EPA-600/2-76-257
September 1976
Environmental Protection Technology Series
PARTICLE EMISSION REACTIVITY
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environments I technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The live series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
envinnmentel degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This, report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield. Virginia 22161.
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EPA-600/2-76-257
September 1976
PARTICLE
EMISSION
REACTIVITY
by
K.P. Ananth, J.B. Galeski, and F.I. Honea
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
Contract No. 68-02-1324, Task 44
ROAPNo. 21ADL-029
Program Element No. 1AB012
EPA Task Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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CONTENTS
Page
List of Figures . iv
List of Table... vi
Acknowledgment vii
Summary .......................... 1
Introduction* ..**.»............*......... 4
Historical Trends in Emissions and Acid Rain 5
Mass Emission Trends * 5
Ambient Pollutant Levels and Trends . ....... 14
Summary of Particulate, S02, NOX Trend Data 27
Acid Rain Trend 28
Size and Composition of Particulate Emissions 32
Particle Size Distribution. 32
Particulate Composition ............ . 37
Summary of Size/Composition of Particulate Emissions 44
Analysis of Interactions Between Particulate and Sulfur-Containing
Gases 45
Method of Analysis of Published Rate Data 46
Absorption-Oxidation of S02 by Particulate 48
Direct Catalytic Oxidation of S02 by Particulate 51
Adsorption by Particulate (Acid Removal Processes) 53
Photolytic Oxidation of S02 56
Comparison of Particulate-S0x Interactions 56
Effect of Particulate Emission Control Devices on Conversion
Reactions 61
Conclusions and Recommendations ................... 63
References* ............................. 65
iii
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LIST OF FIGURES
No. Page
1 Projected Nationwide Sulfur Oxides Emission Trends,
1950-1990 6
2 Projected Particulate Nationwide Trends, 1950-1990 7
3 Projected NOX Nationwide Trends, 1950-1990 8
4 Sulfur Oxides Emissions From Fossil Fuel Combustion, Millions
of Tons Per Year* 10
5 Emissions of Nitrogen Oxides From Fossil Fuel Combustion,
Millions of Tons Per Year 11
6 Fossil-Fueled Power Plant Generating Capacity, 1970 13
7 Nationwide Geographic Variation in Annual S02 Emission
Density 15
8 24-State Region With High SOX Emission 16
9 NASN Urban Sulfur Dioxide Trends. ....... 18
10 NASN Urban and Nonurban Sulfate Trends 19
11 1970 Annual Urban Sulfate Concentrations. .......... 21
12 1970 Annual Nonurban Average Sulfate Concentrations • • . . • 22
13 Not.urban Sulfate Trends in the Northeast (8 Sites) 23
14 Composite Average 24-Hr N0~ Ambient Concentration for Seven
Northeastern Cities, 1967-1969 24
15 National and Northeastern Trends for Total Suspended Par-
ticulates, 1970-1973 25
iv
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LIST OF FIGURES (concluded)
No. ' Page
16 Predicted pH of Precipitation Over the Eastern United States
During the Period 1955-1956 . 29
17 Rainfall Acidity, 1965-1966. . „ . . . , 30
18 Composite Mass Size Distribution of Particulates Collected
Before and After an Electrostatic Precipitator at Coal-Fired
Power Plant. .......... 33
19 Cumulative Particle Size Data Taken at the ESP Inlet Using
Brink Cascade Impactors. ...» 34
20 Cumulative Particle Size Distribution Taken at the ESP Outlet
Using Andersen Mark III Cascade Impactors. 35
21 Fractional Efficiency of ESP Under Three Boiler Load Condi-
tions. 36
22 Outlet Size Distributions (Optical and Diffusional) at 140 Mw
Boiler Load 38
23 S02 Aqueous Oxidation Rates 49
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LIST OF TABLES
No. Page
1 Coal Supply and Demand* .»•»».••••......... 12
2 Comparison of Sulfur Dioxide and Sulfate Trends for Selected
Urban and Nonurban Sites 17
3 Aerosol Metal Concentrations at Various Locations ...... 26
4 Chemical Analyses of Fly Ashes. ....... 39
5 Trac.j Metals in Fly Ash as a Function of Particle Size. ... 41
6 Concentration and Size of Trace Metal Particles in Urban Air. 41
7 Traca Element Stack Emissions From Coal-Fired Power Plants. • 42
8 Upper Value of Expected Concentrations in Power Plant Flue
Gases (Uncontrolled) Compared with Measured Ambient Air
Concentrations 43
9 Summary of Published Rate Data for Oxidation of Dissolved S02
(Absorption/Oxidation Mechanism) 50
10 Sumtrary of Pertinent S02 Oxidation Rate Data for Catalysis by
Metal Salts and Oxides. 52
11 Surface Area and Monolayer Adsorption Capacity of Fly Ash for
803 as a Function of Particle Size for a Typical Size
Distribution. ..... ....... 55
12 Comparison of Some SOo Photochemical Reaction Rate Studies. . 57
13 Estimated Maximum Rates of S02 Oxidation in Flue Gas
Catalyzed by Components Present in Fly Ash 58
14 Comparison of Photolytic Oxidation of S02 with Estimated
Maximum Rates of Catalytic Oxidation 60
vi
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ACKNOWLEDGMENT
This report was prepared for the Industrial Environmental Research
Laboratory-RTF of the Environmental Protection Agency under Contract No.
68-02-1324, Task No. 44. The work was performed in the Environmental
Systems Section of the Physical Sciences Division. Dr. K. P. Ananth was
the project leader and Mr. M. P. Schrag, Head, Environmental Systems
Section was the project manager.
The authors sincerely appreciate the helpful suggestions provided
by Dr. L. J. Shannon and Mr. M. P. Schrag in completing this task.
Vll
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SUMMARY
This task was undertaken by Midwest Research Institute for the
Environmental Protection Agency. The purposes of this task were (a)
to study historical trends of particulate, SOX and NOX emissions and
of acid rain in the northeastern United States; (b) to study particle
size and composition of particulates from power plants; and (c) to ana-
lyze interactions between particulates and sulfur-bearing gases from
coal-fired power plants.
An extensive review and analysis of the literature was conducted
to accomplish the above objectives. Pollutant emission trends were ex-
amined based on mass emissions as well as ambient levels in northeastern
sections of the United States. Acid rain trends were studied based on
available information reporting pH of rainwater for various cities in
the Northeast. Interactions between particulates and sulfur-bearing
gases from coal-fired power plants were analyzed using potential reac-
tion mechanisms and order-of-magnitude calculations for reaction rates
for each mechanism based on reported data.
Particulate mass emissions from industrial and combustion sources
project a rising trend, on a nationwide basis, if no additional controls
are assumed. With application of best controls, the trend projection is
reversed. There is a high concentration of particulate emissions in the
northeastern United States. Data for 1972 indicate that 52% of the total
U.S. particulate emissions are emitted in the northeastern region between
the State of Illinois and the Atlantic seaboard lying to the north of the
Tennessee-Mississippi border. However, particulate concentrations, both
in the nation and in the northeastern region, show a decreasing trend
during the 4-year period from 1970 to 1973.
S02 emissions have been increasing nationwide since 1950. With no
additional controls and a 5% industry growth, this trend is expected to
continue; but if new source performance standards (NSPS) and state imple-
mentation plans (SIP) become effective in 1978, then 862 emissions are
expected to decrease. Northeastern United States has a high concentration
of S02 emissions and the highest ambient levels of sulfates» However,
existing data indicate a downward trend for ambient levels of SC>2 and
sulfates for selected northeastern urban areas and an increasing trend
for sulfates in nonurban areas.
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NOX mass emissions have increased consistently since 1920. Emis-
sions for 1970 are more than three times of that observed for 1920.
Only limited data are available for ambient levels of NOX and this is
further explicated by inherent limitations of measurement techniques
used to obtain these data.
Information on acid rain in northeastern United States is extremely
scarce. The available data indicate that precipitation in this region
has become increasingly acidic over the years. A better assessment of
the trend can only be made when additional information on rainwater
pH and p£ measurement techniques becomes available.
Analysis of fly ash particle size distributions and composition
shows a preferential concentration of certain trace elements such as
Pd, Gd, J!e, As, V, Ni, etc., in the finer particles which are emitted
to the atmosphere. Many of the trace elements are active catalysts for
the oxidc'ition of SC^ to acid.
Interactions between particulates and sulfur-bearing gases from
coal-fired power plants were studied by considering four reaction mech-
anisms, "hese mechanisms are absorption-oxidation of 862 by particulate,
direct catalytic oxidation of S02 by particulate, adsorption/chemical
reaction of S02 by particulate, and photolytic oxidation of S02»
Absorption-oxidation refers to SC^ oxidation catalyzed by particulates
in the presence of significant amounts of water. Direct catalytic oxida-
tion assumes minimal quantities of water, typically one monolayer of
adsorbed water. Both of these mechanisms are acid generation processes.
Adsorption/chemical reaction of S02 by particulate is a removal process
whereby the precursor for acid formation is removed by physical adsorp-
tion or chemisorption. The last mechanism (photolytic oxidation) refers
to 862 conversion in the presence of sunlight.
On the basis of data presented in this report, catalytic oxidation
of S02 by absorption-oxidation and direct oxidation are both plausible
mechanisms by which rapid S02 oxidation could occur. These reactions
could take place at or near ground level in the absence of sunlight
and need to be considered in predicting S02 oxidation in stack plumes.
Although the list of catalytically active elements is far from complete,
vanadium pentoxide is found to have a much higher activity than others
for which data are presently available. SCL removal by adsorption is
ruled out by the analysis and photolytic oxidation of S02, particularly
in the presence of hydrocarbons and other oxidants, warrants further
investigation.
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Whether the increased use of particulate control systems on power
plants or shifts in fuel usage have altered (positively or negatively)
the regional acid rain trends in the northeastern U.S. are unanswered
questions at this time. The substitution of oil for coal as a power
plant fuel and the accompanying probable increase in vanadium (a very
active catalyst for SC^ oxidation) emissions may also work to counter
the expected decrease in acid rain resulting from decreased SC>2 emis-
sions achieved by fuel switching. Further study will be needed to answer
these questions.
Future studies of the acid rain phenomenon should include a sys-
tematic evaluation of the various parameters such as catalyst type,
concentration, temperature, relative humidity, etc., which affect the
kinetics of SC>2 oxidation in order to identify the main rate-controlling
factors. Acid rain formation from nitrates and chlorides should be sep-
arately investigated even though currently available information supports
the hypothesis that only about 25% of the acidity in rain is due to ni-
trates and less than 10% due to chlorides. Refinement of techniques to
predict whether sulfates, nitrates, or chlorides are contributory to the
acidity in a particular rain sample should also be undertaken. The in-
corporation of mathematical modeling to include transportation effects
of pollutants to areas of geographic interest and characterization of
ambient particulates for size distribution and comparison are also rec-
ommended. Finally, meteorological methods which monitor rainfall in
different states and regions of the U.S. should include pH measurement
of rainwater on a routine basis if it is not already being done. This
would eliminate the data gap that presently exists for some sections
of the U.S. and for most sections on a recent basis.
