WO1995034349A1

WO1995034349A1 - Method and apparatus for in situ soil remediation

Info

Publication number: WO1995034349A1
Application number: PCT/US1995/007370
Authority: WO
Inventors: Alexander G. Baldwin; James A. Newburn; John T. Schofield; Richard J. Martin
Original assignee: Thermatrix, Inc.
Priority date: 1994-06-10
Filing date: 1995-06-08
Publication date: 1995-12-21
Also published as: AU2702595A

Abstract

Apparatuses and methods for treating in situ soil that is contaminated with organic compounds whereby the contaminants are removed from the soil using soil vapor extraction techniques (36) possibly coupled with steam injection or air sparging (34) and are thereafter destroyed by flameless oxidation (10) within a porous inert media destruction matrix are disclosed. The resultant heat of oxidation of the organic compounds in the flameless oxidizer can be utilized either to generate steam (66) for steam injection or as hot air for air sparging. The destruction matrix is composed of inert materials that enhance process mixing and provide thermal inertia for process stability, with a resultant minimization of NOx oxidation by-products to levels below those achievable by conventional technologies.

Description

METHOD AND APPARATUS FOR IN SITU SOIL REMEDIATION

Field of the Invention

The field of the present invention is methods and apparatuses for separating organics from soils in situ and thereafter destroying such organics. In particular, the present invention relates to apparatuses and methods for controlled exothermic reaction of organic vapors from soil vapor extraction systems, although it will be appreciated that the invention in its broader application can be applied to any commercial process giving off organic vapors.

Background of the Invention

Soils contaminated with organic chemicals are a widespread problem throughout the world, with millions of cubic meters requiring remediation in the United States alone. For example, concentrated underground organic contaminant plumes are one of the most prevalent ground water contamination sources. A typical source of concentrated plume is a leaking underground storage tank. When the stored liquid escapes from the tank slowly, it can take years for the operator to become aware of the problem. By that time the solvent or fuel can percolate deep into the earth, often into water-bearing regions. Collecting as a separate, liquid organic phase called Non-Aqueous-Phase Liquids ("NAPL's"), these contaminants provide a source that continuously compromises surrounding ground water. This type of spill is one of the most difficult environmental problems to remediate. Attempts to remove such material by pumping the ground water require that a huge amount of water be washed through the system, requiring tens of years. Pumping at some sites for many years has resulted in clea effluent water, but when the pumps were shut off and restarte several years later, the ground water was again contaminateα. Clean-up of such contaminated materials is subject t a wide variety of regulations in the United States, includin those covered under The Comprehensive Environmental Response, Compensation, and Liability Act of 1980 ("CERCLA"), Th Superfund Amendments and Reauthorization Act of 1986 ("SARA") , and The Resource Conservation and Recovery Act ("RCRA") . Th total cost of these clean-up efforts in the United States has been estimated to exceed $200 billion over the next 30 to 40 years.

Generally, contaminants exist in the soil in one o more of the following forms: NAPL's, solutions of organics i water, material adsorbed to the soil, and mixtures of free vapor. Under static conditions, these phases are i equilibrium. The distribution between phases is determined b various physical phenomena controlling the equilibrium.

NAPL's can occur in the soil as pools of contaminants or as residual liquids trapped between soil particles. In the vicinity of the NAPL's, the equilibrium between vapor an liquid phases is governed by Raoult's Law. NAPL's consist of light nonaqueous phase liquids ("LNAPL's") and dense nonaqueous phase liquids ("DNAPL's"). LNAPL's, which include hydrocarbons, ketones, etc., are less dense than water. DNAPL's, which include chlorinated hydrocarbons, are more dense than water.

In many instances the contaminants are dissolved in the pore water that fills the interstices between soil particles. Equilibrium between the contaminant in the aqueous solution and that in the associated vapor is then governed by Henry's Law.

If the contaminant is strongly adsorbed to solid material, the equilibrium between vapor and adsorbed contaminant is likely to be controlled by adsorption isotherm parameters. Adsorption control may be operative for low contaminant concentrations, clayey soils, soils containing large amounts of humus, and soils containing large amounts of solid organic matter that can adsorb the contaminant phase of interest. Soil moisture conditions also affect contaminant adsorption, since water molecules compete for the soil adsorption sites. The amount of time that contaminants have been in the soil may affect the amount of material that is adsorbed, especially when the adsorption processes are slow.

Several factors affect the movement of contaminants in soil and groundwater. Soluble compounds tend to travel farther in soils where the water infiltration rate is high. Chemicals with affinity for soil organic material or mineral adsorption sites will move slowly. Contaminant density and, to a lesser extent, viscosity have an impact on organic liquid movement and the location of the contaminants. LNAPL's will sink through the soil until they reach the capillary fringe where they tend to form pools. DNAPL's will continue to sink below the water table until they encounter an impermeable layer.

Soil Vapor Extraction A number of processes can be used to deal with these problems of contaminated soils. For removing the contaminants in si tu, one current methods is soil vapor extraction ("SVE") . The SVE process is a technique for the removal of volatile organic compounds ("VOC's"), and some semivolatile organic compounds ("SVOC's") , from the vadose zone. The vadose zone is the subsurface soil zone located between the land surface and the top of the water table. SVE is an in situ remediation technique that creates a deliberate movement of air (or steam) through the soil by forcing a vacuum in a soil region, causing the organic compounds to vaporize and be removed with air through a system of wells to a vacuum system on the surface. The SVE approach is most suited to use after any free product or liquid has been recovered by conventional pumping techniques to remove occluded liquid remaining in the interstices of the soil particles.

SVE is a basic approach to organic contaminant recovery as a means of soil and groundwater remediation that has proven to be a preferred alternative to the groundwater pump-and-treat technologies commonly practiced. In conjunction with the site conditions and soil properties, contaminant properties will dictate whether SVE is feasible. SVE is most effective at removing compounds that have high vapor pressure and that exhibit significant volatility at ambient temperatures in contaminated soil. Low molecular weight, volatile compounds are most easily removed by SVE. Compounds exhibiting vapor pressures over 0.5 mm Hg can most readily be extracted using SVE. Trichloroethene, trichloroethane, tetrachloroethane, and many gasoline constituents have been effectively removed by SVE. Compounds that are less suitable for removal include trichlorobenzene, acetone, and other extremely water soluble volatiles, and heavier petroleum fuels. The dynamic process of SVE is characterized as follows: When air is drawn through the soil, it passes through a series of pores, most readily following the paths of low resistance/high air permeability. Air that is drawn through pores that contain contaminated vapor and liquids will carry the vapor away. Contaminants will vaporize from one or more of the condensed phases (organic, aqueous, adsorbed) , replacing the vapors that were carried away in the air stream. The vaporization tends to maintain the vapor-condensed phase equilibrium that was established prior to removal of the contaminants. This process will continue until all of the condensed-phase organics are removed from the regions of higher permeability soil. Contaminants in lower permeability zones will not be removed by advection since the air stream will flow through higher permeability zones. If the contamination is located in a stagnant region some distance from the air flow, the vapor must diffuse to the air stream before it can be carried away. This diffusion process would then limit the rate of contaminant removal by the SVE process. If the rate of diffusion is very slow, it can limit the ability of SVE to remove contaminants in an acceptable time frame.

The soil characteristics of the site have a significant effect on the applicability of SVE. The air permeability of the contaminated soils controls the rate at which air can be drawn through the soil by the applied vacuum. The soil moisture content or degree of saturation is also important. It is usually easier to extract VOC's from drier soils due to the greater availability of pore area, which permits higher air flow rates. Operation of an SVE system can dry the soil by entrainment of water droplets and, to a lesser extent, by evaporation. However, extremely dry soils may tenaciously hold VOC's, which are more easily desorbed when water competes with them for adsorption sites. This phenomenon favors the presence of a certain quantity of moisture in the soil to prevent sorption of contaminants.

Soils with high clay or humic content generally provide high adsorption potential for VOC's, thus inhibiting the volatilization of contaminants. However, the high adsorption potential of clayey soils does not necessarily make SVE inapplicable. The success of SVE in these soils may depend on the presence of more permeable zones (as would be expected in alluvial settings) that permit air flow close to the less permeable material (i.e., clay).

