.4072
0.4226
0.4226
0.4226
0.4226
0.4695
0.0228
0.0 11
0.0 II
0.01 1
0.01 1
0.003
i Tuber and Root
f Sugarbeet
- Artichoke0
|" Peanut
,_ Potatoes
| Other
^Sugarcane
I Sources: Data on annual crop production were taken from USDA (1991 a, 19936, I994b, l995f),FAO (1994) and McFarland (1995). Residue/crop
F rat/os, dry matter contents, and carbon contents were taken from Strehler and Stiitzle (1987) and University of California (1977). Nitrogen contents
Cwere also taken from Strehler and Stiitzle (1987) except where indicated by an asterisk (*).These data were token from Barnard (1990). The per-
| cent of produced residue that is burned is based on Crutzen andAndreae (1990).
1 " The percent of crop residue burned is assumed to be 10 percent
\-!> Because millet is such a small commodity relative to other crops, the USDA no longer tracks its production. These production estimates were taken
f from the FAO (1993, 1994).
PS-Total artichoke production was estimated by assuming that California accounted for 90% of the entire market Artichoke production data for
iJCg/ifbrn/a are based on McFarland (1995).
* -4 Corn refers to maize _
wastes decreased 1 to 7 percent between 1990 and
1991 and then increased 15 to 20 percent in 1992. A
similar pattern again followed between 1992 and
1993, with a 20 to 21 percent decrease in 1993, and
then a 31 to 37 percent increase in 1994. Total emis-
sions are based on actual crop production figures and
reflect general crop production trends. Production for
most crops decreased between 1990 and 1991,
increased from 1991 to 1992, decreased from 1992
to 1993, and then increased from 1993 to 1994. The
large decrease in 1993 relative to 1992 and 1994 is
due, in part, to the decrease in area harvested in 1993
(USDA, 1994c) that was partially caused by flooding
(Dowdy, 1995).
For the 1990-1994 time period, 1994 had the
highest emissions. Emissions for that year included
122 thousand tonnes of CH4 (0.8 MMTCE), 2,562
thousand tonnes of CO, 5 thousand tonnes of N2O
(0.4 MMTCE), and 116 thousand tonnes of NOX.
Emissions estimates for 1993 were the lowest for the
period 1990-1994.
To avoid effects of fluctuations in economic or
climatic conditions on estimates of emissions from
burning of agricultural wastes, the IPCC recommends
using a three-year average for crop production cen-
tered around the year in question. This method was
used to estimate emissions for the years 1990, 1991,
1992, 1993, and 1994. The data and results are con-
Emissions from Agriculture
71
-------
Table IV-9
|M
| Gas/Crop Type
I Cereals
Pulse
I Tuber & Root
a-Sugarcane
Carbon or Nitrogen Released
(Thousand Metric Tonnes)
1992
1990 1991
1993
Emission Conversion Emissions
Ratio Factor (Thousand Metric Tonnes)b
1994 1990 1991 1992 (993 1994
10,941 9,663 12,201 9,145 12,154
3,622 3,742 4,075 3,493 4,767
503 535 538 503 580
706 759 762 781 799
0.005
0.005
0.005
0.005
16/12
16/12
16/12
16/12
73
24
3
5
64
...25.
4
5
81
27
4
5
61
23
3
5
81
32
4
5
-co, ,
Cereals 10,941 9,663 12,201 9,145 12,154
Pulse 3,622 3,742 4,075 3,493 4,767
;:; Tuber & Root 503 535 538 5Q3 ^58JD.
Sugarcane 706 759 762 781 799
0.006 28/12 1,532 1,353 1,708 1,280 1,702
0.006 28/12 1507~ 524 57I 489 667
.0..0Q6.. 28/J 2 70 75 75 70 81
0.006 28/12 99 106 "l07 109 112
:N2O
T Cereals
- Pulse
Tuber & Root
- Sugarcane
TOTAt£SBSi
JNCV
f Cereals
i Pulse
• Tuber & Root
Sugarcane
1 TQTAisiiiiliii
'•
147
185
20
5
tllisSSlII
147
185
20
5
136
191
21
5
Iis553]"iii
136
191
21
5
«
173
208
21
5
iE4l7l!i
173
208
21
5
122
179
20
5
§325 ;||
122
179
20
5
^§jHI
174
244
23
5
• I|J!«li,IIIIIf
1.446,,:;:::::
174
244
23
5
0.007
0.007
0.007
0.007
wanes
0.121
0.121
0.121
0.121
IllliZ
44/28
44/28
44/28
44/28
*£gfillffig?jjm
30/14
30/14
30/K
30/14
isSlTIliSlli
_ " "~1
2
2
0.2
0.05
iBmi
llliillliiB
38
48
5
1
nmu
i
2
0.2.
0.05
35
50
5
1
iicli
2
'. 2\",
0.2
0.05
45 ._..
54
5
1
|||][pg||:«
IE
1
..2" '/
0.2
0.05
•m
ItBiHMlllH
32
46
5
1
jfirm
-
.2 j
3 .. ]
0.3 'v
0.06
•ill
i^ffilJlfiiBB
45 . _
63 ,
6
1
w^n
iiiii!iiiiiiii9ia;:": i; 1
1
• Totals may not add due to rounding
b Burning efficiency (the fraction of dry biomass exposed to burning that actually burns) was assumed to be 93 percent and combustion efficiency
(the fraction of carbon in the fire that fe oxidized^ completely to COj was assumed to be 88 percent (U.S. EPA, / 994a). ' ;
,,,e Jhe source for the factor to convert NOX to full molecular weight isAndreae (1990). The ratio of 30/14 was used because NO is the primary form
of NOX emftted during biomass combustion. *
tained in Tables IV-10 and IV-11 and generate a trend
counterintuitive to that reflected by results using the
annual activity data. Emissions based on a three-year
average increased approximately 6 to 7 percent from
1990 to 1991, decreased about 3 to 4 percent
between 1991 and 1992, increased 8 to 9 percent
from 1992 to 1993, and decreased about 2 to 3 per-
cent between 1993 and 1994. Since these trends run
counter to the results obtained using annual data,
annual activity data, rather than three-year averages,
were used in this Inventory to produce annual emis-
sion estimates.
72 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table IV-10
|aCropType
3 Yr.Avg. Production
(Thousand Metric Tonnes)
res/crop dry fraction
ratio matter carbon
1990
1991
1992
£.
6^
i-i-I
=.-•
f-.
ii'
fe.
ereals
Wheat
Barley
Cornd
Oats
Rye
Rice
Millet*.
Sorghum
^ -61,273;
9,367
194,192
4,715
284'""
7,077
183
15,017
; 65,175
"9,737
210,713
4,331
266
7^457
7 180
17,215
'""62,091
'""'"' 9,561
197,186
3,602
267
7,457
"* 180
16,884
i Pulse
B Soya 52,945 ""'~' 55,364 "54,865
Beans 1,359 1,342 1,184
;Peas 204 162 160
^Lentils 56 62 79
gTuber and Root
_ Sugarbeet 24,448 25,661 25,279
p^ Artichoke^ 59 56 52
fe Peanut 1,893 1,937 1,906
fj:.,"potatoes .?". 17,995 77 18,826 7^ !9,228
p/Other ".'''"' 534 ; "' 7 544 1 521
^Sugarcane " '26355 2i6;§38 17,734
1993
65,171
8,912
2i9,4"34'"
* 3,536
279
8,067
180
17,478
60,052
1,115
122
26,429
52
1,806
19,858
549
28,207
I994a
64,189
8,414
208,791
3,169
273
8,026
180
15,103
60,273
1,159
126
88
26,425
50
1,737
20,140
550
28,539
1.3
1.2
1.0
1.3
1.6
1.4
1.4
1.4
2.1
2.1
1.5
2.1
0.3
0.8
1.0
0.4
0.4
0.8
85.0
85.0
78.0
90.4
90.0
85.0
88.5
88.0
86.7
85.4
90.2
86.7
90.0
90.0
90.1
86.7
86.7
90.0
0.4853
0.4567
0.4709
0.4853
0.4853
0.4144
0.4853
0.4853
0.45
0.45
0.45
0.45
0.4072
0.4226
0.4226
0.4226
0.4226
0.4695
fraction
nitrogen
0.003
0.004
0.0081
0.007
0.007
0.0067
0.007*
0.0085*
0.023*
0.023*
0.023*
0.023*
0.0228
0.011
0.011
0.011
0.011
0.003
Sources: Dataon annual crop production were token from USDA '(1991 a, i993b, 1994b, i995f), FAO (1994) and McFarland (1995). Residue/crop
ratios, dry matter contents and carbon contents, were taken from Strehler and Sttitzle (1987) and University of California (1977). Nitrogen contents
yyere a/so taken from Strehler and Sttitzte (1987) except wnere indicated by an asterisk (*). These data were taken from Barnard (1990). The per-
~tent of produced residue that i's burned /s based on Crutzen andAndreae (1990).
^ Crop production for 1994 are tvvo^ear averages f(.e", 1993 and 1994).
S_Because millet is such a small commodity relative to other crops, the USDA no tonger tracks its production. These production estimates were taken
^from FAO (1993, 1994). "
^Tota/ artichoke production was estimated fay assuming that California accounted for 90 percent of the entire market Artichoke production data for
California are based on McFarland (1995).
5? Corn refers to maize.
, The percentage of crop residue burned is assumed to be 10 percent.
Emissions from Agriculture
73
-------
Table IV-11
Gas/Crop Type
CH4
cereals
pulse
tuber/root
sugarcane
Carbon or Nitrogen Released Emission Conversion Emissions
(Thousand Metric Tonnes) Ratio Factor (Thousand Metric Tonnes)b
1990 1991 1992 1993 1994 ,1990 1991 1992 1993 1994
10,149 10,935 10,336 11,167 10,649
3,654 3,813 3,770 4,112 4J30
503 525 525 540 541
735 742 767 780 790
0.005 16/12 68
0.005 16/12 24
O.Q05 16/12 3
0.005 16/12 5
73
25
3
"j
Of
69
25
74
27
4
5
co
cereals
pulse
tuber/root
sugarcane
10,149 10,935 10,336 11,167 10,649
3,654 3,813 3,770 4,112 4,130
503 525 525 540 541
735 742 767 780 790
0.006 28/12
0,006 28/12
0.006 28/12
512
70
103
534
73
104
528
73
107
576
76
109
71
28
4
5
0.006 28/12 1,421 1,531 1,447 1,563 1,491
578
76
III
N2O
cereals
pulse
;- tuber/root
sugarcane
140
187
20
5
152
195
21
5
143
193
20
5
156 148
210 211
21 21
5 5
0.007
0.007
0.007
0.007
44/28
44/28
44/28
44/28
2
2
0.2
0.05
2
2
0.2
0.05
2
2
0.2
0.05
2
2
0.2
0.05
sSliS
2
2
0.2
0.05
i«J||
cereals
pulse
tuber/root
sugarcane
140
187
20
5
152
195
21
5
143
193
20
5
156
210
21
5
148
211
21
5
O.J2I
0.121
0,121
0.121
30/J4
30/14
30/14
30/14
36
48
.,,5,
1
39
51
5
37
5Q
5
I
41
54
5
38
55
6
I
0 Tbtafe may not add due to rounding.
- b Burning efficiency (the fraction of dry biomass exposed to burning that actually burns) was assumed to be 93 percent and combustion efficiency
•! (the fraction of carbon in the fre that is oxidized completely to COJ was assumed to be 88 percent (U.S. EPA, 1994a).
'fc The source for the factor to convert NOX to full molecular weight is Andreae (1990). The ratio of 30114 was used because NO is the primary form
= of NOX emitted during biomass combustion.
74 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
PartV:
Emissions from Land-Use
Change and Forestry
The biosphere emits and absorbs a wide variety of carbon and nitrogen trace gases, including carbon diox-
ide (CO2), methane (CH4), carbon monoxide (CO), nitrous oxide (N2O), oxides of nitrogen (NOX), and
non-methane hydrocarbons (NMHCs).1 When humans impact the biosphere through land-use change and for-
est management activities, such as clearing an area of forest to create cropland, restocking a logged forest, drain-
ing a wetland, or allowing a pasture to revert to a grassland, the natural balance of these trace gas emissions
and uptake is altered and their atmospheric concentrations adjust. Globally, the most important human activ-
ity that affects the biosphere is deforestation, particularly the clearing of tropical forests for agricultural use.
Deforestation is estimated to be responsible for about 20 percent of the current annual global emissions of CO2
from anthropogenic activities (IPCC, 1992).
In the U.S., however, forest management activities and the regeneration of previously cleared forest area are
believed to be the primary factors affecting current greenhouse gas fluxes from land-use change and forestry, as
the amount of forest land has remained fairly constant over recent decades. The net CO2 flux in 1990, 1991
and 1992 due to these activities is estimated to have been an uptake (sequestration) of 125 MMTCE. This car-
bon uptake represents an offset of about 9 percent of the average annual CO2 emissions from energy-related
activities during this period. Emission estimates are not yet available for 1993 and 1994 because the last
national forest inventory was completed in 1992.
The U.S. land area is roughly 2,263 million
acres, of which 33 percent, or 737 million acres, is
forest land (Powell, et al, 1993). The amount of for-
est land has remained fairly constant over recent
decades, declining by approximately 5 million acres
between 1977 and 1987 (USFS, 1990; Waddell, et
al, 1989), and increasing by about 0.5 million acres
between 1987 and 1992 (Powell, et al, 1993). These
changes represent fluctuations of well under 1 per-
cent of the forest land area, or on average, about 0.1
percent per year. Other major land uses in the U.S.
include range and pasture lands (36 percent), crop-
land (18 percent), urban uses (3 percent), and other
lands (10 percent) (Daugherty, 1991).2 Urban lands
are the fastest growing land use. Given that U.S. for-
est land area changed by only about 0.1 percent per
year over the last 15 years, the major influences on
the net carbon flux from forest land are management
activities and ongoing impacts of previous land-use
changes. These activities affect the net flux of carbon
by altering the amount of carbon stored in forest
ecosystems. For example, intensified management of
forests can increase both the rate of growth and the
eventual biomass density3 of the forest, thereby
1 Nonmethane hydrocarbons (NMHCs) are a subset of nonmethane volatile organic compounds.
2 Other lands include farmsteads, transportation uses, marshes, swamps, deserts, tundra, and miscellaneous other lands.
3 Biomass density, or the amount of biomass in a given land area, includes all the living and dead organic material, both above and
below the ground surface.
Emissions from Land-Use Change and Forestry
75
-------
increasing the uptake of carbon. The reversion of
cropland to forest land through natural regeneration
also will, over decades, result 'in increased carbon
storage in biomass and soils (i.e., in general, forests
contain more carbon than cropland).
Forests are complex ecosystems with several
interrelated components, each of which acts as a car-
bon storage pool, including:
• trees (i.e., living trees, standing dead trees,
roots, stems, branches, and foliage);
• soil;
• the forest floor (i.e., woody debris and tree lit-
ter); and
• understory vegetation (i.e., shrubs and
bushes).
As a result of biological processes (e.g., growth
and mortality) and anthropogenic activities (e.g., har-
vesting, thinning, and replanting), carbon is continu-
ously cycled through these ecosystem components, as
well as between the forest ecosystem and the atmos-
phere. For example, the growth of trees results in the
uptake of carbon from the atmosphere and storage in
living biomass. As trees age, they continue to accu-
mulate carbon until they reach maturity, at which
point they are relatively constant carbon stores. As
trees die and otherwise deposit litter and debris on
the forest floor, decay processes release carbon to the
atmosphere and also increase soil carbon. The net
change in forest carbon is the sum of the net changes
in the total amount of carbon stored in each of the
forest carbon pools over time.
The net change in forest carbon, however, is not
likely to be equivalent to the net flux between forests
and the atmosphere. Because most of the timber that
is harvested and removed from U.S. forests is used in
wood products, removals may not always result in an
immediate flux of carbon to the atmosphere.
Harvesting in effect transfers carbon from one of the
"forest pools" to a "product pool." Once in a prod-
uct pool, the carbon is emitted over time as
- CO2 through either combustion or decay,4 although
the exact rate of emission varies considerably
between different product pools and may in fact
result in effective long-term carbon storage. For
example, if timber is harvested and subsequently used
as lumber in a house, it may be many decades or even
centuries before the lumber is allowed to decay and
carbon is released to the atmosphere. If timber is har-
vested for energy use, subsequent combustion results
in an immediate release of carbon. Paper production
may result in emissions over years or decades.
