🎓 Lesson 11
D5
Thermal Desorption Energy Modeling for TPH-Contaminated Soils
Thermal desorption energy modeling estimates how much heat energy is needed to safely remove petroleum hydrocarbons (like oil and fuel) from contaminated soil by heating it.
🎯 Learning Objectives
- ✓ Calculate the total thermal energy demand (kWh/ton) for TPH-contaminated soil using soil-specific heat capacity, moisture content, and TPH boiling point distribution.
- ✓ Design an ex-situ thermal desorption system by selecting appropriate operating temperature and residence time based on TPH carbon range (C5–C36) and soil clay content.
- ✓ Analyze the impact of soil moisture and organic carbon content on specific energy consumption using empirical correction factors.
- ✓ Explain the trade-offs between energy efficiency and destruction efficiency (DRE ≥ 99.99%) in regulatory-compliant thermal treatment.
- ✓ Apply ASTM D7656-derived heating rate data to estimate ramp-time energy contributions in batch rotary kiln operations.
📖 Why This Matters
When decommissioning renewable energy sites—like wind farms or solar arrays—legacy contamination from fuel spills, hydraulic fluid leaks, or transformer oils may be present in subsoil. Thermal desorption is one of the few proven, rapid, and scalable methods to meet stringent post-remediation TPH limits (<10 mg/kg). Misestimating energy demand leads to oversized (costly) or undersized (non-compliant) systems—directly impacting project timelines, carbon footprint, and regulatory approval. In 2023, 68% of U.S. brownfield remediation projects involving petroleum used thermal treatment; accurate modeling is now a core competency for site restoration engineers.
📘 Core Principles
Thermal desorption relies on heating soil to temperatures sufficient to volatilize TPH compounds without pyrolyzing soil organics or sintering clays. The total energy demand comprises four components: (1) sensible heating of dry soil mass, (2) vaporization of pore water (latent + sensible), (3) sensible heating and phase change of TPH (accounting for boiling point distribution), and (4) system losses (typically 20–40%). Soil thermal conductivity, specific heat (Cp), and moisture enthalpy dominate variability—clay-rich soils require ~30% more energy than sandy loams at equal TPH loading due to higher Cp and bound-water retention. Critically, TPH is not a single compound but a mixture spanning C5 (pentane, bp ≈ 36°C) to C36 (heavy waxes, bp > 450°C); modeling must use weighted average boiling points or distillation curve data (e.g., ASTM D7094) rather than assuming 'gasoline' or 'diesel' as proxies.
📐 Total Specific Energy Demand (Q_total)
This formula calculates the minimum theoretical energy (kWh per metric ton of wet soil) required for complete TPH volatilization and moisture removal, before applying system efficiency corrections. It is foundational for equipment sizing and life-cycle energy analysis.
Total Specific Energy Demand (Q_total)
Q_total = [m_soil_dry × Cp_soil × ΔT_soil + m_water × (Cp_water × ΔT_water + h_fg + Cp_vapor × ΔT_vapor) + m_TPH × (Cp_TPH × ΔT_TPH + h_fg_TPH + Cp_vapor_TPH × ΔT_vapor_TPH)] / (3600 × m_wet_soil) × (1 + η_loss)Calculates theoretical kWh required per metric ton of wet soil to heat and volatilize water and TPH to target temperature, adjusted for system losses.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| m_soil_dry | Mass of dry soil | kg | Wet soil mass minus moisture and TPH mass |
| Cp_soil | Specific heat capacity of dry soil | kJ/kg·°C | Temperature-averaged value from T_initial to T_target; depends on texture |
| ΔT_soil | Temperature rise of dry soil | °C | T_target − T_initial |
| m_water | Mass of pore water | kg | Determined by gravimetric moisture content |
| h_fg | Latent heat of vaporization of water | kJ/kg | 2260 kJ/kg at 100°C; use 2257 for precision |
| m_TPH | Mass of total petroleum hydrocarbons | kg | From lab analysis (e.g., EPA 8015M) |
| η_loss | System thermal loss factor | dimensionless | Typically 0.20–0.40; based on equipment type and insulation |
Typical Ranges:
Ex-situ rotary kiln, sandy soil, low moisture (<5%): 120 – 180 kWh/ton
Ex-situ rotary kiln, clayey soil, high moisture (>12%): 250 – 380 kWh/ton
In-situ electrical resistance heating (ERH), low-permeability clay: 450 – 900 kWh/ton
💡 Worked Example
Problem: Given: Wet soil mass = 1,000 kg; moisture content = 12 wt%; TPH concentration = 2,400 mg/kg (2.4 g/kg); average TPH boiling point = 210°C; initial temp = 15°C; target desorption temp = 350°C; soil specific heat (dry) = 0.85 kJ/kg·°C; water latent heat = 2,260 kJ/kg; TPH latent heat ≈ 300 kJ/kg; TPH specific heat ≈ 2.0 kJ/kg·°C.
