United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 2771
Research and Development
EPA/600/S7-87/019 Sept. 1987
Project Summary
Pilot-Scale Evaluation of LIMB
Technology
R. S. Dahlin, R. Beittel, and J. P. Gooch
In support of EPA's LIMB (Limestone
Injection Multistage Burner) develop-
ment program. Southern Research In-
stitute (SoRI) performed pilot-scale
studies of sulfur capture in the LIMB
process and the effect of LIMB on
particulate properties and electrostatic
precipitator (ESP) performance. The
sulfur capture studies showed that
hydrated lime was generally superior to
limestone as a sorbent for in-furnace
sulfur removal. For both sorbents,
downstream injection was found to be
preferable over near-flame injection.
With hydrated lime, the optimum in-
jection temperature was found to be
about 1200°C, where utilizations as
high as 30% were achieved. The injec-
tion of either sorbent resulted in a large
increase in the electrical resistivity of
the ash, which could severely impact
ESP performance. Laboratory and pilot-
scale studies showed that the resistivity
increase could be offset by flue gas
conditioning using sulfur trioxide (SO3)
or water vapor. With limestone injection,
acceptable resistivity levels were re-
stored by the injection of 30 ppm of
S03.
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 In-
formation at back).
Introduction
EPA's Air and Energy Engineering
Research Laboratory (AEERL) is develop-
ing the Limestone Injection Multistage
Burner (LIMB) process as a low-capital-
cost control option for compliance with
possible acid rain legislation. The purpose
of the LIMB process is to achieve a 50%
reduction in emissions of sulfur oxides
(SOX) at a calcium-to-sulf ur (Ca/S) molar
ratio of 2. This would make the process
applicable to a significant number of
existing coal-fired boilers that would
probably be impacted by acid rain legis-
lation. In a retrofit application of LIMB,
the sorbent (limestone or hydrated lime)
is injected downstream of the burner
zone using a retrofitted system for
sorbent handling and injection. The re-
sulting particulate (calcium sulfate, un-
reacted calcium oxide, and fly ash) is
collected in the existing ESP or baghouse.
Since sorbent utilization is generally low
(~ 12-15% for limestone and 25-30% for
hydrated lime), it is necessary to use
more than (typically twice) the stoichio-
metric requirement of the sorbent. This
results in a significant increase in the
particulate loading that must be handled
in the boiler system and particulate col-
lector. The particle size and electrical
resistivity of the particulate are also af-
fected by the sorbent injection, and these
effects can severely impact particulate
collection efficiency.
Objectives and Scope
The original goal of this research pro-
gram was to study the effect of the LIMB
process on particulate properties and ESP
performance. As part of this work, it was
necessary to evaluate the effectiveness
of various sorbents in terms of sulfur
capture. The latter evaluations prompted
an expansion of the project scope to
include a study of sulfur capture in the
LIMB process. Thus, the project report
covers two areas of LIMB-related re-
search: (1) Studies of sulfur capture in
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the LIMB process, and (2) Studies of the
effect of LIMB on participate properties
and ESP performance. The goal of the
sulfur capture studies was to investigate
the effects of sorbent type, injection
conditions, particle size, and promoters
on sulfur capture. The goal of the par-
ticulate and ESP work was to examine
the effects of LIMB on paniculate loading,
particle size distribution, and electrical
resistivity (three of the major factors in-
fluencing ESP performance). Prior to the
sulfur capture or particulate work, how-
ever, it was necessary to verify the ability
of the SoRI pilot-scale coal combustor to
simulate utility boiler conditions and ash
characteristics. This was necessary to
ensure that the LIMB-related testing done
in this unit could be applied to full-scale
utility boilers. The goal of the verification
effort was to show that the ash produced
by the SoRI combustor was similar to
full-scale ash from the same coal in terms
of particle size distribution, morphology,
electrical resistivity, and chemical com-
position. These comparisons showed ex-
cellent correspondence between the
combustor ash and ash produced from
the same coal in a full-scale unit.
Results and Discussion
Sulfur-capture screening studies of
sorbent type and injection location were
conducted with three sorbents: Vicron
45-3 calcitic limestone (V), Longview
calcitic hydrated lime (L), and Corson
pressure-hydrated dolomitic lime (C).
