Reducing Silica Deposition Potential from Waste Waters in Nesjavellir and Hellisheiði Geothermal Power Stations, Iceland

Abstract

Silica deposition from spent geothermal water can be problematic. Correct fluid handling of amorphous silica supersaturated geothermal waters is very important for successful disposal of the water. In this contribution, treatment of spent geothermal water prior to injection from the Nesjavellir and Hellisheiði Power Plants are described. Utilization on the surface makes the separated geothermal waters supersaturated with respect to amorphous silica. Before disposal in Nesjavellir the waters are retained in a tank or pipe where the silica polymerizes and supersaturation with respect to amorphous silica is reduced from 411 ppm to 168 and 199 ppm SiO2 when the flow rate of separated water is 106 and 163 l/s. Mixing with condensed steam after the retention tank in Nesjavellir further reduces the supersaturation to 63 and 94 ppm SiO2 for the two flow rates. In Hellisheiði Power Plant silica supersaturation with respect to amorphous silica is reduced in a pipe from 206 to 136 ppm SiO2 in the separated water before re-injection. This handling of separated water has been successful in Nesjavellir Power Plant where lower temperature of the separated water compared to Hellisheiði power station result in higher proportion of polymerized silica in the injection water.