Single reactor concepts for both catalytic biomass gasification and pyrolysis oil steam reforming have, up till now, not been able to produce a clean gas for a long period of time. This is mainly caused by the fact that no catalyst has yet been developed which is both mechanically strong and active for a full product gas conversion or is resistant against the ‘heavies’ formation (char and coke) which is accompanied with the initial biomass conversion step. To solve these problems, both in biomass gasification as in pyrolysis oil reforming staged systems have been proposed.

Downstream gasifiers upgrading/cleaning of the gas

Staged gasification was initiated at the University of Zaragoza by Corella et al.45,46 where two fluidized beds were used, one as the biomass gasifier and the second one as a (non-catalytic and catalytic) tar converter. When using a commercial catalyst, initially a clean gas was being produced. After a period of 1-2 hours of successful operation the catalyst started to lose its activity. The heavy tar content of the product gas was identified as being the cause of this deactivation. The introduction of a guard bed with calcined dolomite showed very promising results where no catalyst deactivation was found for a 48 hours on stream.20,47 Simell et al.48 were successful in a similar approach where calcined dolomite limestone was used as a guard material with a subsequent monolith catalytic Ni-alumina bed. Although the processes have shown its technical feasibility for hours (up to 100 h) of run time the processes were not developed commercially due to the unfavorable economics of that time.

The Biomass Technology Group B. V.49 is developing a different approach where the biomass is first pyrolysed (~500°C) after which the gas/vapor mixture is subsequently reformed autothermally with the addition of air. From a temperature of around ~1000°C, the product gas is essentially methane and tar free.