United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-89/006 Aug. 1989
Project Summary
N2O Field Study
Russ Clayton, Alston Sykes, Rudi Machilek, Ken Krebs, and Jeff Ryan
Nitrous oxide (N2O) emissions from
coal-fired utility boilers were mea-
sured at three electric power gen-
erating stations. Six units were test-
ed, two at each site, including sizes
ranging from 165 to 700 MW. Several
manufacturers and boiler firing types
(circular, triple cell, and tangential)
were represented. Continuous emis-
sion monitor (CEM) measurements
were made for nitrogen oxide (NO),
oxygen (O2), carbon dioxide (CO2),
and carbon monoxide (CO). On-line
N20 measurements were made using
a gas chromatograph with electron
capture detection. On-line sulfur
dioxide (SO2) levels were measured
on one unit using a gas chromato-
graph with flame photometric detec-
tion. Stainless steel sample con-
tainers were used to collect flue gas
samples for the evaluation of N2O
formation as a function of time in the
presence of NO, SO2, and water. The
N2O on-line results were in the range
from "not detectable" to 4.6 ppm. The
results did not exhibit apparent differ-
ences related to different boiler types
and load conditions. The stainless
steel container results showed a
trend of immediate N2O formation
when SO2 and water are present. The
N2O concentration in all wet samples
increased dramatically within the first
hour to concentrations ranging from
9 to 120 ppm. The dry sample results
showed a much smaller increase in
N2O, ranging from 5 to 25 ppm during
the 2 weeks after sampling.
This Project Summary was devel-
oped by EPA's Air and Energy Engin-
eering 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
Concern over atmospheric emissions of
nitrous oxide (N20) has been growing
because of increasing ambient concen-
trations and the suspected connection
with stratospheric ozone depletion and
expected worldwide climatic changes.
Combustion sources, such as coal-fired
utility boilers, have been singled out as
possible major contributors to overall
N2O emission levels. Analyses of stack
grab samples in stainless steel (SS) con-
tainers known as "bombs" have shown
emission levels as high as hundreds of
parts per million of N20 for these
sources. Concerned about these emis-
sion values, the Environmental Protection
Agency (EPA) has initiated a national and
international research program to char-
acterize N2O emissions. Recent research,
however, has cast doubt on the validity of
previous measurements of N2O
emissions.
Laboratory results suggest that a rapid
reaction occurs with nitrogen oxide (NO)
in the presence of water (H2O) and sulfur
dioxide (SO2) in the stainless steel con-
tainers to form N20. Preliminary results
from pilot-scale testing indicate that N2O
levels in SS container samples taken
from a coal-burning combustor increase
from <10 to >100 ppm within 1 h.
These tests suggest that previously re-
ported N2O emissions may be incorrect.
Furthermore, they indicate that exposing
the flue gas to desiccant such as P205
prior to filling the SS sample containers
helped to moderate the increase in N2O
concentration, yielding a more represen-
tative result.
To test these observations on full-scale
combustors, EPA proposed a field survey
of N2O emissions from several utility
boilers. Several units representing two
boiler types (i.e., tangentially fired and
wall fired) and various siaes were
selected on the basis of availability,
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location, scheduling, and costs. Schedule
constraints limited sampling locations to
available ports at a point between the
particulate emission control device and
the stack.
A two-fold approach to sampling was
used N20 levels were determined on-line
and compared with samples collected in
the SS containers. Both on-line and SS
container samples were analyzed by gas
chromatography (GC) with electron cap-
ture detection (ECD). The SS container
samples were taken before and after moi-
sture removal (by refrigeration and P205
desiccation) to determine the effect of
moisture on N20 formation. Each SS
container sample was taken and analyzed
for N20 and S02 at selected intervals to
determine and plot the N20 reaction over
time. On-line measurements of carbon
monoxide (CO), carbon dioxide (C02),
NO, oxygen (02), and S02 were also
taken to provide additional operating
characteristics of each boiler unit. Initially,
on-line nitrogen dioxide (N02) and SO2
were also to be measured but were not
because of instrument malfunction On-
line S02 was measured at Unit F, using a
GC with flame photometric detection
(FPD). Continuous emissions monitors
(CEMs), GCs, and associated calibration
gases were housed in two self-contained
vehicles and transported to the selected
sites. Multiple GCs were used to allow
simultaneous on-line and SS sample con-
tainer testing. The test series field work
was conducted October 2 through 13,
1988.
Project Description
Samples were taken from six utility
boilers on three separate sites. All boilers
were burning medium sulfur coal but the
exact sulfur content of coal at each site
was not available. The six sampled units
are shown in Table 1.
The first three units were similar in size
and represented two manufacturers and
two firing types. Units D and F allowed
data comparison on two identical units;
Unit E was a similar type but was four
times larger. This selection of boilers pro-
vided sufficient variability to determine
whether N2O emissions from utility
boilers are significant and are affected by
boiler design criteria.
