Use Byproduct Carbon Dioxide to Control Scale and Hydrogen Sulfide

Abstract

The carbon dioxide which is always present geothermal fluids can be used to beneficially control pH of flashed brine and condensate. Separating steam from geothermal brine removes carbon dioxide, sharply increasing the pH of the brine and causing precipitation of pH sensitive minerals, including calcium carbonate, magnesium silicate and other metal silicates, clays, and metal sulfides. The heat exchangers in a hybrid flash-binary power cycle are especially sensitive to scale deposition from flashed geothermal brine. Scale deposition can be inhibited by separating a small amount of gas rich vapor from the brine before the main stage of steam separation, and combining it with the flashed brine before it enters the heat exchanger. The carbon dioxide thus added to the brine will decrease pH, inhibiting deposition of pH sensitive minerals as scale. Where geothermal steam contains ammonia the condensate is alkaline and hydrogen sulfide dissolves in the condensate, requiring secondary H2S abatement which involves chemical costs and complicates management of cooling water chemistry when the condensate is used for cooling water make-up. Nearly all of the hydrogen sulfide can be removed from the condensate by contacting the condensate with condenser vent gas (which is mostly carbon dioxide) at atmospheric pressure or above to decrease pH, then exposing the condensate to condenser vacuum to strip off the hydrogen sulfide.