Process concepts to produce syngas for Fischer–Tropsch fuels by solar thermochemical splitting of water and/or CO2

Abstract

Process concepts for making synthesis gas by solar thermochemical cycling have been screened. The produced gas is delivered at 70°C, 5bar pressure and with a H2/CO ratio of 2.0 which, with moderate adjustments, fit specifications of Fischer–Tropsch synthesis using a cobalt-based catalyst. The cobalt ferrite/alumina assisted hercynite cycle run adiabatically between 1350 and 1330°C was selected for the study. HYSYS process simulations comprise six process combinations; quenching of hydrogen with CO2; separate solar splitting of water and CO2; vacuum depletion of oxygen in cobalt ferrite; flushing out oxygen with nitrogen produced cryogenically; and flushing with nitrogen from high temperature dense membranes. Calculated energy demand eliminates cryogenic concepts whereas the dense membranes require further study. Most promising is the water-only splitting pathway in combination with CO2 quench and vacuum oxygen depletion. The alternative with separate H2O and CO2 solar splitting benefits from half the amount of inert CO2 in the produced syngas and 7% reduction in heliostat size, but suffers from technical challenges with high temperature heat exchangers and severe coking probability; and calculated 7% higher installed cost. The novel chemical quench method limits the use of high temperature heat exchangers to just one for the depleted oxygen.