technologies will contribute not only to lower fuel costs but also to improved environmental performance for a given power output. DOE's research goals for advanced power systems performance and cost were shown earlier in Table 2-3. Later sections of this chapter include discussions and assessments of goals for individual power generation technologies. Budget data are taken from the FY 1994 and FY 1995 congressional budget requests (DOE, 1993b, 1994d). The committee's comments on DOE's overall strategic objectives for advanced power systems are provided in Chapter 10.
Pulverized coal-fired electric power generation involves reducing coal size to a powder and conveying it with combustion air into a boiler where it is burned. The heat released evaporates water flowing in tubes in the boiler walls to form high-pressure, high-temperature steam, which is used to drive a turbine connected to an electric generator. The steam is then condensed back to a liquid and returned to the boiler to repeat the cycle (called the Rankine cycle). A wide range of coals may be combusted in pulverized coal boilers; however, units designed to burn a variety of coals are more costly than units using a more uniform fuel. Coal cleaning is widely practiced, usually at the mine, to reduce the coal ash and sulfur content and to raise its heating value, thus providing a more uniform fuel supply (see Chapter 5). Pulverized coal combustion has been practiced for many decades, and there is an extensive literature on boiler and system designs.
The overall efficiency of a pulverized coal power generation cycle is affected by many factors, including the thermodynamic cycle design, steam conditions (temperature and pressure), coal grind, combustion air-to-fuel ratio, fuel mixing, air leakage into the system, cooling (condenser) water temperature, and parasitic energy loads for auxiliary equipment such as grinding mills, pumps, fans, and environmental control systems. The net thermal efficiency (conversion of fuel energy to electricity leaving the plant) of U.S. coal-fired generating plants operating today averages 33 percent (EIA, 1993). However, newer state-of-the-art plants with full environmental controls have efficiencies of 38 to 42 percent, the higher values corresponding to new supercritical steam units operating in Europe (Gilbert/Commonwealth, Inc., 1992). Supercritical steam units operate at much higher temperature and pressure conditions than subcritical steam units, thus achieving higher overall efficiency. U.S. experience with early supercritical units installed in the 1960s and 1970s was generally unfavorable because of lack of operator experience and reliability and maintenance problems. Most U.S. coal