James R. Katzer1
Coal was used to produce 51 percent of the electricity generated in the United States in 2004. Domestic coal reserves are far greater than those of oil or natural gas, and costs for using coal to generate electricity are much lower than for oil and natural gas. Thus, coal promises to continue its position as the primary fuel for power generation for the foreseeable future. Pennsylvania, West Virginia, and other states have large resources of coal that could be delivered to New York relatively inexpensively.
Coal can contain high concentrations of ash and substantial amounts of sulfur, in addition to other toxic elements. It thus has the potential for high emissions, but appropriate control technology can reduce these emissions to a very low level.
Large coal-fired power plants are expensive to build and require substantial infrastructure for the delivery and storage of coal and the removal of ash and other captured pollutants. A much larger area is required for a coal plant than for a natural gas combined-cycle (NGCC) plant. Thus, coal plants require careful site selection and design. Even then, their impact on the environment and local communities can be greater than that of nuclear plants.
Pulverized coal combustion is the primary technology used to generate electricity from coal. Flue-gas-treatment technology to control emissions on new coal plants is very effective in reducing criteria emissions to very low levels. Plant generating efficiency can range from about 35 percent to as high as 43 percent for ultrasupercritical steam technology.
Fluidized-bed technology is another approach to coal combustion which, compared with pulverized coal combustion, offers much broader operating flexibility with respect to coal type. It also allows the combustion of a range of other materials mixed with the coal, such as the co-firing of biomass, wood wastes, and so on. Efficiency and emissions control are similar to that of pulverized coal.
Integrated gasification combined cycle (IGCC) involves gasification of coal to produce synthesis gas, cleaning the syngas, and then burning it in a combustion turbine. The power generation block for an IGCC plant is similar to that of an NGCC plant. The syngas-burning combustion turbine is connected to a generator; the steam raised from cooling the turbine exhaust powers a steam turbine. Typical generating efficiency is about 39 percent. The technology is commercial, but issues of operability and availability need further resolution. With IGCC, emissions including mercury and other toxics can be extremely low (unlike the case of pulverized coal with current technology), because the gases are all fully contained at high pressure. Coal ash from the IGCC process is fused and exits as a much less-leachable solid than fly ash. IGCC also allows for co-firing with biomass. Gasification provides for the most effective route to the capture of carbon dioxide for sequestration, and IGCC is projected to produce the lowest-cost power from any technology with carbon dioxide capture.
Whereas coal-fired power plants produce the lowest-cost power (without carbon dioxide capture), the requirements for large sites and extensive infrastructure limit the potential for the New York City area. In addition, air emissions and other environmental and community issues are likely to create considerable opposition to them in heavily populated areas. High capital costs and uncertainty of success in construction are likely to discourage investors. Nevertheless, the potential, particularly of the advanced IGCC technology, is so great that coal should be considered an option, at least for New York’s upstate regions. The remainder of Appendix D-5 explores emissions control, probably the most contentious issue for coal plants.
Typical flue-gas-cleaning configurations for coal-fired power plants are shown in Figure D-5-1. U.S. emissions data are typically given in terms of energy input—for example, pounds per million British thermal units (Btu)—and are thus independent of generating efficiency. This does not drive generating efficiency, as would an emissions limit based on output, such as pounds per megawatt (electric)-hour (MWe-h). Emissions below are presented in milligrams per cubic meter (mg/Nm3). The pulverized coal (PC) emissions are typically for supercritical PC units that are operating at about 39 percent (higher heating value [HHV]). Those for IGCC are for a unit that has 38 to 40 percent efficiency (HHV).