Self-Sustaining Smoldering Combustion: A Novel Remediation Process for Non-Aqueous-Phase Liquids in Porous Media

Abstract

Smoldering combustion, the slow burning process associated typically with porous solids (e.g., charcoal), is here proposed as a novel remediation approach for nonaqueous phase liquids (NAPLs) embedded in porous media. Several one-dimensional vertical smoldering experiments are conducted on quartz sand containing fresh coal tar at an initial concentration of 71 000 mg/kg (approximately 25% saturation) and employing an upward darcy air flux of 4.25 cm/s. Following a short-duration energy input to achieve ignition at the lower boundary, a self-sustaining combustion front is observed to propagate upward at 1.3 x 10(-2) cm/s. The process is self-sustaining because the energy released during NAPL smoldering is efficiently trapped and recirculated by the soil matrix, preheating the NAPL ahead of the reaction front. The smoldering process is observed to self-terminate when all of the NAPL is destroyed or when the oxygen source is removed. Pre- and post-soil analysis revealed that NAPL smoldering reduced the concentration of total extractable petroleum hydrocarbons (TPH) from 38 000 mg/kg to below detection limits (< 0.1 mg/kg) throughout the majority of the column. A comparable experiment in which conductive heating is applied in the absence of smoldering demonstrates a 6-fold reduction in the net energy in the system and residual TPH values of 2000-35 000 mg/kg. A further repeat in which the air supply is prematurely terminated demonstrated that the NAPL smoldering process can be extinguished via external control. A suite of 23 demonstration experiments shows that NAPL smoldering is successful across a range of soil types (including simple layered systems) and contaminants (including laboratory mixtures of dodecane, DCA/ grease, TCE/oil, vegetable oil, crude oil, and mineral oil) as well as field-obtained samples of materials containing coal tar, oil drill cutting waste, and oil sands.

Organic liquid mobility induced by smoldering remediation

“…This concept was recently introduced as a promising remediation approach for contaminated soil [4,5]. Smoldering for the destruction of organic liquid wastes is possible but requires the liquids to be mixed intentionally with an inert porous matrix, such as sand [6].…”

“…This facilitates the smoldering of the liquids embedded in the pores of the matrix, destroying the liquids and leaving clean sand behind [4].…”

Effects of high temperature processes on physical properties of silica sand

“…Exposures of 200 -850°C have been observed in soils during wildfires (Certini, 2005;DeBano, 2000;Mataix-Solera and Doerr, 2004;Rein et al, 2008). Moderate (300-400°C) and high (>450°C) temperature processes, such as hot water extraction, thermal desorption, soil heated vapour extraction, incineration or smouldering are widely used to treat contaminated soils (Araruna Jr et al, 2004;Chang and Yen, 2006;Gan et al, 2009;Kronholm et al, 2002;Lee et al, 2008;McGowan et al, 1996;Pironi et al, 2011;Pironi et al, 2009;Switzer et al, 2009;Webb and Phelan, 1997). Previous studies on the effects of non-aqueous phase liquid (NAPL) contamination on soil properties shows a net reduction in soil stability as NAPL content increases (Khamehchiyan et al, 2007).…”

“…These processes maintain high temperatures in the soil for weeks to months or longer. In contrast, smouldering remediation uses the contaminant itself as fuel for the combustion reaction (Pironi et al, 2011;Switzer et al, 2009). In laboratory studies, the soil particles are exposed to high temperatures on the order of 1000°C for coal tars and 600-800°C for oils for up to 60 minutes.…”

“…Decomposition of soil particles, especially clay minerals, starts at temperatures above 550°C (Certini, 2005). These temperatures are rarely reported for wild and forest fire, but temperatures up to 1200°C can be achieved during smouldering remediation Switzer et al, 2009). …”