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Laboratory and field studies of smouldering technology for aquifer remediation (STAR) for non-aqueous phase liquid (NAPL) sources

Switzer, Christine and Pironi, Paolo and Rein, Guillermo and Fuentes, Andres and Torero, Jose L. and Gerhard, Jason I. (2008) Laboratory and field studies of smouldering technology for aquifer remediation (STAR) for non-aqueous phase liquid (NAPL) sources. In: ConSoil 2008, 2008-06-03 - 2008-06-06.

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Smouldering Technology for Aquifer Remediation (STAR) is a novel technology for the remediation of contamination by non-aqueous phase liquids (NAPLs) in the subsurface. Smouldering is a form of combustion that is flameless. Generally, smouldering is slower and less energetic than flaming combustion. Familiar examples of smouldering involve solid fuels that are destroyed by the reaction (e.g., a smouldering cigarette or peat smouldering after a wildfire). In STAR, the NAPL serves as the fuel within an inert, porous soil medium. Once the NAPL fuel is ignited in the subsurface, the hot gases emitted from the exothermic NAPL smouldering heat the porous media ahead of the reaction, increasing the both reaction efficiency and the flammability limits of many liquid fuels. STAR has the potential to be both self-sustaining and self-targeting. If the energy released by the smouldering reaction, stored and recirculated ahead of the reaction is sufficient to allow the smouldering front to propagate forward, the ignition source may be removed and the smouldering reaction may become self-sustaining. The distribution of NAPL fuel in the subsurface is the path the smouldering front will follow. Because the soil is inert, the smouldering front follows the path of the fuel within the soil and does not spread beyond this path, making the reaction self-targeting. The smouldering reaction is controlled by the delivery of air or oxidant to the subsurface. Once the air or oxidant supply is removed, the reaction stops, making the reaction self-terminating. Results from laboratory and field experiments are very promising. At the laboratory scale, nine NAPL fuels have been tested. NAPLs including petrochemicals, chlorinated solvents and coal tar have been remediated successfully by STAR. Physical and chemical analysis of soils after treatment by STAR suggested little to no residual NAPL contamination remained. More extensive characterisation work has been carried out on coal tar, the most energetic of the NAPL fuels. Parameters such as air flow rate, initial NAPL saturation, soil type and ignition method have been varied to study their impact on the smouldering reaction. Larger scale studies have been carried out in the development of this technology toward field-deployable configurations. Field scenarios for STAR include both in situ and ex situ configurations. The laboratory experiments scale directly to the ex situ field configuration where excavated, NAPL-contaminated soil is placed in a reactor vessel and treated by STAR. For the in situ configuration, the STAR process is initiated in the NAPL pool in the subsurface. Air or oxidant is supplied to support the reaction and force it to follow the NAPL fuel through the subsurface. Successful application of the in situ STAR method has the potential for leaving the treated soil in place, avoiding the costly excavation and disposal processes while protecting the aquifer. Because the contaminant is destroyed in place, treatment costs of recovered NAPL are avoided as well. Field trials are underway for ex situ STAR. This paper presents laboratory, small-scale field experiments and pilot ex situ field trials that have been conducted. Discussion focuses on the key parameters required for successful application of STAR as well as emissions characterisation for combustion byproducts.