United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-88/002 Apr. 1 988
v>EPA Project Summary
Fundamental Combustion
Research Applied to Pollution
Formation—Volume III. Support
Studies: Measurement Studies
W. R. Seeker and M. P. Heap
This is the third volume in a series
documenting research activities con-
ducted under the EPA's Fundamental
Combustion Research (FCR) program,
applied to pollution formation. The FCR
program had three major objectives:
• To generate an understanding of
combustion behavior necessary to
aid in developing control strategies
to minimize NOX emissions from
stationary sources.
• To develop engineering models
which would allow effective utiliza-
tion of a large body of fundamental
information in the development of
new NOX control techniques.
• To identify critical information
necessary for low-NO* combustor
development and to generate it in a
time frame which was consistent
with the needs of EPA's technology
development programs.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering 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 documents findings from
a limited number of studies to evaluate
measurement techniques. FCR program
efforts to evaluate and develop measure-
ment techniques were conducted as part
of a support function to the overall FCR
program. An overview of the entire FCR
program content and the objective of the
program's various components are pre-
sented in Volume I. Volume I also
describes FCR program efforts to quantify
the gas-phase chemistry controlling NOX
formation and destruction during com-
bustion. Volume II is a three-part report
describing studies related to various
aspects of the physics and chemistry of
two-phase systems. Volume Ha ad-
dresses flame combustion processes,
Volume lib addresses devolatilization and
volatile reactions, and Volume Me de-
scribes studies of heterogeneous NO
reduction processes. Volume IV docu-
ments FCR program efforts to develop
engineering analysis computer models.
The prime contractor and EPA's project
officer were responsible for program
planning, management, and synthesis of
the overall combined inhouse and sub-
contract program. Approximately 70% of
the program effort involved subcontracts
to a variety of organizations throughout
the world, ensuring that the FCR program
had the benefit of the best scientific
talent available.
A substantial fraction of the experi-
mental studies reported in Volumes I, II,
and III were conducted through subcon-
tracts or were joint efforts involving both
EER and a subcontractor. Most of the
modelling, however, was performed by
EER directly. Volume III gives results from
three individual research projects. The
first study reported, performed directly by
EER, evaluated potential errors involved
in the chemiluminescent measurement
-------
of NO in combustion products. The
second study, performed through a
subcontract to the University of Utah,
included an assessment of the proce-
dures required to measure nitrogenous
intermediates involved in staged com-
bustion. The final portion of Volume III
documents results from a joint study
conducted by EER and Spectron Devel-
opment Laboratories to assess optical
droplet sizing techniques which might be
applied to studies of pollutant formation
in liquid fuel spray flames. Each study
is presented as a separate report with
individual tables of contents, lists of
figures, conclusions, and references. The
remainder of this Project Summary pre-
sents brief descriptions of the individual
components comprising Volume III.
Chemiluminescent
Measurement of NO in
Combustion Products
The measurement of nitric oxide (NO)
concentrations in combustion products
by chemiluminescence has several
advantages over alternative methods. As
a result, the Chemiluminescent NO
analyzer (CLA) has become a standard
instrument for most laboratory and field
emissions tests. CLAs were used by EER
and all FCR program subcontractors for
NOX measurements reported in this
report series. During the FCR program,
several papers appeared in the literature
questioning the accuracy of CLAs when
calibrated using NO in an N2 background
for the span gas. (The primary concern
was that background gas composition
affected the NO concentration measure-
ment.) Molecular nitrogen represents the
dominant species in traditional combus-
tion exhaust products, but there are also
substantial concentrations of other
species (e.g., water vapor and C02). One
paper indicated that the common practice
of neglecting background gas composi-
tion variation could introduce errors as
large as 28% in indicated NO concen-
tration. This was an issue of considerable
concern to the FCR program and to the
EPA. Accordingly, a research project was
initiated with three objectives:
• Assess the accuracy of commercial
CLAs measuring NO in combustion
products following the calibration and
operating procedures recommended
by the instrument manufacturers.
• Examine methods of correcting CLA-
indicated NO concentrations for the
effects of background gas composition
variations.
• Examine methods to improve calibra-
tion procedures.
