uneconomical, there have been important advances in synthesis processes from industrial and government-funded R&D that allow use of the low H2/CO ratios from advanced gasifiers, increased tolerance for sulfur, and improved design of reactors for the highly exothermic methanation reaction. Processes for direct production of methane by coal pyrolysis and low-temperature catalytic gasification followed by cryogenic separation offer additional pathways.
It has been estimated (COGARN, 1987) that these newer technologies can reduce the cost of stand-alone SNG production by approximately 25 percent. However, the resulting cost will still be higher than projections by the EIA (Energy Information Administration) for natural gas wellhead prices of about $3.50/thousand cubic feet or less in 2010. Thus, development of an economic incentive for large single-product plants is not expected before the late mid- or long-term periods (2021-2040). The DOE coal program does not include major programs devoted to catalytic SNG synthesis. This seems appropriate in view of the long time horizon and the excellent capabilities outside DOE. Advanced low-temperature gasification processes, however, ultimately have the potential to increase efficiency and reduce the cost of manufacturing SNG, liquid fuels, and chemicals.
Separating the methane formed directly in gasification processes by pyrolysis and by reactions in low-temperature gasification can be achieved cryogenically or by diffusion. The latter requires advances in high-temperature selective diffusion membranes.
Methanol has been an important commodity for many years, with uses in the chemical industry and as a solvent. It can be used neat as a motor fuel and, with the requirement for inclusion of oxygenates in gasoline, its use in preparing oxygenated components by reaction with olefins has grown rapidly. Manufacture of methanol from coal is currently more expensive than manufacture from natural gas.
Methanol is made by the catalytic conversion of syngas at about 250 °C (480 °F) at 60 to 100 atmospheres pressure. Both coal and natural gas can be used as syngas sources. The current commercial processes use a fixed-bed catalytic reactor in a gas recycle loop. A wide range of mechanical designs are used to control the heat released from the reaction. New developments in methanol technology include fluidized-bed methanol synthesis and use of a liquid-phase slurry reactor for methanol synthesis. The slurry technology offers improved control of temperatures; it was developed in LaPorte, Texas, in a joint DOE/industry program.
There is relatively little industrial R&D activity on processes using syngas with low H2/CO ratios and the sulfur concentrations achievable with hot gas desulfurization. For use of coal, such a process could be less costly and more efficient than current technology and could be integrated advantageously with electricity generation in a coproduct system.