Use of Hydrogen Peroxide To Abate Hydrogen Sulfide in Geothermal Operations
Authors: Castrantas H.M. 
Published: December 2007

Summary
Environmental restrictions on hydrogen sulfide emissions from geothermal operations in the Geysers high-pressure steam field in northern California has led to the development of hydrogen-peroxide-based abatement processes. One process involves the spray injection of hydrogen peroxide and sodium hydroxide into escaping steam during well drilling operations. Other processes involve peroxide treatment of steam condensate in power plants. High sulfide abatement levels have been achieved with these processes.

Introduction
Geothermal resources, whether vapor or liquid dominated, usually contain hydrogen sulfide. Some representative sulfide values' for operational geothermal fields are given in Table 1. Hydrogen sulfide possesses an objectional odor even at concentrations below 0.5 cm3/m3 (0.5 ppm). At 10 cm3/m3 (ppm), eye irritation occurs, and respiratory tract irritation occurs at 50 to 100 cm3/m3 (ppm). Loss of consciousness and death can occur at concentrations of 500 cm3/m3 (ppm) or more.The expanding development of the Geysers field has led to environmental restrictions limiting the amount of hydrogen sulfide that may be emitted during well drilling, stacking, and power generation.As other geothermal fields are developed (e.g., in the Imperial Valley, CA, New Mexico, Nevada, and the U.S. gulf coast), similar restrictions could be imposed if hydrogen sulfide levels prove high enough to exceed present or future ambient or emission standards.The wellknown use of hydrogen peroxide for sulfide destruction in municipal and industrial wastewater treatment fields stimulated the investigation of hydrogen peroxide for treating geothermal steam and fluids.The investigation led to the successful oxidation of hydrogen sulfide during the drilling of steam wells. This was accomplished by the injection of hydrogen peroxide and sodium hydroxide into the steam, using separate spray nozzles. The process presently is used as required by well drillers in the Geysers field. In a second part of the investigation, on a laboratory scale, hydrogen sulfide in steam condensate from a Geysers field geothermal power plant was oxidized with hydrogen peroxide. In this process, trace amounts of iron salts or excess hydrogen peroxide were used to reduce the reaction time to put it within the required limits. The process was modified by Pacific Gas and Electric Co. to improve sulfide removal in its geothermal power plants.

Description of Geothermal Processes Well Drilling
The hydrogen-peroxide/sodium-hydroxide injections are started when the drill bit penetrates the steam reservoir. Thereafter, the injections continue for about 16 days, the time required to complete the well. This is termed the air drilling phase. Steam production increases from zero at the beginning of the air drilling phase to essentially full capacity of the well at the time of completion. Hydrogen sulfide is a normal constituent of the steam and is released to the atmosphere during the air drilling phase after flowing through a blooie line (a steel pipe) leading to an atmospheric separator (Fig. 1). This is used to suppress noise and remove particulates.

Power Generation
Seven to 12 wells are used to supply the 454 000 kg/h (1 million lbm/hr) of steam for each 55-MW turbine.
JPT
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