Photocatalytic convertor

Drivers using the Leopold II tunnel in Brussels will have noticed a string of ghostly blue lights in recent days. Those would be ultraviolet (UV) lights, which is what you get above ground in the form of sunlight, and it could be part of the answer to cleaning up the city’s air.

Using a technique known as photocatalysis, the UV lamps work with the cement newly applied to the walls of the tunnel over a length of 160 metres. Particles of titanium dioxide (TiO2) in the cement are able, in the presence of UV light, to react with pollutants in the air, principally the oxides of nitrogen known collectively as NOx, rendering them relatively harmless.

The tests are being carried out by PhotoPAQ, a Europe-wide research consortium based in Lyon, France. According to a spokesman for Brigitte Grouwels, minister for mobility in the Brussels-Capital Region, the project will cost €100,000, entirely financed by PhotoPAQ. “Drivers won’t notice any inconvenience,” he said.

Watching the tests closely will be Dr Anne Beeldens, a scientist at the Belgian Road Research Centre (BRRC) in Brussels and an expert on photocatalysis. It may seem odd that an experiment that requires UV light is carried out in a tunnel, but, she explains, the site has advantages.

“At the moment, the air quality in the tunnel is controlled by ventilation. The number of hours that the ventilators run is determined by the quality of the air, so if you can clean the air in such a way that, for example, you require 10% less ventilation, then you save quite a lot of energy. The UV lighting is currently very strong, but one of the points of the test is to find the optimum level, at which point you would certainly see a reduction in the energy used. So even in a tunnel, the savings could be important.”

Grouwels has said that positive results could lead to the technique being extended to other tunnels in the region.

Better testing methods

This is the second time such an experiment has taken place in the Leopold II tunnel, the longest in the country at about 2.5 kilometres, carrying some 65,000 vehicles a day. Tests in 2011, though, produced disappointingly unclear results. “The light intensity wasn’t sufficient, the product wasn’t applied in a uniform manner, and the test area was rather limited,” Beeldens says. “This time we’ve extended the test area to double the length of the previous test, and we’ve improved the way the cement is applied. The light intensity has also been increased.”

Farther back still, in 2004, the BRRC tested photocatalytic paving stones on a stretch of Amerikalei in Antwerp. They have also done tests on industrial estates in Lier and Wijnegem, here the catalytic compound was contained in the top layer of concrete as well as in paving stones.

“In both those cases, it was extremely difficult to obtain reference measurements,” admits Beeldens. “There’s sure to have been an effect, but given that we had no reference measurements, it’s difficult to estimate the extent of the effectiveness. The project in the tunnel allows us to take measurements before and after the catalytic product is in place, which allows us to compare the data correctly.”

Decrease in CO2, increase in NOx

Cleaning the air of NOx will also play a role in reducing the amount of fine particulate matter (PM) in the air, according to Dr Silvia Lenaerts, a senior lecturer in bio-engineering sciences at the University of Antwerp. In her opinion, a concentration on reducing emissions of the greenhouse gas carbon dioxide (CO2) has led to an increase in the number of smaller PM particles, which have a more deleterious effect on human health than the larger particles, currently filtered out of exhaust gases.

The concentration on CO2, she says, “means we have made more efficient engines, and that means higher temperatures in the engine. And when you have higher temperatures in the engine, nitrogen and oxygen from the air react together and form nitrogen oxide (NOx) but also small soot particles. So it’s inherent to making more efficient engines that you make more particles and more NOx.”

While everyone is talking about CO2, then, “levels of particles, NOx and even indirectly ozone are increasing, because we’ve only been looking at one pollutant,” she says. “And in the meantime we’re making two others that are worse than the original problem. But it’s rather complicated technically, and that’s why it’s so hard to get the point across.”

www.brrc.be

This article first appeared in Flanders Today