1、OR-05-1 7-2 Performance of Air Cleaners for Removing Multiple Volatile Organic Compounds in Indoor Air Wenhao Chen Student Member ASHRAE Jianshun S. Zhang, PhD Member ASHRAE Zhibin Zhang, PhD ABSTRACT Fifteen air cleaners, representing different technologies and types of devices, were tested with a
2、mixture of I6 repre- sentative VOCs (I 7 VOCs in tests forproducts associated with ozone generation) in a full-scale stainless steel chamber by using a bull-down ” test method. Their initial performance was evaluated in terms of single-pass efJiciency (ri) and the clean air delivery rate (CADR). Tec
3、hnologies evaluated include sorption Jiltration, ultraviolet-photocatalytic oxida- tion (UV-PCO), ozone oxidation, air ionization (plasma decomposition), and botanical air cleaning. Based on test results, the relative effectiveness of the available technologies tested and the effect ofproduct config
4、uration and VOC prop- erties on the VOCremoval eficiencies are analyzed. The impli- cation of test results on the development of a standard test method for performance evaluation of gas-phase air cleaners is also discussed briejly in this paper. INTRODUCTION Poor indoor air qualiy (IAQ) can signific
5、antly affect peoples health, comfort, satisfaction, and productivity. Air cleaninglpurification devices, which can be an effective strat- egy for improving IAQ in conjunction with source control and ventilation, have held a substantial market for use in resi- dences and ofices for removing various c
6、ontaminants (US EPA 1996). Volatile organic compounds (VOCs) represent a major class of indoor pollutants. Typical sources include new building materials and furnishings, consumer products, main- tenance materials, tobacco smoke, and polluted outdoor air. VOC pollutants may cause offensive odors, sk
7、in and membrane irritations, allergic reactions, and chronic effects including cancer. In recent years, more and more air cleaning devices are advertised in the market for removing chemical pollutants such as VOCs and for odor control. However, there is limited information available regarding their
8、performance beyond the general claims of the manufacturers, and there are no standard methods for testing the removal of gaseous contaminants by air cleaning devices. This paper briefly reviews the available technologies for removing indoor VOCs and presents test results for initial VOC removal effi
9、ciencies of 15 air cleaners (12 portable air cleaners and 3 in-duct devices for typical residential applica- tions), representing different technologies. It discusses the relative effectiveness of the available technologies tested and the effect of product configuration and VOC properties on VOC rem
10、oval efficiencies. OVERVIEW OF TECHNOLOGIES FOR REMOVING INDOOR VOCS Technologies for removing indoor VOC contaminants mainly include sorption filtration, ultraviolet-photocatalytic oxidation (UV-PCO), ozone oxidation, air ionization (plasma decomposition), and botanical air cleaning. Here, “removin
11、g” generally refers to the concentration decrease of target VOC pollutants in indoor air. They can either be physically removed from air by adherence to, and retention on, the solid sorbents (at least temporarily) or be chemically changed to other substances such as CO, andwater as the desired final
12、 products. The former is a reversible process, while the latter is an irre- versible one. Sorption filtration removes gaseous contaminants from indoor air by adsorption on solid adsorbents. It is a traditional and most commonly used technology. Most off-the-shelf commercial products are based on thi
13、s technology. The effec- Wenhao Chen is a graduate student, Jianshun S. Zhang is an associate professor, and Zhibin Zhang is a research scientist in the Department of Mechanical, Aerospace, and Manufacturing Engineering, Syracuse University, Syracuse, NY. 02005 ASHRAE. 1101 tiveness of cleaners base
14、d on adsorption technology depends on the properties and amount of sorbents, the packing (or coat- ing) density of the sorbent layer, the velocity and flow rate of air passing through the sorbent media, the properties of the VOCs, and environmental conditions such as relative humid- ity and temperat
15、ure. Depending on specific application requirements, adsorbents such as activated carbon, zeolite, and activated alumina with various packing density can be used as filtration media. In some cases where specific contam- inant(s) is targeted, the adsorbents can also be impregnated with selected chemi
16、cals that will react with target substances (chemisorption) (ASHRAE 1999). Activated carbon, espe- cially granular activated carbon (GAC), is the most common media for general indoor gaseous pollutant removal purposes (Henschell998; VanOsdel1 and Sparks 1995). Due to the satu- ration effect of adsor
17、bents after long-term use, the complete evaluation of a sorption type device should include evaluation of both the initial performance (e.g., by initial removal ea- ciencies) and the long-term performance (e.g., by break- through time). In addition, possible reemission ofthe adsorbed VOCs is a conce
