United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-89/001 Sept. 1989
&EPA Project Summary
Humidif ication of Flue Gas to
Augment SO2 Capture by Dry
Sorbents
J. P. Gooch, R. Beittel, E. B. Dismukes, and R. S. Dahlin
In a coal-burning power plant,
humidification of the flue gas in a low
temperature duct is a possible way to
increase SO2 removal by dry calcium-
based sorbents. In particular,
humidification may be a desirable
modification of EPA's LIMB process,
which is based on the injection of
limestone or hydrated lime in the
furnace; it could augment SO2 re-
moval by adding post-furnace re-
moval to that occurring in the
injection zone. Southern Research
has investigated certain aspects of
low-temperature S02 removal in
humidified flue gas as part of the
research effort funded by the EPA.
This Project Summary was devel-
oped by EPA's Air and Energy
Engineering Research Laboratory, Re-
search 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
EPA has been developing the dry-
sorbent technology known as LIMB over
the past several years as a means of
lowering S02 emissions from coal-fired
power boilers. The acronym stands for
Limestone Injection, Multistage Burners.
Originally, the process was envisioned as
one based on limestone as the specific
calcium-based sorbent; currently, how-
ever, the process emphasizes the use of
hydrated lime — that is, Ca(OH)2 — as a
more reactive sorbent in place of lime-
stone. LIMB based on either sorbent,
however, depends on the injection of the
sorbent in the furnace at a temperature
above 1100°C, which leads to the
calcination of the sorbent and the
reaction of the resulting CaO with SO2 in
the presence of O2 to produce CaSO4.
The utilization of the sorbent under
these conditions does not exceed 25 to
30% on the mole basis and, thus,
accounts for removal of 50 to 60% of the
S02 present at a practical 2:1 Ca/S mole
ratio. Such utilization levels are accept-
able for retrofit operations in boilers
predating new source performance regu-
lations. Still, increased utilization is
obviously desirable if it can be attained.
Humidification and cooling of the flue
gas in a post-furnace duct, beginning at a
temperature of about 150°C, is one con-
ceivable way to enhance the utilization of
furnace-injected sorbent. At relatively low
temperatures, SO2 can react with the
calcine of limestone or hydrated lime to
produce CaSO3; whereas, at high tem-
peratures in the furnace, it reacts only in
combination with 02 to produce CaSO4.
The critical issue, however, is not temper-
ature and the thermodynamic possibility
of sulfite formation but humidification and
the kinetic enhancement of low-temper-
ature reactions, which would be slow
without humidification.
Humidification is a way to enhance SO2
removal with furnace injection of dry
sorbents and to remove SO2 with direct
injection of Ca(OH)2 in either the dry or
the wet state. Extensive research under
the auspices of the Department of Energy
has been recently performed and is now
continuing on duct-injection processes,
which take two basic forms: (1) separate
injections of dry Ca(OH)2 and a water
spray, and 2) simultaneous injection of a
Ca(OH)2 and water in a slurry. In either
process, the sorbent reaches the
particulate collector (ESP or baghouse) in
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a dry state, after the water has evapo-
rated. Maintaining proximity to vapor
saturation, however, is the key to
effective S02 removal.
Scope of the Investigation
This research project addressed four
topics within the general area of sorbent
reaction with S02 under conditions of
low-temperature flue-gas humidification,
• Pilot-scale investigation of post-
furnace humidification to achieve
sorbent activation. This investigation
was carried out with Southern Re-
search's 1 x 106-Btu/h (300-W) coal
combustor to support EPA's LIMB
demonstration at Ohio Edison's
Edgewater power plant at Lorain, Ohio.
The central issue was to determine
what humidification conditions are
required for sulfated calcine from the
furnace to undergo low-temperature
reaction with S02. Is humidification by
water vapor alone adequate, or is
humidification with a water spray
required because of the need for
physical wetting of the sorbent
particles?
• Pilot-scale investigation of charge-
augmented sorbent humidification
(CASH). Once again, the Southern
Research pilot-scale combustor was
used to address a question relevant to
the Edgewater demonstration. Can
opposite electrical charging of sorbent
particles and water droplets increase
collisions sufficiently to enhance S02
removal?
• Laboratory studies of water-vapor
adsorption by Ca(OH)2 with and with-
out additives. A brief experimental
program was conducted to quantify
the effects of certain additives on
water-vapor uptake by Ca(OH)2. If
Ca(OH)2 is to be used in a duct-
injection process, use of a deli-
quescent additive may be desirable.
Alternatively, if partially sulfated
calcine from furnace-injected sorbent
is to be rehydrated outside the power
plant and reinjected in a duct process,
it may be advantageously treated with
a water-attracting additive.
