In-situ Delivery of Heat by Thermal Conduction and Steam Injection for Improved Dnapl Remediation

Thermal Conduction Heating (TCH) is a major component of In Situ Thermal Desorption (ISTD), a soil remediation technology in which heat and vacuum are applied simultaneously. Heat flows into the soil primarily by conduction from heaters typically operated between 1000 and 1500oF (540 and 815oC). The heaters are installed in wells at regular intervals within the soil. As the soil is heated, water is boiled and dense non-aqueous phase liquid (DNAPL) constituents in the soil are vaporized. The resulting steam and vapors are drawn toward extraction wells for in-situ and aboveground treatment. Compared to fluid injection processes, the conductive heating process is very uniform in its vertical and horizontal sweep. Field project experience at numerous TCH/ISTD sites has confirmed that maintaining target temperatures for several days results in extremely high destruction and removal efficiency of chlorinated volatile organic compounds (CVOCs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and other DNAPLs. Provided that water influx is managed, the effectiveness of the TCH/ISTD process is not limited by the presence of heterogeneous soil conditions or clay, and can be applied effectively below the water table. Despite high pre-treatment soil contaminant concentrations, post-treatment soil concentrations have typically been non-detect, with most of the contaminants (95-99% or more) being destroyed in the soil by hydrolysis, oxidation, and pyrolysis. The TCH process progressively heats soil and soil fluids in a highly predictable way even under heterogeneous and saturated soil conditions, resulting in 100% sweep of the targeted DNAPL zone, and making it possible to offer guarantees of cleanup performance. Due to the invariability of thermal conductivity across a wide range of soil types and conditions, TCH does not demand as detailed knowledge of subsurface conditions as do technologies that depend on delivery of fluids. Nevertheless, TCH alone is not well suited to treatment of high-permeability zones below the water table without adoption of methods to manage groundwater influx. One of the most preferable of such methods is Steam Injection (SI), which can be combined with TCH in a variety of ways that offer synergies due to their complementary thermodynamics and logistics. Analytical and numerical models are of particular value in TCH practice, in part because of the highly predictable nature of conductive heating. A simple example is provided showing how to estimate the heating budget and duration for a given site. INTRODUCTION TCH is an in-situ thermal remediation technique that relies on the use of heating elements, typically electrically powered, suspended inside steel pipes in contact with the soil. The operating temperature of the heating elements is typically between 10001500oF (540-815oC). Vertically oriented heaters are installed in the soil typically in triangular patterns, which at the scale of a well field appear as repeating series of hexagons (Figure 1). In a typical ISTD well field, a ring of “Heater-Only” (H-O) wells surrounds each “Heater-Vacuum” (H-V) well. An H-O well is comprised of a heating element inside of a heater can, the purpose of which is to inject heat into the ground. Wattages can vary depending on the application, but are generally ≤350 W/ft (1,150 W/m) of thermal well to avoid overheating the well materials. Silicon Control Rectifiers on each circuit permit the automatic regulation of the heaters based on the temperature of representative thermocouples. Other thermocouples are installed within the target treatment zone (TTZ) to enable monitoring of heating progress. When the heaters are energized, heat is transferred from the heating elements to the walls of the heating pipes by radiation. Heat transfer from the hot pipes (“heater cans”) through the surrounding soil is primarily by thermal conduction, with convection of fluids, primarily steam, playing a supporting role. Figure 2 provides a qualitative snapshot of the transient distribution of temperature and water saturation around a TCH well. There are three distinct zones, the relative proportions of which will vary depending on soil properties: • The dry conduction zone where pore water has been vaporized, while steep temperature gradients drive conductive heating with an energy flux oriented radially outward from the heater well. • The convective zone characterized by varying water saturations, but a relatively constant temperature equal to the local boiling point of the pore water (212oF [100oC] above the water table, gradually increasing with depth below the water table). It is also referred to as a “heat-pipe” zone (Udell and Fitch 1985; Hiester et al. 2003), where steam generated by boiling transfers heat outward, while water wicks back towards the well by unsaturated flow, driven by a gradient in capillary pressure. This zone will vary in relative proportions over time and depending on soil properties. • The saturated zone, where steam recondenses, and both hot water movement and thermal conduction lead to more distal heat propagation. As heat propagates radially outward from each heater, the cylindrically shaped heated zones expand until they overlap and superimpose. While the heaters are FIGURE 1. Schematic of Typical TCH/ ISTD Well Field. (a) Cross-section showing one Heater-Vacuum (H-V) well and two Heater-Only wells within a larger pattern. (b) Plan view showing thermal well field layout. H-V wells are located in the center of each hexagon, and well spacing typically ranges from 6 to 20 feet (1.8 to 6.1 m). Heater-Only Wells