The U.S. forest carbon flux estimates for the
period from 1990 to 1992 presented in this inventory
are based on historical changes in forest carbon stocks
and projected changes in forest product pools. Forest
carbon stock estimates are derived for two years, 1987
and 1992, based on forest surveys conducted for those
years. The derived forest stock estimates only include
carbon contained in biomass of trees and understory
vegetation. Soil and forest floor carbon stocks have
not been included at this time due to methodological
uncertainties associated with their estimation. The
annual net carbon flux from forest growth in 1990,
1991, and 1992 was assumed to be equal to the aver-
age net annual flux during the period between 1987
and 1992. Carbon fluxes associated with changes in
sizes of product pools were derived using the esti-
mated pool sizes in 1980 and 2000. It was assumed
that the product pool fluxes in 1990,1991, and 1992
were equal to the average annual flux during the
1980-2000 period. The total annual carbon flux from
forests was obtained by summing the carbon flux
associated with forest growth and the flux associated
with changes in product pools.
The inventory methodology described above dif-
fers somewhat from that recommended by the IPCC
(EPCC/OECD/IEA, 1995). Instead of directly invento-
rying carbon stocks and changes in stocks over time,.
the IPCC methodology uses average annual statistics
on land-use change and forest management activities,
and applies carbon density and flux rate data to these
4 Actually, if timber undergoes combustion, some small portion of the carbon - as much as 10 percent of the total carbon released - will
be emitted as CO and CH4 rather than CO2. In addition, if timber products are placed in landfills, about 50 percent of the carbon that
eventually decomposes is oxidized to CO2 and about 50 percent is released as CH4. However, eventually both CO and CH4 oxidize to
CO2 in the atmosphere.
76 n Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
activity estimates to derive total flux values. In addi-
tion, the IPCC assumes that harvested biomass
replaces biomass in existing product pools that
decays in the inventory year (i.e., that product pool
sizes do not change over time), while the U.S. inven-
tory accounts for changes in the amount of carbon
stored in product pools. The EPCC has adopted a
methodology that utilizes average land-use change
statistics and does not include fluxes associated with
product pools because the majority of the world's
countries do not have detailed time-series of forest
inventory statistics and information on product pool
sizes, such as are available in the U.S. and have been
used in preparing this inventory. Although there are
large uncertainties, associated with the data used to
develop the emission estimates presented here, the use
of direct measurements from forest surveys and of
product pool size estimates is likely to have resulted
in more accurate flux estimates than if the basic EPCC
methodology had been employed.
The estimates of forest carbon and product car-
bon stocks used in this inventory to derive carbon
fluxes were provided by Birdsey and Heath (1995).
These estimates are based on a variety of data
sources. The amount of carbon in trees and under-
story vegetation in 1987 and 1992 was estimated
based on empirical data collected by the U.S. Forest
Service (USFS) (waddell, et al.,1989; Powell, et al.,
1993). The data include estimates of timber volume
by tree species, size class, and other categories.
Although the surveys cover only timberland, which is
a subset of the forest land base, they capture the most
productive and intensively managed forest lands.5
The amount of carbon in aboveground and below
ground tree biomass in forests was calculated by mul-
tiplying timber volume by conversion factors derived
from a national biomass inventory (Cost, et al.,
1990). Carbon storage in understory vegetation was
estimated based on simple models (Vogt, et al., 1986)
Table V-1
Year Carbon in Carbon in Carbon in
?=±- Forests Landfills Wood Products
: 1980
(Million Metric Tonnes)
1,236 1,272
13,567
14,057
12000
1,533
1,520
|- Source: (Birdsey and Heath, 1995)
and review of numerous intensive ecosystem studies
(Birdsey, 1992). The total biomass carbon was esti-
mated as a sum of carbon contained in individual
pools. Carbon stored annually in harvested biomass
was assumed to be equal to a sum of the net amount
of carbon deposited in landfills and the net increase in
carbon contained in durable wood products. The
average values of these fluxes were obtained using the
1980 and 2000 sizes of landfill and wood product
pools (Row and Phelps, 1991).
The total amounts of biomass carbon in U.S.
forests in 1987 and 1992 are given in Table V-l. The
increase in forest carbon stocks over time indicates
that, during the examined periods, forests on average
functioned as net sinks of carbon.
The annual net carbon flux in forests in 1990,
1991, and 1992 was estimated by dividing the differ-
ence in total forest carbon storage in 1987 and 1992
by the number of years between the two surveys
(Table V-2):
(14,057- 13,567) _
tonnes of carbon/year
Net carbon fluxes associated with biomass accu-
mulated in landfills and in wood products were esti-
mated based on corresponding pool sizes in 1980 and
2000 (Table V-l). The annual average net fluxes for
these pools were estimated by dividing the difference
between 1980 and 2000 pool sizes by 20 (Table V-2).
5 Forest land in the U.S. includes all land that is at least 10 percent stocked with trees of any size. Timberlands are the most productive
of these forest lands, growing at a rate of 20 cubic feet per acre per year or more. In 1992 there were about 490 million acres of
Timberlands, which represented 66 percent of all forest lands (Powell, et al., 1993). Forest land classified as Timberlands is unreserved
forest land that is producing or is capable of producing crops of industrial wood. The remaining 34 percent of forest land is classified as
Productive Reserved Forest Land, which is withdrawn from timber use by a statute or regulation, or Other Forest Land, which includes
unreserved and reserved unproductive forest land. While this inventory does not quantify the carbon flux on Productive Reserved or
Other Forest Lands, this missing flux is assumed to be relatively minor because trees on these lands grow slowly and the standing stock
of trees is not managed intensively.
Emissions from Land-Use Change and Forestry B 77
-------
The annual total net carbon fluxes from U.S. forests
were estimated by summing the fluxes from forests,
wood products, and landfills (Table V-2).6
There are considerable uncertainties associated
with the estimates of the net carbon flux from U.S.
forests in the present inventory. The first source of
uncertainty is due to the probabilistic rather than the
deterministic nature of forest surveys. These surveys
are based on a statistical sample designed to represent
a wide variety of growth conditions present over
large territories. Therefore, the actual values of car-
bon stored in forests are represented by average val-
ues that are subject to sampling and estimation
errors. However, according to Birdsey and Heath
(1995), these errors are likely to be relatively small.
The second source of uncertainty results from
incomplete accounting of wood products. Only bio-
mass removed from private timberland was used to
assess net carbon fluxes from landfill and wood prod-
uct pools. Inclusion of biomass removed from other
timberland would likely increase the estimated net
sequestered carbon.
The third source of uncertainty is associated with
the fact that the carbon content of the understory
vegetation pool was evaluated using independent
ecosystem studies. In order to extrapolate results of
these studies to all forest lands, it was assumed that
they adequately describe regional or national aver-
ages. This assumption can potentially lead to the fol-
lowing errors: bias from applying data from studies
that inadequately represent average forest conditions,
Table V-2
modeling errors (erroneous assumptions), and errors
in converting estimates from one reporting unit to
another (Birdsey and Heath, 1995).
It should be noted that the current inventory does
not include estimates of soil and forest floor carbon
fluxes. The main reason for excluding these fluxes is
that impacts of forest management activities, includ-
ing harvest, on soil and forest floor carbon are not
well understood. For example, Moore, et al. (1981)
found that harvest may lead to a 20 percent loss of
soil carbon, while little or no net change in soil car-
bon following harvest was reported in another study
(Johnson, 1992). Since forest soils and floors contain
over 60 percent of the total stored forest carbon in
the U.S., this difference can have a large impact on
flux estimates.
The current inventory also does not address
emissions of greenhouse gases other than CO2. It is
known that forest management activities result in
fluxes of other radiatively important gases, such as
CH4, N20, CO, and several NMHCs. However, the
effects of forestry activities on fluxes of these gases
are highly uncertain. Similarly, there are several land-
use changes that are not accounted for in the inven-
tory due to uncertainties in their effects on trace gas
fluxes as well as poorly quantified land-use change
statistics. These land-use changes include loss and
reclamation of freshwater wetland areas, conversion
of grasslands to pasture and cropland, and conver-
sion of managed lands to grasslands and other
unmanaged, non-forest dryland types.
j Year
1990
M99I
1992
I 1992
Forest
Flux
98
98
98
. Landjill ,
Carbon Flux
Wood
Products Flux
"' _ * i i j (Million Metric Tonnes)
12 15
12 IS
12 15
Total
Carbon Flux
„ Total
CO2 Flux
6 The new estimates of the net flux associated with forest growth from 1990-1992 are lower than those provided in the 1994 U.S.
Inventory (U.S. EPA, 1994b) because the forest floor flux was not included in the current inventory. Also, in addition to forest carbon
stock changes between 1987 and 1992, the previous estimates were based on stock changes between 1977 and 1987. Estimates of
changes in carbon stocks prior to 1987 were not used in the current inventory to calculate fluxes because it is believed that this would
add extra uncertainty to the final flux estimates. The total net carbon fluxes in the current inventory exceed those presented in 1994 due
to inclusion of fluxes associated with accumulation of harvested biomass in wood product and landfill pools.
78 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Part VI:
Emissions from Waste
Anaerobic decomposition of organic materials by bacteria in landfills can result in emissions of methane
(CH4), carbon dioxide (CO2), and other greenhouse and photochemically important gases. Currently,
methane emissions from landfills are the largest single anthropogenic source of methane in the U.S., contribut-
ing about 36 percent of total U.S. methane emissions. Large quantities of methane can be also emitted as a result
of anaerobic decomposition processes in wastewater streams with high organic material content. In addition,
the combustion of waste, both in incinerators and by open burning, is also a source of many greenhouse and
photochemically important gases. This section covers methane emissions from U.S. landfills and wastewater
streams, and criteria pollutant emissions from waste incineration. Emissions from each of these sources from
1990 to 1994 are presented in Table VI-1.
Landfills
Organic landfill materials such as yard waste,
household garbage, food waste, and paper can
decompose and produce methane. This decomposi-
tion process is a natural mechanism through which
microorganisms derive energy for growth. Methane
production typically begins one or two years after
waste placement in a landfill and may last from 10 to
60 years. Methane emissions from landfills are the
\ Methjane Emissions
from Wastes by Source: 1994
Waste Water
2%
Emissions from Waste
79
-------
Table VI-1
I Gas/Source
}:;,:: Landfills
S Wastewater
INMVOC
i Waste Combustion
'; co
I Waste Combustion
fNOx
: Waste Combustion
Emissions . ..__ Emissions
(Full Molecular Weight) (Direct and Indirect Effects;
't-r-,. -r-v. Carbon-Equivalent)
(Million Metric Tonnes)
1990 1991 1992, J993 199/£ _ _ 1990, 199J _ 1992_ 1993 I994_
9.9 10.1 9.9 10.0 IQ.2 , ^66,2 ,67t.5 66^2 66.8 68.2
0.2 ..0.2. 0.2 0.2 0.2 1.0 1.0 1.0 1.0 I.I
0.3 0.3 0.3 0.3 0.3 - - -
1.5 1.5 1.6 1.6 1.6
O.I O.I O.I
0.1
^ Note; Totals presented in the summary tables in this chapter may not equal the sum of the individual source categories due to rounding.
~-~—, ^—^-^^.^-^^^^^^^^^^^s MWJ^wwmmfcjjiiLa ^11. '*MU "•"IIHIdUi.aUliMmiPJaMjaiimMiiau^^ ~"'
largest single anthropogenic source of methane in the
U.S. In 1994, methane emissions from U.S. landfills
totaled approximately 10.2 million tonnes (68.2
MMTCE), or about 36 percent of total U.S. methane
emissions. Emissions from U.S. municipal solid waste
(MSW) landfills, which received about 67 percent of
the total solid waste generated in the U.S., account
for about 90 to 95 percent of the total landfill emis-
sions, while industrial landfills account for the
remaining 5 to 10 percent. There are over 6,000 land-
fills in the U.S., with 1,300 of the largest landfills gen-
erating almost all the methane and receiving almost
all the waste.
Between 1990 and 1994, estimates of methane
emissions from landfills remained essentially
unchanged. The relatively constant emissions esti-
mates for the period are actually the result of two
countervailing factors: (1) an increase in the amount
of MSW in landfills contributing to methane emis-
sions (thereby increasing the potential for emissions);
and (2) an increase in the amount of landfill gas col-
lected and combusted (thereby reducing emissions).
Methane emissions from landfills are a function
of several factors, including the total amount of
MSW landfilled over the last 30 years; composition
of the waste in place; the amount of methane that is
recovered and either flared or used for energy pur-
poses; and the amount of methane oxidized in land-
fills before being released into the atmosphere. The
estimated total quantity of waste in place contribut-
ing to emissions increased from about 4,708 million
metric tons in 1990 to 4,971 million metric tonnes in
1994, an increase of 5.6 percent. During this same
period, the estimated methane recovered and flared
from landfills increased as well. In 1990, for example,
approximately 1.5 million tonnes of methane were
recovered and combusted (i.e., used for energy or
flared) from landfills. In 1992, the estimated quantity
of methane recovered and combusted increased to 1.8
million metric tonnes. While 1994 data are unavail-
able, the amount of methane recovered and com-
busted from landfills was expected to have continued
increasing, resulting in relatively constant emissions
estimates between 1990 and 1994.
Over the next several years, the total amount of
MSW generated is anticipated to continue to
increase. The percentage of waste landfilled, however,
may decrease due to increased recycling and com-
posting practices. While the percentage of waste land-
filled could decrease, the composition of the waste
being landfilled could include a higher proportion of
organic material, thereby increasing methane genera-
tion per unit of waste in place (U.S. EPA, 1993a).
80 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Conversely, the quantity of methane that is recovered
and either flared or used for energy purposes is
expected to increase, partially as a result of a new reg-
ulation that will require large landfills to collect and
combust landfill gas in the future.1 The impact of
such shifts in activity on emissions cannot be fully
assessed at this time.
After being placed in a landfill, organic waste
first decomposes aerobically (in the presence of oxy-
gen) and is then attacked by anaerobic bacteria which
convert organic material to simpler forms like cellu-
lose, ammo acids, and sugars. These simple sub-
stances are further broken down through
fermentation into gases and short-chain organic com-
pounds that form the substrates for methanogenic
bacteria. Methane producing bacteria then convert
these fermentation products into stabilized organic
materials and a biogas consisting of approximately
50 percent carbon dioxide and 50 percent methane
by volume. The percentage of carbon dioxide in the
biogas released from a landfill may be smaller
because some CO2 dissolves in landfill water
(Bingemer and Crutzen, 1987).
Emissions of methane into the atmosphere will
equal total methane production from municipal land-
fills adjusted for the methane produced by industrial
landfills, the methane recovered, and the methane
Table VI-2
oxidized before being released into the atmosphere.
These adjustments can be described as follows:
Methane Emissions =
municipal landfill methane generation
plus industrial landfill methane generation
minus methane recovery
minus methane oxidation
Further detail on the methodology used here can be
found in Annex E.
In 1994, landfills in the U.S. were estimated to
have emitted between 8.3 million metric tonnes to
12.0 million metric tonnes, with a central estimate of
10.2 million metric tonnes of methane (68.2
MMTCE; see Table VI-2). This amount represents
about 36 percent of total U.S. methane emissions.
Based on this analysis, landfills are by far the largest
anthropogenic source of methane emissions in the
U.S. Table VI-2 presents the estimates of landfill
methane emissions and uncertainty ranges for 1990
through 1994.
Methane emissions estimates from 1990 to 1994
have remained essentially unchanged. The data indi-
cate a slight increase in emissions from 1990 to 1994
(3 percent), but the uncertainties in the data available
: Landfills
tMedium Landfills
Umall LandfiHs
: Industrial
152
1,137
4,744
N/A
3.0-3.8
"3.5-5.9
'(•
-------
make it difficult to precisely define trends. However,
the estimates clearly indicate that larger landfills in
the U.S. contribute more to overall methane emis-
sions than smaller landfills. In 1994 "large" landfills
accounted for only 2.5 percent of all landfills, but
over 30 percent of total landfill methane emissions,
while a far greater number of small landfills (79 per-
cent) accounted for only about 12 percent of these
emissions (see Table VI-2). Moreover, these percent-
ages have remained essentially unchanged between
1990 and 1994.