The design of SVE systems is normally site-specific. Most site conditions cannot be changed. The extent to which VOC's are vertically and horizontally dispersed in the soil is an important consideration in deciding whether SVE is preferable to other methods. Soil excavation and treatment are probably a more cost effective technique when only a few hundred cubic yards (yd³) of near-surface soils are contaminated. If the spill has penetrated more than 20-30 ft, has spread through an area of several hundred square feet at a particular depth, or has contaminated a soil volume of 500 yd³, however, excavation costs begin to exceed those associated with an SVE system.

The depth to groundwater is also important because SVE is applicable only to the vadose zone. If contaminated soil is below the top of the water table, the level of the water table may be lowered, in some cases, to increase the volume of the unsaturated zone that can be treated. Vapor extraction wells usually consist of slotted pipe placed in permeable packing, such as coarse sand or gravel. For long-term applications, the well casing material is selected to be compatible with the contaminants of concern. The top few feet of the augured column for vertical wells, or the trench for horizontal wells, is grouted to prevent the direct inflow of air from the surface (short circuiting) along the well casing or through the trench.

In some cases, it may also be desirable to install air inlets to enhance air flow through zones of maximum contamination. These wells are constructed similarly to the vapor extraction wells. Inlet wells or vents are passive and allow air to be drawn into the ground.

Vacuum pumps or blowers reduce gas pressure in the extraction wells and induce subsurface air flow to the wells. Ball or butterfly valves are used to adjust flow from or into individual wells. The pressure from the outlet side of the pumps or blowers can be used to push the exit gas through a treatment system. The induced vacuum causes a negative pressure gradient in the surrounding soils. The projected area of soil affected by this pressure gradient is called the zone of influence. The radius of influence ("ROI") is the radial distance from the vapor extraction well that has adequate air flow for effective removal of contaminants when a vacuum is applied to the vapor extraction well. Hence, the ROI and the extent of contamination determine the number of extraction wells required on the site.

Site characteristics such as stratigraphy, the presence of an impermeable surface or subsurface barrier, and soil properties such as porosity and permeability affect the ROI. Air vents and increases in the strength of the applied vacuum can be used to maximize the ROI. Reported ROI values for permeable soils (sandy soils) range from 30 to 120 feet. Good surface seals are required, especially for shallow wells (screened less than 20 feet below surface) , to prevent short circuiting of air flow to the surface. For less permeable soils (silts, clays) or for shallow wells, the ROI is usually less. The ROI in fractured bedrock or in other non-homogeneous stratigraphies will not be symmetrical (i.e., it may extend 200 feet along a fracture but be only 2 or 3 feet wide) .

An optional "impermeable" cap over the treatment site serves several purposes. First, it minimizes infiltration of water from the surface. Infiltration water can fill soil pore spaces and reduce air flows. A cap may increase the system's ROI by preventing short circuiting. Finally, it may also help to control the horizontal movement of inlet air, which can bypass contaminants. Any cap used must be specifically designed for the site. For instance, if a thick layer of gravel exists below an asphalt or concrete cap, there can be short circuiting through the gravel. Plastic membranes, existing buildings and parking lots, and natural soil layers of low permeability, however, may serve the purpose.

A vapor/liquid separator is installed on some systems to protect the blowers and to increase the efficiency of vapor treatment systems. The entrained groundwater and condensate brought up through the system may then have to be treated as a hazardous waste, depending on the types and concentrations of contaminants.

Heterogeneities, such as debris, fill material, and geological anomalies, influence air movement as well as the location of contaminants. The uncertainty in the location of heterogeneities makes it more difficult to position vapor extraction inlet wells. There generally will be significant differences in the air permeability of the various soil strata. SVE may be favorable for a horizontally stratified soil because the relatively impervious layers will limit the rate of vertical inflow of air from the surface and tend to extend the applied vacuum's influence from the point of extraction.

In efforts to improve the efficiency of SVE remediation, supplemental techniques have recently been applied to standard SVE systems. These include the application of air sparging and steam injection. Air sparging allows for the recovery of the less volatile organics and dissolved contaminants and residuals beneath the water table by injection of heated air below the groundwater surface. The injected air enhances volatilization by increasing the water-to-air surface area and heating of the soil matrix. In some cases it may induce upward migration of globules of product with migrating air bubbles.

Steam injection injects steam into the contaminated zone to increase the subsurface temperatures, thereby volatilizing organic compounds with high boiling points. The added heat provided by the steam enhances the volatilization of organic residuals that are in the soil. The steam front mobilizes the heavy residuals and volatilizes the light fractions. Enhanced volatilization and residuals migration effects a faster, more complete mass transfer process that speeds the remediation and reduces cleanup costs. Both of these methods reduce the required remediation operating period compared to conventional soil venting techniques and are discussed in more detail below.

For removing VOC's from the saturated zone, the combination of air sparging and SVE has been shown to be more effective and have lower capital and operating costs than a traditional ground water pump-and-treat system. For removing semi-volatile organic contaminants from the unsaturated and saturated zones, steam injection combined with SVE has been shown to be more effective and requires less cleanup time than excavation and above ground treatment.

Air Sparging Air sparging, also referred to as " in situ air stripping" or " in situ volatilization," is a treatment technology for removing VOC's from the saturated zone. Contaminant-free air is injected into contaminated groundwater to remove contaminants from the saturated zone and effectively capture them with an SVE system. SVE without air sparging can remove contaminants from the saturated zone. However, the transport rates due to diffusion/dispersion of the dissolved contaminants in the aqueous phase to the air-water interface limit the removal effectiveness. This rate of contaminant transport can be significantly increased by the addition of air sparging to an SVE system.

The use of an air sparging system results in a net positive pressure in the subsurface, which must be compensated for by the SVE system to prevent migration to previously uncontaminated areas. Without SVE, uncontrolled contaminated soil vapor flow may enter basements of nearby buildings, potentially creating an explosion or health hazard.

The effectiveness of the air sparging/SVE system can be attributed to two major mechanisms: (1) contaminant mass transport; and (2) biodegradation. Depending on the configuration of the system, the operating parameters, and the types of contaminants found at the site, one of these mechanisms usually predominates or can be enhanced to optimize contaminant removal.

The mass-transfer mechanism consists of movement of contaminants in the subsurface and eventual extraction via an SVE system. Contaminants adsorbed to soils in the saturated zone dissolve into groundwater. The sparged air displaces water in the soil pore spaces and causes the soil contaminants to desorb, volatilize, and enter the saturated zone vapor phase

("SZVP") . The mechanical action of the air passing through the saturated zone increases turbulence and mixing in the groundwater. Dissolved groundwater contaminants also volatilize into the SZVP and migrate up through the aquifer to the unsaturated zone. The SVE system then creates a negative pressure gradient in the unsaturated zone that pulls the contaminant vapors toward the SVE wells.

Aerobic biodegradation of contaminants by indigenous microorganisms requires the presence of sufficient carbon source, nutrients, and oxygen. Air sparging increases the oxygen content of the groundwater, thus enhancing aerobic biodegradation of contaminants in the subsurface. The organic contaminants, especially petroleum constituents, provide the microorganisms with a carbon source. If the rate of biodegradation is to be significantly enhanced, nutrients such as nitrogen and phosphorus usually must be added to the contaminant zone. However, nutrient addition can cause excessive biological growth, which may cause significant fouling of the injection wells and thereby reduce the effectiveness of an air sparging system. 5 Perhaps the single most important design element of an air sparging system is the layout and construction of the well network. The placement of the air sparging and vapo extraction wells must take into account a wide range of groundwater, soil and chemical properties of the site. The

10 physical conditions of the soil/groundwater matrix, such as depth to groundwater, soil particle-size distribution, soil stratification, and soil porosity, as well as the thermodynamic and transport properties, such as chemical/soil partitioning coefficients and pneumatic and hydraulic conductivity of the

15_. soil, all factor into determining the best well configuration for a particular site. There are normally three configuration choices: (1) spaced; (2) nested; or (3) horizontal.

A spaced configuration is generally applied in a square grid pattern with the extraction well in the center and

20 four injection wells at the corners. This pattern works well for sites with highly uniform sandy soils where an effective air flow pattern can be created between the injection and extraction points. The vertical wells are laid out throughout the site covering the zone of contamination.

