H2S sequestration process and sustainability in geothermal systems

Abstract

Reduction of H2S emissions from geothermal power plants is an important environmental, economic, and social challenge that needs to be addressed. The geochemistry associated with H2S injection into a geothermal reservoir with a basaltic host rock followed by sulfide mineralization was investigated experimentally, and reactive transport modeling used to determine the geochemical feasibility of such H2S sequestration. Re-injection of H2S-rich fluids into geothermal systems resulted in the dissolution of basalt followed by the mineralization of the H2S into pyrite and pyrrhotite. At a neutral pH of the injected solution the H2S mineralization was observed to be slow and result in limited H2S mineralization, whereas at acid pH the reaction rates are much faster, and up to ∼62% of the injected H2S mineralized. The rate of H2S mineralization was observed to be limited by Fe leaching rates from basaltic glass, suggesting that the rock dissolution rate is the limiting factor of H2S sequestration in geothermal systems. The long-term effects of H2S-rich fluid re-injection into geothermal systems (>100years) apparently results in increased porosity close to the outlet of the injection well due to rock dissolution, whereas pyrite formation has only limited effects on the porosity of the entire system. The optimized conditions of H2S sequestration are moderate H2S concentrations of re-injection fluids (10–30mmol/kg) with acid pH/25°C ∼2–3, and reaction temperatures of 200–250°C. Higher H2S concentrations result in a lower sequestration proportion due to the limited supply of Fe from the host rocks for sulfide precipitation. Higher temperatures result in the uptake of Fe by minerals other than pyrite, such as epidote, and also lower the proportion of sequestration. H2S re-injection and sequestration in geothermal systems is thought to be a geochemically and economically feasible option, as the method is cheaper than commonly used procedures today.