Energieffektivisering för sänkta returtemperaturer till fjärrvärmenätet på Ryaverket

Sammanfattning

Wastewater treatment plants play an important role in the transition towards a more resource efficient and sustainable society. In addition to treating wastewater, organic material and sewage sludge can be utilized through anaerobic digestion, in which biogas is produced as a renewable alternative to fossil fuels. For this process to operate efficiently, the sludge in the digesters must be maintained at a stable temperature. This means that the process depends on heat, while the heat use must also be an energy efficient as possible. At Ryaverket in western Sweden, owned by Gryaab AB, district heating from Göteborg Energi is used to heat an internal hot water system that, among other functions, supplies heat to the digester process. The hot water system also includes heat recovery from air compressors, where heat from the compressed air is utilized. For several years, the plant had experienced problems with high temperatures to the district heating network. This means that not enough heat is extracted from the district heating water before it is returned to the network. As a result, the return temperature remains too high, which reduces the efficiency of the district heating system and results in additional fees for Gryaab. These fees are applied when the returned water has a higher temperature than the level desired by Göteborg Energi. The aim of this study was to map Ryaverket’s hot water system and identify the factors contributing to the high return temperatures. The work included energy balance calculations and an analysis of operating data to investigate how heat is produced, transferred and used within the system. The results show that the heat demand in the digestor process is mainly ruled by the amount of incoming sludge. At high sludge flows, the heating demand increases, while the sludge-water heat exchangers do not always have sufficient capacity to cool the hot water. This results in high return temperatures from the digestor process. In the current system, the waste heat is added to the return flows to the district heating network, which is also a factor behind the elevated return temperatures. Alternative placements of the heat recovery were therefore investigated, and the results show that the integration is possible under certain conditions. Overall, the study shows that the high return temperature is caused by a combination of high sludge flows, limited heat transfer and the current placement of the heat recovery. The results therefore provide a basis for further investigations into improved waste heat integration and alternative operating strategies, with the aim of reducing the return temperature to the district heating network.