United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S2-89/065 Apr. 1990
&EPA Project Summary
Furnace Sorbent Reactivity
Testing for Control of SO2
Emissions from Illinois Coals
Brian K. Gullettand Frank E. Briden
Research was undertaken to
evaluate the potential of furnace
sorbent injection (FSI) for sulfur
dioxide (SO2) emission control on
coal-fired boilers utilizing coals
indigenous to Illinois. Tests were run
using four coals from the Illinois
Basin and six calcium hydroxide
[Ca(OH)2] sorbents, including one
provided by the Illinois State
Geological Survey (ISGS). The
evaluation included: pilot- and bench-
scale sorbent reactivity testing,
sorbent microstructure
characterization, and injection ash
characterization.
Pilot-scale FSI testing gave SO2
removal greater than 60%, with some
tests (including those with the ISGS
sorbent) exceeding 70% removal for
Ca/S ratios of 2:1. Bench-scale
testing of injection at economizer
tem-peratures (538°C) yielded com-
parable removals of about 55%. X-Ray
diffraction (XRD) tests of the
sorbents showed a strong correlation
between three measured crystallite
micro-structural parameters and
sorbent reactivity in the FSI tests.
Extraction Procedure (EP) toxicity
tests with the sorbent injection ash
gave values well below Resource
Conversation and Recovery Act
(RCRA) limits for regulated metals.
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 documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Emissions of sulfur oxides, principally
sulfur dioxide (SO2), from combustion
sources have increased awareness and
concern in recent years. In particular,
S02 emissions from coal-fired boilers
used by utilities and industries have been
implicated as major contributors to a
growing acid precipitation problem. While
long-term ecological effects of acid
precipitation are being debated, it is clear
that a reduction in SO2 emissions is
greatly desirable. Factors to weigh in
determining an SO2 control technology
are cost, SO2 removal efficiency, and
ease of retrofitting to existing boilers. The
optimum control technology would
balance removal levels with the cost to
the industry or utility (and ultimately the
consumer). One technology that has
received considerable attention is
Furnace Sorbent Injection (FSI), which
offers relatively low capital cost, ease of
retrofitting, and reasonable removal
efficiencies.
A large body of research on FSI is
currently available. The effects of such
fundamental parameters as injection
temperature, sorbent type, particle size,
and SO2 concentration have been
investigated on the pilot-scale. These
investigations, along with on-going full-
scale demonstrations, indicate that SO2
removals of about 60% may be expected
using commercially available calcium hy-
droxide [Ca(OH)2] sorbents. Noted
potential impacts of FSI on the boiler
include increased slagging and fouling,
increased mass loading on particulate
removal systems, and alteration of the
chemical composition of boiler ash.
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This current investigation is designed
to provide data at the pilot-scale on SO2
removal from a combustpr fired with
Illinois Basin coals and injected with a
range of sorbent types. These
comparative data, along with results from
low temperature testing, physical analysis
of the sorbents, and chemical analysis of
the ash will be used to evaluate FSI as a
control technology for facilities using
Illinois Basin coals. Exceptionally high
removal efficiencies could expand the
range of applications for Illinois high
sulfur coal at a lower cost than coal
cleaning or wet flue gas desulfurization
(FGD) alternatives.
The primary objective of the planned
research has been to evaluate FSI as a
potential S02 emission control
technology for coal fired boilers burning- -
Illinois Basin coals. FSI offers the benefits
of being less capital intensive than wet
FGD as well as the ability to be readily
retrofitted to existing facilities with space
limitations. To evaluate FSI potential, the
following objectives have been outlined:
1. Develop a data base of sorbent
SOg removal efficiencies using six
sorbents with four coals at two Ca/S
ratios in the Environmental
Protection Agency's (EPA)
Innovative Furnace Reactor (IFR) at
a high injection temperature
(1.200 °C) regime.
2. Obtain comparative SO2 reactivity
data for the six sorbents at mid-
range temperatures (538°C) in
EPA's Graphite Furnace Reactor
(GFR).
