RDF cofiring combines two or more materials as feed to a boiler. Although other fossil fuels can be used, the combination of RDF and coal has been used by electric utilities because the two fuels are ordinarily burned in a similar manner, using the same generic equipment. The RDF can either be produced at the power plant site or be shipped from another, generally nearby location. It is more frequently produced at another location. Cofiring can permit a community to avoid the substantial cost of building a new dedicated MSW combustion facility.
Preparation of RDF was discussed in the section on RDF. This section focuses on the differences between firing RDF with coal and firing RDF alone.
For firing, RDF is combined with pulverized coal and fed to the fire box. Cyclone and semi-suspension spreader stoker feed systems are often used. The boiler, steam turbine, and electrical generation subsystems are the same as those used in conventional coal-fired electricity generating units. The fuel handling and ash removal portions of the plant are slightly larger than in conventional coal-fired units because cofiring of RDF requires larger fuel volumes and produces larger quantities of ash. The conventional SO(x) and particulate removal systems installed on the original units for burning coal usually require no modification when the plant is converted for cofiring. Electrostatic precipitators are reported to have lower efficiencies in cofiring plants, but the efficiency of bag houses is not affected by cofiring (McGowin, 1991).
At present the characteristics of the fuel mixture and those of the fire box and boiler limit the amount of RDF that can be burned. The boiler output of the unit is generally kept near or above 50% of the nominal unit capacity before RDF is added to avoid flame loss, and the heat supplied by the RDF is kept below 25% of the total needed; 10-15% is more common (McGowin, 1991).
At present, although 40 plants in the United States now make RDF and/or use RDF as a fuel source, only 3 U.S. electric utilities cofire RDF with coal to generate power. The existing cofiring plants, with a combined capacity of 564 MW, have been operating for some years, as shown in Exhibit I. Six other units with a combined capacity of 1,291 MW have operated in the past but have been shut down, primarily for economic reasons (see Appendix B for additional details). The most recent closure was in 1991 (McGowin, 1992). Although cofiring with coal for power production accounts for only 3% of the total RDF consumed, the three operating plants, as well as the six that have been abandoned, provide evidence that the technology can work.
Because of the extensive processing required to make RDF from MSW, RDF preparation plants tend to be fairly large; capacities of 1,000-2,000 tons per day are common. A 500 MW coal-fired power unit operating at an average of 65% of capacity and using RDF for 10% of the heat input will consume the output of a 660 ton per day RDF installation. Efficient use of RDF will require utility plants in the 500 to 1,000 MW range, perhaps configured as multiple units to ensure continuous use of the RDF produced.
Utilities generally have little economic or operational incentive to convert coal-fired boilers to cofiring projects. Utilities have important concerns about the use or introduction of RDF into existing boilers. Some of the concerns are regulatory (see the "Environmental Releases" subsection below), but others are related to performance. Because only about 10-15% of the heat release during normal firing will come from RDF, the major concern of utilities is whether RDF might interfere with plant operations. The potential for additional problems with ash slagging and/or boiler tube erosion is a factor in utility decisions.
The use of RDF for power production displaces coal. Current utility plants have been optimized for the fuel they use. Switching to RDF will lower the efficiency of the boiler by about 2-3% (McGowin, 1991), and the utility will lose power generating capacity. The total saving of coal achieved by cofiring at 10% of the heat content, is about 5.3-5.7% when the energy consumed in preparing the RDF from MSW is included(2). This calculation neglects the costs of producing the coal. Mining and delivery of Eastern coal to the plant will require approximately 0.26 to 0.31 million Btu per ton of coal, or about 1.1-1.3% of the energy content of the coal(3). If these losses are charged to the coal-only unit's efficiency, the overall coal energy savings for cofiring RDF with the coal is about 5%.
The conversion efficiencies for cofiring RDF with coal in a boiler are substantially higher than those for other electricity generation technologies based on MSW. Even if the larger of the energy requirements for RDF processing are assumed, the efficiency of power production when RDF is cofired is higher than that from mass burning of MSW. Cofiring at a 10% heat content level is expected to be approximately 1.4% more efficient in the use of RDF than simple RDF firing(4).
