Self-sustaining smoldering combustion for NAPL remediation: laboratory evaluation of process sensitivity to key parameters.

Smoldering combustion has been introduced recently as a potential remediation strategy for soil contaminated by nonaqueous phase liquids (NAPLs). Published proof-of-concept experiments demonstrated that the process can be self-sustaining (i.e., requires energy input only to start the process) and achieve essentially complete remediation of the contaminated soil. Those initial experiments indicated that the process may be applicable across a broad range of NAPLs and soils. This work presents the results of a series of bench-scale experiments that examine in detail the sensitivity of the process to a range of key parameters, including contaminant concentration, water saturation, soil type, and air flow rates for two contaminants, coal tar and crude oil. Smoldering combustion was observed to be self-sustaining in the range 28,400 to 142,000 mg/kg for coal tar and in the range 31,200 to 104,000 mg/kg for crude oil, for the base case air flux. The process remained self-sustaining and achieved effective remediation across a range of initial water concentrations (0 to 177,000 mg/kg water) despite extended ignition times and decreased temperatures and velocities of the reaction front. The process also exhibited self-sustaining and effective remediation behavior across a range of fine to coarse sand grain sizes up to a threshold maximum value between 6 mm and 10 mm. Propagation velocity is observed to be highly dependent on air flux, and smoldering was observed to be self-sustaining down to an air Darcy flux of at least 0.5 cm/s for both contaminants. The extent of remediation in these cases was determined to be at least 99.5% and 99.9% for crude oil and coal tar, respectively. Moreover, no physical evidence of contamination was detected in the treatment zone for any case where a self-sustaining reaction was achieved. Lateral heat losses to the external environment were observed to significantly affect the smoldering process at the bench scale, suggesting that the field-scale lower bounds on concentration and air flux and upper bound on grain size were not achieved; larger scale experiments and field trials where lateral heat losses are much less significant are necessary to define these process limits for the purposes of field application. This work provides valuable design data for pilot field trials of both in situ and ex situ smoldering remediation applications.

Remediation of trichloroethylene-contaminated soils by star technology using vegetable oil smoldering.

Self-sustaining treatment for active remediation (STAR) is an innovative soil remediation approach based on smoldering combustion that has been demonstrated to effectively destroy complex hydrocarbon nonaqueous phase liquids (NAPLs) with minimal energy input. This is the first study to explore the smoldering remediation of sand contaminated by a volatile NAPL (trichloroethylene, TCE) and the first to consider utilizing vegetable oil as supplemental fuel for STAR. Thirty laboratory-scale experiments were conducted to evaluate the relationship between key outcomes (TCE destruction, rate of remediation) to initial conditions (vegetable oil type, oil: TCE mass ratio, neat versus emulsified oils). Several vegetable oils and emulsified vegetable oil formulations were shown to support remediation of TCE via self-sustaining smoldering. A minimum concentration of 14,000 mg/kg canola oil was found to treat sand exhibiting up to 80,000 mg/kg TCE. On average, 75% of the TCE mass was removed due to volatilization. This proof-of-concept study suggests that injection and smoldering of vegetable oil may provide a new alternative for driving volatile contaminants to traditional vapour extraction systems without supplying substantial external energy.

Volumetric scale-up of smouldering remediation of contaminated materials.

Smouldering remediation is a process that has been introduced recently to address non-aqueous phase liquid (NAPL) contamination in soils and other porous media. Previous work demonstrated this process to be highly effective across a wide range of contaminants and soil conditions at the bench scale. In this work, a suite of 12 experiments explored the effectiveness of the process as operating scale was increased 1000-fold from the bench (0.003m(3)) to intermediate (0.3m(3)) and pilot field-scale (3m(3)) with coal tar and petrochemical NAPLs. As scale increased, remediation efficiency of 97-99.95% was maintained. Smouldering propagation velocities of 0.6-14×10(-5)m/s at Darcy air fluxes of 1.54-9.15cm/s were consistent with observations in previous bench studies, as was the dependence on air flux. The pilot field-scale experiments demonstrated the robustness of the process despite heterogeneities, localised operation, controllability through airflow supply, and the importance of a minimum air flux for self-sustainability. Experiments at the intermediate scale established a minimum-observed, not minimum-possible, initial concentration of 12,000mg/kg in mixed oil waste, providing support for the expectation that lower thresholds for self-sustaining smouldering decreased with increasing scale. Once the threshold was exceeded, basic process characteristics of average peak temperature, destructive efficiency, and treatment velocity were relatively independent of scale.