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 
  • F23C5/00—Disposition of burners with respect to the combustion chamber or to one another; Mounting of burners in combustion apparatus
  • F23C5/08—Disposition of burners
• • 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 
  • F23C6/00—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion
  • F23C6/04—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection
  • F23C6/045—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with staged combustion in a single enclosure
• • 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 
  • F23C7/00—Combustion apparatus characterised by arrangements for air supply
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23D—BURNERS
  • F23D23/00—Assemblies of two or more burners
• • 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 
  • F23C2201/00—Staged combustion
  • F23C2201/10—Furnace staging
• • 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 
  • F23C2201/00—Staged combustion
  • F23C2201/10—Furnace staging
  • F23C2201/102—Furnace staging in horizontal direction
• • 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 
  • F23C2201/00—Staged combustion
  • F23C2201/20—Burner staging
• • 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
  • F23C2202/40—Inducing local whirls around flame

Definitions

• the invention relates to a method of burning fuel and a boiler.
• the flame of the burners is often stabilized by setting the secondary or tertiary air into a tangential motion. This can be achieved by directing the air into the furnace trough a swirler, which gives tangential motion to the air flowing through it.
• the swirler comprises vanes set at a certain angle to change the direction of the air flow.
• the swirl number S of the air is set to give the air a certain tangen ⁇ tial velocity.
• the swirl number S characterizes the ratio of tangential to axial momentum of the air flow. When the swirl number is low, weak swirl is created, and there is little or no flow recirculation. When the swirl number is high, a recirculation zone is formed.
• the purpose of the invention is to provide a method of burning fuel and a boiler, in which the flu ⁇ id dynamics in the furnace is affected so as to cause a flow pattern, which favours fuel rich flue gases to entrain from furnace walls to the center of the fur- nace, thereby reducing corrosion of furnace walls.
• the purpose is also to keep the carbon monoxide and nitro ⁇ gen oxide emissions below the allowable limits.
• the boiler according to the present invention is characterized by what is presented in claim 15. BRIEF DESCRIPTION OF THE DRAWINGS
• Fig. 1 is a schematic perspective view of a boiler comprising the set of burners according to one embodiment at two opposite furnace walls,
• Fig. 2 is a schematic sectional view of the burner according to one embodiment
• Fig.3 is a schematic sectional view of the burner according to a second embodiment
• Fig.4 is a schematic sectional view of the overall flow field at a certain horizontal burner lev- el inside the boiler furnace.
• a method of burning fuel comprising supplying fuel and combustion air into a boiler furnace comprising furnace walls through a set of burners located on a furnace wall at or about a certain hori ⁇ zontal burner level for producing a flame by one or more of the burners.
• the combustion air comprises pri- mary air and secondary air.
• the fuel is supplied through a fuel feed pipe comprising a downstream end.
• the flame of each of the burners is stabilized by providing the downstream end of the fuel feed pipe with a flame stabilizing ring for producing an inner recirculation zone downstream of the flame stabilizing ring.
• Each burner has an air coefficient (SR W i ng , SR cent - er ) and the secondary air supplied through the burners has a swirl number (S W i ng , S cente r) ⁇
• the set of burners comprises at least one center burner and at least one wing burner located on both sides of the at least one center burner. The method further comprises:
• a boiler comprising a boiler furnace comprising furnace walls, and a set of burners located on a furnace wall at or about a certain horizontal burner level.
• Each burner of the set of burners comprises
• a fuel feed pipe for supplying fuel into the boiler furnace and comprising a downstream end;
• a secondary air flow channel for supplying secondary air into the boiler furnace and arranged around the fuel feed pipe;
• Each burner has an air coefficient ( SR W ing , SRcenter ) and the secondary air supplied through the burners has a swirl number (S W i ng , S cen ter ) ⁇
• the set of burners comprises at least one center burner and at least one wing burner located on both sides of the at least one center burner.
• Each of the wing burners is configured to supply combustion air into the boiler furnace in such an amount that the air coefficient ( SR W i ng ) of each of the wing burners is 0.95 or above
• the at least one center burner is configured to supply combustion air into the boiler furnace in such an amount that the air coefficient ( SRcenter ) of the at least one center burner is 0.9 or below .
• Each of the wing burners is configured to provide the secondary air supplied through each of the wing burners a swirl number ( S W i ng ) of 0 - 0.3.
• the at least one center burner is configured to produce an outer recirculation zone around the inner recirculation zone produced downstream of the flame stabilizing ring of the at least one center burner for broadening the shape of the flame of the at least one center burner.
• the method provided herein is a two-stage combustion system where the burners at a certain horizontal burner level are operated such that different air coefficients are provided for the center and the wing burners .
• the shape of the flame of the outermost wing burners is kept nar ⁇ row, whereas the shape of the flame of the innermost center burners is kept broad and short.
• a flow pattern is created that results in more oxygen- rich conditions close to the furnace walls, which fa ⁇ vours reduction of corrosion rate of the furnace walls.
• the flow pattern causes fuel rich flue gases to entrain from furnace walls to the center of the fur- nace .
• the flow pattern is achieved by proper opera ⁇ tion and configuration of the set of burners. Combina ⁇ tion of hybrid burner configuration and operation results in differences in the momentum flows of the wing and center burners.
• the set of burners comprises at least one center burner and at least one wing burner located on both sides of the at least one center burner.
• the set of burners consists of at least one center burner and at least one wing burner located on both sides of the at least one center burner.
• at least one wing burner is located on both sides of the set of center burners.
• the wing burner is thus located between the furnace wall and the outermost center burner at the horizontal burner level. No wing burners are located between the center burners.
• the set of burners comprises adjacent burners at a certain horizontal burner level on one furnace wall.
• the set of burners may consist of for example three, four, five, six, seven or eight burners.
• the at least one center burner consists of one center burner.
• the at least one center burner may consist of for example two, three, four, five or six center burners .
• the set of burners com- prises a center section and a side section on both sides of the center section.
• the center section comprises at least one center burner.
• the side section comprises at least one wing burner.
• the center section consists of at least one center burner.
• the side section consists of at least one wing burner.
• the burners of the set of burners are located at substantially the same horizontal burner level.
• the burners need not be located at exactly the same hori- zontal burner level, but the horizontal level of the burners may differ slightly from each other.
• the set of burners is located on a furnace wall at a certain horizontal burner level.
• the number of horizontal burner levels, each comprising a set of burners on two opposite furnace walls, may be for example one, two, three four or five.
• the structure of the at least one center burner is the same as the structure of each of the wing burners. In one embodiment, the structure of the at least one center burner is different from the structure of each of the wing burners.
