The advantages attributed to the process are recovery of a high fraction of the energy (up to 55%) in the organic fraction of MSW as methane, and production of a compost that can be used as a soil amendment. Anaerobic digestion recovers energy from MSW slowly when compared to combustion, but digestion is very fast when compared to energy recovery by methane generation from landfills. Anaerobic digestion retention times range from 10 to 30 days (see Appendix H. page H-19ff), as opposed to the 2 to 20 years that a landfill requires to release one-half the methane it will generate (Augenstein and Pacey, 1991).
A typical system entails the four basic steps described below.
As in RDF or aerobic compost preparation, MSW is separated into an organic-rich fraction that is then shredded or otherwise comminuted. The organic fraction is mixed with water, which is removed at the end of the process, or with sewage sludge, which provides a source of water.
The mixture is placed in an air-tight reactor for 10-30 days, and a warm temperature is maintained in the reactor. Microorganisms react with the feedstock, and the dry weight of the organics declines by about 50% during the process. The gas produced is collected for use or for upgrading and sale.
The decomposed residue is removed from the reactor and then composted at the plant in a normal, aerobic fashion to stabilize the residue and remove odors. This stage of the composting, which can be done in a vessel or in piles (Larsen Engineers, 1991), takes 2 days (Chynoweth and Le Grand, 1989) to 6 weeks (OWS, 1991).
The gas produced can be used in the digestion plant for fuel to provide process heat and to dry the compost if necessary. The gas, which has about 500-600 Btu per cubic foot, can also be upgraded to pipeline quality methane (1,000 Btu per cubic foot). The compost can be used in municipal projects, sold, or combusted to provide more energy (Larsen Engineers, 1991).
Most U.S. experience has been with pilot and demonstration plants with solids content of about 5% and with large quantities of water added to carry out the reaction. This approach requires large reactors and thus high capital costs. Two pilot and demonstration plants have been operated in the United States-the RefCoM plant at Pompano Beach, Florida, and the Solcon plant at Walt Disney World in Orlando, Florida. Both added sewage sludge to MSW before anaerobic composting. A laboratory-scale prototype under investigation at the University of California at Davis has apparently operated with and without sewage sludge .
The RefCoM plant, which ran from 1977 to 1985, was designed to process more than 200 tons per day of MSW and 5 to 10 tons per day of sewage solids, but it never achieved that capacity (Renewable Energy Systems, 1990). The small Solcon experimental test unit was designed to produce methane directly, thereby avoiding the costs of separating methane from CO(2); the unit achieved a product gas with a methane content of 93%.
Two commercial processes are used in Europe-those of Dranco and Valorga (discussed below). Neither adds sludge, and both operate with much less moisture than either the RefCoM or Solcon plants. In this newer technology, called high-solids anaerobic composting, a solids content of 30% or more is used (Logsdon, 1990).
Appendix H presents more detailed information on these technologies and on individual installations.
In Europe, the French company Valorga operates approximately eight plants in France that treat from 44 to 300 tons per day of mixed MSW using anaerobic digestion. The Belgian company OWS has been operating a pilot plant (27.5 tons per day) since 1984 and has one larger commercial-scale plant nearing completion, with start-up scheduled in 1992. OWS' process is known by the acronym DRANCO C Larsen Engineers, 1991).
The United States has no commercial facility. In addition to the RefCoM and Solcon demonstration plants described above, the University of Florida operates a sequenced batch anaerobic composting (SEBAC) process, which is a high-solids pilot plant (Chynoweth et al., 1990).
The University of California at Davis plans to build a 100 ton per day anaerobic, high-solids pilot plant based on its existing smaller pilot plant. If the 100 ton per day plant is successful, the California Prison Industry Authority proposes to build and operate a 1,000 ton per day MSW processing plant near San Diego that will include anaerobic treatment.
For treating sewage sludge, anaerobic digestion is a fully commercial process, and the process is used at more than 200 plants in the United States (Weston, 1985). Other digestors operate on various biomass feedstocks.
The literature contains one extensive evaluation of the energy balance for an MSW digestor. However, the data used in this modeling study assumed conditions that did not reflect the full operational capacity of the plant, and the accuracy of the model cannot be verified because no commercial facility is operating. The data reported here are the literature values (Chynoweth and Le Grand, 1988 and 1989).
Table 9.1 shows the energy balance for an anaerobic digestion plant like the RefCoM plant-discussed above. Note that about one-half of the energy produced in the plant described in Table 9.1 is substitute natural gas (SNG) for export. Computer-modeled estimates give a range for total, gross Btu values of 2.7 million 4.3 million Btu produced per ton of MSW (see Appendix H). The data base in Exhibit II is based on the available data, but those data have been used with reservations and supplemented with judgment to a certain extent. The assumptions made for this study can be changed in the electronic version of the data base, but the calculations will remain speculative.
Table 9.1 ENERGY BALANCE FOR ANAEROBIC DIGESTION OF MSW Million Btu per Ton MSW input(a) 9 Gross biogas actually generated(b) 4.3 Net synthetic natural gas 3.87 Process heat required 0.167 Process electricity required 0.433 Net excess electricity 0.076 Source: Calculated from Chynoweth and Le Grand, 1988 (a) At 4,500 Btu/lb  (b) 3 million Btu per ton of MSW, plus 1.3 million Btu from sewage sludgeClick here for table in WK1 format.
Capital and operating costs, which have been updated from a 1986 model of a full-scale facility based on the 1984 RefCoM plant, do not include allowance for current regulatory requirements for emissions from residue combustors. Such requirements would add greatly to the costs estimated in 1986. The costs are shown in Table Gas I-2 in Exhibit I. The distribution of costs indicates that the processes ancillary to the digestion are the major cost contributors, as shown in Table 9.2.
Table 9.2 DISTRIBUTION OF COSTS BY PROCESS UNIT FOR THE RefCoM PLANT Process Unit Percent of O&M RDF plant 27.4 Anaerobic digestion 16.2 Gas cleaning 5.0 Residue burning 27.3 Landfill noncombustibles 24.0 Total 100 Source: Chynoweth and Le Grand, 1988Click here for table in WK1 format.
Using a model, costs for the University of Florida's SEBAC process were estimated . The estimated costs are likely to be lower than actual because no plants have been built. Updated costs for plants are included in Exhibit 1.
Although claims have been made that anaerobic composting results in fewer environmental releases than mass burning (Chynoweth and Le Grand, 1988 and 1989), it is difficult to document expected advantages until the technology is proven. Such claims are often based on comparing expected releases from a hypothetical plant with emissions from MSW combustors operating under out-of-date emissions regulations.
Exhibit II sets forth estimates for an anaerobic digestion plant. Note that the data are incomplete and therefore underestimate emissions. Some estimates have been used to compensate for the lack of data, but those estimates are speculative.
Missing Data and Research Needs
No full-scale plant is operating in the United States. Until one is constructed, a consistent set of data to use in documenting energy, environmental releases, and costs will be lacking.
The largest existing facilities-300 tons per day, which operate in France-would be useful for many U.S. communities. The 1,000 ton per day San Diego facility will be valuable in providing information to evaluate all the parameters of the technology.
Cofiring RDF with Coal for Power Production
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