United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-86/035 Feb. 1987
Project Summary
EPA Workshop on N20
Emission from Combustion
(Durham, NC,
February 13-14, 1986)
W. Steven Lanier and Susan B. Robinson
This report summarizes the dialogue and
interaction which took place during an EPA
sponsored workshop addressing nitrous
oxide (N2O) emissions from fossil fuel
combustion. The workshop was held in
Durham, NC, February 13-14,1986. Prior
research studies had identified N20 as a
trace gas potentially contributing to deple-
tion of stratospheric ozone as well as be-
ing a contributor to global climate change
through the greenhouse effect. Prior stud-
ies also suggested that fossil fuel combus-
tion was a major anthropogenic source of
N2O emissions. The workshop was or-
ganized to assess the current understand-
ing of combustion generated N20 emis-
sion and to assist EPA in formulating com-
bustion research activities. The four tech-
nical sessions addressed: (1) measurement
of N2O; (2) N2O formation/destruction
mechanisms during combustion; (3) emis-
sion source prioritization; and (4) emission
control approaches. It appears that the
most significant N2O emitters are utility
and industrial boilers firing coal and heavy
fuel oil. Mobile sources are a minor con-
tributor to atmospheric N20. loading.
Available field test data indicate a direct
correlation between N2O and nitrogen ox-
ide (NOX) emission rates, suggesting that
combustion modifications for NOX control
may also reduce N20 emission. The cur-
rent data base and the understanding of
N20 formation processes are insufficient
to identify control strategies. Workshop at-
tendees suggested that further research
be conducted to improve the data base
and to better understand the chemical
processes controlling IM2O formation and
destruction.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of the
research project that is fully documented
in a separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The U.S. EPA sponsored a workshop on
nitrous oxide (N20) emissions from the
combustion of fossil fuels, held in Durham,
NC, February 13-14, 1986. The workshop
was designed to help EPA identify critical
issues related to combustion emission of
N20 which would serve as the basis for
developing an N20 research program
plan. This report summarizes the dialogue
and interaction which took place during
the workshop.
Concern over N2O emissions is based
on three facts: (1) N2O participates in
reactions which deplete ozone in the strat-
osphere, (2) N2O is a greenhouse gas,
and (3) atmospheric N2O concentration is
increasing. N20 is one of several trace
gases which participate in ozone depletion
reactions. Ozone strongly absorbs ultra-
violet radiation from the sun at wave-
lengths of 240-320 nm. If the strato-
spheric ozone concentration is decreased,
increased quantities of this radiation, re-
ferred to as UV-B, will reach the Earth's
surface with potentially serious effects on
human health, agricultural productivity,
and fisheries.
In addition to participation in ozone
depletion reactions, there is concern for
the fact that N20 absorbs infrared (IR)
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radiation in wavelength ranges which are
normally transparent in the atmosphere.
This IR absorption process reduces cool-
ing of the earth during the night. This is
the greenhouse effect usually associated
with CO2.
The atmospheric concentration of N20
was about 300 ppb in the late 1970s, but
is increasing at a rate of approximately
0.25 percent per year. The increasing con-
centration is of particular concern since
there are no known atmospheric sinks for
N20. In fact, the stratospheric lifetime of
N20 is estimated to be in excess of 100
years.
N20 is emitted to the atmosphere from
both natural and anthropogenic sources.
Figure 1 shows that soil denitrification and
ocean release account for about two-
thirds of the ambient N20 loading. It is
believed that one of the major sources of
anthropogenic N20 is the combustion of
fossil fuel. Since combustion generated
N20 is potentially controllable, the EPA
and others are examining methods which
may be effective in reducing N20 emis-
sions from combustion sources. It should
be emphasized that the scope of the cur-
rent workshop was limited to examining
combustion generated N20; it did not ad-
dress the impact of potential control
strategies on ozone layer protection, global
climate change, or human health. The
workshop was organized to assist EPA in
defining critical combustion issues and in
suggesting appropriate research actions.
The workshop was divided into five ses-
sions: an introductory session containing
an overview of the global N20 situation,
and four technical sessions (N20
measurement techniques, N2O formation/
destruction mechanisms, source prioritiza-
tion, and control approaches).
Introductory Session
The introductory session began with an
overview of EPA's risk assessment studies
relative to global climate change. With the
current levels of trace gases in the atmos-
phere, modification of the global climate
is inevitable, even if current emission levels
are drastically reduced. With current emis-
sion rates it is projected that the global
surface temperature could rise by as much
as 5 °C over the next 50 years. This is as
much as the temperature differential be-
tween the current level and that at the
height of the Ice Age. The potential gravity
of the situation is great, but is often
overlooked by both researchers and the
general public. It should be emphasized
that N20 is only one of the several gases
contributing to the climate change phe-
nomena and that the global warming pro-
Total Natural:
==0.3 Tmol/yr (from strato-
spheric removal models
assuming no other sinks)
Oceans: =0.1 Tmol/yr (from mean
oceanic supersaturation of
<4% and radon calibrated
exchange models)
Approx.