There are several uncertainties associated with
the estimates provided for methane emissions from
landfills. The primary uncertainty surrounding the
estimates is the lack of comprehensive information
regarding the characterization of landfills, in terms of
acres landfilled, moisture content, waste composi-
tion, operating practices at the landfill, and total
waste in place (the fundamental factors affecting
methane production). In addition, there is very little
information on the quantity of methane that is cur-
rently flared at non-energy related projects. Finally,
the statistical model used to estimate emissions is
based on methane generation at landfill facilities that
currently have developed energy recovery projects,
and may not precisely capture the relationship
between emissions and various physical characteris-
tics of all U.S. landfills. To the degree that emission
data and inputs for the model are not representative
of landfills as a whole, error may be introduced.
In addition to methane, carbon dioxide is also
released from landfills. However, carbon dioxide
from landfills is believed to come mainly from
organic materials. Since these materials are assumed
to absorb a similar amount of carbon during the
growing cycle, the net contribution of landfills to the
global carbon dioxide budget is assumed to be zero,
and therefore is not included in the emissions totals.
The same is not true for the methane that may be pro-
duced, however, since the methane is typically only
produced as a by-product of the landfilling process.
For the same reason, carbon dioxide released during
methane combustion is also not counted.
Wastewater
Wastewater can be treated using aerobic and/or
anaerobic technologies, or if untreated, can degrade
under either aerobic or anaerobic conditions.
Methane is produced when organic material in
treated and untreated wastewater degrades anaerobi-
cally, i.e., without the presence of oxygen. Based on
available data, methane emissions from municipal
wastewater in the U.S. were about 1.1 MMTCE in
1994, or about 0.6 percent of total U.S. methane
emissions. Emissions over the period from 1990
through 1994 have remained relatively constant,
increasing only slightly due to a growing volume of
municipal wastewater caused by a rising U.S. popu-
lation. This estimate is based on rough assumptions
of the U.S. municipal wastewater stream drawn from
U.S. EPA (1994a), and at this time data are not suffi-
cient to estimate methane emissions from industrial
wastewater streams. Further research is ongoing at
the U.S. EPA to better quantify emissions from this
source.2
Highly organic wastewater streams such as waste
streams from food processing or pulp and paper
2 EPA's Atmospheric Pollution Prevention Division is currently conducting research to better quantify methane emissions from the U.S.
wastewater stream. The results of this analysis should be available in early 1996.
82 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
plants rapidly deplete available oxygen in the water
stream as their organic matter decomposes. The
organic content, otherwise known as "loading" of
these wastewater streams, is expressed in terms of
biochemical oxygen demand, or "BOD." BOD repre-
sents the amounts of oxygen taken up by the organic
matter in the wastewater during decomposition.
Under the same conditions, wastewater with higher
BOD concentrations will produce more methane than
wastewater with relatively lower BOD concentra-
tions. Most industrial wastewater has a low BOD
content, while food processing facilities such as fruit,
sugar, meat processing plants, and breweries can pro-
duce untreated waste streams with high BOD con-
tent.
Although IPCC-recommended methodologies for
estimating municipal and industrial wastewater
methane emissions exist, the data required by these
methodologies are not easily obtained, especially
industrial wastewater data. Estimates of municipal
wastewater methane for the U.S. provided in this sec-
tion are based on methods outlined in U.S. EPA
(1994a). That report's methodologies, which are sim-
ilar to the proposed IPCC methodologies, are based
on BOD loading in the municipal wastewater flow in
the U.S., resulting in the following equation:
kgCH4
Table VI-3
yr
Population x
kgBODs 365 days 0.22kgCH4
x x
capita/day " yr " kgBODs
x (Fraction Anaerobically Digested)
As shown in Table VI-3, applying this equation
with U.S. population statistics and available waste
stream data results in estimated methane emissions
from municipal wastewater of about 150,000 metric
tonnes (1.0 MMTCE) in 1990. This value increased
slightly to about 160,000 metric tonnes (1.1
MMTCE) in 1994, which was about 0.6 percent of
total U.S. methane emissions in that year. This small
increase was due to an increase in U.S. population
resulting in an increased flow of municipal waste-
water.
P '.."?. -: 1 r.,- \ .^'. *
Methane Emissions f
-^ -
!fe£=7=
-------
mated emissions from waste combustion by applying
activity emission factors (from MSW incineration or
open burning) to collected or estimated local and
regional activities to obtain local and regional emis-
sions, which were then aggregated to obtain national
emissions.
At present, net carbon dioxide emissions from
waste incineration are not included in this inventory
because a large fraction of the carbon in combusted
waste (e.g., food waste) is quickly recycled, typically
on an annual basis as crops regrow or trees are
replanted. Combusted wastes can also contain plas-
tics or other fossil-fuel based products that contribute
to net carbon dioxide emissions. At this time, how-
ever, carbon emissions from the incineration of fossil-
based products are not estimated.
Table VI-4
U,S. NMVDC, CO, &N
frpmV^asre Inaneration:
Source
1990
1994
I"NMVOCS''
Ico
f NOV
J Source: US. EPA (I995b)
0.290
1.530
1991 1992 1993
(Million Metric Tonnes)
0.292 0,296 0.2.98 0.300 ^
1.543" ............................. i"s58 ......................... L57J I.5841
....... (X075 ............................ a075 .............................. O076 ............................. 0.077"
Table VI-5
UlS. NMVOG, CO, & NO
Source
x _
. . .
J^Ju^nicipal Waste Incineration
&Ppen Burning
SKTSlte
Source: US. EPA (I995b)
NMVOCs CO NO
j(Miinon Metric Tonnes)
0.054 0.797 0.029
0.247 0.787 0.047
llfc'JOO,.... ,;..=.= 1.5 SC'
84 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
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R-12 B Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
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ANNEX A
METHOD OF ESTIMATING EMISSIONS OF CO2
FROM FOSSIL ENERGY CONSUMPTION
The "bottom-up" methodology is characterized by six basic steps, which are described
below. This discussion focuses on emission estimates for the year 1994, with the relevant data
presented in Tables A-l through A-6. Emissions estimates for other years were performed using
the same methodology. Relevant data sources and notations are outlined at the end of this
discussion.
METHODOLOGY
Step 1. Determine energy consumption by energy type and sector.
The bottom-up methodology used by the U.S. for estimating CO2 emissions from fossil
energy consumption is conceptually similar to the approach recommended by the IPCC for
countries that intend to develop detailed, sectoral-based emission estimates (IPCC/OECD/IEA,
1995). Basic consumption data are presented in Columns 2-8 of Table A-l, with totals by
energy type in Column 8 and totals by sector in the last row. Fuel consumption data for the
bottom-up approach were obtained directly from the Energy Information Administration (EIA) of
the U.S. Department of Energy, which is responsible for the collection of all U.S. energy data.
All the EIA data were collected through surveys at the point of delivery or use, so they reflect the
reported consumption of fuel by sector and fuel type. Individual data elements came from a
variety of sources within EIA (see below for exact sources). Most information is from published
reports, although some data have been clrawn from unpublished energy studies and databases
maintained by EIA.
By aggregating consumption data by sector (i.e., residential, commercial, industrial,
transportation, electric utilities, and U.S. territories), primary fuel type (e.g., bituminous coal,
natural gas, and petroleum), and secondary fuel type (e.g., gasoline, distillate fuel, etc.), one can
estimate total U.S. energy consumption for a particular year. The 1994 total energy consumption
across all sectors, including territories, and energy types is 76,216 trillion Btu, as indicated in the
last entry of Column 8 in Table A-l. This total includes fuel used for non-fuel purposes and fuel
consumed as international bunkers, both of which are deducted in later steps.
There are three modifications made to consumption in this report that may cause
consumption information herein to differ from figures given in the cited literature. These are the
consideration of unmetered natural gas consumption, synthetic natural gas production, and
ethanol added to motor gasoline. Unmetered natural gas is part of the "balancing item" found in
most EIA gas statistics. This item represents unaccounted for differences between calculated
Annex A • A-1
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supply and consumption, including processes leaks, accounting and reporting problems, and
other data errors. It is assumed that a fraction of this "balancing item" is actually combusted,
despite not appearing in consumption figures: In this report, this additional unmetered portion is
added to each sector's natural gas consumption in proportion to its total gas consumption,
making the numbers reported herein slightly larger than in most EIA sources.
A portion of industrial coal accounted for in EIA combustion figures is actually used to
make "synthetic natural gas" via coal gasification. The energy in this gas enters the natural gas
stream, and is accounted for in natural gas consumption statistics. Since this energy is already
accounted for as natural gas, it is deducted from industrial coal consumption to avoid double
counting. This makes the figure for other industrial coal consumption in this report slightly
lower than most EIA sources.
Ethanol has been add to the motor gasoline stream for several years, but prior to 1993 this
addition was not captured in EIA motor gasoline statistics. Starting in 1993 this ethanol was
included in the gasoline statistics. However, since ethanol is a biofuel which is assumed not to
result in net carbon dioxide emissions to the atmosphere, the amount of ethanol added is
subtracted from total gasoline consumption so as not to include the associated carbon dioxide
emissions. Thus, motor gasoline consumption statistics given in this report may be slightly lower
than in EIA sources.
There are three basic differences between the consumption figures presented in Table A-l
and those recommended in the IPCC emission inventory methodology. First, all consumption
data in the U.S. inventory are presented using higher heating values (HHV) rather than the lower
heating values (LHV) reflected in the IPCC emission inventory methodology. This convention is
followed because all data obtained from EIA are based on HHV.
Second, while EIA's energy use data for the U.S. includes only the 50 U.S. states and the
District of Columbia, the data reported for the U.S. by international agencies, such as the
International Energy Agency (IEA), includes consumption in the U.S. territories. To make the
inventory estimates comparable with these statistics, consumption estimates for U.S. territories
were added to domestic consumption of fossil fuel. Energy consumption data from U.S.
territories are presented in Column 7 of Table A-l. It is reported separately from domestic
sectoral consumption, because it is collected separately by EIA with no sectoral disaggregation.
Third, the domestic sectoral consumption figures in Table A-l include bunker fuels and
non-fuel uses of energy. The IPCC recommends that countries estimate emissions from bunker
fuels separately and exclude these emissions from national totals, so bunker fuel emissions have
been estimated in Table A-2 and deducted from national estimates (see Step 4). Similarly, fossil
fuels used to produce non-energy products that store carbon rather than release it to the
atmosphere are calculated in Table A-3 and deducted from national emission estimates (see Step
3). The carbon content values of bunker fuels and carbon stored in products are reported as
A-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
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"Adjustments" in Columns 17 through 19. The calculation if these adjustments is further
described in Steps 3 and 4 below.
Step 2. Determine the carbon content of all fuels.
Total carbon contained in the energy consumed was estimated by multiplying energy
consumption (Columns 2 through 8 of Table A-l) by fuel specific carbon content coefficients
(Table A-6a, A-6b, and Column 9 of Table A-l) that reflected the amount of carbon per unit of
energy for each fuel. The resulting carbon contents (Columns 10 through 16) are sometimes
referred to as potential emissions, or the maximum amount of carbon that could potentially be
released to the atmosphere if all carbon in the fuels were converted to CO2. Fuel specific and
sectoral totals are given in Column 16 and the last row of Table A-l, respectively. The carbon
content coefficients used in the U.S. inventory were derived by EIA from detailed fuel
information and are similar to the carbon content coefficients contained in the IPCC's default
methodology (IPCC/OECD/IEA, 1995), with modifications reflecting fuel qualities specific to
theU.S.
Step 3. Adjust for the amount of carbon stored in products.
Depending on the end use, non-fuel uses of fossil energy can result in storage of some or
all of the carbon contained in the energy for some period of time. For example, asphalt made
from petroleum can sequester up to 100 percent of the carbon contained in the petroleum
feedstock for extended periods of time. Other non-fuel products, such as lubricants or plastics,
also store carbon, but can lose or emit some of this carbon when they are used and/or burned as
waste after utilization.
The amount of carbon sequestered or stored in non-fuel uses of fossil fuel energy
products was based on data concerning the end uses and ultimate fate of various energy products,
with all non-fuel use attributed to the industrial and transportation sectors. This non-fuel
consumption is presented in Columns 2 and 3 of Table A-3. Non-fuel consumption was then
multiplied by fuel specific carbon content coefficients (Tables A-6a and A-6b, and Column 4 of
Table A-3) to obtain the carbon content of the fuel, or the maximum amount of carbon that could
potentially be sequestered if all the carbon in the fuel were stored in non-fuel products (Columns
5 and 6 of Table A-3). This carbon content was then multiplied by the fraction of carbon actually
sequestered in products (Table A-6a and Column 7 of Table A-3), resulting in the final estimates
of carbon stored by sector and fuel type, which are presented in Columns 8 through 10 of Table
A-3. The portions of carbon sequestered were based on IPCC data (IPCC/OECD/IEA, 1995) and
U.S. specific estimates based on information provided by EIA (Rypinski, 1994).
Annex A • A-3
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Step 4. Subtract carbon from bunker fuels.
According to the decision reached at INC-9, emissions from international transport
activities, or bunker fuel consumption, should not be included in national totals. There is
international disagreement as to which countries are responsible for these emissions, and until
this issue is resolved, countries are to report these emissions separately. However, EIA data
includes bunker fuels (primarily residual oil) as part of consumption by the transportation sector.
To compensate for this, bunker fuel emissions were calculated separately (Table A-2) and the
carbon content of these fuels was subtracted from the transportation sector (Column 17 of Table
A-l). This deduction, together with that of carbon stored in products, resulted in the net carbon
content, or carbon content with adjustments, as presented in Columns 20 through 25 of Table A-
I. The calculations of bunker fuel emissions followed the same procedures used for emissions
due to consumption of all fossil fuels in the U.S. (i.e., estimation of consumption, determination
of carbon content, and adjustment for the fraction of carbon not.oxidized).
Step 5. Account for carbon that does not oxidize during combustion.
Since combustion processes are not 100 percent, efficient, some of the carbon contained in
fuels is not emitted to the atmosphere. Rather, it remains behind as soot, particulate matter, or
other byproducts of inefficient combustion. The estimated fraction of carbon not oxidized in
U.S. energy conversion processes due to inefficiencies during combustion ranges from 0.5
percent for natural gas to one percent for oil and coal. Except for coal these assumptions are
consistent with the default values recommended by the IPCC (IPCC/OECD/EEA, 1995). In the
U.S. unoxidized carbon from coal combustion was estimated to be no more than one percent
(Bechtel, 1993). Column 26 of Table A-l presents fractions oxidized by fuel type, which are
multiplied by the net carbon content of the combusted energy to give final emissions estimates
(Columns 27 - 33 of Table A-l).
Step 6. Summarize emission estimates.
Table A-4 summarizes actual CO2 emissions in the U.S. by major consuming sector (i.e.,
residential, commercial, industrial, transportation, and electric utilities) and major fuel type (i.e.',
coal, natural gas, and petroleum products). Adjustments for bunker fuels and carbon sequestered
in products have been made, as shown in Table A-l. Emissions in Table A-4 are expressed in
terms of million metric tons of carbon equivalent (MMTCE), except in the last column and row,
which shows carbon dioxide emissions on a full molecular weight basis.
Table A-5 summarizes U.S. carbon dioxide emissions by end-use sector. To determine
these estimates, emissions from the electric utility sector were distributed over the four end use
sectors according to their share of electricity consumed. Column 2 presents the fraction of total
A-4 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
U.S. electricity consumed by each of the four end-use sectors. This fraction was then multiplied
by total emissions by fuel type from the utility sector (Columns 2 - 4 of Table A-4), resulting in
the portion of utility emissions attributable to each end-use sector. These end use emissions from
electricity consumption were then added to the non-utility emission estimates taken from Table
A-4 (Columns 2 - 4), resulting in total emissions from each of the four end-use sectors (Column
3-5 of Table A-5).
REFERENCES BY SUBJECT
Some of the major sources used in estimating CO2 emissions from energy consumption
are listed below. They have been grouped by subject for ease of reference and are included
alphabetically with other sources in the reference section of this document. These sources are
often annual or monthly publications. The citations given reflect only the most current issues at
the time of data compilation. Where necessary, data were also taken from previous installments
of the reports.
