EP0017721B1

EP0017721B1 - Low load coal bucket and method of operating a pulverised coal-fired furnace

Info

Publication number: EP0017721B1
Application number: EP19800100740
Authority: EP
Inventors: Angelos Kokkinos; Michael Scott Mccartney
Original assignee: Combustion Engineering Inc
Current assignee: SOCIETE FRANCAISE DES TECHNIQUES LUMMUS
Priority date: 1979-04-13
Filing date: 1980-02-14
Publication date: 1983-07-20
Also published as: EP0017721B2; EP0017721A2; AU5738080A; DE3064180D1; ES8103344A1; AU530834B2; EP0017721A3; IN151051B; ES490436A0

Description

Background of the Invention

The present invention relates to pulverized coal-fired furnaces and, more particularly, to improving the low load operation of fuel burners employed therein.
In view of today's fluctuating electricity demand, typified by peak demand occurring during weekday daytime hours and minimum demand occurring at night and on the weekends, electric utilities have chosen to cycle many of their conventional coal-fired steam generator boilers by operating them at full load during peak demand hours and reducing them to low loads during periods of minimum demand.
As a consequence of this mode of operation, the electric utilities have used large quantities of natural gas or oil to furnish additional ignition energy during low load operation because the current generation of coal-fired steam generator furnaces require stabilization of the coal flames when operating at low loads. The required amount of auxiliary fuel fired for stabilization purposes is significant and, for example, to maintain a 500 megawatt coal-fired steam generator at 10 to 15 percent load during minimum demand periods would require the use of 1700 I (450 gallons) of oil per hour.
One common method of firing coal in conventional coal-fired steam generator boilers is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through burners, termed fuel-air admission assemblies, located in the corners of the furnace. The fuel-air streams discharged from these burners are aimed tangentially to an imaginary circle in the middle of the furnace. This creates a fireball which serves as a continuous source of ignition for the incoming coal. More specifically, a flame is established at one corner which in turn supplies the required ignition energy to stabilize the flame emanating from the corner downstream of and laterally adjacent to it. When load is reduced, the flames emanating from each corner become shorter and, as a consequence, a reduction in the amount of ignition energy available to the downstream corner occurs. As a result, auxiliary fuel such as oil or natural gas must be introduced in each corner adjacent to the pulverized coal-air stream to provide additional ignition energy thereby insuring that a flameout and resultant unit trip will not occur.
Another problem associated with operating a coal-fired burner at low load results from the fact that the pulverizing mills typically operate with a fairly constant air flow over all load ranges. When furnace load is reduced, the amount of coal pulverized in the mills decreases proportionally while the amount of primary air used to convey the pulverized coal from the mills through the admission assemblies into the furnace remains fairly constant. Consequently, the fuel-air ratio decreases. When the load on the furnace is reduced to the low levels desired during minimum demand periods, the fuel-air ratio has decreased to the point where the pulverized coal-primary air mixture has become too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available.
A dual nozzle burner designed to provide a stable pocket is disclosed in United States patent 2,608,168. As disclosed therein, the coal bucket is comprised of two nozzles which are physically spaced apart but nonadjustable with respect to each other. Therefore, two parallel but physically separated coal-air streams are discharged from the coal bucket at all loads. Additionally, both of the coal-air streams exhibit the same coal to air ratio.
As the parallel but separated coal-air streams are discharged into, a low pressure recirculation zone of high turbulence is established between the streams thereby stabilizing ignition for even low volatile coals. A problem which would arise if such a dual nozzle bucket were used at high loads on coals of average or better volatile matter would be that the presence of the high turbulence zone between the separated coal-air streams would result in higher combustion temperatures and more rapid coal-air mixing. Both of these conditions would lead to the increased formation of nitrogen oxide, a major air pollutant.
Accordingly, it is an object of the present invention to provide for separated coal-air streams to be discharged from a single coal bucket at low loads to stabilize ignition without using auxiliary fuel while also providing for a single coal-air stream to be discharged at high loads.

