Wei, Xinyu; (2020) Evaluating alternative routes of using algal biomass for energy production. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Biomass is an excellent renewable energy source for alleviating the energy crisis and reducing the GHG emissions from the combustion of fossil fuels. Since the microalgae can accumulate lipids in the cell, the concept of using algal biomass for biodiesel production has attracted significant attention in recent years. The advantages of using microalgae as a biofuel source are that these organisms have a high growth rate, high lipid content and low requirement for nutrition compared to oil-producing plants. The most important property is that they can absorb CO2 from the atmosphere thus contributing to an overall reduction of GHG emissions. The existing downstream processes of biodiesel production are complex and costly, which makes the product not compatible with other bioenergy sources. Therefore, it is important to develop alternative routes of utilising algal biomass for energy production. This study mainly developed a novel method to deliver algal biomass through the air intake into a diesel engine for energy production and evaluated the environmental impact of the processes. A correlation between engine work produced during aspiration and combustion of algae biomass suspension was found. At constant flowrate and greater than 5% of biomass concentrations, a contribution to energy release during combustion from the aspirated algae could be observed. However, the aspiration of low concentration biomass suspension produced a negative impact on engine performance relative to water-only aspiration. From the perspective of the impact on the environment, the increase of biomass concentration and aspiration flowrates in the life cycle impact assessment of the whole process show a proportional increase in GWP, NOx and water consumption. Contribution analysis indicates that changing the biomass concentration has a greater impact on the environment than changing the aspiration flowrate and the proportion of total impact of the DSP steps increase with the increase of biomass concentration. Moreover, as an alternative way of using algae/diesel emulsion for energy generation, the results of the investigation on stability and viscosity of algae/diesel emulsion show the maximum stable time is 8.5 hours with the relative low viscosity of 49.9 mPa·s. However, the engine test of the stable emulsion was not successful due to the injector failure caused by phase separation of the emulsion under high pressure in the modern diesel engine. Overall, these findings demonstrate the feasibility of utilising the energy content of algal biomass in combustion without lipid extraction, with added benefits of reducing NOx emissions. It therefore gives a foundation for further exploration and development of algal biomass-derived bioenergy production.

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