Equilibrium effects in high-pressure hydrogen production from thermochemical water-splitting cycles

When hydrogen comes into widespread use as a supplemental fuel, a chemical feedstock, and a primary, future energy carrier, it must be available at pressures of the order of 50 atmospheres. Existing studies of hydrogen transmission indicate that pipeline pressures of 50 to 100 atmospheres will yield an energy carrier system with better cost-effectiveness than underground transmission of electricity. Because high capacity hydrogen compressors have a low compression ratio, high supply pressures are required. This requirement will affect product separations steps and process heat load-line matching. The hydrogen production steps of a large number of projected water-splitting cycles were classified according to whether the sum of the mole numbers of gaseous products is larger or smaller than the sum of the mole numbers of gaseous reactants. When product mole numbers are larger, the hydrogen production step occurs at relatively high temperatures (about 600/sup 0/C or higher). When reactant mole numbers are larger, the required temperature is generally low (about 300/sup 0/C or lower). There were few exceptions, though some water-splitting cycles based on organic chemical reactions fit into a temperature range between these two categories. A group of generalized relationships are presented for hydrogen production steps such that ..delta..G, the Gibb's free energy change for the reaction, is zero. For equilibria favored by, and not favored by pressure, a series of relationships between the hydrogen mole fraction and product pressure can be defined. The trade-offs between product stream composition and input heat temperature can be assessed. The degree to which feed stream impurities can be tolerated, given fixed operating variables, can also be determined