United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-86/023 Sept. 1986
Project Summary
Boiler Design Criteria for Dry
Sorbent S02 Control with
Low-N0x Burners
J. P. Clark, A. Kokkinos, D. C. Borio, R. W. Koucky, and C. Y. Sun
A program to develop boiler design
criteria for application of dry sorbent
control technology with low-NOx burn-
ers on tangentially fired pulverized-
coal-burning boilers was conducted
under EPA sponsorship. A comprehen-
sive review of past and current research
in the area of sorbent SOX control was
performed to provide a basis for evalu-
ating the implications of this technol-
ogy on boiler design, cost effective-
ness, and operability. Historical and
projected design trends were analyzed
for all tangentially fired pulverized-coal
utility boilers built by C-E since 1960,
including the effect of coal rank. A can-
didate host unit was selected for con-
sideration as a site for demonstration of
dry sorbent SO2 control. Dry sorbent
process designs, including sorbent
preparation/delivery equipment and
boiler modifications, were developed
and costed for new and retrofit (200,
400, and 600 MWe) high-sulfur coal-
fired units and new (200, 400, and 600
MWe) low-sulfur coal-fired units. SO2
removed was 50% in the boiler for all
cases. Spray dryers were incorporated
on the new units to achieve overall SO2
removal (sorbent injection plus spray
dryer) of 70% (low-sulfur coal) and 90%
(high-sulfur coal). Conventional lime-
stone flue gas desulfurization costs
were developed for comparison. Capi-
tal costs, cost of electricity, and cost
effectiveness per ton of SO2 removed
were, developed.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research 7/7-
angel 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 or-
dering information at back).
Introduction
Proposed legislation in response to
the acid rain question would require at
least a portion of existing oil and coal
burning plants to limit emissions of sul-
fur dioxide (S02). Due to the possibility
of regulation of existing plants, various
alternative sulfur oxide (SOX) removal
processes are being examined in addi-
tion to the tail-end lime or limestone
scrubbing systems predominantly em-
ployed to meet NSPS. Because of the
combined requirements of retrofitabil-
ity and moderate sulfur removal effi-
ciency, the concept of furnace sorbent
injection has received renewed interest
and study as an alternative to tail-end
lime or limestone scrubbing systems.
Early trials in small-scale furnaces and
full-scale utility boilers generally failed
to demonstrate sufficient in-furnace
SO2 removal at reasonable sorbent-to-
sulfur ratios.
The development of advanced low ni-
trogen oxide (NOX) utility boiler com-
bustion technologies may provide new
combustion conditions and lower fur-
nace gas temperatures which may be
more suited for in-furnace absorption of
S02 by limestone or other sorbents. The
combination of these two technologies
has been given the acronym LIMB,
Limestone Injection with Multi-stage
Burners.
This report presents the results of the
EPA contract, "Boiler Design criteria for
Dry Sorbent SO2 Control with Low-N0x
Burners." This study consisted of sev-
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eral tasks: (1) literature survey of state-
of-the-art knowledge of S02 removal by
sorbent injection; (2) review of histori-
cal and projected design trends for C-E
coal-fired utility boilers; (3) selection of
a candidate host site for LIMB demon-
stration; (4) design of sorbent prepara-
tion systems plus definition of boiler
and back-end modifications required to
incorporate LIMB on new and retrofit
units; and (5) economic analyses of
these LIMB-related designs, plus, for
new units, SO2 removal via conven-
tional flue gas desulfurization (FGD)
techniques.
Results and Discussion
Process Design Criteria
Five factors which most directly de-
termine the effectiveness of sorbent in-
jection on a particular boiler were iden-
tified during the literature survey
portion of this contract:
Coal type—Ash compositions vary
considerably. Laboratory tests have
shown that sorbent injection affects the
slagging and fouling behavior of some
coals. Ash reactivity can affect sorbent
capture.
Sorbent type—Ca(OH)2 was deter-
mined to be the most effective sorbent,
followd by limestone. The most impor-
tant physical characteristics are specific
surface area and porosity which deter-
mine the calcination and sintering rates.
Limestone was shown to have a maxi-
mum specific surface area at 1000°C.
CaO absorption falls off sharply above
1200°C, while CaS04 starts to decom-
pose at 1204°C.
Time/temperature effects—S02 ab-
sorption is a strong function of the resi-
dence time within a critical temperature
range. For limestone, this range was
found to be roughly 1204 to 927°C. The
point at which the sorbent is injected
into the boiler is critical in maximizing
the residence time within this tempera-
ture range and minimizing the exposure
to higher temperatures which adversely
affect sorbent performance. Maximum
S02 absorption was obtained when the
sorbent was injected in the area of the
overfire air ports.
Stoichiometry—S02 absorption in-
creases with increasing Ca/S molar
ratios. Ca/S ratios above roughly 4:1
were shown to give only small improve-
ments in S02 removal.
Gas composition—A reducing
atmosphere in the reaction zone does
not affect SO2 absorption. The rate of
S02 absorption is higher for higher con-
centrations of SO2 in the gas stream.
Existing ash collection systems may
be adversely affected by sorbent injec-
tion. Electrostatic precipitator (ESP) per-
formance was shown to drop off consid-
erably due to the change in particulate
resistivity. Wet ash conveying systems
were susceptible to plugging. No ad-
verse effects were identified in the per-
formance of mechanical collectors.
