United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-84-017b Apr. 1984
Project Summary
Development of Criteria for
Extension of Applicability of Low
Emission, High Efficiency Coal
Burners: Second Annual Report
A.R. Brienza, S.L Chen, M.P. Heap, J.W. Lee, W.H. Nurick, D.W. Pershing,
and D.P. Rees
This report describes the second
year's effort under EPA Contract 68-
02-2667, which concerns the develop-
ment of criteria for the evaluation and
applicability of low-emission, high-
efficiency coal burners. The report
describes progress in three major
areas: (1) bench scale studies, (2)
distributed mixing burner development.
and (3) comparison with commercial
practice. The bench scale studies
concern the impact of fuel characteris-
tics on fuel NO formation during the
combustion of pulverized coal. Although
fuel NO emissions generally increase
with increasing fuel nitrogen content,
coals with the same rank and similar
nitrogen contents may produce markedly
different fuel nitrogen levels. Prelimi-
nary results indicate that a simple
procedure, evaluating reactive volatile
nitrogen, can be used to assess the
impact of coal type on fuel NO forma-
tion. Data relating to the development
of the distributed mixing burner have
been reviewed, and data have been
obtained with both single and multiple
burners. No operability problems were
encountered with different fuel types,
but NO emissions were fuel dependent.
Several commercial burners have been
tested satisfactorily in the test facility.
One gave severe flame impingement.
Carbon monoxide emissions were
generally higher and nitrogen oxide
(NO>) emissions were slightly lower
than commercial practice.
This Project Summary was developed
by EPA's Industrial Environmental
Research 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
information at back).
Introduction
NOX formation in pulverized coal flames
can be reduced by using the burner to
control the rate of fuel/air mixing to
minimize fuel nitrogen conversion. This
report describes the second year's
progress on EPA Contract 68-02-2667.
The program is aimed to:
• Expand the fuel capability of low NOX
burners to include the major types of
solid fossil fuels projected for use by
the utility industry.
• Explore additional burner concepts
and configurations that show poten-
tial for improving the emission and
thermal performance of pulverized
coal burners.
• Determine the effects of multiple
burner configurations that are
encountered in utility boilers.
• Provide for direct comparison be-
tween the experimental burners
being developed here and the
current state-of-the-art for commer-
cially available coal burners.
• Arrange for coordination of the
technology development program
with boiler manufacturers and
users.
• Provide testing in support of planned
application of the burner technology.
Figure 1 shows the relationship of the
-------
Planning/Approach
Taskl
Program
Definition
Experimental
Burner Development
Task 2
Screening
Experiments
/
\
Task 3
Single Burner
Experiments
Task 4
Multiburner
Experiments
TaskS
Comparison with
Commercial Practice
Relate to | Technology
Practical Units Transfer
Task?
Data Analysis
and
Criteria Development
Figure 1. Overall program logic.
various tasks that are planned to meet
these objectives. In this, the second year
of the program, efforts were expended on
the following tasks:
• Bench scale fuel screening studies
to assess the impact of fuel properties
on fuel NO formation (Task 2).
• Tests with the distributed mixing
burner and different fuel types
(Tasks 3 and 4).
• Tests with commercial burners to
allow a comparison with commercial
practice.
Bench Scale Studies
In the first year's effort, a bench scale
reactor was constructed with a design
input firing rate ranging from 50 to 150x
103 Btu/hr.* This facility, to be used for
fuel screening studies and concept
development, had several versatile
features including:
• The ability to burn the fuel with
nitrogen-free oxidant mixtures com-
posed of 02, COz, and Ar, to define
fuel NO production.
• Complete control over fuel input rate
and fuel size distribution.
• A modular combustion chamber to
permit probe access and variable
heat extraction rates.
*To convert nonmetric units to their metric equiva-
lents, please use the conversion factors at the back
of this Summary:
Table 1 lists the 25 coals tested to date in
this furnace and their properties. Initially,
the impact of fuel properties on NO
formation under excess air is discussed;
then data obtained to date on the
evaluation of advanced concepts for NO*
control will be presented.
