Optimized H2S Treatment at Pacific Gas and Electric Company's Geysers Geothermal Project Unit 11

Abstract

The geothermal steam at Pacific Gas and Electric Company's' Geysers Geothermal Project contain hazardous constituents of which hydrogen sulfide (H2S) present the greatest problem. Hydrogen sulfide removal processes were developed in the mid 1970's to comply with local air pollution control regulations. These chemical processes consumed large volume of expensive chemicals and generated the major portion of the total waste for the geothermal plant. This report describes the operation of a new process that was applied to Unit 11. This integrated system uses ferric HEDTA chelate to produce zerovalent sulfur in the condensate and the reduced iron chelate is deoxidized in the cooling tower. Thermal incineration and scrubbing of the non condensable gas produces sodium bisulfate which is contacted with the zerovalent sulfur to produce soluble thiosulfate which remain in the geothermal condensate. This process has greatly reduced the consumption of expensive chemicals and virtually eliminated the generation of hazardous wastes at Unit 11. The chemistry and process results will be discussed.