I have a heavily contaminated Site; will STAR work?
Yes. Higher contaminant concentrations contain more energy to support the smoldering combustion process, increasing the self-sustainability of STAR and increasing contaminant destruction efficiency.
What site conditions are required for STAR to work?
Typical STAR sites are those characterized as being impacted by coal tar, creosote, or petroleum hydrocarbons at soil concentrations equal to or greater than approximately 3,000 milligrams per kilogram (mg/kg) Total Petroleum Hydrocarbons (TPH) within silty sand or coarser geologic units. Soil concentrations below this level will be combusted and the process can tolerate “gaps” in contaminant distribution, but this level is required to maintain self-sustaining smoldering combustion (i.e., combustion without the input of external energy). Depth below the water table is an important design consideration but does not impact the suitability of the STAR process (i.e., STAR can be applied both above and below the water table).
Does STAR work beneath the water table?
Yes. STAR has been demonstrated to work below the water table at numerous sites. The STAR process involves the injection of pressurized air and the generation of heat from the smoldering reaction. Together, the air pressure and heat drive off water in the immediate vicinity of the smoldering reaction. In essence, the STAR process proceeds in a “bubble” below the water table.
For what range of petroleum products is this technology applicable?
STAR is well suited to Diesel Range Organic (DRO) compounds and heavier hydrocarbons. However, Gasoline Range Organics (GRO) can be treated with STAR according to the method developed for other high volatility compounds (see below).
Has the technology been tested on any combustible materials other than petroleum or polycyclic aromatic hydrocarbons?
Yes. STAR can be used to treat chlorinated solvents, pesticides and other compounds but sometimes requires the use of the method developed for high volatility compounds (see below).
If zones of lower concentration exist, over how large a distance can combustion “jump”?
The ability of STAR to jump "clean gaps" is the focus of an extensive laboratory study conducted with support from Battelle, the Electric Power Research Institute (EPRI) and the United States Environmental Protection Agency (USEPA). The tolerable “clean gap” distance is a function of reaction temperature, the direction of the gap with regard to air flow and combustion front propagation, and the permeability of the treatment horizon. Our experience suggests that in situ applications at coal tar Sites, along the direction of air flow, the “jump” distance is on the order of a few feet. Transverse to the direction of air flow this distance is approximately one foot.
What is the range of permeability that the technology can be applied to?
STAR is tolerant of heterogeneity relative to many other in situ technologies that involve injection. For example, STAR involves the injection of air into the air-filled porous media behind the combustion front (which is devoid of groundwater and contaminants) and avoids the multi-phase flow injection scenario common to in situ bioremediation or in situ chemical oxidation. STAR can be applied in anything from gravels to sandy silts. Smoldering will occur in clay materials, but the timescales are dependent on air flow velocities and STAR may not be economical in very fine clay materials.
Can STAR address contamination on the capillary fringe as well as in the saturated zone?
STAR has been demonstrated at more than 25 feet below the water table without any physical barrier or other impediment to groundwater flow. However, groundwater is a heat sink and the process becomes more robust and faces fewer technical challenges when applied in the vadose zone or capillary fringe.
How much does STAR cost?
The self-sustaining nature of the process means that the energy requirements for STAR are minimal. The ignition process involves the heating of a few inches of soil around a one foot well screen, and the remainder of the heat energy for the combustion process comes from the materials being combusted themselves. Costs are site-specific, but our experience indicates that STAR is approximately 75% - 80% of the cost of in situ solidification / stabilization (ISS) and approximately 50% of the cost for excavation and treatment (thermal desorption).
What are the key cost factors?
Cost are driven by site-specific factors including: 1) radius of influence (ROI) which governs infrastructure costs (i.e., # or wells / ignition events / etc.); 2) emissions (i.e., volatile mass loading) which impacts vapor treatment costs; and 3) combustion front propagation rate which impacts the timescales of treatment. These three key cost factors are the focus of pre-design evaluations that are often conducted at sites prior to full-scale design and implementation.
What are the key capital elements of STAR?
