Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C9/00—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber
  • F23C9/006—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber the recirculation taking place in the combustion chamber
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C1/00—Combustion apparatus specially adapted for combustion of two or more kinds of fuel simultaneously or alternately, at least one kind of fuel being either a fluid fuel or a solid fuel suspended in a carrier gas or air
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C15/00—Apparatus in which combustion takes place in pulses influenced by acoustic resonance in a gas mass
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D14/00—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid
  • F23D14/20—Non-premix gas burners, i.e. in which gaseous fuel is mixed with combustion air on arrival at the combustion zone
  • F23D14/22—Non-premix gas burners, i.e. in which gaseous fuel is mixed with combustion air on arrival at the combustion zone with separate air and gas feed ducts, e.g. with ducts running parallel or crossing each other
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D14/00—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid
  • F23D14/32—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid using a mixture of gaseous fuel and pure oxygen or oxygen-enriched air
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C2202/00—Fluegas recirculation
  • F23C2202/30—Premixing fluegas with combustion air

Definitions

• the present invention relates generally to a method and an apparatus for heat treatment of materials, preferably metals, and more particularly to heat treatment involving low NOx emissions in open flame and radiant tubes as well as a very high thermal efficiency.
• NOx nitrous oxides
• a further problem when using a conventional oxy-fuel burner for heat treatment of metals using radiant tubes is that the high flame temperature of an oxy-fuel burner will damage the tube material, which of course is unacceptable.
• An object of the present invention is to provide a method and an apparatus for heat treatment of metals wherein the drawbacks of prior art apparatuses are eliminated or at least mitigated.
• a particular object is to provide a method and an apparatus wherein emission of nitrous oxides (NOx) is kept to a minimum while main- taining a high degree of thermal efficiency and providing technology to use oxy-fuel combustion in radiant tubes .
• NOx nitrous oxides
• the method provides for an environ- mentally friendly and at the same time cost effective process.
• the invention is used in combination with radiant tubes, allowing the invention to be used in protective atmosphere processes.
• These atmospheres are normally all atmospheres other than the atmosphere resulting form combustion of hydrocarbons .
• Fig. 1 shows a longitudinal elevation sectional view through an apparatus according to the invention
• Fig. 2 is a transverse cross sectional view from line II-II of Fig. 1;
• Fig. 3 is an overall sectional view of a burner and radiant tube arrangement
• Fig. 4 is a diagram showing typical temperature profiles for different types of combustion
• Fig. 5 is a diagram showing flame and recirculation temperatures during operation of an apparatus according to the invention.
• Fig. 6 is a diagram showing resulting equilibrium NO concentrations as well as two measured results .
• Fig. 1 there is shown a sectional view of a burner arrangement, generally designated 1.
• the burner arrangement comprises an insert 10 having a generally circular cross-section, see fig. 2.
• the insert is arranged to be mounted through a hole in a wall 12 of a furnace (not shown), as is conventional. It is also preferred to arrange a heat insulating material 14 on the hot side of the burner mounting plate 12.
• a fuel supply pipe 16 is centrally provided for supplying fuel, such as natural gas, to a burner reaction zone or flame 26, a portion of which is shown in figure 1.
• the fuel supply pipe is in one end provided with a connector arranged to be connected to a source of fuel (not shown) and in the other end with a fuel nozzle 16a.
• Oxygen is supplied through six equidistant pipes 18 placed at a constant distance from the centre axis of the insert 10, see fig. 2.
• the oxygen supply pipes are in one end provided with a respective connector arranged to be connected to a source of oxygen (not shown) and in the other end with a respective oxygen nozzle 18a having a cross-sectional area of A3.
• the oxygen nozzles are streamlined, thus minimizing turbulence of the oxygen leaving the nozzle, preferably at supersonic velocities. The effect of this will be explained further below.
• An annular exhaust opening 20 is provided outside of the oxygen pipes 18. This opening is provided for accommodating recirculation of hot exhausts resulting from the heating process involving the burner.
• the exhaust opening is separated from the oxygen pipes by a comparatively thin wall, thereby providing for a heat exchange between the hot exhausts from the burner process and the relatively cold oxygen supplied through the oxygen pipes 18.
• This heat exchange provides for a high thermal efficiency, in all cases above 90%, resulting in a very energy efficient process, with low exhaust gas temperatures .
• a circular flame tube 22 at the front end portion of the insert, having a cross- sectional area A2 and a diameter L2.
• the flame tube surrounds the fuel and oxygen nozzles 16a, 18a and extends a distance Ll from the oxygen nozzles.
• the primary function of the flame tube is to form a mixing compartment for oxygen and recirculated exhaust gas in a first mixing step and to direct the resulting jet momentum forwards.
• the tube 22 preferably has a cross-sectional area of more than 100 times the cross-sectional area of each of the gas nozzles 18a.
• equidistant apertures 24 each having an area Al, are provided in the flame tube — each one outside of a respective oxygen nozzle 18a. In that way, an ejector function is obtained whereby hot exhausts from outside of the flame tube 22 is sucked into the same.
