Optimering av fjärrvärmesystemet i Bjurholm

Sammanfattning: On behalf of Umeå Energi, an investigation of the Bjurholm district heating plant has been performed, with the purpose to reduce its oil consumption. The Bjurholm plant consists with a pellet boiler and an oil boiler. The oil boiler is automatically demand-controlled, which means that it is used when the pellet boiler cannot manage the power requirements. The pellet boiler has a nominal capacity of 1500 kW, which is a capacity the pellet boiler could manage 2013 but not today. Since a few years, the boiler experiences low oxygen levels whenever the fuel supply rate is increased, which then terminates the combustion process. In this project, possible reasons for this problem were investigated. In order to investigate why the boiler cannot reach its nominal capacity, four different parameters were scrutinized: the quality of the pellets, the logged measurement values from the sensors in the plant, the boiler efficiency and the air supply. The investigation of the pellets showed that the pellets are of high quality and are unlikely to cause moisture- or ash-related problems. A comparison between the sensor measurement values, for example temperature in the water lines, flue gas temperature, pressure and oxygen level in the boiler, logged February 2013 against February 2019, showed small differences. The boiler efficiency was on average several percent higher 2013 than 2019, but the measurement uncertainty in the calculations is high. The reason for this uncertainty is foremost that Umeå Energi only logs measurement values two to three times per week by hand. This means that the calculations cannot be trusted. Measurements of primary and secondary air supply were performed on one occasion, when the effective boiler capacity was 1068 kW. The results of this project do not fully explain, why the oxygen level in the boiler reaction zone decreases with increased fuel supply but can be used as a basis for further investigations. It can be concluded that, in order to solve the problem, the plant needs to be upgraded. In particular, continues, digital logging of several critical boiler parameters should be enabled. Further work should focus on accurately measuring the primary and secondary air supply rates during operation at higher boiler capacities.