Page 277 - Materials Chemistry, Second Edition

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248 Waste Management Practices: Municipal, Hazardous, and Industrial
9.2 COMBUSTION CONCEPTS
Incineration is almost exclusively carried out as an aerobic thermal destruction process. Aerobic
combustion will result in the most complete transformation of solid waste to ash, gases, and heat
energy. In order to achieve efficient combustion with a minimum of air pollutant emissions, how-
ever, several requirements must be addressed during system design. For example, the correct
amount of air must be available to the combustion chamber. This “stoichiometric air” is needed to
bring the combustion reactions to completion and avoid the generation of any products of incom-
plete combustion (PICs; see below).
In the combustion of an organic material in MSW, the presumed reaction is

(HC) O → CO H O heat (9.1)
x 2 2 2
The process is much more complex, however, since not all the hydrocarbons are converted into car-
bon dioxide and water, and other components of the waste such as sulfur and nitrogen are also oxi-
dized (see below).

EXAMPLE 9.1
Determine the stoichiometric air required to combust (a) char, C; and (b) methane, CH .
4
SOLUTION
(a) First, we will calculate the O required for complete combustion:
2
C O → CO 2H O
2 2 2
Grams 12 32
The stoichiometric O required for complete combustion is 32/16 2 g O /g CH .
2 2 4
We will assume that the O concentration in the incinerator is about equivalent to dry air concen-
2
trations (~ 23.5%), although many incinerators will operate with elevated O levels.
2
The stoichiometric air requirement will be:
2 g / 0.235 8.5 g air/g C

(b) For methane, the equation is

CH 2O → CO 2H O
4 2 2 2
Grams 16 64
The stoichiometric O required for complete combustion is
2
64/16 4 g O /g CH
2 4
The stoichiometric air requirement will be
4 / 0.235 17.0 g air/g CH
4

We can represent the aerobic decomposition of carbonaceous materials in MSW by the reaction

C H O Cl F N S [a b/4 – (c d e – f )/2 g] O → aCO [(b – d – e )/2] H O
a b c d e f g 2 2 2
dHCl eHF f NO gSO
2
Fluorine, chlorine, and sulfur are typically present in small amounts in MSW and are, therefore,
omitted from calculations.