(
Consumption Data
General
EIA, 1995. State Energy Data Report 1993, unpublished full table presentations, DOE/EIA-
0214(93)-annual, Energy Information Administration, U.S. Department of Energy,
Washington, DC.
EIA, 1995. Petroleum Supply Annual 1994, DOE/EIA-0340(94)l&2-annual, Energy
Information Administration, U.S. Department of Energy, Washington, DC.
EIA, 1995. Petroleum Supply Monthly, DOE/EIA-0109(95)-monthly, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
Non-Fuel Use
EIA, 1994. Annual Energy Review 1993, DOE/EIA-0384(93)-annual, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
EIA, 1995. Monthly Energy Review, DOE/EIA-0035(94)-monthly, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
Annex A • A-5
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Bunker Fuels
EIA, 1995. International Energy Annual, unpublished full table presentations, DOE/EIA-
0219(93)-annual, Energy Information Administration, U.S. Department of Energy,
Washington, DC.
EIA, 1993. Fuel Oil and Kerosene Sales 1993. Report DOE/EIA-0535(94)-annual. Energy
Information Administration, U.S. Department of Energy, Washington, DC.
FAA, 1994. Fuel Cost and Consumption, monthly reports, DAI-10, Federal Aviation
Administration, U.S. Department of Transportation, Washington, DC.
ORNL, 1994. Transportation Energy Data Book, ORNL-6798, Edition 14, Oak Ridge
National Laboratory, Oak Ridge, Tennessee. Prepared for Office of Transportation
Technologies, U.S. Department of Energy, Washington, DC.
Unmetered and Synthetic Natural Gas
EIA, 1994. Natural Gas Annual: 1993, DOE/EIA-O131(93)-annual, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
EIA, 1995. Natural Gas Monthly, DOE/EIA-0130(95)-monthly, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
Additional Information
Rypinski, 1994. Memorandum from Arthur Rypinski of the Energy Information
Administration to Bill Hohenstein of U.S. EPA regarding "Unpublished Data for Inventory,"
July 27,1994.
Carbon Content Coefficients and Thermal Conversion Factors
IPCC/OECD/IEA, 1995. IPCC Guidelines for National Greenhouse Gas Inventories, 3
volumes: Vol. 1, Reporting Instructions; Vol. 2, Workbook; Vol. 3, Reference Manual.
United Nations Environment Programme, Intergovernmental Panel on Climate Change,
Organization for Economic Co-Operation and Development, International Energy Agency.
Paris, France.
EIA, 1995. Draft Emissions of Greenhouse Gases In the United States 1989 -1994,
DOE/EIA-0573-annual, Energy Information Administration, U.S. Department of Energy,
Washington, DC. In Press.
A-6 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
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EIA, 1995. State Energy Data Report 1993, DOE/EIA-0214(93)-annual, Energy Information
Administration, U.S. Department of Energy, Washington, DC.
Percents Sequestered
IPCC/OECD/IEA, 1995. IPCC Guidelines for National Greenhouse Gas Inventories, 3
volumes: Vol. 1, Reporting Instructions; Vol. 2, Workbook; Vol. 3, Reference Manual.
United Nations Environment Programme, Intergovernmental Panel on Climate Change,
Organization for Economic Co-Operation and Development, International Energy Agency.
Paris, France.
Rypinski, 1994. Memorandum from Arthur Rypinski of the Energy Information
Administration to Bill Hohenstein of U.S. EPA regarding "Unpublished Data for Inventory,"
July 27,1994.
Marland, G. and A. Pippin, 1990. "United States Emissions of Carbon Dioxide to the Earth's
Atmosphere by Economic Activity," Energy Systems and Policy, 14(4):323.
Fraction Oxidized
IPCC/OECD/IEA, 1995. IPCC Guidelines for National Greenhouse Gas Inventories, 3
volumes: Vol. 1, Reporting Instructions; Vol. 2, Workbook; Vol. 3, Reference Manual.
United Nations Environment Programme, Intergovernmental Panel on Climate Change,
Organization for Economic Co-Operation and Development, International Energy Agency.
Paris, France.
Bechtel, 1993. A Modified EPRI Class II Estimate for Low NOX Burner Technology Retrofit,
Prepared for Radian Corporation by Bechtel Power, Gaithersburg, Maryland. April, 1993.
Annex A • A-7
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I
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Table A-6a: Key Assumptions for Estimating Carbon Dioxide Emissions
Fuel
Residential Coal
Commercial Coal
Industrial Coking Coal
Industrial Other Coal
Coke Imports
Transportation Coal
Utility Coal
U.S. Territory Coal (bit)
Natural Gas
Asphalt & Road Oil
Aviation Gasoline
Distillate Fuel Oil
Jet Fuel
Kerosene
LPG
Lubricants
Motor Gasoline
Residual Fuel
Other Petroleum
AvGas Blend Components
Crude Oil
MoGas Blend Components
Misc. Products
Naphtha (<401 deg. F)
Other Oil (>401 deg. F)
Pentanes Plus
Petrochemical Feedstocks
Petroleum Coke
Still Gas
Special Naphtha
Unfinished Oils
Waxes
Other Wax & Misc.
Carbon Content
Coefficient
(MMTCE/QBtu)
[a]
[a]
[a]
[a]
27.85
NC
[a]
25.14
14.47
20.62
18.87
19.95
[a]
19.72
[a]
20.24
[a]
21.49
18.87
[a]
19.41
20.31
18.14
19.95
18.24
19.37
27.85
17.51
19.86
20.21
19.81
19.81
Fraction
Oxidized
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.995
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
0.99
Fraction Sequestered
-
.
0.75
-
-. .
.
-
-
l.OO1"1
1.00
-
[b]
-
-
0.80
0.50
-
[b]
-
-
-
[b]
[c]
[c]
- . -
0.80
0.00
-
0.00
-
[b]
1.00
Sources: Carbon Coefficients from EIA (1995a). Stored Carbon from Marland and Pippen (1990) and Rypinski (1994).
Combustion efficiency for coal from Bechtel (1993) and for oil and gas from IPCC (IPCC/OECD/IEA, 1995).
Notes: NC = Not Calculated
[a] =These coefficients vary annually due to fluxuations in fuel quality. See Table A-6b for more information.
[b] = Non-fuel use values of distillate fuel, miscellaneous products, residual fuel, and waxes are reported in
aggregate in the "Other Waxes & Misc." category.
[c] = Non-fuel use values of Naphtha (<401 deg. F) and Other Oil (>401 deg. F) are reported in aggregate in the
"Petrochemical Feedstocks" category.
[d] = There are two major non-fuel uses of natural gas: 1. ammonia production in nitrogenous fertilizer
manufacture; and 2. chemical feedstocks. It is assumed that 100 percent of the carbon in natural gas used as a
chemical feedstock is sequestered, while the carbon in that used for ammonia production is oxidized quickly.
A-12 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table A-6b; Variation in Carbon Content Coefficients by Year (MMTCE / QBtu)
Fuel Type
Residential Coal
Commercial Coal
Industrial Coking Coal
Industrial Other Coal
Utility Coal
LPG
Motor Gasoline
Jet Fuel
Crude Oil
1990
25.92
25.92.
25.51
25.58
25.68
17.00
19.41
19.40
20.16
1991
26.00
26.00
25.51
25.60
25.69
16.99
19.41
19.40
20.18
1992
26.13
26.13
25.51
25.62
25.69
17.00
19.42
19.39
20.22
1993
25.97
25.97
25.51
25.61
25.71
16.98
19.43
19.37
20.23
1994
25.97
25.97
25.51
25.61
25.71
17.02
19.43
19.34
20.21
Sources: Carbon Coefficients from EIA (1995a).
Annex A • A-13
-------
-------
ANNEX B
EMISSIONS FROM MOBILE COMBUSTION
Greenhouse gas emissions from mobile sources are reported by transport mode (i.e., road,
rail, air), vehicle type, and fuel type. The emissions estimates for NOX, NMVOCs, and CO (U.S.
criteria pollutants) were taken directly from the U.S. EPA's Draft National Air Pollutant
Emissions Trends, 1900 -1994 (U.S. EPA, 1995b). This EPA report provides emission
estimates for these gases by sector and fuel type using a "top down" estimating procedure
whereby emissions were calculated using basic activity data, such as amount of fuel delivered or
miles traveled, as indicators of emissions.
Estimates for CH4 and N2O emissions from mobile combustion were calculated by
multiplying the appropriate emission factors provided in IPCC/OECD/IEA (1995) by measures
of activity for each source category. National activity data for the source categories were
obtained from a number of U.S. government agency publications. Depending on the category,
these basic activity data included such information as fuel consumption, fuel deliveries, or
vehicle miles traveled (VMT).
Estimates of NMVOCs, NOX, and CO Emissions From Mobile Combustion
Estimates of NMVOCs, NOX, and CO emissions from gasoline and diesel-powered
highway vehicles are reported by U.S. EPA (1995b) and based on annual VMT and distance
based emission factors. The annual VMT data was obtained from the Federal Highway
Administration's (FHWA) Highway Performance Monitoring System database as noted in U.S.
EPA (1995b). The emission factors were calculated using MOBILESa, a model used by U.S.
EPA to estimate exhaust and running loss emissions from highway vehicles. The MOBILESa
model uses information on ambient temperature, vehicle speeds, national vehicle registration
distributions, gasoline volatility, and other variables in order to produce these factors (U.S. EPA,
1995b).
Emissions of NMVOCs, NOX, and CO from aircraft, marine vessels, railroads, and other
non-highway vehicles are also reported by U.S. EPA (1995b). These values were grown from
emissions calculated in the 1985 National Acid Precipitation Assessment Program (NAPAP)
Inventory, based on E-GAS growth factors obtained by Bureau of Labor Statistics codes (U.S.
EPA, 1995b).
Annex B K B-1
-------
Estimates of CH4 and N2O Emissions From Mobile Combustion
Since EPA does not systematically track emissions of CH4 and N2O, estimates of these
gases were determined using a methodology conceptually similar to that outlined by the IPCC in
which activity data for each source category was multiplied by the appropriate emission factors
provided in the IPCC Guidelines for National Greenhouse Gas Inventories (IPCC/OECD/IEA,
1995). The emission factors were derived in part from data used in MOB1LE4, a somewhat
earlier version of EPA's MOBILESa mobile source emissions model, while activity data was
derived from information provided by various government agencies as noted below.
The 1990 activity data for highway vehicles entailed estimates of VMT by vehicle type
and control technology obtained from U.S. EPA's National Vehicle and Fuel Emissions
Laboratory (Brezinski, et al., 1992; Carlson, 1994; Nizich, 1994; U.S. DOT, 1994; and U.S.
EPA, 1995b). For 1991 through 1994, aggregate VMT data were used to adjust the 1990
emissions estimates (Nizich, 1995 and U.S. EPA, 1995b). Activity data for gasoline highway
vehicles are presented in Table B-l, while the breakdown by control technology (assumed
roughly constant for the period 1990 to 1994) is presented in Table B-2. Given the uncertainty
underlying these estimates, an arbitrary uncertainty range of ± 50 percent was assigned to the
resulting emission totals, which are presented in Part I.
Because the travel fraction and control technology data for diesel highway vehicles and
motorcycles were not available from U.S. EPA, emissions estimates for these vehicle types were
conducted in a slightly different manner than gasoline highway vehicles. Rather than
determining a point estimate, they were calculated as a range of values by multiplying the total
VMT by the high (uncontrolled) and low (advanced) emission factors provided for each category
(IPCC/OECD/IEA, 1995). The emission estimates reported in the inventory for diesel vehicles
and motorcycles are the midpoint of these ranges. The data used are included in Table B-3.
Activity data for off-highway vehicles generally took the form of annual fuel
consumption broken down by transportation mode and fuel type. Consumption of distillate
(diesel) and residual fuel oil by marine bunkers, boats, construction equipment, farm equipment,
and locomotives, as well as coal consumption by locomotives, was obtained from EIA (1994e
and 1995e). Aircraft consumption of jet fuel and aviation gasoline was obtained from FAA
(1995 and 1994). Consumption of motor gasoline by boats, construction equipment, farm
equipment, and locomotives was drawn from FHWA (1994). The activity data used for off-
highway vehicles are included in Table B-4.
B-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table B-l. Vehicle Miles Traveled for Gasoline Highway Vehicles
(109 Miles)
Year
1990
1991
1992
1993
1994
Passenger Cars
1492.6
1512.7
1574.6
1602.3
1636.4
Light Duty
Vehicles
462.3
468.9
472.9
493.2
504.1
Heavy Duty
Vehicles
43.3
43.6
43.4
46.0
47.1
Source: VMT data are the same as those used in U.S. EPA (1995b) as reported by Nizich (1995).
Table B-2 . Control Technology Breakdown for Gasoline Highway Vehicles
(percent of total vehicles)
Data Category
3-Way Catalyst
Oxi-3-Way
Catalyst
Oxi-Catalyst
Non-Catalyst
Uncontrolled
Passenger
Cars
45%
32%
18%
2%
4%
Light Duty
Trucks 1
36%
17%
14%
2%
31%
Light Duty
Trucks 2
30%
15%
14%
3%
39%
Heavy Duty
Vehicles
6%
0%
9%
35%
50%
Source: Distribution of control technologies are calculated from U.S. EPA data (Brezinski, 1992).
Table B-3. Vehicle Miles Traveled for Diesel Highway Vehicles and Motorcycles
(106 Miles)
Year
1990
1991
1992
1993
1994
Diesel
20.60
20.90
21.70
22.09
22.56
Light Duty
3.80
3.80
3.90
4.08
4.18
Heavy Duty
112.2
112.90
1115.00
119.61
122.77
Motorcycles
9.57
9.20
9.55
9.89
10.12
Source: VMT data are the same as those used in U.S. EPA (1995b) as reported by Nizich (1995).
Annex B
B-3
-------
Table B-4. Activity Data for Non-Highway Vehicles
Fuel Category
Aircraft "
1990
1991
1992
1993
1994
Marine Bunkers
1990
1991
1992
1993
1994
Boats b
1990
1991
1992
1993
1994
Construction Equip.0
1990
1991
1992
1993
1994
Farm Equip.
1990
1991
1992
1993
1994
Locomotives'1
1990
1991
1992
1993
1994
Fuel Quantity (U.S. gallons unless otherwise noted)
Residual
NA
NA
NA
NA
NA
4,686,071,250
5,089,541,250
5,399,308,500
4,702,411,500
4,702,411,500
1,562,023,750
1,696,513,750
1,799,769,500
1,567,470,500
1,567,470,500
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
25,422
6,845
8,343
4,065
4,065
Diesel
NA
NA
NA
NA
NA
549,251,000
541,910,000
560,042,500
510,936,250
510,936,250
1,647,753,000
1,625,730,000
1,680,127,500
1,532,808,750
1,532,808,750
2,508,300,000
2,447,400,000
2,287,642,000
2,323,183,000
2,323,183,000
3,164,200,000
3,144,200,000
3,274,811,000
3,077,122,000
3,077,122,000
3,210,111,000
3,026,292,000
3,217,231,000
2,906,998,000
2,906,998,000
Jet Fuel
12,986,111,661
11,995,880,426
12,279,912,686
12,326,549,428
12,838,425,825
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
., NA
NA
NA
NA
NA
NA
NA •
Other
353,100,000
353,600,000 .
314,000,000
268,400,000
268,400,000
NA
NA
NA
NA
NA
1,300,400,000
1,709,700,000
1,316,170,000
873,687,000
873,687,000
1,523,600,000
1,384,900,000
1,492,200,000
1,464,599,000
1,464,599,000
812,800,000
776,200,000
805,500,000
845,320,000
845,320,000
28,000
17,000
42,000
18,000
42,000
"NA" denotes not applicable.
Sources: FWHA, 1994; EIA, 1994e; EIA, 1995e; FAA, 1994, and FAA, 1995.
Notes: [a] Other Fuel = Aviation Gasoline.
[b] Other Fuel = Motor Gasoline
[c] Construction Equipment includes snowmobiles. Other Fuel = Motor Gasoline
[d] Other Fuel = Coal (in short tons)
B-4 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
ANNEXC
EMISSIONS OF HFCs, PFCs AND SF6
This annex describes the assumptions and methodologies behind the United States
emissions calculations of hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur
hexafluoride (SF6) from 1990 to 1994. The spreadsheets used to generate the emissions figures
follow the descriptions.