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INTRODUCTION
Increased acidity in rain and snow in the northeastern United States,
Sweden, and other parts of Europe, is of growing concern due to its po-
tential for ecological damage, adverse health effects and corrosion of
materials. It is presently believed that anthropogenic factors are the
major cause for the increased acidity. But one recent report claims
that biogenic sources can contribute up to 50% to the annual mean sul-
fate leve.ls near extensive bodies of (polluted) water.
Acic. rain has been primarily attributed to industrial sulfurous
emission!; which result in increased sulfates in the atmosphere. Another
important: industrial emission is particulates which can act as nuclei
and/or catalysts for the adsorption and reaction of sulfur dioxide to
form suli:ates. Anmonia, nitrates, and chlorides from industrial emis-
sions can also contribute to acidity in rain but to a lesser extent.
The Environmental Protection Agency sponsored the present study
with the following general objectives.
• Review historical trends in SOX emissions, particulate emis-
sions, NOX emissions, and acid rain.
• Review information on size and composition of particulate emis-
sions to identify catalytically active components in the particu-
lates.
Analyze chemical interactions between particulates and sulfur-
bearing gases from coal-fired power plants.
Predict the impact of the above items on acid rain production.
An extensive review and analysis of available literature was con-
ducted t:o provide data needed to achieve the preceding objectives. The
following sections of this report address the salient observations re-
lated to each objective.
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HISTORICAL TRENDS IN EMISSIONS AND ACID RAIN
The initial effort involved a review of the trends and/or relation-
ships between emissions and acid rain. The .results of this activity are
highlighted next.
MASS EMISSION TRENDS
Industrial emissions are estimated on an annual basis by the Environ-
mental Protection Agency (EPA) for inclusion in the Council on Environ-
mental Quality (CEQ) annual report on environmental quality. The first
annual report was issued in August 1970 and included emissions estimates
for 1968.-=' Every 5 years, EPA also publishes a detailed national emis- ,
sions report based on data from the NEDS (National Emissions Data System)."
The emission estimates and trends for sulfur oxides (SOX) emissions
from power plants and from all sources are indicated in Figure 1. The
estimates and trends for particulates and nitrogen oxides (NO ) are
X
indicated in Figures 2 and 3, respectively. The estimates and trends
in the 1970's indicate a possible peaking and decline in SOX emissions. .
The particulate and NO emissions may increase or decrease depending
X
upon emissions control and fuel allocation. In the 1968 to 1974 period,
many industries and electrical utilities switched from coal combustion
to low-sulfur fuel oil to decrease SOX and particulate emissions. This
move, although decreasing emissions, affects the national energy policy
since the United States' supply of low-sulfur oil is restricted compared
to the large reserve of coal. In the next few years, until coal gasifi-
cation or liquefaction is practiced on a large scale, many of these
plants will be required to revert to coal for fuel with a potential
for increase in emissions levels.
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60
50
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co
20
10
All Sources
Power Plants
Projections Assuming
No Control, 5%/year Growth
Projections Assuming
SIP and NSPS in 1978
Note: Discontinuity in Emissions Caused by Change in Data Base.
I I I I
1950
1960
1970 1980
YEAR
1990
2000
Figure 1. Projected nationwide sulfur oxides emission
trends, 1950-199u£^/
-------
o
X
l/>
o
•o
o
70
60
50
1950
I T I I I
• Total Man-Generated
A Total Industrial + Combustion
• Stationary Combustion
o Electrical Utilities
1950 & 1960 Estimated
1960
1970 1980
YEAR
1990
2000
Figure 2. Projected particulate nationwide trends,
195Q-19901.2.4-6/
-------
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X
V,
c
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00
Z
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to
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X
0
Z
ou
50
40
30
20
10'
1 1 1 1 1 1 1 1 1
• Total Man-Generated
~: A Stationary Combustion ~~
• Electrical Utilities
1950 and 1960 Estimated
No Additional Standards (1972) -
,'*
^^^^
jf \
^- — • "" ^-A-A x* Assumed Standards (1977)
fif* ~
<*
01- 1 f 1 * 1 1 1 1 1 1
1950 1960 1970 1980 1990 20C
YEAR
Figure 3. Projected NOX nationwide trends,
1950-19902,5-77
8
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In considering emission sources which may contribute to the acid
rain problem, electric power plants emerge as the major potential con-
tributor. Currently, these power plants contribute 56 to 61% of the
SOX, 11 to 15% of the particulates, and 14% of the NC^ based on data
in Figures 1, 2, and 3. In addition, all stationary combustion sources
including power plants contribute 73.4% of SOX, 26% of particulates,
and 50% of NOX.
A backward projection of fossil-fuel combustion process emissions
from 1970 to 1920 by Land^ places these emissions in clearer histori-
cal perspective. The SOX emissions level as indicated in Figure 4 has
fluctuated, but was only 26% higher in 1970 than in 1920 and was about
the same as in 1942. As indicated in Figure 5, the NOX emissions level
has increased consistently from 1920 to 1970 with the level in 1970
more than three times that calculated for 1920.
The contributions by electric power plants and other stationary
combustion sources become even more important when considering the fore-
casts for electrical energy requirements and the regional pattern of
power plant locations and coal utilization. The consumption of coal and
the major users are summarized in Table 1 for 1974 through 1976. From
this table, power plant consumption of coal is expected to increase
by 7.4% between 1975 and 1976 (compared to 3.8% from 1974 to 1975) and
the total demand is expected to increase by 7.3% (compared to almost
no increase from 1974 to 1975). This increased use of coal may lead
to increased particulate and SO emissions for 1976 over those projected
.X
in Figures 1 and 2. From the coal consumption data in Table 1, power
plants are the major consumer and use 70 to 72% of the total domestic
demand.
Since fossil-fueled power plants are a major source of the industrial
emissions which can cause acid rain conditions, the location of these
plants is an important consideration. A map of the United States indicating
the location of the major fossil-fueled power plants in operation in 1970
is presented in Figure 6. A high percentage of these power plants are lo-
cated in the eastern half of the United States. When the prevailing wind
directions are considered, this distribution of power plants could lead
to a concentration of emissions in the northeastern United States. This
area of the United States also includes most of the high-sulfur coal
power plants and oil-fired power plants in the U.S., whereas most of
the gas-fired plants are located in the Southwest.
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30
25
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a>
c.
o
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to
o
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^
X
O
15
10
1920
I
I
1930
1940 1950
YEAR
Total SOX
Petroleum SCX
1960
Figure 4. Sulfur oxides emissions from fossil fuel combustion,
millions of tons per
10
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25
20
D
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X
I/I
c
c 15
o
on
§ 10
to
ox
1920
Total NO,
1930
1940
1950
1960
YEAR
Figure 5. Emissions of nitrogen oxides from fossil fuel combustion,
millions of tons per year-S'
11
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Table 1. COAL SUPPLY AND DEMAND (MILLIONS OF TONS)
Year
_, -„, Estimated Percentage
User 1974" 1973— 1976J2/ change
Electric utility 390 405 435 + 7.4
Coking ccal 91 83 90 +8.4
General industry 70 64 68 +6.3
Retail 9 7 7 0
Total domestic demand 560 559 600 +7.3
Coal exports 60 65 6.9 + 6.2
Total demand 620 624 669 +7.2
Total production 590 638 664 + 4.1
12
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Figure 6. Fossil-fueled power plant
generating capacity, 1970**'
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The concentration of S02 emissions in the Northeast has been verified
as indicated in Figure 7.Is!/ The estimated SOX industrial emissions trend
for a 24-si:ate region shown in Figure 8 indicates relatively constant
emissions of about 22.5 million tons per year from 1970 to 1985.!£/ This
amount of !>0X emissions represents 70% of the total U.S. industrial SOX
emissions :cor 1970.
Particulate emissions in the 24-state region totaled 11,013,794
tons in 1972 when the total particulate emissions in the U.S. were
19,789,748 tons.— Particulate emissions from electric utilities alone
in the 24-state region totaled 2,287,048 tons. This emission level repre-
sents 68% of the total U.S. particulate emission from electric utilities.—'
It is clear from these values that the 24-state region of the United States
identified in Figure 8 has a higher burden of both particulates and sul-
fur oxides in terms of mass emissions per year.
AMBIENT POLLUTANT LEVELS AND TRENDS
Mass emissions from industrial sources in specific geographical
regions are an indicator of possible pollution problems in a region
or city of the United States. More detailed evaluations of the air pol-
lution levels and trends nationally and in regions or cities is made
by direct measurement of the ambient pollutants by networks of air moni-
toring stations. These networks are operated throughout the United States
by cities, regions, and states. The U.S. EPA systems, including the
National Air Sampling Network (NASN) system and the Regional Air Pol-
lution Study (RAPS), as well as the World Meteorological Organization
network are a few examples of such network systems.
Measured levels and trends of ambient levels of sulfur dioxide and . .
sulfates i'or selected urban and nonurban sites are summarized in Table 2.
These data show a significant increase in sulfates in three nonurban
sites. Thd trend comparing 1964 to 1966 and 1970 to 1973 averages for
most northeastern urban sites as noted are downward for both sulfur
dioxide arid sulfates. The composite trends for sulfur dioxide and sul-
fates are presented in Figures 9 and 10, respectively. The composite
S02 ambient levels show a cyclic seasonal variation with a slight de-
creasing lirend in maximum and minimum levels. The composite mean urban
sulfate levels are almost constant from 1964 through 1970.
14
-------
Ul
> 20 tons/km2
Figure 7. Nationwide geographic variation in annual SO,
-------
\
Figure 8. 24-State region with high SO emission
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Table 2. COMPARISON OF SULFUR DIOXIDE AND SULFATE TRENDS FOR SELECTED URBAN AND NONURBAN
SITES-^
SUe .locations*
S02,
Average,
1964-1966
Average
vrg, .
1970-19733
% Chanee
Sulfate, Hg/ra
Average,
1964-1966
Average, .
1971-19732
7. Chanee
Urban sites
New York, New York
Glassboro, New Jersey
Jersey City, New Jersey
Newark, New Jersey
Philadelphia, Pennsylvania
Charleston, West Virginia
Pittsburgh, Pennsylvania
Youngstown, Ohio
Cincinnati, Ohio
Toledo, Ohio
Detroit, Michigan
Indianapolis, Indiana
East Chicago, Indiana
Chicago, Illinois
St. Louis, Missouri
Nonurban sites
Shenandoah National Park, Virginia
Clarion Company, Pennsylvania
Parke Company, Indiana
411—'
NA-
NA
174
NA ,
21-2
90
62
NA
NA ,
13,
lOS^7
NA .
86^
45—.
ur.
