Formatted Title
The Physics of Thermal Remediation: Don't Forget Henry and Fourier on Your Thermal Projects!
Background/Objectives
Among developments in in situ thermal remediation (ISTR), some seem too good to be true.
Can you really
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Remediate compounds with boiling points above 100⁰C at the boiling point of water?
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“Sustainably” remediate TCE at 60-70⁰C with minimal energy consumption?
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Disperse 5 kW of heat per meter of heater well using thermal conduction or induction to heat faster?
Let’s dive into the physics behind thermal conductive heating to see what is required to achieve certain remediation targets.
Approach/Activities
Volatility data: Boiling points, azeotropic and co-boiling points, as well as Henry’s law constants, are used to simulate the evaporation of some contaminants from a saturated soil at the boiling point of water. The results are compared with data from full scale ISTR projects as well as lab-scale treatability testing.
The heat input limitations are put into perspective using simple heat conduction examples to estimate the heating element temperatures that are required to achieve given heat fluxes in different soil settings.
Results/Lessons Learned
The boiling point of the contaminant is only part of the answer to whether the contaminant can be remediated from the saturated zone at the boiling point of water. In many cases, the contaminant is only present in solution (no free phase) or must be remediated well below the free phase limit. In these cases, the contaminant vapor pressure in equilibrium with the aqueous solution is more relevant than the vapor pressure of the pure compound.
At a former chemical landfill site, chlorobenzenes with boiling point up to 175 ⁰C have been remediated to below 1 mg/kg (>99% removal rate) in the saturated zone by ISTR. Results for bisphenol-A (Bp. 220⁰C) and chloromethyl aniline (Bp. 239 ⁰C) are 50% and 75% removal rate, respectively. Lab-scale treatability studies on soils from PAH- and TPH-contaminated sites show comparable results for these compounds depending on molar weight.
The surface temperature of a thermal conductive heater well are limited by the heater can material, typically mild steel. When exceeding the scaling temperature of approximately 500 ⁰C the service life will dramatically decrease. In order to prevent damaging the heater cans, the heating power output must be limited to match the thermal conductivity of the soil.
The lesson learned is that contaminants with boiling points in the 125-250 ⁰C range can, in many cases, be remediated from the saturated zone by ISTR in spite of the gap between contaminant boiling point and temperature achieved. In order to get a better understanding of the possibilities for thermal remediation and a realistic remediation target, the vapor pressure should be accessed using Henry’s law constants available at relevant temperatures.
It should be kept in mind that ISTR in saturated soil is dependent on the steam generated by boiling groundwater to establish extraction pathways. Therefore significant evaporation is impossible below the boiling point of water and 70 ⁰C will not work in the saturated zone.
The heater output of a 3-inxh heater can is limited to 800-1300 W/m if excessive scaling of the mild steel heater can material is to be avoided.
Clever solutions are important, but unless they work within the limits of physics, they will ultimately lead to failures in the field. So, it is critical that we bring these basic laws into all our solutions!