The Influence of Coal
Properties on Fuel NO
Formation
Pulverized coal was burned under
premixed and diffusion flame conditions
in nitrogen-free atmospheres to evaluate
fuel nitrogen conversion. Figure 2
summarizes the data obtained with the
coals tested to date under premixed and
axial diffusion conditions. Fuel NO
emissions are shown in pounds of NOa
per 10s Btu as a function of percentage
nitrogen in the coal on a dry ash-free
basis for overall 5 percent excess oxygen
in the combustion products. The data
presented illustrate two major points:
1. Fuel NO emissions tend to increase
with increasing fuel nitrogen content.
This is most pronounced for premixed
conditions. However, for any given
nitrogen content there is a consider-
able spread in fuel NO emissions,
indicating that some property other
than total fuel nitrogen content
influences fuel NO formation.
2. Fuel nitrogen conversion decreases
with decreasing fuel/air mixing
rates. Since oxygen availability
controls fuel NO formation, this is
expected.
Some anomalous behavior was observed
associated with fuel/air mixing effects
and fuel nitrogen formation. Some coals
show a very strong dependence of fuel
nitrogen formation on fuel/air mixing
characteristics. However, this is not
always the case; even for coals of the
same rank, the influence of fuel/air
mixing appears to be also coal-dependent.
Two fuels tested to date show very
different characteristics. The anthracite
shows almost no impact of fuel/air
mixing rate. Since anthracite is very low
in volatile content, it can be assumed that
most of the fuel NO formed is produced
from solid nitrogen, and that the impact of
fuel/air mixing characteristics is mini-
mized. Alternatively, the Australian
lignite showed a decrease in fuel NO from
950 to 675 to 175 ppm as the combustion
conditions changed from premixed to
radial diffusion to axial diffusion.
It can be argued that the fraction of the
coal fuel nitrogen that is volatile (i.e.,
emitted with the volatile fractions during
thermal decomposition) has a major
impact on the formation of fuel NO.
Therefore, two simple experiments were
carried out to determine if volatile fuel
nitrogen (determined by either the ASTM
volatile matter procedure or an inert
pyrolysis experiment developed under
EPA sponsorship) can be used to predict
fuel NO formation in pulverized coal
flames. The char residue from an ASTM
volatile test was analyzed for nitrogen
content to determine the fraction of the
nitrogen that had been liberated with the
coal volatiles. The results suggest that
this method of assessing volatile nitrogen
evolution is not even capable of ranking
fuels, and is certainly not suitable as a
predictive tool. Under EPA sponsorship, a
pyrolysis reactor was developed to
investigate the conversion of fuel nitrogen
to HCN under both inert and oxidative
pyrolysis conditions. Gas-phase kinetics
experiments suggest that HCN is a
reasonable model for volatile fuel nitrogen
compounds; therefore, the amount of fuel
nitrogen that is converted to HCN could
indicate the potential for a particular coal
to produce fuel NO. The pyrolysis experi-
ment was operated as a two-stage
system. The fuel was placed in an initial
reactor whose temperature could be
varied, and the pyrolysis compounds
were swept into a secondary reactor
(maintained at 1373 K) with an inert Ar
flow. Experiments have demonstrated
-------
Fuel Symbol
Fuel Source
Proximate Analysis
1% as received)
Moisture
Ash
Volatile Matter
Fixed Carton
Ultimate Analysis
l%Dry)
C
H
N
S
Ash
0
Heating Value
IBtu/lb. Wet)
CLASSIFICATION
(ASTMD388)
O
Haze/ton
PA
5.13
5.74
4.39
84.74
88.45
2.14
0.79
0.47
6.05
2.10
13. 124
Anthracite
O
Upper Cliff
AL
3.0O
9.49
20.44
67.O7
79.32
4.47
1.47
1.3O
9.78
3.66
13.254
Medium
Volatile
Bituminous
O
Rosa
AL
8.02
6.79
21.81
63.38
81.23
4.73
1.54
1.04
7.38
4.08
13.394
Medium
Volatile
Bituminous
0
Black Creek
AL
2.25
4.45
28.28
65.02
8212
5.21
1.79
0.76
4.56
5.56
14.284
Medium
Volatile
Bituminous
0
W.KY
5.43
7.53
37.79
49.25
71.58
5.43
1.55
3.21
7.96
10.27
12.082
High
Volatile
A Bitu-
minous
a
wv
7.57
12.05
29.12
51.26
74.68
4.96
1.38
0.86
13.04
5.08
12.228
High
Volatile
A Bitu-
minous
O
Elkay
WV
0.60
11.61
33.35
54.44
73.96
4.93
1.39
2.47
11.69
5.56
13.115
High
Volatile
A Bitu-
minous
0
Gauley
Eagle
WV
3.72
26.89
26.65
42.74
59.43
4.11
1.35
0.84
27.94
6.33
10.110
High
Volatile
A Bitu-
minous
O
Price
UTfl)
6.39
7.40
38.89
47.32
73.52
5.52
1.44
0.61
7.89
11.29
12.340
High
Volatile
B Bitu-
minous
k.