6«*/
13s
56
28
26
13
30
20
58s'
79
32
-89.1
-
-95.4
-28.6
-37.8
-54.8
-
-
+130
-64.9
-44.8
-
-62.8
26.9
9.3
22.2s
16.4
23.3
25.6
14.3
13.3
13.7
12.0
15.3
14.0
17.7
15.5
15.6
18.3*.
11.0s.
15.8s,
13.4s7
14.9
17.9
19.4
15.4
11.7
13.5
14.7
12.1
19.1
15.4
13.5
-31.2
+18.3
-28.8
-18.3
-36.1
-30.1
+35.4
+15.8
-14.6
+12.5
- 3.9
-13.6
+ 7.9
- 0.6
-13.5
NA
NA
NA
NA
NA
NA
5 <
8.
9.2
11.0
13.6
+55.9
+32.5
+70.0
_a/ Sulfur dioxide averages are computed for 3 years within the 4-year time span. Qualitative trends "are defined
b/
£/
d/
e/
*
over several sub-intervals and are based on statistically significant changes in geometric mean concentrations."
Some 1-year averages do not include enough samples to meet usual criteria for a valid sampling year. Their use
for calculating 3-year means is not a significant problem.
Two-year average.
NA = no data available.
One-year average.
Adapted from Reference 12, including only those urban/nonurban sites within the 24-state area described
in Figure 8.
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00
200
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03
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Seasonal
100
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I
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1964 1965 1966 1967 1968 1969 1970
YEAR
Figure 9. NASN urban sulfur dioxide trends
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CO
15
10
O 5
CO
«.„_. Long Term
Seasonal
1957 1958 1959
1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970
YEAR
Figure 10. NASN urban and nonurban sulfate trends
-------
The ambient sulfate concentration levels for 1970 are mapped for
the United States urban areas in Figure 11 and for nonurban areas in
Figure 12. Both of these maps clearly show that higher concentration
levels exist predominately in the northeastern region of the United
States. A composite mean of the nonurban ambient sulfate levels for
the Northeast in Figure 13 shows a significant increasing trend from
6.5 Hg/tn? j.n 1965 to 10 iig/irP in 1972 as opposed to the nationwide trend
shown in Figure 10.
Other ambient pollutants which may contribute to acid rain formation
include nit.rogen oxides and the metallic oxides in emitted particulates.
Measured data for N0£ are presented in Figure 14 and give an indication
of the magnitude of the N02 ambient level and trend. The data in Figure
14 are composite values of NC>2 for seven cities in the northeastern
United Stal:es and are based on tabulated data published from measurements
at the National Air Sampling Network sites for 1967 through 1969.!£/ Due
to inherent: limitations in the measurement techniques used at that time,
data should be used with caution. The mean data and maximum and minimum
range values as presented in Figure 14 all show a decrease with a mean
decrease o:: 42% in N02 24-hr average concentration from 1967 to 1969.
Ambient total suspended particulate concentrations have been re-
ported for the U.S. and for the northeastern region for the 4-year period
from 1970 i:o 1973.Z^/ These data which are shown in Figure 15 exhibit a
decreasing trend, both for the northeastern region as well as the nation.
Ambient levels of trace metals have also been measured. Some metals
which are present in the atmosphere and their concentrations are listed
in Table 3.—' Unfortunately, trend data for these pollutants are ap-
parently lacking. These metals may be important as catalysts promoting
the formation of sulfates and resultant acid rain. One specific element,
vanadium, is present due to the combustion of coal and even more from
the combus:ion of fuel oil. Vanadium can play an important role as a
catalyst for converting S02 to sulfates. Relative amounts of metals
present in particulate emissions and their concentration as a function
of particle size are discussed in more detail in a subsequent section.
20
-------
A URBAN SITE
• NONURBANSITE
7.0-13.0 M9/m3
£3 >13.0
3/
Figure 11. 1970 Annual urban sulfate concentrations-
21
-------
Figure 12. 1970 Annual nonurban average sulfate concentracion^
3/
22
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CO
GO
LU
$
LL.
_I
D
co
11
10
9
8
5
0
I I | I
65 66
3-Year Mean
Annual Mean
67 68
YEAR
69 70 71 72
Figure 13. Nonurban sulfate trends in the northeast (8 sites
121
23
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Z)
O
x
4
CN
uu
o
UJ
300
200
100
0
Note: Values based on NASN data.
I I
1966
1967
1968
YEAR
1969
1970
Figure 14. Composite average 24-hr NC>2 ambient concentration
for seven northeastern cities, 1967-1969
-------
CO
z
o
(J
o
(J
-------
Table 3. AEROSOL METAL CONCENTRATIONS AT VARIOUS LOCATIONS (u-g/m"" )±=S/
Location and vear(s) sampled
Many U.S. urban stations
(1964-1965) (4)
Northwest Indiana, 25 stations
(June 11-12, 1969) (5)
Columbia, Missouri (1971) (6)
San Francisco, 9 stations
(July 23, 1970) (7)
Corvallis, Oregon, rural
(1972) (8)
Windward Hawaii (1967) (9)
North Atlantic sites
(1970-1972) (10)
South Pole (1970) (11)
Chadron, Nebraska (1973)
Js. M.
< 1-5 1,375-2,600
1,520
0.05-0.2 250-2,000
450
28
0.6
0.15 535
Cd Co
2 < 0.5
0.47-2.6
0.86
0.46-1.7
< 0. 1
0.003-0.62
0.8
0.57 3.3
Cu
90
26-4,000
27-100
< 35
55
0.12-10
36
5.3
Mn JPb jrl
100 790
63-390 400-3,700
30
5-34
2.7
0.10-64
10 0.6
5.7 45 0.22
Zn
670
100-1,540
56
27-500.
30
16
-------
SUMMARY OF PARTICULATE, S02, NOX TREND DATA
Estimated particulate mass emissions from industrial and combustion
sources, on a nationwide basis, appear to have been constant from 1950
to 1968. Beyond 1968, the trend appears to rise until the year 2000,
assuming no additional controls. With best possible controls, these
emissions show a decreasing trend (see Figure 2). There is a high con-
centration of particulate mass emissions in the northeastern United
States. Data for 1972 indicate that 56% of the total U.S. particulate
emissions are emitted in a 24-state region which is to the east of
Illinois and to the north of the Tennessee-Mississippi border. However,
ambient levels of particulates show a decreasing trend both for the
northeast as well as the entire U»S« during the years 1970-1973.
Nationwide, SC>2 mass emissions have been increasing since 1950.
This trend is expected to continue until 1990 assuming no additional
control and 5% industry growth. However, if new source performance stan-
dards (NSPS) and state implementation plans (SIP) become effective in
1978, then there will be a drop in mass emissions before 1980 (see Fig-
ure 1). NASN data for urban ambient concentrations of SC>2 show a cyclic
but decreasing trend since 1964.
Northeastern United States has a high concentration of SC>2 emis-
sions. Also, the ambient sulfate concentrations are highest for this
region both in urban and nonurban areas. However, data for the periods
1964 to 1966 and 1970 to 1973 indicate a downward trend for ambient
levels of S02 and sulfates for selected urban areas in the Northeast.
For nonurban areas in the Northeast, the ambient sulfate concentration
has been increasing from about 7 M.g/m? in 1968 to 10 M/g/nP in 1972.
Trend data for average 24-hr concentrations of N02 are apparently
limited. During 1967 to 1969 the average concentration has decreased
from about 220 ug/nr3 to about 160 n
27
-------
Electric utility plants are predominantly located in the north-
eastern section of the United States. In 1972, particulate emissions
from these plants represented 68% of the total U.S. particulate emis-
sions frcm electric utilities. Also, this region has limited access
to low sulfur coal. Thus if coal-fired power plants continue to be lo-
cated in the northeast region of the United States, adverse impacts on
pollutant trends will occur unless more effective controls are adopted.
ACID RAIN TREND
The acidity of rain has been of concern in Europe since the early
1950*s, ••"•'• when a network of stations for observing the chemical
composition of air and precipitation was established. Analysis of the
network c.ata indicate that the main cause for acidification of precipi-
tation in Europe is the increasing use of fossil fuels. It is also re-
ported that large amounts of sulfuric acid can be transported as far
as a few thousand kilometers..^' A more permanent monitoring system
is currently being established for measuring ambient air pollutants
in Europe under the joint sponsorship of the Economic Commission for
Europe--UN, the World Meteorological Organization and the GEMS program
of the United Nations Environment Program.^'
In contrast to the European interest, most of the major efforts
in studying acid rain in the U.S. are of a more recent origin.^Zz^S'
Cogbill and Likens report,_/ based on earlier papers on the alkalinity
of precipitation in central New York and Tennessee and the acidity of
precipitation in Boston, that acid precipitation has been occurring in
the northeastern United States as early as 1952. The annual average pH
of precipitation for the eastern U.S. during the period 1955 through
1956 is fihown in Figure 16.—' Similarly, using prior chemical data
reported for the U.S. during 1960 through 1966, for North Carolina and
Virginia during 1962 and 1963 and for New England and New York in 1965
through !.966, Cogbill and Likens have predicted pH of precipitation
for the northeastern U.S. during 1965 through 1966..2I' These data are
illustrated in Figure 17. Comparison of Figures 16 and 17 indicates
the same basic regional distribution of acid precipitation in 1965
through !L966 as that observed 10 years earlier. The distribution for
1965 through 1966 shows an enlarged segment particularly in the north-
western ,md southern portions of this region. Also, Figure 17 shows
intensified acidification in New York and New England. Furthermore,
Cogbill and Likens report that the predicted pH for central New York
state was 4.44 during 1955 through 1956 compared to a pH of 4.05 in
1973. It is important to note that the predictions made by Cogbill and
Likens are based on the existence of a stoichiometric relationship for
the major chemical ions in precipitation and a high degree of correlation
(r = 0.95) exists between their predicted values and actual measurements
on rain samples.
28
-------
1955-1956
5.42
5.60
6.00
Figure 16. Predicted pH of precipitation over the eastern
United States during the period 1955-1956.
Isolines are spaced at intervals of 10
T-l /
microequivalents of H+ per liter.—'
29
-------
0 50 100 200 300
31/
Figure 17. Rainfall acidity, 1965-1966 (pH units)1"1
30
-------
30/
Likens and BormanrT"" conclude that precipitation falling in the
northeastern United States during the past 11 years is significantly
more acidic than elsewhere in the U.S. They report that the annual mean
pH of precipitation based upon samples collected weekly during 1970
and 1971 and weighted proportionately to the amount of water and pH
during each period of precipitation was 4.03 at the Hubbard Brook Ex-
perimental Forest in New Hampshire; 3.98 at Ithaca, New York; 3.91 at
Aurora, New York; and 4.02 at Geneva, New York. Also, measurements on
individual rain storms frequently showed values between pH 3 and 4 at
*3 A /
all of these locations.—
The preceding discussion on acid rain trends is based on the limited
information that is presently documented. It appears that precipitation
in the northeastern United States has become increasingly acidic over
the years and further investigation towards a better understanding of
the problem, probable causes, and possible solutions is warranted.
