Developing mass transfer based model for predicting VOCs emission from building materials

Abstract

Modern people spend most of their time indoors. Many of them do not realize that the air they breathe indoors may be more polluted than the air they breathe outdoors. There are a variety of sources of indoor air pollution. Building interior materials have been identified as significant indoor VOC (Volatile Organic Compounds) sources. It has been demonstrated that inhalation of VOC may cause a variety of adverse health effects. Selecting low-emission building products is a more cost-effective solution to improve indoor air quality (IAQ), compared with ventilation method which is energy-intensive in both cold and hot, humid climates. Current standards only require that the VOCs emission rate profile be documented. The problem is that the emission rate thus obtained is test-condition specific, and can only be used to compare and rank the emission strengths between the tested samples under the specific test conditions, but cannot be extended to predict the VOCs concentrations in a real building. To evaluate the actual impact of a particular material on indoor VOCs concentration, modeling methods based upon mass transfer theories are required. A simplified yet physically-based model is developed to predict VOCs emissions from wet materials like paints. To validate the proposed model, the European standard emission test cell called field and laboratory emission cell (FLEC) is adopted, in view of its low cost and good repeatability, and potential to be adopted as a standard in Asia. Locally bought water-based emulsion paint is tested in the FLEC, and experimental results agree well with the model predictions. The proposed model is easy to scale up because the parameters involved have distinct physical meanings. Regarding dry building materials, very few data on the key parameters, particularly the diffusion and sorption parameters, are available now. Different methods have been developed for the experimental determination of these parameters for a variety of VOC/material combinations, but large discrepancies exist due to the limitations of the test methods and data analysis techniques. In this study, FLEC is used to inversely determine the model parameters required for dry building materials, and also to validate the model. Fundamental mass-transfer method is applied to analyze the VOCs emission processes in a standard test cell for dry building materials and then an inverse method is developed and utilized to analyze the recorded FLEC exhaust VOCs concentration profiles to determine the corresponding mass transfer parameters. The inverse parameter estimation method developed in this research is solved by the well-known Levenberg-Marquardt (LM) optimization algorithm and allows accurate determination of several parameters simultaneously. The inverse method is validated regarding the uniqueness, stability and accuracy by using synthetically generated series-of-measurements. Then real measured data for selected VOCs/materials are processed using the proposed method. The VOCs/material properties identified in this way are independent of test conditions. Another important parameter to be determined is the VOCs initial condition within the material which is very likely to be non-uniform. To date, there is only one reported experimental method, which requires special equipments. However, the reported method was destructive to the test sample and possible loss of VOCs might occur during sample preparation and analysis. Further, the obtained VOCs distribution within the material was in a discrete form. In this research, an inverse function estimation method - conjugate method of minimization with the adjoint problem, in conjunction with an environmental chamber or the non-destructive test facility - FLEC, is developed to figure out the VOCs' initial condition within the dry building material. The proposed method can be non-destructive to the test material if using FLEC and the distribution obtained is in a continuous form. On the basis of a single component multi-layer model, a comprehensive VOCs source/sink model is developed and applied to predict the level of VOCs concentration, so that proper selection criteria can be set up based upon the health impact, and applicable IAQ control strategies can be evaluated. Unlike existing models, the comprehensive model developed in this research takes various conditions into consideration and thus can handle different building scenarios. Currently, the model can deal with different ventilation modes, multi-component and multi-layer material, porous/non-porous materials, non-uniform initial VOCs distributions, and transient outdoor air VOCs concentration. It can be used as a decision-making tool for assessing the impact of material emissions on indoor air quality under various ventilation conditions.