Meyland advances drying technology with pilot plant

Recycling to the last grain

In Flanders, we are increasingly recycling our organic waste. In large digestion plants, we turn it into biogas. What remains is called digestate. That, in turn, is a good basis for fertilisers, but to do that we first have to dry it - a very energy-intensive process. That is why, until recently, we preferred to export our digestate.

“That is no longer allowed now,” Nicolas Aldeweireldt, project manager at Meyland, told me. “The amended legislation suddenly created a huge demand for machines to process digestate in the most energy-efficient way. And we jumped on that.”

Meyland took the project very seriously. “Normally, we build machines to customer specifications. Now we first built a pilot plant in our own backyard to optimise the process. Otherwise, digestate drying cannot be made profitable.”

Check every possible parameter

The pilot plant transports the digestate by screws to a fluidised bed dryer. It lands on a perforated, vibrating bedplate. Warm air is then blown under the digestate, and after about eight minutes, the dried digestate exits the dryer. It sounds simple, but it is not.

“We dry the digestate to 10% moisture content,” explains Arne Roeland, project engineer at Meyland. “That is fixed. But the moisture content of the digestate at the entrance can fluctuate greatly. Depending on what products it contains, it loses moisture more or less quickly. Carrot-based digestate reacts completely differently from potato-based digestate. But you can no longer tell from the digestate what was once in it.”

So to get a consistent product, Meyland has to continuously measure and adjust based on those measurements. “We continuously adjust our throughput and air temperature automatically. All transports are frequency-controlled, so we can control the speed of each screw individually. That is overkill for a standard installation, but for a test setup it is a luxury as well as a must,” says Arne. “We can control each component separately and see exactly what the effect is.” A SINAMICS G120C drive takes care of that. A SIMATIC S7-1500 orchestrates the process.

“The PLC collects data and transmits it to our Industrial Edge Controller," says Jeroen Pieters, account manager at Siemens. “That sends everything structured to Meyland’s server as well as to a live dashboard on a WinCC Comfort Panel.”

“Every second we log all the parameters,” Arne confirms. “After each run, our own system gives us a good overview of how the machine performed - energy consumption, how much digestate was dried, etc. We use that data to further refine the process. We can do this on the screen at the machine, but also remotely via the web client. A big advantage, as it means we don’t have to stay with the machine all the time.”

The process optimises itself

After a year and a half of testing, Meyland has already made significant progress. “Based on the temperature at the outlet, we control a PID controller, which continuously adjusts the parameters. Thus, the process continues to optimise itself further. Currently, we are still experimenting with a moisture sensor on the outlet to be even more fine-tuned. And by reusing the dedusted hot air at the outlet, we need to reheat less ambient air.”

The pilot plant operates with a 100-kW gas burner. “We are now in the design phase of a project where we would go up to 8 MW. Even with all the optimisations, that’s still a lot of energy. That is why we build our plants where there is residual heat from industry that we can reuse. This is how we win twice - we recycle a waste product into a new raw material using waste energy.”

“Digestate is waste, but we turn it back into something useful,” Nicolas concludes. “Our dried digestate is the ideal feedstock for fertiliser manufacturers. And so the circle is complete. And with local, circular plants, we can drastically reduce transport and emissions.”