Boosting carbon efficiency of the biomass to liquid process with hydrogen from power: The effect of H2/CO ratio to the Fischer-Tropsch reactors on the production and power consumption

Carbon efficiency of a biomass to liquid process can be increased from ca. 30 to more than 90% by adding hydrogen generated from renewable power. The main reason is that in order to increase the H2/CO ratio after gasification to the value required for Fischer-Tropsch (FT) synthesis, the water gas shift reaction step can be avoided; instead a reversed water gas shift reactor is introduced to convert produced CO2 to CO. Process simulations are done for a 46 t/h FT biofuel production unit. Previous results are confirmed, and it is shown how the process can be further improved. The effect of changing the H2/CO ratio to the Fischer-Tropsch synthesis reactors is studied with the use of three different kinetic models. Keeping the CO conversion in the reactors constant at 55%, the volume of the reactors decreases with increasing H2/CO ratio because the reaction rates increase with the partial pressure of hydrogen. Concurrently, the production of C5+ products and the consumption of hydrogen increases. However, the power required per extra produced liter fuel also increases pointing at optimum conditions at a H2/CO feed ratio significantly lower than 2. The trends are the same for all three kinetic models, although one of the models is less sensitive to the hydrogen partial pressure. Finally, excess renewable energy can be transformed to FT syncrude with an efficiency of 0.8–0.88 on energy basis.