18、rn. UV-PCO removes gaseous contaminants via chemical reactions on semiconductor catalyst surface under UV irradi- ation, More specifically, when a semiconductor material is irradiated by photons with energy that matches or exceeds the band gap energy (Eg) of the semiconductor, an electron is promote
19、d from the valence band (VB) to the conduction band (CB), leaving a hole behind. These photogenerated holes and electrons diffuse to the surface and react with adsorbed water molecules. The resultant hydroxyl radicals are highly reactive species that can oxidize VOCs adsorbed on the catalyst surface
20、 (Hager and Bauer 1999; Jacoby et al. 1996). Hoffmann et al. (1 995) reported that the application of illuminated semicon- ductors for the remediation of contaminants has been success- fully used for a wide variety of compounds (alkanes, simple aromatics, etc.). However, there is lack of widespread
21、commercialization of this technology, and only a few products are available in the US market. The effectiveness of cleaners based on UV-PCO technology depends on the photoactivity of the catalyst, the UV light intensity on the catalyst surface, contact time between the contaminated airflow and catal
22、yst surface, the properties of VOCs, and environmental conditions such as relative humidity and temperature. The most widely used photocatalyst for air purification today is Tio2 with Eg = 3.2 eV. Depending on the type and concentration level of the contaminants treated, generation of harmful interm
23、ediates and by-products may be a concern. Ozone is a strong oxidizer. Theoretically, it can react with many VOCs found indoors. In todays market, some ozone- oxidation-based air purifiers are advertised for regular use in homes and offices for removing chemicals and odors. However, under low VOC and
24、 ozone concentration levels, the reaction rate might be too low to be effective for most indoor VOCs. In case of those VOCs that do react with ozone fast enough (e.g., a subset of VOCs with unsaturated carbon- carbon bonds), reaction with ozone may produce other contaminants, e.g., aldehydes and org
25、anic aerosols (Weschler 2000). In addition, lack of adequate control on the ozone generation level can be a concern of safety because ozone itself is a potent lung irritant and is harmful to people at elevated levels. Air ionizers create charged air molecules upon the appli- cation of an energy sour
26、ce (Daniels 2002). Theoretically, air ionization forms “nonthermal” plasmas-cluster ions (radi- calsbwhich decompose VOCs by a complex series of oxida- tion reactions with eventual products of CO, and water. Destruction efficiency depends on ion density, treatment time, and chemical structures ofVOC
27、s (Yan et al. 1998). Two modes of ionization have often been employed: photon ionization and electronic ionization. However, since indoor air chemistry triggered by the ionization process mainly relies on the natural tendency of the ions and the chemicals that exist in the surrounding air, harmful i
28、ntermediates and by-products may be generated during the process. In addition, many ionizers may produce ozone. Related fundamental studies are limited and experimental data are lacking for the demonstration ofthis technology for indoor environmental applications. Botanical air cleaning refers to th
29、e removal of gaseous contaminants from indoor air by plants and their soils through biological processes. It is a relatively novel idea. There are no commercial products in the US market advertised for this tech- nology, although significant reductions in formaldehyde and other VOCs were reported in
30、 several initial static chamber studies (Godish 2001). DESCRIPTION OF AIR CLEANERS SELECTED FOR EVALUATION For residential houses, two types of devices are readily available: stand-alone (portable) room air cleaners (including desktop units) and in-duct devices (filters). Portable air clean- ers can
31、 be easily operated in a room with flexible time sched- ules but only clean the air in a limited area (e.g., up to several connected rooms without obstructions to airflow). In-duct devices (filters) work for the whole house but need to be installed in the air-handling system and function only when t
32、he air-handling system is operating. Table 1 presents descrip- tions of air cleaners tested in this study. In Table 1, the air cleaners are grouped based on the product type first, then the primary technology for VOC removal, and finally are sorted according to the purchase price within the same tec
33、hnology group. EXPERIMENTAL METHODS Test Facilities All of the tests for characterizing the VOC removal ef- ciencies were carried out in a full-scale environmental cham- ber (Figure I), which has an interior volume of 1920 fi3 (16 ft by 12 ft by 10 ft). The chamber and all its components are made of
