Magnesium-silicate scaling in mixture of geothermal water and deaerated fresh water in a district heating system

Abstract

The low-temperature geothermal fields in Reykjavik utilized by the Reykjavik Municipal District Heating Service are now fully exploited. Additional hot water will be obtained by heating and deaerating fresh water using high temperature geothermal fluid. The heated fresh water will mix with low-temperature geothermal water in the distribution system in Reykjavik. A pilot plant has been set up to investigate magnesium silicate scale formation when mixing of these waters occures. Tests show that the scale formation is dependent on the severity of deaeration of the fresh water and the proportion of geothermal water in the mixture. Increased deaeration and thermal water proportion increase the pH of the mixture, and this promotes scaling. The scale formed is poorly crystalline, near amorphous trioctahedral smectite close to saponite in composition. By using minimum deaeration and traces of geothermal steam to remove the last remaining dissolved oxygen, scaling in the distribution system can by avoided.