Chapter 5:Assessment of Economic and Environmental Cost-benefits of Developed Biorefinery Schemes

In this work modelling and optimisation studies for the evaluation and improvement of several biorefinery schemes were performed. Economic and environmental objectives were considered with the aim to find configurations that maximise profits while minimising the environmental impact. Biorefineries were modelled using a commercial simulator (Aspen Plus) combined with calculations in MatLab. Optimisation studies (including multi-objective optimisation) were carried out using both stochastic (simulated annealing) and deterministic (sequential-quadratic-programming based) approaches. The results show that for a number of schemes improved profits can be found including biodiesel and biogas production in addition to supercritical CO2 extraction from wheat straws. An oil-to-methanol ratio of around 1:13 to 1:14 was found to be optimal for the biodiesel-producing transesterification reaction. The optimum conditions for the biogas case involved recycling most of the digestate, which leads to a high productivity of biogas. Supercritical CO2 extraction was found to be most profitable at around 36,000 tons per year capacity. Multi-objective optimisation was also performed in order to find how the optimal profits change when different constraints are placed on the emissions from each process. For the biodiesel cases these constraints reduced the profits by up to €10 per ton of feed. For biogas production reducing the emissions meant using more of the digestate as fertiliser. For supercritical CO2 extraction reducing emissions requires reducing the yearly capacity, which in turn reduces the profits by €63 per ton. Comparisons of the different schemes are performed using calculations of profits and emissions ‘per ton of feed’ in order to give a fair comparison of biorefinery processes operating at different scales. These comparisons showed that the most profitable schemes were those with the lowest capacity. These lower capacity schemes could be very profitable if they can be scaled up and if there is sufficient feedstock available. Comparison of the emissions for these cases showed that the most profitable schemes also have the highest emissions. This is because these cases involve either high-energy usage (usually with larger quantities of electricity) or combustion.