Regenerative thermal oxidizer

Abstract: An apparatus (10) having an incineration chamber (12) and at least one burner (14,16) for oxidizing fumes is provided. First and second regenerators (18,20) are in fluid communication with the incineration chamber, as is a bypass (22) which introduces unburnt fumes to the incineration chamber without passing them through either of the regenerators. While the fumes are in the bypass, a purging device (24), including a purge fan (56) and accompanying conduits (52,54) and valves (58), introduces a purge gas to either one of the regenerators to force unburnt fumes therefrom. The purged fumes and the purge gas are mixed with the incoming fumes from the bypass in an annular plenum (47) downstream of the purged regenerator before they are introduced to the incineration chamber for oxidation. The flow of incoming fumes to the system may be continuous, even during purging, and the purge fan may also be continuously operated.

Description: This invention relates to regenerative incinerators for thermally oxidizing contaminated fumes as described in the preamble of claim 1 and to a method of oxidizing fumes as described in the preamble of claim 9. Such apparatus and method are known from the prior art document US-A-3870474. Incinerators are frequently employed to destroy harmful emissions resulting from various processes. Frequently, incinerators are used to oxidize light hydrocabron emissions. For example, the finishing line on an aluminium strip coating process may emit toluene, which is directed with the finishing line exhaust to a downstream incinerator where toluene and other harmful emissions are oxidized at high temperatures. The incinerator exhaust is then suitable for introduction to the atmosphere, or it may be recycled to meet other plant energy needs. Incinerators are also applied in conjunction with food processing to control odors, pharmaceutical and fragrance manufacturing, painting and printing and many othter applications. Thermal regenerators, including beds of ceramic materials, may be included in the incinerator design. The regenerative beds greatly increase the overall thermal efficiency of the incinerator (as high as 95%), reducing annual fuel costs and maximizing contaminant destruction rates within the incinerator. The contaminated fumes are typically raised to temperatures of 650°C to 1200°C (1,200 °F to 2,200 °F) within the regenerator before being introduced to the incinerator. The main problem with regenerators is that contaminated fumes are left within the regenerative bed when flow through the system is reversed and the bed is switched from the preheating mode to the exhaust mode. There is a risk that these contaminants may be emitted into the atmosphere with incinerator exhaust. The prior art has generally addressed the problem of residual contaminants by including a purging means with the incinerator to force contaminated fumes from the bed while the bed is between preheating and exhaust cycles. For example, the above-identified US Patent No. 3 870 474 to Houston provides a purging means for a system having three regenerators. A first regenerator preheats contaminated fumes prior to incineration while a second regenerator receives and extracts heat from products of incineration. A third regenerator at the same time receives a purge of treated or purified air to force any untreated or contaminted fumes from the regenerator into the incineration chamber. In another form, this system has two regenerators, and a vacuum surge tank is in fluid communication with each regenerator. When flow in the system is reversed, the vacuum surge tank is placed in fluid communication with the appropriate regenerator by a four-way valve and a surge tank valve, and the contaminants within the regenerator are drawn into the surge tank. The contaminants are then evacuated from the surge tank by a vacuum pump, which places the contaminants back into the contaminant inlet. There are several problems with the vaccuum design. First, the vacuum system presents a risk of emitting untreated, contaminated fumes to the atmosphere when the regenerative cycle in the incinerator is reversed. The four-way valve which controls the flow of perfect synchronization with the valve which admits contaminants into the surge tank. If the surge tank valve is opened an instant later then the reveral of flow, a small amount of contaminants will be emitted through the vent to the atmosphere. Over extended periods of time, this could amount to substantial volumes of untreated fumes exhausted to the atmosphere. Second, repeated application of a strong