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Thermal catalytic oxidation performance study of SWTCO system for the degradation of indoor formaldehyde: Kinetics and feasibility analysis
Building and Environment 108 (2016) 183e193
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Thermal catalytic oxidation performance study of SWTCO system for the degradation of indoor formaldehyde: Kinetics and feasibility analysis Bendong Yu a, Wei He b, *, Niansi Li a, Feng Yang b, Jie Ji a a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, 230026, China Department of Building Environment and Equipment, Hefei University of Technology, Hefei, 230009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 18 August 2016 Accepted 28 August 2016 Available online 31 August 2016
This article proposes a novel application combining the thermal catalytic oxidation with solar-collected wall (SWTCO) in buildings without auxiliary energy. Thermal catalyst MnOx-CeO2 was prepared by the method of modified coprecipitation. The performance and kinetics of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde were investigated. Once-through experiments at different concentrations of 300 e4300 ppb and temperatures 20e100 C were conducted. Moreover, formaldehyde conversion experiments in SWTCO system at several typical indoor concentrations (289 ppb, 587 ppb and 1374 ppb) were performed. Furthermore, a simple system model predicting the degradation of indoor formaldehyde versus the time in SWTCO system was built. Results were as follows: (1) The reaction rate expression considering two parameters i.e. concentration and temperature based on modified L-H model fit the experimental data well; (2) System model calculation results showed initial formaldehyde concentration was an important factor to the formaldehyde degradation capability; (3) SWTCO system could realize high pollutant-removal efficiency and low energy costs simultaneously under higher temperature compared with the system using electrical heating; (4) The purification time constant increased with the horizontal solar radiation and the influence of concentration in typical indoor formaldehyde concentration on it could be neglected. © 2016 Elsevier Ltd. All rights reserved.
Keywords: SWTCO MnOx-CeO2 Solar energy Formaldehyde Kinetics Numerical model
1. Introduction Formaldehyde is a prevalent indoor chemical pollutant in building environment and considered to be carcinogenic and teratogenic. Due to the rapid development of real estate industry especially in China, more and more residential and commercial buildings have been constructed overnight. These newly buildings release large amounts of formaldehyde gas from the decorating materials, plywood, fiberboard, particleboard, and other artificial boards [1]. However, due to tight buildings and high emission materials, there is a general agreement that the concentration of indoor formaldehyde is 2e10 times than that of current legal limiting concentration [2]. Tang et al. [3] reported that the maximum formaldehyde concentration in newly remodeled residential buildings were more than 4472 ppb. The WHO guideline concentration for indoor
* Corresponding author. E-mail address: [email protected] (W. He). http://dx.doi.org/10.1016/j.buildenv.2016.08.030 0360-1323/© 2016 Elsevier Ltd. All rights reserved.
formaldehyde is only 80 ppb [4]. Therefore, indoor formaldehyde is a great threat for indoor air quality and human health. Several strategies such as adsorption, photocatalytic oxidation (PCO), and thermal catalytic oxidation (TCO) have been investigated for the elimination of indoor formaldehyde in recent decades [5e9]. However, the main technical barrier is the secondary pollution of toxicity when degrading volatile organic compounds (VOCs) for photocatalytic oxidation technology [10,11]. Moreover, the band gap of semiconductor greatly limits its application on solar energy. The photoresponse range of commonly used photocatalyst TiO2 is where the ultraviolet light lies in and the ratio of ultraviolet light in solar spectrum is only 5%. Adsorption technology can effectively capture the indoor formaldehyde at low concentration levels. But it is restricted because of the problems such as the limited adsorption capacity and adsorbent regeneration. Catalytic oxidation technology is found to be innovative and promising technique to gaseous formaldehyde abatement and has advantages such as high removal efficiency, low light-off temperature, wide application scope, simple equipment, and no secondary pollution [12].
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Greeks
Nomenclature I A c R T Nu Re Pr m Q K h
l C D f F t x E V r
2
global solar radiation, W/m area, m2 specific heat capacity, J/(kg$K) the universal gas constant, J/(mol$K), or radius of catalyst particles, m temperature, K Nusslet number Reynolds number Prandtl number mass, kg volumetric flow rate of gas, ml/min reaction factor, m/s external mass transfer coefficient, m/s, or heat transfer coefficient, W/(m2$K) thermal conductivity, W/(m$K) concentration of formaldehyde, ppb, or characteristic constant effective diffusion coefficient, m2/s, or the thickness of the air cavity, m external diffusion factor view factor time, s the distance from the air inlet in the air collector, m activation energy or adsorption heat of formaldehyde, J/mol air flow velocity, m/s, or volume of the test chamber, m3 catalytic degradation rate, ppb/(m$s)
The thermal catalytic materials mainly fall into two categories: noble-metal and transition-metal oxide. And noble-metal catalysts have been playing an important role in the field of energy and environment because of their high reaction activity [13,14]. The research on total oxidation of formaldehyde into harmless products CO2 and H2O at low temperature even at ambient temperature has been attracted tremendous interest. The method of supporting noble metals is proved to be effective for improving the activities of catalysts. Zhang et al. [15] prepared various TiO2 supported noble metal catalysts (Au/TiO2, Rh/TiO2, Pd/TiO2 and Pt/TiO2) by the impregnation method. Results showed that the order of the catalysts activity was Pt/TiO2 [ Rh/TiO2 > Pd/TiO2 > Au/TiO2 [ TiO2. And the experimental formaldehyde (100 ppm) could be completely oxidized into CO2 and H2O over Pt/TiO2 in a gas hourly space velocity (GHSV) of 50,000 h1 even at room temperature. Nie et al. [16] prepared Pt/TiO2 catalysts with various Pt loadings (0.05e2 wt%) and found that the amount of noble-metal dispersion was an important factor that influenced the activity of catalyst. Also, other factors such as the type of support, relative humidity and metal valence state have been investigated [17e19]. Similarly, Pt/MnOx-CeO2 showed extremely high activity and stability and realized the totally conversion of formaldehyde (580 ppm) at ambient temperature and no deactivation was observed for a 120 h continuous experiment [7]. High noble-metal content is an important reason for the high activity of these thermal catalysts. However, the high price of noble metals still greatly limits their application in a long time. Transition metal oxides such as MnOx, CeO2, and CuO also behave high catalyst activity to the degradation of formaldehyde. And more importantly, they are cheap and plentiful. Tang et al. [9] prepared MnOx-CeO2 mixed oxides by the method of modified
r F
ε
a s t n
density of catalyst, Kg/m3 solar flux absorbed, W conversion efficiency absorptivity, or thermal diffusivity, m2/s Stefan-Boltzmann constant, W/(m2$K4) transmissivity, or purification time constant dynamic viscosity of air, m2/s
Subscripts 0 original t certain time cat catalyst particles eq equivalent in inlet out outlet m mean or mass transfer s surface g glazing ab absorber plate ca air cavity Abbreviation TCO thermal catalytic oxidation PCO photocatalytic oxidation GHSV gas hourly space velocity CADR Clean air delivery rate, m3/h ppb parts per billion ppm parts per million
coprecipitation and results showed 30% formaldehyde conversion at the temperature as low as 303 K and 100% formaldehyde conversion at 373 K with a 580 ppm feed concentration and space velocity of 21,000 ml/gcat$h. However, the research on TCO in typical indoor contaminant concentration levels such as less than 1 ppm is very limited in the open literature. Most of them focused on the formaldehyde concentration levels in the range of hundreds of ppm. Pei et al. [20] investigated the performance and kinetics of CuO-MnO2 for indoor formaldehyde removal under typical indoor concentration levels (180e1300 ppb). They found that the reaction rate followed the traditional L-H model. However, transition metal oxides are not able to totally degrade formaldehyde at room temperature. In general, the formaldehyde conversion efficiency increases with the temperature of catalyst bed [9,20e22]. And current research on solar air collector showed the temperature of air in the air cavity could approach 60e100 C [23,24]. An interesting idea is the combination of TCO and solarcollected wall (SWTCO). Indoor air with trace amount of formaldehyde is heated by solar-collected wall. Then the heated air enters the catalyst bed. The catalyst bed is heated by the hot air. Formaldehyde is continuously degraded by the heated catalyst bed without added energy. Then the clean air enters room. Moreover, this system can realizes the space heating in winter. This article prepared transition metal oxides MnOx-CeO2 by the method of the modified coprecipitation. To investigate the performance and kinetics of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde, once-through experiments at different feed concentrations of formaldehyde (300e4300 ppb) and temperatures of catalyst bed (20e100 C) were conducted. The kinetics model simultaneously considering concentration and temperature was proposed. Moreover, formaldehyde conversion experiments in
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SWTCO system at several typical indoor concentrations (289 ppb, 587 ppb and 1374 ppb) were performed and a simple SWTCO system model was built. 2. Experimental 2.1. Preparation of MnOx-CeO2 catalyst The preparation method for the MnOx-CeO2 catalysts has been described previously [7,9]. Modified coprecipitation method was applied in this article. Firstly, a mixed solution containing Mn(NO3)2$6H2O, KMnO4 and (NH4)2Ce(NO3)6 was previously stirred and became a uniform suspension solution and sealed. And the specific mole ratio of these three chemical reagents was 3: 2: 5. Secondly, 2 M KOH solution was dropwise added to the abovementioned mixed solution at 50 C until the pH value of the mixture reached 10.5 under vigorous stirring. The precipitate was further aged at 50 C for 2 h. Then the filtration and washing procedures were proceeded. At last, the obtained materials were heated at 110 C for 12 h and calcined at 500 C for 6 h in air. The desired catalysts were obtained. 2.2. Catalyst characterization The porous parameters including BET specific area and pore diameter of the catalysts were measured using ASAP2010 adsorption volumetric adsorption analyzer made by Micromeritics Co. in USA from Instruments' Center for Physical Science, University of Science and Technology of China through the N2 adsorption method. X-ray powder diffraction pattern was recorded with a D/Max2500/PC diffractometer (Rigaku, Japan), University of Science and Technology of China, operated at 40 kV and 250 mA, using nickelfiltered Cu Ka (l ¼ 0.15,418 nm) radiation. 2.3. Activity measurement 2.3.1. Formaldehyde single-pass conversion measurement Fig. 1 shows the schematic of the experimental set-up used for investigating the formaldehyde single-pass conversion over the prepared catalysts. Similar experimental set-up was been described in our previous study [25]. This set-up consisted of three sections: (1) gas-generating section, (2) reaction section, (3) gas detection section. The main difference in this set-up was the reactor. Fig. 2 shows the photo of continuous flow-type fixed bed reactor. It was made of stainless steel tube with the structure of stepped hole. The inner hole was used to be filled in the catalysts, which had an
Fig. 1. Schematic diagram of experimental set-up to study formaldehyde single-pass conversion over the catalysts. (1) Dry air cylinder. (2) Pressure gage. (3) Gas purifier (silica gel and molecular sieve). (4) Bypass valve. (5) Flowmeter. (6) Formaldehyde bottle with bubbler. (7) Thermostatic bath. (8) Three-way valve. (9) Mixed chamber. (10) Temperature controller. (11) Tube furnace (12) Reactor. (13) Formaldehyde detector.