Summary of the Invention

The present invention provides an improved fuel-air admission assembly incorporating a split coal bucket which permits a pulverized coal-fired furnace and, more specifically, a pulverized coal-fired furnace employing the tangential firing method, to be operated at low loads without the use of auxiliary fuel to provide stabilization.
In accordance with the invention, the split coal bucket comprises an upper and a lower coal nozzle pivotally mounted to the coal delivery pipe, the upper and lower coal nozzles being independently tiltable. When the furnace is operating at low loads such as during the minimum demand periods, the primary air and pulverized coal stream discharging from the coal delivery pipe is split into an upper and a lower coal-air stream and independently directed into the furnace by tilting at least one of the nozzles away from the longitudinal axis of the coal delivery pipe. In doing so, an ignition stabilizing pocket is established in the locally low pressure zone created between the spread apart coal-air streams. Hot combustion products are drawn, i.e., recirculated, into this low pressure zone, thus providing enough additional ignition energy to the incoming fuel to stabilize the flame.
Ignition stability is further improved by disposing within the coal delivery pipe means for separating the pulverized coal-primary air mixture received from the main fuel pipe outlet elbow into a first portion and a second portion, the first portion having a higher coal-air ratio than the second portion, and for maintaining said separation between the first and second portions for the length of the coal delivery pipe so that the first portion is directed into the furnace through the upper coal nozzle and the second portion is directed into the furnace through the lower coal nozzle.
As the primary air and pulverized coal mixture is being conveyed from the pulverizers through the main fuel pipe to the furnace, the mixture turns through an angle generally of 90° as it flows from the main fuel pipe through the main fuel pipe outlet elbow into the coal delivery pipe of the fuel-air admission assembly. As the mixture turns through the main fuel pipe outlet elbow, the pulverized coal particles being denser than air tend to concentrate along the outer radius of the main fuel pipe outlet elbow due to centrifugal forces. Therefore, two distinct regions of differing fuel-air ratio are established within the primary air and pulverized coal mixture as it enters the main fuel pipe outlet elbow. A first portion of the primary air-pulverized coal stream along the outer radius of the main fuel pipe outlet elbow has a high concentration of coal because of the more dense coal particles being displaced radially outward due to centrifugal forces as the primary air-pulverized coal stream turns through the main fuel pipe outlet elbow. A second portion along the inner radius of the main fuel pipe outlet elbow conversely has a low coal concentration.
In accordance with the invention, a plate is disposed along the longitudinal axis of the coal delivery pipe with its leading edge orientated across the inlet end of the coal delivery pipe so that that portion of the primary air-pulverized coal stream having a high coal concentration enters the coal delivery pipe on one side of the plate and that portion of the primary air-pulverized coal stream having a low coal concentration enters the coal delivery pipe on the other side of the plate. The trailing edge of the plate is orientated across the outlet end of the coal delivery pipe such that that portion of the primary air-pulverized coal stream having a high coal concentration is discharged from the coal delivery pipe through the upper coal nozzle and such that that portion of the primary air-pulverized coal stream having a low coal concentration is discharged from the coal delivery pipe through the lower coal nozzle.
As the upper coal-air stream is turned upward through the upper coal nozzle, the coal in this already high coal concentration stream tends to further concentrate along the lower surface of the upper coal nozzle because of the density differential between the coal particles and the air resulting in the coal particles being thrown outward by the centrifugal force as the coal-air stream turns upward through the upper coal-air nozzle. Thus, the coal is further concentrated along the lower surface of the coal-air nozzle and consequently is drawn into the low pressure ignition zone to form a subregion therein which has a fuel-air ratio significantly higher than that normally present at low load. This subregion because of its relatively high fuel-air ratio readily ignites thereby providing for the stable ignition of the remainder of the primary air-pulverized coal stream.

Brief Description of the Drawings

Figure 1 is a diagrammatic plan view of a furnace employing the tangential firing method;
Figure 2 is an elevational cross-sectional view, taken along line 2-2 of Figure 1, of a set of three fuel-air admission assemblies, the upper two assemblies having a split coal bucket designed in accordance with the present invention and the lower assembly equipped with a coal bucket typical of the prior art;
Figure 3 is an enlarged cross-sectional view taken along line 3-3 of Figure 4 of the split coal bucket of the present invention;
Figure 4 is an enlarged cross-sectional view taken along line 4-4 of Figure 3 of the split coal bucket of the present invention;
Figure 5 is an elevational cross-sectional view of a single fuel-air admission assembly equipped with a split coal bucket designed in accordance with the present invention with the coal nozzles orientated in the normal full load operating position;
Figure 6 shows an elevational cross-sectional view of a fuel air admission assembly equipped with a split coal-air bucket designed in accordance with the present invention with the coal nozzles tilted apart for stable low load operation;
Figure 7 is a diagrammatic elevational illustration of a fuel-air admission assembly equipped with the split coal bucket of the present invention showing the flame shape and recirculation pattern established during low load operation with the coal nozzles tilted apart;
Figure 8 is an elevational cross-sectional view of a fuel-air admission assembly with a plate disposed along the longitudinal axis of the coal delivery pipe in accordance with the present invention;
Figure 9 is an end view taken along line 9-9 of Figure 8;
Figure 10 is an elevational cross-sectional view of a fuel-air admission assembly with a twisted plate disposed along its longitudinal axis in accordance with the present invention; and
Figure 11 is an end view taken along line 10-10 of Figure 10.

