Process synthesis and design of low temperature Fischer-Tropsch crude production from biomass derived syngas

The production of biofuels via a low temperature Fischer-Tropsch synthesis could potentially increase the utilization of biofuels without having to change the currently used combustion engines. Furthermore, the upgrading process needed to convert the FT-crude obtained just after the synthesis into commercial motor fuels could be done in a state-of-the-art refinery. In addition, current infrastructures would still be suitable for the distribution of the FT-fuels. \nTo gain knowledge about this synthesis, a model has been developed with particular focus on the FT-synthesis of hydrocarbons from biomass derived syngas. The general Biomass-To-Liquid process would also include the upstream gasification process which converts biomass into syngas and the further upgrading of the FT-crude into diesel and gasoline. The main features of this model are: a chain growth probability, α, dependent on temperature, H2 and CO mole fraction, a concomitant production of olefins and paraffins considered and kinetics of the FT-synthesis reaction taken into account.\nThe starting point of the modelled processes is a cleaned syngas which was previously derived from biomass through a gasification process. This syngas is then converted into a FT-crude stream. However, not all of the H2 and CO in the fresh syngas is converted in the FT-synthesis. Therefore, it can be recycled into the reactor to increase the overall conversion. Alternatively, the light hydrocarbons in the syngas obtained after crude condensation can be reformed to H2 and CO, thus increasing the fresh syngas available for the synthesis. To avoid a build-up of inert components, some of the recycled stream is purged. Four different process configurations have been modelled and analysed in this work. They differ by the way the syngas loop is handled (with and without reformer) and by the final utilization of the purge gas (simple combustion in a boiler or used to fuel a gas turbine for power production). \nThis work discusses the results of a parametric study of the different configurations in order to investigate the impact of the reactor operating temperature, pressure and of the desired CO conversion on different indicators. Within this study the product distribution has been investigated according to the characteristics required for products in the carbon ranges of interest. Catalyst amount and reactor volume needed to achieve a certain CO conversion have been calculated as well as efficiencies of the process using different system boundaries. The electricity balance of the processes has also been considered for further evaluation. The results highlight that there is a trade-off between the quality and quantity of FT-crude production and the reactor size which mainly depends on the temperature. With an increase in temperature the reactor volume decreases, however, the amount of long chain hydrocarbons decreases as well and the production of C1-4 is favoured. This gives a less valuable product stream. The same trend is applicable for the system and conversion efficiency of the modelled process. \nDue to the applied model for the chain growth probability (α) of the hydrocarbons, the pressure only has a minor impact except for the electricity consumption. It can be generally concluded that the electricity demand of the FT synthesis process increases with the pressure. It is furthermore shown that the same impact on the electricity consumption can be observed with an increase of the CO conversion within the FT reactor. Considering the impact of an upgrading process for the recirculating gas flow, it can be concluded that the utilisation of a reformer helps to a large extent to reduce the need for a water gas shift prior the synthesis step. However, with the syngas composition considered in this work (similar to that of a biomass indirect gasifier product gas) the reformer’s contribution is not enough to completely avoid this part of the system.\nThe model of the FT-reactor provided by this study can be used in the future to investigate a more complete process where the syngas production, e.g. by biomass gasification, as well as the following upgrading of the FT-crude to motor fuels is also included. The major advantage of this model with respect to other literature models is that kinetic has been taken into account.