Fig. 2. The photo of continuous flow-type fixed bed reactor.
internal diameter of 5 mm and a height of 10 mm. The metal net was used to support the catalysts and ensured the filled catalysts were not flowed away. The stainless steel reaction tube and the air tube were connected through two screw threads. The reaction temperature of the bed was controlled by tube furnace MXG120060 purchased from Shanghai Weihang furnace industry co. in China. Formaldehyde detector ZK-101D purchased from Beijing Zhongke Zhonghuan environmental application technology research center measured the effluent gas stream from the reactor. Measurement uncertainty of measured gas concentration was 5%. And temperature uncertainty of all thermocouples was ±0.5 C.The temperature control range of reactor is 20e100 C. Approximately, 0.15 g of catalyst (40e60 mesh) was used for each reaction. The total flow rate was 400 ml/min, and the gas hourly space velocity (GHSV) was 160,000 ml/(gcat$h). We chose five experimental formaldehyde concentrations i.e. 300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb, respectively. We recorded the final outlet concentration after a steady-state condition was achieved. A fresh catalyst was used for each experiment. 2.3.2. Formaldehyde conversion measurement in SWTCO system Fig. 3 shows the schematic diagram of experimental set-up to study formaldehyde conversion over the catalysts in SWTCO system. And Fig. 4 shows the photo of the simple SWTCO system. The test chamber of length 0.75 m, width 0.75 m and height 1 m, respectively, was made of stainless steel with little adsorption to the gaseous formaldehyde, which was used as simulating the indoor environment. And air collector was also made of stainless steel considering the low adsorption to formaldehyde as well. It consisted three sections: absorber plate, air cavity and glazing. Thermal insulation material were also used for these two parts. The reactor was same as Fig. 2. The mass of used catalysts was 3.125 g. Before every experiment, a certain amount of formaldehyde was injected into the test chamber through a micro-syringe. The initial formaldehyde concentration was measured until the formaldehyde gas in the chamber was mixed uniformity through the rotation of the electric fan. Then the gas in the test chamber was extracted by fan and flowed through air collector and the reactor and eventually return to the chamber. The air flow was 0.5 m3/h. Concentration of formaldehyde in the chamber was measured every 5 min. The thermocouple measured the temperature every minute. Before the formaldehyde was added, to keep only one contaminant i.e. formaldehyde in the chamber, the gas in the chamber entered the gas purifier for enough time. Considering the typical indoor exceeding formaldehyde concentration and the actual gas-generating concentration, we choose three experimental concentrations i.e. 289 ppb, 587 ppb and 1425 ppb, respectively.
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10
ε¼
19 9 7
3
6
12 17 8
16
rR2cat r Cs Dm
(2)
Q ðCin Cout Þ ABET
(3)
Cwp ¼
18 11
1
2
(1)
where Cin and Cout, ppb, are inlet and outlet concentration under the steady state, respectively. The Weisz-Prater criterion was calculated to evaluate the internal mass transfer [20]. The effect of internal mass transfer can be ignored when the criterion is much less than 1.
4
5
Cin Cout 100%; Cin
13
15
14
Fig. 3. Schematic diagram of experimental set-up to study formaldehyde conversion over the catalysts in SWTCO system. (1) Test chamber. (2) Electric fan. (3) Thermocouple. (4) Reactor. (5) Microinjector. (6) Formaldehyde detector. (7) Sampling opening. (8) Air inlet. (9) Air outlet. (10) Three-way valve. (11) Flowmeter. (12) Air collector. (13) Valve. (14) Fan. (15) Gas purifier (silica gel and molecular sieve) (16) Solar radiation. (17) Absorber plate. (18) Glazing. (19) Air flow direction arrows.
r¼
where Cwp is Weisz-Prater criterion, r is the reaction rate, which is defined the amount of formaldehyde oxidation per unit area of the catalyst and per second, ppb$m/s. Q is gas flow rate, ml/min. And ABET is the total surface area of the catalyst, m2. Rcat is the radius of catalyst particles, m, r is the density of catalyst, Kg/m3, Cs is the surface formaldehyde concentration on the catalyst, ppb, and Dm is the effective diffusion coefficient, m2/s. According to the study of Xu et al. [21], the external mass transfer correction factor fm, which value stands for the ratio of reaction resistance in the total resistance including external convective mass transfer resistance and reaction resistance, can well describe the effect of external convective mass transfer. The influence of external mass transfer for reaction of catalytic bed almost eliminate when fm approach to 1.
fm ¼
h m As hm As þ Kapp ABET
(4)
Cs ¼
Cin Cout fm lnðCin Þ lnðCout Þ
(5)
where fm is the external mass transfer correction factor, Kapp is the apparent reaction factor which was defined as the reaction rate per unit surface concentration, m/s, As is the total external surface area of the catalyst, m2. hm is the mean external mass transfer coefficient, m/s, which can calculated by the empirical correlation [26]. Cs is the average surface concentration of catalyst, which is calculated by the logarithmic mean value of inlet and outlet formaldehyde concentrations. The details of calculation can be referred in Xu et al.’s study [21]. 3.2. SWTCO system model Fig. 4. The photo of a simple SWTCO system.
Because the thickness of the glazing is very thin, it is reasonable to ignore the heat transfer in the horizontal. Therefore, the temperature of the glazing can be assumed uniform in the x-direction.
3. Data analysis and system model 3.1. Kinetic data analysis Generally speaking, three processes happen in the catalytic oxidization of formaldehyde including external mass transfer through the external film of catalysts, internal mass transfer and surface reaction [20]. This article focused on the intrinsic reaction kinetics of the prepared catalysts. Therefore, the surface reaction should play a determinative effect to the reaction rate through eliminating the effect of external and internal mass transfer. Using the concentrations of the gases at a steady state, the formaldehyde conversion is calculated according to the following equation:
mg cg
dTg ¼ Ag hconvrad Teq Tg þ Ag hconv;g;ca Tca Tg dt þ Ag hrad;g;ab Tab Tg þ Fg
Fg ¼ Ag I90+ ag
(6) (7)
where Teq is the equivalent temperature, which is considered to be equal to the outdoor air temperature, and hconv-rad has a standard value of 29 W/(m2$K) [27]. Fg is the solar flux absorbed by the glazing. The convective heat transfer coefficient between the glazing and the air in air cavity hconv,g,ca is calculated by the empirical formula of laminar flow forced convection over a plate [28].
B. Yu et al. / Building and Environment 108 (2016) 183e193 1=2
Nux ¼ 0:332Rex Pr 1=3 Re ¼ Va ¼
Va x
na Q As
(8)
Fab ¼ Aab I90+ tg aab
(9)
where tg is the transmissivity of the glazing, aab is the absorptivity of the absorber plate. In the test chamber, we assume that the temperature in it is a constant and the formaldehyde in it is mixed well all the time because the rotation of electric fan. And the temperature of catalyst bed is assumed to be equal to the outlet temperature of air cavity because the amount of catalysts is very little relative to the air [21]. The mass conservation equation of formaldehyde in the chamber is as follows:
(10)
hconv;g;ca ¼
Nux la x
(11)
where the characteristic length in characteristic number is the distance from the air inlet in the air collector. Va is the face velocity of the cross section of air cavity, m/s, la is the thermal conductivity of air, W/(m$K), na is the dynamic viscosity of air, m2/s, As is the cross section air cavity, m. hrad,g,ab is the radiation heat transfer coefficient between the glazing and the absorber plate, which is defined as [27]: 2 4sTm hrad;g;ab ¼ . 1 Fg;ab þ 1 εg εg þ ð1 εab Þ=εab
(12)
where εg and εab is the emissivity of the glazing and the absorber plate, respectively. Tm is the mean temperature of the glazing and the absorber plate. Fg,ab is the view factor for the glazing and the absorber plate, which can be calculated by using the fictitious cavity method showed in Fig. 5 [28]:
Fcd;ab ¼
ðbc þ adÞ ðac þ bdÞ 2ab
(13)
The air per unit length in the air cavity is taken as a control volume. It is given as:
ra Dca
vTca vTca ¼hconv;g;ca Tg Tca þhconv;ab;ca ðTab Tca Þ ra Va Dca vt vx (14)
where D is the thickness of the air cavity, m. It is reasonable to assume the temperature of inlet air in air cavity is a constant because the air in the test chamber has little change. In fact, air condition can keep indoor temperature almost constant. The model of the absorber plate is similar to the glazing:
mab cab
187
dTab ¼Aab hconv;ab;ca ðTca Tab ÞþAab hrad;g;ab Tg Tab þ Fab dt (15)
V
(16)
dC ¼ ðr þ rn ÞA dt
(17)
where V is the volume of the test chamber, m3, C is the formaldehyde concentration in the test chamber, ppb. And rn and r are the adsorption or the leakage rate produced by this system itself and catalytic degradation rate, respectively, ppb/s, A is the total surface area of the catalysts, m2. The detail computational parameters was shown in Table 1. 4. Results and discussion 4.1. BET and XRD measurements The basic physical pore structure parameters for the prepared catalysts are listed in Table 2. These parameters were necessary for the further kinetic analysis. The XRD pattern of MnOx-CeO2 are shown in Fig. 6. The pecks where the red and blue arrows point to belong to MnO2 and CeO2 phase, respectively [29]. 4.2. Kinetics Fig. 7 shows the impact of internal and external mass transfer in the reactor under different formaldehyde concentrations and temperatures. From these results, all Cwp under different Table 1 Detail computational parameters for the SWTCO model.
