United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park. NC 27711
Research and Development
EPA/600/S7-89/006b Feb. 1990
x°/EPA Project Summary
Evaluation of FGD Injection
Sorbents and Additives:
Volume 2. Pilot Plant
Evaluation of High Reactivity
Sorbents
John C. S. Chang and Claus Jorgensen
A mini-pilot test program was
undertaken to investigate potential
new sorbents and processes for dry
SO2 removal. Initial testing showed
that the 85 m3/h pilot plant could be
used successfully to evaluate both
spray dryer and dry injection
processes using traditional calcium
or sodium based sorbents. The major
part of the test program Investigated
the use of flyash or diatomaceous
earth for enhancement of lime with
respect to SO2 removal in a dry
infection process. This part of the
test program verified the silica
enhancement of Ca(OH)2 which
previously has been extensively
studied on a bench-scale reactor.
The pilot program showed that 50 to
90% SO2 removal can be achieved for
a stolchiom
injection of
when usin
+ cyclone
recycle. TC
itric ratio of 1 to 2 by dry
the silica enhanced lime
) a duct + baghouse
configuration or when using a duct
configuration including
e sorbent preparation
procedure was developed throughout
the test program, and an important
result was that the silica enhanced
lime can be prepared as a semidry
sorbent containing 20 to 30%
moisture, suitable for duct injection.
This allows evaporative cooling of the
flue gas as a result of the sorbent
injection, thereby lowering the
approach to saturation and
increasing the SO2 removal.
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
A research program to evaluate an
emerging dry flue gas desulfurization
(FGD) technology was undertaken at the
Environmental Protection Agency/Air and
Energy Engineering Research Laboratory
(EPA/AEERL) in Research Triangle Park,
North Carolina. This research was
conducted on a recently installed mini-
pilot plant, with an 85 m3/h simulated flue
gas flow. The pilot plant features a spray
dryer, a duct section, and a cyclone
separator or a baghouse as its main
components for controlling sulfur dioxide
(SO2) and particulates from a simulated
flue gas stream (see Figure 1).
The initial goal of the program was to
start up the pilot facility and to evaluate
the S02 removal efficiency which can be
achieved in a spray dryer/baghouse test
configuration, using lime as sorbent. The
test results related well to previous
results, reported by other researchers,
which were generated on the basis of
pilot or larger scale testing. The
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Flue Gas Duct (2.0 SEC)
L.
•11
Water
Tank
SO2 Gas Supply —»»
Sorbent Injection Point
Figure 1. Schematic of duct injection pilot plant facility.
Flue Gas Duct (0.8 SEC)
Spray Dryer
. (Gas Humidifier)
Clean-Side
Gas Duct %
n/
Exhaust Duct
(To Stack)
4
Temperature
Control Panel
adequacy of the mini-pilot plant as a
suitable test facility for preliminary
evaluation of new processes and
sorbents therefore seems justified.
Pilot Plant Tests
Testing using the pilot plant for dry
injection studies in combination with flue
gas humidification was accomplished by
spraying water from the rotary atomizer
in the spray dryer, then injecting dry
sorbent in the duct section located
downstream. Specifically, calcium
hydroxide [Ca(OH)2] and sodium
bicarbonate (NaHCOa) have been
injected into the humidified flue gas and
entrained to the fabric filter baghouse for
collection. The normal sorbent residence
time in the gas stream provided by this
equipment configuration ranged from 10
to 30 minutes. The results, specifically in
terms of SO2 removal and sorbent
utilization, can be related to previously
performed bench-scale testing in a
packed bed reactor. These results clearly
demonstrate the limited potential of a
totally dry Ca(OH)2 injection process, as
compared to a spray drying process.
Consequently, the major part of the
test program uses a recently developed
method which enhances lime with silica
in order to promote its reactivity toward
S02. This method, extensively tested in
bench-scale packed bed reactors used
flyash or diatomaceous earth as the silica
source. Under the present pilot program,
lime was slurried with either flyash or
diatomaceous earth at elevated
temperatures of 85 to 95°C for prolonged
periods of time (8 to 16 h). The lime
slurry was then dewatered and the
resulting filter cake dried to produce a
powder. Injection of this "silica
enhanced" lime into the simulated flue
gas stream and its entrainment to the
baghouse for collection clearly
demonstrated an increased S02 removal,
compared to using pure Ca(OH)2.