• the air coefficient ( SR W i ng ) of each of the wing burners is set at 0.95 or above. Each of the wing burners is thus operated at near stoichiometric ratio of air and fuel. As the air coefficient ( SR W i ng ) of each of the wing burners is relatively high, the amount of carbon monoxide formed during combustion at the wing burners is kept low, which for its part helps reducing the corrosion of furnace walls caused by car ⁇ bon monoxide.
• the high air co ⁇ efficient causes elevated nitrogen oxide emissions formed during combustion at the wing burners.
• the total amount of nitrogen oxides produced in the furnace is kept below allowable limits by ad ⁇ justing the air coefficient ( SR cen ter ) of the at least one center burner at 0.9 or below, such that the amount of nitrogen oxides produced in combustion at the center burners is considerably lower than the amount of nitrogen oxides produced in combustion at the wing burners .
• the air coefficient ( SR cen ter ) of the at least one center burner is set at 0.9 or below.
• Each of the center burners is thus operated at substoichiometric ratio of air and fuel.
• the low air coefficient reduces the amount of nitrogen oxides formed during combustion at the center burners.
• the sub ⁇ stoichiometric air coefficient increases the amount of carbon monoxides formed during combustion at the cen- ter burners.
• the center burners are the innermost burners in the horizontal burner row, the formed carbon monoxide is kept away from the furnace walls and does not lead to corrosion of furnace walls to the same extent as carbon monoxide formed near the furnace walls.
• the value of air the coefficient ( SRcenter ) of the at least one center burner is adjusted such that the overall amount of carbon monoxides produced in the furnace is kept below the allowable limits.
• the air coefficient of the center burners ( SRcenter ) is smaller than the air coefficient ( SR W i ng ) of the wing burners.
• the difference in the air coeffi ⁇ cient of the wing and center burners changes the over- all flow field in the furnace.
• underpressure is caused downstream of the center burn- ers as compared to the pressure downstream of the wing burners.
• the underpressure downstream of the center burners causes the flame of the wing burners to bend towards the center of the furnace and away from the furnace walls. This helps in keeping carbon monoxide away from the furnace walls and leads in reduced side wall corrosion.
• the meaning of the term air coefficient is clear to a skilled person.
• the air coefficient or the stoichiometric ratio SR tells how much air is used for the combustion in comparison with the theoretical (stoichiometric) volume of air needed for complete combustion of the fuel. In substoichiometric combus ⁇ tion, the air coefficient SR is under 1, and in super- stoichiometric combustion the air coefficient SR is over 1.
• the air coefficient of a burner tells how much air is supplied though said burner in comparison with the theoretical volume of air needed for complete com ⁇ bustion of the fuel supplied through said burner.
• the amount of combustion air supplied through the burners or the air coefficient (SR) of the burners is adjusted using conventional means known in the field.
• the amount of combustion air supplied through each burner is calculated based on the amount of fuel supplied through the burner and the desired air coef- ficient.
• the amount of combustion air supplied through a burner is adjusted by means of a damper or a control valve.
• the damper or the control valve allows a certain amount of combustion air to flow through it.
• the amount of com- bustion air supplied through a burner is adjusted by directing combustion air to the burners relative to the pressure loss of the burners from a common wind box connected to the burners.
• the pressure loss at each burner can be adjusted e.g. by changing the swirl number of combustion air supplied through the burner.
• the swirl number ( S W i ng ) of the secondary air supplied through each of the wing burners is set at 0 - 0.3. In one embodiment, the swirl number ( S W i ng ) of the secondary air supplied through each of the wing burners is set at 0.01 - 0.3 or 0.1 - 0.3. In one em ⁇ bodiment, the combustion air comprises tertiary air and the swirl number ( S W i ng ) of the tertiary air sup ⁇ plied through each of the wing burners is set at 0 - 0.3 or 0.01 - 0.3 or 0.1 - 0.3.
• each of the wing burners is configured to provide the tertiary air supplied through each of the wing burners a swirl number ( S W i ng ) of 0 - 0.3 or 0.01 - 0.3 or 0.1 0.3.
• the flame stabilizing ring attached to the downstream end of the fuel feed pipe enables reducing the swirl number by stabilizing the flame.
• the swirl number ( S W i ng ) of the secondary air or the tertiary air supplied through each of the wing burners is set at 0 - 0.1.
• each of the wing burners is configured to provide the second ⁇ ary air or the tertiary air supplied through each of the wing burners a swirl number ( S W i ng ) of 0 - 0.1.
• the swirl number ( S W i ng ) of the second ⁇ ary air or the tertiary air supplied through each of the wing burners is set at 0, i.e. no swirl is provid ⁇ ed to the air.
• the secondary air or tertiary air supplied through each of the wing burners located near the furnace walls is in non-swirling motion, or pro ⁇ vided with a weak swirl. Intensive separation of air and fuel at near burner region is avoided, but rather, flows of air and fuel are more axial to avoid bending of fuel to the furnace walls.
• the secondary air flow channel or tertiary air flow channel of each of the wing burners comprises a swirler for directing the secondary air or tertiary air by means of vanes.
• the swirler is configured to provide the secondary or ter ⁇ tiary air supplied through each of the wing burners a swirl number ( S W i ng ) of 0 - 0.3 or 0.01 - 0.3 or 0.1 - 0.3 or 0 - 0.1 or 0.
• the vanes are arranged in an an ⁇ gle ⁇ to the flow of the secondary air or tertiary air.
• the angle ⁇ is set to provide the secondary air or tertiary air supplied through each of the wing burners a swirl number ( S W i ng ) of 0 - 0.3 or 0.01 - 0.3 or 0.1 - 0.3 or 0 - 0.1 or 0.
• the secondary air flow channel or tertiary air flow channel does not contain a swirler.
• the swirl number S characterizes the ratio of tangential to axial momentum of the air flow.
• the con- cept of swirl number is common general knowledge to a skilled person. The calculation of the swirl number can be found, e.g. in Beer, J. and Chigier, N., Combustion Aerodynamics, 1972, pages 109-115.
• the swirl number of the air flow can be changed by directing the air flow through a swirler comprising vanes which are set at an angle to the direction of the air flow.
• the swirler gives tangential motion to the air flowing through it.
• the swirl number can be adjusted by ad ⁇ justing the angle of the vanes and by changing the ve ⁇ locity of the air.
• the vane angle needed to supply the combustion air a certain swirl number S depends on the type of the swirler.
• the swirler is commonly located in the secondary or tertiary air flow channel.
• Each of the set of burners i.e. both the center and the wing burners, is provided with a flame stabilizing ring.
• the fuel feed pipe further comprises an outer wall and an outlet, and the secondary air is supplied through a secondary air flow channel arranged around the fuel feed pipe, the secondary air flow channel comprising an outlet, and the flame stabilizing ring is attached to the out- er wall of the fuel feed pipe such that it surrounds the outlet of the fuel feed pipe and protrudes towards the outlet of the secondary air flow channel.
