Table 2.2 shows overall energy comparisons for the major MSW management strategies. Figure 2.1 shows the relationships for the 11 common strategies in graphical form. The electronic data base prepared for this study allows users to estimate energy balances for integrated strategies consisting of any combination of the waste management technologies covered in this report (see Section 4).
When an integrated MSW management strategy generates fuel energy in excess of the amount that 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 or other sources. Energy saved by recycling is not necessarily in the form of electricity. As a result, the comparisons in this subsection are expressed in Btu, with electricity use converted on the basis of 10,000 British thermal units (Btu) per kilowatt-hour (kWh).
Table 2.1 QUALITATIVE DESCRIPTIONS OF ENERGY BALANCES FOR MAJOR MSW TECHNOLOGIES Technology Energy Transportation Required: Energy for fuel production and truck operations Landfill with gas recovery Required: Construction and operation of the landfill Produced: Methane captured and burned for power production in an internal combustion engine or turbine, or exported Mass burning required: Operation of the mass burn facility, landfilling the ash Produced: Heat for conversion to steam and electricity Preparation and combustion of RDF Required: RDF preparation-energy to operate the combustion facility, landfilling the ash Produced: Heat for conversion to electricity, methane from landfilling unburned residue Collection/separation/recycling Required: Extra transportation energy, energy for separation, energy for transport to remanufacturing and energy for remanufacturing Saved: Energy for mining or logging the virgin material, for processing, for transportation of the raw materials to the point of manufacture, and for manufacture Excluded: Energy savings when the recycled material is recycled again, and displaces a mix of virgin and recycled material; these are small when recycling is extensive. Energy to transport finished products to market is assumed to be the same as for shipment of original products Composting Required: Energy for separate pickup of yard waste (if used), for grinding and aeration, for screening and processing; also for MSW processing if MSW composting is used Produced: None Excluded: Energy for transport of the compost to point of use
Click here for table in WK1 format.
Table 2.2Click here for table in WK1 format.
ENERGY EFFECTS OF COMMON MSW STRATEGIES Energy (Million Btu per Ton of MSW) Strategy No.(a) Required Produced Net Savings Landfill with gas recovery 1 0.08 2.20 2.12 Mass burn 2 1.59 10.3 8.7 Onsite MRF plus mass burn 3 1.40 10.2 8.76 Direct firing of RDF 4 2.16 10.1 7.94 Yard waste composting plus landfill 5 2.33 2.12 -0.21 MRF/C(b) plus landfill 6 0.12 2.80 2.68 MRF/C plus mass burn 7 1.48 10.1 8.58 MRF/C plus direct firing RDF 8 1.99 9.92 7.93 RDF preparation and MSW composting 9 0.54 1.90 1.36 MRF/C plus landfill plus yard waste composting 10 2.37 2.71 0.34 MRF/C plus mass burn plus yard waste composting 11 3.46 9.75 6.29 Source: SRI International based on various sources noted in the data sheets in Exhibit II (a) As listed in Table 1.1 in the Introduction (b) MRF/C designates MRF with curbside collection of recyclables Note: Totals may not add because of rounding
The combustion strategies produce the greatest energy savings and the largest quantities of available electricity. Recovering gas from landfills and burning it to produce heat or electricity is the next most energy-efficient strategy. Recycling achieves some energy savings, but the quantity is smaller. All mass burning strategies and RDF preparation and direct combustion strategies include recycling to some degree. The energy savings associated with the recycling are included in the estimates in Figure 2.1(1). Composting is the only technology that produces no recoverable energy.
ENERGY ANALYSIS FOR STRATEGIES BASED ON THE FIVE MAJOR OPTIONS
(PER TON OF MSW)
Click here to expand figure.
Some of the results in Table 2.2 appear unlikely at first glance. In strategies that include two technologies, mass burning and RDF direct firing, the net energy produced is lower when curb-side recycling is included. In contrast, adding curbside recycling to landfilling results in a higher energy saving than landfilling alone. The reason is that the Btu value of newsprint is about twice the energy saved by recycling the newsprint. The landfill is inefficient at recovering the energy in the discarded paper, and recycling therefore produces a larger energy savings. In a combustor, the energy released from burning the paper is greater than the energy saved by recycling the paper; therefore, removing the paper leads to a net decrease in energy produced (see Section 7 and Exhibit VII). The extra energy recovered from burning the paper even exceeds the total energy saved by recycling the aluminum, glass, and steel, as well as the paper.
Although energy production is not the primary goal of any MSW management strategy, the opportunity to recovery materials and energy is an added benefit. Recovering energy from MSW eliminates the need for some other fuel. Because about 55% of the electricity produced in the United States comes from burning coal (DOE, 1992), that is the fuel that is most likely to be displaced by energy generated from MSW.
Figure 2.2 shows the quantities of electrical energy that could be produced from those strategies that generate a fuel or burn MSW in an electrical generating facility. It compares only the portions of the strategies that involve conversion to heat; energy saved by recycling is excluded (although it is included in the energy balances in Table 2.2), as is energy for collection and transportation. The comparison is based on net kilowatt-hours of electricity generated by each method.
Strategies can be net energy consumers, yet still generate electricity. For example, one strategy includes composting MSW, yet the figure shows that energy is generated. Normal aerobic composing consumes energy for shredding or grinding, aeration, turning, screening, and so on. Composing itself produces no recoverable energy, and the entire strategy produces no net energy. However, about one-half of the organics in the MSW are removed before composing and are landfilled. The organics that were not composted produce part of the methane collected from the landfill. Figure 2.2 therefore shows that the strategy generates electrical power. For curbside collection of yard waste for composting, a larger amount of MSW goes to the landfill and generates more methane.
NET ELECTRICAL ENERGY (PER TON OF MSW)
Click here to expand figure.
(1) Recycling in Strategy 2, Mass Burn, consists of recovering about 3% of the weight of the MSW as ferrous metal after combustion.
3. ENVIRONMENTAL RELEASES
Table of Contents