| Title | Scale Prevention Method by pH Modification Using Advanced Bioreactor |
|---|---|
| Authors | K. Takeuchi, Y. Fujioka, K. Hirowatari, S. Kusaba and H. Suzuki |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | scale prevention, H2S abatement, bioreactor, pH modification |
| Abstract | Silica scale deposition is one of the most serious problems in geothermal power stations using hot water-dominated geothermal systems. The scale causes plugging troubles in surface facilities and reduces the capacity of reinjection wells. A new process was proposed for scale prevention. This process is based on pH modification methods. H2S in geothermal gas is oxidized to H2 SO4 using sulfur-oxidizing bacteria in the bioreactor. H2SO4 is used for pH modification of geothermal brine. We have already reported on the field experiment carried out at Otake geothermal power station using a bubbling tower type of bioreactor. The results showed that the system could be operated stably for over 2,700 hours and was applicable to geothermal power stations, but mass transfer of H2S at gasliquid interface limited H2SO4 production rate. In this study, we replaced the bubbling tower with a packing tower to make the sulfur oxidizing reaction efficient. The packing tower type of bioreactor is filled with Raschig rings made of ceramic. The solution containing the sulfur oxidizing bacteria and nutrients is injected into bioreactor from the top and the geothermal gas is injected from the bottom. H2S is oxidized in the zone where the gas contacts the solution. The geothermal gases are mixed with air and the H2S concentration in mixed gas is 0.2~0.3 %. The advanced system was operated for over 3,000 hours on site. H2SO4 production rate was 0.07 kg-S m-3 h-1 at high H2S removal operations (H2S removal efficiency 95%) and 0.12 kg-S m-3 h-1 at high H2SO4 production rate operations (70%). This rate was 3~6 times larger than the bubbling tower type. From the estimation using the results of this study, the bioreactor volume of 12 m3 was need for controlling the pH of the waste brine from the Otake geothermal power station. |