The objective of this project was to demonstrate and validate Thermal Conductive Heating (TCH) performance in fractured bedrock and develop guidelines for practitioners on how to apply TCH. Specific objectives for the on-site TCH demonstration included:
In Situ Thermal Desorption (ISTD) is the simultaneous application of TCH and vacuum to the subsurface. TCH’s primary application uses thermal heating wells along with extraction wells, which can be placed to almost any depth in virtually any media. During the TCH process, heat is applied to the subsurface using simple electrical heaters installed inside a casing in contact with the soil, so that radiation and thermal conduction heat transfer are effective near the heater. As a result, thermal conduction and convection occur in the bulk of the soil volume. For the TCH demonstration at the Naval Air Warfare Center (NAWC) Trenton site, 15 TCH heater borings were installed in addition to 15 vapor extraction points (next to the heater wells, co-located in the borehole) and eight temperature monitoring points.
During the course of the TCH demonstration, data were collected and compiled to monitor the performance of the TCH system. These data included energy expenditures for the target treatment zone (TTZ) and volumes for water and air removed from the subsurface. An energy balance also was set up and maintained during operation to keep track of energy injected and extracted from the TTZ on a daily basis. The energy balance was used to optimize the thermal treatment.
Bedrock samples were collected from borings within the TTZ in order to evaluate TCH performance both before and after treatment. Three boreholes were cored prior to treatment in order to collect the rock samples and establish baseline conditions. Three boreholes were also cored after treatment to collect a similar set of rock samples. The pre- and post-treatment core locations were located approximately 2-3 feet apart to ensure that the post-treatment cores would not intersect fractures that had been filled with grout from the pre-treatment coring activities.
Demonstration results from the bedrock samples indicate that the average reduction in trichloroethene (TCE) concentrations was 41-69%. However, careful examination of selected points in the rock matrix revealed that the rock matrix did not achieve targeted temperature in all locations (due mostly to contaminated groundwater influx through existing fractures). Since discrete sampling was done at 5-foot intervals, it was possible to identify at which depth there was incomplete heating and correlate that with observed fractures from a video log of the boreholes. Eliminating data from the locations where boiling water temperature was not achieved due to cool water influx, the average reduction was higher, at 94.5%. This chlorinated organic compound mass removal rate is consistent with others findings. For example, in a literature survey conducted by NAVFAC ESC and Geosyntec Consultants under ESTCP project ER-200424, thermal technologies typically achieved levels of dense nonaqueous phase liquid mass removal ranging between 94% and 96%.
The data also show that most rock concentrations were lowered to around 0-5 mg/kg, but that higher concentrations were maintained at distinct depth intervals. These depths correlated reasonably well with the depths showing the highest TCE concentrations prior to heating. The total amount of TCE removed (vapor and liquid) was estimated to be between 530 lbs (based on daily photoionization detector readings) and 680 lbs (based on analytical data).
TCH cost depends primarily on the size and depth of the treated subsurface volume. A secondary parameter is the type of rock or sedimentary deposit, particularly its porosity and heat capacity. These parameters determine the amount of energy necessary to heat the target volume to the treatment temperature. In fractured rock, mineralogy of the rock, organic matter content, fracture rinds and fracture patterns and permeability are also important parameters.
TerraTherm used its proprietary cost model to produce cost estimates for three treatment scenarios with the same design parameters but with different treatment areas and volumes to demonstrate the range of treatment costs dependent upon the treatment volume. Sites are classified as follows: Small - treatment zone approximately 12,500 yd3; Medium - treatment zone approximately 50,000 yd3; and Large - treatment zone approximately 250,000 yd3. The total remediation time frame for each of the three volume scenarios is approximately 200 days. In the cost model scenarios, the cost per cubic yard ranges from $269/cu yd for the Small site scenario to $91/cu yd for the Large site scenario.
The key implications of this work for practitioners include: (1) System design must take into account the induced flow of cool groundwater into the treatment volume; (2) Consider the use of larger-diameter vapor extraction points to reduce the potential for liquid entrainment in the extracted steam; (3) Consider smaller-scale testing prior to full-scale deployment to identify potential problems and refine full-scale designs and operations; (4) Consider longer treatment times and/or higher temperatures than those used at this site, to remove contaminants from difficult regions; and (5) Attention should be given to groundwater influx into the treatment zone in order to determine whether boiling can be achieved and the length of heating time required to achieve boiling.
At the NAWC Trenton site, cooling associated with the substantial water flow through the fractures and the continual influx of contaminants from the bedrock surrounding the TTZ is believed to have limited the remedial efficiency in the bedrock close to such fractures. Use of larger-diameter vapor extraction points or grouting in the heater borings and use of separate vapor extraction points would have significantly reduced the amount of water produced by eliminating the percolation effect seen at the vapor extraction points during operation. This percolation effect is created because the steam cannot bubble through the standing water without pushing it out, and the resulting liquid entrainment induces more flow into the TTZ. Using larger vapor extraction points would have likely limited the water extraction rate to the rate of in situ steam production from the fractures and the matrix, thereby limiting the rate of contaminant and cold water flux into the TTZ and enabling efficient heating and treatment of the TTZ. Another potential remedy for full-scale applications would be the use of steam injection to heat the fractures and minimize groundwater inflow from outside of the TTZ.