Encyclopedia of Environmental Science and Engineering, Volume I and II

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V

VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

The toxic gases produced during combustion and other

chemical processes may be removed by destructive dis-

posal, dispersive dilution or as recoverable side products.

The removal path chosen is at the present time motivated

primarily by economics, but public pressure and aware-

ness of environmental problems also influence the choice.

This section will concern itself with destructive dis-

posal and/or various recovery processes, the subject of dis-

persion being ably handled in the sections on Air Pollution

Meteorology and Urban Air Pollution Modeling. The main

emphasis will be on principles of gaseous reaction and

removal with the description of equipment for air pollu-

tion abatement covered by pollutant. Problems specifi-

cally concerned with the automobile can be found under

Mobile Source Pollution. Although the control principles

to be described below are general, it is usually necessary

to design equipment for each installation because of varia-

tions in physical and chemical properties of effluents;

also, in general, the cost of adding pollution devices to an

existing unit (retrofit) will be higher than if they were placed

in the original design, because of construction difficulty and

downtime.

Although the majority of effluent material from com-

bustion occurs in the gaseous state, it is important to char-

acterize the total effluent stream for control purposes. For

example, the effluent may be condensible at operating

temperature (a vapour) or noncondensible (a gas), but it

usually is a mixture of the two. Particulate matter (solids)

and mists (liquids) are often suspended in the gaseous

stream; if the particles do not separate upon settling they

are called aerosols. The considerations in this section deal

with gas or vapor removal only and not with liquid or solid

particle removal.

SULFUR DIOXIDE, SO 2 , AND TRIOXIDE, SO 3

Sulfur dioxide is generated during combustion of any sulfur-

containing fuel and is emitted by industrial processes that use

sulfuric acid or consume sulfur-containing raw material. The

major industrial sources of SO 2 are sulfuric acid plants, smelt-

ing of metallic ores, paper mills, and refining of oil. Fuel com-

bustion accounts for roughly 75% of the total SO 2 emitted.

Associated with utility growth is the continued long term

increase in utility coal consumption from some 650 million

tons/year in 1975 to between 1400 and 1800 million tons/year

in 1990. Also the utility industry is increasingly converting

to coal. Under the current performance standards for power

plants, national SO 2 emissions are projected to increase approx-

imately 15 to 16% between 1975 and 1990 (Anon. 1978). The

SO 2 emitted from power plants is usually at low concentration

(0.5% by volume). However, a 900 MW unit will emit over

13,000 pounds of SO 2 per hour for a 1% sulfur coal. The SO 2

emitted from industrial processes is at higher concentrations

and lower flow rates. The emitted SO 2 combines readily with

mists and aerosols, thus compounding the removal problem.

Information concerning emissions standards is essential

to pollution control engineering design. The current US fed-

eral SO 2 emissions limits for a stack are 1.2 lb/10^6 BTU for

new oil and gas fired plants. Also, uncontrolled SO 2 emis-

sions from new plants firing solid, liquid, and gaseous fuels

are required to be reduced by 85%. The percent reduction

requirement does not apply if SO 2 emissions into the atmo-

sphere are less than 0.2 lb/10^6 BTU.

Flue gas desulfurization (FGD) methods are catego-

rized as nonregenerable and regenerable. Nonregenerable

processes produce a sludge that consists of fly ash, water,

calcium sulfate and calcium sulfite. In regenerable pro-

cesses, SO 2 is recovered and converted into marketable

by-products such as elemental sulfur, sulfuric acid or con-

centrated SO 2. The sorbent is regenerated and recycled.

The US Environmental Protection Agency believes the

following types of FGD systems are capable of achieving

the emissions limit standards: lime, limestone, Wellman-

Lord, magnesium oxide and double alkali. Due to the pro-

cess economics, utility industry prefer the lime/limestone

systems. Limestone processes constitute about 58% of the

current calcium-based capacity in service and under con-

struction, and 69% of that planned, which amounts to 63% of

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