Process efficiency of biofuel production via gasification and Fischer-Tropsch synthesis

A thermodynamic equilibrium model was used to predict the composition of syngas produced by oxygen-blown biomass gasification at different operating conditions. The effects of temperature, pressure, moisture content, steam to biomass ratio and equivalence ratio (ratio of the amount of oxygen that is fed to the gasifier as a fraction of the oxygen required to achieve full combustion) were studied using sugarcane bagasse and pyrolysis slurry derived from sugarcane bagasse as feed. Taking kinetic limitations into account, the optimum operating conditions to maximise gasification efficiency, or to produce the stoichiometric H2/CO syngas ratio of 2, were determined for each feedstock and integrated with a process model for Fischer-Tropsch liquids production. The maximum overall process efficiency of 51%, of which 40% was in the form of Fischer-Tropsch liquids, corresponded with the maximum gasification efficiency of 75%, based on atmospheric gasification of bagasse with 5% moisture at a temperature of 1100 K, equivalence ratio of 0.25 and steam to biomass ratio of 0.75. Operating the gasifier at a steam:biomass ratio of 2.25 to yield an equilibrium H2/CO ratio of 2 increased the Fischer-Tropsch liquid yield, while inclusion of a shift reactor downstream from the gasifier had the same effect and was apparently more energy efficient. However, maximising the Fischer-Tropsch liquid yield did not necessarily increase the process thermal efficiencies. It was also observed that the thermal process energy efficiencies previously reported for Fischer-Tropsch synthesis from atmospheric biomass gasification with a shift reactor, could be improved by 10.7% by excluding the shift process, although 5% less liquid fuel energy would be produced.