vacuum to the entire system substantially decreases the useful life of various parts of the system, especially the valves. Particularly, the surge tank valve and a flap valve on the exhaust vent would require exceptional durability standard. Finally, the purging means, namely the vacuum surge tank, the surge tank valve and the vacuum pump, present added maintenance requirements and initial installation costs, and they may also be problematic in situations where overall system weight is a concern, such a rooftop installations. Further regenerative incinerator designs may be seen in US Patent Nos. 4 874 311; 4 650 414; 4 474 118; 4 454 826; 4 302 426; 3 895 918; 3 634 026; 3 211 534 and 1 940 371. Additionally, a publication by Proctor and Schwartz, Inc., dated 1971, discloses a regenerative air purification system having two regenerators and a purge valve, which opens briefly to flush residual contaminated gas into the purification chamber. The disadvantage with this system is that the flow of incoming contaminated fumes must be completely halted while purging is taking place. This may require fans for the contaminated fumes and the purge gas to be frequently stopped and started, and it may also include further undesirabel complications for the upstream system from which the contaminants originate. It is therefore an object of the present invention to provide a regenerative incinerator having purging means which do not result in emission of untreated contaminants to the atmosphere when the purging means are activated. It is a further object to provide a regenerative incinerator with purging means that are relatively compact, lightweight and suitable for rooftop installations. It is a still further object to provide a regenerative incinerator having purging means which require low maintenance and which may be continuously operated to avoid frequent stops and starts and to avoid placing frequent sudden stresses on the overall system. Finally, it is an object of the present invention to provide a regenerative incinerator with a plenum for thouroughly mixing contaminated gases with purge gas and for introducing the mixture to an incineration chamber in a manner that ensures maximum destructive efficiency of the incinerator system. In accordance with the present invention, these objects are attained by an apparatus for oxidizing fumes, comprising: an incineration chamber; at least one burner directed into said incineration chamber; a first regenerator in fluid communication with said incineration chamber; a second regenerator in fluid communication with said incineration chamber; said first regenerator preheating unburnt fumes prior to oxidation while said second regenerator extracts heat from oxidized fumes in a first cycle; said second regenerator preheating unburnt fumes prior to oxidation while said first regenerator extracts heat from oxidized fumes in a second cycle, wherein the flow of fumes through said apparatus is reversed; which apparatus is characterized by: a bypass in fluid communication with said incineration chamber during a purge cycle, intermediate said first cycle and said second cycle, without passing the unburnt fumes through said first or second regenerator; means for selectively directing the unburnt fumes either into said bypass during said purge cycle or into said first or second regenerator during said first or second cycle, respectively; and a purging device for selectively introducing a purge gas either one of said first or second regenerators during said purge cycle to purge unburnt fumes therefrom and to direct said purged unburnt fumes to said incineration chamber for oxidation. Preferred embodiments of the apparatus of the invention form the subject matter of the dependent claims 2-8. The invention further relates to a method of thermally oxidizing contaminated fumes with which the above objects may be obtained. To this end, the invention provides a method for oxidizing fumes in an incineration chamber, said method having a first cycle followed by a second cycle, comprising the steps of: (a) introducing unburnt fumes to an inlet; (b) directing the unburnt fumes to a first regenerator in said first cycle wherein said unburnt fumes are preheated; (c) oxidizing the preheated unburnt fumes in the incineration chamber. (d) directing the oxidized fumes to a second regenetor, wherein heat is extracted from said oxidized fumes, (e) after the second regenerator reaches a predetermined temperature, diverting the unburnt fumes in step (a) into said second regenerator to initiate said second cycle and preheat the unburnt fumes; (f) oxidizing the