Test chamber Air cavity Glazing
Symbol
Explanation
Unit
Value
L*W*H L*W*H
Size Size Density Specific heat capacity Absorptivity Transmissivity Emissivity Density Thickness Specific heat capacity Absorptivity Emissivity Density specific heat capacity Thermal conductivity Kinematic viscosity
m m Kg/m3 J/(Kg$K) e e e Kg/m3 mm J/(Kg$K) e e Kg/m3 J/(Kg$K) W/(m$K) m2/s
0.75*0.75*1 0.08*0.01*0.24 2500 840 0.1 0.9 0.9 2700 0.5 900 0.95 0.95 1.18 1100 0.026 1.58*105
rg cg ag
tg 3g
Absorber plate
rab dab cab
aab 3 ab
Air
ra ca
la na
Table 2 Physical parameters of the catalysts. Physical parameters
Fig. 5. Fictitious cavity for calculating the view factors for the glazing and the absorber plate.
Particle size (mesh) Average pore diameter (nm) BET area (m2/g) Particle porosity
40e60 22.6 64.6 0.49
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Fig. 6. XRD pattern of the prepared MnOx-CeO2.
Fig. 8. Temperature dependence of formaldehyde conversion over the prepared catalysts under different formaldehyde concentrations: 300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)).
Fig. 7. Impact of internal mass transfer and external mass transfer in the reactor under different formaldehyde concentrations and temperatures (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)). Fig. 9. Reaction rate at different surface formaldehyde concentrations at temperatures of 20 Ce100 C (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)).
formaldehyde concentrations and temperatures were much less than 1 (in the order of 104) and all fm almost approached to 1, which suggested that the influence of internal and external mass transfer for reaction of catalytic bed in our reaction conditions could be ignored. Fig. 8 shows the formaldehyde single-pass conversion over the prepared catalysts under different temperatures (20e100 C) and various inlet concentrations (300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb). The formaldehyde single-pass conversion ε approached 30%e50% at typical indoor formaldehyde concentration such as 300 ppb and 600 ppb at temperature of 40e60 C. For all curves, the degradation efficiency of formaldehyde increased with the increase of temperature. In all the range of measured temperature, the degradation efficiency of formaldehyde decreased with the increasing concentration. The reaction rate is related to the surface formaldehyde concentration. Fig. 9 shows the reaction rate at different surface formaldehyde concentrations at temperatures of 20 Ce100 C. For the curves below 70 C, reaction rate increased firstly then approached the peck and finally decreased with the increase of formaldehyde surface concentration. At this low temperature range, thermal catalytic had enough reaction activity to degrade the
surface formaldehyde gas when the surface concentration was low. Theoretically, reaction rate approached to a constant when the surface concentration kept increasing due to the limited reaction activity in the range of low temperature. In reality, reaction rate decreased slightly, which might be a result of hardly release of the reaction products in time with the increasing surface concentration. However, for the curves above 70 C, reaction rate presented the monotonically increasing trend as the increase of surface concentration. These because the high reaction activity of thermal catalytic at high temperature range could degrade more formaldehyde timely. This article was aimed at investigating the kinetic model of the catalytic reaction involved two variables i.e. reaction concentration and temperature. To simplify this issue, in current study, we firstly fit the experimental data only considering single variable. Then the kinetic model considering two variables was developed. The surface reaction was the rate-determining step of the catalytic oxidization process when the impact of internal and external mass transfer in the reactor could be ignored based on Fig. 7. Bimolecular Langmuir-Hinshelwod model has been widely applied
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in the research on catalytic reaction kinetics (considering absorbed formaldehyde reacting with adsorbed O2, with competitive adsorption) [20,21]. The experimental data were fitted to L-H model using formula (18) and the results were listed in Table 3. The resulting correspondence between the fitting and data was good for all samples from the correlation coefficient R2.
r¼
k1 k2 Cs
(18)
ð1 þ k2 Cs Þ2
The impact of temperature to the reaction rate mainly be shown in the reaction coefficient and adsorption coefficient. The temperature dependence of these two coefficients can be described by Arrhenius equation:
E k1 ¼ k01 exp 1 RT
(19)
E k2 ¼ k02 exp 2 RT
(20)
Fig. 10. Linear dependence between lnk1 and 1/T using Arrhenius equation.
where k10 and k20 are the pre-exponential factor, E1 and E2 are the activation energy of catalytic reaction and adsorption heat of formaldehyde, respectively, J/mol. And R is the universal gas constant, J/(mol$K), T is the thermodynamic temperature, K. Then the log-linearization of formulae (19) and (20) were as follows:
ln k1 ¼ ln k01
E1 1 R T
(21)
ln k2 ¼ ln k02
E2 1 R T
(22)
Figs. 10e11 show the fittings of reaction coefficient and adsorption coefficient applying formulae (21) and (22) based on the data in Table 3. The resulting correspondence was good for all data from the correlation coefficient R2. And some basic parameters such as k10, k20 , E1 and E2 could be obtained from the slope and intercept of the curves in Figs. 10e11. By applying the Arrhenius equation to correlate the reaction coefficient and adsorption coefficient at different temperatures, the bimolecular L-H kinetic was expressed as [21]:
r¼
Fig. 11. Linear dependence between lnk2 and 1/T using Arrhenius equation.
E1 E2 k2 exp RT Cs k1 exp RT
(23)
2 E2 1 þ k2 exp RT Cs
Fig. 12 shows predicated versus experimental reaction rates for the catalytic oxidation of formaldehyde. The coefficient of determination R2 was 0.98, which indicated the modified model fit well with the experimental data. The reaction activation energy of formaldehyde catalytic reaction on the catalysts MnOx-CeO2 was Table 3 Parameters of formaldehyde catalytic oxidation in bimolecular L-H form. Temperature ( C)
k1 (ppb$m/s)
k2 (ppb1)
R2
20 30 40 50 60 70 80 90 100
0.0003245 0.0004783 0.0008448 0.0012 0.0016 0.0023 0.0029 0.0033 0.0033
0.0008517 0.0006954 0.0005574 0.0005 0.0004 0.0003 0.0003 0.0002 0.0002
0.95 0.98 0.98 0.99 0.99 0.98 0.99 0.99 0.99
Fig. 12. Predicated versus experimental reaction rates for the catalytic oxidation of formaldehyde.
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Pollutant-removal efficiency and energy costs are two important aspects for the design and research of an air cleaner. Clean air delivery rate (CADR), m3/h, which is an assessment of pollutantremoval efficiency, can be expressed as follows:
CADR ¼ mSBET
Fig. 13. Influence of temperature on clean air delivery rate (CADR) and clean air delivery rate per power input (CADR/P) for system using solar energy and system using electric heater. Mass of catalyst (40e60 mesh): 432 g, flow rate 70 m3/h, formaldehyde 300 ppb, environmental temperature 20 C.
estimated as 30.615 kJ/mol, which was larger than that of on the catalysts Pt/MnOx-CeO2 (25.8 kJ/mol) [21]. These indicated that noble metal catalysts could reduce the reaction activation energy of formaldehyde catalytic reaction. However, transition metal oxides were the better candidate considering the price of noble metal catalysts.
r Cin
(24)
where m is the mass of catalyst, SBET is the total BET area of used catalysts, r is the catalytic reaction rate, which can be calculated by equation (23) and Cin is the inlet formaldehyde concentration. CADR is the volume of fresh air generated by an air cleaner per hour. CADR/P, which is defined the clean air delivery rate per power input, is an assessment criteria of energy costs of an air cleaner. And P is the energy input rate of an air cleaner, including fan and electrical heater. For assessing the performance of SWTCO system, we simulated two systems: (1) system using solar energy i.e. SWTCO system; (2) system using electrical heater. A packed catalytic bed of mass of catalysts 432 g (40e60 mesh) in the reactor was used for this calculation. Air flow rate was 70 m3/h and formaldehyde concentration was 300 ppb. Environmental temperature was 20 C. And we assumed that electrical heating energy were all used for heating air flow in the air cleaner. The electrical heating energy is zero when the reaction occurs at room temperature. Fig. 13 shows CADR and CADR/P of two simulated systems under different temperatures. The value of CADR increased with temperature almost linearly, which was consistent with Xu et al.’s research [21]. The value of CADR/P for two systems presented
Fig. 14. Comparisons of experimental data and predicted data in formaldehyde conversion experiments under solar radiation (N: numerical result, E: experimental data, (a): air temperature curve of the air collector outlet, (b) ~ (d): concentration curves of formaldehyde in the test chamber under different initial formaldehyde concentrations). Experiment conditions: (a) time: time: 13:00e16:00, 5th June 2016; (b) initial formaldehyde concentration: 289 ppb, time: 9:00e10:30, 5th June 2016; (c) initial formaldehyde concentration: 587 ppb, time: 11:00e12:30, 4th June 2016; (d) initial formaldehyde concentration: 1374 ppb, time: 13:00e15:00, 5th June 2016.
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different trends. For the system using electrical heater, CADR/ Pfanþheater decreased with temperature exponentially, while for the system using solar energy, CADR/Pfan almost increased with temperature linearly. CADR/Pfanþheater was equal with CADR/Pfan at environmental temperature because of not requiring to be heated at this temperature. Therefore, high pollutant-removal efficiency (high CADR) was at the cost of more energy supply at higher temperature for the system using electrical heater. However, for system using solar energy i.e. SWTCO system, high pollutantremoval efficiency and low energy costs were simultaneously realized under higher temperature because solar energy replaced electrical heater. 4.3. Verification of the numerical results The adsorption or the leakage rate produced by this system itself, rn, in formula (17), could be ignored through a long time measurement. To validate the model, the comparisons of the measured and predicted outlet temperature and formaldehyde conversion versus time are shown in Fig. 14. The dotted red lines in Fig. 14aed are the threshold in Chinese formaldehyde concentration standard (80 ppb). All the formaldehyde in the measuring conditions approached to the standard in 1 h. In all figures (Fig. 14aed), the predicted data were in well agreement with experimental results in the most range of time. Fig. 14a depicts the comparison of measured and predicted outlet temperature of the air collector. And the formaldehyde degradation curves versus time under different concentration i.e. 289 ppb, 587 ppb and 1374 ppb are shown in Fig. 14ced, respectively. From these curves, some errors existed inevitably due to the air leakage, limitation of above-mentioned assumptions and so on. To analyze the error between the simulation and experiment, the root mean square error (RMSE) and mean absolute error (MAE) are employed in model evaluation [30].