Parametric testing was undertaken to
evaluate the effect of varying sorbent
preparation procedures (e.g., changing
the silica/lime ration, the slurrying
temperature, and time period used for
slurrying; or using potential additives like
sodium hydroxide (NaOH) to increase the
solubility of silica). Also investigated were
the effects on the S02 removal efficiency
of changing various operating conditions,
e.g., the approach to saturation (At,),
stoichiometric ratio (SR), S02
concentration in the inlet gas.
Most of these pilot plant results seem
to confirm the effects which had been
anticipated from the bench-scale testing.
The most significant evidence obtained,
however, is the relationship between S02
removal efficiency and SR, which could
not have been determined from bench-
scale testing.
Both the baghouse and the cyclone
separator have been used for particulate
collection of silica-enhanced lime
sorbent. The cyclone separator simulates
the relatively short residence time
provided by an electrostatic precipitator
(ESP) for the sorbent in the flue gas
stream. The highest SO2 removal and
sorbent utilization are achieved when
using a baghouse for particulate
collection, obviously relating to the longer
sorbent residence time in the baghouse.
However, as most of the existing coal
fired utility boilers in the U.S. are
equipped with ESPs, the main goal of the
present work is to study retrofit options
for S02 control for those facilities.
Continued work focused on the use of a
cyclone separator for particulate
collection in the pilot plant.
The concept of recycling the product
collected in the cyclone separator back
into the makeup sorbent consisting of
silica-enhanced lime was now introduced.
Normally, the collected product is only
partly reacted, and therefore contains
unutilized sorbent. The advantages of
reintroducing it into the flue gas streaml
thereby increasing its overall residence
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time, seem obvious. The success of this
approach is also explained by its
t/ersatility as a means of "dewatering"
the makeup slurry (or paste). Dewatering
converts the silica-enhanced lime sorbent
into a semidry, manageable powder, with
20 to 30% moisture, which is suitable for
dry injection. Injecting this semidry
sorbent into the flue gas duct at an At, of
up to 55 °C demonstrates the effect of
flyash drying the sorbent throughout the
duct section, thereby lowering the Ats to
10 to 20° C and achieving significant S02
removal across the short residence time
(2s) duct section prior to collection of dry
product (about 5% residual moisture) in
the cyclone. Finally, the direct use of
recycle solids, rather than pure flyash, as
the silica source for lime enhancement
was also successfully tested.
An overall flow sheet for the
developed process is being proposed
(Figure 2). Based on these mini-pilot
plant results, this particular process is
expected to result in about 50% sorbent
utilization when used in combination with
an ESP for particulate collection. About
50% S02 removal is expected for an SR
of 1. Based on mini-pilot plant results
only, a utilization of at least 80% is
expected if a fabric filter baghouse is
used, which provides a longer sorbent
residence time in the gas stream.
Testing in the immediate future should
undertake the pilot development of
pressure hydration to enhance lime with
flyash/recycle solids during the slaking
process. Bench-scale and preliminary
pilot plant tests have demonstrated that
the increased temperature provided by a
pressure hydrator (up to 150°C) will
significantly reduce the time requirement
of the lime-silica reaction. Eventually the
process should be evaluated on a large
pilot facility, such as AEERL's 3,400 m3/h
pilot plant. This facility should be
operated preferably in a continuous mode
where key operational parameters (e.g.,
SO2 concentration, recycle ratios, Ats,
SR) and their impact on overall SO2
removal could be evaluated.
Conclusions
Conclusions from the present work on
the mini-pilot plant include:
• The mini-pilot plant is an efficient
and effective tool for evaluating dry
FGD processes, including both spray
drying and dry sorbent injection.
• In-duct injection of dry Ca(OH)2 into
a humidified SOg-laden flue gas
stream results in significantly less
S02 removal in the duct/baghouse
system that in a lime spray drying
system.
• In-duct injection of dry NaHC03 at
low temperature (49 to 93°C) and
high relative humidity (up to 60%)
results in very high S02 removal, if
the injected sorbent has a particle
size sufficiently small (less than 10
jam) to allow effective gas
entrainment of the particles and their
deposition on the fabric filter bags. At
low temperature (49°C), the thermal
decomposition of NaHC03 to
NagCOs may not occur to a
significant extent, so that only 1 mole
of NaHCOa is required to react with 1
mole of S02 at a low stoichiometry.
