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

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.

Smoldering Remediation of Coal-Tar-Contaminated Soil: Pilot Field Tests of STAR.

Self-sustaining treatment for active remediation (STAR) is an emerging, smoldering-based technology for nonaqueous-phase liquid (NAPL) remediation. This work presents the first in situ field evaluation of STAR. Pilot field tests were performed at 3.0 m (shallow test) and 7.9 m (deep test) below ground surface within distinct lithological units contaminated with coal tar at a former industrial facility. Self-sustained smoldering (i.e., after the in-well ignition heater was terminated) was demonstrated below the water table for the first time. The outward propagation of a NAPL smoldering front was mapped, and the NAPL destruction rate was quantified in real time. A total of 3700 kg of coal tar over 12 days in the shallow test and 860 kg over 11 days in the deep test was destroyed; less than 2% of total mass removed was volatilized. Self-sustaining propagation was relatively uniform radially outward in the deep test, achieving a radius of influence of 3.7 m; strong permeability contrasts and installed barriers influenced the front propagation geometry in the shallow test. Reductions in soil hydrocarbon concentrations of 99.3% and 97.3% were achieved in the shallow and deep tests, respectively. Overall, this provides the first field evaluation of STAR and demonstrates that it is effective in situ and under a variety of conditions and provides the information necessary for designing the full-scale site treatment.

The influence of porous media heterogeneity on smouldering remediation.

Self-sustained Treatment for Active Remediation (STAR) is a thermal remediation technology that uses smouldering, a flameless form of combustion, for destroying organic contaminants in soil. Injected cold air flowing through the soil to the treatment zone supports the release of sufficient energy to maintain a self-sustained reaction and the propagation of the reaction through the contaminated zone as long as the airflow local to the reaction exceeds a minimum value. However, the distribution and magnitude of air flux vectors can be complex in the heterogeneous environment common at contaminated sites. This research presents the first investigation of smouldering remediation under varying degrees and patterns of permeability heterogeneity. Nine experiments examined smouldering remediation in contaminated layers of varying permeability arranged alone and in contrasting layers in series, in parallel, and in two distinct complex patterns. The results suggest that smouldering can successfully propagate across layer boundaries and through layers in series regardless of their permeability (at least down to 1 x 10-12 m2). However, fine layers were not smouldered for layers in parallel with a permeability ratio ≥ 3:1. Numerical modelling of these cases with a published smouldering model revealed that this occurred due to insufficient airflow in the fine layers in some cases, or conductive heat transfer (thermal coupling) between parallel layers in other cases. The more complex heterogeneity patterns underscored the importance of the connected length of the higher permeability pathway on airflow distribution and therefore on smouldering propagation. Disconnected coarse zones supported smouldering in both coarse and fine zones while connected coarse zones kept smouldering in the coarse pathway while bypassing fine zones. Overall, this research provides unique insights into understanding heterogeneous scenarios to ensure the successful application of smouldering remediation.

Scaling up self-sustained smouldering of sewage sludge for waste-to-energy.

Self-sustained smouldering combustion presents strong potential as a green waste-to-energy technique for a range of wastes, especially those with high moisture content like wastewater sewage sludge. While well-demonstrated in laboratory experiments, there is little known about scaling up this process to larger, commercial reactors. This paper addresses this knowledge gap by systematically conducting and analyzing experiments in a variety of reactors extending beyond the laboratory scale. This work reveals a robust treatment regime; however, it also identifies potential complications associated with perimeter heat losses at scale. Two key impacts, on the smouldering reactions and the air flow patterns, are shown to potentially degrade treatment if not properly understood and managed. Altogether, this study provides novel insight and guidance for scaling up smouldering science into practical, waste-to-energy systems.

Multidimensional validation of a numerical model for simulating a DNAPL release in heterogeneous porous media.

A fixed-volume release of 1,2-DCE, tracked in space and time with a light transmission/image analysis system, provided a data set for the infiltration, redistribution, and immobilisation of a dense non-aqueous phase liquid (DNAPL) in a heterogeneous porous medium. The two-dimensional bench scale flow cell was packed with a spatially correlated, random heterogeneous distribution of six sand types. In order to provide the necessary modelling parameters, detailed constitutive relationships were measured at the local scale for the six sands. These experiments revealed that nonwetting phase (NWP) relative permeability-saturation (k(rN)-S(W)) relationships are strongly correlated to sand type. Trends in the best-fit k(rN)-S(W) parameters reflected a positive correlation between mean grain diameter and the maximum NWP relative permeability, k(rN)(max). Multiphase flow simulations of the bench scale experiment best reproduced the experimental observations, producing excellent matches in both time and space, when the measured, correlated local scale k(rN)-S(W) relationships were employed.

Application of self-sustaining smouldering combustion for the destruction of wastewater biosolids.

Managing biosolids, the major by-product from wastewater treatment plants (WWTPs), persists as a widespread challenge that often constitutes the majority of WWTP operating costs. Self-sustained smouldering combustion is a new approach for organic waste treatment, in which the waste - the combustion fuel - is destroyed in an energy efficient manner after mixing it with sand. Smouldering has never been applied to biosolids. Column experiments, using biosolids obtained from a WWTP, were employed to identify if, and under what conditions, smouldering could be used for treating biosolids. The parameter space in which smouldering was self-sustaining was mapped as a function of key system metrics: (1) sand/biosolids mass fraction, (2) biosolids moisture content, and (3) forced air flux. It was found that a self-sustaining reaction is achievable using biosolids with water content as high as 80% (with a biosolids lower heating value greater than 1.6 kJ/g). Moreover, results suggest that operator-controlled air flux can assist in keeping the reaction self-sustaining in response to fluctuations in biosolids properties. This proof-of-concept demonstrates the potential for smouldering as a new energy efficient biosolids disposal method for very wet (i.e., minimally processed) biosolids that may offer WWTPs significant operating cost savings. This study emphasizes smouldering's usefulness as a novel waste management technique.

The influence of high initial concentration aqueous-phase TCE on the performance of iron wall systems.

Flow-through column tests were conducted to investigate the performance of iron wall remediation systems for the degradation of aqueous-phase trichloroethylene (TCE). Concentration profiles under steady-state transport conditions were generated by measuring TCE concentrations at sample ports located at various locations along the length of the column. The results indicated that a pseudo-first-order model is adequate at describing degradation kinetics for low initial TCE concentrations, but not for higher initial concentrations. The deviation from pseudo-first-order kinetics can be explained by interspecies competition for reactive sites between TCE and a dominant reaction product. A modification of the pseudo-first-order model that accounts for product interference predicts laboratory data for high initial concentration profiles, but deviates slightly as initial concentrations approach the solubility of TCE. The data clearly demonstrate the importance of accurately describing reaction kinetics for the purpose of designing iron wall treatment systems.