A review of the primary measures for tar elimination in biomass gasification processes

Tar formation is one of the major problems to deal with during biomass gasification. Tar condenses at reduced temperature, thus blocking and fouling process equipments such as engines and turbines. Considerable efforts have been directed on tar removal from fuel gas. Tar removal technologies can broadly be divided into two approaches; hot gas cleaning after the gasifier (secondary methods), and treatments inside the gasifier (primary methods). Although secondary methods are proven to be effective, treatments inside the gasifier are gaining much attention as these may eliminate the need for downstream cleanup. In primary treatment, the gasifier is optimized to produce a fuel gas with minimum tar concentration. The different approaches of primary treatment are (a) proper selection of operating parameters, (b) use of bed additive/catalyst, and (c) gasifier modifications. The operating parameters such as temperature, gasifying agent, equivalence ratio, residence time, etc. play an important role in formation and decomposition of tar. There is a potential of using some active bed additives such as dolomite, olivine, char, etc. inside the gasifier. Ni-based catalyst are reported to be very effective not only for tar reduction, but also for decreasing the amount of nitrogenous compounds such as ammonia. Also, reactor modification can improve the quality of the product gas. The concepts of two-stage gasification and secondary air injection in the gasifier are of prime importance. Some aspects of primary methods and the research and development in this area are reviewed and cited in the present paper.