United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-82-051 Sept. 1982
Project Summary
Technology Overview:
Circulating Fluidized-Bed
Combustion
Douglas R. Roeck
The circulating fluidized-bed
combustion (CFBC) process is a
second generation FBC system that is
well underway toward commercializ-
ation in the U.S. The CFB operates at
higher fluidization velocity, lower
mean bed particle size, and higher
recirculation rate than conventional
FBC systems. Probable advantages of
CFBC over the traditional FBC
process include: more flexibility in fuel
selection, reduced number of fuel feed
points, higher combustion efficiency,
better calcium utilization, and lower
NOx emissions. Potential process
limitations that must still be
evaluated, however, include
equipment erosion due to the more
severe operating conditions,
separation of bed material from
effluent gas, severity of cyclone
separation equipment design, and
power requirements for process and
auxiliary equipment operation.
Battelle Development Corp., Lurgi
Corp., and Pyropower Corp. are the
major companies now involved in
demonstrating the commercial
viability of this process in the U.S.
Both Lurgi and Pyropower are basing
their CFB systems on technology that
has already been commercially
demonstrated in Europe. Battelle,
after proving its process on the
laboratory and pilot plant scale, is
building (through its licensee,
Struthers Thermo-Flood Corp.) the
first U.S. commercial plant, which will
generate steam for secondary oil
recovery operations at a Conoco tar
sands facility in Uvalde, TX.
Additionally, TVA has initiated
construction (at its Shawnee Plant) of
a 20 MWe pilot unit, described as a
hybrid CFB-AFBC system.
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).
Background
Circulating fluidized-bed combustion
(CFBC) is a technological offshoot of
conventional FBC, designed to alleviate
some of the potential limitations of
conventional FBC systems, yet
incorporating inherent process
advantages. In comparison with
classical FBC units, the CFB operates at
a higher fluidization velocity (10-30
versus 2-12 ft/sec),* lower mean bed
particle size (50-300 versus 1000-1200
/jm), and higher solid recirculation rate.
In the circulating bed, the entire reactor
contains solids of significantly lower
density than in conventional FBs, and
the degree of gas/solids contact over
the entire reactor height leads to longer
contact times in the CFB, even at the
* Although ERA'S policy is to use metric units in all
its documents, this summary uses certain non-
metric units for the reader's convenience Readers
more familiar with metric units are asked to use the
conversion units listed at the end of this summary
-------
higher gas velocities used. For these
and other reasons, CFBs have several
potential advantages including:
• More flexibility in fuel selection
(coal, wood, peat, etc.).
• Lower number of feed points.
• Higher combustion efficiencies.
• Better sorbent utilization.
• Lower NOx emissions resulting
from staged combustion.
However, several potential problem
areas (depending on specific designs)
may require further investigation and
evaluation.
• Number, severity of design, and
power requirements associated
with auxiliary equipment.
• Equipment erosion due to higher
velocities and greater solids
concentrations.
• Difficulty of separation of bed
material from effluent gas.
Most literature on CFB technology has
been prepared by companies that are
developing and marketing systems for
commercialization. The claims of
improved combustion efficiency,
reduced sorbent requirement, and
lower NOx emissions are tentatively
supported by limited data from test
burns on both commercial (foreign
installations) and pilot-scale CFBC units
(domestic and foreign)
Process Description and
Development Status
The major companies now active in
researching, developing, and
commercializing CFBC technology in
the U.S. include Battelle Development
Corp., Combustion Engineering,
Conoco, Lurgi Corp., Pyropower, Stone
and Webster, and Struthers Thermo-
Flood. Synopses of each company's
CFBC system and experience are
presented in this section. Process
design features and commercialization
status of the systems are described,
as well as a brief discussion of foreign
CFBC technology.