• the flame stabilizing ring blocks a part of the outlet of the secondary air flow channel.
• the fuel feed pipe further comprises an outer wall and an outlet
• the secondary air flow channel comprises an outlet
• the flame stabilizing ring is attached to the outer wall of the fuel feed pipe such that it surrounds the outlet of the fuel feed pipe and pro- trudes towards the outlet of the secondary air flow channel .
• Fuel carrier gas and fuel supplied through the fuel feed pipe flow on one side of the flame sta ⁇ bilizing ring and secondary air supplied through the secondary air flow channel flows on the other side of the flame stabilizing ring.
• the flame stabilizing ring blocks a part of the outlet of the secondary air flow channel.
• a part of the secondary air flow collides with the flame stabilizing ring, whereby the flow field of the air is changed.
• An inner recirculation zone is formed downstream of the flame stabilizing ring.
• the inner recirculation zone is formed by the reverse flow of combustion air back to the burner.
• the inner recirculation zone is delimited in radial direc ⁇ tion, i.e.
• the flame stabi- lizing ring in the direction perpendicular to the central axis of the fuel feed pipe, by the flame stabi- lizing ring. Behind, i.e. upstream of, the flame sta ⁇ bilizing ring in the secondary air flow channel a reduced pressure field is provided, which causes stabi ⁇ lization of the flame, or at least enhances the sta ⁇ bility of the flame.
• the flame ignites better by means of the flame stabilizing ring than without the flame stabilizing ring.
• the fuel is ignit ⁇ ed within the recirculation flow generated inside the flame stabilizing ring.
• the flame stabilizing ring changes the flow field of the flame so as to keep the flame narrow.
• the flame stabilizing ring ignites the fuel right in the vicinity of the burner nozzle.
• the fuel is ignited within the recirculation flow generated inside the flame stabilizing ring.
• flue gas tempera ⁇ tures in the furnace upper part decrease by about 20 - 50 °C, which increases boiler efficiency. Due to en ⁇ hanced burning in the burner zone, flue gases are at lower temperature when entering the superheaters and also the temperature distribution within the flue gas ⁇ es is more uniform. Consequently, the material temper ⁇ atures of the superheater and reheaters will be kept lower and more uniform. Experiences show that this will result in remarkable reduction of material damag ⁇ es in the heating surfaces.
• the diameter of the cross-section of the flame stabilizing ring increases in the direction towards the center of the boiler furnace.
• the flame sta- bilizing ring broadens in the direction of the furnace walls.
• the flame stabilizing ring may be substantially truncated cone shaped, so as to open toward the center of the boiler furnace and the furnace walls.
• the flame stabilizing ring may also have another shape, as long as it changes the flow field of the flame to the de ⁇ sired direction.
• the shape of the flame stabilizing ring may be staggered in such a way that the end of the flame stabilizing ring secured to the fuel feed pipe is perpendicular to the center axis of the fuel feed pipe and the flame stabilizing ring takes a turn at a distance from its point of attachment toward the center of the boiler furnace.
• the flame stabilizing ring comprises an annular section widening in a direction away from the outlet of the fuel feed pipe.
• the flame stabilizing ring may comprise a number of tooth- like projections which radially extend into the fuel feed pipe.
• the wall thickness of the widening annular section of the flame stabilizing ring is steadily reduced toward the free edge of the flame stabilizing ring.
• the up- stream end of the fuel feed pipe is thinned, and the flame stabilizing ring comprises a uniform annular section which can be fitted around the thinned end of the fuel feed pipe and secured by a locking ring.
• the flame stabilizing ring is made of heat-resistant steel.
• the flame stabilizing ring may consist of one or more sections. Said tooth-like pro ⁇ jections can be made of heat-resistant steel or heat- resistant ceramic material.
• the shape of the flame of the at least one center burner is broadened by producing an outer recirculation zone around the inner recirculation zone produced downstream of the flame stabilizing ring of the at least one center burner.
• the outer recircula- tion zone is formed by the reverse flow of combustion air back to the burner.
• the outer recirculation zone improves ignition and broadens the flame.
• the broaden- ing of the flame helps in reducing the amount of ni ⁇ trogen oxides.
• the flames of the center burners are broader and shorter in the axial direction, i.e. in the direction of the central axis of the fuel feed pipe, than the flames of the wing burners.
• the air to fuel ratio at the near burner region is kept optimal, and the volatile components and nitrogen are allowed to be ef ⁇ ficiently released from the fuel before mixing of fur- ther combustion air to the fuel. A high temperature flame is thereby produced.
• efficient nitrogen oxide reduction is achieved in the boiler furnace.
• the outer recirculation zone forms around the inner recirculation zone.
• the outer recircula ⁇ tion zone surrounds the inner recirculation zone.
• the outer recirculation zone carries the combustion air supplied outside the flame stabilizing ring to the center of the boiler furnace, wherein the combustion air is heated. The heated combustion air is then pulled by the recirculating flow back to the near burner region, bringing heat to the near burner region and producing efficient combustion at the center burners .
• sufficient air staging effect is achieved at near burner region, that is, separation of flows of fuel and combustion air.
• the outer recirculation zone improves the air staging effect. Mixing of combustion air, which is supplied outside the flame stabilizing ring, with the fuel occurs later, and high-temperature reducing conditions are provided at near burner region of each of the center burners.
• the fuel is pulverized fuel. In one embodiment, the fuel is pulverized coal. The fuel may also be other type of fuel, e.g pulver ⁇ ized wood pellet or pulverized biomass. In one embodi- ment, the fuel is supplied trough a fuel feed pipe. In one embodiment, the fuel is supplied together with a carrier gas. In one embodiment, the carrier gas is air. In one embodiment, the carrier gas is primary air. In one embodiment, the carrier gas is a mixture of air and flue gas.
• the burner comprises a fuel feed pipe.
• a secondary air flow channel is ar ⁇ ranged around the fuel feed pipe.
• the secondary air flow channel is delimited by the fuel feed pipe and a first tube coaxially arranged around the fuel feed pipe.
• the cross-section of the secondary air flow channel is annular.
• a tertiary air flow channel is arranged around the secondary air flow channel.
• the ter- tiary air flow channel is delimited by the first tube and a second tube coaxially arranged around the first tube.
• the cross-section of the tertiary air flow channel is annular.
• the method of burning fuel comprises setting the air coefficient ( SR W i ng ) of each of the wing burners at 0.95 - 1.1.
• each of the wing burners is configured to supply com ⁇ bustion air in such an amount that the air coefficient ( SR W in g ) of each of the wing burners is 0.95 - 1.1.
