Study of Activation of Coal Char

This is the third report on a project whose aim is to explore in a fundamental manner the factors that influence the development of porosity in coal chars during the process of activation. It is known that choices of starting coal, activating agent and conditions can strongly influence the nature of an activated carbon produced from a coal. Interest in this phase of the project turned to characterization of one particular char. Results have been published on Pittsburgh No. 8 char using an entirely different porosity characterization method. The interpretation of the results in that other study is not entirely consistent with what has been observed in this study. In particular, the results of the present study seemed to indicate the opening up of existing porosity, as opposed to creation of new porosity. It is difficult to infer much, based upon the porosity characterizations alone. Instead, attention was turned to the correlation of porosity with reactivity, which can provide a clue as to whether there was actually full accessibility of all of the observed porosity. The conclusion is that the pores are not all fully accessible, and that different oxidizing gases behave differently. The suggestion is that measured porosity is not all accessible to reactants. Also, attempts to correlate reactivity of chars with surface area are likely to be problematic, if different gases behave differently in this regard

Study of Activation of Coal Char

This is the second report on a project whose aim is to explore in a fundamental manner the factors that influence the development of porosity in coal chars during the process of activation. It is known that choices of starting coal, activating agent and conditions can strongly influence the nature of an activated carbon produced from a coal. This work has again confirmed that there is a fundamental difference in char structure that is reflective of the source of the chars. What is new in the present results is a strong indication that this difference is seen, irrespective of the conditions of char preparation. Results were compared for utility combustion chars, all of which were prepared under the very high intense heating conditions of utility boilers, and the laboratory-prepared chars prepared at orders of magnitude lower heating rates. The chars were of very similar nature regardless of the heating conditions that led to their preparation (and despite major differences in level of burnoff). On the other hand, the results from the examination of the laboratory char results do again suggest that the activation conditions play some role in determining porosity, though their effect is decidedly less important than the role of the parent material. This is true despite an enormous range of reactivity exhibited by the activating agents

Study of Activation of Coal Char

This is the fourth report on a project whose aim is to explore in a fundamental manner the factors that influence the development of porosity in coal chars during the process of activation. It is known that choices of starting coal, activating agent and conditions can strongly influence the nature of an activated carbon produced from a coal. Interest in this phase of the project turned to characterization of one particular char. Results have been published on Pittsburgh No. 8 char using an entirely different porosity characterization method. The interpretation of the results in that other study is not entirely consistent with what has been observed in this study. In particular, the results of the present study seemed to indicate the opening up of existing porosity, as opposed to creation of new porosity. It is difficult to infer much, based upon the porosity characterizations alone. Instead, attention was turned to the correlation of porosity with reactivity, which can provide a clue as to whether there was actually full accessibility of all of the observed porosity. The conclusion is that the pores are not all fully accessible, and that different oxidizing gases behave differently. The suggestion is that measured porosity is not all accessible to reactants. Also, attempts to correlate reactivity of chars with surface area are likely to be problematic, if different gases behave differently in this regard

A two-dimensional analytical model of petroleum vapor intrusion

In this study we present an analytical solution of a two-dimensional petroleum vapor intrusion model, which incorporates a steady-state diffusion-dominated vapor transport in a homogeneous soil and piecewise first-order aerobic biodegradation limited by oxygen availability. This new model can help practitioners to easily generate two-dimensional soil gas concentration profiles for both hydrocarbons and oxygen and estimate hydrocarbon indoor air concentrations as a function of site-specific conditions such as source strength and depth, reaction rate constant, soil characteristics and building features. The soil gas concentration profiles generated by this new model are shown in good agreement with three-dimensional numerical simulations and two-dimensional measured soil gas data from a field study. This implies that for cases involving diffusion dominated soil gas transport, steady state conditions and homogenous source and soil, this analytical model can be used as a fast and easy-to-use risk screening tool by replicating the results of 3-D numerical simulations but with much less computational effort

Risk Assessment Tool for Chlorinated Vapor Intrusion Based on a Two-Dimensional Analytical Model Involving Vertical Heterogeneity

At contaminated sites, it is common to observe volatile organic compounds rising from subsurface sources and migrating into overlying buildings through cracks or other openings present in foundation slabs and basement walls. Such process, called vapor intrusion, is usually the most critical exposure pathway at sites contaminated by chlorinated solvents. In this study we present a chlorinated vapor intrusion tool implemented in Microsoft (R) Exce (R) using Visual Basic for Applications and integrated within a graphical interface that helps users to visualize two-dimensional (2D) soil gas concentration profiles and indoor air concentration in scenarios involving subsurface vertical heterogeneity. Vertical heterogeneity in soil gas concentration can be induced by a variable soil moisture profile caused by layering (geological barriers) or by the presence of the capillary fringe. This tool can be used in the risk assessment procedure to assess expected indoor air concentrations in conjunction with other lines of evidence, such as the evaluation of the 2D soil gas concentration profiles below and beyond the building footprint. Moreover, this tool allows users to predict indoor air concentration by employing either the traditional perimeter crack entry pathway or the empirical subslab-to-indoor attenuation factor. After a brief description of the developed tool, we show some practical applications to highlight the potential benefits in using this tool compared with the U.S. Environmental Protection Agency (U.S. EPA) tool that implements a simplified form of the Johnson and Ettinger model. Based on our testing, we found that the two-layer approach (capillary fringe and vadose zone) employed in the U.S. EPA tool, can lead to an overestimation of subslab vapor concentrations by more than an order of magnitude

Estimating the oxygenated zone beneath building foundations for petroleum vapor intrusion assessment

Previous studies show that aerobic biodegradation can effectively reduce hydrocarbon soil gas concentrations by orders of magnitude. Increasingly, oxygen limited biodegradation is being included in petroleum vapor intrusion (PVI) guidance for risk assessment at leaking underground storage tank sites. The application of PVI risk screening tools is aided by the knowledge of subslab oxygen conditions, which, however, are not commonly measured during site investigations. Here we introduce an algebraically explicit analytical method that can estimate oxygen conditions beneath the building slab, for PVI scenarios with impervious or pervious building foundations. Simulation results by this new model are then used to illustrate the role of site-specific conditions in determining the oxygen replenishment below the building for both scenarios. Furthermore, critical slab-width-to-source-depth ratios and critical source depths for the establishment of a subslab "oxygen shadow" (i.e. anoxic zone below the building) are provided as a function of key parameters such as vapor source concentration, effective diffusion coefficients of concrete and building depth. For impervious slab scenarios the obtained results are shown in good agreement with findings by previous studies and further support the recommendation by U.S. EPA about the inapplicability of vertical exclusion distances for scenarios involving large buildings and high source concentrations. For pervious slabs, results by this new model indicate that even relatively low effective diffusion coefficients of concrete can facilitate the oxygen transport into the subsurface below the building and create oxygenated conditions below the whole slab foundation favorable for petroleum vapor biodegradation. (C) 2016 Elsevier B.V. All rights reserved