408 FLUIDIZED BED COMBUSTION
Preliminary studies indicated that the reaction did pro-
ceed and could be an effective method for NO x control.
Nineteen metal sulfi des were used. All but one reduced NO
to N 2 at temperatures between 194F and 1202F. However,
a weight loss did occur indicating that an undesirable side
reaction was taking place—probably the formation of SO 2.
Some metal SO 4 was formed in most of the tests. However,
the alkaline earth sulfi des were determined to be the most
stable.
It was also found that the temperature at which the reduc-
tion reaction occurs can be lowered if certain catalysts such
as NaF and FeCl 2 are mixed with the sulfi des. Reaction tem-
perature was again reduced when the sulfi de/catalyst combi-
nation was impregnated on alumina pellets. Tests were also
conducted involving synthetic fl ue gas containing 1000 ppm
NO, 1% O 2 , 18% CO 2 and the remainder N 2. Using this gas
in combination with the CaS showed that NO was signifi -
cantly reduced above temperatures of 1112F, by using the
sulfi de/catalyst combination. The results of the experiments
showed that between 0.372 and 0.134 grams of NO were
reduced per gram of metal sulfi de. Between 0.76 and 0.91
grams was achieved when using the impregnated alumina
pellets. The authors recommended that more research be
done to evaluate the economical implications of using these
materials.
Several other interesting facts known about NO x con-
trol and found in the literature are that increasing fl uidizing
velocity decreases NO x , NO x is not signifi cantly affected by
excess air, and NO x production increases at lower tempera-
ture, especially below approximately 1500F.
For conventional coal-fi red boilers the most common
approach to control NO x and SO x simultaneously is the
combination of selective catalytic reduction (SCR) and wet-
limestone or spray dryer fl ue gas desulfurization (FGD). The
SCR process converts NO x to N 2 and H 2 O by using ammo-
nia as a reducing agent in the presence of a catalyst. The
catalytic reactor is located upstream from the air heater and
speeds up the reaction between the NO x and the ammonia,
which is injected into the fl ue gas in vapor form immediately
prior to entering the reactor. The reduction reactions are as
follows:^21443222 U46NH NO O N H O(^42) 322 22U36NH NO O N H O.
It can be seen that the amount of NO 2 removed primarily
depends on the amount of NH 3 used. Although SCR technol-
ogy has proved to be an effective means to reduce NO x with
removal results as high as 90% in some European facilities,
the U.S. does not consider the technology economically fea-
sible. In addition to the high cost there are the undesirable
effects of unreacted NH 3 , by-product SO 3 and increased CO
production to consider. There are also catalyst deactivation
problems caused by contamination by trace metals in the fl y
ash and by sulfur poisoning. The Japanese have improved
on the design of catalysts and their arrangement within the
reactor. However, these modifi cations are still too new to
evaluate their merit.^22 U.S. industry also feels that more data
has to be generated for the medium to high sulfur coals most
commonly used in this country. Since characteristics such as
high sulfur, low fl yash alkalinity and high iron content are
common in U.S. coal, and these qualities do infl uence SO 3
production, SCR would not appear to be one of the likely
options for U.S. industry at least in the near future.
Exxon has developed a process called “Thermal deNO x ”
which makes use of ammonia injection into the fl ue gas at
temperatures of between 1600 and 2200F. This process is
claimed to remove NO x by up to 90%. years, CFB’s have
become the dominant FBC choice in industry. The most
common problems that have been associated with bubbling
beds include erosion of the inbed tubes. This can be reduced
through the use of studding, fi ns, etc. as previously men-
tioned in this report. However, CFB’s are also prone to ero-
sion, i.e., the waterwalls, as well as the refractory lining.
Agglomeration is another common problem associated with
bubbling beds. Sand can fuse in localized hot spots to form
clinkers or “sand babies” especially when the fuel has a
high concentration of alkali compounds.^9 In severe cases,
agglomerations can cause the bed to defl uidize, block air
ports, and make bed material removal more diffi cult. Sulfur
removal is more diffi cult with bubbling beds. In general,
large quantities of double-screen stoker coal must be used
to attain the high sulfur removal rate displayed by CFB’s.
Most overbed feed bubbling beds in existence must use
coals which contain less than 10% fi nes. This can often be
quite costly. As previously mentioned, underbed feed also
has problems associated with it. Since low fl uidizing veloci-
ties are required with underbed feed, the bed plan area must
be larger and, subsequently, contain a higher density of feed
ports. This serves to complicate the already unreliable feed
system. In order to utilize the sorbent better, the recycle
ratio has to be increased. However, above a certain recycle
ratio, and in-bed tubes might have to be removed in order
to maintain combustor temperature, compromising the CFB
design.
NO x control is better with CFB’s than with bubbling
beds. This is because of the aforementioned stage combus-
tion which is physically unachievable in bubbling beds due
to the large bed plan area and low fl uidizing velocity. On
average, 0.1 lb/million Btu less NO x is produced by CFB’s
than by bubbling beds.
As of the present, there are no federal regulations gov-
erning CO emissions. However, some states have promul-
gated regulations. As would be expected with overbed feed
bubbling bed combustors, the CO emissions are high. While
emissions of over 40 ppmv are common with bubbling beds,
CFB’s are usually under 100 ppmv.^3 This is due to better
circulation and recycle. There is not much data on CO emis-
sions for underbed feed bubbling beds. However, it evidently
reduces CO more than does overbed. Unfortunately, with
CFB’s there is a trade-off between SO 2 /NO x and CO. Staged
combustion will increase CO emissions as the primary to
secondary air ratio becomes smaller. SCR/SNR specifi c to
CO also may cause an increase in NO x.
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