United States
EnvironmentaLErotection
Agency
Air and Energy Engineering
Research Laboratory
Research Trangle Park, NC 27711
Research and Development
EPA/600/S9-89/089 August 1990
v°/EPA Project Summary
EPA/IFP European
Workshop on the Emission of
Nitrous Oxide from Fossil Fuel
Combustion
Jeffrey V. Ryan and Ravi K. Srivastava
This report summarizes the pro-
ceedings Of an Environmental Pro-
tection Agency (EPA/lnstitut Francais
du Petrole (IPP) cosponsored
workshop addressing direct nitrous
oxide (N2O) emission from fossH-fuel
combustion. The third in a series, the
workshop was held at the IFP in
Rueil-Malmaison, France on June 1-2,
1988.
Increasing atmospheric N2O
concentrations have been linked to
depletion of atmospheric ozone (O3)
and to global climatic warming. The
combustion of fossil fuels has been
identified as a potential major
anthropogenic source of N2O. This
workshop had two goals: (1) to
exchange Information among various
international research and industrial
groups that are involved in N2O
chemistry, modeling, and N2O
measurement; and (2) to develop a
network for coordinating future
related efforts.
The five technical sessions
addressed: stratospheric O3
depletion and global climate change,
mechanisms of N2O formation and
destruction during combustion, N2O
measurment techniques, full-scale
field data, and practical conclusions
based on general discussion.
A sampling artifact discovered
during EPRHundred research
revealed that N2O can be generated
in a sample container in the
presence of nitrogen oxides (NOX),
sulfur dioxide (SO2), and water. This
artifact potentially discredits much of
the N2O emissions data collected
from samples containing the above
compounds when stored for some
time prior to analysis. Recent
sampling techniques that minimize
the artifact have produced data from
stationary sources that indicate that
direct emission of N2O from fossil -
fuel combustion may not be a major
contributor to the measured annual
Increase. Limited data also indicate
that some specific sources (e.g., fluid
bed combustors) may be high N2O
emitters. A standardized sampling
protocol would help validate current
data.
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
This report summarizes activities at a
workshop on direct nitrous oxide N20
emissions from fossil fuel combustion,
held in Rueil-Malmaison, France, on June
1-2, 1988. The workshop was the third in
a series on N2O emissions and was Co-
sponsored by the U.S. Environmental
Protection Agency (EPA) and the Institut
Francais du Petrole (IFP). Its primary
focus was to assess and evaluate the role
that combustion of fossil fuels plays in
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directly emitting N2O into the
atmosphere. This subject is significant
because the ambient levels of N2O have
been reported to be increasing and
because N20 has been linked to
stratospheric ozone (O3) depletion and
global climatic warming. Another focus
was to establish an international network
for exchanging information related to
N20 emissions.
The workshop brought together a
complementary group of experts involved
in various aspects of N2O research.
Specifically discussed were the role of
N20 in stratopheric O3 depletion and
global climate change, the mechanisms
of N2O formation/destruction (involving
both homogeneous and heterogeneous
reactions), and the status of N20
measurement techniques. Presentations
were also made on emissions data
collected from a variety of stationary,
sub- and full-scale facilities, as well as on
data from mobile sources. After these
presentations/discussions, conclusions
were reached regarding the current
status and future direction of N20
research.
This report documents both the formal
and informal material presented at the
workshop, and it covers vigorous
diccussion periods that followed formal
presentatations. It is an account of the
proceedings and summarizes the
workshop's content chronologically.
Atmospheric Concerns
Representatives of EPA, NASA, OECD,
and Max Planck Institute presented
overviews of each agency's program
relating to stratospheric ozone depletion
and global climate change.
