The town of Differdange has one obsession, namely cutting its carbon footprint to zero by 2030. To get there, it is now trialling what it calls a very promising solution, unprecedented in Luxembourg. Biochar-enriched asphalt has just been laid on Rue de l'Hôpital.
For road users, nothing really changes, Philippe Reuter, head of the Environmental Department at the municipality of Differdange, readily admits. By simply looking at it, he says, you cannot tell the difference from standard asphalt, apart from it being a slightly darker shade.
Even so, the new asphalt freshly rolled out in Rue de l'Hôpital is the first of its kind to be laid in Luxembourg, and Reuter describes it, with real enthusiasm, as a very promising solution. The reason for the choice, he says, comes down to the carbon footprint.
With Rue de l'Hôpital due for a redesign, Differdange decided to launch a pilot project that could benefit other municipalities down the line. The idea, Reuter explains, was to integrate biochar into the asphalt.
Biochar is a porous material very rich in carbon. It is produced by pyrolysis, a thermal decomposition at high temperature in the absence of oxygen, of residual biomass. By locking carbon away in a stable form for the very long term, it acts as a carbon sink.
The full-scale experiment on Rue de l'Hôpital was put to the town by the BAU Foundation, formerly the Council for Economic Development in Construction, and by the asphalt specialist Julien Cajot. Since Differdange is one of the 100 cities in the NetZeroCities mission aiming for carbon neutrality by 2030, it is multiplying initiatives to bring down its environmental footprint, and agreed to run the test.
Environment and Mobility Alderwoman Zenia Charlé said that by weaving biochar directly into the structure of Rue de l'Hôpital, the town was turning simple road infrastructure into a genuine lever for decarbonisation, thanks to concrete solutions that can be put into practice straight away.
In concrete terms, 200 tonnes of asphalt enriched with 3% biochar were compacted over 1,650 square metres, the equivalent of nearly a quarter of a football pitch. The volume amounts to six tonnes of carbon sequestered, roughly equal to one person's annual carbon footprint.
The chosen street is in fact an open-air laboratory. This carbon-enriched asphalt has already been laid on a cycle path in the Netherlands, but here the forces exerted by cars and lorries will be monitored closely over time. The Environmental Service will observe over two, four, and five years how the biochar asphalt behaves, whether it cracks, deforms or runs into other problems, and try to work out whether any issues come from the biochar itself, Reuter explains.
Following the trial period, Differdange will decide whether to deploy the decarbonisation solution more widely across its municipal road network.
The role of a pioneering municipality, Charlé said, is to clear the path and test the building materials of tomorrow. Faced with the climate emergency, she argued, traditional methods can no longer be relied on alone. She also pointed to the public-private partnership behind the initative, which she said was shifting the paradigm for the benefit of residents.
A great deal is asked of people, Reuter added, from sorting waste to using gentler forms of transport, so the municipality also has to look to its own turf to improve its carbon balance.
As for the cost, the biochar asphalt comes in 2 to 3% more expensive, Reuter said, with the construction work itself costing the same. The extra outlay, Reuter explained, is down to the integration of the biochar.
