Abstract:
An improved staged combustion method useful with oxy-fuel combustion and in a furnace which contains a charge, wherein substoichiometric combustion and low velocity injection of fuel and primary and secondary oxidant are carried out in an orientation which forms a reducing atmosphere proximate the charge surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention claims priority to U.S. provisional patent application Ser. No. 60/937,768, filed Jun. 29, 2007, the entire contents of which are incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates generally to staged combustion within a furnace which contains a charge to be heated by heat generated by the combustion. 
       BACKGROUND OF THE INVENTION 
       [0003]    In many industrial heating processes fired with fuel and oxidant, products of fuel combustion interact or react with furnace charge and often cause undesirable effects. For example, fuel rich flame impinging over glassmelt in a glass melting furnace is known to cause color change in glass product due to redox change of the glassmelt exposed to the fuel rich flame. In a steel reheat furnace oxide scale is formed during heating resulting in loss of metal or surface defects. In the direct reduction of iron oxide by the process disclosed in U.S. Pat. Nos. 6,592,649 and 6,602,320, a mixture of iron ore, coal particles and flux material is agglomerated into balls and laid on a rotary hearth furnace, heated and reduced to produce iron nuggets. Iron oxide is preheated, reduced by carbon from coal, and melted to form iron nuggets. In the melting zone, the reduced charge material is heated by gas burners to 1300 to 1500 C to form nuggets and to separate from slag. In the reduction zone, rapid evolution of CO gas from the iron reduction reaction prevents oxidizing gases (CO2, H2O, and O2) in the furnace atmosphere from oxidizing the charge material. In the melting-nugget-forming zone, little CO is evolved from the charge material and reduced iron nuggets are susceptible for re-oxidation by furnace combustion products (CO2, H2O and excess O2). Prior art has disclosed partially solving the re-oxidation problem by charging extra coal particles in the bed of the charge material to protect the iron nuggets from re-oxidation. After devolatization, a bed of char is formed. 
         [0004]    There are drawbacks with this approach. Even if the iron nuggets are formed on a bed of excess coke particles, the top surface of each nugget is exposed to the furnace atmosphere. The melting process requires a significant amount of heat which is typically provided by combustion of natural gas with air. The reactions of CO2 and H2O with carbon are endothermic and consume heat and increase the consumption of natural gas. It is desirable to prevent re-oxidation of the iron nuggets. 
         [0005]    Nitrogen oxides (NOx) 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. 
         [0006]    Staged combustion has been used to reduce NOx generation, particularly when the oxidant is a fluid having an oxygen concentration which exceeds that of air. In staged combustion, fuel and oxidant are introduced into a combustion zone in a substoichiometric ratio and combusted. Due to the excess amount of fuel available for combustion, very few of the oxygen molecules of the oxidant react with nitrogen to form NOx. Additional oxygen is provided to the combustion zone to complete the combustion in a second downstream stage. Because the secondary oxygen is first diluted with furnace gases before it mixes with the unburned fuel, the combustion in the second stage does not occur at very high temperatures, thus limiting the amount of NOx formed. 
         [0007]    Using a deeply staged combustion process the furnace atmosphere near the hearth area can be made either more reducing (U.S. Pat. No. 5,755,818) or more oxidizing (U.S. Pat. No. 5,924,858) by vertically stratifying the furnace atmosphere. For the direct reduction of iron, a reducing atmosphere near the hearth area is desirable. Although this technology has been used commercially in glass melting furnaces where hearth areas are controlled to have a more oxygen rich atmosphere, the degree of atmosphere stratification was limited due to the relatively high momentum required for this method. More recently a technology to fully control the furnace atmosphere by providing an inert protective atmosphere (such as nitrogen) in the lower half of a directly fired furnace was described in U.S. Pat. Nos. 5,609,481, 5,563,903, 5,961,689 and 6,572,676. The process was applied for aluminum remelting and reduced dross formation by 80% in a full scale furnace (13 ft wide×23 ft long×8 ft high). Although the process could be applied in a direct reduction furnace to create a reducing atmosphere in the lower half of the furnace and oxidizing atmosphere in the upper half of the furnace, the large number of special low velocity burners required for the process makes the process more complex to operate. A cost effective and better stratification method is desirable for the direct reduction process, glass melting furnaces and other industrial furnaces where combustion atmosphere interacts with the furnace charge. 