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X RMSE ¼ t ðXE XN Þ2 n
Fig. 15. Formaldehyde degradation curves in the test chamber under different initial formaldehyde concentrations calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, outdoor temperature ¼ 30 C, GHSV ¼ 160,000 ml/ (gcat$h), horizontal solar radiation ¼ 400 W/m2).
(25)
i¼1
MAE ¼
n 1X jXE XN j n
(26)
i¼1
From the data in Table 4, the errors were acceptable from the perspective of a simplified approach. Therefore, the proposed model can be used to investigate the formaldehyde degradation of SWTCO system.
Fig. 16. Formaldehyde degradation curves in the test chamber under different horizontal solar radiations calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, outdoor temperature ¼ 30 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h)).
4.4. Parametric study Figs. 15e17 show the effect of different factors on the formaldehyde degradation curves in the test chamber calculated by the proposed system model. The researchful factors included initial formaldehyde concentration, horizontal solar radiation and
Table 4 Comparisons between the simulated and experimental results. Experiments
289 ppb 587 ppb 1374 ppb
Concentration/ppb Temperature/ C Concentration/ppb Temperature/ C Concentration/ppb Temperature/ C
Root mean square error (RMSE)
Mean absolute error (MAE)
16 0.21 36 0.32 46 0.45
15 0.18 32 0.21 38 0.32
outdoor temperature. From these curves, some information could be obtained. The complete degradation time increased with the increase of initial formaldehyde concentration. The complete degradation time of formaldehyde was less than 1 h when initial formaldehyde concentrations were less than 1400 ppb. The formaldehyde degradation capability of system enhanced with the increase of the horizontal solar radiation and outdoor temperature. However, outdoor temperature seemed to be an unimportant factor to the formaldehyde degradation capability relative to the others. The increase of these two factors leaded to the temperature increase of the catalytic bed, which was an important factor to increasing the catalytic reaction rate. Fig. 18 shows the comparison of formaldehyde degradation curve between the systems using solar energy and without using solar energy calculated by the system model. We could easily find that the system using solar energy approached to the threshold earlier than the system without using solar energy. And Dt is the
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Fig. 17. Formaldehyde degradation curves in the test chamber under different outdoor temperatures calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h), horizontal solar radiation ¼ 400 W/m2).
Fig. 18. Comparison between the formaldehyde degradation curves of the systems using solar energy and without using solar energy calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h), horizontal solar radiation ¼ 400 W/m2).
Fig. 19 shows the values of Dt and tnoSE under different formaldehyde concentrations and horizontal solar radiations calculated by the system model. From these histograms, we could intuitively find that the time difference Dt increased with the horizontal solar radiations for all concentrations levels, which resulted from high horizontal solar radiations leaded to high temperature of catalyst bed and then caused the increase of catalytic reaction rate. Fig. 20 shows the effect of formaldehyde concentration and horizontal solar radiation on the purification time constant in the test chamber calculated by the proposed system model. The purification time constant presented a basically increasing trend with the increase of formaldehyde concentration. And the influence of concentration in typical indoor formaldehyde concentration such as a few times of the threshold in Chinese IAQ standard (300 ppb, 600 ppb and 900 ppb) on purification time constant could be neglected. And the purification time constant increased with the increasing horizontal solar radiation. Horizontal solar radiation had a greater effect on purification time constant in the low range of solar radiation (100e300 W/m2).
Fig. 19. Time difference under different formaldehyde concentrations and horizontal solar radiations calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, GHSV ¼ 160,000 ml/(gcat$h), outdoor temperature ¼ 30 C).
time difference between the two systems when the formaldehyde concentration approach to the threshold in Chinese formaldehyde concentration standard, which was marked in Fig. 18. To evaluate the contribution of the applied solar energy in SWTCO system, we proposed a purification time constant t:
t¼
Dt tnoSE
(27)
where tnoSE is the needed time approaching to the standard for the system without using solar energy, min. The purification time constant represents the energy-saving potential of SWTCO system. More economic and health benefit can be obtained directly because SWTCO system makes indoor formaldehyde concentration approach to the threshold faster comparing with the system without using solar energy, which means SWTCO system can work fewer hours and make people be exposed in high formaldehyde concentration environment for fewer hours.
Fig. 20. Concentration dependence of purification time constant under different horizontal solar radiations calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, GHSV ¼ 160,000 ml/(gcat$h), outdoor temperature ¼ 30 C).
B. Yu et al. / Building and Environment 108 (2016) 183e193
5. Conclusion Once-through experiments of thermal catalyst MnOx-CeO2 showed great potential for indoor environment application (25% degradation efficiency, 30 C, 300 ± 21 ppb). This degradation efficiency was less effective than some precious metal catalysts, its performance investigation still be meaningful because of the price and the combining of the thermal catalytic oxidation with solarcollected wall (SWTCO). Kinetics analysis of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde showed that the catalytic reaction rate followed the bimolecular L-H kinetic model well and the reaction rate expression considering two parameters i.e. concentration and temperature based on modified L-H model fit the experimental data well. High pollutant-removal efficiency and low energy costs were simultaneously realized under higher temperature for SWTCO system. In addition, formaldehyde conversion experiments in SWTCO system showed that this system could make indoor formaldehyde approach to the threshold in Chinese IAQ standard in 1 h (289 ppb, 587 ppb and 1374 ppb). Furthermore, the proposed system model could well predict the degradation of indoor formaldehyde versus the time under solar radiation. Model calculation results indicated initial formaldehyde concentration was an important factor to the formaldehyde degradation capability relative to horizontal solar radiation and outdoor temperature, and outdoor temperature seemed to be an unimportant factor. The purification time constant presented an increasing trend with the increase of horizontal solar radiation. And the influence of concentration in typical indoor formaldehyde concentration on purification time constant could be neglected. In real residential buildings, the performance measurement of formaldehyde or other VOCs degradation for SWTCO system should be conducted further because of the complexity of real operational conditions. However, this article proposes an interesting application combining the thermal catalytic oxidation with solar-collected wall in buildings without auxiliary energy. And the feasibility analysis of SWTCO system is an important first step in the plan of practical application of this novel system in residential buildings. Acknowledgments This research was supported by the grants from the Twelfth Five-year Science and Technology Support Key Project of China (No. 2012BAJ08B04) and also Program for Dongguan Innovative Research Team Program (No. 2014607101008). References [1] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev. 110 (2010) 2536e2572. [2] H. Plaisance, A. Blondel, V. Desauziers, P. Mocho, Characteristics of formaldehyde emissions from indoor materials assessed by a method using passive flux sampler measurements, Build. Environ. 73 (2014) 249e255. [3] X. Tang, Y. Bai, A. Duong, M.T. Smith, L. Li, L. Zhang, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environ. Int. 35 (2009) 1210e1224. [4] Organization WH. Air Quality Guidelines for Europe, 2000. [5] E. Gallego, F. Roca, J. Perales, X. Guardino, Experimental evaluation of VOC removal efficiency of a coconut shell activated carbon filter for indoor air quality enhancement, Build. Environ. 67 (2013) 14e25.