25 Nested wells are extraction and sparging wells placed in the same borehole. The advantage of this configuration is that drilling costs may be reduced. The disadvantages are: (1) care must be taken to properly grout the borehole to prevent short-circuiting of air; and (2) the pressure gradient is

30 primarily in the vertical direction. This configuration works well for sites with highly stratified silty soils where the vertical permeability is significantly less than the horizontal permeability.

Trenches or horizontal wells are formed by installing

35 perforated pipe and gravel pack in a trench or by using a new advanced drilling techniques for horizontal well installation.

. The horizontal configuration provides a more uniform pressure gradient at specific depths over a wider range than a series of vertical wells. Trenches are particularly well suited for sites with shallow aquifers less than 10 feet below grade. Horizontal wells are well suited for contaminant plumes resulting from leaking pipelines.

The ROI around the sparging and extraction wells is the zone in which the vapor flow is induced toward the well. The ROI is determined by pressure gradients and/or changes in the chemical composition at distances away from the well. Soil permeability, among other factors, will affect ROI -- soils with high permeabilities would have larger ROI's than soils with low permeabilities (all other factors remaining the same) . Air injection pressure, flow rate, and the depth of injection below the water table will also affect the ROI. The ROI is used to determine the well spacing and number of wells needed for the site. Consideration should also be given, however, to the travel time of the contaminant from the outer perimeter of the ROI to the travel extraction well, since restrictive travel times may impede the cleanup and mitigate for overlapping the well influence areas.

In general, high permeability sandy soils will result in a higher ROI and consequently higher flow rates than low permeability silty soils. The ROI for a sparging well can range from 10 to 100 feet from the injection point, while the ROI for an extraction well can range from 25 to 300 feet from the extraction point. The air injection flow rate is always less than the extraction flow rate in order to capture the injection air in the extraction system. Typical systems operate with injection air flows on the order of 10%-20% of the extraction flow rates.

The implementation of an air sparging system must also take into consideration the dynamic changes that may occur in the subsurface. The introduction of air below the water table will cause an increase in the groundwater elevation, an effect known as mounding. This effect, if not properly controlled, may cause further migration of contaminants away from the treatment area. Sparging can also cause dissolved minerals to precipitate, thereby impeding the flow of air through the subsurface. Careful operation of the air injection rate or the use of nitrogen can avoid this problem. Steam Injection Steam injection, also called "soil heating" or "steam venting, " is an in situ treatment technology for remediation of organic contaminants in the subsurface. Steam is injected into a contaminated subsurface to thermally recover volatile and semivolatile liquids in conjunction with water and SVE. Steam injection is coupled with an SVE system and a water extraction system in order to capture the contaminants that are liberated from the porous soil. The use of steam injection results in the migration of vapors in the steam zone and the flow of contaminant liquids ahead of the steam condensation front.

The effectiveness of the steam injection/recovery system can be attributed to two major mechanisms: (1) vaporization of volatile and semivolatile contaminants; and (2) displacement of liquids. Depending on the configuration of the system, the steam injection rate, additional operating parameters, and the types of contaminants found, these mechanisms can be optimized for maximum contaminant removal.

The operation of a steam injection system begins with the simultaneous injection of steam and the extraction of liquids and vapors. As the steam is initially injected into the subsurface, the ambient soils remove the latent heat of vaporization from the steam and it condenses. Following additional steam injection, the steam condensate front moves outward from the injection point and an isothermal steam zone is evident. The zone beyond the steam condensate front is referred to as a variable temperature zone.

After the injected steam breaks through to the extraction wells, steam injection continues until the contaminant concentrations approach the cleanup goals. At that point, steam injection is stopped while the vapor extraction system continues to operate. The continued SVE operation will result in the further vaporization of the residual contaminants in the pore spaces and the drying out of the treated soils. As steam input stops, a drop in steam zone pressure slightly reduces the boiling point of any residual water or contaminants (such as that held by capillary forces) , forcing them to boil and convert to removable vapor.

The volume affected by the^"steam zone depends on the steam injection rate. The system is generally characterized by a high vapor volumetric flow rate, typically in the range of 1,000-2,000 lb/hr., and temperatures can be raised to greater than 115°C.

Low-boiling-point liquids in the range of 90-150°C will generally be mobilized ahead of the steam condensate front in the variable temperature zone and accumulate in both the vapor and liquid phases. Organic contaminants with low vapor pressures (i.e., C₁₅ hydrocarbons and greater) may remain in the pore spaces within the isothermal steam zone. Continued steam flow, however, will subsequently evaporate these contaminants or enhance their migration toward the collection wells.

The removal of residual petroleum at a contaminated site can be accomplished over the entire contaminated area or sequentially in small areas. Although energy intensive to operate, a steam injection system is applied for only a fraction as long as conventional remediation techniques -- on the order of weeks as opposed to months or years for traditional remediation methods.

The major factors affecting ROI of a steam injection system are soil permeability, steam injection pressure, and steam flow rate. In general, high permeability sandy soils will result in a higher ROI than low permeability silty soils. The ROI for a steam injection well can range from 25 to 100 feet from the injection point, and the ROI for an extraction well can range from 25 to 300 feet from the extraction point.

The ROI for the steam injection system will determine the well spacing and number of wells needed for the site. Based upon a square injection well grid, the maximum well spacing is the square root of two times the ROI. If a faster cleanup time is required, the injection wells should be spaced closer together in order to heat the subsurface in less time. Based upon a steam injection rate of 1,000 lb/hr. in a 10 foot layer of the vadose zone and a maximum ROI of 40 feet, the treatment zone is fully developed in about 16 days. Steam injection has been used to remove gasoline and diesel oil in both the unsaturated and saturated zones. In general, the amount of steam required in the saturated zone is on the order of 4-5 times greater than that required in the unsaturated zone. This additional heat is required to physically displace, heat, and vaporize the groundwater. However, steam injection cleanup times and costs are still only a fraction of those required for pump-and-treat systems. Post-Extraction Treatment SVE, whether or not combined with air sparging or steam injection techniques, is used with other technologies in a treatment train since it transfers contaminants from soil and interstitial water to air and the entrained and condensed water waste streams. These streams require further treatment. Treatment of the contaminated air in typical SVE processes today includes either adsorption using activated carbon, condensation, or oxidation of the VOC's, catalytically or by incineration. Other methods, such as biological treatment, ultraviolet oxidation, and dispersion have also been used in SVE systems. To date, the type of treatment chosen has generally depended on the composition and concentration of contaminants. For example, in cases where the concentration and/or the boiling point of the VOC's are low, condensation is economically impractical as compared to the capital and operating costs of adsorption or oxidation.

Carbon adsorption is the most commonly employed vapor treatment process and is adaptable to a wide range of VOC concentrations and flow rates. Skid-mounted, offsite- regenerated, carbon-canister systems are generally employed for low gas volumes and onsite-regenerated bed systems are employed for high gas volumes and cleanups of extended duration. Adsorption on granular activated carbon, however, is often unsuitable when the quantity of the contaminant is large, or the VOC's are not readily adsorbed because such situations lead to rapid saturation of the carbon.

Thermal destruction of contaminant vapors by incineration or catalytic oxidation is effective for a wide range of compounds. Catalytic oxidation is effective on hydrocarbon vapors. Recently developed catalysts also permit the efficient destruction of halogenated compounds (bromides, chlorides, or fluorides) . Conventional flame-based combustion technologies, however, offer only adequate destruction efficiencies while generating secondary pollutants such as NO_x. Other thermal oxidation systems such as those employing catalysts have demonstrated that effectiveness is greatly diminished at chlorinated hydrocarbon concentrations of greater than 100 ppm.

Flame-based destruction processes also pose serious performance, regulatory, and public acceptance issues. Incineration is difficult to control and can result in the formation of highly undesirable products such as dioxins, furans and oxides of nitrogen.

For example, standard combustors are particularly undesirable when dealing with chlorinated hydrocarbons. A free flame also results, in some instances, in incomplete combustion and uncontrollable production of undesirable side products. Because combustors typically operate at flame temperatures on the order of 3500°F, significant amounts of unwanted N0_X are often produced. The high temperatures also raise significant safety issues.

Condensation can be used to separate the effluent VOC's from the carrier air. This is usually accomplished by refrigeration. The efficiency of this technique is determined by the effect of temperature on the vapor pressure of the VOC's present. Condensation is most efficient for high concentrations of vapors. The technology becomes less efficient as the clean up progresses and vapor concentrations drop. It may be ineffective during the last stages of the clean up. Since vapors are not completely condensed, a carbon adsorption or other additional treatment step may be required to remove residual vapors from the effluent stream.

