On-site reduction of nitrogen oxides at an emission hotspot using actively vented photocatalytic reactors in a highway tunnel

This study presents our solution of an active nitric oxides (NOx) control method for large traffic tunnels. A titanium dioxide (TiO2) coated carrier material is assessed using lab-based photocatalysis experiments, leading to a coating with high photocatalytic activity (deposition speed of 1.4 cm/s for nitrogen monoxide, NO). The coating is tested on several carrier materials to maximize the interaction between the reactive surface and the pollution molecules in the air. Several reactor prototype geometries and carrier materials are simulated and tested on a pilot plant scale. A coated PU-foam with 3 cm thickness and porosity of five pores per inch proved to be the most effective carrier material, while a reactor design with vertically flowed stacks of the foam carrier is capable of optimally exploiting the potential of the photocatalytic coating for high volume flows. With data from on-site measurements of the atmospheric conditions and pollution in the highway tunnel 'Rudower Hohe' in Berlin, Germany, we could build a simulated tunnel setup of our reactors within the tunnel. An estimate based on these simulations assumes a reduction potential of 25% of the NOx mass generated in the tunnel. In conclusion, actively vented TiO2 surfaces are controversial yet could achieve high removal rates while simple to clean or exchange.