Ash made from combustion takes the form of fly ash or bottom ash. Fly ash is your fine particles taken away in the form of smoke whereas bottom ash is your course non-combustible and unburned substance that remains after the burn is complete. The type and amount of pollutants at the fly and bottom ashes depend upon what waste is burnt and completeness of the combustion procedure.
The completeness of combustion is determined by all of the following factors:
Temperature
The temperature generated is a function of the heating value of the waste and auxiliary fuel, incinerator or burn unit design, air distribution and combustion management. Total combustion requires high temperatures. Usually, temperatures that exceed 650oC with a holding time of 1-2 seconds can cause complete combustion of most food and other common household waste. Segregation of waste is necessary when using methods that don’t routinely achieve those temperatures. Double chamber incinerators, which can be made to burn complex mixtures of waste, hazardous waste and biomedical waste, has to offer a temperature greater than 1000oC and a holding time of one moment to ensure complete combustion and decrease dioxin and furan emissions. When these high temperatures and holding times are achieved, waste will be wholly burnt and ashes, smoke and pollutant concentrations will likely be minimized.
Because exhaust gas temperatures vary from nearby to greater than 1000°C whenever a batch waste incinerator is used, optional air pollution control systems with evaporative cooling towers and scrubbers are rarely suggested. But it could be required to use these systems with big constant feed incinerators if further cleaning of exhaust gas is required by regulatory authorities. Holding time, otherwise called retention or residence time, is the length of time available to ensure the total mixing of air and fuel, and thus the total burning of waste. Low temperatures, very low heating values of this waste and reduced turbulence require that the holding time be raised to complete the combustion procedure. This will help in attaining the high temperatures at which waste can be completely burned. The total amount of mixing is affected by the form and size of the burn chamber and how the air is injected. Passive under-fire ventilation achieved during open burning does not lead to adequate turbulence for the burning of a huge variety of waste. Also, it’s important not to overfill the burn chamber as airflow might 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. The greater the burn temperature, holding time and turbulence that are achieved, the less effect the composition of this waste gets on completeness of the burn.
Because exhaust gas temperatures vary from ambient to greater than 1000°C each time a batch waste incinerator is used, optional air pollution control systems with evaporative cooling towers and scrubbers are seldom recommended. However, it may be necessary to employ these systems with large continuous feed incinerators if additional cleaning of exhaust gas is required by regulatory authorities.
Holding Time
Complete combustion takes time. Holding time, otherwise known as retention or residence time, is the length of time available to ensure the complete mixing of air and fuel, and thus the complete 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.
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.