5.2.1 Known Catalyst Types for Biomass Gasification

5.2.1.1 The Synthesis Gas

The main product of biomass gasification is the synthesis gas. The synthesis gas is produced in the presence of steam. The following reactions are observed during biomass gasification.

C + H2O! CO + H2(syngas) AH°298 = 323.1kJ/mol (5.1)

AH°298 = —394kJ/mol (5.2)

AH°298 = 282.1 kJ/mol (5.3)

The first reaction, between carbon and steam, is strongly endothermic, pro­ducing carbon monoxide (CO) and hydrogen (H2). When the coke bed has cooled to a temperature at which the endothermic reaction can no longer proceed, the steam is then replaced by a blast of air.

The reactions (5.2) and (5.3) take place, producing an exothermic reaction— forming initially carbon dioxide—raising the temperature of the coke bed— followed by the second endothermic reaction, in which the latter is converted to CO. The overall reaction is exothermic, forming ‘‘producer gas’’. Steam can then be re-injected, then air etc., to give an endless series of cycles until the coke is finally consumed. Producer gas has a much lower energy value, relative to syngas, primarily due to dilution with atmospheric nitrogen. Pure oxygen can be substituted for air to avoid the dilution effect, producing gas of much higher calorific value.

The synthesis gas can be used for power/heat generation or further transformed into diesel range hydrocarbons by Fischer-Tropsch synthesis. Since products of synthesis gas conversion by the Fischer-Tropsch reaction contain olefins and oxygenates, there is considerable interest in combining a Fischer-Tropsch metal, such as Fe, Co or Ru, with ZSM-5 to form a bifunctional catalyst. These catalysts exhibit improved selectivity for a gasoline-range product, and synthesis gas can be converted to gasoline-range hydrocarbons in one step.