• the air coefficient ( SR W i ng ) of each of the wing burners is 0.95 - 1.1, the amount of carbon monoxide produced in combustion at the wing burners is kept low enough so as not to cause extensive corrosion of fur ⁇ nace walls.
• the method of burning fuel comprises setting the air coefficient ( SR W i ng ) of each of the wing burners at 1.0.
• each of the wing burners is configured to supply com ⁇ bustion air in such an amount that the air coefficient ( SR W in g ) of each of the wing burners is 1.0.
• the method of burning fuel comprises setting the air coefficient ( SR cen ter ) of the at least one center burner at 0.6 - 0.9. In one embod- iment, the method of burning fuel comprises setting the air coefficient (SR ce n ter ) of the at least one cen ⁇ ter burner at 0.7 - 0.8. In one embodiment, the method of burning fuel comprises setting the air coefficient ( S Rcenter ) of the at least one center burner at 0.75. In one embodiment, the at least one center burner is configured to supply combustion air in such an amount that the air coefficient ( S R ce n ter ) of the at least one center burner is 0.6 - 0.9.
• the at least one center burner is configured to supply com ⁇ bustion air in such an amount that the air coefficient ( S Rcenter ) of the at least one center burner is 0.7 - 0.8. In one embodiment, the at least one center burner is configured to supply combustion air in such an amount that the air coefficient ( S R ce nter ) of the at least one center burner is 0.75.
• the air coeffi ⁇ cient ( S Rcenter ) of the at least one center burner is 0.6 - 0.9, the amount of nitrogen oxides produced in combustion at the center burners is kept low enough so as to keep the overall nitrogen oxide emissions below allowable limits.
• over-firing air (OFA) system may be modified accordingly as unburned fuel (unburned carbon in ash, CO) is more concentrated to the center of boiler furnace .
• the overall air coeffi- cient SR in the combustion zone before overfire air ⁇ port is below 0.85 - 0.9.
• the method of burning fuel comprises producing the outer recirculation zone by setting the swirl number (S cen ter ) of the secondary air supplied through the at least one center burner at 0.6 1.5.
• the at least one center burner is configured to provide the secondary air sup ⁇ plied through the at least one center burner a swirl number (S cen ter ) of 0.6 - 1.5.
• the secondary air is thus provided with a relatively strong swirl, i.e. the tan ⁇ gential velocity of the secondary air is increased.
• the tangentially flowing secondary air broadens the flame of the center burners.
• an outer re ⁇ circulation zone stabilizing the flame is formed around the inner recirculation zone. Broadening the shape of the flame also improves the combustion.
• Suf ⁇ ficient swirl and low air ratio at each of the center burners result in shorter flame of the center burners as compared to the flame of the wing burners.
• the secondary air flow channel of the at least one center burner comprises a swirler for directing the secondary air by means of vanes.
• the vanes are arranged in an angle ⁇ to the flow of the secondary air.
• the at least one center burner is configured to produce the outer recircula ⁇ tion zone by setting the angle ⁇ to provide the sec ⁇ ondary air supplied through the at least one center burner a swirl number ( S ce n ter ) of 0.6 - 1.5 or 0.9 - 1.5 or 1.1 - 1.3.
• the swirl number ( S ce n ter ) of the secondary air supplied through the at least one center burner is set at 0.9 - 1.5 or 1.1 - 1.3.
• the at least one center burner is config ⁇ ured to provide the secondary air supplied through the at least one center burner a swirl number ( S ce n ter ) of 0.9 - 1.5 or 1.1 - 1.3.
• the combus- tion air comprises tertiary air, and the swirl number ( S center ) of the tertiary air supplied through the at least one center burner is set at 0.6 - 1.5.
• the at least one center burner is config ⁇ ured to provide the tertiary air supplied through the at least one center burner a swirl number ( S ce n ter ) of 0.6 - 1.5.
• the combustion air com ⁇ prises tertiary air
• the swirl number ( S ce n ter ) of the tertiary air supplied through the at least one center burner is set at 0.9 - 1.5 or 1.1 - 1.3.
• the at least one center burner is config ⁇ ured to provide the tertiary air supplied through the at least one center burner a swirl number ( S ce n ter ) of 0.9 - 1.5 or 1.1 - 1.3.
• the outer recirculation zone may be produced by using a different structure for the center burners than for the wing burners.
• the sec ⁇ ondary air flow channel further comprises a downstream end and an outer wall and the combustion air supplied through the at least one center burner comprises tertiary air, and the tertiary air is supplied into the boiler furnace through a tertiary air flow channel arranged around the secondary air flow channel, the ter- tiary air flow channel comprising an outlet
• the method comprises producing the outer recirculation zone by providing the downstream end of the secondary air flow channel with an air guiding device attached to the outer wall of the secondary air flow channel such that is surrounds the outlet of the secondary air flow channel and protrudes towards the outlet of the tertiary air flow channel.
• the secondary air flow channel further comprises a downstream end and an out ⁇ er wall
• the at least one center burner comprises a tertiary air flow channel arranged around the sec- ondary air flow channel for supplying tertiary air into the boiler furnace, the tertiary air flow channel comprising an outlet
• the at least one center burner is configured to produce the outer recircula ⁇ tion zone by providing the downstream end of the sec- ondary air flow channel with an air guiding device attached to the outer wall of the secondary air flow channel such that it surrounds the outlet of the sec ⁇ ondary air flow channel and protrudes towards the out ⁇ let of the tertiary air flow channel.
• the air guiding device is substantially truncated cone shaped.
• the air guiding device is an expanded portion of the outer wall of the secondary air flow channel.
• the diameter of the cross-section of the air guiding de- vice increases towards the center of the boiler fur ⁇ nace.
• the air guiding device broadens in the direction of the outlet of the tertiary air flow channel.
• the air guiding devise blocks part of the outlet of the tertiary air flow channel. A part of the tertiary air collides with the air guiding device, whereby the flow field of the air is changed.
• the air guiding device turns the tertiary air to flow radially outwards and delays mixing of the tertiary air with the fuel.
• the flame is broadened resulting in efficient combustion.
• An outer recirculation zone stabilizing the flame is formed around the inner circulation zone.
• the air guiding device is arranged in an angle a to the direction of the center axis of the fuel feed pipe and the angle a is 25 - 45 degrees. In one embodiment, the angle a is 30 - 40 de- grees. In one embodiment, the angle a is 35 degrees. According to CFD calculations, good fluid dynamics is achieved when the angle a is 25 - 45 degrees.
• the swirl number (S cen ter) of the secondary air supplied through the at least one center burner is set at 0 - 0.5. In one embodiment, the swirl number (S cen ter) of the secondary air supplied through the at least one center burner is set at 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0. In one embodi ⁇ ment, the swirl number (S cen ter) of the tertiary air supplied through the at least one center burner is set at 0 - 0.5. In one embodiment, the swirl number (Scenter) of the tertiary air supplied through the at least one center burner is set at 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0.