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[6] L. Zhong, F. Haghighat, Photocatalytic air cleaners and materials technologieseAbilities and limitations, Build. Environ. 91 (2015) 191e203. [7] X. Tang, J. Chen, X. Huang, Y. Xu, W. Shen, Pt/MnO xeCeO 2 catalysts for the complete oxidation of formaldehyde at ambient temperature, Appl. Catal. B Environ. 81 (2008) 115e121. [8] X. Tang, J. Chen, Y. Li, Y. Li, Y. Xu, W. Shen, Complete oxidation of formaldehyde over Ag/MnO xeCeO 2 catalysts, Chem. Eng. J. 118 (2006) 119e125. [9] X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang, et al., MnO xeCeO 2 mixed oxide catalysts for complete oxidation of formaldehyde: effect of preparation method and calcination temperature, Appl. Catal. B Environ. 62 (2006) 265e273. [10] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Photocatalytic purification of volatile organic compounds in indoor air: a literature review, Atmos. Environ. 43 (2009) 2229e2246. [11] J. Zhao, X. Yang, Photocatalytic oxidation for indoor air purification: a literature review, Build. Environ. 38 (2003) 645e654. [12] B. Bai, Q. Qiao, J. Li, J. Hao, Progress in research on catalysts for catalytic oxidation of formaldehyde, Chin. J. Catal. 37 (2016) 102e122. [13] W. Yu, J. Tao, X Yu, S. Zhao, S-T. Tu, H. Liu, A microreactor with superhydrophobic PteAl2O3 catalyst coating concerning oxidation of hydrogen offgas from fuel cell. Appl. Energy (2016) http://dx.doi.org/10.1016/ j.apenergy.2016.01.048. [14] T.A. Wierzbicki, I.C. Lee, A.K. Gupta, Recent advances in catalytic oxidation and reformation of jet fuels, Appl. Energy 165 (2016) 904e918. [15] C. Zhang, H. He, A comparative study of TiO 2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature, Catal. Today 126 (2007) 345e350. [16] L. Nie, J. Yu, X. Li, B. Cheng, G. Liu, M. Jaroniec, Enhanced performance of NaOH-modified Pt/TiO2 toward room temperature selective oxidation of formaldehyde, Environ. Sci. Technol. 47 (2013) 2777e2783. [17] S.S. Kim, K.H. Park, S.C. Hong, A study on HCHO oxidation characteristics at room temperature using a Pt/TiO 2 catalyst, Appl. Catal. A General 398 (2011) 96e103. [18] N. An, W. Zhang, X. Yuan, B. Pan, G. Liu, M. Jia, et al., Catalytic oxidation of formaldehyde over different silica supported platinum catalysts, Chem. Eng. J. 215 (2013) 1e6. [19] D.W. Kwon, P.W. Seo, G.J. Kim, S.C. Hong, Characteristics of the HCHO oxidation reaction over Pt/TiO 2 catalysts at room temperature: the effect of relative humidity on catalytic activity, Appl. Catal. B Environ. 163 (2015) 436e443. [20] J. Pei, X. Han, Y. Lu, Performance and kinetics of catalytic oxidation of formaldehyde over copper manganese oxide catalyst, Build. Environ. 84 (2015) 134e141. [21] Q. Xu, Y. Zhang, J. Mo, X. Li, Indoor formaldehyde removal by thermal catalyst: kinetic characteristics, key parameters, and temperature influence, Environ. Sci. Technol. 45 (2011) 5754e5760. [22] J. Peng, S. Wang, Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures, Appl. Catal. B Environ. 73 (2007) 282e291. [23] T. Zhang, Y. Tan, H. Yang, X. Zhang, The application of air layers in building envelopes: a review, Appl. Energy 165 (2016) 707e734. [24] G. Colangelo, E. Favale, P. Miglietta, A. de Risi, Innovation in flat solar thermal collectors: a review of the last ten years experimental results, Renew. Sustain. Energy Rev. 57 (2016) 1141e1159. [25] B. Yu, N. Li, W. He, J. Ji, S. Zhang, H. Chen, Multifunctional solar wall for dehumidification, heating and removal of formaldehyde: part 1. System description, preparation and performance of SiO2/TiO2 adsorbent. Build. Environ. 100 (2016) 203e214. [26] N. Wakao, T. Funazkri, Effect of fluid dispersion coefficients on particle-tofluid mass transfer coefficients in packed beds: correlation of Sherwood numbers, Chem. Eng. Sci. 33 (1978) 1375e1384. [27] W. He, Z. Hu, B. Luo, X. Hong, W. Sun, J. Ji, The thermal behavior of Trombe wall system with venetian blind: an experimental and numerical study, Energy Build. 104 (2015) 395e404. [28] T.L. Bergman, F.P. Incropera, A.S. Lavine, Fundamentals of Heat and Mass Transfer, John Wiley & Sons, 2011. [29] Guo R-t, Chen Q-l, Ding H-l, Wang Q-s, Pan W-g, Yang N-z, et al., Preparation and characterization of CeOx@ MnOx coreeshell structure catalyst for catalytic oxidation of NO, Catal. Commun. 69 (2015) 165e169. [30] Z. Hu, W. He, X. Hong, J. Ji, Z. Shen, Numerical analysis on the cooling performance of a ventilated Trombe wall combined with venetian blinds in an office building, Energy Build. 126 (2016) 14e27.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Thermal catalytic oxidation performance study of SWTCO system for the degradation of indoor formaldehyde: Kinetics and feasibility analysis Bendong Yu a, Wei He b, *, Niansi Li a, Feng Yang b, Jie Ji a a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, 230026, China Department of Building Environment and Equipment, Hefei University of Technology, Hefei, 230009, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 June 2016 Received in revised form 18 August 2016 Accepted 28 August 2016 Available online 31 August 2016
This article proposes a novel application combining the thermal catalytic oxidation with solar-collected wall (SWTCO) in buildings without auxiliary energy. Thermal catalyst MnOx-CeO2 was prepared by the method of modified coprecipitation. The performance and kinetics of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde were investigated. Once-through experiments at different concentrations of 300 e4300 ppb and temperatures 20e100 C were conducted. Moreover, formaldehyde conversion experiments in SWTCO system at several typical indoor concentrations (289 ppb, 587 ppb and 1374 ppb) were performed. Furthermore, a simple system model predicting the degradation of indoor formaldehyde versus the time in SWTCO system was built. Results were as follows: (1) The reaction rate expression considering two parameters i.e. concentration and temperature based on modified L-H model fit the experimental data well; (2) System model calculation results showed initial formaldehyde concentration was an important factor to the formaldehyde degradation capability; (3) SWTCO system could realize high pollutant-removal efficiency and low energy costs simultaneously under higher temperature compared with the system using electrical heating; (4) The purification time constant increased with the horizontal solar radiation and the influence of concentration in typical indoor formaldehyde concentration on it could be neglected. © 2016 Elsevier Ltd. All rights reserved.
Keywords: SWTCO MnOx-CeO2 Solar energy Formaldehyde Kinetics Numerical model
1. Introduction Formaldehyde is a prevalent indoor chemical pollutant in building environment and considered to be carcinogenic and teratogenic. Due to the rapid development of real estate industry especially in China, more and more residential and commercial buildings have been constructed overnight. These newly buildings release large amounts of formaldehyde gas from the decorating materials, plywood, fiberboard, particleboard, and other artificial boards [1]. However, due to tight buildings and high emission materials, there is a general agreement that the concentration of indoor formaldehyde is 2e10 times than that of current legal limiting concentration [2]. Tang et al. [3] reported that the maximum formaldehyde concentration in newly remodeled residential buildings were more than 4472 ppb. The WHO guideline concentration for indoor
* Corresponding author. E-mail address: [email protected] (W. He). http://dx.doi.org/10.1016/j.buildenv.2016.08.030 0360-1323/© 2016 Elsevier Ltd. All rights reserved.
formaldehyde is only 80 ppb [4]. Therefore, indoor formaldehyde is a great threat for indoor air quality and human health. Several strategies such as adsorption, photocatalytic oxidation (PCO), and thermal catalytic oxidation (TCO) have been investigated for the elimination of indoor formaldehyde in recent decades [5e9]. However, the main technical barrier is the secondary pollution of toxicity when degrading volatile organic compounds (VOCs) for photocatalytic oxidation technology [10,11]. Moreover, the band gap of semiconductor greatly limits its application on solar energy. The photoresponse range of commonly used photocatalyst TiO2 is where the ultraviolet light lies in and the ratio of ultraviolet light in solar spectrum is only 5%. Adsorption technology can effectively capture the indoor formaldehyde at low concentration levels. But it is restricted because of the problems such as the limited adsorption capacity and adsorbent regeneration. Catalytic oxidation technology is found to be innovative and promising technique to gaseous formaldehyde abatement and has advantages such as high removal efficiency, low light-off temperature, wide application scope, simple equipment, and no secondary pollution [12].
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Greeks
Nomenclature I A c R T Nu Re Pr m Q K h
l C D f F t x E V r
2
global solar radiation, W/m area, m2 specific heat capacity, J/(kg$K) the universal gas constant, J/(mol$K), or radius of catalyst particles, m temperature, K Nusslet number Reynolds number Prandtl number mass, kg volumetric flow rate of gas, ml/min reaction factor, m/s external mass transfer coefficient, m/s, or heat transfer coefficient, W/(m2$K) thermal conductivity, W/(m$K) concentration of formaldehyde, ppb, or characteristic constant effective diffusion coefficient, m2/s, or the thickness of the air cavity, m external diffusion factor view factor time, s the distance from the air inlet in the air collector, m activation energy or adsorption heat of formaldehyde, J/mol air flow velocity, m/s, or volume of the test chamber, m3 catalytic degradation rate, ppb/(m$s)
The thermal catalytic materials mainly fall into two categories: noble-metal and transition-metal oxide. And noble-metal catalysts have been playing an important role in the field of energy and environment because of their high reaction activity [13,14]. The research on total oxidation of formaldehyde into harmless products CO2 and H2O at low temperature even at ambient temperature has been attracted tremendous interest. The method of supporting noble metals is proved to be effective for improving the activities of catalysts. Zhang et al. [15] prepared various TiO2 supported noble metal catalysts (Au/TiO2, Rh/TiO2, Pd/TiO2 and Pt/TiO2) by the impregnation method. Results showed that the order of the catalysts activity was Pt/TiO2 [ Rh/TiO2 > Pd/TiO2 > Au/TiO2 [ TiO2. And the experimental formaldehyde (100 ppm) could be completely oxidized into CO2 and H2O over Pt/TiO2 in a gas hourly space velocity (GHSV) of 50,000 h1 even at room temperature. Nie et al. [16] prepared Pt/TiO2 catalysts with various Pt loadings (0.05e2 wt%) and found that the amount of noble-metal dispersion was an important factor that influenced the activity of catalyst. Also, other factors such as the type of support, relative humidity and metal valence state have been investigated [17e19]. Similarly, Pt/MnOx-CeO2 showed extremely high activity and stability and realized the totally conversion of formaldehyde (580 ppm) at ambient temperature and no deactivation was observed for a 120 h continuous experiment [7]. High noble-metal content is an important reason for the high activity of these thermal catalysts. However, the high price of noble metals still greatly limits their application in a long time. Transition metal oxides such as MnOx, CeO2, and CuO also behave high catalyst activity to the degradation of formaldehyde. And more importantly, they are cheap and plentiful. Tang et al. [9] prepared MnOx-CeO2 mixed oxides by the method of modified
r F
ε
a s t n
density of catalyst, Kg/m3 solar flux absorbed, W conversion efficiency absorptivity, or thermal diffusivity, m2/s Stefan-Boltzmann constant, W/(m2$K4) transmissivity, or purification time constant dynamic viscosity of air, m2/s
