WO1995030453A1

WO1995030453A1 - Method and apparatus for thermal desorption soil remediation

Info

Publication number: WO1995030453A1
Application number: PCT/US1995/004914
Authority: WO
Inventors: Alexander G. Baldwin; James A. Newburn; John T. Schofield
Original assignee: Thermatrix, Inc.
Priority date: 1994-05-06
Filing date: 1995-05-03
Publication date: 1995-11-16
Also published as: IL113593A0; AU2360395A

Abstract

An apparatus (10) and method for treating non-combustible solids that are contaminated with organic compounds whereby the contaminants are removed from the solids using heat treatment under a comparatively inert atmosphere and are thereafter destroyed by flameless oxidation within a porous inert media destruction matrix (14). The resultant heat of oxidation of the organic compounds in the flameless oxidizer (10) can be utilized in the initial step of thermally removing the contaminants from the solids. In addition, the resulting safe gases from the flameless oxidizer (10) can be used as an inert carrier gas in the thermal desorption step. The destruction matrix (14) is composed of inert ceramic materials that enhance process mixing and provide thermal inertia for process stability, with a resultant minimization of NOx oxidation by-products to level below those achievable by conventional technologies.

Description

METHOD AND APPARATUS FOR THERMAL DESORPTION SOIL REMEDIATION

Field of the Invention

The field of the present invention is methods and apparatuses for separating organics from solids and soils and thereafter destroying such organics. In particular, the present invention relates to an apparatus and method for controlled exothermic reaction of organic vapors from thermal desorbers, 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 and sludges contaminated with organic chemicals are a widespread problem throughout the world, with millions of cubic meters requiring remediation in the United States alone. Clean-up of such contaminated materials is subject to a wide variety of regulations in the United States, including those covered under The Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) , The Superfund Amendments and Reauthorization Act of 1986 (SARA) , and The Resource Conservation and Recovery Act (RCRA) . The total cost of these clean-up efforts has been estimated to exceed $200 billion over the next 30 to 40 years.

A number of processes can be used to deal with these problems of contaminated soils and sludges. One such overall technique is thermal remediation. Thermal remediation treatment, however, can be further split into two general categories: (1) incineration; and (2) thermal desorption or recovery.

Processes such as thermal desorption and in-situ soil vapor extraction (SVE) allow for the controlled separation of organics from solids and soils. In these processes, the organic material is volatilized to produce an organic vapor, which thereafter must be removed or otherwise treated. This is in contrast to direct incineration, which involves heating solid material to destruction temperatures in an oxidizing environment where volatilization and combustion of organics takes place simultaneously.

Thermal desorption has been successfully demonstrated for the treatment of soils and solids contaminated by organic compounds. Treatment of soils contaminated with organic compounds, dioxin, polynuclear aromatic hydrocarbons (PAHs) , polychlorinated biphenyl's (PCBs) , and low level mixed wastes using thermal desorption is known. Recognizing the cost- competitive nature of thermal desorption, many remediation companies have diversified their capabilities to include thermal desorber processes. Others are converting existing fluidized bed incinerators to thermal desorbers emphasizing thermal desorption as the preferred thermal treatment process.

In addition, the dilemma presented by mixed wastes or soils contaminated with both radioactive compounds and organics has long challenged the U.S. Department of Energy and others operating nuclear sites. These soils are difficult to remediate and neither nuclear waste disposal sites nor hazardous waste disposal sites are permitted to accept mixed wastes. Thermal desorption has proven to be an effective method in remediating mixed wastes.

With thermal desorption, the process removes organic contaminants by indirectly heating the soils and solids to temperatures sufficient to vaporize the hazardous components. In these systems, important variables include soil temperature, the time at that temperature, and the particle size being treated. The soil is typically heated to no higher than 550°C, and frequently in the absence of oxygen. The thermal desorber, acting as a separator, removes the organic contamination, leaving a residue that contains inerts, radioactive material (when present) , and metals in the soil. Once the treated soil has been stabilized to prevent any metal salts in the soil from dissolving in water, the stabilized material can be characterized and disposed of or handled as a low-level radioactive waste.