HFCs:
HFC-23 emissions were assumed to equal three percent of HCFC-22 production. U.S.
HCFC-22 production (Kozel, 1995) was as follows:
• 1990 138,823 metric tonnes
1991 142,641
1992 149,526
1993 132,165
• 1994 139,44-4 (preliminary)
Emissions of HFC-125, HFC-134a, HFC-152a, and HFC-227 were taken from the latest
available information in the U.S. EPA's Vintaging Framework computer model. Values
for HFC-125 and HFC-227 prior to 1994 were not available because these chemicals
were not assumed to enter the market as substitutes until 1994.
PFCs:
CF4 and C2F6 emissions are primarily by-products of aluminum production. The
respective emission factor ranges were estimated to be 0.01 to 1.2 kg CF4 per metric
tonne of aluminum produced and 0.001 to 0.12 kg C2F6 per metric tonne of aluminum
produced (Jacobs, 1994). For this analysis, estimates were provided for the low, high and
average emission factors for each chemical. U.S. aluminum production (Bureau of
Mines, 1995e) was as follows:
• 1990 4.048 million metric tonnes
1991 4.121
1992 4.042
1993 3.695
1994 3.299
Annex C WL C-1
-------
U.S. production of SF6 was estimated to be 6,000,000 Ibs annually for the period 1990 to
1994 (Wagner, 1994). U.S. production was assumed to equal U.S. consumption, i.e., no
imports or exports. Eighty percent of SF6 consumption was assumed to be used in heavy
electrical equipment, while the remaining 20 percent was assumed to be used in metal
industries, e.g., aluminum degassing, magnesium casting (Wagner, 1994). Emissions
from electrical equipment were set at 1 percent of existing stock annually (Norwegian
Institute for Air Research (NIAR), 1993). While leakage rates may be higher for older
equipment, the 1 percent rate has been assumed for all equipment at this time. EPA is
currently conducting additional research on this matter; estimates will be updated as new
information becomes available. All SF6 used in the metal industries was assumed to be
emitted in the year of production. This assumption, too, may change as further research is
conducted. Use of SF6 in electrical equipment was assumed to begin in 1973 (NIAR,
1993). The GWP of 24,900 corresponds to a 100 year time horizon.
C-2 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
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ANNEXD
ESTIMATION OF METHANE EMISSIONS FROM ENTERIC
FERMENTATION IN CATTLE AND FROM ANIMAL MANURE
MANAGEMENT
This annex presents a detailed explanation of the methodologies and data used to estimate
methane emissions from enteric fermentation in cattle and from animal manure management.
This information is provided in order to enable the reader to verify the emission estimates
presented in Part 4 of the inventory. Emissions estimates are based on the analysis presented in
U.S. EPA (1993a). Information from U.S. EPA (1993a and 1994b) is included in this annex to
serve as a reference point for the updated emissions estimates presented in this report.
Methane Emissions From Enteric Fermentation in Cattle
To estimate methane emissions from enteric fermentation in cattle, detailed analyses of
rumen digestion and animal production were performed using a mechanistic model of cattle
digestion. This model, originally described in Baldwin et al. (1987), explicitly models the
fermentation of feed within the rumen, and estimates the amount of methane formed and emitted
as a result. Since the original model of Baldwin et al. was developed for application to lactating
cows, it was revised to enable evaluations of a wider range of animal types, sizes, and stages of
maturity, as well as a wider range of diets.
To apply the model, representative cattle types and diets for five geographic regions of
the U.S. were defined. The cattle type categories represent the different sizes, ages, feeding
systems, and management systems that are typically found in the U.S. Representative diets were
defined for each category of cattle, reflecting the diversity of diets that are found in each of the
five regions (Figure D-l). Each cattle type within each region was evaluated using the model,
resulting in emission factors (kilograms CH4/head/year) for each type in each region.
The following animal types were defined for the cattle population:
Dairy Animal Types
Replacement heifers 0-12 months of age1
Replacement heifers 12-24 months of age
Mature dairy cows (over 24 months of age)
1 "Replacements" are the offspring that are retained to replace mature cows that die or are removed from the herd (culled)
each year.
" ~ ~ Annex D • D-1
-------
Figure D-l: Geographic Regions Used in the Analysis
West*
South
Central
' Includes Alaska and Hawaii
Source: U.S. EPA (1993a)
Beef Animal Types
Replacement heifers 0-12 months of age
Replacement heifers 12-24 months of age
Mature beef cows (over 24 months of age)
Weanling system heifers and steers2
Yearling system heifers and steers3
Mature bulls
In "weanling systems", calves are moved directly from weaning to confined feeding programs. This system represents a
very fast movement of cattle through to marketing for slaughter. Weanling system cattle are marketed at about 420 days of age
(14 months).
"Yearling systems" represent a relatively slow movement of cattle through to marketing for slaughter. These systems
include a wintering over, followed by a summer of grazing on pasture. Yearling system cattle are marketed at 565 davs of age
(18.8 months).
D-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Due to their small number, mature dairy bulls were not evaluated. Dairy calves that are
not kept as replacements are generally fed for slaughter. Therefore, these animals were included
in the total for weanling and yearling system heifers and steers (i.e., heifers and steers grown for
slaughter). Tables D-l and D-2 summarize the size, age, and production characteristics used to
simulate each of the representative animal types..
A total of 32 different diets were defined to represent the diverse feeds and forages
consumed by cattle in the U.S. Fourteen diets were defined for dairy cattle: six for dairy cows
and four each for replacement heifers 0-12 months and 12-24 months. Eighteen diets were
defined for beef cattle: three each for beef cows, replacements 0-12 months, weanling system
heifers and steers, and yearling system heifers and steers; four for replacements 12-24 months;
and two for beef bulls.
Table D-l. Representative Animal Characteristics: Heifers and Cattle Fed for Slaughter
Animal Type
Replacement Heifers
Dairy Replacement Heifers:
0-12 months
Dairy Replacement Heifers:
12-24 months
Beef Replacement Heifers:
0-12 months
Beef Replacement Heifers:
12-24 months
Initial
Weight
(kg)'
170
285
165
270
Final
Weight
(kg)
285
460
270
390
Initial Age
(days)
165
365
165
365
Final Age
(days)
365
730
365
730
Other
—
Pregnant
—
Pregnant
Feedlot Fed Cattle for Slaughter
Yearling Systemb
Weanling System'
170
170
480
480
165
165
565
422
fed to 26-27%
carcass fat
fed to 29-30%
carcass fat
1 All weights reported as empty body weight.
b Includes 260 day stocker period principally on forages and a 140 day feedlot period with a high grain ration.
c Includes a 257 day feeding period, initially at 30 to 50 percent concentrate (125 days), followed by 132 days of a high
grain ration.
Source: U.S. EPA (1993a)
Annex D M D-3
-------
Table D-2. Representative Animal Characteristics: Dairy Cows and Beef Cows
Animal Type
Initial and Final
Weight
(kg)a
Lactation/Dry
Periods
(days)
Milk Production/
Lactation
(kg)
Other
Dairy Cows 550 305/60 5,570-7,190" Pregnant
BeefCows 450 205/160 1,400 Pregnant
Beef Bulls 650 NA NA NA
* All weights reported as empty body weight.
b Milk production per lactation varies by region.
Source: U.S. EPA (1993a)
Table D-3. Dairy Cow Diet Descriptions
Description
ME (Meal/kg)
Lactating Cow Diets
Dietl
50% alfalfa hay,
50% corn-SBM"
concen-trate
!2.61
Diet 2
60% alfalfa hay,
40% corn-
cotton-seed
meal concen-
trate (15% CP)b
2.56
Diet3
69% corn silage,
16% corn meal,
14% SBM
2.65
Diet 4
50% alfalfa hay,
50% barley-
SBM
concentrate
2.57
Diet 5
40% timothy
hay, 45% corn
meal, 15%
SBM- cane mo-
lasses concen-
trate
2.69
Diet 6
Early timothy
hay supple-
mented to
14.5% CP
2.41
Regional Distribution of Diets0
North Atlantic 33% 33% 33%
South Atlantic 40% 30% 30%
North Central 50% 50%
South Central 33% 33% 33%
West 75% 25%
1 SBM = soybean meal
b CP = crude protein
c Regional distributions show the extent to which each diet is simulated to be used in each region. The percentages for
each region sum to 100 percent.
Source: U.S. EPA (1993a)
To derive emission factors for each of the cattle types in each region, the extent to which
each diet is used in each region was specified for each cattle type. For example, in the North
Atlantic region, it was estimated that one third of the mature dairy cows are fed Dairy Cow Diet
1, one third Dairy Cow Diet 3, and one third Dairy Cow Diet 5 (Table D-3). The specification of
the regional diet mixes was based on comments from cattle experts in different regions
throughout the U.S. and on data on regional feed availability.
D-4 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Estimates of methane emissions from enteric fermentation presented in this report and in
the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1993 (U.S. EPA 1994b) are
derived using the methodology developed in EPA 1993a, however emissions estimates discussed
in this report are based on revised dairy cow emission factors and animal population data. To
provide a reference point for the updated emissions estimates, a discussion of previous emission
estimates is provided in Box D-l.
To update emission estimates for 1990 through 1993, and to calculate 1994 emissions, the
"national" emissions factors developed in the EPA report were multiplied by the applicable
"national" animal populations and the resulting emissions by animal type were summed across
animal types to estimate total annual methane emissions. The above method was adopted for all
animal types except dairy cows. For dairy cows, regional emissions factors were developed to
reflect increasing milk production per cow. The regional emissions factors were applied to the
regional animal populations to estimate total dairy cow emissions. Regional emission factors and
updated estimates of emissions from dairy cows for 1990 through 1994 are presented in Table D-
4.
Unlike the EPA report, the update does not account for regional shifts in animal
populations (except dairy cows). Dairy cow populations for the 5 regions defined in the EPA
report were used to determine total emissions from dairy cows. The data reflects a shift in dairy
cows away from North Central and toward the West. Since the publication of U.S. EPA 1994b,
USDA has revised all animal population data for 1990 to 1994. The emissions estimates for
1990 to 1994 presented in Part 4 of this report reflect these revisions.
iJRfefcsrencePoint J*>;rUpdatedEnflsslonEstimates'" -,\vfl ~ ^, -^
/'"* While-estinStes of emissions from entericH:e^
revised^aary-cdw emission facfoi^and,moreDecent animal population data, emissions estimates and r ,;„
^relevant statistics;presented in TT s .F.PA* 1004K serveUs a .valuable relerence-oomfefer the,analvsis,of='-'
updated emissions, estimates.
.JEilTjrV- \J-J7«%Jta,f* JUJLlw ilw'giV/JUtt*~A/JLlJUU3t»iV«£ *«A*^w«kt> *.V/JL JUI.M.VMJL«.^ v*vfc*i_j W*T»^ ^m^ji.-** TWW»»J..**WI.WW*». —^ „—j^jp.—^ Q,
catde.digestion model'to^ average;djfet arid th%=%yerage»annua! milfprodjictioir petheSd in e^H!
region (Table D%). For the othejcdmj^ c|ftle t^pe|"and allio0th*6 beef cattle types, 'enflsiibn factors
wgre simulated for/each of the define! £liet>typ%s, and then»usingsjhe diet percehtag^issignel for each
-region, weighted*average emission factors were cMculated'fqr.each animal Hype in eacfi rpgtbn.^Th^ . ^
'sitatisScs usedln thes^stauialbnsfa^i the^resulfant regiSnal ^ussioniactdfs ate suinmarizedln -'
, .
estimate na&n^'Snissions.for'each cattle^gpe,- the.r,egfEqgi emi§sion|actpts-were <
ui&phed by regio4ail)opiilations"6f e^ach'tyj^e ^^^
the average 1990 regional p'optdatfons wereifefeein "from published^tatistics (Schoeff ,andj£astaldo
: nils1on factofor'liie feedlot edTcattle
., /t, -
ri^ 365 ""J,,
'' ""
systeC"caWe) "are based on:the_entire m
daysforJhS&lsystems. TheSf^'&e yearling system and ^^mg'sysfejtri-cattlepopulaiions\weEe
derivea^om^l^O'slaalhter'statpticV'^SDA, |992b; CF Resources, 1591). IsTational emissions from, ,
tie,entire ca^^opulation^^ estimated bf ^mining the emission estimates:for;all '
Annex D HI D-5
-------
Table D-4. Dairy Cow Data and Estimates3: By Region and US Total
Region: U.S. Total
Milk Prod (10A61bs):
Population (OOO's):
ProdnVCow (Ibs/yr):
Prodn./Cow (kg/day):
Emissions Factor:
Total Einissions(Tg):
Region: North Atlantic
Milk Prod (10*6 Ibs):
Population( OOO's):
Prodn./Cow (Ibs/yr):
Prodn./Cow (kg/day):
Emissions Factor:
Total Emissions( Tg):
Region: South Atlantic
Milk Prod (10A61bs):
Population (OOO's):
Prodn./Cow (Ibs/yr):
Prodn./Cow (kg/day):
Emissions Factor:
Total Emissions (Tg):
Region: North Central
Milk Prod (10A61bs):
Population (OOO's):
Prodn./Cow (Ibs/yr):
Prodn7Cow (kg/day):
Emissions Factor:
Total Emissions (Tg):
Region: South Central
Milk Prod (10A61bs):
Population (OOO's):
Prodn./Cow (Ibs/yr):
Prodn,/Cow (kg/day):
Emissions Factor:
Total Emissions (Tg):
Region: West
Milk Prod (10A61bs):
Population (OOO's):
ProdnVCow (Ibs/yr):
ProdnJCow (kg/day):
Emissions Factor:
Total Emissions (Tg):
1990
147,722
10,007
14,761
18.34
114.8
1.15
1990
5,727
1,775
14,493
18.01
116.2
0.206
1990
9,705
708
13,698
7.02
127.7
0.090
1990
61,605
4,412
13,964
17.35
104.8
0.462
1990
14,081
1,121
12,559
15.61
116.2
0.130
1990
36,604
1,991
18,385
22.85
130.5
0.260
1991
147,695
9,883
14,945
18.57
115.7
1.14
1991
26,060
1,729
15,072
18.73
118.8
0.205
1991
9,752
702
13,889
17.26
128.7
0.090
1991
60,570
,284
14,138
17.57
105.6
0.452
1991
13,800
1,101
12,537
15.58
116.1
0.128
1991
37,513
2,067
18,150
22.56
129.4
0.267
1992
150,884
9,714
5,532
19.30
118.3
1.15
1992
26,819
1,716
15,630
19.42
121.3
0.208
1992
9,957
682
14,599
18.14
132.3
0.090
1992
60,722
4,148
14,639
18.19
107.7
0.447
1992
13,945
1,081
12,895
16.02
117.8
0.127
1992
39,441
2,087
18,900
23.49
132.7
0.277
1993
150,594
9,679
15,559
9.34
118.5
1.15
1993
26,504
1,704
15,553
19.33
121.0
0.206
1993
9,826
674
14,569
18.10
132.2
0.089
1993
59,036
4,041
14,610
18.16
107.6
0.435
1993
14,207
1,078
13,175
16.37
119.2
0.129
1993
41,021
2,181
18,805
23.37
132.3
0.289
" Dairy milk production data for 1990-1994 from: USDA Economic Research Service,
February 27, 1995; Dairy cow population data from NASS data-sets.
1994
153,622
9,614
15,979
19.86
120.4
1.16
1994
26,410
1,692
15,605
19.39
121.2
0.205
1994
9,758
659
14,810
18.40
133.4
0.088
1994
57,980
3,913
14,817
18.41
108.4
0.424
1994
14,370
1,082
13,285
16.51
119.8
0.130
1994
45,104
2,268
19,887
24.71
137.1
0.311
Dairy Outlook.
