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 is nothing but 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 maximum 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 sulphur 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. Firstly there is the quality of 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.
Although an entrained-flow process might have an apparent attraction in being able to generate a clean, tar-free gas as required for chemical applications, and the low melting point of the ash would keep the oxidant demand low, the aggressive quality of the molten slag speaks against such a solution, whether using a refractory or a cooling membrane for containment protection. Furthermore the short residence times of entrained-flow reactors require a small particle size, to ensure full gasification of the char. No method of size reduction has yet been found, which will perform satisfactorily on fibrous biomass.
A number of fixed-bed processes have been applied to lump wood, but they are limited to this material. They would not work on straw, miscanthus or other materials generally considered for large-scale biomass production unless these were previously bricketted. Furthermore in a counter-flow gasifier, the gas would is heavily laden with tar. The alternative of co-current flow could reduce the tar problem substantially, but the necessity to maintain good control over the blast distribution in the bed restricts this solution to units of very small size.
With this background it is probably not surprising that most processes for biomass gasification use fluid beds and aim at finding a solution to the tar problem outside the gasifier. In co-firing applications where the syngas is fired in an associated large-scale fossil fuel boiler, the problem can be circumvented by maintaining the gas at a temperature above the dewpoint of the tar. This has the added advantage of bringing the heating value of the tars and the sensible heat of the hot gas into the boiler.
There are many biomass processes at various stages of development. Summaries are given in, for example, Kwant (2001) and Ciferno and Marano (2002). The selection chosen here represents generally those that have reached some degree of commercialization.