Model-based Optimisation of Integrated Algae Biorefinery

Abstract

With conventional energy reserves diminishing and atmospheric pollution increasing due to excessive usages of conventional fuel products, we need to switch to cleaner replacements of energy urgently. In this respect, microalgae have many advantages over other bio-energy options. Microalgae can yield very high lipid content, which can be processed to biodiesel. However, the major concern for large scale application of microalgae as an energy-resource is the relatively high cost associated with the entire process. The presence of several batch processes, which are inherently dynamic in nature, provides a significant challenge. Thus, optimisation study needs to be carried out to ensure efficient and effective application of diverse potential of microalgae in an economical way. Model-based optimisation provides the necessary tools to achieve this goal. In this work, we have studied various alternatives for algae growth and harvesting steps. An economic model has been formulated with cost data of various alternatives. The model was applied to a case of fixed daily biodiesel demand of 230550 kg, with the objective of net annualised life cycle cost minimisation. The optimisation model developed using the software of General Algebraic Modeling System (GAMS) problem was solved using the CPLEX solver. The optimal process sequence obtained was growth in 1095 numbers of ponds of 100000 m 2 for 20 days with chicken waste as nutrient, followed by settling in a series of two settling tanks without the aid of any flocculant. 19 primary tanks of 25000 m 3 capacity each and one secondary tank of 24000 m 3 capacity were selected by the model. The subsequent centrifugation of the resulting solution was carried out by 43 centrifuges of capacity 0.01009 m 3 /sec. (160 gallons per minute) each. The computed net cost of growth and harvesting steps of biodiesel production was 1.446 $/kg (1.258 $/l).