| With more and more attention of indoor air quality (IAQ), the photocatalysis technology, as an advanced high-tech purification technology, is paid great attention in the world due to the fact that it can degrade a broad range of volatile organic compounds (VOCs) to H2O and CO2 at room temperature and atmospheric pressure. Formaldehyde extensively exits in indoor environments, and it is one of representative indoor contaminant. In this study, the photocatalytic degradation of formaldehyde in heating, ventilating and air-conditioning (HVAC) systems was studied, which has a vital significance for improving IAQ and was supported by the Special Foundation of Nanometer Technology (No. 0552nm002) from the Shanghai Municipal Science and Technology Commission. The research included that the optimum of influence factors including the thin-film thickness of catalysts and the light intensity, the design and optimum of a new-type photocatalytic air-purifier, the improvement of formaldehyde degradation by the hybrid process of vacuum ultraviolet (VUV) and photocatalysis, and the prediction of performance demands of purifiers. The research provided a theoretical basis for the application of photocatalysis technology in HVAC systems. The contents and conclusions are as follows:(1) Optimum of the thin-film thickness of catalystsThe optimum of the thin-film thickness of catalysts is a specific question for the immobilized catalyst. Based on the reaction kinetics and the mass transfer theory, a model was developed to predict the optimal thin-film thickness of catalysts for photocatalytic degradation of formaldehyde by a TiO2-coated foam nickel at the continuous flow mode. The results showed the reaction rate of formaldehyde increased firstly and then reached a plateau with the increase of the thin-film thickness of catalysts. The attenuation coefficient of ultraviolet (UV) light in catalyst layers had great effect on the optimal thickness of catalysts and made the optimal value change at the order of magnitude. Shorter the wavelength was, smaller the attenuation coefficient was. This model can explain the effect of catalyst thin-film thickness on photocatalytic reaction rate and the effect of model parameters on the optimal thin-film thickness of catalysts.(2) Optimum of light intensitiesThe UV light is the driving force of photocatalytic reactions, and it directly influences the photocatalytic reaction rate. At the same time, it is also a kind of energy, so its appropriate use has vital significance for saving energy. Light energy losses in the heterogeneous photocatalysis process were analyzed. A new idea of using appropriate light intensities was presented to decrease the recombination of electron-hole pairs for improving the utilization ratio of light energy. A model was presented to predict light intensities for the photocatalytic degradation of indoor gaseous VOCs based on the degradation mechanism. A method was obtained to determine the appropriate light intensity by simplifying the model base on the definition of an excess coefficient. The use of the method was explained by the degradation data of formaldehyde from literature and experiments. The results showed that the derived light intensities according to our method were appropriate. Good degradation performance and high utilization ratio of light energy can be attained simultaneously. The appropriate light intensity depends on the species and concentration of VOCs, the properties of the catalysts and supports and the wavelength of UV light.(3) Design and optimum of a new-type photocatalytic air-purifier used in HVAC systems.The flow velocity in HVAC systems is high, so how to increase the resident time of VOCs and decrease the pressure loss of purifiers are two important questions in the design process of photocatalytic purifiers. A novel photocatalytic purifier, available to HVAC systems, was researched based on the effects of the windward area on the face velocity and the pressure drop. The resistant of the purifier was tested. The performance of the purifier was investigated by degrading formaldehyde at an indoor concentration level. The optimum step of purifier size was presented. The experimental results showed that the purifier had uniform light intensities, low pressure loss and high energy efficiency. This configuration allows for much larger reaction area on a limited cross section and the large-scale application by increasing the number of reaction cells connected in parallel. It is flexible enough to adapt to different HVAC systems with different air velocities by changing the windward area. The light source can be rationally selected according to the optimum step of purifier size for decreasing the energy consumption.(4) Experiments on the improvement of the removal of formaldehyde by the hybrid process of VUV and photocatalysisVUV, an economical method, was combined with photocatalysis to improve the removal of formaldehyde. The technological and economical feasibility of the hybrid process were investigated experimentally and theoretically. The results showed that the hybrid process can be technically feasible and economically attractive for the decomposition of gaseous formaldehyde. The outstanding merit of the hybrid process is the complementation of advantages of VUV and photocatalysis. The removal efficiency of formaldehyde can be improved markedly by VUV on the basis of photocatalysis. Ozone produced by VUV can be decomposed by photocatalysis. The hybrid process was more economical than the photocatalysis process with a cost reduction of about 44% for removing per kg formaldehyde when the air flow rate was 518 m3/h and the formaldehyde concentration was 0.6 mg/m3. Because the indoor materials can decompose ozone, the fresh air can dilute the ozone concentration, and the photocatalytic purifier can decompose ozone, the allowable concentration of ozone produced by the hybrid purification system is higher than the maximum value recommended by the WHO. Therefore, the hybrid system can be applied to the engineering.(5) Predictions of the performance demand, operation time and maximum energy consumption demand of photocatalytic purifiers.The effects of the efficiency of photocatalytic purifiers on the indoor concentration and emission rate of formaldehyde were investigated by considering the emission of building materials. According to these studies, the performance demand and the operation time of purifiers were predicted and analyzed. The maximum energy consumption demand of photocatalytic purifiers were predicted based on the comparison between the ventilation and photocatalysis. The results showed that the removal efficiency was proportional to the power function of purification times. The effect of the diffusion coefficient on the linear coefficient was large. When the interior diffusion process controlled the VOC emission and the decrease rate of the indoor VOC concentration with purifiers was no larger than that without purifiers, the application of purifiers will not improve the removal rate of indoor VOC and its unique action is that controlling the indoor VOC to low level. But, whether the purifier should stop operating depends on the effect of the VOC dose on the human's health. The energy consumption of ventilation presents the demand of the maximum energy consumption to photocatalytic purifiers based on the same purification time. The maximum energy consumption depends on the set value of indoor air temperature, the outdoor air temperature, the outdoor contaminant concentration and so on. This research provided theoretical basis for the design and application of photocatalytic purifiers.
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