3. Characterize sorbent microstructure
properties using x-ray diffraction
(XRD) techniques in an effort to
correlate these properties with
sorbent S02 removal efficiencies.
4. Determine the potential for leaching
of toxic metals from FSI ash using
the EPA's Extraction Procedure
(EP) toxicity test.
Experimental Procedures
The four coals used in testing were
Illinois Basin Coal Sample Program
(IBCSP) #1. #2, #6, and #9. Sorbents
chosen for testing included three
commercially available calcium
hydroxides (Marblehead, Linwood, and
Snowflake), a dolomitic hydroxide
(Kemidol), a surfactant modified calcium
hydroxide (lignosulfonate modified
Marblehead), and an alcohol calcium
hydroxide provided by the Illinois State
Geological Survey (ISGS). The ISGS
sorbent was tested by combining equal
parts of each of the 10 batches provided.
This insured that adequate sorbent was
on hand for FSI testing in the IFR.
Individual batches were tested on a
limited basis in the other reactor
systems.
Testing in the IFR consisted of
determining baseline SO2 concentrations
in the flue gas while burning each of the
coals at feed rates sufficient to yield a
firing rate of approximately 49,600 KJ/h
(47,000 Btu/h). After determining a stable
SO2 concentration, sorbent was injected
at various Ca/S ratios between 1:1 and
2:1 and the SO2 level monitored until
equilibrium was achieved. The final SO2
removal percentage was determined as
the average of duplicate tests. The test
matrix consisted of testing each coal with
all six sorbents using duplicate runs (4
coals x 6 sorbents x 2 duplicates x 2
_Ca/S ratios-=_ 96 tests).- „ ,—
Current supply to the electrically
heated GFR was regulated to yield a
temperature profile with a peak of near
538°C while declining rapidly with
residence time (or distance) in the
reactor. Flow rates sufficient to give a
residence time of 0.75 s between 538
and 427°C with an SO2 concentration of
3,000 ppm were used. Each sorbent was
injected under differential conditions with
respect to SO2 concentration and
conversion to calcium sulfite (or sulfate)
determined on solid samples collected by
a cyclone separator.
Each of the six sorbents was analyzed
using XRD. The Warren-Averbach
method of peak analysis for separation of
the crystallite size and strain components
was used to determine major
microstructure properties.
The individual values of the sorbent
microstructural properties were related to
IFR-determined reactivities by regression
functions to test the hypothesis that
various combinations of these properties
could predict^sorbent reactivity.
Tbxicfty""test's"' were performed tin ash
taken from the IFR baghouse during each
of the baseline coal tests excluding coal
#1 for which insufficient sample was
collected. Analyses on eight Resource
Conservation and Recovery Act (RCRA)
regulated metals (antimony, barium,
cadmium, chromium, lead, mercury,
selenium, and silver) and pH were carried
out using methods outlined in EPA
method 1310. Ash from sorbent injection
using one coal (IBCSP #6) and all six
sorbents (Ca/S = 2:1) was also tested to
determine the impact of FSI on disposal
of ash.
Tests were run on the Short Time
Differential Reactor (STDR) using 4 mg of
sorbent exposed to process gas
consisting of 3,000 ppm SO2 in 5 percent
O2 and a N2 balance, preheated to
538°C. The reactor is designed to allow
fixed bed sample exposure times in the
range of 0.3 to 5 s, while maintaining
conditions differential with respect to SO2
concentration.
Results and Discussion
Figure 1 shows several data trends.
SO2 capture levels for the IBCSP #2 coal
are substantially lower for all sorbents
tested (with the possible exception of the
Marblehead hydroxide) than for the other
coals. It is interesting to note that while
the sulfur content of the IBCSP #2 coal
(3.23%) is bracketed by the other coals,
it differs from them in one important
aspect: unlike the other coals tested,
pyritic sulfur accounts for most of the
-.sulfur_-present_in_IB_CSP__#2, ,giviog_a,
pyritic/organic sulfur ratio of 2.53:1
compared to values less than 1:1 for the
other coals. No explanation for the
apparent adverse effect of a high
pyritic/organic sulfur ratio on FSI is
currently available. Results for FSI testing
on the IFR are compiled in Figure 1. The
data presented are estimated SO2
removal percentages at Ca/S of 2:1,
calculated by extrapolating linearly from
the mean removals at both Ca/S ratios
run for each coal/sorbent combination.
