sc/EPA
United States
Environmental Protection
Agency
Industrial Environmental Resear
Laboratory
Research Triangle Park NC 2771
Research and Development
EPA-600/S7-81-075 Aug. 1981
Project Summary
Environmental Aspects of
Fluidized-Bed Combustion
J. M. Robinson, R. J. Kindya, C. W. Young, R. R. Hall, and P. Fennelly
This report is a summary of the
complete project report entitled
"Environmental Aspects of Fluidized-
Bed Combustion." The report is or-
ganized according to environmental
media and specific pollutants of con-
cern. Emissions data and results of
biological testing of FBC emission
stream samples, where available, are
presented and discussed. This report
represents work completed or data
available through late 1979.
Comprehensive emissions data from
FBC processes are limited. Those data
which are available have, in general,
been obtained through sampling and
analysis of effluent and emissions
streams from bench-top or other pilot-
scale units under controlled operating
conditions. Conclusions drawn re-
garding FBC environmental impacts
must be considered preliminary and
should be verified in future testing.
Data that are available have indicated
that FBC technology is a viable alter-
native to conventional coal combus-
tion. Adverse impacts on health or the
environment appear to be minimal and
are, at worst, no different from impacts
associated with conventional coal
combustion systems. Future testing
and emissions analysis are needed to
further quantify and assess these
impacts.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal 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
Multimedia (air, water, and land)
environmental impacts of fluidized-bed
combustion (FBC) are discussed in this
report in the context of available emis-
sions data and current or proposed key
research. To demonstrate the potential
benefits and possible disadvantages of
the two alternative approaches to using
FBC technology, the environmental
aspects of atmospheric and pressurized
FBC systems are discussed concurrently.
Figure 1 is a schematic diagram of an
FBC unit that illustrates potential emis-
sion/effluent/solid waste sources. The
major source of air emissions is the
boiler flue gas. Minor fugitive emission
sources include material transfer oper-
ations and coal and sorbent storage
piles. Liquid effluent sources include
boiler blowdown, cooling tower blow-
down, boiler water treatment waste-
water, equipment cleaning waste, and
runoff or leachates from materials
storage and solid waste disposal areas.
The liquid waste sources are not ex-
pected to be much different from those
associated with conventional power
plants or industrial boilers; however,
flue gas desulfurization (FGD) liquid
waste can be avoided with FBC tech-
nology because in situ sulfur dioxide
(SOz) reduction is possible. Also, the
quantities of leachate and runoff from
materials and solid waste storage areas
may be greater than those of conven-
tional systems because more sorbent is
used in FBC than in FGD. Solid wastes
emanate from collection of flue gas
particles, which consist mostly of coal
ash, but more importantly, from with-
drawal of spent bed material, which is
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Feed Water Steam
Liquid
Effluent
Treatment
Air Emissions
A
Water
Coal
.Limestone
Primary
Paniculate
Control
Stack Gas
Final
Paniculate
Control
Fly Ash
Total
Air
Emissions
Air
Emissions
Waste
Leachate
Figure 1. FBC flow diagram.
partially sulfated limestone or dolomite.
Typically, for a high sulfur coal with a
limestone (calcium) to sulfur (Ca/S)
ratio of about 4.0, FBC solid wastes are
one-third fly ash and two-thirds spent
bed material.
Air Emissions
Sulfur dioxide control in FBC is a dual-
purpose technology that incorporates
gaseous pollutant reduction with com-
bustion for raising steam. The fuel being
used is burned in the presence of a
sorbent material, usually limestone or
dolomite, for in situ desulfurization.
An excess of sorbent, beyond that
theoretically required for a specific
percentage of SO2 removal, is needed
because the dense sulfate layer, which
forms first on the outside of the particles,
retards the diffusion of S02 gas into the
interior of the sorbent particles. The
keys to low Ca/S requirements are that:
the diameter of the particles be small
(which requires an appropriately low
gas velocity); the pores be large enough
to avoid blockage with sulfate near the
mouth of the pore; and gas residence
time be increased.
Higher gas residence times can be
obtained by increasing the bed depth,
decreasing the superficial velocity, or
both. Increasing the bed depth increases
the fan power requirements, whereas
decreasing the air velocity decreases
the power density, requiring larger bed
areas and more complex feed systems.
Thus, there is a tradeoff between capital
(plant area and feed systems) and oper-
ating costs (sorbent use, fan power). The
effects of operating conditions on SO2
removal are summarized in Table 1.
Available experimental data on desul-
furization in FBC have been obtained
from bench-scale to small industrial-
scale units, as well as by thermogravi-
metric analysis and mathematical pro-
jections. In general, experimental
results have shown that atmospheric
fluidized-bed combustion (AFBC) can
achieve 90 percent S02 reduction with
Ca/S ratios as low as 2.6 using a high
reactivity sorbent. Reductions in S02
emissions from pressurized fluidized-
bed combustion (PFBC) of 90 percent
have been observed with a Ca/S ratio of
less than 1, although the typical range is
between 1 and 2.