• The reactivity of Ca(OH)2 with
respect to dry S02 removal can be
enhanced by slurrying lime with
silica, such as flyash or
diatomaceous earth, at elevated
temperatures (85 to 95°C) for
prolonged periods (12 to 16 h).
• Slurrying flyash with Ca(OH)2 at a
weight ratio of 3:1 and injecting the
resultant dry sorbent into humidified
flue gas (Ats = 11 to 17°C)
containing 1,500 ppm S02 resulted
in an S02 removal of 50 to 80% over
the SR range of 1 to 1.8, in the 0.8s
gas retention time duct-baghouse
system (Figure 3).
• Without product recycle,
approximately 50% Ca(OH)2
utilization occurred in the tests cited
above. With product recycle the
utilization increased to at least 70 to
80% at the same conditions.
• Tests using the same silica-
enhanced sorbents injected into the
2s gas retention time duct/cyclone
system without recycle, at similar
test conditions, provided S02
removal efficiencies of only about
30% of those above.
• Introducing recycle of the sorbents
by mixing the cyclone catch solids
into the prepared slurry of silica-
enhanced Ca(OH)2 prior to duct
injection avoided the need for
separate drying/dewatering of the
enhanced sorbent slurry. The overall
Ca(OH)2 utilization also increased.
• The recycle solids provided the silica
for Ca(OH)2 enhancement in lieu of
flyash during slurrying at elevated
temperatures.
• When using sorbent recycle, the
duct injection/cyclone system (2.0 s
retention time) resulted in S02
removals of about 50% at an SR of
1, and 50 to 90% at SRs in the range
of 1 to 2 (Figure 4).
• The results suggest that a dry
injection process using silica-
enhanced lime sorbents may be an
attractive retrofit option for controlling
S02 emissions from an existing coal-
fired power plant equipped with an
ESP.
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Raw Flue Gas: 40 scfm
0.6 Ib/h S02 ^—-—
2.1 Ib/h Ash ———
Water
Discharge
3.3 Ib/h
Make up
Water
9.0 Ib/h
Sorbent: 28.5 Ib/h
Slurry Solids
3.11b/h
Recycle Solids
14 Ib/h
NOTE: 1 Ib = 0.454 kg
1 scfm = 1.70 sm3/h
Cleaned Flue Gas
0.5 Ib/h SO2
Disposal
3.1 Ib/h
0.70 Ib/h
Figure 2. Continuous mini-pilot process using lime enhanced by recycle solids.
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IUV
90
SO
*. 70
|60
•58 50
30
20
10
n
I I I I I
S02 Inlet: 1500 ppm O
4f,= 11-17°C D,*x'
Flyash/Ca(OH)2 Ratio: O D
O 3:1 XX DO^^**
4fs= 11"C(200F)/ &»
X° A A Pure Hy*a^t
x ^»**^
~4f, = 17°C(3B°F) ^^ l->"
.-•—•'"""""**"
— -"*"T i i i i
-
-
-
„.-—""**
—
_
i
0.5 r.O J.5 2.0 2.5 3.0
Stoichiometric Ratio
Figure 3. Removal of S02 in duct injecfon/baghouse pilot plant facility. Ftyash enhanced lime
used as dry sorbent.
IOU
90
80
70
geo
Iso
f
0 40
9)
«30
20
10
0
i i i ID
A xx
>'"
O XXX A
° X
n ° .4 -
' ° 4f, = 5-J7°C
O O J0~* v O Flyash/Ca(OH)2 = 3:1
fS Recycle/Cake = 2:1
9'' O Recycle/Ca(OH)2 = 4.5:1
j* . Recycle Cake = 2:1
A A Recycle/Ca(OH)2 = 4.5:1
O Recycle/ Slurry = 2:1
1 1 I 1
-
0.5 7.0 J.5 2.0 2.
Stoichiometric Ratio
Figure 4. Removal of SO2 in duct injection/cyclone pilot plant facility (2.0 s retention time);
flyash-enhanced lime used as dry sorbent with recycle.
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John C. S. Chang and Claus Jorgensen are with Acurex Corp., Research
Triangle Park. NC 27709.
Charles B. Sedman is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of FGD Dry Injection Sorbents and
Additives: Volume 2. Pilot Plant Evaluation of High Reactivity Sorbents,"
(Order No. PB 89-214 134/AS; Cost: $21.95, 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/S7-89/006b
000085833 PS
U S EMVXR PROTECTION AGEMCY
REGION 5 LIflEARY
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CHICAGO XL 60604
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