United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/016 Sept. 1987
Project Summary
Evaluation of Boiler Design
Modifications for Enhanced
LIMB Application
D. E. Ryan and R. Smolensk^
Under EPA sponsorship, a study was
conducted to evaluate the technical
and economic impact of designing
wait-fired pulverized-coal units to
incorporate dry sorbent sulfur control
technology (LIMB). Conventional (non-
Li MB) units were set up at three sizes
(200. 400, and 600 MW) to burn a low
sulfur (0.48%) subbituminous fuel.
LIMB units were then set up to achieve
70% SOX removal using dry sorbent
injection. Standard cost estimating
procedures were used to evaluate the
cost differentials between the conven-
tional and LIMB units. These cost data
were used to establish removal cost
trends as a function of calcium-to-
sulfur (Ca/S) mole ratios and boiler
size. The study concluded that it is
technically feasible to achieve 70%
sulfur removal in a unit burning low
sulfur fuel using only dry sorbent
injection. The results also showed that
Ca/S ratios between 2.8 and 3.5 can
be accommodated without radical
alterations to state-of-the-art boiler
configurations.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering 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 and Summary
The Environmental Protection Agency
is engaged in a research program to
develop improved control technologies
for the emission of sulfur and nitrogen
oxides (SOX and NOX) from the combus-
tion of fossil fuels. These emissions are
important due to their magnitude and
apparent link to acid rain. The cause and
potential solutions of the acid rain
problem are still being debated; however,
control of SO, and NOxfrom power plants
is a major element in all proposed control
strategies.
The choice of control techniques for
acid rain will likely include a mix of
technologies to achieve the mandated
pollutant reduction at a cost that will
minimize the impact on the nation and
on specific geographical areas. Commer-
cially available options include: coal
switching, coal cleaning, and various
types of flue gas desulfurizatipn (FGD)
systems. In addition, analysis indicates
that low capital cost technology for
retrofit to existing boilers would provide
further flexibility in a control strategy
even at moderate (e.g., 50%) S02 removal
rates. One such technology is LIMB
(acronym for Limestone Injection Multi-
stage Burner), which is based qn the
injection of dry sorbents into the boiler
for direct capture of SOa from the
combustion gases. LIMB combines sor-
bent injection for SO« control with the
use of low-NOx burners.
The EPA LIMB program was initiated
in 1981, although work in related areas
had been previously conducted by the
EPA and others. The LIMB program is
structured to provide an understanding
of the controlling factors in the LIMB
process and to establish a basis for the
private sector commercialization of the
technology.
This engineering study was performed
for the E PA as part of the Agency's
ongoing effort to investigate all the
potential uses of sorbent injection for the
control of SOX emissions.
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This investigation was performed to
determine the technical and cost differ-
entials within the scope of the study
between a conventional utility boiler and
a utility boiler designed to incorporate the
LIMB process. Comparisons were made
using nominal 200, 400, and 600 MWe
units. The scope of the study considers
only the boiler shell; auxiliary equipment
(e.g., fans, precipitators, flue gas desul-
furization equipment) is not addressed.
Both the conventional and LIMB units
were designed to fire a low sulfur (0.48%
by weight), subbituminous western coal.
The overall procedure was: (1) a conven-
tional boiler was designed for each of the
three plant sizes to be studied; (2) a LIMB
boiler was designed which gave the same
output as its conventional sister unit (the
LIMB unit included the alterations
required to allow for 70% of the sulfur
introduced into the boiler via the fuel to
be removed by sorbent injection without
additional downstream gas processing);
and (3) a cost analysis was performed
to determine the cost differentials
between the LIMB and the conventional
units.
Conclusions
The conceptual design work performed
to date demonstrates that it is technically
feasible to design a LIMB unit burning
low-sulfur coal to accommodate Ca/S
ratios between 2.8 and 3.5 without
radically altering state-of-the-art boiler
configurations. In addition, it was deter-
mined that, based on an economic
analysis utilizing "LIMBCOST" (an EPA
computer program), the limiting factors
when optimizing with respect to the
Ca/S ratio are thermodynamic (i.e., the
lower limit on the Ca/S ratio was set
by establishing the largest possible
residence time cavity and still have a high
enough gas temperature entering the
convection pass to complete the required
heat transfer to surfaces downstream of
the cavity).
The conceptual design work also
indicated that of the two principal LIMB
design criteria (sorbent residence time to
980°C and quench rate to 870°C),
residence time dominated unit configu-
ration considerations. When the design
residence time is achieved, acceptable
quench rates typically follow.
Recommendations
The work performed to date suggests
a need for continuing validation of some
of the cost assumptions used in this
study, to reflect actual experience as
LIMB technology matures. The optimiza-
tion work performed as part of this study
used an AEERL in-house cost estimating
program called "LIMBCOST." This pro-
gram includes a number of algorithms
and parameter values for: (1) sorbent
injection system (materials handling)
costs; (2) base unit capital costs; (3) fuel
costs; (4) sorbent costs; (5) waste dispo-
sal costs; (6) costs of other consumables;
and (7) maintenance and operating costs.
Verifying these cost assumptions was
beyond the scope of this study. Therefore,
these study results can be considered
initial estimates. In addition, no compar-
ison was made to determine the trade-
off between LIMB and backend sulfur
removal. Any true optimization would
have to consider this issue.
From a technical standpoint, further
investigations should consider the injec-
tion process and verification of the
capture data provided by the EPA. These
data are expected to be generated during
the EPA's demonstration programs.
However, for purposes of this study, it
was assumed that: (1) the nozzles could
indeed get the sorbent into the flue gas
stream; (2) complete mixing was subse-
quently achieved after injection; and (3)
the expected capture data provided by the
EPA are valid.
Planned demonstration testing will
provide the data to validate these
assumptions and (if necessary) appro-
priately modify the results of this study.
Objectives and Approach
The work done in developing this report
focused on three primary objectives:
1. Demonstrating the feasibility of
designing boilers at 200, 400, and
600 MW to be compatible with the
LIMB process.
2. Evaluating the cost differentials
between these LIMB compatible
units and boilers designed without
the LIMB process incorporated (i.e.,
conventional units).
3. Establishing cost trends for these
units as a function of both megawatt
rating and Ca/S ratio.
The five-step approach used during
this study consisted of:
1. Establishing design criteria for the
boilers. This included defining steam
conditions, fuel type, and the LIMB
parameters to be used.
2. Designing conventional boilers at (
200, 400, and 600 MW which used
all of the design parameters estab-
lished in step 1, except the LIMB
design parameters.
3. Designing units with the same
megawatt ratings used in step two,
but this time incorporating LIMB
parameters into the design.
4. Using standard cost estimating
procedures to establish cost differ-
entials between the conventional
units and their sister LIMB units.
5. Using these cost differentials to
establish and evaluate cost trends as
a function of megawatt rating and
design Ca/S ratio.
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D. E. Ryan and R. Smolensk! are with Babcock & Wilcox Co., Barberton, OH
44203.
David G. Lachapelle is the EPA Project Officer (see below).
The complete report entitled "Evaluation of Boiler Design Modifications for
Enhanced LIMB Application." (Order No. PB 87-199 634/AS; Cost: $13.95,
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/S7-87/016
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