Oxygen-enriched combustion method

Abstract: A process and apparatus for oxygen-enriched combustion of a fuel in an industrial furnace in which a preheated primary oxidant from a heat exchanger and a fuel to be combusted are introduced into a combustion chamber and ignited, a secondary oxidant having an oxygen concentration in excess of the concentration of oxygen in air is introduced into the combustion chamber downstream of the flame, and the resulting combustion products are exhausted. In accordance with one embodiment, the secondary oxidant comprises preheated oxidant aspirated from the heat exchanger using industrial-grade oxygen, resulting in an oxygen-enriched mixture being injected into the combustion chamber downstream of the flame. As a result, NO x emissions are reduced without adverse effects on the overall furnace operation.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process and apparatus for oxygen-enriched combustion of a fuel in an industrial furnace. Description of the Prior Art It is generally known that oxygen-enriched combustion, usually implemented by increasing the oxygen concentration of the combustion air or by direct injection of oxygen into the flame, increases production rate as well as furnace thermal efficiency. However, to provide such increases in production rate and furnace thermal efficiency, oxygen-enriched combustion increases the peak flame temperature, a condition which directly conflicts with known methods for reducing the formation of NO x . In addition, oxygen-enriched combustion is known to raise furnace refractory temperatures, particularly in the furnace zones where temperatures are already very high, thereby adversely affecting the service life of the furnace. It is generally known that to control the amount of NO x emissions generated by a combustion process, it is necessary to reduce the amount of oxygen available for NO x formation. It is also generally known that peak flame temperatures in the combustion process must be maintained below the level required for substantial formation of NO x . Many approaches to producing such conditions to reduce NO x emissions generated in combustion processes, including oxygen-enriched combustion and staged combustion, have been used, investigated, or proposed. U.S. Patent 4,761,132 teaches a process and apparatus for oxygen-rich combustion in which a first portion of fuel and an oxygen-rich gas are introduced into a cracking chamber and cracked to produce a cracked products mixture. The cracked products mixture, a second portion of fuel, and an oxidizer having sufficient oxygen for substantially completing combustion of the combustible portion of the cracked products mixture and the fresh fuel are subsequently introduced into a combustion chamber for completion of the combustion process. Staged combustion using air staging is also taught by U.S. Patent 4,358,268 in which a preheated fuel-rich combustion mixture is fed into an industrial furnace system having a heating chamber, the mixture having been preheated in a first regenerator. Flue gases from the heating chamber are exhausted through a second regenerator into which is fed secondary combustion air to more completely oxidize the flue gases. A portion of the hot flue gases leaving the second regenerator is recirculated to the heating chamber and the remaining flue gases, having been fully oxidized, are vented to the atmosphere. The flow of flue gases within the heating chamber is periodically reversed such that the fuel-rich combustion mixture is heated in the second regenerator and the flue gases from the heating chamber are exhausted through the first regenerator to which secondary combustion air is added. Such reversal of flows is typical of the operation of regenerative-type industrial furnaces, including regenerative glass-melting furnaces. U.S. Patent 3,207,493 teaches a regenerative furnace having two combustion air inlets and a separate outlet for combustion products, reversing valves for alternately connecting the cold ends of the regenerators with the cold air supply and the exhaust flue, and operative to connect the hot end of one regenerator alternately with the combustion air inlets and the hot end of the other regenerator with the combustion product outlet of the furnace chamber. In air staging for regenerative glass melters, in particular, fuel is generally burned at the firing port(s) with a portion of hot combustion air, primary air, at fuel-rich conditions, and the remaining air, secondary air, is added so that it mixes with the fuel-rich flame after a substantial amount of heat has been removed. The fuel-rich combustion retards NO x formation due to both lower oxygen availability and lower flame temperatures. Also, because the secondary air mixes with the flame only after heat has been removed, the secondary peak flame temperatures are also lowered. Publication EP-A-306 657 is considered to be the most relevant prior art. It discloses a regeneration-type industrial glass melting furnace for oxygen-enriched combustion which