Gasification of Biomass: The Consequences of Equilibrium

A model minimising Gibbs Free Energy is used to examine the thermodynamic limits of performance of a gasifier for biomass and other alternative fuels. The minimisation of free energy is highly flexible in that it allows a large number of species to be examined. Such an equilibrium model gives insight into the differences in the behaviour of coal and biomass in gasifiers. Biomass differs from coal in terms of heating value, ash, volatile and carbon contents and the amount of elemental oxygen. The model has been used to explore, entirely from a thermodynamic viewpoint: (i) the off-gas compositions, (ii) the impact of process variables on the heat balance and when gasification is complete, (iii) the effect of different gasification agents on process performance and (iv) optimisation of the calorific value of the hot and cold gas produced. Dried sewage sludge was used as a typical biomass fuel for these simulations. For biomass fuels with a low calorific value, it is shown that co-gasification with a support-fuel of higher calorific value, for example coal, is more practicable than gasification of the biomass alone.