preheated unburnt fumes of step (e) in the incineration chamber; and (g) directing the oxidized fumes of step (f) to said first regenerator wherein heat is extracted from the oxidized fumes, said method being characterized by a purge cycle between said first and second cycles, said purge cycle comprising the steps of: (h) after the heat extracting regenerator of the first cycle reaches a predetermined temperature diverting the unburnt fumes introduced into said inlet into; a bypass to initiate said purge cycle, thereby placing the unburnt fumes downstream of the first regenerator without passing them through said first regenerator; (i) introducing a purge gas to said first regenerator to purge the unburnt fumes therefrom and to preheat the purge gas; (j) mixing the purge gas with the unburnt fumes from said bypass downstream of said first regenerator and upstream of the incineration chamber; (k) introducing said mixture to the incineration chamber to oxidize the unburnt fumes; and (l) after the unburnt fumes are completely purged from said first regenerator, diverting the unburnt fumes introduced into said inlet from said bypass to said second regenerator to initiate said second cycle. Claim 10 describes a preferred way of performing the method of the invention. Further aspects and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings. Fig. 1 is a schematic view of a regenerative incineration system operating in a first cycle in accordance with the present invention; Fig. 2 is a schematic view of the system of Fig. 1 operating in a first purge cycle; Fig. 3 is a schematic view of the system of Fig. 1 operating in a second cycle; Fig. 4 is a schematic view of the system of Fig. 3 operating in a second purge cycle; Fig. 5 is a cross-section of a burner having an annular plenum in accordance with the present invention; and Fig. 6 is a front view of the burner of Fig. 5. Fig. 1 shows an apparatus 10 for oxidizing fumes which has an incineration chamber 12 with a pair of burners 14, 16 directed into the incineration chamber 12. A pair of regenerators 18,20 are associated with the burners 14,16 and are in fluid communication with the incineration chamber 12. A bypass 22 is also in fluid communication with the incineration chamber 12 to introduce unburnt fumes 60 to the incineration chamber without passing them through either of the regenerators 18,20. A purging device 24 purges unburnt fumes from the regenerator 18,20 prior to reversal of flow through the apparatus 10. While each of the regenerators 18,20 is being purged, the fumes previously passing through that regenerator are diverted to the bypass 22 and introduces into the incineration chamber 12. This ensures that the flow of incoming fumes 60 through an inlet 26 of the apparatus 10 may be constant and that no unburnt fumes will escape from the apparatus 10 through an exhaust 28 into the atmosphere during purging. Specifically, the incineration chamber 12 is lined with a fibrous ceramic material (not shown), and it is generally sized to accommodate a throughput of, for example, 280 cubic metres (10,000 cubic feet) per minute. Referring to Fig. 5, the first burner 14 has a concentric duct 30 and a port block 32 which is intermediate the duct 30 and the incineration chamber 12. A fuel line 34 termines in a nozzle 36 adjacent the upstream end of the port block 32. A fuel line sleeve 38 receives a pilot air/gas mixture, which is admitted through a pilot inlet 40. A cooling sleeve 42 encloses the fuel line sleeve 38, with cooling air admitted through a cooling inlet 44. A duct inlet 46 admits a first portion of the incoming fumes into the duct 30. the burner 14 is sized to accommodate a maximum fuel rate of 1.0548 x 10⁹J (one million BTUs) per hour with a corresponding combustion air requirement of 7.1 cubic metres (250 cubic feet) per minute. The structure and sizing of the second burner 16 is identical to that for the first burner 14. Each burner 14, 16 also includes an annular plenum 47 which is concentric with the duct 30 and the port blok 32. The plenum 47 also has a plenum inlet 51 for receiving a second portion of the incoming fumes. Apertures 49, duct 30 and a pair of downstream lines 53,55 which append from the duct inlet 46 and the plenum inlet 51 should be sized to provide adequate combustion air to the burner without "flame-out", with the excess fumes and combustion air passing through the plenum. For example, the ratio of the sum of the cross-sectional areas of the apertures 49 to the cross-sectional area of the duct 30 may be approximately 40;1, so that approximately 97.5% by volume of the unburnt fumes