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Cement kiln waste disposal operation

  In addition to these primary fuels, various combustible waste materials have been fed to kilns. These alternative fuels (AF) include:

  Used motor-vehicle tiresSewage sludgeAgricultural wasteLandfill gasRefuse-derived fuel (RDF)Chemical and other hazardous waste

  Cement kilns are an attractive way of disposing of hazardous materials, because of:

  the temperatures in the kiln, which are much higher than in other combustion systems (e.g. incinerators),

  the alkaline conditions in the kiln, afforded by the high-calcium rawmix, which can absorb acidic combustion products,

  the ability of the clinker to absorb heavy metals into its structure.

  A notable example is the use of scrapped motor-vehicle tires, which are very difficult to dispose of by other means. Whole tires are commonly introduced in the kiln by rolling them into the upper end of a preheater kiln, or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures (1000–1200 °C) cause almost instantaneous, complete and smokeless combustion of the tire. Alternatively, tires are chopped into 5–10 mm chips, in which form they can be injected into a precalciner combustion chamber. The steel and zinc in the tires become chemically incorporated into the clinker, partially replacing iron that must otherwise be fed as raw material.

  A high level of monitoring of both the fuel and its combustion products is necessary to maintain safe operation.

  For maximum kiln efficiency, high quality conventional fuels are the best choice. However, burning any fuels, especially hazardous waste materials, can result in toxic emissions.[15] Thus, it is necessary for operators of cement kilns to closely monitor many process variables to ensure emissions are continuously minimized. In the U.S., cement kilns are regulated as a major source of air pollution by the EPA and must meet stringent air pollution control requirements.

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