ASHRAE LV-11-C052-2011 Air Cleaning by Photo Catalytic Oxidation An Experimental Performance Test.pdf

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1、g53g68g74g76g69g3g46g68g71g85g76g69g72g74g82g89g76g70 and g37g76g81g74g69g76g81g74g3g54g75g76 are PhD students in the Department of Energy and Environment, Division of Building Services Engineering, Chalmers University of Technology, Gothenburg, Sweden. g47g68g85g86g3g40g78g69g72g85g74 is a project

2、manager at CIT Energy Management and an associated professor in the Department of Energy and Environment, Division of Building Services Engineering, Chalmers University of Technology, Gothenburg, Sweden. g36g81g87g82g81g76g68g3g42g76g79g79g76g74g68g81 is a head manager at ALPHA Environmental Inc., E

3、merson, New Jersey, USA. g36g76g85g3g38g79g72g68g81g76g81g74g3g69g92g3g51g75g82g87g82g3g38g68g87g68g79g92g87g76g70g3g50g91g76g71g68g87g76g82g81g29g3g36g81g3g40g91g83g72g85g76g80g72g81g87g68g79g3g51g72g85g73g82g85g80g68g81g70g72g3g55g72g86g87g3g53g68g74g76g69g3g46g68g71g85g76g69g72g74g82g89g76g70g3g3

4、g3 g3 g47g68g85g86g3g40g78g69g72g85g74g15g3g51g75g17g39g17g3Student Member ASHRAE g36g81g87g82g81g76g68g3g42g76g79g79g76g74g68g81 g3 g3 g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g3g37g76g81g74g69g76g81g74g3g54g75g76g3g36g37g54g55g53g36g38g55g3g3The paper presents the results from an experimental

5、 evaluation of a novel air cleaner based on UVC radiation at 253.7 nm in combination with a titanium dioxide (TiO2) catalytic converter. The wavelength of the UV light is selected with the purpose of enhancing the capability of the device to deactivate micro-organisms. Furthermore, the selected UV-w

6、avelength is expected not to cause any substantial generation of ozone. The air cleaner, denoted PCOC3, consists of three photo catalytic oxidation (PCO) chambers connected in series. The device is equipped with a pre-filter for removal of airborne particles (MERV 11). The air cleaner is intended fo

7、r in-duct use, i.e. integration in central ventilation systems. In a subset of the experiments ozone was generated by an electrical spark generator placed upstream of the tested air cleaner and measured downstream by a direct reading instrument based on UV-spectroscopy. In another subset of experime

8、nts selected VOCs were injected into the test-rig and the decay was measured after the VOC injection had been stopped. The measurements showed no signs of any ozone being generated by the PCOC3. Instead the results indicate that ozone is captured by the device at a removal rate corresponding to 67 a

9、ir changes per hour in the test-rig used, which, in turn corresponds to a single pass efficiency of about 15%. Also the studied VOCs were found to be removed by the PCOC3, but at substantially lower rates; in the range of a few air changes per hour only. g44g49g55g53g50g39g56g38g55g44g50g49g3g3Photo

10、 catalytic oxidation (PCO) is a process where a semiconductor, upon adsorption of a photon, acts as a catalyst in producing reactive radicals, mainly hydroxyl radicals, which in turn can oxidize organic compounds and mineralize them. In this way organic molecules are decomposed to form carbon dioxid

11、e, water and mineral acids as final products (Goswami, 2003). Both the hydroxyl free radical (OH) and the superoxide anion radical (O2-) are highly reactive species that oxidize VOCs. However, the hydroxyl free radical is the most reactive of all reactive oxygen species (ROS) and is the primary oxid

12、ant in PCO reactions, in which pure or doped metal oxide semiconductors (e.g., TiO2, ZNO, CdS, Fe(III)-doped TiO2) are commonly used as the photo catalyst. PCO reactions with TiO2, which is the most common catalyst, have been described by Goswami (2003) and Zhao and Yang (2003). Utilizing PCO to rem

13、ove trace-level organic compounds in air has recently received considerable attention since this technology has a potential to be applied to air purification in office buildings, factories, homes, cars, spacecraft and special LV-11-C052426 ASHRAE Transactions2011. American Society of Heating, Refrig

14、erating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.laboratories. T

15、he applicability of novel air cleaning technologies including PCO has been evaluated by LBL (2009). Several detailed reviews of PCO have been done, (Daniels, 2007, Zhao, Yang, 2003). The overall conclusion of the reviews is that PCO with TiO2 as catalyst and UV light is a promising technology that i

16、s self-regenerating and potentially cost effective. The performance of a PCO prototype and commercialized systems were considered for destruction of individual VOCs and mixtures of VOCs (Goswami, 2003, Hodgson et al., Jo et al., Pershin et al., 2004, Sun et al., 2006). Reported VOC single-pass conve

17、rsion efficiencies varied widely, from 20% to almost 100%. Efficiencies in the higher end of this range were reported when the PCO devices were exposed to individual VOCs. Hodgson et al. (2007) concluded that for a prototype device evaluated with realistic mixtures of VOCs, conversion efficiencies t