D-6 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-5. Regional Estimates of Methane Emissions from Mature Dairy Cows Statistics
for the Average Animal Modeled
Feed consumed
per year (kg DM)
ME" consumed per
year (Meal)
Diet ME (Meal/kg)
Average feed
digestibility (%)b
Methane emissions
per year (kg/cow)
Milk Production
per cow per year
(kg)
Methane emissions
per kg of milk
produced (g/kg)
N. Atlantic
5735
15,224
2.65
68
116.2
6574
17.8
S. Atlantic
5460
13,421
2.46
66
127.7
6213
20.56
N. Central
5805
15',012
2.59
66
104.8
6334
16.55
S. Central
5182
12,975
2.50
64
116.2
5696
20.40
West
6032
15,190
2.52
66
130.5
8339
15.65
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a
reference point for updated emissions estimates in this report (See Box D-l). Regional diets are weighted
averages of the diets shown in Table D-3.
a ME = metabolizable energy
b Digestibility is reported as simulated digestible energy divided by gross energy intake.
Source: U.S. EPA (1993a)
Annex D • D-7
-------
Table D-6. Regional Estimates of Emissions from Dairy Replacement Heifers: 0-12 Months
Statistics for the Average Animal Modeled
Diet Description
Feed consumed per
year (kg DM)
MEC consumed
(Meal)
DietME(McaJ/kg)
Average feed
digestibility (%)d
Methane emissions
(kg/head/yr)
Dietl
Alfalfa hay
1116
2623
2.35
62
21.4
Diet 2 Diet 3
75% alfalfa High quality
hay, 25% grass forage
concen.a (CP=18%)b
1080 967
2684 2613
2.48 2.70
65 67
20.0 20.1
Regional Distribution of Diets (%)e
North Atlantic
South Atlantic
North Central
South Central
West
25%
33%
25%
15%
50%
60%
67%
50%
85%
25% 25%
Diet 4
Corn silage
with protein to
14% CP
904
2432
2.69
69
14
Emissions
(ke/head/vr)
15% 19.5
20.5
25% 18.9
20.3
20.7
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a reference
point for updated emissions estimates in this report (See Box D-l).
" Concentrate of com meal and soybean meal
b CP = crude protein
c ME = metabolizable energy
d Digestibility is reported as simulated digestible energy divided by gross energy intake.
c Regional distribution of diets shows the extent to which each of the four diets is used in each region. The
emissions estimates are the weighted average emissions using these percentages.
Source: U.S. EPA (1993a)
D-8 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-7. Regional Estimates of Emissions from Dairy Replacement Heifers:
12-24 Months Statistics for the Average Animal Modeled
Dietl Diet 2 Diet 3 Diet 4
75% alfalfa Grass forage Corn silage
Diet Description Alfalfa hay hay, 25% of declining with protein to
concen." quality6 14% CPC
Feed consumed per
year (kg DM)
MEC consumed (Meal)
Diet ME (Meal/kg)
Average feed
digestibility (%)"
Methane emissions
(kg/head/yr)
Regional Distribution of Diets
North Atlantic
South Atlantic
North Central
South Central
West
3184 3018 3172 2540
7419 7437 7183 6801
2.33 2.46 . 2.25 2.68
62 64 58 67
63.0 57.3 61.4 47.9
.„ ., Emissions
(/0) Offl/head/vr)
25% 50% 25% 58.4
25% 10% 45% 20% 58.7
33% 33% 33% 57.4
20% 80% 61.7
50% 25% 25% 61.2
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a reference
point for updated emissions estimates in this report (See Box D-l).
a Concentrate of corn and cottonseed meal
b High quality grass forage for 100 days (ME=2.8 Meal/kg). Intermediate quality grass forage for 100 days
(ME=2.5 Meal/kg). Lower quality grass forage for 165 days (ME=2.1 Meal/kg).
0 CP = crude protein
d ME = metabolizable energy
c Digestibility is reported as simulated digestible energy divided by gross energy intake.
f Regional distribution of diets shows the extent to which each of the four diets is used in each region. The
emissions estimates are the weighted average emissions using these percentages.
Source: U.S. EPA (1993a)
Annex D • D-9
-------
Table D-8. Regional Estimates of Methane Emissions from Beef Cows
Statistics for the Average Animal Modeled
Diet Description
Feed consumed per
year (kg DM)
MEd consumed
(Meal)
Diet ME (Meal/kg)
Average feed
digestibility (%)'
Methane emissions
(kg/head/yr)
Dietl
Pasture for
7 mos; mixed
hay for 5 mos°
3029
7370
2.43
63
63.4
Diet 2
Pasture of
varying
qualityb
3172
7731
2.44
63
71.7
Diet 3
Pasture with 4
mos of
supplement0
2700
7047
2.61
65
53.7
Regional Distribution of Diets (%)'
North Atlantic
South Atlantic
North Central
South Central
West
80%
20%
60%
10%
10%
80%
90%
80%
20%
40%
10%
Emissions
(ke/head/vr)
60.5
70.0
59.5
70.9
69.1
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and
serve as a reference point for updated emissions estimates in this report (See Box D-l).
" Seven months of pasture declining in quality as the seasons progress. Five months of mixed
hay, grass with some legumes.
b Pasture quality varies with the seasons.
c Pasture with four months of supplementation using a mixed forage (80 percent) and
concentrate (20 percent) supplement.
d ME = metabolizable energy
e Digestibility is reported as simulated digestible energy divided by gross energy intake.
' Regional distribution of diets shows the extent to which each of the three diets is used in each
region. The emissions estimates are the weighted average emissions using these percentages.
Source: U.S. EPA (1993a)
D-10 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-9. Regional Estimates of Emissions from Beef Replacements: 0-12 Months
Statistics for the Average Animal Modeled
Dietl
Legume pasture
Diet Description with
supplement3
Feed consumed per year
(kg DM)
M£c consumed (Meal)
Diet ME (Meal/kg)
Average feed digestibility
Methane emissions
(kg/head/yr)
Regional Distribution of Diets (%)
North Atlantic
South Atlantic
North Central
South Central
West
984
2443
2.48
65
18.1
e
50%
50%
33%
40%
50%
Diet 2 DietS
Very high Corn silage
quality grass supplemented to
(18%CP)b 14% CP
1011 922
2614 2454
2.58 2.66
68 68
27.2 15.8
Emissions
(kg/head/vr)
20% 30% 19.2
50% 22.7
33% 33% 20.4
60% 23.6
50% 22.7
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a
reference point for updated emissions estimates in this report (See Box D-l).
a Concentrate = 25 percent of ration
b CP = Crude protein
0 ME = metabolizable energy
d Digestibility is reported as simulated digestible energy divided by gross energy intake.
° Regional distribution of diets shows the extent to which each of the three diets is used in each region.
The emissions estimates are the weighted average emissions using these percentages.
Source: U.S. EPA (1993a)
Annex D BD-11
-------
Table D-10. Regional Estimates of Emissions from Beef Replacement Heifers: 12-24 Months
Statistics, for the Average Animal Modeled
Diet Description
Feed consumed per
year (kg DM)
MEC consumed (Meal)
DietME(McaI/kg)
Average feed
digestibility (%)'
Methane emissions
(kg/head/yr)
Dietl
Varying
quality grass
forage"
2454
6356
2.59
67
66.9
Diet 2
Varying
quality grass
forageb
2675
6524
2.49
66
71.0
Diet 3
Varying
quality grass
with winter
. supplement0
2359
5990
2.54
66
56.5
Regional Distribution of Diets (%)g
North Atlantic
South Atlantic
North Central
South Central
West
50%
80%
33%
50%
40%
33%
20%
33%
50%
10%
33%
33%
Diet 4
Varying
quality grass
with winter
supplementd
2305
6000
2.60
67
54.8
Emissions
(ke/head/vr)
63.8
67.5
33% 60.8
67.7
64.8
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a reference
point for updated emissions estimates in this report (See Box D-l).
1 165 days of high quality grass followed by 200 days of intermediate quality grass.
b 120 days of high quality grass followed by 125 days of intermediate quality grass — grass hay provided for 120
days during winter
c 120 days of high quality grass followed by 125 days of intermediate quality grass — medium quality alfalfa with a
corn:soybean meal concentrate (25 percent) provided for 120 days during winter
d 120 days of high quality grass followed by 125 days of intermediate quality grass — corn silage supplemented to
14 percent CP provided for 120 days during winter
c ME = metabolizable energy
' Digestibility is reported as simulated digestible energy divided by gross energy intake.
* Regional distribution of diets shows the extent to which each of the three diets is used in each region. The
emissions estimates are the weighted average emissions using these percentages.
Source: U.S. EPA (1993a)
D-12 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-ll. Regional Estimates of Emissions from Feedlot Fed Cattle: Yearling System
Statistics for the Average Animal Modeled
Dietl
Diet 2
Diet 3
All diets include forages during the stocker phase
Diet Description followed by high grain diets during feedlot feeding3
Feed consumed per year
(kg DM)
MEb consumed (Meal)
Diet ME (Meal/kg)
Average feed digestibility (%)c
Methane emissions (kg/head/yr)
Adjustment for ionophores and
hormone implants
Methane emissions (kg/head/yr)
Regional Distribution of Diets (%)"
North Central
South Central
West
2865
7588
2.65
67
50.0
90%
45.0
30%
20%
2775
7383
2.66
67
54.1
90%
48.7
20%
50%
2755
7366
2.67
68
52.9
90%
47.6
Emissions
(kg/head/vr)
50% 47.0
100% 47.6
30% 47.6
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a
reference point for updated emissions estimates in this report (See Box D-l).
a All three diets include a high quality mixed hay (legume and grass) for the first winter (90 days). The
three diets then include:
Diet 1: mixed pasture (legume and grass) to 425 days of age; 50 percent alfalfa:50 percent concentrate for
40 days; 10 percent alfalfa:90 percent concentrate for 100 days.
Diet 2: grass pasture to 425 days of age; 50 percent alfalfa:50 percent concentrate for 40 days; 10
percent alfalfa:90 percent concentrate for 100 days.
Diet 3: grass pasture to 425 days of age; 70 percent corn silage:30 percent concentrate for 40 days; 10
percent alfalfa:90 percent concentrate for 100 days.
b ME = metabolizable energy
c Digestibility is reported as simulated digestible energy divided by gross energy intake.
d Regional distribution of diets shows the extent to which each of the four diets is used in each region. The
emissions estimates are the weighted average emissions using these percentages. Only the three regions
with feedlots are shown.
Source: U.S. EPA (1993a)
Annex D M D-13
-------
Table D-12. Regional Estimates of Emissions from Feedlot Fed Cattle: Weanling System
Statistics for the Average Animal Modeled
Diet Description
Feed consumed per year (kg
DM)
MEb consumed (Meal)
Diet ME (Meal/kg)
Average feed digestibility (%)c
Methane emissions (kg/head/yr)
Adjustment for ionophores and
hormone implants
Methane emissions (kg/head/yr)
Regional Distribution of Diets (%)d
North Central
South Central
West
Dietl
Diet 2
Diets
All diets include mixed rations with increasing
amounts of high grain concentrates3
1935
5232
2.70
68
31.2
85%
26.5
20%
50%
40%
1763
5184
2.94
71
25.3
85%
21.5
20%
50%
30%
1742
5059
2.90
71
25.4
85%
21.6
Emissions
(ke/head/vr>
60% 22.6
24.0
30% 23.5
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a
reference point for updated emissions estimates in this report (See Box D-l).
* The following diets were simulated:
Diet 1: 60 percent alfalfa:40 percent concentrate for 125 days; 10 percent alfalfa:90 percent concentrate
for 132 days.
Diet 2: 50 percent alfalfa:50 percent concentrate for 125 days; 10 percent alfalfa:90 percent concentrate
for 132 days.
Diet 3: 69 percent com silage:31 percent concentrate for 125 days; 10 percent alfalfa:90 percent
concentrate for 132 days.
b ME = metabolizable energy
c Digestibility is reported as simulated digestible energy divided by gross energy intake.
d Regional distribution of diets shows the extent to which each of the four diets is used in each region. The
emissions estimates are the weighted average emissions using these percentages. Only the three regions
with feedlots are shown.
Source: U.S. EPA (1993a)
D-14 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-13. Methane Emissions From Enteric Fermentation in U.S. Dairy Cattle
Region/Animal Type
North Atlantic
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
South Atlantic
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
North Central
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
South Central
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
West
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
National Total
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
Total
Emissions Factor
(kg/head/yr)
19.5
58.4
117.5
20.5
58.7
126.5
18.9
57.4
109.4
20.3
61.7
114.8
20.7
61.2
119.3
19.6
58.8
114.6
80.4
Population
(000 Head)
712
712
1,795
268
268
710
1,987
1,987
4,497
405
405
1,156
833
833
1,972
4,205
4,205
10,130
18,540
Emissions
(Tg/yr)
0.014
0.042
0.211
0.005
0.016
0.090
0.038
0.114
0.492
0.008
0.025
0.133
0.017
0.051
0.235
0.082
0.247
1.161
1.490
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a reference point for
updated emissions estimates in this report (See Box D-l).
Source: U.S. EPA (1993a)
Annex D m D-1'5
-------
Table D-14. Methane Emissions From Enteric Fermentation in U.S. Beef Cattle
Region/Animal Type
North Atlantic
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
South Atlantic
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
North Central
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
Weanling System Steers/Heifers'"
Yearling System Steers/Heifers
South Central
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
Weanling System Steers/Heifers
Yearling System Steers/Heifers
West
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
Weanling System Steers/Heifers
Yearling System Steers/Heifers
Bulls: Nationally
National Total
Replacements 0-12 months
Replacements 12-24 months
Mature Cows
Weanling System Steers/Heifers
Yearling System Steers/Heifers
Bulls
Total"
Emissions Factor
(kg/head/yr)
19.2
63.8
61.5
22.7
67.5
70.0
20.4
60.8
59.5
22.6
47.0
23.6
67.7
70.9
24.0
47.6
22.7
64.8
69.1
23.5
47.6
100.0
22.3
65.0
66.7
23.1
47.3
100.0
47.5
Population
(000 Head)3
87
87
337
594
594
3,418
1,546
1,546
10,592
2,963
11,852
2,079
2,079
12,359
1,164
4,656
1,229
1,229
6,772
1,133
4*532
2,200
5,535
5,535
33,478
5,260
21,040
2,200
85,398C
Emissions
(Tg/yr)
0.002
0.006
0.021
0.013
0.040
0.239
0.032
0.094
0.630
0.067
0.557
0.049
0.141
0.876
0.028
0.222
0.028
0.080
0.468
0.027
0.216
0.220
0.124
0.360
2.234
0.122
0.994
0.220
4.054
Note: Statistics and emissions estimates presented in this table are from U.S. EPA 1994b and serve as a reference point for
updated emissions estimates in this report (See Box D-l).
" Population for slaughter steers and heifers in each region is the number slaughtered annually.
b The emissions from Yearling and Weanling System steers and heifers are assigned to the regions in which they are
managed in feedlots.
c The national population is estimated using the average annual population of Yearling and Weanling System cattle:
38.65 million. See text.
d Total may not add due to rounding.
Source: U.S. EPA (1993a)
D-16 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
METHANE EMISSIONS FROM ANIMAL MANURE MANAGEMENT
Estimates of 1990 methane emissions from animal manure management were derived using the
approach of Safley et al. (1992a). This approach is as follows:
(1) Estimate annual methane emissions for each animal type i and manure system/ in each state k:
TMijk = NfcXTAMjX VS; x B^ x MCFjk x WS%ijk
Where TMijk = annual methane emissions for each animal type i and manure
management system j in each state k
Nik = number of animals of type i in state k
TAMj = typical animal mass of animal i
vSj = average annual volatile solids production per unit of animal mass for
animal i
B0i = maximum methane producing capacity of the manure of animal i
MCFjk = the methane conversion factor of the manure system j in the state k
WS%ijk = the percent of animal i's manure managed in manure system/ in state
k
(2) Estimate total annual methane emissions for animal i by summing annual emissions over all applicable
manure management systems./ and states k.
(3) Estimate total annual methane emissions from all animals by summing over all animal types i.
Emissions estimates in this study differ slightly from previous estimates (EPA 1993a, EPA 1994b)
because of the following:
• 1990 animal population data used to calculate point estimates of emissions in EPA 1993a
were approximated based on 1987 population data and growth rates.
« In the current estimate, data was taken from the USDA National Agricultural Statistics
Service (NASS) data sets to calculate 1990 emissions. The population numbers published
by the NASS differ from those approximated in EPA 1993a. This produced different
emissions values than those presented in EPA 1994b, which used the emissions data from
EPA 1993a. The contrasting population numbers are presented in Table D-15.
