406 FLUIDIZED BED COMBUSTION
continuous; this is to allow the temperature rise to be more
uniform.^14
One of the larger commercial units in the U.S. is located
in Colton, California and was installed for Cal-Mat Co. The
25 MW CFB was constructed because electric utility rates
were rising and the availability of power was uncertain. The
company manufactures cement—a process requiring much
electricity. Since the company had easy access to coal and
limestone as well as a large quantity of heat from its kilns,
CFB technology became an effective solution to their energy
needs. Bottom ash waste and fl yash could also be used in the
cement-making process.
As could be expected, the air pollution controls instituted
by the state of California are very strict. However, a permit
was granted to CalMat in a relatively short period of time
because of the fi ne performance demonstrated by this unit.
The exhaust gases were found to contain SO 2 at 30 lb/hr., NO x
at 57 lb/hr. and CO at 24 lb/hr.^15 There were initial problems
with equipment and systems, however, these were eventually
eliminated. Bed retention and temperature control problems
have also been resolved through modifi cations of the air fl ows
and nozzles.
PFBC’s have been installed in Sweden, the U.S. and
Spain.
Two PFBC modules of 200 MW each have been installed
in Vartan, Stockholm. The fi rst unit is due for start-up in late- The Swedish emission standards are very strict and
include special restrictions on noise and dust since the units
are located very close to a residential area.
A 200 MW combined cycle PFBC will be installed by
American Electric Power (AEP) at its Tidd Power Plant at
Brilliant, Ohio. Test results from joint studies proved PFBC
technology to have environmental benefi ts surpassing those
of traditional boilers with fl ue gas desulfurization systems
(FGD), selective catalytic reduction, etc.
A 200 MW PFBC will be installed by Empresa Nacional
de Electricidad S.A. (ENDESA) at its Escatron Power Plant
as a retrofi t for an existing unit. 90% sulfur removal and a
NO x decrease of 30% are expected. Many different technolo-
gies were considered but PFBC was chosen because of the
high sulfur/ash/moisture black lignite coal that they burn.^16
NO X /SO 2 FORMATION AND CONTROLFossil fuels naturally contain sulfur in varying percentages.
As fuel is burned the sulfur combines with oxygen to form
SO x , and primarily, SO 2. When emitted into the atmosphere
this SO 2 can combined with water vapor to form sulfuric
acid (and sulfurous acid to a lesser extent). This is a part of
the basic mechanism by which acid rain is created.
In order to control sulfur dioxide emissions, the oldest
and still most common method used is to react the gas with
limestone or a similar calcium based material. Crushed lime-
stone (CaCO 3 ) can be fed continuously to a conventional
coal boiler or fl uidized bed where it calcines to lime (CaO)
and then reacts with SO 2 in the presence of oxygen to formcalcium sulphate (CaSO 4 ). This material precipitates to the
bottom of the combustor and is removed.
Coal particle size has a defi nite impact on desulfuriza-
tion. Bed composition also has an effect on sulfur removal.
A typical bed might be composed of coarse partially sul-
fated limestone and ash (produced by combustion). The par-
ticle size of the coal and limestone would probably be equal
however combustor operation conditions such as fl uidizing
velocity will dictate the particle size.
An alternate scenario might be to pulverize the limestone
and introduce it to a bed composed of ash or some other type
of refractory material. Fines naturally have shorter residence
times than do coarse materials and, hence, would probably
have to be recycled to increase effi ciency.
A series of experiments were carried out by Argonne
National Laboratory^17 using three different types of lime-
stones to test their effects on sulfur capture during com-
bustion. The average particle size of the limestone was
500–600 micrometers. The Ca : S ratio was 2.3–2.6 and
the combustion temperature was 1600F. S O 2 removal was
74 to 86%. The test proved that the amount of SO 2 removal
was relatively independent of the type of limestone used.
The test also proved that particle size did not have much of
an effect on SO 2 removal. The explanation offered for this
observation was that although larger particles are less reac-
tive than smaller particles, the increased residence time in
the combustor of larger particles compensates for the lower
reactivity.
Dolomite was also evaluated for SO 2 capture. In two
experiments, Tymochtee dolomite was added to a bed
composed of alumina at Ca : S ratios of 1.5 and 1.6. The
average particle size was 650 micrometers. The SO 2 remov-
als were 78% and 87% respectively. MgO is contained
within the dolomite matrix and is believed to keep the par-
ticles more porous such that sulfation is greater, especially
in larger particles.
Combustion temperature had a marked effect on SO 2
removal in these experiments. Dolomite No. 1337 was
most effective in reducing SO 2 at 1480F. Limestone No.
1359 was most effective in the range of 1500–1550F. Both
sorbents achieved approximately 91% SO 2 removal. The
average particle size was approximately 500 micrometers.
Pulverized limestone No. 1359 with an average particle size
of 25 micrometers was most effective in the range of 1550–
1600 F. The extent of calcination is more dependent upon
bed temperature for fi nely pulverized limestone. The greater
the calcination, the greater the reactivity with SO 2.
An unusual fi nding occurred in that Tymochtee dolomite
was observed to be most effective in SO 2 removal at 1800F.
For all of the other sorbents the SO 2 removal was very poor
at this temperature. Explanations for this phenomenon have
been proposed. One explanation suggests that above a certain
temperature the sorbent’s pores close thereby ending sulfona-
tion. Depending upon the sorbent’s structure and composition,
this temperature would be different for each sorbent. Another
explanation involves the effect of fl uidized bed gas circulation
on bed chemistry. An emulsion phase and a gas bubble phaseC006_001_r03.indd 406C006_001_r03.indd 406 11/18/2005 2:28:15 PM11/18/2005 2:28:15 PM