A comparison of direct and indirect liquefaction technologies for making fluid fuels from coal

Abstract

Direct and indirect liquefaction technologies for making synthetic liquid fuels from coal are compared. It is shown that although direct liquefaction conversion processes might be more energy-efficient, overall system efficiencies for direct and indirect liquefaction are typically comparable if end-use as well as production efficiencies are taken into account. It is shown that some synfuels derived via indirect liquefaction can outperform fuels derived from crude oil with regard to both air-pollutant and greenhouse-gas emissions, but direct liquefaction-derived synfuels cannot. Deployment now of some indirect liquefaction technologies could put coal on a track consistent with later addressing severe climate and other environmental constraints without having to abandon coal for energy, but deploying direct liquefaction technologies cannot. And finally, there are much stronger supporting technological infrastructures for indirect than for direct liquefaction technologies. Prospective costs in China for some indirect liquefaction-derived fuels are developed but not costs for direct liquefaction-based synfuels, because experience with the latter is inadequate for making meaningful cost projections. Especially promising is the outlook for the indirect liquefaction product dimethyl ether, a versatile and clean fuel that could probably be produced in China at costs competitive with crude oil-derived liquid fuels. An important finding is the potential for realizing, in the case of dimethyl ether, significant reductions in greenhouse gas emissions relative to crude oil-derived hydrocarbon fuels, even in the absence of an explicit climate change mitigation policy, when this fuel is co-produced with electricity. But this finding depends on the viability of underground storage of H2S and CO2 as an acid gas management strategy for synfuel production. Many “megascale” demonstration projects for underground CO2 storage and H2S/CO2 co-storage, along with appropriate monitoring, modeling, and scientific experiments, in alternative geological contexts, are needed to verify this prospect. It is very likely that China has some of the least-costly CO2 sources in the world for possible use in such demonstrations. It would be worthwhile to explore whether there are interesting prospective demonstration sites near one or more of these sources and to see if other countries might work with China in exploiting demonstration opportunities at such sites.