Capture of industrial CO2-rich off-gas through optimized cultivation of microalgae

Abstract

The utilization of microalgae to treat carbon dioxide (CO2)-rich industrial off-gas has been suggested as both beneficial for emissions reduction and economically favourable for the production of microalgal products, such as lipids to produce biodiesel. Common sources of off-gases include coal combustion (2- 15% CO2), cement production (8-15% CO2), coke production (18-23% CO2) and ore smelting (6-7% CO2). However, industrial off-gas also commonly contains other acid gas components (typically nitrogen oxides (NOX) and sulphur dioxide (SO2)) and metals that could inhibit microalgae growth and productivity. To utilize industrial off-gas effectively in microalgae cultivation systems, a number of solutions have been proposed to overcome potential inhibitions. These include genetic modification to improve specific cellular characteristics, chemical additions, bioreactor designs and operating procedures, and bioprospecting to identify suitable acid tolerant strains. In all this work, one commonly overlooked consideration is the inoculation density of the microalgae culture, which can have significant influence on growth, and possibly lipid production withing the cell. This thesis examines inoculation density and the utilization of bioprospected (acidophilic or acid tolerant) strains to combat the inhibitions caused by acidic conditions, simulating those created through the addition of industrial off-gas. Inoculation density optimization has shown to significantly affect the growth and biomass production of microalgae. It was found that, for most microalgae strains tested, an inoculation density of 100 ml L-1 resulted in optimal growth and biomass productivity. It was, however, determined that there was no correlation between inoculation density and cellular lipid content. The most successful strain was a bioprospected Coccamyxa sp. strain. While the inoculation density affects the growth, the extent is clearly species dependent. There is a growing need for CO2 capture, and results of this thesis lead to several possible future directions for further development, including; development of an evaluation matrix for target compounds, repeat experiments with direct application of industrial off-gas, analysis of lipid quality, increasing volumes to test at pilot scale (and eventually, industrial scale), and direct testing of an optimized cyclical harvest approach