Biomass-mixed, pulverized coal-fired burner and fuel combustion method

Abstract

A biomass-mixed, pulverized coal-fired burner is provided, capable of burning biomass fuel as auxiliary fuel in large quantities and burning only pulverized coal when the biomass fuel is not sufficiently available. The biomass-mixed, pulverized coal-fired burner includes a biomass fuel jet nozzle that extends axially along the biomass-mixed, pulverized coal-fired burner, a fuel jet nozzle that is open midway in the biomass fuel jet nozzle, a secondary air nozzle that surrounds the fuel jet nozzle, and a tertiary air nozzle that surrounds the secondary air nozzle. A pulverized coal component in a fuel stream as a mixture of the pulverized coal fuel stream and the biomass fuel stream is distributed with a higher concentration on an outer circumferential wall side and a biomass fuel component in the fuel stream is distributed inside of the pulverized coal fuel component.

Claims

1. A biomass-mixed, pulverized coal-fired burner, comprising: a biomass fuel jet nozzle that supplies biomass fuel conveyed by biomass fuel primary air as a biomass fuel stream; a fuel jet nozzle including a fuel conveying pipe that introduces pulverized coal fuel conveyed by pulverized coal fuel primary air as a pulverized coal fuel stream to thereby form a flow path for the pulverized coal fuel stream, and a fuel jet port through which the pulverized coal fuel stream is jetted together with the biomass fuel supplied inside of the fuel conveying pipe from the biomass fuel jet nozzle; a secondary air nozzle having a secondary air jet port that surrounds an opening in the fuel jet port, the secondary air jet port jetting a secondary air swirl flow; and a tertiary air nozzle having a tertiary air jet port that surrounds the secondary air jet port, the tertiary air jet port jetting a tertiary air swirl flow, wherein the biomass fuel jet nozzle has a biomass fuel jet port that supplies the biomass fuel into an inside of the fuel conveying pipe included in the fuel jet nozzle, the fuel jet nozzle includes: a fuel swirl vane disposed inside the fuel conveying pipe, the fuel swirl vane changing a fuel stream as a mixture of the pulverized coal fuel stream and the biomass fuel stream into a whirling swirl flow such that a pulverized coal fuel component in the fuel stream is distributed with a higher concentration on an outer circumferential wall side of the fuel conveying pipe and a biomass fuel component in the fuel stream is distributed inside of the pulverized coal fuel component; a flame stabilizer disposed at a pipe end of the fuel jet port, the flame stabilizer having a conical funnel-shaped widening ring with an opening at an expanding end, the expanding end pointing in a direction away from the pipe end of the fuel jet port, the widening ring comprising a micro step formed at an intermediate portion thereof; and a fuel baffle plate disposed on a pipe inner wall at a position upstream of the flame stabilizer, the fuel baffle plate restricting a swirl of the fuel stream jetted from the fuel jet port, the fuel stream jetted from the fuel jet port is formed so that the pulverized coal fuel stream envelopes the biomass fuel stream, and the secondary air jetted from the secondary air jet port forms a buffer stream between the fuel stream and the tertiary air swirl flow.

2. The biomass-mixed, pulverized coal-fired burner according to claim 1, wherein the biomass fuel jet nozzle includes a biomass fuel bent section disposed upstream of the biomass fuel jet port, and the fuel jet nozzle includes a pulverized coal fuel bent section disposed upstream of the fuel swirl vane.

3. The biomass-mixed, pulverized coal-fired burner according to claim 2, wherein the biomass fuel primary air and the pulverized coal fuel primary air are supplied such that the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm.sup.3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

4. The biomass-mixed, pulverized coal-fired burner according to claim 2, wherein the biomass fuel primary air is supplied in such a quantity that a velocity of a fuel conveying stream in the biomass fuel jet nozzle falls within a range between 14.5 m/s and 22 m/s.

5. The biomass-mixed, pulverized coal-fired burner according to claim 4, wherein the biomass fuel primary air and the pulverized coal fuel primary air are supplied such that the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

6. The biomass-mixed, pulverized coal-fired burner according to claim 1, wherein the biomass fuel primary air is supplied in such a quantity that a velocity of a fuel conveying stream in the biomass fuel jet nozzle falls within a range between 14.5 m/s and 22 m/s.

