Difficulty in effective removal of syngas tars continues to be one of the biggest barriers to commercialization of gasification-based technologies for power, fuels and chemicals production. Tar is a mixture of condensable organic compound resulted from thermal degradation of biomass and is composed of mostly oxygenated aromatic hydrocarbons (Abu El-Rub, Bramer and Brem 2004). Benzene is generally not considered a tar compound because it is in gaseous form at temperature above 100°C and it does not create clogging problem. Syngas tar content generated from biomass gasification varies from 1 to 100 g/m3 depending on the type of gasifier, biomass properties and gasification conditions (Milne, Evans, and Abatzoglou 1998). Removal of tar from syngas is accomplished through either cracking the tar with high temperature (>600°C) in presence of catalysts (hot gas cleaning) or condensing the tar with solvents such as water, alcohols and oil in a scrubbing unit (cold gas cleaning). Cracking tar results in CO, H2 and other light gases leading to improved syngas composition. However, use of high temperature and catalysts increases the operational cost. Similarly, scrubbing tar with solvents also results in contaminated solvents which need to be treated for recycling. Cost effective and environmental friendly gas technologies are needed for effective removal of syngas tar.

Other contaminants in the syngas include NH3 and H2S. NH3 and H2S are especially problematic if the syngas is to be used for catalytic conversion into fuels and chemicals. Levels of contaminants that can be tolerated by the downstream applications depend on the specific application. For conversion of syngas into fuels and chemicals such as Fischer-Tropsch (FT) hydrocarbon, methanol, and ammonia, the level of sulfur-based contaminants must be below 1 ppm to prevent poisoning of catalysts. NH3 for FT process is acceptable up to 10 ppm Physical and chemical scrubbing systems are commercially available to remove sulfur and nitrogen-based contaminants (Spath and Dayton 2003).