The combustion, or burning, of solid waste proceeds through a succession of stages. Water is first driven in the unburned waste by heat produced from material burning nearby or from an auxiliary burner. Since the waste heats up, carbon and other substances are released and converted into burnable gases. This is known as gasification. These gases are then able to combine with oxygen. If the temperature inside the burn chamber is high enough and maintained for a long enough period of time, the hot gases are fully converted into water vapour and carbon dioxide, which is then discharged into the air. If the temperature inside the burn chamber is not high enough and the burn time is too brief, complete conversion of the burnable gases does not happen and visible smoke is released into the air. Another result of burning at low temperatures is that the creation of pollutants which weren’t initially present in the waste. This practice is called de novo synthesis.
Ash produced from combustion takes the form of fly ash or bottom ash. The type and amount of pollutants at the fly and bottom ash depend upon what waste is burned and completeness of the combustion process.
The completeness of combustion is determined by all the following factors:
Temperature
The temperature generated is a function of the heating value of the waste and auxiliary fuel, incinerator or burn unit layout, air supply and combustion control. Total combustion requires high temperatures. Normally, temperatures which exceed 650oC with a holding time of 1-2 seconds will cause complete combustion of the majority of food and other common household waste. Segregation of waste is required when using procedures that don’t routinely achieve these temperatures. Double chamber incinerators, which are intended to burn complex mixtures of waste, hazardous waste and biomedical waste, must provide a temperature higher than 1000oC and a holding time of one second to ensure complete combustion and minimize dioxin and furan emissions. When these high temperatures and holding times are achieved, waste will be completely burned and ash, smoke and pollutant concentrations will be minimized.
Because exhaust gas temperatures differ from ambient to greater than 1000°C every time a batch waste incinerator is used, optional air pollution control systems with evaporative cooling systems and scrubbers are seldom recommended. However, it might be necessary to employ these systems with large constant feed incinerators if additional cleaning of exhaust gas is required by regulatory authorities.
Holding Time
Total combustion takes time. Holding time, otherwise called retention or residence time, is the duration of time available to ensure the complete mixing of gas and air, and therefore the total burning of waste. Low temperatures, low heating values of the waste and reduced turbulence require that the holding time be increased to complete the combustion process. This will help in attaining the high temperatures where waste could be completely burned. The amount of mixing is affected by the form and size of the burn chamber and the way the air is injected. Passive under-fire ventilation achieved during open burning does not result in sufficient turbulence for the burning of a wide variety of waste. Also, it is important not to overfill the burn chamber as airflow could be blocked and the amount of turbulence further decreased. The more advanced incineration designs give effective turbulence through the forced introduction of air directly into hot zones. The higher the burn temperature, holding time and turbulence which are achieved, the less effect the composition of the waste has on completeness of the burn.
Turbulence
The turbulent mixing of burnable gases with sufficient oxygen is needed to promote good contact between the burning waste and incoming air. This will help in achieving the high temperatures at which waste can be completely burned. The amount of mixing is influenced by the shape and size of the burn chamber and how the air is injected. Passive under-fire ventilation achieved during open burning does not result in sufficient turbulence for the burning of a wide variety of waste. Also, it is important not to overfill the burn chamber as airflow may be blocked and the amount of turbulence further reduced. The more advanced incineration designs provide effective turbulence through the forced introduction of air directly into hot zones.
Composition of the Waste
The heating value, wetness and chemical properties of the waste affect the combustion process and the pollutants that are contained in the resulting smoke and ash. The higher the burn temperature, holding time and turbulence that are achieved, the less effect the composition of the waste has on completeness of the burn.
