FIGURE G-7 Depiction of the most promising sulfur thermochemical cycles for water splitting. Courtesy of Charles Forsberg, Oak Ridge National Laboratory.

tion in the capital cost may result from improved catalytic materials and higher hydrogen production capacity in each facility.

Calcium-Bromine-Iron Cycle The calcium-bromine-iron (Ca-Br, or UT-3) cycle involves solid-gas interactions that may facilitate the reagent-product separations, as opposed to the all-fluid interactions in the SI cycle, but it will introduce the problems of solids handling, support, and attrition. This process is formed of the following reactions (Doctor et al., 2002):

The thermodynamics of these reactions have been found favorable. However, the hydrogen production efficiency of the process is limited to about 40 percent, owing to the melting point of Ca-Br₂ at 760°C (Schultz et al., 2002).

Other Cycles Argonne National Laboratory’s Chemical Engineering Division is studying other cycles like the copper-chlorine thermochemical cycle. The energy efficiency of the process is projected to be 40 to 45 percent (ANL, 2003). This work is currently being investigated only by ANL, at a bench-scale R&D level, and no pilot demonstra-