TABLE OF CONTENTS When looking at biomass conversion it is instructive to look at coal conversion, as there are many similarities. This is not so surprising, since biomass can be considered as a very young coal. For coal gasification the minimum temperature required is about 900 C, as is demonstrated in the old water-gas process in which the temperature during the steam run was allowed to drop from the maximum of 1300 C to 900 C. About the same minimum temperature of 800 900 C is required to gasify the most refractory part of almost any biomass. In other words, the temperature required for the complete thermal gasification of biomass is of the same order of magnitude as for coal. This high temperature, in combination with the impurities, whether sulfur or ash components, is why indirectly heated coal and biomass gasification processes in which external heat has to be transferred via a metal surface have not yet achieved any commercial success.
On the other hand, there are a number of significant differences between coal gasification and biomass gasification, which are directly attributable to the nature of the feedstock. First, there is the quality of the biomass ash, which has a comparatively low melting point, but in the molten state is very aggressive. Secondly, there is the generally high reactivity (see Figure 3.3) of biomass. Furthermore, particularly with vegetable biomass, there is its fibrous characteristic. Finally, there is the fact that, particularly in the lower temperature range, biomass gasification has a very high tar make (Milne
