Enhancements in Biomass-to-Liquid processes: Gasification aiming at high hydrogen/carbon monoxide ratios for direct Fischer-Tropsch synthesis applications

The Fischer-Tropsch process is one of the possible routes to produce synthetic liquid fuels and chemicals employed on large scale in Gas-to-Liquid and Coal-to-Liquid processes, after appropriate transformation of respective fossil sources into syngas. However, syngas can be produced by biomass gasification as well. One of the main problems is the need to ensure high H₂/CO ratio to allow the Fischer-Tropsch reaction to occur, operation commonly achieved by resorting to a Water-gas shift step. The present work reports an experimental study on biomass gasification. The aim was to set up a more efficient Biomass-to-Liquid process, which ensures a high H₂/CO ratio syngas suitable for Fischer-Tropsch synthesis directly from biomass gasification. In this way, it is possible to remove the Water-gas shift section and consequently make biofuels production more attractive, reducing costs and plant complexity. Two types of biomass have been tested: softwood from forest residues and fast growing crops. Moreover, a bubbling fluidized bed reactor and an indirect gasifier with internal circulating fluidized bed have been used. Gasification tests have been conducted by running gasifiers with both inert and catalytic bed materials. The results have shown that, using the direct gasifier with catalytic bed and a proper configuration, H₂/CO molar ratio of 2 can been obtained even at low temperature and low steam/biomass ratio, whereas this is not the case in the indirect one. However, tests suggest that an inverted configuration would make it possible to obtain values near two also with indirect gasifiers, resulting in a new interesting approach. - Highlights: • MILENA gasifier potentialities were explored in comparison with a conventional BFB. • Aim was to find the configurations to maximize benefits and process simplification. • Biomass from two classes were tested in inert and active beds at various conditions. • High H₂/CO ratio can be obtained via a fine tuning of operating conditions for BFB. • An "inverted" MILENA could also avoid WGS besides ASU unit improving BtL efficiency.