Abstract:
An improved apparatus for on-line coal flow control in vertical spindle mills comprising a plurality of independently adjustable flow control elements and positioning rods that adjust the positioning of those flow control elements. Each flow control element is positioned within the discharge turret of the vertical spindle mill along the outer wall of the discharge turret proximate the entrance to its corresponding coal outlet pipe. The adjustable rods are seated on the side or top of the discharge turret of the coal pulverizer and are connected to the flow control element horizontally or vertically as the case may be. The flow control elements can be independently rotated by +/−90 degrees about the positioning rod axis, moved back and forth in the horizontal plane, and can also be moved up and down in the vertical plane. Therefore, each flow control element has three degrees-of-freedom: one rotational and two linear displacements. The apparatus improves boiler performance by making it possible to operate the boiler with reduced pollutant levels (e.g. NOx, CO) and increased combustion efficiency. Automated computer control of the control surfaces is contemplated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 11,385,016 filed Mar. 20, 2006 now U.S. Pat. No. 7,549,382, which was a continuation in part of U.S. patent application Ser. No. 10/936,401 filed Sep. 8, 2004 now U.S. Pat. No. 7,013,815, which was a continuation-in-part of U.S. patent application Ser. No. 10/258,630 (now U.S. Pat. No. 6,789,488), filed Oct. 24, 2002, which is from International PCT Application PCT/US01/12842 filed Apr. 20, 2001, corresponding to U.S. Patent Application Ser. No. 60/199,300, filed 24 Apr. 2000 and Ser. No. 60/265,206, filed: 1 Feb. 2001, which are each incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to pulverized coal boilers and, more particularly, to a mechanism for directing coal flow to the corresponding outlet pipes of the vertical spindle mill with negligible effect on the pre-existing primary air flow distribution, the mechanism comprising an array of individually adjustable flow control elements positioned inside the discharge turret of the vertical spindle mill. 
     2. Description of the Background 
     Coal fired boilers utilize pulverizers to grind coal to a desired fineness so that it may be used as fuel for burners. In a typical large pulverized coal boiler, coal particulate and primary air flow from the pulverizers to the burners through a network of fuel lines that are referred to as coal pipes. Typically, raw coal is fed through a central coal inlet at the top of the pulverizer and falls by gravity to the grinding area at the base of the mill. Once ground (different types of pulverizers use different grinding methods), the pulverized coal is transported upwards, using air as the transport medium. The pulverized coal passes through classifier vanes within the pulverizer. These classifier vanes may vary in structure, but are intended to establish a swirling flow within the rejects cone to prevent coarse coal particles from flowing into the discharge turret of the pulverizer. The centrifugal force field set up in the rejects cone forces the coarse coal particles to drop back down onto the grinding surface until the desired fineness is met. Once the coal is ground finely enough, it is discharged and distributed among multiple pulverized coal outlet pipes and into respective fuel conduits where it is carried to the burners. Each coal pulverizer is an independent system and delivers fuel (pulverized coal) to a group of burners. 
     In a conventional coal pulverizer  100  as shown in  FIG. 1  (A &amp; B), raw coal  101  is dropped into coal inlet port  102  and by force of gravity falls through coal chute  103  until it reaches the grinding mechanism  104 . The grinding mechanism  104  grinds the coal into fine pieces. Air  105  flows into air inlet port  106  through a nozzle ring on the outside perimeter of the grinding mechanism  104 , feeding primary air into the pulverizer. This creates a stream of low-velocity air that carries the particles of pulverized coal upward where they enter classifier vanes  109  that establish a swirling flow within a reject cone  120 . The centrifugal force field set up in the reject cone  120  prevents coarse pieces of coal  110  from entering the discharge turret  108 . The coarse pieces of coal  110  fall by force of gravity back into the grinding mechanism  104 . Once the pulverized coal  107  enters the discharge turret  108  it is distributed between the multiple equal diameter pulverized coal outlet pipes  111  ( FIG. 1  shows six pulverized coal outlet pipes  111  at the top). The pulverized coal  107  is then carried by connected fuel conduits to a boiler where it is burned as fuel. 
       FIG. 2  is a simplified cross-section of the vertical spindle pulverizer as in  FIGS. 1A &amp; 1B  with four outlet pipes, and  FIG. 3  is a top view of  FIG. 2 . Poor balance of pulverized coal  107  distribution between pulverized coal outlet pipes  111  is commonly experienced in utility boilers. This can be due to various reasons, such as system resistance of each individual fuel conduit, physical differences inside the pulverizer, and coal fineness. The unbalanced distribution of coal among the pulverized coal outlet pipes adversely affects the unit performance and leads to decreased combustion efficiency, increased unburned carbon in fly ash, increased potential for fuel line plugging and burner damage, increased potential for furnace slagging, and non-uniform heat release within the combustion chamber. In addition, it is critical for low NOx (Nitric Oxides) firing systems to precisely control air-to-fuel ratios in the burner zones to achieve minimum production of NOx. The relative distribution of coal between the pulverized coal outlet pipes is monitored by either measuring the pulverized coal flow at the individual pulverized coal outlet pipes or measuring the particular flame characteristics of burning fuel discharged from the each of the burners. 
