Mineral Deposition From Icelandic Geothermal Waters: Environmental and Utilization Problems

Abstract

Summary Scales of silica, calcite, and iron-bearing minerals have caused problems in exploiting high-temperature fluids in some geothermal fields in Iceland. Calcite scales are encountered only when the first level of boiling is within wells. Silica scales are confined mainly to waste waters discharged to the atmosphere. Iron-bearing minerals in the Krafla field wells are related to emission of volcanic gases into the geothermal system. The theory of scaling conditions and possible solutions to the problem are reviewed. Introduction Geothermal reservoirs provide about 30% of the annual energy consumption in Iceland. To date, the main emphasis has been on house heating. Greenhouse farming based on geothermal heat, however, has been substantial, and geothermal water and steam have been used on a small scale for industrial purposes. In all, 30 district heating systems serving 39 communities are in operation. Several of these systems are still in the late stages of construction. Expansion is planned to include another two villages. One system is in an early stage of construction and another is planned in the near future. A district heating service in the Vestmann Islands (4,500 inhabitants) uses heat from lava that erupted in 1973. In Oct. 1980, about 69% of Iceland's inhabitants (115,000) used geothermal for house heating. When the planned system is in operation and the present systems have been constructed and expanded as scheduled, this figure will rise to 76% (170,000) and 78% if the Vestmann Islands system is included. The peak-load energy requirements for all the district heating systems then will reach nearly 600-MW thermal.Greenhouses cover almost 140 000 m2, requiring about 40-MW thermal as a peak load. Energy requirements for industrial uses are around 25-MW thermal-the most noteworthy use being drying of diatomaceous earth by geothermal seawater. A pilot plant extracting salt from geothermal seawater is being operated at Reykjanes. A 3-MW electric power plant has been in operation for 9 years at Namafjall in northern Iceland, and a 60-MW plant is under construction in the nearby Krafla geothermal field.More than 90% of the water currently used for home heating is extracted from geothermal reservoirs with temperatures ranging from 55 to 140 degrees C. These waters are usually very low in dissolved solids and possess a high pH. Insignificant deposits of amorphous silica have been observed in distribution systems and radiators conducting these waters. They tend to be found where local rusting occurs. Some corrosion has been observed in those systems where chloride concentrations exceed about 500 ppm.Increasing exploitation of high-temperature (200 to 340 degrees C) geothermal reservoirs during the last decade has brought about environmental and utilization problems associated with mineral deposition from the thermal water. The major element chemistry of water in geothermal reservoirs is governed mainly by temperature-dependent equilibria between solutes and alteration minerals. This explains why the magnitude of mineral deposition from the hot water is related directly to the reservoir temperatures. Deposition is caused by the physical and chemical changes that accompany cooling and boiling of the geothermal water. Identification of Problem Three types of deposits from geothermal waters have been observed. JPT P. 181^