Figure 1 is a simplified schematic of
a CLA NO analyzer. A flow of ozone is
mixed with the sample gas in the reaction
chamber. The ozone and NO in the sam-
ple gas (or span gas) rapidly react to form
nitrogen dioxide in either an excited state
(NO$) or a ground level state (NO2). The
yield of NO$ is about 10% at ambient
temperatures. The excited molecules can
decay to ground state giving off light of'
a characteristic frequency (chemilumi-
nescence) or can collide with any third
body (M) and decay to ground state
without chemiluminescence (quench-
ing). The relative importance of the
Chemiluminescent and quenching reac-
tions depends upon the temperature, and
the amount and type of molecules
available for quenching (i.e., the back-
ground gas composition). The intensity
of chemiluminescence, measured with a
photomultiplier tube (PMT), is directly
proportional to NO concentration. The
relation between PMT output and NO
concentration is determined by calibrat-
ing the instrument using a known
concentration of NO in a background gas
to adjust the amplifier gain. A span gas
which is NO free is used to adjust zero
offset and to account for PMT dark cur-
rent effects.
The previously noted literature reports
indicating substantial CLA NO measure-
ment error were primarily concerned
with the third body quenching efficiency
of major combustion product gases (e.g.,
H20 and CO2) relative to that of IM2. These
reports presented experimental evidence
showing the strong impact of background
gas composition on the quenching
process. Background gas composition
can also influence other important
features of CLA operation. A potential
influence of backgorund gas composition
is its impact on sample gas flow rate to
the reaction chamber. It is important to
recognize that the output from the CLA's
NO + M
Sample
Reaction Chamber
Optical Filter
Photomultiplier Tube
Lighttight Housing
Vent
L
Oi+Os
Ozone
Generator
Power Supply.
Amplifier,
and Readout
Figure 1. Chemiluminescent measurement of NO.
2
-------
PMT is actually proportional to the
number of NO molecules entering the
reaction chamber, not simply the NO
concentration. For that reason, any
process that impacts the flow rate of
sample gas to the reaction chamber will
also affect the NO concentration indi-
cated by the CLA. Commercial CLAs
typically control flow rates by passing the
sample gas (and span gas) through a
capillary and regulating the capillary
pressure drop. Note, however, that for
a constant pressure drop, the flowrate
through a capillary is inversely propor-
tional to the gas viscosity. Variations in
sample gas composition which alter the
average viscosity will result in variation
in sample flowrate and the indicated NO
concentration. This potential source of
CLA measurement bias was not consid-
ered in the above noted reports dealing
with the quenching process. Fortunately,
for the major non-N2 constituents of
typical combustion products, biases due
to quenching and viscosity effects are in
opposite directions.
To accomplish the program objectives,
two commercially available CLAs were
zeroed and spanned with N2 and NO in
N2, respectively, and then used to
measure known concentrations of NO in
a variety of background gases. The gas
mixtures were prepared by blending high
purity gases and mixtures of NO in carrier
gases in a precision flow metering
system. The gas mixtures simulated
combustion products from a wide range
of fuel compositions and combustor
operating conditions. The NO concentra-
tions indicated by the CLAs were then
compared with the actual NO concentra-
tions to determine the effects of back-
ground gas composition. Two commer-
cial CLAs selected for evaluation: a
Beckman Model 951 and a Thermal
Electron Corporation (TECO) Model 10A.
The major distinction between the
instruments is that the Beckman is
designed to operate with the reaction
chamber at atmospehric pressure, while
the TECO utilizes a subatmospheric
pressure (nominally 9 torr1) reaction
chamber. These pressure differences
were expected to highlight the influence
of the quenching process on NO mea-
surement.
The experimental program consisted of
three series of tests. The Series 1 sample
gas consisted of NO in a background of
N2 and varying percentages of another
gas. Figure 2 illustrates the response of
both instruments to varying (but large)
concentrations of O2, C02, CO, and CH4
in the background gas. In typical com-
bustion system exhausts, the 02 concen-
tration will be less, than 20% (assuming
combustion in air), the CO and CH«
concentrations should be in the ppm
range, and the C02 concentration will be
on the order of 10%. As shown, the
impact of these sample gases on CLA
operation is to decrease the indicated NO
concentration. The atmospheric pressure
reaction cell of the Beckman instrument
showed a slightly stronger bias.
Series 2 simulated background gases
that would be produced in actual com-
bustion systems(asopposedto the binary
mixtures examined in Series 1). Back-
ground gas mixtures were prepared
simulating the products of complete
combustion for a range of fuels at
variable excess air levels. The fuels
considered were methane and pure
carbon, representing the extremes in
fuel-carbon/hydrogen ratio. The fuel/air
ratio simulated ranged from stoichiomet-
ric all the way to an infinite excess air
level. Figures 3A and 3B show the re-
sponse of the TECO and Beckman instru-
ments to sample gases containing 200
ppm NO. Figure 3C shows the response
of both instruments to different NO levels
as a function of theoretical air but holding
the fuel type constant. These figures
illustrate that the anticipated error from
the TECO instrument will be less than
5% for all conditions analyzed. For the
Beckman instrument, the influence of
other background gases was found to be
much stronger, with errors as large as
11% indicated in the 100% theoretical
air range.
