Facilities for mass burning include smaller units (25-300 tons per day of MSW) that are fabricated in a shop before installation on site (modular units), as well as larger field-erected plants (200-3,000 tons per day). Figure 5.1 presents a block diagram of the flows of MSW through both types of facility. A variety of grates, boiler designs, feeding arrangements, and air pollution controls are possible. Individual systems are described in Appendix B. This subsection describes typical units in both size ranges.
In typical large, field-erected facilities, packer trucks empty waste into a large pit in a building. The MSW is retrieved by a crane and dropped into a hopper that feeds the combustor.
Field-erected combustors have a variety of designs to move the waste on grates through the furnace as the MSW burns. The grates move under the waste by reciprocating, rocking, rolling, moving as an endless belt, or rotating as a large tilted cylinder. Air is forced up through the grate to support combustion.
In a typical modular facility, packer trucks unload inside a building on a tipping floor, where front-end loaders are used to move the waste. At some facilities, oversized waste, such as household appliances, and tires are separated for disposal before combustion begins. The frontend loaders will break down some furniture and boxes by driving on them or by using their buckets. The MSW is then pushed onto a conveyor for feed to the combustor.
Figure 5.1 BLOCK DIAGRAM FOR A TYPICAL MASS BURN FACILITY
Click here to expand figure.
Modular designs create combustion in two chambers. The solid MSW is fed into a starved-air chamber (one that contains too little air for complete combustion) that gasifies part of the waste. The gas is burned in a second chamber with excess air at high temperature for additional heat recovery and organics destruction. Large plants have a single combustion chamber that uses excess air.
In both modular and field-erected units, the heat of combustion may be transferred to water or steam in tubes that form the chamber of the combustor or the grate in a rotary combustor. These tubes are called "water walls," and they are highly efficient at heat recovery. Another variation is to combust the waste in a refractory lined firebox and recover the heat in a waste heat boiler located farther from the point of combustion.
Finally, in both types of units, the exhaust gases are cleaned. Combinations of devices used for cleaning may include:
Both mass burning and RDF combustion are mature technologies. The first mass burn plant in the United States that generated electricity for sale was built in New York City in 1902. In addition, combustion plants with capacities greater than 250 tons per day have been evaluated more carefully and completely than any other MSW management alternative.
Direct combustion ranks second to landfilling as an MSW management technique in the United States, accounting for disposal of 17% of all MSW (Kiser, l991a). Of the 176 municipal waste combustors (MWCs) operating in the United States in 1991, 149 are mass burn plants (Kiser, l991a). Of those, 60 are large, field-erected plants and another 50 are smaller, modular plants. The other 39 mass burn plants recover no energy.
Because MSW receives minimal or no preprocessing before mass burning, essentially no energy is required. When mass burning is preceded by separation of recyclables at an MRF, the energy requirements for materials recovery are assigned to the MRF.
The total energy produced by mass burning is higher than that of any other technology except shred-and-burn RDF. Energy production from mass burning is often comparable to that for shred-and-burn RDF, and it may be higher.
Net Energy Balance
For new, larger mass burn facilities, a reasonable estimate of net energy recovery is 525 kilowatt-hours (kWh) per ton of MSW, with a variance of +75 kWh per ton; that is, 3.8 pounds of MSW generate 1 net kWh (see Appendix A, Attachment 11, page 11-4). Plants that cogenerate steam and electricity have proportionally higher useful energy recovery, but it is more difficult to find an appropriate mix of users for the steam.
The efficiency of small, modular plants is only two-thirds as great as that of a large, field erected plant because the smaller plants use a smaller turbine generator, have lower carbon burnout, have higher radiative heat losses, and operate at lower heat transfer temperatures (see Appendix A, page A-24). The average net electricity production, in kilowatt-hours per ton, is also two-thirds as great for a modular plant as it is for a larger plant (see Appendix A, Attachment 11, pages 11-4 and 11-5; also Berenyi and Gould, l991a). Most of the small, modular plants have been designed for steam production only.
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