For the purposes of this discussion, the following definitions were used for gasification and pyrolysis. Gasification is a high-temperature process that is optimized to produce a fuel gas with a minimum of liquids and solids. Gasification, which is more proven than pyrolysis, consists of heating the feed material in a vessel with or without the addition of oxygen. Water may or may not be added. Decomposition reactions take place, and a mixture of hydrogen and CO are the predominant gas products, along with water, methane, and CO2. Pyrolysis is a medium to high temperature (500-1000íC) process for converting solid feedstocks into a mixture of solid, liquid, and gaseous products. Pyrolysis to maximize production of liquid fuels and chemical feedstocks directly from a feedstock requires careful reaction control and fast heating and cooling rates to prevent the liquids that do form from breaking down to gases.
Although gasification/pyrolysis plants for MSW that were operated in the 1970s experienced many technical problems, the application of this technology to MSW offers potential advantages. One is that in some locations, nearby industrial users could burn the gas for process heat. Other claimed advantages include reductions in metal volatilization and particulates compared with MSW combustion technologies. The studies that found those advantages, however, were based on comparisons with direct firing of MSW or RDF before modern mandated air pollution controls were developed and installed (see Appendix D). No data to support such claims were found for this study. The higher efficiencies inherent with combined cycles for power generation may revive interest in pyrolysis and gasification. Compared with low-temperature gasification processes such as anaerobic digestion, high-temperature gasification is likely to convert a larger fraction of the organics into a fuel gas. Anaerobic digestion converts about 40-55% of the contained energy in the biodegradable part of the feed into energy in the methane product; typical gasifiers (e.g., methane from coal or lignite) achieve about 75% conversion of the energy in the solid (including plastics) to the energy in the product gas(6). Performance on lignite does not translate directly into performance on MSW because the handling characteristics and chemical compositions of the materials are quite different. Many of the smaller gasifiers appear to perform functions similar to those that occur in two-stage, starved-air modular combustors (a proven combustion technology).
Preparation of MSW for gasification varies greatly with the process. Some processes, e.g., the Andco-Torrax (which may still be in use in Japan and France), require minimal preparation. Other processes, e.g., the Purox process, required RDF. Pyrolysis intended for direct production of liquid fuels or chemicals requires very rapid heat transfer and the preparation of RDF with a minimum of inert solids.
Although the U.S. MSW pyrolysis plants were closed in the late 1970s, gasification projects based on biomass, coal, and lignite have proceeded. Several gasifiers produced by Texaco, Lurgi, and other companies are operating commercially on coal, and a few are processing biomass(7). A successful 100 megawatt-electricity (MWe) integrated gasification/combined cycle gasifier demonstration power plant was sponsored by EPRI. That pilot plant was larger in power output than most of the largest municipal waste combustors (MWCs) in the United States. The Great plains Gasification Project, which operated on lignite, was a short-lived technical success; it was abandoned in large part because natural gas prices fell dramatically just as it was coming on line (this plant is being converted to liquid fuel production via Fischer-Tropsch). Sasol I and II, which are very large coal-to-gasoline conversion plants in South Africa, use Lurgi gasifiers successfully for converting coal to synthetic petroleum products via Fischer-Tropsch reactions.
Although none of these projects were intended to use MSW feedstocks, they made significant progress in proving gasification and pyrolysis technology in general. The integrated gasification/combined cycle work indicated that gasifiers can be clean ways to handle fuel feedstocks that have many impurities.
No commercial plants that gasify or pyrolyze MSW are operating in the United States today. The history of U.S. efforts to develop gasification and pyrolysis processes is discussed in the subsection on technical status and in Appendix D. Attempts to apply these technologies to MSW were made in the 1970s, but the plants failed to achieve acceptable technical or economic performance, and all have been shut down.
One gasifier designed for 400 tons per day of MSW may still be operating in France; a 400+ ton per day fluid bed gasifier and a 150 ton per day gasifier may still be operating in Japan. The most recent references, published in 1988, appear to report on work done in the early 1980s (see Appendix D). A 200 ton per day MSW gasifier is reported to be under construction in Italy (Dhargalkar, 1991). No current data on these possibly operational facilities were found. The old data are fragmentary and anecdotal or simply descriptive of the earlier projects; they do not provide a basis for estimating energy efficiency, emissions, or costs for the plants.
