Modelling of a 50 MWth on-sun reactor for SCWG of algae: Understanding the design constraints

Thermochemical conversion of algae, using solar-assisted supercritical water gasification (SCWG), into liquid fuels is a promising route for reducing the greenhouse gas (GHG) emissions associated with the transportation sector. This study attempts to combine CST and SCWG, and identify the key constraints in designing a solar cavity receiver/reactor for on-sun SCWG of algae. A detailed model of a tubular plug-flow receiver/reactor, operating at 24 MPa and 400–600°C with a solar input of 50 MWth is presented here. An optimised polar heliostat field is designed using SolarPILOT, for developing a pseudo-steady state model for summer solstice noon conditions in Geraldton, WA, Australia. Monte-Carlo Ray Tracing is used to determine the flux distribution on the reactor tubes and also to determine the view-factor matrix. The radiosity calculations are coupled with the hydrodynamic model, which consists of equilibrium prediction, mixture property calculation, and heat/mass transfer in the reactor. A parametric evaluation of the steady state performance and quantification of the losses through wall conduction, external radiation and convection, internal convection, frictional pressure drop, mixing and chemical irreversibility, is presented. The model additionally incorporates material constraints based on the allowable stresses for a commercially available Ni-based alloys for the cavity reactor.