• the at least one center burner is configured to provide the secondary air sup ⁇ plied through the at least one center burner a swirl number (S cen ter) of 0 - 0.5. In one embodiment, the at least one center burner is configured to provide the secondary air supplied through the at least one center burner a swirl number (S cen ter) of 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0. In one embodiment, the at least one center burner is configured to provide the ter ⁇ tiary air supplied through the at least one center burner a swirl number (S cen ter) of 0 - 0.5.
• the at least one center burner is configured to provide the tertiary air supplied through the at least one center burner a swirl number (S cen ter) of 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0.
• the secondary air flow channel or tertiary air flow channel of the at least one center burner com ⁇ prises a swirler for directing the secondary air or tertiary air by means of vanes arranged in an angle ⁇ to the flow of the secondary air or tertiary air, and the angle ⁇ is set to provide the secondary air or tertiary air supplied through the at least one center burner a swirl number ( S cen ter ) of 0 - 0.5 or 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0.
• the secondary air flow channel or tertiary air flow channel does not contain a swirler.
• At least a part of the set of burners comprises a core air duct coaxially ar ⁇ ranged inside the fuel feed pipe.
• core air is supplied through the core air duct.
• no air is supplied through the core air duct.
• only little air is supplied through the core air duct.
• the core air duct acts as a bluff body that enhances the flame stabili ⁇ zation.
• each of the wing burners comprises a core air duct coaxially arranged inside the fuel feed pipe.
• the at least one center burner comprises a core air duct coaxially arrange inside the fuel feed pipe.
• both each of the wing burners and the at least one center burner comprises a core air duct coaxially ar ⁇ range inside the fuel feed pipe.
• the velocity of the sec ⁇ ondary air at the outlet of the secondary air flow channel of each of the burners is set at 40 - 60 m/s . In one embodiment, the velocity of the secondary air at the outlet of the secondary air flow channel of each of the burners is arranged to be 40 - 60 m/s. In one embodiment, the cross-sectional area of the sec ⁇ ondary air flow channel of each of the burners at the outlet of the secondary air flow channel is arranged to be such that the velocity of the secondary air is 40 - 60 m/s. In one embodiment, the velocity of the tertiary air at the outlet of the tertiary air flow channel of each of the burners is set at 40 - 60 m/s .
• the velocity of the tertiary air at the outlet of the tertiary air flow channel of each of the burners is arranged to be 40 - 60 m/s.
• the cross-sectional area of the tertiary air flow channel of each of the burners at the outlet of the tertiary air flow channel is arranged to be such that the velocity of the tertiary air is 40 - 60 m/s.
• the velocity of both the secondary air at the outlet of the secondary air flow channel and the tertiary air at the outlet of the tertiary air flow channel of each of the burners is set at 40 - 60 m/s .
• the swirl number of the air is increased and both the inner and the outer recirculation zones are improved.
• the recirculation flow upstream of the flame stabilizing ring is en- larged.
• ignition of the flame is improved.
• the velocity of the secondary or the tertiary air may be increased by narrowing the flow path of the air. High- velocity secondary or tertiary air creates strong turbulence, which leads to efficient mixing of combustion air and fuel and quick ignition and hot flame.
• the velocity of the secondary air or tertiary air at the outlet of the secondary or tertiary air flow channel depends on the volume flow rate of the air and on the cross-sectional area of the secondary or tertiary air flow channel at the outlet of the flow channel.
• the air pressure drop met by the burner is typi ⁇ cally 150 mm 3 ⁇ 40. This pressure drop will be handled by the force draft fan, which provides secondary air and tertiary air into the boiler.
• the volume flow rate of the air depends on several factors.
• the stoichiometric amount of air needed for combustion by each burner depends on the size of the burner.
• the size of the burner used in the method is typically 30 - 120 MW. Having the knowledge of the desired air coefficient, i.e. the amount of air used for combustion in comparison with the theoretical (stoichiometric) volume of air needed for complete combustion of the fuel, the skilled person is able to determine the volume flow rate of air used for combus- tion.
• Air is brought into the boiler as primary air, secondary air and, optionally, tertiary air.
• Primary air is supplied into the boiler furnace along with the fuel, e.g. pulverized coal.
• the amount of primary air corresponds to the amount of air needed for combustion of the fuel's volatile matter, and is typically 20 - 25 % of the total amount of air sup ⁇ plied through the burner.
• the volume flow rate of primary, secondary and tertiary air can be determined e.g. as follows.
• the air coefficient for primary air supplied through the burner may be, for example, 0.2 - 0.3, de ⁇ pending on the amount of volatile matter in the fuel.
• the remaining amount of air corresponding to an air coefficient of 0.6 - 0.7 and needed to achieve the to ⁇ tal air coefficient of 0.9 is supplied as secondary air or secondary and tertiary air.
• approximately 1/6 to 1/3 of the total amount of secondary and tertiary air is second ⁇ ary air, and the rest is tertiary air.
• shield air is supplied from under the at least one center burner through at least one air nozzle located below the at least one center burner.
• the boiler comprises at least one air nozzle located below the at least one center burner for supplying shield air into the boiler furnace from under the at least one center burner.
• one air nozzle is located below each of the center burners.
• shield air is supplied from under the at least one center burner located at the lowest horizontal burner level.
• the boiler comprises at least one air noz ⁇ zle located below the at least one center burner at the lowest horizontal burner level.
• one air nozzle is located below each of the center burners at the lowest horizontal burner level.
• the purpose of the shield air is to protect the furnace walls from the corrosion-causing effect of carbon monoxide by reducing the amount of carbon monoxides.
• the shield air reduces the amount of carbon monoxide formed at the center burners .
• the set of burners com ⁇ prises at least one center burner and two wing burners located on both sides of the at least one center burn- er.
• the boiler comprises at least one center burner and two wing burners located on both sides of the at least one center burner.
• the set of burners consists of at least one center burner and two wing burners located on both sides of the at least one center burner. In case the number of center burners is high, e.g. four, two wing burners may be provided on both sides of the set of center burners in order to achieve the desired overall flow field.
• the method is for opposite wall firing.
• the boiler is for op ⁇ posite wall firing.
• burners are located at opposite walls of the boiler furnace.
• multiple sets of burners are locat ⁇ ed on two opposite furnace walls at different horizon ⁇ tal burner levels one on the other. There may be for example four horizontal burner levels, or any other number of burner levels.
