Miljøprojekt Nr. 1177 2007 - Termisk assisteret vakuumventilation

Termisk assisteret vakuumventilation

Summary and conclusions

A remediation based on a combination of soil vapour extraction and conductive heating of the soil was designed. Heating was performed by installing a number of AC-powered heating elements in the soil. The spread of energy by this method is primarily conductive heating. The two parameters controlling the energy spreading are the heat capacity of the soil and its thermal conductivity. Both parameters vary depending on the soil’s water content. Compared to permeability, the thermal parameters vary very little, typically by less than one order of magnitude. This implies a very small variation in temperature around the heating elements compared to variations in the flow of water or soil gas, which occurs in high-permeable regions in the soil.

Based on the thermodynamic laws of heat transfer, a simple spreadsheet model was developed to describe the spread of energy in the soil and to determine optimal distances between heaters, temperature of heaters and warm-up time. Five meters was found to be an optimal distance between the elements in the actual setup, where a heater temperature of 500-600°C was chosen. The total amount of energy was calculated to be in the range of 300 kWh/m³ soil.

The actual treatment plant was based on the calculations. It consisted of a number of cooling devices, a vacuum pump, GAC filters for water and air treatment and a neutralizer. From the start we assumed that hydrochloric acid would be produced from oxidation/pyrolysis of PCE, this was handled in the neutralizer by adding potassium hydroxide.

The heaters were made by Heatex Ltd. an English based company, which had experience from chemical plants etc. The design was performed by Heatex on the basis of specifications from Hedeselskabet.

The treatment plant as well as the heaters were installed at Ravnsbjergvej 8, Alsønderup. Hillerød. The soil at the site was contaminated with PCE in relatively high concentration. The contamination was located in an approximately 75 m² area down to 10 m below ground surface, near and under two buildings. The soil was clayey till with variable clay content.

The remediation was started, but after only 24 hours defects in the heater elements were observed. At this early stage temperature increase was only observed very near the elements. No significant remediation had actually taken place at this stage. The heaters and treatment plant were shut down. After some correspondence between manufacturer, consultant and county the heaters were withdrawn from the soil. Visual examination revealed that the heater elements were corroded. This had led to short circuiting and light arcs in the soil with temperatures above 1000°C, which harmed the elements as well as the rest of the construction. At the time of this report it is still unclear who will have to pay for the faulty design.

Due to this it was not possible to test the technology in a Danish environmental setting. During the litigation process another technology was chosen to do the actual clean-up.

Based on this very preliminary trial of conductive heating combined with SVE the following conclusions are drawn:

Great care has to be taken in the design and choice of materials used in the heaters themselves as well as the rest of the treatment plant. In this trial corrosion of the heater elements was the basic defect that led to complete breakdown of the heaters. Due to this the technology was not tested completely in a Danish setting.
The technology is rather expensive and uses high amounts of electrical energy. For VOCs an energy amount of app. 300 kWh/m³ soil was estimated to attain an adequate temperature. A US remediation reported an energy consumption of 360 kWh/m³. The unit costs depend heavily on the size of the site. American data shows costs in the range of DKK 2,000-3,000 per tonne soil (€ 270-400 /tonne) for small sites. This site has a projected cost of app. DKK 3,600 /tonne soil (€ 480 /tonne). The difference can be ascribed to higher machinery, energy and labour costs in Denmark compared to the US as well as the fact that the total cost of the treatment plant was assigned to this remediation solely.
The technology is well documented from 17 sites in the US. Both VOCs (TCE, PCE, TCA etc.) and SVOCs (PAHs, pentachlorophenol, diesel) have been removed with efficiency above 99 %.
The technology faces a couple of potential problems beyond the price. US patents control the number of companies that offer the technology, thereby limiting competition. Also potential geotechnical implications may occur during heating. Lab experiments carried out as a part of this trial showed a volume decrease of up to 3 % of the soil when it dried out, even when the soil was re-watered afterwards. It is unclear whether or not this occurs at field scale. A measuring programme in this trial was planned but was not completed. No geotechnical issues have been described from the American sites.
Overall Hedeselskabet evaluates the technology positively, especially for organic contaminants in deeper, low-permeable geologies. In order to obtain a greater rate of success, new tests should be performed with American heating elements.