Burner and Method for Forming a Flame in a Furnace by a Burner

Abstract

The invention relates to a burner for forming a flame in a furnace by means of gaseous fuel and combustion air (I), simultaneously recirculating flue gases in the furnace, wherein the burner comprisesa burner frame and a combustion head connected to the same by its cylindrical body, wherein the end portion of the combustion head facing the furnace extends at its first end to the furnace, and the second end of the end portion is connected to said cylindrical body,a primary gas pipe for supplying gaseous fuel (G; G1) to the furnace, wherein the primary gas pipe extends inside the combustion head in its longitudinal direction, particularly inside of the end portion of the combustion head, and the orifice of the primary gas pipe opens into or close to the furnace, and a primary air supply pipe also extends inside the combustion head and surrounds said primary gas pipe, and the orifice of the supply pipe opens into the furnace or to the orifice of the end portion of the combustion head,a flow duct for main combustion air (Imain) extending inside the combustion head and surrounding said primary air supply pipe at least inside of the end portion, anda set of main gas rods, comprising a set of elongated primary gas rods extending in the longitudinal direction of the combustion head, for supplying the furnace with fuel, wherein the main gas rods are arranged at least partly outside the cylindrical body of the combustion head, extending close to the joint between the cylindrical body and the end portion so that they are surround said body at regular intervals around the circumference of the body,and wherein in connection with the main gas rods, at the end portion of the combustion head, several ejector ducts are provided for recirculating flue gas from the furnace into the gaseous fuel (G; G2) coming from the main gas rods, and for conveying the produced flue gas-flue mixture (S+G2) further to the furnace.

1 . A burner for forming a flame (F) into a furnace by means of gaseous fuel (G) and combustion air (I), simultaneously recirculating flue gases (S) in the furnace, wherein the burner comprises
 a burner frame and a combustion head connected by its cylindrical body to the burner frame, wherein an end portion of the combustion head facing the furnace extends at its first end to the furnace, and a second end of the end portion is connected to said cylindrical body,   a primary gas pipe for supplying gaseous fuel (G; G 1 ) to the furnace, wherein the primary gas pipe extends inside the combustion head in its longitudinal direction, particularly inside (T) of the end portion of the combustion head, and an orifice of the primary gas pipe opens into or close to the furnace, and a primary air supply pipe also extends inside the combustion head and surrounds said primary gas pipe, and an orifice of the supply pipe opens into the furnace or to the orifice of the end portion of the combustion head,   a flow duct for main combustion air (Imain) extending inside the combustion head and surrounding said primary air supply pipe at least inside (T) of the end portion, and   a set of main gas rods, comprising a set of elongated primary gas rods extending in the longitudinal direction of the combustion head, for supplying the furnace with fuel,   wherein the main gas rods are arranged at least partly outside the cylindrical body of the combustion head, extending close to a joint (L) between the cylindrical body and the end portion so that they are surrounding said body at regular intervals around a circumference of the body,   and   wherein in connection with the main gas rods, at the end portion of the combustion head, several ejector ducts are provided for recirculating flue gas (S) from the furnace into the gaseous fuel (G, G 2 ) coming from the main gas rods, and for conveying the produced flue gas-flue mixture (S+G 2 ) further to the furnace,   characterized in that the end portion of the combustion head has the shape of a truncated cone and comprises ejector ducts and air ducts alternating in a direction of a circumference of said end portion, free ends of the ducts opening in the direction of the furnace, and a top end of a duct groove of each ejector duct being also open in the direction of the furnace, and a lower end of the duct groove (of each air duct being also open in the direction of the central axis of the combustion head,   wherein the end portion having the shape of the truncated cone expands at an angle of 3 to 30 degrees, seen from the direction of the joint (L) between the cylindrical body of the combustion head and the end portion of the combustion head.   
     
     
         2 . The burner according to  claim 1 , characterized in that the ejector duct at the end portion of the combustion head comprises a groove bottom in the longitudinal direction of the combustion head, wherein a groove wall extends on either side upward from the groove bottom, and the duct groove of the ejector duct left between the groove walls is open at its top facing the furnace. 
     
     
         3 . The burner according to  claim 2 , characterized in that the bottom groove of the ejector duct at the end portion of the combustion head extends at an angle of 3 to 20 degrees downward, i.e., towards the longitudinal central axis (P) of the combustion head, when said groove bottom is seen from the joint between the cylindrical body of the combustion head and the end portion of the combustion head, encircling the combustion head. 
     