• the method of burning fuel and the boiler de ⁇ scribed herein may provide significant advantages over the prior art. At least some of the embodiments de- scribed herein provide a method and a boiler by which the presence of carbon monoxide close to the furnace walls is greatly reduced. As a result, the corrosion of furnace walls is reduced and boiler availability is improved. At the same time, at least some of the em- bodiments described herein provide a major reduction of nitrogen oxide emissions, with no significant change in carbon monoxide emissions. In addition, at least some of the embodiments described herein reduce the furnace gas exit temperature (FEGT) . The reduced FEGT improves boiler efficiency. A stable flame is achieved without swirl generation.
• FEGT furnace gas exit temperature
• Reduced nitrogen oxide emissions result in reduced consumption of ammonia used in reducing nitro ⁇ gen oxides, and longer lifetime of the catalyst mate- rial.
• a method and a boiler, to which the invention is re ⁇ lated, may comprise at least one of the embodiments of the invention described hereinbefore.
• FIG. 1 is a schematic perspective view of a boiler according to one embodiment.
• the boiler of figure 1 is used for opposite wall firing.
• the boiler comprises a boiler furnace 1 and burners 3 at four horizontal burner levels 4 for supplying fuel and com ⁇ bustion air into the boiler furnace 1.
• Each set of burners 3 comprises two center burners 8 and one wing burner 9 on one side of the two center burners 8 and another wing burner 9 on the other side of the two center burners 8.
• the outermost burners in a row are wing burners 9 and the two inner ⁇ most burners in a row are center burners 8.
• the burners of the boiler of figure 1 at a certain horizontal burner level 4 are operated such that different air coefficients are provided for the center 8 and wing burners 9.
• the wing burners 9 are configured to supply combustion air in such an amount that the air coefficient ( SR W i ng ) of the wing burners 9 is 0.95 or above, or in the range 0.95 - 1.1.
• the cen ⁇ ter burners 8 are configured to supply combustion air in such an amount that the air coefficient ( S R cen ter ) of the center burners 8 is 0.9 or below, or in the range 0.6 - 0.9 or 0.7 - 0.8.
• the wing burners 9 are configured to provide the secondary air supplied through the wing burners 9 a swirl number ( S W i ng ) of 0 - 0.3 or 0.01 - 0.3 or 0.1 - 0.3 or 0 - 0.1 or 0. That is, either a weak swirl or no swirl is given to the secondary air supplied though the wing burners 9.
• the center burners 8 are configured to pro ⁇ cute an outer recirculation zone around the inner re- circulation zone produced downstream of the flame sta ⁇ bilizing ring.
• the outer recirculation zone broadens the shape of the flame of the center burners 8.
• the structure of the center burners 8 may be the same as the structure of the wing burners 9 or different from the structure of the wing burners 9. Due to the operation and/or construction of the burners 3, an overall flow field is achieved which causes the flame of the wing burners 9 to bend towards the center of the boiler furnace 1 and away from the fur ⁇ nace walls 2. The amount of carbon monoxide near the furnace walls 2 is thus reduced.
• the overall flow field at a certain horizontal burner level is shown in figure 4.
• the number of horizontal burner levels 4 may vary depending on the boiler construction. Also, the number of center 8 and wing burners 9 in a set of burners 3 may vary.
• the boiler of figure 1 comprises two air noz- zles 20 below the lowest horizontal burner level 4 at two opposite furnace walls 2.
• One air nozzle 20 is lo ⁇ cated below each center burner 8 in the lowest horizontal burner level 4.
• the number of air nozzles 20 may also be other than two.
• Shield air is supplied in- to the boiler furnace 1 through the air nozzles 20. The shield air is used to protect the furnace walls 2 from corrosion caused by carbon monoxide by reducing the amount of carbon monoxides.
• FIG. 2 is a schematic sectional view of the burner 3 according to one embodiment. This burner may be used as a center burner 8 or as a wing burner 9.
• the burner 3 comprises a fuel feed pipe 5 and a secondary air flow channel 12 arranged around the fuel feed pipe 5.
• a mixture of pulverized fuel and carrier gas is supplied into the boiler furnace through the fuel feed pipe 5.
• the fuel-air mixture is supplied into the boiler furnace through an outlet 11 of the fuel feed pipe 5.
• Secondary air is supplied in ⁇ to the boiler furnace through the secondary air flow channel 12.
• the cross-section of the secondary air flow channel 12 is annular.
• the fuel feed pipe 5 com- prises an outer wall 10, a downstream end 6 and an outlet 11.
• the secondary air flow channel 12 comprises an outlet 13.
• a flame stabilizing ring 7 is attached to the downstream end 6 of the fuel feed pipe 5.
• the flame stabilizing ring 7 is attached to the outer wall 10 of the fuel feed pipe 5.
• the flame stabilizing ring 7 surrounds the outlet 11 of the fuel feed pipe 5 and protrudes towards the outlet 13 of the secondary air flow channel 12.
• the flame stabilizing ring 7 thus blocks a part of the outlet 13 of the secondary air flow channel 12.
• a part of the secondary air supplied through the secondary air flow channel 12 collides with the flame stabilizing ring 7 and the flow field of the air is changed.
• the flame stabilizing ring 7 causes formation of an inner recirculation zone downstream of the flame stabilizing ring 7.
• the flame stabilizing ring 7 improves ignition and keeps the flame narrow.
• the flame stabilizing ring 7 causes stabiliza ⁇ tion of the flame.
• the flame stabilizing ring 7 of the burner 3 of figure 2 is staggered in such a way that the end of the flame stabilizing ring 7 secured to the fuel feed pipe 5 is perpendicular to the direction of the central axis of the fuel feed pipe 5.
• the flame stabiliz- ing ring 7 takes a turn at a distance from its point of attachment toward the center of the boiler furnace
• the flame stabilizing ring may also be of another shape, such as a truncated cone, as long as it changes the flow field in the way desired.
• the secondary air flow channel 12 may consist of only one channel or of two channels, as in figure
• the secondary air flow channel 12a, 12b of the burn- er 3 of figure 2 is provided with an annular partition wall 21 that divides the secondary air flow channel into an inner secondary air flow channel 12a and an outer secondary air flow channel 12b.
• Both the inner 12a and the outer secondary air flow channel 12b may be provided with a swirler 22, i.e. a swirl generator.
• the inner and outer secondary air flow channels 12a, 12b do not, however, necessarily have to contain a swirler 22.
• the swirler 22 in the outer secondary air flow channel 12b is shown in figure 2.
• the swirl- ers located in the inner and outer secondary air flow channels 12a, 12b may apply a rotational or curling movement to the secondary air flow entering the inner or the outer secondary air flow channel 12a, 12b.
• the swirler 22 may be configured to give the secondary air supplied through the secondary air flow channel 12a, 12b a swirling motion.
• the swirler has a conventional structure generally known in the field.
