INCINERATOR FOR COMPLETE OXIDATION OF IMPURITIES IN A GAS STREAM

Abstract: A device and method for oxidizing gaseous materials, the device comprising a combustion chamber (230), a flue gas outlet (216) operating in conjunction with means for applying negative pressure to the outlet, means for introducing process gas and a fuel (214, 220) into the top (222) of the combustion chamber (230) means for effecting heat exchange (234) between oxygen-containing gas and the combustion chamber (230) to preheat the oxygen-containing gas, means for introducing the preheated gas into the top (222) of the combustion chamber (230), and means for mixing the preheated gas with the process gas and fuel (256) so that the mixture can burn in the combustion chamber (230) to form a flue gas. The heat exchanger (234) may concentrically surround the combustion chamber (230). The preheated oxygen containing gas may swirl in a first direction as it enters the combustion chamber (230) and be mixed with process gas and fuel swirling in the opposite direction. Particulates may be removed (250) from the combustion chamber (230).

Description: INCINERATOR FOR COMPLETE OXIDATION OF IMPURITIES IN A GAS STREAM BACKGROUND OF THE INVENTION The present invention relates to an apparatus and method for completely incinerating oxidizable impurities in a process gas, including a nonflammable gas stream. It is particularly suitable for treatment of waste gases from semiconductor fabrication processes, and other gases containing highly toxic, pyrophoric, or otherwise undesirable impurities. Semiconductor manufacturing processes utilize a variety of chemicals, many of which have extremely low human tolerance levels. Such materials include gaseous hydrides of antimony, arsenic, boron, germanium, nitrogen, phosphorous, silicon, selenium and other chemical elements. Typically used compounds include arsine, diborane, germane, hydrogen selenide, phosphine, silane, stibine and many others including halogenated derivatives of the above hydrides, hydrogen chloride, tungsten hexafluoride and a wide variety of metal-organic compounds as well. Acceptable exposure levels for certain of these materials are as low as 50 parts per billion in air that may be inhaled. A significant problem has been the removal of these materials from the effluent gas streams of semiconductor manufacturing processes. While virtually all U.S. semiconductor manufacturing facilities utilize scrubbers or other related means for treatment of their effluent gases, that technology is not capable of removing all toxic or otherwise unacceptable impurities. Although the levels of arsine, phosphine and other similarly toxic materials in typical process gas streams are relatively low, and can be further reduced by scrubbers, these levels are, under various state and federal standards, still above acceptable levels. On the other hand, if it were possible to effect substantially complete oxidation of these impurities, those oxidation products could be reduced to acceptable levels by conventional scrubber technology. One solution to this problem, then, is to incinerate the process gas to oxidize the toxic materials, converting them to less toxic form. Conventional incinerators, however, typically achieve less than complete combustion. This is particularly „ so when the impurity to be incinerated is present in relatively low quantities. The problem is compounded when the process stream to be treated is composed primarily of a nonflammable gas such as nitrogen bearing the undesirable ~ impurities. While some of the oxidizable impurities are removed by conventional incinerators, at least a portion of' those impurities can escape the combustion zone, thereby creating at least the potential of toxic discharge into the atmosphere. Another problem with utilization of conventional incinerators is the size of the incinerator and the temperature of the flue gas exiting the incinerator. Many semiconductor fabrication facilities have heretofore used scrubbers as the primary means for removing the impurities in question. In a large number of cases, recent more stringent air pollution regulations have necessitated a reduction in emissions. Even after an incinerator is installed, the scrubber will need to treat the flue gas of the incinerator to remove the oxides of the incinerated impurities. Thus, the incinerator will be installed, in many instances, as a retrofit device. The ducting and other process gas handling - - equipment in many existing facilities is not designed fo high temperature gas, and flue gas from conventiona incinerators can melt plastic ducting and/or create fir hazards. Because retrofit applications are contemplated, the siz of the incinerator is also a major concern. Desirably, th incinerator can fit and utilize existing ducting withou requiring substantial additional space. A further limitation of conventional incinerators i their lack of ability to both mix sufficient fuel with nonflammable process stream in order to render it flammabl and then to also achieve complete combustion of this fue enriched gas. The choice of such fuel gas to be mixed with nonflammable process gas is also important from th perspective of maintaining low operating costs, and th design must reflect this choice of fuel if proper burnin characteristics are to be achieved. Another problem with existing incinerators is thei inability to handle the formation of solid oxides. Th oxidation products of certain gaseous impurities