31
-------
SIZE AND COMPOSITION OF PARTICULATE EMISSIONS
It i s important to evaluate the size and composition of particulate
emissions; since they can contribute to the formation of acid rain by
acting afi catalysts or as adsorption media for 303. These mechanisms
and their approximate reaction rates are described in the next section.
This discussion will primarily address the size and composition of fly
ash partJ.culates from coal-fired power plants along with information
on trace element concentrations in air.
PARTICLE SIZE DISTRIBUTION
Par1:icle size distributions for emissions from coal-fired power
plants a):e readily available in the literature.32"35/ pigure IQ was
obtained from a recent paper and shows the composite mass size distri-
bution o:: particulates collected before and after an electrostatic pre-
cipitator (ESP) at a power plant generating 105 Mw. The stack gas flow
rate was 257,000 dry standard cubic feet per minute, equivalent to 7,273
m /min. 'It is clear from Figure 18 that the mass median diameter for the
outlet particles is significantly lower than that of the inlet, indicating
a higher capture efficiency for the larger particles. Figures 19 and 20
are othe:: sets of particle size data for power plant fly ash obtained
at an ES? inlet and outlet, respectively. The presentation of cumulative
mass in Cerms of particulate concentrations in these figures enables
calculation of concentrations directly for different particle size ranges.
The fractional efficiency curve for the ESP obtained using data from
Figures 19 and 20 is shown in Figure 21.
32
-------
40.0
LJJ
N
LJJ
_l
u
u
Q
O
on
10.0
1.0
Inlet (2 Samples)
MMD = 17/urn
Ave. Cone. = 3,070,000// g/m3
Outlet (2 Samples)
MMD =4.9/urn
Ave. Cone. =9160^g/m3
O
0.1
I
0.01 0.1 1 10 50
CUMULATIVE % MASS < PARTICLE SIZE
90
98
Figure 18. Composite mass size distribution of particulates
collected before and after an electrostatic precipitator
at coal-fired power pla
33
-------
lO.OOOi-
00
Q
N
CO
1,000
LO
CO
CO
CO
100
r)
u
101
0.1
1.0
10
100
PARTICLE DIAMETER, yum
Figure 19. Cumulative particle size data taken at the ESP inlet
using Brink Cascade Irapactors (average of 27 runs).
Particle density =2.6
34
-------
1000
u
00
Q
\
OJ
N
oo1 100
to
z
<
OO
uo
.o—•
oo
10
13
O 140 mw
D100 mw
A 77 mw
0.1
1.0
10
100
PARTICLE DIAMETER ,/um
Figure 20. Cumulative particle size distribution taken at the ESP
outlet using Andersen Mark III Cascade Impactors.
Particle density =2.6 gm/cm3.-^.'
35
-------
u>
o
1
Z
LLJ
a.
U.VI
0, 1
0.5
1
2
5
10
20
30
40
50
60
70
80
90
95
98
99
1
—
_
H A
n
o D
0
—
—
—
—
\—
1 1 1 1 1 1 1 1 1
A
A
A A
D A
0 0 Q °
0 0
1 1 1
r —|
MM
* * Sl s .
D 0
a
o
_o
^
A
•
«
1 1 1' 1 1 1 1 1
A A A 9
A • • " "
!*••**-
> _
g —
0140
D 100
A 77
Mw
Mw
Mw
^-Extrapolated Data
—
Impactors
• 140
• 100
A 77
_
I
0.01
Mw
Mw
Mw
1
0.1
1 II
1 1 M
1.
—
__
1 1 1 1 1 1 1 1
77.77
99 9
99.5
99
98
95
90
80
70
60
50
40
30
20
10
5
2
1
0 10
55
U
z
LU
u
LO-
LL.
LLJ
Z
O
u
LLJ
_I
_l
O
u
PARTICLE DIAMETER, /im
Figure 21. Fractional efficiency of ESP under three boiler load conditions.
377
-------
Typically, mass collection efficiency is lower for the smaller par-
ticles. On a number basis, this means that there are many more smaller
particles being emitted to the atmosphere than larger ones. A simple
comparison of 1-jJ.m particles with 0.01-^im particles shows that there
are 10^ smaller particles for each l-M,m particle, using the r^ dependence
of n.* This feature is also illustrated in Figure 22, which shows mea-
sured outlet particle size distributions for a coal-fired boiler operated
at 140 MW. Using the same concept, the ratio of the surface areas has an
r2 dependence. This ratio is 10^ when the area of a l-|a,m particle is
compared to another of size 0.01 |j,m.
The relative proportion of particulates and S02 emitted to the
atmosphere may also need to be considered since this factor could be
important in evaluating their interaction. However, at this point we
have not been able to establish the importance of this ratio.
PARTICULATE COMPOSITION
In addition to providing surface area for some of the acid rain
forming reactions, particulates also act as catalysts in sulfate form-
ing reactions. Vanadium and cadmium are good examples of cataysts con-
tained in particulates which enhance S02 conversion. Other metals as
oxides including iron, calcium, sodium, magnesium, and lead also react
with S02 to form metal sulfates. Whether or not these metals play an
important role in the formation of acid rain depends on their concen-
tration in the atmosphere and their accessibility within the particles
for reaction with the S02»
Table 4 shows the major elements observed in analyzing 28 samples
of fly ash.ifi.' Ashes 1 through 15 came from western coals, while ashes
16 through 28 were produced from eastern coals. By far the major consti-
tuents are Si02» A^Oo, CaO, and Fe^Og for western coals and Si02, A1203,
and ?&2®3 for eastern coals. Due to higher concentrations of CaO and
Na20, fly ash from western coals is usually more alkaline than eastern
coals. The major elements are reported as oxides in weight percent to
enable conversion to molecular fractions from which molecular percent
oxides and atomic percent cations could be determined.^.'
For particles of radius ro, density p, and numbering no, the mass
is given by 4/3 Trr^ x no x p and for particles of same density
but having radius rp the number n-^ is given by:
or
37
-------
CO
u
o
N
00
Q
LU
S
y
o
5 105
Z
<
3
104
* 103
LU
CO
13
Z
LU
102
Z)
r>
u
0.01
140 Megawatt Tests
Sedimentation
Diameters
PSL
Diameters
0.1
PARTICLE DIAMETER,
1.0
Figure 22
Outlet size distributions (optical and diffusional) at
140 Mw boiler
38
-------
Table 4. CHEMICAL ANALYSES OF FLY
vO
Ash
No.^
1
2
3
4
5
6
7
8
. 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Expressed
Li20
0.04
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.01
0.01
0.01
0.02
0.01
0.01
0.05
0.04
0.05
0.02
0.02
0.01
0.04
0.03
0.07
0.05
0.04
0.05
0.06
Na20
0.09
0.24
0.25
0.34
0.32
0.53
1.30
1.07
1.77
3.10
2.10
2.40
0.38
1.47
1.84
0.48
0.49
0.32
0.23
0.51
0.35
0.33
0.38
0.40
0.16
0.51
1.55
0.42
K20
0.30
0.89
0.89
0.77
0.90
1.10
0.72
0.70
1.13
0.80
1.00
1.00
2.36
0.68
0.20
2.75
3.12
2.30
2.80
2.80
2.36
3.88
3.34
3.10
2.60
3.80
2.80
3.70
MgO
5.40
3.30
1.88
1.60
1.80
1.70
2.50
2.23
1.93
0.90
0.99
0.93
2.59
1.73
12.75
0.88
1.09
0.98
0.86
0.93
1.66
1.57
1.29
1.20
0.93
1.30
1.15
1.30
CaO
21.50
23.50
11.10
11.60
12.70
12.20
8.60
8.30
6.36
10.40
12.30
13.60
11.41
7.33
31.00
0.87
2.48
4.60
2.20
0.26
3.72
0.77
1.04
1.40
0.87
0.68
2.50
1.10
in weight percent as oxides
Fe^
4.30
5.53
3.71
3.60
3.90
4.20
4.65
4.66
4.61
3.40
4.60
4.80
6.03
5.33
11.20
5.50
13.24
23.70
23.00
17.90
16.10
10.01
9.70
8.80
8.40
4.90
3.85
9.90
A1203
22.80
21.20
23.60
25.30
25.80
25.10
18.20
17.70
24.60
28.20
26.50
26.20
19.64
23.10
14.80
27.80
26.40
21.20
21.00
21.90
17.80
27.50
25.90
29.00
31.00
30.20
25.00
30.30
Si02
40.15
40.50
55.60
56.80
55.00
55.10
59.00
61.00
53.70
52.00
52.00
49.20
55.29
53.10
22.00
50.45
50.70
42.70
47.70
51.00
43.30
51.40
29.90
52.40
55.10
53.00
56.75
52.40
Ti02
1.30
2.13
1.56
0.83
0.89
0.83
2.20
1.53
1.49
0.88
0.93
0.93
0.95
4.17
0.60
1.85
1.62
1.40
1.80
1.40
1.27
1.79
1.98
1.70
2.30
2.00
1.55
1.70
^ 5
0.34
0.54
0.14
0.17
0.17
0.18
0.19
0.16
1.06
0.18
0.21
0.24
0.60
0.90
0.39
0.23
0.28
0.34
0.52
0.33
0.36
0.32
0.32
0.40
0.36
0.17
0.28
0.51
so2k/
1.70
1.83
0.32
0.17
0.32
0.24
1.05
0.77
0.79
0.26
0.36
0.46
0.58
1.11
4.80
0.70
0.57
0.69
1.30
0.96
0.64
0.37
0.42
0.29
0.23
0.36
1.12
0.45
L01
0.33
0.20
0.74
0.60
0.65
0.60
0.45
0.50
1.49
0.45
0.40
0.45
0.35
0.44
0.41
7.70
1.00
5.50
21.0
1.80
10.30
1.50
4.40
3.20
1.40
2.50
1.70
1.30
aj Ash Nos. 1-15 are for western coals and 16-28 are for eastern coals.
b/ Total sulfur in ash.
-------
39—42/
Several sources report trace element concentrations in fly ash. " ' '
Of these, References 39 and 42 are of special value. Reference 39 indicates
trace metals in fly ash as a function of particle size and also their con-
centration and size in urban air. These are shown in Tables 5 and 6,
respectively.
Table 5 shows that particles 1.5 M/m in diameter contain greater
quantities of cadmium, chromium, manganese, and lead than larger-sized
particles. Higher concentrations of iron are associated with particles
>1.5 (J-m. These data indicate preferential concentration of some elements
as a function of particle size. Metals which tend to concentrate in
the submicron particles can escape capture by most conventional emission
control devices which exhibit relatively poor efficiency in this size
range. Airborne metals found in concentrations of the order of 1 jig/m-*
or greater include iron, lead, zinc, magnesium, copper, and vanadium
as shown in Table 6. All of these are present in particulate emissions
from coal-fired power plants.
