A study on VOC source and sink behavior in porous building materials - analytical model development and assessment

Abstract

Building materials can strongly affect indoor air quality. Porous building materials are not only sources of indoor air pollutants such as volatile organic compounds (VOC) but they are also strong sinks of these pollutants. The knowledge of VOC transfer mechanisms in these materials is an important step for controlling the indoor VOC concentration levels, and for determining the optimum ventilation requirements for acceptable IAQ. This study provides a theoretical investigation of primary and secondary VOC source and sink behavior of porous building materials. A new analytical model was developed based on the fundamental theories of mass transfer mechanisms in porous materials. The proposed model considers both primary and secondary source/sink behavior for the first time. The former refers to the transfer of gas-phase and/or physically adsorbed VOC, while the latter refers to the generation or elimination of VOC within the solid because of chemical reactions like oxidation, hydrolysis, chemical adsorption, etc. The proposed model was assessed with experimental data, namely emission tests of carpets and sorption tests of wood chipboard. It was demonstrated that, unlike the existing analytical models, the proposed analytical model could simultaneously account for the effect of air velocity on both VOC source as well as sink behavior. Case studies were then carried out for secondary VOC source behavior. Due to the lack of experimental studies on mechanisms of secondary behavior, hypothetical generation functions were implemented. It was demonstrated that the proposed analytical model is suitable for describing various mechanisms involved in the secondary behavior due to the little limitations imposed on the generation/elimination term. When VOC generation takes place at the material-air interface, the simulation shows that although the primary emission is not affected by air velocity, the secondary emission, however, is clearly affected. This behavior agrees with the available experimental findings on secondary emissions. PRACTICAL IMPLICATION: The analytical model presented in this paper can predict both primary and secondary VOC source (emission) or sink (sorption) behavior of porous building materials. Since the model considers diffusion and adsorption/desorption within the material, and convection over the material surface, the simulation using the model can readily provide the effects of material properties and airflow properties on the primary and/or the secondary behavior, hence, it can provide a better understanding on the mechanisms. This will enable us to keep the indoor VOC concentration within a desirable level.

General analytical mass transfer model for VOC emissions from multi-layer dry building materials with internal chemical reactions

“…Case (c) produces the lowest normalized emission factor because all generated VOCs must penetrate through the entire thickness of the material first and the normalized emission factor is zero before the VOCs from the bottom reach the surface. These results are consistent with those obtained by Lee et al [25]. From Figure 8, it can be seen that the emission rate with only secondary VOC sources is low initially, while according to an earlier study [13] the emission rate with only primary sources decreases with time.…”

“…Models for analyzing VOC emissions from these materials are useful for addressing these problems. VOC emission models in the literature fall into two general categories [4]: empirical or semi-empirical models *Corresponding author (email: wangxinke@mail.xjtu.edu.cn) [5][6][7], and mass transfer based models [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The former are often simple and easy to use and their parameters are determined by fitting experimental data to the predefined model.…”

“…The mass transfer model developed by Little et al [8] is considered the first mass transfer model for analyzing VOC emissions from dry building materials. Many researchers subsequently developed various mass transfer based models to describe more complicated problems, such as emissions from one-[9-15], two-[16], or multi-layer materials [17][18][19][20][21], materials with two emission surfaces [22], and porous materials [23][24][25][26][27]. Secondary VOC emissions from building materials are recognized as an important aspect of indoor VOC exposure [28].…”

Predicting emission characteristics of volatile organic compounds from wet surface coatings

“…In a few diffusion models, external convective mass transfer resistance was neglected and only internal diffusion within the material was taken into account [11,12]. More complete models were developed by taking into account combined effects of internal and external mass transfer resistances [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. In these models, the material is treated as a homogenous medium that can be characterized by an effective diffusion coefficient.…”

“…The initial concentration of VOC was assumed to be either constant [13][14][15][16][17][18][19][20][21][22]24,25,28] or nonuniform [23,26,27]. To predict the concentration of VOCs in air, macroscopic mass balances were generally used [16][17][18][19][20][21][23][24][25][26][27][28], on the other hand, in a few studies, two-or three-dimensional momentum and mass transfer equations were solved [13][14][15]22]. Most of these models assume that equilibrium relationship is linear and the diffusivity of VOC in the coating material is constant.…”