Cofiring of RDF with coal has been done commercially for a long time. Reasonably complete and reliable data are available on this option, although air emissions have been less well characterized than those from MSW combustion. For gasification/pyrolysis and anaerobic digestion, the data are speculative and incomplete.
Table 9.3
LESS COMMON STRATEGIES PRESENTED IN THE DATA BASE
12 RDF production for cofiring with coal
Collection and transportation of MSW in a packer truck
RDF preparation and metal recovery
Combustion of the RDF (cofiring with coal)
Landfilling RDF rejects
Landfilling ash in a monofill
13 RDF production for gasification
Collection and transportation of MSW in a packer truck
MSW preparation for gasification
Gasification of the prepared MSW
Landfilling ash in a monofill
14 Anaerobic digestion of MSW, plus curbside collection of recyclables, plus landfilling
Collection and transportation of MSW in a packer truck
Collection and transportation of curbside-separted recyclables in a
multi-compartment truck
MRF operations
RDF preparation and metal recovery
Anaerobic digestion of RDF
Landfilling RDF and MRF rejects
15 Curbside collection with mixed recyclables to MRF, plus yard waste composting,
plus RDF for cofiring
Collection and transportation of MSW in a packer truck
Collection and transportation of curbside-separated recyclables in a
multi-compartment truck
Collection and transportation of curbside-separated yard waste in a packer truck
MRF operations
Yard waste composting
RDF preparation and metal recovery
Combustion of the RDF (cofiring with coal)
Landfilling RDF, MRF, and composting rejects
Landfilling ash in a monofill
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Table 9.4 shows estimates of energy production and use found for the less commonly used technologies. Again, note that the quality of the data varies widely, and all the data for anaerobic digestion and gasification/pyrolysis reflect pilot plant experience.
Table 9.4
ENERGY EFFECTS OF LESS COMMONLY USED MSW STRATEGIES
Energy (Million Btu per Ton of MSW)
Strategy No.(a) Required Produced Net Savings
RDF production plus cofiring
of RDF with coal(b) 12 2.16 10.2 7.94
Gasification 13 2.76 8.57 5.81
Anaerobic digestion 14 0.51 3.88 3.36
MRF/C(c) plus cofiring of RDF
plus yard waste composting(b) 15 4.16 9.7 5.54
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) Contribution of RDF only; energy from coal is excluded. (Firing RDF with coal increases
the combustion efficiency of the RDF by 1.4%, as indicated in Section 9.)
(c) MRF/C designates MRF with curbside collection of recyclables.
Notes: Totals may not add because of rounding.
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When RDF is cofired with coal, the combustion efficiency of the mixture is lower than the efficiency of coal alone, but the RDF does displace some coal. The size of the savings depends on the quality of the RDF and the quality of the coal it displaces. The energy reported in this section refers to the contribution of the RDF alone; the extra coal is not considered.
Sewage sludge is added in some anaerobic digestion processes. The sludge is an additional source of organics that are digested into methane, and that methane is counted in the estimates of the total energy produced by the process. The yield of energy from a ton of MSW is therefore overstated, but only by 5-10%.
NOTES:
(1) Anaerobic composting, a similar process, was defined in the introduction to Section 8.
(2) Estimates based on individual process energy requirements [888] indicate that the processing of MSW to RDF requires from 27 to 39 kilowatt-hours (kWH) per short ton of RDF produced. At an energy equivalent of 10,000 Btu per kWh, that quantity is 2.3-3.3% of the energy content of the RDF. (If the RDF plant heat-to-electricity conversion rate of 15,450 Btu per kWh is used, the energy for RDF preparation ranges from 3.54 to 5.1% of the energy recovered.) The remainder of the loss in net electricity produced reflects the lower efficiency in the conversion of heat energy to electricity by the cofired unit, compared to a coal-only unit.
(3) This estimate assumes underground mining and transportation by unit train for a distance of 180 miles or by truck for a distance of 50 miles (Kinderman et al., 1975).
(4) Assuming standard heat content and heat rate values.
(5) That estimate assumes coal cost at $1.60 per million Btu and standard electric utility financing.
(6) Texaco achieved 76.6% efficiency in converting energy in feedstock to energy in product gas; Dow claimed 76% conversion efficiency, and Great Plains claimed 72% (SRL 1985).
(7) Plants in Georgia, Florida, Missouri, and Oregon are gasifying wood chips or whole-tree chips. One large plant generates gas at a rate sufficient to produce 20 million Btu per hour. The gas from some of these operations is used in a clay kiln (CEC, 1991).
10. MISSING DATA AND RESEARCH NEEDS
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