in the wast gas from semiconductor and other processes are solids. Fo example, silanes oxidize to silica (sand) . Accumulations o silica in most incinerators represents a substantia maintenance problem when those devices are used in t treatment of silane-containing gas streams. In addition to converting toxic materials to their les toxic oxides, controlled incineration is desirable when t waste gas stream contains potentially flammable or explosi materials. Some semiconductor process gas streams, f example, contain substantial quantities of hydrogen gas organic solvents, presenting obvious explosion hazard Similarly, the presence of pyrσphoric gases such as silan can also present an explosion hazard. (Such pyrophor materials spontaneously ignite in air.) An additional hazard potential exists when pipes leadi to the incinerator contain premixed combustible mixtur which might be ignited by the incinerator flame and then propagate a flame backwards into the pipe with concomitant explosion potential. When conventional flame arrestors are used to prevent such flashbacks from occurring, they tend to plug up very quickly because of the solid oxide particles which are typically present in such mixtures, particularly those containing silane. Accordingly, one object of the present invention is to provide an incinerator having increased ability to oxidize virtually all oxidizable components in a waste gas stream. Another object of the present invention is to provide controlled combustion of flammable or explosive components of a waste gas. Still another object of the present invention is to provide an incinerator that is compact and can merge easily with existing ductwork. Another object of the present invention is to provide an incinerator which converts a nonflammable gas stream into a flammable stream through the mixing in of additional fuel gas such that the entire mixture is burned and not merely heated in the incinerator. A further object is to provide an incinerator which can effectively make use of inexpensive fuel in carrying out the conversion of nonflammable to flammable mixtures. Yet another object is to provide an incinerator that has an effluent flue gas which is sufficiently cool to be compatible with existing ducting. Another object is to provide an incinerator with improved capabilities for handling solid particulate combustion products. An additional object of the present invention is the prevention of flashbacks without the associated plugging which typically occurs with use of conventional flame arrestors. A further object is to reduce the tendency for all reactions to accelerate within the inlet pipe close to the lame by cooling the inlet section. Of particular interes among these reactions are those whose solid products o reaction tend to deposit in the pipes and clog them, as wel as those reactions which accelerate flame speed a temperature is raised. The incinerator of the presen invention employs active cooling of the inlet pipe. Yet another object of the present invention is t increase the flow speed of gases introduced therein, such a nitrogen, which further reduces the likelihood of flashback by exceeding the normal flame propagation speed in the inle section and by tending to dilute incoming combustibl mixtures to below their fla mability limits. Sufficien additional fuel to render the mixture flammable again i subsequently added downstream. The incinerator of the present invention is also able t burn oil mists that are generally non-combustible in othe units, such as fluorinated oils used in vacuum pumps. Suc vacuum pump oil mists tend to interact with silanes, halogen and other related compounds which are present in the pum exhaust. If the oil mists are not burned, these chemica interactions can lead to formation of hazardous soli materials in plant ductwork including cumulative depositio of certain explosive polymers. This problem is eliminated b the incinerator of the present invention, which is capable o burning such oil mists before they create a potential hazard Finally, another object of the present invention is t provide a unit with low maintenance requirements. These and other objectives are fulfilled by th incinerator of the present invention. SUMMARY OF THE INVENTION Thus, in accordance with one aspect of the presen invention, there is provided a device for oxidizing gaseou materials, comprising a combustion chamber in the device, th device and the combustion chamber both having a top and bottom, the device further having a gas outlet and bein __„, 12636 -6- adapted to operate in conjunction with means for applying negative pressure to the outlet, means ' for introducing process gas and a source of fuel into the top of the combustion chamber, the means located sufficiently below the top of the combustion chamber that solid combustion byproducts do not substantially deposit in the combustion chamber above the means for introducing process gas, means for introducing oxygen carrying gas into the top of the combustion chamber, means for mixing the oxygen carrying gas with the process gas and fuel so that the mixture can burn in the combustion chamber to form a flue gas, means for permitting the flue gas to exit from the top of the combustion chamber, and means for directing the flue gas from the combustion chamber out of the outlet of the device. In one embodiment, the device further comprises a removable tray beneath the combustion chamber for collecting solid products of