7. The biomass-mixed, pulverized coal-fired burner according to claim 6, wherein the biomass fuel primary air and the pulverized coal fuel primary air are supplied such that the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

8. The biomass-mixed, pulverized coal-fired burner according to claim 1, wherein the biomass fuel primary air and the pulverized coal fuel primary air are supplied such that the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm.sup.3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

9. A fuel combustion method comprising: burning biomass fuel and pulverized coal fuel using a biomass-mixed, pulverized coal-fired burner comprising: a biomass fuel jet nozzle that supplies biomass fuel conveyed by biomass fuel primary air as a biomass fuel stream; a fuel jet nozzle including a fuel conveying pipe that introduces pulverized coal fuel conveyed by pulverized coal fuel primary air as a pulverized coal fuel stream to thereby form a flow path for the pulverized coal fuel stream, and a fuel jet port through which the pulverized coal fuel stream is jetted together with the biomass fuel supplied inside of the fuel conveying pipe from the biomass fuel jet nozzle; a secondary air nozzle having a secondary air jet port that surrounds an opening in the fuel jet port, the secondary air jet port jetting a secondary air swirl flow; and a tertiary air nozzle having a tertiary air jet port that surrounds the secondary air jet port, the tertiary air jet port jetting a tertiary air swirl flow, wherein the biomass fuel jet nozzle has a biomass fuel jet port that supplies the biomass fuel into an inside of the fuel conveying pipe included in the fuel jet nozzle, the fuel jet nozzle includes: a fuel swirl vane disposed inside the fuel conveying pipe, the fuel swirl vane changing a fuel stream as a mixture of the pulverized coal fuel stream and the biomass fuel stream into a whirling swirl flow such that a pulverized coal fuel component in the fuel stream is distributed with a higher concentration on an outer circumferential wall side of the fuel conveying pipe and a biomass fuel component in the fuel stream is distributed inside of the pulverized coal fuel component; a flame stabilizer disposed at a pipe end of the fuel jet port, the flame stabilizer having a conical funnel-shaped widening ring with an opening at an expanding end, the expanding end pointing in a direction away from the pipe end of the fuel jet port, the widening ring comprising a micro step formed at an intermediate portion thereof; and a fuel baffle plate disposed on a pipe inner wall at a position upstream of the flame stabilizer, the fuel baffle plate restricting a swirl of the fuel stream jetted from the fuel jet port, the fuel stream jetted from the fuel jet port is formed so that the pulverized coal fuel stream envelopes the biomass fuel stream, and the secondary air jetted from the secondary air jet port forms a buffer stream between the fuel stream and the tertiary air swirl flow.

10. The fuel combustion method according to claim 9, wherein the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

11. The fuel combustion method according to claim 9, wherein the biomass fuel primary air is supplied in such a quantity that a velocity of a fuel conveying stream in the biomass fuel jet nozzle falls within a range between 14.5 m/s and 22 m/s.

12. The fuel combustion method according to claim 11, wherein the biomass-mixed, pulverized coal-fired burner is operated in a range that, with fuel containing 60% by weight of biomass fuel, is sandwiched between a first straight line and a second straight line as follows: the first straight line extending from A/C 1.0 relating to mixed fuel containing therein pulverized coal and biomass to A/C 1.8 relating to the mixed fuel at a load factor of 100% of the biomass-mixed, pulverized coal-fired burner; and the second straight line extending from A/C 1.0 relating to the mixed fuel to A/C 3.2 relating to the mixed fuel at a load factor of 50% of the biomass-mixed, pulverized coal-fired burner, wherein the A/C relating to mixed fuel is a ratio of a total of biomass fuel primary air flow rate and coal fuel primary air flow rate (Nm3/h) to mixed fuel including pulverized coal fuel and biomass fuel (kg/h).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic cross-sectional view showing a biomass-mixed, pulverized coal-fired burner according to an embodiment of the present invention.

(2) FIG. 2 is a diagram showing a relation between burner load and A/C representing an operating range of the biomass-mixed, pulverized coal-fired burner according to the embodiment.

(3) FIG. 3 is a portion of FIG. 1 showing a micro-step.

MODES FOR CARRYING OUT THE INVENTION

(4) An embodiment of the present invention will be described below with reference to the accompanying drawings.

(5) FIG. 1 is a schematic cross-sectional view showing a biomass-mixed, pulverized coal-fired burner according to an embodiment of the present invention.