Currently, contaminated residuals are produced from the application of SVE technology. These may include recovered 5 condensate (contaminated water and possibly supernatant organics) , spent activated carbon from off-gas treatment, nonrecovered contaminant in the soil, soil tailings from drilling, and air emissions after treatment. Contaminated water requires treatment in accordance with the State/National

10 Pollution Discharge Elimination System ("SPDES/NPDES") permit levels prior to surface water discharge, or in accordance with pretreatment requirements prior to discharge to a publicly owned treatment works ("POTW"). When contaminated water is recovered by the SVE process, it can usually be treated with

15. carbon adsorption or air stripping followed by discharge to surface waters, POTW, or by onsite reinjection. If this is not feasible, the contaminated water can be pumped into a holding tank. This holding tank can be emptied by a tank truck that periodically hauls the contaminated water to an appropriate

20 treatment and disposal facility. Soil tailings from the drilling operation may be contaminated. They can be placed in covered piles and treated onsite by adding vent connections to the SVE system. The soil tailings can also be collected in drums or dumpsters and sent for offsite treatment. Any spent

25 activated carbon should be disposed of in accordance with regulations and policy.

Thus, it can be seen that there is a need for a practical means of removing organics from contaminated soils in situ that avoids the various difficulties and inefficiencies of

30 the prior art. There is a need for a system that has the advantages of soil vapor extraction while reducing or eliminating the problems arising from the need to adsorb, condense, incinerate, or otherwise dispose of the volatilized contaminants. There is a further need for such a system to

35 result in high destruction and removal efficiency ("DRE") of the organics in a cost-effective manner. Summary of the Invention

The present invention is directed to methods and apparatuses for treating soil in situ that is contaminated with organic compounds whereby the contaminants are removed from the soil using soil vapor extraction technology and are thereafter destroyed by oxidation within a porous inert media destruction matrix contained as part of a flameless oxidizer. The resultant heat of oxidation of the organic compounds in the flameless oxidizer can be utilized to generate at least some steam for use in a steam injection system that supplements the SVE system, thereby reducing energy costs and improving efficiency. .Alternatively, the oxidized gasses can be used directly for hot air sparging.

The combination of enhanced SVE processes with the flameless oxidation technology provides an integrated, closed loop, energy efficient remediation process offering significant advantages. These advantages include shorter on-site remediation schedules, higher contamination removal efficiencies, "near zero" emissions, and reduced remediation costs. By combining conventional heat recovery devices with the flameless oxidation process, the greater benefits of enhanced soil venting using hot air sparging and steam injection are economically implemented as an integrated system. The high performance destruction characteristics inherent to the flameless oxidizer coupled with the enhanced SVE process exceeds the regulatory criteria in a consistent, reliable, and cost effective manner.

The destruction matrix is composed of inert ceramic materials that enhance process mixing and provide thermal inertia for process stability. Such a destruction matrix is designed to produce DRE's of greater than 99.99%, with less than 10 ppmV CO and less than 2 ppmV N0_X. The thermal oxidizer/destruction matrix is designed to operate in a flameless manner at temperatures of 1550-1800°F, below the normal flammability limits of the volatiles to be destroyed. The appropriate conversion may be obtained at lower temperatures and residence times than those required in a conventional incinerator. There is also inherent safety in th use of a process in which there are no open flames, and i which the mixture of gasses to be introduced into the matrix i relatively cool, outside the flammability limits of th constituents, and, therefore, not explosive under ambien conditions. Problems of flameouts are avoided. Moreover, fro a practical viewpoint, all of these features should result i the ability to obtain required government permitting mor easily. Accordingly, it is an object of the present inventio to provide methods and apparatuses capable of meeting existin regulations for the destruction of organic contaminant contained within soils using an in situ system.

It is another object of the present invention t provide methods and apparatuses for destruction of organi contaminants contained in soils in si tu while minimizing N0 oxidation by-products to levels below those achievable b conventional technologies.

It is an additional object of the present inventio to utilize the heat generated on oxidation of the extracte organic compounds to assist in extraction of additional organi contaminants.

Other and further objects and advantages will appea hereinafte .

Brief Description of the Drawings

Fig. 1 is an embodiment of a flameless oxidizer a might be used in the process and apparatus of the presen invention.

Fig. 2 is a flow diagram detailing another embodimen of an apparatus of the present invention that allows for ai sparging of the oxidizer unit off-gas.

Fig. 3 is a flow diagram detailing one embodiment o an apparatus of the present invention that recycles heat fro the oxidizer unit to a boiler. Detailed Description of the Preferred Embodiments It has now been discovered that a combination of successfully demonstrated SVE technologies with an innovative high performance flameless oxidation process results in an integrated, closed loop soil remediation unit offering operational simplicity, near zero emissions, and reduced costs. The proposed integrated soil remediation system is designed to operate at reduced temperatures, utilizing the hot, inert off- gas (void of products of incomplete combustion ("PIC's")) from the flameless oxidizer to heat steam for injection into the soil, thus providing less expensive, yet superior and more reliable, performance as an alternative to incineration.

Significant research into the phenomena of oxidation within porous inert media ("PIM") has recently been undertaken. Because PIM oxidation can occur outside the normal premixed fueled/air flammability limits, the technology can be called

"flameless." In this regard U.S. Patent Nos. 4,688,495

(Galloway) and 4,823,711 (Kroneberger et al . ) disclose early work on matrix oxidation technology. In addition, U.S. Patent Nos. 5,165,884 (Martin et al . ) and 5,320,518 (Stilger et al.) discuss in significant detail the technology involved in a flameless oxidizer. Each of these patents is incorporated herein by reference.

As a treatment technology, such a flameless oxidizer process exhibits most of the advantages of conventional or catalytic thermal combustion, while avoiding many of the disadvantages. Like flame-based thermal combustion, organics are oxidized to harmless product gasses (C0₂, H₂0) or easily neutralized acid gasses (HC1, S0₂) . No waste or residues are created, and the process is suitable for a wide range of compounds or mixtures. Unlike thermal incineration, where the mixing and reaction are interdependent with the flame, these are decoupled in the inventive system, allowing greater flexibility and control, and the elimination of PICs. Additionally, no catalysts are necessary.

The basis for the oxidation process is a "destruction matrix" that fosters the conditions necessary for stable, flameless oxidation of organic compounds, outside thei respective flammability limits. The three primary attribute of the destruction matrix that permit flameless oxidation ar its interstitial geometry (which enhances mixing) , its therma inertia (which promotes stability) , and its surfac characteristics (which augment heat transfer) . The therma properties of the matrix allow the mixing zone to be nea ambient temperature where the fume enters while the reactio zone, further downstream, is at the appropriate oxidatio temperature.

These attributes lead to several performance- an safety-related advantages in practical applications. Amon these are the ability to establish a stationary reaction zon (wherein the rate of fume oxidation is much faster than in th post-flame region of an incinerator) ,- the ability t accommodate rapid process fluctuations (as with batch chemica reactor discharges) ; the capability for wide process turndow (for cost effective adaptation to changing conditions) ; th suppression of flashback (by virtue of the matrix's hig surface area and heat absorption capability) ; and a high leve of manageability and control (compared to a flame) .

Turning in detail to the drawings, where like number designate like components, Fig. 1 illustrates an embodiment o one such flameless oxidizer as might be used in the process an apparatus of this invention. Typically, the flameless oxidize (10) will consist of a suitable matrix bed containment shel (12) that is filled with a quantity of heat resistant materia creating a matrix bed (14) . The types of matrix materials use should have high heat conductance by radiation, convection, an conduction. The heat transfer properties of the system ar dependent on the ratio of radiative to convective heat transfer.

The matrix bed (14) may be sized for any desired flo stream by altering the matrix flow cross-section, height, material, void fraction, outlet temperature, and supplementa heat addition, if desired. Preferred matrix materials ar ceramic balls or saddles, but other bed materials an configurations may be used, including, but not limited to, other random ceramic packings such as pall rings, structured ceramic packing, ceramic or metal foam, metal or ceramic wool and the like. Generally, the void fraction of the matrix bed will be between 0.3 and 0.9. In addition, the material in the matrix bed will typically have a specific surface area ranging from 40 m²/m³ to 1040 m²/m³.