• the implication of that will be explained in the following in connection with the description of the method according to the invention.
• fuel and oxygen are supplied from a respective source to the respective pipes.
• the fuel can be any gaseous fuel, such as, natural gas, propane, coke oven gas, etc having a combustible content, or any liquid fuel, such as light to heavy fuel oil, emulsions containing carboneous substances , etc ..
• oxygen is in this context meant a gas with an 0 2 content exceeding 80% by volume, and preferably exceeding 99.5%, i.e., essentially pure oxygen.
• the oxidizing gas comprises other elements usually found in air, such as nitrogen and argon. It is preferred that the oxidizing gas comprises less than 5.5% by volume nitrogen and less than 4.5% by volume argon.
• a pilot burner (not shown) is provided in the insert 20.
• y is molefraction of Hydrogen (H) in fuel.
• T rec i ro temperature of exhaust when recirculated
• a mass flow of fuel
• b mass flow of oxygen
• c mass flow of recirculated exhaust.
• the recirculation factor, c/b is typically above 10.
• the exhausts created in the process can contain among 15 other things nitrogen, which, together with oxygen, forms unwanted NOx gases, mainly NO, which in nature is transformed into N0 2 .
• the oxygen is supplied at high velocities, preferably at supersonic velocities, such as Mach 0.5 or above.
• the oxygen is preferably injected to form a free jet to a distance of at least 15 nozzle diameters.
• These velocities together with the configuration of the apertures 24 in the flame tube and the shape of the oxygen nozzles 18a and positions thereof relatively to the respective hole, create an ejector effect sucking the flame exhausts into the flame tube, as indicated by arrows in fig. 1.
• the oxygen nozzles are Laval shaped and aerodynamic .
• the oxygen containing mixture resulting from the primary recirculation Prior to being ignited, the oxygen containing mixture resulting from the primary recirculation is further mixed with the fuel and more exhaust gas in a second recirculation step. Due to the supersonic speed of the injected oxygen, preferably having a velocity of Mach 0.5 or above, this mixing is also effected at that hing velocity. This mixture is thus ignited and forms an extended reaction zone. It is thus realised, that the combination of among other things the parameters Al , A2 , A3, Ll, and L2 is of vital importance to effect the proper mixing of oxygen and exhausts. In a preferred embodiment, the following combination is provided: A1»A3, A2 ⁇ ⁇ Al (area of all apertures 24), and/or Ll ⁇ L2.
• the higher oxygen content the higher temperature.
• the high theoretical flame temperatures obtained with oxy-fuel burners could be disadvantageous in certain heat treatment processes wherein the material to be heated must be brought to very uniform temperatures.
• the flame temperature is lowered to desired temperatures while the high NOx promoting temperatures are avoided, see Fig. 4.
• a secondary recirculation of exhausts is also provided just down-stream of the flame tube 22, as indicated by the arrows in fig. 1.
• This additional dilution of the oxygen content further helps to lower the temperatures of the flame.
• the visible portion of the reaction zone 26 starts at a distance from the fuel nozzle, allowing for the recirculation of the exhausts prior to ignition.
• the effect of the recirculation factor, as defined above, and of the temperature of the recirculated exhaust gases on the resulting flame temperature can be seen in Fig. 5.
• a second preferred embodiment of a burner arrangement according to the invention will now be described with reference to fig. 3, wherein a burner and radiant tube arrangement is shown.
• a burner insert 10 is provided in an aperture in a furnace wall 12.
• a radiant tube, generally designated 30, is provided in front of the burner insert 10 and having a diameter exceeding that of the flame tube 22.
• the use of a radiant tube is known per se and the radiant tube 30 com- prises an outer cylindrical tube 32 having a first open end facing the furnace wall 12 and a second closed end opposite of the first end.
• an inner tube 34 having a diameter less than the inner diameter of the outer tube 32 so as to create an inner, essentially circular channel 36 and an outer, annular channel 38.
• the inner tube is kept in position in any suitable way, such as by means of flanges (not shown) extending outwardly there from.
• the inner tube 34 is positioned with its first end ending a dis- tance L3 from the front end of the flame tube 22 of the burner and with its second end ending spaced apart from the closed end wall of the outer tube 32, thereby providing a recirculation path for exhausts.
• the reaction takes place in the inner channel 36. Exhausts created in the combustion process are guided through the inner channel, turn at the closed end of the outer cylinder 32, and return in the opposite direction through the outer annular channel 38.
• the radiant tube forms an essentially closed system.
• the exhausts returning to the burner 10 are guided either through the openings 24 in the flame tube, forming a primary recirculation path, or through the gap formed between the flame tube 22 and the inner tube 34, forming a secondary recirculation path.
• the proportion of exhausts of the first and secondary recirculation paths is determined e.g., by the parameters Al, A2, Ll, L2, and L3 as well as by the velocity of the oxygen and the fuel.
• the burner arrangement according to the invention allows very high degree of recirculation of exhausts. This in turn allows for extremely low NOx emissions; figures showing as low as 0-25 mg/MJ NOx have been obtained during test runs, depending on the N2 content. An oxygen content below 15% in the reaction zone has been found feasible.
• a very high thermal efficiency is obtained, resulting in a cost-effective process.

