Encyclopedia of Environmental Science and Engineering, Volume I and II

VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1237

Fluidized Bed Combustion (FBC) According to the

operating pressure of the beds, there are two classifications

of FBC—atmospheric pressure and pressurized. The former

is appropriate for both utility and industrial heating applica-

tions. The latter is for electricity-generating plants. In FBC,

crushed coal is fed to a bed of fine limestone or dolomite

solvent particles which is fluidized by hot air. Water which

is circulated through tubes immersed in the bed is converted

into steam by the heat released during combustion. The bed

temperature is in the range of 1500–1600F. The raw lime-

stone (CaCO 3 ) will calcine to lime (CaO) at the normal bed

operating temperature. The lime will react with SO 2 released

during coal combustion and form gypsum (CaSO 4 ) on the

surface of the particles. The necessary sulfur capture is

achieved by continuously feeding limestone and maintain-

ing proper bed inventory.

A fluidized bed offers very high heat transfer rates with

resulting low combustion temperatures. In addition, the

limestone or dolomite absorbs much of the sulfur in the fuel.

Thus, fluidized bed combustion promises lower SO 2 and

NOx emissions, coupled with high overall efficiency gener-

ating steam or electricity. Also it allows for the use of lower

grade fuels. A comparison of EPA permissible limits with

NOx and SO 2 emissions from the 100,000 lb saturated steam/

hr FBC unit at Georgetown University is shown in Table 16

(Gamble, 1980). FBC produces more particles in the offgas

than conventional systems, since there is no slag to carry

them off and an offgas dust collector of high efficiency is

required.

FLUE GAS DENITRIFICATION

There are two major classifications of flue gas denitrification:

dry and wet. (Dry techniques tests to date include selective

catalytic reduction.)

Dry Processes (SCR) These include nonselective cata-

lytic reduction, selective noncatalytic reduction, adsorption

and electron-beam radiation. The selective catalytic reduc-

tion of NOx to nitrogen and water by injection of ammonia

in the presence of a catalyst has been developed and com-

mercialized by a number of Japanese companies because of

Japan’s special need. In selective catalytic reduction of NOx

with ammonia as the reductant, the main reactions are

4NH 3  4NO  O 2 → 4N 2  6H 2 O

4NH 3  2NO 2  O 2 → 3N 2  6H 2 O.

The first reaction predominates when the NO concentra-

tion is about 90–95% of the NOx in combustion flue gas. The

application of SCR techniques to coal fired sources does not

have long commercial experience, because of catalyst pore

plugging problems in the presence of particulate fines. Two

commercial units with a fixed bed “parallel passage reac-

tor” to remove the NOx with efficiency of 98–99% have been

reported. Japan Gas Co. (JGC) claimed that this configuration

allows NOx reduction while minimizing dust plugging and

pressure drop. Moving bed reactors have also been developed

(Ricci, 1977).

Exxon’s Thermal De NOx system is a typical selective

noncatelytic reduction process. The technique uses ammo-

nia, but no catalyst and operates in a fairly narrow tempera-

ture range 1700–1800F. The reaction temperature can be

reduced to around 1300–1400F by introducing hydrogen.

Due to the narrow reaction temperature range, it is diffi-

cult to maintain optimum reaction temperature. However,

Exxon’s system claims low denitrification costs and no cata-

lyst plugging problem. Mitsui Petrochemical Industries, Ltd.

was the first one to commercially apply the process. The

NOx reduction at full load on a 120 tons/hr heavy fuel oil

fired steam boiler is about 50%.

As previously described, the molecular sieves can be

used to reduce the NOx emissions from nitric acid plants.

NOx reduction using activated carbon adsorption is tested

in pilot plant. The Activated carbon can (1) adsorb NO 2 to

form nitric acid (2) promote the oxidation of NO to NO 2

(3) catalyze reduction NO to N 2. Chemical impregnations,

such as those of copper and vanadium, enhance carbon’s

catalytic effect on NOx/NH 3 reaction. NH 3 is injected into

the column. The carbon in the columns adsorb SO 2 , NOx

reacts with ammonia to form N 2 and water.

Wet Processes Wet processes fall into four catego-

ries, absorption-reduction, oxidation-absorption-reduction,

absorption-oxidation, and oxidation-absorption. The clas-

sification system for wet NOx removal processes is shown

in Figure 20 (Fawcett et al., 1977). Due to the high capital

and operating costs, and the formation of NO 3 containing

wastewater, the wet NOx removal processes haven’t been

able to match the commercial gains posted by selective

catalytic reduction and other dry techniques. The details of

dry and wet methods for NOx removal have been reviewed

by Fawcett et al. (1977). Also, Siddiqi and Tenini (1981)

reviewed the commercial flue gas treating applications. The

flue gas treating applications in the US and Japan shown in

Table 17.

FLUORIDES

Not all fluoride emissions are gases. The system employed

must have the capability of handling particulates, dispersed

in huge quantities of air. Phosphoric acid production is a

typical fluoride producer.

The process begins with the grinding of rock phosphate

(typically Ca 10 F 2 (PO 4 ) 6 ). This is naturally a source of particu-

lates, as is the drying of the pulverized phosphate. Acidulation

TABLE 16

Comparison of FBC stack emission with EPA limit (Gamble, 1980)

EPA limit FBC stack emission

SO 2 lb/10^6 BTU 1.2 0.48

NOxlb/10^6 BTU 0.7 0.3

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