The usual procedure in commercial
CLA operation is to draw the sample gas
through an ice bath (or through a dryer)
to remove water from the sample and
to reduce the sample dewpoint to below
room temperature. As part of Series 2,
experiments were conducted which
varied the sample dewpoint in the range
of 275-294 K. The impact of sample
dewpoint on indicated NO level was
found to be small for the TECO instru-
ment (1-2%), but substantial for the
Beckman instrument (up to 6%).
Series 3 evaluated the impact of using
CLAs spanned on NO in N2 to measure
the NO concentration from combustion
experiments using artificial oxidizers. As
part of the FCR program and EPA's
inhouse research program, experiments
were conducted in which argon or an
argon/CO2 mixture replaced molecular
nitrogen in the combustion air. Such
artificial oxidizer tests were used to
evaluate the contribution of fuel bound
nitrogen to exhaust NO. This particular
combustion situation was expected to
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Figure 2.
Binary background gases on impact of chemiluminescent NO measurement.
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result \n large instrument errors since
the anticipated bias from argon quench-
ing effects are in the same direction as
the bias from variation in sample flow
to the reaction cell. Results from Series
3 are shown in Figure 4 and clearly
demonstrate the anticipated result. For
the TECO instrument, the indicated
concentration was low by 14-18%. For
the Beckman instrument, the measure-
ment error is a relatively strong function
of excess air level, but errors as large
as 20% are observed. Based on these
examinations of CLA bias for argon-
substituted combustion air, it is sug-
gested that the CLA response be carefully
calibrated and experimental results
corrected for the instrument bias. Alter-
nately, special calibration gases with
argon/C02 mixtures as the background
gas should be used.
Measurement of Nitrogenous
Intermediates in a Staged-
Combustion System
A small FCR program subcontract was
issued to the University of Utah to assess
various techniques being used to meas-
ure nitrogenous intermediates from
staged combustion processes. The pri-
mary focus of the study was evaluation
of specific ion electrodes for the meas-
urement of hydrogen cyanide (HCN) and
ammonia (NH3) concentrations gener-
ated in staged combustion flames. Three
important introductory notes are in order.
First, HCN and NH3 are formed as
intermediate species in the process of
converting fuel bound nitrogen into
either NO or N2. These intermediates are
not expected as constituents in the
exhaust gas of traditional or staged
combustion processes. The second intro-
ductory note is that this study was not
intended as an exhaustive evaluation of
nitrogenous intermediate measurement
techniques. Within the FCR program and
the combustion research community in
general, specific ion electrodes are
widely used to determine the concentra-
tion of HCN and NHa. There was, how-
ever, wide variation in the details of hovt
samples were collected and the proce
dures used for sample analysis. The
study conducted at the University of UtaF
was focused on evaluation of the various
procedures being employed by other FCF
research organizations. Finally, it shoulc
be noted that emphasis is placed or
extracting samples from within the flam<
zone of a furnace fired with coal o
residual fuel oil. This is an extreme!'
hostile environment which impacts hov
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100 200 300 400 500
1000^ oo 100 200 300 400 500
Theoretical Air f%)
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Figure 4. Influence ofOz/Ar andOz/COz/Ar artificial oxidizers on measurement of NO from combustion tests using chemiluminescent analyzers.
the sampling probes must be designed
and operated if the probe is to survive
mechanically.
The study was performed through
separate analysis of NH3 and HCN
measurement procedures and included
evaluation of sample collection, sample
preparation, and sample analysis. Spe-
cific ion electrode for NH3 measurement
was evaluated first. The initial portion of
the study centered on procedures for
using the specific ion electrode itself.
Effects of sample solution temperature,
pH effects, and ionic strength effects on
NH3 concentration measurement were
all evaluated. In general, results from this
evaluation confirmed the electrode
manufacturer's operating instructions
and suggestions.
To collect a sample for analysis it is
necessary to drive the NH3 in the sample
gas into solution. Figure 5 illustrates the
probe and sampling system found to be
most appropriate for that purpose. The
top portion of Figure 5 is a schematic
of the spray probe which was used for
furnace sampling at the University of
Utah. The probe body is water-cooled and
of stainless steel to withstand the high
temperature environments from which
sample gases must be extracted. A small
tube is on the centerline of the probe.
