The solar thermal gasification of coal — energy conversion efficiency and CO2 mitigation potential

The steam-gasification of coal (peat, lignite, bituminous, and anthracite) into syngas is investigated using concentrated solar energy as the source of high-temperature process heat. The advantages of the solar-driven process are threefold: (1) the discharge of pollutants is avoided; (2) the gaseous products are not contaminated by combustion byproducts; and (3) the calorific value of the fuel is upgraded. A second-law analysis is carried out for a blackbody solar cavity-receiver/reactor operated at 1350 K and subjected to a mean solar flux concentration ratio of 2000. Two technically viable routes for generating electricity using the gasification products are examined: (1) syngas is used to fuel a 55%-efficient combined Brayton–Rankine cycle; and (2) syngas is further processed to H2 (by water-gas shift reaction followed by H2/CO2 separation) which is used to fuel a 65%-efficient fuel cell. The maximum exergy efficiency, defined as the ratio of electric power output to the thermal power input (solar power+heating value of reactants), reaches 50% for the combined cycle route and 46% for the fuel cell route. Both of these routes offer a net gain in the electrical output by a factor varying in the range 1.7–1.9, depending on the coal type and the power generation route, vis-à-vis the direct use of coal for fueling a 35%-efficient Rankine cycle. Specific CO2 emissions amounts to 0.49–0.56 kg CO2/kWhe, about half as much as the specific emissions discharged by conventional coal-fired power plants. Solar/coal hybrid processes, such as the one examined in this paper, offer important intermediate solutions towards a sustainable energy supply system.