SolarThermal Splitting of Water to Hydrogen with Co-doped-hercynite - Testing and Characterisation / Heat integration
MetadataVis full innførsel
By using concentrated solar energy, it is possible to split water to hydrogen and oxygen, or eventually CO2 to CO and oxygen, by using redox materials at temperatures between 1200 1500 °C. One of the best redox materials studied so far, Co-doped-hercynite, has been synthesised by the incipient wetness impregnation (IWI) technique, characterised by surface area/pore measurements (BET/BJH) and X-ray diffraction (XRD), and tested by thermogravimetric analysis (TGA). Successfully tested and characterised material had theoretical production capacities of H2 or CO per mass Fe similar to the ones of other Co-doped-hercynite produced earlier by more advanced techniques. Activity was confirmed for 12 subsequent isothermal redox cycles at 1400 °C. Results and considerations imply that the total yield per mass Fe is dependent on the concentration of Co. Under certain conditions, oxidation was observed to start at about 500 °C, and reduction to start at as low as at around 1000 °C, which is the lowest reduction temperature recorded for Co-doped-hercynite. During CO2 splitting cycles, weight change fluctuations seemed to occur during oxidation, which could be related to simultaneous reduction activity, and another observation indicated carbonization. Observations were also in accordance with theory regarding increasing formation rate of Co alumina compounds when calcination occurs under inert atmosphere. A kinetic study to determine the reduction reaction model was also performed, where the D3 and D4 model seemed to be the best fits. Results from X-ray diffraction strongly indicated formation of α-alumina for all samples made and calcined at only 1000 °C overnight. Apart from this, the expected phases were identified for all samples at the different calcination temperatures, and the XRD pattern of cycled material was found to be as good as identical to a previously reported cycled Co-doped-hercynite material. BET showed an increasing surface area and pore volume the higher metal loadings, but this have to be confirmed by further studies. Samples prepared by calcination at 300 °C is believed to be at least about 200 m3/g, and at least about 8 m3/g when calcined at 1000 °C in air. In addition to experiments, evaluation, testing and further development of the method used for heat integration of a particularly cost efficient process design, studied in the specialisation project prior to this master thesis, was performed. An efficient heat integration is particularly crucial for the profitability of solar thermal splitting processes. The primary cost driver for the process is likely to be the cost of heliostats, and in this supplementary work, an automated techno-economic procedure to balance heat recovery against the cost of heliostats was accounted for.