In 1973, Battelle began work to
improve conventional FBC technology
for burning coal As a result, a second-
generation FBC process -- a Multisolid
Fluidized-Bed Combustion (MS-
FBC) system -- was developed and
patented. The MS-FBC system, shown in
Figure 1, features an entrained bed of
small or light particles (typically sand or
limestone) and a permanently fluidized
dense bed (typically iron ore or silica)--
both in the combustor. The light
entrained bed penetrates up through
the dense bed and is elutriated from the
combustor column. The entrained bed,
the heat carrier in the process, is then
collected in a cyclone and sent to an
external heat exchanger. The cooled
entrained bed material is then returned
to the combustor. The system can burn
either high sulfur coal or coke, or
combinations of solid and liquid fuels.
Development work by Battelle has
been conducted in 6-in. I.D. (0.4 x 106
To
connective
boiler and
paniculate
removal
Btu/hr) and 10-in. I.D. (1.0x106 Btu/hr)
pilot plant units. Testing has been
carried out on high-sulfur coal from
Ohio, Illinois, and Pennsylvania and
limestone from Ohio and Virginia. SO2
levels of 1.2 lb/106 Btu have
been achieved with Ca/S mole ratios of
1.5-22 while burning 4% sulfur coal.
The effect of entrained bed recycle rate
and Ca/S mole ratio on SO2 emissions
in the Battelle pilot plant units is shown
in Figure 2.
The MS-FBC is being marketed for
various industrial steam generation
applications wherein the specific
sequence of the heat transfer steps and
the particular operating conditions
would be optimized for the require-
ments of any given plant. Commercial-
ization of the MS-FBC has been initiated
in conjunction with Struthers Thermo-
Entrained
bed
To ash
disposal
To steam drum
Circulating
water
:
Fluidizing AIR
air
External
heat
exchanger
* Coal
O Dense bed
° Limestone
Ash
• Carbon
Figure 1. Battelle multisolid fluidized-bed combustor.
-------
Flood Corp. of Winfield, KS. Struthers
has concluded a license agreement with
Battelle which gives Struthers exclusive
worldwide rights to the design for use
on secondary oil recovery steam
generators. Figure 3 shows the
Battelle/Struthers oil field steam
production configuration. A 50 x 106
Btu/hr unit has been installed and is
presently undergoing start-up at a
Conoco plant in Uvalde, TX. This steam
generator is designed to burn a wide
variety of solid fuels including
petroleum coke, coal, and lignite for
steam injection being utilized in a tar
sand reservoir. Steam, at an outlet
pressure of 2,450 psia, will be produced
from feedwater at temperatures of 70-
200°F.
Lurgi Corp., with over 30 years
experience in the design and
construction of high-temperature FB
processes and hardware (some 350
conventional bubbling-bed systems
worldwide), began developing CFBC
technology around 1960. Their initial
application was for calcination of
aluminum trihydrate to cell-grade
alumina, the Lurgi-VAW process; the
first commercial plant went on-line in
1970. Based on their experience with
roasting and combustion in convention-
al FBs, and with the operation of CFB
alumina calciners, Lurgi began
developing CFBC technology as an
alternative approach to coal
combustion. This work led to several
novel process design concepts, one of
which is shown in Figure 4. In this
system, fine-sized coal (average particle
size of 200-300 yum) and limestone
(about the same size) are fed
3,000
I
CO
§
/,000
85 Percent
Capture
100 ppm
0
O MSFB-0.4, Coal, R = 2,500 Ib/hr ft2
MSFB-1, Coal, R = 8.000 Ib/hr ft2
MSFB-1 Delayed Coke,
R = 8.000 Ib/hr ft2
O MSFB-1, Fluid Coke.
R = 3,000 Ib/hr ft2
1 2 3
Ca/S Ratio, moles/mole
Figure 2. Sulfur capture in MS-FBC-effect of entrained bed recycle rate (R).
pneumatically to the lower part of the
reactor while the combustion air is
introduced at two levels.