In the preferred embodiment of Fig. 1, two types of heat resistant material are used. In the lower portion of the flameless oxidizer (10) , a bed of ceramic balls acts as a mixing zone (16) . This mixing zone (16) would typically have an interstitial volume of about 40%. Above this bed of balls, a bed of ceramic saddles is utilized to create a reaction zone

(18) . This reaction zone (18) would typically have an interstitial volume of about 70%.

A preheater apparatus (30) is configured at the base of flameless oxidizer (10) . This preheater (30) initially passes hot gas through the matrix bed (14) in order to preheat both the ceramic ball mixing zone (16) and the ceramic saddle reaction zone (18) to normal operating temperatures. In one alternative embodiment, heating elements (not shown) , which are preferably electric, can surround this containment shell (12) to provide the system with preheating and proper temperature maintenance during operation. The entire thermal oxidation assembly will preferably be designed so as to minimize heat loss to the environment, while ensuring that all exposed surfaces remain below those temperatures acceptable for a Class I, Division 2, Group D area. (The National Electrical Code categorizes locations by class, division, and group, depending upon the properties of the flammable vapors, liquids, or gasses that may be present and the likelihood that a flammable or oxidizable concentration or quantity is present. The Code requires that the surface temperature of any exposed surfaces be below the ignition temperature of the relevant gas or vapor.)

Inlet gasses (20) from an upstream SVE system enter the flameless oxidizer (10) through inlet (22) . While shown in Fig. 1 entering through separate inlet (22) , inlet gasses (20) could enter through the same inlet as that used for preheater (30) , thereby eliminating the need for a separate inlet (22) . In addition, depending upon process conditions, and as needed 5 to provide sufficient heat values so as to maintain a self- sufficient operating environment within the flameless oxidizer, additional air and/or natural gas or other fuel may be added to this inlet stream (20) . There will typically, but not necessarily, be a plenum (24) , preferably made of a

10 heat-resistant material such as a perforated plate, at the bottom of the matrix bed (14) to prevent the heat resistant material (16) from entering the piping below the matrix bed.

In the normal flow pattern, where the oxidizer input stream (20) enters the flameless oxidizer (10) near the bottom,

15. this plenum (24) will also act to evenly distribute incoming gasses and further mix these gasses prior to entering the matrix bed (14) . Nevertheless, while Fig. 1 indicates that the input stream (20) enters the flameless oxidizer (10) at the bottom and that the gaseous products (26) exit at the top, and

20 this is the preferred embodiment, the present invention can be operated in an alternate configuration wherein the gasses enter at the top and exit at the bottom.

Within the reactor vessel (10) , during normal processing, the fume stream (20) first enters the mixing zone

25 (16), which is at ambient temperature. Upon entering the mixing zone (16) , and thereafter the reaction zone (18) , the inlet gasses will be raised to oxidation temperatures of 1400- 3500°F (760-1925°C) , and preferably 1550-1800°F (845-980°C) . The emissions are then maintained at these temperatures for a

30 sufficient residence time to ensure substantially complete destruction. In normal operation, it is contemplated that this residence time will be less than 2.0 seconds, and preferably less than 0.2 seconds.

After undergoing intimate mixing in the matrix

35 interstices of the mixing zone (16) , the reactant mixture enters the reaction zone (18) where oxidation and heat release occur. As the gasses heat up, they expand, and this expansion is preferably accommodated by an increase in matrix void volume in reaction zone (18) , such as through the use of ceramic saddles within the reaction zone versus ceramic balls within the mixing zone. The result of this heating is the creation of a flameless oxidation zone within the matrix bed (14) whereby the VOC's are ignited and oxidized to stable products, such as water and carbon dioxide. The oxidation zone is observed as a steep increase in bed temperature from ambient temperature on the inlet side of the zone to approximately the adiabatic oxidation temperature of the mixture on the outlet side of the zone. This rapid change takes place over a distance of usually several inches in a typical oxidizer, with the actual distance being dependent upon feed concentrations, feed rates, gas velocity distribution, bed material, and bed physical properties, type of specific feed materials, etc. Heat losses in the direction of flow also will have an effect on the length of the oxidation zone. The rapidity of the change allows for use of a very compact reactor. The temperature of the oxidation is dependent upon feed concentrations, feed rates, gas velocity distribution, bed physical properties, type of specific feed materials, heat losses, heat input from the heaters, etc.

By decoupling the mixing from the oxidation, one of three critical parameters (turbulence, the others being time and temperature) is removed from the design equation. Accomplishing the mixing prior to the reaction achieves two beneficial results. First, thorough mixing of the fume and air is ensured, negating the possibility of poorly mixed parcels leaving the system unreacted. Second, the uniformity of the reactant stream also helps to establish the uniformity of the reaction zone. Together, these factors allow the processing rate to be turned up or down, without regard to fluid mechanics constraints, over a much wider range. After thorough destruction in the flameless oxidizer

(10) , the product gasses (26) then leave the reactor through port (28) to any needed post-treatment devices (e.g., an acid gas scrubber) or to the atmosphere, as will be further discussed below.

Thus, the basics of the preferred embodiments of the flameless oxidizer of the present invention have been disclosed. Many variations on, and additions to, these basic embodiments are also possible.

The existence of a uniform, stationary, intramatrix reaction zone perpendicular to the flow axis is the fundamental condition of this flameless oxidation process. In the zone, the reactant gasses are efficiently preheated up to the oxidation temperature by the hot matrix surface, whereupon they are oxidized exothermally. They quickly release their heat back to the matrix, to maintain its local temperature. The unique heat transfer properties of the matrix bed (14) are what allows this stable reaction to occur at organic concentrations well below the lower flammability limit of the constituents.

The reaction zone covers the entire flow section of the flameless oxidizer (10) , ensuring that all reactants pass through this highly reactive region. The presence of a large pool of active radicals (H, OH, etc.) in this domain allows the oxidation reactions to occur at rates up to two orders of magnitude faster than the simple thermal decomposition reactions that occur in the post-flame region of a conventional incinerator or thermal oxidizer. Since the inventive process takes advantage of the active radical chemistry (e.g., C_mH_n + O = C_^H_n.-_L + OH) that is characteristic of combustion chain reactions, the reaction time required to destroy the vast majority of organic molecules is less than 0.1 seconds. This runs counter to the conventional incineration process with the majority of organic molecules being destroyed in the "post- flame" region, where the population of active radicals is low, and slower thermal decomposition reactions (e.g., C_mH_n + M = -H.,._! + H + M) govern the chemistry.

These exceptionally fast kinetics eliminate the need for additional residence time, because the reactions proceed to completeness in tens of milliseconds. Therefore, in order to assure high destruction efficiencies, the appropriate constraint in such a flameless oxidizer is design capacity flow rate, rather than residence time. Because maximum flow is determined by device geometry and reaction zone properties, this constraint is device dependent, and not generic, as is residence time for flame-based technologies.

The flameless technology is extremely effective at destroying chlorinated organic compounds. Chlorinated compounds are difficult to destroy by flames because of their narrow flammability range. The present methods, however, effectively convert the chlorine to HCl that is easily removed in a scrubber following the oxidizer.

Furthermore, the existence of a uniform reaction zone also minimizes the formation of PICs, which are most commonly formed in the post-flame region of an incinerator, where the organic fragments are more likely to combine with each other than they would if the radical population was higher.

The uniform reaction zone also eliminates the regions of very high temperatures and the step temperature gradients that exist in a flamed device. The present invention's ability to control the maximum reaction temperature to be equivalent to the averagre reaction temperature, virtually eliminates the formation of thermal N0_X and CO. In a typical system according to the present invention, the DRE of the organic vapors has been shown to be greater than 99.99%. Because the present invention typically operates at temperatures (1550-1850°F) significantly below those present in standard combustors (about 3500°F) , there is less production of the undesirable NO_x by¬ products. Typical NO_x concentrations in the outlet stream are less than 2 ppmv and CO is generally undetectable. Extensive testing of this technology has been undertaken in determining the DRE attainable in the treatment of various hydrocarbons and halogenated hydrocarbons. These test results are summarized in Table 1. Table 1

Summary of Test Conditions and Results -^■ Volatile

As a totally flameless system, the technical challenges and stigma associated with incineration or other flame-based destruction technologies are avoided. The system's flameless nature will ease the permitting process as well as acceptance by the general public.

