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Techno-economic and environmental assessment of milking of microalgae for renewable hydrocarbon production

Chaudry, SofiaORCID: 0000-0003-1508-8626 (2018) Techno-economic and environmental assessment of milking of microalgae for renewable hydrocarbon production. PhD thesis, Murdoch University.

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Abstract

Fossil fuels are the finite resources of fuels produced naturally from the remaining of living organisms in millions of years. Most of the world's energy demand is fulfilled by the fossil fuels in the form of heat and electricity generation, as well as transportation fuels. The increased use of fossil fuel to fulfill the rising energy demand of the world is not only depleting their finite resources but also causing the environmental damage due to the accumulation of CO2 into our atmosphere, a major cause of global warming.

Microalgae are one of the potential resources which offer solution for CO2 mitigation and global energy demand. They produce energy-rich contents (mainly oils/lipids) which can be converted to fuel. Various pathways of producing fuel from microalgae have been proposed in the literature. However, all of these pathways are very cost and energy intensive making the algal fuel production unfeasible on the commercial scale.

A unique species of microalgae, Botryococcus braunii, is known to produce long chain hydrocarbons similar to the fossil oil in contrast to the other algal species which produce lipids. The B. braunii hydrocarbons can be utilized to produce high quality fuel or fuel based products. However, the slow growth rate of B. braunii makes it an unlikely candidate for the conventional algal fuel production process due to the cost and energy intensive nature of the process. One of the unique characteristics of B. braunii is the repeated production of hydrocarbons after their non-destructive extraction - the process is called milking. Recycling of microalgae as in case of milking for repeated production of hydrocarbons not only reduces the fertilizer consumption of the process required for the growth of microalgae but also reduces the requirement of fast growth of microalgae.

This study targets to determine the potential of milking of B. braunii as a pathway of renewable hydrocarbon production on a commercial scale by conducting the techno-economic and the environmental assessment of the process. The process modeling, superstructure optimization, plant design and economics, and life cycle analysis are the various methods used for the analysis of the milking process. Aspen Custom Modeler, GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) and Microsoft Excel are the software used for this work. The superstructure is developed for the milking process using all possible technology options for each stage of the process. The process models are developed for each flowsheet configuration in the superstructure. The final flowsheet has been selected by the superstructure optimization with an objective function to maximize the net energy gain of the process. The layout of the plant has been proposed for the commercial-scale production of B. braunii hydrocarbons. The economics of the process has been established using discounted cash flow analysis. Greenhouse gas (GHG) emissions and the total fossil energy consumption of the process have been estimated using life cycle analysis methods. The minimum sales price for B. braunii hydrocarbons is estimated to be US$3.20/L for 30-year project life. The life cycle energy return on investment is found to be 1.04 MJ produced/MJ fossil energy consumed, and the GHG emissions of the process are estimated to be -0.90 kg CO2-eq/kg B. braunii hydrocarbons. The results show that in spite of slow growth rate, B. braunii has no less potential for renewable fuel production than any other fast-growing algal species. Given the current fossil fuel prices are very low, the economics of microalgal biorefinery, in general, is not very attractive today. Keeping in view the global need of CO2 mitigation and the rising energy demand in parallel to continued research and development in the field, it is anticipated that the future of microalgal based biorefinery is not very far.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor(s): Bahri, Parisa and Moheimani, Navid
URI: http://researchrepository.murdoch.edu.au/id/eprint/41578
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