A listing of trace element stack emissions from tests at four dif-
/ O /
ferent ccal-fired power plants is shown in Table 7.-t=.' Values in the
table represent fractions in coal, expressed as percent, released to
the atmosphere. Values in Column A were calculated from analyses of
ashes collected from the unit whereas values in Column B were calculated
from analyses of fly ash and flue gas sampled from the stack. A "0"
entry signifies that the calculated value was £ Oj a "-" entry signi-
fies that a numerical value could not be calculated from the experi-
mental df.ta.—' Mass balances reported by groups identified with the
four separate studies were used in calculating, for each element, the
fraction emitted to the atmosphere.—
Table 8 presents measured air concentrations of specific elements
and compares these values with the upper value of expected concentrations
observed in stack emissions from coal-fired systems.— Ambient concen-
trations shown in Table 8 do not take into account elements that may
be present in vapor form, and this could result in erroneous comparisons
for these elements unless appropriate correction is made.
40
-------
Table 5. TRACE METALS IN FLY ASH AS A FUNCTION OF
PARTICLE SIZE12/
a/
Elementr*
Al
B
Be
Cd
Cr
Cu
Fe
Mm
Ni
Pb
V
Concentration,
25 Urn
67,000
300
2
S 5
130
150
40,000
200
300
300
200
12.5 um
54,300
500
1
£ 5
130
150
59,000
240
200
200
200
10 urn
57,300
500
2
£ 5
130
200
43,500
290
200
300
300
ppm
3.5 urn
63,600
500
2
£ 5
300
200
35,500
390
300
300
200
1.5 um
59,300
500
2
100
300
200
32,300
500
300
500
200
_a/ Sample collected from a coal-fired steam power plant and analyzed
by neutron activation and spark source mass spectrometry.
Table 6.
CONCENTRATION AND SIZE OF TRACE METAL
PARTICLES IN URBAN
Metal
Concentrat ion
(ue/m3)
MMDa/
dim)
Particles
< 1 |o,m
Fe
Pb
Zn
Cu
Ni
Mn
V
Cd
Ba
Cr
Sn
Mg
0.6-1.8
0.3-3.2
0.1-1.7
0.05-0.9
0.04-0.11
0.02-0.17
0.06-0.86
0-0.08
0-0.09
0.005-0.31
0-0.09
0.42-7.21
2.35-
0.2-
0.58-
0.87-
0.83-
1.34-
0.35-
1.54-
1.95-
1.5-
0.93-
4.5-
57
43
79
78
67
04
25
1
26
1.9
1.53
7.2
12-35
59-74
14-72
16-61
28-55
13-40
41-72
22-28
20-31
45-74
28-55
17-23
_a/ The mass median diameter (MMD) represents the ap-
proximate "average" aerodynamic particle size, i.e.,
50% of the particles are above this size and 50% are
below.
41
-------
Table 7. TRACE ELEMENT STACK EMISSIONS FROM COAL-FIRED
POWER PLANTS
Element
SI i
AJ;
Ba
Valmontr
14
14
Chalk Point^
A
32
33
6
Widows Creekr
-A- _!_
88
40
44
26
26
Allerf§/
0
8
0
_B_
27
3.4
0.31
B
B::
' 01
Cl
Cr
C3
GJ
F
Fe
Fb
%
Kb
Ki
£e
1e
n
i\n
Ti
li
\'
7n
'.'r
a/
b/
-
-
-
-
-
-
19
-
4.6
28
89
27
-
80
-
-
-
-
-
-
24
3.3
Adapted from Re:
These data were
82
95
35
58
43
97
0
66
19
60
40
12
87
-
-
-
39
11
30
-
0
-
0
0
-
100
2.3
97
1.2
0.64
-
20
43
25
35
4.6
0
13
0
45
0.70
2.5
0.81
56
12
84
26
-
44
69
4.7
32
-
29
30
-
2
0
0
22
-
0.71
0.67
1.4
2.3
cj
&J
_f/
98
10
11
34
0
84
17
52
17
25
23
17
:ained from a 180 Mw pulverized coal-fired unit. The
unit was fitted with an economizer, a mechanical dust collector, and
both an electrostatic precipitator and a wet scrubber which had been
installed in parallel.
These data were obtained from a 355 Mw wet bottom pulverized coal-fired
unit. The unit was fitted with an economizer and an electrostatic pre-
cipitator. The study was not designed particularly to produce a mass
balance.
These data were obtained from a 125 Mw dry bottom pulverized coal-fired
unit fitted with an economizer and a mechanical dust collector. The
unit was not fitted with an electrostatic precipitator.
These data were obtained in a 290 Mw crushed coal-fired cyclone unit
fitted with an economizer and a high efficiency electrostatic pre-
cipitator.
These estimates of trace element emissions apply strictly to pulverized
coal-fired units only. They were made after examining the mass balances
from the Valmont, Chalk Point, and Widows Creek units, all of which
burn pulverized coal, and other trace element studies not involving
mass balances.
42
-------
Table 8. UPPER VALUE OF EXPECTED CONCENTRATIONS
IN POWER PLANT FLUE GASES (UNCONTROLLED)
COMPARED WITH MEASURED AMBIENT AIR
CONCENTRATIONS!/
El ement
Aluminum
Antimony
Arsenic
Barium
Beryllium
Boron
Bromine
Cadmium
Calciumk/
Chlorine
Chromium
Cobalt
Copper
Fluorine
Iron
Lead
Manganese
Mercury
Molybdenum
Nickel
Selenium
Silver
Tellurium
Thallium
Tin
Titanium
Uranium
Vanadium
Yttrium
Zinc
Zirconium
Upper value of
expected concentration
in flue gases for coal
2,000,000
1,000
5,000
10,000
1,000
10,000
5,000
1,000
1,000,000
500,000
5,000
2,000
10,000
10,000
2,500,000
10,000
10,000
100
5,000
5,000
500
100
100
1,000
1,000
100,000
500
5,000
1,000
10,000
10,000
Measured air concentrations
(lig/m3)
0.2-2.2
Avg. 0.001 max. 0.160
Avg. 0.02
0.002-1.5
Avg. 0.0005 max. 0.008
No data
0.0056-1.0
0.002-0.37, 0.001-0.05
0.02-0.39
0.07-2.9
Avg. 0.015 max. 0.33
Avg. < 0.0005 max. 0.060
Avg. 0.09 max. 10.0
No data
Avg. 1.58 max. 22.0
Avg. 0.79 max. 8.50
Avg. 0.10 max. 9.98
0.0001-0.210
Avg. < 0.005-0.78
Avg. 0.032 max. 0.690
0.003-0.009
0.0002-0.004
No data
No data
Avg. 0.02 max. 0.50
Avg. 0.04 max. 1.10
No data
0.1-1.0
No data
Avg. 0.67 max. 58
0.0005-0.08
.a/ Adapted from Reference 43.
b/ Calcium in fly ash is usually present in the combined form primarily
as glassy aggregates which are relatively inert.
43
-------
SUMMARY OF SIZE/COMPOSITION OP PARTIGULATE EMISSIONS
Particle size distribution data for fly ash from coal-fired power
plants show a mass median diameter of about 5 |im at the outlet of the
control device compared to 17 p,m at the inlet, illustrating the fact
that emission controls have a higher collection efficiency for larger
particles. On a number basis, this means that there are many more smaller
particles being emitted to the atmosphere, thus resulting in an increased
probability for atmospheric reactions.
Analysis of fly ash particulate compositions indicates that the
major constituents are Si02, A1203, CaO, and Fe2°3 in the western coals
and Si02» ^203, and ?&2®3 ^n t*ie eastern coals. Trace elements such as
Pb, Cd, V, Se, As, Ni Cr, etc., are also present in fly ash. Furthermore,
these trace materials exhibit pronounced concentration increases with de-
creasing particle sizes.
44
-------
ANALYSIS OF INTERACTIONS BETWEEN PARTICULATE AND
SULFUR-CONTAINING GASES
Particulate-S0x interactions are complex in nature and do not appear
to be fully understood. Three major processes were selected by which par-
ticulates could interact with sulfur-containing gases to lower the pH
of rainfall. Order-of-magnitude calculations for reaction rates for
each process based on existing information for size distribution, chem-
ical composition, physical properties and measured chemical reactivity
of fly ash from coal-fired power plants were attempted. A fourth mech-
anism which can directly oxidize S0£ in the presence of light was also
considered. The four mechanisms and their definitions, as used here,
are listed below.
1. Absorption-oxidation of S02 by particulate,
2. Direct catalytic oxidation of SO by particulate,
3. Adsorption/chemical reaction of SO by particulate, and
4. Photolytic oxidation of SO .
The term "absorption-oxidation" in this discussion refers to catalysis
by particulates in the presence of significant amounts of water so that
S02 solubility, liquid phase diffusion, and droplet size are the key param-
eters in determining the overall S02 conversion rate along with the cata-
lytic activity of fly ash.
Direct catalytic oxidation of S02 by particulate refers to the oxi-
dation of S02 catalyzed by fly ash and other suspended particulates in
the presence of relatively small quantities of adsorbed water. At low
relative humidities up to about 40%, fly ash and other suspended parti-
culate would probably be covered with only a fraction of a monolayer
of adsorbed water. Under these conditions, effects of SOo solubility,
pH and bulk diffusion on the overall reaction rate would be quite dif-
ferent than in the first mechanism defined above.
45
-------
Adscrption/chemical reaction of SC^ by particulate is a removal
process vnlike the two mechanisms defined above which are acid generation
processes. SC>2 removal by this mechanism can occur either as physical
adsorption or as chemisorption.
Photolytic oxidation of SC^ may be defined as SC^ conversion in
the presence of sunlight. This is an acid generation mechanism.
A review of existing rate data disclosed that there were so many
different, forms of reported rate data and different reaction configur-
ations that an order-of-magnitude comparison of any two given experi-
ments was; usually not possible without significant data reduction. The
following discussion includes the method of analysis which was used
in analysing published rate data, and an individual discussion of each
mechanism.
METHOD 01-' ANALYSIS OF PUBLISHED RATE DATA
A simple procedure of recalculating the published data in terms
of "differential" or instantaneous forward reaction rates was adopted.
The data were normalized to unit mass of the catalytic solid, as is
the usual practice in heterogeneous rate studies..-!-!/ It was possible
using thi.s procedure to then directly compare experimental results ob-
tained from both steady-state flow and batch reaction experiments.
The differential reaction rate was calculated from reported experi-
mental data according to the method given by Levenspi el .•=*•*' Two different
equation;; were used, depending on whether a batch (static) or flow reactor
system were used. The molar rates calculated in this manner are directly
comparabi.e regardless of which type of experimental reactor was used.
For a flow reactor, the differential rate is calculated from the following
equation::
r
rSOo
2
AV
46
-------
3
where Tg^ = differential rate of conversion of SC^, mmoles/m -min*
Fgg = molar feed rate of SC^, mmoles/min
AX = X - X.
f i
where Xr and X* are fractions of SC^ converted at points f and i of
the reactor.