and oxygen introduced to the incineration chamber 12 will pass through the plenum 47, and approximately 2.5% will pass through the burner. Finally, the plenums 47 and the burners 14,16 may have a lining 59 of refractory material. Referring back to Fig. 1, each burner 14,16 has an associated regenerator 18,20 in fluid communication with the burner. Each regenertor 18,20 contains a ceramic bed (not shown) having a matrix of highly heat-absorbent material. In operation, the regenerator 18 preheats unburnt fumes 60 while the burner 14 is in the firing mode, and the regenerator 20 extracts heat from oxidized fumes 70 while the burner 16 is in the exhaust mode. The flow through the apparatus 10 is periodically reversed, with the regenerator 20 preheating unburnt fumes and the regenerator 18 extracting heat from oxidized fumes, as discussed in further detail below. The bypass 22 is in fluid communication with both the inlet 26 and the incineration chamber 12. A pair of fume bypass valves 48, 50 are positioned at the opposite end of the bypass 22 from the inlet 26. When one of the fume bypass valves 48, 50 is opened, the bypass 22 provides a direct passage for incoming unburnt fumes 60 to a location downstream of the regenerators 18, 20 so that fumes may be introduced directly to the incineration chamber without passing through either regenerator. The purging device 24 includes a pair of purge conduits 52, 54 and a purge fan 56 in fluid communication with the purge conduits 52, 54. A purge valve 58 selectively admits a purge gas from the purge fan 56 to either one of the purge conduits 52, 54. The purge gas may be either clean air or products of incineration. When clean air is used, it is preferable to include a centrifugal-type purge fan 56, while an axial-type fan is preferred with products of incineration. The purge fan 56 is in fluid communication with the exhaust 28 so that the fan may be continuously run without the need to start and stop every time purging is required. In operation, unburnt contaminated fumes 60 enter the inlet 26 from an upstream source, such as the finishing line on an aluminum strip coating process. Typical strip coating exhaust contains an unacceptable amount of toluene at less than 15% of its lowest explosive limit. The unburnt fumes 60 then come to a Y-juncture 62 where, by reason of the valve configuration, the unburnt fumes are directed through an inlet valve 64 into the regenerator 18 as shown in Fig. 1. Particularly, the fume bypass valves 48, 50 are closed as is an inlet valve 66. The unburnt fumes 60 typically enter the inlet 26 at a temperature of approximately 40-200°C (100-400°F). In the regenerator 18, the temperature of the unburnt fumes 60 is raised so that preheated fumes 68 exit the regenerator 18 at approximately 700-760°C (1300-1400°F). The flow of preheated fumes is then split by the varying diameters of the conduits 53,55 appending the duct inlet 46 and the plenum inlet 51. Thus, a first portion of the preheated fumes 68 enters the incineration chamber 12 through the duct 30, and a second portion enters the plenum 47 to be introduced to the incineration chamber 12 through the apertures 49. The preheated fumes 68 are then oxidized in the incineration chamber 12 by the burner 14. Specifically, volatile organic compount ("VOCs"), mainly hydrocarbon emissions such as toluene, are oxidized to carbon dioxide and water. To achieve thorough incineration of all VOCs, it is desibrable to maintain a temerpature of approxiamtely 870°C (1600°F) within the incineration chamber, while maintaining the fumes within the regenerator for a one-half second residence time. Separate combustion air need not be fed to the burners 14,16 as long as the fumes 68 contain a minimum of 16% oxygen. Oxidized fumes 70 exit the incineration chamber 12 through the burner 16. They enter the regenerator 20 at approximately 870°C (1600°F) and exit the regenerator as cooled fumes 72 at approximately 150°C (300°F). Thus, the bulk of the heat in the oxidized fumes 70 is absorbed by the ceramic matrix material in the regenerator 20. The cooled fumes 72 are then suitable for emission to the atmosphere through the exhaust 28. As shown in Fig. 1, the purge gas 57 flows through purge conduit 52 and mixes with the cooled fumes 72 in the exhaust. Thus, the purge fan 56 may be continuously operated. The first cycle lasts approximately 20-30 seconds, or until the ceramic bed in the second regenerator 20 has reached a predetermined maximum temperature. At this time, flow through the apparatus 10 is ready to be reversed in accordance with conventional regenerative burner