(6) Reference is made to FIG. 1. The biomass-mixed, pulverized coal-fired burner 1 according to the embodiment includes a biomass fuel jet nozzle 20 disposed at a center thereof. The biomass-mixed, pulverized coal-fired burner 1 further includes a fuel jet nozzle 30, a secondary air nozzle 40, and a tertiary air nozzle 50 disposed coaxially in sequence around the biomass fuel jet nozzle 20. It is noted that an auxiliary fuel nozzle 10 that supplies auxiliary or starting liquid or gas fuel may be disposed on a pipe axis of the biomass-mixed, pulverized coal-fired burner 1.

(7) The biomass fuel jet nozzle 20 supplies biomass fuel conveyed by biomass fuel primary air to an intermediate position of the fuel jet nozzle 30. The biomass fuel jet nozzle 20 includes a biomass fuel introducing pipe 21, a biomass fuel reflecting plate 22, a biomass fuel conveying pipe 23, and a biomass fuel jet port 24.

(8) The fuel jet nozzle 30 jets pulverized coal fuel conveyed by pulverized coal fuel primary air, together with the biomass fuel introduced to the intermediate position of the fuel jet nozzle 30, into a furnace. The fuel jet nozzle 30 includes a pulverized coal fuel introducing pipe 31, a pulverized coal fuel reflecting plate 32, a fuel conveying pipe 33, and a fuel jet port 34. The biomass fuel is supplied to a pipe axis portion of the fuel conveying pipe 33 by way of the biomass fuel jet port 24. A pulverized coal fuel stream is supplied along a pipe wall of the fuel conveying pipe 33.

(9) The fuel jet nozzle 30 further includes a fuel swirl vane 35 at an intermediate portion of the fuel conveying pipe 33 downstream of the biomass fuel jet port 24. The fuel swirl vane 35 comprises a plurality of swirl vanes disposed in a flow path for the fuel in the fuel conveying pipe 33. The swirl vanes are inclined relative to the pipe axis. The swirl vanes rotate the fuel stream that flows therein about the pipe axis and use a centrifugal force to make a fuel concentration lower at a center side and higher on an outer circumferential side and make a concentration distribution substantially uniform in a circumferential direction.

(10) The fuel stream that is a mixture of the pulverized coal fuel stream and the biomass fuel stream contacts the fuel swirl vane 35 and forms a swirl flow having fuel components distributed according to their specific gravity. Specifically, under the centrifugal force, the fuel stream that has flowed past the fuel swirl vane 35 has a higher concentration of a pulverized coal fuel component on the pipe wall side of the fuel conveying pipe 33 with a biomass fuel component being distributed inside of the pulverized coal component.

(11) The fuel jet nozzle 30 further includes a fuel baffle plate 36 disposed on a pipe inner wall at a position upstream of the fuel jet port 34 located at a leading end of the fuel conveying pipe. The fuel baffle plate 36 comprises a plurality of flat plates, each flat plate being disposed at substantially equal intervals in the circumferential direction and extending along the pipe axis. The fuel baffle plate 36 can reduce a swirl force of the fuel stream that flows therethrough to thereby change the swirl flow substantially into an axial flow. The number, size, and inclination relative to the pipe axis of the flat plates constituting the fuel baffle plate 36 may be determined as appropriate according to the swirl force of the fuel stream and a spread angle after jetting.

(12) The fuel jet nozzle 30 further includes a fuel flame stabilizer 37 disposed at the fuel jet port 34. The fuel flame stabilizer 37 has a funnel-shaped widening ring that widens a jet stream outwardly. As shown in FIG. 3, the widening ring has a micro-step 37a formed at an intermediate portion thereof, the micro-step stagnating the jet stream and generating a reverse flow in the jet stream, thereby improving ignition performance and flame holding performance.

(13) The fuel stream jetted into the furnace from the fuel jet port 34 is formed by an action of the fuel swirl vane 35 such that the biomass fuel stream is enveloped by the pulverized coal fuel stream.

(14) The secondary air nozzle 40 is disposed so as to surround the fuel jet nozzle 30. The secondary air nozzle 40 includes a secondary air introducing pipe 41, a secondary air conveying pipe 42, and a secondary air widening ring 43. The secondary air nozzle 40 draws swirling secondary air from a spirally formed wind box not shown and supplies the secondary air into the furnace by way of a secondary air supply port formed around the fuel jet port 34. The secondary air is supplied to the outside of the fuel stream jetted from the fuel jet port 34 via the secondary air widening ring 43 disposed at the secondary air supply port.

