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|Title:||Thermodynamic Possibilities of Pure Hydrogen Production by a Nickel or Cobalt - based Redox Chemical Looping Process at Lower Temperatures|
|Authors:||SVOBODA KAREL; POHORELY MICHAEL; WIECZOREK KAMILA; BAXTER DAVID; SCHOSGER JEAN-PIERRE|
|Citation:||Proceedings of the 34th International Conference of the Slovak Society of Chemical Engineering vol. ISBN: 978-80-227-2640-5 p. 011-1, 011-14|
|Publisher:||Slovak Society of Chemical Engineering|
|Type:||Articles in periodicals and books|
|Abstract:||The development of hydrogen based energy and transport systems needs reliable and inexpensive methods of relatively pure hydrogen production, storage and supply. Water splitting, as a relatively simple process for hydrogen production can be performed by various reactive transition metals. In such a process a metal is oxidized by water vapour (steam) under release of hydrogen. For repeated production of hydrogen by this method the resulting metal oxide(s) must be reduced again. Nickel and cobalt oxide based systems can be reduced at relatively lower temperatures (400 – 1000 K) to reactive metals (Ni or Co), which are able to react with steam and produce hydrogen. The reduction step can be based on CH4, syngas or CO reaction with nickel or cobalt oxide. Attention was concentrated on convenient conditions for reduction of the relevant oxides to metals in a range 400 – 1000 K and also on possible formation of undesired soot, carbides and carbonates as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of nickel and cobalt oxides by hydrogen, CO or even methane at such temperatures is feasible. The oxidation of Ni or Co by steam and production of hydrogen is thermodynamically the more difficult step at temperatures 400 – 900 K. It seems that formation of Ni or Co-ferrite or Ni/Co-aluminum spinels could be used for a higher hydrogen equilibrium yield. Oxidation of mixed Ni-Fe and Co-Fe metals or alloys by steam without segregation caused by preferential oxidation of Fe is critical for the ferrites. For process based on Ni/Co-aluminum spinel the reduction step to metal is critical in the cyclic process below 1000 K. Possible effects of nano-scale structure of the system on feasibility of the ferrite or aluminum spinel redox cycles are discussed. Under strongly reducing conditions with high CO concentrations/pressures formation of particularly Ni3C is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO2 containing gases enhance the formation of soot, carbides and carbonates.|
|JRC Institute:||Energy, Transport and Climate|
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