Small holes at the end of this tube admit
a spray of water or other collecting
solution into the sample gas flow. This
spray serves to both cool the sample gas
and drive the NH3 into solution. The
combined sample gas and liquid from the
spray are directed to a series of impingers
at the probe outlet. Sampling system
components downstream of the impin-
gers measure the sample gas flow rate.
Extensive evaluations were conducted to
evaluate the influence of spray compo-
sition, type of bubblers or impingers, type
and quantity of contacting solution in the
impingers, and effects of sample flow
rate and NH3 concentration in the sample
gas. These evaluations indicated that
distilled deionized (DD) water for the
spray collection efficiency of 97% was
achieved (sampling 5 L/min of 550 ppm
NH3, balance N2) with the spray probe
itself. Adding fritted bubblers on the
probe outlet increased the collection
efficiency to 98%. Changing the spray
and bubbler solution from DD water to
0.1 N H2SO4 increased the collection
efficiency to 99%.
The final portion of the ammonia probe
study included assessment of potential
interference effects from other species
in the sample gas. Strong interferences
were found for two gases: when the
sample gas contained dimethylamine, a
strong positive interference was ob-
served, indicating that the electrode is
not totally specific to NH3 but also senses
NH species; and a strong negative
interference was found for SOa. Further
investigation indicated that a substantial
-------
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Spray Jets
Cooling
Water
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Cooling
Water
In
Spray
u
Gas and
Solution
Out
Solution
In
Waterspray Probe
Spray Pump
Cooling Coils
Knockout
IT Drum
Glass-Wool
Filters
Dessicant
Dryer
Impingers
Figure 5. Waterspray probe and samplimg system.
Temperature
Gauge
Dry Gas Meter
Pressure
Gauge
Sample Pump
To System Exhaust
fraction (not quantified) of the SOs
interference was the result of gas-phase
reactions between NH3 and S02 prior to
the sample gas reaching the sampling
probe. The need for further evaluation
of this interference is indicated.
Evaluation of the HCN measurement
system proceeded similarly to the eval-
uation of the NH3 probe. The specific ion
electrode evaluation included assess-
ments of the impacts of solution temper-
ature, pH effects, and ionic strength
impacts on the electrode response. As
with the NH3 electrode, the cyanide
electrode performs in much the fashion
indicated by the instrument manufac-
turer. However, an observed hysteresi:
effect suggested that the cyanide elec
trode be conditioned with a solution o
low cyanide concentration in comparisoi
with the solution on which measure
ments are to be made.
The optimal sample collection systen
for HCN was found to be the same a
-------
that for the NH3 system shown in Figure
5. A variety of spray solutions and
contacting solutions in the impingers
were investigated. Using DD water, 0.1
N H2SO4, and 1.0 M NaOH, all resulted
in HCN collection efficiencies of only
about 85%. Evaluations of interfering gas
species in the sample flow found that the
only major interference came from S=.
Addition of Bi+3 to the impingers was
found effective for removal of both the
S= ion and its interfering effect.
A final portion of the University of Utah
study reports on experiments to measure
nitrogenous intermediates in a staged
combustion system. Experiments were
conducted in a refractory tunnel furnace
and burned natural gas doped with
surrogate fuel nitrogen compounds in a
staged combustion configuration.
Spray Characterization
An FCR experimental research project
was jointly conducted by EER and Spec-
Iron Development Laboratories (SDL) to
evaluate three laser-based techniques
for measuring characteristics of sprays.
A number of optical techniques have
been developed or proposed which
promise information on spray parame-
ters such as the spatial distribution of
drop size distribution and the velocity of
Transmitting Optics
various drop size classes. The three
techniques evaluated in this FCR project
included processes based on interfer-
ometry, holography, and laser diffraction.
Interferometry, a point measurement
technique, can simultaneously deter-
mine the diameter and velocity of a
droplet passing through the point sam-
pling volume. Droplet diameter and
velocity distribution information can be
obtained at a given probing location,
while spatial distribution of size distri-
bution and velocity is obtained by
sequentially examining multiple loca-
tions in the spray. Holography provides
an instantaneous, three-dimensional
picture of the droplet field and was
included in the evaluation as a cross-
check for the interferometry and diffrac-
tion techniques. The diffraction tech-
nique examined provided a line-of-sight
measurement of drop size distribution
and did not provide information on
droplet velocity. It was included in the
evaluation because the technique rapidly
provides processed data and because it
had been used for spray nozzle charac-
terization in a previous EPA study
conducted at the International Flame
Research Foundation (IFRF) in IJmuiden,
Holland.
