LO-CAT® Plants At A Geothermal Power Facility Operating Well After 15 Years

By Gary J. Nagl, Vice President, Gas Technology Products Division Merichem Chemicals & Refinery Services LLC

Geothermal power plants are environmentally attractive since they employ a renewable energy source, which unlike wind and solar is continuous. However, geothermal steam contains varying amounts of non-condensable gases (NCG) such as carbon dioxide (CO2, hydrogen sulfide (H2S), mercury vapor (Hg) and in some cases benzene (C6H6), which if not disposed of properly may cause environmental, health and safety problems. This article describes the operating history of three geothermal power facilities, which have been operating for over 15 years while meeting strict California emission regulations.

Terra-Gen Power LLC operates a geothermal power facilities located on the China Lake Naval Weapons Center in southeastern California. As illustrated in Figure 1, hot brine is extracted from high pressure, underground wells. The brine is flashed and the produced steam is passed through condensing turbine/generator sets, which produce approximately 250 MW of electrical power, which is sent into the local power grid. This is enough power to service approximately 250,000 homes. The unflashed brine and condensate from the turbine/generator sets are reinjected into the geothermal reservoir and the non-condensable gas (NCG), consisting mostly of carbon dioxide (CO2), is sent to treatment facilities. Initially, the NCG was reinjected back into the reservoir; however, over time this practice began to affect the performance of the reservoir and the practice was stopped. It was then decided to remove the hydrogen sulfide, H2S, and mercury, Hg, from the NCG and exhaust the remaining gas, CO2 and water vapor, to the atmosphere. In 1993, a former owner of the facility began an investigation into various H2S and mercury removal systems. This investigation resulted in the selection of the LO-CAT® process for H2S removal, with a sulfided, activated carbon media upstream of the process for mercury removal. These systems were initially installed at the Navy I and Navy II facilities. A third system was later added at Navy II due to larger gas loading than previously anticipated. The LO-CAT process is licensed by the Merichem Chemicals & Refinery Services LLC.