United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-88/023 Dec. 1988
&EPA Project Summary
Effects of Sorbent Injection for
Sulfur Dioxide Removal on
Particulate Control Systems for
Coal-Fired Boilers
J. P. Gooch, J. L. DuBard, M. G. Faulkner, G. H. Marchant, Jr., R.
S. Dahlin, and R. Beittel
Various studies were undertaken to
quantify the effects of dry SO2
sorbent injection on ESP operation
with a coal-burning utility boiler.
The specific dry sorbent operation of
interest was EPA's LIMB process.
The combination of spent sorbent
and fly ash has a higher resistivity, a
higher mass concentration, and a
finer particle-size distribution than
the ash alone; all of these factors
diminish the effectiveness of ESP.
Also investigated was chemical
conditioning to reduce the resistivity
concern, the only one of the three
concerns stemming from sorbent
injection that can be readily
mitigated. Other topics studied were:
the recycle, disposal, and utilization
of waste ash-sorbent mixtures; the
selection and modification of
sorbents to improve SOg capture in
the furnace; and the reactivation of
spent sorbent by humidification to
achieve supplemental post-furnace
capture of SO2-
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
Limestone Injection, Multistage Burners
(LIMB) is being developed by EPA as a
way to control SC>2 emissions from
coal-fired boilers. This technology is
one of several based in dry-sorbent
injection that are expected to be
adaptable in retrofit applications to
boilers that predate new-source
performance standards. It provides only
partial reduction of SOj emissions at
economical operating costs; however, it
requires a lesser capital investment than
more efficient conventional processes,
such as wet scrubbing, and is thus a
candidate for application where old
boilers continue in service.
One concern m implementing LIMB
technology is the difficulty introduced in
maintaining acceptable levels of
particulate emissions. The concentration
of suspended solid matter that must be
removed is greatly increased; moreover,
the fraction of the solids consisting of
small particles (and thus especially
difficult to remove) is substantially
increased. These changes in suspended
solids will have unfavorable
consequences in any particulate-control
device. Their impact is most serious,
however, in terms of the performance of
electrostatic precipitators (ESPs), since
most of the boilers where LIMB may be
applied operate in conjunction with ESPs
for particulate control.
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The primary concern of this research
was the problem of operating ESPs
successfully in utility plants equipped
with LIMB. Three of the five major tasks
addressed this central issue. A
secondary concern was to provide
technical support of the full-scale
demonstration of LIMB that EPA is
undertaking at the Edgewater plant of
Ohio Edison at Lorain, Ohio. The two
remaining tasks were in this area.
Project Scope and Objectives
The five technical tasks that provided
the organization structure of this project
were:
1. Development of a predictor of the
electrical resistivity of mixtures of fly
ash and sorbent that result from
sorbent injection into coal-fired
boilers.
2. Determination of the effect of high
mass loadings of ash/sorbent
mixtures on ESPs.
3.Designing and implementing a
pilot-scale program of ESP with
furnace sorbent injection.
4. Investigation of recycle, disposal,
and utilization of LIMB ash/sorbent
mixtures.
5. Studies with a pilot-scale
combustor to characterize sorbents
that may be applicable to the LIMB
process.
Tasks 1-3 addressed different
aspects of the concern of ESP
performance in conjunction with a
LIMB-equipped furnace: the high
electrical resistivity of the ash/sorbent
mixture resulting from the LIMB
operation; the substantially elevated
concentration of particulate matter to be
collected; and the long-term
maintenance of a pilot-scale ESP
attached to a moderate- or full-size
furnace. An objective under Task 5 was
the evaluation of sulfur tnoxide gas or
water vapor as a means of lowering the
high resistivity of an ash/sorbent mixture.
Task 4 addressed issues of importance
to the LIMB demonstration at Edgewater.
Can spent sorbent in the form of calcium
oxide be rejuvenated by hydration and
reinjected as calcium hydroxide to
achieve more complete utilization? Can
spent sorbent be disposed of in a landfill
without contaminating groundwater? A
third question, less related to Edgewater
than to the broader future application of
LIMB, can a commercial market be found
for LIMB wastes?
Task 5 addressed a number of
additional issues of concern at
Edgewater: the selection of a sorbent
from those available in the market; the
enchancement of sorbent activity with
lignosulfonate (a dispersing agent) as an
additive; and the reactivation of spent
furnace-injected sorbent by humid-
ification of flue gas in a low-temperature
duct, to achieve post-furnace capture of
SOa This task also addressed the issue
of chemical conditioning of ash/sorbent
mixtures to lower their resistivity.
Results and Discussion
Task 1. Development of a resistivity
predictor for ash/sorbent mixtures. The
first step was to obtain a variety of
samples of ash/sorbent mixtures and
characterize them with respect to mineral
composition, particle size, surface area,
density, and electrical resistivity. Efforts
were then made to establish correlations
between resistivity and one or more of
the other properties. In particular, an
attempt was made to establish a
relationship between resistivity and the
sum of lithium and sodium contents,
which is the basis of an earlier correlation
for ordinary fly ash; this attempt failed.
During the project, it was discovered that
the procedure used for resistivity
measurements altered the surface area
of the ash/sorbent mixtures and thus
altered the resistivity.
The need for a revised procedure for
measuring resistivity was thus evident.
Work toward that end will be described
in the report of another project. The effort
to develop the resistivity predictor,
however, remains unfulfilled; the pros-
pects for completing this effort
successfully cannot be evaluated from
the data now available.