Description of the Preferred Embodiment

While the present invention may be applied, in spirit and in scope, to a number of different firing methods employed in conventional pulverized coal-fired steam generator boiler furnaces, it may be best described when embodied in a pulverized coal-fired furnace employing the tangential firing method as illustrated in Figure 1. In the tangential firing method, fuel and air are introduced to the furnace through fuel-air admission assemblies 10 mounted in the four corners of furnace 1. The fuel-air admission assemblies 10 are orientated so as to deliver the pulverized coal and air streams tangentially to an imaginary circle 3 in the center of furnace 1 so as to form a rotating vortex-like flame termed a fireball therein.
As shown in Figure 2, a plurality of fuel-air admission assemblies 10 are arranged in the comers in a vertical column separated by auxiliary air compartments 20 and 20'. One or more of these auxiliary air compartments, such as compartment 20', is adapted to accommodate an auxiliary fuel burner, which is used when starting and warming up the boiler and which may be used when necessary to provide additional ignition energy to stabilize the coal flame when operating at low loads.
Each fuel-air admission assembly 10 comprises a coal delivery pipe 12 extending therethrough and opening into the furnace, and a secondary air conduit 14 which surrounds coal delivery pipe 12 and provides a flow passage so that the secondary air may be introduced into the furnace as a stream surrounding the primary air-pulverized coal stream discharged from coal delivery pipe 12. Each coal delivery pipe 12 is provided with a tip, termed a coal bucket, which is pivotally mounted to the coal delivery pipe 12 so that the coal bucket may be tilted about an axis 16 transverse to the longitudinal axis of coal delivery pipe 12.
A typical prior art single nozzle coal bucket 28 is shown in Figure 2 mounted to the coal delivery pipe of the lower fuel-air admission assembly. Coal bucket 28 can be tilted upward or downward about axis 16 in order to direct the pulverized-coal primary air mixture into the furnace at an upward or downward angle as a means of controlling the position of the fireball within the furnace as a means of controlling the temperature of the superheated steam leaving the generator (not shown) in the manner taught by U.S. Patent 2,363,875 issued November 28, 1944, to Kreisinger et al. for "Combustion Zone Control".
In accordance with this invention, coal bucket 28 is replaced with a split coal bucket 30 shown in Figure 2 pivotally mounted to the coal delivery pipes 12 of the upper two fuel-air admission assemblies. Each split coal bucket 30 comprises an upper coal nozzle 32 and a lower coal nozzle 34, both of which are independently tiltable about axis 16 transverse to the longitudinal axis of coal delivery pipe 12. By tilting the upper coal nozzle 32 upward, a first portion of the primary air and pulverized coal mixture discharging from coal delivery pipe 12 may be selectively directed upwardly into the furnace as an upper coal-air stream. Similarly, by tilting the lower coal nozzle downward a second portion of the primary air and pulverized coal mixture discharging from the coal delivery pipe 12 can be selectively directed downwardly into the furnace as a lower coal-air stream. Means 50 and 60 are provided for independently tilting the upper and lower nozzles of the split coal bucket 30.
In the preferred embodiment, as shown in Figures 3 and 4, an upper air nozzle 40 is rigidly mounted on the upper surface of the upper coal nozzle 32 to provide an upper air pathway 42 for directing a first portion of the secondary air passing from the secondary air conduit 14 into the furnace along the path essentially parallel to the upper coal-air stream. Similarly, a lower air nozzle 44 is rigidly mounted to the bottom surface of the lower coal nozzle 34 to provide a lower air pathway 46 for directing a second portion of the secondary air passing from the secondary air conduit 14 into the furnace along a path essentially parallel to the lower coal-air stream. Additionally, lateral air pathways 48 are provided on the sides of both the upper coal nozzle 32 and the lower coal nozzle 34 for directing the remainder of the secondary air into the furnace along a path flanking and essentially parallel to the upper and lower coal-air streams. Further, barrier plates 52 are suspended from the bottom of the upper coal nozzle 32 into the lateral air pathways 48 of the lower coal nozzle 34 in order to prevent the secondary air from entering the low pressure zone established between the upper and lower coal-air streams when the upper and lower coal nozzles are tilted apart.
Also disposed within the upper coal nozzle 32 and the lower coal nozzle 34 are flow baffles 36 and 38 respectively. Flow baffle 36 comprises a foreshortened flat plate aligned substantially parallel to the direction of the flow through the upper coal nozzle 32 thereby defining within the upper coal nozzle 32 an upper flow channel 54 and a lower flow channel 56. When the upper coal nozzle is tilted upward, as shown in Figure 6, the flow baffle 36 causes a major portion of the pulverized coal and primary air entering the upper coal nozzle 32 to flow through the lower flow channel 56. Similarly, the flow baffle 38 comprises a foreshortened flat plate aligned substantially parallel to the direction of flow through the lower coal nozzle 34 thereby defining within the lower coal nozzle 34 an upper flow channel 55 and a lower flow channel 57. When the lower coal nozzle is tilted downward, the flow baffle 38 causes a major portion of the pulverized coal and primary air entering the lower coal nozzle 34 to flow through the upper channel 55. So disposed, flow baffles 36 and 38 do not in any way affect the flow of the primary air-pulverized coal stream through coal nozzles 32 and 34 when said nozzles are orientated parallel to the longitudinal axis of the coal delivery pipe 12, as is typical at high loads. However, during load operation when at least one of the coal nozzles 32 and 34 is tilted away from the longitudinal axis of the coal delivery pipe 12, the corresponding flow baffle causes a major portion of the primary air-pulverized coal stream passing therethrough to flow through the flow channel bordering upon the low pressure ignition stabilizing zone.
