Status of the Less Common Options

Cofiring RDF with Coal

A cofiring facility at Ames, Iowa, has been operating longer than any dedicated RDF boiler. RDF refiring is the most technologically proven of the less common MSW management options covered in the report. RDF can be effectively mixed with coal and burned in existing coalfired utility boilers to produce electricity. Cofiring is an effective way to burn the RDF, which has a lower sulfur content than coal. A municipality that finds a utility willing to coflre can avoid the expense of acquiring a new combustor, boiler, air pollution control equipment, and steam turbine and generator. Several utilities now cofire RDF with coal. The disadvantages are that the coal boilers must be derated, and RDF handling is difficult.

Anaerobic Digestion

Anaerobic digestion is a biological process similar to the decomposition that takes place in a landfill. It is applicable to the organic matter in MSW. Its advantage over landfilling is more efficient methane formation; anaerobic digestion of the organic material from 1 ton of MSW can produce 2 to 4 times as much methane in less than 3 weeks as the same ton of MSW in a landfill produces over 2-7 or more years. After minimal further treatment, the residue from the anaerobic processing can be used like compost for soil conditioning, or as fuel. New plants with recently developed technology and improved operating characteristics have been apparently successful in Europe, but no commercial anaerobic digestion plants are currently operating in the United States.

Gasification/Pyrolysis

Gasification/pyrolysis can be used to produce a fuel gas or synthesis gas consisting principally of carbon monoxide and hydrogen (once called "town gas") from MSW. The fuel is compatible with existing boilers or furnaces. The process operates at a high temperature and in the absence of air. Under special conditions, a liquid fuel or chemical feedstock could also be formed. The process has been used commercially with coal and wood chips. It was used with MSW in the United States in the 1970s, but all those plants have been shut down because of operating and financial problems. Some gasification/pyrolysis plants were built and operated in Europe in the early 1980s.

Life-Cycle Energy and Environmental Releases from Common Integrated Strategies

Because communities commonly combine more than one MSW management option into an integrated strategy for handling their waste, life-cycle analyses were conducted for common integrated strategies. Key steps in those strategies are shown in Figure ES.1. The rest of this subsection summarizes the findings of those lifecycle analyses. Note that the analyses do not include a differential credit for emissions from displaced or avoided energy. Examples include the coal displaced by burning MSW for fuel and the substitution of fossil fuel used in paper remanufacturing for the renewable fuel used for virgin paper manufacture.

Energy Savings from the Various Options

For every MSW management strategy, energy is needed for collection (e.g., to pick up and deliver the MSW) and processing (in a landfill, an MRF, or a combustion plant). The life-cycle analysis in this study compared both the energy needed for each major strategy and the energy that is produced by the strategy, if any.

When an integrated MSW management strategy generates fuel energy in excess of the amount the entire strategy requires, the energy is reported as a net Btu savings. Usually the excess energy (which is referred to as "exported energy") is generated and sold as electricity, and it therefore displaces the need to generate the same amount of electricity from a virgin fuel, most commonly coal, which provides 55% of U.S. electrical power, or from some other source (e.g., hydropower or nuclear).

Figure ES.1
STRATEGIES BASED ON THE FIVE MAJOR OPTIONS

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Figure ES.1
STRATEGIES BASED ON THE FIVE MAJOR OPTIONS (Concluded)

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The results of the energy comparisons for the major strategies are shown in Figure ES.2. The estimates indicate the energy balance for 1 ton of MSW at the curb over the 20-year period. For strategies that include recycling, the energy required for and saved by remanufacturing and reusing the recyclable products is included in the analysis. Energy for transportation of the separated recyclables is a small fraction of the energy required for remanufacture of glass and metal. Transportation energy is quantified in the report, but it is not included in the comparisons shown in Figures ES.2 and ES.3.

To determine the amounts of energy used and saved for remanufactured materials made from the separated recyclables, the products had to be identified. For this analysis, the following assumptions were made:

Energy savings for remanufacturing aluminum, steel, and glass have been well documented. However, energy data for manufacturing paper products from virgin timber and used paper vary widely(4).

The combustion strategies produce the greatest energy savings and the largest quantities of exportable electricity. Recovering gas from landfills and burning it to produce heat or electricity is the next most energy-efficient strategy. Recycling achieves smaller energy savings. Composting is the only option that neither produces nor saves energy.

Figure ES.3 shows the quantities of electrical energy that could be produced from those strategies that generate a fuel or burn MSW. The illustration compares only the portions of the strategies that involve conversion to heat for electricity generation; energy saved by recycling is excluded (although it is included in the energy balance in Figure ES.2), as is energy used for collection and transportation. The patterns of energy savings in the two figures are quite similar.

Figure ES.2
ENERGY ANALYSIS FOR STRATEGIES BASED ON THE FIVE MAJOR OPTIONS
(PER TON OF MSW)

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Figure ES.3
NET ELECTRICAL ENERGY (PER TON OF MSW)

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Air Emissions

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