Yang, X., Chen, Q., Zhang, J.S, Magee, R., Zeng, J. and Shaw,C.Y. 2001. Nuerical siulation of VOC eissions fro dry aterials, Building and Environent, 36(10), 1099-1107. Nuerical siulation of VOC Eissions fro Dry Materials X. Yang 1*, Q. Chen 1, J.S. Zhang 2, R. Magee 2, J. Zeng 2, and C.Y. Shaw 2 1 Building Technology Progra, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cabridge, MA 02139, USA 2 National Research Council of Canada, Institute for Research in Construction, Indoor Environent Progra, M-24, Montreal Road, Ottawa, Ontario K1A 0R6, Canada * Current address: Departent of Civil, Architectural, and Environental Engineering, University of Miai, Coral Gables, FL 33124-0630, USA, Fax: (305) 284-3492 Abstract This paper presents a nuerical approach for siulating volatile organic copound (VOC) eissions fro dry aterials. The approach has been used to exaine the VOC eissions fro two particleboards. The eission study for the particleboards shows that a fairly good agreeent of VOC concentrations between the odel prediction and experiental data can be achieved by pre-calculating the partition coefficient (K a ) and aterial age (AGE) and adusting the diffusion coefficient (D ) and initial concentration (C 0 ). Further, the study finds that K a only affects short-ter eissions while D influences both the short-ter and long-ter eissions. 1. Introduction Nuerous field and laboratory studies have found that coonly used dry aterials such as wood products, floor coverings (carpet, vinyl), wall coverings (wallpaper, fabric), ceiling aterials (acoustic tiles, subfloors), and insulation aterials (fiberglass, rigid foa) eit a variety of volatile organic copounds (VOCs). Eissions fro dry aterials are iportant to indoor air quality because of their large surface area and peranent exposure to indoor air. Therefore, they should be studied and rated so that only those with low eission rates are used in buildings. In the past, VOC eissions fro several hundred types of dry aterials have been tested. These have been done ainly for screening purposes, i.e., identifying the aor VOCs eitted fro a particular source and its tie-varying eission dynaics. A aority of the tests use a sallscale test chaber under controlled environental conditions (e.g., 23 C teperature, 50% relative huidity, and 1 air exchange per hour). A large body of easureent results can be found in the literature, and recently, have been included in the eission database of indoor aterials [1, 2]. In addition to screening aterials, the easureents should also:
(1) facilitate aterial ranking. Industries use the eission data to rank a aterial as a high, ediu, or low eission source. This inforation can be given to aterial anufacturers for iproving their products as well as to building designers to select the least toxic or non-toxic aterials. (2) help us understand eission characteristics. Contrary to wet aterials, the eission rates of dry aterials are usually sall and decay slowly. This eans that their eissions can last uch longer than wet aterials. The data would also help us to better understand eission echaniss (e.g., diffusion-controlled instead of evaporation- controlled). (3) provide data for indoor air quality studies. Because liiting all the aterials to zero- or loweissions is either ipractical or quite expensive, inforation on how to transfer the data fro an environental chaber to buildings is needed for indoor environent design. The first two obectives can be easily achieved. The last one, however, cannot be inferred directly fro the easured data due to the following two reasons: (1) To obtain useful eission data for dry aterial, the test period ust be sufficiently long. The real easureents cannot cover the entire eission life of a aterial. Material eissions beyond the period of easureent reain unknown. (2) A chaber test usually easures a aterial saple. While in buildings, both the geoetry (e.g., the thickness of the aterial) and boundary conditions ay be different fro the test saple. Further, the environental conditions in buildings ay not be the sae as those in a test chaber. Even though airflow ay have a negligible effect, the diffusion process ay be significantly affected by other factors, such as teperature. Hence, the easured