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Combustion Of Fluid Fuel (AREA)
• Gas Burners (AREA)
• Pre-Mixing And Non-Premixing Gas Burner (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=20289091

Family Applications (1)

Country Status (14)

Families Citing this family (37)

Family Cites Families (6)

• 2002
  • 2002-09-25 SE SE0202836A patent/SE0202836D0/xx unknown
• 2003
  • 2003-09-25 MX MXPA05003198A patent/MXPA05003198A/es not_active Application Discontinuation
  • 2003-09-25 EP EP03798637A patent/EP1543271A1/en not_active Withdrawn
  • 2003-09-25 US US10/528,546 patent/US20050239005A1/en not_active Abandoned
  • 2003-09-25 RU RU2005109915/06A patent/RU2005109915A/ru not_active Application Discontinuation
  • 2003-09-25 BR BR0314741-0A patent/BR0314741A/pt not_active IP Right Cessation
  • 2003-09-25 AU AU2003263721A patent/AU2003263721A1/en not_active Abandoned
  • 2003-09-25 JP JP2004539712A patent/JP2006500543A/ja active Pending
  • 2003-09-25 KR KR1020057004970A patent/KR20050062556A/ko not_active Application Discontinuation
  • 2003-09-25 CA CA002501062A patent/CA2501062A1/en not_active Abandoned
  • 2003-09-25 WO PCT/SE2003/001492 patent/WO2004029511A1/en not_active Application Discontinuation
  • 2003-09-25 PL PL03374499A patent/PL374499A1/xx not_active Application Discontinuation
• 2005
  • 2005-03-22 ZA ZA2005/02379A patent/ZA200502379B/en unknown
  • 2005-04-21 NO NO20051955A patent/NO20051955L/no not_active Application Discontinuation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events