(15) The tertiary air nozzle 50 is disposed so as to surround the secondary air nozzle 40. The tertiary air nozzle 50 includes a tertiary air introducing pipe 51, a tertiary air throat 52, a tertiary air widening ring 53, and a tertiary air swirl vane 54. The tertiary air nozzle 50 draws swirling tertiary air from the spirally formed wind box not shown and supplies the tertiary air to the outside of the fuel stream by way of a tertiary air supply port formed so as to surround the secondary air supply port. Swirl strength of the tertiary air can be adjusted with the tertiary air swirl vane 54 disposed at a draw-in port.

(16) It is noted that the secondary air exists between the fuel stream and the tertiary air to thereby assume a buffer stream that retards interference therebetween.

(17) The auxiliary fuel nozzle 10 includes an auxiliary fuel conveying pipe 11 disposed at an axial position of the biomass-mixed, pulverized coal-fired burner 1 and an auxiliary fuel jet port 12. The auxiliary fuel nozzle 10 assumes a fuel supply pipe used for supplying auxiliary or starting liquid or gas fuel when a pulverized coal system fails. The addition of the auxiliary fuel nozzle 10 enhances operating stability.

(18) Additionally, the biomass-mixed, pulverized coal-fired burner 1 according to the embodiment further includes, though not shown, a pilot burner and a flame detector.

(19) The biomass fuel jet nozzle 20 and the fuel jet nozzle 30 in the embodiment require an amount of primary air that results in the biomass fuel flowing at a flow velocity of 14.5 m/s or higher to ensure that the biomass fuel does not stagnate in the horizontally disposed piping. Preferably, however, the flow velocity of the biomass fuel stream is held below about 22 m/s, because excessively high flow velocities degrade ignition performance and flame holding performance.

(20) The biomass fuel jet nozzle 20 includes the biomass fuel conveying pipe 23 disposed in a horizontal direction and the biomass fuel introducing pipe 21 connected substantially perpendicularly to the biomass fuel conveying pipe 23 via a bent section 28. The biomass fuel stream flowing from the biomass fuel introducing pipe 21 collides against the flat biomass fuel reflecting plate 22 disposed at the bent section 28 and is thereby bent substantially at 90.

(21) The bent section 28, if formed with a bent pipe, causes the introduced biomass fuel stream to be smoothly bent. Thus, heavy fuel particles in the stream tend to reside on an outer circumferential side of the bent pipe due to a centrifugal force, so that a fuel distribution inside the pipe becomes uneven circumferentially at an outlet of the bent pipe. The nozzle according to the embodiment causes the biomass fuel stream to collide with the flat biomass fuel reflecting plate 22 to thereby disturb the stream, thereby enhancing uniformity of the fuel distribution in the circumferential direction inside the pipe.

(22) The biomass fuel stream conveyed by the primary air flows past the bent section 28 provided with the biomass fuel reflecting plate 22, which reduces unevenness in the circumferential direction. The biomass fuel stream is then supplied to the intermediate position of the fuel conveying pipe 33 from the biomass fuel jet port 24.

(23) The fuel jet nozzle 30 in the embodiment includes the fuel conveying pipe 33 disposed in a horizontal direction and the pulverized coal fuel introducing pipe 31 connected substantially perpendicularly to the fuel conveying pipe 33 via a bent section 38. The pulverized coal fuel stream conveyed by the primary air and flowing from the pulverized coal fuel introducing pipe 31 collides against the flat pulverized coal fuel reflecting plate 32 disposed at the bent section 38 and is thereby bent substantially at 90. This can enhance uniformity of the fuel distribution in the circumferential direction inside the pipe.

(24) The pulverized coal fuel stream undergoes, together with the biomass fuel stream supplied midway in the fuel conveying pipe 33, an adjustment of a fuel concentration distribution in the fuel stream by the fuel swirl vane 35 disposed downstream in the fuel conveying pipe 33.

(25) The fuel swirl vane 35 comprises a plurality of swirl vanes disposed in the flow path in the fuel conveying pipe 33. The swirl vanes are inclined relative to the pipe axis. The swirl vanes change the fuel stream that flows therein into a swirl flow whirling around the axis, thereby causing a component having high specific gravity to reside heavily on the outer circumferential side and making the concentration distribution substantially uniform in the circumferential direction.

(26) The pulverized coal fuel stream and the biomass fuel stream, which have been changed into a swirl flow by the fuel swirl vane 35, are mixed with each other to become a fuel stream that is conveyed to the downstream side, the fuel stream having the pulverized coal component gathering at a portion thereof close to the outer surface and the biomass fuel component residing thereinside.

