Abstract:
A process for incinerating combustible materials including the steps of: delivering combustible material and inlet gases to a primary combustion chamber, the inlet gases having an oxygen content of at least 50 vol %; burning the combustible material with the oxygen of the inlet gases in the primary combustion chamber producing flue gases and solid particulates as thermal decomposition products of the burnt combustible material; passing the flue gases and particulates to a secondary combustion chamber where further combustion occurs; cooling the flue gases exiting the secondary combustion chamber; returning a portion of the cooled flue gases to at least one of the combustion chambers where the cooled gases moderate the temperature in the at least one chamber; and passing the remaining portion of cooled flue gases on to a flue gas purification system where pollutants in the flue gases and particulates are substantially converted to benign compounds or removed entirely before the flue gases are emitted into the atmosphere.

Description:
The present application is a 35 USC 371 national phase application from and claims priority to international application PCT/IL02/00503, filed 24 Jun. 2002, established under PCT Article 21(2) in English, which claims priority to Israeli patent application Ser. No. 143993, filed 26 Jun. 2001, which applications are incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to a method for incinerating combustible materials, particularly waste materials, including hazardous and bio-hazardous waste materials. 
   BACKGROUND OF THE INVENTION 
   The disposal of waste is a serious problem to governments, especially municipal governments. The waste disposal process is regulated by increasingly stricter standards since some wastes are toxic. In the case of industrial waste, there are even more problematic materials, such as petrochemicals, PCBs (polychlorinated biphenyls), etc. than in common, non-industrial waste. Additionally, medical and other biological waste is often hazardous and requires complete sterilization and decomposition. 
   Previously, other methods of waste disposal were more attractive than incineration. Landfills, for example, were used instead of incineration since the cost of disposing waste at a landfill was far less than that of incineration. However, increasingly more severe environmental standards have made landfills less attractive, primarily because of the increased awareness that toxic chemicals, over long periods of time, percolate through the ground contaminating aquifers. Similarly, the ever increasing quantity of waste make landfills and other methods physically impractical. 
   Accordingly, destructive, degradative processes such as incineration have become more popular. Destructive techniques like incineration must efficiently turn waste into innocuous end-products. This is a particularly acute problem in incineration where burning hazardous waste requires high temperatures so that the resulting decomposition products are environmentally benign. The high temperatures needed and the large quantities of waste involved require the development of incinerators that are economically and environmentally efficient. The emissions from such products are generally gaseous and must comply with standards set by international and governmental agencies. Similarly, solid and particulate wastes of incineration, such as slag, bottom ash and fly ash, must be neutered to remove harmful effects to the environment. 
   Examples of recently proposed incineration methods and incinerators can be found in U.S. Pat. Nos. 5,752,452 and 5,179,903, and WO 96/24804, Abboud. U.S. Pat. No. 5,179,903 and WO 96/24804 describe recycled flue gases which are augmented with oxygen, U.S. Pat. No. 5,752,452 describes a system with lances which inject oxygen into a heating zone at a velocity of at least 350 ft/sec. 
   However, despite improvements in incinerators and incineration processes, capital and maintenance costs are still very high. In addition, effluents emitted into the environment still require further reduction. 
   SUMMARY OF THE PRESENT INVENTION 
   An object of the present invention is to provide a process which maximizes the rate of incineration and throughput in waste incinerators while minimizing gas emissions and solid waste produced. 
   It is a further object of the present invention to provide an economical incineration process for use with industrial, consumer and biological wastes, including hazardous waste. 
   It is yet another object of the present invention to minimize the size of the required incinerator and flue gas purification system, thereby minimizing the required investment and maintenance costs. 
   A further object of the present invention is to provide an economical, environmentally friendly process which can be applied to large industrial installations, such as electricity generating plants, which burn large quantities of fossil fuels. 
   There is thus provided in accordance with the present invention a process for incinerating combustible material including the step of delivering combustible material and inlet gases to a primary combustion chamber, the inlet gases having an oxygen content of at least 50 vol. %. This is followed by burning the combustible material with the oxygen of the inlet gases in a primary combustion chamber producing flue gases and solid particulates as thermal decomposition products of the burnt combustible material. The flue gases and particulates are then passed to a secondary combustion chamber where further combustion occurs. The flue gases exiting from the secondary combustion chamber are cooled. A portion of the cooled flue gases is returned to at least one of the combustion chambers where the cooled gases moderate the temperature in the at least one chamber. Finally, the remaining portion of the cooled flue gases is passed on to a flue gas purification system where pollutants in the flue gases and particulates are substantially converted to benign compounds or removed entirely before the flue gases are emitted into the atmosphere. 
