Abstract:
A method and apparatus for repressing NOx formation in twinned regenerative burner pairs includes inducing a stream of hot flue gas, preferably containing enriched products of combustion, from the main hot flue gas exhaust stream and vitiating the preheated combustion air with the hot flue gas in the firing burner. An interconnecting duct communicating with the twinned burner pair includes a coaxial gas nozzle for injecting a high kinetic energy gas stream into the exhausting hot flue gas to induce a portion of the hot flue gas into the interconnecting duct to pass the hot flue gas to the firing burner for vitiation purposes.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to generally regenerative type burners for heating a furnace, and more particularly to regenerative burners with minimized NOx formation in the ultimate combustion effluents. This invention is related to co-pending application Ser. No. 168,892 entitled &#34;Low NOx Regenerative Burner&#34; filed Mar. 16, 1988, the contents of which are incorporated by reference herein. 
     Regenerative-type burners for furnaces are well-known in the art in varied forms and designs, but they share the common feature whereby heat storage units are provided to withdraw and store heat from hot combustion effluents known as flue gas, with subsequent transfer of the heat to preheat incoming combustion air. The earliest regenerative-type furnaces were symmetrical arrangements having both burner(s) and heat storage units (often solid structural arrays of &#34;checker chamber&#34; bricks) in place on each of two sides of the furnace. Firing of such a regenerative furnace began with the burner(s) on one side, with concomitant storage of the heat present in the combustion effluents by the heat storage units on the second side. After optimal heating of the heat storage units, or the &#34;checker chamber,&#34; the air flow in the furnace was reversed to draw combustion air in through the checker chamber, thus preheating the combustion air. Ducts in the checker chamber thus alternately conveyed combustion products and combustion air, and the burners functioned alternately as burners and as flues. 
     Modern regenerative systems do not involve complete symmetrical furnaces but instead include specialized regenerative burners employed, typically, in pairs. Each of the paired regenerative burners is equipped with heat storage units, ordinarily in the form of compact regenerative beds, through which combustion air passes en route to the burner. Because the burners are employed in pairs, one burner is fired at a time while the other functions as a flue and heat storage bed. Then every 20-120 seconds or so, flow in the furnace is reversed and the burners &#34;exchange&#34; functions, that is, the first-fired burner becomes the flue gas exhaust/heat storage bed as the second burner fires. A system exemplary of one paired burner arrangement is found in U.S. Pat. No. 4,522,588. 
     A persistent problem with regenerative systems involves the extremely high NOx concentrations inevitably present in the combustion effluents, produced as a result of the extremely high air preheats and flame temperatures, as well as through fuel bound nitrogen. As a result, regenerative systems which historically enjoyed industry-wide acceptance now cannot meet the emissions standards in an ever-increasing number of localities and/or process conditions. There is a need for low NOx burner concepts which can be broadly adapted to the specific applications by altering the flame temperature to meet NOx emission requirements. A need therefore persists for regenerative burner systems which provide the heat-regenerative function of prior art systems yet provide for significant NOx reduction. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprises a paired heat regenerative burner system suitable for direct firing into a furnace or for use on an enclosed radiant tube burner assembly. The burner pair includes two identical right and left burner units which individually and alternately operate in a heating (firing) or exhaust/heat reclaiming mode. For ease of description herein the right hand burner will be assumed to be in the firing mode and the left hand burner in the regenerative mode. Each of the burners have chambers which are connected at a respective combustion end to a spaced pair of first ducts which communicate with the interior of the furnace or to the tube of a U-shaped radiant tube burner and alternately act to emit hot combustion gases into the furnace or to exhaust hot flue gases from the furnace. Each of the burners have a heat regenerator media bed associated therewith which are respectively connected to a combustion air inlet duct/flue gas outlet duct. A vitiation duct interconnects the two burners on the furnace side of the regenerative media beds and communicates with the interior chamber portion of each burner. A nozzle is positioned at each of the burner chambers to alternately inject a high velocity gas stream into the interconnecting duct. The high velocity stream entrains a portion of the flue gas containing hot products of combustion (&#34;POC&#34;) exiting the furnace at the left bank burner and causes the entrained hot flue gas to vitiate the preheated combustion air in the right hand burner chamber. The balance of the hot flue gases pass through the left bank regenerative media bed to transfer its heat thereto for later transfer to a reverse flowing stream of combustion air (when the left bank is in the firing mode). A fuel nozzle or other injector means introduces a fuel stream to the mixture of hot POC containing flue gas and preheated combustion air at the right bank burner chamber whereby suppressed or reduced NOx formation in the burner flame is obtained. The regenerator sizing is significantly reduced while cycle efficiency is also increased due to the fact that the preheat combustion air is vitiated with a hot POC containing flue gas at very high temperatures, for example, between about 1800° to 2000° F. The lowered oxygen content of the vitiated combustion air lowers flame temperatures and minimizes NOx formation while the high BTU content of the hot vitiating POC containing flue gas contributes to higher furnace efficiencies. 