data fro an environental chaber ay not be valid in buildings. Appropriate eission odels are needed to solve the above probles. At present, ost existing odels for dry aterials assue that eissions are exclusively doinated by internal diffusion. These existing odels, so-called diffusion odels, use Fick s law to solve VOC diffusions in a solid under siple initial and boundary conditions. For exaple, Dunn [3] calculates diffusioncontrolled copound eissions fro a sei-infinite source. Little et al. [4] siulates the VOC eissions fro new carpets using the assuption that the VOCs originate predoinately in a unifor slab of polyer backing aterial. These odels, though based on the sound ass transfer echaniss of VOC species, still have liitations: (1) They presue that the only ass transfer echanis is the diffusion through the source aterial. The odels neglect the ass transfer resistance through the air phase boundary layer and also the air phase concentration on eissions. Although this ay be true for soe dry aterials, the assuption as a general one has not been well ustified. (2) They tend to solve the VOC diffusion proble analytically. These odels usually apply only to a 1-D diffusion process with siple boundary and initial conditions. In practice, eissions can be 3-D with coplicated initial and boundary conditions. 2
Other than the above odels, Yang et al. [5] developed a 2-D nuerical odel to siulate VOC eissions fro an styrene-butadiene rubber (SBR) carpet. Their study considers the VOC ass transfer in both the aterial and the air phase boundary layer, but neglects the partition coefficient at the aterial-air interface. Use of the odel is thus liited to aterials with extreely sall diffusivity or long-ter eission evaluations. The above indicates that, although there is general agreeent that eissions fro indoor sources can be described by fundaental ass transfer theories, a generalized odel that has been validated with experiental data for detailed eission study is not yet available. This suggests a need for developing advanced odels to fully characterize building aterial eissions and their ipact on IAQ. This paper presents a new strategy to developing a coprehensive ass transfer odel without neglecting the echaniss affecting dry aterial eissions. The odel is a general one that applies to broad purposes, fro obtaining useful aterial properties based on environental chaber test data to studying aterial eissions in buildings. 2. The atheatical odel Consider a aterial source that has one surface exposed to the air. VOC eissions fro the source include the ass transfer in three different regions [5]: The solid aterial The aterial-air interface The bulk air 2.1. Material layer For a dry aterial with hoogeneous diffusivity, several researchers [4-8] have used the following diffusion odel to describe the VOC ass transport within the aterial: C τ = (D C ) (1) where: C = VOC concentration in the solid aterial, μg/ 3 τ = tie, sec x = coordinates ( = 1,2,3) D = diffusion coefficient of the VOC in the solid aterial, 2 /s Here, the dependence of D on the VOC concentration is usually neglected because the VOC concentrations in a dry source are usually very sall (e.g., the ratio of the initial VOC concentration to the aterial density is uch less than the threshold 1%, as suggested by Schwope et al. [9]). The dependence of D on teperature can be assued to follow an Arrhenius type [5]: 3
D E d = D,0 exp( ) (2) RT For a coposite aterial coposed of two or ore layers of hoogeneous aterials, applying Eq. (1) to each layer yields: C τ, i = ( D, i C, i ) (3) where i represents the i th layer of the coposite aterial. At the interface between the two layers, the VOC ass balance and the concentrations on the two sides take the for: D C C, i, k, i = D, k (4) C i K ikc, k, = (5) where i and k indicate two adacent aterials. 2.2. Material-air interface At the aterial-air interface, a VOC phase change occurs fro the aterial side to the air side. For low concentrations, the equilibriu condition at the aterial-air interface ay be described by [8]: where K a = the diensionless aterial-air partition coefficient. C a = the VOC concentration on the air side, g/ 3 2.3. Air C = K a C a (6) The VOC ass transfer fro the aterial-air interface to the abient air is deterined by velocity distributions as well as the flow type (e.g., lainar or turbulent). For siplicity, the following discussion is confined to lainar flow (e.g., flow in a sall-scale chaber without a ixing fan). For an incopressible, lainar, and Newtonian flow, the conservation equations for continuity, oentu (using Boussenisseq approxiation for buoyancy), energy, and VOC species can be generalized as: 4