   Additionally, there is provided in accordance with the present invention a process which further includes the step of monitoring the value of at least one parameter in at least one combustion chamber, the parameter being a function of the thermal decomposition of the combustible material in at least one combustion chamber. This is followed by comparing the value of the at least one monitored parameter with at least one predetermined value for that parameter, the comparison being effected by a control device. Finally, the result of the comparison is communicated to a means for controlling the portions of cooled flue gases returned to the at least one combustion chamber and the flue gas purification system. The means for controlling the portions adjusts the relative sizes of the two portions accordingly. 
   Additionally, in accordance with a preferred embodiment of the present invention the at least one parameter in the monitoring step is temperature. The temperature can be monitored in the primary combustion chamber or in the secondary combustion chamber or in both chambers. 
   Further, in accordance with a preferred embodiment of the present invention, the at least one parameter in the monitoring step is the concentration of carbon monoxide or the concentration of oxygen or the concentration of both simultaneously. These concentrations can be measured in the effluent of the secondary combustion chamber. 
   In accordance with a preferred embodiment of the present invention, the means for controlling the amount of cooled gases are valves. 
   Additionally, in accordance with a preferred embodiment of the present invention, the inlet gases of the delivering step are delivered in two high concentration oxygen streams, one inlet gas stream positioned adjacent to the burning waste and the other above the flames of the burning waste, the amount of oxygen from each stream controlled so that the temperature of the burning waste is maintained at a temperature that does minimal damage to the floor of the primary combustion chamber, while ensuring complete combustion of the waste and an oxygen volume % in the system&#39;s effluent within regulatory limits. 
   Further, in accordance with a preferred embodiment of the present invention the oxygen content of the inlet gases is at least 80 vol. %. 
   Additionally, in a preferred embodiment of the present invention, the oxygen content of the inlet gases is at least 90 vol. %. 
   Further, in a preferred embodiment of the present invention, the oxygen content of the inlet gases is between about 90 vol. % and 95 vol. %. 
   Additionally, in accordance with a preferred embodiment of the present invention, the burning step in the primary combustion chamber is effected at a temperature from about 1100° C. to about 2000° C. 
   In another preferred embodiment of the present invention, the burning step in the primary combustion chamber is effected at a temperature from about 1200° C. to about 1750° C. 
   Additionally, in a preferred embodiment of the present invention, the burning step in the primary combustion chamber is effected at a temperature from about 1300° C. to about 1500° C. 
   Further, in yet another embodiment of the present invention, combustion in the secondary combustion chamber of the first passing step is effected at a temperature from about 850° C. to about 1500° C. 
   In another embodiment of the present invention, combustion in the secondary combustion chamber of the first passing step is effected at a temperature from about 950° C. to about 1350° C. 
   Additionally, in yet another embodiment of the present invention, combustion in the secondary combustion chamber of the first passing step is effected at a temperature from about 1050° C. to about 1200° C. 
   In another embodiment of the present invention, the process further includes the step of adding at least one reduced nitrogen compound into the second combustion chamber to destroy nitrogen oxide gases. Typically, the at least one reduced nitrogen compound can be ammonia or urea. 
   Further, in a preferred embodiment of the present invention, the process further includes the step of separating solid particulates from the flue gases after the gases are cooled. 
   Additionally, in a preferred embodiment of the invention, the at least one combustion chamber of the returning step is the primary combustion chamber. 
   Finally, in a preferred embodiment of the present invention, the cooled flue gases are returned to the primary combustion chamber proximate to the flame produced by burning combustible material in that chamber. In another embodiment, the cooled flue gases are returned to the primary combustion chamber proximate to the bottom ash and slag. 
   In yet another preferred embodiment of the present invention, the at least one combustion chamber of the returning step is the secondary combustion chamber. 
