| Title | Direct mixing of geothermal water into heated groundwater to improve water quality in the district heating system of the Reykjavik capital area |
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| Authors | Arna Pálsdóttir, Jóhann Garðar Einarsson, Sigurður Rúnar Rúnarsson, Baldur Brynjarsson |
| Year | 2023 |
| Conference | World Geothermal Congress |
| Keywords | district heating, geochemistry, water chemistry, corrosion |
| Abstract | The Hellisheidi Geothermal Power Plant has produced hot water for the Reykjavik capital area since 2010. The power plant is co-generative, producing electricity in steam turbines and hot water for district heating through heat exchange between the geothermal fluid and cold groundwater. The cold groundwater is saturated with oxygen when it is pumped out of the ground and becomes supersaturated after going through the heat exchangers. Deaeration of the water is necessary after passing through the heat exchangers as the presence of dissolved oxygen in the water causes corrosion in the carbon steel pipes of the district heating system. The deaeration process is both physical and chemical and takes place in a set of deaerators. The chemical deaeration is achieved with the injection of a small amount of steam. Geothermal steam contains hydrogen sulfide (H2S) which reacts with the remaining oxygen after the physical deaeration as well as maintaining a high enough H2S level in the water in the district heating system to react with any oxygen that may be introduced into the system. However, H2S is acidic and thus steam injection lowers the pH of the heated groundwater. From the commencement of hot water production at the Hellisheidi Geothermal Power Plant, keeping a steady pH and H2S level has been problematic. To keep from corroding the pipes in the district heating system in Reykjavik the pH must be above 8,0. The average from the start of production is 7,9 and the pH has in certain instances been measured at 7,0. A project to adjust this chemical balance in the heated groundwater started as a research project in 2018 and came into operation on a full scale in February 2022. In this project a small amount of the separator water (the liquid part of the geothermal fluid after steam and water have been separated) is mixed with the heated groundwater after deaeration. The separator water contains 20-30 mg/kg of H2S and has a pH of ~9.2. It is fairly buffered compared to the heated groundwater and becomes dominating in the mixture even though the mixing ratio never exceeds 2%. Through this mixing the pH and H2S can be adjusted simultaneously to an appropriate level. In addition to adjusting the chemical balance this mixing increases the production of hot water at the Hellisheidi Geothermal Power Plant by 20-40 l/s at peak capacity, reduces the need for geothermal water re-injection, and the steam injection can be lowered significantly, resulting in more available steam for electricity production. |