Method for operating a furnace

Abstract: A method for operating a furnace (32) comprising generating NOx with burners (13,20) at a value exceeding the equilibrium value by combustion at a distance from the flue (34) and generating NOx with burners (1,12) at a lesser value, preferably at a value below the equilibrium value, by combustion close to the flue (34).

Description: Technical Field This invention relates generally to a method for the operation of furnaces according to the preamble of claim 1, and more particularly to the reduction of nitrogen oxides (NOx) emitted from a furnace. Background Art Many industrial processes employ furnaces wherein fuel and oxidant are combusted to generate heat which is used to heat a charge within the furnace. Among such industrial processes one can name glassmaking wherein the charge is glassmaking materials or molten or solid glass, steelmaking wherein the charge is steel or iron and aluminum melting wherein the charge is aluminum ingots or scrap. Nitrogen oxides are a significant pollutant generated during combustion and it is desirable to reduce their generation in carrying out combustion. It is known that combustion may be carried out with reduced NOx generation by using technically pure oxygen or oxygen-enriched air as the oxidant as this reduces the amount of nitrogen provided to the combustion reaction on an equivalent oxygen basis. However, the use of an oxidant having a higher oxygen concentration than that of air causes the combustion reaction to run at a higher temperature and this higher temperature kinetically favors the formation of NOx. It is known that different burners and burner systems will, in operation, generate NOx at different rates. For example, all other things being equal, combustion using a concentric burner arrangement will generally result in greater NOx formation than will combustion using a staged burner arrangement. Accordingly, in a furnace employing a plurality of burners, one may reduce NOx emissions from the furnace by replacing one or more of the high NOx burners with low NOx burners. However, such a conversion is costly and it is desirable to reduce the number of burners in a multiburner furnace which must be converted from high NOx to low NOx. US-A-4 117 075 which forms the preamble of claim 1 discloses a burner operating method using auxiliary combustion, wherein a front and a middle burner effect complete combustion by using a first fuel and air as the oxidant, whereas the last downstream burner effects additional combustion of the combustion product emanating from the upstream front and middle burners by using a reducing fuel, e.g. carbon monoxide or hydrogen, in order to reduce the NOx content of the exhaust gas. It is an object of this invention to provide a method for operating a furnace which can minimize the number of low NOx burners used for any particular target level of NOx emissions from the furnace. Summary of the Invention The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by: A method for operating a furnace to reduce the level of NOx emitted from the flue of the furnace as defined in claim 1. As used herein the term "flue" means a passage capable of passing furnace gases out from the furnace, generally to the ambient atmosphere. As used herein the term "equilibrium value" means the concentration of nitric oxide plus nitrogen dioxide that would result if the concentration of nitrogen and oxygen in the furnace combustion products were held at the furnace gas temperature for an infinite period of time. Those skilled in the art are familiar with how to calculate equilibrium values using published equilibrium constants. As used herein the term "high NOx burner" means a burner which carries out a combustion reaction wherein NOx is generated above the equilibrium value. Examples of high NOx burners can be found in U.S. Patent No. 5,267,850 and U.S. Patent No. 5,256,058. As used herein the term "low NOx burner" means a burner which carries out a combustion reaction wherein NOx is generated below that of a high NOx burner and can be generated below the equilibrium value. Examples of low NOx burners can be found in U.S. Patent No. 4,378,205, U.S. Patent No. 4,907,961 and U.S. Patent No. 5,209,656. As used herein the terms "nitrogen oxides" and "NOx" mean the sum of nitric oxide (NO) and nitrogen dioxide (NO 2 ). Brief Description of the Drawings The sole Figure is a simplified plan view representation of one furnace which may be used in the practice of the invention. Detailed Description This invention comprises the recognition that the NOx level generated in a combustion reaction is not static within a furnace but rather is dynamic within the furnace and, furthermore, that the nonstatic condition is in the direction toward the equilibrium value. The longer the combustion reaction gases from a combustion reaction remain in a furnace, the closer to the NOx equilibrium value they will come prior to exiting the furnace through the flue. The rate at which the gases approach the equilibrium value depends on the temperature of the furnace. The invention achieves the best results when the furnace temperature exceeds 1482°C (2700°F). In a multiburner furnace the level of NOx emitted from the flue may be disproportionately reduced by the practice of the invention over that which is achieved by the mere replacement of one or more high NOx burners with low NOx burners. If a low NOx burner is positioned in the furnace at a significant distance from the flue and it generates NOx levels below the equilibrium value, the furnace gases resulting from the combustion will have a long residence time within the furnace as they proceed toward the flue, and this long residence time will result in an increase in the level of NOx as the below equilibrium value of the NOx generated by the combustion dynamically progresses toward the equilibrium value. Moreover, if a high NOx burner is positioned in the furnace near the flue, the furnace gases resulting from the combustion will have a short residence time within the furnace which will not allow a significant NOx reduction. However, if a high NOx burner is positioned in the furnace at a significant distance from the flue, the furnace gases resulting from the combustion will have a long residence time within the furnace as they proceed toward the flue, and this long residence time will result in a decrease in the level of NOx as the above equilibrium value of the NOx generated by the combustion dynamically progresses toward the equilibrium value. The dynamic progression, either an increase or a decrease, toward the equilibrium value does not continue appreciably outside the furnace zone because it is the furnace conditions of heat, fluid flows and reactant concentrations which cause the dynamic progression toward equilibrium to take place. The fuel useful in the practice of this invention may be any gas or other fuel which contains combustibles which may combust in a furnace or combustion zone. Among such fuels one can name natural gas, coke oven gas, propane, methane, oil and pulverized coal. The oxidant useful in the practice of this invention is any fluid containing sufficient oxygen to combust with fuel in a furnace or combustion zone. The oxidant may be air. Preferably the oxidant is a fluid having an oxygen concentration of at least 30 volume percent oxygen, most preferably at least 90 volume percent oxygen. The oxidant may be technically pure oxygen having an oxygen concentration of 99.5 percent or more. The furnace which may be used in the practice of this invention is generally an industrial furnace. The invention will have particular utility in the operation of a glassmaking furnace due to the high temperatures involved. A furnace includes burner ports wherein a burner may be placed and wherein some of all of the combustion may take place. The invention will be described in greater detail with reference to the Figure and the following example and comparative example. The example is presented for illustrative purposes and is not intended to be limiting. Referring now to the Figure, there is shown in simplified form glassmelting furnace 32 where glassmaking material is passed into the furnace through charger ports 30 and 31. The glassmaking materials pass through the furnace from left to right as indicated by the glass flow arrows and in doing so are melted by heat from combustion carried out by burners 1-20 which are shown in representative form by the -arrows. The melted glass passes out from the furnace through throat passage 33. The combustion reaction gases flow above the glassmaking materials in the opposite direction as indicated by the gas flow arrow and out of furnace 32 through flue 34. A furnace similar to that illustrated in the Figure was operated with all twenty burners being high NOx burners of the type illustrated in U.S. Patent No. 5,267,850. The fuel used was natural gas and the oxidant used was a fluid comprising 93 mole percent oxygen. The burners generated NOx at the rate of 0.129 kg/GJ (0.30 lb/million BTU). The NOx generation rate equilibrium value was 0.090 kg/GJ (0.21 1b/million BTU). The NOx emissions were measured at the flue and found to be 0.715 kg NOx/t (1.43 lb NOx/ton) glass. Burner numbers 1-12 were changed to low NOx burners using natural gas as fuel and a fluid comprising 93 mole percent oxygen as oxidant and the furnace was again operated to produce molten glass. The low NOx burners were operated in the manner described in U.S. Serial No. 153,505, filed November 17, 1993 and generated NOx at the rate of 0.0288 kg/GJ (0.067 1b/million BTU). Based on laboratory results of the operation of the two different burners, it was expected that the replacement of 12 of the 20 high NOx burners with low NOx burners would reduce the NOx emissions at the flue by 45 percent. However, when the invention was actually carried out, the measured NOx emissions at the flue were only 0.25 kg NOx/t (0.5 lb NOx/ton) glass, a 65 percent reduction. Heretofore it has been conventional practice to employ one type of burner throughout a multiburner furnace. If conditions required a burner changeover, typically all of the burners were changed. The invention employs an unconventional arrangement wherein two different burner types are used in a multiburner furnace. Moreover, by the practice of this invention, one can attain a greater degree of NOx reduction for any given level of burner conversion from high NOx to low NOx in the operation of a multiburner furnace than would otherwise be possible. Although the invention has been described in detail with reference to a certain preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the scope of the claims.

