System Study Towards the Integration of Indirect Biomass Gasification, Methanol and Power Production

Abstract

The transition from a fossil based economy to a greener, bio-based economy remains challenged by the gap in technological maturity between bio-based processes and conventional fossil energy. Decreasing the price difference between fossil fuels and bio-fuels remains a major constraint. With 98% of the transportation sector now depending on fossil fuels (oil) as the primary fuel, methanol produced from CO2 and other greener sources is now growing in interest as a more sustainable fuel. This thesis focused on investigating the process biomass gasification, its challenges and its integration with methanol and power production. A number of gasification technologies were studied and the Indirect gasification technology was identified to be technologically advantageous to conventional gasification technologies. The Fast Internally Circulating Fluidised Bed (FICFB) gasification technology of the University of Technology, Vienna was modeled in Aspen Plus®. Different sets of kinetics were studied to improve the accuracy of the developed model in comparison with the data from the 100kW pilot plant at Gussing. The composition of the product gas from the developed model was then validated with the experimental data from the pilot plant if the FICFB gasifier, and found to be accurate up to 3.5%. The principle of Absorption Enhanced Reforming (AER) was then simulated in the developed gasifier by changing the bed material used to dolomite. With dolomite-CO2 sorption kinetics validated with literature, the AER principle in the gasifier predicted a 4% increase in hydrogen composition in the product gas obtained in addition to a slight increase in the cold gas efficiency of the system.
Different processing steps required to convert biomass to methanol were then identified and modeled in the same Aspen Plus® model. The identified blocks, Gasifier, Gas Cleaning Unit, Methanol Synthesis and the Energy network were optimised by performing a number of sensitivity studies. The now optimised model was then used as a common tool to study the effect of choosing different technologies and parameters within the blocks on the overall process behaviour. Four different case studies were defined, each varying from each other by a difference in technology of one of the blocks. Sankey plots for each of these cases were drawn to visualize the energy losses in such complicated systems. Results of incorporating AER on the end methanol yield and the overall efficiency of the process were studied as one of the cases. Dolomite (AER) although very encouraging as a bed material during gasification, was shown to be detrimental to the methanol synthesis process when used as a common catalyst/bed material for the water gas shift reactor. Two different cases of IGCC (Integrated Gasification Combined Cycle) systems were modeled and studied by varying the gas turbine technology employed. A new technology Inverted Brayton Cycle (IBC) gas turbine was simulated against a standard IGCC system. The Inverted Brayton Cycle system was shown to closely efficient to a standard gas turbine system working on product gas from the gasifier. The scale of biomass gasification was then identified as a significant parameter, which would determine the suitable choice of gas turbine technology to be employed. The thesis concluded by discussing some of the more influential parameters observed during the course of this study and recommending further optimization and mitigation steps corresponding to each of these identified losses. The thesis served its purpose by developing a quantitative tool to compare and validate different technological solutions to improve the process of producing methanol and power from biomass.