Processes that break the carbon-containing material down into gaseous products by gasification and then use those to produce liquid fuels are referred to as indirect processes to distinguish them from “direct” processes that break coal down into liquid products without going through gaseous intermediates.
For the indirect route of principal interest, solid feedstock is gasified by reacting it with sufficient oxygen to increase its temperature to a point where steam can react with the remaining carbonaceous material to produce syngas, a mixture of carbon monoxide (CO) and H2. Next, the syngas is cleaned to remove contaminants—such as particles, sulfur, ammonia, and mercury—and further processed to adjust the ratio of H2 to CO by using the water–gas shift reaction. The clean syngas is then used to make either a single product, such as fertilizer or methanol, or multiple products, such as fuels, H2, steam, and electric power.
Gasification has been used commercially around the world for nearly a century by the chemical, refining, and fertilizer industries and for more than 35 years by the electric-power industry. More than 420 gasifiers are in use in some 140 facilities worldwide, including 19 plants in the United States. Gasification technologies can also be used on the vast Canadian oil-sand deposits to gasify coke or bitumen to produce H2 and to produce a substitute natural gas from America’s abundant coal resources (Furimsky, 1998). The gasification process can convert combined feedstocks, such as coal and biomass, in the same gasifier at the same time. Thermochemical conversion would use nonfood biomass feedstocks—such as lignin, cellulose, and plastic wastes—and thus would not raise issues of competition between the markets for fuel and food.
Broadly speaking, two technologies for converting synthesis gas to liquid transportation fuels have been proved on a commercial scale:
Fischer-Tropsch (FT) technology. This technology was developed in Germany in the 1920s, and commercial plants constructed there in the middle 1930s were later used to produce transportation fuel in World War II. FT technology was commercialized in the South African Synthetic Oil Corporation (Sasol) complexes beginning in the middle 1950s. The process involves the catalytic conversion of the H2 and CO in synthesis gas into fuel-range hydrocarbons, such as diesel or gaso-