EPA believes that human activities are
responsible for increases in atmospheric
gases that are causing global climate
change through warming (a result of the
greenhouse effect) and stratospheric
ozone (03) depletion. NASA believes that
in the last 100 years, the trace gas
composition of the atmosphere has
changed more than in the previous 4.5
billion years, owing to increased human
activity. Carbon dioxide (C02),
chlorofluorocarbons (CFCs), and methane
(CH4) are anthropogenic gases known to
participate in reactions contributing to
climatic warming and ozone depletion.
Nitrous oxide (N20) is another trace
gas that contributes to both stratospheric
ozone depletion and climatic warming.
Ice core data show steady levels of N2O
until the start of the industrial revolution
and that currently, ambient
concentrations are increasing at an
annual rate of about 0.2 percent.
Although natural sources are thought to
comprise over 50 percent of total
emissions, combustion of fossil fuels and
biomass burning are considered to be
major anthropogenic contributors.
N2O contributes to the greenhouse
effect through the absorption of infrared
radiation. Global temperatures have
already increased 0.6 °C since 1880,
and the four hottest years on record have
occurred in the 1980s. The modeled
predictions of global temperature, using
recorded data, suggest that
temperatures may rise by as much as
5°C worldwide by the year 2030. About
10 percent of this warming may be due
to increased N2O concentrations. Many
global features are expected to be
adversely affected by drastic climatic
changes. Global warming will melt polar
ice caps, change important weather
patterns, and tax human health and
society.
Some scientists suggest that ozone,
the only atmospheric gas that absorbs
ultraviolet radiation, may be second only
to oxygen as the earth's most important
gas. A 50 percent reduction in
stratospheric ozone has been observed
over a ten-year period over the South
Pole. This depletion probably represents
the largest geophysical perturbation ever
measured over so short a period.
It has been determined that N2O plays
a key role in stratospheric ozone
depletion. A catalytic loss mechanism
involving the oxides of nitrogen (NOX)
destroys stratospheric ozone. It is
interesting to note that the main source
of NO in the stratosphere originates from
N20.
Currently, the N20 data base is
insufficient. There is a critical need to
foster research directed at developing a
better inventory of N2O emissions
sources and contributions, and to
improve understanding of both the
combustion processes related to N20
emissions and control options.
Accelerated research is vital, because
critical decisions are being made to
control NOX, new combustion devices
are being designed and built, and
developing nations are growing and
contributing to the N2O problem, all with
insufficient information.
Mechanisms of N2O Formation
and Destruction During
Combustion
Basic Kinetics
Nancy Brown (Lawrence Berkeley
Laboratory) described her ongoing
investigation of the formation/destruction
chemistry of N20 in premixed, laminar,
lean, atmospheric-pressure flames.The
study involves both experiments and
modeling .
For the experiments, CH4/air/nitrogen
compound flames and
H2/O2/argon/nitrogen compound flames
were chosen. The H2 flames were chosen
to provide results without the
complexities of carbon chemistry and to
allow a comparison of results to model
predictions. Experimental variables were
bulk flow rate (28-40 Lpm), to include the
effects of heat transfer to the burner,
equivalence ratio (0.75-0.9), and nitrogen
additive (NH3, NO, N20, and N2).
Thermochemistry, probe, and quenching
effects were also studied.
For modeling, she used Sandia codes
Premix with full chemistry and transport,
Chemkin with full chemistry,and
Thermochemistry-1987 Chemkin release.
For modeling, actual measured
temperature profiles were used rathe
than solutions of the energy equation.
She concluded that agreement
between the experiment and modeling
was satisfactory near 1,700 K, but the
chemistry at temperatures less than
1,100 K needs more clarification. Other
conclusions of her research are:
• N2O destruction results in N2
production.
• Destruction reactions need to be
determined over a wide range of
temperatures and product yields.
• More information on N2O production
reactions is needed.
• Probe effects need to be eliminated
because they could have a
significant effect on N20
measurements.
Gas Phase Kinetics
John Kramlich, (Energy and
Environmental Research Corporation)
focused on the emissions of N20 from
industrial flames. He discussed were the
issues of N20 formation and emission in
coal and oil flames and the effects of
firing configuration and pollution control
strategies on these emissions. This
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study has involved both experiments and
modeling.