Future research in FBC S02 control
will emphasize the confirmation of S02
control capability in large-scale units. In
addition, to verify theoretical predictions
and provide guidance for future system
design, the influence of gas residence
time and sorbent particle size in large
demonstration units must be docu-
mented. Cost/benefit tradeoffs associ-
ated with maximizing or minimizing
these parameters must also be defined.
Other investigations that are of prime
importance are those focusing on th<
assessment of limestone characteristic;
and availability as well as sorben
regeneration and alternative sorbents.
Nitrogen Oxide Control in FBC
Nitrogen oxide (NOX) emissions frori
fluidized-bed combustion of coal an
generally lower than those from con
ventional systems. Design and operating
factors that influence the formation anc
reduction of NO« in FBC are listed ir
Table 2, which indicates the genera
trend of the effect of FBC operating
conditions on NO. emissions.
The kinetics of NOX reduction are no
well defined to date. The low NO
emissions from FBC are inherent to the
system. A quantitative correlation
beyond the trends shown in Table 2, i;
not predictable at this time.
Research on characterization of th<
mechanisms of NOX control in FBC arc
proceeding. An alternative operating
mode that can be used to reduce NO
emissions even further is two-stage
combustion. In conventional systems
the application of staged combustior
has resulted in 30 to 50 percent NO
reduction. Further testing is required tc
Table 1. Operating Conditions Affecting SOz Removal
Variable increased Effect on SO2 removal efficiency
Sorbent reactivity
Gas phase residence time
Sorbent particle size
Pressure
increase
increase
decrease
increase
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Table 2. Operating Conditions Affecting /VOX Emissions from FBC
Variable increased Effect on /VO, emissions
Temperature
Gas phase residence time
Excess air
Pressure
Fuel nitrogen content
Coal particle size
increase
decrease
increase
decrease
increase
decrease
define the NO* control potential of two-
stage combustion in FBC systems.
Other techniques that could be con-
sidered for further NOX control in FBC
include flue gas recirculation and am-
monia/urea injection. Further testing is
required to determine the incremental
NOX reduction that can be expected
under these optional operating condi-
tions.
Paniculate Emissions from
FBC Systems
Particles emitted from a fluidized-bed
boiler are a combination of fly ash from
the coal, unburned carbon, and elutriated
sorbent. Particulate emissions from a
fluidized-bed boiler will be controlled in
a manner similar to that of conventional
boilers. A primary particulate control
device, usually a cyclone, is used to
collect larger particles that have the
most significant carbon content. This
primary cyclone catch is often recircu-
lated to the boiler to attain greater
combustion efficiency and improve SOz
reduction efficiency. An alternative
method for increasing carbon use is to
feed the collected material to a secondary
FBC chamber called a carbon burnup
cell (CBC).
The particles that pass through the
primary cyclone are collected in a final
control device. Final particulate control
has not yet been demonstrated on AFBC
units; however, conventional particle
control devices should be adequate to
meet present and planned emission
standards. If high levels of control are
desired, electrostatic precipitators
(ESPs) or fabric filters will most likely be
applied. If lower efficiency is acceptable,
a multitube cyclone may be sufficient.
Pressurized systems may require greater
control efficiency because of strict
limitations on the amount of particulate
matter that can be tolerated in the flue-
gas-driven turbine that supplies about
20 percent of the PFBC output. There is
some conflict of opinion on allowable
levels, and the effect of particulate level
on turbine life needs to be further
investigated. Research efforts to demon-
strate the capability of various final
particulate control devices applied to
FBC systems are planned as part of
ongoing technology demonstration
programs.
Trace Element Emissions to
Air from FBC
Because fluidized-bed combustion is
carried out at temperatures well below
those of conventional combustion sys-
tems, it is possible that trace element
emissions will differ. The presence of
sorbent material in the combustion bed
adds another factor whose contribution
to trace element emissions is unknown.
There are limited trace element emis-
sions data from FBC units. To date, the
most comprehensive sampling and
analysis of FBC emissions has been the
EPA-sponsored Level 1 environmental
assessment conducted at the Exxon
Mini-plant PFBC. Trace element analysis
of particulate material entrained in flue
gas was conducted on size-fractionated
samples obtained using a Source As-
sessment Sampling System train. Both
spark source mass spectrometry and
atomic absorption spectrometry were
used for elemental analysis. Similar
'programs are planned at the B&W-
EPRI/Alliance, Ohio, AFBC facility and
at the Georgetown University atmo-
spheric fluidized-bed boiler.
Solid Waste Generation
from FBC
Solid residue from the fluidized-bed
process consists of spent bed material
(largely calcined and sulfated sorbent
withdrawn from the combustor) and a
mixture of fly ash, spent sorbent, and
unburned carbon collected in the par-
ticulate control-device.
The amount of solid waste generated
is a function of the fuel and sorbent
characteristics and the level of S02
control and particulate control. The
major variables and their general effect
on the amount of residue generated are
listed in Table 3.
In most cases, disposal of solid waste
from FBC systems is expected to be
performed by landfilling the material.
