Dense Nonaqueous Phase Liquids (DNAPLs)
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Thermal Processes: In Situ
Steam Injection and Extraction
Steam injection and extraction (also known as steam-enhanced extraction, or SEE) involves the introduction of steam into injection wells and the removal of mobilized groundwater, contaminants, and vapor from the recovery wells. Initially, when steam is injected into the subsurface, it gives up its latent heat of vaporization to the soil. As the steam loses heat, it condenses into a hot water phase that moves radially into the soil, displacing air and water in front of it. Continued input of steam eventually causes the soil near the well to reach steam temperatures, creating a steam front that begins to propagate away from the well. This process creates a moving front consisting of ambient-temperature water/air that is pushed by a variable-temperature zone of warm to very hot water. The water in turn is pushed out by the pressure of the steam moving into the steam-temperature zone. The movement of the ambient-temperature water may displace NAPLs, a process that is aided by viscosity reduction when the hot water reaches the NAPL. The arrival of the steam phase vaporizes compounds remaining as residual saturation or adsorbed to the soil. These vapors are transported to the leading edge of the steam zone where they condense, forming a contaminant condensate bank. The condensate bank may have a tendency to sink, and it is important in the design of the system to ensure its capture. Schmidt et al. (2002) and Kaslusky and Udell (2002) have found that co-injection of air with the steam helps prevent downward contaminant migration.
Contaminant removal thus occurs by several mechanisms. The most important mechanism in a given situation depends on the volatility of the contaminant. For VOCs, vaporization and co-boiling are the most important recovery mechanisms, while for SVOCs, displacement as a NAPL and viscosity reduction may be more important. Recovery wells are used to capture both liquids and gases and transport them to a surface facility for treatment.
The applicability of steam injection to a particular site is determined by the permeability of the soil, the depth at which the contaminants reside, and the type and degree of heterogeneity, as well as the contaminant type. The permeability of the soil must be high enough to allow sufficient steam to be injected to heat the entire source zone. Higher injection rates can be achieved by increasing the injection pressure; however, in general, pressures should not be higher than 1.65 pounds per square inch per meter of depth, or the overburden pressure will be exceeded, and fracturing to the surface can be expected (Davis 1998). Shallow treatment areas are difficult to heat with steam, and collection of all the vapors generated may be challenging; an impermeable surface cover may help in this regard.
Heterogeneity of the Subsurface and Soil Type: The soil type affects the ability of the steam to remove contaminants in two ways. The permeability determines how fast a steam front can move into and through the soil. Low permeability soil may not allow steam to move through it at an economical rate or may require unsupportable pressures to do so. The other aspect of soil that affects contaminant removal is its reactivity with contaminants. Silica-based sands are not particularly reactive, and contaminants can be removed easily. Smectite clays and soil rich in organic matter both have the ability to bind some organic compounds and prevent their full removal at steam temperatures.
When there is heterogeneity, steam tends to channel to the more permeable layer, or in the case of discontinuous layers, bypass the less permeable one. If this happens, heating in the less permeable soil usually is done by conductive methods. To fully heat low permeability zones by conduction, steam should be injected on both sides of the low permeability zone, and the zone should be less than about 10 feet in thickness.
Injection and Extraction Well Placement and Operation: The most effective design of steam injection and extraction systems is to surround the contaminated zone with four to six injection wells and to extract the contaminants from the center. If the area to be treated is large, repeating patterns of injection and extraction wells may be used to cover the entire area (Davis 1998). The propagation of the steam front is a balance between the injection well's ability to add heat and the surrounding soil's ability to absorb it. When the rate of heat loss to the soil surrounding the front equals the system's ability to input it, the steam front will stop growing. The extraction well needs to be closer to the injection well than this point of equilibrium. Distances between wells have been reported as close as 1.5 meters and as far apart as 18 meters.
The wells and couplings handle very hot streams of vapors, and their construction is a concern. Plastics, such as PVC, are generally not appropriate; hence, steel is typically used. Since the injection wells are both very hot and pressurized, couplings should be carefully designed and an appropriate sealant for the annulus should be chosen. Cement grouts used in conventional water well completion may not withstand the pressure, heat, and expansion/contraction of the well casing itself and could crack, causing the release of steam to the surface.
A characteristic of steam treatment in a source zone under saturated conditions that needs to be considered is override. Override occurs when there is a difference in density between two fluids, such as that between steam and ambient temperature water. The resulting interface tends to move the steam out and up, causing the top of the steam front to be considerably further from the injection well than the bottom. This situation can lead to untreated spots near the bottom of the injection well. Override cannot be completely eliminated, but it can be minimized within the constraints of the site hydrogeology by using high injection rates.
Pressure cycling of the steam injection generally is utilized as part of the system operation. Cycling is the process where, after breakthrough of steam at the extraction wells, the steam injection system is shut down while allowing the extraction process to continue. The loss of pressure thermodynamically destabilizes the system, forcing the temperature to drop to restore stability. The system loses heat by evaporation of residual moisture and the contaminants that are collected by the extraction wells. Davis (1998) and Davis et al. (2005) report on several studies where repeated cycling has resulted in increased contaminant concentrations in the extracted vapors.
Because of the sensitivity of steam flow to heterogeneous subsurface conditions, more site characterization efforts may be required than for other heating technologies. One type of steam delivery system combines steam injection with an in situ auger mixing system. In this application, steam is applied through specially designed augers while the soil is being mixed. The steam strips the volatile contaminants from the moving soil and brings them to the surface, where they are captured in a shroud or bell device and transported to a treatment system.
Cost data for steam injection and extraction are limited. U.S. DOE (2000) reported cleanup at approximately $39 per cubic yard, but their system also employed electrical heating. The technology is mature and well established; however, few vendors use it for environmental remediation.
This discussion is taken from Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.
In-Situ Delivery of Heat by Thermal Conduction and Steam Injection for Improved DNAPL Remediation
R. Baker and G. Heron.
Proceedings of the 4th International Conf. on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 24-27, 2004. Battelle, Columbus, OH.
Presents an overview of how in situ thermal conductance works and how steam can be used to improve its performance particularly in the saturated zone.
This document is not meant to provide detailed information that would allow the design of a steam injection remediation project, but rather provides design considerations to familiarize remediation workers with what is involved in the design process.
Full-Scale Removal of DNAPL Constituents Using Steam-Enhanced Extraction and Electrical Resistance Heating
G. Heron, S. Carroll, and S. Nielsen.
Ground Water Monitoring & Remediation, Vol 25 No 4, p 97-107, 2005
Discusses the full-scale remediation of Area A of the Northeast Site at the Young-Rainey STAR Center, Largo, FL. The site was contaminated with TCE, cis-1,2-DCE, methylene chloride, toluene, and petroleum hydrocarbons. The hydrogeology consists of a fine-grained sand aquifer underlain by Hawthorne clay at 30 ft depth. The upper 5 ft of this clay formed part of the remediation volume, as DNAPL was present in this layer. Steam-enhanced extraction and electrical resistance heating were used to remediate the site.
A Theoretical Model of Air and Steam Co-Injection to Prevent the Downward Migration of DNAPLs During Steam-Enhanced Extraction
S. Kaslusky and K.S. Udell.
Journal of Contaminant Hydrology, Vol 55, p 213-232, 2002
Removal of NAPLs From the Unsaturated Zone Using Steam: Prevention of Downward Migration by Injecting Mixtures of Steam and Air
R. Schmidt, J. Gudbjerg, T.O. Sonnenborg, and K.H. Jensen.
Journal of Contaminant Hydrology, Vol 55, p 233-260, 2002