Theme: 01. Remediation Technology Innovations

Topic Area: 01r. Thermal Conductive Heating: Best Practices and Lessons Learned

Formatted Title:

Investigating the Effectiveness of the Dry-Out and Two-Phase Zones around Heaters for Vapor Extraction during Thermal Conduction Heating

Background/Objectives:

Thermal conductive heating (TCH) is an effective technique for the treatment of contaminated soils and groundwater to enable the redevelopment of impacted land for beneficial use. The success of a TCH project at sites impacted by volatile contaminants relies on the production of gas (water and contaminant vapor) and the subsequent capture and treatment of these fluids. If the vapor is not captured, it may migrate outside of the treatment zone and condense, leading to potential rebound effects. During TCH below the water table, vapor generation occurs adjacent to the heater, creating a dry-out (steam) zone and a two-phase (steam and liquid water) zone. Common conceptual models assume that these zones promote and enhance vapor capture by increasing gas permeability near the heaters, and may be sufficient to support the upwards migration of steam and contaminant vapor towards gas extraction points in the vadose zone. However, no laboratory studies have investigated the effectiveness of the dry-out and two-phase zone for gas extraction, particularly in heterogeneous settings where TCH is most often applied. Additionally, no laboratory studies have tracked the migration of liquid water through the two-phase zone during TCH in a heterogeneous setting. Therefore, the objectives of this study were to investigate the evolution of the gas saturation around a thermal conduction heater in heterogeneous porous media and to understand the gas and water pathways through the adjacent zones.

Approach/Activities:

A series of laboratory heating experiments were completed to investigate the evolution of gas around a thermal conduction heater. A 240 W heater was emplaced in a glass-walled 40 × 40 × 1 cm³ flow cell packed with different configurations of water-saturated coarse (d₅₀= 1.1 mm), medium (d₅₀= 0.7mm), and/or fine (d₅₀= 0.1mm) sands. Experiments consisted of either: (1) homogeneous coarse sand, (2) homogeneous fine sand, (3) coarse sand overlain by medium sand, or (4) coarse sand overlain by fine sand. Thermocouples were installed throughout the flow cell to monitor temperature. During the experiments, steam was produced adjacent to the heater and a visual light transmission technique was used to quantify the evolution of gas saturation using digital images. To investigate liquid water migration around the heater, additional experiments were conducted with a constant head boundary on one side of the flow cell and a visual dye tracer.

Results/Lessons Learned:

To the authors’ knowledge, this is the first study that has used light transmission to analyze the evolution of gas saturation adjacent to a conductive heater. The dry-out, two-phase, and water-saturated zones were clearly identified in the homogeneous coarse sand and homogeneous fine sand experiments. The width of the two-phase zone was greater in the fine sand than in the coarse sand because a larger area was required to transfer steam through the lower permeability sand. The experiments completed in the homogeneous sand packs showed distinct differences to the experiments conducted in the heterogenous sand packs with regard to the extent of the two-phase zone. In the heterogeneous experiment with medium sand underlain by coarse sand, gas pooling and lateral migration occurred underneath the capillary barrier, despite the two sands having similar intrinsic permeabilities (k_coarse/k_medium= 2). In the heterogeneous experiment with fine sand underlain by the coarse sand, an even greater extent of gas pooling and lateral migration occurred beneath the capillary barrier. Although in both heterogeneous experiments a two-phase zone was created around the entirety of the thermal conduction heater, the permeability and/or extent of this zone in the upper sand was not sufficient to transport all the gas upwards, which led to lateral steam migration. The migration of steam under capillary barriers has important implications for heat and mass transfer out of the target heated zone. Incorporating the gas dynamics observed in this study into numerical simulations will provide more accurate predictions of vapor system requirements at TCH sites. Additionally, the refined conceptual model of steam migration subject to heterogeneity adjacent to a heater during TCH will allow for a better understanding of temperature and pressure data collected at sensors underneath capillary barriers.

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Kevin Mumford