STARx Hottpad for smoldering treatment of waste oil sludge: Proof of concept and sensitivity to key design parameters

Abstract

Growing stockpiles of waste oil sludge (WOS) are an outstanding problem worldwide. Self-sustaining Treatment for Active Remediation applied ex situ (STARx) is a treatment technology based on smoldering combustion. Pilot-scale experiments for the STARx Hottpad prove this new concept for the mobile treatment of WOS mixed intentionally with sand or contaminated soil. The experiments also allowed for the calibration and validation of a smoldering propagation numerical model. The model was used to systematically explore the sensitivity of Hottpad performance to system design, operational parameters, and environmental factors. Pilot-scale (~1.5 m width) simulations investigated sensitivity to injected air flux, WOS saturation, heterogeneity of intrinsic permeability, and heterogeneity of WOS saturation. Results reveal that Hottpad design is predicted to be successful for WOS treatment across a wide range of scenarios. The operator can control the rate of WOS destruction and extent of treatment by increasing the air flux injected into the bed. The potential for smoldering channeling to develop was demonstrated for the first time. Under certain conditions, such as WOS saturations of 80%, high heterogeneity of WOS saturations, or moderate to high heterogeneity of soil permeability, smoldering channeling was predicted to accelerate to the point that remedial performance was degraded. Field-scale simulations (~10 m width) predicted successful treatment, with WOS destruction rates an order of magnitude higher than the pilot-scale and treatment times increasing only linearly with bed height. This work is a key step toward the design and effective operation of field STARx Hottpad systems for eliminating WOS.

Heat losses in a smouldering system: The key role of non-uniform air flux

“…5 would approach a steady-state condition when radial heat losses balance the energy released from smouldering. Here, like nearly all other applied smouldering experiments in the literature and all smouldering reactors used in practice (e.g., [3,12,45]), the energy balances are far from this late-time condition.…”

“…A pilot smouldering reactor demonstrated the ex situ treatment of coal-tar contaminated soil [44], which was similar in size as another smouldering reactor studied for deriving liquid fuel from waste tires [9]. Following [44], a modular, scalable, batch treatment system was developed for contaminated soils and organic sludges [3,12,45].…”

“…Table 1 presents the experimental conditions, illustrating that the variables adjusted were column diameter, fuel concentration, and applied air flux. The experimental conditions reflect both common values used in applied smouldering research (e.g., [30,54,65,80,82]) and application (e.g., [3,28,45]). The GAC was mechanically mixed with coarse grain sand (1.180 ≤ mean grain diameter ≤ 2.000 mm, porosity (D) = 0.37, bulk density to desired fuel concentrations (example photo in the Supplementary Materials, Fig.…”

Processes defining smouldering combustion: Integrated review and synthesis

“…Smouldering has been applied commercially for in situ (i.e., on site, below ground) remediation of industrially polluted soil [47][48][49][50][51][52][53][54]. Commercial smouldering systems have been developed to treat waste industrial liquids, such as crude oil tank bottoms, in above ground systems [55]. In addition, smouldering has been proposed for managing sewage sludge (in some cases termed biosolids), the dominant waste from industrial waste water treatment plants [56], and for recovering energy from pine bark waste [57].…”

Remediation of PFAS-Contaminated Soil and Granular Activated Carbon by Smoldering Combustion

“…Smoldering can be self-sustaining after ignition, meaning no external energy input is needed to convert the carbonaceous fuel to primarily heat, water, and carbon dioxide; charcoal burning in a traditional barbeque is an example. Smoldering is a commercially available remediation approach for hydrocarbon-contaminated soils and is applied as both an in situ and ex situ technique. − Smoldering is a highly energy- and cost-efficient treatment method because the contaminant is the fuel. The reaction propagates through the soil as a hot, thin front in the direction of airflow, oxidizing the contaminant and leaving clean, organic-free soil behind.…”