combustion. In another embodiment, the device further comprises means in the combustion chamber for directing the mixture as it is burning outward toward the wall of the combustion chamber. In one preferred embodiment, the mixture directing means has a conical configuration and has an apex at the A top thereof. Another variation of the device further comprises a mechanical scraper for removing solid products of reaction from the mixture directing cone and from the walls of the combustion chamber. A further variation comprises an additional mechanical scraper for removing solid products of combustion from the process gas/fuel inlet and mixing means. In accordance with another aspect of the present invention, there is provided a device for oxidizing gaseous materials, comprising a combustion chamber in the device, the device and the combustion chamber having a top and a bottom and a vertical axis, a preheater chamber located radially outside of the combustion chamber adapted to preheat oxygen- containing gas, a mixing zone at the top of the combustion chamber, means for introducing oxygen-containing gas into the preheater chamber, means for introducing oxygen-containi gas from the preheater, fuel, and impurity-containing proce gas into the mixing zone, a first swirler for swirling t oxygen-carrying gas entering the mixing zone in a fir direction, a second swirler for swirling the process g entering the mixing zone in a second direction, an opening the bottom of the combustion chamber through.which flue g can exit the chamber, and means for removing accumulations particulate products of combustion from the incinerator. Furthermore, the device may advantageously include a vertic flue located concentrically outside of the preheater chamb for directing the flue gas exiting the chamber upward and o of the top of the device. In another embodiment, the devi may also include means for cooling the flue gas via mixi the flue gas with cooling air. The device may also include a conduit supplying proce gas into the chamber, and means for introducing sufficie nonflammable gas into the conduit to raise the flow rate gas in the conduit above the flame propagation speed for t gas in the conduit. Another embodiment further compris means for cooling the conduit via directing the path of fu gas over the outer surface of the conduit. In anoth variation, the device further comprises means interposed the combustion chamber for directing the reacting mixt radially outwardly. In another aspect of the invention, t removing means comprise mechanical scrapers movable insi the combustion chamber and inlet means in contact with surfaces within these means. In accordance with yet another aspect of the pres invention, there is provided a method for oxidizing materi in a gas stream, comprising the steps of introducing fuel process gas into a mixing zone, introducing preheated oxyg carrying gas into the mixing zone and mixing it with the f and process gas to form a burning mixture, directing burning mixture downwardly through a combustion chamber forming a flue gas which exits the combustion chamb preheating the oxygen carrying gas prior to its entry into the mixing- chamber with heat from the oxidation of the fuel, and automatically removing accumulated solid combustion products from walls of the chamber. In another embodiment, the preheating step comprises passing the oxygen-carrying gas upward over the outside of the combustion chamber. Yet another aspect includes a method wherein the mixing step comprises swirling the preheated gas entering the mixing zone in a first direction and swirling the fuel and process gas entering the mixing zone in a second, opposite direction, so that turbulent mixing occurs when the swirling gases collide. The method may also include the step wherein one of the swirling gases is introduced into the mixing zone radially inwardly of another of the swirling gases, and at a point sufficiently below the top of the combustion chamber that solid products of combustion do not accumulate on the top. Alternatively, the method further comprises the step of cooling the flue gas by mixing it with cooling air. In another embodiment, the method further comprises the step of preventing flashback into a conduit supplying process gas into the combustion chamber by reducing the cross sectional area of the conduit and by introducing sufficient nonflammable gas into the conduit upstream of the chamber so that the flow rate of gas in the conduit is greater than the flame propagation speed in the resulting gas mixture. The method also may include the step of cooling the flue gas by mixing it with cooling air, and then directing the cooled flue gas through a particulate removal device, such as a wet cyclone or a filter for removal of entrained solids, and optionally through a scrubber. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of the incinerator of the present invention. Figure 2 is a cross section of the preferred embodiment. Figure 3 is a perspective cut-away view of a combustion chamber/preheater assembly. Figure 4 is an enlarged cross sectional view of a inlet/ nozzle scraper. Figure 5 is an enlarged cross sectional view of a inlet/nozzle scraper assembly and a chamber scraper assembly, taken in the same manner as the cross section of. Figure 2. