FLUIDIZED BED COMBUSTION 405
joule is the limit for lignite which is mined in North Dakota,
South Dakota, or Montana and which is burned in a cyclone
fi red unit.
When different fossil fuels are burned simultaneously
in any combination, the nitrogen oxide emission standard is
calculated by the following formula:PSNOxwxyz
wxyz
()()()()
.260 86 130 300Where PSNOx is the prorated standard in ng/joule heat input
for nitrogen oxides (except nitrous oxide) derived from all
fossil fuels or fossil fuels and wood residue fi red, w is the
percentage of total heat input derived from lignite, x is the
percentage of total heat input derived from gaseous fossil
fuel, 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 (except lignite).
There is no standard for nitrogen oxides when burning
gaseous, liquid, or solid fossil fuel or wood residue in com-
bination with a fossil fuel that contains 25%, by weight, coal
refuse. Coal refuse is defi ned as “the waste products of coal
mining, cleaning and coal preparation operations (e.g., culm,
gob, etc.) containing coal, matrix material, clay, and other
organic and inorganic material.”^6
The NO x emission standards for West Germany and
Japan are even more stringent than those of the U.S.^7 For
new and existing West Germany boilers of over 110 MW,
the limit is 0.16 lb./MBtu (6% O 2 ). For Japanese boilers built
after 1987, the limit is 0.33 lb./MBtu.PROMINENT FBC INSTALLATIONS IN THE U.S.Recently, in order to reduce SO 2 emissions, Northern States
Power Company (NSP) converted its Black Dog pulverized
coal-fi red boiler to that of a bubbling bed combustor. This
unit is the largest of its kind in the world; its capacity is 130
megawatts.
NSP received a new Emissions Permit from the Minnesota
Pollution Control Agency (MPCA) for the upgraded unit.
The emissions standards set forth in this permit are less strin-
gent than those of the federal standards for particulate matter
and SO 2. In the event that utilities should become regulated,
the operating parameters of the system or the system itself
would have to be modifi ed.^10
The most recent literature available to the author (April
1988) stated that limestone was being added to the bed in
order to lower SO 2 emissions suffi ciently to help NSPS stan-
dard. The control of particulate matter was diffi cult at the
onset. However, this problem was resolved by changing the
bed material to an inert fi red-clay material. NO x emissions
requirements have easily been met.
The Tennessee Valley Authority (TVA) has built a 160
MW bubbling bed combustor for the utility’s Shawnee steam
plant in Paducah, Kentucky. It has been operating sporadi-
cally since autumn of 1988.A pilot plant (20 MW) was completed in 1982 and had
brought forth some very promising results. With a Ca : S ratio
of 2 to 2.5 (typical range) and a recycle ratio of 2 to 2.5 the
SO 2 retention was approximately 90%.^11 This result has been
matched by the scaled-up plant. The pilot plant has both an
underbed and overbed feed system. Overbed feed does not
produce as great a combustion effi ciency as that achieved by
the underbed method. This would be expected due to the lack
of control over fi nes in the feed. NO x emissions were less than
0.25 lb/million Btu.^11 The NSPS for NO x is 0.7 lb/million Btu
for solid fuel.
The original underbed feed system was determined to
be inadequate because of plugging and erosion problems.
The system was redesigned and proved to be successful. The
feed system is one of pressurized bottles mentioned earlier
in this report under “Bubbling Bed Combustors.”
As stated in the “Introduction,” fl uidized bed combus-
tion can be used for many different types of industrial pro-
cesses. An example of this is the installation of the direct
alkali recovery system at Associated Pulp and Paper Mills’
Burnie, Tasmania mill.
In this process, sodium carbonate (residual) found in
soda-quinone black liquor (a waste product) reacts with
ferric oxide to produce sodium ferrite in the combustor (bub-
bling bed). The sodium ferrite is then contacted with water to
yield sodium hydroxide (desired) and ferric oxide. The ferric
oxide is returned to the combustor to be reused. It is interest-
ing to note that most of the steam produced in this process
is created from the extraction of heat from the exhaust gas
and not from bed tubes. The exhaust gas is cleaned via a
fabric fi lter and the dust collected is palletized. The pellets
are later used in the process. The fl uidizing air is heated from
the heat extracted from the hot sodium ferrite after it has
been removed from the combustor.
Since there is no sulfur involved in this process the
exhaust gas is easily cooled, thereby allowing greater pro-
duction of high-pressure steam.^13
The title for the world’s largest CFB probably belongs
to the nuclear generating station owned by Colorado-Ute
Electric Association. The original 25-year-old plant was
replaced because it was uneconomical to operate. The capac-
ity of the new plant is 110 MW.
In May of 1988 on EPRI (Electric Power Research
Institute) assessment began and is scheduled to continue
until May of 1990. As of April 1988, the unit was reported
to be easy to operate, responsive to load variations, and
easily restarted following a trip. However in 1989 opera-
tional diffi culties were reported. SO 2 emissions standards
were expected to be easily met and NO x emissions were
well under the limit. Final determination of the optimum
Ca : S ratio still needed to be determined. Particulate matter
emissions are expected to be less than 0.03 lbs/million. Btu
because of the addition a new baghouse to the existing three
baghouses.
Some valuable information has been learned from the
unit thus far, e.g., control of coal feed size has been impor-
tant in maintaining the bed quality and agglomerations can
be avoided if the feed is started in short bursts prior to beingC006_001_r03.indd 405C006_001_r03.indd 405 11/18/2005 2:28:15 PM11/18/2005 2:28:15 PM