FLUIDIZED BED COMBUSTION 407
exist in any gas-solid fl uidized bed. Excess gas which is not
needed for fl uidization circulates back and forth between the
two phases. This gas does not react with the sorbent until it
reaches the emulsion phase. All of the oxygen in the lower
portion of the emulsion phase reacts with the fuel to from CO.
As the bubbles rise through the bed, air exchanges between
the bubbles and the emulsion phase. The upper portion of the
emulsion phase contains excess oxygen. The following reac-
tion was thus proposed as one which takes place in the lower
portion of the combustor:CaSO^42 ^4 COUCaS^4 CO. (1)One or more of the following reactions were proposed to
occur in the upper portion of the combustor:(^)
CaS 1 O 22 CaO SO
1
2
U
(2)
CaS344CaSO^4 U CaO SO^2
(3)
CaS2O^2 UCaSO^4. (4)
Reactions 2 and 3 would limit the unit’s ability to remove
sulfur because of the regeneration of SO 2. This regeneration
of SO 2 is so dependent upon temperature that it could very
possibly be an explanation as to why SO 2 removal generally
suffers at high temperatures.
Nitrogen also occurs naturally in fossil fuels. This nitro-
gen reacts with oxygen during combustion and later forms
acid rain in very much the same manner as with sulfur.
Oxides of nitrogen (NO x ) are also responsible for the cre-
ation of “smog.” As nitrogen dioxide (NO 2 ) absorbs light of
certain wavelengths it dissociates photochemically to form
nitric oxide (NO) and atomic oxygen. This atomic oxygen
is very reactive and readily combines with O 2 to from ozone
(O 3 ). Ozone in turn oxidizes hydrocarbons in the air to form
aldehydes. Ozone and the aldehydes are components of
smog. NO 2 is the reddish-brown gas which can often be seen
on the horizons of cities such as Los Angeles.
The principal oxide of nitrogen formed during com-
bustion is nitric oxide. Nitrogen in the fuel combines with
oxygen in the fl uidizing air as follows:
1
2
1
2
NONO. 22 U
The kinetics of NO decomposition are slow enough so that
equilibrium levels are not achieved. Various experiments
conducted by Argonne National Laboratory as well as by
other researchers have proven that most of the nitrogen
forming NO x is from the fuel and not from the air. This
has been easily demonstrated by substituting an inert gas
(such as argon) for nitrogen in the fl uidizing air and then
comparing the results to those of combustion with standard
fl uidizing air.
As previously mentioned in this report two-stage com-
bustion is an effective method of decreasing NO x emissions.
As with SO 2 reduction bed composition has an important
effect on NO x. It has been determined through experimenta-
tion and experience that limestone also decreases NO x emis-
sions. Skopp and Hammons^18 observed that when using a
limestone bed two factors were changing with time which
could have been responsible for decreasing NO emissions:
the CaSO 4 concentration in the bed was increasing and so
was the SO 2 concentration. The increase in CaSO 4 sug-
gested that it could be a selective catalyst for reduction of
NO. The increasing SO 2 concentration suggested that there
might be a reaction occurring between it and the NO which
was lowering the NO. This was investigated by conduct-
ing experiments using synthetic NO—SO 2 —N 2 gas mix-
tures. The results showed that no reaction in the gas phase
occurred. There was also no reaction between the NO and
SO 2 over CaSO 4. However, there was a reaction occurring
over a bed of 20% sulfated lime. This reaction was found
to have a negative temperature dependence. The following
mechanism was proposed by Skopp and Hammons^18 as an
explanation for their results:
CaOSO^23 UCaSO
CaSO^33 ^2 NOUCaSO (NO)^2
CaSO (NO^32 ) UCaSO^42 N O
(^)
NO 222 UN O
1
2
.
Esso researchers investigated the possibility of NO being
produced by CO catalyzed by CaSO 4. The rate of this reac-
tion was found to increase with increasing temperature.
Argonne researchers^17 investigated the use of metal
oxides, among them, aluminum oxide (Al 2 O 3 ), zirconium
oxide (ZrO 2 ) and cobalt oxide (Co 3 O 4 ). At the time these
experiments were conducted, the literature had indicated that
these metal oxides were effective in reducing or catalytically
decomposing NO. The results showed that the addition of
Al 2 O 3 and ZrO 2 did nothing to reduce NO formation during
combustion in a fl uidized bed. The addition of Co 3 O 4 actu-
ally increased rather than decreased the formation of NO.
A study was conducted by McCandless and Hodgson^20
for the U.S.E.P.A. on the use of metal sulfi des as a way to
reduce NO emissions. The following is well known as the
“Thiogen” process and has been used in the recovery of
sulfur from SO 2
CaS 2SO 242 UCaSO S
4CaS64 3SO 232 U CaSO S.
Based on this process it was determined that the following
reaction might also be possible
CaS4NOUCaSO^42 2N.
C006_001_r03.indd 407C006_001_r03.indd 407 11/18/2005 2:28:15 PM11/18/2005 2:28:15 PM