The flameless oxidation process itself is inherently energy efficient. Such a system also enhances energy efficient operation of the entire system of the present invention by utilizing the heat generated through oxidizing the VOC's to generate steam that can be used in a steam injection enhancement of the standard SVE process. If the fume contains sufficient organics (enthalpy content approximately 30 BTU/scf or more) , the reaction can be self-sustaining, and no supplementary fuel or heat is required (although sizing considerations may be a factor when comparing the amount of heat needed for sufficient steam generation -- a topic discussed further below) . This behavior is contrary to the operation of a flame-based oxidizer, where the main flame is fueled exclusively by a clean, stable fuel source such as natural gas, regardless of the fume enthalpy content.

Indeed, if the recuperative techniques within the flameless oxidizer, such as those set forth in U.S. Patent No. 5,320,518 (Stilger et al . ) , which has been incorporated herein by reference, are used, it is possible to establish a self- sustaining reaction with a stream having an enthalpy content as low as 10 BTU/scf.

The process is typically controlled by simple temperature control. Temperature elements (32) as shown in Fig. 1, can be connected to a programmable control system (not shown) to regulate the flow of supplementary fuel or air in the respective cases of lean or rich fume streams.

The flameless oxidizer reactor vessel is normally insulated for personnel safety and heat retention. Depending on unit size, the matrix can retain heat for 24 hours or more, which helps to reduce operating costs. The matrix also acts as a heat sink, to buffer any possible fluctuations in fume flow, concentration, and composition. During the delay period after a spike or step change in flow or concentration begins to affect the matrix temperature, the supervisory control system is able to take the appropriate corrective action (adding supplementary fuel or air) to maintain temperature.

The heat capacity and geometry of the matrix also provide an important safety benefit -- an inherent flame arresting capability. In the event that a flammable mixture enters the reactor, the cold (mixing) region (16) of the matrix bed (14) would prevent the backward propagation of a flame upstream. The heat capacity of a unit volume of matrix is typically two or three orders of magnitude greater than the maximum exothermicity in an equivalent volume of flammable gas.

Furthermore, the matrix interstices provide both the high quench surface area and tortuous pathways for flow interruption that are intrinsic to commercial flame arrestors. By using only inert ceramics, the matrix is not subject to poisoning or thermal deactivation, as are catalytic materials. Also, the high initial and replacement cost of noble metal coated packings is avoided. Alternatively, whil the present invention contemplates the use of heat resistan bed materials without catalysts, a combined inert bed an catalyst may be used to enhance process characteristics such a 5 reaction rate, if so desired. Catalyst could be impregnate onto the heat resistant materials to alter the oxidatio properties. Use of a catalyst may allow for the use of lowe operating temperatures.

The types of materials in the matrix bed (14) may b

10 varied so that the inner body heat transfer characteristics, the radiative characteristics, the forced convectiv characteristics, and the inner matrix solids thermall conductive characteristics may be controlled within the bed. This may be done by varying the radiative heat transfe

15_. characteristics of the matrix bed (14) by using different sizes of heat resistant materials (16, 18) to change the mean free radiative path or varying the emissivity of these materials, varying the forced convection heat transfer characteristics of the matrix bed (14) by varying its surface area per unit

20 volume, or geometry, varying the thermally conductive heat transfer characteristics of the matrix bed (14) by using heat resistant materials (16, 18) with different thermal conductivities, or changing the point to point surface contact area of the materials in the bed. These properties may be

25 varied either concurrently or discretely to achieve a desire effect.

In addition to changing the properties of the matri bed (14) itself, an interface, or several interfaces, can be introduced into the bed where one or more of the heat transfer

30 properties of the bed are discretely or concurrently changed on either side of the interface and wherein this variation serves to help stabilize the reaction zone in that location and acts as an "oxidation zone anchor." This may be done, for example, by introducing an interface where void fractions change across

35 the interface within the matrix bed (14) , such as is represented in Fig. 1 by mixing zone (16) and reaction zone (18) . The interface may change the mean free radiative path across the interface independent of the void fraction. By changing heat resistant materials, the emissivity may change across the interface within the matrix bed. Changing the area per unit volume of the heat resistant materials across an interface, the forced convective heat transfer characteristics may change as the gas is passed across the interface.

The matrix bed cross-section perpendicular to the flow axis may be configured in a circular, square, rectangular, or other geometry. The area of the cross-section may be intentionally varied (i.e., as a truncated cone or truncated pyramid) to achieve a wide, stable range of reactant volumetric flow rates at each given matrix burning velocity.

Turning now to the integration of this flameless oxidizer technology within an overall system of soil vapor extraction and volatiles destruction, different embodiments employing the same fundamental concepts are shown in Figs. 2 and 3.

The equipment typically required for enhanced SVE is consists of a matrix of screened injection/sparging wells (34) and extraction wells (36) spaced between 50 and 75 feet apart in a grid pattern across the contaminated zone. Extraction wells (36) are connected to a vacuum pumping system (38) to remove the contaminated fume from the soil.

The collected contaminated fume (40) is routed through a knockout vessel (42) . After the organic contaminants are passed through the knockout vessel (42) , a gas stream containing the VOC's (44) is pumped via the pumping system (38) to the flameless oxidizer (10) . Additional air (46) is introduced upstream of the pumping system (38) , if needed, to control the amount of vacuum at the wells (36) .

Although only shown in the embodiment of Figure 2, either embodiment could also utilize a stripper column (43) to reclaim any VOC's that had been condensed in the knockout vessel (42) . Such a stripper column (43) would be fed the liquid drain stream (41) which would pass through the stripper (43) countercurrently to an air stream (45) . The air stream (45) would re-volatilize the contaminants into stream (46) which is then combined with the volatile stream (44) from the knockout vessel (42) . Clean water would exit the stripper (43) in stream (47) .

As discussed above, supplemental fuel or air (48) can be added to the inlet stream before entering the flameless oxidizer (10) in combined stream (20) . Optionally, a flame arrestor (not shown) can be located just upstream of the flameless oxidizer (10) .

Prior to being vented to the atmosphere or being re- used within the system, the gaseous products from the flameless oxidizer (the off-gas) (26) may be fed through additional gas cleaning systems as needed. These may include a gas scrubber (50) in the case of chlorinated or sulfonated contaminants. Such a scrubber (50) may be provided with caustic in stream (52) and results in the production of salt (54) that may be easily disposed of and an exhaust (56) that may be vented to the atmosphere. In certain situations, such additional processing will be unnecessary.

In the embodiment of Figure 2, the hot exhaust gas (26) from the flameless oxidizer (10) , usually at 600-1200°F, is then re-injected into the sparging wells (34) , either directly in stream (58) or after scrubbing in stream (60) , to provide for a more complete recovery operation. It should be noted that additional opportunities also exist to optimize the separation process utilizing techniques such as horizontal sparging and extraction wells.

The exhaust gas (26) can be used for air sparging because essentially all of the VOC's have been destroyed. Furthermore, to the extent that the exhaust gas (26) contains HCl, such gas might be beneficial to the soil.

An alternative embodiment is shown in Fig. 3. While similar to the design of Fig. 2 in many respects, this embodiment utilizes the off-gas (26) to generate steam. In this embodiment, boiler feed water stream (62) is passed through waste heat boiler (64) . Off-gas stream (26) is passed through the opposite side of the waste heat boiler (64) wherein heat is transferred to the boiler feed water stream (62) , generating a steam stream (66) that can then be injected into well (34) . Cooled off-gas stream (68) can then be vented or fed to additional gas cleaning equipment as in the embodiment of Figure 2. In addition, the off-gas stream, whether directly from the waste heat boiler in stream (68) or after additional gas cleaning in stream (56) , can be passed through an air-air heat exchanger (70) . Such a stream would transfer heat to incoming air stream (72) prior to such air being inlet to the system in stream (46) . The further cooled exhaust (74) can then be vented. Such a variation will minimize condensation within, and liquid effluent from, the knockout drum (42) .

In either of these embodiments, recuperative techniques within the flameless oxidizer, such as those set forth in U.S. Patent No. 5,320,518 (Stilger et al . ) , which has been incorporated herein by reference, can be used. It is believed, however, that, when the embodiment of Figure 3 is utilized, a straight through flameless oxidizer should be used in order to obtain the hottest possible off-gas (26) so that the thermal driving force into the waste heat boiler (64) is high and residual heat loss out of the ultimate exhaust is low.