Price
UT(II)
7.41
8.83
38.84
44.92
72.24
5.75
1.55
0.76
9.54
10.16
11.877
High
Volatile
B Bitu-
minous
0
Utah
(Coal)
4.20
9.62
42.97
43.21
69.36
5.32
1.50
1.04
10.05
12.73
11.718
High
Volatile
B Bitu-
minous
e
Utah
(Char)
2.70
16.83
10.29
70.18
75.39
1.28
1.22
1.12
17.30
3.69
11.185
—
a
Cadiz
OH
4.29
18.05
34.87
42.79
62.08
4.44
1.07
7.40
18.86
6.15
11.038
High
Volatile
B Bitu-
minous
Fuel Symbol
Fuel Source
Proximate Analysis
/% as received)
Moisture
Ash
Volatile Matter
Fixed Carbon
Ultimate Analysis
(% Dry)
C
H
N
S
Ash
0
Heating Value
(Btu/lb. Wet)
CLASSIFICATION
(ASTMD380)
Farmington
NM
7.76
17.96
34.95
39.33
63.06
4.65
1.40
O.81
19.47
10.61
10.391
High
Volatile C
Bituminous
Four
Corners
NM
5.24
24.06
35.94
34.76
55.99
4.71
1.23
1.03
25.39
11.65
9.425
High
Volatile C
Fruit/and
NM
11.02
20.72
31.66
36.60
58.71
4.21
1.30
0.93
23.29
11.56
9.344
High
Volatile C
Bituminous Bituminous
Co/strip
MT
21.27
9.58
30.82
38.33
67.52
4.36
1.38
0.63
12.17
13.94
9.169
Sub-
Hardin
MT
22.70
11.26
31.26
34.88
65.54
4.15
0.95
0.79
14.57
14.00
8.603
Sub-
Hardin
MT
20.49
8.62
33.24
37.65
67.88
4.65
0.99
1.07
10.84
14.57
9.229
Sub-
bituminous bituminous bituminous
B
B
B
Shell
TX
25.23
10.28
35.31
29.18
60.99
4.49
1.13
1.O2
13.74
18.63
8.131
Sub-
bituminous
C
Scranton
NO
34.96
7.50
28.85
28.69
64.61
4.17
0.83
1.52
11.53
17.34
6.446
Lignite
A
Beulah
ND
33.10
7.12
28.65
31.13
65.29
3.96
0.99
1.14
10.64
17.98
7.245
Lignite
A
Beulah
ND
34.63
4.97
27.02
3338
66.15
4.20
0.96
0.37
7.60
20.72
7.245
Lignite
A
Savage
MT
36.36
4.61
28.48
30.55
64.99
4.04
1.00
0,42
7.25
22.30
6.995
Lignite
A
Morwell
Australia
9.07
3.38
48.79
38.76
66.25
5.01
0.65
0.28
3.72
24.09
10.051
(wet)
—
Saar
Germany
216
8.04
36.23
53.57
76.13
5.25
1.37
0.83
8.21
8.21
13.657
(dry)
—
almost quantitative conversion of nitrogen
in pyridine to HCN at 1373 K. Thus, in this
experiment, the fraction of fuel nitrogen
capable of conversion to HCN could be
assessed as a function of first-stage
pyrolysis temperature. The results shown
in Figure 3 compare the fraction of fuel
nitrogen converted to NO for the three
combustion conditions as a function of
the fraction of the fuel nitrogen converted
to HCN under pyrolysis conditions at
1373 K. It can be seen that the pyrolysis
data agree qualitatively with the experi-
mental results obtained in the premixed
and radial diffusion flames. The Savage
lignite (A) gives the highest percentage
conversion of fuel nitrogen to NO under
combustion conditions, and also has the
highest fraction of volatile nitrogen yield
in the inert pyrolysis study.
