A Perspective on Syngas from Coal

Abstract

Abstract Syngas, a mixture of hydrogen and carbon monoxide, has been produced from coal for more than 100 years. But today most syngas is produced from noncoal feedstocks, by catalytic steam reforming of natural gas and naphtha or partial oxidation of heavy hydrocarbons such as petroleum resid. Three types of syngas, characterized by their H2/CO ratio, are needed. Low ratio, H2/CO = 0·4–0·8, syngas can be used in recently developed processes such as the Liquid Phase Methanol synthesis and the Shell Fischer-Tropsch wax synthesis; moderate ratio, H2/CO = 0·8–1·5, syngas is used in the Tennessee Eastman coal based synthesis of methanol and acetic anhydride; high ratio, H2/CO = 1·8–2·5, syngas is used in traditional methanol synthesis and the Fischer-Tropsch synthesis at Sasol. Different types of gasifiers are available for the production of syngas. These include Lurgi fixed-bed dry bottom and slagging gasifiers, agglomerating fluidized-bed gasifiers, single and two-stage entrained slurry feed gasifiers, and single-stage entrained dry feed gasifiers. Each produces a syngas with a different H2/CO ratio, different amounts of hydrocarbons in the syngas, at different exit temperatures, using various steam and oxygen to coal feed ratios. The use of low rank coals, which have very low mining costs and are suitable for gasifiers which do not handle agglomerating coals, are also considered. The cost of syngas from subbituminous coal is shown to be relatively insensitive to the H2/CO ratio produced and may soon be competitive with natural gas-based syngas in some parts of the country due to the increasing demand for and cost of natural gas. Recent DOE sponsored research on three topics on the production of syngas from coal, coal gasifiers for the direct production of high hydrogen content syngas, advanced methods to separate hydrogen from syngas at elevated temperatures and biological conversion of coal to syngas, are also discussed.