   Finally, the present invention can be used with combustible material which is waste, including hazardous waste, or fuels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
       FIG. 1  is a flow diagram illustrating a preferred embodiment of the process of the present invention; 
       FIG. 2A  is a schematic view of an incinerator operative in accordance with the present invention; 
       FIG. 2B  is a schematic view of a typical purification system which can be used with an incinerator operative in accordance with the present invention; and 
       FIG. 3  is a schematic diagram illustrating another preferred embodiment of the process of the present invention. 
   

   Similar elements in the Figures are numbered with similar reference numerals. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference is now made to  FIG. 1 , which shows a flow diagram of a preferred embodiment of an incineration process generally referenced  110 , in accordance with the present invention. Process  110  is particularly preferred when used to incinerate industrial, commercial and/or biological waste. The description herein below, as well as the accompanying Figures, describe the process in terms of such waste. However, while the above process  110  has been discussed as a process for the incineration of waste, the system can also be used to burn any fuel, producing energy in a clean, cost efficient manner. In lieu of municipal or industrial waste, process  110  can be used to burn fuels such as natural gas, fuel oil, and coal. These fuels, however, are to be viewed as non-limiting examples. 
   Process  110  includes a primary combustion chamber (PCC)  12  into which waste is fed  51  via a conduit (not shown). Inlet gases containing at least 50 vol. %, preferably at least 80 vol. %, and most preferably at least 90 vol. % oxygen, usually between about 90 vol. % to 95 vol. % oxygen, are also passed  53 , via a conduit (not shown), into PCC  12 , typically in the region immediately proximate to the burning waste. The waste is burned in an excess of the stoichiometric amount of oxygen. The waste is burned in PCC  12  at temperatures maintained between about 1100 to 2000° C., preferably between about 1200 to 1750° C., and even more preferably between about 1300 to 1500° C. Because of the high oxygen concentrations used in primary combustion chamber  12 , a significant percentage of the material burned undergoes complete oxidation. Oxygen lancing and other methods to introduce supplementary oxygen are therefore not required. 
   Flue gases mixed with small solid particulates resulting from incineration rise from PCC  12  and pass  55 , via a conduit (not shown), into a secondary combustion chamber (SCC)  14 . Partially combusted flue gases are further combusted in SCC  14  to more completely oxidized gases using the residual oxygen arriving from PCC  12 . In SCC  14 , the temperature is maintained within the range of from about 850 to 1500° C., preferably from about 950 to 1350° C., and even more preferably from about 1000 to 1200° C. 
   Optionally, materials which destroy nitrogen oxide gases (NOx) are fed  57 , via a conduit (not shown), into SCC  14 . Typically, these materials are reduced nitrogen compounds such as ammonia or urea which convert the NOx gases formed in PCC  12  and SCC  14  into nitrogen and water. Since the amount of nitrogen comprising the inlet gases passed  53 , via a conduit (not shown), into PCC  12  is small, the amount of NOx present in the system is not great. In some embodiments, the materials which destroy nitrogen oxide gases may be employed without a catalyst; in other embodiments, a catalyst may be required. Preferably, PCC  12  and SCC  14  are contained in a single structure, but each can be located in separate structures, when necessary. 
   The flue gases are conveyed  59  via a conduit (not shown) to a heat exchanger  22 . Typically, heat exchanger  22  may be a boiler which removes heat from the flue gases. The energy removed, usually as steam, is conveyed  61 , via a conduit (not shown), to an energy converter  18 , often a turbogenerator. Alternatively, any heat recovery system from which electricity or steam can be withdrawn  63  can be employed. Any electricity generated or steam removed can be returned to the incineration plant or distributed to outside consumers. 
   After emerging from heat exchanger  22 , the flue gases have a temperature of about 230 to 270° C., preferably about 250° C. The gases are transferred  65  via a conduit (not shown) to a particulate separator  26 , typically a cyclone separator, which via a conduit (not shown), removes  69  fly ash  27  from the flue gases. The removed fly ash  27  is collected, “bagged” and sent to a toxic waste disposal site. The use of a particulate separator  26  at this stage of process  110  is optional. Alternatively, particulates can be removed exclusively in flue gas purification system  29  discussed below. As another alternative, purification system  29  can include a particulate remover which supplements particulate separator  26 . 