Furthermore, when the data from the
three other coals (IBCSP #1, #6, and #9)
are viewed collectively, the S02 removal
by individual sorbents does not differ
radically from coal to coal. For each, the
relative standard deviation of the mean
SO2 removal percentage (standard
deviation of mean removal divided by the
mean) is less than 10%. This could
indicate that the pyritic/organic sulfur
ratio of each coal is the largest coal-
specific factor in FSI performance using
the same sorbent.
The commercially available Ca(OH)2
~ sofb'eTits^tinwoadr-Marbreheadr-and
Snowflake) yield about the same values
for SO2 removal percentages when
excluding the data from IBCSP #2. The
sorbents hydrated under special
conditions (the lignosulfonate modified
Marblehead and the ISGS alcohol
hydroxide) clearly exhibit superior
performance. Past tests attribute the
enhanced performance of the modified
Marblehead to its ability to resist sintering
at the high temperatures seen in FSI.
The performance of the ISGS sorbent
may be related to its very small particle
size. Recent tests have demonstrated the
importance of sorbent particle size to
sulfur capture. Mixing studies have
shown that, in many instances, sorbent
injection takes place under conditions
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' Kemidol
Unwood Marblehead Modified Snowflake
Marbtehead
Sorbent
Coals: •• /BCSP#t EH /8CSP#2
Figure 1. IFR Sorbent Reactivity.
/BCSP#6
likely to result in limitations on mass
transfer rates of S02 to the reacting
particle. In such a regime, ultimate
sorbent reactivity will be inversely related
to the size of the reacting particle.
The overall impression of the
applicability of FSI as a SO2 control
technology for Illinois coals is positive.
Except for IBCSP #2, which gave lower
results for unknown reasons discussed
earlier, SO2 removals for each of the
coal/sorbent tests approached or
exceeded 60% at a Ca/S ratio of 2:1.
Indeed, tests with the specially modified
sorbents routinely exceeded 70%. These
test results strongly recommend FSI as a
cost effective means of controlling S02
emissions from coal-fired combustors.
Results from economizer temperature
»(53S°G) sorbent injection testing on the
GFR are shown in Table 1. The data
show a clear inverse relationship to
sorbent particle size as measured using
the sedigraph; as particle size decreases,
the conversion of the sorbent to the
calcium sulfite product in the GFR
increases. Again, this indicates mass
transfer resistances acting to control the
rate of reaction, rather than other
potentially faster mechanisms such as
inherent chemical kinetics. Removing
these resistances may show a faster true
rate of reaction.
Results from testing in the STDR with
an SO2 concentration of 3,000 ppm using
ISGS BH-29 sorbent are shown in Figure
2. Similar conversions were obtained with
Linwood hydroxide over the same time
Table 1. Results from Economizer Injection
Tests on GFR
Mean Conversion
Sorbent (%)*
Marblehead 9.8 ±0.7
Modified Marblehead 11.0 + 1.1
Snowflake 11.7 ±1.0
Linwood 15.2 ±2.9
ISGSBH-20 17.7 + 1.0
ISGSBH-24 17.6 ±1.1
ISGS BH-29 19.9+2.2
Kemidol 15.3+2.5
C)Data obtained from minimum of 10 runs at
538°C, resisdence time =0.75 s, 3,000 ppm
So2, 5% O2, A/2 balance.
range. These results predict an SO2
removal of roughly 55% for a 1 s
residence time and Ca/S ratio of 2:1
when injecting sorbent at or near 538° C.
