Abstract:
A method of and an apparatus for operating a pulverized coal-fired boiler system, which includes at least one coal pulverizer in flow connection with at least two burners disposed on a boiler enclosure by a coal feed system having a coal pipe junction with a riffle enclosure including a plurality of parallel partition walls forming multiple flow channels from an upstream coal pipe to each of at least two downstream coal pipes and a plurality of individually pivotable vanes upstream of the plurality of partition walls, and controlling the distribution of coal between the downstream pipes by pivoting at least one of the vanes. Preferably a coal flow rate is monitored in the downstream coal pipes and at least one of the vanes is pivoted in response to a measured distribution of the coal flow rate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method of and an apparatus for operating a pulverized coal-fired boiler system including an on-line adjustable coal flow distributing device. The invention particularly relates to controlling the balance of flows of pulverized coal from a coal pipe to multiple coal pipes by using an adjustable riffle in a coal pipe junction. A riffle is an enclosure including a series of parallel flowchannels with rectangular cross sections, leading coal from an inlet pipe alternatively to each of a plurality of outlet pipes.  
         [0003]     2. Description of the Related Art  
         [0004]     In a pulverized coal-fired boiler, particulate coal and primary air flow from one or more pulverizers to multiple coal burners disposed on the boiler enclosure, i.e., on the walls, corners, roof and/or arches, of the boiler. Coal is fed to the burners through a coal feed system comprising multiple coal pipes extending from the pulverizers to the burners. Usually, the number of coal pipes connected to the pulverizers is less than the number of coal pipes connected to the burners, and the coal feed system comprises several coal pipe junctions, where an upstream coal pipe is divided into two or more downstream coal pipes.  
         [0005]     It is generally known that controlled, usually uniform, distribution of coal and primary air between the individual burners is important for achieving a desired temperature distribution in the boiler as well as reduced emissions to the environment and a low level of unburned carbon in the ash. A sufficiently uniform distribution of primary air to the coal pipes can usually be obtained using adjustable orifices or flow restrictors in the coal pipes, to balance the overall flow resistances of the coal pipes. However, achieving a uniform coal flow distribution to the coal pipes is often a difficult task. A reason for this is that coal has a tendency to flow in the coal pipes as so-called “ropes,” or relatively narrow, concentrated streams of coal. Such ropes propagate in certain portions of the cross sections of the coal pipes. Often, there is only one rope within a coal pipe, which may change its position depending, e.g., on the air and coal flow rates. When reaching a coal pipe junction, the rope may be directed mainly to one downstream coal pipe, and the distribution of coal to the burners may therefore become highly unbalanced.  
         [0006]     A method used for trying to balance the coal streams is to provide a venturi with an internal swirler in the coal pipe at a position upstream of the junction, so as to break up the coal rope. However, such a swirler increases the pressure drop in the coal pipe. Also, in cases in which an unbalanced distribution of coal is observed, such a swirler cannot be adjusted to correct the flows.  
         [0007]     In trying to divide a coal particle stream uniformly from a coal pipe into multiple downstream pipes, the pipe junction is often provided with a riffle, i.e., an enclosure comprising a series of parallel flow channels with rectangular cross sections, leading coal from the inlet pipe alternatively to each of the outlet pipes. Typically, a riffle comprises eight to sixteen channels having a width from about one to about four inches. By using very narrow channels in the riffle, the coal rope can be expected to hit the inlet openings of more than a few channels and a relatively good coal flow balance may be achieved. However, channels that are too narrow cause a high flow resistance. Therefore, the channels are often of such a size that the rope may hit mainly a few channels only. However, in this configuration, a too large imbalance may be caused. For the foregoing reasons, there is a need to improve the balancing of coal flow in a riffle without severely increasing the flow resistance.  
         [0008]     U.S. Pat. No. 6,789,488 shows a device for balancing pulverized coal flows having adjustable flow control elements disposed upstream of a riffle. The flow control elements comprise an array of, e.g., tear-drop shaped elements mounted on a positioning rod. The transverse position of all parallel flow control elements can simultaneously be adjusted by sliding the positioning rod in or out of the coal pipe. Such a device is, however, quite a complicated construction and prone to wearing.  
