It was difficult to find data on the quantities of recyclable materials recovered from MSW during RDF operations. A few plants have been well characterized, but no broad data base exists.
Plants that precede mass burning with mixed waste processing are beginning to be operated. However, few data were found on operating results for those plants, on quantity and quality of materials removed, and on the markets for the products.
Although regulations on existing operating MSW combustors have become more restrictive (ErR, 1987c; FR 1989a; FR l991a; and the timetable set by the Clean Air Act Amendments of 1990), periodic evaluations of older plants might show that emissions have been reduced as new guidelines governing older plants have been implemented.
Far less information is available on stack emissions from smaller modular mass burn plants. After 1993, when the new guidelines on plants with capacities of less than 250 tons per day go into effect, assessments of emissions from smaller plants will become more available.
Few data on air emissions from the RDF preparation areas or tipping areas of a mass burn plants have been reported. Some of the air is used for combustion, but some is vented.
Long-term studies may also reflect the changing composition of the waste stream. Technological changes over time influence the nature of the waste that is discarded. Examples of such changes include the recent substantial reductions in the mercury in alkaline cells (from 1% to less than 0.1%), the growing popularity of zinc-air cells as replacements for mercury batteries in hearing aids, and the elimination of some metals from inks. In addition, new laws in Europe and California are requiring elimination of lead from the 2 billion wine bottle closures produced each year (Andre and Karpel, 1991).
Some sources have referred to the possibility that free carbon in the flyash portion of the ash might absorb some metals, such as mercury, as well as organics. The role of free carbon as an absorbent might be worth investigating.
No data were found to indicate whether significant reductions in emissions can be achieved by removing retrievable, and possibly recyclable, materials from the MSW prior to combustion.
Methods for reducing emissions from smaller modular mass burn units are needed. Better combustion control is needed for smaller modular combustors to allow them to maintain optimized combustion conditions during charging of new MSW.
The available data were insufficient to support an evaluation of the water emissions from RDF preparation, if any is discharged. Data on the various blowdown streams from combustion operations were also unavailable, perhaps because those streams are entirely consumed in the ash quench tank.
Few data were found on the amount of bulky waste removed before mass burning. Nor were data available on the amount of bulky material that is sold as scrap or on the fate of bulky materials that have no scrap value.
Engineering estimates that compare typical sizes of ash monofills with those of raw MSW landfills are available; however, comparisons of the actual depth and acreage of existing landfills and monofills were not found. Similarly, no studies provide guidance concerning land use for ash monofills after closure.
Research into beneficial uses of stabilized ash is frequently based on the relatively extensive research on the uses for flyash from coal-fired utilities. Some studies have evaluated use of the ash as a component of bituminous highway material. Other research is under way on uses in masonry block construction materials. Some processes vitrify or melt the ash into a glass that is extremely inert to leaching and can often be used beneficially as aggregate (see Appendix A, page A-80, and DeCesare, l991). Alternative uses for ash could save landfill space.
Problems with using historical cost data as predictors of future costs have been discussed. Unless the costs to be compared are built up from consistent base costs using the same assumptions, valid comparisons of generic technologies and systems cannot be made.
The available data show no consistent variations by region. In addition, local factors will alter the costs of individual technologies greatly, and may overwhelm any suspected regional differences. The range of available data is insufficient for estimating effects of local technologies, however.
(1)In 1990, the United States burned 726 minion tons of coal for electricity generation; that coal supplied about 55% of the nation's electricity needs (DOE, 1991); also see Appendix B. page B-33.
(2)"Shred-and-burn" plants are a technical exception to this statement. They are considered RDF plants because they do shred the MSW before combustion; however, like mass burn units, shred-and-burn plants remove almost no combustibles before combustion. Five shred-and-burn plants were operating in the United States in 1991.
(3)To standardize the presentation of costs, all published estimates have been updated to a mid-1991 time frame using SRI International PEP Cost Index. Unit capital costs and O&M Costs are presented in dollars per ton of MSW as collected. If information on individual cost items was unavailable in the literature, estimates based on reasonable assumptions were used. The bases for the data are described in detail in Exhibit I.
(4) Approximately one-half of these products are recyclable as scrap metal (wTe, 1992).
(5)All the integrated strategy examples in this report compare other technologies with a strategy of landfilling alone because no strategy can eliminate the need for a landfill; thus, all integrated strategies will involve adding other technologies to landfilling.
(6)Some RDF plants recover less than 50% of MSW as RDF.
(7)These estimates are based on the Dade County Resource Recovery Project in Florida(capacity, 3,000 tons per day), the Penobscot Energy Recovery Co. in Maine(capacity, 750 tons per day), and the SEMASS facility in Massachusetts(capacity 1,900 tons per day).
(8) To standardize the presentation of costs, all published estimates have been updated to mid-1991 using SRI International's PEP Cost Index. Unit capital costs and O&M costs are presented in dollars per ton of MSW as collected. If information on individual cost items was unavailable in the literature, estimates based on reasonable assumptions were used. The bases for the data are described in detail in Exhibit I.
(9) All the integrated strategy examples in this report compare other technologies with a strategy of landfilling alone because no strategy can eliminate the need for a landfill; thus, all integrated strategies will involve adding other technologies to landfilling.
6. SANITARY LANDFILLLS
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