Task 2. Investigation of high mass
loadings in ESPs. This task was
addressed through both experimental
and theoretical approaches. The
experimental effort centered on the use
of a specially instrumented ESP that was
operated in conjunction with a pilot-
scale combustion facility, which
produced an ash of high resistivity (2 x
1012 ohm-cm), an elevated concentra-
tion (higher by a factor of three), and a
particle-size distribution with a much
greater proportion of fine material The
primary observations were that: there
was an ESP malfunction owing to the
high ash resistivity, and there was a
strong space-charge suppression of the
corona (observable when the ast
resistivity was lowered with 803) due t(
the fineness of the particle-siz*
distribution. The data were used in the
existing ESP model to project the effec
on ESP emissions of adding the LIME
process to a typical boiler, shown ir
Figure 1.
The theoretical study consisted o
development of a revised version of the
ESP model, which is responsive to £
wider range of ESP operating conditions
Comparisons of the revised model with z
limited number of field data sets indicate
that the revised model functions satis-
factorily as a predictive tool
Nevertheless, there was an indicated
need for certain further refinements in the
model.
Task 3. Performance of a pilot-scale
program of ESP performance with
furnace sorbent injection. Arrangements
for pursuing this task at Base Gagetown
of the Canadian armed forces in New
Brunswick were completed, and certain
background data were obtained.
Ultimately, however, the pilot facilities
assembled at Gagetown were dismantled
at EPA's direction before the planned
measurement program could be
completed. A crucial factor that led to this
action was the abnormally high carbon
content of the ash produced in the
Gagetown burners, which would have had
a serious impact on ESP performance,
regardless of the impact produced by
sorbent injection.
Task 4. Studies of LIMB ash recycle,
disposal, and utilization. The results of
these studies demonstrate that LIMB ash
can be reactivated by hydration and can
then be reinjected to achieve further
utilization for SO2 capture A major
stumbling block to this approach,
however, is the excessive mass loadings
that will occur unless the sorbent can be
separated from the fly ash. Potential
separation methods that deserve further
study are based on a cyclone or an ESP,
a dense-phase classifier, or a steam
elutriator.
Both laboratory and field tests showed
that LIMB ash can be safely handled and
used for landfilling. Cementitious
reactions in the moist ash produce a
nearly impermeable product, which has a
minimal potential for groundwater
contamination. LIMB ash cannot be
classified as a hazardous waste under
any of the RCRA criteria.
Synthetic aggregates represent one of
the most promising uses for LIMB ash
Such aggregates are likely to conform
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vapor alone cannot activate the solid
appreciably. Physical wetting, on the
other hand which occurs when dry
entrained sorbent particles and water
spray droplets collide, has the
potential for strongly activating the
solid. The practical limitation of the
wetting process is the infrequency of
particle-droplet collisions, which
allows most of the sorbent particles
to remain dry and hence unreactive.
• Charge-augmented sorbent
humidification (CASH) was studied
briefly. In this process, sorbent
particles were charged negatively
and water droplet positively, to
enhance the collision frequency.
Further study of the concept was
pursued under another project. In the
final analysis, no beneficial result of
the process seems to be in the
offing.
The effects of SOa gas and water
vapor were compared as
conditioning agents, to alleviate the
concern with high resistivity of
ash/sorbent mixtures in an ESP. At
concentrations of about 60 ppm,
SOa was found to be largely
ineffective - much less promising
than had been indicated by prior
studies. At addition levels of around
5% by volume (which gave
approaches to adiabatic saturation of
11°C), water vapor, on the othe
hand, produced a much mor<
promising result - a reduction ii
resistivity from 1 x 1013 to 1 x 101
ohm-cm. The effect of water vapo
is shown in Figure 2.
Recommendations
Further research should be done tc
complete or augment work initiate(
during this project. Examples o
objectives not yet satisfied are the
development of a resistivity predicto
(Task 1) and the long-term evaluation o
a pilot-scale ESP operating with LIME
particulate matter generated in <
moderate- or full-scale combusto
(Task 3). Efforts to complete Task 1 have
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0 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.
C 40 60 84 112 144 182 227 283 352 44
f 103 141 183 233 291 359 441 541 666 82
Temperature
Figure 2. Resistivity of ash/sorbent mixture as a function of temperature and humidification.
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been made under another project, but
have not yet reached a satisfactory end
point. The long-term ESP test is now
rescheduled under another project as
part of the LIMB demonstration at
Edgewater. Further developments of the
revised ESP Model from Task 2 have
been made under another project.
Certain findings under Tasks 4 and 5
can be implemented at Edgewater if
required: The LIMB wastes may be
safely disposed of in a landfill; the
Ca(OH)2 sorbent selected for Edgewater
may be treated with lignosulfonate
additive, either in solid or liquid form, to
achieve enhanced activity; and the
concern with high electrical resistivity of
the waste-ash/sorbent mixture may be
dealt with adequately with water-vapor
conditioning.
Certain results under Tasks 4 and 5
indicate that certain modifications to
LIMB (to enhance the process) may
require further work. Further work is
needed to make the recycle or
commercial utilization of LIMB wastes
practical and economical. Additional
studies are needed to permit effective
post-furnace reactivation of spent
sorbent by humidification; the data now
available, however, reveal that the
primary limitation on the effectiveness of
humidification is the infrequency of
collisions of sorbent particles and water
droplets. The charge-augmented
humidification process that was
investigated seems to offer no benefit.
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