After volatilization, the organic vapors in the off- gas are presently typically treated by either oxidation in a high temperature combustion chamber/incinerator or by condensation and conventional treatment of the small amount of resultant condensate such as by capturing on a carbon substrate. Examples of known systems for thermal desorption that use a following combustion technique include those described in U.S. Patent Nos. 5,282,695 (Crosby et al . ) , 5,228,803 (Crosby et al . ) , 4,974,528 (Barcell) , 4,961,391 (Mak et al . ) , 4,925,389 (DeCicco et al . ) , 4,815,398 (Keating et al.), 4,766,822 (DeCicco et al . ) , and 4,746,290 (DeCicco et al . ) . Examples of known systems for thermal desorption that use a following condensation technique include those described in U.S. Patent Nos. 5,098,481 (Monlux) , and 5,228,803 (Crosby et al . ) .

As compared to direct soil incineration, indirect heating of soils and solids as a means of separating out contaminants offers several advantages. Multiple temperature control zones along the heating chamber are possible. The residence time of the solids can be readily varied. Furthermore, the composition and rate of any purge gas used can be adjusted and controlled. Indirect heating prevents contact between the contaminants, the direct flame, and the combustion products. Because the volume of the off-gasses from an indirect system is much less than in an incinerator, downstream equipment is comparatively small and solids entrainment is minimized. This is one reason why condensation is a viable option for collecting the contaminants for either recovery or further treatment. On the other hand, thermal desorption techniques have their own problems, particularly in their need for further processing after the contaminants are volatilized. Examples of problems that arise when condensation is used for post- volatilization treatment include disposal issues surrounding both the carbon used for adsorption and the recovered liquid organic wastes. As such, the direct destruction of waste organics into benign products such as water, carbon dioxide, and salts is frequently preferable as a final solution. The use of destruction technologies in the processing of volatilized contaminants typically involves the thermo- chemical reformation of the organic compounds into such oxidized products. While this is desirable as a final solution, flame-based destruction process can 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 NO_x are often produced. Nitrous oxide (N₂0) and ammonia (NH₃) are often by-products of NO_x removal techniques . The high temperatures also raise significant safety issues.

The difficulties and expense of obtaining operating permits for hazardous waste treatment processes utilizing flame based technologies, either for direct soil incineration or for incineration of volatilized contaminants, is also well known. Alternatively, the encumbrances of dealing with contaminated carbon wastes or off-site liquid waste disposal inherent with the condensing option increases the costs of that type of system and affects operational factors negatively. Furthermore, the relatively low temperatures associates with thermal separation can be the optimum temperature for converting PAHs and chlorinated phenolics into dibenzo furans and dioxins.

Thus, it can be seen that there is a need for a practical means of removing organics from contaminated soils and solids that avoids the various difficulties and inefficiencies of the prior art. There is a need for a system that has the advantages of thermal desorption while reducing or eliminating the problems arising from the need to condense, incinerate, or otherwise dispose of the volatilized contaminants. There is a further need for such a system to result in high destruction and removal efficiency (DRE) of the organics while handling a broad range of contaminated soils in a cost-effective manner.

Summary of the Invention The present invention is directed to a method and apparatus for treating soils and solids that are contaminated with organic compounds whereby the contaminants are removed from the solids using heat treatment under a comparatively inert atmosphere 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 in the initial step of thermally removing the contaminants from the solids. In one embodiment, the resulting safe gasses from the flameless oxidizer can be used as an inert carrier gas in the thermal desorption step.

Thus, the system and process of the present invention offers operational simplicity, near zero emissions, heat recovery and reuse, and reduced costs. 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 12 ppmV NO_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 the use of a process in which there are no open flames, and in which the mixture of gasses to be introduced into the matrix is relatively cool, outside the flammability limits of the constituents, and, therefore, not explosive under ambient conditions. Problems of flameouts are avoided. Moreover, from a practical viewpoint, .ll of these features should result in the ability to obtain required government permitting more easily.

