Page 285 - Materials Chemistry, Second Edition

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256 Waste Management Practices: Municipal, Hazardous, and Industrial
9.4.5 ACID GASES
As shown in Equation 9.1, the combustion of an organic material results in the generation of car-
bon dioxide and water and other components. Gases such as SO ,NO , and HCl may be produced
x x
by incinerators at rates of several pounds per ton of waste charged. These gases are collectively con-
sidered “acid gases” because they dissolve readily in water to form the corresponding strong acids.
Sulfur occurs in tires, wallboard, and plant tissue (yard waste). During combustion, sulfur is
converted into the corresponding oxides. Sulfur dioxide is a primary pollutant as it is emitted
directly from MSW burning and concurrent sulfur oxidation,

S O → SO 2 (9.3)
2
SO can cause direct respiratory irritation and damage materials such as stone and metal. Sulfur
2
emissions are converted into a secondary pollutant when sulfur dioxide reacts with water vapor and
oxygen in the atmosphere, producing sulfur trioxide:

2SO O → 2SO 3 (9.4)
2
2
The SO combines with water to form sulfuric acid,
3
SO H O → H SO 4 (9.5)
2
2
3
This product is corrosive to skin and mucosa and is linked with several respiratory ailments.
Sulfuric acid is the primary component of acid rain, and will damage materials and is hazardous
to biota. A wide range of values of SO has been measured in stack emissions. As much as 0.68
2
to 1.4 kg (1.5 to 3 lb) SO has been measured per ton of MSW charged, which can substantially
2
alter the pH of local precipitation. Natural, uncontaminated rain has a pH of about 5.6 to 5.7,
but the pH of acid rain can be as low as 2. The deposition of atmospheric acid on freshwater
aquatic systems prompted the U.S. EPA to recommend a limit of 10 to 20 kg SO 2 / ha/year.
4
Nitrogen (N) also occurs in food and yard waste. The product of N combustion is nitrogen
oxides, NO :
x
N O → 2NO (9.6)
2 2
2NO O → 2NO (9.7)
2 2
Nitrogen dioxide is an important component of photochemical smog. The formation of photo-
chemical smog begins with the production of nitrogen oxides whether from automobiles, industrial
facilities, or MSW combustion. Hydrocarbons are also emitted into the atmosphere from various
sources such as automotive and industrial sources (including incineration). The various constituents
react with sunlight to yield ozone (O ), a secondary pollutant, which in turn reacts with hydrocar-
3
bons to form a wide range of compounds including aldehydes and organic acids. Table 9.2 lists
some of the major reactions involved in the formation of photochemical smog.
The concentrations of NO generated from incineration, however, are often low due to the rel-
2
atively low temperatures at which incineration occurs. Nitrogen can also be converted into HNO ,
3
another component of acid rain, by a series of reactions abbreviated below:
1
2NO H O → → 2HNO (9.8)
2 2 2 3
Chlorine occurs in MSW in paints, dyes, polyvinylchloride (PVC)-based products, and bleached
paper. During combustion gaseous hydrogen chloride, HCl, is produced which condenses with