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention i illustrated in the Figures. Shown in Figures 1-5 is a updraft system suitable for use in handling lightweigh particulate combustion by-products, including those material that tend to accumulate on burner components. With reference to Figure 1, the incinerator 210 of th present invention is preferably a vertically oriented device, having a housing 212, a process gas inlet 214, and a flue ga outlet 216. The housing 212 may further contain an apertur 201 through which a pneumatically activated bypass valve (no shown) is connected to a compressed gas line within housin 212, and apertures 203 for the attachment of incoming fue gas, pressurized gases and electrical lines (not shown) t appropriate structures contained within the housing 212 Control panel 200 may advantageously contain various on/of and reset buttons, status lights and a display windo providing operating parameter information, next ste instructions and fault diagnostic messages as appropriate Louvers 266 provide access for cooling air to enter housin 212. In operation, process gas containing oxidizabl impurities is introduced into the incinerator 210 through th process gas inlet 214, where it is combined with fuel ga (such as natural gas) entering the fuel gas inlet 220 (show in Figure 2) and with combustion air or other oxygen containing gas, and is oxidized, generating a flue gas. Th flue gas exits the housing 212 of the incinerator 210 throug the flue gas outlet 216, which is preferably situated at th 12636 -10- top 222 of the incinerator 210. The preferred embodiment illustrated in Figures 2-5 includes a number of features that facilitate operation when treating materials that generate particulate by-products and includes features which remove such particulates from interior surfaces of the incinerator. With reference to Figure 2, the general design of the incinerator 210 of the preferred embodiment is illustrated. In this design, the incinerator 210 has a housing 212 and a process gas inlet 214 substantially similar to those of Figure 1. Within the housing 212 is contained a combustion chamber 230. Exhaust gases leaving the combustion chamber 230 of this embodiment of the incinerator 210 exit at a flue gas outlet 216. In most instances, the incinerator 210 is also provided with a pilot light chamber 263. Process gas containing oxidizable impurities is introduced into the incinerator 210 through the process gas inlet 214, where it is combined with fuel gas (such as natural gas) entering the fuel gas inlet 220 and with combustion air or other oxygen-containing gas, and is oxidized, generating a flue gas. The flue gas exits the housing 212 of the incinerator 210 through the flue gas outlet 216 located on top of flue gas plenum 215. In operation, process gas containing impurities enters the housing 212 through the process gas inlet 214. This inlet 214 preferably carries the process gas radially inward to the central axis of the incinerator 210, which runs vertically through the center of the incinerator 210. The process gas inlet 214 then directs the process gas downwardly along the central axis of the incinerator 210. At this point, fuel gas enters through fuel gas inlet hole(s) 223 and is mixed with the process gas. The path of fuel gas from inlet 220 to inlet hole(s) 223 may be directed by a jacket over the outer surface of the process gas inlet 214 thereby providing convective cooling to the inside wall of process gas inlet 214. Additionally, nitrogen for flashback protection may be introduced into the process gas inlet 214 through inlet 218. A total gas flow above 1 scfm in inlet 214 will exceed the flame propagation speed at ordinary temperatures for most combustible mixtures except those containing premixed hydrogen and oxidizer. The downwardly directed mixture of fuel and process gas is further thoroughly mixed with air as it enters combustion chamber 230. This mixing is preferably accomplished by means of a ring nozzle which is represented in Figure 2 as element 224. A sight tube 233 (shown in Figure 3) for attachment of a UV detector is located at the top of combustion chamber 230 adjacent to ring nozzle 224. The UV detector (not shown) functions as a control system to monitor the presence of flame within the combustion chamber. In one embodiment, the ring nozzle 224 is also connected to a pressurized air line 226 which imparts a swirl to the combined fuel gas and process gas. The ring nozzle 224 is advantageously an annular nozzle surrounding the pathway of the process gas as it leaves the process gas inlet 214 in a downward direction along the vertical axis of the incinerator 210. The ring nozzle 224 may comprise a plurality of sonic jets directing air or other pressurized gas around and into the process gas to effect mixing of the process gas, the fuel gas, and the air from the ring nozzle 224. To the extent that any combustion is occurring at the ring nozzle 224, the high velocity within the sonic air jets (which will also function effectively when subsonic) prevents plugging of the jet orifices. Such plugging might otherwise occur through reaction between the injected oxygen and silane in the process gas to form solid reaction products in the jets. It is preferred that the sonic jets are directed not only in a spiral direction, but also are canted downward. This tends to draw the process gas into the combustion chamber 230, with the sonic jets acting as an ejector pump, preventing bac pressure from being exerted on the process gas inlet lin 214. The ring nozzle 224 preferably injects air throug 12636 -12- orifices distributed all the way around the