As a result of favorable tests
conducted in their 14-in. I.D. pilot unit in
Frankfort, West Germany, Lurgi has
been awarded (along with Combustion
Engineering) a contract by TVA to
perform preliminary design of 200 and
800 MWe utility boilers using the Lurgi
CFBC process. Design parameters for
the 200 MWe system are shown in
Table 1. Lurgi is in the process of
commercializing CFB technology in the
U.S., although there are no such units
yet installed. However, one commercial
CFBC unit is being built by Lurgi at the
Vereinigte Aluminumwerke (VAW) in
Lunen, West Germany. This unit will
have a capacity of 84 MWt, will produce
high pressure steam (convective
section)and will reheat 2.8 x 106 Ib/hr
molten salt heat carrier from 650 to
800°F (FB heater section). On
equivalent terms the unit (if designed
for steam production only) would
produce 220,000 Ib/hr. The unit will
burn high-ash coal waste (50% ash by
weight, dry basis) and is scheduled for
commissioning in mid-1982.
The Pyropower Corp., San Diego, CA,
is also promoting CFB technology in the
U.S., based on research by its parent
organization, Ahlstrom Co., Helsinki,
Finland. FBC research has been a major
project at the Hans Ahlstrom
Laboratory--the R&D Department of the
Company's Engineering Division in
Karhula, Finland—since 1969. Aware of
the limitations of conventional FBs,
Ahlstrom developed the Pyroflow CFB
system in 1976. Pyropower offers two
basic systems for steam generation: (1)
for low-to-medium-pressure steam
applications, a convective boiler bank is
required because all of the evaporative
duty cannot be done in the combustion
chamber (a superheater is at the inlet to
the boiler bank, an economizer for
heating incoming feedwater is at the
boiler bank outlet); and (2) for medium-
to-high-pressure steam applications, all
evaporation will be done in the
combustion chamber and superheating
will be done in the convection zone of
the boiler (an economizer is also in the
convection zone). Depending on the fuel
to be used, an air heater may also be
included m the second configuration.
Table 2 lists Pyropower's commercial
CFB installations since 1976. Since the
first CFB system was developed at
Ahlstrom, 10 additional systems in sizes
up to 200,000 Ib steam/hr have been
-------
Secondary
Air
Blower
Coal Feed
Limestone
Feed
Convection
Section
(Economizer)
External
Heat
Exchanger
Primary
Air
Blower
Entrained
Bed Recycle
Blower
80% Steam
Product to
Injection Well
Baghouses
Induced
Draft
Blower
Figure 3. MS-FBC for oil field steam injection.
Steam drum
Coal
Pneumatic
Feeding
Superheated
-steam ,—°>
Boiler
feed
water
To
stack
Electrostatic
precipitator
8
°0
Bun
+f
j
y^l
\ \ \
H
\
i 1 Sect
I 9 air b
Ash disposal
«' i
Fluidized\
evaporator
•>ndary
lower
Ast
1
\ /
\
? disposal
1 '
Air preheater
Fan
Secondary
air blower
O ©
'{ [Prii
Primary
air blower
Figure 4. Circulating fluid bed boiler.
sold for commercial operation. One
system has operated for 2 years with an
availability of over 95%.
Pyropower is now offering Pyroflow
systems to the North American
market. To support this effort, they
initiated a testing program in 1979 in
conjunction with the Electric Power
Research Institute (EPRI) and TVA.
Preliminary results from combustion
tests on several U.S. fuels are shown in
Table 3
Several other groups and organiza-
tions, both in the U.S. and abroad, are or
have been involved in research related
to CFBC technology. In the U.S., Com-
bustion Engineering, Conoco, and
Stone and Webster are involved in a
joint venture for developing a Solids
Circulation Boiler for industrial
application. This concept is basically
opposite to that employed in other CFBC
configurations in that coal is combusted
in the dense (bubbling) bed while heat
exchange occurs in the dilute
(entrained) bed. Otfier work in the U.S.
has involved EPRI and TVA, as
mentioned previously, and the
Westinghouse R&D Center.