         [0008]    In order to carry out effective combustion with oxidant having a higher oxygen concentration than that of air, the fuel and/or oxidant must be provided into the furnace at a relatively high velocity in order to achieve the requisite momentum. The combustion reactants must have a certain momentum in order to assure adequate mixing of the fuel and oxidant for efficient combustion. The high momentum also causes the combustion reaction products to more effectively spread throughout the furnace to transfer heat to the furnace charge. Momentum is the product of mass and velocity. An oxidant having an oxygen concentration which exceeds that of air will have a lower mass than air on an equivalent oxygen molecule basis. For example, an oxidant fluid having an oxygen concentration of 30 mole percent will have about 70 percent the mass of an oxidatively equivalent amount of air. Accordingly, in order to maintain the requisite momentum, the velocity of the combustion reaction, i.e. the velocity of the fuel and/or oxidant of the combustion reaction, must be correspondingly higher. 
         [0009]    Accordingly, it is an object of this invention to provide an improved staged combustion method wherein fuel and oxidant combust in a combustion reaction having the requisite momentum, with the charge being protected from deleterious contact with combustion reaction products while still ensuring good heat transfer from the combustion reaction to the charge. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    One aspect of the present invention is a method for carrying out combustion comprising: 
         [0011]    (A) injecting into a furnace which contains a charge, at a point above the charge, fuel and primary oxidant in a substoichiometric ratio not exceeding 70 percent of stoichiometric, said primary oxidant being a fluid comprising at least 50 mole percent oxygen, both of said fuel and primary oxidant being injected into the furnace at a velocity of 100 feet per second or less; 
         [0012]    (B) combusting fuel and primary oxidant within the furnace to produce heat and combustion reaction products including unburned fuel; 
         [0013]    (C) injecting secondary oxidant into the furnace above the injection point of the fuel and primary oxidant, said secondary oxidant being a fluid comprising at least 50 mole percent oxygen, at a velocity of 100 feet per second or less; 
         [0014]    (D) establishing a fuel rich gas layer proximate the charge, said fuel rich gas layer being more reducing to the charge than the secondary oxidant; and 
         [0015]    (E) combusting secondary oxidant with unburned fuel to provide additional heat and combustion reaction products within the furnace. 
         [0016]    Another aspect of the invention is a method for carrying out combustion comprising: 
         [0017]    (A) injecting into a furnace which contains a charge, at a point above the charge, fuel and primary oxidant in a substoichiometric ratio not exceeding 70 percent of stoichiometric, said primary oxidant being a fluid comprising at least 50 mole percent oxygen, both of said fuel and primary oxidant being injected into the furnace at a velocity of 100 feet per second or less; 
         [0018]    (B) combusting fuel and primary oxidant within the furnace to produce heat and combustion reaction products including unburned fuel; 
         [0019]    (C) injecting secondary oxidant into the furnace below the injection point of the fuel and primary oxidant, said secondary oxidant being a fluid comprising at least 50 mole percent oxygen, at a velocity of 100 feet per second or less; 
         [0020]    (D) establishing a oxygen rich gas layer proximate the charge, said oxygen rich gas layer being more oxidizing to the charge than the combustion reaction products within the furnace; and 
         [0021]    (E) combusting secondary oxidant with unburned fuel to provide additional heat and combustion reaction products within the furnace. 
         [0022]    As used herein the term “products of complete combustion” means one or more of carbon dioxide and water vapor. 
         [0023]    As used herein the term “products of incomplete combustion” means one or more of carbon monoxide, hydrogen, carbon and partially combusted hydrocarbons. 
         [0024]    As used herein the term “unburned fuel” means material that comprises one or more of fuel which has undergone no combustion, products of incomplete combustion of the fuel, and mixtures thereof. 