   Two valves (not shown) located between particulate separator  26  and flue gas purification system  29  divide the flue gases into two portions. The percentage of flue gas that is recycled  73  through a conduit (not shown) and the percentage of flue gas passed  71  via a conduit (not shown) directly on to a flue gas purification system  29  for further purification is determined by some parameter(s) of PCC  12  and/or SCC  14 . Typically, the parameter is its (their) temperature(s) or the concentration of carbon monoxide and/or oxygen on the downstream side of SCC  14 . 
   The flue gases that are passed on  71  via a conduit (not shown) for further purification reach flue gas purification system  29 , details of which are not shown. The exact nature of purification system  29  depends on the waste being incinerated, the gases and particulates emitted, and the environmental standards which must be met. Typically, flue gas purification system  29  contains a particulate remover, which supplements optional particulate separator  26 , discussed above, and sometimes serves as the sole particulate remover in process  110 . Generally, the particulate remover in purification system  29  traps finer particles than optional particulate separator  26 . Typically, purification system  29  also contains a scrubber to neutralize acid gases. Other apparatuses commonly used for purifying effluent gases can be added as needed to attain the required effluent emission standards before the gases are expelled  81  to the atmosphere. 
   Another portion of the flue gases is recycled  73  via a conduit (not shown) to PCC  12 . Typically, the recycled, cooled flue gases are returned  73 A via a conduit (not shown) to PCC  12  directly above the flame, thereby removing heat from PCC  12  and transferring it to heat exchanger  22  via SCC  14 . In another embodiment, the recycled flue gases can be returned  73 B via a conduit (not shown) directly to SCC  14 . In yet another embodiment, the flue gases can also be recycled  73 C via a conduit (not shown) through bottom ash and slag  17  lying at the floor of PCC  12 . Finally, in other embodiments, the cooled flue gases can be returned to both PCC  12  and SCC  14 . Because PCC  12  operates at temperatures in excess of 1300° C., the bottom ash becomes vitrified  75  when cooled. Some ash is carried  79  by convection to SCC  14 . Cooled slag and vitrified bottom ash  17  are periodically removed  77  to a slag and bottom ash receptacle (not shown) for disposal. 
   Reference is now made to  FIG. 2A  which shows a schematic view of an incinerator system  210  operated in accordance with the process  110  of the present invention shown in FIG.  1 . The system  210  permits a better understanding of process  110  presented in FIG.  1 . The system shown in  FIG. 2A , however, is presented by way of example only and should not be considered as limiting. 
   System  210  includes a primary combustion chamber  12  into which waste  19  is fed from a waste feed  10 . There is an inlet gas feed array  15  which delivers inlet gases for combustion, the gases typically being composed of at least 90 vol. % oxygen. Waste  19  is burned in primary combustion chamber (PCC)  12 . The inlet gases are brought from array  15  proximate to the burning waste in PCC  12 . The high concentration of oxygen in the inlet gases fed to primary combustion chamber  12  accelerates the rate of combustion of waste  19 . The temperature in PCC  12  is also significantly higher than temperatures generated when air alone is used. The higher temperatures attained easily crack and shatter solids, facilitating their incineration. Materials that do not burn in air, or do so only incompletely, burn easily in inlet gases with a high oxygen content, often to near completion. Since the oxygen concentration used in the process of the present invention is so high, burning is much more complete and there is no need for selectively introducing lanced oxygen. Because the rate of combustion is faster than in currently used incinerators, primary combustion chamber  12  can be made smaller while throughput will be greater than in prior art incinerators. 
   PCC  12  has a bottom grating comprised of slats, which are preferably adapted to be rotatable or otherwise movable so as to rotate or otherwise agitate the burning waste. The grating can be made from, or covered with, ceramic materials which protect it from the elevated temperature of combustion. Typically, every other grating slat is moved periodically, turning over the burning waste, permitting more thorough and rapid combustion. The lower parts of the walls of PCC  12  must also be protected from the heat, usually using ceramic tiling as shields. Alternatively, the walls and the grating can be cooled with water flowing through adjacent water pipes. It is readily apparent to one skilled in the art that instead of grating slats at the bottom of primary combustion chamber  12 , the floor of chamber  12  can include rotating metal cylindrical rollers or any other means that can periodically move and/or rotate the burning waste. 