τ ( ρφ) + ( ρu φ) = ( Γ φ φ ) + S φ (7) where ρ = air density, kg/ 3 φ = 1 for ass continuity φ = u ( = 1, 2, and 3) for three coponents of oentu (u,v,w) φ = T for teperature φ = C for VOC concentrations x (=1,2,3) = coordinate. In a cartecine coordinate, x 1 =x, x 2 =y, x 3 =z u (=1,2,3) = three coponents of air velocity. In a Cartesian coordinate, u 1 = u (velocity in x direction), u 2 = v (velocity in y direction), u 3 = w (velocity in z direction) Γ φ = effective diffusion coefficient for φ = source ter for φ S φ The φ, Γ φ and S φ for different flows are shown in Table 1. 2.4. Boundary conditions In addition to the above equations, appropriate boundary conditions are also needed. Although the boundary conditions for different probles vary, the four types of coon boundary conditions of friction and VOC are inlet, outlet, walls, and axis of syetry. (a) Inlet Air velocity (V) and copound concentration (C) are specified for the air supply inlet as: V=V supply C=C 0 (8) where V supply is air velocity at the supply inlet, and C 0 the inlet concentration (usually 0). (b) Outlet A pressure is given and zero gradient of copound concentration in the direction noral to the return outlet: P=P return C = 0 (9) where P return is the pressure at the return outlet and x is the coordinate noral to the outlet. 5
(c) Walls Walls include solid walls of a test chaber (or roo) and surfaces of the eission aterial. If V i is air velocity parallel to a wall and x is the coordinate noral to the wall surface, the shear stress τ w at a wall is expressed by: τ w V i = μ (10) The boundary condition for concentration at the eitting aterial surface is: C D = x C Da (11) x and at other non-eitting surfaces: C = 0 (12) (d) Axis of syetry At the axis of syetry, we have: C Vi = 0 = 0 (13) where V i is the air velocity perpendicular to the axis of syetry, x. The initial conditions for VOC are given as follows. In the air, the initial copound concentration is zero. In the solid aterial, we consider two different types of initial conditions: C = C 0 for a new aterial: (14) C = C 0 F(x, AGE) for an aged aterial: (15) where: C 0 = initial concentration of copound in the solid slab, μg/ 3 AGE = age of the aterial, day F(x, AGE) = function used to describe the initial concentration profile in the solid 6
The key to using the above atheatical odel for a dry aterial is to first obtain the physical properties of the aterial. These properties are a) the diffusion coefficient, D, b) the partition coefficient, K a, and c) the initial concentration, C 0. If the aterial is not totally new, the age of the aterial, AGE, is also needed. AGE specifies the initial condition of a dry aterial. AGE = 0 eans a unifor initial concentration in the aterial while a non-zero AGE indicates a nonunifor initial concentration. Once these paraeters are deterined, they can be used to study the eissions under different environental and boundary conditions (e.g., in a building). Before discussing the ethods for obtaining the aterial properties, it is iportant to understand the influences of these key paraeters on eissions fro a dry source. The following section exaines such influences. 3. The influence of C 0, D, K a, and AGE on dry aterial eissions In this section, we present results using an exaple to study the sensitivity of dry aterial eissions to C 0, D, K a, and AGE. A dry aterial with a diension of 0.212 0.212 0.0159 3 is placed in a sall chaber of 0.5 0.4 0.25 3, as shown in Figure 1. The chaber is ventilated by 1 ACH and the inlet concentration is 0. The only eitting area fro the dry aterial is fro the top surface (0.212 0.212 2 ), and the other faces of the aterial are well sealed. The physical properties of the aterial as a reference case are given as follows: C 0 = 5.28 10 7 μg/ 3 D = 7.65 10-11 2 /s K a = 3289 AGE = 0 The above paraeters were obtained fro those of a particleboard that is exained later. This section will exaine the influence of each individual paraeter (all the other paraeters reain the sae as those of the reference case) on the resulting VOC concentration at the chaber exhaust. 