Fuels - Hydrogen From Water Thermochemical Cycles - General

Abstract

The single-step thermal dissociation of water is at a first glance the simplest reaction to split water. Nevertheless because of its unfavorable thermodynamics the reaction is one of the most challenging with respect to a realization in a process. Although water thermolysis is conceptually simple, the need of a high-temperature heat source above 2500 K for achieving a reasonable degree of dissociation and the need of an effective technique for separating H2 and O2 at high temperatures to avoid an explosive mixture of hydrogen and oxygen imposes high barriers to a technical realization. (1) H₂O = H₂ + ½ O₂ Therefore different so called thermochemical cycles were proposed to divide the single water thermolysis reaction, into several steps. Water-splitting thermochemical cycles avoid the H2/O2 separation problem and reduce the temperature level. The cycles were originally developed to use high temperature heat from nuclear power plants for hydrogen productions but recently concentrated solar radiation becomes more important as a renewable heat source.