Its an exciting time. In-Situ Thermal Remediation (ISTR) shows great promise and has proven to be an extremely effective method for cleaning a wide range of Contaminants of Concern (COC’s) at contaminated sites. Most recently, gas-fired soil heating technologies are entering the market with promises of much cheaper natural gas heating, faster setup and heating times, less heaters required (and therefore less drilling), and a ‘huge cost savings’ opportunity to clean your site. REALLY? Lets take a look at the real truth about current ‘gas fired’ soil heating technologies and their actual benefits for ISTR operations.
First, not all gas-fired heating technologies are the same. We’ve been hearing for years that “a burner is a burner, and all burners and oxidizers are the same - - just get the best price you can”. NOT TRUE. Energy efficiency, combustion technology, method of heating, heat recovery, and effective heat transfer to the soil must be properly designed and engineered into every system, especially for ISTR soil heating. The cost benefits of natural gas or propane can be lost with a poorly designed heating process. So, here’s a brief discussion on several ‘gas-fired’ soil heating technologies currently on the market:
“Induced Air Draft” Heating Burner Technology. One or more companies currently using gas-fired ISTR technologies utilize ‘induced air draft’ burners. You can see this by the many burners sticking out of the ground fairly close together, and have a seemingly endless supply of above ground ducting and insulation all over the cleanup site. ‘Induced Air’ burner technology is where the flame is generated and supported by being ‘drawn’ down a central piping section with the hot combustion gases doing a 180 degree turn back up through the annular space between the center pipe and outer well casing. In this design, the flame and heat is drawn down through a single heating well or pulled through a second vertical heating well, then through a cooling heat exchanger, then pushed out to the atmosphere. Sounds energy efficient? Based on the data reviewed by several associates, it appears that this heating process exhausts from 58-75% of the natural gas heat directly to the atmosphere - - and if you’re using propane at approximately $22/MM btus, you’re now spending up to 75% in more energy costs than if you were using electric downwell heaters. Yee-ouch!
There are several other interesting features about ‘induced air draft’ gas-fired burners that many people may not be aware of: (1) typically weak flamed and very sensitive to downstream piping pressures, (2) typically only capable of less than stoichiometric combustion temperatures from 600 – 1100F, thereby limiting their effective heat to soil capabilities and incapable of flame destroying VOC’s or CVOC’s to RCRA emissions requirements, (4) typically require a downstream cooling heat exchanger to reduce well exhaust temperatures prior to entering the centrifugal suction blower, which is an additional net energy cost to the project, (5) typically only operate within ‘laminar flow’ conditions, which can severely limit heat transfer to the soil, and (6) they appear to require as many heater wells with burners as their electric heater competitors in order to heat the soil. This begs the question: Is ‘induced air’ gas-fired burner technology really better or more energy efficient than electric heating?
“Forced Air” Heating Burner Technology. There are a wide range of forced air burners on the market which have been used for oxidizers, boilers, and heaters throughout industry for many years. For ISTR applications, a ‘forced air burner’ with high pressure output is required. The ‘pressure burner’ is required to drive the flame and hot combustion gases through 50 ft or more of downstream piping in the soil, and it must be resistant to high downstream back pressures in order to push the heat through the entire heating network reliably. The benefits are tremendous over ‘laminar flow’ and ‘induced air’ heating technologies: (1) better and faster heat transfer to the soil, (2) higher temperatures to 1700+F from burner to the soil, (3) the ability to effectively use the COC’s as a fuel and meet RCRA destruction requirements and transfer heat to the soil to augment the ISTR cleanup, (4) eliminates the need for costly aboveground emissions control equipment or those huge vapor condensing machines, and (5) have the ability to recover valuable heat from the well exhaust gases to reduce burner fuel consumption and operating costs. It’s been our finding that ‘induced air’ heating technology can achieve none of the above.
The SOIL-THERM ‘forced air’ DWR heating technology operates using natural gas or propane, and can withstand downstream back pressures in excess of 15+ psig at combustion temperatures to 1800F. With high heat, high pressure, high Reynolds numbers, heat transfer to the soil is optimized in a manner and through a steel (CS or SS) piping network that allows heat transfer through one hundred feet of downwell steel piping with only a single SOIL-THERM Jet-THERM gas-fired burner. In designing these systems, SOIL-THERM carefully examined the well exhaust heat and came up with a cost-effective means of recovering the well exhaust using shell and tube heat exchangers. This heat is then transferred to preheat the ‘forced air burners’ combustion air to 400 - 600F, thereby recovering approximately 60% of the waste heat and lowering fuel operating costs by 25-35%. This results in an effective heat to soil percentage from 65 – 80%, depending on the transfer efficiency rating of the heat exchanger and pore volume of the soil. This means almost all of the valuable heat used by the forced air burner is going into the soil to heat and drive out the COC’s. Low cost and powerful.
As we find so many times in our lives, 'Things Aren't Always What They Seem'. Just because a process uses natural gas doesn’t mean it’s a better ISTR technology with guaranteed lower cleanup costs and more effective results. Careful diligence, review of process data, and carefully reviewing and understanding burner heating principles is essential to getting the real benefits of ‘gas-fired’ for your project.