   Slag and bottom ash  17  from PCC  12  are cooled and emptied into an ash and slag receptacle (not shown) via a slag channel  16 . Because of the high temperatures (&gt;1300° C.) in the primary combustion chamber  12 , the bottom ash  17  is vitrified when cooled and encapsulated in a glass-like crust. The encapsulation insulates and neutralizes harmful materials making them usable for civil engineering projects such as road beds without the need for further processing. 
   Gases and fly ash emitted from the burning waste as well as residual oxygen from PCC  12  enter a secondary combustion chamber  14  where additional combustion occurs. An array  30  of nozzles in the wall of primary combustion chamber  12  injects cooled, recycled flue gases into PCC  12 ; typically these recycled gases enter PCC  12  immediately above flames  11 . The cooled, recycled flue gases entering from array  30  have a typical temperature of approximately 250° C. and they maintain the temperature in primary combustion chamber  12  at a predetermined temperature, generally about 1300 to 1500° C. Similarly, they cool the gases rising from PCC  12  into SCC  14  to temperatures between about 1000 to 1300° C. 
   Optionally, ammonia or urea are added to the flue gas in SCC  14  reducing the nitrogen oxide gases produced in PCC  12  and SCC  14  to nitrogen and water. PCC  12  and SCC  14  can be constructed as any one of several types of chambers, such as rotary kiln, fixed hearth or other types of ovens. 
   The gases continue on from secondary combustion chamber  14  to an heat exchanger  22 , typically a boiler. Heat exchanger  22  removes heat from the flue gases, generally forming steam which is led to a turbogenerator (not shown). The turbogenerator can be connected to an electric grid from which electricity can be delivered directly to consumers or returned to the incineration plant for use within the plant. Alternatively, the steam itself, or a mixture of steam and electricity generated by the heat exchanger/boiler  22  and turbogenerator (not shown) respectively, can be sold. By the time the gases and fly ash emissions from the burnt waste reach an optional blower  24 , the temperature of the gases has been reduced to approximately 250° C. 
   The fly ash that passes through optional blower  24  enters an optional cyclone separator  26  which precipitates the bulk of the fly ash passing through blower  24 . The cyclone separator  26  may be any cyclone separator commercially available used to separate particulates from gases. A single cyclone or multiple cyclones can be used. 
   It should be noted that there is a significant reduction in the amount of fly ash produced by the process of the present invention. The reduction in fly ash is a direct consequence of the very high percentage of oxygen introduced with the inlet gases. The high percentage of oxygen reduces the total amount of inlet gases provided to primary combustion chamber  12 , which in turn leads to a smaller volume of carrier gas for ash generated by incineration. More of the ash produced remains as bottom ash. Since fly ash traps poisonous materials found in flue gases, such as dioxins and heavy metals, the law requires that fly ash be gathered and delivered to a toxic disposal dump. Any reduction in fly ash therefore results in a reduction in waste treatment expense. 
   The bulk of the emitted waste gases, the flue gas, is returned via a recycling line  28  to primary combustion chamber  12 . The recycled flue gas is at a temperature of approximately 250° C. and enters PCC  12  through array  30  in the wall of primary combustion chamber  12 . Generally, the gases enter the chamber proximate to and above flames  11 . The cooled recycled flue gas functions as a coolant keeping the temperature in primary combustion chamber  12  at the predetermined temperature, typically 1300-1500° C. Typically, the recycled flue gases reenter the system directly into PCC  12  above flames  11  therein; optionally they can also be recycled directly to SCC  14  or into the bottom ash and slag  17  on the floor of PCC  12 . Typically, an array of conduits is used for reintroducing the recycled flue gas, but in other embodiments, a single point of entry for the recycled flue gases may be employed. 
   Part of the flue gases from blower  24  enters a cleaning line  32 . Valves  31 A and  31 B determine how much, and when, flue gases enter cleaning line  32  and recycling line  28 . Using 90 vol. % oxygen and a typical mix of Israeli municipal waste, the mixture of flue gases generated and entering these lines has a typical approximate composition of oxygen 6 vol. %, nitrogen 5 vol. %, CO 2  43 vol. % and steam 46 vol. %. If the inlet gases fed to primary combustion chamber  12  had been air (approximately 21 vol. % oxygen) and not a gas mixture containing at least 90 vol. % oxygen, the nitrogen content of the flue gases entering cleaning line  32  and recycling line  28  would have risen to approximately 66 vol. %. 
