United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-93/088 July 1993
EPA Project Summary
Development of Sampling and
Analytical Methods for the
Measurement of Nitrous Oxide
from Fossil Fuel Combustion
Sources
Jeffrey V. Ryan and Shawn A. Karns
The combustion of fossil fuels is sus-
pected to contribute to measured in-
creases in ambient concentrations of
nitrous oxide (N2O). Accurate and reli-
able measurement techniques would
help to assess the relative contribution
of fossil fuel combustion N2O emissions
to the increase in ambient concentra-
tions. The characterization of NO emis-
sions from fossil fuel combustion
sources has been hindered by the lack
of suitable and acceptable grab sam-
pling and on-line monitoring method-
ologies. Grab samples have been
shown to be compromised by a sam-
pling artifact where N2O is actually gen-
erated in the sample container in the
presence of SO2, NOx, and moisture.
On-line monitoring techniques are lim-
ited and, of those available, instrument
costs are often prohibitive, detection
levels are often insufficient, and the
techniques are often susceptible to in-
terferences present in combustion pro-
cess effluents. The report documents
the technical approach and results
achieved while developing a grab sam-
pling method and an automated, on-
line gas chromatography method
suitable to characterize N^O emissions
from fossil fuel combustion sources.
The two methods developed have been
documented as EPA/Air and Energy
Engineering Research Laboratory
(AEERL) Recommended Operating Pro-
cedures (ROPs).
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Nitrous oxide (N2O) has been a great
concern to the combustion community
largely because the combustion of fossil
fuels has been proposed as a potential
contributor to measured increases in ambi-
ent N20 concentrations. Currently, atmo-
spheric N2O concentrations are increasing
at nearly 1 ppbv annually from a present
level of approximately 303 ppbv. This in-
crease is significant because N2O is con-
sidered a "greenhouse" gas and is
associated with stratospheric ozone deple-
tion. Studies tracking atmospheric in-
creases of carbon dioxide (CO2) over time
reveal that the increases of both N2O and
CO2 occur similarly and that increases of
both these anthropogenic pollutants corre-
late well with increases in industrial activ-
ity.
Early efforts to characterize N2O emis-
sions from fossil fuel combustion sources
focused on identifying a relationship be-
tween nitrogen oxides (NOX) and N2O emis-
sions. Data were nominally collected in a
"piggy back" manner, where N20 grab
samples were collected during NOX perfor-
mance tests. Considerable data exist on
N2O emissions from diverse combustion
sources and techniques firing on various
fossil fuels. Much of the data reported on
N2O measurements from fossil fuel com-
bustion sources were obtained using grab
sampling methods conducive to a sam-
pling artifact that biases the measured val-
Printed on Recycled Paper
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ues of actual emissions. The grab sam-
pling artifact is a situation where N20 is
actually generated in grab sample contain-
ers in the presence of NOX, sulfur dioxide
(SO2), and moisture (H2O). N2O generation
approaching 200 ppm in grab sample con-
tainers has been observed.
Realizing that accurate and reliable N2O
measurements were essential to emissions
characterization research, the Combustion
Research Branch (CRB) of EPA's Air and
Energy Engineering Research Laboratory
(AEERL) initiated a program to concur-
rently develop grab sampling and on-line
monitoring methodologies suitable for char-
acterizing N2O emissions from various com-
bustion sources and processes. As a result
of this program, two AEERL Recommended
Operating Procedures (ROP) were gener-
ated. ROP No. 45: "Analysis of Nitrous
Oxide from Combustion Sources" details a
gas chromatography/electron capture de-
tector (GC/ECD) method suitable for grab
sample analysis as well as on-line monitor-
ing. ROP No. 56: "Collection of Gaseous
Grab Samples from Combustion Sources
for Nitrous Oxide Measurement" details a
grab sampling method suitable for collect-
ing gaseous grab samples from combus-
tion sources to screen N2O emissions. This
report documents the approach and re-
sults obtained while developing these pro-
cedures.
Analytical Method
The GC/ECD backflush method devel-
oped was found to be suitable for measur-
ing N2O from a variety of combustion
sources and applications. In addition, the
method was found to be equally suitable
for on-line monitoring or grab sample analy-
sis. Analytical interferences, present in com-
bustion process effluents, were negated
through the use of a backflushing tech-
nique. The backflushing method uses a
single, 10-port valve to divert/direct the
flow of carrier and sample gas streams
through the chromatographic system. This
technique employs a precolumn to isolate
the analyte of interest from slower eluting
interferences such as SO2 and moisture.
Once the analyte of interest has eluted
from the precolumn to the secondary ana-
lytical column, the carrier gas flow through
the precolumn is reversed, flushing the
undesirable components from the
precolumn. Other common flue gas com-
ponents—oxygen, carbon monoxide (CO),
CO2, NOX, total unburned hydrocarbons
(THCs), and ammonia (NH3)—were found
not to interfere with the analytical proce-
dure.
