A MEMBRANE REACTOR CONCEPT FOR SIMULTANEOUS PRODUCTION OF PURE HYDROGEN AND SYNTHESIS GAS OVER CERAMIC MATERIALS

摘要

The thermochemical water splitting, in particular when combined to solar energy supply, is one of the most promising routes for long-term sustainable hydrogen production. The technology however has major drawbacks such as the batchwise non-isothermal character of the process and the high temperatures required for material regeneration. In this paper experimental demonstration is provided of a novel concept for hydrogen production by a two-step thermochemical water splitting process simultaneously with synthesis gas production by a stepwise redox methane oxidation reaction. It is based on a dense membrane reactor which ensures isothermal and continuous production of high purity hydrogen. Perovskite materials having the formula La1-xSrxFeO3 are investigated as potential redox materials for the membrane preparation since they exhibit mixed type conductivity for the necessary transfer of anions, vacancies and electrons. Their ability to accommodate large concentrations of vacancies in their structure and to reversibly pick up and deliver oxygen at high temperatures was studied by thermogravimetric oxidation/reduction experiments which indicated that the materials are able to loose and uptake reversibly oxygen from their lattice and the weight loss is proportional to Sr content. Conventional reactor experiments indicated that the materials can be used as catalysts in a redox process where water is dissociated giving rise to the production of pure hydrogen. Dense, disc shaped membranes of the materials were synthesized and placed in a membrane reactor. Experiments at 1000°C revealed the possibility of performing the reduction and oxidation steps simultaneously and isothermally on each side of the membrane reactor. A steady-state situation was thereby achieved where hydrogen was continuously produced on one side while the material was simultaneously regenerated by a carbon containing species, on the other side. The created oxygen vacancy gradient formed the driving force for a continuous flux of vacancies from the membrane reduction surface to the membrane oxidation surface. The system is also able to operate on partial pressure based desorption without the need of a carbon containing reductant, so that a process towards hydrogen production, based only on renewable hydrogen source such as water, can be established. The hydrogen production rate under enhanced oxygen ions removal by a reductant is estimated to be up to 145 cm3 H2 (STP)m~(-2) min~(-1) while under unforced reduction conditions a hydrogen production rate of 47.5 cm3 H2 (STP)m~(-2) min~(-1) was measured.^