United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85-012 May 1985
&ER& Project Summary
Bench-Scale Process
Evaluation of Reburning and
Sorbent Injection for In-Furnace
NOx/SOx Reduction
S. B. Greene, S. L Chen, W. D. Clark, M. P. Heap, D. W. Pershing, and W. R.
Seeker
A study was initiated to investigate
in-furnace NOX/SOX reduction tech-
niques through the combined use of
returning and limestone injection. Re-
burning is a multistage combustion
modification technique in which fuel is
added downstream of the main firing
zone to produce a fuel-rich zone where
NO from the main firing zone is reduced.
Burnout air is added farther down-
stream to provide for complete burnout
of the reburning fuel. Sorbent injection
involves injection, into the furnace, of
calcium-based materials onto which
SO2 can be absorbed. The absorbed
sulfur is removed from the flue gas with
the particulate as calcium sulfate.
Tests have been carried out at bench
scale (20.5 kW) to investigate the
impact of process variables on the
effectiveness of the combined technol-
ogy. Under the best conditions, up to
80% NOX reduction and 60% removal of
SOx at a calcium/sulfur ratio of 2 have
been achieved. The impact of each
variable in each zone was investigated
independently. The dominant parame-
ters were found to be the reburning
condition and the primary NO level. The
time, temperature, and stoichiometric
requirements of the reburning zone
influenced NO, reduction efficiency in a
manner consistent with a kinetically
controlled process; i.e., higher temper-
atures and longer residence times at an
optimum stoichiometry of 0.9 were
favorable. SO, reduction was most in-
fluenced by the location of injection of
the sorbent; in particular, injection with
the burnout air was optimum.
This Project Summary was developed
by EPA's Air and Energy Engineering
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 infor-
mation at back).
Introduction
This project addresses the combined
technologies of reburning for NOX reduc-
tion and sorbent injection for SO, capture.
The goal of the project was to provide an
authoritative assessment of the applica-
tion of these technologies to U.S.-de-
signed pulverized-coal-fired boilers. Re-
burning involves the addition of down-
stream fuel to form a rich zone in which
NO formed upstream is reduced. The
concept of NO reduction by flames has
been known for over a decade and is now
being applied in Japan in large furnaces.
The reburning process can be divided into
three zones:
• Primary Zone: This main heat release
zone accounts for about 80 to 90% of
the total heat input to the system. The
zone is operated under overall fuel-
lean conditions, although the burners
might be Iow-N0x distributed mixing
burners. The level of NOX exiting from
this zone is the level to be reduced in
the reburning process.
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• Reburning Zone: The reburning fuel
(normally about 1 0 to 20% of the total
fuel requirements) is injected down-
stream of the primary zone to create a
fuel-rich reduction zone. The reactive
nitrogen entering this zone comes from
two sources: the primary NO level and
thefuel nitrogen in the reburning fuel.
These fuel nitrogen species apparently
react with the hydrocarbon fragments
from the reburning fuel to produce
intermediate species such as NH3 and
HCN, while some is converted to Nz
and some is retained as NO. The
products of this reduction zone are the
reactive nitrogen species such as NO,
char nitrogen, NHs, and HCN, which
will be referred to as total reactive
nitrogen. To optimize the NO reduction
by reburning, it is necessary to min-
imize thetotal reactive nitrogen exiting
the reburning zone.
• Burnout Zone: In the burnout zone, air
is added to produce overall lean condi-
tions which oxidize all the remaining
fuel and convert the total fixed nitrogen
to either NO or Na.
Reburning can be combined with fur-
nace sorbent injection in an attempt to
reduce both NOX and SOX. Calcium-con-
taining sorbents, typically either calcitic
or dolomitic limestones, lime, or hydrated
lime, are injected directly into the furnace
and absorb SO2. The reaction is a gas/
solid reaction of the form:
CaO(s) + S0z(g) - CaSOafs)
Oz(g) - CaS04(s).
The solid calcium sulfate, CaS04, formed
is removed with the fly ash particulate.