To eliminate the need for manual valve
switching, an air actuator, controlled by the
GC electronic hardware, was used. Electri-
cally energized solenoid valves were used
to control actuator operation. The valving
system was automated by interfacing the
GC and integrator to a timed event control
module that converted digital commands
from the integrator to time controlled elec-
trical switches. The integrator could be
programmed to turn the solenoid valves on
or off at specific times. The solenoid valves,
when actuated, allowed compressed air to
pressurize the air actuator. When pressur-
ized, the air actuator rotates the 10-port
valve to the desired position.
The developed system is also capable
of unattended, continuous operation, by
incorporating the programmed timed events
into a separate BASIC program capable of
loop functions. At the end of the analytical
run, the system is capable of automatically
re-initiating the sequence of timed events.
Using this method for on-line monitoring
allows a semicontinuous measurement
approximately every 8 min. The system
can be easily incorporated into most con-
tinuous emission monitoring sample deliv-
ery/conditioning systems, with the only
requirements being the removal of particu-
late and moisture from the sample stream
by a refrigeration condenser. The sample
stream should be diverted to the analytical
system before further moisture condition-
ing by a desiccant. Figure 1 is a schematic
diagram of the automated system.
N2O is quantified from the linear rela-
tionship between N2O concentration and
integrated peak area. A least squares lin-
ear regression of the calibration variables
is used to establish the linear relationship.
To improve quantitative accuracy as well
as expand the linear range of quantitation,
alternative mathematical approaches to lin-
earize detector response were investigated.
The linear regression approach enables
the determination of quantitative bias on
an absolute basis. With this approach, er-
ror can be reported as less than a certain
concentration, often reported as a percent-
age of full scale or as deviation from the
true or known value. A problem arises in
that the estimated bias for low concentra-
tions will be very large relative to the mea-
sured or true concentration. By performing
a linear regression of natural log (In) trans-
formed calibration variables, error is ca-
pable of being reported on a relative basis.
Table 1 compares the relative bias of
calculated concentrations (relative to the
true concentration) using both quantitative
approaches. The linear regression of the
transformed calibration variables was ef-
fective in minimizing the relative error of
calculated concentrations. Less than 10%
bias was observed over the entire quanti-
tative range as opposed to as much as
700% relative bias for the non-transformed
quantitative approach.
The automated, on-line GC/ECD sys-
tem was evaluated extensively on a num-
ber of diverse EPA/AEERL fossil fuel
combustion test facilities. Initially, the ana-
lytical system was used exclusively during
the development of the N2O grab sampling
method. On-line and grab sample mea-
surements were performed on gases gen-
erated by the Flue Gas Simulation System
(FGSS). The on-line concentrations mea-
sured were compared to grab sample mea-
sured concentrations to assess artifact
generation. For AEERL's Gas Cleaning
Technology Branch (GCTB), the N2O moni-
toring system was used to measure N2O
emissions resulting from the combustion of
various coals during parametric SO2 re-
moval testing. The N2O concentrations
measured 0.5-10 ppm.
During these tests, quality control span
checks were performed nearly every hour
over the 8-h test period. The quality control
checks were used to assess method ana-
lytical bias and precision over the course
of the entire test period. The average bias
observed (2.9%) was well within the tar-
geted level of less than 15%. Similarly, the
precision observed (2.7%), expressed as
percent relative standard deviation (RSD),
was well within the targeted level of less
than 10%.
Grab Sampling Method
The discovery of the N2O sampling arti-
fact attenuated the need for a standard-
ized, reliable sampling method in order to
accurately assess the N2O emissions from
fossil fuel combustion sources. Realizing
that consistently eliminating the sampling
artifact entirely would be extremely diffi-
cult, if not impossible, the AEERL/CRB
believed that minimizing N2O generation to
consistent levels within aged sample con-
tainers would be suitable to screen for high
N2O-emitting combustion sources. Specifi-
cally, AEERL researchers felt that consis-
tently minimizing N2O generation within
grab sample containers to less than 10
ppm over a 1-2 week period would be
more than acceptable to screen for high
N2O-emitting fossil fuel combustion
sources. The screening technique could
then be used to direct on-line monitoring
efforts.
The screening of intended fossil fuel
combustion sources would require the vol-
untary cooperation of commercial and re-
search combustion facilities alike.
Therefore, the grab sampling equipment
and technique must be easy to use and
pose minimal imposition to those partici-
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To CEMs
12:fr
Analytical':
Column —'-
A/2O
Span
Gas
Air
or
House
Air
P5
Carrier
Gas
Sample Event
Control Module
Integrator
(') 1 ft = 0.305 m
Figure 1. Automated, on-line GC/ECD N2O monitoring system.
