Tar Control and Treatment Methods
Tar formation is a side reaction in biomass gasification. When the gas cools, tar condensation can foul filters and can make deposits in pipes, making tar control a high priority in biomass gasification. In fluidized-bed gasifiers the tar concentration in biomass-derived syngas is typically in the order of 10 g/m3. In these gasifiers, fouling is not a significant problem as long as all the tar is present in the gas phase. Additionally, tar content in the syngas can be controlled by means of tar prevention and treatments inside the gasifier. The primary measures for reduction and elimination of tar in biomass gasification processes are discussed in two review articles by Devi et al. [9] and Han et al. [29].
A number of techniques are known to reduce tar concentration in the syngas produced. Some common techniques are: optimization of the gasifier design, optimization of the operation conditions, addition of catalytic bed materials, and controlling the biomass properties. The use of catalyst promoters in the bed material is also a popular method for controlling the tar content in syngas. Metallic elements such as Ni or Co and their metal oxide are added as promoters to the typical bed catalyst materials dolomite, limestone, olivine sand, bauxite, natural alumina, clay minerals and iron ore. Internal reforming of tars by inclusion of a catalytic hot gas filter in the freeboard of the gasifier and at the gasifier outlet is a smart technology for tar reduction. Monolith reactors or ceramic blocks containing a honeycomb structure with a thin layer of catalytically active material on the channel walls such as a Ni-based coating, have also been used in the internal reforming of tars [2].
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