process gas inlet 214. The mixture of fuel gas and process gas exiting the ring nozzle 224 travels downward into a combustion chamber 230 located vertically below the ring nozzle 224 along the vertical axis of the incinerator 210. There the fuel gas and process gases are combined with combustion air that has been preheated by means of a heat exchanger 234. The heat exchanger 234 in the illustrated embodiment comprises a jacket 236 located radially outwardly from and circumferentially surrounding the outer wall 240 of the combustion chamber 230. The combustion air is preferably directed into the heat exchanger 234 by means of a fan 242, which is preferably located at the bottom 244 of the incinerator 210. The fan 242 pushes outside air up through a bifurcated air pipe 243 into the heat exchanger 234. The combustion air leaves the pipe 243 and travels into the heat exchanger 234, which is preferably annular in cross section. The air then moves upward through a swirl inducing ring 254 in the heat exchanger 234. The swirling air then reverses its upward component of direction 180 degrees as it enters the top of the combustion chamber 230. Turbulent mixing of the preheated combustion air and the fuel gas and process gas occurs in a mixing zone 256 in the top of the combustion chamber 230. The hot combustion air entering the mixing zone is swirling in the opposite direction of the fuel/process gas stream stirred by the sonic mixing jets, and the relative angular momentums of the swirling streams are such that little or no swirling is present in the downwardly-traveling gas mixture within and exiting the combustion zone. Particulate matter exiting the bottom of the combustion chamber 230 is normally entrained in the upwardly moving cooling air flow within flue gas plenum 215. Any dense material falling out of the air flow is collected in a catch tray 250 located beneath the combustion chamber and supported atop base support 259. The various support mechanisms beneath the combustion chamber 230 are of an ope construction to allow cooling air to pass upwardly past the and past catch tray 230 into flue gas plenum 215. During startup of the incinerator, and as otherwis needed, an igniter 260 (which can be a conventional spar plug) is used to ignite an air/fuel gas mixture create within pilot chamber 263 to produce a continuous pilot flam extending from pilot chamber 263 into the combustion chambe 230. (Pilot air inlet 221 and pilot fuel gas inlet 225 ar shown in Figure 3) . The pilot flame assures ignition of th process gas/fuel/air mixture present within the combustio chamber and is located on the blind side of the optical pat provided by sight tube 233 for the UV detector. This insure that the UV detector will see no flame should the mai combustion flame extinguish but the pilot flame continue t burn. Located just below the mixing zone 256 in the top of th combustion chamber 230 is a flow director 262 supported by fixed rod 280. The flow director 262 urges the downwardly moving burning gas leaving the precombustion zone 256 in radially outward direction. The flow director 262 ma advantageously be conical in configuration, as illustrated i Figure 2, with its apex located in mixing zone 256 an situated symmetrically about the vertical axis of th incinerator 210. The flue gas generated by the combustion process exit down and out of the combustion chamber 230. The flue ga then reverses direction 180 degrees as it is entrained in an mixes turbulently with a large volume of cooling air draw upwardly through flue gas plenum 215. In one preferre embodiment, the incinerator is adapted to operate wit negative pressure applied to the flue gas outlet. Thi negative pressure, for example, is applied to the ductwor into which the flue gas travels as it leaves the incinerato 210 by a conventional scrubber or other device. Suc negative pressure may be used to further cool the flue gas b 0/12636 _ _ drawing outside cooling air into the incinerator through louvers 266 on the housing 212 of the incinerator 210. These louvers 266 are preferably located below the combustion chamber 230. Convective cooling of the housing 212 is assured by the provision for a gap between collar 211 (attached to the external ducting) and the smalle concentric .cylindrical portion of flue gas outlet 216, in that some of the cooling air also flows through this gap and must reach it from below by moving upwardly through the space between the inside surfaces of housing 212 and the outer surface of flue gas plenum 215. As a result, the oute housing 212 of the incinerator 210 remains cool. In addition, the continuous flow of air through this gap prevents any accumulation of undesirable gas inside the uppe portions of housing 212 should any internal leaks occur. Another aspect of the preferred embodiment of the incinerator 210 that is particularly significant is the * provision of scrapers for removal of particulates from interior surfaces of the incinerator. As illustrated in Figures 2,4,5, there is a first scraper 310, an inlet/nozzle scraper, located within tube 312. This inlet/nozzle scraper 310, as shown in Fig. 4, comprises a body 314, two centering rings 316 located annularly over the body 314 for maintainin it centered within the tube 312, a lower post 318 extendin downwardly from the body 314, and a cap 320 threaded into th top of the body 314 and compressing a spring 323 inside of the body 314, which in turn biases a piston 324 downwardly. The piston 324 