Stoichiometry

The starting basis of thermal incineration is the complete combustion of a hydrocarbon to carbon dioxide and water in air as follows:

This equation accounts for all major atoms. If the VOC contains Cl, S, or N in appreciable amounts, these components must be accounted for in the stoichiometry; usually HCl, SO2, and NO are combustion products. If the components of a mixture are known, equations can be written for each species; if the components are not known, an apparent chemical formula can be based on the weight percents of each combustible element (e.g., C, H, N, and S).

Any oxygen in VOCs (alcohols and ketones) is subtracted from the stoichiometric oxygen requirements. The formation of thermal NO (from nitrogen in the air) is not accounted for in stoichiometric combustion equations; only when nitrogen is present in the fuel-VOC mixture is NO formation (from fuel or VOC N) accounted for stoi-chiometrically.

For dilute gas streams, achieving high-DRE and low-PIC emissions (not accounted for in the combustion stoi-chiometry) requires auxiliary fuel to maintain a minimum temperature and residence time.

Using excess air is common practice. This use is expressed in terms of the air/fuel ratio (mass based) or the equivalence ratio, defined as ^ as follows:

(VOC/oxygen/ctual (VOC/oxygen)stoichio.

SAMPLE CALCULATION #1

A hazardous chlorinated hydrocarbon is combusted in air according to the following stoichiometry:

Calculate the stoichiometric HCl mole fraction for x = 6, y = 3, and z = 2. If the combustion is conducted in a way that leads to 4 oxygen mole % in the stack gas, what is the equivalence ratio; if, under these conditions, the DRE is 99.99%, what is the emitted C6H3Cl2 concentration?

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