3
AV = volume increment (m )
For a static or batch reactor experiment, the reaction time is
equivalent to reactor length in a fixed-bed flow reactor. The differ-
ential molar rate is calculated from the following equation.^'
ro = _ _ (2)
S°2 vAt
where Nq~ = initial number of mmoles of S02, At = tf - t^, and
AX and V have the same representation as that presented
above.
A number of field studies have been made in which concentrations
of SC>2, SOo, and sulfates were measured in the power plant plume. Much
of these data have been summarized in recent EPA studies as part of .
project Midwest Interstate Sulfur Transport and Transformation (MISTT).—
There is wide variation in the rates of SC>2 sulfation observed in these
studies (from about 0 to 50%/hr) and it is difficult to relate the oxi-
dation rate to fundamental rate parameters: temperature, relative humid-
ity, and sunlight intensity.-^' The complexities involved in attempting
to determine SC^ oxidation rates from field data have been discussed
by Hales et al.ll/
Due-, to the above reasons, the analysis of SC>2 oxidation catalyzed
by particulates was limited, as far as possible, to studies performed
under controlled, laboratory conditions.
* mmoles denotes millimoles.
47
-------
ABSORPTION-OXIDATION OF S02 BY PARTICULATE
This phenomenon refers to catalysis by particulate in the presence
of large amounts of water and is frequently known as "precipitation
scavenging,"—' i.e., oxidation in the liquid phase in cloud droplets,
or during periods of actual rainfall. However, kinetics derived from
this assutied mechanism have also been used to predict sulfation rates
in power plant plumes during periods of moderate relative humidity,
as shown in the recent analytical study by Freiburg.-^
There is direct evidence for the activity of fly ash from coal-
fired plants in catalyzing the oxidation of SC>2 to SOj in the liquid
phase.—' The fly ash samples tested in the referenced study included:
(a) a sample of aged NBS fly ash (from Potomac Electric Power Company),
and (b) samples of fly ash collected from the pulverized coal-fired
Salem Harbor Station. Catalytic activities for S02 oxidation in aqueous
solution were found to be comparable for the various fly ash samples
tested, regardless of aging. The initial activity of fly ash was approxi-
mately 11.1% of the activity of ferrous chloride as originally determined
by Junge and RyaruLP.' and later reconfirmed by GGA.A2/ The results are
depicted in Figure 23.
In I'igure 23, the solution pH was measured in the bulk aqueous
phase using conventional pH electrodes (i.e., by measurement of bulk
solution conductivity). Total solution volume was approximately 30 cc.
As indicated in the figure, the water-to-fly ash ratio was approximately
167 litej.-s/g.
The mechanism by which gaseous S02 can be converted to sulfate in
droplets was first reported by Junge and Ryan.^2/ They described the
oxidation of S02 in solution as nonphotolytic in nature and strongly
inhibited by hydrogen ions. They measured S02 conversion rates in aqueous
solutions of MnCl2» CuCl2» FeC^* CoC^, NaOH, NaCl, and distilled water.
HCl or citric acid was used for pH adjustment. In a recent studylS' the
activity of FeC^ was measured by an experimental technique similar to
that of Junge and Ryan. The results of the earlier study were confirmed
and the activity of several fly ash samples for oxidation of dissolved
S02 was also determined. A summary of differential rate data recalculated
from data reported by several investigators is presented in Table 9.
48
-------
4.0 3.0 2.5
PH
2.4 2.2
2.1
2.0
400
E
\
O)
CN
cT
LO
200
T
T
FeCI2, 1 mg/l
(17 ppm SO2)
•~~ Junge and Ryan
(4.2 ppm SO2«
FeCI2, 1 mg/l)
NBS Fly Ash (6 mg/l)
O No Catalyst
I
TIME (hrs)
Figure 23. SO aqueous oxidation rate
49 /
49
-------
Table.9. SUMMARY OF PUBLISHED RATE DATA FOR OXIDATION OF
DISSOLVED SO (ABSORPTION/OXIDATION MECHANISM)
Metal ion
(or metal salt)
MnSO y
MnSO<£/
J^ QC_/
H,0£{
c/
MnCl,
CuClj
FeCl2
CoCl2
NH^OH
Nad
H,0
Fa?*
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
Fe3*
Fe3+
Fe3+
Fe3+
Fe3+
Fe*
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
F*3+
Fe3+
F 95%.
zl Distilled water droplets (400-900 p.) at 94Z relative humidity.
A_l pH decreased from 7 to about 2 during the reaction.
el A dilute mixl ure of SCL in air was bubbled into a 30-cc volume of distilled water and catalyst.
l_l Vapor-phase concentration of S02 was not determined; concentration in the liquid phase varied from 1.0-1.5 moles/m .
£/ Reaction temperature was not reported; however, it is obvious from the discussion that the experiment was performed
under laboratory ambient conditions. _
h/ Differential conversion rates estimated from concentrations of SO^ measured after 3-hr reaction time.
ND = Not determined.
50
-------
From Table 9, it is evident that oxidation rates for SOo in bulk
aqueous solution are at least three orders-of-magnitude faster in the
presence of most soluble salts and undissolved fly ash than in distilled
water. Ammonium hydroxide also has a significant influence in accelerating
the oxidation rate. The oxidation rate is highly dependent upon solution
pHj however, there is insufficient information to permit a direct com-
parison between any two salts at this time. The overall range of values
is probably representative of the expected range for the catalytic ma-
terials for which quantitative data could be found. Comparative rate
data for SC^ oxidation catalyzed by particulate in the absence of a
bulk HoO phase are presented next.
DIRECT CATALYTIC OXIDATION OF S02 BY PARTICULATE
This mechanism refers to S02 oxidation catalyzed by fly ash and
other suspended particulates in the presence of small quantities of
adsorbed water as opposed to absorption/oxidation discussed above which
is in the presence of large amounts of water.
54/
An order-of-magnitude calculation made using the BET theory1^
indicates that at 25 C, fly ash and other suspended particulates would
be covered with less than a monolayer of water at relative humidities
up to 60 to 70%. At 90% relative humidity, the predicted coverage is
approximately five monolayers. One other sourcejl^/ estimates that at
27 C, monolayer coverage would occur at approximately 40% of saturation.
There were relatively few data on the rate of S02 oxidation under
these conditions even though the data, when converted to differential
conversion rates, were typically higher than for oxidation of dissolved
S02 by one or two orders-of-magnitude on a unit weight basis. Comparative
rate data for a number of cations commonly found in coal fly ash as
either minor or trace elements are presented in Table 10.
Rate data included in Table 10 are based only on reported data
in which particulates were heated to combustion temperature prior to
testing. Rate measurements included were either made at steady state
or corrected for the anomalously high initial activity which results
when a large ratio of particulate to S02 is used in the measurements.
Measurements of S02 oxidation in the presence of various particu-
lates have been reported by Urone et al*J&' They were not included here
because (a) it was not possible to distinguish between initial and equili-
brium rates, and (b) it was not possible to determine what fraction of
the S02 concentration decrease resulted from S02 chemisorbed on the
51
-------
Table 10. SUMMARY OF PERTINENT S02 OXIDATION RATE DATA FOR
CATALYSIS BY METAL SALTS AND OXIDES
'Ni
Total
catalyst
weight
Catalyst (g)— /
MnS04d-/ 15 e 5
MnCl2d/ 6.375
CuSO^/ h/
NaCll/ h/
Sn02£/ 59.6
BaO6-' 58.4
Gr2°3-/' 62'8
BaO + SnQ§/ 59.2
Cr00o + BaO6-' 56.0
v n e f/ Q 9
V n Hi- C « •*- / 7 » ^
v2°5f»g/ 8»8
weight
oxide/salt
(mg)Ii/
0.51
0.255
0.15
0.36
4,768
4,672
5,024
4,736
4,480
736
704
so2
concencrac ion
(influent)
(ppm)
3.3
3.3
14.4
14.4
3,058
3,058
3,058
3,058
3,058
3,058
4,329
Effective
1-tictC.l- J.U11
volume
(cc)
i/
23. 9r.
23 <£f
i/
78 -2T/
78 2—
101.l45
99.145
106.54,
100.445
95.o45
15.645
14.91
Feed rate
(mmole/min)
h/ 6
6.24 x 10"
h/
h/
0.312
0.307
0.288
0.290
0.288
0.277
0.215
Conversion
(%)
36.5
5.2
6.8
5.2
19.2
19.2
17.1
14.6
14.0
2.1
0.6
Differential
****** " — ~ — — — —
rate .
3 c/
(nmole/min-m -g)"~
k/
185.4-'
53.2
36 •5W
15.2^X
124.3
127.4
91.9
89.2
94.9
506.0
123.3
I/
y
i/
i/
k/
Weight of catalyst and support.
Weight of active metal salt/oxide. __
For definition, see Eq. (1) and (2).
Volume per unit time (molar feed volume times fractional conversion), mmole/min. ,
Hydrous aerosols supported by Teflon beads. Reaction conditions--23°C, 740 mm Hg, relative humidity = 95%.
Metal oxides supported on Fuller's Earth; conditions—-23.9°C, 1 atm, S02 = 2.28 torr (0.37., by volume), O2 =
21.28 torr (2.8% by volume), N2 = 377.89 torr (82815% by volume), C02 = 111.72^torr (14.70% by volume),
NOX = 0.38 torr (0.05% by volume), and feed concentration = 0.125 graoles/m , "
Vanadium oxide promoted with 20% Se02.
Reaction conditions are the same as described in Note e, except 148.9°C.
Data either not determined or not reported.
Calculated as the product of the mean residence time times the gas flow rate.
Assuming a packed density of 36.816 lb/ft3.
Estimated from reported first-order rate constants.
-------
surface of the particulate investigated. For example, high conversion
rates were reported in the presence of alumina and calcium aluminum
oxide particulates. In the case of alumina, the initial reaction in-
volves reaction of SC^ with the particulate itself, rather than for-
mation of gaseous 803. Initial products are solid sulfates, sulfites,
and sulfides which remain bonded to the solid surface up to very high
temperatures.~' These factors have apparently not been considered
in Reference 58.
Although not usable for quantitative purposes, the study of Urone
et al.—' yields some extremely useful qualitative results. Signifi-
cant oxidation rates were measured for Fe20-j, (^03, PbO, PbC^, and
^2^5* ^ was also found that at an intensity of ultraviolet light
approximately seven times greater than expected under ambient condi-
tions, there was relatively little enhancement of reaction rates over
any of the particulates tested. In contrast to photolytic oxidation,
Urone et al., reported relatively little effect of NC^, hydrocarbons, ,
and relative humidity on the oxidation rates catalyzed by particulates.