Figure 1 summarizes the study with a
mechanism overview and admits that
predominant generation of N20 occurs in
the post-flame zone from conversion of
HCN to N2O. Presence of HCN in the
post flame can be credited to
devolatilization of char-N and to free HCN
transported from the edge of volatile
flame in stratified flow. Some N20 is
produced in the flame zone but is rapidly
destroyed to form N2.
Heterogeneous Reactions
Gerard de Soete (IFP) defined a
heterogeneous reaction as one between
two different phases, as in gas to solid.
Heterogeneous reactions implying N2O
as a reactant or as a product are
numerous. Three examples are:
1. Formation of N20 from bound N
during heterogeneous char oxidation.
2. Heterogeneous reduction of N2O on
char-bound C atoms during
heterogeneous char oxidation.
3. Formation of N20 during catalytic
NO reduction (coupled with catalytic
N2O reduction), which may play a
larger role in N20 emissions from
mobile sources.
A heterogeneous reaction rate can be
expressed by:
where k, is is the rate constant, enm (a
fraction) expresses the dependence of
the covering degree, which is
proportional to active sites in the solid
involved in the reaction, and Py is the
dependence of partial pressure of one or
more reactant gaseous species.
Thus the study, focusing on
heterogeneous reactions that imply N2O
as a reactant or product, involved two
tasks: (1) identification of the adsorption
and desorption reactions composing the
heterogeneous mechanisms and their
reaction rates, VjS; and (2) a time
resolved study of the reactions identified
in Task (1). This allowed determination of
the fractions leading to determination of
the rate constants, kfs.
The formation of N20 during NO
catalytic reduction plays a major role in
emissions from mobile sources using
three-way catalysts.
Passing N2O and NO over a typical
metal catalyst results in:
1. Reduction of NO at a much lower
temperature than N20, resulting in
the formation of N2O as a function
of temperature.
2. Intermediate formation of N2O
when NO is fed as a function of
temperature.
The above indicates that transformation
of N2O into NO is a minor reaction. In all
circumstances using graphite, N20 will
be reduced at a lower temperature than
will NO, as opposed to when a metal
catalyst is used with a reducing agent.
Since N2O reduces at a lower
temperature than NO, little N20 is found
when NO is reduced. Reduction of NO
could result in formation of N20;
however, this reduction of NO at the
temperature where N20 can be formed
occurs so fast that the N2O cannot be
seen in the gas. For graphite, this is true
in both the presence and absence of
reducing gases. The situation is the same
during heterogeneous reduction of N2O
and NO on char-bound carbon atoms.
Shown in Figure 2 are the
experimentally determined overall
reaction rates of C oxidation and, NO,
N20, and HCN formation as a function of
temperature for Eschweiler char. The
subscript e in this figure implies that
these reaction rates were obtained under
adsorption/desorption equilibrium rates.
The figure suggests a rough
proportionality between Vc, VNO. and
VN,O.
For a Cedar Grove char, plotted in
Figure 3 are fractions of nitrogen
converting to NO and to N20 as functions
of fraction of carbon burnt. These results
have been obtained for all fractions of
carbon burnt and for temperatures up to
1,400 K. As a rough approximation, the
fractions of N converted to NO and N20
seem to be proportional to the fractions
of carbon burnt. Thus it seems that a
constant fraction of N is formed into NO
and another constant fraction of N is
converted into N20 in a heterogeneous
way. This is true for all char burnout
fractions and at all temperatures up to
1.400K.
Therefore, the main reaction path of
N20 formation from bound nitrogen,
during char combustion, at temperatures
below 1,400 K, is probably not a
formation from HCN, occurring in the
gas phase, but a heterogeneous reaction
directly linked to char oxidation. This is
suggested by reaction order with respect
to oxygen of one for Vc (overall
combustion rate), VNO (overall NO
formation rate), and VN o (overall N20
formation rate), and similar temperature
dependence of their overall rate.
