Typical Processes. All RDF processes typically begin with shredding MSW to a finer size; many then separate the fuel fraction from the residue. In plants where no additional preparation is included, the operation is called a "shred-and-burn" RDF facility. Frequently, however, the separated fuel fraction is further processed to recover metals and sometimes glass. The normal sequence of RDF preparation is shredding, air classifying/screening, magnetic separation, and sometimes eddy current separation for nonferrous metal recovery. Many variations of the process have been developed, each of which has certain advantages. Appendix B provides detailed information on these processes.
Figure 5.8 shows a schematic flowsheet for a typical process to make RDF. A typical plant of the type represented in the schematic could process 500 to more than 2,000 tons per day of MSW (see Appendix B. page B-67).
In 1990, 18 RDF facilities prepared fuel for combustion in dedicated boilers (see Appendix B for descriptions of combustion processes). Processes used to prepare RDF for that purpose vary according to the desired product quality, which affects yields and therefore equipment selection. Appendix B describes the major process equipment differences.
Status of Development. Over the past 2 decades, RDF process technology has undergone a number of changes. The earlier years were characterized by technologically complex plants that had poor reliability and high costs; many of those plants failed. After this initial experience, most RDF plants used simple processes with minimal shredding and separation ("shred and burn") that proved to be reliable. Today, the industry is slowly returning to increasingly sophisticated processes that include separation to enhance recycling opportunities and eliminate materials in the waste stream that could become hazardous emissions from the combustor.
RDF PROCESSING SYSTEM DESIGN (HARTFORD, CONNECTICUT)
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One of the most dangerous problems in preparation of RDF is the possibility of an explosion during shredding. Process and equipment improvements have significantly reduced the severity of the problem, but not eliminated it. Improved designs for commonly used shredders and their enclosures have been able to minimize the number of explosions and reduce their destructiveness. Explosion-suppression systems have been effective in preventing many solvent ignition and dust explosions. New equipment has contributed to progress; for example, slow-speed shear shredders cause far fewer explosions than the usual higher speed mills. Preprocessing of MSW before shredding has also been effective in removing potentially dangerous materials and explosives from the feed to the shredders.
Dedicated RDF combustors include heat recovery systems and pollution controls. Designs vary in the feed, grate, and furnace system. RDF can be fired in suspension, or partly in suspension and partly on a grate, or entirely on a grate, as a mass burning system does. Design choices are based on size, equipment suppliers, and whether other fuels may be used.
Fluidized bed combustors are also used to burn RDF. Of the three operating fluidized bed plants that burn RDF, all cofire other materials with the RDF. Appendix C provides detailed descriptions of fluidized bed combustor systems and plants for MSW. A more complete discussion of RDF combustion systems is included in Appendix B.
Dedicated RDF preparation and combustion plants are a fully developed and proven MSW management technology that is directly competitive with large mass burn plants. In 1991, about 40 plants that produce RDF, burn RDF, or both were operating in the United States (Kiser, 1991b). Of the total of 29,000 tons per day of RDF made in the United States in 1990, an overwhelming majority (89%) was directly fired alone for energy recovery.
Five plants make densified (pelletized, cubed, compressed) RDF, called d-RDF, for use in other facilities, as reported in Appendix B (page B-50). Most of these are small plants that process about 100 tons per day of MSW. d-RDF is expensive to make because of high processing costs and equipment wear.
Applications and Markets Whenever RDF is prepared in one facility for firing at another, a key commercial consideration is the need for a strong contract or a close financial relationship between the preparers of RDF or d-RDF and the final users. In some instances in which the relationship was loose, the user has refused to continue to accept the RDF, and the result was the failure of RDF as an MSW disposal strategy.
d-RDF is often difficult to market. Unless potential customers are willing to pay the additional cost of the densified material for its special properties, plan have little incentive to make d-RDF. The largest user, Ottertail Power, substitutes d-RDF for coal at about 5% of its heat load (RRR, 1992; Berenyi and Gould, 1991a).
Energy Requirements for RDF Preparation
The energy required for RDF preparation is about 0.031-0.046 million Btu per ton of MSW. Appendix A provides additional details on the estimated energy requirements.
Energy Produced by RDF Combustion
As a fuel, RDF has roughly one-half the Btu value of the same weight of coal. RDF also has a higher ash and chloride content, and a lower sulfur content.
A plant like the one shown previously in Figure 5.8 will typically convert 75-85% of the weight(6) and 80-90% of the Btu value of the MSW into RDF . The RDF typically contains 10-17% ash and has a Btu range of 4,800-6,400 Btu per pound. A value of 5,900 Btu per pound was used in the calculations for this study (see Appendix B, pages B-5, B-69, and B-47). When it is used as fuel for electric power production, RDF typically produces 1 kWh for 15,460 Btu (2.6 pounds of RDF).
Net Energy Balance
Thermal efficiency of three new operating RDF plants is about 455 net kWh per ton of MSW, with a range of +100 kWh per ton (7). According to studies of actual performance, the electrical efficiency achieved by burning RDF, like the efficiency achieved by mass burning, depends more on the nature of the raw MSW fuel than it does on the combustion plant design. Boiler design and operating characteristics greatly affect these efficiencies, and neither mass burning nor RDF direct combustion is consistently more efficient.
