Rotary Kiln Incinerator

The rotary kiln is often used in solid/liquid waste incineration because of its versatility in processing solid, liquid, and containerized wastes. The kiln is refractory-lined. The shell is mounted at a slight incline (about 5 degrees) from the horizontal plane to facilitate mixing the waste materials. A conveyor system or a ram usually feeds solid wastes and drummed wastes. Liquid hazardous wastes are injected through a nozzle(s). Noncombustible metal and other residues are discharged as ash at the end of the kiln. Rotary kilns are also frequently used to burn hazardous wastes.

Rotary kiln incinerators are cylindrical, refractory-lined steel shells supported by two or more steel trundles that ride on rollers, allowing the kiln to rotate on its horizontal axis. The refractory lining is resistant to corrosion from the acid gases generated during the incineration process. Rotary kiln incinerators usually have a length-to-diameter ratio (L/D) between 2 and 8. Rotational speeds range from 0.5-2.5 cm/s, depending on the kiln periphery. High L/D ratios and slower rotational speeds are used for wastes requiring longer residence times. The kilns ranged from 2-5 meters in diameter and 8-40 meters in length. The burners for the kilns ranged from 10 million British Thermal Units (BTU) per hour to 100 million BTU per hour.

Rotation rate of the kiln and residence time for solids are inversely related; as the rotation rate increases, residence time for solids decreases. Residence time for the waste feeds varied from 30 to 80 minutes, and the kiln rotation rate ranged from 30 to 120 revolutions per hour. Another factor that has an effect on residence time is the orientation of the kiln. Kilns are oriented on a slight incline, a position referred to as the rake. The rake typically is inclined 5 degrees from the horizontal.

Rotary kiln incinerators are designed with either a co-current or a counter-current chamber. In the counter-current design, waste is introduced at the end opposite the burner and flows down the rake toward the burner, while combustion gases are drawn up the rake. In a co-current design, the waste feed is introduced at the burner end and flows down the rake, while the combustion gases are also drawn down the rake. Most rotary kiln incinerators were of the co-current design, which provides for more rapid ignition of the waste feed and greater gas residence time for combustion than does the counter-current design.

Hazardous or nonhazardous wastes are fed directly into the rotary kiln, either continuously or semi-continuously. Devices such as arm feeders, auger screw feeders, or belt feeders can be used to feed solid wastes. Hazardous liquid wastes can also be injected by a waste lance or mixed with solid wastes. Drums and cartons of hazardous waste may be fed directly into the kiln but are often shredded first. Rotary kiln systems typically include secondary combustion chambers of afterburners to ensure complete destruction of the hazardous waste. Operating kiln temperatures rang from 800°C to 1300°C in the secondary combustion chamber or afterburner depends on the type of wastes. Liquid wastes are often injected into the kiln combustion chamber.

The advantages of the rotary kiln include the ability to handle a variety of wastes, high operating temperature, and continuous mixing of incoming wastes. The disadvantages are high capital and operating costs and the need for trained personnel. Maintenance costs can also be high because of the abrasive characteristics of the waste and exposure of moving parts to high incineration temperatures.

Cement kiln incinerator is an option that can be used to incinerate most hazardous and non-hazardous wastes. The rotary kiln type is the typical furnace used in all cement factories. Rotary kilns used in cement industry are much larger in diameter and longer in length than the previously discussed incinerator.

The manufacture of cement from limestone requires high kiln temperatures (1400°C) and long residence times, creating an excellent opportunity for hazardous waste destruction. Further, the lime can neutralize the hydrogen chloride generated from chlorinated wastes without adversely affecting the properties of the cement. Liquid hazardous wastes with high heat contents are an ideal supplemental fuel for cement kilns and promote the concept of recycling and recovery. As much as 40% of the fuel requirement of a well-operated cement kiln can be supplied by hazardous wastes such as solvents, paint thinners, and dry cleaning fluids. The selection of hazardous wastes to be used in cement kiln incinerators are very important not only to treat the hazardous wastes but also to get some benefits out of it as alternative fuel and alternative raw material without affecting both the product properties as well as gas emissions. However, if hazardous waste is burned in a cement kiln, attention has to be given to determine the compounds that may be released as air emissions because of the combustion of the hazardous waste. The savings in fuel cost due to use of hazardous waste as a fuel, may offset the cost of additional air emission control systems in a cement kiln. Therefore with proper emission control systems, cement kilns may be an economical option for incineration of hazardous waste.

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