         [0009]     It would, therefore, be advantageous to provide a simple and durable coal pipe junction design that enables control of the distribution of coal flows in a pulverized coal feed system. Such a design would result in improved operation of the pulverized coal boiler system, reduced pollutant emissions, and improved combustion efficiency.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of the present invention is to provide a riffle configuration in a simple and reliable method of and apparatus for efficiently controlling the balance of multiple coal flows in a pulverized coal fired boiler system, without significantly increasing the pressure drop of the system.  
         [0011]     Another object of the present invention is to provide an automatic, on-line control in an efficient method of and apparatus for controlling the balance of multiple coal flows in a pulverized coal fired boiler system.  
         [0012]     According to an aspect, the present invention provides a method of controlling the distribution of coal in a pulverized coal-fired boiler system having at least one coal pulverizer and at least two burners disposed on a boiler enclosure for receiving pulverized coal from the coal pulverizer. The method includes providing a coal feed system for supplying coal from the at least one coal pulverizer to the at least two burners. The coal feed system has a coal pipe junction with a riffle enclosure comprising a plurality of parallel partition walls forming multiple flow channels from an upstream coal pipe to each of at least two downstream pipes. The method also includes providing a plurality of pivotable vanes upstream of the plurality of partition walls and pivoting at least one of the pivotable vanes to control the distribution of coal among the at least two downstream coal pipes.  
         [0013]     According to another aspect, the present invention provides a coal feed system for use in a pulverized coal-fired boiler system having at least one coal pulverizer for pulverizing coal to be supplied to at least two burners disposed on a boiler enclosure. The coal feed system includes a coal pipe junction and a plurality of pivotable vanes. The coal pipe junction includes a riffle enclosure having a plurality of partition walls forming multiple flow channels from an upstream coal pipe, through which the pulverized coal is supplied from the coal pulverizer, to each of at least two downstream coal pipes, which supply the pulverized coal to the at least two burners. The plurality of pivotable vanes are disposed upstream of the plurality of partition walls to control distribution of coal from the upstream coal pipe among the downstream coal pipes.  
         [0014]     By adjusting one or more of the individual vanes disposed upstream of the partition walls from an inline position to a position at least partially blocking the inlet to one or more flow channels, the flow of coal and air through the channels can be reduced. As an object of the present invention, while coal is transported in the coal pipes as concentrated ropes, which typically hit a few of the flow channels only, a considerable portion of the coal may be redistributed to flow to the neighboring channels by partially or fully blocking the inlets to one or at most a few of-the channels. By adjusting the vanes properly, the coal flow can be distributed evenly to the outlet pipes. In some cases, there is also a need to specifically bias the coal distribution, for example, to address emissions or slagging problems. By adjusting the vanes properly, it is also possible to obtain such a biased coal distribution.  
         [0015]     The pivotable vanes in accordance with the present invention allow balancing of the outlet coal flows without having to use flow channels in the riffle that are so narrow that a significant pressure drop in the riffle is created. In most cases, the device in accordance with the present invention renders it possible to balance the coal flow distribution by turning one or at most a few vanes in the riffle. Thus, most of the vanes can be maintained inline with the flow, a pressure drop does not severely increase, and the velocity or distribution of primary air, which is spread to all of the channels, is not significantly affected.  
         [0016]     In another preferred embodiment of the present invention, a coal flow rate is measured in the downstream coal pipes and at least one of the vanes is pivoted, in response to a measured coal flow imbalance, to achieve the required coal flow distribution. The coal flow data is preferably received from a real-time, on-line coal flow measuring system, such as a so-called ECT (Electric Charge Transfer) system. An automated control system advantageously adjusts the vane(s).  
         [0017]     The coal pipe junction preferably comprises two or more outlet openings, one to each of the corresponding downstream coal pipes. When the junction has only two outlet openings, the riffle enclosure advantageously comprises parallel flow channels leading alternately to each of the outlet openings. By disposing a pivotable vane upstream of every second of the partition walls between the flow channels and a fixed partition wall extension between the pivotable vanes, it is possible to close or to reduce the flow to any one of the channels, and thereby to balance the coal flows by redistributing a portion of an unfavorable high coal flow from a channel to its neighboring channel.  
         [0018]     When the riffle comprises N outlet openings, where N is larger than two, a fixed partition wall extension is preferably disposed upstream of every N th  partition wall and an individually pivotable vane is preferably disposed upstream of all other partition walls. By pivoting N-I adjacent vanes, it is then usually possible to balance the coal flow to the N outlet openings. To provide the highest controlling flexibility, it is also possible to dispose an adjustable vane upstream of each of the partition walls, although such a riffle is somewhat more complicated than the ones with fixed wall extensions between the vanes.  
