Biomass Gasifier 'Tars': Their Nature, Formation, Destruction, and Tolerance Limits in Energy Conversion Devices

The gasification of biomass to produce a fuel that can be used in boilers, internal combustion engines, turbines, and fuel cells is a topic of research, development, and demonstration for large-scale industrial and small-scale rural power applications. One much-discussed problem is the so-called 'tars' carried over in the raw gas stream from gaifiers. These tars range from heavy, condensibleorganics that plug or coke process elements (e.g., fuel valves), to light olefins and aromatics that could poison downstream catalysts or electrodes. This paper summarizes the chemical nature of tar, the tolerance levels of vatious devices for its organic makeup, and studies that have been carried out to thermally and catalytically remove tar, particularly in hot-gas cleanup. An important goalof this review is to summarize the progress that has been made in reducing or altering it in various gasifier applications.