Thus, based on this study, the
probable mechanisms involved in
formation of N2O from bound nitrogen
during heterogeneous char oxidation are
shown in Figure 4. The parentheses in
this figure imply solid bound groups.
Overall Chemical Information from
Laboratory Combustor
Experiments
Jost Wendt (University of Arizona)
gave results of ongoing DOE-funded
research at that institution. The work
focused on optimizing reburning
configurations for NOX control. A
secondary focus of the work was on N20
measurements. N2O was measured by
on-line GC.ECD following a sample
conditioning system.
The experiments are being conducted
on a premixed, down-fired combustor at a
firing rate of 27 kW to study NO and N20
emissions from the combustion of
Beulah lignite and Utah #2 bituminous
coals. NO2 levels for both coals were
found to be insignificant at less than 2
ppm for stoichiometric ratios (SR)
ranging from fuel rich to fuel lean.
Measurements of NO and N20 as a
function of residence time at fixed fuel
lean SRs, with combustion gas
temperatures in the range 850-1,290 K
for Beulah coal and 1,050-1,525 K for
Utah coal, showed N20 concentrations to
be less than 5 ppm and the N2O/NO
concentration ratio to be very small.
Results, so far, have also indicated that
there is little N20 formation under reburn
conditions using natural gas as the
secondary fuel.
William Linak (EPA/AEERL), presented
N20 emission data from a down-fired,
60,000 Btu/h (18kW) laboratory-scale
coal combustor with a variable swirl
burner. The purpose of these
experiments was to change NO
emissions through changes in the overall
turbulent diffusion flame type and thus to
determine if a correlation between N2O
and NO emissions was valid. Bomb
samples were drawn from a post-
combustion region under stack type
conditions. In an attempt to vary the
NOX concentration, burner swirl and flame
shapes were altered by changing axial
and radial air ratios and locations of the
injector tube. A Utah bituminous coal
and a Montana subbituminous coal were
used in these experiments, the first
having a higher organic content but lower
moisture level than the other. For each
coal, data were collected at three (fuel
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In Post-flame
Devolatilization HCN
Char-N »•
N2O
Free HCN
NH3
N2O
At Edge of Volatile Flame
Char Contains Char Nitrogen
Stratified Flow May Contain HCN
In Volatile Flame Zone
Devolatilization
s
^
Figure 1. Mechanism overview.
lean) stoichiometric ratios and two flame
shapes — one axial and one radial. Gas
samples were collected in stainless-steel
containers after refrigeration drying and
were analyzed for N2O by GC/ECD. The
widely scattered data show N2O
emissions of 10-250 ppm. Experiments
using on-line N20 analyses by GC/ECD,
indicated N2O emissions consistently
below 10 ppm. There seems to be no
correlation between N2O emissions and
NOX emissions.
John Kramlich (EERC) described his
fluid bed model as a stirred reactor for
bed particles releasing volatiles such as
HCN and CO, and as a plug-flow reactor
for the gases moving up through the bed.
Initial model results show that NO
concentration initially increased because
of a leaner stoichiometry, reached a
maximum of about 200 ppm, and then
decreased in bed region with relatively
less lean stoichiometries. The final
stoichiometry was taken to be 1.25. The
results also show that N20 emissions at
the end of a residence time of 0.5 s,
corresponding to the beginning of
freeboard part, were reasonably high
(about 250 ppm). The model predictions
for variations in excess air, bed
temperature, and fuel nitrogen speciation
are shown in Figure 5. The stoichiometry
in the bed was lean, so an increase in
excess air did not have much effect on
N20 emissions, which are very sensitive
to temperature in the range 1,000-1,200
K. NH3 does not produce much N2O;
HCN produces reasonably high levels of
N2O; and a combination of NH3 and HCN
produces intermediate N2O levels. All
these parametric effects, explored by the
model, agree with N2O formation/
destruction chemistry understood so far.