The above ground equipment used to implement the technology is similar to that used in Air Sparge (AS) / Soil Vapor Extraction (SVE) systems and includes compressors for sub-surface air delivery, blowers for ground surface vapor collection, and vapor phase activated carbon or a thermal oxidizer for vapor treatment. The specialized equipment associated with the STAR process includes the use of 2-inch diameter, carbon steel direct push ignition wells with a stainless steel screen, temporary in-well heaters to initiate the process (and associated control system), and subsurface multi-level thermocouple bundles to track the combustion process.
STAR Evaluation Process
How do I know if STAR is appropriate for my Site?
The typical STAR evaluation/application process consists of the following steps:
- Site Screening. Site data, including geologic logs, cross-sections, soil analytical data, water table depth and groundwater flow data, are evaluated to pre-screen sites for suitability with respect to the STAR process. Typical STAR sites are those characterized as being impacted by coal tar, creosote, or petroleum hydrocarbons at soil concentrations equal to or greater than approximately 3,000 milligrams per kilogram (mg/kg) Total Petroleum Hydrocarbons (TPH) within silty sand or coarser geologic units. Soil concentrations below this level will be combusted and the process can tolerate “gaps” in contaminant distribution, but this level is required to maintain self-sustaining smoldering combustion (i.e., combustion without the input of external energy). Depth below the water table is an important design consideration but does not impact that suitability of the STAR process (i.e., STAR can be applied both above and below the water table).
- Treatability Testing. If the Site Screening process indicates that the STAR technology may be suitable for the Site, a treatability study is recommended to confirm that the impacted soils are capable of supporting a self-sustaining smoldering combustion reaction and to determine peak combustion temperatures, anticipated smoldering combustion propagation velocities, potential treatment levels (concentration reductions in soils), and to identify off-gases that may require treatment during field implementation. A typical treatability study can be performed on a lump sum basis for $15,000 USD per sample. A multi-sample discount can be applied for treatability studies involving three or more samples. Combustion tests typically required two weeks to complete (per sample), followed by two weeks for laboratory analysis of samples, followed by one week to complete a brief treatability study report (five weeks total from receipt of samples).
- Pre-Design Evaluation Planning. If treatability testing confirms the suitability of the STAR technology for the Site, the next step is to conduct a pre-design evaluation. In preparation for that activity, civil, mechanical, electrical, and layout drawings, a subcontractor bid-package, and a Work Plan will be prepared describing the goals, objectives, and methods of the pre-design evaluation.
- Pre-Design Evaluation. A single well ignition test is used to gather key process data and evaluate operational limitations for use in the design of a full-scale STAR system at the subject site. Process data and operational limitations to be evaluated include: 1) smoldering propagation velocity (which governs the timescales of operation and the requirements for process support equipment [e.g., compressors and blowers operation]); 2) ignition well radius of influence (ROI; which governs well spacing and process monitoring equipment and infrastructure requirements); 3) vapor phase mass loading (which governs the sizing and type of vapor phase treatment); 4) contaminant mass destruction rate; and 5) achievable soil treatment levels. These process data / operational limits will be evaluated throughout the pre-design evaluation by monitoring subsurface temperatures with multi-level thermocouple arrays, monitoring combustion gas concentrations and volatile compound concentrations in the collected vapors, by recording process parameters such as heater operation, air injection flow rates and pressures, collected vapor temperatures, flow rates and vacuums, and by collecting ‘before’ and ‘after soil and vapor samples. Depending on the scale and objectives of the pre-design evaluation, the variability of each of the key design parameters can also be evaluated by implementing the STAR process at one or more wells across the proposed treatment area or within different geologic units. Typical costs for the pre-design evaluation are on the order of $200,000 to $250,000 USD, depending on site-specific conditions and project-specific objectives.
- Full Scale Design and Application. Following the pre-design evaluation, a full scale design for implementation of the STAR technology at the Site will be developed that consists of the preparation of civil, mechanical, electrical, and layout drawings and the preparation of a subcontractor bid-package. Upon completion of the design Savron will install and operate the STAR system with assistance from subcontractors as appropriate.
What are typical residual total organic carbon levels that are achieved?
Contaminant destruction within the combustion zone is complete (i.e., all organic matter destroyed). Some, low-level residuals (<3,000 mg/kg TPH) may persist on the periphery of the treatment zone, but contamination at this level is typically addressed through Monitored Natural Attenuation (MNA).
Are there secondary contamination issues?
