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

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.