° The updated population numbers, along with revised MCFs, have also produced new
emissions estimates for the years 1991 to 1993.
Annex D H D-17
-------
Table D-15. 1990 Animal Population Estimates: Projected levels vs. Actual levels
Dairy Cattle
Beef Cattle
Swine
Poultry
Other
1990 Populations
(projected)"
T1000 head)
14,335
89,293
55,299
1,368,166
15,444
1990 Populations (actual)"
(1000 head)
14,143
86,065
53,807
1,703,037
19,116
" Source: U.S. EPA 1993a
"Source: USDA, 1994a,c,d
Table D-16 presents the annual increases in the national census of beef cattle, swine, and poultry,
along with the respective emission levels for each for each year of the study. The total population of animals
in the "other" category decreased over the five year span, without an effect on emissions.
To estimate methane emissions from manure, twenty types of animals were defined for the U.S., and
data were collected on the populations of each animal type in each state, their typical animal mass, and their
average annual volatile solids production per unit of animal mass. The cattle populations and weights are equal
to those used in the previous section of this annex to estimate emissions from enteric fermentation.4
The maximum amount of methane that can be produced per kilogram of volatile solids, or the
maximum methane producing capacity of each animal's manure (B0), varies by animal type and diet.
Appropriate B0 values were chosen from the scientific literature depending on the typical diet of each animal
type. For animal types without B0 measurements, the B0 was estimated based on similarities with other animals
and the experience of the authors of Safley etal. (1992a). Table D-18 lists the values selected for the analysis.
The extent to which the maximum methane producing capacity of each animal's manure is realized,
or the methane conversion factor (MCF), depends upon the management system and climate conditions in
which the manure is managed. Ten categories of manure management systems were identified for the U.S.,
and based upon estimates in the scientific literature and research sponsored by U.S. EPA, MCFs for each
system were identified (Table D-19). The MCF for each management system in each state was calculated by:
• estimating the average monthly temperature in each climate division of each state;5
• estimating the MCF value for each month using the average temperature data and the MCF values
listed in Table D-19;
• estimating the annual MCF by averaging the monthly division estimates; and
4 Tables D-l and D-2 (in the enteric fermentation section) list weights on an empty body weight basis. These values were
converted to live weight for purposes of estimating emissions from animal manure management.
5 The average temperature in each climate division of each state was calculated for the normal period of 1951 to 1980 using
the National Climatic Data Center time-bias corrected Historical Climatological Series Divisional Data (NCDC, 1991).
D-18 H Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
• estimating the state-wide MCF by weighting the average MCF for each division by the fraction of the
state's dairy population represented in each division.6
Table D-20 summarizes the resultant MCF estimates by management system for each state.
Livestock manure management system usage in the U.S. was determined by obtaining information
from Extension Service personnel in each state. The U.S. was divided into eleven geographic regions based
on similarities of climate and livestock production. For states that did not provide information, the regional
average manure system usage was assumed. The results are summarized in Table D-21.
Shifts in Manure management practices in seven states also impact methane emissions. These changes
are presented in Table D-22. These factors are derived from the change in total weighted MCFs for these
states, calculated by multiplying the state MCF factor for a given management system by the degree to which
that system is utilized. From this compilation of state data, national figures were calculated.
Information on shifts in manure management, particularly towards lagoon manure management, was
ascertained based on analyses of the industry trends towards larger confinement facilities, which necessitate
automated management systems. Discussions with industry experts and facility owners supplemented this
information.
6 The dairy populations in each climate division were estimated using the dairy population in each county (Bureau of the
Census, 1987) and detailed county and climate division maps (NCDC, 1991). Using the dairy population as a weighting factor
may slightly over or underestimate the MCFs for other livestock populations.
Annex D BD-19
-------
Table D-16. Methane Emissions from Manure Management: 1990-1994
Dairy
Dairy Cows
Dairy Heifers
Beef
Fecdlot Steers
Feedlot Heifers
Fecdlot Cows/other
NOF Bulls
NOF Calves
NOF Heifers
NOF Steers
NOF Cows
Swine
Breeding pigs
Markets < 60 Ibs
Markets 60-119
Ibs
Markets
120-1791bs
Markets 180+ Ibs
Poultry
Hens > 1 yr
Pullets laying
Pullets > 3 mo.
Pullets < 3 mo.
Chickens
Broilers
Other (Lost)
Other (Sold)
Turkeys
Other
Ewes > 1 yr
Rams/Weth > 1 yr
Ewes < 1 yr
Rams/Weth < 1 yr
Sheep on Feed
Goats
Horses
Total
1990
Population
(1,000
head)
14,143
10,007
4,135
86,065
7,336
3,458
90
2,103
23,621
10,326
7,287
31,844
53,807
6,883
18,372
11,661
9,367
7,523
1,703,037
119,551
153,916
34,222
38,945
6,546
1,172,830
6,971
41,672
128,384
19,116
7,961
369
1,491
381
1,154
2,545
5.215
1990
Emissions
(Tg)
0.75
0.58
0.17
0.20
0.03
0.02
0.00
0.01
0.02
0.02
0.01
0.10
0.95
0.31
0.08
0.15
0.19
0.22
0.26
0.05
0.06
0.01
0.01
0.00
0.10
0.00
0.01
0.03
0.06
0.00
0.00
0.00
0.00
0.00
0.03
0.03
2.21
1991
Population
(1,000 head)
13,980
9,883
4,097
87,267
7,976
3,841
102
2,099
23,665
10,356
7,206
32,022
56,535
7,239
19,320
12,348
9,778
7,850
1,767,513
117,178
162,943
34,272
42,344
6,857
1,227,430
7,278
39,707
129,505
18,864
7,799
361
1,464
373
1,177
2,475
5,215
1991
Emissions
(Tg)
0.75
0.59
0.16
0.20
0.03
0.02
0.00
0.01
0.02
0.02
0.01
0.10
0.99
0.32
0.09
0.16
0.20
0.23
0.27
0.05
0.06
0.01
0.01
0.00
0.10
0.00
0.01
0.03
0.06
0.00
0.00
0.00
0.00
0.00
0.03
0.03
2.28
1992
Population
(1,000 head)
13,830
9,714
4,116
88,548
7,617
3,608
96
2,132
24,067
10,728
7,523
32,776
58,553
7,269
19,948
12,823
10,180
8,334
1,832,308
121,103
163,397
34,710
45,160
7,113
1,280,498
7,025
41,538
131,764
18,657
7,556
350
1,432
366
1,093
2,645
5,215
1992
Emissions
(Tg)
0.79
0.62
0.17
0.21
0.03
0.02
0.00
0.01
0.02
0.02
0.01
0.10
1.04
0.33
0.09
0.16
0.21
0.24
0.27
0.06
0.06
0.01
0.01
0.00
0.11
0.00
0.01
0.03
0.06
0.00
0.00
0.00
0.00
0.00
0.03
0.03
2.37
1993
Population
(1,000 head)
13,767
9,679
4,088
90,321
8,032
3,878
101
2,146
24,369
10,868
7,464
33,464
56,919
7,212
18,426
12,758
10,323
8,201
1,895,851
131,688
158,938
33,833
47,941
7,240
1,338,862
6,992
39,606
130,750
18,021
7,140
331
1,349
348
1,032
2,605
5,215
1993
Emissions
(Tg)
0.80
0.63
0.17
0.21
0.03
0.02
0.00
0.01
0.02
0.02
0.01
0.11
1.03
0.32
0.08
0.16
0.22
0.24
0.28
0.06
0.06
0.01
0.01
0.00
0.11
0.00
0.01
0.03
0.06
0.00
0.00
0.00
0.00
0.00
0.00
0.03
2.39
1994
Population
(1,000 head)
13,686
9,614
4,072
92,623
8,223
3,938
100
2,218
24,145
10,999
7,604
34,396
60,028
7,594
19,525
13,403
10,850
8,656
1,971,404
134,876
163,628
32,808
44,875
7,319
1,403,508
12,744
40,272
131,375
17,552
6,775
314
1,277
332
1,044
2,595
5,215
1994
Emissions
(Tg)
0.84
0.66
0.17
0.22
0.03
0.02
0.00
0.01
0.02
0.02
0.01
0.11
1.14
0.36
0.09
0.18
0.24
0.27
0.29
0.06
0.06
0.01
0.01
0.00
0.12
0.00
0.01
0.03
0.04
0.00
0.00
0.00
0.00
0.00
0.03
0.03
0
D-20 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-17. U.S. Animal Populations, Average Size, and VS Production
Animal Type
Feedlot Beef Cattle
Other Beef Cattle
Dairy Cattle
Swine
Poultry'
Other
Steers/Heifers
Calves
Heifers
Steers
Cows
Bulls
Total
Heifers
Cows
Total
Market
Breeding
Total
Layers
Broilers
Ducks
Turkeys
Sheep
Goats
Donkeys
Horses and Mules
Population"'11
N,
10,088,000
36,040,000
5,535,0000
2,162,000
33,478,000
2,200,000
79,205,000
4,205,000
10,130,000
14,335,000
48,259,000
7,040,000
55,299,000
355,469,000
951,914,000
7,000,000
53,783,000
10,639,000
2,396,000
4,000
2,405,000
Typical
Animal
Mass
(TAMi)c
Kg
415
180
360
360
500
720
410
610
46
181
1.6
0.7
1.4
3.4
70
64
300
450
Manure per dayd
(kg/day per 1000 kg mass)
Total
Manure
58
58
58
58
58
58
86
86
84
84
64
85
107
47
40
41
51
51
Volatile
Solids
VSj
7.2
7.2
7.2
7.2
7.2
7.2
10
10
8.5
8.5
12
17
18.5
9.1
9.2
9.5
10
10
a Population data for swine, poultry, and sheep from USDA (1989a-f). Goat and horse population data from Bureau of
Census (1987). Population data for cattle are the same as those used to estimate emissions from enteric fermentation in
cattle in 1990. Population data as of January 1,1988 for poultry, and sheep and as of December 1,1987 for swine,
goats, and horses. Cattle populations represent an average for 1990.
b Broiler/turkey populations estimated yearly based on number of flocks per year (North 1978; Carter 1989).
c Source: Taiganides and Stroshine (1971).
" Source: ASAE (1988).
Source: U.S. EPA (1993a)
Annex D H D-21
-------
Table D-18. Maximum Methane Producing Capacity Adopted For U.S. Estimates
Animal Type, Category
Cattle:
Swine:
Poultry:
Sheep:
Goats:
Horses, Mules, and
Donkeys:
Beef in Feedlots
BeefNotinFeedlots
Dairy
Breeder
Market
Layers
Broilers
Turkeys
Ducks
In Feedlots
Not in Feedlots
Maximum Potential Reference
__ . , .—_ . JX.CI.Ci C11VC
Emissions (B0)
0.33
0.17
0.24
0.36
0.47
0.34
0.30
0.30
0.32
0.36
0.19
0.17
0.33
., Hashimoto etal. (1981)
Hashimotos al. (1981)
Morris (1976)
Summers & Bousfield (1980)
Chen (1983)
Hill ( 1 982 & 1984)
Safley e/ a/. (1992a)
Sa&eyetal. (1992a)
Safley et al. (\992a)
Safley etal. (1992a)
Safley etal. (1992a)
Safley etal. (1992a)
Ghosh (1984)
Source: U.S. EPA (1993a)
Table D-19. Methane Conversion Factors for U.S. Livestock Manure Systems
MCFs based on
laboratory measurement
Pasture, Range, Paddocks'
Liquid/Slurry"
Pit Storage < 30 days"
Pit Storage > 30 days'
Drylot"
Solid Storage'
Daily Spread'
MCF measured by
long term Held monitoring
Anaerobic Lagoons0
MCFs estimated by Safley et al.
Litter4
Deep Pit StackinG*
MCFat30°C MCFat20°C
2 % 1.5 %
65 % 35 %
33 % 18 %
65 % 35 %
5 % 1.5%
2% 1.5%
1 % 0.5 %
Average Annual MCF
90%
Average Annual MCF
10 %
5%
MCF at 10°C
1%
10%
5%
J0%
1 %
1%
0.1%
Source: U.S. EPA (1993a)
1 Hashimoto (1992)
b Based on Hashimoto (1992).
c Safley et al. (1992a) and Safley and Westerman (1992b).
d Safley et al. (1992a).
D-22 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-20. Methane Conversion Factors for U.S. Livestock Manure Systems
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New Hampshire
New Jersey
New Mexico
New York
North Carolina
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Wvomine
Pasture, Range
& Paddocks
1.4%
1.4%
1.3%
1.2%
0.9%
0.9%
1.2%
1.5%
1.4%
0.8%
1.1%
1.0%
0.9%
1.1%
1.2%
1.4%
0.8%
1.1%
0.9%
0.8%
0.8%
1.4%
1.1%
0.7%
1.0%
1.2%
0.8%
1.0%
1.2%
0.9%
1.3%
0.7%
1.0%
1.4%
1.1%
0.9%
1.0%
1.3%
0.8%
1.3%
1.4%
0.9%
0.8%
1.2%
1.0%
1.2%
0.8%
0.8%
Brylot
.1.9%
1.9%
1.8%
1.4%
1.0%
1.0%
1.4%
2.4%
1.8%
0.8%
.3%
.2%
.1%
.5%
.5%
2.1%
0.8%
1.2%
1.0%
0.9%
0.8%
1.9%
1.4%
0.8%
1.1%
1.4%
0.8%
1.1%
1.3%
0.9%
1.5%
0.7%
1.1%
1.9%
1.1%
1.0%
1.1%
1.7%
0.9%
1.6%
2.1%
1.0%
0.8%
1.4%
1.0%
1.3%
0.8%
0.8%
Solid
. Storage
1.4%
1.4%
1.3%
1.2%
0.9%
0.9%
1.2%
1.5%
1.4%
0.8%
1.1%
1.0%
0.9%
1.1%
1.2%
1.4%
0.8%
1.1%
0.9%
0.8%
0.8%
1.4%
1.1%
0.7%
1.0%
1.2%
0.8%
1.0%
1.2%
0.9%
1.3%
0.7%
1.0%
1.4%
1.1%
0.9%
1.0%
1.3%
0.8%
1.3%
1.4%
0.9%
0.8%
1.2%
1.0%
1.2%
0.8%
0.8%
Daily
Spread
0.4%
0.4%
0.4%
0.3%
0.2%
0.2%
0.3%
0.6%
0.4%
0.2%
0.3%
0.3%
0.2%
0.3%
0.3%
0.5%
0.2%
0.3%
0.2%
0.2%
0.2%
0.4%
0.3%
0.2%
0.2%
0.3%
0.2%
0.3%
0.3%
0.2%
0.3%
0.2%
0.2%
0.4%
0.2%
0.2%
0.2%
0.4%
0.2%
0.3%
0.5%
0.2%
0.2%
0.3%
0.2%
0.3%
0.2%
0.2%
Liquid/
Slurry
29.0%
28.9%
27.6%
21.9%
18.2%
18.5%
22.6%
38.6%
29.0%
15.5%
22.8%
21.5%
20.7%
24.7%
23.8%
32.5%
15.5%
21.0%
18.1%
17.0%
18.0%
29.3%
24.1%
15.8%
20.8%
22.1%
16.3%
20.6%
21.3%
18.1%
24.5%
16.8%
20.2%
28.7%
16.2%
18.7%
18.7%
27.3%
19.1%
24.8%
31.7%
17.4%
16.6%
22.5%
15.5%
21.4%
17.0%
15.9%
Other Systems: Pit Storage for less than 30 days is assumed to have an MCF equal to 50 percent of the MCF for
Liquid/Slurry. Pit Storage for more than 30 days is assumed to have an MCF equal to liquid/slurry. Anaerobic lagoons are
assumed to have an MCF of 90 percent; litter and deep pit stacks an MCF of 10 percent.
Source: U.S. EPA (1993a)
Annex D • D-23
-------
Table D-21. Livestock Manure System Usage for the U.S.
