United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-89/005 Feb. 1990
ve/EPA Project Summary
Bench-Scale Studies to Identify
Process Parameters Controlling
Reburning with Pulverized Coal
D. W. Pershing, M. P. Heap, and W. R. Seeker
The report addresses the
evaluation of a technology which is a
combination of two technologies
used to control the atmospheric
emission of NOX by stationary
sources: (1) combustion modification
(controls flame temperature and
maximizes fuel-rich residence time to
minimize NOX formation); and (2) flue
gas cleaning (uses a reducing agent
with or without a catalyst to remove
NOX from combustion products). The
combined technology uses fuel as a
reducing agent to remove NOX. The
process (referred to as in-furnace
NOX reduction, reburning, and staged
fuel injection) can be applied to many
types of combustion systems. In
fact, reburning is the process which
allows the "in-furnace NOX reduction"
to take place.
This Project Summary was devel-
oped by EPA's Air and Energy
Engineering 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 summarizes the results of a
small theoretical and experimental study
which was undertaken as part of the
EPA's Fundamental Combustion
Research Program to investigate in-
furnace NOX reduction (reburning). In
simple terms, the reburning concept
involves the use of a heat release zone
(via staged fuel addition) to convert NO
formed earlier in the main combustion
zone into some form which will ultimately
produce N2. The process takes place in
two discrete zones.
1. NO Reduction Zone. Here fuel is
added to produce CH radicals
which reduce part of the NO from
the main combustion zone to N2,
HCN, and NH3.
2. X/V Oxidation Zone. Here the final
combustion air is added and a
percentage of the total fixed
nitrogen (TFN) pool (HCN + NO
+ NH3) and char nitrogen (if any)
are oxidized to NO as the
remaining fuel fragments burn to
C02 and H20).
Thus, NO can produce N2 in both
zones and the key to reburning is to
provide the species and temperatures
which allow this to happen. The first zone
forms N2 but also converts NO into
species which can also be converted to
N2 in the second zone.
Data Available
Figure 1 shows data obtained with coal
firing, demonstrating the overall potential
of the reburning concept (referred to as
Mitsubishi Advanced Combustion
Technology, or MACT). This figure also
shows data obtained in the boiler
simulator furnace (BSF) at Energy and
Environmental Research Corporation.
Both sets of data indicate that it is
possible to achieve extremely low levels
(50 ppm NOX at 6% 02) under ideal
combustion conditions; in general, it has
been possible to reproduce the Japanese
results under similar test conditions in the
U.S. However, the application of
reburning to large-scale commercial
systems in the U.S., burning a wide
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range of bituminous and low-rank coals,
is not as simple as Figure 1 suggests. In
the reburning process NO can produce
N2 in both zones and, to optimize the
process, it is necessary to provide the
species and temperatures which maxi-
mize the rate of N2 formation. Some NO
is reduced to N2 in the first zone, but the
remaining NO either remains as NO or is
converted into species which are capable
of being reoxidized in the second stage.
To investigate the optimization and
application of reburning, experimental
studies were conducted in two facilities: a
5.7 cm ID, 7.38 MJ/hr (2kW), back-fired,
laboratory reactor firing doped gaseous
fuels and a 15.2 cm ID, 73.8 MJ/hr (21
kW), tunnel furnace firing pulverized coal.
The reburning fuels used in this
investigation included propane, North
Dakota lignite, bituminous coals from
Utah and Alabama, and Australian coal.
Conclusions
The principal conclusions of the work
relate to both the NO reduction zone and
the XN oxidation zone.
NO Reduction Zone
1. The optimum rich-zone stoichio-
metry (SR2) is approximately 0.9
because of the tradeoff between
NO reduction and increased
concentration of easily oxidizable N
species. Figure 2 compares the
data from several sources and
attests to the consistency of the
overall conclusion. At stoichio-
metries leaner than 0.9, the initial
NO is not reduced as effectively
perhaps because of a lack of CH
radicals. At rich-zone stoichio-
metries below approximately 0.9,
large amounts of TFN species
(particularly HCN and NH3) are
produced and ultimately oxidized to
NO in the final stage. As Figure 2
indicates, this problem is greatly
enhanced with coal, where the
reburning fuel nitrogen becomes
more significant as additional
reburning fuel is added.
2. The primary zone stoichiometry
(SR,) has little influence on the
exhaust NO at the optimum
reburning conditions. Increasing
SR1 only slightly decreases
effectiveness of the reburr
concept in spite of a large inc«
in the available reactive nitroge
the rich zone (due to increz
reburning fuel).