Model predictions were also made on
N20 emissions from coal and natural gas
reburning. The results suggest that
application of carefully picked gas
reburning conditions may decrease both
NO and N2O emissions.
Alan Williams (Leeds University)
summarized his own investigation. His
measurements reveal that N20 is about
10-20% of the total Nox for fluidized bed
and drop-tube combustion, with grate
combustion producing higher levels than
systems with sustained high temperature
in the post-.flame region . He suggested
the mechanism of N2O formation as
outlined in Figure 6 where coal
pyrolizes to give tar + gases + char, all of
which can produce N2O. His kinetic
calculations reveal that the N20
contribution from the interaction of NH3
+ NO was only about 5 ppm. N20
yields from both tar and char combustion
were each about 10% of the NO
produced. These results suggest that at
each stage of combustion, either single
or staged, the N20 is about 10% (or
possibly higher) of the NO ( + N02)
produced, and production of N20 is a
strong function of temperature
John Smart (International Flame
Research Foundation) presented data on
his experiments on a 4 MW horizontal,
swirled pulverized-coal-fired furnace.
The preliminary results indicated that
HCN and NH3 found in the flame at the
0.1m axial location were essentially gone
by the 0.2 m location. N20 values were
low at the 0.1 m location, although at 0.2
m near peak values were achieved. N20
and NOX in furnace exhaust (at about
1,070 CC) were 43 and 720 ppm, (at 3.6
% 02). The data suggest N20 formation
from fuel-bound N, but further work is
needed to characterize this hypothesis.
N2O Measuement Techniques
The second session began by
addressing the need to develop
standardized sampling and analytical
techniques. Given the measured
increase in atmospheric N20 levels, it
can be concluded that a significant
anthropogenic source exists. The relative
importance of candidate sources such as
agriculture, biomass burning and fossil
fuel combustion and others is unclear at
this time.Therefore, a reliable, accurate
method for identifying and analyzing
these sources is crucial. A standardized
sampling and analytical protocol is critical
in verifying the reliability of N2O data
gathered worldwide. A wide variance in
sampling methodology is evident. Grab
samples have been collected in many
types of containers, including glass,
stainless-steel, and Tedlar bags. Sample
moisture content also varies greatly.
Furthermore, the development of an on-
line real time analyzer would eliminate
long analytical delays in research
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Eschweiler Char
35 to 50 Microns
P02 ' 29 Wa
(differential)
--6
h-8
1 1.2
Figure 2. Reaction rates for Eschweiler char.
1.4
1.6
-------
0.6
0.4
0.2
Cedar Grove Char
P = 1650 Pa
0.5
figure 3 . Cedar Grove char conversion of nitrogen.
Oxidation of (-C) and (-H)
Nitrogen Species Involving Reactions
Carbon Reactions:
O2 + 2(-C) 1+ 2(-CO)
(-CO) ^ CO * free site
2(-CO) ^» CO2 * (-C ; * free site
(-CO
C0
(-CO)
CO
CO2
Hydrogen Reactions:
2(-CH) ^ H2+2(-C)
H2 + (-CO) 3-^ H20 * (-C;
H2O + (-C) "H H2+ (-CO)
Formation of NO:
O2 * (-c; * (-CN) 1-% (-co; * (-c/vo;
(-c/vo; 2£ NO-I-(-C)
(-CN) + (-CO;^J NO +2(-C)
Formation of N2Q:
2(-CNO)2-2* (-CO) * N2O
NO + (-CN)2£ N2O + (-C)
(-CNO) * (-CN) 2-$N2) + 2 (-C)
Heterogeneous Reduction of
NO and N2O
NO + 2(-C) ^» (-CO; + (-CN)
2(-CN) N -^ 2 •» 2(-C;
A/O + (-CN) -^ /V20 *2 (-C;
N20 + (-C)1%N2 + (-CO)
(-CO)
CO2
18
N2O
w2o * (-co; ^ 2/vo * (-c;
NO + (-c; * (-CH; -»(-co; * (CH/V;
Formation /Destruction of HCN:
(-CH) + (-CN) ¥ HCN * (-C;
«2 * r-c; * (-c/v; 2-% (-CH) + (-CHN)
(-CHN) ^» HCN + free site
HCN + (-CN) 2* N2 + (-CH)
(HCN-* NO-I-...?)