   Valves  31 A and  31 B are connected to a control system which monitors a parameter, typically the temperature, of the gases exiting primary combustion chamber  12  and/or secondary combustion chamber  14 . If the temperature is higher than required, a larger percentage of the flue gases is recirculated to the primary combustion chamber; if the temperature in the primary combustion chamber is lower than required, the amount of flue gases that is returned is decreased. If, for example, the temperature in PCC  12  is 1750° C. and the temperature in SCC  14  is 1100° C., the approximate percentage of flue gases recycled is 60 vol. % while 40 vol. % are passed via line  32  directly to the flue gas purification system  310  shown in FIG.  2 B and discussed below. 
   Typically, a device, for example a thermocouple, is used to measure the temperature inside PCC  12  and/or SCC  14 , while a temperature controller compares the measured PCC  12  and SCC  14  temperatures, with one or more temperature set points. The controller then opens or closes the two valves accordingly, returning the required amount of recycled flue gases to PCC  12  and/or SCC  14 . The recycling of cooled flue gases ensures better control of temperature in primary combustion chamber  12  than when recycling is absent. It also increases the degree of combustion of the flue gases. 
   Reference is now made to  FIG. 2B , where a schematic view of an exemplary purification and scrubbing system  310  of the incinerator plant is shown. The configuration of devices in  FIG. 2B  are shown merely by way of example and the scope of the present invention is not intended to be limited thereby. 
   Cleaning line  32  continues into the purification system  310  of the plant where the amount of effluent solid and flue gases is reduced. These gases and solids are led into an electrostatic precipitator (ESP)  34  which complements or functions in place of cyclone separator  26  discussed above. In ESP  34  much of the remaining fly ash is removed. In ESP  34 , fly ash particulates are charged by a high voltage source and drawn to a conductive plate of opposite charge where the particulate&#39;s charge is dissipated. The ash is then precipitated and collected. 
   The flue gases are then sent via a line  42  to a scrubber heat exchanger  36  which removes heat from the system. The gases enter the lower part of a scrubber  40  where the temperature is less than 100° C. and much of the water vapor in the flue gases condenses. In scrubber  40 , drops of a basic solution containing calcium hydroxide, sodium hydroxide, sodium carbonate, potassium carbonate or some other such alkaline compound are injected. These neutralize acid gases such as sulfur dioxide and any residual nitrogen oxides not destroyed by ammonia or urea optionally added in secondary combustion chamber  14 . The scrubbed gases then reenter heat exchanger  36 , via a line  38 , where they are reheated using the heat previously withdrawn from the flue gases before these gases entered the lower part of scrubber  40 . The reheated gases then enter a line  46 , where an activated carbon injector  44  injects carbon into line  46 , so that contaminants, among them dioxins and furans, are adsorbed. The carbon also traps other contaminants including heavy metal and heavy metal oxide particulates. 
   The injected activated carbon and gas effluents advance through line  46  and are deposited onto a fabric filter  50 , which removes the injected active carbon from the flue gases. Residual gases such as oxygen and nitrogen are then led through a line  48  to a stack  52  where they are emitted into the air, usually with the assistance of a blower  49  located at the bottom of the stack. 
   When the inlet gases contain at least 90% oxygen, the amount of effluent gases emitted from stack  52  is about 5 times less than the amount emitted by currently used incinerators. Typically, approximate percentages of the emitted gases using the process of the present invention are 6 vol. % oxygen, 5 vol. % nitrogen, 20 vol. % water vapor and 70 vol. % carbon dioxide. 
   The reduction in nitrogen and the large amount of completely oxidized carbon in the form of carbon dioxide are a direct result of the use of inlet gases with a very high oxygen content followed by recycling of flue gases into the primary combustion chamber. The reduction in water vapor is a consequence of the condensation of a large percentage of the vapor in scrubber  40  discussed above. 
   It should be apparent to one skilled in the art that the exact configuration of devices used to clean the effluent after it enters cleaning line  32  is to a degree variable and/or optional. Other types of scrubbers and filters known in the art can be used. Similarly, some of the devices discussed above may be absent entirely while others not shown can be added. Cleaning devices at different plants would be expected to vary depending on the nature of the waste being burned and the environmental standards which must be met. 