Subscripts 0 original t certain time cat catalyst particles eq equivalent in inlet out outlet m mean or mass transfer s surface g glazing ab absorber plate ca air cavity Abbreviation TCO thermal catalytic oxidation PCO photocatalytic oxidation GHSV gas hourly space velocity CADR Clean air delivery rate, m3/h ppb parts per billion ppm parts per million
coprecipitation and results showed 30% formaldehyde conversion at the temperature as low as 303 K and 100% formaldehyde conversion at 373 K with a 580 ppm feed concentration and space velocity of 21,000 ml/gcat$h. However, the research on TCO in typical indoor contaminant concentration levels such as less than 1 ppm is very limited in the open literature. Most of them focused on the formaldehyde concentration levels in the range of hundreds of ppm. Pei et al. [20] investigated the performance and kinetics of CuO-MnO2 for indoor formaldehyde removal under typical indoor concentration levels (180e1300 ppb). They found that the reaction rate followed the traditional L-H model. However, transition metal oxides are not able to totally degrade formaldehyde at room temperature. In general, the formaldehyde conversion efficiency increases with the temperature of catalyst bed [9,20e22]. And current research on solar air collector showed the temperature of air in the air cavity could approach 60e100 C [23,24]. An interesting idea is the combination of TCO and solarcollected wall (SWTCO). Indoor air with trace amount of formaldehyde is heated by solar-collected wall. Then the heated air enters the catalyst bed. The catalyst bed is heated by the hot air. Formaldehyde is continuously degraded by the heated catalyst bed without added energy. Then the clean air enters room. Moreover, this system can realizes the space heating in winter. This article prepared transition metal oxides MnOx-CeO2 by the method of the modified coprecipitation. To investigate the performance and kinetics of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde, once-through experiments at different feed concentrations of formaldehyde (300e4300 ppb) and temperatures of catalyst bed (20e100 C) were conducted. The kinetics model simultaneously considering concentration and temperature was proposed. Moreover, formaldehyde conversion experiments in
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SWTCO system at several typical indoor concentrations (289 ppb, 587 ppb and 1374 ppb) were performed and a simple SWTCO system model was built. 2. Experimental 2.1. Preparation of MnOx-CeO2 catalyst The preparation method for the MnOx-CeO2 catalysts has been described previously [7,9]. Modified coprecipitation method was applied in this article. Firstly, a mixed solution containing Mn(NO3)2$6H2O, KMnO4 and (NH4)2Ce(NO3)6 was previously stirred and became a uniform suspension solution and sealed. And the specific mole ratio of these three chemical reagents was 3: 2: 5. Secondly, 2 M KOH solution was dropwise added to the abovementioned mixed solution at 50 C until the pH value of the mixture reached 10.5 under vigorous stirring. The precipitate was further aged at 50 C for 2 h. Then the filtration and washing procedures were proceeded. At last, the obtained materials were heated at 110 C for 12 h and calcined at 500 C for 6 h in air. The desired catalysts were obtained. 2.2. Catalyst characterization The porous parameters including BET specific area and pore diameter of the catalysts were measured using ASAP2010 adsorption volumetric adsorption analyzer made by Micromeritics Co. in USA from Instruments' Center for Physical Science, University of Science and Technology of China through the N2 adsorption method. X-ray powder diffraction pattern was recorded with a D/Max2500/PC diffractometer (Rigaku, Japan), University of Science and Technology of China, operated at 40 kV and 250 mA, using nickelfiltered Cu Ka (l ¼ 0.15,418 nm) radiation. 2.3. Activity measurement 2.3.1. Formaldehyde single-pass conversion measurement Fig. 1 shows the schematic of the experimental set-up used for investigating the formaldehyde single-pass conversion over the prepared catalysts. Similar experimental set-up was been described in our previous study [25]. This set-up consisted of three sections: (1) gas-generating section, (2) reaction section, (3) gas detection section. The main difference in this set-up was the reactor. Fig. 2 shows the photo of continuous flow-type fixed bed reactor. It was made of stainless steel tube with the structure of stepped hole. The inner hole was used to be filled in the catalysts, which had an
Fig. 1. Schematic diagram of experimental set-up to study formaldehyde single-pass conversion over the catalysts. (1) Dry air cylinder. (2) Pressure gage. (3) Gas purifier (silica gel and molecular sieve). (4) Bypass valve. (5) Flowmeter. (6) Formaldehyde bottle with bubbler. (7) Thermostatic bath. (8) Three-way valve. (9) Mixed chamber. (10) Temperature controller. (11) Tube furnace (12) Reactor. (13) Formaldehyde detector.
Fig. 2. The photo of continuous flow-type fixed bed reactor.
internal diameter of 5 mm and a height of 10 mm. The metal net was used to support the catalysts and ensured the filled catalysts were not flowed away. The stainless steel reaction tube and the air tube were connected through two screw threads. The reaction temperature of the bed was controlled by tube furnace MXG120060 purchased from Shanghai Weihang furnace industry co. in China. Formaldehyde detector ZK-101D purchased from Beijing Zhongke Zhonghuan environmental application technology research center measured the effluent gas stream from the reactor. Measurement uncertainty of measured gas concentration was 5%. And temperature uncertainty of all thermocouples was ±0.5 C.The temperature control range of reactor is 20e100 C. Approximately, 0.15 g of catalyst (40e60 mesh) was used for each reaction. The total flow rate was 400 ml/min, and the gas hourly space velocity (GHSV) was 160,000 ml/(gcat$h). We chose five experimental formaldehyde concentrations i.e. 300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb, respectively. We recorded the final outlet concentration after a steady-state condition was achieved. A fresh catalyst was used for each experiment. 2.3.2. Formaldehyde conversion measurement in SWTCO system Fig. 3 shows the schematic diagram of experimental set-up to study formaldehyde conversion over the catalysts in SWTCO system. And Fig. 4 shows the photo of the simple SWTCO system. The test chamber of length 0.75 m, width 0.75 m and height 1 m, respectively, was made of stainless steel with little adsorption to the gaseous formaldehyde, which was used as simulating the indoor environment. And air collector was also made of stainless steel considering the low adsorption to formaldehyde as well. It consisted three sections: absorber plate, air cavity and glazing. Thermal insulation material were also used for these two parts. The reactor was same as Fig. 2. The mass of used catalysts was 3.125 g. Before every experiment, a certain amount of formaldehyde was injected into the test chamber through a micro-syringe. The initial formaldehyde concentration was measured until the formaldehyde gas in the chamber was mixed uniformity through the rotation of the electric fan. Then the gas in the test chamber was extracted by fan and flowed through air collector and the reactor and eventually return to the chamber. The air flow was 0.5 m3/h. Concentration of formaldehyde in the chamber was measured every 5 min. The thermocouple measured the temperature every minute. Before the formaldehyde was added, to keep only one contaminant i.e. formaldehyde in the chamber, the gas in the chamber entered the gas purifier for enough time. Considering the typical indoor exceeding formaldehyde concentration and the actual gas-generating concentration, we choose three experimental concentrations i.e. 289 ppb, 587 ppb and 1425 ppb, respectively.
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10
ε¼
19 9 7
3
6
12 17 8
16
rR2cat r Cs Dm
(2)
Q ðCin Cout Þ ABET
(3)
Cwp ¼
18 11
1
2
(1)
where Cin and Cout, ppb, are inlet and outlet concentration under the steady state, respectively. The Weisz-Prater criterion was calculated to evaluate the internal mass transfer [20]. The effect of internal mass transfer can be ignored when the criterion is much less than 1.
4
5
Cin Cout 100%; Cin
13
15
14
Fig. 3. Schematic diagram of experimental set-up to study formaldehyde conversion over the catalysts in SWTCO system. (1) Test chamber. (2) Electric fan. (3) Thermocouple. (4) Reactor. (5) Microinjector. (6) Formaldehyde detector. (7) Sampling opening. (8) Air inlet. (9) Air outlet. (10) Three-way valve. (11) Flowmeter. (12) Air collector. (13) Valve. (14) Fan. (15) Gas purifier (silica gel and molecular sieve) (16) Solar radiation. (17) Absorber plate. (18) Glazing. (19) Air flow direction arrows.
r¼
where Cwp is Weisz-Prater criterion, r is the reaction rate, which is defined the amount of formaldehyde oxidation per unit area of the catalyst and per second, ppb$m/s. Q is gas flow rate, ml/min. And ABET is the total surface area of the catalyst, m2. Rcat is the radius of catalyst particles, m, r is the density of catalyst, Kg/m3, Cs is the surface formaldehyde concentration on the catalyst, ppb, and Dm is the effective diffusion coefficient, m2/s. According to the study of Xu et al. [21], the external mass transfer correction factor fm, which value stands for the ratio of reaction resistance in the total resistance including external convective mass transfer resistance and reaction resistance, can well describe the effect of external convective mass transfer. The influence of external mass transfer for reaction of catalytic bed almost eliminate when fm approach to 1.
fm ¼
h m As hm As þ Kapp ABET
(4)
Cs ¼
Cin Cout fm lnðCin Þ lnðCout Þ
(5)
where fm is the external mass transfer correction factor, Kapp is the apparent reaction factor which was defined as the reaction rate per unit surface concentration, m/s, As is the total external surface area of the catalyst, m2. hm is the mean external mass transfer coefficient, m/s, which can calculated by the empirical correlation [26]. Cs is the average surface concentration of catalyst, which is calculated by the logarithmic mean value of inlet and outlet formaldehyde concentrations. The details of calculation can be referred in Xu et al.’s study [21]. 3.2. SWTCO system model Fig. 4. The photo of a simple SWTCO system.
Because the thickness of the glazing is very thin, it is reasonable to ignore the heat transfer in the horizontal. Therefore, the temperature of the glazing can be assumed uniform in the x-direction.
3. Data analysis and system model 3.1. Kinetic data analysis Generally speaking, three processes happen in the catalytic oxidization of formaldehyde including external mass transfer through the external film of catalysts, internal mass transfer and surface reaction [20]. This article focused on the intrinsic reaction kinetics of the prepared catalysts. Therefore, the surface reaction should play a determinative effect to the reaction rate through eliminating the effect of external and internal mass transfer. Using the concentrations of the gases at a steady state, the formaldehyde conversion is calculated according to the following equation:
mg cg
dTg ¼ Ag hconvrad Teq Tg þ Ag hconv;g;ca Tca Tg dt þ Ag hrad;g;ab Tab Tg þ Fg
Fg ¼ Ag I90+ ag
(6) (7)
where Teq is the equivalent temperature, which is considered to be equal to the outdoor air temperature, and hconv-rad has a standard value of 29 W/(m2$K) [27]. Fg is the solar flux absorbed by the glazing. The convective heat transfer coefficient between the glazing and the air in air cavity hconv,g,ca is calculated by the empirical formula of laminar flow forced convection over a plate [28].