When combined with reburning, sorbents
can be injected in a number of locations:
with the primary fuel, with the reburning
fuel, with the burnout air, or downstream
of all the reburning zones.
The U.S. EPA has a major program to
investigate the combined in-furnace NO,/
SOX reduction technologies for pulverized-
coal-fired furnaces. This program will
attempt to define the process require-
ments and application techniques and
will ultimately include estimates of capital
and operating costs for application to new
and existing boiler designs. The first
phase of that program, reported here, is
an experimental effort carried out at
bench-scale to investigate the impact of
process design variables on the effective-
ness of the combined technologies. The
bench-scale testing has provided funda-
mental insight into the chemical process-
es that control NOX/SOX reduction. The
continuing effort will include pilot-scale
tests at 10 x 106 Btu/hr (3 MW> on
reburning/limestone injection perform-
ance in order to provide information on
process scaling and hardware. The final
task will be an engineering and economic
assessment of the application of the
technology as defined by the experimental
program for the inventory of U.S. coal-
fired furnaces.
Control Temperature Tower
The process studies were carried out in
a refractory-lined Control Temperature
Tower (CTT), shown schematically in
Figure 1. The CTT has a total firing rate of
between 18 and 24 kW (60,000 and
80,000 Btu/hr) in the main combustion
chamber, which is 20.3 cm in diameter
and includes a long quarl entry to promote
flame stabilization and to provide for one-
dimensional plug flow. The time/ temper-
ature profile along the furnace could be
manipulated by using back-fired heating
sections within the insulating refractory.
The back-fired sections consisted of natu-
ral gas burners fired into refractory
channels in the direction opposite to the
main chamber. The high temperature
gases pass through the channels sur-
rounding the main chamber (see the
c
Removable/
Choke
Back-Fired
Heating
Channel
Removable —
Cooling
Coil
Back- Fired —
Heating
Channel
Removable
Cooling —
Coil a
(\
1 \
— - — a
1
— ^.
k »
» -
H
— fci
— *t
Flue Gas - ,^.
Sampling
Location
I
i
f
radical cross-sectional view in Figure 1)
and minimize the temperature decay t
along the furnace. A more rapid temper- '
ature decline can be achieved by leaving
the back-fired channels off or by inserting
cooling coils around the main chambers.
The tower is equipped with numerous
ports along the axis of the reactor which
allow the installation of zone separation
chokes, fuel and air injectors, cooling
coils, and sampling probes.
The CTT was configured into three
zones: (1) the primary zone, formed using
a premixed burner fired on pulverized
coal or propane doped with various levels
of H2S and NO, under lean conditions
(typically 10% excess air); (2) the reburn-
ing zone, formed by injecting the reburn-
ing fuel (either coal or doped gas) at
various flowrates to control the reburning
zone stoichiometry; and (3) the burnout
zone, in which air was injected to bring
the overall stoichiometry to typically 25%
excess air. The parameters in each zone
were examined separately in terms of
how they influenced the exhaust level of
NOX. The test series was performed by
establishing the level of NOX from the
primary and then increasing the amount
of reburning fuel addition and burnout air
correspondingly to decrease the reburn-
ing zone stoichiometry and maintain the
Back-Fired
Heating Channel
To Stack
Figure 1. Cross-sectional views of the control temperature tower.
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overall burnout zone stoichiometry. In
this manner, the residence time and
temperature in the reburning zone were
maintained relatively constant while the
reburning zone stoichiometry was varied.
Process Parameters
The process zones and parameters
investigated were:
• Primary Zone
—Stoichiometry
—Fuel type
—NO level
—SOx level
• Reburning Zone
—Stoichiometry
—Mixing rate of reburning fuel
—Reburning fuel type(propane, hydro-
gen, CO, and coals)
—Nitrogen content of reburning fuel
—Temperature (peak temperature and
temperature profile)
—Residence time
—Limestone sorbent type and Ca/S
ratio
—Transport media for reburning fuel
(air or inert)
• Burnout Zone
—Temperature
—Excess air
—Air mixing rate
—Limestone sorbent type and Ca/S
ratio
The impact of each variable was invest-
igated independently in terms of the
reduction efficiency of NOX and SOz
capture by calcium.