         [0019]     The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic view of a pulverized coal-fired boiler system.  
         [0021]      FIG. 2  is a schematic cross-sectional front view of a coal pipe junction with a riffle according to a first preferred embodiment of the present invention.  
         [0022]      FIGS. 3A and 3B  are schematic cross-sectional side views of a coal pipe junction with a riffle according the preferred embodiments of the present invention.  
         [0023]      FIG. 4  is a schematic cross-sectional front view of a coal pipe junction with a riffle according to another preferred embodiment of the present invention.  
         [0024]      FIG. 5  is a schematic cross-sectional side view of a coal pipe junction with a riffle according to the preferred embodiment depicted in  FIG. 4 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Referring now in greater detail to the drawings,  FIG. 1  schematically shows an overall view of a pulverized coal-fired boiler system  10 . Coal  12  and air  14  are introduced into a pulverizer  16  in which the coal is pulverized into a size suitable for combustion in each of two burners  18 ,  20  disposed on the sidewalls of a boiler  22 . In other types of boilers, the burners  18 ,  20  can be disposed on the corners, roof, and/or arches of the boiler. Pulverized coal and primary air are conveyed along a coal feed system  24  from the pulverizer  16  to the burners  18 ,  20 , where the coal is combusted by primary air and secondary air  26 . Exhaust gases produced in the combustion process are discharged from the boiler through a discharge gas channel  28 .  
         [0026]     The coal feed system  24  comprises, in the simple pulverized coal firing boiler system shown in  FIG. 1 , an upstream pipe  30 , connected to the pulverizer  16 , and two downstream pipes  32  and  34 , connected to the burners  16  and  18 , respectively. The upstream pipe  30  is connected to the downstream pipes  32 ,  34  by a junction  36 , where the initial flow of pulverized coal and primary air is divided among the two downstream pipes  32 ,  34 . In practice, pulverized coal firing boilers often comprise several pulverizers each having one or more outlets, each of which may be connected to two or more burners. Accordingly, a coal feed system of such a more complicated pulverized coal firing boiler system may comprise several coal pipe junctions, which each split coal flow from an inlet pipe, or upstream pipe, carrying coal from an outlet of a pulverizer into two or more outlets pipes, or downstream pipes. For example, a coal pipe junction, in accordance with the present invention, may be connected to two outlet pipes, as the one shown in  FIGS. 1-3B , or to more than two outlet pipes, as the one shown in  FIGS. 4 and 5 .  
         [0027]     In order to have an even temperature distribution in the boiler  22 , as well as high combustion efficiency and low emissions, flows of the primary air and coal to the burners  18 ,  20  should usually be well-balanced. However, especially because coal tends to flow in the coal feed system  24  as concentrated ropes, there is a risk of an uneven distribution of coal into the coal pipes  32 ,  34 , downstream of the junction  36 . On the other hand, in some cases, there is a need to specifically bias the coal distribution, for example, to address emissions or slagging problems. In order to achieve any of these goals, the junction  36  advantageously comprises an adjustable riffle according to the present invention so as to control the coal flow distribution between the pipes  32 ,  34 , as will be discussed below in more detail.  
         [0028]      FIG. 2  schematically depicts a front cross-sectional view of an embodiment of the coal pipe junction  36 , and  FIGS. 3A and 3B  depict side cross-sectional views thereof. As illustrated, the junction  36  has one inlet opening  38  and two outlet openings  40 ,  42 , to which the upstream pipe  30  and the downstream pipes  32 ,  34 , respectively, are to be connected. The coal pipe junction  36  is here shown as having the inlet opening  38  at its lower end and the outlet openings  40 ,  42  at the upper end. However, the orientation of the junction  36  may be different. For example, the inlet opening  38  may be at the upper end of the junction, or on a side. The coal pipe junction  36  of  FIGS. 2,3A  and  3 B also includes a riffle enclosure  44  having vertical side walls  46 , an open, horizontal lower end  48 , and partially-open upper ends  50 ,  52 , which are slanted towards the sides of the enclosure.  
