Synthesis Gas Production with an Adjustable H₂/CO Ratio through the Coal Gasification Process: Effects of Coal Ranks And Methane Addition

Abstract

With the decline of oil reserves and production, the gas-to-liquids (GTL) part of Fischer–Tropsch (F-T) synthesis technology has become increasing important. Synthesis gas (H2 + CO) with a stoichiometric ratio (H2/CO) at 2 or ranging from 1 to 2 is generally used in major synthesis-gas-based chemicals production. There are growing interests in the development of an alternative technology, other than the expensive natural-gas-based catalytic process, for cost-effective production of synthesis gas with a flexible hydrogen/carbon monoxide (H2/CO) ratio. Direct production of synthesis gas using coal as a cheap feedstock is attractive but challenging due to its low H2/CO ratio of generated synthesis gas. Three typical U.S. coals of different ranks were tested in a 2.5 in. coal gasifier to investigate their gasification reactivity and adjustability on H2/CO ratio of generated synthesis gas with or without the addition of methane. Tests indicated that lower-rank coals (lignite and sub-bituminous) have higher gasification reactivity than bituminous coals. The coal gasification reactivity is correlated to its synthesis-gas yield and the total percentage of H2 and CO in the synthesis gas, but not to the H2/CO ratio. The H2/CO ratio of coal gasification was found to be correlated to the rank of coals, especially the H/C ratio of coals. Methane addition into the dense phase of the pyrolysis and gasification zone of the cogasification reactor could make the best use of methane in adjusting the H2/CO ratio of the generated synthesis gas. The maximum methane conversion efficiency, which was likely correlated to its gasification reactivity, could be achieved by 70% on average for all tested coals. The actual catalytic effect of generated coal chars on methane conversion seemed coal-dependent. The coal-gasification process benefits from methane addition and subsequent conversion on the adjustment of the H2/CO ratio of synthesis gas. The methane conversion process benefits from the use of coal chars due to their catalytic effects. This implies that there were likely synergistic effects on both.