A techno-economic, sustainability and experimental assessment of the direct methanation of biodiesel waste glycerol

Crude glycerol from biodiesel production is a potential feedstock for energy in many parts of the world. A concept that is yet to be explored is the conversion of glycerol to a methane rich gas known, in this case, as bio-SNG, by taking advantage of steam reforming and methanation reactions in a single reactor. This process is known as direct methanation. Direct methanation is a type of low temperature steam reforming and is performed at temperatures lower than those commonly used for hydrogen production from organic feedstock. When applied to glycerol, it is termed ‘GLT-SR’ in this thesis. GLT-SR allows glycerol to be transformed into a gaseous energy vector that can be converted to energy on-site at the biodiesel refinery to offset fossil natural gas usage, for instance, in the energy intensive bean or seed crushing stage. The present work identifies the feasibility of a GLT-SR process using a design process framework and included process modelling in Aspen Plus, technoeconomic and environmental energy life cycle impacts analysis and laboratory scale experiments using pure glycerol to avoid any unknown impacts of contaminants contained in crude glycerol. Based on the thermodynamic analysis and process model, the optimum conditions favouring methane production and energy efficiency were 8 atm, with a feed molar steam to carbon ratio of 2.56 and an inlet temperature of 474 K. When compared to natural gas, bio-SNG from soybean based glycerol had the potential to decrease global warming potential with a trade-off of increased eutrophication, terrestrial ecotoxicity and freshwater aquatic ecotoxicity potential. The economic analysis based on biodiesel plants located in the USA, determined that a gas price of USD$6-7 per million BTU was necessary to achieve acceptable rates of return and coincided with the states of Missouri and Arkansas. A gasification rig was constructed and laboratory experiments confirmed that reducing the temperature were essential to maximising methane production. At steady state roughly 90% of the glycerol was converted to carbon gases with the most effective conditions achieving 66% of the CH4 conversion at equilibrium at 673 K (400 °C), steam to carbon ratio 2.5, pressure 1 atm and weight hourly space velocity of 0.54 hr-1. Thus current process conditions showed the process was operating away from the desired equilibrium for maximum CH4. Based on economics and environmental analysis, the process is feasible but would rely on an optimised process to maximise CH4 production and further trials to determine the impact of glycerol contaminants