Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions
Graphical abstract
Highlights
► Role of key design and dimensioning parameters on PCO performance. ► 20-m3 room-sized chamber using TiO2-coated quartz felt in realistic indoor conditions. ► Efficient removal of VOC mixture that includes formaldehyde and acetaldehyde. ► Contact time between compounds and TiO2 surface is a critical parameter. ► Kinetic model identifies optimal operation conditions.
Introduction
An estimated 10% of the total energy consumed by the U.S. commercial building stock is used to condition ventilation air [1], [2]. Hence, significant energy savings can be achieved in the built environment, among other approaches, by tightening the building envelope and reducing ventilation rates. These strategies require parallel implementation of countermeasures to preserve acceptable indoor air quality, including source controls and the operation of advanced air cleaning technologies. For example, achieving 50% reduction in outdoor air ventilation in a typical US building would require a pollutant removal efficiency of 15–20% in order to prevent increased occupant exposures to volatile organic compounds (VOCs) [3], [4].
Photocatalytic oxidation (PCO) is a promising technology for indoor air purification [5], [6], [7]. It can decompose a broad spectrum of VOCs containing multiple chemical functionalities, including several that are poorly removed by other methods. For example, formaldehyde is not efficiently removed by air cleaning methods based on adsorption (e.g., activated carbon containing media) [3]. Other advantages include relatively low cost and maintenance of PCO systems and the use of non-toxic catalysts. Multiple factors have been observed to affect the efficiency of PCO air cleaners for indoor air applications [8]. Significant research efforts have been devoted to the optimization of photocatalytic reactor design [9], [10], [11], [12], [13], [14] and supported TiO2-based catalysts [15], [16], [17], [18], [19]. However, the formation of partially oxidized byproducts, and specifically volatile aldehydes such as formaldehyde and acetaldehyde, is a concern that remains to be fully addressed. Such byproducts are generated in the photocatalytic oxidation of various VOCs, as shown in experiments performed with full-scale chamber settings [20], [21], [22] and bench-scale experiments [23].
Advancement of PCO air cleaning technologies requires identifying the optimal tests that will challenge the system under realistic conditions that include low (part-per-billion) pollutant concentrations, the presence of a VOC mixture representative of typical indoor contaminants and the use of flow conditions that are consistent with those encountered in buildings, e.g. face velocities typical of heating, ventilation and air conditioning (HVAC) systems. There is an international consensus on the need for reliable standardization of testing methods for air cleaning technologies.
The goal of this study was to evaluate the performance of a PCO air cleaner prototype under realistic conditions. We have carried out experiments to evaluate the effectiveness of the PCO unit in reducing chamber concentrations of target pollutants while minimizing or avoiding the formation of secondary byproducts. Key system parameters were investigated, including the effect of the recirculating airflow rate (or the recycle ratio), the effect of the type of irradiation and the geometry of the PCO media.
Section snippets
PCO media
We used two different types of PCO filters, flat and pleated, fabricated with QUARTZEL® photocatalytic media. The media consisted of TiO2-coated quartz fibers of 9 μm diameter pressed into a 20-mm thick felt. TiO2 particles were deposited on quartz fibers through a sol–gel process, and had a BET specific surface area of 120 m2 g−1.The areal weight of the uncoated felt was 80 g m−2; it increased to 120 g m−2 after the media was coated with titania. Due to the different geometries corresponding to the
Results and discussion
The initial and final chamber concentrations determined in each experiment performed with the flat PCO media (Experiments #2 to #5) and the pleated media (Experiments #6) are reported in Table 3. Acetone is included in the table because it is the only compound that was present in chamber background air at significant levels. No other byproduct was observed to be formed by the PCO reaction, except for very small (<0.2 μg m−3) amounts of propanal and butanal. Recent studies report the formation of
Conclusions
Specific concerns about PCO performance are associated with the formation of volatile aldehydes and other partially oxidized byproducts that can be released back to indoor air. Our results highlight the vital role of design and dimensioning parameters in achieving complete removal of primary pollutants and eliminating or reducing yields of secondary pollutants. Control of these parameters is critical for the safe utilization of PCO systems. In particular, we observed that a sufficiently long
Acknowledgements
The authors thank W.J. Fisk, M.G. Apte, O. Rosseler, M. Sidheswaran (LBNL) and Prof. P. Pichat (EC Lyon) for helpful suggestions. We also thank R. Maddalena, M. Russell, A. Montalbano (LBNL) and A. Durand (SGQ) for experimental assistance. LBNL is a U.S. Department of Energy laboratory under Contract DE-AC02-05CH11231. The authors also acknowledge Prof. J.-M. Herrmann, to whom this special issue is dedicated. In particular, M. Sleiman expresses his deepest gratitude to Prof. Herrmann, for his
References (35)
- et al.
Building and Environment
(2012) - et al.
Applied Catalysis B-Environmental
(2011) Applied Catalysis B-Environmental
(2010)- et al.
Building and Environment
(2010) - et al.
Chemical Engineering Science
(2007) - et al.
Applied Catalysis B-Environmental
(2010) - et al.
Catalysis Today
(2007) - et al.
Separation and Purification Technology
(2009) - et al.
Applied Catalysis B-Environmental
(2010) - et al.
Journal of Hazardous Materials
(2011)
Applied Clay Science
Applied Catalysis B-Environmental
Applied Catalysis B-Environmental
Chemical Engineering Journal
Applied Catalysis B-Environmental
Journal of Photochemistry and Photobiology A: Chemistry
Applied Catalysis B-Environmental
Cited by (111)
Optimization of photocatalytic oxidation reactor for air purifier design: Application of artificial neural network and genetic algorithm
2023, Chemical Engineering JournalMesoporous molecular sieve-based materials for catalytic oxidation of VOC: A review
2023, Journal of Environmental Sciences (China)Large-scale evaluation of microorganism inactivation by bipolar ionization and photocatalytic devices
2023, Building and EnvironmentAdvanced oxidation processes for air purification
2022, Hybrid and Combined Processes for Air Pollution Control: Methodologies, Mechanisms and Effect of Key Parameters