The environmental impact of this method
of disposal is under investigation. The
primary sources of potential environ-
mental degradation are the leachate
formed by rainwater runoff and percola-
tion after landfilling, and the heat
release from the material upon initial
contact with water, because of hydration
of the CaO in the waste. Limited data on
leachate generation and transport/
transformation phenomena are available.
One of the most definitive evaluations
of the potential contamination from FBC
waste was performed by Westinghouse
Research Laboratories. Leachates were
generated using distilled, deionized
water in laboratory shake tests for a
variety of FBC wastes. The resulting
leachate concentrations were then
compared with drinking water standards
(National Interim Primary Drinking
Water Regulations, and U.S. Public
Health Service's Drinking Water
Standards).
Conclusions from this study included:
• No water pollution is expected from
the leaching of those trace-metal
ions for which drinking water stan-
dards exist because the leachate
itself meets drinking water stan-
dards.
• The total dissolved organics are
below detection limits.
• Potential problems with the leach-
ates are the high concentrations of
calcium (Ca), sulfate (SO*), pH, and
total dissolved solids (TDS) that are
above drinking water standards.
• The addition of 20 wt percent fly
ash to the spent sorbent improves
leachate quality. Thus, codisposal
of spent sorbent and ash can re-
duce the adverse environmental
impact.
• The environmental impact is re-
duced by room-temperature process-
ing with water to eliminate a po-
tential heat release problem.
These conclusions have been corrob-
orated by results of other studies. In
general, data indicate that FBC solid
residue will not be a hazardous pollutant.
FBC solid waste could be a useful
byproduct. Because of the high amount
of unused lime (CaO) in the waste, its
uses as a cement supplement, agricul-
tural additive, building material, and
road aggregate have all been explored,
and results are promising. As larger
quantities of waste become available
from operation of larger demonstration
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Table 3. Operating Conditions Affecting
Variable increased
Quantity of Solid Waste Generated
Effect on quantity of waste
1
Ca/S molar feed ratio
Sorbent reactivity
Gas phase residence time
Sorbent particle size
Percent SOa removal
Percent coal sulfur
Percent coal ash
increase
decrease
decrease
decrease
increase
increase
increase
plants, the resource recovery possibilities
can be more thoroughly assessed.
Water Effluents from FBC
Although no supportive data are cur-
rently available, water discharges from
FBC units are expected to be similar to
effluents from conventionally fired
boilers. The sources include boiler
feedwater treatment wastes, boiler
blowdown, cooling tower blowdown,
fireside and waterside operational
cleaning waste, and drainage from
materials storage piles.
The major differences between the
two technologies are expected to be
runoff or leaching from materials storage
and spent solids disposal sites and
equipment cleaning wastes. Other
streams should be similar because they
involve components and operations that
are equivalent for the two systems.
Radioactive Pollutants
Associated with FBC
Radioactive contaminants, contained
in emissions to the atmosphere or in
solid waste residues, can be released to
the environment from combustion of
coal. Under the Clean Air Act Amend-
ments of 1977, the Environmental
Protection Agency (EPA) is instructed to
review all pertinent data to determine
whether emission of radioactive pol-
lutants into ambient air poses a threat to
public health. EPA is empowered to
establish, implement, and enforce limi-
tations, standards of performance, or
other requirements sufficient to mitigate
the potential hazards from such emis-
sions. The EPA Office of Radiation
Programs (ORP) is conducting this re-
view of all relevant, available informa-
tion. In support of ORP, samples collected
by GCA/Technology Division during
environmental assessment programs at
the Exxon Miniplant PFBC were anal-
yzed for selected isotopes. The data
show higher radioisotope concentra-
tions in the relatively fine particles
collected by the final control devices.
Cascade impactor measurements show
that the particles entering the final
control device are 95 percent by mass
less than 4 /urn and 50 percent less than
2 //m. Secondary cyclone catch typically
has a volume median diameter (deter-
mined by Coulter Counter) of 17 (im and
the third cyclone catch typically has a
median diameter of 4-6 /urn.
Data illustrating the flow of the
isotopes through the system show that
most of the radioisotopes are emitted as
part of the spent combustion bed solids
or in the cyclone catch. Radioisotopes
escaping the plant in the flue gases at
Exxon (or in a commercial plant) would
probably be 1 to 20 percent in the final
particulate control device catch. This
area requires further investigative
effort.
Bioassay Testing of FBC
Emissions
Conclusive results from bioassay
testing of FBC emission/effluent streams
are not yet available because of limited
testing, different and in some cases
incomparable analysis methods, and
continually changing protocols and
procedures for performing bioassays.
The data that are available generally
indicate mutagenic response to airborne
particulates and ecological effects test
response to coal, spent solids, and
collected fly ash material. In general,
based on limited comparative results,
health and ecological effects from FBC
waste streams are not expected to be
greater than those resulting from con-
ventional combustion.
J. M. Robinson, R. J. Kindya, C. W. Young, R. R. Hall, and P. Fennelly are with
GCA Corporation, Technology Division, Bedford, MA 01730.
John O. Millikan is the EPA Project Officer (see below).
The complete report, entitled "Environmental Aspects of Fluidized-Bed Com-
bustion," (Order No. PB 81-217 630; Cost: $9.50. 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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