Energy recovery from thermal pressure hydrolysis
The incoming raw sludge is not sterile, so the first step is hygienization or disinfection. Lindum A/S is using thermal pressure hydrolysis for this purpose (steam pressure heating), meaning that the media are getting heated up to a minimum of 133°C, for 20 minutes. The maximum temperature is set to 160°C, requiring a pressure of 5-6 bar. Once the heating is finished, the temperature is to be reduced quickly to 40°C, breaking down the cell structure and fibers to smaller particles. The thermal pressure hydrolysis process assures a quicker and more efficient break down of structures difficult to digest by bacteria. Organic material gets digested better and quicker this way. The installation of the Läckeby sludge-water heat exchanger has contributed to an efficient and cost-saving thermal pressure hydrolysis process.
Circular solutions in the green economy
Lindum A/S is one of the major players in Norway in the area of waste collection and waste processing. Lindum A/S is constantly searching for green solutions and technologies to reuse as much as possible the waste collected and processed. A very good example is the biogas Lindum is producing from manure, sludge and household waste. Lindum is spending a lot of time and money on their search for green solutions and is proud to play a main role in Norway’s cricular economy.
The Lackeby solution
In 2016, Läckeby was contacted by Lars Erik Smith from COWI, a leading norvegian company, active in the field of construction, environment, energy and infrastructure. It was Covi’s task to find a solution to use existing process water as cooling medium for the sludge and then reuse the heated water back into the process. As the available process water was not as cold as first expected and seen a limited space in the Lindum facilites, Läckeby wasn’t able to reach the requested 40°C in one step, but we got much closer than what Covi had calculated at the start. After cooling, the sludge gets mixed with water before entering the digester tank, allowing us to reach the requested 40 °C temperature. Our solution, together with Lindum’s previous good experience with our heat exchangers delivered several years ago, gave us the order to supply our heat exchanger. We came up with a Läckeby Heat exchanger HSW 40-120-6, to be installed in 2017.
Energy recovery result
Hydrolyzed sludge with a DS content of 12 – 14 % has a similar viscosity as municipal sludge with approx. 6% DS. The hydrolyzed sludge coming from the flash tank enters the HSW 40-120-6 with a temperature of 107°C and a flow rate of 8 m3/h. Using water of 56°C and a flow rate of 5.9 m3/h, we end up with a sludge temperature of 73.3°C, which is well within the expectations. The process was started up just after the summer of 2017. At the beginning there were some problems with a sudden increase of pressure drop, measured after the heat exchanger, causing the process to stop. However, after investigations, they found the problem to be in the flash tank when adding new effluent. Once this issue was solved, the heat exchanger has been doing a prefect job.