B. Yu et al. / Building and Environment 108 (2016) 183e193 1=2
Nux ¼ 0:332Rex Pr 1=3 Re ¼ Va ¼
Va x
na Q As
(8)
Fab ¼ Aab I90+ tg aab
(9)
where tg is the transmissivity of the glazing, aab is the absorptivity of the absorber plate. In the test chamber, we assume that the temperature in it is a constant and the formaldehyde in it is mixed well all the time because the rotation of electric fan. And the temperature of catalyst bed is assumed to be equal to the outlet temperature of air cavity because the amount of catalysts is very little relative to the air [21]. The mass conservation equation of formaldehyde in the chamber is as follows:
(10)
hconv;g;ca ¼
Nux la x
(11)
where the characteristic length in characteristic number is the distance from the air inlet in the air collector. Va is the face velocity of the cross section of air cavity, m/s, la is the thermal conductivity of air, W/(m$K), na is the dynamic viscosity of air, m2/s, As is the cross section air cavity, m. hrad,g,ab is the radiation heat transfer coefficient between the glazing and the absorber plate, which is defined as [27]: 2 4sTm hrad;g;ab ¼ . 1 Fg;ab þ 1 εg εg þ ð1 εab Þ=εab
(12)
where εg and εab is the emissivity of the glazing and the absorber plate, respectively. Tm is the mean temperature of the glazing and the absorber plate. Fg,ab is the view factor for the glazing and the absorber plate, which can be calculated by using the fictitious cavity method showed in Fig. 5 [28]:
Fcd;ab ¼
ðbc þ adÞ ðac þ bdÞ 2ab
(13)
The air per unit length in the air cavity is taken as a control volume. It is given as:
ra Dca
vTca vTca ¼hconv;g;ca Tg Tca þhconv;ab;ca ðTab Tca Þ ra Va Dca vt vx (14)
where D is the thickness of the air cavity, m. It is reasonable to assume the temperature of inlet air in air cavity is a constant because the air in the test chamber has little change. In fact, air condition can keep indoor temperature almost constant. The model of the absorber plate is similar to the glazing:
mab cab
187
dTab ¼Aab hconv;ab;ca ðTca Tab ÞþAab hrad;g;ab Tg Tab þ Fab dt (15)
V
(16)
dC ¼ ðr þ rn ÞA dt
(17)
where V is the volume of the test chamber, m3, C is the formaldehyde concentration in the test chamber, ppb. And rn and r are the adsorption or the leakage rate produced by this system itself and catalytic degradation rate, respectively, ppb/s, A is the total surface area of the catalysts, m2. The detail computational parameters was shown in Table 1. 4. Results and discussion 4.1. BET and XRD measurements The basic physical pore structure parameters for the prepared catalysts are listed in Table 2. These parameters were necessary for the further kinetic analysis. The XRD pattern of MnOx-CeO2 are shown in Fig. 6. The pecks where the red and blue arrows point to belong to MnO2 and CeO2 phase, respectively [29]. 4.2. Kinetics Fig. 7 shows the impact of internal and external mass transfer in the reactor under different formaldehyde concentrations and temperatures. From these results, all Cwp under different Table 1 Detail computational parameters for the SWTCO model.
Test chamber Air cavity Glazing
Symbol
Explanation
Unit
Value
L*W*H L*W*H
Size Size Density Specific heat capacity Absorptivity Transmissivity Emissivity Density Thickness Specific heat capacity Absorptivity Emissivity Density specific heat capacity Thermal conductivity Kinematic viscosity
m m Kg/m3 J/(Kg$K) e e e Kg/m3 mm J/(Kg$K) e e Kg/m3 J/(Kg$K) W/(m$K) m2/s
0.75*0.75*1 0.08*0.01*0.24 2500 840 0.1 0.9 0.9 2700 0.5 900 0.95 0.95 1.18 1100 0.026 1.58*105
rg cg ag
tg 3g
Absorber plate
rab dab cab
aab 3 ab
Air
ra ca
la na
Table 2 Physical parameters of the catalysts. Physical parameters
Fig. 5. Fictitious cavity for calculating the view factors for the glazing and the absorber plate.
Particle size (mesh) Average pore diameter (nm) BET area (m2/g) Particle porosity
40e60 22.6 64.6 0.49
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Fig. 6. XRD pattern of the prepared MnOx-CeO2.
Fig. 8. Temperature dependence of formaldehyde conversion over the prepared catalysts under different formaldehyde concentrations: 300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)).
Fig. 7. Impact of internal mass transfer and external mass transfer in the reactor under different formaldehyde concentrations and temperatures (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)). Fig. 9. Reaction rate at different surface formaldehyde concentrations at temperatures of 20 Ce100 C (Reaction conditions: GHSV ¼ 160,000 ml/(gcat$h)).
formaldehyde concentrations and temperatures were much less than 1 (in the order of 104) and all fm almost approached to 1, which suggested that the influence of internal and external mass transfer for reaction of catalytic bed in our reaction conditions could be ignored. Fig. 8 shows the formaldehyde single-pass conversion over the prepared catalysts under different temperatures (20e100 C) and various inlet concentrations (300 ± 21 ppb, 600 ± 33 ppb, 900 ± 35 ppb, 1400 ± 62 ppb, 2500 ± 96 ppb and 4300 ± 160 ppb). The formaldehyde single-pass conversion ε approached 30%e50% at typical indoor formaldehyde concentration such as 300 ppb and 600 ppb at temperature of 40e60 C. For all curves, the degradation efficiency of formaldehyde increased with the increase of temperature. In all the range of measured temperature, the degradation efficiency of formaldehyde decreased with the increasing concentration. The reaction rate is related to the surface formaldehyde concentration. Fig. 9 shows the reaction rate at different surface formaldehyde concentrations at temperatures of 20 Ce100 C. For the curves below 70 C, reaction rate increased firstly then approached the peck and finally decreased with the increase of formaldehyde surface concentration. At this low temperature range, thermal catalytic had enough reaction activity to degrade the
surface formaldehyde gas when the surface concentration was low. Theoretically, reaction rate approached to a constant when the surface concentration kept increasing due to the limited reaction activity in the range of low temperature. In reality, reaction rate decreased slightly, which might be a result of hardly release of the reaction products in time with the increasing surface concentration. However, for the curves above 70 C, reaction rate presented the monotonically increasing trend as the increase of surface concentration. These because the high reaction activity of thermal catalytic at high temperature range could degrade more formaldehyde timely. This article was aimed at investigating the kinetic model of the catalytic reaction involved two variables i.e. reaction concentration and temperature. To simplify this issue, in current study, we firstly fit the experimental data only considering single variable. Then the kinetic model considering two variables was developed. The surface reaction was the rate-determining step of the catalytic oxidization process when the impact of internal and external mass transfer in the reactor could be ignored based on Fig. 7. Bimolecular Langmuir-Hinshelwod model has been widely applied
B. Yu et al. / Building and Environment 108 (2016) 183e193
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in the research on catalytic reaction kinetics (considering absorbed formaldehyde reacting with adsorbed O2, with competitive adsorption) [20,21]. The experimental data were fitted to L-H model using formula (18) and the results were listed in Table 3. The resulting correspondence between the fitting and data was good for all samples from the correlation coefficient R2.
r¼
k1 k2 Cs
(18)
ð1 þ k2 Cs Þ2
The impact of temperature to the reaction rate mainly be shown in the reaction coefficient and adsorption coefficient. The temperature dependence of these two coefficients can be described by Arrhenius equation:
E k1 ¼ k01 exp 1 RT
(19)
E k2 ¼ k02 exp 2 RT
(20)
Fig. 10. Linear dependence between lnk1 and 1/T using Arrhenius equation.
where k10 and k20 are the pre-exponential factor, E1 and E2 are the activation energy of catalytic reaction and adsorption heat of formaldehyde, respectively, J/mol. And R is the universal gas constant, J/(mol$K), T is the thermodynamic temperature, K. Then the log-linearization of formulae (19) and (20) were as follows:
ln k1 ¼ ln k01
E1 1 R T
(21)
ln k2 ¼ ln k02
E2 1 R T
(22)
Figs. 10e11 show the fittings of reaction coefficient and adsorption coefficient applying formulae (21) and (22) based on the data in Table 3. The resulting correspondence was good for all data from the correlation coefficient R2. And some basic parameters such as k10, k20 , E1 and E2 could be obtained from the slope and intercept of the curves in Figs. 10e11. By applying the Arrhenius equation to correlate the reaction coefficient and adsorption coefficient at different temperatures, the bimolecular L-H kinetic was expressed as [21]:
r¼
Fig. 11. Linear dependence between lnk2 and 1/T using Arrhenius equation.
E1 E2 k2 exp RT Cs k1 exp RT
(23)
2 E2 1 þ k2 exp RT Cs
Fig. 12 shows predicated versus experimental reaction rates for the catalytic oxidation of formaldehyde. The coefficient of determination R2 was 0.98, which indicated the modified model fit well with the experimental data. The reaction activation energy of formaldehyde catalytic reaction on the catalysts MnOx-CeO2 was Table 3 Parameters of formaldehyde catalytic oxidation in bimolecular L-H form. Temperature ( C)
k1 (ppb$m/s)
k2 (ppb1)
R2
20 30 40 50 60 70 80 90 100
0.0003245 0.0004783 0.0008448 0.0012 0.0016 0.0023 0.0029 0.0033 0.0033
0.0008517 0.0006954 0.0005574 0.0005 0.0004 0.0003 0.0003 0.0002 0.0002
0.95 0.98 0.98 0.99 0.99 0.98 0.99 0.99 0.99
Fig. 12. Predicated versus experimental reaction rates for the catalytic oxidation of formaldehyde.
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Pollutant-removal efficiency and energy costs are two important aspects for the design and research of an air cleaner. Clean air delivery rate (CADR), m3/h, which is an assessment of pollutantremoval efficiency, can be expressed as follows:
CADR ¼ mSBET
Fig. 13. Influence of temperature on clean air delivery rate (CADR) and clean air delivery rate per power input (CADR/P) for system using solar energy and system using electric heater. Mass of catalyst (40e60 mesh): 432 g, flow rate 70 m3/h, formaldehyde 300 ppb, environmental temperature 20 C.
estimated as 30.615 kJ/mol, which was larger than that of on the catalysts Pt/MnOx-CeO2 (25.8 kJ/mol) [21]. These indicated that noble metal catalysts could reduce the reaction activation energy of formaldehyde catalytic reaction. However, transition metal oxides were the better candidate considering the price of noble metal catalysts.
r Cin
(24)
where m is the mass of catalyst, SBET is the total BET area of used catalysts, r is the catalytic reaction rate, which can be calculated by equation (23) and Cin is the inlet formaldehyde concentration. CADR is the volume of fresh air generated by an air cleaner per hour. CADR/P, which is defined the clean air delivery rate per power input, is an assessment criteria of energy costs of an air cleaner. And P is the energy input rate of an air cleaner, including fan and electrical heater. For assessing the performance of SWTCO system, we simulated two systems: (1) system using solar energy i.e. SWTCO system; (2) system using electrical heater. A packed catalytic bed of mass of catalysts 432 g (40e60 mesh) in the reactor was used for this calculation. Air flow rate was 70 m3/h and formaldehyde concentration was 300 ppb. Environmental temperature was 20 C. And we assumed that electrical heating energy were all used for heating air flow in the air cleaner. The electrical heating energy is zero when the reaction occurs at room temperature. Fig. 13 shows CADR and CADR/P of two simulated systems under different temperatures. The value of CADR increased with temperature almost linearly, which was consistent with Xu et al.’s research [21]. The value of CADR/P for two systems presented
Fig. 14. Comparisons of experimental data and predicted data in formaldehyde conversion experiments under solar radiation (N: numerical result, E: experimental data, (a): air temperature curve of the air collector outlet, (b) ~ (d): concentration curves of formaldehyde in the test chamber under different initial formaldehyde concentrations). Experiment conditions: (a) time: time: 13:00e16:00, 5th June 2016; (b) initial formaldehyde concentration: 289 ppb, time: 9:00e10:30, 5th June 2016; (c) initial formaldehyde concentration: 587 ppb, time: 11:00e12:30, 4th June 2016; (d) initial formaldehyde concentration: 1374 ppb, time: 13:00e15:00, 5th June 2016.
