“We thought that if we could minimise the energy that drives the heat pump while also maximising energy output from the process, it would pay off,” explains Tore Hulgaard, Technical Manager at Ramboll Energy.
Traditionally, the flue gas condensation process at waste-to-energy plants involves three steps, during which a so-called wet scrubber removes pollutants from the gas. In the third step the scrubber can recover heat by using a heat exchanger to create direct condensation from water that is approximately 50 degrees Celsius– the temperature of the water when it returns from the district heating system.
Recovering heat when the scrubbing temperature is below the district heating temperature requires a heat pump, which recovers the heat by means of a cold circuit – the reverse of a refrigerator. This process, the fourth step, is usually highly energy-intensive and thus results in considerable electricity loss.
But Tore Hulgaard and his colleagues figured that if you built the turbine system to extract the steam from the middle of it, where the temperature is still just enough to drive the heat pump, less electricity would be lost.
They also discovered that the heat pump could cool the flue gas down to 25 degrees Celsius. That produces 5-6 kWh of heat for every kWh of power put into the system.
“These optimisation techniques boost energy production by around 20%, which probably makes the system the most energy efficient of its kind in the world. At the same time, this fourth step removes even more pollution from the gas,” Tore Hulgaard points out.
These facts and figures convinced the owners of Amager Bakke, a waste-to-energy plant currently under construction in Copenhagen. They installed Ramboll’s system earlier this year, and other waste-to-energy plants are following suit.
“It’s good not only for the plant owners but also for society as a whole: You could argue that the system is resource efficient and carbon neutral, because no additional fuel is used to recover the heat from the flue gas condensation,” Tore Hulgaard says.