Gas Emission Control - Mathematics and Statistics for the Environment - page 478

16.5.1.5 Combustion Limits
Not all mixtures of fuel and air are able to support combustion. Incinerators usually operate at
organic vapor concentrations below 25% of the lower explosive limit (LEL) or lower flammability
limit (LFL). Combustion does not occur readily in this range. The explosive or flammable limits
for a mixture are the maximum and minimum concentrations of fuel in air that will support com-
bustion. The upper explosive limit (UEL) is defined as the concentration of fuel that produces a
non-burning mixture due to a lack of oxygen. The lower explosive limit (LEL) is defined as the
concentration of fuel below which combustion will not be self-sustaining.

16.5.1.6 Flame Combustion
When mixing fuel and air, two different mechanisms of combustion can occur (USEPA, 1981, p.
3-7). When air and fuel flowing through separate ports are ignited at the burner nozzle, a luminous
yellow flame results. The yellow flame results from thermal cracking of the fuel. Cracking occurs
when hydrocarbons are intensely heated before they have a chance to combine with oxygen. The
cracking releases both hydrogen and carbon, which diffuse into the flame to form carbon dioxide
and water. The carbon particles give the flame the yellow appearance. If incomplete combustion
occurs from flame temperature cooling or if there is insufficient oxygen, soot and black smoke
will form. When the fuel and air are premixed in front of the burner nozzle, blue flame combus-
tion occurs. The reason for the different flame is that the fuel-air mixture is gradually heated. The
hydrocarbon molecules are slowly oxidized, going from aldehydes and ketones to carbon dioxide
and water. No cracking occurs and no carbon particles are formed. Incomplete combustion results
in the release of the intermediate partially oxidized compounds. Blue haze and odors are emitted
from the stack.

16.5.1.7 Heat
The fuel requirement (for burners) is one of the main parameters of concern in incineration systems.
Moreover, the amount of fuel required to raise the temperature of the waste stream to the tempera-
ture required for complete oxidation is another area of concern. The burner fuel requirement can
be estimated based on a simple heat balance of the unit and information concerning the waste gas
stream. The first step in computing the heat required is to perform a heat balance around the oxida-
tion system. From the first law of conservation of energy:

Heat in = Heat out + Heat loss (16.18)

Heat is a relative term that is compared at a reference temperature. In order to calculate the heat
that exits the incinerator with the waste gas stream, the enthalpies of the inlet and outlet waste gas
streams must be determined. Enthalpy is a thermodynamic term that includes the sensible heat and
latent heat of a material. The heat content of a substance is arbitrarily taken as zero at a specified ref-
erence temperature. In the natural gas industry, the reference temperature is normally 16°C (60°F).
The enthalpy of the waste gas stream can be computed from Equation 16.19:

H = C p ( T - T 0 ) (16.19)

where
H = Enthalpy (J/kg, Btu/lb).
C p = Specific heat at temperature T (J/kg, °C; Btu/lb, °F).
T = Temperature of the substance (°C or °F).
T 0 = Reference temperature (°C or °F).