Studies on the Thermochemical Cycle for Heat/Chemical Energy Conversion by a hybrid use of Natural Gas/Unutilized Heat

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Thermochemical redox Cycles for converting methane to calorie-upgraded chemical fuels of H_2, CO and C_2-hydrocarbon were investigated for the purpose of efficiently converting unutilized heat, such as concentrated solar heat, to chemical fuels below 1273 K.The following thermochemical cycles using a metal-oxide redox system were examined.1. Methane reforming/Water Decomposition CycleCH_4+M_xO_y→M_xO_<y-m-n-1>+(1-n)CO+nCO_2+(2-m)H_2+mH_2OM_xO_<y-m-n-1>+(m+n+1)H_2O→M_xO_y+(m+n+1)H_2The reactive working materials of WO_3/ZrO_2 and Ni_<0.39>Fe_<2.61>O_4/ZrO_2 were prepared for this redox cycle. Especially, the cycle using Ni_<0.39>Fe_<2.61>O_4/ZrO_2 could be repeated below 1173K, yielding 40-60% methane conversion. The syngas produced in the first reaction step has a H_2/CO molar ratio of about two, which is suitable for methanol production. Also almost pure hydrogen was obtained in a separate second step.2. Methane Coupling/Water Decomposition Cycle2CH_4+M_xO_y→M_xO_<y-2>+C_2H_4+2H_2OM_xO_<y-2>+2H_2O→M_xO_y+2H_2The reactive working material of SnO_2/Fe_3O_4/SiO_2 was prepared for this cycle. methane was also highly selectively converted to C_2-hydrocargon and H_2 by a single-step reaction mode of co-feeding methane and steam over SnO_2/Fe_3O_4/SiO_2 at 1173K.In comparison with earlier studies on methane coupling using steam as an oxidant, our experimental data gave the higher C_2-formation rate with a high C_2-selectivity.