NOx Reduction by Reburning
Table 1 summarizes the influence of
the process variables on effectiveness of
the reburning process to reduce NOX. The
dominant parameters were found to be
the reburning zone condition and the
primary NOX level. An optimum stoichi-
ometry of 0.9 was found to exist inde-
pendent of the primary zone stoichiometry
for a wide variety of fuels and conditions.
The time and temperatures within the
reburning zone influenced the NO reduc-
tion efficiency in a manner consistent
with a kinetically controlled process.
Higher temperatures and longer resi-
dence times were favorable. The most
dramatic impact was with the primary
NO* level, as shown in Figure 2. At high
levels of primary NOx, the reduction
efficiency was relatively independent of
NO* level or reburning fuel type, and NOx
reductions of 70% were achieved. At
lower levels of primary NO, «200 ppm).
Table 1. Influence of Process Variables on Reburning Effectiveness for NO, Reduction
Parameter Impact
Primary Zone
Stoichiometry
Fuel type
NO level
Reburning Zone
Stoichiometry
Mixing rate of reburning fuel
Fuel type
Temperature
Residence time
Transport media
Burnout Zone
Excess air
Air mixing rate
Temperature
• No effect except will require more
reburning fuel for burner operation.
• No direct effect. Can influence through
temperature and /VOX level
entering reburning zone.
• Strong effect. More difficult to
reduce lower levels of primary NO*
• Optimum at overall stoichiometry of 0.9.
• Faster mixing preferred,
• Hydrocarbon fuels more effective; fuel
nitrogen content detrimental at lower
primary NO* level.
• Reduction increases with increasing
temperature (1316 - 1593°C).
• Strong impact, increasing with time
(100-750 msec).
• Inert transport (oxygen free) is desirable
since less reburning fuel is required
to attain optimum stoichiometry.
• Not important except for burnout.
• Not important.
• Not important unless temperature is
dropped to 927°C where selective
NH \-NOn reactions can take place to
enhance reduction.
140
120
c 100
^ 60
1 40
20
i ^ i r
Utah Coal
Indiana Coal
100 200 300 400 500 600 700 800 900 1000 1100 1200
(NO Jr. ppm (Dry. 0% 0?)
Figure 2. Impact of primary /VOX on effectiveness of reburning.
3
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the reduction efficiency decreased with
decreasing NOX. The reduction was less
for nitrogen-containing reburning fuels.
Reburning with coals gave similar reduc-
tions to gaseous fuels except at primary
NO* levels below about 700 ppm. A wide
variety of coals were tested including all
ranks from lignite, subbituminous, and
bituminous, to anthracite. All coals were
found to behave similarly except at low
primary NOX levels where the coal nitro-
gen content became important. Some
operational problems were encountered
with lower volatile coals due to burnout
limitations.
Combined NOX/SOX Reduction
Sulfur capture by calcium-containing
sorbents was investigated when the
sorbents were injected in the primary,
along with the burnout fuel and with the
burnout air. These studies were con-
ducted under conditions that were found
to favor N0« reduction by reburning. The
dominant parameters found to influence
sulfur capture by sorbent injection were:
• Sorbent type (limestone, dolomite, or
hydrate lime).
• Sorbent injection location (injection
temperature).
• Temperature profile in sulfation temper-
ature region (1250-950°C)
• Calcium/sulfur ratio.
The first two parameters were found to
change the surface area of the sorbent
after calcination which determines its
ability to subsequently uptake sulfur.
Figure 3 shows the sulfur capture data for
one particular reburning condition as a
function of specific surface area of the
sorbents after calcination. The surface
area was measured by extracting solid
samples when sorbents were injected
into the furnace in the absence of sulfur,
since sulfation closes the surface area.
There was found to be an almost linear
relationship of sulfur capture with specific
surface area and calcium/sulfur ratio.
