Page 326 - Wastewater Solids Incineration Systems

Appendix A Combustion Fundamentals 287

fuel or oxygen (or air). In practice, to ensure completeness of combustion, more
oxygen is provided than the theoretical stoichiometric amount. This extra oxygen is
referred to as “excess oxygen” or “excess air” (XSA or XS air) when air is used as the
source of oxygen for combustion.
Excess air is the fractional amount supplied in addition to the air required for sto-
ichiometric combustion. Excess air cannot be determined without a measurement of
the unreacted oxygen in the flue gas unless flow rates and heating values are known
for all fuels entering a furnace, with the total airflow rate. Excess air is calculated by
the formula:
XS air %O /(20.9% %O )
2, dry 2, dry
where %O the flue gas oxygen concentration (volume, dry basis).
2,dty
This simplified formula is accurate for high carbon fuels but introduces a slight
error for high hydrogen fuels (e.g., natural gas). However, the overall accuracy is suf-
ficient for it to be accepted by the U.S. Environmental Protection Agency for emission
calculations.

3.3 Higher and Lower Heating Values

In U.S. customary units, the heating value of a fuel—sometimes referred to as the
heat of combustion—is measured in British thermal units (Btu 1.055 kJ). A Btu is
defined as the amount of heat needed to heat 1 lb of water 1°F. The terms higher
heating value (HHV) and lower heating value (LHV) represent different accounting
means by which the heat of vaporization of the water vapor formed by the combus-
tion of hydrogen in the fuel is accounted. Some texts refer to HHV as gross heat and
LHV as net heat. The HHV, which includes the heat of vaporization, is the most
common unit used in the United States. For a 60°F base temperature, the difference
between HHV and LHV for a bone-dry fuel is 1060 Btu/lb of water vapor formed
by the combustion of hydrogen in the fuel (Btu/lb 2.326 kJ/kg).

3.4 Common Auxiliary Fuels
There are many auxiliary fuels that can assist in the combustion of sludge. How-
ever, natural gas and no. 2 fuel oil are the most common. Natural gas is just what
its name implies, a natural gas that varies slightly in composition throughout the
United States. The main constituent of natural gas is methane (CH ); therefore, cal-
4
culations will assume that natural gas is pure methane with an HHV of 23 875
Btu/lb. This measure is equivalent to an HHV of 1009 Btu/cu ft (60°F and 14.696
3
psia) (Btu/cu ft 37.26 kJ/m ). For computations, natural gas is typically