Conversion of coal to a gaseous fuel that can be cleaned and used in homes and commercial installations has been practiced for over 200 years. It was a major industry in the United States and abroad until the 1940s. During World War II, the manufacture of liquid fuels was practiced by Germany to provide military fuel, and in this context significant advances were made in gasification technology that formed the basis for present-day gasifiers. The increasing availability of natural gas and petroleum in the United States and other industrialized countries resulted in the replacement of coal-based town gas with natural gas or heating oil. The oil embargo of 1973 and predictions of impending natural gas shortages, however, resulted in major industry and government programs in the United States and Europe to develop gasification systems for production of SNG from coal. This effort led to pilot plant studies incorporating many of the major engineering approaches for development of superior gasification technologies. However, when petroleum and gas prices fell and it became clear that domestic resources were adequate to provide low-cost natural gas at least through the year 2000, the incentive for the construction of facilities for SNG production was eliminated, leaving a relatively few surviving commercial coal gasification systems. These were primarily aimed at manufacture of high-value products, such as methanol, ammonia, and chemicals. Today's emphasis on increased power generation efficiency, and the availability of high-performance gas turbines and fuel cells, have created a strong incentive for development of high-efficiency gasification systems specifically designed to provide fuel for power generation. These systems can differ from systems optimized to produce highly purified synthesis gas for catalytic conversion to chemicals and clean fuels in that dilution by methane and nitrogen is acceptable and a higher level of impurities can be tolerated.
The status of gasification processes of current interest that are either commercially available or have reached the stage of major pilot plant development is shown in Table 6-1.
Gasification processes can be divided into three major classes: entrained-flow, fluidized-bed, and moving fixed-bed. All involve operating pressures up to several hundred psi. For entrained-flow systems, powdered coal is generally first gasified with a mixture of steam and oxygen (or air) in a zone where the main part of the molten slag is collected. The high-temperature products require quenching or cooling prior to cleanup, with resulting loss of thermal efficiency. Entrained-flow gasification systems produce little methane, are relatively compact, and,