A solar thermochemical fuel production system integrated with fossil fuels

Thermochemical cycling (TC) is a promising means of harvesting solar energy. In a typical two-step TC, a redox- Active metal oxide (e.g., cerium dioxide, or ceria) serves as a reaction intermediate that is initially reduced at a high temperature (e.g. 1500°C, by concentrating sunlight) and then yield fuels (e.g. H₂ or CO₂) by dissociating H₂O or CO₂. Isothermal TC recently emerged as an important special case of two-step TC due to the relatively ease of heat recovery from the fluid phase (rather than the solid phase as in traditional non-isothermal TC) and thus potentially high attainable efficiencies. Such advantages shall result in much alleviated thermal (and thus mechanical) shock in reactor design and less irreversibility in concentrated solar energy utilization. The high efficiency of isothermal TC is contingent upon effective gas-phase heat recovery, which however is immature at temperatures above 1100°C. Instead of direct heat recovery via heat exchanger, we propose a novel approach of simultaneously recovering waste heat and unreacted gas (e.g. H₂O or CO₂) downstream the reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH₄). Such comprehensive utilization and the syngas produced enable the establishment of a polygeneration system for simultaneous power and methanol production with the possibility of eliminating the self-generation power-plant that is conventionally needed for methanol production. A scheme is conceptually proposed based on the type of splitting reaction in isothermal TC, and optimization of the polygeneration system is discussed with solar conversion efficiency as the objective. Fossil fuel consumption for the production of a unit mass of methanol is about 22GJ/ton, lower than typical values in current industrial processes. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream.