IN-SITU THERMAL DESTRUCTION (ISTD) OF MGP WASTE IN A FORMER GASHOLDER: DESIGN AND INSTALLATION
ABSTRACT:
In Situ Thermal Desorption (ISTD), also known as In-Situ Thermal Destruction, is being used to remediate a gasholder containing residual coal tar at a former manufactured gas plant (MGP) site in Massachusetts. When the gasholder was decommissioned, it was backfilled with soil and debris. The walls and base of the holder are intact, enclosing a volume of approximately 2,013 cubic yards (CY) (1,539 m3).
Water was present at a depth of approximately 3 ft (0.9 m) below ground surface (bgs) within the gasholder. Based on limited soil investigations within the gasholder, residual coal tar was present throughout the soils and the bottom 4 ft (1.2 m) of soil was saturated with coal tar DNAPL. ISTD is being used to remediate the gasholder in a step-wise fashion to achieve Massachusetts soil cleanup standards that are protective of human health and groundwater, with respect to polycyclic aromatic hydrocarbons (PAHs), benzene, petroleum hydrocarbons and other compounds.
Laboratory results on a sample of the tar indicate that a modest, 100°F (56°C) increase in temperature results in a 20-fold decrease in viscosity. Thus, raising the soil temperatures will increase the fluidity and recoverability of the tar. Initially the heaters will operate at low temperatures, during which water and recoverable tar will be removed from the gasholder via two liquid extraction wells. Once the water and recoverable tar have been removed, the heaters will be ramped up to their full operating temperature and the soil will be heated to a minimum target temperature of 617°F (325°C). Soil in close proximity to the heater wells will become superheated, which will result in substantial insitu destruction of the organic compounds. Air will be injected around the thermal wells to prevent coke from forming close to heater-vacuum (H-V) wells and obstructing subsurface vapor flow.