Formation/Destruction of NH3
H2 + (-CHN) ^ NH3 + (-C;
30
NH3+ (-CO) + (-CN) -» N2 + H2O +
CH+ (-C)
(NH3 -» HCN....?)
(NH3-+NO...?)
Figure 4. Formation of N2 from bound nitrogen during heterogeneous char oxidation.
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400
300
200
TOO
Excess Air
Fluid Bed
Bed
Temperature
Fuel Nitrogen
Speciation
0 10 20 30 40 1000 1100 1200
Percent K
NH3 HCN NH3+HCN
Figure 5. Predictions under varied conditions-
Coal Particle
*F 1
Cyanogens
i
t
Oxycyanogens
\
p
A/Hj, NHz, NH
i
p
1
NO
>
V
r
•4-
1
f
Figure 6. NOxand N2O formation from coal..
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programs as well as the problems
associated with grab sampling
Limited methods of N20 measurement
are available. Quantitation is usually
done by gas chromatography using an
electron capture detector (GC/ECD).
Although suitable, a disadvantage of GC
analysis is that a true real time
measurement is not possible and that a
high skill level is required to perform the
analysis.
Larry Muzio (Fossil Energy Research
Corporation) reported on the status of
the development of an on-line infrared
N2O analyzer. As a result of his research
efforts, a prototype production model
capable of measuring low ppm
concentrations with high precision is
currently being manufactured and will be
field tested soon.
Muzio (FERCo) and Kramlich (EERC)
presented startling results of a
collaborative study of interferences from
SO2 with respect to N20, in sample
containers under certain conditions.
While conducting natural gas tests using
FERCo's combustor, high N2O was
measured while doping with ammonia (to
generate NOX) and SO2 N20 samples
were collected in glass flasks using an
extraction system where moisture
removal was not utilized prior to
collection. It was discovered that, while
doping with 2,500 ppm S02,N20
concentrations on the order of 300 ppm
were observed, while the condition
without S02 doping measured only about
1 ppm N20.
Muzio and Kramlich began to suspect
that a reaction in the sampling system
was the source of the N20 artifact. At this
time, an attempt was made to generate
N2O in the sample container. A synthetic
mixture containing N2, 02 C02 and NO
was placed in the flasks, one contaning
deionized H2O and no SO2 with the other
containing 1,500 ppm SO2 and dilute
sulfuric acid. Over 150 ppm N20 was
generated from the SO2/sulfuric-acid
containing mixture while virtually no N20
was measured in the mixture lacking
these components. The test was
performed again without adding S02.
This time no N20 generation was
observed, isolating the importance of S02
in the artifact mechanism (see Figure 7).
Tests were designed to determine if
the removal of S02 or moisture could
enable a sample of uncompromised
integrity to be collected. Sodium
hydroxide and sodium carbonate were
found to be effective in neutralizing SO2
while having no evident effect on known
N20 levels. Passing the sample gas
through an ice bath condenser reduced
the NO/SO2 interaction considerably,
although some N20 generation was
observed.
Muzio stressed the impetus of the
artifact discovery in regard to current
emission data bases and to the
contribution from fossil fuel combustion
to the global N20 budget.
