AEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81 -171 c May 1982
Project Summary
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Development of Criteria for
Extension of Applicability of
Low-Emission, High-Efficiency
Coal Burners: Third Annual
Report
R. Payne, J. Lee, P. Case, S. Chen, and D. Pershing
A brief summary of progress on this
program since its initiation in 1977 is
shown in Figure 1, along with the experi-
mental efforts proposed for fiscal year
1981. The major thrust of the program in
1980 was in the small-scale efforts to
define the impact of fuel properties and
operating conditions on NOX emissions.
Initial studies were also undertaken to
assess the viability of using dry sor-
bents for SOX control. The progress of
the program's pilot-scale studies has
been hindered because the large water-
tube simulator (LWS) and small water-
tube simulator (SWS) facilities were not
available after January 1980. Furnace
deterioration necessitated complete re-
building of these units before further
testing could .be accomplished. This re-
building effort was completed in January
1981 and testing was commenced.
For the small-scale fuel studies, 28
coals covering all ranks were tested
under a wide variety of conditions to
ascertain the impact of coal properties
on the fate of fuel nitrogen. Significant
accomplishments in this part of the pro-
gram include:
• Potential sources of NOX during coal
combustion are: (1) fixation of nitro-
gen in the combustion air; (2) con-
version of fuel NO in the volatiles;
and (3) conversion of fuel NO in the
solid phase. Under typical condi-
tions, oxidation of nitrogen chemi-
cally bound in the coal is the major
source of NOX emissions. The con-
version of nitrogen which remains
in the solid phase during devolatili-
zation is inherently low. Hydro-
carbon volatiles do, however, appear
to enhance this conversion. The
conversion of nitrogen evolved
with the volatiles appears to be
higher than that of char nitrogen,
but significantly less than suggested
by gas- and liquid-phase results.
The solid phase is believed to play a
major role in suppressing volatile
nitrogen conversion.
> The bench-scale test results confirm
the pilot-scale concept that de-
creasing the initial air/fuel ratio
decreases fuel NOX formation. The
initial fuel-oxidant contacting rate
appears to have little influence on
heterogeneous nitrogen oxidation;
however, it has a strong effect on
volatile nitrogen oxidation. With
premixed or rapidly mixed systems,
fuel NO formation increases with
increasing fuel nitrogen content;
however, other fuel properties also
significantly affect the fate of fuel-
bound nitrogen during combustion.
In particular, fuel nitrogen conver-
sion is proportionally greater with
coals containing a high fraction of
volatile reactive nitrogen.
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Program Objectives
Areas of study concentration
Oct 1977 - Oct 1980
Key results
1. Expand fuel capability of
distributed mixing burner
IDMB)
Define impact of fuel
properties and rank on NO*
• Major effort in small- scale
controlled experiments
• Limited pilot-scale efforts
• Small scale
• Optimum staging condition is dependent on fuel
properties
• No emissions are related to TFN as well as
other fuel and process variables
• S02 measurement methods have been defined
• Pilot scale
• Impact of fuel properties on NO* under typical
burner combustion conditions more complex than
simply fuel nitrogen content related
2. Explore additional concepts
No effort experimentally
• Need for inboard tertiary air identified for
retrofit boilers - aerodynamic staging
3. Burner configurations
Front wall fired
• Single burners
• Burner scale
• Multi-burners
Single burners
• Developed design data base for DMB (NO*)
• Identified in-situ dry sorbent as having
potential for combined NOx/SOx combustion
modification control
Multi-burners
• NO* characteristics similar to single burners
• Emissions (NO*) correlate with total heat input
• Tertiary air ports location critical to CO
levels but insensitive to NOx
Scale
• Scaling by velocity results in higher NO emissions at
higher heat input (might be related to furnace effects)
4. Compare DMB with
commercial burners
• Evaluation of commercial
burners in L WS
• General flame characteristics similar to field operation
• NOx levels 50-100 ppm lower than field
• CO levels » than field
• Stability and performance similar to field
5. Industry coordination
• Setup broad participation
in review panels
• Held three technical panel meetings
• Held two technology transfer panel meetings
6. Support field application
of DMB technology
Identified several field/
commercial problem areas
and limits on general
acceptability of results
• Retrofit of existing boilers with tertiary air
ports not economical
• Need for flexibility in location of tertiary air ports-
• More detailed design criteria required showing
operating limits for DMB and performance data
• Need for testing with wide range of fuels
Figure 1. Program executive summary of key results.
