For synthesis reaction, a high degree of gas purity is needed, so the gas must be cleaned of particulates and other contaminating gases. The raw syngas may contain three principal types of impurity: (1) solid particulates (unconverted char, ash); (2) inorganic impurities (halides, alkali, sulfur compounds, nitro­gen); and (3) organic impurities (tar, aromatics, carbon dioxide).

At high temperatures, the equilibrium shifts toward hydrogen-producing hydrogen-rich gas. The ash in the biomass appears as slag. At low temperatures, the ash remains in the product gas as dry ash. Cleaning has two aspects: remov­ing undesired impurities and conditioning the gas to get the right ratio of H2 and CO for the intended use. This use determines the level of cleaning and conditioning. Table 9.1 presents examples of product-gas specifications for different end uses.

Cleanup Options

For cleaning the gas of dust or particulates, there are four options: (1) cyclone, (2) fabric or other barrier filter, (3) electrostatic filter, and (4) solvent scrubber. Among organic impurities, tar is the most undesirable. The three main options for tar removal are

• Scrubbing with an organic liquid (e. g., methyl ester)

• Catalytic cracking by nickel-based catalysts or olivine sand

• High-temperature cracking

Inorganic impurities are best removed in sequence because some removal processes produce other components that need to be removed as well. First,

water quenching removes char and ash particles. Next, hydrolysis removes COS and HCN by converting them into H2S and NH3. The ammonia and halides can be washed with water, followed by adsorption of H2S, which can be removed with the wash water. Solid or liquid adsorbents are used to remove carbon dioxide from the product gas.