Ash produced from combustion takes the form of either fly ash or bottom ash. Fly ash is your fine particles taken away in the form of smoke while bottom ash is your course non-combustible and unburned material that remains after the burn is complete. The type and amount of pollutants at the fly and bottom ash depend upon that which waste is burnt and completeness of the combustion procedure. Complete combustion demands high temperatures. Usually, temperatures that exceed 650oC with a holding time of 1-2 seconds will cause complete combustion of most food and other ordinary household waste. Segregation of waste is required when using methods that don’t routinely achieve those temperatures. Double chamber incinerators, which are made to burn complicated mixtures of waste, toxic waste and biomedical waste, has to offer a temperature higher than 1000oC and a holding time of one moment to ensure complete combustion and minimize dioxin and furan emissions. When these elevated temperatures and holding times are attained, waste will be completely burnt and ash, smoke and pollutant concentrations will be reduced.
Since exhaust gas temperatures differ from ambient to larger than 1000°C every time a batch waste incinerator is used, optional air pollution control systems with evaporative cooling towers and scrubbers are seldom recommended. However, it could be necessary to employ these systems with big continuous feed incinerators if additional cleaning of exhaust gas is needed by regulatory authorities.
Holding Time
Complete combustion takes time. Taking time, otherwise known as retention or home time, is the length of time available to ensure the total mixing of gas and air, and thus the total burning of waste. Low temperatures, low heating values of the waste and reduced turbulence demand that the holding time be raised to complete the combustion procedure. This will aid in attaining 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 the way the air is injected. Passive under-fire ventilation attained during open burning doesn’t result in adequate turbulence for the burning of a wide variety of waste. Additionally, it’s important not to overfill the burn chamber as airflow may be blocked and the quantity of turbulence further reduced. The more innovative incineration designs give powerful turbulence through the forced introduction of air directly into hot zones. The higher the burn temperature, holding time and turbulence that are attained, the less impact the composition of the waste has to completeness of their burn.
The temperature generated is a function of the heating value of the waste and auxiliary fuel, incinerator or burn unit design, air supply and combustion control. Complete combustion requires high temperatures. Generally, temperatures that exceed 650oC with a holding time of 1-2 seconds will cause complete combustion of most food and other common household waste. Segregation of waste is required when using methods that don’t routinely achieve these temperatures. Dual chamber incinerators, which are designed to burn complex mixtures of waste, hazardous waste and biomedical waste, must provide a temperature higher than 1000oC and a holding time of at least 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 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.
