Electrochemical desulfurization of geothermal fluids under high temperature and pressure

Electrochemical removal of sulfide ions was achieved in salt water using graphite anodes in an autoclave under high temperatures and pressures, simulating geothermal fluids. The reaction products were characterized using microscopy and X-ray photoelectron spectroscopy (XPS). At low temperatures the reaction rate is quite small. It decreases rapidly with time down to a negligibly small value, which increases only slightly with temperature. The reaction produces elemental sulfur, which was seen under the microscope and identified using XPS. It passivates the electrode and hence diminishes its activity. Above about 115 A degrees C, much higher removal rates can be sustained for much longer times, while the increase of temperature has a much stronger effect on the reaction rate. Under this condition, elemental sulfur was no longer detected among the reaction products, while the electrode retained its activity for continuous operation. The XPS spectra at high temperatures reveal the presence of oxygen bearing sulfur species, such as sulfates. The melting of sulfur (at 115 A degrees C) has a much stronger effect on the efficiency of the process than the transition of orthorhombic to monoclinic sulfur (at 95 A degrees C). A Clausius-Clapeyron's analysis reveals that the melting point of sulfur inside the autoclave is nearly equal to its normal melting point.