Control of Magnesium Silicate Scaling in District Heating Systems

Abstract

Magnesium-silicate scaling has been encountered in several geothermal district heating systems in Iceland. The scaling occurs in heated freshwater that has undergone thermal deaeration and also when geothermal and fresh waters are mixed. The solubility and precipitation rate of amorphous magnesiumsilicate in district heating water was studied experimentally in the temperature interval of 60 to 120 The scale forming mineral was found to be amorphous magnesium-silicate, and the solubility to be controlled by the concentration of magnesium, silica and the of the water. The rate of mineral formation was found to be dependent on supersaturation, temperature and flow conditions. The results have made possible the prediction of scaling when geothermal water is mixed with deaerated fresh water and have led to the abandonment of earlier plans by Reykjavik District Heating to mix geothermal water from low temperature fields and heated freshwater from the high-temperature geothermal field of Nesjavellir. Determination of optimal of deaerated freshwater and effective control of the scaling in the Sudurnes District Heating system has also been made possible by this study.