To evaluate the measurement tech-
niques, a special spray chamber was
constructed from a 0.61 m diameter
Plexiglas tube to simulate the geometry
of EPA's firetube package boiler simu-
lator. Commercial spray nozzles could be
located on the chamber centerline, while
an adjustable quantity of air was provided
co-axial to the nozzle. Initial testing was
restricted to a small twin-fluid atomizer
(Sonicore model 052 H) which had been
used in several combustion evaluation
studies. This Sonicore nozzle had also
been evaluated as part of the previously
noted IFRF spray characterization study.
The laser interferometry study utilized
optical hardware and signal processing
equipment provided by SDL (see Figure
6). Light from a helium-neon (HeNe) laser
was split into two beams by a diffraction
grating and then crossed within the spray
chamber. At the beam intersection, an
interference fringe pattern is estab-
lished. When a droplet passes through
this probe volume, light is scattered and
collected on a photomultiplier tube.
Analysis of the resultant electrical signal
provides information on the droplet
diameter and velocity. Data from multiple
droplets passing through the probe
volume are stored on a computer disk
and analyzed to define histograms of
droplet size and velocity.
The laser holography system, also
provided by SDL, consisted of an HeNe
L-\,
DC: Diffraction Grating
LZ, /.»' Transmitting Lenses
Lt. LS: Collecting Lenses
M: Mirror
PMT: Photomultiplier Tube
PV. Probe Volume
0: Collection Angle
Collecting Optics
Figure 6. Laser interferometry system.
-------
laser for alignment and a ruby laser to
take the hologram. The ruby laser beam
was split into information and reference
beams. Both beams were expanded,
spatially filtered, and collimated. The
information beam went through the fuel
spray, while the reference beam
bypassed the test section. The two beams
were then mixed on a holoplate. The
optical configuration employed produced
a probe volume on the order of 10 cm3.
A shutter in front of the holoplate holder
allowed light to enter only during record-
ing and minimized the influence of
extraneous light sources.
The laser diffraction system employed
was the Malvern Model 1800 purchased
by EPA for the previously mentioned IFRF
study. The basic measurement system
consists of an HeNe laser whose beam
is expanded to 6.0 mm diameter and
passed through the spray flow field. Light
from the beam is diffracted by droplets
in the spray. The angle of diffraction
depends on the droplet diameter,
increasing with decreasing droplet size.
As illustrated in Figure 7, the diffracted
light is collected by a Fourier transform
lens and focused on a 30-ring detector.
The distribution of light energy across the
30 rings is read by a minicomputer and
fit to a Roslin-Rammler distribution. The
sweep time of the detector is approxi-
mately 13 msec. A representative droplet
size distribution is obtained by averaging
the results from about 100 to 200
sweeps.
Figure 8 illustrates typical results from
the evaluation in a plot of spray weight
density distribution versus droplet
diameter for the three measurement
techniques. These data were all collected
at the same axial location in the spray
with the same nozzle operating condi-
tions. The diffraction and holography
techniques are in reasonable agreement:
the major source of deviation is attributed
to failure of the holography system to
detect droplets smaller than about 15-
20 Aim in diameter. Both the diffraction
and holography techniques give results
in marked contrast to the interferometric
technique. This discrepancy is attributed
to the limited dynamic range of the
interferometric technique. The technique
is capable of measuring droplets within
only a 1 decade variation range in
diameter. As noted in the figure, the
diffraction technique detected droplets
ranging from <10 to >200//m diameter.
The interferometer tests were conducted
to measure droplets in the range of
approximately 10-100 /urn. Centering of
the dynamic range is adjustable, but
8
Fourier
Transform
Lens
Detector
Beam
Expander
Minicomputer
Teletype
Figure 7. Laser diffraction diagnostic technique.
0.5
I I I I I
0.2
0.1
Figure 8.
0 20 40 60 80 100 120 140 160 180 200 220
Diameter, ^m
Interferometric, diffraction, and holography data—weight density distribution for
Sonicore spray nozzle.
biasing resulting from the limited
dynamic range appears to be a major
drawbacktothetechnique.lt is important
to note that SDL felt that many of the
apparent shortcomings of the interfer-
ometer technique were a matter of data
interpretation. An appendix in the report
documents the evaluation position taken
by SDL.
-------
W. Seeker and M. Heap are with Energy and Environmental Research Corp..
Irvine, CA 92718.
W. Steven Lanier is the EPA Project Officer (see below).
The complete report, entitled "Fundamental Combustion Research Applied to
Pollution Formation: Volume III. Support Studies: Measurement Studies,"
(Order No. PB 88-168 968/AS; Cost: $38.95. subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
*U.S.Government Printing Office: 1988 — 548-158/67096
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