extends through the center of the body 314 to the bottom thereof. Scraper 310 further comprises one o more extensible scraper blades 322 extending verticall downward and radially outward from the body 314, which ar spring loaded to extend radially outward against the insid of the tube 312 by means of compressed spring 323 and pisto 324. In the illustrated embodiment, multiple blades 322 ar shown, albeit it is to be understood that the number o blades may vary without altering the spirit and scope of th invention. Post 318 is provided to dislodge accumulate particulate material at the top of the flow director 262. A cap 320, scraper 310 is attached to a lead screw/flexibl drive device used for rotating, raising and lowering th inlet/nozzle scraper 310 up and down within tube 312, a shown in Figures 2 and 5. Figure 5 shows the two typica extreme positions of movable inlet/nozzle scraper 310. The rotating, raising and lowering of inlet/nozzl scraper 310 is accomplished as follows: a flexible driv cable 284 is attached to scraper 310 at one end and to a lea screw drive mechanism 299 at the other end. A fixed body 28 houses a nut through which the lead screw 288 turns whe rotated by a reversing drive motor 290. As the lead scre 288 advances through fixed nut 292 it simultaneously rotate and advances flexible drive cable 284 which is attache firmly to the end of the lead screw 288. Attached firmly t the other end of drive cable 284 is the scraper 310 whic then rotates and advances with the drive cable. Drive moto 290 is mounted on the lower end of the lead screw an advances with it. Additionally, a small flow of inert or no reactive gas such as nitrogen is introduced into the fixe body 286 through inlet 298 in order to purge the drive cabl conduit and keep it free of reactive or corrosive proces gases. This purge gas enters tube 312 at the top and flow past the parked inlet/nozzle scraper 310 thereby cooling it keeping its guide rings 316 open and preventing process ga from entering the portion of tube 312 above the plane of th process gas inlet 214. This upper portion of tube 312 i which the scraper resides most of the time is unavoidabl heated by radiation from the hot combustion chamber. Figure 5 indicates how the drive motor housing an mounting are constrained from rotating by guide rods 29 which slide along a vertical guide bar 296 as the moto advances up or down with the rotating lead screw. Revers of rotation and of advancement direction takes place wh electrical limit switches are tripped at the ends of t travel and the motor 290 is reversed. As rotating inlet/nozzle scraper 310 advances downwar through tube 312 and past the mouth 213 of the process gas inlet tube 214 it dislodges deposited solids from the inne wall of the tube, thereby preventing gradual buildup o solids and eventual plugging of inlet mouth.213, tube 312 o ring nozzle 224. As the inlet/nozzle scraper , 310 continues to move down, extensible blades 322 are biased in such a manner that, when they are extended beyond the ring nozzl 224, they are able to open up radially outwardly and scrap particulate material lying very near to the sonic mixing jet (not shown) of the ring nozzle 224. This is necessary t preserve good mixing in mixing zone 256 where any physical interference between the incoming process gas/fuel mixtur and the mixing jets can significantly degrade performance. When inlet/nozzle scraper 310 is withdrawn back into tub 312, blades 322, due to their biasing, may easily be draw back into ring nozzle 224 and tube 312. An inlet/nozzl scraping cycle is completed when the scraper returns to it parking position at the top of the tube 312 where it reside above the inlet gas flow. One cycle, from top to nozzle an back again, takes about forty seconds to complete. The cycl may be repeated as often as desired, typically once ever half hour. A second, or chamber, scraper 340 is positioned withi combustion chamber 230 and, as shown in Figures 2 and 5, comprises a center support bearing 342 located on the central axis of the combustion chamber 230 below the bottom of sai chamber; a long radially extending support spoke 344 connected to the bearing 342 which supports a long uprigh scraper blade 346 extending vertically into the combustio chamber 230 against the inside wall thereof, preferabl extending from bottom to top; a short radially extendin support spoke 348 connected to the bearing 342 which support the bottom end of a short upright bar 350, which is furthe connected at its top end at an angle of about 60° to -17- scraper blade 352 extending vertically along the outside o the conical flow director 262. Center support bearing 342 i rotatably attached at its lower end to a chain driv mechanism (not shown) , distal to the central pivotal axis Bearing 342 is fitted with a sprocket connected by a driv chain to a sprocket on a motor located below motor 290. Thi chamber scraper motor runs continuously and turns chambe scraper 340 slowly at several revolutions per minute. During the operation of the incinerator of the preferre embodiment, the inlet/nozzle scraper 310 and the chambe scraper 340 function to remove particulate materials fro within the combustion chamber 230. As noted previously, th inlet/nozzle scraper 310 removes particulates from within th tube 312, from the jets of the ring nozzle 224, and from th apex of the flow director 262. Additionally, scraper blade 346 and 352 rotate continuously. Scraper blade 346 remove particulates from