The concept of a georeactor in geothermal environment has been proposed for direct utilization of geothermal energy resources. Chemical processes in the georeactor must accommodate a range of operating conditions, since the georeactor utilizes natural resources that are sometimes difficult to control. A hydrogen generating system, in which hydrogen is produced from hydrogen sulfide by using a photocatalysis, was examined as a case study for use of the georeactor. Combination of hydrogen generation by solar energy from hydrogen sulfide, and recycling of waste sulfur and its compounds using geothermal energy is a potential, environmental energy supply system. There are a few previous studies of sulfur reduction reactions with water. Self-oxidation and reduction reactions of sulfur are the considered to be suitable reactions for the thermal condition of georeactor which is around 200°C. Hence we try to apply self-oxidation and reduction reactions of sulfur to the georeactor. In our previous studies, self-oxidation and reduction reactions of sulfur proceeded in neutral or alkaline solution. In neutral solution, hydrogen sulfide was formed as reduction species, and sulfuric acid was formed as oxide. On the other hand the reaction formed several sulfur oxides such as thiosulfuric acid and other complicated sulfur species with strong alkaline solution. Total amount of generated hydrogen sulfide by the reactions in strong alkaline solution was larger than that by the reactions in neutral solution. However it was difficult to use such strong alkaline solution for self-oxidation and reduction reactions of sulfur in the georeactor as hydrogen generator because of no cost performance for keeping pH of strong alkaline solution and for pipeline erosion. Therefore we applied seawater which is alkalescent solution and can be purveyed at a low price. In order to study self-oxidation and reduction reactions of sulfur seawater related to the georeactor, we have conducted hydrothermal experiments of the reaction between sulfur and seawater using a batch-type autoclave. The experimental results showed that the maximum conversion rate of hydrogen sulfide with self-oxidation and reduction reactions of sulfur in seawater is almost 50% and the highest concentration of generated hydrogen sulfide is 0.026 mol/l.
- Direct use
- Hydrogen generator
- Self-oxidation and reduction reaction
- Sulfur recycle