Source
Strength
(Tmol/yrj
0.3
Terrestrial: =0.2 Tmol/yr (by difference,
includes soil sources and
sinks) 0.2
Anthropogenic: =*0.14 Tmol/yr {from ob-
served rate of increase and
box model calculations)
Note: 1 Tmol= W^g moles
0.1
0.0
Natural
Anthro-
pogenic
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Table 1. Comparison of Measurement Systems for Different Applications
Chromatography
Measurements Standards
ECD N2O/CO2
FTIR
Diode IR
NDIR
Ambient
Stack
Plumes
Flame
Xs XX
X
XX X X
X
X
X
aStandards available only for ambient measurements. Standards are needed for
all measurements.
140
120
100
n 80
§: eo
S
40
20
VfOV? steamer equipped with MHIPM burner
O Coal/oil-mixture -fired industrial b oiler
O Oil/refinery-gas-fired crude oil heater
O'Coal/water-slurry-fired industrial boiler
A Oil/refinery-gas-fired industrial boiler
^ Coal/plastic/water-fired commercial boiler
I> Coal-fired commercial boiler
O Coal/water-slurry-fired industrial boiler
OfO/? steamer equipped with the EPA tow NO, burner
100 200 300 400 500
NO,, ppm, 3% Oi. dry
600
Figure 2.
A/20 emissions from combustion sources as a function of NO, emission (Source:
Environmental Assessment of an Enhanced Oil Recovery Steam Generator
Equipped with a Low-NO, Burner. Volume 1. Technical Results. EPA/600/7-
86/003a [NTIS PB 86-159837], February 1986).
N2O:NOX molar ratio of 22 percent. The
control strategy implication of this data
trend is that combustion modification
techniques for NOX reduction may also be
effective in reducing N20 emissions. An
important workshop discussion topic was
consideration of this trend. It was noted
that essentially all the data were taken
from different units and did not examine
N2O emissions under normal and low-
NOX conditions. In the absence of such
data, a control strategy based on im-
plementing low NOX control is not scien-
tifically justifiable. That is to say, additional
research is required to establish a control
strategy. The need for additional R&D is
amplified by two sets of pilot scale data
indicating conflicting N2O emission
results. One set of data indicates that the
N2O:NOX emission ratio remains constant
as NOX is reduced by combustion modifi-
cation. The second set of data indicates
that the exhaust N20 concentration re-
mains constant as the NOX emission rate
is reduced by a factor of 6 by applying
staged combustion.
There was extensive discussion of
chemical, mixing, and thermal phenomena
which are important in controlling N20
formation and destruction processes. It
was generally concluded that current
knowledge does not provide a sufficient
understanding of N20 control in the com-
bustion process. Many workshop partici-
pants felt that additional fundamental
research to develop a "working
hypothesis" of N20 formation and
destruction was an important first step in
an EPA research program.
The second session also provided im-
portant information defining the dominant
sources of N2O emissions. Figure 3
shows results from utility boiler field tests
in which the N2O:C02 molar ratio in the
boiler exhaust is correlated with the
nitrogen to carbon molar ratio in the fuel
being fired. This figure suggests a con-
stant fractional conversion of fuel bound
nitrogen to N20, which implies that coal-
and heavy-oil-fired boilers are the most
important N20 emission sources. This
trend further supports the analysis which
defined large industrial and utility boilers
as the largest contributors to anthropo-
genic N2O emissions.
Source Prioritization
The third technical session focused on
mobile versus stationary source emissions
of N20 and was included to aid EPA in
determining priorities for R & D activities.
The global projections referred to earlier
indicate N20 emission growth in the third
world as well as in the U.S.—both in the
energy and transportation sectors. It is im-
portant from both a U.S. EPA and a global
perspective to define whether one sector
is much more important than the other, or
whether it is necessary to address both
areas.
As shown in Table 2, limited data are
available from mobile sources. However,
available data show a large variation in
emission rates: from 5 to 147 mg N2O
per mile.* Assuming an average emission
rate of 100 mg/mile and estimating 1.6 x
1012 total vehicle miles traveled per year
in the U.S., the total annual U.S. NZ0
emission rate from mobile sources is on
the order of 1.6 x 105 metric tons. It has
been estimated that the total an-
thropogenic N2O emission rate is 3.0 x
106 metric tons per year, which implies
that U.S. mobile sources account for only
about 5.5 percent of the anthropogenic
total.
