Production of hydrogen by direct thermal decomposition of water

Abstract

This paper reports the first results of a study on the feasibility of producing hydrogen by direct thermal splitting of water by use of concentrated radiation. Relative amounts of H, O, OH, H 2, O 2 and H 2O have been computed between 1500 and 4000 K at thermodynamic equilibrium. In addition, the rate of the overall reaction has been estimated assuming a kinetic model: 90% of equilibrium concentrations are reached after about 10 −2, 10 −3 and 10 −4s at 2200, 2500 and 3000 K respectively. The dissociation experiments have been carried out in an image furnace, simulating the future use of a solar furnace. Water is injected through a zirconia nozzle heated at the focus. The hot jet containing active species is then quenched by turbulent cold jets in order to minimize recombinations. Several shapes of zirconia nozzles and quenching devices have been tested in order to maximize the net production of hydrogen, which reaches 1.7 STP l. h −1 in continuous operation. Modelling of the hot nozzle allows the calculation of the gas temperature and shows that thermodynamic equilibrium conditions are reached under certain conditions. Parallel experiments show that cooling rates up to 10 6 Ks −1 can be obtained by quenching.