Detailed modelling of a double fluidised bed steam gasifier processing woody biomass and solid recovered fuel mixtures

Double fluidized bed gasification is based on the circulation of an inert bed between two reaction sections: the gasification reactor, where a solid feedstock, generally biomass. is converted into a syngas by steam or air oxidation; the combustor or riser, where residual char from the thermal decomposition of the feedstock is oxidized by air (in some cases with additional fuel) to provide the energy contribution for the gasification reactions and ensure autothermal operation of the system. In this work, detailed modelling of a dual fluidized bed steam gasification reactor is performed in Aspen Plus by incorporating: (1) gas, char and tar production during thermal decomposition of the feedstock according to experimental correlations developed and taken from the literature, (2) heterogeneous and homogeneous reaction kinetics in the gasifier bed and in the freeboard. The model has been validated by comparison with experimental results on different mixtures containing solid recovered fuels and woody biomass. The model is quite accurate in predicting the gas products composition for different feedstocks mixtures (root mean square error of 12% for CO, CO2, H2 and CH4) and enables coupling with different downstream liquid fuels synthesis processes (synthetic natural gas, methanol, dimethyl-ether, Fischer-Tropsch products etc.).