Accordingly, it is an object of the present invention to provide a method and apparatus capable of meeting existing regulations for the destruction of organic contaminants contained within soils os solids.

It is another object of the present invention to provide a method and apparatus for destruction of organic contaminants contained in solids or soils while minimizing NO_x oxidation by-products to levels below those achievable by conventional technologies.

Other and further objects and advantages will appear hereinafter.

Brief Description of the Drawings

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

Fig. 2 is a flow diagram detailing one embodiment of the apparatus of the present invention using an indirectly heated desorber unit and recycling heat from the oxidizer unit to the desorber.

Fig. 3 is a flow diagram detailing another embodiment of the apparatus of the present invention using a direct fired desorber unit. Fig. 4 is a flow diagram detailing a further embodiment of the apparatus of the present invention using an indirect fired desorber unit and recycling heat from the oxidizer unit to the desorber through direct contacting of the oxidizer off-gasses with the solids or soils to be remediated.

Detailed Descrir>tion of the Preferred Embodiments

It has now been discovered that a combination of successfully demonstrated thermal desorber technologies with an innovative high performance flameless oxidation process results in an integrated, closed loop waste processing unit offering operational simplicity, near zero emissions, and reduced costs. The proposed integrated waste processing system is designed to operate at reduced temperatures, utilizing the hot, inert off- gas (void of products of incomplete combustion (PICs) ) from the flameless oxidizer to heat the contaminated feed, 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 t3chnology. In addition, U.S. Patent No. 5,165,884 (Martin et al . ) , along with presently pending application Serial No. 07/945,218 (Martin et al . ) , filed September 15, 1992, now allowed and issue fee paid November 26, 1993, discuss in significant detail the technology involved in a flameless oxidizer. The issued Martin et al . patent, the Martin et al . application, and the Galloway and Kroneberger et al . patents are all hereby 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 (HCl, 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 their respective flammability limits. The three primary attributes of the destruction matrix that permit flameless oxidation are its interstitial geometry (which enhances mixing) , its thermal inertia (which promotes stability) , and its surface characteristics (which augment heat transfer) . The thermal properties of the matrix allow the mixing zone to be near ambient temperature where the fume enters while the reaction zone, further downstream, is at the appropriate oxidation temperature. These attributes lead to several performance- and safety-related advantages in practical applications. Among these are the ability to establish a stationary reaction zone

(wherein the rate of fume oxidation is much faster than in the post-flame region of an incinerator) ; the ability to accommodate rapid process fluctuations (as with batch chemical reactor discharges) ; the capability for wide process turndown

(for cost effective adaptation to changing conditions) ,* the suppression of flashback (by virtue of the matrix's high surface area and heat absorption capability) ; and a high level of manageability and control (compared to a flame) .

Turning in detail to the drawings, where like numbers designate like components, Fig. 1 illustrates an embodiment of one such flameless oxidizer as might be used in the process and apparatus of this invention. Typically, the flameless oxidizer (10) will consist of a suitable matrix bed containment shell (12) that is filled with a quantity of heat resistant material creating a matrix bed (14) . The types of matrix materials used should have high heat conductance by radiation, convection, and conduction. The heat transfer properties of the system are dependent on the ratio of radiative to convective heat- transfer. The matrix bed (14) may be sized for any desired flow stream by altering the matrix flow cross-section, height, material, void fraction, outlet temperature, and supplemental heat addition, if desired. Preferred matrix materials are ceramic balls or saddles, but other bed materials and 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 thermal desorption process, 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 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 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, 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 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 (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 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 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 emissions compounds 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 + 0 = C-.H-.._! + 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 = C_n-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 method, however, effectively converts 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 average reaction temperature, virtually eliminates the formation of thermal NO_x and CO. The oxidation of the organic vapors is more complete than flame combustion. According to the present invention, the organic vapors are also heated by heat generated by the reaction, further increasing the completeness of the reaction. 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 Organic Compound Destruction