         [0025]    As used herein the term “stoichiometric” means the ratio of oxygen to fuel for combustion purposes. A stoichiometric ratio of less than 100 percent means there is less oxygen present than the amount necessary to completely combust the fuel present, i.e. fuel-rich conditions. A stoichiometric ratio greater than 100 percent means there is more oxygen present than the amount necessary to completely combust the fuel, i.e. excess oxygen conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a simplified cross-sectional representation of one embodiment of the invention wherein the gas layer above the charge is reducing. 
           [0027]      FIG. 2  is a simplified cross-sectional representation of one embodiment of the invention wherein the gas layer above the charge is oxidizing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The invention will be described in detail with reference to Figure, in which is shown industrial furnace  1  which contains a charge  2 . Any industrial furnace or one or more zones of an industrial furnace which is heated by one or more burners may be used in the practice of this invention. Examples of such furnaces include a steel reheating furnace wherein the charge is steel, an aluminum melting furnace wherein the charge is aluminum, a glass melting furnace wherein the charge comprises glassmaking materials, and a cement kiln wherein the charge comprises cement. 
         [0029]    Preferred examples are charges which are either susceptible to oxidation or reduction under the conditions that prevail when combustion is occurring in the furnace. A particularly preferred example that is susceptible for oxidation is a charge comprising iron in its reduced form, or iron in its reduced form mixed with carbonaceous matter such as coke or charcoal. A particularly preferred example that is susceptible for reduction or redox changes is a charge comprising oxidized molten glass. 
         [0030]    Fuel  6  and primary oxidant  7  are provided into furnace  1  at point  3  above charge  2  such as through burner  4 . The fuel and primary oxidant may be injected into furnace  1  separately or together in a premixed condition. The fuel and primary oxidant may be provided into furnace  1  through a plurality of burners. Any suitable oxy-fuel burner may be employed in the practice of this invention. One particularly preferred oxy-fuel burner for use in the practice of this invention is the fuel jet burner disclosed in U.S. Pat. No. 5,411,395 to Kobayashi et al. which is incorporated herein by reference. 
         [0031]    The fuel may be any gas or other fluid which contains combustibles which may combust in the combustion zone of the furnace. Among such fuels one can name natural gas, coke oven gas, propane, methane and oil. 
         [0032]    The primary oxidant is a fluid having an oxygen concentration of at least 50 volume percent oxygen, preferably at least 90 volume percent oxygen. The primary oxidant may be commercially pure oxygen having an oxygen concentration of 99.5 percent or more. 
         [0033]    The fuel and primary oxidant are provided into furnace  1  at flow rates such that the stoichiometric ratio of primary oxygen to fuel is less than 70 percent and preferably is within the range of from 5 to 50 percent of stoichiometric. 
         [0034]    Both of the fuel and primary oxidant are injected into furnace  1  at a velocity of 100 feet per second (fps) or less. Preferably the fuel is provided at a velocity of 50 to 100 fps. Preferably the primary oxidant is provided at a velocity of 2 to 50 fps. These velocities, low relative to prior art practices, impart the requisite low momentum to the combustion reactants. The fuel and primary oxidant combust within furnace  1  in a combustion reaction  5  to produce heat and combustion reaction products. Combustion reaction products may include products of complete combustion but, owing to the defined substoichiometric primary oxygen to fuel ratio, will include unburned fuel. The incomplete combustion of the fuel with the primary oxidant enables the combustion of fuel and primary oxidant to proceed at a substantially lower temperature than would otherwise be the case, thus reducing the tendency of NOx to form. The combustion reaction products may also include some residual oxygen because of incomplete mixing and short residence time during the combustion reaction although it is possible that the concentration of oxygen within the combustion reaction products is zero. 
         [0035]    In the embodiment of the invention illustrated in  FIG. 1 , in order to establish a reducing gas layer over the charge surface, secondary oxidant  8  is provided into furnace  1  through lance  10  above point  3 . Preferably, in this embodiment the secondary oxidant is injected into the furnace at a point that is further from the upper surface of the charge  2  than point  3  is. The secondary oxidant may be provided into the furnace from a point vertically above the fuel and primary oxidant, or from a point offset from the vertical, such as by an angle of up to 45 degrees. 