   The inlet gases used to burn waste in primary combustion chamber  12  of the process discussed herein above should typically contain at least 80 vol. %, preferably at least 90 vol. %, but generally between 90 vol. % and 95 vol. %, oxygen. This level of oxygen content (90-95 vol. %) is readily attained by using a vapor pressure swing adsorption (VPSA) device, such as the one produced by Praxair Inc. A VPSA device absorbs nitrogen from air and passes the rest of the gases, mainly oxygen, to primary combustion chamber  12  at relatively low cost. VPSA separates nitrogen from air by molecular sieving. Nitrogen is adsorbed at low pressures in the sieve and then removed by vacuum. Presently, this method is the most economical way to obtain gas fractions having such high percentages of oxygen. Any attempt to use higher concentrations of oxygen to increase the performance of the incinerator would increase the cost of producing the inlet gas because it would require distillation of liquefied air. 
   The use of VPSA as discussed above or, alternatively, the related pressure swing adsorption (PSA) process to produce inlet gases containing a high percentage of oxygen should be viewed as non-limiting. Devices employing membrane technology also can be used to produce inlet gases with higher than atmospheric oxygen content but these typically are only 40 to 60 vol. %. 
   Since there is likely to be a reduction by a factor of about 5 in effluent gases at the incinerator&#39;s stack when the inlet gases of the incinerator include at least 90% oxygen (based on absolute amount of weight per ton of waste), there is a concomitant reduction in the size and cost of the apparatus required to clean up effluent gases. Similarly, costs of the incinerator are reduced because of the faster combustion and higher throughput. In addition, because of the reduction in fly ash and the vitrification of bottom ash in the system, waste disposal costs are reduced. Finally, because nitrogen forms a much smaller portion of the inlet gases, energy lost in heating nitrogen is reduced. This energy may be retrieved for profitable use elsewhere. 
   Reference is now made to  FIG. 3  which schematically shows another embodiment of the present invention.  FIG. 3  includes a primary combustion chamber (PCC)  12 , a secondary combustion chamber (SCC)  14 , and their control systems  120 ,  122  and  124 . It also includes an inlet gas feed array  15 , an auxiliary inlet gas feed array  115  and a recycled gas flue array  30 , positioned in the aforementioned chambers. 
   In this, as in previous embodiments, a high oxygen concentration is fed into PCC  12  proximate to the burning waste at the floor of PCC  12 . Oxygen is delivered through inlet gas feed array  15 , which, because of the high concentration of oxygen delivered, generates very high temperatures near the burning waste  11 . These temperatures may adversely effect the structure of PCC  12  and can require different, more heat resistant, more costly materials from which to construct PCC  12 . 
   In order to reduce combustion temperatures in the bottom region of PCC  12 , the present embodiment contemplates limiting the total amount of oxygen supplied to the primary chamber by inlet gas feed array  15 . Limiting the oxygen introduced by array  15 , but not the high concentration of the oxygen, reduces the temperature at, or near, the floor of PCC  12 . 
   With the reduction in total amount of oxygen introduced through inlet gas feed array  15 , some waste, and the flue gases generated therefrom, may be incompletely oxidized. In order to ensure that all the waste and flue gases are substantially completely burned, there is positioned in PCC  12  a second gas feed array carrying a high concentration of oxygen to PCC  12 . This second array, herein denoted as an auxiliary inlet gas feed array  115 , supplies a high concentration of oxygen, typically in excess of 90%, over the burning coals and into the flue gases rising therefrom. The oxygen fed through auxiliary inlet gas feed array  115  produces substantially complete combustion of the flue gases generated by the burning waste in PCC  12 , while permitting operation of PCC  12  at lower temperatures. Even if oxygen provided by auxiliary inlet gas feed array  115  increases the temperature of the exiting flue gases, little increase in temperature results in the burning waste adjacent the floor of PCC  12  and little damage to the floor of PCC  12  occurs. 
   The temperature of the exiting flue gases is moderated by recycled gases introduced from an array of nozzles  30  through valves  130 , the nozzles generally located in the wall of SCC  14  or in the upper region of PCC  12 . The temperature of the exiting flue gases is measured by a thermocouple, pyrometer or other temperature monitoring instrument  142 B connected to a temperature control unit  120  which controls the operation of valves  130 . 