• the swirler conventionally comprises vanes and the posi ⁇ tion of the vanes is adjusted to give the combustion air a certain swirl number S.
• the burner is configured to provide the sec ⁇ ondary or tertiary air a certain swirl number.
• the de- sired swirl number depends on whether the burner 3 is used as a center burner or a wing burner.
• the annular partition wall 21 may also extend further in the direction of the center of the boiler furnace, in which case the burner may comprise a sepa- rate secondary air flow channel 12 and a tertiary air flow channel (not shown in figure 2) .
• FIG. 3 is a schematic sectional view of the burner according to a second embodiment. This burner may be used as a center burner 8.
• the burner 3 of figure 3 comprises a fuel feed pipe 5 and a secondary air flow channel 12 arranged around the fuel feed pipe 5.
• a mixture of pulverized fuel and carrier gas is supplied into the boiler furnace through the fuel feed pipe 5.
• the fuel-air mixture is supplied into the boiler furnace through the outlet 11 of the fuel feed pipe 5.
• Secondary air is supplied into the boiler fur ⁇ nace through the secondary air flow channel 12.
• the cross-section of the secondary air flow channel 12 is annular.
• the fuel feed pipe 5 comprises an outer wall 10, a downstream end 6 and an outlet 11.
• the secondary air flow channel 12 comprises an outer wall 15, a downstream end 14 and an outlet 13.
• the annular partition wall 21 extends to the outlet region of the secondary air flow channel 12.
• the burner 3 thus comprises separate secondary 12 and tertiary air flow channels 16 instead of inner and outer secondary air flow channels as in the burner of figure 2.
• the tertiary air flow channel 16 is arranged around the secondary air flow channel 12. Tertiary air is supplied into the boiler furnace through the outlet 17 of the tertiary air flow channel 16.
• a flame stabi ⁇ lizing ring 7 is attached to the downstream end 6 of the fuel feed pipe 5.
• the flame stabilizing ring 7 is attached to the outer wall 10 of the fuel feed pipe 5.
• the flame stabilizing ring 7 surrounds the outlet 11 of the fuel feed pipe 5 and protrudes towards the out ⁇ let 13 of the secondary air flow channel 12.
• the flame stabilizing ring 7 thus blocks a part of the outlet 13 of the secondary air flow channel 12.
• a part of the secondary air supplied through the secondary air flow channel 12 collides with the flame stabilizing ring 7 and the flow field of the air is changed.
• the flame stabilizing ring 7 causes formation of an inner recirculation zone downstream of the flame stabilizing ring 7.
• the flame stabilizing ring 7 improves ignition and keeps the flame narrow.
• the flame stabilizing ring 7 causes stabilization of the flame.
• the flame stabiliz- ing ring may be of any shape, such as a truncated cone, as long as it changes the flow field in the way desired .
• Both the secondary 12 and the tertiary air flow channel 16 may comprise a swirler 22 for setting the secondary and the tertiary air in tangential mo ⁇ tion.
• the swirler has a conventional structure gener ⁇ ally known in the field.
• the swirler of the secondary air flow channel 12 is not shown in the figure.
• the burner of figure 3 When the burner of figure 3 is used as a center burner, the burner may be configured to provide the secondary and/or the tertiary air supplied through the center burner a swirl number ( S cen ter ) of 0 - 0.5 or 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0.
• the vane an- gle of the swirler 22 is set to give the secondary and tertiary air a swirl number of 0 - 0.5 or 0.2 - 0.3 or 0.01 - 0.5 or 0.1 - 0.5 or 0.
• the secondary 12 and the tertiary air flow channels 16 do not necessarily have to contain a swirler 22.
• An air guiding device 18 is attached to the downstream end 14 of the secondary air flow channel 12 and to the outer wall 15 of the secondary air flow channel 12.
• the air guiding device 18 surrounds the outlet 13 of the secondary air flow channel 12 and protrudes towards the outlet 17 of the tertiary air flow channel 16.
• the air guiding device 18 turns the tertiary air supplied through the tertiary air flow channel 16 to flow radially outwards and delays mixing of the tertiary air with the fuel.
• An outer recircula ⁇ tion zone is produced around the inner recirculation zone formed downstream of the flame stabilizing ring 7. The outer recirculation zone broadens the shape of the flame and improves ignition.
• the air guiding device 18 of the burner of figure 3 is substantially a truncated cone shaped and is arranged in an angle a to the direction of the cen ⁇ ter axis of the fuel feed pipe 5.
• the angle a is 25 - 45 degrees.
• FIG 4 is a schematic sectional view of the overall flow field at a certain horizontal burner lev ⁇ el inside the boiler furnace 1.
• Two opposite furnace walls 2 comprise a set of burners 8,9 for supplying fuel and combustion air into the boiler furnace 1.
• Each set of burners comprises two center burners 8 and one wing burner 9 located on one side of the center burners 8 and another wing burner 9 located on the other side of the center burners 8.
• the wing burners 8 may have the structure shown in figure 2 and the cen ⁇ ter burners 8 may have the structure shown in figure 2 or 3.
• Each of the burners 8,9 is provided with a flame stabilizing ring (not shown) .
• the shape of the flame of the wing burners 9 is kept narrow and long.
• the swirl number ( S W i ng ) of the secondary air and/or the tertiary air supplied through the wing burners 9 is kept at 0 - 0.3 or 0.01 - 0.3 or 0.1 - 0.3 or 0 - 0.1 or 0.
• the shape of the flame of the center burners 8 is kept broad and short. This is achieved by produc ⁇ ing an outer recirculation zone around the inner re- circulation zone produced downstream the flame stabi ⁇ lizing ring.
• the outer recirculation zone may be produced by setting the swirl number ( S cen ter ) of the sec ⁇ ondary air and/or the tertiary air supplied through the center burners 8 at 0.6 - 1.5 or 0.9 - 1.5 or 1.1 - 1.3.
• the outer recirculation zone may be produced by providing the center burners with an air guiding device, i.e. to use the burner structure shown in figure 3. Formation of the outer recircula ⁇ tion zone broadens the shape of the flame of the cen- ter burners.
• the wing burners 9 are configured to supply combustion air in such an amount that the air coeffi- cient ( SR W in g ) of the wing burners 9 is 0.95 or above, or in the range 0.95 - 1.1.
• the center burners 8 are configured to supply combustion air in such an amount that the air coefficient ( SR cen ter ) of the center burn- ers 8 is 0.9 or below, or in the range 0.6 - 0.9 or 0.7 - 0.8.
• an underpressure zone is caused in the center part 23 of the boiler furnace 1.
• the under- pressure zone causes the flames of the wing burners 9 to bend towards the center part 23 of the boiler fur ⁇ nace 1.