reduces NO x emissions. The glass melting furnace of said citation operates with a single combustion zone in which fuel and first preheated combustion air are introduced through a fuel and inlet port at one end of the melter into the combustion chamber and the combustion products are exhausted through an exhaust port disposed at the same end of the melter as the fuel and air inlet port. Secondary preheated combustion air is introduced substantially uniform along the length of the flame to maintain substantially uniform combustion characteristics in the whole combustion chamber including flame temperature and under stoichiometric conditions. SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a process and apparatus for oxygen-enriched combustion in a regenerative industrial furnace, which reduces NO x emissions from regenerative industrial furnaces, with no adverse effects on production rate, product quality, furnace thermal efficiency, and other air pollutant emission levels. It is yet another object of this invention to provide a process and apparatus for oxygen-enriched combustion which reduces NO x emissions and provides improvements in product quality, production rate, furnace thermal efficiency, and other air pollutant emission levels. It is yet another object of this invention to provide a process and apparatus for oxygen-enriched combustion which can be applied to state-of-the-art, natural gas fired, high performance regenerative furnaces, in particular, glass melting furnaces. It is yet another object of this invention to provide a process and apparatus for reducing NO x emissions from regenerative industrial furnaces which can be retrofit to existing regenerative industrial furnaces. These and other objects are achieved with a process in accordance with claim 1. Further advantages of particularly preferred embodiments of the process of this invention as claimed in claims 2-5 are described in the following specification. The industrial furnace for oxygen-enriched combustion and preferred embodiments are claimed in claims 6-10. BRIEF DESCRIPTION OF THE DRAWING These and other features and aspects of this invention will be better understood from the figure taken in conjunction with the following detailed description, wherein: Fig. 1 is a vertical cross-sectional view of a regenerative sideport fired furnace incorporating features of this invention. DESCRIPTION OF PREFERRED EMBODIMENTS A typical regenerative furnace incorporating features of this invention is shown in Fig. 1. Regenerative furnace 10 comprises regenerators 11, 18, air ports 12, 24, burners 21 disposed in a sidewall of air ports 12, 24 for sideport firing, and combustion chamber 25 in which are disposed load 20, and flame 19 above load 20. It is apparent that regenerative furnaces can be fired with burners 21 positioned above air ports 12, 24, that is, overport firing, or with burners 21 positioned below air ports 12, 24, that is, underport firing. This invention is equally applicable to sideport, overport, and underport fired regenerative furnaces. It will also be apparent to those skilled in the art that this invention is also applicable to other types of industrial furnaces including recuperative furnaces and furnaces equipped with other types of combustion air and fuel ports. In operation, oxidant, preferably in the form of combustion air, is introduced into the base of regenerator 11 in which the combustion air is preheated. It is apparent that other oxidants including oxygen-enriched combustion air may be used in the combustion process. From regenerator 11, combustion air flows through primary combustion air port 12 and into combustion chamber 25 where it mixes with fuel, preferably natural gas, from burner 21 to form flame 19. Heat, primarily in the form of radiation, is transmitted from flame 19 to load 20. It is apparent that the amount of material in load 20 which can be processed depends upon the amount of heat transferred from flame 19 to load 20. Thus, the more heat that is transferred from flame 19 to load 20, greater is the amount of material which can be processed. It is also apparent that load 20 is required for removal of heat from flame 19 to maintain peak flame temperatures generally below the temperature required for significant NO x formation. Combustion products generated from the combustion of fuel and primary combustion air are exhausted as flue gases from combustion chamber 25 through exhaust port 24 from which they enter regenerator 18. As the hot flue gases pass through regenerator 18 and are exhausted to a stack, heat is transferred from the hot flue gases to regenerator 18. When the temperature in regenerator 18 reaches a certain level, the flows within regenerative furnace 10 are reversed, with primary combustion air being introduced into the base of regenerator 18. As the primary combustion air passes through regenerator 18, heat is transferred from the regenerator to the combustion air which subsequently passes through port 24 and into combustion chamber 25. During this firing