The first combustion tests of CWMs—also known as coal-water slurries (CWSs)—were conducted in the United States, Germany, and the former Soviet Union in the 1960s. There was active development of CWMs in the United States in the 1980s, with emphasis on developing technologies to prepare mixtures with desirable physical and chemical properties, demonstrating retrofit in existing boilers, and developing specialized equipment for handling and transporting slurries. During this period, a number of private companies were actively involved in, or planned to enter, the CWS business. All have subsequently gone out of business or abandoned commercialization of slurries as oil prices declined in the early 1980s.
Areas for further performance improvements in COMs depend on advanced coal beneficiation to further reduce sulfur and ash content and improved additives or other means of increasing the weight percentage of coal in the mixture. CWSs also are a potential alternative to premium fuels (oil and gas) being used in industrial and utility boilers and were offered commercially in the early 1980s. Cost studies suggest that slurries could be prepared and used economically with oil prices around $25 to $30/bbl, given a production facility of sufficient scale and the infrastructure required to handle the fuel. Such studies also indicate that slurries are economical if the differential in cost between heavy oil and slurry is $1.50 per 106 Btu (Addy and Considine, 1994). Present oil price forecasts, however, make it unlikely that coal-based substitutes will be competitive in the near to mid-term. Nevertheless, one Pennsylvania utility (Penelec) is currently investigating cofiring its pulverized coal utility boilers with a CWS to provide 20 to 40 percent of fuel needs (Battista et al., 1994). This technology would allow the utility to purchase and utilize fine upgraded coal while reducing NOx emissions with no boiler derating.
Much of the current work on coal-liquid mixtures is being funded, at least in part, by DOE. Activities range from fundamental research on mixture preparation and properties, through bench-scale preparation and combustion, to commercial-scale demonstrations. The emphasis in all these programs is on CWSs rather than COMs.
Fundamental research on CWSs is being conducted at Adelphi University, Carnegie Mellon University, and Texas A&M University under the Coal Utilization Science program of DOE's AR&TD activity. Topics under investigation include the combustion system atomization processes, modeling, and measurement of viscosity and surface properties. The Pennsylvania State University is conducting a superclean CWS program with support from DOE and the Commonwealth of Pennsylvania to determine the capability of firing such slurries in an industrial boiler designed for firing heavy fuel oils, with no adverse impact on