Claims: A method for operating a furnace to reduce the level of NOx emitted from the flue of the furnace comprising: (A) combusting fuel and oxidant in a first combustion reaction wherein NOx is generated within the furnace; (B) combusting fuel and oxidant in a second combustion reaction wherein NOx is generated within the furnace at a value lower than that of the first combustion reaction, said second combustion reaction occurring nearer to the flue than said first combustion reaction; (C) passing NOx generated by said first and second combustion reactions out from the furnace through the flue; characterized by the NOx being generated in the first combustion reaction at a value exceeding the NOx concentration that would result if the concentrations of nitrogen and oxygen in the furnace combustion products were held at the furnace gas temperature for an infinite period of time and the NOx being generated in the second combustion reaction at a value lower than the NOx concentration that would result if the concentrations of nitrogen and oxygen in the furnace combustion products were held at the furnace gas temperature for an infinite period of time. The method of Claim 1 wherein there is carried out a plurality of first reactions. The method of Claim 1 wherein there is carried out a plurality of second reactions. The method of Claim 1 wherein there is carried out a plurality of first reactions and a plurality of second reactions. The method of Claim 4 wherein all of the second reactions occur nearer to the flue than do any of the first reactions. The method of Claim 1 wherein the furnace is a glassmaking furnace. The method of claim 1 wherein the temperature within the furnace exceeds 1482°C (2700°F).

Inventors: SNYDER WILLIAM JOSEPH, SNYDER, WILLIAM JOSEPH
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1995
Main classification: F23C6
IPC codes: F23C6, F23C5, F23C99, C03B5, F23D23