Outside the U.S., three groups, all in
Sweden, have been investigating CFB
technology. At the Lund Institute of
Technology, a reactor concept that has
been demonstrated to work in the
gasification of black shale has been
developed. At Gotaverken in Goteborg,
Sweden, construction has nearly been
completed on an 8 MWt demonstration
CFBC that will burn coal (with peat and
wood as alternate fuels) and will provide
steam for the company's shipyard. At
Studsvik Energiteknik AB in Nykoping,
Sweden, experience with a 250 kW fast
FB experimental model designed for
cold flow and combustion experiments
has led to development of a 2.5 MW
prototype module.
Due to the lack of commercial
experience (in the U.S.) with CFB
technology, capital and operating costs
are not well-defined. However, several
studies have tentatively concluded that
capital costs for a CFB boiler would be
about the same as those for a
conventional FBC unit and that
operating costs for the CFB may be
slightly less. For example, a conceptual
design study fro EPRI indicates that the
capital costs for an atmospheric CFB
-------
Table 1. Design parameters for 200 MWe CFBC conceptual design study.
CFBC
Combustion temperature
Excess air ratio
Fluid/zing velocity
Average carbon content of ash
Combustion efficiency
Ca/S mole ratio
Sulfur removal efficiency
CFB pressure drop
1560°F
1.2
19 ft/sec
1 percent
99.4 percent
1.5
90 percent
104 in. W.C.
Heat transfer coefficient to CFB tube walls30 Btu/ft2-hr-°F
Number of coal feed points
Number of limestone feed points
Solids entrainment from CFB furnace
Mean coal feed size
Mean limestone feed size
1 per 50 MWe
1 per 100 MWe
0.15 Ib/ft3 gas
300-500 fim
250-400 fjm
Metric Conversion
Readers more familiar with metric
units are asked to use the following
factors to convert the nonmetric units
used in this summary.
Non-metric Multiplied by Yields metric
Btu
°F
ft
ft2
ft3
in.
in.2
Ib.
1055
5/9(°F-32)
0.3
0.09
28.3
2.54
6.45
0.45
J
°C
m
m2
1
cm
cm2
kg
Cyclones
Axial velocity
Recycling cyclones efficiency
Secondary cyclones efficiency
FB Heat Exchanger
Fluid/zing velocity
Heat transfer coefficient to immersed
tube surface
FB heat exchanger pressure drop
10.5 ft/sec
96 percent
85 percent
3 ft/sec
70 Btu/ft2-hr-°F
36 in. W.C.
may actually be less than conventional
FBCs due to reduced combustor size,
but that any cost advantage for a
pressurized CFB would be questionable.
This same study showed that the overall
efficiency of an electric utility powe'r
plant should be increased by at least 1 %
over a pulverized coal boiler—using an
ACFB boiler, and by at least 3%--using a
PCFB boiler. Another study examined
the economics of conventional stoker
firing as compared to the Battelle MS-
FBC and conventional FBC systems. The
results of this analysis, although
showing a slight economic advantage in
terms of total steam cost for the MS-
FBC, are judged to be very similar, given
the overall accuracy of the component
cost estimates.
Conclusions
The concept of CFBC, after having
been successfully demonstrated on a
commercial scale in Europe, is taking on
renewed interest in the U.S. as a result
of active marketing efforts by three
companies. Battelle Development,
Lurgi, and Pyropower are all primarily
responsible for the development of this
novel FBC technology in the U.S.
Additional work that has helped
stimulate interest has been performed
by or in conjunction with EPRI, TVA,
Combustion Engineering, Conoco,
Stone and Webster, and Westinghouse.
Based on European experiences of both
Pyropower and Lurgi, it would seem
likely that the industrial market would
be more easily penetrated than, say, the
utility market for a variety of reasons.