2
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• Detailed studies on the optimization
of a staged combustion system sug-
gest that the stoichiometry produc-
ing minimum NOX omissions is a
function of both fuel composition
and primary-zone conditions. As
first-stage stoichiometry is de-
creased, the NO formed in the first
stage decreases, but other oxidiz-
able gas nitrogen species increase
as does nitrogen retention in the
char.
• The distribution of the total fixed
nitrogen (TFM) species (NO, NH&
HCN) leaving the first stage is
strongly dependent on coal compo-
sition. In general, the first stage NO
percentage decreased significantly
with decreasing coal rank from
anthracite to lignite. However, the
percentage of NH3 grew with de-
creasing rank which may give
greater NO emissions in the second
stage. HCN was greater than NH3
in all bituminous coals, but less
than NH3 with all subbituminous
and lignite coals.
• The distribution of the first-stage
fuel nitrogen emissions had a signifi-
cant impact on the second-stage
exhaust NO emissions. The mini-
mum second-stage NO emissions
depend on competition between the
first stage NO and increased gas
and solid-phase nitrogen species.
• During staged combustion, increas-
ing the rate of heat extraction from
the first stage, or fuel-rich zone, de-
creases the decay of TFN species,
but dramatically decreases TFN
conversion in the second stage.
Thus, first-stage heat extraction
has the net effect of reducing ex-
haust NO emissions.
This effort is now complete, and
future small-scale studies will concen-
trate on the dry sorbent for SOX control.
Previous effort suggested that careful
control of the temperature/time/stoichi-
ometry history in the burner zone was
essential for effective utilization of a dry
sorbent for removal of SO2 in the furnace.
Based on these results, a study was ini-
tiated to verify and develop the technical
information to extend the results to an
operating boiler system. The effort to
date has concentrated on verifying the
measurement techniques under both
gas-phase and heterogeneous combus-
tion conditions. The results suggest
that:
• Uniform dispersion of limestone in
the combustion zone is critical to
obtaining high sorbent utilization.
• Proper control of temperature/time/
stoichiometry can produce signifi-
cant SO2 removal from combustion
products.
• Experimental measurement methods
can be designed which provide suf-
ficient accuracy for reasonable sul-
fur balances.
The major thrust of the experimental
efforts during the next fiscal year is to
provide critical data addressing problems
and concerns expressed by manufac-
turers and users which could limit their
acceptability of the DMB demonstration
results and burner concept. These areas
are: (1) applying the distributed-mixing
concept to a burner without outboard
air to meet retrofit application; (2) de-
termining the impact of a wide range in
coal characteristics on boiler operation,
maintenance, and performance; and (3)
defining limits with respect to perfor-
mance and flame stability. These ef-
forts will be closely coordinated with
the companion EPA demonstration pro-
grams 68-02-3130 (utility boilers),
68-02-3127 (industrial boilers), and
RFP DU 80-A162 (large utility boilers;
under negotiation) to ensure that the
most relevant pilot-scale experiments
are conducted.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The EPA's overall program in research,
development, and demonstration of
Iow-N0x wall-fired burners is shown in
Figure 2. Since initiation of this effort in
the early 1970s, EPA has recognized
the importance of combustion modifica-
tion through burner design as the most
cost-effective NOX control method. The
Combustion Research Branch of EPA's
lERL^RTP initiated the first study at the
IFRF (68-02-0202) to better understand
the influence of burner design on NOX
formation. The results of this study
showed that fuel/air contacting, largely
controlled by burner design, is the key
to controlling NOX emissions. This result
then led to the definition and demon-
stration of EPA's first distributed-mixing
burner (DMB) in 1972. Under two sepa-
rate contracts, the EPA then pursued
the development of this concept. In the
first study (68-02-1488), the basic
burner design criteria were studied; units
were scaled up to 100 x 106 Btu/hr and
multiple-burner interaction was investi-
gated. The most significant result of
this study was demonstration that
under furnace-simulated conditions the
system could operate at less than 0.2 Ib
NOX/106 Btu. As a logical extension of
this effort, EPA then undertook this
contract (68-02-2667) to extend the
burner criteria to cover a wider range of
fuels, as well as to study other burner
configurations in addition to providing
on-going support to EPA demonstration
programs 68-02-3127, 68-02-3130,
and RFP DU 80-A162. Consequently,
contract 68-02-2667, in addition to its
prime R/D objective, must also act as
the bridge between the R/D studies and
resolution of practical field application
problem areas.
