Evaluation of an ultraviolet photocatalytic oxidation air cleaner. Impact on volatile organic compounds and ozone

Abstract

The technology of ultra violet photocatalytic oxidation (UV-PCO) in indoor air quality studies is relatively new and to test the VOCs and ozone removal efficien-cies of each new device is essential. A lot of research has been done in laboratory applications; often tests have been done at low airflow rates – from 1 to 100L/min (i.e. a long contact time) – with rather high VOCs concentrations. In building ap-plications, the parameters are completely different: short contact time 100 to 350ms, high airflow rates, and with typical indoor concentration (levels < 20-30ppb in most of the cases) for VOCs. This report presents a set of laboratory tests of a commercially available UV-PCO unit. The study deals with the decom-position of VOCs and ozone under 254nm and 185nm UV wavelengths, the influ-ence of relative humidity of the air, the contact time (i.e. airflow rate) and the identification of by-products generated with rather low VOCs concentration injec-tions. 254nm UV wavelength seems to be good in removing ozone, with a single pass efficiency reaching 60%, in specific conditions: a relative humidity of 35% and long residence time (350ms). Some studies claim that the addition of ozone with 185nm UV lights, enhances the VOCs removal efficiency with long contact time (> 2s). In our case, the 185nm UV wavelength or a combination of 254nm + 185nm UV wavelengths, generates too high concentration of ozone for building applications (>50ppb). The addition of 185 nm UV-light did not seem to improve the VOCs removal efficiency. However, the addition of ozone, obtained by the 185nm UV wavelength irradiation, is suspected to enhance transformation of sty-rene into formaldehyde and possibly also benzaldehyde as by-products. Another part of the report assesses the air flow resistance of the tested UV-PCO unit. The pressure drops were verified and are quite low in comparison with common sorption or active carbon air cleaners, a key point to reduce the energy consumption in HVAC systems. An evaluation of Chalmers test-rig and instru-mentation equipment has been performed, in order to help the new users in their use of these devices in other projects.