Optimization of Staged
Combustion Conditions
It is generally recognized that staged
combustion offers the most cost-effective
control technique for minimizing fuel NO
formation. The basis for staged combus-
tion as an NOX control technique involves
the competition between two reaction
paths: one forms N2, and the other
NO from the fuel-nitrogen species. The
path producing N2 is favored under fuel-
rich conditions. Thermodynamic limita-
tions indicate that the total fixed nitrogen
species (NO, NH3, HCN) are minimum at
about 65 percent of the theoretical air
requirements for complete combustion
for most hydrocarbon fuels, and that this
minimum concentration is of the order of
10 ppm under combustion conditions.
Thus, it could be concluded that if all the
nitrogen species were in the gas phase
then this thermodynamic limitation
would represent the minimum NO
achievable under staged combustion
conditions, since it is reasonable to
assume total conversion of these small
TFN concentrations during second-stage
burnout. Gas-phase kinetics would also
indicate that the approach to thermody-
namic equilibrium is accelerated at high
temperatures, even though the minimum
TFN concentration would tend to increase.
Staged combustion also has an impact on
the formation of thermal NO because the
heat release process is extended in time
allowing enthalphy loss, thus reducing
peak temperatures.
The primary control options in a staged
system are:
-------
/ .u
1.4
1.2
1.0
0.8
s
5 0.6
° O 4
^
0 0.2
« n
g/..
Q) - i
3 1.2
U.
/.O
0.8
0.6
0.4
0.2
O
„ o i2
- Premixed ^
a ^ 0 °
- r * ° 4°
^
70x/03fltu/"r
" V' 5% O2 (Stack)
Open Symbols— Bituminous
-
-
-
^
-
_
Half Shaded Symbols — Subbituminous
Solid Symbols— Lignite
, * — Anthracite
\\i\\\\l
. Axial Diffusion
.
-
* * A* Q*o
t • fc
vO
O Q (f>
-
r
\ \ \ i i i i i
-
.
-
-
-
-
-
0.6 0.8 1.0 1.2 1.4
Fuel Nitrogen, lb/10e Btu
Figure 2. Fuel NO emissions as a function
of fuel nitrogen content and
mixing conditions.
• How long and at what temperature
must the fuel products remain under
fuel-rich conditions?
• What is the optimum stoichiometry
or distribution of stoichiometries in
fuel-rich zone?
Two series of experiments have been
carried out: one involved three-stage
combustion; and the other, heat extrac-
tion from the primary and/or secondary
zone to assess the various options.
Experiments were carried out in which
the fuel-rich stage was divided into two
sections: the first fuel-rich stage stoichi-
ometry was maintained constant (as was
the overall excess air level), and the
stoichiometry of the second fuel-rich
stage was varied. Exhaust NO emissions
were reduced by about 15 percent when
operating under three-stage conditions
compared to two-stage operation. In
another experiment the bench scale
reactor was modified to allow removable
space cooling coils to be added to the fuel-
rich first stage, or the second stage of a
two-stage combustion system. Maximum
emissions were obtained without cooling,
and minimum emissions were obtained
when heat was extracted from the
primary section. This is surprising since a
reduction in temperature of the first stage
would be expected to increase the
concentration of total fixed nitrogen at
15 0.4
.C
w
Q
as
(£>
O 0.3
s
1
0.2
0.1
- Premixed
Radial Diffusion\
-tt-
Axial Diffusion
0.1
-if-
-If—• •—
0.2 0.1 0.2 0.1
Fraction of Fuel N Converted to HCN at 1373 K
0.2
Figure 3.
Fuel nitrogen conversion can be correlated by HCN produced during pyrolysis for
well mixed combustion conditions.
the exit of the first stage, thus increasing
(not decreasing) final emissions.
Commercial Burner Testing
Since a primary goal of the program is
to demonstrate the distributed mixing
burner (DMB) technology in field operating
boilers, some assurance is required that
the DMB, which has been demonstrated
only in the large watertube simulator
(LWS) research furnace, will also perform
in the field as predicted. Also, the basis
for the prototype design is the LWS
furnace, not a steam raising system.
