Thermochemical hydrogen generation.

The basic concepts for thermochemical hydrogen generation processes have been summarized in this article. A useful set of criteria has been described for the screening and selection of potentially viable, multistep, closedcycle thermochemical processes for hydrogen generation. Three illustrative, new closed-cycle processes have been discussed, indicating potential, overall thermal efficiencies ranging from approximately 40 to 60 percent. Combined thermochemical-electrolytic schemes also warrant further consideration. Principal technical problems in the development of such thermochemical closed-cycle and mixed-cycle processes are expected to include primarily materials compatibility, reaction kinetics, separation techniques, and heat-exchanger systems. As natural gas supplies decline and prices rise, new open-cycle thermochemical processes based on water and other fossil fuel feedstocks will be the first important new technology in supplying the growing hydrogen needs of industry for at least the next two decades. Conventional electrolysis technology does not appear to be a competitor for large-scale supplies in this century unless very low off-peak electrical power rates become available, although electrolysis will be the best technique for some small-scale uses. Further analysis will be required to determine if closed-cycle thermochemical or mixed-cycle methods will displace electrolysis or other methods as the principal technology for the production of hydrogen on a large scale for the longer term.