Indirect liquefaction converts coal, biomass, or mixtures of coal and biomass to liquid fuels by first gasifying the feedstocks to produce syngas, then cleaning it and adjusting its H2-to-CO ratio (whereupon it is called synthesis gas) and catalytically converting the synthesis gas using Fischer-Tropsch (FT) technology into high cetane, clean diesel, and some naphtha (which can be upgraded to gasoline). The synthesis gas can also be converted into methanol using commercial technology, and methanol-to-gasoline (MTG) technology can then be used to produce high-octane gasoline from the methanol (Figure 5.2). These technologies can be integrated with those that compress the CO2 emitted during production and store it underground—for example, in deep saline aquifers. Unlike ethanol, the gasoline and diesel produced via FT and MTG are fully compatible with the existing infrastructure and vehicle fleet.
Gasification has been used commercially worldwide for nearly a century by the chemical, refining, and fertilizer industries and for more than 10 years by the electric power industry. More than 420 gasifiers are currently in use in some 140 facilities worldwide, with 19 plants operating in the United States. Application to coal-to-liquid-fuel systems, and to combined coal-and-biomass gasification, will lead to further improvements in the technology so that it might become more robust and efficient by 2020. Gasification of biomass alone has been commercially demonstrated but requires added operational experience to render it more robust.
FT technology was first commercialized by the South African firm Sasol in the mid-1950s. Sasol now produces more than 165,000 bbl/d of transportation fuels from coal, and it has built large plants based on conversion of natural gas into synthesis gas, which is then converted into diesel and gasoline by FT. As with several other ready-to-deploy technologies, FT will likely undergo significant process improvements by 2020. For example, more robust and efficient technology for producing liquid transportation fuels, and significant catalyst improvements for coal applications, can be expected.
In technologies based on methanol synthesis, synthesis gas is converted to methanol using available commercial technology; plants as large as 6000 tons per day are currently operating. The methanol can be used directly or upgraded into high-octane gasoline using the proprietary MTG catalytic process developed by ExxonMobil and commercialized in New Zealand in the late 1980s.4 Standard