Although indirect, the approach taken
was to evaluate commercial burner
operation in the LWS and compare it
against field experience. This approach
provides some assurance that, if the
burner operates satisfactorily in the LWS,
it will also operate satisfactorily in the
field.
The burners selected for testing under
this program are:
• Babcock and Wilcox dual-register
burner.
• Peabody Engineering Corporation
standard pulverized-coal burner
with low-NOx modification.
• Steinmuller low-NOx burner (similar
to burners currently used at the
Weiher plant in West Germany).
All the burners were designed for a
nominal firing rate of 50 x 106 Btu/hr,
and the installation in the LWS was
similar to that in the field. To date, the
B&W and Peabody burners have been
tested with the baseline Utah bituminous
coal. The Steinmuller burner will soon be
tested with a bituminous coal from
Germany (similar to coal used in the field)
and baseline Utah coal. In support of the
industrial demonstration program (EPA
Contract 68-02-3127), an 80 x 106
Btu/hr, Foster Wheeler intervane burner
(pre-NSPS) was also evaluated; results
are included in this report for complete-
ness. All of the burners gave stable
flames and performed satisfactorily.
However, the Babcock and Wilcox dual-
register burner produced a flame that
was too long for the LWS firing depth and
gave severe flame impingement on the
back wall. Thus, it is very difficult to
compare field and LWS experience
because impingment prevented complete
heat release of the input coal. Several
general conclusions can be drawn from
the studies with commercial burners:
• The flames observed in the LWS
were judged, by experienced engi-
neers, to be very similar to those
observed in the field.
• CO levels were consistently higher
than measured in the field.
• NO levels were slightly lower than
measured in field operating units.
Conclusions
• Bench Scale Studies. These studies
concerned the development of a
data base on the effect of coal type
on NO formation and the definition
of optimum conditions for minimum
NO formation under idealized fuel/
4
-------
air contacting. Although fuel NO
emissions increased with increas-
ing coal nitrogen content, coals with
the same rank and similar fuel nitro-
gen content produce markedly dif-
ferent fuel NO levels. Preliminary re-
sults suggest that a simple proce-
dure (that evaluated reactive volatile
nitrogen) could be used to assess
the impact of coal type on NO pro-
duction. Studies on the optimization
of the conditions in the fuel-rich
stage of a staged system indicate
that minimum gas-phase nitrogen
species (HCN + NH3 + NO) does not
necessarily give minimum emis-
sions after burnout.
• Distributed Mixing Burner Develop-
ment. Data relating to the develop-
ment of the distributed mixing
burner have been reviewed and
summarized. Single and multiple
burner data have been obtained for
two different fuels. The distributed
mixing concept can be operated with
different fuels, and it appears that
emission levels are sensitive to fuel
type. However, no attempt was
made to reoptimize the burner for
different fuels.
• Comparison with Commercial Prac-
tice. Several commercial burners
have been tested in the research
facility. All the burners performed
satisfactorily and produced stable
flames. However, one burner gave a
flame length that caused severe
flame impingement on the rear wall
of the test furnace. Since severe
impingement cannot be tolerated in
the field, comparison with commercial
practice is invalid. Carbon monoxide
emissions produced with the com-
mercial burners were slightly higher
than those usually found in field
operating units, and NOX emissions
were slightly lower.
Conversion Factors
Readers more familiar with metric
units may use the following factors to
convert the nonmetric units used in this
Summary:
Nonmetric
Btu/hr
Btu/lb
lb/106 Btu
Times Equals Metric
A. ft. Brienza. S. L Chen. M. P. Heap, J. W. Lee. W. H. Nurick, D. W. Pershing, and
D. P. Rees. are with Energy and Environmental Research Corp., Irvine, CA 92714.
G. Blair Martin is the EPA Project Officer (see below).
The complete report, entitled "Development of Criteria for Extension of
Applicability of Low Emission, High Efficiency Coal Burners: Second Annual
Report," (Order No. PB 84-163 898; Cost: $19.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 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
2.93
2.33
430
W,
J/0
ng/J
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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Kfc'ol
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U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/922
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