(27) The fuel baffle plate 36 is disposed on the pipe inner wall at an end of the fuel conveying pipe 33 immediately upstream of the fuel jet port 34. The fuel baffle plate 36 reduces a swirl force of the fuel stream conveyed through the fuel conveying pipe 33, thereby reducing the spread angle of the fuel stream jetted from the fuel jet port 34. Meanwhile, the fuel stream is spread into the furnace by the funnel-shaped opening in the fuel flame stabilizer 37 so as to be mixed well with the secondary air or the tertiary air.

(28) The fuel baffle plate 36 comprises a plurality of flat plates, each flat plate being disposed at substantially equal intervals in the circumferential direction and extending substantially in parallel with the pipe axis. The number, size, orientation, and the like of the flat plates constituting the fuel baffle plate 36 may be determined as appropriate according to the swirl force of the pulverized coal fuel stream and the spread angle after jetting.

(29) In the jetted fuel stream, the pulverized coal fuel is distributed so as to envelope the biomass fuel. Even after the fuel stream is released into the furnace, a condition is maintained in which the pulverized coal fuel covers the biomass fuel like a sheath, so that the biomass fuel burns in a condition of being enveloped by a pulverized coal flame. This achieves reliable ignition and flame holding performance of the biomass fuel.

(30) The secondary air and the tertiary air are mixed with the fuel stream that spreads from the fuel jet port 34 into the furnace and function as part of combustion air to burn the pulverized coal fuel and the biomass fuel.

(31) The secondary air is supplied as a buffer stream into an inside of the tertiary air stream supplied in a large quantity. The supplied secondary air delays the pulverized coal fuel stream to meet a tertiary air swirl flow. A condition in which the fuel concentration is high is thereby sustained. Thus, the secondary air has actions of achieving stable ignition performance and improving flame holding performance. In addition, combustion time with low oxygen conditions is ensured, so that NOx can be reduced even more effectively.

(32) In the biomass-mixed, pulverized coal-fired burner 1 shown in FIG. 1, swirling air is drawn in from the spirally formed wind box in order to form a tertiary air swirl flow around the fuel jet port 34. Additionally, the tertiary air swirl vane 54 is disposed near the draw-in port of the tertiary air introducing pipe 51 of the tertiary air nozzle 50 from the wind box. The tertiary air swirl vane 54 allows the swirl strength to be adjusted. As with the tertiary air, the secondary air, when drawn from the spirally formed wind box, becomes a swirl flow. The burner may include a swirl vane, though not shown, as necessary.

(33) In the biomass-mixed, pulverized coal-fired burner 1 according to the embodiment, the biomass fuel is supplied to the inside of the pulverized coal fuel. As a result, the biomass fuel is readily ignited in the flame of the pulverized coal burned earlier and the biomass fuel flame is stably held. This results in minor restrictions on a mixing ratio of the biomass fuel and the pulverized coal fuel, allowing a large amount of biomass fuel to be burned. Under a condition of a short supply of biomass fuel, the biomass-mixed, pulverized coal-fired burner 1 may be used as a pulverized coal-fired burner that burns only the pulverized coal. It is noted that, when only the pulverized coal is burned, preferably, a small amount of air is supplied to the biomass fuel jet nozzle 20 in order to prevent the pulverized coal fuel from flowing back to the biomass fuel conveying pipe 23.

(34) The conventional pulverized coal-fired burner generally requires that coal be pulverized in order to enhance combustion efficiency, the coal being typically pulverized into fine particles of commonly 200 m or less, preferably about 70 m, for use with the conventional pulverized coal burner.

(35) When, for example, only the pulverized coal fuel that has been processed such that fuel particle diameters of 74 m or less account for 80% is burned, it has been determined that the biomass-mixed, pulverized coal-fired burner according to the embodiment can burn the pulverized coal such that a load factor to a rated value falls within a range of 40% to 100%, if A/C (fuel conveying air flow rate (Nm.sup.3/h) to fuel (kg/h): unit Nm.sup.3/kg) is adjusted to fall within a range of 1.7 to 3.0.

(36) With the biomass fuel, however, electric power for pulverization increases sharply at smaller grain sizes involved in pulverizing the material, aggravating economy. In addition, the biomass fuel is easier to burn than the coal for the same particle diameter, which allows the pulverized grain size to be made larger. As a result, preferably, the biomass fuel is pulverized to a grain size distribution of substantially 2 mm or under.

