Solar gasification of coal, activated carbon, coke and coal and biomass mixtures☆

Abstract Subbituminous coal, activated carbon, coke and a mixture of coal and biomass were gasified using direct solar irradiation in a 23-kW solar furnace located at the U.S. Army White Sands Missile Range, White Sands, New Mexico. The sunlight was focused directly on the coal (or alternate fuel) bed being gasified through a window in the reactor. Steam or CO 2 (in different experiments) was passed through the solar-heated coal bed where it reacted with the coal and thus formed a combustible product gas that contained the energy content of both the coal and the sunlight. More than 40 per cent of the sunlight arriving at the focus external to the reactor was chemically stored as fuel value in the product gas. Since there were considerable solar losses because of the reflectivity of the window and the window aperture being smaller than the focal-spot size, it is estimated that in excess of 60 per cent of the solar energy that entered the reactor was chemically stored. The product-gas production rate increased with increased solar power, and when steam was used for gasification, the product-gas composition and thus heating value were almost independent of solar power. A typical moisture-free gas composition was 54 per cent H 2 , 25 per cent CO, 16 per cent CO 2 , 4 per cent CH 4 and 1 per cent higher hydrocarbons. Activated carbon and a uniform mixture of coal and biomass were also gasified with similar efficiencies but slightly different product-gas compositions. Coke showed a lower solar conversion efficiency. Solar gasification offers several advantages over conventional oxygen-blown gasifiers: (1) commercial grade oxygen is not required, (2) almost twice as much gas per ton of coal can be achieved because no coal is burned to provide process heat and because the gas contains energy from both coal and the sun, and (3) the system has very low thermal inertia and is insensitive to thermal shock, making it very adaptable to rapidly changing solar conditions such as passing clouds.