Tests were performed with both coal firing
and S02-doped natural gas firing. The
injection location was varied from the
burner (B) to furnace section 4 (S-4),
which is near the furnace outlet. The
corresponding gas temperatures and
sulfur captures (at Ca/S = 2) are:
Coal Firing
Injection
Location
B
S-3
S-4
Temperature
°C
ND
1237
1132
Capture, %
V L C
35 37 ND
40 61 81
32 S3 ND
Natural Gas Firing
Injection Temperature Capture, %
Location °C V L C
B
S-3
S-4
1477
1332
1126
30 29 ND
24 40 80
28 45 ND
At equivalent Ca/S ratios, the sorbents
are ranked in performance: pressure-
hydrated dolomitic lime > calcitic hydrated
lime > limestone. This may be misleading,
however, since the dolomitic lime also
contains 1 mole of magnesium per mole
of calcium. Thus, a much greater weight
of dolomitic lime is required to achieve
the same Ca/S ratio. When compared at
equivalent mass injection rates, the
calcitic and dolomitic hydrated limes give
virtually identical performance, despite
the fact that the magnesium is inert and
does not react with S02 under furnace
conditions. This suggests that the
magnesium acts to facilitate the reaction
between the CaO and SO2. For both the
limestone and the hydrated lime, the
optimum injection temperature was about
1237°C with coal firing. Since this was
optimum for both the limestone and
hydrated lime, the other injection loca-
tions were not tested with the pressure-
hydrated dolomitic lime.
The dependence of sulfur capture on
the sorbent particle size was investigated
using size-fractionated samples of ash/
sorbent mixtures collected isokinetically
at the exit of the combustor system.
Chemical analyses of these fractions
showed that sorbent utilization was a
strong function of particle size for both
the limestone and the hydrated lime. The
data obtained with both sorbents injected
atS-4(~1132°C)are:
Particle size, fim 0.5 1.0 2.0 5.0 10 20
Utilization, %
Vicron limestone 35 32 22 15 14 ND
Longview hydrate 25 26 25 18 15 13
Sorbent utilization decreases with in-
creasing particle size for both sorbents.
This points out one advantage of hydrated
lime over limestone: the mass median
particle size is much smaller (~2 vs. 15
Mm). It also illustrates the potential per-
formance gains from ultrafine grinding of
the limestone, to the extent it is practical.
The promotion of sulfur capture by the
use of a sodium-based additive was in-
vestigated using 5 wt% of sodium bi-
carbonate premixed with the Vicron
limestone. With natural gas firing and
sorbent injected through the burner, the
sulfur capture (at Ca/S = 2) was almost
doubled by the promoter (32% vs. 60%
capture). However, this effect was largely
eliminated when fly ash was added to the
system to simulate coal firing and when
similar tests were conducted during coal
firing. This suggests that the volatilized
sodium is being lost to the fine fly ash
particles, so that it is not available for
promotion with coal firing.
The effect of sorbent injection on the
electrical resistivity of the ash was deter-
mined through in situ resistivity measure-
ments in the pilot-scale combustion
system and through IEEE laboratory tests
in controlled environments. The baseline (
(without sorbent injection) resistivity
values ranged from 2x10s to 1010 ohm-
cm in the presence of 22 to 40 ppm of
naturally occurring S03. When limestone
was injected, and virtually all of the S03
was removed « 0.2 ppm remaining), the
in situ resistivity was increased to about
1012 ohm-cm. In laboratory tests per-
formed at the same temperature (~
150°C), the resistivity was 9x1012 ohm-
cm in the absence of any S03. With 5
ppm of S03 in the laboratory test cell, the
resistivity was reduced to 5x108 ohm-cm,
illustrating the extreme sensitivity to
residual S03 levels. Higher levels of S03
are required to produce this effect in the
combustor system due to the much
shorter exposure time (days in the labora-
tory vs. seconds in the combustor system).
The ability to restore acceptable SO3 levels
and resistivity values was demonstrated
using a catalytic S03 generator and in-
jection system. The results of these
studies are:
SO3 injected, ppm 0 10
Resistivity, ohm-cm I.SxIO'2 2x10"
20 30 40
3x10'° 3x10* 2x10s
These results indicate that resistivity can
be restored to acceptable levels at rea-
sonable S03 injection rates. This was
also true with hydrated lime injection,
although more SO3 was required, and
the amenability of the ash to conditioning
was much more sensitive to sorbent
injection location. With injection at S-3
(~ 1237°C), 120 ppm of SO3 was required
to reduce resistivity to 1010 ohm-cm.
