Biomass To Liquids: Thermo-Economic Analysis and Multi-Objective Optimisation

Biomass is the major source of renewable carbon which can be used to substitute fossil carbon in fuels and chemical products. This thesis addresses the modelling, design and thermoeconomic evaluation and optimisation of processes converting Biomass to Liquids (BtL) for the effective exploitation of a renewable, yet limited, resource. The focus is on thermochemical conversion, through gasification and Fischer-Tropsch (F-T) synthesis, producing drop-in fuels which could be used in the current infrastructure. Given the number of potential technological options and process configurations, a methodology is developed for a systematic comparison in terms of several performance indicators. The analysis is carried out through a framework combining thermochemical modelling, economic evaluation, process integration and multi-objective optimisation. Particular attention is given to the representation of biomass and its coherent modelling in terms of its elemental composition. Experimental data from previous studies is used to develop new thermochemical models for torrefaction and for fluidized bed gasification. Other technologies included in this study are air and steam drying, entrained flow gasification, tar reforming, high temperature tar cracking, water and gas quench and radiant panels, hot and cold gas cleaning, water gas shift, high temperature steam and steam/carbon dioxide electrolysis and F-T synthesis and upgrading. The final comparison, through the multi-objective optimisation, provides a better understanding of the trade-offs of different technological options and process variables in terms of competing economic and thermodynamic objectives. For 200MWth biomass input plant capacities, production costs are in the range of 1.0-1.4 euro/l for technologies producing up to about 0.5 kJFT /kJth and close to being neutral in terms of electricity balance. For technologies using electrolysis the conversion can increase to 0.8 kJFT /kJth with production costs of 1.8 euro/l. The electricity storage capacity, in this case, is of 0.5 kJe/kJFT , corresponding to a net electricity requirement of about 0.4 kJe/kJth. This work stems from the collaboration between LTB (Biomass Technology Laboratory) of CEA/Liten in Grenoble, France, and IPESE (Industrial Process and Energy Systems Engineering) of EPFL in Lausanne, Switzerland.


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