The environmental regulations for any new cofired unit that uses 30% or less by weight of RDF are not subject to the standards applied to municipal waste combustors. They fall instead under the requirements for fossil-fuel-fired installations (CPR, l991d; FR, l991a). New cofired units will be expected to conform to the standards established by the Clean Air Act Amendments of 1990 (P.L. 101-549, especially 104 STAT 2584) for acid gas emissions. These generally will call for emissions lower than 1.20 pounds S0(2) and 0.45 pound NO(X) per million Btu.
Compared with firing coal alone, RDF cofiring is expected initially to produce smaller quantities of SO(2), but larger amounts of particulates and hydrogen chloride, as well as larger quantities of ash that contains metals (e.g., lead and cadmium) and organics. However, conventional pollution control technologies are expected to permit RDF cofiring units to meet federal emissions standards (McGowin, 1991).
Because of the low fuel value and density of RDF, its use would substantially increase truck traffic to the power plant. Thus, it would increase transportation-related environmental emissions in the neighborhood.
The Department of Energy (DOE) and the Electric Power Research Institute (EPRI) have developed guidelines for model cofiring projects in terms of design criteria, capital and operation and maintenance (O&M) costs, and other factors (Fiscus, 1988a-c). A summary of the results, derived from a later report (McGowin, 1991), is provided in Appendix B. The three example plants considered in the DOE and EPRI study were used with the data for two operating RDF-coal-fired plants to develop the cost estimates for this study. Details of those estimates, normalized to reflect 1991 costs, are provided in Exhibit I.
The data provided by EPRI for 1984 conditions show that the additional capital cost associated with building a new plant to cofire RDF are only slightly greater than the costs for a coal-only installation. Unit capital costs also vary with plant size and may differ by as much as 5-6% according to the coal to be burned. The additional cost for a cofired unit using an Eastern coal is estimated at $22 per kilowatt (1.8%) for a plant consisting of two 200-MW units and $17 per kilowatt (1.4%) for a plant consisting of two 500-MW units. Under financing and other conditions characteristic of utility operations, these differences in capital costs will increase the cost of electricity attributable to capital by 0.8 and 0.5 mill per kWh (10-year levelized values), respectively, for the smaller and the larger cofiring plants operating at a 65% capacity factor.
Retrofitting a coal plant with two 50-MW units to accept RDF as well is relatively more costly. The additional capital cost is $40 per kilowatt, and the resulting increase in capital charges is 1.2 mills per kWh.
The O&M costs are higher for all the example retrofitted cofiring plants, but they decrease as the size of unit increases. The higher costs estimated for the cofired units arise in part from the impurities in RDF that may increase corrosion and erosion in the boiler, cause slagging, and require additional maintenance.
Overall the retrofitted cofiring plant has a higher electricity cost of 1.2 mills per kWh generated when it is compared to a coal-only unit. On the other hand, the electricity cost per kilowatt-hour is lower for the new plants at 0.3 and 0.5 mill per kWh for the 200 and 500 MW units, if the costs of RDF fuel are excluded from the calculations.
McGowin and Hughes (undated) have also calculated the value of MSW fuel as RDF to a retrofitted 250 MW coal-cofired unit and to a dedicated MSW and RDF-only installation of the same size. Because of the larger capital cost of the entirely new facilities, the overall electricity cost is higher when MSW is disposed of in these dedicated units. To break even when the RDF is cofired in a retrofitted facility, the utility must be paid $1.65 per ton of MSW ($1.96 per ton of RDF)(5). The equivalent tipping fee was $41.65 per ton of MSW, with allowance for sale of electricity at $0.05 per kWh. The tipping fee was estimated at $115 per ton of MSW for RDF, and $74.40 per ton for direct-fired MSW.
Missing Data and Research Needs
Barriers to Widespread Use
The most significant barriers to wider use of cofiring as an MSW management strategy include:
Among the major technical problems are those of compatibility between specific coals and RDF mixtures; the influence of these combinations on ash slagging and fire box performance must be considered separately for each coal and RDF mixture used. Continued study of long-term performance and possible needs for operational modifications will be important in determining the future for cofiring installations.
The emissions from cofiring may also differ from those of coal-only plants; the possible effects of those differences are a technical, as well as a regulatory, concern.
Direct comparisons between the emissions from cofiring and those from coal-only installations are impossible at present because no data are available on:
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