3.1. The influence of C 0 The atheatical odel (Eqs. (1) - (15)) indicates that the VOC concentration in the chaber air depends linearly on the C 0. This can be confired by the results given in Figure 2. The figure shows that when C 0 increases fro 5.28 10 7 μg/ 3 to 1.056 10 8 μg/ 3 (D, K a, and AGE reain the sae as those in the reference case), the concentration also increases by a factor of 2. 3.2. The influence of D Figure 3 illustrates the effect of D on the chaber concentration with tie for a constant C 0 of 5.28 10 7 and K a of 3289 (AGE = 0). The values of D vary between 10-12 2 /s and 10-10 2 /s. 7
Results show that D significantly influences both the peak concentration and the decay rate of the concentration curve. A higher D results in a higher peak concentration and a faster decay rate. 3.3. The influence of K a Figure 4 shows the effect of K a on the chaber concentration with tie for a constant C 0 of 5.28 10 7 and D of 7.65 10-11 (AGE = 0). The values of K a vary between 1 and 10,000. Siulation results show that the influence of K a is two-fold. First, increasing the K a decreases the eission rate at early ties and results in a slower depletion rate of the source. However, the influence of a change in K a is virtually insignificant below a value of about 1,000. On the other hand, it was observed that although the initial eission and depletion rates vary significantly for different K a, the chaber concentration after soe tie is alost identical. This suggests that for a dry source with sall diffusivity, K a ay only affect early-stage eissions. To theoretically prove the above prediction, consider a dry aterial of thickness L that is exposed to air and eits VOCs. For siplicity, consider a 1-D proble only. The governing equation describing the transient diffusion through the aterial reads: C (y, τ) = D τ 2 C 2 y (y, τ) 0<y<L, τ >0 (16) where C (y, τ) = the VOC concentration at y and tie τ y = the coordinate in which direction that the VOC diffusion in the aterial takes place At the aterial-air interface (y = L): C (y = L, τ) = K C (y = L, τ) (17) a a C (y = L, τ) q(y = L, τ) = D (18) y where q(y, τ) is the VOC eission rate (g/ 2 s) at the aterial-air interface and tie τ. At y = 0 we assue an adiabatic surface whereby: C (y = 0, τ) = 0 y (19) The initial concentration in the solid aterial is given as unifor: C 0 = (y, τ = 0) = C const 0<y<L (20) 8
Divide both sides of Eqs. (16) - (19) by C 0 and let: C (y, τ) θ (y, τ) = (21) C 0 and C (y = L, τ) a θ a (y = L, τ) = (22) C 0 Laplace transfor Eqs. (16) - (19): s d θ 2 θ (y,s) 1 = D 2 0<y<L, τ>0 (23) dy (y,s) θ y = L,s) = K θ (y L,s) (24) ( a a = dθ (y = L,s) Q(y = L,s) = D dy (25) dθ (y = 0,s) = 0 dy (26) where s is tie in the Laplace doain. The solution to Eq. (23) that satisfies Eq. (26) is: 1 s θ (y,s) = + A cosh(y ) 0<y<L (27) s D where A is a constant to be deterined by boundary conditions At y = L, we have fro Eq. (27): and 1 s θ (y = L,s) = + A cosh(l ) (28) s D dθ (y = L,s) = A sinh(l dy s D ) (29) Fro Eqs. (25) and (29): 9
Fro Eqs. (24) and (28), we can deterine A as: s Q(y = L,s) = A D s sinh(l ) (30) D 1 K aθa (y = L,s) A = s (31) s cosh(l ) D Fro Eqs. (30) and (31), we have the eission rate Q(y = L,s) in the Laplace doain: Q(y s 1 = L,s) = D s tanh(l )[K aθa (y = L,s) ] (32) D s Eq. (32) indicates that the total eission in the Laplace doain, Q(y = L,s), is coposed of two parts: due to the surface concentration, K a θ a (y = L,s), and due to a step function, 1/s. Initially (τ=0), K 1 θ a (y = L,s) so Q(y = L,s) = 0. s a = As τ > 0, the ter K a θ a (y = L,s) in Eq. (32) decreases as ore VOCs are eitted out but the ter 1/s reains constant. This eans the contribution of the ter K a θ a (y = L,s) to eissions becoes less. At the point when K a 1 θ a ( y = L, s) <<, s the effect of the surface concentration (hence K a ) becoes negligible. The above proves that the K a does not affect the long-ter eission of dry aterials. Physically, the VOC concentration at the aterial surface approaches zero after sufficient tie has elapsed. 