Operating dry scrubber systems in con-
junction with sorbent injection may of-
fer economic advantages in some cases
by allowing units firing high sulfur coals
to meet NSPS limits.
Process Design
Data for all C-E tangentially fired
pulverized-coal-burning utility boilers
built since 1960 were reviewed to deter-
mine historical and projected design
trends for potential retrofit applicability
of dry sorbent injection for S02 control.
From this review, three units (200, 450,
and 560 MWe) were identified as candi-
dates for demonstration of dry sorbent
injection. These units were used to de-
fine the process equipment and boiler
modifications which would be required
to achieve 50% SO2 removal in the
boiler in a retrofit application with high-
sulfur coal.
Similarly, generic process designs
were defined for new tangentially fired
pulverized-coal-burning boilers at 200,
400, and 600 MWe with low- and high-
sulfur coals. An SO2 removal of 50% in
the boiler with sorbent injection was de-
fined, with spray dryers added to
achieve NSPS-required overall S02 re-
movals of 70% (low-sulfur coal) or 90%
(high-sulfur coal).
The sorbent preparation and injection
system in all cases was designed for on-
site pulverization of limestone to 90%
—325 mesh via dedicated roller mills.
The sorbent was transported via dilute
phase flow (1.5 kg air/kg limestone) to
multiple nozzles located in the upper
furnace area with a discharge velocity of
61 m/sec. Calcium-to-sulfur mole ratios
were defined as 2:1 for high-sulfur coal
units and 4:1 for low-sulfur coal units.
The boiler modifications required to
incorporate sorbent injection included
boiler nozzle penetrations (a retrofit
cost only), increased soot blower capac-
ity (new and retrofit units), and surface
modifications (new units only). For new
high sulfur units, reducing the S03 con-
centration in the gas stream permitted a
reduction in the gas outlet temperature.
which resulted in costs associated with
incorporating larger Ljungstrom air
heaters.
Gas cleanup requirements were
handled differently for retrofit and new
units. Multiclone collectors were added
upstream of the ESPs to reduce the
solids loading to the existing ESPs. S03
conditioning was incorporated to offset
the resistivity increase associated with
the high calcium content. Baghouses
were incorporated on new units as part
of the spray dryer systems, but were not
included as a sorbent-injection-related
cost. Incremental ash removal capacity
was added for all units as required.
For the six new unit cases, limestone
plus forced oxidation scrubber systems
were defined for the purpose of com-
paring the costs associated with con-
ventional FGD to the costs associated
with dry sorbent injection.
Process Economics
Capital and operating costs were de-
veloped for the LIMB and FGD process
equipment for the new and retrofit
cases described above. These costs
were developed according to proce-
dures outlined in the EPRI Technical As-
sessment Guides, and are expressed in
December 1985 dollars for a January
1986 start-up.
Retrofit units
The costs of retrofitting sorbent injec-
tion to the existing units considered in
this study, including sorbent prepara-
tion and delivery, boiler and back-end
modifications, and low-NOx burners,
ranged from 55.9 to 78.8 $/kW. The ef-
fect on first year costs of electricity
ranged from 4.15 to 5.72 mills/kW-hr
(15-year levelized costs ranged from
5.98 to 8.23 mills/kW-hr).
New high-sulfur units
The cost of incorporating LIMB for
50% SO2 removal plus spray dryers to
achieve 90% overall SO2 removal on the
new high-sulfur units considered in this
study ranged from 93.0 to 111.6 $/kW.
The effect on first year costs of electric-
ity ranged from 6.20 to 7.05 mills/kW-hr
(30-year levelized costs ranged from
11.05to 12.38mills/kW-hr).
The cost of incorporating conven-
tional limestone FGD equipment to
achieve 90% S02 removal ranged from
221.3 to 320.7 $/kW. The effect on first
year costs of electricity ranged from
11.17 to 14.84 mills/kW-hr (30-year lev-
elized costs ranged from 18.03 to 23.02
mills/kW-hr).
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New low-sulfur units
The cost of incorporating LIMB for
50% SO2 removal plus spray dryers to
achieve 70% overall SO2 removal on the
new low-sulfur units considered in this
study ranged from 44.1 to 60.3 $/kW.
The effect of first year costs of electricity
ranged from 2.29 to 3.03 mills kW-hr
(30-year levelized costs ranged from
3.73 to 4.88 mills/kW-hr).
The cost of incorporating conven-
tional limestone FGD equipment to
achieve 70% S02 removal ranged from
151.1 to 228.2 $/kW. The effect on first
year costs of electricity ranged from
5.99 to 8.82 mills/kW-hr (30-year lev-
elized costs ranged from 8.53 to 12.35
mills/kW-hr).
J. Clark. A. Kokkinos, D. Borio, R. Koucky. and C. Sun are with Combustion
Engineering. Inc., Windsor. CT 06O95.
David G. Lachapelle is the EPA Project Officer (see below).
The complete report, entitled "Boiler Design Criteria for Dry Sorbent SOx Control
with Low-NOi Burners," (Order No. PB86-216 736/AS; Cost: $22.95, 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
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EPA/600/S7-86/023
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