Full Scale Field Data
The third session of the workshop
included a series of presentations on data
collected from both mobile and stationary
sources. The presentations included
data from full-scale stationary utilities in
the United States and Europe. The
presentations provided N20 emission
values measured from different research
facilities as well as industrial equipment
to provide a better representation of N20
discharge levels and to assess the role of
fossil fuel combustion on global N2O
levels. Because of the sampling artifact
discussed in the previous session, each
presenter was asked to include
information regarding sampling and
analytical techniques used, moisture and
sulfur content of the sample, and any
efforts taken to remove these
constituents.
European data represented about 70
different facilities ranging in firing
capacities from 0.5 to 800 MW, with most
facilities in the 100-600 MW range. Data
from circulating and stationary fluidized
bed combustors (FBCs) were included in
the presentations. Data from coal
combustors comprised a significant
portion of the presentations, although
emission data from peat, distillate oil,
wood, refuse, and natural gas combustion
were also discussed.
The European data, excluding those
from FBCs, were considerably lower than
U.S. data presented from similar
facilities. Figure 8 shows data collected
from full scale units using various NOX
reduction methods while firing European
brown coal (RWE) that are plotted along
with data presented at the Boulder,
Colorado workshop. Table 1 is a
compilation of data presented for easy
comparison. Other data collected using
on-line techniques also exhibited
extremely low N20 values, often less
than 10 ppm.
Discussion To Arrive at Practical
Conclusions
The remainder of the workshop was
designated for open discussion of
material presented and for reaching
practical conclusions on the direction o
future research efforts.
Steven Lanier (EERC) offered the
following conclusions to the participants
for discussion:
1. A critical need exists to develop and
validate sampling and analysis
procedures.
2. The GC/ECD method of analysis
appears adequate, but the
development of a continuous
emission monitor for N2O is
encouraged.
3. Following validation, repeat sampling
of utility boiler and fuel classes,
which have previously indicated high
N20 emissions, is urgently needed.
4. Prediction of global N2O increases
based on AP42 NOX factors is not
scientifically justifiable.
S.Mobile sources are a minor
contributor to the N2O emissions.
6. European data indicate that direct
N2O emissions from coal-fired
boilers are a minor contributor of
N2O.
7.Continued evaluation of N2O
emissions from fluidized bed
combustors is encouraged.
8. Other combustion sources such as
catalytic crackers should be
evaluated.
9. No clear consensus exists as to how
NOX combustion modifications will
affect emissions
A vigorous discussion period followed
Lanier's conclusions, showing general
agreement with them. It was suggested
that the possibility of NO's being
converted to N2O in the atmosphere be
explored as well. It was also suggested
that the contribution from other
combustion sources should also be
evaluated, biomass burning in particular.
The participants agreed most strongly
that a suitable and valid sampling
protocol should be developed.
K. Hein (RWE) summarized the final
conclusions reached from the
discussion:
I.New information on N20 formation
and destruction mechanisms in stack
gas samples precludes earlier coal-
and oil-fired boiler data.
2. Assuming that the European data are
valid, the direct emission of N2O
from coal-fired boilers is a minor
contributor to observed global
increases in N2O.
3. Sampling, handling, and
measurement techniques must be
thoroughly studied to assess NO
conversion to N2O in sample
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200
TOO -
Base Gas Mixture:
N2 86.2%
O2 4.3%
CO2 9.4%
NO 600 ppm
-
Liquid: H2O H2SO4(PH=2)
SO2 0 7500
Figure 7. Synthetic gas mixtures in glass sample flask. 1. Title.
200
150
Q.
o"
too
50
250
500 750
ppm
QAcurexJEPA
OHarvard
OMIT
&EPFU
VTU Delft (PFBC)
• RWE
1000
1250
Figure 8. Comparison with previous results.
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containers characterizing catalytic
effects.
4. Repeat and extended measurements
of NOX and emissions from different
combustion equipment types, fuel,
etc., are needed, with special
attention to FBC units as high
emitters of N2O.
5. All other combustion sources,
including biomass burning, should be
included in the evaluation of global
N20 emissions.