3.4. The influence of AGE The AGE is the tie that it takes for a aterial with unifor initial concentration to reach the sae initial VOC distribution as the aterial tested. In order to deterine the ipact of AGE on the subsequent eissions, a nuerical siulation is conducted for a period of AGE by assuing the aterial is in the sall-scale chaber. The concentration distributions in the aterial (which are not unifor) are then used as the initial condition for a new siulation. Results of the new siulation give the eissions by considering the aterial age. Figure 5 gives the predicted VOC concentrations in the chaber outlet for different AGE. Results show that the eission rates for a saller AGE are higher at the beginning. Meanwhile, 10
the figure also shows that the effect of AGE tends to diinish after a period. This indicates that the AGE affects only the early stage eissions. 4. Identification of aterial properties The above sensitivity analysis indicates that in order to predict aterial eissions, all the aterial properties (C 0, D, K a, and AGE) ust be properly obtained. In case these properties cannot all be accurately given, special attention should be paid to the C 0 and D because they deterine both the short-ter and long-ter eission characteristics. The ost direct way of obtaining the aterial properties is by experiental easureents. Recently, Bodalol et al. [7] developed a ethod to easure the D and K a of dry sources. The ethod is proising in that it can provide a database of VOC transport properties for building aterials. However, the easureents require a specially designed diffusioeter consisting of two stainless steel chabers (with the test specien in between). Conducting the experients is expensive and tie consuing (it takes a long tie for VOCs to diffuse through the specien). Moreover, Bodalol [7] estiated that the easureent equipent has an uncertainty of ±18% for both D and K a. When the easured property values were used to predict eissions fro the sae aterial and VOCs, a uch larger siulation error was observed [7]. Due to the liitations of the direct easureent approach, aterial properties can also be estiated by the use of eission chaber data (concentration vs. tie curve) together with eission odeling through curve fitting. Since any chaber easureents have been conducted and published, obtaining aterial properties fro the existing eission data is uch cheaper than obtaining it fro direct easureents. A aor proble for this ethod is that it ay involve obtaining two or ore coupled unknowns using one set of data. However, results fro the previous section indicate that the influence of C 0 on eissions can be separated fro that of D, K a, and AGE. Aong the last three, K a and AGE can be estiated in light of the following: (a) Most aterials tested are stored in a sealed tedlar bag fro the tie they are anufactured until they are tested. In such a case, AGE = 0. (b) The short-ter eissions of a dry aterial are not sensitive to the value of K a within a certain range (e.g., 1-1,000) and the long-ter eissions are independent of K a. This allows the use of soe approxiations to estiate K a, as deonstrated below. If both AGE and K a can be pre-deterined, the curve fitting will be for D only. In this way, the uncertainties of estiating two or ore coupled paraeters using one set of data can be eliinated. In suary, the following procedures can be followed to obtain the aterial properties (D, K a, C 0 ) and aterial age (AGE) based on the easured eission data (C vs. τ) for a single saple of dry aterial. 11