Remediation utilizing the embodiment of Figure 3 would begin with pumping of the extraction wells (36) to depress the water table in the center of the pattern, followed by steam injection through injection wells (34) at the desired pressure. Injection pressure is controlled by depth, and would be lower in shallow applications. As steam is forced into the formation, the earth is heated to the boiling point of water and the advancing pressure front displaces ground water toward the extraction well (36) . Near the steam-condensate front, organics are distilled into the vapor phase, transported to the steam condensation front, and condensed there.

The system may include a submersible liquid pump (not shown) to facilitate removal of all steam condensate to an oil water separator that recycles the treated condensate as a supplement to the boiler feed water supply. VOC-contaminated steam vapor (40) is routed through a condenser/knockout drum (42) upstream of the vacuum blower (38) , typically controllin the well vacuum at negative 1.5-7.5 psig depending on th design conditions. Because the extraction fluid (40) is characteristically two-phase, some steam will condense in the knockout drum (42) and will be drained in stream (41) .

The off-gas (26) from the oxidation unit (10) is discharged directly to a fire tube type waste heat recover unit (64) producing 10-100 psig steam for injection into the soil substrate. After the injected steam breaks through to the extraction wells (36) , vacuum extraction becomes the primar removal mechanism, although steam is continuously injecte until the contaminant concentrations approach the cleanup goals. At that point, steam injection is stopped while the vapor extraction system continues to operate. The continued SVE operation will result in the further vaporization of the residual contaminants in the pore spaces and the drying out of the treated soils with heated air as a drop in the steam zone pressure slightly reduces the boiling point of any residual water or contaminants forcing them to boil and convert to removable vapor.

Example As a particular working example of the inventive embodiment of Figure 3, 5500 lb/hr of steam at 100 psig and 338°F is fed to an injection well (34) . At steady state, 1000 scfm of saturated steam containing 5000 ppm of trichloroethylene ("TCE") is removed from extraction well (36) at approximately 180°F, an amount equalling about 2750 lb/hr. Note that the relatively low temperature of the extracted steam is because the vacuum allows boiling at such lower temperatures. Note, too, that the flow of steam entering the ground is greater than the flow of steam leaving the ground due to some loss of water underground. This stream is fed to the knockout drum (42) where approximately 1380 lb/hr of liquid water is removed. Ambient air at a rate of 3275 scfm is fed through heat exchanger (70) where it is heated to about 220°F and is then combined with the vapor stream (44) from the knockout drum (42) . Thus, the stream leaving the vacuum blower (38) contains

3275 scfm of air, 495 scfm of water, and 5 scfm of TCE.

Approximately 165 scfm of methane is added in stream (48) so that the total inlet stream (20) to the flameless oxidizer (10) is 3940 scfm.

The off-gas (26) from the flameless oxidizer (10) will be made up of air, C0₂, water, and HCl, and will normally be at a temperature of approximately 1800°F. To produce the required amount of steam, 5500 lb/hr of ambient water is fed to the waste heat boiler (64) where it is converted to the 338°F, 100 psig steam that is injected in well (34) . After passing through the waste heat boiler (64) the off-gas temperature is reduced to approximately 340°F.

Approximately 435 lb/hr of NaHC0₃ is added at (52) to this off-gas stream prior to entering the dry gas scrubber (50) . The scrubber (50) produces 400 lb/hr of salt (NaCl) (54) and 1700 acfm of gas to be passed through heat exchanger (70) , wherein it gives up heat to the incoming air supply (72) before being vented. It should be noted that the additional methane that is added in stream (48) is needed in order to produce the required amount of steam in this embodiment. In addition, the flameless oxidizer (10) will have to have been designed to be larger than is necessary simply to destroy the given amount of TCE. However, using the low-NO_x fuel oxidation that occurs in the flameless oxidizer to run the waste heat boiler (64) is extremely beneficial in NO_x non-attainment areas such as California.

Each of these configurations offers at least one major advantage. The treatment process is not classified as an incinerator, which greatly facilitates permitting. It can be shown that the integrated processing system is scalable to an economical throughput capacity with system performance and operational reliability exceeding that of an incineration system at lower unit operating costs. Further, the utilization of waste heat to assist in volatilizing the organic contaminants provides energy efficiency and can reduce operating costs, depending upon the VOC system involved. In summary, apparatuses and methods for extracting and thereafter destroying hazardous organics from in situ soil using a flameless oxidation system have been described. The oxidation temperature and residence times in the present oxidizer are lower than those of a conventional incinerator, thereby providing a high conversion of reactants to products with a minimum of unwanted by-products such as N0_X. Use of such a flameless oxidation process within a soil vapor extraction system, particularly one that utilizes steam injection or hot air sparging, leads to efficiencies of cost and energy.

The present invention has been described in terms of several preferred embodiments. However, the invention is not limited to the embodiments depicted and described, but can have many variations within the spirit of the invention. For example, while dry gas scrubbers are shown and described, it would be obvious to those of skill in the art that other standard gas cleaning systems can be utilized within the framework of the present invention. As another example, the dry gas scrubber could be located upstream of the waste heat boiler. This would allow for the use of lower specification boiler tubes in the waste heat boiler.

Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but rather by the appended claims and their legal equivalents. Having thus described the invention, what is desired to be protected by Letters Patent is presented by the following appended claims.

Claims

What is claimed is:

1. A method for removing volatile contaminants from soil in si tu comprising the steps of:

(a) extracting volatile contaminants from the in situ soil using one or more extraction wells coupled to a vacuum pump, whereby such volatile contaminants are volatilized into a process gas stream;

(b) heating at least a portion of a matrix bed of heat resistant material within a flameless oxidizer to a temperature above the autoignition temperature of the volatilized contaminants; and

(c) feeding the process gas stream through the matrix bed, whereby the volatilized contaminants are oxidized into gaseous products in a flameless reaction zone.

2. The method of claim 1 further comprising the step of injecting at least a portion of the oxidized gaseous products into the in si tu soil using an air sparging injection well to assist in driving the volatile contaminants towards the extraction wells.

3. The method of claim 1 wherein at least a portion of the oxidized gaseous products is used to generate steam in a heat exchanger.

4. The method of claim 3 wherein the generated steam is injected into the in si tu soil to assist in driving the volatile contaminants towards the extraction wells.

_*

5. The method of claim 1 wherein the flow of the process gas stream through the matrix bed is established so that the heat from the reaction zone is used to preheat the volatilized contaminants as they enter the matrix bed.

6. The method of claim 1 comprising the further steps of monitoring the temperature of the matrix bed and controlling the position of the reaction zone within the matrix bed in response thereto.

7. The method of claim 1 comprising the further step of admixing air, oxygen, supplemental fuel, or both with the process gases prior to feeding the process gases to the matrix bed.

8. The method of claim 7 wherein at least a portion of the oxidized gaseous products is used to generate steam in a heat exchanger that is then injected into the in situ soil to assist in driving the volatile contaminants towards the extraction wells, and wherein the amount of air, oxygen, and supplemental fuel added are established such that the oxidized gaseous products contain sufficient heat to generate the full steam requirements needed for injection.

9. The method of claim 6 wherein the step of controlling the position of the reaction zone within the matrix bed is achieved by supplying controlled volumes of air, fuel, or oxygen to the matrix bed in addition to the volatilized contaminants.

10. The method of claim 1 further comprising the step of treating the oxidized gaseous products in a scrubber prior to venting such gaseous products to the atmosphere.

11. The method of claim 1 wherein at least a portion of the oxidized gaseous products is used to preheat an air stream that is combined with the process gas stream prior to its entry into the flameless oxidizer.

12. The method of claim 1 wherein the matrix bed temperature is maintained between about 1400°F (760°C) and about 3500°F (1925°C) in the reaction zone.

13. The method of claim 1 wherein the process gas stream includes one or more hydrocarbons selected from the group consisting of oxygenated hydrocarbons, halogenated compounds, aminated compounds, and sulphur-containing compounds.

14. The method of claim 1 wherein the oxidized gasses have a N0_X content less than about 2 parts per million by volume and a carbon monoxide content less than about 10 parts per million by volume, on a dry basis, adjusted to 3% oxygen.

