Landfilling of MSW is the preferred disposal method. But the shortage of suitable sites and the regulations and controls now applicable to the construc­tion and operation of new landfills, the operation of existing landfills, and the closure methods and subsequent monitoring requirements have led to renewed interest in incineration. MSW disposal by open-air burning and incineration in small — and large-scale facilities without energy recovery has been practiced for many years. Some small — and many large-scale MSW incineration systems now incorporate energy recovery systems for steam and thermal energy. Some produce electric power as discussed below. MSW incineration plants with energy recovery span a large throughput range—about 50 to 4000 t/day of MSW

In the United States, three main technologies are used for waste-to-energy facilities: mass burn for MSW, modular mass burn, and RDF (Berenyi, 1995). Many of the modern plants in operation are based on European combustion hardware and utilize a waste-heat boiler or a waterwall system to produce steam. In mass burn technology, which is used in the majority of facilities, MSW is combusted as received or with minimum processing to reduce the size of the pieces and clumps present in the mixture and to separate some of the material. At most locations, large appliances, car batteries, and hazardous materials are removed at the tipping floor. Most mass burn plants use waterwall incineration technologies; some use refractory-lined furnaces, rotary combus­tors, and a few other configurations. Modular mass burn facilities often use one or more small-scale combustion units to process smaller quantities of MSW than the waterwall systems. Steam is commonly generated from the hot flue gases in many modular plants using a two-chamber furnace design. Final combustion occurs in the second chamber. For plants based on RDF technolo­gies, MSW is first shredded and then separated into the combustible fraction or RDF, and selected recyclables, such as the ferrous fraction, aluminum, and glass. RDF is usually burned in semisuspension or suspension-fired furnaces or cofired with other fuels. It is also cofired in minor amounts with coal to produce steam in some of the larger power plants or with dewatered municipal biosolids in some plants. Separation of the RDF and recyclables is accomplished with various combinations of magnets, eddy-current devices, air classifiers, trommel screens, rotary drums, flotation devices, and pulping devices (see Chapter 6). Some processes involve the production of powdered RDF or pellet­ized material for use as fuels.

Numerous industrial solid wastes are disposed of in incinerators that have energy recovery capability. Most of these systems are smaller than MSW incin­erators. The compositions of specific industrial wastes are more uniform than those of MSW, but the range of waste categories is so broad that special hardware and furnaces must sometimes be used. Rotary kilns, multihearth furnaces, and fluidized-bed incinerators have been employed for industrial waste incineration systems.