Step 1: Analyze the eission data and identify the copound(s) eissions to be studied. Identify physical properties of the copound(s), such as olecular weight, vapor pressure, etc. Step 2: Estiate AGE by tracking the aterial history and storage ethod. Step 3: Pre-deterine the aterial-air partition coefficient. Recently, Bodalal et al. [7] easured D and K a for several solid aterials and found that although D depends heavily on both aterial and copound properties, the partition coefficient for different aterials can be approxiated based solely on the vapor pressure of the copound. Figure 6 shows the correlations for different aterials and copounds given by Bodalal [7]. When the aterial and copound to be studied do not atch the data available, the following correlation (for plywood) ay be used: 0.91 K a = 10600 / P (33) where P is the vapor pressure of the copound in Hg. Step 4: Establish the coputational doain, boundary and initial conditions. Step 5: Use nuerical siulation to obtain D by adusting its value until the predicted chaber concentration agrees with the easured data. Since the initial concentration C 0 (which is unknown by now) does not affect the shape of the eission curve, at this stage we can assue an arbitrary value, C 0,ini and copare the relative concentration (C/C ax ) between the odel prediction and the data. A least-square analysis is usually eployed to obtain a best fit between the siulation results and the data. The D is obtained when a best fit is achieved. Step 6: Obtain the initial concentration, C 0. Notice that the chaber concentration is proportional to C 0 for the sae K a and D ; the value of C 0 can be obtained by: C ax,data C 0 = C0,ini (34) Cax,si where C ax,data and C ax,si are the axiu concentration by easureent and siulation, respectively. In the following, the use of the above procedures are illustrated with two particleboard saples. 5. Application exaple: VOC eissions fro particleboard Particleboard is a panel product ade of wood particles bounded together under heat and pressure, usually using an ureaforaldehyde (UF) resin adhesive. The adhesive produces a water-resistant bound. The bound is layered with the finer particles in the surface layers and coarser particles in the core layer. 12
Particleboards have long been identified as VOC eitters [10]. Recently, we easured the VOC eissions fro two new particleboards. In the experients, a sall-scale chaber of 0.5 0.4 0.25 3, illustrated in Figure 1, was used for easuring the particleboard eissions. Two different speciens of particleboard (PB1 and PB2) were tested. A saple holder was used in all the tests, liiting the exposure of the test speciens to a single face. Details of the test speciens and their geoetrical diensions are shown in Table 2. The test conditions were: Teperature: 23 ± 0.5 C Relative huidity: 50 ± 0.5 % Air exchange rate: 1.0 ± 0.05 h -1 Loading ratio: 0.729 2 / 3 The tests lasted 96 hours for PB1, and 840 hours for PB2. The VOCs were screened by GC/MS and quantified by tube sapling and analyzed by GC/FID. Maor copounds identified for the two different particleboards were the sae: hexanal, α.pinene, caphene, and lionene. For each particleboard tested, the eissions of TVOC and two aor copounds: hexanal and α.pinene were siulated. Since both particleboards tested were new, the aterial age AGE = 0. K a was calculated using Eq. (33), and D and C 0 were obtained by curve fitting, as shown in Table 3. Figures 7 and 8 copare the predicted VOC concentrations and the data using the properties listed in Table 3. An exaination of the aterial property data revealed several interesting features: (1) The VOC eission profiles between the two particleboards are very different. However, the property data in Table 3 indicate that the two particleboards actually shared the sae partition coefficient (K a ) and diffusion coefficient (D ). The only difference is the initial concentration, C 0. The high initial concentration of the PB2 sees to be connected to the high percentage of scavenger (see Table 2) used during the anufacturing process. This suggests that an effective way to reduce eissions fro the aterial is to use as little scavenger as possible to reduce C 0. (2) For the two types of particleboards studied, K a and D for TVOC can be represented by those of hexanal, the ost abundant copound in the particleboards. (3) For both the two particleboards exained, eissions of α.pinene decayed faster than TVOC and hexanal. Hence, the D for α.pinene is larger than that of hexanal (1.2 10-10 vs. 7.65 10-11 ). This finding however is contrary to the olecular diffusion theory. Based on the diffusion theory, copounds with larger olecular weight (α.pinene) should have a saller diffusion coefficient. The reason for this unusual phenoenon is not clear. 6. Conclusions This paper presents a nuerical approach for siulating VOC eissions fro dry aterials. The approach uses the following paraeters to describe eission characteristics: the initial VOC 13