15. The method of claim 1 wherein the heat resistant material is chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic raschig rings.

16. The method of claim 1 wherein the matrix bed comprises at least two layers of heat resistant material wherein the layers are comprised of differently sized heat resistant material and the process gas stream passes through the layer of smaller sized materials first.

17. The method of claim 1 further comprising the step of filtering out solids or liquids from the process gas stream prior to feeding the process gas stream through the matrix bed.

18. The method of claim 1 wherein the heat resistant material of the matrix bed comprises a catalyst.

19. The method of claim 1 wherein a destruction and removal efficiency of the volatilized contaminants of at least 99.99% is achieved.

20. In a method for remediating soil in situ comprising the steps of extracting volatile contaminants using one or more extraction wells coupled to a vacuum pump, whereby such volatile contaminants are volatilized into a process gas stream and are thereafter treated to remove or destroy the volatilized contaminants, the improvement comprising:

(a) heating at least a portion of a matrix bed of heat resistant material within a flameless oxidizer above the autoignition temperature of the volatilized contaminants; and

(b) feeding the process gas stream through the matrix bed, whereby the volatilized contaminants are oxidized into gaseous products in a flameless reaction zone.

21. The method of claim 20 further comprising the step of injecting at least a portion of the oxidized gaseous products into the in situ soil using an air sparging injection well to assist in driving the volatile contaminants towards the extraction wells.

22. The method of claim 20 wherein at least a portion of the oxidized gaseous products is used to generate steam in a heat exchanger.

23. The method of claim 22 wherein the generated steam is injected into the in situ soil to assist in driving the volatile contaminants towards the extraction wells.

24. The method of claim 20 wherein the flow of the process gas stream through the matrix bed is established so that the heat from the reaction zone is used to preheat the volatilized contaminants as they enter the matrix bed.

25. The method of claim 20 comprising the further steps of monitoring the temperature of the matrix bed and controlling the position of the reaction zone within the matrix bed in response thereto.

26. The method of claim 20 comprising the further step of admixing air, oxygen, supplemental fuel, or both with the process gases prior to feeding the process gases to the matrix bed.

27. The method of claim 25 wherein the step of controlling the position of the reaction zone within the matrix bed is achieved by supplying controlled volumes of air, fuel, or oxygen to the matrix bed in addition to the volatilized contaminants.

28. The method of claim 20 wherein at least a portion of the oxidized gaseous products is used to preheat an air stream that is combined with the process gas stream prior to its entry into the flameless oxidizer.

29. The method of claim 20 wherein the matrix bed temperature is maintained between about 1400°F (760°C) and about 3500°F (1925°C) in the reaction zone.

30. The method of claim 20 wherein the oxidized gasses have a N0_X content less than about 2 parts per million by volume and a carbon monoxide content less than about 10 parts per million by volume, on a dry basis, adjusted to 3% oxygen.

31. The method of claim 1 wherein the matrix bed comprises at least two layers of heat resistant material wherein the layers are comprised of differently sized heat resistant material and the process gas stream passes through the layer of smaller sized materials first.

32. A method for removing volatile contaminants from soil in situ comprising the steps of: (a) extractingvolatile contaminants from the in situ soil using one or more extraction wells coupled to a vacuum pump, whereby such volatile contaminants are volatilized into a process gas stream;

(b) heating at least a portion of a matrix bed of heat resistant material chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic raschig rings within a flameless oxidizer to a temperature above the autoignition temperature of the volatilized contaminants;

(c) feeding the process gas stream through the matrix bed, whereby the volatilized contaminants are oxidized into gaseous products in a flameless reaction zone;

(d) monitoring the temperature of the matrix bed and controlling the position of the reaction zone within the matrix bed in response thereto by supplying controlled volumes of air, fuel, or oxygen to the matrix bed in addition to the volatilized contaminants;

(e) utilizing at least a portion of the oxidized gaseous products to generate steam in a heat exchanger;

(f) injecting the generated steam into the in situ soil to assist in driving the volatile contaminants towards the extraction wells; and

(g) utilizing at least a portion of the oxidized gaseous products to preheat an air stream that is combined with the process gas stream prior to its entry into the flameless oxidizer.

33. An apparatus for removing volatile contaminants from soil in situ comprising:

(a) one or more extraction wells comprising a vacuum pump having an inlet and an outlet capable of creating a gaseous flow; (b) a flameless oxidizer having:

(i) an inlet in flow communication with the outlet of the vacuum pump;

(ii) an outlet for reaction gaseous products; and

(iii) a section located between the inlet and the outlet of the flameless oxidizer including a matrix bed of heat resistant material; and

(c) a heater for heating at least a portion of the section including a matrix bed of heat resistant material to a temperature exceeding the decomposition temperature of_^ the volatilized contaminants.

34. The apparatus of claim 33 further comprising one or more injection wells in flow communication with the outlet of the flameless oxidizer.

35. The apparatus of claim 33 further comprising: a heat exchanger having a cool side with an inlet and an outlet and a hot side with an inlet and an outlet wherein the outlet from the cool side of the heat exchanger is in flow communication one or more injection wells and wherein the inlet to the hot side of the heat exchanger is in flow communication with the outlet for reaction gaseous products from the flameless oxidizer.

36. The apparatus of claim 33 further comprising: a heat exchanger having a cool side with an inlet and an outlet and a hot side with an inlet and an outlet wherein the outlet from the cool side of the heat exchanger is in flow communication inlet of the flameless oxidizer and wherein the inlet to the hot side of the heat exchanger is in flow communication with the outlet for reaction gaseous products from the flameless oxidizer.

37. The apparatus of claim 33 wherein the section of the flameless oxidizer is constructed so that it can thermally destroy volatilized contaminants without use or creation of a flame.

38. The apparatus of claim 33 further comprising one or more temperature sensors for sensing the temperature of the matrix bed.

39. The apparatus of claim 33 further comprising means for controllably adding air, oxygen, supplemental fuel, or both to any gas flow created by the vacuum pump prior to the entry of such gas stream into the flameless oxidizer.

40. The apparatus of claim 33 further comprising scrubber in flow communication with the outlet of the flameles oxidizer.

41. The apparatus of claim 33 wherein the hea resistant material is chosen from the group consisting o ceramic balls, ceramic saddles, ceramic pall rings, or cerami raschig rings.

42. The apparatus of claim 33 wherein the matrix be comprises at least two layers of heat resistant materia wherein the layers are comprised of differently sized hea resistant material and wherein the section of the flameles oxidizer is configured to create a flow pattern from the inle to the outlet that causes any flow to pass through the layer o smaller sized materials first.

43. The apparatus of claim 33 wherein the hea resistant material of the matrix bed comprises a catalyst.

44. The apparatus of claim 33 wherein the matrix be has a void fraction from 0.3 to 0.9.

45. The apparatus of claim 33 wherein the materia in the matrix bed has a specific surface area from 40 m²/m³ t

1040 m²/m³.

46. The apparatus of claim 33 wherein the one or mor extraction wells further comprise one or more filters fo filtering out solids or liquids in flow communication with th vacuum pump and the flameless oxidizer.

47. An apparatus for removing volatile contaminant from soil in situ comprising:

(a) one or more extraction wells comprising a vacuu pump having an inlet and an outlet capable of creating gaseous flow; (b) a flameless oxidizer having:

(i) an inlet in flow communication with the outlet of the vacuum pump;

(ii) an outlet for reaction gaseous products; (iii) a section located between the inlet and the outlet of the flameless oxidizer including a matrix bed of heat resistant material chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic raschig rings and constructed so that it can thermally destroy volatilized contaminants without use or creation of a flame; and

(iv) one or more temperature sensors for sensing the temperature of the matrix bed; (c) a heater for heating at least a portion of the section including a matrix bed of heat resistant material to a temperature exceeding the decomposition temperature of the volatilized contaminants;

(d) means for controllably adding air, oxygen, supplemental fuel, or both to any gas flow created by the vacuum pump prior to the entry of such gas stream into the flameless oxidizer;

(d) a heat exchanger having a cool side with an inlet and an outlet and a hot side with an inlet and an outlet wherein the outlet from the cool side of the heat exchanger is in flow communication one or more injection wells and wherein the inlet to the hot side of the heat exchanger is in flow communication with the outlet for reaction gaseous products from the flameless oxidizer.