Metals will not be remediated by the STAR process (other than mercury which can be volatilized, captured, and treated). The metals that remain in soil may undergo some mineralogical changes and recent work at the University of Strathclyde is considering this issue.
How quickly can the reaction be terminated?
We have demonstrated hundreds of times in the lab and in the field that terminating the injection of air terminates the smoldering reaction instantly. If additional engineering controls are required as a safeguard, nitrogen can be injected to asphyxiate the reaction. It has also been demonstrated that the process self-terminates when the fuel source (i.e., contaminants) are exhausted.
Is there a minimum organic concentration in soil or BTU content that is required?
BTU is a metric that only has value when discussing flaming combustion; however, empirical studies have demonstrated that total petroleum hydrocarbon (TPH) concentrations on the order of 3,000 milligrams per kilogram (mg/kg) are required to maintain STAR in a self-sustaining manner. Lower concentrations will smolder, but this minimum “fuel content” is required to maintain self-sustaining conditions.
What is more important TOC or BTU?
BTU has limited value when discussing smoldering combustion. We tend to refer to TPH or total organic carbon (TOC) concentrations when discussing the applicability of STAR.
Is there a depth limitation (too shallow or too deep)?
There are no depth limitations. STAR is ideally suited to applications well below the water table and in the vadose zone. Near water table applications will involve a slight modification to the implementation approach which could affect well spacing / cost.
At what organic concentration/BTU content will combustion stop?
Self-sustaining smoldering combustion will cease at concentrations below 3,000 mg/kg but smoldering (non-self-sustained) will continue to lower concentrations.
Does pyrolysis or carbonization also occur? Can they be controlled if present?
Smoldering is a two-step combustion process: pyrolysis and oxidation. Therefore, pyrolysis does occur in all smoldering reactions; however, as long as sufficient oxidant (air) is injected, the process will continue on to the oxidation step to complete the combustion reaction.
How important are vapor controls?
Vapor controls will be required at most sites for volatile organic carbons and carbon monoxide for reasons of health and safety and permitting.
What permits may be required?
Air discharge permitting (primarily benzene and carbon monoxide).
What has to be monitored? And for how long?
Typically we monitor for benzene and carbon monoxide. The duration / frequency of monitoring will depend on the jurisdiction for the permit.
Are there issues with vapors?
Data collected to date reveals that vapors from the STAR process are primarily carbon dioxide and water with smaller quantities of carbon monoxide. Depending on the contaminant being treated, some much lower quantities of contaminant vapors may be generated during the combustion process; however, these are easily managed using standard vapor collection and treatment technologies (e.g., activated carbon or thermal oxidation).
Is there a concentration at which natural soil organics may ignite and continue smoldering combustion (e.g. peat or other high organics content material)?
The smoldering of peat is a well-known phenomenon and is a problem that has been extensively studied by some of the members of our Academic STAR Team (Jose Torero, University of Queensland). STAR will combust ANY organic material in the subsurface including natural organic matter; therefore, the presence of significant peat layers is something that requires careful consideration during the design phase of work at a Site.
In shallow porous soil or fractured rock with outcrops, is there a concern that natural oxygen flux may be sufficient to continue to support combustion even after air injection has ceased (i.e., Centralia)?
This may be an issue in vadose zone treatment areas but will not occur during STAR implementations below the water table. The groundwater acts as a medium to prevent the influx of ambient air and acts as a heat sink to prevent continued / unplanned smoldering. In the vadose zone, engineering controls can be put in place to mitigate the potential for continued smoldering following implementation (e.g., waterflooding to remove heat, nitrogen injection to asphyxiate the reaction, physical barriers, etc.)
Can STAR ever become a rapid combustion process (i.e., with flames)?
No. Within a porous matrix, smoldering is the only type of combustion that can occur. However, if there are significant void spaces in the subsurface (e.g., karst, unidentified underground storage tanks), then there is a potential for flaming if combustible gases accumulate sufficiently within these features.
What are the key human health exposure issues?
The primary human health exposure issues relate to emissions. Typically, a small fraction (<1%) of remediated mass is released as volatile components for subsequent capture and treatment (primarily BTEX and naphthalene); in addition, the smoldering combustion process converts contaminants to carbon monoxide and carbon dioxide. Therefore, the vapor collection and treatment system must be designed to mitigate any potential human health exposure issues related to volatile vapors and carbon monoxide.