     In one preferred form of the invention, the injected stream of gas is injected tangentially relative to the stream of hot flue gas exiting the furnace to impart a swirling motion to the flue gas to create an enriched layer of POC which is then entrained in the vitiating stream. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a twinned heat regenerative burner system according to the present invention; 
     FIG. 2 is a schematic view in front elevation of the embodiment of the invention depicted in FIG. 1; 
     FIG. 3 is a schematic plan view a further embodiment of the present invention in the form of twined burners having a modified form of fuel delivery; 
     FIG. 4 is a cross-sectional view of the fuel injector manifold taken along lines IV--IV of FIG. 3; and 
     FIG. 5 is a partial cross-sectional view of the gas injector left bank burner chamber and interconnecting duct taken along line V--V of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is now made to the drawings in which identical elements are designated by identical reference numerals throughout the various figures and mirror images of the same elements are designated by primed numerals. In FIGS. 1-2, a left bank of a regenerative burner 4 is shown, with an identical right bank mirror image, identified in primed numbers. The right bank burner system is generally designated by reference numeral 4&#39;. The left and right regenerative burner pair 4 and 4&#39; is mounted within a wall 8 of a furnace 6 for supplying heat to the interior chamber 10 thereof. The left bank and right bank burners 4, 4&#39; are adapted to alternately fire combustion products directly into the furnace chamber 10 as shown in the drawings. The invention is also suitable for use in connection with an alternately fired, continuous U-shaped, radiant tube regenerative burner system, the radiant tube partially shown in phantom lines and designated by reference numeral 15 in FIG. 1. In both instances, i.e., direct fired and radiant tube fired, the invention provides a self-vitiating low NOx regenerative burner pair. As appreciated in the art, a regenerative type burner is significantly more efficient in recovering waste heat from hot flue gases than the common recuperator style preheaters previously used in connection with radiant tube burners. 
     In the firing mode of operation shown in FIGS. 1 and 2, hot flue gases containing products of combustion (&#34;POC&#34;) leave the furnace interior 10 under the influence of a forced or induced draft initiated by a remotely located fan means (not shown) and exit via a first duct 12 to then enter a burner chamber 14 of the burner 4 which is in a non-firing, exhaust mode. A medium pressure or a high pressure energy source, which may include air, POC or gaseous fuel, is pumped through a feed conduit 20 and discharged preferably at a high velocity through a nozzle 22, communicating therewith, into the burner chamber 14. The nozzle 22 is aligned co-axially with the longitudinal axis of an interconnecting duct 24. Duct 24 communicates at its ends with the burner chambers 14 and 14&#39; of the spaced-apart left and right bank burners 4 and 4&#39;, respectively. The interconnecting duct 24 is preferably tangentially offset from the sidewalls of burner chambers 14 and 14&#39; as shown in FIGS. 2 and 5. The high velocity gas stream which is emitted from the nozzle 22 creates an induction of a portion of the hot flue gas entering the chamber 14. The portion of hot flue gas so induced is entrained into the high velocity gas stream and passes through the interconnecting duct 24. A high kinetic energy is imparted to the gas stream exiting the nozzle 22 which is sufficient to move the subject gas stream and its entrained portion of flue gas from the left burner bank 4 to the right burner bank 4&#39; via interconnecting duct 24, without the need for any auxiliary fans or blowers. 
     The major portion of the hot flue gas which exhausts from the furnace 6 at first duct 12 and enters the chamber 14, passes downwardly through a left heat storage bed or regenerator 16, of known construction, which extracts the sensible heat from the flue gas and stores the heat for later preheating of combustion air when the firing cycle is reversed. As best seen in FIG. 2, cooled flue gas leaves the regenerator bed 16 by way of a duct 18, and is exhausted through a port 26 of a valve assembly 30. A rotatable valve plate 32 directs the cooled flue gas to the exhaust port 26 of valve 30 and simultaneously introduces cool combustion air therein by way of a port 28. The combustion air port 28 may be opened directly to the atmosphere, in a negative draft mode, or it may be connected to a positive pressure, fan driven system, all in a known manner. The cool combustion air passes through the valve 30 and enters a duct 18&#39; whereupon it is passed upwardly through a previously heated regenerator bed 16&#39;. Stored heat in the regenerator bed 16&#39; is given up to preheat the incoming combustion air which then enters the right hand burner bank 4&#39; at chamber 14&#39;. The high velocity gas stream from nozzle 22 and its entrained hot flue gas, exiting interconnecting duct 24, enters the burner chamber 14&#39; and mixes with the preheated combustion air therein to vitiate or dilute the combustion air stream by lowering the oxygen level thereof. The degree of vitiation to be achieved is controlled by the mass and velocity of the high velocity gas stream introduced through the feed conduit 20 and nozzle 22. By way of example, vitiation of the combustion air stream is controllable within a range of between about 15% to about 21% as measured in the mixed air and gas stream exiting the burner chamber 14&#39; of the right bank burner 4&#39;. 
