The potential of microalgae as a renewable energy source through synthetic fuel production has been actively researched in the recent years. However, most economic viability studies have discredited algal biomass due to the prohibitively expensive cost of algae production. In this work, a techno-economic sensitivity analysis of liquid fuels production via solar-driven non-catalytic supercritical water gasification (SCWG) of microalgae is presented. The system consists of four major parts â heliostat field for capturing the solar energy, the gasification reactor, post-processing of syngas in water-gas shift and methane reforming reactors, and production of liquid fuels using Fischer-Tropsch process. SCWG of wet algal biomass is carried out in a tubular continuous flow reactor, modelled as a series of parallel tubes inside a cavity receiver. The base case scenario has been standardized using 50 MWth input from the concentrated solar thermal plant. The plant design is optimized using exergy accounting in each unit operation and extensive heat exchange network optimization. Three scenarios for syngas refining have been considered - using steam methane reforming, partial oxidation and dry reforming, and autothermal reforming. The capital investment, operating costs, and total product values are calculated considering an operating duration of 30 years for the plant, and the data are reported based on the 2016 cost year.
The findings of this study have been compared with a recently published non-solar catalytic SCWG process, wherein a significant fraction of the product synthetics natural gas is used to provide the process heat for the gasification reactor. This substantially reduces the yield of product gas per unit mass of the expensive algae feedstock. The merits of using a renewable source (such as the sun) for driving the endothermic process are evaluated here. The current work highlights that from an economic standpoint the focus of process design should be on three aspects â maximizing the overall carbon efficiency, minimizing exergy losses and effective utilization of the captured solar energy.