United States
Environmental Protection
Agency
Air and Energy Engineering Researc
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/002 Mar. 1985
&EPA Project Summary
Particulate Emissions and
Control in Fluidized-Bed
Combustion: Modeling and
Parametric Performance
A. Y. Ranadive, R. A. Newby, and D. F. Ciliberti
A model was developed to describe
the physical characteristics of the par-
ticulates emitted from fluidized-bed
combustion (FBC) systems and to eval-
uate data on FBC particulate control
systems. The model, which describes
the particulate emissions profile from
FBC, considers the attrition of the bed
material and the recycling of the elu-
triated fines in addition to other FBC
phenomena. For a given combustor
design and set of operating conditions,
the particle profile program projects the
mass rate and size distribution of the
solids in the bed draw-off and carry-
over streams, as well as those emitted
from particle removal devices of any
given configuration.
Examples of particulate emission and
control are given for both atmospheric-
pressure and pressurized FBC systems.
The effect of calcium-to-sulfur ratio,
rate of sorbent attrition, superficial gas
velocity, and recycle cyclone efficiency
was examined by using the model. A
manual for the particle profiles program
comprising a quick-reference user's
section, as well as a more detailed
descriptive section, is provided in the
full report.
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 infor-
mation at back).
Introduction
The fluidized-bed combustion (FBC) of
coal offers the potential for a more
thermally efficient, environmentally supe-
rior, and more economical way to use coal
resources to generate electric power and
to produce industrial steam than do
conventional methods of coal utilization.
Many different FBC concepts have
been proposed having significantly dif-
ferent emission behavior and control
requirements. FBC concepts can be used
for atmospheric (AFBC) or pressurized
(PFBC) operation and for industrial or
utility applications.
The conventional FBC boiler concept
integrates coal combustion, sulfur con-
trol, and temperature control in a single
component. In both AFBC and PFBC
boilers, coal and sulfur sorbent (limestone
or dolomite) are fed into a FB reactor,
where the combustion of coal occurs,
accompanied by the release of heat, ash,
and sulfur as sulfur oxides (SOX). The
sorbent in the bed reacts with the sulfur
to form a solid product, thereby reducing
the release of the sulfur pollutant to the
atmosphere. Most of the ash, together
with some sorbent and char, is carried
overhead and out of the reactor with the
off-gas. The bulk of these particulate
emissions may be captured in a cyclone
and recycled to the bed to achieve higher
carbon and sorbent utilization. Heat is
extracted from the reactor by immersing
the heat transfer surface in the bed to
generate steam. For industrial applica-
tions the steam so generated finds use as
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process heat or as process feedstock. For
utility applications the steam is expanded
through a steam turbine for power genera-
tion. Additionally, in the case of PFBC for
utility applications, the hot pressurized
gas from the reactor is cleaned of partic-
ulate matter bypassing it through a train
of particle removal devices and then
expanding it through a gas turbine for
power generation. Particle emissions
from PFBC processes utilizing gas turbine
expanders for power generation must
therefore be controlled at levels that are
acceptable for reliable turbine operation.
Paniculate control is, therefore, a vital
aspect of FBC technology.
Results and Recommendations
The particulate control problem as it
relates to FBC is threefold: (1) the particle
emission characteristics of FB combus-
tors need to be determined; (2) acceptable
levels of particle loadings and size distri-
butions, from the standpoints both of
environmental constraints and, for PFBC,
of the requirements for reliable gas
turbine operation, need to be established;
and (3) an appropriate train of particulate
control devices needs to be identified for
reducing the particle emissions to ac-
ceptable levels.
The present work addresses two of
these areas of concern. A particle profile
model, incorporating the FB combustor
and particulate control subsystems, has
been developed that permits rational and
consistent projections of solid sizes and
loadings from the FB combustor and
throughout the FBC system. The model
considers particle attrition and the recy-
cling of elutriated fines, in addition to
other FBC phenomena, and is able to
simulate gas cleaning equipment of any
desired configuration.
The particle profile model is illustrated
by providing some initial perspective on
particle emission and control for AFBC
and PFBC systems. The results of para-
metric studies with respect to calcium-to-
sulfur (Ca/S) ratio, rate of sorbent attrition,
superficial gas velocity, and recycle cy-
clone efficiency are described.
The particle profile model developed is
highly useful for evaluating FBC design
and operating conditions that relate to
particulate emissions from the combus-
tor. Particle control devices and systems
integrated with FB combustors can be
assessed for their ability to satify envi-
ronmental standards and/or gas-turbine
protection requirements. Particle attrition
(sorbent and ash) represents the greatest
uncertainty in the model. This could be
clarified by:
• Further studies to develop increased
understanding of attrition and elutria-
tion.
• Comparison of model projections with
available plant data to establish its
validity and to identify and characterize
controlling phenomena.
The application of the particle profile
program to particle emission and control
for AFBC and PFBC systems results in the
following conclusions:
• The final particulate control device is
of primary importance, since this filter
stage determines the particle size
distribution and loading released to
the environment or presented to the
turbine.
• FBC design and operating conditions
(and resulting Ca/S ratio) have little
effect on the size distribution of emit-
ted particles for the final-stage control
devices examined here.
• The selection of coal, sorbent, and FBC
design will significantly affect the
particle loading to the environment or
to the turbine for a given final-stage
device performance. The coal-ash par-
ticle size distribution, the sorbent attri-
tion rate, and the attrited sorbent
particle size distribution are important
performance factors.
• The design of the FBC to minimize
sorbent consumption (Ca/S) and par-
ticle attrition is important for mini-
mizing the quantity of particles emitted
to the environment or turbine.
• The recycle cyclone performance
strongly influences the rate of recy-
cling solids. The use of a high-effi-
ciency recycle cyclone results in high
solid recycle rates, but does not affect
the particulate emission from the final
cleaning device. The relation between
recycle rate and FBC combustion and
sulfur removal performance is not
generally predictable. To minimize
both the recycled solids handling re-
quirement and the erosion of convec-
tive heat transfer surface above the
bed, a medium-efficiency, rather than
a high-efficiency, recycle cyclone
should be used.
• For AFBC, commercial, final-stage
particulate control devices (i.e., fabric
filters and electrostatic precipitators)
are available that will perform satis-
factorily to meet environmental stand-
ards. Some technical problems remain
to be dealt with for both fabric filters
and electrostatic precipitators. Staged
cyclones will be insufficient to meet
environmental standards except in
cases of unusual coals and sorbents.
• For PFBC, many technical and eco-
nomic considerations can result ir
either the environmental standards oi
the turbine protection requirements
being the most stringent and control
ling the selection and design of <
particulate control system. Final-stag*
particulate control devices are bein<
developed for this application, bu
their performance and economic feas
ibility are not yet established
A. Y. Ranadive. R. A. Newby. andD. F. Ciliberti are with Westinghouse Research
and Development Center, Pittsburgh, PA 15235.
John O. Milliken is the EPA Project Officer (see below).
The complete report, entitled "Particulate Emissions and Control in Fluidized-Bed
Combustion: Modeling and Parametric Performance." (Order No. PB 85-152
973/AS; Cost: $14.50, 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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