the interior cylindrical wall of th combustion chamber 230, while scraper blade 352 remove particulates from the conical surface of the flow directo 262. As particulates are dislodged, they drift out th bottom of chamber 230 and are either entrained into th upwardly moving cooling air within plenum 215 or else fal out of the flow and into catch tray 250. As illustrated i the Figures, in the preferred embodiment, the chamber scrape support and drive apparatus, i.e., the spokes, chain, bearin and motor, are located below the combustion chamber and flo reversal plane as characterized by the arrows indicatin direction of gas flow, in order to avoid excessive heating o fouling with particulates, and to minimize intrusion into th combustion chamber of mechanical surfaces which can provide foothold for particulate deposition leading to eventua blockage of the chamber exit. During the operation of a typical incinerator 21 having a 2 inch process gas inlet 214 and a combustio chamber volume of 1/3 cubic foot, process gases comprising 0 3 scfm N2 with 0-1 scfm H2 or 0 2 and smaller amounts o ι vr_ on / ι __ WO 90/12636 _^8- oxidizable impurities such as arsine, silane, phosphine, diborane and halogenated hydrocarbons are combined with sufficient natural gas or other gaseous fuels to sustain combustion (typically from 0-2.5 scfm) _ Compressed air is 5 introduced into the swirl nozzle 224 at a rate of from 0.5 to 3 scfm, 80 psig. The preheated air from the fan 242 enters the combustion zone at a rate of from about 10-40 scfm, at a preheated temperature of about 400 degrees C. The negative pressure applied to the flue gas outlet draws air and gas out 10 of the incinerator at a rate of about 400-600 scfm. The incinerator 210 houses a control system with electronic controls, plumbing, automatic valves, various sensors, power supplies and other appurtenances (not shown) generally located in the space below catch tray 250 and 15 mounted to vertical panels in the manner illustrated by the mounting of fan 242. The controls are operated from control panel 200. The general nature and functions of the control system will be described briefly. A microprocessor (such as an Intel 8051, Intel Corp., 20 Santa Clara, CA) works in conjunction with a UV detector/controller (e.g., an Allen Bradley Fireye®, Waltham, Mass.) to operate a number of normally closed solenoid valves, sensors, switches and the like, and to provide status information to the operator. Input gas pressures, operating 25 temperatures, fan pressure, duct draw and natural gas back pressure are sensed and input to the microprocessor. Resident firmware performs a self test and turns on the UV detector/controller if all is well. If the UV detector/controller loses power or fails for any reason the 30 spring loaded fuel gas valve closes, the combustion air fan stops, there is no spark, combustion ceases and the incinerator shuts down. The UV detector/controller has direct control of the essential safety functions (fuel, air, ignition) . If the UV detector/controller shuts the system 35 down it cannot be restarted by the microprocessor until the UV controller has been reset manually. The microprocessor prompts the operator that the manual reset is needed and tells when pressing the reset button is likely to b successful. Alternatively, if any of the inputs to the microprocessor indicate a fault, the firmware can instruc the UV detector/controller to shut the system down and th microprocessor can simultaneously take action to notify th operator about the nature of the specific problem and what t do next. For example, if an internal blockage were t restrict the entrance of natural gas into the combustor, th back pressure would begin to rise. Long before the bac pressure approaches the line pressure, the back pressur switch will trip and the natural gas valve will close, thereby protecting the gas line from possible back streaming. The microprocessor will sound an audible alarm and place th message "fuel pressure low" in the display window along wit instructions about the next action to be taken. Simultaneously the UV detector will sense a flame ou condition and its controller will shut the incinerator down. If chamber or exhaust temperatures exceed preset limit during operation, the microprocessor will instruct the U controller to shut the system down. If any compressed ga pressure falls below its preset limit the microprocessor wil close the valve for that gas and similarly call for syste shut down. In all such cases the operator is provided wit an interlock signal which can be used to automatically shu down his process or, alternatively, the control system wil switch a compressed gas solenoid to actuate a pneumatic valv in order to divert the process gas into another operatin unit (valve normally diverted) . In the event of a scrape failure . the microprocessor sounds the alarm and indicate which scraper is out but does not divert the flow or shu down the process, since there is little immediate danger an there is a high probability that the batch can be complete successfully before plugging of the incinerator become severe. Although the present invention has been described in th 12636 .__. context of certain preferred embodiments, it will be understood that modifications are possible without departing from the spirit of the invention. Thus, it is intended that the scope of this patent be measured by the claims that follow, and not be limited to these preferred embodiments.

