UC Riverside

The development of sustainable fuels from biomass has become important due to the depletion of fossil fuels and concerns of global climate change. Microalgae offer several advantages as sources for bioenergy; their rapid growth rate, high productivity and lipid content, ability to cultivate in places other than on farmlands, or arable land, and their ability to capture CO₂ from flue gas which can be used as a source for photosynthesis. Microalgae are able to produce a wide range of biofuels including biodiesel, methane, ethanol, hydrogen, and synthetic fuels using different conversion technologies. High energy requirements and the cost for dewatering/drying and extraction are major drawbacks. The steam hydrogasification (SHR) process is attractive as an alternative for the conversion of microalgae to biofuels because the SHR can handle wet microalgae biomass without drying or extraction and been shown to have high carbon conversion efficiencies.

The overall objective of this thesis is to investigate the feasibility of using whole microalgae directly (wet biomass) and/or using the microalgae residue after lipid extraction to produce low carbon sustainable fuel using the CE–CERT process. Steam hydrogasification can convert microalgae to a high energetic synthesis gas in the presence of steam and hydrogen. The carbon in the synthesis gas then can be converted to synthetic fuels and electricity.; The performance of steam hydrogasification of microalgae was investigated by varying parameters including gasification temperature, H₂/C and steam/biomass ratios. It was found that the operating conditions at a gasification temperature 750 °C, a steam/biomass ratio of 2 and H₂/C ratio of 1 could provide richer in methane production and high carbon conversion (65%) using microalgae as a feedstock. The hydrogen in the product gas was sufficient to maintain a self–sustained supply back to the SHR. One ton per day of microalgae biomass is expected to produce FT fuel of 1.06 barrel with an overall thermal efficiency of 27%.

The utilization of the microalgae residue from traditional transesterification and the effect of different lipid content of different microalgae using the steam hydrogasification process was investigated also. It was found that the SHR could use microalgae residue to reduce the algae waste and recover energy of about 4.9 MJ/kg dry microalgae residue. A higher lipid content would enhance the performance of the SHR in terms of carbon conversion and production of methane that resulted in more FT fuel.

Life cycle energy and greenhouse gas (GHG) emissions for the production of Fischer–Tropsch (FT) fuel derived from microalgae using the CE–CERT process were calculated and compared to microalgae to biodiesel production using transesterification. It was found that life cycle energy requirements for the microalgae biofuel production using CE–CERT process were significantly lower (about 50%) compared to the transesterification process. The life cycle analysis showed that the lowest energy consumption to be 1.96 MJ/MJ fuel (40 wt% of lipid content) compared to microalgae to biodiesel production. CE–CERT technology reduces the GHG emissions by 50–64% compared to production of conventional diesel fuel.