Compounds Lowest Inlet Highest Inlet Minimum %

Concen. Concen. DRE*

(ppmv) (ppmv)

Benzene 1,719 8,406 99.99

Carbon 0.67% 1.15% 99.99 Tetrachloride weight weight

Dichloro- 5,000 18,000 99.99 methane

Isopropanol 400 600 99.99

Methyl 10,000 30,000 99.99 Chloride

Monomethy- 16,000 31,000 99.99 lamine

Paint Solvent 3.87 mg/liter 5.32 mg/liter 99.99 Mixture

*Note: Detection Level Limited

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 wastes to volatilize the organics compounds in the thermal desorber. If the fume contains sufficient organics (enthalpy content greater than 30 BTU/scf) , the reaction can be self-sustaining, and no supplementary fuel or heat is required. This behavior is contrary to the operation of a flame-based oxidizer, where the ain flame is fueled exclusively by a clean, stable fuel source such as natural gas, regardless of the fume enthalpy content. The ability to operate without a separate fuel source represents a substantial energy savings for applications with non-lean fumes.

Indeed, if the recuperative techniques within the flameless oxidizer, such as those set forth patent application Serial No. 07/945,218 (Martin et al . ) , filed September 15, 1992, now allowed and issue fee paid November 26, 1993, 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 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, while the present invention contemplates the use of heat resistant bed materials without catalysts, a combined inert bed and catalyst may be used to enhance process characteristics such as reaction rate, if so desired. Catalyst could be impregnated onto the heat resistant materials to alter the oxidation properties. Use of a catalyst may allow for the use of lower operating temperatures.

The types of materials in the matrix bed (14) may be varied so that the inner body heat transfer characteristics, the radiative characteristics, the forced convective characteristics, and the inner matrix solids thermally conductive characteristics may be controlled within the bed. This may be done by varying the radiative heat transfer characteristics of the matr?.x 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 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 varied either concurrently or discretely to achieve a desired effect. In addition to changing the properties of the matrix bed (14) itself, an interface, or several interfaces, can be introduced into the bed where one or more of the heat transfer 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 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 thermal desorption and volatiles destruction, different embodiments

■ employing the same fundamental concepts are shown in Figs. 2-4.

In each embodiment, waste feed material (34) , either solid or sludge, is fed into a rotary thermal desorber (36) with solid feed equipment selected to handle widely variable feed stocks and soil types. While rotary thermal desorbers are shown and described, it would be obvious to those of skill in the art that other standard thermal desorption systems can be utilized within the framework of the present invention. After the organic contaminants are volatilized within the thermal desorber (36) , they are fed in stream (37) through one or more solids filters and/or knock-out pots (38, 40) , such as primary cyclone filters and/or a baghouse, in order to remove any entrained solids . While this is not a requirement of the present inventive technique, it is preferable. Thereafter, the volatilized contaminants are delivered to the flameless oxidizer (10) , as described above, in inlet stream (20) . As discussed above, supplemental fuel or air can be either premixed with the inlet stream in stream (48) before entering the flameless oxidizer (10) , as shown in Figs. 3 and 4, or otherwise added to the flameless oxidizer (10) in a stream (50), as shown in Fig. 2. Optionally, a flame arrestor (not shown) can be located just upstream of the flameless oxidizer (10) .

The solids ash (42) left behind when the volatiles are removed in the desorber (36) , is typically removed and combined with the solids (44) collected in the filters (38, 40) and is thereafter subjected to normal treated solids handling (46) and disposal (47) as would be understood by those in the art. In many instances, this soil can be returned directly to the original site.

Prior to being vented to the atmosphere, the gaseous products from the flameless oxidizer (the off-gas) (26) may be fed through additional gas cleaning systems (52) as needed. These may include scrubbers in the case of chlorinated or sulfonated contaminants.

