In thermal oxidizers and incinerators, one big issue is where to introduce the waste in the combustion chamber. Let’s take a look at the concerns on this:
Waste gases and waste liquids that are not capable of burning as a stable fuel are usually introduced downstream of the burner. However, even though a waste does not have sufficient heating value to sustain stable combustion, it can still be classified as either endothermic or exothermic, for a specific operating temperature.
Endothermic & Exothermic Wastes
Waste is considered to be endothermic if the hydrocarbon content (heating value) is small and much more than a minimum amount of auxiliary fuel must be burned to maintain the required operating temperature in the combustion chamber.
If the heating value is high enough that a cooling media must be added to control the maximum operating temperature, the waste is considered exothermic.
In other words, if the waste contains more heat content than the energy required to raise its mass, as well as the mass of air required for oxidation of the waste hydrocarbons, to the operating temperature of the oxidizer, then it is considered exothermic.
A waste can be exothermic at a lower operating temperature, but endothermic for a higher operating temperature.
Pressure Drop Considerations
Waste liquids are normally available at higher pressure and can be sprayed into the system using hardware (atomizers, etc.) similar to those used for liquid fuels.
Waste gases, on the other hand, are normally available only at lower pressures, so injection hardware must be designed for the lower pressure drop.
Why Injecting Downstream of Flame?
The reason for introducing wastes containing low organic content, whether liquid or gas, downstream of the burner flame zone, is to avoid impairing oxidation of the burner fuel. Impaired fuel oxidation can result in the formation of carbon monoxide, unburned hydrocarbons, or worse, soot.
Use of Waste Stream as Combustion Air
If the waste is air that is only slightly contaminated with organics, it may be used as the combustion air source and introduced directly through the burner’s air passages.
The direct injection of the waste through the burner’s air passage is typically used in catalytic systems because the waste is generally slightly contaminated air.
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