The SnO2/Sn Carbothermic Cycle for Splitting Water and Production of Hydrogen

Abstract

The carboreduction in SnO2 to produce Sn and its hydrolysis with steam to generate hydrogen were studied. The SnO2/C/Sn system has several advantages compared with the most advanced cycle considered so far, which is the ZnO/C/Zn system. The most significant one is the lower reduction temperatures (850–900°C for the SnO2 versus 1100–1150°C for the ZnO). The rate of carbothermal reduction was studied experimentally. SnO2 powder (300 mesh, 99.9% purity) was reduced with beech charcoal and graphite using a thermogravimetric analysis apparatus and fixed bed flow reactor at a temperature range of 800–1000°C. Optimal temperature range for the reduction with beech charcoal is 875–900°C. The reaction time needed to reach conversion of SnO2 close to 100% is 5–10 min in this temperature range. The transmission electron microscopy results show that after cooling, the product of carboreduction contains mainly metallic Sn with a particle size of 1–3 μm. The hydrolysis step is crucial to the success of the entire cycle. Reactions between the steam and solid tin having as powder structure similar to the reduced one were performed at a temperature range of 350–600°C. Results of both the reduction and hydrolysis reactions are presented in addition to thermodynamic analysis of this cycle.