ADSORPTION BY PARTICULATE (ACID REMOVAL PROCESSES) ,
Relatively little data are available which are directly applicable
to acid removal processes. The laboratory studies by Bienstock, Field,
and Meyers are pertinent in that they attempted to study the rate and
mechanism of S02 adsorption from simulated flue gases."u»ni/ Their simu-
lated flue gas was a mixture of gases--0.3% sulfur dioxide, 13% carbon
dioxide, 6% oxygen, 6.7% water vapor, and 74% nitrogen. This gas mixture
was pumped through fixed beds of oxides of magnesium, cobalt, and cop-
per; alkalized alumina; and activated charcoal. They found that sulfur
dioxide was more efficiently removed from the mixture at 330 C than at
130 C. Adsorption increased with temperature and water vapor.^2l2i/
The preceding brief discussion indicates that particulates may
function as adsorbents in removing S02 from the atmosphere. Regardless
of whether the S02, §03, sulfites, and sulfates are irreversibly removed
from the atmosphere in the form of insoluble, chemisorbed species, the
maximum removal capacity of fly ash is determined by the monolayer ad-
sorption capacity. Order-of-magnitude calculations to determine the
monolayer adsorption capacity and fractional removal capacity of fly
ash as a function of particle size, geometric surface area, and probable
discharge rates of fly ash and SC^ are presented next. The discussion
assumes that none of the fly ash can serve to neutralize acidic species
by dissolving in rainwater.
53
-------
The maximum acid removal capacity of fly ash by chemisorption is
based on monolayer adsorption capacity which is directly proportional
to the specific surface area5 in area per unit weight.^ The predicted
variation in surface area and adsorption capacity with particle size
is depict. ed in Table 11» The particle size distribution is for a tangen
tially fired, pulverized coal-fired boiler.^2,' 'In Table 11, specific
fly ash £,urface area is calculated using an assumed particle density
of 2.3 g/cc and the geometric surface area, calculated according to
the following widely used equation.^2'
S = 6/pd
2
where S = specific surface area, m /g
3
p = particle density, g/cm
d = particle diameter, |j,m
The specific surface area calculated using Eq. (3) has been compared
with experimentally determined surface area data for submicron aerosols
obtained using both BET and electron microscope methods and is found
to yield excellent agreement Jl2'
The chemisorption capacity is calculated by assuming monolayer ad-
sorption capacity and hexagonal close packing of SO^ (as H2SO,) in the
adsorbed surface layer. The total monolayer capacity of the selected
fly ash Ls approximately 4.4 micromoles of I^SO^ (or 503) per gram of
fly ash. By comparison, complete combustion of a 4% sulfur coal con-
taining L0% ash by suspension-firing generates approximately 14.7 x 10
micromoles of l^SO^ (or 803) per gram of fly ash. Typical data for coal-
fired plants (i.e., less than 4% sulfur) would indicate that the average
coal-fired plant generates about 0.58 g of sulfur compounds per gram
of fly ash with a mass ratio of S02:S03 of 53.0 (30 ppm SO-).-5-5-/ On
this basis, the ratio of total sulfur compounds in the flue gas would
be 9,168 micromoles/g, and the ratio of SO, to fly ash would be 136.35
micromoles/ge This latter value exceeds the predicted monolayer capacity
of 4.4 micromoles/g by more than one order-of -magnitude.
On the basis of this information, it is considered unlikely that
fly ash from coal-fired combustion sources can play a significant role
in removing acid sulfates from the atmosphere via chemisorption.
54
-------
Table 11. SURFACE AREA AND MONOLAYER ADSORPTION CAPACITY OF
FLY ASH FOR S03 AS A FUNCTION OF PARTICLE SIZE FOR A
TYPICAL SIZE DISTRIBUTION
01
Median particle
size
Specific particle
Weight surface^/
(Hm) fraction2'
a/
b/
34
9
6
3
1
0
0
0
.64
.80
.00
.45
.61
.83
.43
.16
Source: Ref.
S = 6/pd; S =
0.
0.
0.
0.
0.
0.
0.
0.
0.
6333
1000
0718
0622
0956
0144
0064
0102
9999
62.
surface area,
Total surface
i
(m^/g) (m^/s)
0
0
0
0
1
3
6
16
m /g>
.075
.266
.435
.756
.620
.143
.067
.304
p = particle
0.0475
0.0266
0.0312
0.0470
0.1548
0.0453
0.0388
0.1663
0.5575
density, g/cc;
Adsorption capacity
for S03 (as H2S04)
(a
0.
0.
0.
0.
1.
0.
0.
1.
4.
mole/s)
376
211
247
372
227
358
308
317
416
x
x
X
X
X
X
X
X
X
d = particle
io-6
10"
10"
10"
10"
10
io:6
10 °
-6
10
di-
ameter, |J-m (Reference: Hillenbrand, L. J., "Chemical Composition of Particu-
late Air Pollutants, EPA-R2-73-216, March 1973).
£/ Assumed particle density for coal fly ash was 2.3 g/cc.
-------
PHOTOLYTI3 OXIDATION OF S02
In order to place the measured rates of S02 conversion by parti-
culates into perspective, a comparison was made with reported photolytic
reaction rate data contained in the literature. Table 12 summarizes
reported rates of photolytic oxidation of SO including the rate cal-
culated in this study.
The maximum expected conversion rate was calculated using reported
solar intensity and quantum yield data with assumed values for mixing
height and light fraction absorbed. Urone et al.Z2' report a maximum
intensity of 7.70 x 1014 photons/cm2-sec for the 3100 to 4000 A" region,
and Sethi2L/ reports a quantum yield of 2.63 x 10" 3 molecules/photon.
If 50% of the incident light is absorbed and the mixing height is 1,000
m, then the conversion rate is 1.01 x 10"" mmole/min-m . Conversion
rates reported in Table 12 are for strict photolytic reactions and can
vary if the reactions take place in the presence of hydrocarbons and
other photochemical oxidants.^2'
COMPARISON OF PARTICULATE-SOX INTERACTIONS
Catalytic data summarized in Tables 9 and 10 can be used to predict
S02 oxidation rates in the presence of fly ash particulates provided the
composition ranges of the catalytically active elements are known. As
the rate data in Tables 9 and 10 indicate, there are a large number of
elements which may be active as catalysts in the form of oxides, chlorides,
or sulfai:es and their catalytic activities can vary by over an order of
magnitude. Also, for conditions shown in Tables 9 and 10, the gas phase
oxidation rate is higher than the oxidation rate for dissolved S02« This
result i 3 pertinent to the analysis of sulfation occurring at or near
ground 1=vel where ambient temperatures are higher and relative humidities
lower than that observed in clouds.
Usiig rate data contained in Table 10 for different catalysts, the
maximum axpected SC^ oxidation rates in a typical power plant stack have
been calculated. These values are shown in Table 13. A stack temper-
ature of 149°C (300°F) and a residence time of 5.9 sec (250,000 cfm,
14-ft stack diameter and 160-ft stack height) were assumed in the calcu-
lation The expected maximum concentrations for the catalysts indicated
in Table 13 were selected, at random, to include 90% of the reported
data.—-' Coal composition data were compiled from approximately 20
literature reports.
, 56
-------
Table 12. COMPARISON OF SOME S02 PHOTOCHEMICAL
REACTION RATE STUDIES
Relative
[S02] humidity
(ppm)
Rate of SO,
/
conversion
5-30
0.2-0.6
10-20
y
1,000
1,000
10-50
32-91 (
50
50
0
0
50
c/
(1.7-3.3) x 10
0.11
1.4 x 10
-3
-3
-4
Differential
conversion rate
O
(mmole/min-m )
(3.47-40.5) x 10
(9.0-26.9) x 10
(5.72-11.4) x 10
8.0 x 10 . (24,000-98,000) x 10
3.9 x 10
4.6 x 10
c/
-4
-4
159.4 x 10
188.0 x 10
-6
-6
-6
-6
-6
-6
Reference
64
65
66
67
68
68
1.01 x 10-6 This study
£/ Recalculated based on reported data by adjusting experimental condi-
tions to the maximum expected UV light intensities under ambient
conditions. (Data taken from Ref. 69.)
b_/ Sulfur dioxide concentrations were reported in terms of partial pres-
sures—56-230 mm Hg.
c/ Not determined.
57
-------
Table !.3. ESTIMATED MAXIMUM RATES OF S02 OXIDATION IN FLUE GAS
CATALYZED BY COMPONENTS PRESENT IN FLY ASH
Range of
concentrations
in coal
Catalyst
Sn02
BaO
Cr2°3
V205
Expected
maximum
concentration
in flue gas
Differential
reaction rate
—' (ug/Nm3)-i (mmole/g-sec-m3)
Calculated
maximum S02
conversion
in stack
0-404 1,000
0-3,000 10,000
0.89-1,690 5,000
0.38-500 5,000
1-2,000 10,000
0.02-800 10,000
30.29-^
31.04*'
22.39-^
123.3
1.3
13.5
4.9
26.8
19.6
3.9
aj Based on 10.0 Nm3/kg (coalj assuming 100% of coal ash is converted
to fly ash} upper values for metal concentrations in coal include
approximately 90% of reported data.
b/ Conveision rate data from Table 10 adjusted for flue gas temperature
"" of 149°C (300°F) and for time in seconds.
cj Basis: 4% sulfur in coalj 10.0 Nm3 flue gas per kilogram coal; stack
"" gas temperature 149°C (300°F)J stack residence time, 5.9 sec.
58
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It is evident from Table 13 that there are a number of components
in fly ash which are sufficiently active to convert up to 27% of the
S02 to sulfate (or SO^) in the stack at or near their maximum concen-
trations. It is unlikely that the maximum concentration of more than
one active metal would occur in the same coal sample so that the highest
conversion level of 26.8% (for vanadium pentoxide) will probably never
be exceeded under actual conditons. One report claims that the average
S02 conversion in flue gas from coal-fired plants is only about 1.5
Table 14 presents an order-of-magnitude comparison between catalytic
(i.e., gas phase) and photolytic reaction rates shown in Tables 10 and
12, respectively. Catalytic rates listed correspond to maximum reported
particulate concentrations. Zero-order kinetics were assumed for the
rate dependence on SC^ partial pressure, i.e., no dilution effect was
considered. The catalytic rate calculation further assumes (a) first-
order rate dependence on CL, and (b) negligible inhibition by sulfate
products. Metal oxide and/or sulfate concentrations used in Table 14
are based on reported ambient measurements summarized from National
79I
Air Sampling Network Data and other sources.^-='
At the maximum reported ambient concentrations, all of the cata-
lytically active materials for which quantitative rate data could be
found are predicted to exhibit conversion rates higher than for photo-
lytic oxidation. The highest conversion rate was calculated for vanadium
pentoxide (3.61 x 10"^ mmole/min-m^) which is a common component of fly
ash from both coal, and in higher concentrations, from oil-fired power
plants. On the basis of these data, the S02 oxidation rate in the presence
of reported ambient levels of V2®5 exceeds the reported .photolytic oxi-
dation rates (1.01 x 10"" mmole/min-m ) by more than three orders of
magnitude.