As stated previously, the thermal desorber (36) can be any one of a variety of known desorber designs. In the

■ embodiment of Fig. 2, the desorber (36) is a indirectly heated rotary kiln desorber that has an externally heated sealed rotating cylinder with the feed material tumbling inside and the heat source on the outside. To make efficient use of the heat contained within the off-gas (26) , these gaseous products are fed the external side of the desorber (36) , where they give up heat to the soil within and assist in volatilizing the organic contaminants.

Depending upon the make-up of the various vapor and solids streams, it may be necessary to supply supplemental heat to the desorber (36) to sufficiently volatilize the contaminated solids.

Since the thermal desorber (36) of Fig. 2 is externally heated, the off-gas (26) does not contact the waste material being processed. The off-gas proceeds to any supplemental gas cleaning systems (52) in stream (54) and, thereafter, is vented in stream (56) .

In such a desorber system, typically a relatively inert carrier gas stream (58) , such as N₂ at a nominal volume of 50 scfm or greater, would be fed to the interior of the desorber (36) to assist in carrying the volatilized organic contaminants out of the desorber in stream (37) to the downstream oxidation process. Additional air vapor will also typically leak into the desorber (36) during normal operation of such a rotary desorber.

In the embodiment of Fig. 2, the fume stream (37) exiting the desorber (36) is converted to inert components with near zero secondary pollutants in the flameless oxidizer (10) and is then used to externally heat the rotary chamber. In a typical system of this configuration, the volatilized stream

(37) from the desorber (36) will be at a temperature ranging from 250°F up to nominally 1000°F, the flameless oxidizer matrix bed (14) will reach 1800°F so that the off-gasses are also at approximately 1800°F (in a flow of approximately 1500 scfm) . All organics are destroyed completely in the stationary reaction zone within the flameless oxidizer (10) , sustained by a lean premix of combustible organics and air using supplemental stream (50) , which, for example, might contain a gas to supply 2.5 MM BTU/hr with an air stream of approximately 1200 scfm.

Once cooled through heat exchange in passing through the outside of the thermal desorber (36) , the off-gas stream

(54) will be at approximately 350-1200°F. Final off-gas treatment is accomplished in a reduced-size gas cleaning system (52) . The process vent stream (54) to the gas cleaning system will be comprised of combustion products that are essentially void of volatile organic compounds and PICs with NO_x levels of approximately 2-12 ppm (corrected to 3% 0₂) . Various types of air pollution control devices can be employed depending on the constituents to be removed.

It is also possible to utilize one or more varieties of waste heat recovery systems in conjunction with the present invention. For example, a heat recovery add-on device could be placed to recover heat either from the effluent of the desorber

(36) or from the gaseous products stream (26) . Recovered heat could be used, for example, to generate steam that could then be used in indirect heating of the thermal desorber (36) .

The heated solids (in this example at approximately 950°F) are discharged from the thermal desorber as a powdered or granular dry material (42) . For most applications, water would be mixed with the exiting solids to cool them and to prevent dusting. This water will normally be blowdown from any final off-gas treatment.

This configuration offers at least two major advantages. The gas cleaning system components are reduced in size and the relatively small amount of off-gas allows the vent stream to be cleaned to high standards at a lower cost. The other major advantage is that neither the separation process or the treatment process is classified as an incinerator, which greatly facilitates permitting. It can be shown that the integrated waste 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 volatilize the organic contaminants provides energy efficiency and reduced operating costs.