• Mathematical modeling of humidifica-
tion and S02 removal processes. A
mathematical model was developed to
treat two processes: (1) the collision of
sorbent particles and water droplets
from a spray nozzle, and (2) the
capture of S02 by the wetted sorbent
particles. Interaction parameters con-
sidered included particle and droplet
sizes, relative velocities, proximity to
saturation residence time, and Ca/S
ratio.
Procedures and Results
Mechanisms of post-furnace
humidification. The first of the pilot-scale
investigations listed above was
concerned with how flue gas should be
humidified to achieve effective capture of
S02 in a post-furnace duct. Three
humidification procedures were con-
sidered: (1) addition of water vapor alone,
with separate cooling of the gas by
conductive or convective processes; (2)
addition of a water spray, resulting in
simultaneous humidification and cooling
of the gas by evaporation; (3) reinjection
of sorbent in an aqueous slurry. The third
procedure would provide chemical reacti-
vation of the sorbent — conversion of
CaO to the more reactive Ca(OH)2 — and
a humidified and cooled reaction environ-
ment. To evaluate these procedures, a
large batch of partially sulfated calcine
was generated by burning S02-doped
natural gas in the combustor, injecting
Ca(OH)2 in the furnace, and collecting the
resulting solid on a fabric filter at the
point of gas discharge to the atmosphere.
The solid was then reinjected either as a
powder in a low-temperature duct that
was humidified with water vapor of spray,
or in a slurry. Reaction conditions were
adjusted to provide reaction with S02 at
the same temperature, humidity level,
and approach to saturation.
The molar extent of conversion of CaO
to CaSO4 prior to reinjection of the solid,
was 18%. The molar increments in con-
version produced by further, low-
temperature reaction with S02 were:
Increment in
Humidification method conversion, %
Cooling to enhance the
effect of water vapor
already present 1
Cooling and
humidification with a
water spray 6
Slurrying with liquid water
and soray injection
33
Very little reaction of S02 with the
calcine, producing an increment in
conversion of only 1%, occurred when
water vapor already present was cooled.
A sixfold increase in reaction, giving an
increment of 6% occurred when the dry
solid was subjected to the effects of
water spray. An increment in excess
30% occurred with slurry injection. Cor
parison of the results obtained with tl
water spray and the slurry sprs
indicates that the extent of lov
temperature reaction was controlled t
the fraction of sorbent particles actual
colliding with water droplets and becor
ing physically wet. The added increme
due to the water spray (5%) divided t
the increment due to the slurry sprj
(33%) indicates a collision efficiency
15% (5/33 x 100 = 15).
Further experiments were conducte
with fresh Ca(OH)2 injected in the lo\
temperature duct in place of the partial
sulfated calcine. These experimen
yielded:
Increment in
Humidification method conversion, %
Cooling to enhance the
effect of water vapor
already present 9.6
Cooling and
humidification with a
water spray 12
Slurrying with liquid water
and soray injection 28
The results with the hydrated lirr
differ from those with the partial
sulfated calcine primarily in terms of tr
effects produced by cooling alone and t
the combination of spray humidificatic
and cooling. Cooling alone gave
utilization of nearly 10% with tr
hydrated lime but an increment of ju
1% with the calcine. The presence
water spray, on the other hand, increase
the utilization of hydrated lime only fro
10 to 12% but increased that of tf
calcine from 1 to 6%. Slurrying ga\
about the same result with both sorbenl
a utilization of about 30% totally with tt
hydrated lime or about 30% incremei
tally with the calcine.
The primary conclusion from th
investigation, insofar as the Edgewat
demonstration is concerned, is that wat
vapor alone is not effective for activatir
the removal of S02 by partially sulfate
calcine at low temperature. Humidificatic
of the sorbent with a water spray is mo
effective, but even its effectiveness
limited by the infrequency of collisioi
between the sorbent particles and wat
droplets. The rapid and extensive rate
reaction of sorbent in a slurry, even aft
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very brief contact between sorbent and
vater, reveals that the key to successful
numidification is increasing the frequency
of sorbent-droplet collisions.
Water vapor alone is more effective for
activating hydrated lime than for activa-
ting partially sulfated calcine. Clearly,
however, an increased frequency of colli-
sions between Ca(OH)2 particles and
water droplets would be even more
effective.
Charge-augmented sorbent humidifica-
tion. In studies of CASH, the usual proce-
dure was to charge sorbent particles
negatively and water droplets positively.