         [0029]     Disposed within the riffle enclosure  44  is a series of vertical partition walls  54 ,  54 ′, which form, together with the side walls  46 , a series of parallel flow channels  56 ,  56 ′. Every second of the flow channels  56 ,  56 ′ has a slanted upper end portion open towards one of the outlet openings  40 ,  42  and a slanted upper end portion closed towards the other of the outlet openings  40 ,  42 , while the rest of the flow channels have the open and closed slanted end portions reversed. Thus, in  FIGS. 3A and 3B , for example, the flow channels a 1 -a 5  are open to the outlet opening  40  and closed to the outlet opening  42 , and the flow channels b 1 -b 5  are open to the outlet opening  42  and closed to the outlet opening  40 .  
         [0030]     In some cases, there is a need to specifically bias the coal distribution, for example, to address emissions or slagging problems. Thus, in the present invention depicted in  FIG. 3A , a vane  58 , which can be pivoted about its rear (downstream) edge by a lever  60 , is disposed adjacent to the lower (upstream) end of every second of the partition walls  54 ,  54 ′. (As shown in  FIG. 3 , each of these every second of the partition walls is referenced by numeral  54 .) The lever  60  can advantageously be closed to a desired position by a locking pin  62 . In a preferred embodiment, each of the vanes  58  is independently pivotable. According to an alternative embodiment, however, more than one of the vanes  58  can be pivoted by connecting shafts  64  connected to the more than one vanes  58 , correspondingly, and an automatic control system  66 . The control system  66  preferably comprises means  68  for monitoring the coal flow in the downstream pipes  32 ,  34 . Based on measurements by such means, for example, based on an observed coal flow imbalance, one or more of the vanes  58  are turned so as to balance the flows. The coal flow monitoring method can be based on the so-called ECT (Electric Charge Transfer) technology, or some other coal flow monitoring method known to persons skilled in the art, for example, based on an optical, microwave or sampling-based method.  
         [0031]     Also in  FIG. 3A , fixed wall extensions  70  are advantageously disposed adjacent to the lower ends of the partition walls  54 ,  54 ′ not associated with a pivotable vane  58 . (As shown in  FIG. 3 , the wall extensions are disposed adjacent to the partition walls referenced by numeral  54 ′.) The extensions preferably extend downwards substantially as far as the vanes  58 . Thus, the vanes  58  can advantageously be used to partially or fully close the inlet to either one of the adjacent flow channels  56 ,  56 ′ by turning the leading edge of the vane towards an adjacent wall extension  70  or side wall  46 . In  FIG. 3A , the vane  58  at the lower end of the partition wall  54  between the channels a 1  and b 1  is pivoted to nearly close the inlet to the channel b 1 .  
         [0032]     If the coal flow to the coal pipe junction  36  forms only one coal rope, the rope may have a width extending mainly to the inlets of only a few of the channels  56 ,  56 ′. When, for example, originally  20 % of the coal impinges on the channel a 1 , 70% on the channel b 1 , and 10% on the channel a 2 , only 30% of the coal is directed to the outlet opening  40 , while the rest, 70%, is directed to the outlet opening  42 . To correct this situation, the vane  58  between the channels a 1  and b 1  is pivoted toward the channel b 1  to partially block channel b 1 . When the vane  58  is pivoted so that the portion of coal flowing to the channel a 1  is increased to 40% and the portion flowing to the channel bI is decreased to 50%, the total coal flow downstream of the junction  36  is balanced. While the balancing is made by tilting one vane  58  only, the distribution of primary air to the outlet pipes  40  and  42  is not significantly changed.  
         [0033]     The vane to be turned for balancing the coal flows can be found by testing them one-by-one, and monitoring the changes of the coal streams. If an on-line coal stream monitoring system is not available, it is in some cases also possible to adjust the flows indirectly by some other parameters, such as the temperatures in the boiler. If the coal is flowing in the upstream pipe as a single rope or, more generally, in a form having only one maximum, it should theoretically always be possible to balance the flows by tilting one vane only. However, in some cases, when the density of the coal flow has more than one maximum point, the optimal balance of coal flows can be obtained by tilting two or more vanes.  
         [0034]     According to the present invention, the vanes  58  are preferably controlled independently. Moreover, it is possible, especially if the riffle comprises a large number of narrow channels, that, for example, the pivoting of two neighboring vanes is connected. However, as a whole, the vanes preferably are independently controlled at least in the sense that the vanes in different portions of the inlet opening  38  of the riffle enclosure  44  can be controlled independently. According to a most preferred embodiment of the present invention, each of the vanes  58  can be controlled fully independently, but in other embodiments, two or more vanes  58  may be pivoted together.  
