FIGURE 5.2 Steps involved in the biochemical conversion of biomass and the thermochemical conversion (indirect route only) of coal, biomass, or combined coal and biomass into liquid transportation fuels.

Biochemical conversion typically uses enzymes to transform starch (from grains) or lignocelluloses into sugars (saccharification), which are then converted into ethanol by microorganisms (fermentation). Thermochemical conversion includes indirect liquefaction, which uses heat and steam to convert biomass or coal into carbon monoxide and hydrogen (synthesis gas). The synthesis gas can then be catalytically converted into liquid fuels such as diesel and gasoline. The CO₂ from the fermentation process in biochemical conversion or from the offgas streams of the thermochemical processes can be captured and geologically stored. Direct liquefaction of coal (not shown in Figure 5.2), which involves adding hydrogen to slurried coal at high temperatures and pressures in the presence of suitable catalysts, represents another route from coal to liquid fuels, but it is less developed than is indirect liquefaction.

The biochemical conversion of starch (from grains) to ethanol, as depicted on the left side of Figure 5.2, has been commercially deployed. But while this pro-