United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/025 Aug. 1985
SERA Project Summary
Bench Scale Studies of
Limestone Injection for S02
Control
P. L Case, L Ho, M. P. Heap, R. Payne, and D. W. Pershing
This report describes research carried
out in one task of an EPA program
entitled. The Development of Criteria
for Extension of Applicability of Low
Emission, High Efficiency Coal Burners.
The task involved using a series of
bench-scale facilities to determine the
process parameters controlling the cap-
ture of sulfur species by calcium-based
sorbents when pulverized coal is burned
under low NO* conditions.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory. Research Trian-
gle 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).
Objectives
Although coal is the most abundant
source of fossil fuel energy in the U.S., its
use poses several serious problems for
society, not the least of which is the
emission of atmospheric pollutants. The
combustion of coal results in the forma-
tion of sulfur and nitrogen oxides (SO«
and NOx), which have been identified as
precursors of acid precipitation. Although
evidence is not conclusive, it is generally
recognized that the environmental dam-
age caused by acid precipitation can be
mitigated by reducing emissions of these
precursors from coal-fired power plants.
Consequently, there is a need to develop
efficient and cost effective control tech-
niques for these pollutants for retrofit
applications. This report addresses the
use of calcium based sorbents for in situ
sulfur capture under combustion condi-
tions that minimize NOx formation from
pulverized coal combustion.
The use of calcium based sorbents to
remove S02 from the combustion prod-
ucts of coal and oil flames is not a new
concept. Considerable effort was expend-
ed during the late 1960s and early 1970s
to develop the dry limestone injection
process. These activities were curtailed
because sulfur capture was not particu-
larly efficient and injecting limestone
caused considerable operating problems
(e.g., fouling in the convective sections
and plugging of the air heater). Recent
studies in Germany indicate that this
process could be a viable means of reduc-
ing SOa emissions if the sorbent were
injected under low NOx conditions. In the
earlier studies, poor sulfur capture was
attributed to thermal deactivation of the
sorbent (deadburning) and poor sorbent
dispersion throughout the boiler. These
problems may be overcome if the sorbent
is injected with one of the air or fuel
streams in a low NO, burner. This would
solve the problem of dispersion since all
the fuel and air must mix if combustion is
to be completed. Thermal deactivation
could be reduced because low NOX condi-
tions minimize peak flame temperature.
Objectives of the bench scale studies
described in this report were:
1. To determine if the reduced peak
flame temperatures and extensive
fuel-rich zones typical of low NO*
combustion conditions are condu-
cive to sulfur capture by dry sorbent.
2. To determine the optimum time/-
temperature stoichiometry history
of the sorbent for sulfur capture
during coal combustion.
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3. To determine if captures reported
in several pilot scale tests are pos-
sible in the current boiler popula-
tion.
4. To provide preliminary information
on the impact of calcium based sor-
bents on boiler operability.
Approach
When the task was initated, insuffi-
cient information was available to allow a
complete definition of the bench scale
studies. The questions raised in previous
experiments and the large number of
process variables suggested that a
phased approach was necessary. This
method of planning and experimentation
allowed the investigation of processes
and parameters which proved most impor-
tant and enabled a selection of test condi-
tions which would develop an under-
standing of the sulfur capture process in
the most cost effective manner. The pro-
gram organization is shown schemati-
cally in Figure 1. The task was initated by
conducting a series of program definition
tasks in an existing refractory tunnel fur-
nace, concurrent with the design and
construction of a boiler simulator furnace
(BSF). Most of the experimental investi-
gations were carried out in the BSF. Fig-
ure 1 shows that the BSF experiments
were divided into three categories: (A)
combustion parameter screening, (B)
thermal history/sulfur chemistry studies,
and (C) fuel and sorbent studies. The data
from these experiments were used to
plant the pilot scale studies carried out in
other tasks of the same program and to
define the needs for future experiments.
The BSF, simulating the thermal his-
tory of the products of combustion in a
wide variety of pulverized coal fired boil-
ers, was designed to satisfy the following
criteria:
• Independent control of wall tempera-
ture throughout the furnace and con-
vective sections.