Table 1. Comparison of Relative Bias Using Differing Mathematical Approaches
Linear Regression
(Untransformed Variables)
Linear Regression
(Transformed Variables)
N2O Known
ppm
0.5I
0.97
1.99
5.03
9.85
19.4
40.4
80.1
128
N2O Calc.
ppm
-3.11
-2.13
-0.40
4.58
11.35
23.18
45.74
83.36
123.68
% Bias
-705. 1
-319.6
-120.1
-8.9
15.2
19.5
13.2
4.1
-3.4
N2O Calc.
ppm
0.47
0.99
2.02
5.36
10.41
20.11
40.45
77.74
120.79
% Bias
-8.6
2.1
1.5
6.6
5.7
3.7
0.1
-2.9
-5.6
pating in screening surveys. Specifically,
the grab sampling method should not re-
quire a great degree of sampling exper-
tise. In addition, the grab sample should
be obtainable in a manner compatible with
commonly employed continuous emission
monitoring (CEM) sample delivery systems.
SO2, NOX, and H2O, components
present in most fossil fuel combustion pro-
cess emissions, were identified as the key
artifact reactants. The selective removal of
any or all of these reactants was targeted
as an approach to eliminating the sam-
pling artifact. The use of a desiccant alone,
phosphorus pentoxide (P2O5), was effec-
tive in drastically reducing the artifact gen-
eration but was unable to eliminate it
completely. A calcium hydroxide/sand mix-
ture was found to be extremely effective at
neutralizing high concentrations of SO2.
The method developed was designed
to be used compatibly with continuous
emission monitoring sample delivery/con-
ditioning systems or as a stand-alone pro-
cedure. Specifically, the developed method
uses reactant-specific dry sorbents to re-
move the gaseous components, SO2 and
H2O, to the degree that H2O generation in
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stored sample containers (1-2 weeks) is
consistently minimized to less than 10 ppm.
Sequentially, SO2 is neutralized and H2O
removed from a fossil fuel combustion pro-
cess flue gas sample stream before enter-
ing a Teflon-lined stainless steel container.
The neutralization of SO2 requires H2O in
the flue gas stream. Therefore, the flue
gas sample must be collected upstream of
any moisture conditioning devices such as
condensers and/or desiccants that may be
present in CEM sample delivery/condition-
ing systems.
The flue gas sample is extracted from
the combustion source using a vacuum
pump which pushes the gaseous-sample
through the two-cartridge, solid sorbent
system and ultimately through the grab
sample container or "sample bomb." The
sampling system was designed to be used
upstream of any pollution control equip-
ment or on combustion facilities where
pollution control equipment did not exist.
The grab sampling system is shown in
F:igure 2.
During the development of this sam-
pling method, tests were conducted to de-
termine the fossil fuel combustion process
flue gas nitric oxide (NO), SO2, and H2O
concentration ranges where N2O genera-
tion in aged (1-2 week) samples would be
consistently minimized to less than 10 ppm.
This method was found suitable for use on
combustion systems with flue gas concen-
trations in the following ranges:
SO2: 0-2,500 ppm
NO: 0-1,000 ppm
H2O: 5-25% (by volume)
These flue gas concentration ranges
were verified under simulated and actual
combustion conditions. An example of the
method's effectiveness to minimize N2O
generation is shown in Figure 3. Flue gas
components of CO, CO2, and THCs, typi-
cally present in fossil fuel combustion pro-
cess streams, were found not to interfere
with sampling method performance. Other
common flue gas components, such as
hydrogen chloride (HCI) and NH3, were not
evaluated and may act as interferences.
Inlet
From Stack
Outlet
H2O Sorbent
Sample Container
Meter
Sample Pump
Figure 2. Sorbent/sample container schematic.
Flue Gas Simulation System
A/;?O Artifact Generation of Samples Collected
200
150
§: so
165 ppm
0.8 ppm
0.5 ppm
I Without Sorbents
\Sample Flow = 11 LPM
Sample Bombs Aged 6 Days
• With Sorbents (X10)
Sample Flow = 9 LPM
On-Line N#O Cone.
(X10)
Sorbent (10:1 Sand, Ca(OH)2 and P&$
1,200 ppm SO2, 600 ppm NO, 10% Moisture
NgO Initial = 0.5 ppm
Figure 3. Comparison of on-line N2O concentration to N2O sample container generation with and without use of sorbents.
•fru.S. GOVERNMENT PRINTING OFFICE: 1*93 - 730-071/80024
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J.V. Ryan and SA /Cams are with Acurex Environmental Corp., Research
Triangle Park, NC 27709.
William P. Linak is the EPA Project Officer (see below).
The complete report, entitled "Development of Sampling and Analytical Methods
for the Measurement of Nitrous Oxide from Fossil Fuel Combustion Sources,"
(Order No. PB93-194330; Cost: $17.50; subject to change) will be available
only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-93/088
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