Construction of a photocatalytic de-polluting field site in the Leopold II tunnel in Brussels

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Highlights

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    Life+ project PhotoPAQ aimed at demonstrating effect of photocatalytic materials.

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    A photocatalytic de-polluting field site was set up in Leopold II tunnel, Brussels.

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    Conception and implementation of measuring campaigns are discussed.

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    Severe de-activation was observed under the polluted tunnel conditions.

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    Recommendations for future applications of photocatalytic technology are given.

Abstract

Within the framework of the European Life+-funded project PhotoPAQ (Demonstration of Photocatalytic remediation Processes on Air Quality), which was aimed at demonstrating the effectiveness of photocatalytic coating materials on a realistic scale, a photocatalytic de-polluting field site was set up in the Leopold II tunnel in Brussels, Belgium. For that purpose, photocatalytic cementitious materials were applied on the side walls and ceiling of selected test sections inside a one-way tunnel tube. This article presents the configuration of the test sections used and the preparation and implementation of the measuring campaigns inside the Leopold II tunnel. While emphasizing on how to implement measuring campaigns under such conditions, difficulties encountered during these extensive field campaigns are presented and discussed. This included the severe de-activation observed for the investigated material under the polluted tunnel conditions, which was revealed by additional laboratory experiments on photocatalytic samples that were exposed to tunnel air. Finally, recommendations for future applications of photocatalytic building materials inside tunnels are given.

Introduction

The de-pollution performance of photocatalytic cement-based materials containing titanium dioxide (TiO2) has been assessed in numerous studies over the past decade, e.g. (Ângelo et al., 2013, Boonen and Beeldens, 2013, Maggos et al., 2008, Maury Ramirez et al., 2010, Ohama and Van Gemert, 2011), illustrating their potential for urban pollution control. However, in addition to application on outdoor building façades and road surfaces, these materials – irradiated by artificial UV light to activate them – might also contribute to a significant reduction of air pollution in road tunnels. Although road tunnels in urban areas are usually well ventilated with regular air renewal inside, previous research has shown that they suffer from strongly elevated concentrations of air pollutants such as nitrogen oxides (NOx), volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs); these are associated with emissions from road traffic, especially during rush hours (Indrehus and Vassbotn, 2001, Larsson et al., 2007, Vanderstraeten et al., 1991). Moreover, air from tunnels is often ventilated to the ambient atmosphere without effective de-pollution measures and, therefore, contributes to a large extent to air pollution in the areas nearby the ventilation exit. Hence, by cleaning the air inside the tunnel using e.g. TiO2 photocatalysis technology, not only a significant improvement for tunnel users could be obtained, but also a better air quality for the outside surroundings. So far, to the authors' best knowledge only one study has been conducted on this topic and it was limited to one type of pollutant, i.e. NOx (Guerrini, 2012).

The European Life + project PhotoPAQ, Demonstration of Photocatalytic remediation Processes on Air Quality (PhotoPAQ, 2010–2014), was aimed at demonstrating the usefulness of photocatalytic construction materials for air purification purposes in the urban environment. Eight partners from five different European countries formed the consortium that undertook this project. For the needs of the project, and in view of the aforementioned findings, an extensive three-step field campaign was organized in the Leopold II tunnel in Brussels, Belgium during the period June 2011–January 2013. In particular, two different photocatalytic cementitious coating materials were applied on the side walls and the ceiling of selected sections in the tunnel branch running along the Basilica–Midi axis. The air-purifying surfaces were activated by a dedicated UV lighting system installed inside the tunnel. During the associated field campaigns, the effect of the photocatalytic coatings on air pollution (including NOx, VOCs, particulate matter, etc.) in the tunnel section was rigorously assessed. In the present article, the construction of the photocatalytic de-polluting field site and the implementation of the field campaigns are elaborated in detail. In addition, the results of some supplementary laboratory experiments are presented; these were performed on photocatalytic samples exposed to tunnel air, in order to investigate possible surface passivation phenomena under the prevailing tunnel conditions. The actual results for NOx abatement are discussed in more detail elsewhere (Gallus et al., 2015).

Section snippets

Setup and requirements for the tunnel field campaigns

Carrying out a monitoring campaign in a tunnel environment has the advantage that local pollution and meteorological conditions are far less variable than in an outdoor environment, thus allowing an easier interpretation of the collected data. For the location of the test site in the tunnel, some general prerequisites had to be met:

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    the site should provide the maximum measurable effect of the photocatalytic material;

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      high surface-to-volume ratio of the photocatalytically active test area,

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      high

First tunnel campaign

During the first main field campaign organized in the Leopold II tunnel the selected test section (Fig. 3) was located between security recesses 43 and 49, with a length of about 90 m measured on the road surface. It starts at a discontinuity in the tunnel ceiling (site 1), after which the cross section remains fairly constant (about 4.8 m × 8.4 m) and ends just before an extraction point (site 2, “first campaign”), to avoid the influence of air renewal on the measurements.

Second tunnel campaign

During the second main campaign a similar strategy was followed as in the September 2011 campaign, with the following differences:

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    increased length of the test section (160 m instead of 70 m);

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    higher UV-A irradiance, with a targeted level of at least 4 W/m²;

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    use of a more active material newly prepared by Italcementi (TX Active Skim Coat Boosted, defined here as TX-Boosted) to provide a higher photocatalytic activity, and much less subject to de-activation at 4 W/m2 (see below).

In addition, prior

Conclusions and perspectives

The Leopold II field campaigns conducted by the PhotoPAQ team proved to be a unique real-world and fully comprehensive assessment of the effect of photocatalytic air-purifying materials on air pollution inside a tunnel environment. Although the final results were not as expected, a lot of useful information was gathered on air pollution dynamics in tunnels and the setting up of a tunnel field campaign.

Furthermore, recommendations for the proper use of photocatalytic materials can be made, such

Acknowledgements

The PhotoPAQ consortium gratefully acknowledges the financial support of the European Commission through the Life + -grant LIFE 08 ENV/F/000487 PHOTOPAQ, as well as the support and participation of the Brussels Regional Public Service – Brussels Mobility in the tunnel field campaigns.

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