404 FLUIDIZED BED COMBUSTION
turbine must be fl exible. An effective turbine should be able
to accept low gas temperatures, be minimally affected by
unremoved fi nes in the gas, compensate for low load condi-
tions, and allow the gas velocity through the hot gas clean
up (HGCU) system and excess air to remain near constant
over much of the load range. Most FBC systems incorpo-
rate a free-wheeling low pressure and constant velocity high
pressure shaft design to accomplish the aforementioned
requirements. The HGCU system generally consists of one
or several cyclones. Sometimes a back-end fi lter at conven-
tional pressuers and temperatures is used in addition.
The gas turbine accounts for approximately 20% of a
FBC’s total power output while the steam turbine creates
the remainder.^5 The steam turbine is powered from steam
created via combustor tubes and is totally independent
of the exhaust gas and gas turbine. Steam turbine perfor-
mance is therefore only affected by fuel/feed conditions.
Two types of fuel feeding are generally used for FBC’s—
dry and wet. For fuels with high heating values the fuel is
mixed with water to create a paste (20–25% water). With
this method there is naturally no need for coal drying, and
evaporated water creates additional mass fl ow through the
gas turbine. Dry fuel feeding is more benefi cial with low
heating value fuels.FEDERAL AIR EMISSIONS STANDARDSThe standards of performance for fossil-fuel-fi red steam gen-
erators (constructed after August 17, 1971) were last revised
by the federal government as of July 1, 1988.
Regulated facilities include fossil-fuel-fi red steam gen-
erating units of more than 73 megawatts (heat input rate
250,000,000 Btu/hr.) and fossil-fuel and wood-residue-fi red
steam generating units capable of fi ring fossil fuel at a heat
input rate of more than 73 megawatts. Existing fossil-fuel-fi red
units which have been modifi ed to accommodate the use of
combustible materials other than fossil fuels are regulated in a
different manner.
Within 60 days after the maximum production rate is
attained by a regulated facility, the facility must conduct
performance tests and provide the E.P.A. with the results of
the tests. The tests must also take place before 180 days after
the initial start-up of a facility. Each test is specifi c and used
for the determination of such things as nitrogen oxide emis-
sion. These test methods and procedures may be found in 40
C.F.R. (Code of Federation Regulations) Part 60.46.^6 After a
performance text is completed, a facility must not discharge
pollutants into the atmosphere at levels greater than those
established and listed in the federal regulations.
Gases may not contain more than 43 nanograms of par-
ticulate matter per joule heat input (0.10 lb. per million Btu)
where particulate matter is defi ned as a fi nely divided solid
or liquid material, other than uncombined water as measured
by the reference methods specifi ed in 40 C.F.R. Part 60.46.
These gases must also not exhibit greater than 20% opac-
ity except for one six-minute period per hour of not more
than 27% opacity. Opacity is defi ned as “the degree to whichemissions reduce the transmission of light and obscure the
view of an object in the background.”
Less stringent standards have been developed for the
three following facilities.^6
The Southwestern Public Service Company’s Harrington
Station No. 1 in Amarillo, Texas must meet an opacity of
not greater than 35%, except that a maximum of 42% opac-
ity is permitted for not more than six minutes in any hour.
The Interstate Power Company’s Lansing Station Unit No.
4 in Lansing, Iowa must meet an opacity of not greater than
32%, except than a maximum of 39% opacity is permitted
for not more than six minutes in any hour. The Omaha Public
Power District’s Power Station in Nebraska City, Nebraska
must meet an opacity of not greater than 30%, except that a
maximum of 37% opacity is permitted for not more than six
minutes in any hour.
Gases may not contain more than 30 nanograms per
joule heat input (0.80 lb. per million Btu) of sulfur dioxide
(SO 2 ) derived from liquid fossil fuel or liquid fossil fuel and
wood residue. 520 ng/joule heat input (1.2 lb. per million
Btu) is the maximum allowable SO 2 discharge from gases
derived from solid fossil fuel or solid fossil fuel and wood
residue.
When different fossil fuels are burned simultaneously in
any combination, the SO 2 emission standard is calculated by
the following formula:PSso 2 (( ) ( ))/(y 340 zyz 520 )where PSso 2 is the prorated standard in ng/joule heat input
derived from all fossil fuels or fossil fuels and wood resi-
due fi red, y is the percentage of total heat input derived from
liquid fossil fuel, and z is the percentage of total heat input
derived from solid fossil fuel.
The SO 2 emission standard for Units 1 and 2 at the
Central Illinois Public Service Company’s Newton Power
Station must comply with the 520 ng/joule requirement if
the units individually comply with the 520 ng/joule require-
ment or if the combined emission rate from both unites does
not exceed 470 ng/joule (1.1 lb/million Btu) combined heat
input to both units.
It is interesting to note that the federal SO 2 emission limit
for West German coal fi red boilers is 2.5 lB./Mbtu (avg.) for
boilers of between 18 and 110 MW and 0.51b./MBtu (avg.)
for boilers of over 110 MW.^7
Gases may not contain more than 86 ng/joule heat input
(0.20 lb/million Btu) of nitrogen dioxide (NO 2 ) derived
from gaseous fossil fuel. 129 ng/joule heat input (0.30
lb/million Btu) is the maximum allowable NO 2 discharge
from gases derived from liquid fossil fuel, liquid fossil fuel
and wood residue, or gaseous fossil fuel and wood residue.
300 ng/joule (0.70 lb/million Btu) is the maximum allow-
able NO 2 from solid fossil fuel or solid fossil fuel and wood
residue (except lignite or a solid fossil fuel containing 25%,
by weight, or more of coal refuse). 260 ng/joule (0.60 lb/
million Btu) is the maximum allowable NO 2 from lignite
or lignite and wood residue with the exception that 340 ng/C006_001_r03.indd 404C006_001_r03.indd 404 11/18/2005 2:28:15 PM11/18/2005 2:28:15 PM