53/
Reported residence times,* as summarized by Freiburg5^ range from
about 8 to 120 hr or more. Calculated S02 residence times, as shown in
Table 14, are based on average reported ambient concentrations of catalytic
compounds and range from 67.4 to 380.5 hr if the primary air standard of
80 pig/nm is used as the initial S02 concentration. However, if the maxi-
mum reported ambient levels of catalytic compounds are used, the SC>2
residence time is of the order of a few minutes.
On the basis of data presented here, catalytic oxidation mechanisms
of S02 by trace and minor components in fly ash in the presence of sig-
nificant amounts of water and in the presence of small amounts of water
are both plausible mechanisms by which rapid S02 oxidation could occur.
These could take place at or near ground level and in the absence of
sunlight. There is insufficient quantitative data for either catalytic
Residence time for assumed zero order kinetics is defined as the
ratio of concentration to rate constant,
59
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Table 14. COMPARISON OF PHOTOLYTIC OXIDATION OF S02 WITH
ESTIMATED MAXIMUM RATES OF CATALYTIC OXIDATION
Catalyst
Sn02
BaO
Cr2°3
V205
Photochemical
reported ambient
concent rat ions
in air
(iLK/Nm3)
0.02 (avg.)-O.SO
0.002-1.5
0.015 (avg.)-0.33
0.05-1.0
0.10 (avg.)-9.98
0.09 (avg.)-lO.O
-
Differential
reaction rate
3 a/
(mmole/g-min-m )—
124.3
127.4
91.9
506.0
185.4
36.5
-
Estimated
maximum
atmospheric rate
0 i /
(mmole/min-nr)— '
-4
4.43 x 10
1.36 x 10~
-4
2.16 x 10
3.61 x 10"
1.85 x 10~
-4
3.65 x 10_.-
6j /
..-*. A xv -S'
average SO
residence
time
(hr)-£'
70.0
-
127.0
-
67.4
380.5
1,250.0
.§/ Data from Table 10 (23.8-25 C).
JD/ Maximum rate calculated by using the maximum reported elemental concentrations in
ambient air and assuming (a) first-order rate dependence on oxygen, (b) zero-
order dependence on S02» and (c) no inhibition of reaction rate by SO^.
£/ Calculated from average reported ambient metal concentrations by assuming an initial
S02 concentration in ambient air of 80 ug/Nm3 (1.25 x 10~6 gmole/Ito3).
d/ Data from Table 12.
-------
mechanism presented here to determine the relative importance of oxidation
in solution (e.g., in cloud droplets or in raindrops) versus the oxidation
rate on "dry" particulate ("direct oxidation mechanism"). However, data
presented here indicates that both mechanisms need to be considered in
predictions of SCL oxidation rates in stack plumes. Although a large num-
ber of minor and trace elements can be demonstrated to have significant
catalytic activity for SC>2 oxidation, the list of such elements is far
from complete. The activity of vanadium pentoxide for this reaction is
tentatively concluded to be much higher than for other metals, and this
may have significant implication in terms of fuel substitution strategies.
Finally, additional study is required to determine the probable length
of time fly ash of various size fractions remains suspended and the
relative catalytic reactivity of these different size fractions.
EFFECT OF PARTICULATE EMISSION CONTROL DEVICES ON CONVERSION REACTIONS
Components of fly ash such as Na20, IGjO, and CaO are alkaline in
nature and fly ash which is collected in precipitators is also usually
alkaline with a pH in the range of 6.5 to 10. 5.Z2/ It has been suggested
that the use of emission control devices on coal-fired power plants
causes the removal of these alkaline materials and thereby removes the
"neutralizing" agents in acid-forming reactions in the atmosphere.
30/
Likens and Hermann"" suggested that the use of particulate emis-
sion control systems removes the alkaline substances, consequently per-
mitting appreciable quantities of S0« to be converted to acid. Gordon
et al. have studied the acidity of rainfall around the Chalk Point Power
Plant and report that pH values from sampling stations ranged from 3.0
to 5.7 with modal values between 3.6 and 4.0. — ' Gordon et al. propose
that the acid rain arises from the conversion of S02 and possibly N02
in the plume to I^SO^ and HNO^. They also suggest that these acids are
partially neutralized by species such as NH^ and basic metal oxide
particulates.
At the present time, it is difficult to determine the quantitative
effect of the removal of the alkaline metal oxides on acid rain forma-
tion processes. First, atmospheric reactions are complex in nature and
insufficiently defined to precisely determine whether and to what extent
these alkaline materials would prevent acid-forming reactions or neutra-
lize the acid after it is formed. Secondly, if these alkaline materials
are indeed captured in the control device then it is logical to assume
that they are larger in size than those emitted to the atmosphere. If
this is the case, then with no controls the alkaline materials may not
have a long enough atmospheric residence time to influence reactions
61
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or alternatively, would settle out closer to the power plants due to
their size. The net result would be a decrease in their capability to
"neutralize'1 acid-forming reactions in the atmosphere which are usually
catalyzed by fine particulates which have a concentration of active
trace elements and which can be dispersed over long distances. Also,
coals used in the northeast are, most likely, eastern coals which are
less alkaline than western coals. Therefore, this factor must also be
considered In evaluating the "neutralizing" capacity of fly ash. In
any case, ti.ie collection of any alkaline material by the control device
clearly removes an agent which can neutralize an acid. Whether this
removal results in significant shifts in the atmospheric reactions lead-
ing to acid rain cannot be conclusively documented.
62
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CONCLUSIONS AND RECOMMENDATIONS
This study has examined pollutant emission trends and acid rain
trends in the northeastern United States. Characteristics of particulates
from power plants and interactions of particulates and sulfur-bearing
gases have also been investigated.
Review and analysis of the technical literature confirms that parti-
culate and sulfur oxide emissions are concentrated in the northeastern
section of the United States. Historical trend data, however, do not
show significant increases in these pollutants, which may be a result
of increased emphasis on control of pollutants in recent years. Acid
rain trend data are extremely limited but do seem to support an increase
in acidity in rainwater over much of New England and sections of New
York.
Characterization of particulates from power plants indicates a
mass median diameter of about 5 (J-m for particulates at the control de-
vice outlet. Also, mass collection efficiencies are less than about
85% for 1-l^m particulates and fall off rapidly for fractions in the
submicron region. Chemical characterization of particulate streams shows
the existence of catalytically active materials such as oxides and sul-
fates of iron, vanadium, chromium, manganese, etc. Data also indicate
that some of these materials tend to concentrate in smaller particles
and thus escape capture by most commonly used control systems. All of
this focuses attention on the need to enhance control efficiencies for
particulates in the submicron region and may provide the reason as to
why reduction in mass emissions alone is not adequate to combat the
problem.
Particulate-SOo interactions have been reviewed via four oxidation
mechanisms for acid aerosol generation. Results indicate that oxidation
of S0? catalyzed by active metal oxides in particulates from power plants
both in the presence of significant quantities of t^O and in the absence
of large quantities of H^O may be the more important reactions. S02
removal by adsorption is ruled out in the analysis. Photolytic oxidation
of S02 catalyzed by hydrocarbons and other oxidants is a distinct pos-
sibility and warrants further investigation.
63
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Shifts to fossil fuels other than coal may also make contributions
to the acid rain problem, especially the use of fuel oil containing
vanadium. Vanadium pentoxide, a product from fossil fuel combustion,
has significant catalytic activity for SC>2 oxidation. The emission of
vanadium may work to counter the expected decrease in acid rain resulting
from decieased S02 emissions achieved through fuel switching strategies.
Futv.re studies in this area should include a systematic evaluation
of the various parameters such as catalyst type, concentration, temperature,
relative humidity, etc., which affect the kinetics of SO oxidation in order
to identify the main rate-controlling factors. Presently available experi-
mental di.ta do not consider these variables in a systematic manner and
are not amenable to direct comparison. Acid rain formation from nitrates
and chlorides should be separately investigated even though currently
available information supports the hypothesis that only about 25% of
the acidj.ty in rain is due to nitrates and less than 10% due to chlorides.
Refinement of techniques to predict whether sulfates, nitrates or chlor-
ides are contributory to the acidity in a particular rain sample should
also be undertaken. The incorporation of mathematical modeling to include
transportation effects of pollutants to areas of geographic interest is
also recommended. Finally, meteorological methods which monitor rainfall
in different states and regions of the U.S. should include pH measurement
of rainwater on a routine basis if it is not already being done. This
would eliminate the data gap that presently exists for some sections
of the U. S. and for most sections on a recent basis.
64
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67
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36. Leej R. E., Jr., H. L. Crist, A. E. Riley, and K. E. MacLeod, "Con-
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68
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48. Freiberg, J., "The Iron Catalyzed Oxidation of S02 to Acid Sulfate
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60. Bienstock, D., J. H. Field, and J. H. Meyers, "Process Developments
in Removing Sulfur Dioxide from Hot Flue Gases. 1. Bench-Scale
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Control Assoc., ,21(7):418 (1971).
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Hazardous Pollutant Emissions from Coal- and Oil-Fired Utility
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71
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-7(>-257
2.
3. RECIPIENT'S ACCESSION-NO.
4.TITLE ANDSUBTITLE
PARTICLE EMISSION REACTIVITY
5. REPORT DATE
September 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
K.P. Ananth, J.B. Galeski, and F.I. Honea
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORQANIZATION NAME AND ADDRESS
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1324, Task 44
12. SPONSORING AGEMCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 2-6/76
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES T£RL-RTP task officer for this report is B.C. Drehmel, Mail
Drop 61, 919/549-8411, Ext 2925.
6. ABSTRACT ,
The report gives results of an extensive review and analysis of the literature
aimed at: studying historical trends of particulate, SOx, and NOx emissions and of
acid rain in the northeastern U.S.; studying size and composition of particulates from
power plants; and analyzing interactions between particulates and sulfur-bearing
gases from coal-fired power plants. Particulate mass emissions from industrial and
combustion sources project a rising trend, nationwide, if no additional controls are
assumed. (With application of best controls, the trend projection is reversed.) SO2
emissions have been increasing nationwide since 1950; without additional controls and
with a 5% industry growth, this trend is expected to continue. (If new source perfor-
mance standards and state implementation plans become effective in 1978, SO2 emis-
sions are expected to decrease.) NOx mass emissions have increased consistently
since 1920; those for 1970 were more than 3 times those for 1920. The limited data
available for ambient NOx levels is further complicated by inherent limitations of
measurement techniques used to obtain the data. Information on acid rain in north-
eastern U.S. is extremely scarce. Available data indicates that precipitation in this
region has become increasingly acidic over the years. A better assessment of the
trend can be made only when better rainwater pH information becomes available.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Rain
Acidity
Dust
Sulfur Oxides
Nitrogen Oxides
Catalysis
Electric Power
Plants
Coal
Combustion
Air Pollution Control
Stationary Sources
Acid Rain
Particulate
13B
04A
07D
11G
07B
10B
2 ID
2 IB
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. i
..PAGES
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-7:)
72
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