     
         4 . The burner according to  claim 2 , characterized in that the groove bottom of the ejector duct at the end portion of the combustion head tapers or has a constant width in the direction from the joint (L) between the cylindrical body of the combustion head and the groove bottom of the ejector duct at the end portion of the combustion head towards the free end of the groove bottom of said ejector duct facing the furnace. 
     
     
         5 . The burner according to  claim 3 , characterized in that the groove bottom of the ejector duct at the end portion of the combustion head is, at the joint (L 1 ) between said ejector duct and the cylindrical body of the combustion head, flush with the shell of the cylindrical body. 
     
     
         6 . The burner according to  claim 2 , characterized in that the groove walls of the ejector duct at the end portion of the combustion head extend from the groove bottom of the ejector duct obliquely upward, i.e. away from the central axis of the combustion head, towards the furnace, so that the planes extending via these groove walls are placed at an angle A of 1 to 30 degrees to each other. 
     
     
         7 . The burner according to  claim 2 , characterized in that the width of the duct groove of the ejector duct at the end portion of the combustion head, measured at the joint (L 1 ) between the groove bottom of said ejector duct and the cylindrical body, is 0.2 to 1 times the length of the circumference of the shell of the cylindrical body, divided by the number (n) of ejector ducts at the end portion of the combustion head. 
     
     
         8 . The burner according to  claim 2 , characterized in that the length of the duct groove of each ejector duct is 3 to 15 times the width of the same duct groove of the ejector duct measured at the joint (L 1 ) between the groove bottom of said ejector duct and the cylindrical body. 
     
     
         9 . The burner according to  claim 1 , characterized in that each air duct at the end portion of the combustion head comprises a groove bottom in the longitudinal direction of the combustion head, with a groove wall extending on either side downward from said groove bottom, i.e. in the direction of the central axis (P) of the combustion head, wherein the duct groove of the air duct left between the groove walls is open towards the inside of the end portion of the combustion head, encircling the central axis (P) of the combustion head at the end portion. 
     
     
         10 . The burner according to  claim 2 , characterized in that the groove walls of each ejector duct at the end portion of the combustion head simultaneously constitute the groove walls of the two adjacent air ducts. 
     
     
         11 . The burner according to  claim 9 , characterized in that the groove bottom of each air duct at the end portion of the combustion head extends at an angle of 3 to 30 degrees upward, i.e. away from the longitudinal central axis (P) of the combustion head, when said groove bottom of the air duct is seen from the joint between the cylindrical body and the body, encircling the combustion head. 
     
     
         12 . The burner according to  claim 9 , characterized in that the inside of the end portion encircling the longitudinal central axis (P) of the combustion head expands so that the distance between the groove bottom of each air duct at the end portion of the combustion head and the longitudinal central axis (P) of the combustion head is 3 to 30% greater at the end of the groove bottom of said air duct on the side of the furnace than at the joint between the groove bottom of the same air duct and the cylindrical body. 
     
     
         13 . The burner according to  claim 1 , characterized in that the main gas rods extend through the flange between the furnace and the burner frame into the furnace, and extend in the longitudinal direction of the cylindrical body of the combustion head to a distance from said flange, close to the joint (L 1 ) between the body of the combustion head and the bottom of the ejector duct at the end portion of the combustion head. 
     
     
         14 . The burner according to  claim 1 , characterized in that the burner further comprises air ejectors provided in connection with the flow duct for main combustion air (Imain) in the body of the combustion head, arranged to absorb flue gas from the furnace. 
     
     
         15 . The burner according to  claim 14 , characterized in that the air ejectors in the body of the combustion head and the ejector ducts at the end portion of the combustion head are arranged, with respect to each other, in such a way that the location for supplying flue gas to the ejector ducts is upstream of the openings leading to the air ejectors in the body, seen from the direction of the orifice of the end portion of the combustion head. 
     
     
         16 . The burner according to  claim 15 , characterized in that flue gases can be conveyed to each air ejector via the flue gas duct of the air ejector, from that area (B) in the longitudinal direction of the body of the combustion head where the main gas rods extend in the furnace, outside the body of the combustion head. 
     
