German researchers have been developing a new technique to permanently bind carbon dioxide within municipal solid waste incineration (MSWI) ash, and turn it into a usable substitute for sand, gravel, or even concrete components.
MSWI ash consists of up to 95% bottom ash, and fly ash, which is produced from burning municipal solid waste. It is considered hazardous waste due to high concentrations of heavy metals, toxic dioxins, furans and soluble salts.
Different building material samples. Image: refer GmbH.
To tackle this issue, researchers from the Cologne University of Applied Sciences (TH Köln) and RWTH Aachen University, have started exploring whether this harmful residue ash could be repurposed into a climate-friendly raw material.
Led by the Bergischer Waste Management Association (BAV), the project focuses on a natural reaction known as carbonation, in which the minerals inside MSWI ash chemically react with carbon dioxide (CO2).
An eco-friendly solution
Germany produces about six million tons of bottom ash from municipal waste incineration each year. This byproduct is already processed in specialised facilities to recover materials and remove contaminants.
It is ultimately converted into a cleaned finished ash through targeted screening and sorting steps. “The mineral components it contains are capable of absorbing and permanently binding CO2, a process known as carbonation,” says Björn Siebert, PhD, a civil engineering professor at TH Köln.
Siebert elaborated that the project’s goal is to develop a practical carbonation process in order to assess whether the resulting material can be used in road construction or concrete production.
The facility at the Leppe Waste Management centre in Lindlar, Germany. Image: refer GmbH.
To test the idea outside if the lab, BAV is building a new technical pilot plant at the Leppe Waste Management centre near Lindlar, Germany. The plant will trial multiple carbonation methods on ash supplied by industry partner refer GmbH.
Axel Wellendorf, PhD, a mechanical engineering professor at TH Köln, says they plan to pursue at least two different approaches, each with its own advantages and drawbacks.
“Wet carbonation under water allows greater CO2 uptake but requires energy for subsequent drying,” says Prof Wellendor. “In contrast, moist carbonation with limited moisture forms a relatively dense carbonated layer on the ash surface, which stores less CO2 because it cannot penetrate into the interior.”
Construction and concrete production
The team will first test the carbonation processes, and then build a flexible pilot plant which allows them to adjust the methods under real conditions. It will then focus on how to utilise the resulting material.
“Our goal is to substitute raw materials that are otherwise extracted with high energy input,” says Siebert. “In unbound form, our carbonated ash could replace gravel or sand in road construction or earthworks.”
Municipal solid waste incineration (MSWI) ash. Image: refer GmbH.
At the same time, the researchers are also studying whether the ash could work as a concrete binder. This application requires strict quality control, which is why the project includes tests on selective crushing and further material processing to ensure consistent particle size and composition.
“If we succeed in providing carbonated ash that meets the relevant standards and environmental requirements, this would be an important step for the circular economy and climate protection,” Prof Wellendorf concluded in a press release.