Most of the available literature on catalytic gas cleaning for biomass gasification involves nickel catalysts. The nickel catalysts are typically supported on materials such as a-alumina, magnesia, magnesium aluminum spinel and calcined zirconia. They perform best as secondary catalysts located in a separate reactor downstream, which can be operated under different conditions than those of the gasifier. Tar contents of 4mg/m3N and less have been reported (Aznar, 1993). The Ni-based catalysts designed for steam reforming of heavy hydrocarbons seems to be active for tar removal (Arauza, 1997).

In principle no tar is formed as long as the Ni-catalyst is active, deactivation is due to sulfur poisoning, carbon fouling and (thermal) sintering of the nickel particles. The problem with sulfur poisoning increases with pressure and secondary Ni-based fixed beds tends to deactivate easier than the secondary fluidized ones.

Carbon deposition, and hence, catalyst deactivation may be reduced by introducing a guard bed of dolomite or by adding dopants to the catalyst, such as lanthanum (Sutton, 2001). By using a guard bed of dolomite, the removal of tar up to 95% can be achieved, followed by the adjustment of the gas composition and final tar cracking using a second catalytic nickel bed.

The nickel-based catalysts are commercially available and effective in the removal of hydro­carbons and adjustment of the gas composition to syngas quality. However, the nickel catalysts have potential drawbacks vis-a-vis cost, intolerance to oxygen breakthrough and disposal.