Animal
Non-Dairy Cattle
Dairy
Poultry11
Sheep
Swine
Other Animals'
Anaerobic
Lagoons
<1%
11%
4%
0%
29%
0%
Liquid/Slurry
and Pit
Storage
<1%
21%
3%
0%
44%
0%
Daily
Spread
0%
41%
0%
0%
0%
0%
Solid
Storage
& Drylot
10%
18%
0%
0%
20%
0%
Pasture,
Range &
Paddock
89%
0%
<1%
92%
0%
89%
Litter,
Deep Pit Stacks
and Other
0%
8%
93%
8%
7%
11%
Note: Totals may not add due to rounding.
1 Includes liquid/slurry storage and pit storage.
b Includes chickens, turkeys, and ducks.
c Includes goats, horses, mules, and donkeys.
Source: Safley et al. C1992a).
D-24 n Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Table D-22. Methane Conversion Factor Changes from 1990 to 1992
DAIRY
State
AZ
FL
NV
NC
ND
TX
TX
UT
1990,1991
1992+
MCF
1990,1991
1992+
MCF
1990,1991
1992+
MCF
1990,1991
1992+
MCF
1990,1991
1992
MCF
1990,1991
1992+
MCF
SWINE
State
1990,1991
1992+
MCF
1990,1991
1992+
MCF
Lagoon
10%
50%
90.0%
2%
30%
90%
1%
40%
90.0%
5%
20%
90.0%
0%
1%
90.0%
25%
25%
90.0%
Lagoon
35%
45%
90.0%
25%
75%
90.0%
Liquid
Slurry
0%
0%
24.0%
0%
0%
36.9%
1%
10%
13.0%
35%
20%
20.3%
20%
1%
10.0%
60%
10%
29.4%
Dry
Lot
20%
30%
2.1%
75%
25%
1.0%
Daily
Spread
0%
0%
0.4%
10%
10%
0.6%
8%
0%
0.3%
50%
50%
0.3%
10%
8%
0.2%
15%
15%
0.5%
Pit St.
<1 mnth
15%
15%
14.7%
0%
0%
5.4%
Solid
Storage
0%
0%
1.4%
0%
0%
1.5%
90%
50%
1.2%
10%
10%
1.3%
70%
90%
0.7%
0%
50%
1.4%
Pit St.
>1 mnth
30%
10%
29.4%
0%
0%
10.8%
Other
90%
50%
1.0%
88%
60%
1.0%
0%
0%
0.0%
0%
0%
0.0%
0%
0%
0.0%
0%
0%
0.0%
Other
0%
0%
20.0%
0%
0%
0.0%
Weighted
MCF*WS
0.0990
0.46
0.0274
0.28
0.0213
0.38
0.1189
0.22
0.0251
0.02
0.4022
0.26
Weighted
MCF*WS
0.43
0.46
0.23
0.68
Change
Factor
4.60
10.09
17.76
1.88
0.66
0.65
Change
Factor
1.08
2.91
Annex D • D-25
-------
Point estimates of emissions were calculated using the previously described data. Emissions were
estimated for each animal type by summing annual emissions over all applicable manure management systems
and states. Total annual methane emissions from all animals were estimated by summing over all animal types.
Uncertainties in the point estimates result from uncertainties in the data used to make these estimates,
in particular:
• The estimated MCF values for pasture, range, drylots, solid storage, and paddocks are based on dry
manure. This may underestimate the MCFs for regions with significant rainfall. Because a large
fraction of animal manure is managed in these systems; total emissions may be underestimated.
• The methane producing potential of liquid/slurry and pit storage manure systems may be greater than
assumed. These systems are widespread, so total emissions may be underestimated.
The greatest uncertainty results from the MCF assumptions. Therefore, "high" and "low" case
emission estimates were defined based on varying the MCFs used for the various manure management systems
in the base case:
• High Case. The MCFs for liquid/slurry, pit storage, litter, and deep pit stacking systems were assumed
to be double the base case. The MCFs for solid systems (except litter and deep stack pits) were
assumed to be five times the base case. The MCFs for anaerobic lagoons were the same as the base
case.
• Low Case. The MCFs for each of the major solid systems (pasture/range, solid storage, and drylots)
were assumed to be 80 percent of the base case. The MCFs for liquid/slurry and pit storage were
assumed to be 90 percent of the base case. The MCFs for litter and deep pits were assumed to be half
the base case. The MCFs for anaerobic lagoons, estimated using a lagoon methanogenesis model
prepared for U.S. EPA7, were 40 to 100 percent of the base case.
These assumptions are summarized in Table D-23.
Table D-23. Base, High, and Low Case Emission Estimate Assumptions
Management System
MCF
High Case
Low Case
Pasture, Range, Paddock, Drylot, Daily
Spread
Liquid/Slurry, Pit Storage
Litter, Deep Pits
Anaerobic Lagoons
Five Times Base Case
Two Times Base Case
Two Times Base Case
Same as Base Case
80 percent of Base Case
90 percent of Base Case
50 percent of Base Case
Model Estimates 40 to 100 percent
of Base Case
Source: U.S. EPA (1993a)
7 The model estimates methane production based on loading rates, lagoon characteristics, and climate. The model estimates
are "conservative" because the model focuses on the amount of methane that can be recovered reliably for use as an energy
source.
D-26 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
ANNEXE
METHANE EMISSIONS FROM LANDFILLS
Municipal Solid Waste Landfill Methane Generation
Municipal solid waste (MSW) landfill methane generation is estimated by the Waste In
Place - 30 (WIP-30) model developed by U.S. EPA (henceforth known as the EPA model),
which uses statistical relationships between landfill gas (LFG) recovery rates and landfill waste
quantities (U.S. EPA, 1993a). The model coefficients are based on measured methane recovery
rates at approximately 100 LFG energy recovery projects in the U.S. TKelotal waste in place
was divided into 7 landfill size classes and point estimates of emissions from each landfill size
class were derived using regression coefficients developed in the model.
The EPA model assumes that the methane producing lifetime of waste is 30 years.
Therefore, the estimate of total waste landfilled over the last 30 years is used as the quantity of
MSW contributing to methane emissions. For example, in 1990, the total MSW contributing to
methane emissions would be the total MSW landfilled for the period 1961 to 1990. Therefore,
for 1994, total MSW contributing to methane emissions is the total MSW landfilled for the
period 1965 to 1994. The amount of MSW landfilled in 1994 is estimated to be 195 million
tonnes. Using these estimates, the EPA model gives a 30 year waste in place estimate of 4,971
million tonnes for 1994 (see Table E-l).
Table E-l. Municipal Solid Waste Contributing to Methane Emissions: 1990-1994
(Million Tonnes of Waste)
Total MSW Generated3
Percent of MSW Landfilled3
Total MSW Landfilled
MSW Contributing to CH4 Emissions15
1990
264
71%
189
4,708
a Source: Biocycle 1991, 1992, 1993, 1994, and 1995. The data,
converted to metric tons.
" These are estimates of MSW in place for the past 30 years (U.S.
1991
255
76%
194
4,771
1992
265
72%
190
4,838
1993
278
71%
197
4,901
1994
290
67%
195
4,971
originally reported in short tons, have been
EPA, 1993a).
Annex E 9 E-1
-------
For the purposes of analyzing methane emissions from landfills in the U.S., the
population of landfills was characterized in terms of size (i.e., waste in place) and climate (arid
and non-arid1).
• Size. The EPA model defined 7 landfill size classes based on the amount of waste
in place, with class 7 having the largest amount of waste in place, and class 1, the
smallest. In this analysis, the different landfill classes were grouped as large,
medium, and small. Classes 5, 6, and 7 were grouped as "large" landfills, classes
3 and 4 represented "medium" landfills, and class 2 was defined as "small"
landfills. Approximately 3000 Class 1 landfills were excluded from this analysis
as the quantity of waste in place contained in these landfills were a negligible
fraction of total waste in place.
« Climate. The analyses indicate that about 13 percent of the waste in landfills can
be considered to be in arid climates (U.S. EPA, 1993a). The methane emissions
estimates reflect that the emissions from waste in arid climates are lower than
waste in non-arid climates. Moisture can facilitate faster methane generation.
Industrial Landfill Methane Generation
Industrial landfills receive waste from factories, processing plants, and. other
manufacturing activities. Since there is no information available on methane generation at
industrial landfills, the approached used is to assume that industrial methane generation equals
about 7 percent of municipal landfill methane generation (U.S. EPA, 1993a).
Methane Recovery
To estimate LFG recovered per year in the U.S., data on current and planned LFG energy
recovery projects in the U.S. were obtained from Government Advisory Associates (GAA). The
GAA database, considered to be the most comprehensive source of information on LFG energy
recovery in the U.S., contains 1990 and 1992 estimates for LFG energy recovery. The data set
used in this analysis indicates that 1,200 and 1,440 thousand tonnes of methane were recovered
nationally by MSW landfills in 1990 and 1992, respectively. In addition, a number of landfills
are believed to recover and flare methane without energy recovery and were not included in the
GAA database. To account for the amount methane flared without energy recovery, the estimate
of gas recovered is increased by 25 percent (U.S. EPA, 1993a). Therefore, net methane recovery
from landfills is assumed to equal 1,500 thousand tonnes in 1990 and 1,800 thousand tonnes in
1992. The 1990 estimate of methane recovered is used for 1991 and the 1992 estimate, presented
in Table E-2, is used for 1992 through 1994.
A comprehensive census of landfills in the U.S. does not exist, making the landfill characterization somewhat uncertain.
See EPA (1993a) for a description of the landfill population data used in the analysis.
E-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
Methane Oxidation
Methane migrating through the top layer of the soil over the landfill can be oxidized by
micro-organisms. Landfills that recover methane practically eliminate migration of methane
through the soil, thereby minimizing the amount of methane that is oxidized. The amount of
oxidation that occurs is uncertain and depends on the characteristics of the soil and the
environment. For purposes of this analysis, it is assumed that 10 percent of the methane
produced is oxidized in the soil.
Table E-2. Landfill Gas Recovered Per Year: 1992-1994
LFG
Recovered3
(ftVday)
A
410,823,840
Methane Recovered6
(103 tonnes/yr)
B
B=Axl9.2x0.5x365xlO-9
1,440
Other
Recovery0
(103 tonnes/yr)
C
C=Bx0.25
360
Total Methane
Recovered
(103 tonnes/yr)
D
D=B+C
1,800
a Landfill gas recovered is estimated by aggregating total landfill gas processed for
operational landfills which utilize gas for energy recovery (GAA, 1994).
b Conversion of LFG recovered from ft'/day to 103 tonnes/yr assumes a methane density
of 19.2 g/ft3 and a methane concentration of 50%.
c The GAA data used to estimate LFG recovered does not include all landfills in the U.S.
A small number of landfills are believed to recover and flare methane without energy
recovery which are not included in the GAA data set. An estimated 25% of the
estimated landfill gas recovered for energy use is assumed to be recovered and flared
without energy recovery (U.S. EPA, 1993a).
Annex E B E-3
-------
-------
ANNEXF
SULFUR DIOXIDE: EFFECT ON
RADIATIVE FORCING AND SOURCES OF EMISSIONS
Sulfur dioxide emitted into the atmosphere through natural and anthropogenic processes
affects the Earth's radiative budget through photochemical transformation into sulfate particles
that (I) scatter sunlight back to space, thereby reducing the radiation reaching the Earth's surface;
(ii) possibly increase the number of cloud condensation nuclei, thereby potentially altering the
physical characteristics of clouds; and (iii) affect atmospheric chemical composition (e.g.,
stratospheric ozone, by providing surfaces for heterogeneous chemical processes). As a result of
these activities, the effect of SO2 on radiative forcing may be negative (IPCC, 1992).
Additionally, since SO2 is short-lived, it may make no long-term contribution to radiative forcing
(IPCC, 1994). Because the effects of SO2 are uncertain and potentially opposite from the other
criteria pollutants, SO2 emissions have been presented separately below in Tables F-l and F-2.
The major source of SO2 emissions in the U.S. is the burning of sulfur containing fuels,
mainly coal. Metal smelting and other industrial processes also release significant quantities of
SO2. As a result, the largest contributor to overall U.S. emissions of SO2 are electric utilities,
accounting for about 70 percent in 1994. Coal combustion accounted for approximately 96
percent of SO2 emissions from electric utilities in the same year. The second largest source is
industrial fuel combustion, which produced about 14 percent of 1994 SO2 emissions. Table F-2
provides SO2 emissions disaggregated by fuel source.
Sulfur dioxide is important for reasons other than its effect on radiative forcing. It is a
major contributor to the formation of urban smog and acid rain. As a contributor to urban smog,
high concentrations of SO2 can cause significant increases in acute and chronic respiratory
diseases. In addition, once SO2 is emitted, it is chemically transformed in the atmosphere and
returns to earth as the primary contributor to acid deposition, or acid rain. Acid rain has been
found to accelerate the decay of building materials and paints, as well as cause the acidification
of lakes and streams and damage trees. As a result of these harmful effects, the U.S. has
regulated the emissions of SO2 in the Clean Air Act of 1970 and in the amendments of 1990.
The U.S. EPA has also developed a strategy to control these emissions via four programs: (1) the
National Ambient Air Quality Program, which protects air quality and public health on the local
level; (2) New Source Performance Standards, which set
emission limits for new sources; (3) the New Source Review/Prevention of Significant
Deterioration Program, which protects air quality from deteriorating, especially in clean areas;
and (4) the Acid Rain Program, which addressees regional environmental problems often
associated with long-range transport of SO2 and other pollutants.
Annex F M F-1
-------
Table F-l. Emissions of Sulfur Dioxide: 1990
(Million Metric Tonnes)
Source
Fossil Fuel Combustion
Electric Utilities
Industrial
Commercial
Residential
Transportation
Industrial Processes
Metals Processing
Chemical and Allied
Manufacturing
Asphalt Manufacturing
Agriculture, Food,
Kindred Products
Wood, Pulp, Paper, and
Publishing
Mineral Products
Solvent Use
Waste Incineration
Fossil Fuel Production,
Distribution and Storage
Total
1990
18.55
14.42
2.82
0.37
0.17
0.76
1.360
0.600
0.400
0.001
0.002
0.120
0.230
0.001
0.03
0.40
20.35
Emissions
1991
18.27
14.32
2.64
0.37
0.17
0.76
1.330
0.570
0.400
0.001
0.002
0.120
0.220
0.001
0.03
0.39
20.02
1992
18.03
13.99
2.72
0.38
0.17
0.77
1.360
0.590
0.410
0.001
0.002
0.130
0.240
0.001
0.03
0.38
19.81
1993
17.72
13.78
2.67
0.38
0.17
0.72
1.390
0.610
0.410
0.001.
0.002
0.130
0.240
0.001
0.03
0.38
1994
17.31
13.49
2.75
•0.38
0.17
0.52
1.430
0.630
0.410
0.001
0.002
0.130
0.250
0.001
0.03
0.37
Note: Totals may not add to the sum of the individual source categories due to independent rounding.
Table F-2. Emissions of SO2 from Fossil Fuel Combustion by Fuel Source: 1990
(Million Metric Tonnes)
Fuel Source
Fuel Oil
Natural Gas
Wood1
Internal
Combustion
Other Fuels'"
Total
1990
15.672
2.179
0.322
0.006
0.033
0.336
18.548
Emissions
1991
15.285
2.295
0.322
0.006
0.037
0.330
18.275
1992
15.205
2.133
0.318
0.006
0.034
0.337
18.033
1993
14.880
2.144
0.317
0.006
0.034
0.340
17.721
14.715
1.893
0.316
0.006
0.036
0.350
Notes: Totals may not add to the sum of the independent source categories due to independent rounding.
* Residential sector only.
" Other fuels include: LPG, waste oil, coke oven gas, coke, and wood from sectors other than the residential sector.
F-2 • Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994
-------
ANNEXG
IPCC REPORTING TABLES
This annex contains a series of tables which summarize the emissions and activity data
discussed in the body of this report. These tables conform to guidelines established by the IPCC
(IPCC/OECD/IEA, 1995; Vol. 1) for consistent international reporting of greenhouse gas
emissions inventories. The format of these tables does not always correspond directly with the
calculations discussed the body of the report. In these instances, the data have been reorganized
to conform to the IPCC tables. As a result, a few slight differences may exist between the figures
presented in the IPCC tables and those in the body of the report. These differences are merely an
artifact of the variation in format and total U.S. emissions are unaffected.
Annex G • G-1
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\S Land Use Change and Forestry
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