34、 stainless steel to minimize the adsorptioddesorption of contaminants by the chamber itself. It has a dedicated 1102 ASH RAE Transactions: Symposia Table 1. Summary of Tested Products Device No. P1 Product Type Portable Purchase Type of Air Cleaning Technologies (1) Special high grade of activated c
35、arbon filter and (2) allerm relief filter Price (Stated by Manufacturer) $120 In-duct P2 P3 P4 P5 Primary Technology for VOC Removal Sorption filtration $158 $299 Activated carbon pre-filter and (2) HEPA filter HEPA filter, (2) plasma deodorization unit, and (3) activated charcoal filter Pre-filter,
36、 (2) cotton retaining filter, (3) 6.5 lbs. of carbon- zeolite mixture impregnated with potassium iodide, and (4) HEPA filter media Aluminum mesh pre-filter, (2) HEPA filter, (3) polyester fiber filters treated with an anti-microbial solution, and (4) activated charcoal filter $300 $315 UV-PCO UV-PCO
37、 + Air ionization Air ionization P8 P9 P10 P11 P12T P13 Ozone oxidation Botanical air cleaning $399 $699 High-intensity UV lamp and photo-catalytic semi-conductoi Photo-ionization module, including UV lamp and tri-metallic catalyst, and (2) electron generator Needlepoint ionization - use 16 stainles
38、s steel, ion-producing electrodes to produce a high intensity of negative ions and generate ozone as a by-product $150 $200 Photoplasmdphotochemistry Biofiltration - plant and a proprietaiy lightweight soil Granular activated carbon and activated alumina impregnated with potassium permanganate $33 S
39、orption filtration P151 JV-PCO + Sorption fil- tration Two honeycomb monoliths coated with titanium dioxide cat alyst and an array of 3 germicidal UV lamps in between UV-PCO P6 1 $360 1 Two layers of nonwoven, polyester filter media impregnated with activated carbon and (2) HEPA filter P7 I $470 I P
40、re-filter, (2) electronic cell, and (3) activated carbon post- filter P14 I $485 1 (1) High-intensity UV lamps and photo-catalytic semi-con- ductor and (2) pleated activated carbon filter Flow Rate* (CFW 300 335 240 160 250 200 320 110 14 -t 8 22* NIAtt NIA NIA Measurement methods can be found later
41、 in section of flow rate measurements. Results shown here are for maximum speed level for each air cleaner. t Product P10 is an ionizer with no fan unit but generates tiny ionic breeze. Accurate measurement of the flow rate, although possible, was difficult using our current expen- mental setup and
42、therefore was not conducted. * Product P12 and PI5 are prototypes of innovative products. tt “NA” means “Not Applicable” because product P13, P14, and P15 are in-duct devices and their flow rates are controlled by the air-handling system. The measured pressure drop under test conditions was 0.022 in
43、. w.g., 0.05 in. w.g., and 0.01 in. w.g. for product P13, P14, and P15, respectively. The initial resistance from manufacturer is only available for product PI3 and the given initial resistance is 0.18 in. w.g. at 300 Wmin. Product PI2 is an indoor plant system but has a small fan to deliver air mov
44、ement through the soil and the plant root system. *I HVAC system for the control of the airflow rates and environ- mental conditions in the chamber. A detailed description of this chamber facility and its performance evaluation can be found in Hermann et al. (2003). Test Procedures ASHRAE has sponso
45、red several research projects to investigate feasible test methods for determining the effective- ness and capacity of gas-phase air filtration equipment for indoor applications (Ostojic 1985; VanOsdeIl 1994). A draft standard (ASHRAE Standard 145.1P) has been proposed. It mainly addresses the media
46、 performance and therefore cannot be used for rating the overall performance of an air cleaner. To our best knowledge, no standard test methods have been estab- lished for testing the effectiveness of gaseous portable or resi- dential in-duct air cleaners. AHAM has developed an ANSI- approved standa
47、rd (AHAM 2002) for testing the particulate removal efficiencies of portable air cleaners. Under this stan- dard, the effectiveness of the room air cleaner is quantified by the clean air delivery rate (CADR). This concept has been applied to testing the initial gaseous contaminant removal effi- ASHRA
48、E Transactions: Symposia 1103 O T -+ Tine Figure 1 Full-scale chamber Figure 2 Schematic of pull-down test method. ciencies of portable air cleaners by previous researchers (Daisey and Hodgson 1989; Niu et al. 1998). Since most of the selected products were portable air cleaners, a “pull-down” test
49、method, similar to that used by Daisey and Hodgson (1989) and Niu et al. (1998), was used in this study. It consisted of three test periods under full-recircu- lation condition: injection period (1 h), static period (1 h), and dynamic period (12 h) (Figure 2). The injection of a known amount of contaminants into the experimental system, followed by a static period, resulted in stable initial high concentration levels. The time when the air cleaner was turned on was defined as time zero, at which the dynamic period began. Using the measured concentration decay rate from the dynam
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