An alternative embodiment is shown in Fig. 3. While similar to the design of Fig. 2 in many respects (as discussed previously) , this embodiment utilizes a direct-fired desorber

(36) . In such a system, fuel is directly burned within the main chamber of the thermal desorber (36) and the contaminated solids are directly exposed to this flame. If needed, an additional carrier gas stream (58) is passed through the desorber (36) to entrain the volatilized organic contaminants. In one specific example using the configuration of Fig. 3, 80 ton/hr of contaminated soils can be fed to the thermal desorber (36) . The thermal desorption process gas (37) exits the desorber (36) at approximately 400°F and at a flow of approximately 21,500 average cfm (acfm) . This stream (37) contains approximately 2.3% oxygen, 1000 ppm organic compounds and 5 lb/hr NO_x. After solids filtering, the stream (37) enters the flameless oxidizer (10) at a nominal flow rate of 13,500 scfm, 20,000 acfm at approximately 350°F. Air and natural gas (if necessary to sustain the thermal treatment) are added in stream (48) and the blended mixture is fed at a combined flow rate of 23,500 scfm to three recuperative flameless oxidizers (10) , of which only one is shown, operated at 1600°F. The treated exhaust gas (26) contains less than 5.5 lb/hr NO_x, and exits at about 700°F and at a rate of about 53,000 acfm, where it is exhausted to the atmosphere.

Approximately 60% of the heat energy produced during the flameless oxidation treatment is used to pre-heat the incoming gas mixture to the oxidizer, resulting in a fuel gas cost savings of hundreds of thousands of dollars.

The solids (42) discharged from the desorber (36) will be at approximately 450°F and, after further handling, will be disposed (47) at approximately 250°F.

Still another alternative embodiment is shown in Fig.

4. While similar to the design of Fig. 2, this embodiment

• recycles some or all of the oxidizer off-gas (26) to the thermal desorber (36) . In such a system, rather than simply using the off-gas (26) in a tube and shell type heat exchange, the off-gas (26) is fed in stream (60) directly through the main chamber of the rotary kiln thermal desorber (36) . In this arrangement, the off-gas (26) accomplishes two goals. First, it provides heat for volatilizing the organic contaminants in the solids. Second, it acts as a carrier gas to entrain the volatilized compounds and carry them downstream to the flameless oxidizer. In doing this, because of its low oxygen content, the gas is simultaneously used to inert the rotary separator in which the thermal desorption of the organic compounds is carried out.

In such a configuration, it is possible that additional heat (62) will be required to be input into the thermal desorber (36) in order to sufficiently volatilize the organic contaminants.

This configuration is believed to be the preferred configuration in that it eliminates the need for any additional inert carrier gas feed to the system. Further, it minimizes the need for any additional, external heat input to the system. The heat produced by the exothermic oxidation of the organic compounds is used in whole, or in part, to: (a) accomplish the removal of the organic contaminants from the solids in the desorption unit (36) ; and/or (b) to preheat the incoming volatilized compounds to the flameless oxidizer (10) .

It is also possible, as would be readily understood by those of skill in the art, to design a system that allows for the gaseous products (26) to be used partially for indirect heating of the thermal desorber (36) and partially for direct heating and use as a carrier gas. For example, this could be accomplished using a modification of the system shown in Fig. 2 wherein a split-off stream is taken from gaseous products stream (26) prior to its entry into the indirect heating portion of the thermal desorber (36) and combining this split- off stream with carrier gas stream (58) .

In one specific example using the configuration of Fig. 4, 5 ton/hr of contaminated soils can be fed to the thermal desorber (36) . The thermal desorption process gas (37) exits the desorber (36) at 500-850°F. This stream (37) , after solids filtering, enters the flameless oxidizer (10) , which is of recuperative design, at a flow rate of about 700 acfm, with around 2% oxygen. Air in the amount of about 250 scfm is added in stream (48) . The treated exhaust gas (26) exits at 700- 1000°F and at a rate of about 1700 acfm. Approximately 400 acfm of this exhaust, containing around 3-5% oxygen, is recycled in stream (60) to the thermal desorber (36) . In summary, apparatus and methods for thermally desorbing and thereafter destroying hazardous organics 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 NO_x. Use of such a flameless oxidation process within a thermal desorption system 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, the present flameless thermal treatment technology can be used in conjunction with an in-situ thermal desorption process. In this technology integration, PCBs can be removed from contaminated soil by the use of an electrically-heated, impervious thermal blanket operated under slight negative pressure. Preliminary test data confirms that such a system can be used to achieve the absence of detectable PCBs and hydrocarbons in the off-gas.