Are there ecological issues?
None identified to date; however, it should be noted that ALL organic material is destroyed within the combustion zone.
Are dioxins and furans formed?
No. A study was recently conducted by USEPA and Battelle examining the potential for dioxin / furan formation as a result of the STAR process for contaminated soils from a former wood treating facility. The test sample was contaminated with pentachlorophenol (PCP) and creosote and the study demonstrated that dioxins and furans were not created but were actually destroyed by the STAR process. The study also confirmed that dioxins and furans were not volatilized into the vapor phase.
What analysis has been done to date on collected vapors?
We traditionally analyze vapors for combustion gases (carbon monoxide and carbon dioxide) to verify the occurrence of smoldering and to estimate the mass of contaminants destroyed by the process, volatile organic compounds (VOCs), and semi-volatile organic compounds (SVOCs). The most common and largest proportion of compounds detected in vapors include benzene, toluene, ethylbenzene, xylene, and naphthalene (i.e., light hydrocarbons).
What is the post treatment temperature? What post treatment cooling times have been seen?
The combustion zone during STAR operation is very narrow with a pre-heating zone in front (on the order of six to 12 inches) and a cooling zone behind. Temperatures within the soils quickly reduce to the temperature of the injected air (nominally ambient) within a few hours following combustion as the process relies on the efficient transfer of heat from the combustion zone to the pre-heating zone to maintain self-sustaining conditions. Elevated temperatures only remain in the subsurface if the process is terminated prematurely (i.e., if air flow is shut off while combustion is occurring); however, the band of heat is narrow and below water table applications will see groundwater influx and rapid cooling.
What are the Health and Safety concerns?
Health and Safety concerns for the implementation STAR are similar to those encountered with a thermal remedy [e.g., in situ thermal desorption (ISTD) and electric resistive heating (ERH)]. STAR is a high temperature remedy, but unlike ISTD and ERH, there is no extended period of energy being applied (i.e., electricity). A second difference from ERH and ISTD is that instead of heating the entire contaminant zone, the STAR region being heated at any one time is highly localized. Movement of the localized treatment region is controlled and tracked during STAR application.
What are the chances of generating a 'runaway' reaction or an explosion?
None. Explosions are gas-phase combustion reactions involving gasified fuel and excess oxygen. STAR involves smoldering combustion which occurs on and within the surface of the fuel (i.e., the liquid contaminant). As such, its progress is dictated by the rate that oxygen can diffuse into the liquid and, thus, is necessarily oxygen-limited. Experiments have demonstrated that the rate of progress can be controlled by the rate of oxygen delivery up to a maximum and that further increases (i.e., excess oxygen) only cool (i.e., reduces) the reaction. Moreover, the radii of even the largest pores are below the critical length required for a flame to exist. Finally, experiments have repeatedly shown that when the air delivery is terminated, STAR stops immediately.
How can STAR be used to treat high volatiliy compounds such as chlorinated solvents and gasoline range organices?
High volatility compounds are difficult to ignite because they tend to volatilize at a rate that’s faster than the heating rate (i.e., the compounds are volatilized before they reach the ignition temperature). In order to treat high volatility compounds with STAR, a surrogate fuel can be injected into the treatment area such as emulsified vegetable oil (EVO). In this scenario, the EVO is the primary fuel for combustion and the heat of the combustion process is used to drive off the volatile contaminants for subsequent above ground collection and treatment. This is similar to electric resistive heating (ERH), except the energy for the desorption / volatilization process comes from vegetable oil (STAR) as opposed to electricity (ERH). Recent work has shown that approximately 30% of the chlorinated solvent mass is combusted, and that approximately 70% is volatilized for subsequent capture and treatment.
How does STAR compare to other technologies?
STAR is a fast, energy efficient, sustainable technology that is capable of completely remediating a Site. A treatability test conducted by Savron can tell you if your Site is a candidate for the application of STAR.
What are the current areas of development?
We are exploring the use of STAR to treat chlorinated solvents, pesticides, gasoline range and diesel range organics on the in situ side, and are continuing to develop our reactor systems to treat a range of compounds (primarily waste oils, lagoon sludge, etc.) on the ex situ side.