A theoretical study on VOC source and sink behavior of porous building materials

摘要

The indoor air quality (IAQ) of a building can be significantly affected by the building materials. Porous building materials are not only sources of indoor air pollutants such as volatile organic compounds (VOC) but also strong sinks of those 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 has investigated theoretically VOC source and sink behavior of porous building materials. A novel 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 is generation or elimination of VOC within the solid due to 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 the proposed analytical model could simultaneously account for the effect of air velocity on both VOC source and sink behavior unlike the existing analytical models. A parametric study was carried out to investigate the effects of air velocity and material properties including diffusion coefficient, sorption partition coefficient, porosity, thickness and length on the primary VOC source/sink behavior, and these effects were quantified. Due to the lack of knowledge on the secondary source/sink behavior, five hypothetical cases were considered, and the model predictions agree with experimental findings on the secondary emissions available in the literature. The validity of four main assumptions imposed on the developed analytical model, was investigated through the development of numerical conjugate mass transfer models. The considered assumptions are (1) constant VOC concentration at the solid-fluid interface along the solid plate length; (2) quasi-steady convection mass transfer in fluid; (3) one-dimensional diffusion in solid; and (4) Henry (linear) sorption isotherm. The limits of the proposed analytical model due to each of the mentioned assumptions were clearly defined hence providing a range for the validity of the novel analytical model.