Physicochemical and thermodynamic investigation of the UT-3 hydrogen production cycle: A new technological assessment

The previous study of the UT-3 thermochemical cycle was based on laboratory-scale tests to assess the feasibility of each step. In order to enclose the reactants in the experimental vessel the study was carried out on pellets that had to be prepared according to a specific protocol. It would appear difficult to design and operate a cost-effective industrial process using this technique, and it could be worthwhile to consider another approach. This paper identifies problem areas in the cycle as expressed below:(A)CaO+Br2→CaBr2+12O2,(B)CaBr2+H2O→CaO+2HBrCaBr2+H2O→CaO+2HBr,(C)Fe3O4+8HBr→3FeBr2+4H2O+Br2Fe3O4+8HBr→3FeBr2+4H2O+Br2,(D)3FeBr2+4H2O→Fe3O4+6HBr+H23FeBr2+4H2O→Fe3O4+6HBr+H2.A thermodynamic approach predicts that the first and third reactions will occur easily, but a physicochemical approach predicts some difficulties due to sintering of the solid reactants. With regard to the second and fourth reactions, thermodynamics would dictate operation at low pressure and high temperature, in contradiction with the volatility of the bromides. One solution could be to ensure superstoichiometric conditions, which assumes a large quantity of gas. Based on these considerations, and on the fact that all the steps of this thermodynamic cycle are gas–solid reactions, a fluidized bed process is proposed with continuous feed of the gaseous reactants. The efficiency of the process is discussed.