Gaseous Pollutants - Incineration

Control of gaseous pollutants from stationary sources

Incineration, also known as combustion, is most used to control the emissions of organic compounds from process industries. This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor. Incomplete combustion will result in some pollutants being released into the atmosphere. Smoke is one indication of incomplete combustion. Equipment used to control waste gases by combustion can be divided in three categories: direct combustion or flaring, thermal incineration and catalytic incineration. Choosing the proper device depends on many factors, including type of hazardous contaminants in the waste stream, concentration of combustibles in the stream, process flow rate, control requirements, and an economic evaluation.

A direct combustor or flare is a device in which air and all the combustible waste gases react at the burner. Complete combustion must occur instantaneously since there is no residence chamber. Flares are commonly used for disposal of waste gases during process upsets, such as those that take place when a process is started or shut down. A flare can be used to control almost any emission stream containing volatile organic compounds. Studies conducted by EPA have shown that the destruction efficiency of a flare is about 98 percent.

Figure: Thermal incinerator general case

In thermal incinerators the combustible waste gases pass over or around a burner flame into a residence chamber where oxidation of the waste gases is completed. For thermal incineration, it is important that the vapor stream directed to the thermal incinerator have a constant combustible gas concentration and flow rate. These devices are not well-suited to vapor streams that fluctuate, because the efficiency of the combustion process depends on the proper mixing of vapors and a specific residence time in the combustion chamber. Residence time is the amount of time the fuel mixture remains in the combustion chamber. Often, supplementary fuel is added to a thermal incinerator to supplement the quantity of pollutant gases being burned by the incinerator. Energy and heat produced by the incineration process can be recovered and put to beneficial uses at a facility. Thermal incinerators can destroy gaseous pollutants at efficiencies of greater than 99 percent when operated correctly.

Figure: Catalytic incinerator

Catalytic incinerators are very similar to thermal incinerators. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst is a substance that enhances a chemical reaction without being changed or consumed by the reaction. A catalyst promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95 percent are possible using a catalytic incinerator. Higher efficiencies are possible if larger catalyst volumes or higher temperatures are used. Catalytic incinerators are best suited for emission streams with low VOC content.