Gas Dynamics during Bench-Scale Electrical Resistance Heating of Water, TCE and Dissolved CO2
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In situ thermal treatment (ISTT) applications require successful gas capture for the effective remediation of chlorinated solvent dense non-aqueous phase liquid (DNAPL) source zones. Gas production and transport mechanisms during bench-scale electrical resistance heating (ERH) experiments were examined in this study using a quantitative light transmission visualization method. Processed images during water boiling indicated that gas bubble nucleation, growth and coalescence into a connected steam phase occurred at critical gas saturations of Sgc = 0.233 ± 0.017, which allowed for continuous gas transport out of the heated zone. Critical gas saturations were lower than air-water emergence gas saturations of Sgm = 0.285 ± 0.025, derived from the inflection point of ambient temperature capillary pressure-saturation curves. Coupled electrical current and temperature measurements were identified as a metric to assess gas phase development. Processed images during co-boiling of pooled trichloroethene (TCE) DNAPL and water indicated that discontinuous gas transport occurred above the DNAPL pool. When colder zones were introduced, condensation prevented the development of continuous steam channels and caused redistribution of DNAPL along the vapour front. These results suggest that water boiling temperatures should be targeted throughout the subsurface (i.e., from specific locations of DNAPL to extraction points) during ERH applications. Because convective heat loss and non-uniform power distributions have the potential to prevent the achievement of boiling temperatures, a thermal enhancement was developed where dissolved gas delivered to the target heated zone liberates from solution at elevated temperatures and increases gas production. Processed images of ERH-activated carbon dioxide (CO2) exsolution indicated that discontinuous gas transport occurred above saturations of Sg = 0.070 ± 0.022. Maximum exsolved gas saturations of Sg = 0.118 ± 0.005 were sustained during continuous injection of the saturated CO2 solution into the heated zone. Estimated groundwater relative permeabilities of krw = 0.642 ± 0.009 at these saturations are expected to decrease convective heat loss. Discontinuous transport of exsolved gas at sub-boiling temperatures also demonstrated the potential of the enhancement to bridge vertical gas transport through colder zones. In conclusion, sustained gas saturations and transport mechanisms were dependent on the mechanism of gas production and effects of condensation.