practice. Referring to Fig. 2, a first purge cycle is schematically represented. The first purge cycle immediately follows the first cycle and precedes reversal of flow through the apparatus 10. The inlet valve 64 is closed while the fumes bypass valve 48 is opened so that the unburnt fumes 60 are directed around the regenerator 18 without passing therethrough. Simultaneously, the purge valve 58 is actuated to direct purge gas 57 from the purge fan 56 into the purge conduit 54, which is in fluid communication with the regenerator 18. The purge gas 57 enters the ceramic bed of the regenerator 18 and pushes the residual unburnt fumes from the bed. Additionally, the purge gas 57 is itself preheated within the regenerator 18 so that the thermal efficiency of the apparatus 10 is not substantially compromised, even during the purge cycle. To further adjust for loss of heat due to bypassing of the unburnt fumes, the firing rate of the burner 14 may be adjusted upward during the first purge cycle to maintain temperatures within the incineration chamber 12. The purge gas 57 and the unburnt fumes 60 mix downstream of the first regenerator 18, thereby raising the temperature of the bypassed unburnt fumes 60. As stated above, preferably 97.5% of this mixture will enter the plenum 47, and the swirling motion within the plenum serves to further mix the purge gas with the unburnt fumes before they are introduced to the burner 14 through the apertures 49. The purge cycle preferably lasts 2-5 seconds. Referring to Fig. 3, after the unburnt fumes 60 have been completely purged from the first regenerator 18 and oxidized by the first burner 14, the flow through the apparatus 10 is reversed by simultaneous closure of fume bypass valve 48 and opening of inlet valve 66. Thus, a second cycle is initiated which is basically a mirror image of the first cycle, discussed above. Again, after 20-30 seconds or until the regenerator 18 has reached a predetermined maximum temperature, a second purge cycle, depicted in Fig. 4, is initiated. The inlet valve 66 is closed while the fume bypass valve 50 is opened, and the purge valve 58 is actuated to direct purge gas 57 into the purge conduit 52. The regenerator 20 is purged and the preheated purge gas mixes with the bypassed unburnt fumes 60 substantially as described in connection with the first purge cycle above. The mixture is oxidized in the incineration chamber 12 by burner 16, and the first cycle is reinitiated.

Claims: An apparatus (10) for oxidizing fumes, comprising: an incineration chamber (12); at least one burner (14,16) directed into said incineration chamber (12); a first regeneration (18) in fluid communication with said incineration chamber (12); a second regenerator (20) in fluid communication with said incineration chamber (12); said first regenerator (18) preheating unburnt fumes (60) prior to oxidation while said second regenerator (20) extracts heat from oxidized fumes (70) in a first cycle; said second regenerator (20) preheating unburnt fumes (60) prior to oxidation while said first regenerator (18) extracts heat from oxidized fumes (70) in a second cycle, wherein the flow of fumes (60,68,70,72) through said apparatus (10) is reversed; characterized by : a bypass (22) in fluid communication with said incineration chamber (12) during a purge cycle, intermediate said first cycle and said second cycle, without passing the unburnt fumes (60) through said first or second regenerator (18,20); means (48,50,64,66) for selectively directing the unburnt fumes (60) either into said bypass (22) during said purge cycle or into said first (18) or second (20) regenerator during said first or second cycle, respectively; and a purging device (74) for selectively introducing a purge gas (57) to either one of said first (18) or second (20) regenerators during said purge cycle to purge unburnt fumes (60) therefrom and to direct said purged unburnt fumes (60) to said incineration chamber (12) for oxidation. The appratus (10) of claim 1, characterized in that said purging device (24) includes at least two conduits (52,54), each conduit (52,54) in fluid communication with one of said regenerators (18,20) and at least one valve (58), said valve (58) selectively directing said purge gas (57) to either one of said two conduits (52,54). The apparatus (10) of claim 2, characterized in that said purging device (24) includes a purge fan (56) in fluid communication with said conduits (52,54). The apparatus (10) of claim 3, characterized in that said conduits (52,54) are also in fluid communication with an exhaust (28) so that said purge fan (56) may be continuously