H2S Injection and Sequestration into Basalt - The SulFix Project

Atmospheric H₂S emissions are among major environmental concern associated with geothermal energy utilization. It is therefore of great importance for the geothermal power sector to reduce H₂S emissions. Known solutions for H₂S neutralization are both expensive and include production of elemental sulfur and sulfuric acid that needs to be disposed of. Icelandic energy companies that utilize geothermal power for electricity production have decided to try to find an environmentally friendly and economically feasible solution to reduce the H₂S emission, in a joint venture called SulFix. The aim of SulFix project is to explore the possibilities of injecting H₂S dissolved in water into basaltic formations in close proximity to the power plants for permanent fixation as sulfides. The formation of sulfides is a natural process in geothermal systems. Due to basalt being rich in iron and dissolving readily at acidic conditions, it is feasible to re-inject the H₂S dissolved in water, into basaltic formations to form pyrite.

To estimate the mineralization rates of H₂S, in the basaltic formation, flow through experiments in columns were conducted at various H₂S concentrations, temperatures (100 – 240°C) and both fresh and altered basaltic glass.

The results indicate that pyrite rapidly forms during injection into fresh basalt but the precipiation in altered basalt is slower. Three different alteration stages, as a function of distance from inlet, can be observed in the column with fresh basaltic glass; (1) dissolution features along with precipitation, (2) precipitation increases, both sulfides and other secondary minerals and (3) the basalt looks to be unaltered and little if any precipitation is observed. The sulfur has precipitated in the first half of the column and thereafter the solution is possibly close to be supersaturated with respect to the rock.

These results indicate that the H₂S sequestration into basalt is possible under geothermal conditions. The rate limiting step is the availability of iron released from the dissolving rock. The rapid precipitation of secondary phases in the column suggests the possibility of decreased porosity in the vicinity of the injection well.