The Development of Criteria for Ex-
tension of Applicability of Low-Emission,
High-Efficiency Coal Burners program
was initiated in October 1977. The
overall program is structured to meet
six objectives:
1. Expand fuel capability of DMB to
include fuels proposed for use by
utilities.
2. Explore additional burner concepts.
3. Determine effects of burner con-
figurations on performance/emis-
sions characteristics.
4. Provide direct comparison between
DMB and commercial burners.
5. Ensure appropriate technology
transfer to manufacturers and
users.
6. Provide test support for field
demonstrations.
The program is divided into the seven
tasks shown in Figure 3. Task 1 —Pro-
gram Definition—is designed to plan the
three key elements of the program: (1)
fuels selection; (2) experimental plan;
and (3) measurements of protocol. The
remainder of the program is structured
to carry out these efforts. Tasks 2 through
5 are experimental efforts designed to
progressively move from bench-scale
process studies (Task 2 —Fuel Screening
Experiments) to single- (Task 3 —Single-
Burner Experiments) and multiple-burner
configurations at pilot scale, which
incorporate the distributed-mixing con-
cept, and then to compare the operating
and emissions characteristics of the
DMB against commercial technology
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(Task 5 —Comparison to Current Tech-
nology). Throughout the program, results
are continually reviewed with manufac-
turers and users to secure their advice
and recommendations before the finali-
zation of the testing program as part of
Task 6 —Industry Coordination. All of
the information is then analyzed in Task
7 —Data Analysis and Criteria Develop-
ment—and DMB design guidelines are
established.
Bench-Scale Studies to Assess
the Impact of Coal Type on
NOX Formation
Twenty-eight different coals as well
as several simulated coals (i.e., char/
propane and char/propane/ammonia or
nitric oxide) were tested under excess
air and staged conditions to study the
effects of fuel properties on the fate of
fuel-bound nitrogen during combustion.
Detailed measurements of first-stage
and exhaust species concentrations
were performed for 14 different coals
to investigate the parameters controlling
a staged combustion process.
Results point to the following conclu-
sions and implications.
Excess Air
NO formation during pulverized coal
combustion can be attributed to the ni-
trogen in both the volatiles and the
char. The conversion of nitrogen which
remains in the solid phase during devol-
atilization is inherently low. The conver-
sion of nitrogen evolved with the vola-
tiles appears to be higher than that of
the char nitrogen. The presence of hy-
drocarbon volatiles, however, enhances
the conversion of char nitrogen. On the
other hand, the presence of solid phase
is believed to suppress the conversion
of volatile nitrogen. The initial fuel/air
contacting rate controls the oxygen avail-
ability for both fuel nitrogen fractions. It
appears that the volatile nitrogen con-
version increases with increasing mixing
rate, but the conversion of char nitrogen
is less sensitive to change in the initial
air distribution.
Thus, coals which evolve more volatile
nitrogen are expected to produce higher
NO emission under well mixed conditions.