     
         17 . The burner according to  claim 14 , characterized in that each air ejector is arranged to absorb at least 1% of flue gas per mbar of static pressure loss produced by the ejector into the main combustion air (Imain), calculated from the total content (wt-%/wt-%) of main combustion air. 
     
     
         18 . The burner according to  claim 14 , characterized in that the content of flue gas to be absorbed into the main combustion air by each air ejector is further adjusted so that the maximum temperature of the air-flue gas mixture downstream of the air ejector, in the flow direction of the air-flue gas mixture (I+S), is lower than 500° C., preferably lower than lower than 300° C. 
     
     
         19 . The burner according to  claim 14 , characterized in that at least one opening is provided for each air ejector in the body, and the openings provided for the air ejectors in the assembly of air ejectors in the body and connected to the flue gas ducts constitute a row of 6 to 60 openings encircling the body of the combustion head at the same distance from the vertical plane extending via the orifice of the combustion head. 
     
     
         20 . The burner according to  claim 14 , characterized in that each air ejector comprises one or more partly hollow blade, including an entry-side blade arranged to accelerate the flow of combustion air steadily on said entry-side blade, wherein this entry-side blade extends into the flue gas duct of the air ejector, and downstream of the flue gas duct the same or a different blade extends as an exit-side blade arranged to steadily decelerate the flow of air-flue gas mixture (S,I) on said exit-side blade. 
     
     
         21 . A method for forming a flame (F; F 1 , F 2 ) into a furnace by a burner defined in  claim 1 , by means of gaseous fuel and combustion air (I), simultaneously recirculating flue gases (S); wherein
 for forming a primary flame (F 1 ), primary gas is supplied into a primary gas pipe via a part of said primary gas pipe on the side of the burner frame, and primary air is also supplied by means of a primary air supply pipe extending inside the combustion head and into the furnace,   and   for forming a main flame (F 2 ), a flow of main combustion air (Imain; Is+lt) is conveyed from inside of the combustion head to the furnace, and a flow of main gas (G 2 ) is conveyed via main gas rods, whereby the main gas (G 2 ) is conveyed into each main gas rod via its part on the side of the burner frame, wherein flue gas from the furnace is mixed into the flow of main gas (G 2 ) by means of ejector ducts at the orifice of the end portion of the combustion head,   characterized in that the flame (F; F 1 +F 2 ) produced in the furnace, formed by the primary flame (F 1 ) and the main flame (F 2 ), and the distribution of fuel, combustion air and internally recirculated flue gas at the opening of the combustion head, obtained by the shape of the combustion head, results in an inert return flow (PI) of flue gases with low contents of oxygen and unburned fuel components.   
     
     
         22 . The method according to  claim 21 , wherein the main gas (G 2 ) is methane and wherein the inert return flow (PI) of flue gases returning back to the area of the combustion head in the furnace contains less than 5000 ppm of carbon monoxide, less than 5000 ppm of methane, and has an average oxygen content lower than 5 wt-% when the total air factor I is 1.15. 
     
     
         23 . The method according to  claim 21 , characterized in that the inert flue gas (PI) is conveyed to ejector ducts in connection with main gas rods. 
     
     
         24 . The method according to  claim 21 , characterized in that main gas jets from the main gas rods are directed from the main gas rods to the ejector ducts at an angle (D) downward, i.e. directed at an angle towards the longitudinal axis of the combustion head, when said ejector duct is seen from the joint between the cylindrical body and the orifice, the angle (D) being in the same direction as or in a different direction from the tilt angle (B) of the groove bottom of the ejector duct. 
     
     
         25 . The method according to  claim 21 , characterized in that the flow of main combustion air (Imain; Is+It) is directed to the flow duct for main combustion air (Imain) which surrounds the primary air supply pipe and is divided into a tertiary air flow (It) flowing in air ducts within said end portion and a secondary air flow (Is) flowing outside the air ducts. 
     
     
         26 . The method according to  claim 25 , characterized in that the secondary air flow (Is) passes through the inside of the end portion of the combustion head. 
     
     
         27 . A burner-boiler assembly, wherein a frame of the burner as defined in  claim 1  is mounted in a boiler so that the combustion head of the burner extends into the furnace,
 characterized in that power density of the burner relative to a cross-sectional area of the furnace is at most 10,000 kW/m2.