More work is needed to accurately
quantify the fundamental rate of the
sorbent/SO2 reaction under economizer
injection conditions using reactors like
the STDR prior to predicting potential
SO2 removal levels. The effects of
parameters such as S02 concentration,
sorbent surface area, and sorbent
porosity on reaction have not been
thoroughly investigated.
It has been proposed that crystallite
size can affect the gas-solid reactions by
modifying the interface between the two
phases. It is further proposed that crystal
lattice strain could contribute to reactivity
ISGS
Mix
/SCSP#9
60
S? 50
1,0
§ 30
§ 20
•8
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within the distribution of dim-ensions
measured in a crystallite.
In the next stage, regression equations
were derived for relating the XLB factors
two at a time to the observed IFR
reactivities. For the 15 pairs of factors,
the correlation coefficients varied from
0.13 to 0.79. The best pair of estimators
was maximum column length and the
strain at maximum column length with a
correlation coefficient of 0.79. This value
is considered quite significant, con-
sidering its derivation was subject to coal
and furnace variability.
Since the increase in correlation was
vastly improved by using two factors, the
third stage was to use three factors for
the analysis. For triplets, the correlation
coefficient varied from 0.40 to 0.99. It
would appear that it is possible to-almost—
completely characterize the
microstructural relation to reactivity with
three XLB factors. The best correlation
coefficient of 0.99 was derived from the
average column length, modal column
length, and strain at maximum column
length.
Those three XLB factors appear to be
the best estimators of reactivity from the
number of samples analyzed to date.
Future studies of other sorbents could
further establish the reliability of this
method and its application toward ranking
sorbent reactivity without undergoing
large-scale testing.
Values for all of the regulated metals
are below the RCRA limits. Sorbent
injection would appear to stabilize many
of the metal species, particularly arsenic
and cadmium. While the final pH values
are below RCRA limits, they are high
enough to elicit some concern. Methods
for stabilizing the ash or neutralizing
leachate from, the ash may bear
investigation.
Conclusions and
Recommendations
Pilot-scale testing of the SO2 removal
potential of FSJLwittx. Illinois Basin coals
" "show@d~thatTemoval in excess-of-60%"-
can be readily achieved using
commercially available sorbents and a
Ca/S ratio of 2:1. The ISGS alcohol
sorbent and the Marblehead
lignosulfonate modified sorbent gave
removals in excess of 70%. Lower
removals were noted for the coal high in
pyritic sulfur (as opposed to organic
sulfur). Further investigation is necessary
to verify and explain this phenomenon.
The EPA authors, Brian K. Gullett (also the EPA project officer) and Frank E.
Brlden, are with the U.S. EPA's Air and Energy Engineering Research
Laboratory, Research Triangle Park, NC 27711.
Brian K. Gullett is the EPA Project Officer (see below).
The complete report, entitled "Furnace Sorbent Reactivity Testing for Control of
SO2 Emissions from Illinois Coals," (Order No. PB90-150 330/AS; Cost:
$17.00, 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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency ~~ — - -•"=- -^^*^^s^-^s^
Research Triangle Park, NC 27711
The greatest removals were seen using
the ISGS alcohol hydroxide. It is believed
that its performance is enhanced by its
small particle size and the resultant
mixing benefits.
Testing of sorbent injection at
economizer temperatures (538°C)
showed that removals of roughly 55% at
a Ca/S ratio of 2:1 can be expected.
However, not much is currently known
about fundamental reaction kinetics for
this mid-temperature sorbent/SO2
reaction. To more accurately predict the
full-scale performance of injecting
sorbent in this temperature region, the
effects of more temperature, SOz
concentration, and sorbent characteristics
on reactivity need to be clarified.
XRD tests indicated that the sorbent
-microstr-ucturaLcharacteristLcs_oLaver.age_
column length, modal column length, and
strain at maximum column length can
provide a basis for prediction of sorbent
performance in FSI applications.
Analyses of the FSI ash showed that it
could be considered nonhazardous in
terms of RCRA limits for leaching of
heavy metals. The pH of the leachate is a
concern, however, because of its alkaline
nature.
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-89/065
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