The coal delivery pipe 12 receives at its inlet end a mixture of primary air and pulverized coal from a source, not shown, such as a pulverizer. In operation, coal is dried and crushed in the pulverizer, and the pulverized coal is conveyed from the pulverizer to the furnace through a main fuel pipe 128 which terminates in a main fuel outlet pipe elbow 140 aligned with the inlet end of the coal delivery pipe 12. As the primary air and pulverized coal mixture is being conveyed from the pulverizers through the main fuel pipe to the furnace, the mixture turns through an angle, generally but not necessarily of 90°, as it flows from the main fuel pipe through the main fuel pipe outlet elbow 140 into the inlet end of the coal delivery pipe 12 of the fuel-air admission assembly 10.
As the mixture turns through the main fuel pipe outlet elbow 140, the pulverized coal particles being denser than air tend to concentrate along the outer radius of the main fuel pipe outlet elbow 140 due to centrifugal forces. Therefore, two distinct regions of differing fuel-air ratio are established within the primary air-pulverized coal stream as it travels through the main fuel pipe outlet elbow 140. A first portion of the primary air-pulverized coal stream, traveling along the outer radius of the main fuel pipe outlet elbow 140, has a high concentration of coal because of the more dense coal particles being displaced radially outward due to centrifugal forces as the primary air-pulverized coal stream turns through the main fuel pipe outlet elbow 140. A second portion, traveling along the inner radius of the main fuel pipe outlet elbow 140, conversely has a low coal concentration.
In accordance with the invention, a partition plate 130 is disposed along the longitudinal axis of the coal delivery pipe 12 so as to establish an upper flow pathway 136 and a lower flow pathway 138 therethrough. The leading edge 132 of the partition plate 130 is orientated across the inlet end of the coal delivery pipe 12 so that the first portion of the primary air-pulverized coal stream traveling along the outer radius of the main fuel pipe outlet elbow 140 enters the upper flow pathway 136 of the main coal delivery pipe 12; and the second portion of the primary air-pulverized coal stream traveling along the inner radius of the main fuel pipe outlet elbow 140 enters the lower flow pathway 138 of the coal delivery pipe 12. In the preferred embodiment, the trailing edge 134 of the partition plate 130 is orientated across the outlet end of the coal delivery pipe 12 such that the upper flow pathway 136 communicates with the upper coal nozzle 32 and the lower flow pathway 138 communicates with the lower coal nozzle 34 so that the first portion of the primary air-pulverized coal stream, having a high coal concentration, is discharged from the coal delivery pipe 12 through the upper coal nozzle 32, and the second portion of the primary air-pulverized coal mixture, having a low coal concentration, is discharged from the coal delivery pipe 12 through the lower coal nozzle 34.
Accordingly, the partition plate 130 separates the primary air-pulverized coal stream received from the main fuel pipe outlet elbow 140 into a first portion and a second portion, the first portion having a higher coal-air ratio than the second portion, and maintains the separation between the first and second portions for the length of the coal delivery pipe 12 so that the first portion is directed into the furnace through the upper coal nozzle 32 and the second portion is directed into the furnace through the lower coal nozzle 34 as shown in Figures 8 and 10.
In the embodiment illustrated in Figures 8 and 9, the main fuel pipe 128 travels vertically upward along the furnace and terminates in the main fuel pipe outlet elbow 140 which turns the primary air-pulverized coal stream from the vertical to the horizontal through a 90° angle. Accordingly, the pulverized coal is naturally concentrated in the upper half of the primary air-pulverized coal stream entering the coal delivery pipe 12. In this case, the partition plate 130 comprises a simple flat plate disposed along the longitudinal axis of the coal delivery pipe 12 since the concentrated pulverized coal stream may not be turned as it passes through the coal delivery pipe 12 in order to direct it through the upper coal nozzle 32.
In order to accommodate approaches of the main fuel pipe which are not directly upward, the partition plate 130 comprises a warped plate. As illustrated in Figures 10 and 11, the main fuel pipe 128 travels upward along the furnace and terminates in the main fuel pipe outlet elbow 140 which turns the primary air-pulverized coal mixture through a 90° angle to the horizontal but in a plane orientated at an angle to the vertical. The pulverized coal is concentrated along the outer half of the main fuel pipe outlet elbow 140 which, in this case, is not coincident with the upper half of the coal delivery pipe 12. Thus, the partition plate 130 is warped so that its leading edge 132 is orientated across the inlet end of the coal delivery pipe 12 such that the high coal concentration portion of the primary air-pulverized coal stream is directed along the upper flow pathway 136 to the upper coal nozzle 32 and the low coal concentration portion of the primary air-pulverized coal stream is directed along the lower flow pathway 138 to the lower coal nozzle 34.
