Biomass gasification

At first biomass gasification was primarily envisioned for heat and power production. Nowadays, the production of liquid fuels and chemicals via synthesis gas is also regarded as an interesting route. The developments in the coal and oil industry have led to three archetype (biomass) gasifiers, viz.: fixed bed, fluid bed and entrained flow. From extensions of these archetypes and combinations of them, several derived systems were developed such as slagging fixed beds, circulating fluid beds, twin reactors (indirect gasifiers), etc.1 Gasifiers operated below 900°C (low-temperature gasifiers) generate so-called fuel gas including tars. Tars are the Achilles heel of this technology; these poly-cyclic components cause, among other problems, fouling (condensation) in downstream units.9

Operation above 1300°C (high-temperature gasifiers) results in synthesis gas. Intermediate gasification temperatures of 900-1300°C are unfavorable because the ashes in the feed become partly molten/partly solid — a situation that is almost impossible to handle in a reactor. Both fuel gas and synthesis gas need cleaning (removal of e. g. S, Cl, and alkalis) before entering a catalytic downstream conversion step. Biomass gasification is basically the same technology as coal and oil gasification, except gasification (reforming) processes for very wet feeds which are developed especially for biomass. Differences are: (i) the oxygen content of biomass, (ii) the differences in ash (mineral) composition and amount, and (iii) the reactivity. The differences in reactivity become clear when analyzing the main gas-producing step: in coal gasification, gas is produced by the heterogonous reaction of solid carbon with H2O and/or CO2, while for a solid biomass the majority of the gas comes directly from depolymerization/devolatilization reactions of the feedstock. Complete reviews on biomass gasification and the associated problems are those of Beenackers and Van Swaaij,10 Maniatis,11 Knoef,12 and Stassen et al?