ASHRAE AB-10-016-2010 Decomposition of Formaldehyde by TiO2 Nanocatalyst Filters in a Heating Ventilation Air-Conditioning System.pdf

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1、2010 ASHRAE 507ABSTRACT This paper investigates the effectiveness of TiO2photo-catalytic oxidation of formaldehyde under different heating ventilation air conditioning (HVAC)-related air conditions. Formaldehyde, at a specific concentration, was injected into an enclosed HVAC test chamber, and circu

2、lated under UV irradiation though TiO2 nanofilm-coated stainless steel air filter. After two hours of HVAC operation under preset temperature, relative humidity (RH), and air velocity condi-tions, the formaldehyde photocatalyst decomposition effec-tiveness was 6587%. At constant RH and air velocity,

3、 photocatalysis increased with an increase in air temperature. At 4060% RH, and constant temperature and air velocity, the photocatalytic degradation rates were 7889%. Under constant temperature and RH conditions, and air velocities of 0.41.3 m/s (1.34.3 ft/s), the formaldehyde decomposition efficie

4、ncy was 7086%. Therefore, in a sealed system, when one or all of air temperature, RH, and air velocity are increased, the formaldehyde photodegradation rate will be increased.INTRODUCTIONDue to deficiencies in natural ventilation or air exchange in the indoor environment of buildings, mechanical ven

5、tilation equipment may be installed to maintain quality standards, and health and comfort levels of the indoor air. However, when excessive indoor related materials or chemical-based products are present in the indoor environment, harmful chemical substances may be produced (Chiang, 2006), including

6、 vola-tile organic compounds (VOCs) from solvent-based paint and formaldehyde from wooden or other construction materials. These substances are just two indoor factors that may affect human health. A common pollutant of the indoor environment is Volatile organic compounds (VOCs) and formaldehyde (Zh

7、ang, 2007), which may originate from smoking by building occu-pants and/or from construction or related materials. In studies of the relationships between temperature, relative humidity (RH), and formaldehyde concentration indoors, it has been reported that formaldehyde can be removed by photocataly

8、sts (Yang, 2007). However, Matthews et al. (Matthews, 1986)showed a seasonal pattern, with formaldehyde concentrations in summer 69 times higher than in winter. Yu et al. (2006) studied air change rates (ACR), RH, and photocatalytic filters in a simplified heating ventilation air-conditioning (HVAC)

9、 system. Their results showed that first-order decay of toluene and formaldehyde ranged from 0.381 to 1.01 h-1under differ-ent total ACR, from 0.34 to 0.433 h-1under different RH, and from 0.381 to 0.433 h-1for different photocatalytic filters. However, increasing outdoor airflow rate increased the

10、cool-ing load of the HVAC system (Amal, 2001). In order to effec-tively solve the above problem, it is possible to increase purification equipments of indoor is needed, as to reduce the volume of external air induction, and then to save energy and enhance indoor air quality.Recently, a lot of resear

11、ch into the removal of pollutants by photocatalysts has been performed (Ao, 2004). The photocatalyst titanium dioxide (TiO2) has many advantages related to its high level of photocatalytic activity, high photocatalytic efficiency, low operation temperature, high specific surface area, good chemical

12、stability, and thermal stability, TiO2used for reaction are inexpensive. Studies have shown that it completely oxidized pollutant into CO2and H2O (Michael, 1996). An experiment Decomposition of Formaldehyde by TiO2Nanocatalyst Filters in a Heating Ventilation Air-Conditioning SystemChing-Song Jwo Ch

13、ien-Chih Chen Ho ChangSih-Li Chen Shin-Jr HoChing-Song Jwo is a professor and Shin-Jr Ho is a student in the Department of Energy and Refrigerating Air Conditioning Engineering, and Chien-Chih Chen is a doctoral candidate and Ho Chang is a professor in the Graduate Institute of Mechanical and Electr

14、ical Engineering at the National Taipei University of Technology, Taipei, Taiwan, Republic of China. Sih-Li Chen is a professor in the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China.AB-10-016 2010, American Society of Heating, Refrigerating and Ai

15、r-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.508 ASHRAE Transactionsu

16、sing a TiO2photocatalyst-attached air cleaner showed formal-dehyde removal efficiency of 7080%, and toluene removal effi-ciency of 3540% (Zhang, 2003). Esswein et al. reported that an ozone-attached air cleaner had poor formaldehyde removal effi-ciency (Esswein, 1994). Moreover, Ao et al (2005). rep

17、orted that the volume of pollutants absorbed by activated carbon decreases with a rise in temperature. Furthermore, when RH is high, re-emission of formaldehyde adhered to activated carbon woven cloth and to the surface of the filter wall may occur. Therefore, high temperature conditions may not be

