Emission Control of Gas Effluents from Geothermal Power Plants

Abstract

Geothermal steam at the world's five largest power plants contains from 0.15 to 30% noncondensable gases including CO₂, H₂S, H₂, CH₄, N₂, H₃BO₃, and NH₃. At four of the plants the gases are first separated from the steam and then discharged to the environment; at the fifth, the noncondensables exhaust directly to the atmosphere along with spent steam. Some CO₂ and sulfur emission rates rival those from fossil-fueled plants on a per megawatt-day basis. The ammonia and boron effluents can interfere with animal and plant life. The effects of sulfur (which emerges as H₂S but may oxidize to SO₂) on either ambient air quality or longterm human health are largely unknown. Most geothermal turbines are equipped with direct contact condensers which complicate emission control because they provide two or more pathways for the effluents to reach the environment. Use of direct contact condensers could permit efficient emission control if coupled to processes that produce saleable quantities of purified carbon dioxide and elemental sulfur.