The combustion, or burning, of solid waste proceeds via a series of stages. Water is first driven from the unburned waste from heat produced from material burning or from an additional burner. Since the waste pops up, carbon and other materials are released and converted to burnable gases. This is referred to as gasification. These gases are then able to combine with oxygen. If the temperature within the burn space is high enough and maintained for a long period of time, the hot gases are completely converted into water vapour and carbon dioxide, which is then released into the atmosphere. If the temperature within the burn space isn’t high enough and the burn is too brief, full conversion of the burnable gases will not occur and visible smoke is released into the atmosphere. Another consequence of burning at low temperatures would be the creation of pollutants which weren’t originally present in the waste. This process is called de novo synthesis. Dioxins, furans and other complicated chemical pollutants could be formed through this process.
Ash produced from combustion takes the form of either fly ash or bottom ash. Fly ash is your fine particles carried away in the form of smoke whereas bottom ash is the course non-combustible and unburned material that remains after the burn is complete. The kind and amount of pollutants in the fly and bottom ashes depend upon that which waste is burned and completeness of the combustion process. Total combustion demands high temperatures. Generally, temperatures which exceed 650oC using a holding time of 1-2 seconds can cause complete combustion of most food and other ordinary household waste. Segregation of waste is necessary when utilizing methods which don’t routinely achieve those temperatures. Dual chamber incinerations, that can be made to burn complicated combinations of waste, hazardous waste and biomedical waste, has to provide a temperature greater than 1000oC and a holding time of one moment to ensure complete combustion and minimize dioxin and furan emissions. Whenever these elevated temperatures and holding times are achieved, waste will be wholly burned and ashes, smoke and pollutant concentrations will be minimized.
Because exhaust gas temperatures vary from nearby to greater than 1000°C whenever a batch waste incineration is used, optional air pollution control systems using evaporative cooling towers and scrubbers are rarely recommended. But it could be necessary to use these systems with large continuous feed incinerations if further cleaning of exhaust gas is required by regulatory authorities.
Holding Time
Total combustion takes some time. Holding time, otherwise called retention or residence time, is the duration of time available to ensure the total mixing of air and fuel, and so the total burning of waste. Low temperatures, very low heating values of the waste and reduced turbulence demand that the holding time be increased to complete the combustion process. This will aid in achieving the high temperatures where waste could be totally burned. The total amount of mixing is influenced by the form and size of the burn chamber and how the atmosphere is injected. Passive under-fire ventilation achieved during open burning does not lead to sufficient turbulence for the burning of a wide variety of waste. Additionally, it is important not to overfill the burn chamber as airflow might be blocked along with the amount of turbulence further reduced. The more innovative incineration designs provide powerful turbulence throughout the forced introduction of air directly into hot zones.
Composition of the Waste
The heating value, wetness and chemical properties of the waste has an effect on the combustion process and the pollutants that are contained in the resulting smoke and ashes. The greater the burn temperature, holding period and turbulence which are achieved, the less impact the composition of this waste gets on completeness of the burn.
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.