     During the firing mode shown in the drawings, a fuel is introduced through a fuel conduit 5&#39; positioned within the right bank burner 4&#39;. A small quantity of ambient bled air or POC is preferably passed through feed conduit 20&#39; and nozzle 22&#39; during the right hand firing mode in order to cool those components. Simultaneously, when in the right bank firing mode, the left bank fuel conduit 5 is also preferably cooled by a small flow of ambient purge air or POC therethrough. 
     Primary combustion is initiated in the burner chamber 14&#39; of the right bank burner 4&#39; and propagates through the duct 12&#39; into a combustion chamber which can be the open interior 10 of the furnace or the interior of the radiant tube 15 connected to the duct 12&#39;. A layer of refractory insulation 34, 34&#39; envelopes the ducts 12, 12&#39; to protect the support structure 8 or the radiant tube 15 from the high heat of combustion generated, within the ducts 12 and 12&#39;. A layer of insulation 36 is also preferably applied around the burner banks 4, 4&#39;, regenerator beds 16, 16&#39;, and interconnecting duct 24, to minimize heat losses. An outer protective metal surface 38 is also applied to protect the insulation layer 36 against inadvertent damage. 
     The right hand firing mode cycle is completed as the flue gases containing POC exit the furnace 6 through the combustion chamber of first duct 12 on the left burner bank 4. As previously described, a portion of these flue gases are induced into the interconnecting duct 24 with the balance passing through the heat regenerator bed 16. The cooled flue gas is vented to a wsste stack communicating with exhaust port 26 which is either connected to a suction provided by a negative draft system or vented to the stack when the combustion air port 28 is under the influence of a positive pressure air supply system. 
     After a given period of time elapses, the firing direction is reversed from the right hand mode shown to a left hand mode. The direction of combustion air and flue gas wthin the ducts 18, 18&#39; and 24, as well as in the combustion/exhaust ducts 12, 12&#39;, is reversed from that shown in FIGS. 1 and 2. When the left bank burner 4 is in the firing mode, the valve plate 32 is rotated to the position indicated by phantom lines 32&#39;, which directs the cool combustion air upwardly through duct 18 for preheating in regenerator bed 16. In this mode, valve plate 32&#39; concurrently directs the cooled flue gas now exiting from duct 18, to the waste stack via the exhaust port 26. 
     A slightly modified apparatus 40 embodying the principals of the present invention is depicted in FIGS. 3-5. The apparatus 40 operates generally in the same manner as described, with the exception that the fuel stream is introduced tangentially into the swirling stream of flue gases exiting from the interconnecting duct 24. As seen in FIG. 5, the high velocity gaseous jet from nozzle 22 induces a flow of a portion of the flue gas from chamber 14 into the interconnecting duct 24. The tangential offset of the duct 24 with the sidewall of chamber 14 and co-axial alignment of nozzle 22 with duct 24 creates a swirling motion in the flue gas within chamber 14 which provides an outer layer of highly enriched hot POC which is induced to pass into the duct 24 by the high velocity gas stream from nozzle 22. This swirling POC enrichment feature is also achieved in the embodiment of FIGS. 1-2. As seen in FIG. 4, a plurality of fuel conduits 5&#39; supply a plurality of fuel streams tangentially to the chamber 14&#39; to also impart a desired swirling motion to the combustion mixture. The fuel is supplied to the conduits 5&#39; by way of an inlet duct 9&#39; and a communicating, annular manifold 7&#39; which surrounds the burner chamber 14&#39;. 
     The advantages of the present invention are readily appreciated by those skilled in the art in that the flue gas/POC which is recirculated back into the combustion process to vitiate the combustion air is at a temperature approximately equal to the gases exiting the furnace chamber. As a consequence, the sizing of regenerator 16, 16&#39; can be significantly smaller than if vitiation were used on the stream entering the regenerator. In addition, the cycle efficiency when compared to using vitiated air directly into the regenerator is increased due to the fact that the self-vitiating flue gas/POC stream is already at temperatures between about 1800° to 2000° F. when injected into the burner chambers 14, 14&#39;. These obvious economic advantages are achieved while the desired ecological goal of NOx suppression is also realized. 
     Although the invention has been described in connection with specific materials and specific embodiments, the invention is to be limited only insofar as is set forth in the accompanying claims.