18、ypically exceeded the minimum required to counteract the VOC concentration increase predicted to arise from a 50% reduction of the ventilation air flow rate. However, the device resulted in a net generation of formaldehyde and acetaldehyde from the partial oxidation of ubiquitous VOCs. Other studies

19、 done with mixtures of VOCs reported similar by-products due to incomplete oxidation. The oxidant used in PCO is 20% of oxygen present in air. Because the concentration of oxygen in air is so much larger than the (total) concentration of gaseous indoor air pollutants, it is not necessary to employ a

20、dditional oxidants such as ozone or hydrogen peroxide (Tompkins et al. 2005). Nevertheless, using UV lamps in the PCO process may imply generation of ozone depending on lamp type (Jeong et al. 2005). Moreover, some researchers used ozone-producing lamps intentionally to enhance destruction of VOCs i

21、n the PCO reactors as ozone is a powerful oxidant and very reactive gas (Jeong et al. 2005, Zhang et al. 2003, Zhang and Liu, 2004, Yang et al. 2007, Huang, 2009). For example, Zhang and Liu (2004) showed by experimental studies that the addition of ozone significantly increases photo catalytic degr

22、adation of hexane. The reaction rates of hexane increased linearly with an increase of ozone addition. They also reported a positive effect of ozone on the regeneration of TiO2 photo catalyst. However, as a consequence of its high reactivity, ozone is a very toxic material. It causes headaches, thro

23、at dryness and damage to mucous membranes at levels as low as 0.1 1 ppm and can be life threatening at levels higher than this (Mills et al. 2003). An example of a far-reaching indoor air quality guideline regarding ozone is the one published by FiSIAQ (2001), suggesting a maximum of 10-40 ppb, depe

24、nding on indoor air quality class selected. It is therefore, extremely important to ensure that harmful amounts of ozone are not generated in PCO process. This paper reports the results from an experimental evaluation of a novel air cleaner based on UVC radiation at 253.7 nm in combination with a ti

25、tanium dioxide catalytic converter. The wavelength of the UV light is selected with the purpose of enhancing the capability of the device to deactivate micro-organisms. Furthermore, the UV-wavelength is expected not to cause any substantial generation of ozone. In this paper the tested air cleaner i

26、s denoted PCOC3, since the unit has three photo catalytic oxidization chambers coupled in series. Each of the three oxidization chambers has a UV-light source and a TiO2 converter. g48g40g55g43g50g39g54g3g55g72g86g87g16g85g76g74g86g3The experiments were carried out in two laboratories using similar

27、test-rigs. Testing was done at Alpha Environmental, Inc. and at Chalmers University of Technology, Dept. of Energy and Environment, Division of Building Services and Engineering. A simple schematic of the test-rigs is shown in Figure 1. 2011 ASHRAE 427Figure 1 Schematic arrangement of the test-rigs.

28、 The systems are essentially similar in being closed loop test-rigs with the tested PCOC3-unit connected to straight rectangular ducts, as indicated by Figure 1. The internal cross section of the PCOC3-unit is 2 ft (0.6 m) wide by 1.7 ft (0.5 m) high. The Alpha test-rig is a closed loop with a volum

29、e of 65.9 ft3(1.9 m3) with a fan generating a flow rate of 423 cfm (0.2 m3/s), which means that the PCO-unit was tested at a face velocity of 127 ft/min (0.7 m/s) A volatile organic vapor generator was coupled upstream of the PCOC3-unit. Samples were withdrawn before and after the photo catalytic un

30、it and analyzed by infrared spectroscopy (IR), The test-rig at Chalmers has a volume of 282 ft3(8 m3) with a fan generating an air flow of 2118 cfm (1 m3/s), which means that in this case the PCOC3-unit was tested at a face velocity of about 600 ft/min (3 m/s). In the preliminary studies done at Alp

31、ha Environmental, Inc., single organic compounds were introduced into the test-rig. The concentration was measured by a Miran IR Analyzer after the photo catalytic unit. To estimate the removal rate, the test was run with the UV light source energized. The concentration was recorded as a function of

32、 time, and the concentration decay after the VOC generation had been stopped was analyzed. The test was re-run with the UV lamp off, and the rate of VOC removal determined in this case was attributed to VOC removal other than photo catalytic conversion, e.g. adsorption and leakage from the test syst

33、em. The preliminary study at Chalmers comprised measurements of ozone by UV-spectrometry. These tests were conducted according to the same principle as the VOC measurements, i.e. injection of ozone upstream of the PCOC3-unit and by analyzing the concentration decay recorded with the UV-lamp on and w

34、ith the lamp off, and comparing the two decay rates obtained. g48g72g68g86g88g85g72g80g72g81g87g3g72g84g88g76g83g80g72g81g87g3IR Spectroscopy was done using a Miran Sapphire Infrared spectrophotometer using its internal calibration and compound library. The instrument was used according to the direc

35、tions of the manufacturer ThermoFisher Scientific Corporation. Ozone concentrations were measured by UV-spectroscopy using an Environics Series 300 instrument. This instrument enables the ozone concentration to be determined with a detection limit of 1 ppb. The physical parameters of the test-rig we