Table 2 shows that the N2O emission
rate from noncatalyst equipped cars was
only 5-6 mg/mile, an order of magnitude
less than the assumed average emission
rate used in the above calculation. Since
it is unlikely that other nations, such as
third world countries, will require auto-
motive catalysts, it appears that the
mobile source sector will remain an insig-
nificant contributor to atmospheric N20
loading. Before a firm conclusion may be
reached, however, it is essential that the
*1 mi = 1.61 km.
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16
18
20
Figure 3.
6 8 10 12 14
Fuel-N/Fuel-C, mole/mole x 10~3
Relationship between N2O emissions and fuel-nitrogen contents of natural gas. No.
6 fuel oil. and coal-fired combustion systems (Source: S. C. Wofsy et al.. Harvard
University. 1986).
Table 2. N20 Emission Data for Various Vehicle Types
Vehicle Type
Noncatalyst Car
Catalyst Cars
Diesel Trucks/Buses
Gasoline Trucks
No. of
Samples
1
22
4
2
N2O Emission
Rates, mg/mi
5-6
7-137
31-147
48-97
N2O emission rate data base be
significantly expanded, particularly for
noncatalyst equipped vehicles. To expand
that data base, it was suggested that
N2O measurements be incorporated into
existing vehicle emission monitoring and
certification testing. With regard to expan-
sion of the noncatalyst equipped auto data
base, it was suggested that other coun-
tries could assume responsibility through
international cooperation coordinated by
the United Nations.
Results from field tests of different sta-
tionary sources fired with different fuels
are shown in Figure 4. These results incor-
porated those shown in Figure 2, show-
ing a general correlation between N20
and NOX emissions. With the expanded
data set there is a substantial increase in
data scatter, underlining the need for ad-
ditional research before reliable emission
factors can be developed.
Control Approaches
The purpose of the final technical ses-
sion was to identify combustion control
approaches or strategies which could be
used to reduce N2O emissions. Since
many potential control methods had been
considered during the previous workshop
sessions, this session was mainly a sum-
mary discussion. The concept of reburn-
ing was introduced as a possible NOX
control strategy which could also be ap-
plicable to N2O emission control. How-
ever, it was emphasized that not all NOX
controls would automatically be effective
in reducing N20 emissions, and that
careful study must be made to determine
each method's effectiveness. "Piggyback-
ing" of N2O measurements onto current
test programs was suggested as a cost-
effective method to augment the N20
emission data base.
Workshop Summary
The various workshop sessions iden-
tified many important data trends which
may be used to form working hypotheses,
but also underlined the fact that the ex-
isting data base was insufficient to reach
scientifically defensible conclusions. By
way of summary, it would appear thai
N20 emissions from combustion of nitro-
gen bearing fuels is far greater than f rorr
"clean" fuels. Mobile sources appear tc
be negligible N20 emitters relative to sta
tionary sources, but the data base foi
mobile sources is extremely limited (par
ticularly for vehicles without catalysts)
The available stationary source field dat<
show a direct correlation between N0:
and N2O emissions. This indicates that in
stalling low-NOx burners or other NO
control technologies may also be effectivi
for N20 control. It should be emphasized
however, that some pilot-scale results an
in direct conflict with that indication, li
fact, one of the main workshop conclu
sions is a call for additional fundament?
and bench-scale research to clarify thi
discrepancy.
Depletion of the ozone layer and th
greenhouse effect are clearly importar
environmental concerns, but immediat
regulatory response appears unlikely. 0
that basis, prudent programmatic action
could include:
• Addition of N20 measurement to e;
isting field evaluation programs 1
help build the data base.
• Initiation of fundamental combustic
research to identify how N20 emi
sions are impacted by NOX contr
combustion modifications.
• Formation of a working group •
coordinate research activities at ER
NASA, DOE, and NOAA.
These actions could be accomplished wi
minimal resources and would place tl
• EPA in a better position if control tec
nology or regulatory actions were requin
in the future.
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•s
«
o
I
o'
5?
130
120 -
110 -
100 -
90 -
80 -
70 -
60 •
50 -
40 -
30 -
20 -
10 -
O
a
a D
50
150
4SO
550
Figure 4.
250 350
NOf, ppm, 3% Oi. dry
NzO andNOi emissions from various sources (Source: C. Castaldini, Acurex, 1986).
W. S. Lanier and S. B. Robinson are with Energy and Environmental Research
Corporation. Chapel Hill. NC27S14.
Joseph A. McSorley is the EPA Project Officer (see below).
The complete report, entitled "EPA Workshop on N^O Emission from Combustion
(Durham. NC. February 13-14. 1986)."(Order No. PB 87-113 742/AS; Cost:
$ 18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
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