18、favorable to the adsorp-tion of pollutants (Wark, 1998). However, Formic acid is the most commonly found intermediate from the conversion of formalde-hyde by photocatalytic and photooxidation (Ao, 2004; Veyret, 1989), photocatalysis in aqueous phase (Shin, 1996) and gaseous phase (Yang, 2000). In ge

19、neral it has been found that using photo-catalysts to remove indoor VOCs is quite effective (Fujishima, 1972; Ao, 2003; Obee, 1995; Jardim, 1994; Peill, 1996). In this study a small-sized, simulated air-circulation chamber system, was used to simulate an indoor air quality in an enclosed environment

20、, and measure formaldehyde removal efficiency of a photocatalyst-attached air filter. The TiO2photocatalyst was combined with a stainless steel air filter, using a water-soluble polymer binder to firmly fix the TiO2film on the surface of the stainless steel filter. The intention of this filter was t

21、o remove VOC nanoparticles from indoor air as they pass though a HVAC system. Such filtering, if effective, would prevent the particles from entering the nostrils, depos-iting in the lung, and resulting in pulmonary fibrosis or other pathologies to the human occupants of the building. Another advant

22、age of a TiO2-attached air filter is that it can be reused after being washed with pressurized clean air. When fiber-based filters are tested, air and dust passing though a loaded filter have been reported to allow the propagation of patho-genic bacteria, resulting in formaldehyde and acetone produc

23、-tion (Hans, 1999). This study used a stainless steel-based air filter to avoid the propagation of pathogenic bacteria within the filter (ASHRAE, 2005).EXPERIMENTAL DETAILSThe commercial TiO2photocatalyst used was Ti-1125A (QFnano Co., Ltd, Taiwan, Specific surface area (BET): 7515 m2/g (366,262.137

24、3,252.43 ft2/lbm), Al2O3-content: 0.003%, SiO2-content: 0.006%). Fig. 1 shows the X-ray diffraction (XRD) analysis of the TiO2nanofluid. The nanoparticles were anatase TiO2, and TEM analysis indicated good nanoparticle dispersion with a mean particle size of below 10 nm (32.8 nft). After using water

25、-soluble polymer binder to affix the TiO2coating on the stainless steel filter, the filter was installed in a temperature, RH, air velocity-adjust-able HVAC system (Fig. 3). The experiments simulated an enclosed indoor air-conditioning environment that was polluted by formaldehyde. Experiments were

26、conducted with variations in temperature, RH, and air velocity conditions. Measurement of formaldehyde concentrations used a Formaldemeter 400 (PPM Technology, Caernarfon, Wales, UK) with a detection range of 010 ppm (037.85 gal/L), a detection limit of 0.01 ppm (0.0378 gal/L), and a precision of 10

27、% at 2 ppm (7.57 gal/L). The meter was suitable for use over a temperature range of 540C (41104F), and a rela-tive humidity range of 4060%. The study first determined the background adsorption decline of formaldehyde in the test HVAC system and determined the portion of the decline caused by natural

28、 adsorption of formaldehyde by the materi-als within the testing system. Subsequently, ultraviolet (UV) Figure 1 (a) XRD and TEM structural analysis chart of the (b) TiO2used in the experiments.(a) (b)2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.o

29、rg). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 509irradiation was added to determine the effects of UV

30、photo-catalysis of the formaldehyde pollutant using a filter without a TiO2 coating. Next, a direct photolytic experiment was completed using a TiO2-attached stainless steel filter irradi-ated with UV light. These separate experiments were used to determine formaldehyde adsorption by the HVAC system

31、s materials, as well as by UV light only, and by UV light on a TiO2photocatalysis on formaldehyde concentrations.Presetting of ParametersThe American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) standards suggest values for indoor environments temperature and RH during w

32、inter and summer (ASHRAE, 2005). In summer, the most comfortable temperature range was 2527C (7780.6F) dry-bulb (DB), and the most comfortable RH was 4555%. In winter, the most comfortable temperature range was 20-22C (6871.6F) DB, and the most comfortable RH was 4555%. Therefore, our tests were per

33、formed under the common summer and winter indoor temperatures. Under a fixed RH of 50% and a fixed air velocity of 0.7m/s (2.3ft/s), the study investigated the reaction efficien-cies of the photocatalytic decomposition of formaldehyde at temperatures of 18(64.4), 20(68), 22(71.6), 24(75.2), 26(78.8)

34、 and 28C (82.4F). The study also investigated the influence of different RH values (40, 50 and 60%) on the photocatalytic removal efficiency of formaldehyde. For determination of photocatalytic decomposition effec-tiveness, the UV light should sufficiently irradiate the surface Figure 2 Images befor

