Jianshun Zhang

A mass transfer model for simulating volatile organic compound emissions from ‘wet’ coating materials applied to absorptive substrates

Abstract A numerical model has been developed to simulate volatile organic compound (VOC) emissions from ‘wet’ coating materials. The model considers the VOC mass transfer process in the air and material–air interface, and diffusion in the material film and also in the substrate. Our numerical simulations confirmed that the emissions from wet materials applied to an absorptive substrate are dominated by evaporation at the beginning followed by internal diffusion, which had been hypothesized based on previous experimental data. The numerical model has been validated based on the data from small-scale environmental chamber tests.

Comparison of models for describing measured VOC emissions from wood-based panels under dynamic chamber test condition

Measured emission factors are the experimental data used to represent emission characteristics of volatile organic compounds (VOCs) from testing materials under dynamic chamber test conditions. A simple empirical model that describes the measured emission factors will be very useful for practical purposes. In this study, a power law model was compared with a widely used first-order exponential decay model in their ability to describe measured emission factors of wood-based panel materials. It was demonstrated that the power law model is a better choice than the first-order model for describing emission characteristics for short-term (less than 100 h) experimental data. The power law model was also more superior in predicting long-term (up to 900 h) emission factors.

Critical review of catalytic oxidization and chemisorption methods for indoor formaldehyde removal

Formaldehyde, an irritant and cacinogent to humans, is one of the most concerning indoor gaseous pollutants because it is often found in buildings and poses a potential health risk to occupants even at a very low concentration level. Chemisorption and catalytic oxidization are two promising methods for indoor formaldehyde removal. This review covers the following aspects of the two formaldehyde removal methods: reaction mechanism, activity test method, materials, performance, and effect of environmental conditions (temperature, relative humidity, concentration level, and velocity) on the removal performance. Results show that a supported noble metal (e.g., Pt) and metal oxide (e.g., MnO2) are the most effective catalysts, but usually require a high temperature for complete decomposition of formaldehyde. An amino group containing activated carbon is the most commonly used chemisorbent. The effect of the noble metal loading and the preparation method of the noble metal catalyst are also discussed. Possible applications in a building HVAC system are discussed along with needed future research.

Ozone reaction with clothing and its initiated VOC emissions in an environmental chamber

Human health is adversely affected by ozone and the volatile organic compounds (VOCs) produced from its reactions in the indoor environment. Hence, it is important to characterize the ozone-initiated reactive chemistry under indoor conditions and study the influence of different factors on these reactions. This investigation studied the ozone reactions with clothing through a series of experiments conducted in an environmental chamber under various conditions. The study found that the ozone reactions with a soiled (human-worn) T-shirt consumed ozone and generated VOCs. The ozone removal rate and deposition velocity for the T-shirt increased with the increasing soiling level and air change rate, decreased at high ozone concentrations, and were relatively unaffected by the humidity. The deposition velocity for the soiled T-shirt ranged from 0.15 to 0.29 cm/s. The ozone-initiated VOC emissions included C6-C10 straight-chain saturated aldehydes, acetone, and 4-OPA (4-oxopentanal). The VOC emissions were generally higher at higher ozone, humidity, soiling of T-shirt, and air change rate. The total molar yield was approximately 0.5 in most cases, which means that for every two moles of ozone removed by the T-shirt surface, one mole of VOCs was produced.

Stochastic study of hygrothermal performance of a wall assembly—The influence of material properties and boundary coefficients

An accurate description of material properties and boundary conditions is the prerequisite to achieve reliable results of a hygrothermal performance simulation. However, the uncertainties widely exist in those input variables, e.g., due to the inhomogeneous nature of materials and discrepancies in material manufacturing and measurement processes, the material properties are subjected to considerable variation. Therefore, the simulation result may not be only a single value, but in a range of possibilities. In this article, a stochastic approach is developed and implemented by using uncertainty analysis which assesses the uncertainties of a models input variables on that of the output variables, and sensitivity analysis, which identifies the most influential input variables. The hygrothermal performance of one typical wall assembly in North America is studied by applying this approach, considering the uncertainties of material properties of each individual layer and the uncertainties of the interior and exterior boundary coefficients. The influence of wall orientation is also presented. The results show that the effect of a single input variable on an output variable is not constant but varies with time. The key variables that affect the results are also different over time.

New Indices to Evaluate Volatile Organic Compound Sorption Capacity of Building Materials (RP-1321)

The material to air equilibrium partition coefficient (Ke) is often used to represent sorption capacity of building materials. However, it does not represent the sorption dynamics (i.e., the sink effect) of volatile organic compounds (VOCs) inside a porous material, which depends not only on the partition, but also on the in-material diffusion rate and convective mass transfer rate through the boundary layer. Based on fundamental mass transfer theory for VOC sorption by building materials, this paper proposes VOC sorption mass (M(t)) and sorption saturation degree (SSD) as new evaluation indices for sorption capacity and dynamics of building materials under given constant inlet concentration. It is found that SSD can be characterized by dimensionless sorption mass (m*), which is a function of dimensionless air change rate (N*), dimensionless mass capacity (Θ), and Fourier number for mass transfer (Fom). Two cases, one with constant inlet VOC concentration and the other a hypothetical case under a no-ventilation condition, are simulated to illustrate material sorption capacity. This evaluation method can clarify the difference between air saturation state and material saturation state and would be useful for modeling the impact of material sorption on indoor air quality.

Determination of partition and diffusion coefficients of formaldehyde in selected building materials and impact of relative humidity.

