Since coal combustion emits 20 to 75 percent more CO2 per unit of energy than other fossil fuels, it has been a major focus of attention and a key target for greenhouse gas reductions. Coal presently accounts for about 35 percent of total CO2 emissions globally. Emissions are projected to increase significantly in the next century, especially in developing countries such as China. Based on current estimates of natural resources, coal is the only fossil fuel with carbon reserves sufficient to dramatically increase the current carbon content of the atmosphere if burned on a widespread scale (Edmonds, 1994). Thus, there is substantial interest in the long-term trend in CO2 emissions from coal combustion and other conversion processes. Various studies have examined the potential to reduce coal-related greenhouse gas emissions (e.g., NRC, 1992).
Coal-fired electric power plants and fuel conversion processes are subject to state and federal regulations to protect the quality of surface waters, ground water, and drinking water. Stream quality standards for specific receiving waters are established by state and local jurisdictions, while the NSPS are the primary federal vehicle limiting aqueous discharges. The principal environmental concerns are thermal discharges to waterways (which are prohibited for new plants) and various chemical emissions, including heavy metals, organics, suspended solids, and other aqueous constituents found in power plant waste streams. In recent years there has been increasing attention to a large number of hazardous or toxic trace chemical species and a general tightening of effluent emission standards at existing as well as new facilities (Rubin, 1989).
Reauthorization of the Clean Water Act is expected soon, with the potential for more stringent effluent standards for coal-based electric power plants. While water-related environmental controls have not had the visibility or economic impact of air pollution controls, future restrictions could nonetheless have significant consequences for power plant siting and cost. Some advanced power generation and fuel conversion technologies, which produce a variety of aqueous discharges not found in conventional pulverized coal plants, may merit special scrutiny. Overall, the research and development (R&D) implication of current trends is that water-related environmental issues also may require additional attention to preserve or increase options and lower the cost of complying with current and future restrictions.
Water quality issues also affect other parts of the coal fuel cycle, especially coal mining and beneficiation. Acid mine drainage from coal extraction and effluents from coal preparation plants have historically been among the most serious water-related environmental problems associated with coal use. Research needs on advanced treatment technologies and improved process design to minimize or eliminate water-borne pollutants thus extend across the fuel cycle.