• a flow pattern is created that results in more oxygen-rich conditions close to the furnace walls, which favours reduction of corrosion rate of the fur- nace walls.
• the flow pattern causes fuel rich flue gases to entrain from furnace walls to the center of the furnace .
• the fuel and combustion air were supplied at four horizontal burner levels through a set of burners at two opposite furnace walls.
• the fuel was pulverized coal.
• the set of burners comprised four burners.
• the burners comprised a core air duct, a fuel feed pipe coaxially arranged around the core air duct, an inner secondary air flow channel arranged around the fuel feed pipe, and an outer secondary air flow channel arranged around the inner secondary air flow channel .
• each of the burners was stabi ⁇ lized by providing the burners with a flame stabiliz ⁇ ing ring attached to the downstream end of the fuel feed pipe and surrounding the outlet of the fuel feed pipe.
• Shield air was supplied from two air nozzles lo ⁇ cated under the two innermost burners for protecting the furnace walls from the corrosion-causing effect of the carbon monoxide.
• each of the set of burners was operated at substantially the same swirl number S. Also, each of the set of burners was operated at substantially the same air coefficient SR. The swirl number of each of the burners was 1.0 and the air coefficient of each of the burners was 0.90.
• the set of burners comprised two center burners and one wing burner located on both sides of the center burners.
• the air coefficient SR W i ng of both wing burners was set at 1.0.
• any other value of SR W i ng may be chosen in the range 0.95 - 1.1, such as 0.95 or 1.05 or 1.1.
• the air coefficient SR cen ter of both the center burners was set at 0.75.
• any other value of SRcenter may be chosen in the range 0.6 - 0.9, such as 0.6 or 0.7 or 0.8 or 0.9.
• the swirl number S W i ng of the secondary air supplied through each of the wing burners was set at 0 - 0.3.
• any value of S W i ng in the range 0 - 0.3 may be chosen, such as 0 or 0.1 or 0.2 or 0.3.
• the swirl number S cen ter of the secondary air supplied through each of the center burners was set at 0.6 - 1.5. Any value of S cen ter in the range 0.6 - 1.5 may be chosen, such as 0.6 or 0.75 or 0.9 or 1.1 or 1.3 or 1.5.
• the corrosion rate of the side furnace walls was high, when the con- ventional method of burning fuel and a conventional boiler was used.
• the nitrogen oxide emissions were ap ⁇ proximately 950 mg N02 /m3n (dry 6% O 2 ) .
• the carbon monoxide emissions were approximately 35 mg C o/m3n (dry 6% O 2 ) .
• the heat transfer rate to furnace walls prior to SH2 was approximately 730 MW.
• the fur ⁇ nace exit gas temperature prior to SH2 was 1234 °C.
• the corrosion rate of the side furnace walls was low, when the meth ⁇ od and the boiler described herein was used.
• the ni- trogen oxide emissions were approximately 415 mg N02 /m3n (dry 6% O 2 ) .
• the carbon monoxide emissions were approximately 75 mg C o/m3n (dry 6% O2) .
• the heat transfer rate to furnace walls prior to SH2 was ap ⁇ proximately 750 MW.
• the furnace exit gas temperature was 1200 °C.
• the corrosion rate of the side furnace walls was significantly reduced when the method and the boiler described herein was used as compared to the conventional method of burning fuel in a conventional boiler. Nitrogen oxide emissions were significantly reduced, while the carbon monoxide emis ⁇ sions were not significantly influenced.
• the furnace exit gas temperature was decreased by approximately 35 °C.
• the center burners comprised an air guiding device attached to the downstream end of the secondary air flow channel, and arranged in an angle of 25 - 45 degrees in relation to the center axis of the fuel feed pipe.
• the air guiding device sur ⁇ rounds the outlet of the secondary air flow channel and blocks a part of the outlet of the tertiary air flow channel, thereby delaying mixing of the tertiary air with the fuel.
• each of the burners was stabi ⁇ lized by providing the burners with a flame stabiliz- ing ring attached to the downstrean end of the fuel feed pipe and surrounding the outlet of the fuel feed pipe.
• Shield air was supplied from two air nozzles lo ⁇ cated under the two innermost burners for protecting the furnace walls from the corrosion-causing effect of the carbon monoxide.
• each of the set of burners was operated at substantially the same swirl number S. Also, each of the set of burners was operated at substantially the same air coefficient SR. The swirl number of each of the burners was 1.0 and the air coefficient of each of the burners was 0.90.
• the set of burners comprised two center burners and one wing burner located on both sides of the center burners.
• the air coefficient SR W i ng of both wing burners was set at 1.0. Also any other value of SR W i ng may be chosen in the range 0.95 - 1.1, such as 0.95 or 1.05 or 1.1.
• the air coefficient SR cente r of both the center burners was set at 0.9. Also any other value of SR ce n te r may be chosen in the range 0.6 - 0.9, such as 0.6 or 0.7 or 0.8.
• the swirl number S W i ng of the sec- ondary air supplied through each of the wing burners was set at 0 - 0.3.
• any value of S W i ng in the range 0 - 0.3 may be chosen, such as 0 or 0.1 or 0.2 or 0.3.
• the swirl number S center of the secondary air supplied through each of the center burners was set at 0 - 0.5.
• Any value of S cente r in the range 0 - 0.5 may be chosen, such as 0 or 0.1 or 0.25 or 0.4 or 0.5.
• the corrosion rate of the side furnace walls was high, when the con ⁇ ventional method of burning fuel and a conventional boiler was used.
• the nitrogen oxide emissions were ap ⁇ proximately 950 mg N02 /m3n (dry 6% O 2 ) .
• the carbon monoxide emissions were approximately 35 mg C o/m3n (dry 6% O2) .
• the heat transfer rate to furnace walls prior to SH2 was approximately 730 MW.
• the fur- nace exit gas temperature prior to SH2 was 1234 °C.
• the corrosion rate of the side furnace walls was low, when the meth ⁇ od and the boiler described herein was used.
• the ni ⁇ trogen oxide emissions were approximately 510 mg N0 2/m3n (dry 6% O 2 ) .
• the carbon monoxide emissions were approximately 5 mg C o/m3n (dry 6% O2) .
• the heat transfer rate to furnace walls prior to SH2 was ap- proximately 770 MW.
• the furnace exit gas temperature was 1189 °C.
• the corrosion rate of the side furnace walls was significantly reduced when the method and the boiler described herein was used as compared to the conventional method of burning fuel in a conventional boiler. Nitrogen oxide emissions were significantly reduced, while the carbon monoxide emis ⁇ sions were not significantly influenced.
• the furnace exit gas temperature was decreased by approximately 45 °C.

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• 2016
  • 2016-06-08 EP EP16904532.5A patent/EP3469258A4/de not_active Withdrawn
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