Data obtained under excess air condi-
Burner
research
IFRF burner
design criteria
68-02-0202
Delayed mixing
burner design
development
Design and scale-up
of burner concept
68-02-1488
Extend applicability
of design concept
68-02-2667
Field boiler
demonstration
:j:j: Determination oi' :
jSSi commercial applicability \
Application to
industrial boilers
68-02-3127
Application to
utility boilers
68-02-3130
Application to
large utility burners
DU80-A162
1970
1974
1978
Government fiscal year
1982
1986
Figure 2. EPA low NOx burner RD&D programs.
4
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tions indicate that, with a premixed or
rapidly mixed system, fuel NO formation
increases with increasing fuel nitrogen
content; however, other fuel properties
also significantly affect the fate of coal
nitrogen. Attempts to correlate these
data (on the basis of the nitrogen lost in
the ASTM volatile determination or with
such basic fuel properties as percent
nitrogen, percent volatiles, or coal rank)
have been successful.
An experiment has been developed in
another EPA-supported program to de-
fine the reactive volatile nitrogen fraction
of coals. Six of the very different coals
tested in the bench scale reactor were
pyrolyzed under inert conditions at 1370
K, and the volatile nitrogen species
which were converted to HCN were
measured. Figure 4 shows that the data
obtained in this quartz reactor pyrolysis
experiment agree qualitatively with the
experimental results obtained with the
premixed burner. The Savage lignite
had the highest percentage conversion
of fuel nitrogen to NO in the combustion
experiments, and also the highest frac-
tional volatile reactive nitrogen yield in
the inert pyrolysis study. Likewise, the
Beulah lignite had the lowest in both
experiments. The slope of the correla-
tion decreases with decreasing mixing
rate (premixed or radial diffusion), and
the axial diffusion data do not correlate
well. This may be because the pyrolysis
experiment does not characterize char
nitrogen. Therefore, it can be concluded
that, under excess air conditions, fuel
nitrogen conversion is proportionally
greater with coals containing a high
fraction of volatile reactive nitrogen.
Staged Combustion
Detailed measurements of first stage
and exhaust species concentrations
suggest that a staged combustion sys-
tem must be optimized with respect to
first-stage stoichiometry and residence
time, fuel properties, and heat extraction
rate. As first-stage stoichiometry is de-
creased, the NO formed in the first
stage decreases, but other oxidizable
gas-nitrogen species increase, as does
nitrogen retention in the char. Total fixed
nitrogen (TFN = NO + NH3 + HCN)
generally increases with increasing fuel
nitrogen and correlates well with excess-
air exhaust emissions. Increasing the
residence time in the fuel-rich stage
allows TFN species to decay toward
low equilibrium values and thus reduces
exhaust NO emissions.
The distribution of the TFN species
leaving the first stage is strongly depen-
dent on the coal composition. Of the 12
coals tested in detail, only two produced
high HCN concentrations. Figure 5
shows the percent of input fuel nitrogen
exiting the first stage as NO, NH3/ or
Program planning
Task 1
Program definition
Fuels selection
Experimental plan
Measurements protocol
Experimental effort
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Task 2
Fuel screening experiments
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Task3
Single-burner experiments
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Task 4
Multiple-burner experiments
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Task 5
Comparison to current technology
" Key program elements
Technology transfer
Task 6
Industry coordination
Design guidelines
Task?
Data analysis and criteria
development
Figure 3. Program structure for development of criteria for extension of applicability of
low-emission/high efficiency coal burners.
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HCN at SR, = 0.70. The German bitu-
minous coal (Saar) correlates well with
American bituminous coals. The anthra-
cite and the Utah char (classified as
semianthracite according to the criteria
outlined in ASTM Method D 388) pro-
duced relatively large amounts of first
stage NO. This could be due to the in-
creased oxygen partial pressure in the
early stages of combustion. With low
volatile fuels, the rate of consumption
was reduced to decreased hydrocarbon
evolution. For all the bituminous coals
except the Indiana bituminous both NH3
and HCN usually increased with increas-
ing fuel volatility. HCN concentrations
were higher than NH3 for all the bitumi-
nous coals. However, for subbituminous
and lignite coals' HCN concentrations
were significantly less than NH3 levels
at all stoichiometries.