concentrations in the aterial (C 0 ), the solid-phase diffusion coefficient (D ), the aterial-air partition coefficient (K a ), and the age of the aterial (AGE). The paraeters were obtained by fitting the predicted VOC concentrations with the sall-scale chaber data. Our study finds that different paraeters have different ipacts on eissions. The eission rate is in proportion to C 0. D influences both short-ter and long-ter eissions. A higher D results in a higher initial eission rate and a faster decay rate. On the other hand, K a and AGE affect only the short-ter eissions of the particleboard. It has virtually no ipact to long-ter eissions. The eission studies for two different particleboard saples show that a fairly good agreeent of VOC concentrations between the experiental data and odel prediction can be achieved by pre-deterining K a and AGE and adusting D and C 0. These aterial properties obtained can be used to study aterial eissions in buildings, and help to reduce the aterial eissions by reforulating the products. Acknowledgeents This investigation is supported by the US National Science Foundation (Grant CMS-9623864) and National Research Council of Canada. References [1] European Coission (EC), European Data Base on Indoor Air Pollution Sources in Buildings, Final Report, 1997. [2] National Research Council (NRC), MEDB-IAQ: Material Eission Database and a singlezone IAQ odel a tool for building designers, engineers, and anagers, IRC/NRC CMEIAQ Final Report 4.2, Ottawa, Canada, 1999. [3] Dunn, J.E., Models and statistical ethods for gaseous eission testing of finite sources in well-ixed chabers, Atospheric Environent, 1987, 21(2), 425-430. [4] Little, J.C., Hodgson, A.T., and Gadgil, A.J., Modeling eissions of volatile organic copounds fro new carpets, Atospheric Environent, 1994, 28(2), 227-234. [5] Yang, X., Chen, Q. and Bluyssen, P.M., Prediction of short-ter and long-ter volatile organic copound eissions fro SBR bituen-backed carpet under different teperatures, ASHRAE Transactions, 1998, 104(2), 1297-1308. [6] Christianson, J., Yu, J.W. and Neretnieks, I., Eission of VOC s fro PVC-flooring - odels for predicting the tie dependent eission rates and resulting concentrations in the indoor air, Proceedings of Indoor Air 93, 1993, 2, 389-394. 14
[7] Bodalal, A., Fundaental ass transfer odeling of eission of volatile organic copounds fro building aterials, Ph.D. Thesis, Departent of Mechanical and Aerospace Engineering, Carleton University, Canada, 1999. [8] Yang, X., Study of building aterial eissions and indoor air quality, Ph.D. Thesis, Departent of Architecture, Massachusetts Institute of Technology, Cabridge, MA, 1999. [9] Schwope, A.D., Lyan, W.J., and Reid, R.C., Methods for assessing exposure to cheical substances, Vol. 11: Methodology for estiating the igration of additives and ipurities fro polyetric aterials, U.S. Environental Protection Agency, Office of Toxic Substances, Washington, DC, EPA/560/5-85-015, 1989. [10] Matthews, T.G., Reed, T.J., Tiberg, B.J., Daffron, C.R. and Hawthorne, A.R., Foraldehyde eissions fro cobustion sources and solid foraldehyde resin containing products: Potential ipact on indoor foraldehyde concentrations and possible corrective easures, Proceedings of An Engineering Foundation Conference on Manageent of Atospheres in Tightly Enclosed Spaces, Santa Barbara, ASHRAE, 1983, 23-43. 15
Table 1 Values of φ, Γ φ and S φ in Eq. (7). φ Γ φ S φ 1 0 0 U i (i=1,2,3) μ p ρg iβ(t T0 ) i T μ/pr S T C μ/sc S c Where μ is lainar viscosity, Pa.s p is air pressure, Pa g i is coponent i of the gravitation vector, /s 2 β is theral expansion factor, 1/K T 0 is reference teperature Pr= 0.71 is the Prandtl nuber Sc is the olecular Schidt nuber Table 2 Test speciens of the particleboard. Specien Type/Grade Length/width/height Manufacturing details () PB1 Industrial 0.212 0.212 0.0159 Single opening line; 100s press UF Resin: 11.5% face, 8.9% core Scavenger: 15.0% face, 5.0 % core PB2 Industrial 0.212 0.212 0.0159 Multi-opening line; 125s press UF Resin: 11.2% face, 9.4% core Scavenger: 35% face, 20 % core Wax: 1.2% face, 0.9% core 16