• Fuel consumption rate nominally 80
Ib/hr (36 kg/hr).
• Accommodate various firing systems.
• Variable preheat on all combustion air
streams.
• Simulation of particulate dropout
throughout the radiant and convective
sections.
• Ability to rapidly and easily clean inte-
rior sections.
• Potential for near adiabatic operation
by minimizing heat loss.
Program
Definition
Preliminary
Shakedown
Experiments
BSF*
Design
BSF
Construction
Boiler
Simulator
Furnace.
I
(A)
Combustion
Parameter
Screening
\
(B)
Thermal History,
Sulfur Chemistry
Studies
i
(C)
Fue/&
Sorbent
Studies
Results from
Other
Investigations
\\
Pilot
Scale
Studies
Identification
of Controlling
Processes
Figure 1. Organization of bench scale sulfur studies.
• Potential to extract heat at various
locations.
• Ease of access for sampling probes.
However, foremost in the design was
recognition that the times and tempera-
tures experienced by the sorbent should
cover as wide a range as possible to
encompass those encountered in actual
boilers. Figure 2 is a schematic of the BSF
system, showing the sorbent and coal
feed and air supply systems and the gen-
eral arrangement of thefurnacesections.
The BSF was designed to simulate the
various parts of a boiler. Thus, the radiant
furnace simulates the main heat release
zone; the postflame cavity, the region
above the main heat release zone before
the furnace exit; and air-cooled tubes, the
superheater, reheat, and air heater sec-
tions of the convective pass.
Significant Results
Task results are in three groups: com-
bustion parameter screening, sulfur chem-
istry and thermal environment, and fuel
and sorbent effects.
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Sorbent
Feeder
O Standard Gas Sample Locations.
Continuous NO, SOz. Oa CO, CO2
Laminar Flow Elements (LFE)
4/r Heaters. Propane Fired
Probe Access— 6" to 10" Spacing,
Entire Furnace Axis
Loss in Weight
Coal Feeder
Propane or
Baghouse
Removable Water-
Cooled Pipes (3)
Convective
Section
Figure 2. Functional schematic of boiler simulator furnace.
Combustion Parameter
Screening Studies (Series A)
The screening experiments defined the
role of various combustion parameters in
the sulfur capture process. The experi-
ments were conducted in two sections,
according to the method of combustion
air staging used to simulate low NOX
combustion systems. In the first series of
experiments (internally staged), a sub-
scale distributed mixing burner was used
and the parameters investigated included
firing rate, heat extraction, burner zone
stoichiometry, excess air level, sorbent
injection location (with the fuel or staged
air), and staged air velocity. The second
series of experiments (externally staged)
were designed to investigate sulfur cap-
ture under fuel-rich conditions using a
.system which provided for a relatively
' long (1 -2 sec) residence time under fuel-
rich conditions. In this second series of
tests, the parameters investigated in-
cluded firing rate, sorbent injection, heat
extraction rate, rich zone stoichiometry,
excess air level, and tertiary air location.
The results of the screening studies
indicated that thermal environment had
the major impact on sulfur capture by dry
sorbents injected directly into the fur-
nace. The burner-staged experiments
showed the influence of thermal envi-
ronment most clearly: a reduction in peak
furnace temperature due to cooling in the
radiant furnace or to a reduction in load
increased sulfur capture significantly.
Changes in other parameters (e.g., sor-
bent location or injection velocity) had
secondary effects on sulfur capture. The
externally staged experiments indicated
that sulfur capture was less dependent
on overall heat extraction rate.
A main point of interest in the param-
eter studies was to establish the impor-
tance of rich zone capture, since it was
speculated that reactions involving re-
duced sulfur species (e.g., H2S or COS)
could be captured more easily than SO2
Figure 3 shows overall sorbent utilization
as a function of the stoichiometry of the
fuel-rich zone for coal and propane doped
with H2S to give the same exhaust S02
level. I n this instance, the sorbent (Vicron)
was added with air in the fuel-rich zone at
the base of the postflame section. Rich
zone stoichiometry is expressed in terms
of the gas phase for both the gaseous and
solid fuel. It can be argued that the differ-
ence in effectiveness with the two fuels
is associated with coal: under fuel-rich
conditions, much of the sulfur remains
with the char and is not available for cap-
ture. When all sulfur is in the gaseous
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30
20
.§'
I
a
10
Q Coal
/\ Propane + HiS
0.4
Figure 3.