The "Integrated Strategy Example" later in this section presents the net energy balance for using RDF in an MSW management strategy.
Figure 5.9 summarizes the capital cost estimates for 15 operating integrated RDF production/combustion facilities(8). The estimates are based on detailed data included in Exhibit I. The average unit capital cost is $98,000 per ton per day of design capacity. A comparison study that gave capital costs for RDF plants completed in different periods provided a range of costs from $75,000 to $102,000 per ton per day of design capacity (Kiser, 1990).
Figure 5.10 shows the O&M cost estimates for the plants for which data are available. The average O&M cost for the facilities is $36 per ton of MSW processed, with a range of $13 to $67 per ton.
Note, however, that these averages are based on wide ranges. Because these ranges are so wide and the number of data points is so small, the data in Figures 5.9 and 5.10, as well as the more detailed cost data provided in Exhibit I, are useful only as order-of-magnitude estimates of the possible costs of new RDF preparation and combustion facilities.
Air pollution control systems are required for direct combustion of RDF, and existing RDF combustion facilities emit smaller quantities of organics, particulates, and metals than the most recent EPA regulations allow. Typical emissions from RDF combustion are summarized in Table 5.5.
Some organics are separated before combustion and landfilled. The landfilled portion of the MSW undergoes anaerobic digestion and produces landfill gas. The amounts released are discussed in the "Integrated Strategy Example" described in the next subsection. Emissions for the entire strategy are shown later.
The major potential source of water pollution with RDF is from leachate that might escape untreated from a landfill containing the ash. Emissions from ash landfills are discussed in Section 6.
Another potential source of water emissions with RDF is the 10-20% of the MSW that is removed during processing of RDF and is landfilled instead. The air and water emissions for the rejected wastes are similar to the emissions from unprocessed MSW, but adjusted for the smaller volume. These are quantified in the "Integrated Strategy Example" later.
RDF SPREADER STOKER-FIRED ELECTRICITY PRODUCTION PLANTS
EFFECT OF PLANT CAPACITY ON CAPITAL INVESTMENT(a)
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Table 5.5 AIR EMISSIONS FROM RDF FACILITIES (Pounds per Ton MSW)(a) Mid CT (startup 1988) SEMASS (Startup 1988)(c) H Power (startup 1990)(d) Material Range Mean(b) Range Mean Range Mean Water 999(e) C0(2) 1479(e) CO 0.56-3.17 1.35 SO(2) 0.24-2.51 1.17 NO2 2.48 HCI 0.08-1.04 0.28 Total PCDD/PCDF(f) 0.2-10.7x10(-9) 4x10(-9) 2.9-3.4x10(-7) 3.1x10(-7) 1-1.6x10(-7) 1.3x10(-7) Particulates 0.026-0.052 0.039 -- -- 0.02-0.04 0.030 Metals Sb ND 1.4-4.1x10(-2) 2.0x10(-4) NA NA As ND 2.9-7.0x10(-5) 4.5x10(-5) NA NA Cd ND 1.1-1.8x10(-4) 1.3-1.7X10(-4) NA NA Cr 0.54-1.57x10(-4) 0.93x10(-4) 2.7-4.2x10(-1) 2.7x10(-4) NA NA 104NA NA Cu ND Pb 2.28-4.78x10(-4) 3.4x10(-4) 2.4-5.0x10(-3) 3.7x10(-3) 2.9-7.9x10(-6) 5.4x10(-6) Hg 0.21-1x10(-4) 0.69x10(-4) 2.4-20x10(-4) 8.4x10(-4) 0.56-1.8x10(-5) 7.3x10(-6) 104 8.4x Ni 0.16-2.28x10(-4) 0.8x10(-4) 1.1-1.4x10(-4) 1.1x10(-4) NA NA Zn 1.28-2.93x10(-4) 2.12x10(-4) (a) The NSPS regualtions are in different units; for example, the dioxin/furan limit is 30 ng/dscm. An appropriate conversion is shown in Table 5.1. Note that NSPS applies to new plants; these existing plants need to conform with less stringent regulatory "guidelines." (b) Source: Kilgroe and Brna, 1990; Hartman, 1991(a); Hartman, 1991(b). Tests were made under a variety of conditions, but load conditions were "slightly derated" in all cases. (c) Eastmount, 1991 (d) Entropy, 1991 (e) Based on calculationsss of carbon, hydrogen, and water content . (f) Dioxins/furans measured as total-tetra through octa-chlorinated dibenzo-p-dioxinsand dibenzofurans, and not as toxic equivalents. ND = Not detected; NA = Not analyzedClick here for table in WK1 format.
About 10-20% of the MSW is removed during processing of RDF. The rejected material is wet or heavy organics and dirt, with a heating value of only 3,000 Btu per pound, and it is discarded to a landfill. When the volume of material rejected during RDF preparation is combined with the volume of ash generated from burning, RDF produces a larger volume of landfilled material than mass burning, in spite of the improved recycling that occurs during RDF preparation.
Integrated Strategy Example: RDF Preparation and Combustion with Electricity Generation, Ash Disposal in a Monofill, and Landfilling Organic Rejects
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