The partition and effective diffusion coefficients of formaldehyde were measured for three materials (conventional gypsum wallboard, “green” gypsum wallboard, and “green” carpet) under three relative humidity (RH) conditions (20%, 50%, and 70% RH). The “green” materials contained recycled materials and were friendly to environment. A dynamic dual-chamber test method was used. Results showed that a higher relative humidity led to a larger effective diffusion coefficient for two kinds of wallboards and carpet. The carpet was also found to be very permeable resulting in an effective diffusion coefficient at the same order of magnitude with the formaldehyde diffusion coefficient in air. The partition coefficient (K ma) of formaldehyde in conventional wallboard was 1.52 times larger at 50% RH than at 20% RH, whereas it decreased slightly from 50% to 70% RH, presumably due to the combined effects of water solubility of formaldehyde and micro-pore blocking by condensed moisture at the high RH level. The partition coefficient of formaldehyde increased slightly with the increase of relative humidity in “green” wallboard and “green” carpet. At the same relative humidity level, the “green” wallboard had larger partition coefficient and effective diffusion coefficient than the conventional wallboard, presumably due to the micro-pore structure differences between the two materials. The data generated could be used to assess the sorption effects of formaldehyde on building materials and to evaluate its impact on the formaldehyde concentration in buildings. Implications: Based on the results of this study, the sink effects of these commonly used materials (conventional and “green” gypsum wallboards, “green” carpet) on indoor formaldehyde concentration could be estimated. The effects of relative humidity on the diffusion and partition coefficients of formaldehyde were found to differ for materials and for different humidity levels, indicating the need for further investigation of the mechanisms through which humidity effects take place.

Evaluation of filter media performance: Correlation between high and low challenge concentration tests for toluene and formaldehyde (ASHRAE RP-1557)

To guide the selection of gas phase filtration media in the air cleaning devices, it is important to understand and estimate the media performance under usage concentrations. Filters for improving indoor air quality are typically subject to low volatile organic compounds (VOCs) concentration levels (e.g., ∼50 ppb), while the current standard tests per ASHRAE 145.1 (ANSI/ASHRAE 2008). are performed at relatively high challenge concentrations (∼1–100 ppm level). The primary objective of this study was to determine if media that perform well at the high concentration test condition would also perform well under the low concentration. The secondary objective was to investigate if and how existing models of filtration by media bed can be applied to extrapolate the results from the high concentration tests to the low concentration condition. Experiments and simulations were carried out at both high concentrations (100 ppm for toluene and 1 ppm for formaldehyde) and low concentrations (0.05 ppm for toluene and formaldehyde) for six selected filtration media. The results show that (1) the high concentration test data were able to differentiate the relative performance among the media at the low concentration properly, confirming the validity of using ASHRAE 145.1 (ANSI/ASHRAE 2008) for relative performance comparison; (2) significant initial breakthrough observed at high concentration tests of large pellet media was not present at the low concentration tests, indicating the dependency of the adsorption capability of the sorbent media on the concentration level as well as the possible “by-pass” effects (i.e., not all the VOC molecules in the air stream had the same chance to contact with the sorbent media); and (3) existing models need to be improved by incorporating the concentration dependency of the partition coefficient and the by-pass effect in order to predict the breakthrough curve at low concentrations properly. Such an improved model was proposed, evaluated with the measured data, and was found to be promising for physical sorbent, but requires further development for chemical, catalytic sorbent and large pellet sorbent. The study provides previously unavailable experimental data and new insight into the behavior of the filtration media for volatile organic compounds as well as evidence in support of the application of ASHRAE Standard 145.1 (ANSI/ASHRAE 2008) for media performance evaluation.

Modeling ozone penetration through the wall assembly using computational fluid dynamics

Outdoor pollutants, such as ozone, can penetrate into the indoor environment through the wall assemblies and influence the indoor air quality (IAQ). Ozone can also react with chemical compounds (i.e., d-limonene, α-pinene, β-pinene, styrene, etc.) within the wall assemblies to create secondary emissions causing IAQ concerns. This study developed a modeling framework for predicting ozone penetration through building envelope systems. The transport process of ozone through leakage paths was numerically simulated by two approaches: (1) a species transport model plus a chemical reaction model and (2) a user-defined scalar (UDS) transport model plus a user-defined deposition model. A simplified method to simulate ozone transport through fibrous media was also developed in a UDS model based on the analysis of two deposition mechanisms: (1) transport-limited deposition and (2) surface uptake of the fiber. The model was successfully applied to a typical residential wall assembly, assuming crack heights of 1 mm (0.04 in.) in the vertical direction and 3 mm (0.12 in.) in the horizontal direction and with fiberglass insulation width of 0.14 m (5.5 in.). Theoretical analysis showed that gas phase reaction between ozone and unsaturated volatile organic compounds emitted from wall materials in leakage paths and fiberglass insulation could be neglected compared with the surface reaction. The calculated penetration factors through leakage paths by the species transport model and UDS transport model were in good agreement. Fiberglass insulation media greatly reduced ozone penetration by more than 60% under almost all circumstances when reaction probability was larger than 10−8.

On the control of environmental conditions using personal ventilation systems

The concept of personal control is examined as this relates to the conceptual design of personal ventilation systems based upon the bodys physiological and psychological homeostatic requirements. Literature on the effects of personal control of environmental conditions is reviewed. The concept of a “just noticeable difference in discomfort” that underpins the desire to initiate some personal control action is discussed. The concept of personal control is symmetrical for thermal conditions (hot or cold) but, apart from some pleasant fragrances, is asymmetric for indoor air quality (where less pollution is better). Factors that may influence the exercise of personal control will be summarized. Finally, important information for the design of PVS systems that are both energy efficient and effective in optimizing the micro-environmental conditions for workplace employees will be presented.