(37) In the biomass-mixed, pulverized coal-fired burner 1 according to the embodiment, the pulverized coal fuel resident outside the fuel stream to be jetted into the furnace is burned with the secondary air and the tertiary air and the biomass fuel resident inside the fuel stream is ignited and its flame is stably held in the pulverized coal flame. A pulverizing mill dedicated to the biomass fuel is employed to process the biomass fuel into granular particles having a grain size different from that of the pulverized coal. The biomass fuel particles are conveyed by an air stream independent of the pulverized coal and supplied to the biomass-mixed, pulverized coal-fired burner 1.

(38) As such, the biomass fuel can be burned with high efficiency under an optimum combustion condition over a wide range of mixed fuel burning ratios.

(39) FIG. 2 shows a relation between burner load and A/C (a value of the fuel conveying air flow rate divided by the amount of fuel loaded) when the fuel contains 60% by weight of the biomass (40% by weight of the pulverized coal) in the biomass-mixed, pulverized coal-fired burner 1 according to the embodiment. In FIG. 2, the abscissa represents a burner load factor (%) relative to a rating and the ordinate represents total A/C (Nm.sup.3/kg) relating to the mixed fuel containing therein pulverized coal and biomass. Additionally, in FIG. 2, o denotes a case in which the flame was steady with favorable ignition and flame holding performance in the combustion experiment, and x denotes a case in which the combustion was poor with degraded ignition and flame holding performance. The shaded area in FIG. 2 represents a recommended operating range.

(40) Referring to FIG. 2, the biomass-mixed, pulverized coal-fired burner 1 according to the embodiment is determined to be industrially applicable in a recommended operating range that, with fuel containing 60% by weight of the biomass fuel, is sandwiched between a straight line extending from total A/C 1.0 to total A/C 1.8 at a load factor of 100% and a straight line extending from total A/C 1.0 to total A/C 3.2 at a load factor of 50% and drawn in view of a plot position of the poor combustion condition, the recommended operating range having an upper edge partitioned by an upper limit line under which the flame holding performance is ensured, the upper limit line being drawn to pass the upper end point of the straight line at the load factor of 100% and the upper end point of the straight line at the load factor of 50% and drawn to circumvent the x mark at which the combustion is poor, and having a lower edge partitioned by a straight line.

(41) It is noted that a range with a load factor of less than 50% is not recommended, in which steady ignition or steady flame holding cannot be obtained because of a low fuel concentration in the biomass fuel.

(42) In FIG. 2, the broad solid curve represents a conveyance limit flow velocity of 14.5 m/s at which the fuel does not stagnate in the fuel conveying pipe 33 disposed horizontally. Preferably, the actual burner is operated in the darker shaded area above the broad solid curve. It is noted that the conveyance limit flow velocity varies according to a mounting position of the fuel conveying pipe 33.

(43) A biomass-mixed, pulverized coal-fired boiler capable of combustion at a high mixed fuel burning ratio of biomass can be provided by applying the biomass-mixed, pulverized coal-fired burner according to the present invention to a new or existing boiler. The biomass-mixed, pulverized coal-fired boiler to which the biomass, pulverized coal-fired burner of the embodiment is applied burns a large volume of woody biomass fuel to thereby save coal consumption and reduce the amount of CO2 emissions derived from fossil fuels. Since the biomass-mixed, pulverized coal-fired burner burns the biomass fuel in a reducing atmosphere, the amount of NOx in exhaust gases can be reduced.

DESCRIPTION OF REFERENCE NUMERALS

(44) 1: biomass-mixed, pulverized coal-fired burner 10: auxiliary fuel nozzle 11: auxiliary fuel conveying pipe 12: auxiliary fuel jet port 20: biomass fuel jet nozzle 21: biomass fuel introducing pipe 22: biomass fuel reflecting plate 23: biomass fuel conveying pipe 24: biomass fuel jet port 28: biomass fuel bent section 30: fuel jet nozzle 31: pulverized coal fuel introducing pipe 32: pulverized coal fuel reflecting plate 33: fuel conveying pipe 34: fuel jet port 35: fuel swirl vane 36: fuel baffle plate 37: fuel flame stabilizer 38: pulverized coal fuel bent section 40: secondary air nozzle 41: secondary air introducing pipe 42: secondary air conveying pipe 43: secondary air widening ring 50: tertiary air nozzle 51: tertiary air introducing pipe 52: tertiary air throat 53: tertiary air widening ring 54: tertiary air swirl vane