When you consider precision heating, simplicity, reusability, low maintenance, carbon footprint, and safety, TerraTherm’s electric heaters beat gas heaters on all counts. Here’s why;
TerraTherm evaluated electric and gas-fired heaters early on, and concluded that in-well electric heaters are the better choice. We compared the two because many of TerraTherm’s Thermal Conduction Heating (TCH) ISTD patents are broadly written with respect to the means of heating, and would permit the use of any of a number of potential heating means. We concluded that electric heaters are the best choice for the following reasons:
TerraTherm’s electric heaters deliver a pre-determined wattage over their entire length within a heater well (comprising a “line source” of heat). In some cases, TerraTherm’s electric heaters are boosted to deliver a precise increase in wattage at the bottom or top if needed.
- Gas heaters, by contrast, require a burner (“point source” of heat), and the distribution of combustion gases down-well. Inevitably there are heat losses between the burner and flue gas exhaust point. Even with a coaxial (pipe-within-a-pipe) design, the temperature difference between the burner and the bottom of the well leads to uneven heating of the subsurface, which translates directly into slower and less efficient cleanup.
TerraTherm’s electric heaters are field proven (13 years of field deployment), fabricated of simple materials, reusable, and require little to no maintenance. TerraTherm installs its electric heaters in small-diameter casings that are generally direct-pushed into the ground, creating no soil cuttings to dispose of. Nearly 100% of the electricity purchased is converted to heat, where it’s wanted. They operate with > 99.9% uptime.
- Gas heaters are much more complicated, and can foul from deposits caused by incomplete combustion. Gas heaters are not necessarily more efficient than electric heaters, particularly if the combustion is not optimally controlled and/or if the combustion gases vary because some are drawn from the subsurface. Thus, while gas may be seen as an inexpensive fuel source, gas heaters do not necessarily have a lower carbon footprint than electric heaters.
TerraTherm’s electric heaters are safe. Electricity is precise and highly controllable. TerraTherm’s electric heaters are separated from the contaminants, and operated in a dry, sealed space inside the well casing. Electric current flows only through the heating element, not through the ground as with Electrical Resistance Heating. TerraTherm’s electric heaters are monitored and controlled automatically, and have a perfect safety record.
- With gas heaters, safety is more of an issue. Flames can have a tendency to get out of control. The auto-ignition temperature of natural gas (methane) is 595°C (1103°F). So once the heater is hot the fuel may ignite before desired. Introducing contaminant vapors into the gas burner is problematic, because it can lead to incomplete combustion and the production and release of toxic byproducts, including dioxins and furans. If contaminant vapors are being burned, the emissions from each gas heater well, like those from a boiler or thermal oxidizer, need to be closely monitored to ensure that they remain below standards. If chlorinated contaminants are burned in the gas flames, hydrochloric acid is produced. In some cases each heater boring can be seen as a potential emission source, for which stack testing becomes a consideration.
TerraTherm’s electric heaters are highly sustainable. Electricity usage represents only about 10-30% of the cost of a thermal remediation project, with the lower percentages being typical of CVOC sites. Since TerraTherm’s project cycles are short and predictable, total energy use is well defined and the need for repeated applications or long-term operations and maintenance is eliminated. Electric power for the heaters can be supplied from renewable sources if desired, as was the case with a recent large-scale project in Denmark. Furthermore TerraTherm is working with leading researchers in Life Cycle Assessment to develop ways of regulating power output to favor lower-cost nighttime rates. We tested this approach at a recent project with demonstrated cost savings.
- With gas heaters, CO2 is generated at each burner. Other greenhouse gases (e.g., NOx, CO) may also be generated. In the event of incomplete combustion or inadvertent release, methane, a greenhouse gas many times more potent than CO2 can be generated as well. The need for emissions control bears closer scrutiny. Natural gas burners cannot be fueled by a sustainable energy source.