Techno-economic assessment of solar thermal and alternative energy integration in supercritical water gasification of microalgae

Abstract

Supercritical water gasification (SCWG) as a pathway for biofuel production, presents an attractive option for processing assorted biomass feedstock due to its ability to process wet feedstock with higher hydrogen yield. However, SCWG requires significant thermal energy input to reach its high operating temperatures leading to overall process inefficiency. Although previous studies have found microalgae-based SCWG process to be uneconomic, investigations on the choice of energy integration and process-energy configuration options to make the process economically viable has not been considered. As such, this work focused on the economic viability of integrating energy alternatives such as solar thermal options (solar tower and parabolic trough collector), natural gas (NG), and electricity in a microalgae-based SCWG fuel plant. The syngas fuel plant of 19,000 t/y was modelled in Aspen Plus using nine different energy configurations of solar and conventional energy. The economic results of the energy integrations delivered minimum fuel selling prices (MFSPs) of 52–73 AUD/GJ. The most economic options for supplying energy to the plant were shown to be NG alone or using solar parabolic trough combined with supplementary NG to further boost the feedstock temperature in the SCWG reactor. The annual variable operational cost was found to contribute 77% towards MFSP and was highly dependent on the price of microalgae and hydrogen. At higher capital cost, MFSP was affected by the discount rate and the cost of energy which are susceptible to the cost of solar energy infrastructure or price of conventional energy. Sensitivity and switch value analysis showed that natural gas and parabolic trough-natural gas energy integrations could deliver lower MFSP at considerable reductions in annual operational cost and the solar energy components of the annual capital cost. The choice of energy option and process-energy configuration was demonstrated to play a significant role in making SCWG process economical. While solar tower technology may not be viable for high temperature process plant energy utilization, combining solar parabolic trough with natural gas was determined to be a more promising outcome for lower energy cost. Higher prices of conventional energy, carbon tax credit and government policies could make the SCWG technology with algae feedstock economically viable and 100% renewable in the future.