36、re also determined. The test-rigs temperatures were periodically measured by a Hanna HI 8757 K thermocouple thermometer. Air flow was measured with a Sper Scientific Anemometer Model # 428 ASHRAE Transactions840003. Static pressure and/or air velocity were checked with a Fluke 922 Air Flow and Stati

37、c Pressure meter, and by a Swema Air 300 instrument equipped with sensors of models SWA 31 and SWA10. g53g40g54g56g47g55g54g3g36g49g39g3g39g44g54g38g56g54g54g44g50g49g3A sample analysis for benzene, measured by IR-spectroscopy, is outlined below. These measurements were carried out in the Alpha test

38、-rig operated at a face velocity of 127 ft/min (0.7 m/s). The test-rig has an internal volume of 1.9 m3. At Time 0 benzene was loaded into the test-rig with the PCOC3-unit operating with UV-light on. At about 15 minutes the injection of benzene stopped and the decrease in concentration was recorded

39、as shown by Figure 2. This data was transformed into a logarithmic concentration vs. time plot, see Figure 3. g51g38g50g38g22g3g37g72g81g93g72g81g72g3g57g50g38g3g44g81g77g72g70g87g76g82g81g3g68g81g71g3g53g72g80g82g89g68g79g3g47g76g74g75g87g3g50g810200004000060000800001000001200001400001600000:00:00

40、0:07:12 0:14:24 0:21:36 0:28:48 0:36:00 0:43:12 0:50:24 0:57:36g55g76g80g72g83g83g69g3g37g72g81g93g72g81g72Figure 2 Release and removal of benzene with the PCOC3 UV lamp on. The measurements were carried out in the Alpha test-rig operated at a face velocity of 127 ft/min (0.7 m/s). The test-rig has

41、an internal volume of 1.9 m3. 2011 ASHRAE 429g51g38g50g38g22g3g37g72g81g93g72g81g72g3g3g47g76g74g75g87g3g50g81g3g47g81g11g69g72g81g93g72g81g72g12g3g89g86g3g87g76g80g72y = -121.98x + 13.059R2= 0.9949.51010.51111.5120:00:00 0:07:12 0:14:24 0:21:36 0:28:48 0:36:00 0:43:12g55g76g80g72g79g81g3g11g69g72g8

42、1g93g72g81g72g12Figure 3 Plot of benzene removal rate plot. The slope of the decay curve indicates a removal rate corresponding to 5.1 air changes per hour. The measurements were carried out in the Alpha test-rig operated at a face velocity of 127 ft/min (0.7 m/s). The test-rig has an internal volum

43、e of 1.9 m3. Figure 3 indicates a removal rate equivalent to 5.1 air changes per hour (ach). Performing the same series of calculations for the removal observed with the UV-light off indicates the loss rate of the test-rig due to non photo catalytic mechanisms, e.g. air leaks and adsorption on the T

44、iO2-converter and other interior surfaces. The loss rate was in this case found to be equivalent to 2.9 air changes per hour. This results in a net removal rate of 2.2 air changes per hour for benzene under the conditions of the test. This net removal rate indicates the marginal effect from turning

45、on the UV-light. The testing was expanded to include ethyl benzene, xylenes, toluene, ethanol hexane and formaldehyde using the same IR analytical approach and calculation. The results are summarized below: Table 1. Removal Rates of Sample VOCs as Determined by IR Analysis. The Measurements Were Car

46、ried Out in The Alpha Test-Rig Operated at a Face Velocity of 127 ft/min (0.7 m/s). The Test-Rig Has an Internal Volume of 1.9 m3. VOC-Species Initial Concentration at Start of Decay (ppm) Removal When UV-Light Is on (ach) Removal When UV-Light Is off (ach) Net Removal Rate (ach) Ethyl benzene 27 6.

47、8 4.9 1.9 Benzene 140 5.1 2. 2.2 Xylenes 125 6.8 5.6 1.Toluene 22 5.8 3.6 2.2 Ethanol 400 6.5 5.8 0.72 Hexane 470 7.3 6.3 0.97 Formaldehyde 17 6.5 6.2 0.30 g3It is acknowledged that air leakage as well as adsorption on and desorption from the interior surfaces of the test-rig most likely have influe

48、nced the measured decay curves. This has not yet been evaluated further. However, all of the studied VOC-species showed a faster decay rate when the UV-light was on, compared to the case with UV-light off. Thus, the results 430 ASHRAE Transactionsindicate that the studied PCOC3-unit removed VOCs by

49、photo catalytic mechanisms. g50g61g50g49g40g3The ozone measurements were carried out in the Chalmers test-rig operated at a face velocity of 600 ft/min (3 m/s). The test-rig has an internal volume of 8 m3. Ozone was generated at a constant rate resulting in a steady-state concentration of 29 ppb when the PCOC3 was completely removed from the test-rig (both UV-luminaries and the TiO2-converters removed). Concentrations were also measured when the PCOC3 was installed, both with and without UV-light. In all experiments a pre-filter of class MERV 11 was installed immediately upst