Getting the Circular Economy Rolling With a Rotary Kiln

This article was originally published in Thinkers & Makers, a magazine from Akkodis featuring the smartest minds and innovative projects that are driving the future of technology and engineering.

An innovative thermolysis process refined and optimized by Akkodis can put waste to good use, turning it into condensates, raw materials, heat, and power using a rotary kiln able to light a fire without oxygen.

Imagine not having to dig into the ground to find precious metals and other raw materials. Instead, you feed waste into a custom-built oven. With the right amount of indirect heat, it produces new materials, which can be used in a factory or sold to the highest bidder.

Even the nastiest types of waste can be used. Scrap tires, dry sewage sludge, contaminated soil, chicken manure – take your pick. All these waste materials contain value in some form, whether as liquid or solid substances or in gaseous form.

Dry sewage sludge from leather tanning and dying, for instance, contains up to 10 % chromium. Feed it into a custom-built rotary oven, and the end result is a condensate with a 35 % chromium content. That is high-yielding compared to mining, where good chromium ore contains only 5 %.

Trash to Treasure

“Here in Germany, we have around 5,000 old waste dumps that have been closed and covered,” said, a leading expert in rotary kiln pyrolysis technology and member of the chemical Dirk Gerlach plant engineering group at Akkodis Leipzig. 

“When they were in operation, waste sorting was non-existent, so these dumps contain large amounts of raw materials that could be processed and reused.”

He points to battery recycling as a possible use case. With a scarcity of metals for car batteries, harvesting new raw materials from waste has obvious potential. 

Different Chemistry

If you ask Gerlach to explain the technology behind the waste incineration plants, he is an expert in designing, and you’ve already made your first rookie mistake. Because, as he patiently points out, what happens in one of his rotary kilns is fundamentally different from a conventional incineration process. According to him, the difference is oxygen.

“Conventional incineration has a surplus of oxygen. But inside the kiln, there is oxygen deficiency, which gives you a completely different chemistry. You can run various processes in our plant, with low or high temperatures and with or without steam. Depending on how you set the process parameters, that enables you to generate liquid or solid raw materials or gas,” he said. 

While conventional waste incineration turns waste into energy, the pyrolysis and thermolysis processes Gerlach is dealing with turn waste into products, such as hydrogen, methane, sulfur, heating oil, acetic acid, and more. All these have the potential to be used in the new circular economy the world is looking for: Waste is collected and sorted, fed into the thermolysis and carbon capture plant, which then produces new CO2-free raw materials. 

“I predict the waste-to-resources market will step up the pace even further. As CO2 taxes are rising, the demand for technology to reduce CO2 footprint is growing. And that is where we can help.”

Dirk Gerlach, Rotary Kiln Pyrolysis Technology Expert, Akkodis Germany

Innovative Combinations 

According to Gerlach, pyrolysis and thermolysis are well-known technologies that have existed for decades. He and his colleagues are now combining existing components and processes to create an innovation that could be labeled a Swiss army knife of waste-to-resources technology.

To achieve that versatility, Gerlach unites the two worlds of water and fire. In the fire world of a furnace, you have temperatures up to 1.200°C. In the water world of a chemical plant, the typical operating temperatures are below 500°C and the pressures are upwards of 0.5 bar. 

Gerlach merges the two extremes into a new rotary kiln thermolysis design that can produce gas, coke, raw materials, and more, making the technology suitable for various purposes.

“It can be combined with a conventional coal-fired power plant,” Gerlach said. “These plants cannot process plastic waste or biomass. But by connecting them to a thermolysis plant, you can use these materials to produce gas and coke, which can be fed into the coal boiler.” 

Research Meets Reality

The broader topic of thermolysis still needs more research, but the Akkodis next-generation thermolysis technology is nearly ready for full-scale production. For operational flexibility, the team has designed standard facilities in three different sizes: MINI, MIDI, and MASTER.

Gerlach and his colleagues are participating in a development project focusing on producing raw materials in Bavaria. The H2-Reallabor Burghausen project converts chemical waste to new materials. A prototype of the kiln is under construction, and it should start operations in early 2025.

To secure this and other publicly funded research for thermolysis, the chemical plant engineering group at Akkodis Leipzig has collaborated with Akkodis Research, which links publicly funded research activities and Akkodis Centers of Expertise. Akkodis Research combines applied research with the short-term innovation needs of Akkodis clients.

Thinking outside the Kiln

Gerlach is noting a growing interest from many industrial actors as more companies look for ways to reduce their carbon footprint.

However, some challenges to the broader adoption of thermolysis remain. Building a precise business case is difficult because of uncertainties regarding the price of the material going into the kiln and the market value of the substances it produces.

New raw materials produced from scrap tires could be recycled into new tires. Still, if companies could broaden their vision of potential value streams, these materials might also command greater value for other purposes. 

Gerlach, however, believes the market needs to evolve to keep up with technological innovations. He is confident that evolution is underway.

“I predict the waste-to-resources market will step up the pace even further. As CO2 taxes are rising, the demand for technology to reduce CO2 footprint is growing. And that is where we can help.”

The MIDI Plant Design

The MIDI plant design is a standard turnkey unit for 24/7 operations with a 1 t/h capacity. It is equipped with four-step condensation for various condensate qualities and coke post-processing. Including all additional services and storage, the plant site measures 130 x 150 m.

The MIDI system is modular and uses 40’ HQ containers for a semimobile operation. After 3-5 years, when an operation has been completed, the plant can be moved to another location.

The design input is 1 t/h, and four lines can be stacked as part of one system with a fully automatic control system (AutomationX). The system’s inputs and end products can change as markets evolve to focus on solids (carbon or ore), condensates (oils and acetic acid), or syngas (CO/H2 or CH4/CO2).