35、e and after using a water-soluble polymer binder to apply a TiO2coating to a stainless steel air filter: (a) original stainless steel filter (optical microscope, magnification = 40) (b) TiO2coated stainless steel filter (optical microscope, magnification = 40) (c) TiO2coating on the stainless steel

36、filter (SEM image, magnification = 1000).(a) (b)(c)2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in eithe

37、r print or digital form is not permitted without ASHRAEs prior written permission.510 ASHRAE Transactionsof the photocatalyst, and the organic pollutant needs sufficient contact time with the photocatalyst surface. Thus, air velocity in the test system can have an influence on the time during which

38、organic substances are in contact with the photocatalyst surface. Therefore, this study determined the effects of air velocity in the test system. Here, we studied the influence of weak (0.4 m/s)(1.3 ft/s), medium (0.7 m/s)(2.3ft/s) and strong (1.3m/s)(4.3ft/s) air speeds, on the effectiveness of ph

39、otocat-alytic degradation of formaldehyde.Preparation of Stainless Steel Photocatalytic FiltersDue to the photochemical nature of photocatalysts, photocatalysis only occurs on surfaces that receive light. If a photocatalyst is embedded within a fixed area or in a fluidized bed, effectiveness of the

40、photocatalyst can be reduced due to shadowing of the available light. This study selected hole-punched stainless steel filters as the base plate for the photo-catalyst. That selection avoided adsorption on the base plate (Jardim, 1994), and allowed the added TiO2film to be distrib-uted evenly and co

41、ating speedy. The filter preparation proce-dure was as follows:a. The hole-punched stainless steel filter (0.21m (0.689 ft) 0.21m (0.689 ft) 1 mm (0.00328 ft) thick; model, SUS304), with pore diameter of 2.0 mm (0.00656ft) and distance between pores of 1 mm (0.00328ft), was cleaned using ethanol and

42、 DI-water.b. A 1 wt. % solution of water-soluble polymer binder (QFnano Co., Ltd, Taiwan) was prepared, and evenly applied by spray onto the stainless steel metallic filters, which were then baked at 50C (122F) for 10 min to increase viscosity, strengthen the binder coating layer, and allow TiO2nano

43、particles to firmly affix to the stain-less steel.c. The TiO2nanofluid (TiO2in DI-water) was blended to have a TiO2concentration of 3 wt. %, and was then vibrated using a supersonic vibrator (DC150H, DELTA) for 1020 min at beaker to ensure even distribution of TiO2within the solution.d. The vibrated

44、 TiO2nanofluid was then poured into a sol-vent tank, and the prepared stainless steel base plate was submerged in the tank for 510 min to allow the sus-pended TiO2nanoparticles to adhere to the filter surface. The TiO2-stainless steel filter was then sintered at 100C (212F) for 10 min.e. To ensure s

45、imilarity of test filters, a microbalance (resolu-tion 0.001g, Precisa) was used to weigh five TiO2catalyst coated stainless steel filters base plates both before and after TiO2coating. The allowable difference among the five was 0.2 g (441lbm) 5%. In addition, scanning electronic microscopy (SEM) w

46、as used to observe the distribution of TiO2particles on the base plates. As shown in a representative SEM image (Fig. 2(c), TiO2particles were evenly distributed and formed an even layer of nanoparticles on the base plate.f. UV light irradiation was produced by five UVC lamps (5W (17.05Btu/hr), PHIL

47、IPS) with a combined luminous intensity of 2.35 mW/cm2 (7.445 Btu/hr/ft2). Apparatus for Simulation of an Air-Conditioning SystemA schematic diagram of the experimental apparatus is shown in Fig. 3. The volume of the test chamber was 0.29 m3 (10.24 ft3). The apparatus comprised a fan, pre-heater, ev

48、ap-orator, re-heater, humidifier, and a pre-filter. This enclosed HVAC chamber was a modification of an existing tempera-ture- and humidity-adjustable air-conditioning system (model, P.A. Hilton Ltd., Stockbridge, UK). The modifica-tions (temperature, RH, and air velocity) allowed accurate control o

49、f temperature, RH and air speed; required to meet our experimental needs. Temperature control was performed using a proportionalintegralderivative controller (TTM 100, TOHO Electronics Inc., Kanagawa, Japan) was attached to a solid state relay (SSR)-controlled heater. In this way, linear temperature increases could be acquired. After mutual temperature chasing by the heater and cooler, a preset stable temperature value can be reached in the test condition. The controls allowed a temperature control error of 0.1C (32.2F) to be a