“Everything's good for something.” That's the philosophy of Matthias Preußner, who leads the Energor company and is its public face. Energor is based in Friedberg in Hessen to the north of Frankfurt (Germany) – and its business is simply to use food waste to generate heat and electrical power. For the past 20 years, it has been doing so using CHP plants from Rolls-Royce (or its predecessors). Energor currently operates two mtu gas-powered 8V 4000 GS CHP modules, each with 800 kW output.
Food waste neither looks nor smells appetizing – especially when it's been mashed into a pulp inside a tank. Yet food waste can be valorized – something that Matthias Preußner's father already recognized back in the early nineties. At the time, Matthias was still a child and father Gerd a farmer. Food leftovers, as was customary at the time, were fed to the pigs. But Gerd also wondered what he could do with the leftovers if his pigs were not so hungry….
From farming food to farming fuel
The idea was Gerd Preußner's brainchild: To use food leftovers to make biogas for powering a CHP module that would then produce heat and electricity. That idea became reality with the founding of Energor GmbH in 1995. Over the many years that followed, Preußner turned his farm into a food waste recycling enterprise. Today, his son Matthias is at the helm, supported by Gerd and 30 employees.
The Preußners no longer rear pigs. But a fair number of tractors can still be spotted far out in the fields. “We still cultivate some 200 hectares,” explained Preußner. And because the maize harvest is just around the corner, his father is spending the day out on the land. But the family's main occupation is power generation. Each year, they produce three million cubic meters of biogas from 18,000 tons of food waste. That biogas is used in their twin mtu CHP modules to generate a total of 7.5 million kWh – enough to cover the energy demands of a village of almost 2,000 households.
Trucks collect food waste
Each morning, a fleet of trucks leaves the site, driving all around the vicinity within a radius of 50 km. Painted bright red, the trucks are hard to miss on the streets of Friedberg. They stop at schools, nurseries, hospitals, care homes and restaurants – anywhere where food catering causes leftovers. Full bio-waste bins are carried away, empty ones left behind. “It couldn't be more convenient for our customers. We're basically the binmen, but we come each day if need be,” explained Matthias Preußner.
As the morning advances, the red trucks return one by one, passing through the rather grand gates of the Preußner farm, past the parent's house, and towards the collecting tank. As the bins are emptied into the tank, the day's spoils come into view. The stench is overpowering, and the sight is not a pretty one either, with the food leftovers – capsicums, eggs, coffee filters, meat, pasta, rice, potatoes and whatever else has not been eaten up – churned up together in the separating mill. Also added are foodstuffs past the use-by date – bakery waste, baking mixtures, drinks, and even confectionery.
The resulting mixture is brownish in color and viscous. Does it have a special name? “It doesn't actually,” said Matthias Preußner, going on to explain that what's important is its flowability, which he can influence with other ingredients. “If the mixture's too runny, we just add a bit of stale flour,” grinned the amateur chef.
Biogas produced from food waste fuels CHP modules
The biomass is subject to a sanitation process taking one hour at 70°C. Non-organic substances are removed, and then it is transported to the fermenter, in which carbohydrates, proteins and fats are converted to methane by means of micro-organisms. Iron hydroxide is also added during fermentation to eliminate the hydrogen sulfide.
It takes around another 100 days in the fermenter before the food waste begins to produce biogas. This is then transported via active carbon filters to one of the two mtu CHP modules which uses it to deliver 800 kW of heat and electrical power – and with great reliability.
"We had a replacement engine within six weeks"
Matthias Preußner is not only full of praise for the technical superiority of the plant, but for Rolls-Royce service, that has come to his assistance on many occasions. He particularly recalls an incident with the engine in early 2023. “We flooded the engine room with Ad Blue,” he joked, suddenly becoming more reflective: “The Ad-Blue tank above the engine came from a third-party manufacturer, and it wasn’t properly secure. We arrived one morning to find the whole of the CHP plant covered in Ad Blue. The engine was a goner.” He was able to continue using the twin CHP module, but one unit was not sufficient for utilizing all the biogas. Energor, however, had the protection of a service contract, and Matthias Preußner remains grateful to the Rolls-Royce service team to this day: “We got the replacement in under six weeks,” he said. And energy production got back into full swing.
The Preußners feed the electrical power into the public utility grid, but use the heat themselves – to provide hot water for their personal use and for cleaning out the food waste bins. Thermal energy is also harnessed for sanitizing the waste, drying maize crops and wood, and melting fats. The latter is another pillar in the Preußner business, which not only collects food waste, but also frying fat, which it puts into purpose-built containers. The fat is purified, processed and sold on to manufacturers of, for example, HVO.
“Everything's good for something,”' says Matthias Preußner. It's a philosophy that Energor very clearly espouses, and even applies to the by-product of biogas production – dung – which Preußner's father duly returns to the fields. Quite as it should be in a circular economy.
In autumn 2023, Rolls-Royce launched a new mtu CHP module on the market that sets new standards in terms of efficiency, power density and life-cycle costs. The 12-cylinder version is already available to buy – it boasts over 1,500 kW output, peak efficiency of 44.1% and overall efficiency of 90%. Further information is available here.