3. The nitrogen content of
reburning fuel has only a s
effect on the reburning efficienc
optimum combustion condith
but at lower rich-zone stoic
metric ratios, it can be of IT
importance. Figure 3 shows
obtained with doped prop
flames and indicates that at
zone stoichiometries be
approximately 0.8, the nitre
content of the reburning fuel gn
influences the effectiveness of
reburning concept. This ii
particular importance becaus
large-scale utility system w
inevitably have a distributioi
rich-zone stoichiometries ac
the combustion chamber in
reburning zone.
4. The effectiveness of rebur
depends strongly on
250
200
150
o
§ too
I
o
3 50
Rate of Combustion:
- Fuel:
No. of Burner Units:
BSF Data
H50kg/h
Taiheiyo Coal
3 (Stages) x 4 (Corners)
MACT = Mitsubishi Advanced Combustion Technology
BSF = Boiler Simulator Furnace
\ I I i I
I
0 1 23456
O2 Concentration in Exhaust Gas, percent
Figure 1. NOX removal effect for coal firing.
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ID
Q.
90
00
70
60
50
40
30
20
10
Model Prediction (500 ppm)
Wendt (WOO ppm = NO/, Secondary Reduction Only)
_.._ Takahashi et al. (Gas Phase, 100 ppm NO)
EER Gas-Phase Data (540 ppm,
EER Propane/Beulah Bench Scale (633 ppm)
I _ I _ I _ I _ I
0.60 0.70 0.80 0.90 T.O
Stoichiometry of the NO Reduction Zone
Figure 2. Effectiveness of reburmng-subscale data.
1.1
1.2
relationship of the initial NO
concentration to the amount of
reburning fuel nitrogen added to
achieve an overall rich-zone
Stoichiometry of 0.9. Figure 4
shows that, even with optimum
combustion conditions at low initial
NO levels, reburning may actually
increase exhaust NO emissions.
With an initial NO level of
approximately 150 ppm and a
typical coal as the reburning fuel,
the reactive nitrogen available in
the rich zone increases almost
fivefold when the reburning fuel is
added to acheive an overall
Stoichiometry of 0.9.
5. Coal composition is important
because it influences char burnout,
the initial NO level, and the
freburning fuel nitrogen content and
speciation. Many coals commonly
used in the U.S. for power
generation may prove to be
relatively unsuitable for reburning,
particularly in retrofit applications,
because it will not be possible to
effectively burn out the coal char in
the available time.
XN Oxidation Zone
1. The TXN (total fixed nitrogen,
including char N) conversion
depends on the XN speciation, the
XN concentration, the hydrocartbon
content at the rich-zone exhaust,
and the thermal environment.
2. Low XN conversions can be
achieved by tailoring the
temperature profile to obtain
selevtive reduction of NO by NHj
species in the final, oxygen rich
stage. Figure 5 summarizes the
results obtained in the tunnel
furnace and shows the dramatic
influence of thermal environment on
the effectiveness of the reburning
concept for two initial NO levels.
This effect is believed to be directly
related to a large decrease in the
TFN conversion in the final stage of
the reburning process.
In summary, the overall processes
controlling the reburning phenomena
have been relatively well-identified and
characterized. Although the elementary
reactions are not fully understood, the
key parameters have been identified and
the overall mechanisms defined. Further
work could define the influence of mixing
rates and establish the potential impacts
of applying reburning to boilers and
furnaces.
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(NO)., = 633 ppm
170
160
150
140
1 130
I 120
\ 1W
S. 700
90
80
70
60
50
40.
0.6
0.7
0.8
SR2
0.9
1.0
Figure 3. Effect of SR2 and reburning-fuel nitrogen content (tunnel furnace).
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* NHj. N * 1.04%
120
a 80
40
20
SR, = 1.1
SR2 = 09
SR3 = 1.2S
o-
I
200 400 600 800 1000 1200
(NO)p, ppm
Figure 4. Influence of returning fuel type and primary NO level.
CjHg/Seu/an
2400 2600 2800 3000
600
500
400
300
200
too
I I T
_ Open - Normal
Closed - Secondary Cooling
SR, = 1.1
SR3 = 1.25
0.6
0.7
0.8 0.9
SR2
Figure 5. Effect of secondary cooling on exhaust NO emissions.
1.0
1.1
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D. W. Pershing, M. P. Heap, and W. R. Seeker are with Energy and Environmental
Research Corp., Irvine, CA 92714-4190.
W. Steven Lanier is the EPA Project Officer (see below).
The complete report, entitled "Bench-Scale Studies to Identify Process Parameters
Controlling Returning with Pulverized Coal," (Order No. PB 89-200 81 Oi'AS; Cost
$21.95, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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