Claims: .__. WHAT IS CLAIMED IS: 1. A device for oxidizing gaseous materials, comprising: a combustion chamber in said device, said device and said combustion chamber both having a top and a bottom, said device further , having a gas outlet and being adapted to operate in conjunction with means applying negative pressure to said outlet; means for introducing process gas and a source of fuel into the top of said combustion chamber, said means located sufficiently below the top of said combustion chamber that solid combustion byproducts do not substantially deposit in said combustion chamber above said means for introducing process gas; means for introducing oxygen carrying gas into the top of said combustion chamber; means for mixing said oxygen carrying gas with said process gas and fuel so that said mixture can burn in said combustion chamber to form a flue gas; means for permitting said flue gas to exit from the top of said combustion chamber; and means for directing said flue gas from said combustion chamber out of the outlet of said device. 2. The device of Claim 1, further comprising a removable tray beneath said combustion chamber for collecting solid products of combustion. 3. The device of Claim 1, further comprising means in said combustion chamber for directing said mixture as it is burning outward toward the wall of said combustion chamber. 4. The device of Claim 3, wherein said mixture directing means has a conical configuration and has an apex at the top thereof. 5. The device of Claim 1, further comprising a mechanical scraper for removing solid products of combustion from the mixture directing cone and from the walls of said combustion chamber. 6. The device of Claim 1, further comprising a mechanical scraper for removing solid products of reaction from the process gas/fuel inlet and mixing means. 7. A device for oxidizing gaseous materials, comprising: a combustion chamber in said device, said device and said combustion chamber having a top and a bottom and a vertical axis; a preheater chamber located radially outside of said combustion chamber adapted to preheat oxygen-containing gas; a mixing zone at the top of said combustion chamber; means for introducing oxygen-containing gas into said preheater chamber; means for introducing oxygen-containing gas from said preheater, fuel, and impurity-containing process gas into said mixing zone; a first swirler for swirling said oxygen- carrying gas entering said mixing zone in a first direction; a second swirler for swirling said process gas entering said mixing zone in a second direction; an opening at the bottom of the combustion chamber through which flue gas can exit said chamber; and means for removing accumulations of particulate products of combustion from said incinerator. 8. The device of Claim 7, further comprising a vertical flue located concentrically outside of said preheater chamber for directing said flue gas exiting said chamber upward and out of the top of said device, and means for cooling said flue gas via mixing said flue gas with cooling air. 9. The device of Claim 7, further comprising a conduit supplying process gas into said chamber, and means for introducing sufficient nonflammable gas into said conduit to raise the flow rate of gas in said conduit above the flame propagation speed for the gas in said conduit. 10. The device of Claim 9, further comprising means for cooling said conduit via directing the path of fuel gas over the outer surface of said conduit. 11. The device of Claim 7, further comprising means in said combustion chamber for directing said mixture radially outwardly. 12. The device of Claim 7, wherein said removing means comprise mechanical scrapers movable inside said combustion chamber and inlet means in contact with the surfaces within these means. 13. A method for oxidizing materials in a gas stream, comprising the steps of: introducing fuel and process gas into a mixing zone; introducing preheated oxygen-carrying gas into said mixing zone and mixing it with said fuel and process gas to form a burning mixture; directing said burning mixture downwardl through a combustion chamber and forming a flue gas which exits said combustion chamber; preheating said oxygen carrying gas prior t its entry into said mixing chamber with heat fro said oxidation of said fuel; and automatically removing accumulated soli combustion products from walls of said chamber. 14. The method of Claim 13, wherein said preheatin step comprises passing said oxygen-carrying gas upward ove the outside of said combustion chamber. 15. The method of Claim 13, wherein said mixing step comprises swirling the preheated gas entering said mixing zone in a first direction and swirling the fuel and process gas entering said mixing zone in a second, opposite direction, so that turbulent mixing occurs when said swirling gases collide. 16. The method of Claim 15, wherein one of said swirling gases is introduced into said mixing zone radially inwardly of another of said swirling gases, and at a point sufficiently below the top of said combustion chamber that solid products of combustion do not accumulate on said top. 17. The method of Claim 13, further comprising the step of cooling said flue gas by mixing it with cooling air. 18. The method of Claim 13, further comprising the step of preventing flashback into a conduit supplying process gas into the combustion chamber by reducing the cross-sectional area of the conduit and by introducing sufficient nonflammable gas into said conduit upstream of said chamber that the flow rate of gas in said conduit is greater than the flame propagation speed in the resulting gas mixture.

Inventors: ANDERSON LAWRENCE B, FRIELER CLIFF, HAMMON TIMOTHY EDWARD, ANDERSON, LAWRENCE, B., FRIELER, CLIFF, HAMMON, TIMOTHY, EDWARD
Applicants: CUSTOM ENGINEERED MATERIALS IN, CUSTOM ENGINEERED MATERIALS, INC.
Application year: 1990
Main classification: B01D53
IPC codes: B01D53, F23G7, F23J1, F23M9