The sorbent was injected in the furnace
as Ca(OH)2 with either gas- or coal-firing;
it was subsequently charged on passing
through a low-temperature duct. Two
types of particle chargers were investi-
gated: a disc-type ionizer or a wire-pipe
ionizer. Water droplets were dispersed in
the gas stream downstream from the
particle charger; the spray nozzle was
maintained at a high positive potential
and thus the droplets were charged at the
time of dispersion.
A substantial part of the effort devoted
to CASH involved equipment modifica-
tions. For example, the disc-type ionizer
first used to charge sorbent particle was
limited by sparking at low currents; the
vire-pipe ionizer was developed to
overcome these shortcomings. Further-
more, considerable effort was required to
overcome current leakage in the spray
unit and ensure adequate charging of the
water droplets. Measured charge den-
sities were m the range of 2 to 9 ^C/g for
the sorbent particles and 0.2 to 2 iiC/g for
the water droplets.
The usual procedure for determining
the effect of CASH on SO2 removal was
to operate the sorbent and water injection
apparatus for 20-30 minutes and then
activate the two charging devices for a
comparable period. The SO2 concentra-
tion at the exit of the humidification zone
was monitored continuously during both
modes of operation. The effect of CASH
was expected to appear as an increase in
the slope of SO2 versus time; generally
speaking, however, the effects, if any,
were miniscule. Additionally, solids were
occasionally collected with and without
the charging devices in operation, and
the samples were then analyzed to
determine the extent of reaction. Compar-
ison of S/Ca ratios in the solids indicated
that no difference was produced by
charging.
In summary, no favorable effect of
1ASH on sorbent utilization was evident.
he reasons for this disappointing result
are not clear. A possible explanation is
that the attractive force between
oppisitely charged sorbent particles and
water droplets is small; it decreases with
the square of the separating distance and
is small in comparison with other elec-
trical forces except when the separating
distance is of the order of the particle-
dimensions. The other electrical forces
arise from the lack of precise electrical
balance between positive and negative
charges, thus creating a net "space
charge" field. In the absence of zero as a
net charge in the cloud of particles and
droplets, an electrical force operates on
each, bringing about a separation and
deposition on the container walls.
Adsorption isotherms of water vapor.
Adsorption isotherms of water vapor on
Ca(OH)2 prepared with and without addi-
tives were determined at 60°C using a
commercial surface-area analyzer, which
gives the absolute pressure of water
vapor in equilibrium with a solid. The data
were displayed in plots of a) the weight
ratio of water absorbed to solid substrate
(Ag/g) versus b) the relative partial pres-
sure of water vapor (p/p°, the fraction of
the saturation value). The additives inves-
tigated were LiCI, NaCI, Na2CO3, and
CaCI2, mole/mole for the 1:1 salts and
0.05 mole/mole for the 2:1 salts. Three of
the salts (NaCI, Na2C03, and CaCI2) have
received significant attention as deli-
quescent additives in Ca(OH)2 to achieve
enhanced S02 capture; the fourth salt
(LiCI) has not been investigated in this
sense but it, like CaCI2, is more strongly
deliquescent material in the pure state
than either NaCI or Na2C03.
LiCI gave evidence of behaving as a
deliquescent additive, as expected,
throughout the range humidity values
investigated (p/po = 0.2-0.9). CaCI2, on
the other hand, appeared to become
deliquescent only at a much higher
humidity threshold than expected (p/p°
= 0.6-0.7). Evidently, the deliquescence
of CaCI2 in the pure state is inhibited by
the presence of Ca(OH)2, which converts
the normal chloride to a basic chloride.
NaCI, as expected, exhibited delique-
scence only in a range of high humidities
(p/p° > 0.75). Na2CO3 also exhibited
weak deliquescence, which suggests in-
complete transformation to highly deli-
quescent NaOH through the double-
decomposition reaction: Ca(OH)2 +
Na2C03 -» CaC03 + 2NaOH
Deliquescence is clearly not the only
mechanism by which so-called deliques-
cent additives enhance SO2 capture by
Ca(OH)2. None of the three compounds
found beneficial — NaCL, Na2CO3, and
CaCI2 — provides significantly enhanced
water-vapor pickup by Ca(OH)2 at
humidity levels where they do provide
significantly enhanced S02 pickup. Deli-
quescence by some additives may be a
contributing factor in SO2 capture, but
some other unidentified mechanism must
be important also and must be dominant
for these three salts.
Mathematical modeling. A mathe-
matical model was developed to help
understand the complex set of mech-
anisms involved in low-temperature
desulfurization processes. The "first-gen-
eration" model treats the process in two
steps: (1) the "activation" of the sorbent
is modeled in terms of the collisions
between sorbent particles and water
droplets; and (2) the capture of S02 by
the activated sorbent is modeled in terms
of gas-phase diffusion of SO2 by the
sorbent/droplet ensemble, chemically
enhanced absorption into the liquid
phase, and liquid-phase mass transfer of
the reacting species.