         [0036]    In the embodiment of the invention illustrated in  FIG. 2 , in order to establish an oxidizing gas layer over the charge surface, secondary oxidant  8  is provided into furnace  1  through lance  10  below point  3 . Preferably, in this embodiment the secondary oxidant is injected into the furnace at a point that is between the upper surface of the charge  2  and point  3 . The secondary oxidant may be provided into the furnace from a point vertically below the fuel and primary oxidant, or from a point offset from the vertical, such as by an angle of up to 45 degrees. 
         [0037]    The secondary oxidant is in the form of a fluid having an oxygen concentration of at least 50 mole percent, preferably at least 90 mole percent. The secondary oxidant may be commercially pure oxygen. Secondary oxidant  8  is provided into furnace  1  at a velocity of 100 fps or less, and preferably at a velocity which the range of from 50 to 100 fps or even as low as 20 fps to 50 fps. It is important to the practice of this invention that the oxidant have an oxygen concentration significantly greater than that of air. For a given amount of fuel consumption, the total volume of gases passed through the furnace lessens as the oxygen concentration of the oxidant increases. This lower volume flux through the furnace, at the velocities required for the staged combustion practice of this invention, enables the establishment of the gas layer proximate the charge having a different composition than the contents in the rest of the furnace. 
         [0038]    Secondary oxidant gas layer  9  has an oxygen concentration which exceeds that of the combustion reaction products within combustion reaction  5 . Although any suitable oxygen lance may be used to inject the secondary oxidant into the furnace in the practice of this invention, it is preferred that the secondary oxidant be injected into the furnace using the gas injection lance disclosed in U.S. Pat. No. 5,295,816 to Kobayashi et al. which is incorporated herein by reference. 
         [0039]    The secondary oxidant is provided into the furnace at a flowrate such that, when added to the primary oxidant, establishes a stoichiometric ratio with the fuel of at least 90 percent, and preferably within the range of from 100 to 110 percent. When the stoichiometric ratio of the primary and secondary oxidant to the fuel is less than 100 percent, the remaining oxygen needed to achieve complete combustion of the fuel within the furnace may be provided by infiltrating air. Preferably, the momentum ratio of the fuel and primary oxidant stream to the secondary oxidant stream is about 1.0 although some divergence from unity is acceptable, such as a momentum ratio within the range of from 0.3 to 3.0 or less. 
         [0040]    Heat generated in combustion reaction  5  radiates to the charge to heat the charge. This heat radiates from combustion reaction  5  to the charge directly or indirectly through complex radiative interactions with surrounding furnace gases and walls. Very little heat is passed from the combustion reaction to the charge by convection in high temperature furnaces. 
         [0041]    In the embodiment of the invention illustrated in  FIG. 1 , because of the position at which the secondary oxidant is provided into the furnace, there is formed a relatively reducing gas layer which interacts with charge  2  in a manner which differs from the interaction which would occur were the furnace atmosphere homogeneous. In the embodiment of the invention illustrated in  FIG. 2 , because of the position at which the secondary oxidant is provided into the furnace, there is formed a relatively oxidizing gas layer which interacts with charge  2  in a manner which differs from the interaction which would occur were the furnace atmosphere homogeneous. 
         [0042]    Downstream of combustion reaction  5  the secondary oxidant and the unburned fuel will mix, such as in region  11  within furnace  1 , thus serving to prevent the secondary oxidant from directly interacting (reacting) with the oxidizable components of the charge in the embodiment of the invention illustrated in  FIG. 1 , or serving to prevent the products of incomplete combustion from directly interacting (reacting) with the reducible components of the charge in the embodiment of the invention illustrated in  FIG. 2 , to complete the combustion of the fuel, and to provide additional heat and combustion reaction products within the furnace. 
         [0043]    The combustion reaction products in furnace  1  are generally exhausted through a flue port located in the coldest area of the furnace in order to maximize the furnace fuel efficiency. When the present invention is used in a zone of a furnace with multiple zones, the combustion reaction products may be exhausted to the adjacent zone. The elevation of the flue port also influences the degree of furnace atmosphere stratification. Preferably the combustion reaction products in furnace  1  are exhausted from the furnace from a point not below point  3  where fuel and primary oxidant are provided into the furnace, such as from flue  12 .