Summary
Information presented at this workshop
suggests that N20 emissions have a
significant effect on both stratospheric 03
depletion and global climate change.
Studies indicate an annual increase in
N2O in the atmosphere of 0.2-0.3
percent, and isolating the source of the
increase is essential to prevent further
worldwide climate change. There is still
some question as to whether the
combustion of fossil fuels is directly
responsible for the increase. More
research is needed to find reliable and
accurate techniques for establishing a
data base of N20 emissions from
combustion.
The mechanisms and conditions
involved in N2O combustion formation
and destruction are not well
characterized. N2O formation ispossible
in post-flame, temperature-dependent,
gas phase reactions. Material presented
at the workshop suggests that volatile
HCN exposed to temperatures between
1,150 and 1,500 K can be converted to
N20 The possibility of this is supported
by models and experiments.
Heterogeneous N2O ormation
mechanisms are also possible, but the
amount of N20 they produce is less
thanobserved high stack emissions. The
use of catalysts for NOX reduction
methodologies has not been well
characterized for N2O formation. The
production of N20 is temperature
dependent and does not seem to be
significant in these reactions.
Most of the information presented at
the workshop was related to N2O
measurement techniques. Muzio's
presentation on N2O generation in
sample containers questions the validity
and accuracy of much of the reported
data. A verifiable sampling technique is
essential to assess the role of fossil-fuel
combustion in rising concentrations of
atmospheric N20. Muzio pointed out that
NOx, S02, and N2O are involved in the
artifact scenario. The removal of either of
these components could halt the
generation of N2O. However,
considerable research is still needed in
this area, because these mechanisms
may also occur in the atmosphere. If this
is so, positive identification of the
reaction mechanism is crucial, which
opens an entirely new avenue of
research. Although sampling questions
still exist, the analytical methodologies
employed in quantifying grab samples
are adequate. The development of an on-
line continuous N2O monitor would be
invaluable in combustion research.
Substantial data was presented on the
N20 direct-emission levels from various
European stationary utilities employing
several different types of combustion
techniques. Fuels other than coal were
included in these presentations.
Considerably lower N20 emissions were
realized in relation to United States data.
In the presentations, methods of
sampling were also described. Much of
the European data include extensive
moisture removal in sampling. A
refrigeration condenser was used, and
desiccants were employed. Several
presentations included on-line
measurements, which showed very low
N2O concentration levels on non-FBC
combustors. The data presented on
FBCs showed significantly higher levels
of N2O emissions, which supports
predictions by computer modeling. The
emission of N20 from mobile sources
appears to be very dependent on driving
styles. The conversion of NOX to N2O is
directly related to catalyst temperature,
which is proportional to driving speed or
revolutions per minute. Lower
temperature catalysts produce higher
N20 emissions.
A general discussion period completed
the 2-day workshop, put in perspective
the information presented, and enabled
participants to set priorities for research
and goals. Standardization of a practical
and accurate sampling protocol was
deemed most essential. There is no way
to assess N20 emissions from fossil-fuel
combustion unless an accurate
measurement technique is available.
Once a suitable sampling protocol has
been installed, the utility boilers, catalytic
crackers, and other combustion sources
should be retested The combustion of
fuels other than coal and heavy distillates
should be considered, particularly
biomass burning, where limited sampling
efforts have shown significant emissions.
A general consensus was reached that
direct emissions of N2O from the
combustion of fossil fuels is possibly not
the major source of N20 in the
atmosphere, but more research should
be conducted to support this conclusion.
10
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R. J. Ryan and R. Srivastasta are with Acurex Corp.,P.O. Box 13109, Re
Triangle Park, NC 27709
Joseph A. McSor/ey is the EPA Project Officer (see below).
The complete report, entitled "EPA/IFP European Workshop on the Emisss
Nitrous Oxide From Fossil Fuel Combustion (Order No.PB90-126038
$23.00 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
CResearch Triangle Park, NC 27711 Rin
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S9-89/089
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