Environmental Engineering FOURTH EDITION

Air Pollution Control 40 1

L

Fuel & air - 1500°F Temp. r - Clean gas

Dirty gas -

Incineration

Catalyst

Fuel & air Clean gas

Dirty gas

Catalytic combustion

Figure 20-15. Comparison between incineration and catalytic combustion.

0 Desulfurization. Sulfur may be removed from heavy industrial oil by a number

of chemical methods similar to those used to lower the H2S content of crude oil.

In coal, sulfur may be either inorganically bound, as pyrite @e$), or organically

bound. Pyrite can be removed by pulverizing the coal and washing with a detergent

solution. Organically bound sulfur can be removed by washing with concentrated

acid. Preferred methods are coal gasification, which produces pipeline-quality

gas, or solvent extraction, which produces low-sulfur liquid fuel.

0 Tall stacks. Although the use of a tall stack does result in reduced ground-level

concentration of gaseous pollutants (see Eqs. 18.5 and 18.6), it is not considered to

be a pollution control measure. U.S. federal regulations do not sanction pollution

control by use of tall stacks alone.

0 Flue-gas desulfurization. The off-gases from combustion or other SO2-producing

processes are called flue gases. SO2 may be cleaned from flue gas by chemical

processes, some of which are discussed later in this chapter. The reaction of

SO2 to form sulfuric acid or other sulfates is the most frequently used flue-gas

desulfurization method. The method has two limitations: (1) the flue gas must be

cleaned of particulate matter before entering an acid-producing plant and (2) acid

formation is energetically favorable only for a fairly concentrated gas stream

(about 3% - 30,000 ppm- S02). The reactions for acid formation are

SO2 -I- io2 + so3

so3 + H20 +

A double-contact acid plant can produce industrial grade 98% sulfuric acid. The

nonferrous smelting and refining industry has made the most use of this con-

trol method. Analogous reactions can be carried out to produce (NH4)2S04, a

fertilizer, and gypsum, CaS04.

The SO2 in flue gas from fossil-fuel combustion is too dilute to permit trapping as
acid or commercial fertilizer or gypsum. Coal combustion off-gases are also too dirty.