cycle, the flame is generated by a burner (not shown) proximate port 24 and the resulting combustion products are exhausted through port 12 and regenerator 11. To reduce NO x emissions from industrial furnaces, in particular, regenerative furnaces, without adverse effects on or with improvements in product quality, production rate, furnace thermal efficiency, and other air pollutant emission levels, it is necessary to limit the amount of oxygen initially available for combustion and still provide sufficient flame temperatures for effective heat transfer from flame 19 to load 20. In accordance with one embodiment of this invention, these conditions are achieved using oxygen-enriched combustion in which the amount of primary oxidant, preferably combustion air, introduced into combustion chamber 25 through primary combustion air port 12 is maintained at about 0.85 to about 1.15 of the stoichiometric ratio for complete combustion of the fuel, forming a primary combustion zone within combustion chamber 25, and secondary oxidant in the form of oxygen, oxygen-enriched ambient air or oxygen-enriched hot air is injected into combustion chamber 25 downstream of flame 19, forming a secondary combustion zone. In this way, the majority of the combustion, that is, combustion in the primary combustion zone, is relatively low in oxygen concentration to inhibit NO x formation and any resulting combustibles are burned out at lower temperatures near exhaust port 24. Burning natural gas under near stoichiometric conditions produces a highly luminous flame which promotes effective heat transfer from flame 19 to load 20. Preheated secondary combustion air for injection into combustion chamber 25 downstream of flame 19 is aspirated through line 14 from top portion 13 of regenerator 11 using aspirator/ejector 15. This preheated secondary combustion air is enriched with oxygen to increase furnace-specific production rate and combustibles oxidation and injected through line 16 into combustion chamber 25 downstream of flame 19. The use of oxygen-enriched secondary combustion air increases secondary combustion zone temperatures to increase heat transfer to the load in the furnace and improves combustible burnout of combustibles in the exhaust gases. However, the increase in secondary combustion temperatures does not significantly increase the amount of NO x emitted from the furnace, as would be expected in known oxygen-enriched combustion processes. By comparison, injecting ambient air downstream of flame 19, a technique known to those skilled in the art, reduces NO x emissions from the furnace, but it also reduces furnace productivity and furnace thermal efficiency. In accordance with a preferred embodiment of this invention, preheated secondary combustion air is aspirated from regenerator top 13 using industrial grade oxygen having oxygen concentrations from about 35% to about 100% which is preferably available at high supply pressures through line 17. The furnace is operated with near-stoichiometric primary combustion air, providing between about 85% and about 115 %, preferably between about 90% and about 105%, of the total amount of oxygen required for complete combustion, and a small amount of high-velocity ambient or preheated oxygen-enriched secondary combustion air, providing preferably between about 5% and about 40% of the total amount of oxygen required for complete combustion of the fuel, is injected near exhaust port 24 to burn out any residual carbon monoxide and total hydrocarbons. An industrial furnace for oxygen-enriched combustion in accordance with this invention comprises combustion chamber 25, means for preheating an oxidant, preferably air, in the form of regenerator 11 in communication with combustion chamber 25, burner 21 for introducing a fuel into combustion chamber 25 and primary combustion air port 12 for introducing at least a portion of the preheated oxidant into combustion chamber 25. The furnace, in accordance with this invention, further comprises means for withdrawing a portion of preheated oxidant from regenerator 11 in the form of aspirator/ejector 15 in communication with regenerator top 13 of regenerator 11. The withdrawn preheated oxidant is introduced through line 16 in communication with aspirator/ejector 15 through exhaust port 24 into combustion chamber 25 downstream of flame 19. Regenerator 18 is provided for exhausting combustion products from combustion chamber 25. To retrofit the process of this invention to an existing regenerative industrial furnace merely requires the addition of line 14 in communication with regenerator top 13, aspirator/ejector 15 in communication with line 14 and positioned to aspirate preheated oxidant from regenerator 11, line 17 which supplies an aspirating fluid, preferably industrial grade oxygen, to aspirator/ejector 15 and line 16 in communication with aspirator/ejector 15 and combustion chamber 25 through which preheated, secondary oxidant, preferably preheated oxygen-enriched, secondary combustion air is introduced into combustion chamber 25.