         [0035]      FIG. 3A  shows the vanes  58  positioned so that only a very small gap  72  is formed between each of the vanes and the lower ends of the adjacent partition walls  54 . However, in some cases, especially when the optimal vane position includes turning of several vanes from the vertical direction, it may be useful to have a larger spacing therebetween. The spacing may preferably have an extent of from about 0.2 to about 3 times the width of the channels, and even more preferably from about 0.5 to about 1 times the width of the channels. Such a spacing does not have a significant effect on the coal flows, but the spacing may help to control the balance of the primary air flow.  
         [0036]     When the vanes  58  are in an inline position, the flow of air and pulverized coal does not cause any severe stress to the vanes  58 . However, when a vane is in a coal flow-diverting position, pulverized coal will impinge on the vane. Therefore, the vanes  58  are preferably made of wear-resistant materials to extend the useable life of the vanes  58 .  
         [0037]      FIGS. 4 and 5  show schematically, as another preferred embodiment of the present invention, a coal pipe junction  74  having one inlet opening  76  and three outlet openings  78 ,  80 ,  82 . The riffle enclosure  84  comprises a series of three different types of parallel flow channels  86 ,  88 ,  90 , which each direct coal flow to one of the outlet openings  78 ,  80 ,  82 . The flow channels a 1 -a 3  direct coal, for example, to outlet opening  78 , channels b 1 -b 3  to outlet opening  80 , and channels c 1 -c 3  to outlet opening  82 . As is shown in  FIG. 5 , a fixed extension  94  is arranged adjacent to the lower end of every third of the partition walls  92 . For example, a fixed extension  94  is arranged adjacent to the partition wall  92  disposed between the channels c 2  and a 3 . Meanwhile, a pivotable vane  96  is arranged adjacent to the lower end of each of the other partition walls. For example, a vane  96  is arranged adjacent to the partition wall  92  disposed between the channels a 3  and b 3 .  
         [0038]     Balancing the coal flows to the pipes connected to the three outlet openings  78 ,  80  and  82  of the coal pipe junction  74  by pivoting the vanes  96  is somewhat more complicated than in the case of the junction with two outlet openings discussed above with reference to  FIGS. 2 and 3 A. However, it is in principle always possible to turn two adjacent vanes  96  to distribute a coal flow having one concentration maximum evenly to all of the outlet pipes, without having a significant effect on the balance of the primary air flows. If, for example, originally 20% of coal impinges on the channel b 1 , 70% on the channel c 1  and  10 % on the channel a 2 , it is possible to tilt the vane  96 ′, arranged between the channels b 1  and c 1  toward the fixed wall extension  94 ′ arranged between the channels cl and a 2 , so that the inlet to channel c 1  is more than half closed, and then tilt the vane  96 ″ arranged between the channels a 1  and b 1  in the same direction (i.e., toward the fixed wall extension  94 ′), so as to direct a portion of the flow now directed to channel b 1  to channel a 1 . Thereby, it is possible to divide the coal stream equally among the outlet pipes  78 ,  80 ,  82 , and the primary air flow maintains sufficient balance.  
         [0039]     If is naturally also possible to make a junction with an adjustable riffle which directs coal flow to more than three outlet openings. In an adjustable riffle according to the present invention, it is possible to have independently adjustable vanes adjacent to only some of the partition walls, as in the embodiments shown in  FIGS. 2, 3A ,  4 , and  5 , or to have such vanes adjacent to each of the partition walls, as shown in  FIG. 3B . The larger number of independently adjustable vanes renders possible a more flexible adjustment of the coal flows, but it also increases the costs of the device and makes the determination of optimal adjustment more complicated.  
         [0040]     In  FIGS. 2-5 , the adjustable vanes  58 ,  96  are mounted so that they can be pivoted around their rear, i.e., downstream, edges. However, it is also possible in some embodiments of the present invention to mount the vanes pivotable about their leading, i.e., upstream, edges. This alternative, which can be especially useful if all the partition walls are equipped with an adjustable vane, may make the coal flow control somewhat more complicated, but may lead to very good balancing of the flows of both coal and primary air.  
         [0041]     While the invention has been described by way of examples of what are at present considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of the features and applications included within the scope of the invention, as defined in the appended claims.