A GC/ECD was used for the on-line
N20 measurements, while CEMs were
used for measuring the fixed gases
Samples were also collected m SS con-
tainers to evaluate the stability of N20 in
grab samples The grab samples were
analyzed in the field at 1 and 4 h intervals
after collection and shipped back to the
EPA/RTP laboratory for subsequent
analyses at 40, 168, and 336 h periods
after collection
Sampling Procedures
Specific boiler operation data (i.e.,
firing rate, 02) were taken from trans-
ducers already in place. Figure 1 illus-
trates the CEM system. Sample gas was
drawn through a heated filter and passed
through a heated sample line to a tee.
One stream was directed to the S02
monitor The second flow stream contin-
ued to a low-contact-time refrigerated
moisture removal system. The sample
passed through a sample pump, silica gel
desiccator, and was directed into a
distribution manifold. A slipstream of the
sample passed through a second pump
and into the on-line GC. Individual
sample flows were directed to CEMs as
required.
Stainless steel container samples were
collected in both a wet and dry gaseous
environment for locations shown in Figure
1. The wet sample was collected from the
heated portion of the sample system,
upstream of any water removal device.
The dry sample was collected from a
point downstream of the refrigerated
condenser but ahead of the silica gel
desiccant dryer and passed through a
container filled with PaOs before entering
the container. A pump, operating at a
flowrate of > 1 L/min, was used to draw a
sample into the container. This flowrate
was maintained through the sample
container for at least 5 min before clos
the valves on both ends of the contaim
Quality Assurance (QA)
During each test a quality control ((
check sample was analyzed to ve
instrument calibration. The QC chi
sample was not a calibration point. 1
levels of QC checks were used, 15.1 <
104 ppm All QC check results w
within the ± 20% acceptable limits of
QA plan, including the sample line che
The highest deviation was 10.5%.
An independent audit was conduc
by Research Triangle Institute. RTI s
plied a cylinder of N20 in N2- 1
concentration of N20 in this cylinder v
not known to the sampling team. 1
cylinder was analyzed by direct inject
into the GC and also through the sam
loop connected to the sample conditii
ing system. The sampling location
given in Figure 1 The measured cono
tration for both injection methods vi
38 7 ppm, indicating no sampling systi
bias.
Results and Conclusions
On-Une Results
The N20 on-line results, summarized
Table 2, were in the range from "i
detectable" to 4.6 ppm. Table 3 summ
izes the CEM average concentratioi
The results showed no trends with va
ing boiler type, or operating and lo
conditions. The detection limits for N
varied depending on the ECD used: t
ECDs were required for each unit test*
Each ECD exhibited different detecti
limits that varied throughout the test. T
variability was due to sample effects a
signal/noise ratio changes. These d<
agree with recent EPA in-house studi
that show N2O levels ranging from 1.2
5.4 ppm from the combustion of vario
coal types.
Because very little on-line SO2 de
were obtained, no conclusions can
made. Data were collected only on Unil
Table 1.
Unit
A
B
C
0
E
F
Boilers tested
Date
1 0/05/88
f 0/07 '88
70/08/88
70/70/88
10/1 r/88
10/12/88
for emissions
Size (MW)
250
250
250
765
700
765
Class
Pre-NSPS
Pre-NSPS
Pre-NSPS
Pre-NSPS
Pre-NSPS
Pre-NSPS
Manufacturer
B&W
B& W
Riley
CE
CE
CE
Firing Type
Circular
Triple Cell
Circular
Tangential
Tangential
Tangential
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Stack
. Sample &
Bias Cal
Valve
GC Wet
Samplf
Port
Probe
•»
t-
Pump l^T]
Heated
Filter
Heated
Sample
I ine
Rcfri(/er,ttt'
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Table 3
Unit
A
B
C
D
E
F
OEM Summary -
CO2(%)
14.8
13.5
14.3
11.7
13.1
119
Average Concentrations
NO (ppm)
386
513
559
354
374
319
CO (ppm)
-
13.3
8.6
2.2
30.7
3.1
02 (%)
4.6
7.1
6.1
8.3
6.0
8.1
"Missing data.
between 11:54 a.m. and 2:46 p.m. Initial
concentration was measured at 778 ppm
and gradually increased to 1004 ppm. A
total of 25 injections were made, with an
average concentration of 924 ppm
SS Container Results
SS container N20 wet sample results
demonstrate that N20 formation with S02
present is immediate. All wet samples
increased dramatically within the first
hour to concentrations ranging from 9 to
120 ppm. The N20 dry sample results
show a much smaller increase to 5-25
ppm over the 336 h time period. These
results verify the same phenomena ob-
served in the laboratory studies for both
the wet and dry samples for all fuels
(coal, fuel oil, natural gas).
Russ Clayton, Alston Sykes, Rudi Machilek, Ken Krebs, and Jeff wen are with
Acurex Corp., Research Triangle Park, NC 27709.
William P. Linak is the EPA Protect Officer (see below).
The complete report, entitled "N20 Field Study," (Order No. PB 89-166 623/AS;
Cost: $15.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Protect Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-89/006
8°S°Si!lH PROTECTION AGEMCY
CHICAGO
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