Claims: A process for oxygen-enriched combustion of a fuel in a combustion chamber of an industrial furnace comprising: preheating an oxidant required for combustion of a fuel in a heat exchanger; introducing said fuel and at least a portion of said preheated oxidant as a primary preheated oxidant of near stoichiometric primary combustion air into said industrial furnace, producing a fuel/oxidant mixture and igniting said fuel/oxidant mixture producing a flame in said combustion chamber at a first combustion zone providing a first combustion temperature and introducing downstream of flame (19) a secondary preheated oxidant having an oxygen concentration greater than the oxygen concentration in air into a second combustion zone downstream of said flame and said primary combustion zone, providing a second combustion temperature increased in respect to the first combustion temperature, wherein said secondary oxidant is oxygen or a mixture of oxygen and air. A process for oxygen-enriched combustion of fuel in accordance with Claim 1, wherein said preheated air is aspirated from said heat exchanger using oxygen. A process for oxygen-enriched combustion of fuel in accordance with Claim 1, wherein said secondary oxidant comprises between about 5% to about 40% of the total amount of oxygen required for complete combustion of said fuel. A process for oxygen-enriched combustion of fuel in accordance with Claim 1, wherein said fuel is natural gas. A process for oxygen-enriched combustion of fuel in accordance with Claim 1, wherein said preheated primary oxidant comprises between about 85% and about 115% preferable between about 90% and 105% of the total amount of oxygen required for complete combustion of said fuel. An industrial furnace (10) for oxygen-enriched combustion of fuel in accordance with the process of Claim 1, comprising a combustion chamber (25) with two air ports (12,24) of which one is an exhaust port (24) with means (11,18) for preheating a primary oxidant and means (21) for introducing a fuel and said preheated primary oxidant into said combustion chamber; means (14,15,16) for introducing a secondary oxidant and said exhaust port (24) for exhausting products of combustion from said combustion chamber (25) characterised in that said means (14,15,16) for introducing a secondary oxidant are near exhaust port (24) downstream of the flame (19) in the combustion chamber (25) defining a primary combustion zone and a secondary combustion zone, said secondary combustion disposed downstream of said primary combustion zone. An industrial furnace in accordance with Claim 6, wherein said means for preheating said primary oxidant comprises a regenerator (11). An industrial furnace in accordance with Claim 7, wherein said regenerator (11) is connected to said combustion chamber (25) by said air port (12). An industrial furnace in accordance with Claim 6, wherein said preheated primary oxidant is introduced into said combustion chamber (25) through said air port (12) and said fuel is introduced into said combustion chamber through at least one burner (21) positioned over, under and at a sidewall of said air ports (12, 24). An industrial furnace in accordance with Claim 6, wherein means for withdrawing a portion of said preheated primary oxidant comprises an ejector (15) and an aspirator in communication with said means (11) for preheating said primary oxidant and is positioned to aspirate said portion of said preheated oxidant from said means (11) for preheating said primary oxidant. An industrial furnace in accordance with Claim 7, wherein the furnace (10) is a regenerative industrial furnace having a combustion chamber (25) and at least two regenerators (11, 18) in communication with said combustion chamber, said regenerators alternately functioning as an oxidant preheater and an exhaust gas port.

Inventors: ABBASI HAMID A, KHINKIS MARK J, ABBASI, HAMID A., KHINKIS, MARK J.
Applicants: INST GAS TECHNOLOGY, INSTITUTE OF GAS TECHNOLOGY
Application year: 1993
Main classification: F23C6
IPC codes: F23C6, F23L7, F23C7, F23L15, C03B5