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different trends. For the system using electrical heater, CADR/ Pfanþheater decreased with temperature exponentially, while for the system using solar energy, CADR/Pfan almost increased with temperature linearly. CADR/Pfanþheater was equal with CADR/Pfan at environmental temperature because of not requiring to be heated at this temperature. Therefore, high pollutant-removal efficiency (high CADR) was at the cost of more energy supply at higher temperature for the system using electrical heater. However, for system using solar energy i.e. SWTCO system, high pollutantremoval efficiency and low energy costs were simultaneously realized under higher temperature because solar energy replaced electrical heater. 4.3. Verification of the numerical results The adsorption or the leakage rate produced by this system itself, rn, in formula (17), could be ignored through a long time measurement. To validate the model, the comparisons of the measured and predicted outlet temperature and formaldehyde conversion versus time are shown in Fig. 14. The dotted red lines in Fig. 14aed are the threshold in Chinese formaldehyde concentration standard (80 ppb). All the formaldehyde in the measuring conditions approached to the standard in 1 h. In all figures (Fig. 14aed), the predicted data were in well agreement with experimental results in the most range of time. Fig. 14a depicts the comparison of measured and predicted outlet temperature of the air collector. And the formaldehyde degradation curves versus time under different concentration i.e. 289 ppb, 587 ppb and 1374 ppb are shown in Fig. 14ced, respectively. From these curves, some errors existed inevitably due to the air leakage, limitation of above-mentioned assumptions and so on. To analyze the error between the simulation and experiment, the root mean square error (RMSE) and mean absolute error (MAE) are employed in model evaluation [30].
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X RMSE ¼ t ðXE XN Þ2 n
Fig. 15. Formaldehyde degradation curves in the test chamber under different initial formaldehyde concentrations calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, outdoor temperature ¼ 30 C, GHSV ¼ 160,000 ml/ (gcat$h), horizontal solar radiation ¼ 400 W/m2).
(25)
i¼1
MAE ¼
n 1X jXE XN j n
(26)
i¼1
From the data in Table 4, the errors were acceptable from the perspective of a simplified approach. Therefore, the proposed model can be used to investigate the formaldehyde degradation of SWTCO system.
Fig. 16. Formaldehyde degradation curves in the test chamber under different horizontal solar radiations calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, outdoor temperature ¼ 30 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h)).
4.4. Parametric study Figs. 15e17 show the effect of different factors on the formaldehyde degradation curves in the test chamber calculated by the proposed system model. The researchful factors included initial formaldehyde concentration, horizontal solar radiation and
Table 4 Comparisons between the simulated and experimental results. Experiments
289 ppb 587 ppb 1374 ppb
Concentration/ppb Temperature/ C Concentration/ppb Temperature/ C Concentration/ppb Temperature/ C
Root mean square error (RMSE)
Mean absolute error (MAE)
16 0.21 36 0.32 46 0.45
15 0.18 32 0.21 38 0.32
outdoor temperature. From these curves, some information could be obtained. The complete degradation time increased with the increase of initial formaldehyde concentration. The complete degradation time of formaldehyde was less than 1 h when initial formaldehyde concentrations were less than 1400 ppb. The formaldehyde degradation capability of system enhanced with the increase of the horizontal solar radiation and outdoor temperature. However, outdoor temperature seemed to be an unimportant factor to the formaldehyde degradation capability relative to the others. The increase of these two factors leaded to the temperature increase of the catalytic bed, which was an important factor to increasing the catalytic reaction rate. Fig. 18 shows the comparison of formaldehyde degradation curve between the systems using solar energy and without using solar energy calculated by the system model. We could easily find that the system using solar energy approached to the threshold earlier than the system without using solar energy. And Dt is the
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B. Yu et al. / Building and Environment 108 (2016) 183e193
Fig. 17. Formaldehyde degradation curves in the test chamber under different outdoor temperatures calculated by the system model (Reaction conditions: indoor temperature ¼ 25 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h), horizontal solar radiation ¼ 400 W/m2).
Fig. 18. Comparison between the formaldehyde degradation curves of the systems using solar energy and without using solar energy calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, initial formaldehyde concentration ¼ 600 ppb, GHSV ¼ 160,000 ml/(gcat$h), horizontal solar radiation ¼ 400 W/m2).
Fig. 19 shows the values of Dt and tnoSE under different formaldehyde concentrations and horizontal solar radiations calculated by the system model. From these histograms, we could intuitively find that the time difference Dt increased with the horizontal solar radiations for all concentrations levels, which resulted from high horizontal solar radiations leaded to high temperature of catalyst bed and then caused the increase of catalytic reaction rate. Fig. 20 shows the effect of formaldehyde concentration and horizontal solar radiation on the purification time constant in the test chamber calculated by the proposed system model. The purification time constant presented a basically increasing trend with the increase of formaldehyde concentration. And the influence of concentration in typical indoor formaldehyde concentration such as a few times of the threshold in Chinese IAQ standard (300 ppb, 600 ppb and 900 ppb) on purification time constant could be neglected. And the purification time constant increased with the increasing horizontal solar radiation. Horizontal solar radiation had a greater effect on purification time constant in the low range of solar radiation (100e300 W/m2).
Fig. 19. Time difference under different formaldehyde concentrations and horizontal solar radiations calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, GHSV ¼ 160,000 ml/(gcat$h), outdoor temperature ¼ 30 C).
time difference between the two systems when the formaldehyde concentration approach to the threshold in Chinese formaldehyde concentration standard, which was marked in Fig. 18. To evaluate the contribution of the applied solar energy in SWTCO system, we proposed a purification time constant t:
t¼
Dt tnoSE
(27)
where tnoSE is the needed time approaching to the standard for the system without using solar energy, min. The purification time constant represents the energy-saving potential of SWTCO system. More economic and health benefit can be obtained directly because SWTCO system makes indoor formaldehyde concentration approach to the threshold faster comparing with the system without using solar energy, which means SWTCO system can work fewer hours and make people be exposed in high formaldehyde concentration environment for fewer hours.
Fig. 20. Concentration dependence of purification time constant under different horizontal solar radiations calculated by the system model. (Reaction conditions: indoor temperature ¼ 25 C, GHSV ¼ 160,000 ml/(gcat$h), outdoor temperature ¼ 30 C).
B. Yu et al. / Building and Environment 108 (2016) 183e193
5. Conclusion Once-through experiments of thermal catalyst MnOx-CeO2 showed great potential for indoor environment application (25% degradation efficiency, 30 C, 300 ± 21 ppb). This degradation efficiency was less effective than some precious metal catalysts, its performance investigation still be meaningful because of the price and the combining of the thermal catalytic oxidation with solarcollected wall (SWTCO). Kinetics analysis of MnOx-CeO2 for catalytic oxidation of indoor formaldehyde showed that the catalytic reaction rate followed the bimolecular L-H kinetic model well and the reaction rate expression considering two parameters i.e. concentration and temperature based on modified L-H model fit the experimental data well. High pollutant-removal efficiency and low energy costs were simultaneously realized under higher temperature for SWTCO system. In addition, formaldehyde conversion experiments in SWTCO system showed that this system could make indoor formaldehyde approach to the threshold in Chinese IAQ standard in 1 h (289 ppb, 587 ppb and 1374 ppb). Furthermore, the proposed system model could well predict the degradation of indoor formaldehyde versus the time under solar radiation. Model calculation results indicated initial formaldehyde concentration was an important factor to the formaldehyde degradation capability relative to horizontal solar radiation and outdoor temperature, and outdoor temperature seemed to be an unimportant factor. The purification time constant presented an increasing trend with the increase of horizontal solar radiation. And the influence of concentration in typical indoor formaldehyde concentration on purification time constant could be neglected. In real residential buildings, the performance measurement of formaldehyde or other VOCs degradation for SWTCO system should be conducted further because of the complexity of real operational conditions. However, this article proposes an interesting application combining the thermal catalytic oxidation with solar-collected wall in buildings without auxiliary energy. And the feasibility analysis of SWTCO system is an important first step in the plan of practical application of this novel system in residential buildings. Acknowledgments This research was supported by the grants from the Twelfth Five-year Science and Technology Support Key Project of China (No. 2012BAJ08B04) and also Program for Dongguan Innovative Research Team Program (No. 2014607101008). References [1] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev. 110 (2010) 2536e2572. [2] H. Plaisance, A. Blondel, V. Desauziers, P. Mocho, Characteristics of formaldehyde emissions from indoor materials assessed by a method using passive flux sampler measurements, Build. Environ. 73 (2014) 249e255. [3] X. Tang, Y. Bai, A. Duong, M.T. Smith, L. Li, L. Zhang, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environ. Int. 35 (2009) 1210e1224. [4] Organization WH. Air Quality Guidelines for Europe, 2000. [5] E. Gallego, F. Roca, J. Perales, X. Guardino, Experimental evaluation of VOC removal efficiency of a coconut shell activated carbon filter for indoor air quality enhancement, Build. Environ. 67 (2013) 14e25.
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