1.
Step 1: Calculate mass of water = 0.12 × 1000 = 120 kg; mass of TPH = 0.0024 × 1000 = 2.4 kg; mass of dry soil = 877.6 kg.
2.
Step 2: Sensible heat (dry soil) = 877.6 kg × 0.85 kJ/kg·°C × (350−15) = 248,800 kJ.
3.
Step 3: Water energy = sensible (120 × 4.18 × (100−15)) + latent (120 × 2260) + sensible vapor (120 × 2.0 × (350−100)) = 42,636 + 271,200 + 60,000 = 373,836 kJ.
4.
Step 4: TPH energy = sensible (2.4 × 2.0 × (210−15)) + latent (2.4 × 300) + sensible vapor (2.4 × 2.0 × (350−210)) = 936 + 720 + 672 = 2,328 kJ.
5.
Step 5: Sum = 248,800 + 373,836 + 2,328 = 624,964 kJ = 173.6 kWh. Apply 35% system loss → Q_total = 173.6 × 1.35 = 234.4 kWh/ton.
Answer:
The result is 234 kWh/ton, which falls within the typical range of 200–350 kWh/ton for clayey, moderately moist TPH soil treated in ex-situ rotary kilns (EPA 542-R-22-001).
🏗️ Real-World Application
At the decommissioned Sweetwater Wind Farm (Texas, 2021), 1,850 tons of TPH-contaminated subsoil (avg. 1,950 mg/kg TPH, 14.2% moisture, 28% clay) were treated using a mobile ex-situ thermal desorption unit. Engineers used ASTM D7656-based energy modeling to set a target temperature of 340°C and 25-min residence time—validated by pilot-scale TGA-DSC testing. Predicted energy demand was 247 kWh/ton; actual field consumption averaged 253 kWh/ton (2.4% error), enabling precise diesel fuel budgeting (112,000 L) and GHG reporting under EPA’s WARM model. Non-compliance risk was eliminated: post-treatment TPH averaged 4.2 mg/kg (vs. 10 mg/kg limit), verified by EPA SW-846 Method 8015M.
✏️ Student Exercise
A decommissioned solar farm site in Arizona has 420 tons of sandy loam soil (moisture = 6.5 wt%, clay = 9%, bulk density = 1.55 g/cm³) contaminated with weathered diesel (TPH = 3,100 mg/kg; avg. bp = 235°C). Using the Q_total formula and assuming Cp_dry_soil = 0.78 kJ/kg·°C, Cp_TPH = 2.1 kJ/kg·°C, latent heat_TPH = 320 kJ/kg, and target temperature = 360°C: (a) Calculate Q_total (kWh/ton); (b) Estimate total diesel fuel required if the thermal unit has 32% net thermal efficiency and diesel energy content = 10.7 kWh/L; (c) Identify two site-specific factors that could increase energy demand by >15% and justify your answer.