Scaling and Corrosion in an Experimental Geothermal Power Plant
Authors: Bishop H.K. , Bricarello J.R. 
Published: December 2007

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Introduction

The San Diego Gas and Electric Co./U.S. DOE Geothermal Loop Experiment Facility (GLEF) located near Niland in the Imperial Valley, CA, has operated on a high-salinity (about 200,000 mg/L) geothermal fluid from the Salton Sea Field since May 1976. The conversion process being tested is a four-stage binary system (Fig. 1). The geothermal fluid is flashed at successively lower pressures in open drum-type separators. Generated steam pressures in open drum-type separators. Generated steam passes through a scrubber and into a tube and shell heat passes through a scrubber and into a tube and shell heat exchanger, where the heat is transferred to a secondary, working fluid. Steam condensate can be cascaded and mixed with the brine in the next stage, collected and combined with the cooled brine and reinjected, or used for cooling water make-up. The working fluid is condensed after expansion across a throttling valve that simulates a turbine. Waste heat is removed with a conventional spray pond, With a turbine generator (to be installed later) this facility will become a 10-Mw power plant. Water was used as the working (binary) fluid plant. Water was used as the working (binary) fluid during start-up and initial operation. Isobutane, which is the expected preferred working fluid for a binary power plant, will replace water. plant, will replace water. The GLEF operated for about 3,000 hours with frequent interruptions for inspection and facility modification. During this period, the facility operated on one production well (either Magmamax 1 or Woolsey 1). production well (either Magmamax 1 or Woolsey 1). Flow rates were 400,000 lb/hr, about one-half the GLEF design capacity. The temperature and pressure of the fluid entering the first stage is 370 deg. F and 165 psia. Exit temperature of the brine is 200 deg. F. All unflashed brine is reinjected in the reservoir.

Brine Chemistry

The geothermal fluid available from this reservoir is a hypersaline brine containing about 200,000 mg/L total dissolved solids (TDS), mostly chlorides (Table 1). These chlorides remain in solution during the heat extraction process and subsequently are injected back in the reservoir. Certain minor species, however, such as silica, lead, and iron have limited solubility and, as the brine is cooled during the heat extraction process, they precipitate from solution and deposit on pipe and vessel precipitate from solution and deposit on pipe and vessel surfaces.

The principal noncondensable specie is carbon dioxide. Small amounts (up to 6 mg/L) of hydrogen sulfide also are found in this geothermal brine. Ammonia is also present in the geothermal brine and has a significant present in the geothermal brine and has a significant effect on brine chemistry.

The pH of the brine (5.6 to 5.8) is such that a carbonate type of precipitate normally is not observed in spite of the high carbon dioxide level (up to 0.3 wt %).

In this process, the available energy in the geothermal brine is extracted as steam. Drum separators and scrubbers can produce high-quality steam with a TDS content of less than 10 mg/L. However, noncondensable gases (carbon dioxide and hydrogen sulfide) also accompany this steam. Part of the ammonia in the geothermal brine also is driven off with the steam. When these noncondensable gases are removed, the pH of the brine increases to about 6.0.

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