Generation of synthesis gas for fuels and chemicals production

Abstract: Many scientists believe that the oil production will peak in the near future, if the peak has not already occurred. Peak oil theories and uncertain future oil deliveries have stimulated interest in alternative sources of fuel and chemicals. This interest has been enhanced by concerns about energy security and about the climate change caused by emissions of carbon dioxide. The result has been increased interest in substituting fossil fuels with renewable energy sources such as wind, solar and biomass. However, this has proved particularly difficult in the transportation sector. The most likely source of renewable hydrocarbon fuels for transportation is biomass. It comes in many forms, none of which are suitable for direct use in internal combustion engines and gas turbines. Thus the biomass has to be refined to convert its energy into a more usable form. The most versatile conversion of biomass is thermochemical conversion via gasification and downstream synthesis, which allows the production of both fuels and chemicals. In the biomass gasification process, a gasifier converts the solid biomass into a gaseous product known as producer gas. The producer gas contains the desired components carbon monoxide and hydrogen, but it also contains water, carbon dioxide, lower hydrocarbons, tars and impurities that need to be removed from the gas. Reforming the tars and hydrocarbons in producer gas is difficult because of the amount of sulphur present. This thesis investigates the use of reverse-flow reactors to reform the tars and hydrocarbons in biomass generated producer gas.. Reverse-flow reactors operate by periodically reversing the direction of flow to enable high levels of heat recovery. The high heat recovery enables non-catalytic reformers to be operated at efficiencies near that of catalytic reformers. The operation of reverse-flow reactors is investigated experimentally in a tar-cracking reactor using dolomite as bed material and also theoretically using computer models. The investigations show that reverse-flow reactors have great potential, offering a chemically robust alternative to conventional reformers when operating on sulphur-containing biomass-generated producer gas. Furthermore, operation of reverse-flow tar crackers using dolomite as bed material is an efficient and viable solution for tar removal and syngas boosting. The producer gas also contains ammonia in varying amounts depending on the gasifier’s operating parameters and feedstock. Ammonia can be a poison for catalysts and, if the producer gas is burnt, will produce elevated levels of NOX in the flue gas. The selective catalytic oxidation of ammonia in synthesis gas was thus also investigated by experiments on a model synthesis gas. This thesis also covers mass and energy balance calculations to determine the efficiency and economics of synthetic fuels and chemicals plants. Several possible plant configurations were investigated, both stand-alone and integrated. The integration of a pulp and paper mill with a fuel synthesis plant is a very likely scenario as the biomass logistics are already located on-site. Another possible integration scenario involves steel plants, where large quantities of energy-rich gases are handled as off-gases in coke production. Utilisation of this off-gas coupled with biomass gasification was also investigated. In the stand-alone plants, the difference between reverse-flow reformers and conventional non-catalytic reformers was investigated as front-ends to well-head gas upgrading to produce crude oil via the Fischer-Tropsch synthesis. Furthermore a well-to-wheel comparison of synthetic natural gas, methanol, ethanol, dimethyl ether, Fischer-Tropsch diesel and synthetic gasoline was performed. The comparison used woody biomass as feedstock and computed mass and energy balances for complete plants from gasifier to fuel as well as for lignocellulosic ethanol production by fermentation. Efficiency in regard to feedstock to travel distance (Well-to-Wheel) and the cost of transportation was also investigated. Ammonia is one of the most valuable chemicals for modern agriculture. Current production is almost entirely based on fossil fuels. Thus small-scale production of ammonia from renewable feedstocks was also investigated.