Due to high energy demand, renewable energy sources are developing rapidly. In the last decade, microalgae have been studied extensively as an alternative source of biomass to replace fossil fuels. The main advantages of using microalgae for biogas production are high photosynthetic efficiency, low water consumption, their rapid growth, CO2 consumption and relatively low surface area for cultivation. They can be cultivated in open ponds or closed photobioreactors (PBR). Anaerobic digestion of microalgae is an established biological process for biogas production. Refined biogas can be used to produce thermal and electric energy. The final product consists of methane (40-75%), carbon dioxide (15-60%) and other gas traces. Biogas production follow several steps, including cultivation, harvesting, pretreatment and final anaerobic digestion. Anaerobic digestion, which takes place under controlled conditions without oxygen, is carried out in the digestate in four steps: (1) hydrolysis, (2) acidogenesis, (3) acetogenesis and (4) methanogenesis. Because of significant differences in biomethane yield due to the diverse structure of microalgae, selection of microalgae species is very important for process optimisation. The efficiency of the process is affected by mixing, temperature, pH level, carbon to nitrogen ratio (C/N), inhibitors, organic biomass loading and hydraulic retention time (HRT). The main challenges at the industrial level that need to be improved are low C/N ratio and tough microalgae cell wall biodegradability, which can be overcome by effective thermal, mechanical, chemical or biological pretreatment. The aim of the thesis is to describe the process of biogas production, to determine the factors affecting anaerobic digestion and to compare different pretreatments of microalgae biomass.
|
