Nickel-based catalysts have been widely used for syngas production in the petrochemical industry. These types of catalysts are very effective for the catalytic hot gas cleanup during biomass gasification. Elimination of tar is also accomplished by Ni-based catalysts with a high rate. The mechanism of tar elimination can be summarized as follows [15]. Adsorption of hydrocarbons (C1-C7) and water onto the nickel surface is the first step in tar removal. Then, the OH radicals migrate to the metal sites at suitable temperatures and this leads to the oxidation of the intermediate hydrocarbon fragments and surface carbon to CO + H2.

High tar levels on the generated gases lead to coke deposition on the nickel surface and deactivation occurs restricting the routine use of the catalyst. Regeneration of the catalysts might have a positive effect on removal of coke.

Simell et al. investigated the effect of different process parameters on sulfur poisoning of nickel catalysts in tar (toluene), ammonia and methane decomposi­tion [16]. Removing sulfur from the gas mixture leads to the recovered catalyst activity for tar removal. Not only sulfur, but also chlorine and alkali metals might show a poisoning effect.

Ni-based catalysts have also been used for the production of hydrogen-rich product gas as proposed by Wang et al. [17]. They produced significant amounts of hydrogen from acetic acid and hydroxyacetaldehyde in the presence of a Ni-based catalyst. In addition, noble metal catalysts such as Ru, Pt and Rh are considered to be the most important catalysts in hot gas cleaning processes. They are highly effective to remove tar and to help improve the content of syngas. However, they are more expensive than nickel-based catalysts.

For example, nickel-based catalysts were reported as very effective for tar conversion in the secondary reactor at around 700-800°C, resulting in about 98% tar removal from product gas [18]. Asadullah et al. [19] used Rh/CeO2/M (M5 SiO2, Al2O3, and ZrO2) type catalysts with various compositions for the gasification of cellulose in a fluidized bed reactor at 500-700°C. Compared with the conventional nickel and dolomite catalysts and other compositions of Rh/CeO2 catalyst, Rh/CeO2/ SiO2 with 35% CeO2 was found to be the best catalyst with respect to the carbon conversion to gas and product distribution. Addition of steam contributed to the complete conversion of cellulose to gas even at 600°C. Moreover, although they directly used the catalyst in the primary reactor, tar formation was not observed. This is an encouraging result because even if the use of catalyst in the primary reactor offers the benefit of simplification of the overall process, there are very few studies focusing on the direct use of catalysts in the primary bed due to severe catalyst deactivation. Ni-based catalysts are regarded as popular and also very effective for hot gas cleaning [20]. The recent advancement of nanocatalysts has made it possi — bleto upgrade the produced syngas and to reduce the tar formation in gasification of biomass. In a direct gasification of sawdust, Li et al. [21] used nano-Ni catalyst (NiO/ g-Al2O3), and demonstrated that their catalyst can significantly improve the quality of the produced gas and meanwhile efficiently eliminate the tar generation.