Waste incinerators are used to destroy solids, sludges, liquids, and tars. Depending upon the physical, chemical characteristics of the waste and the refractory resistant (to heat: difficult to melt; also to authority)
volatilize vaporize; become gaseous oxidize react with oxygen handling they require, different incinerator designs will be applied. Solids, sludges, and tars are incinerated in fixed-hearth and rotary kiln incinerators. Liquids may also be burned in these systems and used as support fuel. In many plants where liquids are the primary wastes, liquid injection incinerators are used. Boilers, process furnaces, cement kilns, and lightweight aggregate kilns also utilize the energy available from liquid wastes and burn liquid wastes as well as the fossil fuels (natural gas and oil).
Fixed-Hearth Incinerators. Fixed-hearth incinerators are used extensively for medical and municipal waste incineration. Fixed hearths can handle bulk solids and liquids. A controlled flow of "underfire" combustion air (70 to 80 percent of the theoretical air required) is introduced up through the hearth on which the waste sits. Bottom ash is removed by dumping into a water bath.
Unburned combustibles and high levels of carbon monoxide and hydrogen exit above the hearth. These volatiles are oxidized in the combustion zone where overfire air provides sufficient excess air and residence time at temperature to ensure complete burnout. The three Ts of combustion and oxygen provide high combustion efficiency. Natural gas or oil is supplied to maintain temperatures as high as 2,000°F. In some large municipal waste combustors, called waste-to-energy plants, heat recovery boilers are used to generate steam for electric generation. These plants are also referred to as trash-to-steam plants. All incinerator systems are now regulated by exhaust emissions. Air pollution control systems are installed to control emissions of particulate matter including metals and ash, hydrocarbons including dioxins and furans, and acid gases created from the combustion of wastes containing chlorine, sulfur, phosphorous, and nitrogen compounds.
Rotary Kiln Incineration. Solid wastes as well as liquid wastes generated by industry are destroyed by on-site and commercial-site rotary kiln incinerator systems. The rotary kiln is a cylindrical refractory-lined shell that is rotated to provide a tumbling and lifting action to the solid waste materials. This exposes the waste surface to the flames from fuel burning as well as liquid waste burning in the rotating kiln. Flames will also be generated over the surface of waste solids exposed to the heat and incoming air. Pumpable sludges and slurries are injected into the kiln through nozzles. Temperatures for burning vary from 1,300 to 2,400°F. Lower temperatures are often necessary to prevent slagging of certain waste materials.
The rotary kiln provides excellent mixing through a rotating-tumbling action that distributes heat evenly to all the waste materials contained within it. The kiln is the primary combustion chamber (PCC) where organic compounds in the wastes are volatilized and oxidized as air is introduced into the kiln. The unburned volatiles enter the secondary combustion chamber (SCC) along with the hot products of combustion from the PCC where additional oxygen is introduced and ignitable liquid wastes or fuel can be burned. Complete combustion of the volatized waste from the PCC, liquid wastes and fuel occurs in the SCC.
Liquid Injection. The chemical industries generate liquid wastes that contain toxic organics. Typical wastes from the agricultural and pharmaceutical plants may contain compounds such as chlorinated benzenes, vinyl chloride, toluene, phosphorous, and naphthalene. On-site liquid injection incinerators are used to destroy these wastes. Liquid injection incinerators are refractory-
lined chambers, generally cylindrical in shape and equipped with a primary combustor and often secondary injection nozzles for high-water-content waste materials. The liquids are atomized through nozzles, exposed to high temperature fuel burner flames, vaporized, superheated, and when combined with air in a turbulent zone attain temperature levels from 1,800 to 3,000°F. Residence time in the chamber is based on the flow volume of these combined products of combustion (fuel, air, and liquid wastes) in actual cubic feet per second. The physical volume of the chamber in cubic feet determines the total time of these gases in the chamber. This time may vary from 0.5 seconds up to 2.5 seconds. The toxic organic components of the liquid waste are oxidized to carbon dioxide, water vapor, oxygen, nitrogen, and acid gases. Acid gases formed are cleaned from the exhaust stream by wet scrubbers, thus allowing clean products to leave the exhaust stack. Incineration has resulted in the ultimate answer to the disposal of these waste materials.
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