Table 3 Physical properties of particleboard eissions (a) PB1 Copound TVOC Hexanal α.pinene D ( 2 /s) 7.65 10-11 7.65 10-11 1.2 10-10 C 0 (μg/ 3 ) 5.28 10 7 1.15 10 7 9.86 10 6 K a 3289 3289 5602 AGE 0 0 0 (b) PB2 Copound TVOC Hexanal α.pinene D ( 2 /s) 7.65 10-11 7.65 10-11 1.2 10-10 C 0 (μg/ 3 ) 9.86 10 7 2.96 10 7 7.89 10 6 K a 3289 3289 5602 AGE 0 0 0 17
Figure caption Figure 1 A sall-scale chaber for easuring dry aterial eissions. Figure 2 The influence of the initial concentration of the source on the resulting VOC concentration in the chaber air. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, partition coefficient K a = 3289, and aterial age AGE = 0. Figure 3 The influence of diffusion coefficient on the resulting VOC concentration in the chaber air. The curve is predicted with the initial concentration C 0 = 5.28 10 7 μg/ 3, partition coefficient K a = 3289, and aterial age AGE = 0. Figure 4 The influence of the aterial-air partition coefficient on the resulting VOC concentration in the chaber air. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, initial concentration C 0 = 5.28 10 7 μg/ 3, and aterial age AGE = 0. Figure 5 Coparison of VOC concentrations at the chaber outlet with different aterial age, AGE. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, partition coefficient K a = 3289, and initial concentration C 0 = 5.28 10 7 μg/ 3. Figure 6 Measured partition coefficient of different aterials (Bodalal, 1999). Figure 7 Coparison of easured and siulated VOC concentrations eitted fro PB1: (a) TVOC, (b) Hexanal, (c) α.pinene. Figure 8 Coparison of easured and siulated VOC concentrations eitted fro PB2: (a) TVOC, (b) Hexanal, (c) α.pinene. 18
Building and Environent Outlet Inlet 0.25 0.4 0.5 Figure 1 A sall-scale chaber for easuring dry aterial eissions. 19
12000 C (ug/ 3 ) 8000 4000 C0=5.28E7 C0=1.06E8 0 0 20 40 60 80 Tie (h) Figure 2 The influence of the initial concentration of the source on the resulting VOC concentration in the chaber air. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, partition coefficient K a = 3289, and aterial age AGE = 0. 20
C (ug/3) 6000 5000 4000 3000 2000 D=7.64E-11 D=1E-10 D=1E-11 D=1E-12 1000 0 0 20 40 60 80 Tie (h) Figure 3 The influence of diffusion coefficient on the resulting VOC concentration in the chaber air. The curve is predicted with the initial concentration C 0 = 5.28 10 7 μg/ 3, partition coefficient K a = 3289, and aterial age AGE = 0. 21
20000 C (ug/ 3 ) 16000 12000 8000 K=1 K=100 K=1000 K=3289 K=10000 4000 0 0 20 40 60 80 Tie (h) Figure 4 The influence of the aterial-air partition coefficient on the resulting VOC concentration in the chaber air. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, initial concentration C 0 = 5.28 10 7 μg/ 3, and aterial age AGE = 0. 22
C (ug/ 3 ) 6000 5000 4000 3000 2000 AGE=0 AGE=0.5 day AGE=1 day AGE=2 day 1000 0 0 20 40 60 80 Tie (h) Figure 5 Coparison of VOC concentrations at the chaber outlet with different aterial age, AGE. The curve is predicted with the diffusion coefficient D = 7.65 10-11 2 /s, partition coefficient K a = 3289, and initial concentration C 0 = 5.28 10 7 μg/ 3. 23
Partition coefficient (Ka) 20000 16000 12000 8000 4000 experient (Vinyl) Correlation (Vinyl) experient (OSB) Correlation (OSB) experient (Plywood) Correlation (Plywood) experient (Particleboard) Correlation (Particleboard) 0 0 10 20 30 40 50 Vapor pressure P (Hg) Figure 6 Measured partition coefficient of different aterials (Bodalal, 1999). 24
5000 4000 Data Siulation C (ug/ 3 ) 3000 2000 1000 C (ug/ 3 ) 0 1200 800 400 0 20 40 60 80 100 Tie (h) (a) Data Siulation 0 0 20 40 60 80 100 Tie (h) (b) 800 600 Data Siulation C (ug/ 3 ) 400 200 0 0 20 40 60 80 100 Tie (h) (c) Figure 7 Coparison of easured and siulated VOC concentrations eitted fro PB1: (a) TVOC, (b) Hexanal, (c) α.pinene. 25
12000 9000 Data Siulation C (ug/ 3 ) 6000 3000 0 0 200 400 600 800 Tie (h) (a) 2500 C (ug/ 3 ) 2000 1500 1000 Data Siulation 500 C (ug/ 3 ) 0 2500 2000 1500 1000 0 100 200 300 400 500 600 700 800 Tie (h) (b) Data Siulation 500 0 0 100 200 300 400 500 600 700 800 Tie (h) (c) Figure 8 Coparison of easured and siulated VOC concentrations eitted fro PB2: (a) TVOC, (b) Hexanal, (c) α.pinene. 26