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 or other waste material comprising the steps of:

(a) heating the soil or other waste material in a thermal desorber to a first temperature sufficient to volatilize the volatile contaminants but below the temperature at which such volatile contaminants would oxidize or combust, 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 second 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 using at least a portion of the oxidized gaseous products to heat additional soil or waste material in the thermal desorber.

3. The method of claim 2 wherein the oxidized gaseous products are used to provide at least a portion of the heat used to heat the additional soil or waste material through indirect heat exchange.

4. The method of claim 2 wherein the oxidized gaseous products are used to provide at least a portion of the heat used to heat the additional soil or waste material by directly contacting the soil or waste material and wherein such oxidized gaseous products simultaneously act as a carrier gas to carry the volatilized contaminants to the matrix bed in the process gas stream.

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 gasses prior to feeding the process gasses to the matrix bed.

8. 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.

9. 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.

10. The method of claim 1 further comprising the step of recovering heat from one or both of the oxidized gaseous products prior to venting such gaseous products to the atmosphere or the process gas stream prior to feeding the process gas stream to the matrix bed.

11. 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.

12. 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.

13. The method of claim 1 wherein the oxidized gasses have a NO_x content less than about 12 parts per million by volume and a carbon monoxide content is less than about 10 parts per million by volume, on a dry basis, adjusted to 3% oxygen.

14. 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.

15. 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 pr cess gas stream passes through the layer of smaller sized materials first.

16. 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.

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

18. The method of claim 1 wherein a destruction and • removal efficiency of the volatilized contaminants of at least

99.99% is achieved.

19. The method of claim 1 further comprising the step of providing a carrier gas stream to the thermal desorber to entrain the volatilized contaminants so as to form the process gas stream.

20. In a method for remediating soil or other waste material comprising the steps of heating the soil or other waste material in a thermal desorber to a first temperature sufficient to volatilize the volatile contaminants but below the temperature at which such volatile contaminants would oxidize or combust, 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 using at least a portion of the oxidized gaseous products to heat additional soil or waste material in the thermal desorber.

22. The method of claim 21 wherein the oxidized gaseous products are used to provide at least a portion of the heat used to heat the additional soil or waste material through indirect heat exchange.

23. The method of claim 21 wherein the oxidized gaseous products are used to provide at least a portion of the heat used to heat the additional soil or waste material by directly contacting the soil or waste material and wherein such oxidized gaseous products simultaneously act as a carrier gas to carry the volatilized contaminants to the matrix bed in the process gas stream.

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 step of admixing air, oxygen, supplemental fuel, or both with the process gasses prior to feeding the process gasses to the matrix bed so as to control the position of the reaction zone within the matrix bed.

26. 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.

27. The method of claim 20 wherein the oxidized gasses have a N0_X content less than about 12 parts per million by volume and a carbon monoxide content is less than about 10 parts per million by volume, on a dry basis, adjusted to 3% oxygen.

28. The method of claim 20 wherein the heat resistant material is chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic raschig rings.

29. The method of claim 20 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.

30. The method of claim 20 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.

31. The method of claim 20 wherein a destruction and removal efficiency of the volatilized contaminants of at least 99.99% is achieved.

32. A method for removing volatile contaminants from soil or other waste material comprising the steps of: (a) heating the soil or other waste material in a thermal desorber to a first temperature sufficient to volatilize the volatile contaminants but below the temperature at which such volatile contaminants would oxidize or combust, 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 and comprising at least two layers 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 within a flameless oxidizer above the autoignition temperature of the volatilized contaminants; (c) filtering out solids or liquids from the process gas stream;

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

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

(f) using at least a portion of the oxidized gaseous products to heat additional soil or waste material in the thermal desorber.