   Using two high concentration oxygen sources, inlet gas feed array  15  and auxiliary inlet gas feed array  115 , allows for substantially complete combustion of the waste at generally lower temperatures in, or proximate to, the burning waste located at, or near, the bottom of PCC  12 . 
   The amounts of oxygen brought into PCC  12  and needed to maintain relatively low combustion temperatures there can be controlled in several ways. Temperature control can be effected by monitoring the oxygen concentration in the effluent emerging from the system&#39;s stack  52 . As described above, flue gas concentrations entering the atmosphere must meet strict regulatory requirements. An oxygen monitoring instrument  132  can be inserted into, or positioned near, the outlet of stack  52  to monitor the oxygen vol. % of the effluent. Data relating to the concentrations thus measured are then fed to an oxygen concentration control unit  122 . When the oxygen concentration in the effluent emerging from stack  52  is lower than required by regulations, the amount of oxygen provided by auxiliary oxygen feed array  115  is increased; when the amount of oxygen is higher than required by regulations, the amount of oxygen supplied by auxiliary oxygen feed array  115  is reduced. 
   As an alternative to an oxygen monitoring instrument  132  positioned at the outlet of stack  52 , oxygen can be monitored by measuring oxygen content of the recycled gases delivered by recycled gas flue array  30  and entering either PCC  12  or SCC  14 . The percentage oxygen content at stack  52  is related to the oxygen content in the recycled gases arriving from recycled gas flue array  30 . Therefore, the composition of the recycled gases entering either PCC  12  or SCC  14  can be used to determine the over or under abundance of oxygen at stack  52 . 
   In yet other embodiments of the present invention, two oxygen monitoring instruments can be used to determine the oxygen content exiting stack  52 . One instrument  132  can be positioned at stack  52  while the other can be located at the point where recycled flue gases are delivered by array  30 . 
   An alternative method for controlling the system is by monitoring the temperature in PCC  12 . At least one thermocouple or pyrometer  142 A is placed near, or at, the flames  11  of the burning waste. The results of these temperature measurements then are fed into a control unit  124 , the burning waste temperature control unit, and compared to a predetermined temperature setting. The amount of oxygen provided to PCC  12  by both gas inlet arrays  15  and  115  then is adjusted to maintain a predetermined temperature setting at flames  11  by operating valves  126  and  128 , respectively. By controlling temperature, the effluent oxygen concentration at stack  52  is also kept within regulatory limits. 
   It should be readily apparent to one skilled in the art that there is a reciprocal relationship between the amount of oxygen being supplied through valves  126  and  128  of inlet gas feed array  15  and auxiliary inlet gas feed array  115 , respectively. When more oxygen is required at array  15 , generally less oxygen is required at array  115  for a given required flame temperature. 
   When the temperature of the burning material is too high, valve  126 , controlled by burning waste temperature control unit  124 , reduces the flow of oxygen from inlet gas feed array  15  above the burning coals. Control unit  124  is separate from another control unit, the temperature control unit  120 , which monitors temperature at the exit of the secondary combustion chamber (SCC)  14 . This temperature, as discussed above, is effected by means of two valves  31 A and  31 B ( FIG. 2A ) which determine the amount of recycled cooled flue gases returned to PCC  12  and SCC  14  or sent to stack  52  by recycling line  28  ( FIG. 2A ) or cleaning line  32  (FIG.  2 A), respectively. 
   It can readily be seen that the temperature of the burning coals as measured by measuring instrument  142 A and controlled by control unit  124  through valve  126  and gas feed array  15 , the oxygen monitoring instrument  132  at stack  52  and its oxygen control unit  122  through valve  128  and auxiliary gas feed array  115 , and temperature monitoring instrument  142 B through temperature control unit  120  and valve  130  of recycled flue gas array  30  form three control loops which are functionally interconnected. Generally, changes in one have a discernible effect in the other two control loops. 
   The embodiment shown in  FIG. 3  moderates and controls temperature better than in currently available furnaces. This embodiment with its auxiliary oxygen feed array  115  and recycled flue gas array  30 , the latter positioned either in the walls of secondary combustion chamber (SCC)  14  or the upper region of PCC  12 , permits moderation of the temperature at every stage of the combustion process. Furnace temperatures, irrespective of the type of the furnace used, can be maintained so that damage to PCC  12  is minimized. 
   It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined solely by the claims that follow.