Page 302 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 285

7.3.4 Auxiliary Air
If the waste air stream has a low oxygen content (below 13% to 16%), then
auxiliary air would also be used to raise the oxygen level to ensure flame
stability of the burner. If the exact composition of the waste air stream is
known, one can determine the stoichiometric amount of air (oxygen) for
complete combustion. In general practices, excess air is added to ensure
complete combustion. The following example illustrates how to determine
the stoichiometric amount of air and excess air for combusting a landfill gas.

Example 7.10: Determine the Stoichiometric Air and Excess
Air for Combusting Landfill Gas

A landfill gas stream (Q = 200 scfm) is to be treated by an incinerator. The
landfill gas is composed of 60% by volume CH and 40% CO . The gas is
4
2
to be burned with 20% excess air at 1,800°F. Determine (1) the stoichiomet-
ric amount of air required, (2) the auxiliary air required, (3) the total influ-
ent flow rate to the incinerator, and (4) the total effluent flow rate from the
incinerator.

Solution:
(a) The influent flow rate of methane = (60%)(200 scfm) = 120 cfm
The influent flow rate of carbon dioxide = (40%)(200 scfm) = 80 cfm
The reaction for complete combustion of methane is:

CH 4 + 2O 2 → CO 2 + 2H O
2
The stoichiometric requirement of oxygen
= (120 scfm)(2 moles of O per mole of CH )
2
4
= 240 scfm
The stoichiometric requirement of air
= (oxygen flow rate) ÷ (oxygen content in air)
= (240 scfm) ÷ (21%) = 1,140 scfm

(b) The total auxiliary air = (1 + 20%)(1,140 scfm) = 1,368 cfm
The flow rate of nitrogen in the auxiliary air
= (79%)(1,368) = 1,080 scfm

(c) The total influent flow rate
= 120 (methane) + 80 (carbon dioxide) + 1,368 (air)
= 1,568 scfm