The typical prior art coal bucket comprises a single coal nozzle 28, having one or more extended rather than foreshortened baffle plates, surrounded by air pathways as in the present invention. The pulverized coal and primary air passing through the coal delivery pipe was discharged into the furnace through the single coal nozzle as a single coal-air stream. As indicated earlier, when the furnace was operated at low load, ignition became unstable; and supplemental fuel such as natural gas or oil had to be fired in order to provide sufficient additional energy to stabilize the ignition of the single coal-air stream.
In accordance with the present invention, stable ignition at low loads is insured by providing a split coal bucket having independently tiltable upper and lower coal nozzles. In normal operation at higher ratings where ignition stability is not a problem, the upper and lower coal nozzles are disposed parallel to each other as shown in Figure 5. In this configuration, the pulverized coal and primary air discharged from the coal delivery pipe 12 is effectively introduced into the furnace as a single coal-air stream, albeit a first portion is directed through the upper coal nozzle 32, a second portion through the lower coal nozzle 34, and a third portion through the gap therebetween. Thus, at these higher loads the flame pattern established is essentially identical to that associated with the single coal bucket of the prior art, and the characteristics of the tangential firing method are maintained.
However, when the furnace is operated at low loads, the upper coal nozzle 32 is tilted upward and the lower coal nozzle 34 is tilted downward as shown in Figure 6. The pulverized coal and the primary air discharged from the coal delivery pipe 12 through the coal bucket is split into an upper coal-air stream 80 and a lower coal-air stream 90. As illustrated in Figure 7, the upper coal-air stream 80 is directed upward through the upper coal nozzle 32 as it is introduced into the furnace and the lower coal-air stream 90 is directed downward through the lower coal nozzle 34 as it is introduced into the furnace. A low pressure zone 70, which serves as an ignition stabilizing region, is created between the diverging upper and lower coal-air streams. Air and coal and coal particles are drawn into the low pressure region 70 from the lower surface of the upper coal-air stream 80 and the upper surface of the lower coal-air stream 90 and ignited. The ignition is stabilized because a portion of the hot combustion products formed during ignition are recirculated within this low pressure ignition stabilizing zone 70, thereby providing the necessary ignition energy for igniting coal particles which are subsequently drawn into the region from the upper and lower coal-air streams.
Stable ignition is further insured because the fuel-air ratio within the ignition stabilizing zone 70 is increased which in turn reduces the amount of energy necessary to initiate ignition. As the upper coal-air stream is turned upward through the upper coal nozzle, the coal in this already high coal concentration stream tends to further concentrate along the lower surface of the upper coal nozzle 32 because of the density differential between the coal particles and the air resulting in the coal particles being thrown outward by the centrifugal forces. The coal, being concentrated along the lower surface of the coal-air stream, is consequently drawn into the low pressure ignition zone to form a subregion therein which has a fuel-air ratio significantly higher than that normally present at low loads. This subregion because of its relatively high fuel-air ratio readily ignites thereby providing for the stable ignition of the remainder of the primary air-pulverized coal stream.
This novel split nozzle low load coal bucket design stabilizes ignition to an extent which heretofore could not be obtained during the low load operation of pulverized coal-fired furnaces without firing supplemental fuel such as natural gas or oil. Tests conducted on a 75 MW tangentially-fired pulverized coal unit retrofitted with the split nozzle low load coal bucket of the present invention for experimental purposes confirmed this statement. Before the unit was retrofitted with the new low load coal bucket, stable ignition without the use of auxiliary fuel was possible only at loads above approximately 40 percent. With the use of the low load coal bucket as described herein, the regime of stable ignition without the use of auxiliary fuel was extended down to 25 percent load. Such an extension of the stable ignition regime on coal- firing will greatly increase the flexibility of coal-fired steam generator operation and significantly reduce the consumption of oil and natural gas on coal-fired units.
Although described and illustrated hereinabove in terms of an upper and lower nozzle, the split coal bucket of the present invention contemplates split coal buckets with the nozzles arranged in other configurations, such as side by side, so long as at least one of the nozzles may be independently tilted away from the longitudinal axis of the coal delivery pipe.

