Thermodynamic analysis of a solar thermochemical cycle-based direct coal liquefaction system for oil production

Two-step thermochemical cycling based on metal oxide is a promising means of harvesting solar energy, in which hydrogen could be obtained through water splitting via the reduction and oxidation reactions driven by concentrated solar heat. A new kind of solar thermochemical cycle-based direct coal liquefaction system for oil production is proposed and analyzed in this paper. The coal gasification part of the traditional coal liquefaction system for hydrogen generation is replaced by thermochemical cycling process. In addition, fuel coal consumption in the coal-fired boiler for the steam generation of the traditional direct coal liquefaction system could be reduced through the heat recovery of gas mixture or oxygen carrier of the thermochemical cycle. To reveal the characteristics of the new system, the reduction of pollutant emissions, energy efficiency and relative coal saving ratio are used as the key parameters. A large amount of SO₂, NO_x and CO₂ in this coupling system could be reduced as the coal-fired boiler part for steam generation is replaced by the heat recovery from solar thermochemical cycling. The new proposed system is optimized with the reduction temperature and different gas or solid heat recovery rate allocation of the thermochemical cycles, in order to improve the energy efficiency and simplify the system device. Solar-to-hydrogen efficiency of the two-step thermochemical cycle part could be improved by proper heat recovery and extra pure oxygen will be obtained by the reduction reaction of the thermochemical cycling part. The theoretical energy efficiency of ∼47% and the relative coal saving ratio of ∼42.6% for the proposed system (the reduction and oxidation temperatures are 1500 °C and 821 °C with 70% solid heat recovery rate from oxygen carrier for water splitting) may indicate a meaningful route for oil production by direct coal liquefaction.