The results of the modeling study show
that, when lime and water are injected
separately, the S02 removal efficiency is
governed by the degree of sorbent
wetting by collisions between sorbent
particles and water droplets. The
sorbent/droplet collision rate is max-
imized by making the droplet size and
velocity as large as possible. In actual
practice, however, the droplet size must
be well below the optimum value in order
to ensure complete evaporation and
avoid wetting of the duct walls. If the
humidification process could be modified
to allow larger droplets, it would be
possible to significantly improve on the
sulfur capture achieved.
The sulfur capture is strongly influ-
enced by approach to saturation, Ca/S
ratio, residence time, and droplet size
and velocity. To maximize S02 removal,
the approach should be as close as
possible. However, it may be difficult to
consistently maintain dry duct walls at
approaches of less than 11°C. Increasing
the sorbent injection rate, or Ca/S ratio, is
also beneficial, but reaches a point of
diminishing returns due to increased
operating cost and particulate loadings.
The effect of the latter on ESP perform-
ance must also be considered.
Increasing the residence time is bene-
ficial only up to the point of complete
droplet evaporation. Conversely, the resi-
dence time must be at least sufficient to
allow complete evaporation. For more
retrofit situations, the residence time will
be fixed. This means that the droplet size
must be selected to make maximum use
of the available time without allowing
droplets to reach the ESP. The model
can be used to predict the optimum
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droplet sizing for a given duct residence
time.
Conclusions and Plans for
Further Work
The significance of the principal
findings are discussed above in the
concluding paragraphs on each of the
four experimental tasks. Conclusions,
stated more succinctly, are:
• Insofar as enhancement of S02 cap-
ture at the Edgewater demonstration
of LIMB is concerned, humidification
by water spray will be required. A
spray of pure water would suffice if
the frequency of collisions between
sorbent particles and water droplets
could be increased. Otherwise,
isolation of partially sulfated sorbent
followed by rehydration and rein-
jection in a spray will be necessary.
• Charge-augmented sorbent humid-
ification (CASH) - entailing the charg-
ing of sorbent particles and water
droplets negatively and positively,
respectively - appears unlikely to
enhance sorbent reaction with S02
under post-furnace conditions.
• So-called deliquescent additives that
are known to enhance S02 capture
by Ca(OH)2 under low-temperature
conditions clearly must operate to
some extent by mechanism other
than deliquescence.
• The mathematical model gives an
improved appreciation of factors that
are crrtical to S02 capture by
Ca(OH)2 when the sorbent particles
are subject to wetting. The model
gives an improved rational basis for
upgrading the performance of duct
processes for S02 removal.
During the forthcoming Edgewater
demonstration of LIMB, with Ca(OH)2
injected in the furnace, the flue gas will
be humidified before it enters the
electrostatic precipitator. This will be
done to treat the electrical resistivity of
the mixture of sorbent-ash solids and
improve the efficiency of precipitation.
Relatively small increases in SO2
removal are expected, with the size of
the increases depending primarily on
the droplet/sorbent collision efficiency.
In a pilot-scale adjunct to the full-scale
Edgewater operation, humidification will
be tested in conjunction with the low-
temperature duct injection of a modified
calcium-based solid. This low-temper-
ature process, developed by EPA, is
known as ADVACATE; it employs the
product of hydrating LIMB sorbent-ash
mixtures under pressure and generating
a more reactive calcium-based sorbent.
Subsequently, the Department of
Energy will use the Edgewater facilities
to demonstrate a commercial process
based on separate duct injections of
Ca(OH)2 and water. Meanwhile, DOE will
be sponsoring a broad research pro-
gram on generic duct-injection pro-
cesses, ranging from laboratory-through
pilot-scale studies with ultimate full-scale
applications in mind.
A specific need during the further
research is elucidation of the role of
additives in the category known as
deliquescent materials, which may or
may not be actually deliquescent under
the conditions of use, as this report
shows. Further studies are needed to
elucidate the mechanisms of action so
that both the compounds and the
conditions of use may be selected more
intelligently.
J. P. Gooch, R. Be/He/, £. 8. Dismukes, and R. S. Dahlin are with Southern
Research Institute, Birmingham, AL 35255.
Louis S. Hovis is the EPA Project Officer (see below).
The complete report, entitled "Humidification of Flue Gas to Augment SO2 Capture
by Dry Sorbents," (Order No. PB 89-169 841/AS; Cost: $15.95, 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 Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-89/001
000085833 ||TEC«011 »GE»Cl
CHICAGO
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