In the plants that may be operating on MSW, several sources report conversion of 70-80% of the energy in the feed to energy in the output gas (see Appendix D, page D-38, and [831,834]). Such conversion estimates typically refer to the efficiency of the gasifier alone, and not to ancillary preparation and processing equipment, if any. The net energy output in a 400 ton per day plant ranges from 5 million to 8 million Btu per ton of MSW (see Exhibit II, "Basic Gasification").
Because of the low volumetric energy content of gas produced when air is used in gasification of solid fuels, the gas is often converted on site to electricity. MSW, as a solid fuel, could instead be directly combusted to produce electricity, without the gasification step. However, gasification and/or pyrolysis may have efficiency advantages when used in conjunction with combined cycle electrical power generation (Larson and Williams, 1990).
The assumptions about energy consumption and production for gasification/pyrolysis made in the data base in Exhibit II are derived from the data found in the literature, but adjustments were made to conform with reasonable assumptions about the performance of the facilities. Even with those adjustments, the estimates used in this report are highly uncertain, and additional data are needed.
Data on emissions from gasification/pyrolysis plants are scarce. Fragmentary data on the emissions from plants in Japan have been published (see Appendix D and [108,834]), but the data are incomplete, and the feed for the Japanese plants is different from U.S. MSW. The available data are insufficient to establish whether gasification would reduce emissions compared with those from modern direct combustion facilities with currently mandated air pollution control techniques. Because of lower gas flows, however, gas clean-up may be more economical.
Emissions from pyrolysis would include vent gas, flare gas, emissions from burning the gas, ash or slag, and possibly water from scrubbing the gas. If a liquid fuel is made, it might contain carcinogens. Although pyrolysis of wood can be controlled to prevent formation of carcinogens (Elliott, 1988), no data on similar results on MSW are available. In Japan, one of the features that favored selection of pyrolysis was that the residue that contained metals in the MSW was expected to be vitrified to a nonleachable slag that could be used or disposed of safely. No published reports indicate whether that expectation was realized.
The assumptions about environmental releases for gasification/ pyrolysis made in the data base in Exhibit II are also derived from the data found in the literature, but adjustments were made to conform with reasonable assumptions about the performance of the facilities, in some cases by drawing analogies with related technologies. Even with those adjustments, the estimates used in this report are highly uncertain.
Estimates of the costs of pyrolysis facilities are highly speculative because no commercial plants have been built in the United States. For completeness, updates of previously published costs are included in Table I.2 in Exhibit I, but it is not clear that those estimates are meaningful. Because the plants did not achieve adequate technical performance, the costs of those plants provide little indication of what it would cost to construct a new facility that would operate effectively.
Like anaerobic digestion, the greatest value of gasification and pyrolysis appears to lie in their ability to provide a clean, transportable fuel for use in another location for a purpose other than electricity generation. The most promising application of the fuel would probably be for industrial use to generate process heat. Pyrolysis produces a more transportable and storable form of energy than low-Btu gas or steam, but it probably could not compete economically with direct combustion for steam when the latter would be a feasible alternative. Pyrolysis liquids may be a source of valuable chemicals, although laboratory studies have only recently begun.
An important need is documentation for full-scale operating plants in the United States. Until such plants exist, actual operating data on energy requirements, environmental releases, costs, and product properties for gasification/pyrolysis on a basis consistent with those of other MSW technologies will not be available. That need might be partly met by data on plants that are operating in other countries, if they were available. However, current data on the few large plants in Japan and France are missing, and very little is known about the new plant under construction in Italy.
The prospects for this technology will remain limited until research and development are successfully completed and the resulting data are projected to be of economic interest for the United States. Without a demonstration plant, reliable data on the environmental impacts cannot be gathered, and gasification/pyrolysis cannot be assumed to be cleaner than direct combustion or other alternatives. Construction of a demonstration plant seems unlikely because economic incentives do not appear to exist at this time to justify the large-scale investment needed for renewed efforts to apply gasification/pyrolysis technology to MSW.
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