Superstructure optimization of microalgae pretreatment for direct combustion or gasification

Abstract

Pretreatment of microalgae is a vital step preceding thermochemical valorization, necessitating an adjustable approach for its design. This study focuses on the development of a methodology for optimizing this stage by the arrangement of key unit operations, such as washing, drying, and torrefaction, with the primary goal of enhancing energy efficiency. Employing a precise methodology, we utilized simplified mathematical models that incorporated mass and energy yields for a range of technologies. Central to our approach was the application of Mixed Integer Non- Linear Programming (MINLP) to determine the optimal organization of these unit operations within a comprehensive superstructure, accounting for potential interconnections. Methodological rigor was demonstrated through a comparative analysis, contrasting our MINLP-based results with a systematic examination encompassing all possible unit operation combinations. We explored three distinct study cases, investigating the effects on optimal configurations of variations in the initial moisture content of raw biomass and introducing additional constraints on the final ash content and Lower Heating Value (LHV) of the pretreated biomass. Remarkably, results from both approaches exhibited consistent agreement across all cases, validating the optimization methodology. This validation underscores the potential for future studies in microalgae thermochemical valorization, with an emphasis on integrating an optimized pretreatment stage. In summary, our study not only sheds light on the intricacies of microalgae pretreatment but also highlights the versatility of an optimization-based approach in shaping a more energy-efficient biomass conversion process.