Despite this large injection rate, less than
8 ppm of SO3 remained in the gas phase
at the exit of the system, suggesting that
almost all of the SO3 was adsorbed on
the particulate.
The effect of sorbent injection on the
particulate size distribution was evaluated
through in situ measurements made in
the pilot-scale system using cascade
impactors, an optical counter, and an
electrical mobility analyzer. With lime-
stone as the sorbent, burner injection
produced fine (0.1-1.0 /im) particle con-
centrations that were an order of magni-
tude higher than with downstream
injection at S-4. This suggests that the
limestone decrepitates at the higher
temperatures associated with burner
injection. With downstream injection at
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IS-4, the particle size distributions ob-
tained with limestone and hydrated lime
were similar, despite the much finer size
of the original hydrated lime.
The effect of sorbent injection on ESP
performance was predicted using the
resistivity values and the particle size
data in the EPA/SoRI model of electro-
static precipitation. The results for the
baseline (no sorbent) and the limestone
injection cases are:
from the burner, away from the flame
zone. This study suggests that the opti-
mum injection temperature is about
1237°C, although a higher injection
temperature may be needed in a full-
scale boiler to allow for mixing effects.
A retrofit application of LIMB tech-
nology can have a devastating impact on
ESP performance, especially for older
plants with undersized ESPs. In these
Total paniculate loading, mg/m3 (gr/ft3)
Mass median panicle size, p.m
Resistivity, ohm-cm
SCA, nf/rrf/sec (ft'/kacfm)
Average applied voltage, kV
Current density, nA/crrf
Predicted collection efficiency, %
Baseline
6,876(3.00)
14
2x10 10
44.3(225)
41.7
26.3
99
Limestone
15,586(6.8)
16
2x10 12
44.3(225)
29.7
1.50
93
The predicted degradation in ESP per-
formance corresponds to a factor of 16
increase in emissions. If the original
resistivity and electrical operating condi-
tions are restored by flue gas condition-
ing, the collection efficiency can be
brought back to about 99%, but the emis-
sions would still be higher by a factor of
2.3 due to the higher inlet loading.
Further improvements to the ESP in-
ternals would be required to restore
original emission levels with such a small
ESP. Larger units may have excess
capacity that would allow emissions to
be controlled at a comparable level with-
out further modifications.
Conclusions and
Recommendations
Based on the sulfur capture data from
this study, it appears that hydrated lime
must be used in lieu of limestone to meet
the performance objective of 50% SO2
control at Ca/S = 2. The studies of size-
fractionated samples suggest that further
improvements in hydrated lime per-
formance may be possible by fractionating
out the smallest particles. The practicality
of this concept has not yet been evaluated.
Although limestone performance im-
proves with decreasing particle size, it
does not seem feasible to attain 50%
removal by ultrafine grinding due to the
power requirements and cost. Improve-
ment of sorbent performance by the use
of a sodium-based additive does not
appear feasible due to the apparent loss
of the volatilized sodium on the fly ash
particles. Additives that enhance the
specific surface area of the sorbent may
be advantageous. Whatever sorbent is
selected, it should be injected downstream
plants, it appears likely that flue gas
conditioning and modification of the ESP
internals will be necessary to restore
acceptable performance. The use of water
sprays may be advantageous to gain the
benefits of cooling, conditioning, and
reducing the gas flow. This would provide
an increase in the effective specific col-
lection area (SCA) to complement the
reduction in resistivity. Other potential
remedial measures would include the
installation of a cold-pipe precharger
section, conversion of the ESP to an
ESOX process, or even the use of a wet
ESP. Enlargement of the SCA is not
feasible unless it is accompanied by the
use of conditioning or other modifications.
Space limitations also make this difficult
at many older plants. Pilot testing of
various ESP modifications is required to
select the optimum remedial measures
for a LIMB retrofit. Such pilot test should
include an evaluation of electrode de-
posits and rapping requirements.
/?. S. Dahlin R. Beittel, and J. P. Gooch are with Southern Research Institute,
Birmingham. AL 35255-5305.
Samuel L, Rakes is the EPA Project Officer (see below).
The complete report, entitled "Pilot-Scale Evaluation of LIMB Technology,"
(Order No. PB 87-224 630/AS; Cost: $18.95. 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 Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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EPA/600/S7-87/019
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