Coal tar contamination at manufactured gas plant (MGP) sites and hydrocarbon-impacted soils associated with the oil and gas industry are complex problems that only a handful remedies are capable of addressing in a cost effective and timely manner. STAR – based upon in situ smouldering combustion – is an innovative approach that has significant potential for the remediation of sites impacted by such non aqueous phase liquids (NAPLs).
Combustion is the exothermic oxidation of a fuel. In the case of a carbon-based compound, the products are primarily carbon dioxide, water, and energy. The combustibility of NAPLs is a characteristic that has been successfully exploited through the ex situ incineration of NAPLs and contaminated soil (Howell et al., 1996). Incineration is achieved primarily via flaming combustion which involves the gasification of a fuel and its exothermic oxidation in the gas phase. Incineration of NAPLs by flaming combustion is energy inefficient (i.e., high heat losses); as a result, incineration requires the continuous addition of fuel and, often, supplemental energy. Smouldering combustion, in contrast, is the exothermic oxidation of a condensed phase (i.e., solid or liquid) occurring on the fuel surface (Ohlemiller, 1996). Smouldering is limited by the rate of oxygen transport to the fuel’s surface, resulting in a slower and lower temperature reaction than flaming (Rein, 2009).
Importantly, smouldering can be self-sustaining (i.e., no energy input required after ignition) when the fuel is embedded in or itself a porous medium. Self-sustaining smouldering occurs because the solid acts as an energy sink and then feeds that energy back into the unburnt fuel, creating a very energy efficient reaction (Howell et al., 1996). AAPG Search and Discovery Article #90172 © CSPG/CSEG/CWLS GeoConvention 2010, Calgary, Alberta, Canada, May 10-14, 2010 2 NAPL smouldering is different from existing thermal remediation techniques. In situ thermal remediation requires the continuous input of energy in order to primarily volatilize and, in some cases, thermally degrade (pyrolize) and mobilize (via viscosity reductions) the organic phase.
All of these processes are endothermic and remediation continues as long as externally supplied energy input is sustained throughout the NAPL-occupied porous medium. In contrast, NAPL smouldering has the potential to create a combustion front that (i) initiates at a single location with the NAPL-occupied porous medium, (ii) initiates with a one-time, short-duration energy input, (iii) propagates through the NAPL-occupied medium in a self-sustained manner, and (iv) destroys the NAPL wherever the front passes. NAPL smouldering is different from in situ combustion for enhanced oil recovery in that the latter is designed to generate heat and pressure that will mobilize the entrapped oil toward recovery wells. NAPL smouldering, in contrast, may benefit from avoiding the recovery (and thus treatment) of NAPL and/or water. This work presents an overview of the scientific principles behind STAR, and summarizes the five years of proof-of-concept research that has been successfully conducted to date. Furthermore, the design and results of the first STAR pilot study, focusing on an in situ application of the technology at a former cresol manufacturing facility in New Jersey, will be presented.