The likelihood that industrial plants
would have more interest in utilizing
alternative fuels such as peat, wood
waste, and sludges, and the more
critical aspects associated with utility
plant operation would be two reasons
why industrial applications may see
more widespread use of CFB technol-
ogy. On the other hand, reported
advantages of the process relative to net
plant efficiency and turndown
capabilities are factors which could
provide significant economic benefits
for utility applications. The 20 MWe
hybrid CFB-AFBC unit being built at
TVA's Shawnee steam electric
generating plant should provide the cost
and performance data to better define
these benefits.
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Table 2. Pyroflow® circulating fluidized-bed units in operation or under construction by Pyropower
Customer
Hans Ahl strom Laboratory
Karhula. Finland
Ah/strom Co.
Pihlava, Finland
Savon Voima Co.
Suonenjoki, Finland
Kemira Co.
Valkeakoski, Finland
Ahlstrom Co.
Kauttua, Finland
Hyvinkaa Lampovoima Co.
Hyvinkaa. Finland
Skelleftea Kraft Co.
Skelleftea, Sweden
Town of Ruzomberok
Ruzomberok, Czechoslavakia
Hylte Bruk Co.
Hylte Bruk, Sweden
Alko Co.
Kosken Korva, Finland
Kemira Co.
Finland
Start-up
1976
January 1979
September
1979
1980
March 1981
Fall 1981
Fall 1981
Fall 1981
Fall 1982
1982
1983
Gulf Oil Exploration and January 1983
Production Co. Bakersfield, CA
Production Co. Bakersfield,
(USA)
CA
Table 3. Preliminary results of fuel tests for
Fuels
Varied
Peat, wood wastes,
supplementary coal
Peat, wood wastes,
and coal
Zinciferous sludge
Peat, wood wastes,
and coal
Coal, primary; peat
oil, alternate
Peat, wood wastes,
and coal
Sewage and
industrial sludge
Peat, primary;
coal, alternate
Peat
Peat, coal, and
coal wastes
Coal
Application
Pilot plant
and Cogeneration for
board mill
District heating
Incineration
Cogeneration
or District heating
District heating
Incineration
Cogeneration
Process steam
Cogeneration
Process steam for
enhanced oil
recovery
Corp.
Size
2 MWt
5.67 kg/s - 15 MWt
(45,000 Ib/hr steam)
7.0 MWt
0.71 kg/s (5650 Ib/hr)
(21. 5% dry)
25 kg/s - 65 MWt
(200,000 Ib/hr steam)
25 MWt
7.0 MWt
1. 1 1 kg/s (8800 Ib/hr)
(26% dry)
18.27 kg/s - 50 MWt
(145,000 Ib/hr steam)
7 kg/s- 16 MWt
(56,000 Ib/hr steam)
19.5 kg/s - 52 MWt
(155,000 Ib/hr steam)
50 x 706 btu/hr
enhanced oil
input
North American market.*
Fuel
Subbituminous 80 percent Ohio No. 6
Parameters Coal Ash Fuel Coal
Sulfur content, % by
wt. in dry matter
Nitrogen content, % by
wt. in dry matter
Ca/S molar ratio
(average)
S02 retention, %
NOx, ppm (v)
Combustion efficiency, %
0.9
1.1
2.3
84.0
2.5 5. 1
0.3 1.5
2.3 1.8
98.0 90.0
170.0 200.0 280.0
98.0
98.5 98.5
Petroleum
Coke
3.5
1.8
2.4
90.0
100.0
97.0
*A/I tests run at 20-30% excess air.
-------
Douglas R. Roeck is with GCA/Technology Division, Bedford, MA 01730.
John O. Mi/liken is the EPA Project Officer (see below).
The complete report, entitled "Technology Overview: Circulating Fluidized-Bed
Combustion." (Order No. PB 82-240 185; Cost: $9.00, 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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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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