Injection of the sorbent with the burnout
air was preferred due to the relatively low
injection temperature which produced
higher specific surface areas. Dolomite
was found to achieve the highest captures
of the sorbents tested, which could be
attributed to its high specific surface
area. In general, better capture was ob-
tained with coal as the reburning fuel
than with gas. The causes of this en-
hancement with coal are uncertain.
Process Models
A complete process model has been
developed for both NO, reduction by
reburning and sulfur capture by limestone
injection. These models allow the data to
be interpreted and used to estimate
reburning efficiency and sulfur capture in
practical applications of the technologies.
In both cases, the models are semi-empir-
ical approaches, developed in modular
fashion on the individual subprocesses.
The reburning process model treats
separately the fuel-rich secondary flame,
the post-flame reduction, and the lean
combustion of the nitrogen species enter-
ing the burnout zone. The fuel-rich sec-
ondary flame module is a simple correla-
tion of the speciation of nitrogen com-
pounds which occurs when reburning
fuel is mixed with the primary products.
The post-flame reduction module is based
on the combined effects of rich-gas-phase
homogeneous reduction and heterogen-
eous reduction of NO on carbonaceous
particulate, and describes the changes in
total reactive nitrogen. The lean burnout
module predicts final NO concentrations
from the conversion of reactive nitrogen
species. An empirical correlation is used
which was originally developed to predict
NO concentrations emitted from the lean
stage of rich/lean-staged combustion.
Submodels are required for coal-fired
reburning which describe nitrogen spec-
iation in pyrolysis and char nitrogen
processing.
The sulfation model involves dividing
the process into two steps: sorbent acti-
vation and sorbent sulfation. Sorbent
calcination is assumed to occur instan-
taneously, leaving an active stone. The
specific surface area of the stone is
determined by the peak injection temper-
ature. A correlation has previously been
developed which allows specific area to
be estimated for a broad range of temper-
atures and sorbent types. The sulfation
model treats the active sorbent particles
as spheres enveloping a random distribu-
tion of cylindrical pores which provide the
overall particle surface area. The reaction
is initiated at the pore surface, and a layer
of reaction product (calcium sulfate) forms
around the pore. This reaction product
layer separates the pore surface and the
solid reactant surface. The gas (S02) must
diffuse through the product layer to the
surface of the solid reactant (CaO) where
60
50
£ 40
I
c
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the sulfation reaction takes place. The
model also considers boundary layer
diffusion and pore diffusion of the gas
reactant. This model can successfully
predict the capture data for the full range
of conditions investigated in the bench-
scale evaluation.
Conclusions
The process chemistry of the reburning
technology has been examined in some
detail and found to be similar to staged
combustion processes. These data taken
at bench-scale indicate that reburning for
NOx control is a viable technique that can
achieve up to 70% NO* reduction under
optimum conditions. The parameters
which most influence the efficiency of the
process are those conditions in the re-
burning zone and the initial NO level.
Levels below 100 ppm appear to be
difficult to obtain with reburning. The
optimum injection for limestones is with
the burnout air. The reactivity of the
limestone is determined by its surface
area after calcination. To obtain high SO2
capture levels will require stones with
surface areas greater than 20 mVg which
was found only for dolomitic stones in the
present study.
These studies have concentrated on
the chemistry of the reburning process
under ideal conditions; i.e., rapid mixing
and distinct zones. Activity in the next
phase will be designed to investigate the
impact of scale and finite rate mixing.
These tests will be carried out at a firing
rate of 3 MW (10 million Btu/hr) in a
specially designed dispersion furnace.
S. B. Greene, S. L. Chen, W. D. Clark, M. P. Heap, D. W. Pershing, and W. R. Seeker
are with Energy and Environmental Research Corp., Irvine, CA 92718.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Bench-Scale Process Evaluation of Reburning and
Sorbent Injection for In-Furnace A/0«/SO« Reduction," (Order No. PB 85-185
890/AS; Cost: $22.00, subject to change/ will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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:]985/559 111/10850
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