operated throughout said first, purge and second cycles. The apparatus (10) of any one of the preceding claims, characterized in that said at least one burner (14,16) includes a concentric duct (30) in fluid communication with said incineration chamber (12), a port block (32) concentric with said burner (14,16) and intermediate said burner (14,16) and said incineration chamber (12), and an annular plenum (47) having a plurality of apertures (49) radially spaced from the longitudinal axis of said burner (14,16), said apertures (49) coterminus with said port block (32) and in fluid communication with said incineration chamber (12), said burner (14,16) and said plenum (47) also in fluid communication with said bypass (22) and with either one of said first (18) or second (20) regenerators. The apparatus (10) of claim 5, characterized in that a first portion of the fumes (60) is introduced into said incineration chamber (12) through said apertures (49), and a second portion is introduced through said duct (80), with the ratio of the first portion to the second portion being substantially equivalent to the ratio of the combined cross-sectional areas of the apertures (49) to the cross-sectional area of the duct (30). The apparatus (10) of claim 6, characterized in that the ratio of the sum of the cross-sectional areas of said apertures (49) to the cross-sectional area of said duct (30) is approximately 40:1, so that approximately 97.5% by volume of the unburnt fumes (60) and oxygen introduced to said incineration chamber (12) from said bypass (22) or said regenerators (18,20) will pass through said aperatures (49) in said plenum (47) and approximately 2.5% will pass through said burner (14,16). The apparatus (10) of any one of claims 5-7, characterized in that said burner (14,16) and said plenum (47) are lined with refractory insulating material (59). A method for oxidizing fumes (60) in an incineration chamber (12), said method having a first cycle followed by a second cycle, comprising the steps of: a) introducing unburnt fumes (60) to an inlet (26); b) directing the unburnt fumes (60) to a first regenerator (18) in said first cycle wherein said unburnt fumes (60) are preheated; c) oxidizing the preheated unburnt fumes (68) in the incineration chamber (12). d) directing the oxidized fumes (70) to a second regenerator (20), wherein heat is extracted from said oxidized fumes (79), e) after the second regenerator (20) reaches a predetermined temperature, diverting the unburnt fumes in step (a) into said second regenerator (20) to initiate said second cycle and preheat the unburnt fumes (60); f) oxidizing the preheated unburnt fumes (68) of step (e) in the incineration chamber (12); and g) directing the oxidized fumes (70) of step (f) to said first regenerator (18) wherein heat is extracted from the oxidized fumes (79), said method being characterized by a purge cycle between said first and second cycles, said page cycle comprising the steps of: h) after the heat extracting regenerator of the first cycle reaches a predetermined temperature diverting the unburnt fumes (60) introduced into said inlet (26) into; a bypass (22) to initiate said purge cycle, thereby placing the unburnt fumes (60) downstream of the first regenerator (18) without passing them through said first regenerator (18); i) introducing a purge gas (57) to said first regenerator (18) to purge the unburnt fumes (60) therefrom and to preheat the purge gas (57); j) mixing the purge gas (57) with the unburnt fumes (60) from said bypass (22) downstream of said first regenerator (18) and upstream of the incineration chamber (12); k) introducing said mixture to the incineration chamber (12) to oxidize the unburnt fumes (60); and l) after the unburnt fumes (60) are completely purged from said first regenerator (18), diverting the unburnt fumes (60) introduced into said inlet (26) from said bypass (22) to said second regenerator (20) to initiate said second cycle. The method of claim 9, characterized in that the mixing of step (j) takes place in an annular plenum (47) which is concentric with the longitudinal axis of a burner (14,16) that is directed into the incineration chamber (12), said annular plenum (47) having a plurality of apertures (49) radially spaced from the longitudinal axis of said burner (14,16) which admit a first portion of said mixture into the incineration chamber (12).

Inventors: NUTCHER PETER B, WALDERN PETER J, NUTCHER, PETER B., WALDERN, PETER J.
Applicants: PROCESS COMBUSTION CORP, PROCESS COMBUSTION CORPORATION
Application year: 1992
Main classification: F23G7
IPC codes: F23G7, F23D14