Claims

1. A coal-air admission assembly having a coal delivery pipe (12) for discharging a mixture of primary air and pulverized coal into a furnace in a stream parallel to its axis, and a first and a second coal nozzle (32, 34) pivotally mounted to the discharge end of said coal delivery pipe so as to split the primary air and pulverized coal stream discharging from said coal delivery pipe into a first and a second coal-air stream, characterized in that said first and second coal nozzles (32, 34) are tiltable independently of each other about an axis transverse to the longitudinal axis of said coal delivery pipe so that said first and second coal-air streams may be selectively directed into the furnace in parallel relationship at higher furnace ratings and into the furnace in angular relationship away from each other at low furnace ratings.

2. A coal-air admission assembly as claimed in Claim 1 further characterized by: separation means (130) disposed within said coal delivery pipe (12) for separating the coal-air mixture entering the coal delivery pipe into a higher coal-air ratio portion and a lower coal-air ratio portion, and maintaining the separation between the higher and lower coal-air ratio portions for a substantial portion of the length of said coal delivery pipe so that the higher coal-air ratio portion is directed into the furnace through said first coal nozzle (32) and the lower coal-air ratio portion is directed into the furnace through said second coal nozzle (34).

3. A coal-air admission assembly as claimed in Claim 2 further characterized in that said separation means comprises a partition plate (130) disposed along the longitudinal axis of said coal delivery pipe so as to establish a first and a second flow pathway therethrough, said partition plate having a leading edge (132) orientated across the inlet end of said coal delivery pipe and a trailing edge (134) orientated across the outlet end of said coal delivery pipe such that the higher coal-air ratio portion of the coal-air mixture entering the coal delivery pipe flows through the first flow pathway to discharge into the furnace through said first coal nozzle and the lower coal-air ratio portion of the coal-air mixture entering the coal delivery pipe flows through the second flow pathway to discharge into the furnace through said second coal nozzle.

4. A method of operating a pulverized coal-fired furnace including the steps of passing a mixture of pulverized coal and primary air to the furnace, splitting the pulverized coal and primary air mixture into a first and a second coal-air stream prior to discharge into the furnace, discharging the first coal-air stream into the furnace through a first coal nozzle and the second coal-air stream into the furnace through a second coal nozzle, characterized in that said first and second coal nozzles are aligned parallel to each other during furnace operation at normal rating so as to discharge said first and second coal-air streams into the furnace in parallel relationship immediately adjacent each other, and further characterized in that at least one of said first and second coal nozzles is tilted away from the other during furnace operation at low rating so as to discharge said first and second coal-air streams into the furnace in angular relationship away from each other.

5. A method of operating a pulverized coal-fired furnace as claimed in Claim 4 further characterized in that the pulverized coal and primary air mixture is split into a first coal-air stream having higher coal-air ratio than the coal-air ratio of the mixture prior to splitting and a second coal-air stream having a lower coal-air ratio than the coal-air ratio of the mixture prior to splitting.