33. An apparatus for removing volatile contaminants from soil or other waste material comprising: (a) a thermal desorber comprising:

(i) a chamber into which the soil or other waste material may be placed, either continuously or in a batch manner; (ii) a heat source capable of heating the soil or other waste material in the chamber to a temperature sufficient to volatilize the volatile contaminants but below the temperature at which such volatile contaminants would oxidize or combust; and

(iii) an outlet;

(b) a flameless oxidizer having:

(i) an inlet in flow communication with the outlet of the thermal desorber; (ii) an outlet for reaction gaseous products; and

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

(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; and

(d) a means for creating a flow from the thermal desorber through the flameless oxidizer.

34. The apparatus of claim 33 wherein the heat source in the thermal desorber is a direct fired burner that directly fires any soil or waste material within the chamber.

35. The apparatus of claim 33 wherein the thermal desorber further comprises means for indirectly heating the chamber by heat exchange and wherein the outlet for reaction

• gaseous products from the flameless oxidizer is in flow communication with the means for indirectly heating the chamber.

36. The apparatus of claim 33 wherein the outlet for reaction gaseous products from the flameless oxidizer is in flow communication with the chamber such that reaction gaseous products can directly contact any soil or other waste material within the chamber.

37. The apparatus of claim 33 wherein the section of the flameless oxidizer is configured to create a flow pattern from the inlet to the outlet that allows heat from the matrix bed to preheat the volatilized contaminants as they enter the matrix bed.

38. 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.

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

40. The apparatus of claim 33 further comprising means for controllably adding air, oxygen, supplemental fuel, or both to the flow between the outlet of the thermal desorber and the matrix bed.

41. The apparatus of claim 33 further comprising a scrubber in flow communication with the outlet of the flameless oxidizer.

42. The apparatus of claim 33 wherein the heat resistant material is chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic ■ raschig rings.

43. The apparatus of claim 33 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 wherein the section of the flameless oxidizer is configured to create a flow pattern from the inlet to the outlet that causes any flow to pass through the layer of smaller sized materials first.

44. The apparatus of claim 33 further comprising one or more filters for filtering out solids or liquids between the outlet of the thermal desorber and the inlet of the flameless oxidizer and in flow communication with each.

45. The apparatus of claim 33 wherein the heat resistant material of the matrix bed comprises a catalyst.

46. The apparatus of claim 33 further comprising means for providing a carrier gas stream to the chamber of the thermal desorber.

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

48. The apparatus of claim 33 wherein the material in the matrix bed has a specific surface area from 40 m²/m³ to 1040 m²/m³.

49. The apparatus of claim 33 further comprising means for feeding soil or other waste material to the chamber of the thermal desorber.

50. An apparatus for removing volatile contaminants from soil or other waste material comprising: (a) a thermal desorber comprising:

(i) a chamber into which the soil or other waste material may be placed, either continuously or in a batch manner;

(ii) a heat source capable of heating the soil or other waste material in the chamber to a temperature sufficient to volatilize the volatile contaminants but below the temperature at which such volatile contaminants would oxidize or combust; and (iii) an outlet; (b) means for feeding soil or other waste material to the chamber of the thermal desorber; (c) a flameless oxidizer having:

(i) an inlet in flow communication with the outlet of the thermal desorber;

(ii) an outlet for reaction gaseous products; and (iii) a section located between the inlet and the outlet including a matrix bed of heat resistant material wherein the heat resistant material is chosen from the group consisting of ceramic balls, ceramic saddles, ceramic pall rings, or ceramic raschig rings and wherein the matrix bed comprises at least two layers of differently sized heat resistant material and wherein the section of the flameless oxidizer is configured to create a flow pattern from the inlet to the outlet that causes any flow to pass through the layer of smaller sized materials first;

(d) 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; (e) means for controllably adding air, oxygen, supplemental fuel, or both to the flow between the outlet of the thermal desorber and the matrix bed; and

(f) means for creating a flow from the thermal desorber through the flameless oxidizer wherein the outlet for reaction gaseous products from the flameless oxidizer is in flow communication with the chamber such that reaction gaseous products can directly contact any soil or other waste material within the chamber.