0.6 0.8 1.0
First-Stage Stoichiometry Ratio (Gas Phase)
1.2
Capture and retention of sulfur under external staged conditions for coal and
propane doped with H*S, sorbent added with air at base of postflame section.
uncooled.
stage, calcium utilization under fuel-rich
conditions approaches 30 percent.
Sulfur Chemistry and Thermal
Environment Studies (Series B)
These experiments clarified the effect
of thermal environment on sulfur cap-
ture, utilizing two series of experiments
to:
1. Investigate sulfur evolution and
speciation in the fuel-rich zone of a
staged combustion system.
2. Define the influence of a wide range
of thermal environments on sulfur
capture under both internal and
external staged conditions.
Results from these tests include:
• Under fuel-rich conditions, sulfur spe-
cies appear to be far from equilibrium.
• Even with gaseous fuels, sulfur cap-
ture in the fuel-rich zone does not
appear to significantly impact overall
sulfur capture.
• With coal, capture in the fuel-rich
zone is limited because of the reduced
sulfur driving force when a large frac-
tion of the sulfur remains in the solid
phase.
Fuel and Sorbent Effects
(Series C)
The influence of fuel and sorbent prop-
erties on sulfur capture was investigated
in the third series of bench-scale experi-
ments. The sorbent parameters, which
were varied, included particle size and
composition. Sulfur capture was meas-
ured with a coal/water slurry and a
lignite, as well as the baseline bituminous
Indiana coal.
Figure 4 compares sulfur capture for
the four fuels investigated with one sor-
bent. The capture is worst with the Indi-
ana coal and very similar for the lignite,
doped propane, and the coal/water slur-
ry. The dotted line shown in Figure 4 cor-
responds to 50 percent capture at a calci-
um/sulfur ratio of 2 with 2,500 ppm SO2.
The influence of sorbent type on sulfur
capture was fuel dependent. With the
Indiana coal, dolomite was by far the
most effective sorbent; whereas, with
doped propane, captures with dolomite
and Vicron were similar. The reasons for
the interaction of fuel and sorbent effects
are not now known.
Conclusions
Results of the experiments in the boiler
simulator furnace indicated that the
parameters of major importance are
thermal environment, calcium/sulfur
ratio, and sorbent composition. Thermal
environment (local temperature) was
shown to have a strong effect on the utili-
zation of dry sorbents injected directly
into the furnace. The burner-staged
experiments showed the influence of
temperature most clearly: a reduction in
peak flame temperature, due to cooling in
the radiant zone or to a reduction in load,
increased sulfur capture significantly.
Changes in other combustion parameters
(e.g., sorbent location and injection veloc-
ity) had secondary effects on sulfur
capture.
The effect of thermal history under
physically-staged conditions was less
definitive. Changes in both load and
thermal cooling in the radiant furnace
were complicated by changes in furnace
temperature due to the physical staging
itself, since physically staging the coal
flame significantly altered the furnace
temperature profile. With external stag-
ing near stoichiometric conditions, the
first (fuel rich) stage, which included the
entire radiant furnace, and the second
stage region (the first part of the post-
flame cavity) were hotter than under
burner-staged conditions. First stage
Stoichiometry and sorbent location both
influence sulfur capture under externally-
staged conditions; however, the results
suggest that these influences may be
more strongly related to thermal than to
chemical changes. Further, the data do
not support the concept that sorbent utili-
zation is much higher under fuel-rich
conditions.
The influence of sorbent type (Vicron
45-3, Vicron 15-15, Michigan marl, hy-
drated lime [Ca(OH)2], and dolomite) on
sulfur capture was studied at five differ-
ent combustion conditions: Indiana No. 3
coal fired in the distributed mixing burner
with and without external cooling, Indi-
ana No. 3 coal under externally-staged
conditions, and propane doped with HaS.
In general, dolomite gave good capture
under all conditions. Hydrated lime ap-
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