In general, NO at the exit of the first
stage appears to decrease with decreas-
ing coal rank from anthracite to lignite.
Conversely, the relative amounts of NH3
grow with decreasing rank. The percent
of initial nitrogen leaving the first stage
as HCN was below 4 percent for most
of the coals tested.
Second-stage TFN conversion de-
creases as the TFN distribution shifts in
favor of HCN and NH3, and as the hy-
drocarbon content of the second-stage
reactants increases. The percentage
conversion of char nitrogen to NO in the
second stage is low ( <20 percent).
First-stage cooling decreases the TFN
decay in the fuel-rich zone and, hence,
increases the TFN and char nitrogen
carry-over into the second stage. Re-
duced second-stage flame temperatures
have little effect on solid-phase nitrogen
conversion, but they dramatically de-
crease gas-phase TFN conversion due
to a shift in controlling flame chemistry.
The exhaust NO emissions depend on
first stage stoichiometry, on the amount
and speciation of the TFN entering the
second stage, and on the amount of char
nitrogen. Thus the exhaust NO data can
be correlated with a model of the form:
NO exhaust = X, (NO) + X2 (NH3 +
HCN) + X3 (CN + NOTh where
XT = conversion of NO,
X2 = conversion of NH3 and HCN,
X3 = conversion of char nitrogen, and
NO-yh = thermal NO formation
in the second stage flame. The TFN con-
centrations were correlated in ppm, dry
0% O2 based on the detailed measure-
ments at the end of the first stage. The
char nitrogen (CN) was used as equiva-
lent ppm based on stoichiometric con-
version of the remaining nitrogen.
As noted previously, NO conversion
in the second stage flame did not appear
to be a strong function of NO concentra-
tion; however, it was found to depend
on first-stage stoichiometry (probably
specifically on the hydrocarbon content
of the second stage flame). Data from
pairs of Utah char/C3Hg/NH3 experi-
ments indicated that, as the SR was re-
duced from 0.9 to 0.6, NO conversion
dropped from approximately 89 percent
conversion to 51 percent with no change
in HCN or NH3 concentrations. There-
fore, NO conversion was modeled as:
XT = a-SR
where a = correlation constant.
0.4
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0.3
0.2
Beulah
0.1 1—,,-JL
0.1
Savage
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Radial diffusion
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Axial diffusion
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0.2 0.1 0.2 0.1
Fraction of Fuel N converted to HCN at 1373 K
0.2
Figure 4. Fuel NO correlation.
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Figure 6 indicates that the conversion
of HCN and NH3 decreases with increas-
ing TFN content; hence, a reciprocal
model was used for these conversions:
X, =
1
+ 7(NH3 + HCN)
A multivariable linear regression pro-
gram was used to calculate the required
parameters. The resulting correlation
was:
N0ex = 0.8(SR) (NO) +
1
. (NH3 + HCN)
Char nitrogen conversion was assumed
to be approximately constant: X3 = 5
as was the contribution of thermal NO
formation,
2 + 0.007(NH3 + HCN)
+ 0.2 CN + 5.
Figure 6 shows the results of this corre-
lation plotted against the measured ex-
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1
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700
600
500
400
300
200
100
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\
o
100
200
300 400 5OO
NO (calculated), ppm
Figure 6. Exhaust NO can be calculated by NO™ = 0.8(SRJ(NO) +
+ 0.2 (Char N) + 5.
600
700
HCN +NH3
2 + 0.007 (HCN+NHJ
R. Payne, J. Lee, P. Case, S. Chen, and D. Pershing are with Energy and
Environmental Research Corp., Santa Ana, CA 92705.
Dennis C. Drehmel is the EPA Project Officer (see below).
The complete report, entitled "Development of Criteria for Extension of Applica-
bility of Low-Emission, High-Efficiency Coal Burners: Third Annual Report,"
(OrderNo. PB82-197 153; Cost: $18.00, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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