     The distribution of primary air throughout the coal piping network is controlled by the flow resistances of the various coal pipes. Because of differences in pipe lengths and numbers and types of elbows in each fuel line, the different coal pipes from a pulverizer will usually have different flow resistances. It is known that fixed or adjustable vanes may be used to directly divert the coal flow distribution among the outlet pipes  111 . The following references describe the use of vanes to modify coal flow distribution. 
     U.S. Pat. No. 4,570,549 to N. Trozzi shows a Splitter for Use with a Coal-Fired Furnace Utilizing a Low Load Burner. 
     U.S. Pat. No. 4,478,157 to R. Musto shows a Mill Recirculation System. 
     U.S. Pat. No. 4,412,496 to N. Trozzi shows a Combustion System and Method for a Coal-Fired Furnace Utilizing a Low Load Coal Burner. 
     Finally, U.S. Pat. No. 2,975,001 issued on Mar. 14, 1961 to Davis discloses an apparatus for dividing a main stream of pulverized coal between two branch streams. (Col. 1, lines 50-52). The apparatus may be used alone or in conjunction with a conventional slotted riffle. (Col. 1, lines 70-73). The apparatus is comprised of a combination fixed and tiltable nozzle. (Col. 1, lines 50-58). The fixed nozzle is attached to the main duct leaving the pulverizer and concentrates the coal and air flow (see claims 1-5). The concentrated coal and air flow is then directed into the tiltable nozzle with the highest concentration of coal necessarily being at the nozzle centerline. The tiltable nozzle is then “tilted” in order to direct the concentrated coal and air flow into one or the other branch stream. Guide vanes may be mounted inside the tiltable nozzle; however, this patent does not disclose adjustable guide vanes. (Col. 1, lines 58-60). 
     All of the foregoing references teach a form of direct diversion of the coal flow, but this likewise causes direct diversion of the air flow. It is impossible using direct diversion to increase or decrease the flow of coal into a particular outlet pipe without effecting primary air flow, or vice versa. 
     In contrast to an adjustable baffle approach which makes it difficult to simultaneously balance coal and primary air flow rates, the present invention makes it possible to increase or decrease the coal flow in any one of the above-described outlet pipes  111  without affecting the pre-existing air flow distribution among the outlet pipes by changing the position and/or orientation of the control vane in the region of high particle concentration. This unique approach makes it possible to balance the coal flow distribution among the outlet pipes, while eliminating the need to readjust the air flow distribution among the outlet pipes after achieving the desired coal flow rate distribution. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a main object of the present invention to provide an improved apparatus for on-line coal flow control in vertical spindle mills and, specifically, for the on-line balancing and control of pulverized coal flow into the multiple pulverized coal outlet pipes of pressurized vertical spindle mills. 
     It is another object to eliminate coal flow imbalances at crucial points in a pulverized coal boiler system using an on-line adjustment capability that does not disturb any pre-existing primary air flow balance among the multiple coal pipes, thereby reducing pollutant emissions and improving combustion efficiency. 
     It is another object to simplify the coal flow balancing process and eliminate the need of adjustments to the primary air flows between the outlet pipes after achieving the desired coal flow rates between the coal pipes. 
     It is still another object to maintain a balanced coal flow distribution among the pulverized coal outlet pipes despite mill load changes, eliminating or automating the need for re-adjusting the flow control element positions as the mill coal loading changes. 
     It is still another object to provide an improved apparatus for on-line coal flow control in vertical spindle mills that can readily be installed within an existing pressurized vertical spindle pulverizer (within the discharge turret). 
     It is still another object to provide an improved apparatus for on-line coal flow control in vertical spindle mills that contributes no significant pressure drop to the flow system. 
     In accordance with the present invention, an improved apparatus for on-line coal flow control in vertical spindle mills is described which comprises a plurality of independently adjustable flow control elements and a means for adjusting the positioning and/or orientation of those flow control elements. Each flow control element is positioned within the discharge turret of the pulverizer at some appropriate vertical distance from the entrance to the coal outlet pipes. Each flow control element includes an independently adjustable rod seated on the side of the discharge turret of the coal pulverizer and connected to the flow control element horizontally or, alternately, seated on the top of the discharge turret and connected to the flow control element vertically. The flow control elements can be independently rotated by +/−90 degrees about the a horizontal radial axis with respect to the turret, and can also be moved back and forth in the horizontal plane as well as up and down in the vertical plane. Therefore, each flow control element has three degrees-of-freedom: one rotational and two linear displacements. A combination of rotational and linear movements is used to control the coal flows in each pulverized coal outlet pipe, and the flow control elements have neutral positions at which the pre-existing coal and primary air flow distributions between the pulverized coal outlet pipes are undisturbed. 
     The foregoing apparatus provides on-line balancing and control of pulverized coal flows into the multiple pulverized coal outlet pipes of a pulverizer, thereby improving boiler performance by making it possible to operate the boiler with reduced pollutant levels (e.g. NOx, CO) and increased combustion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof when taken together with the accompanying drawings in which: 
         FIG. 1  is a prior art vertical spindle mill,  FIG. 1A  showing a cut-away view and  FIG. 1B  a cross-section. 
         FIG. 2  is a simplified cross-section of the prior art vertical spindle mill as in  FIGS. 1A &amp; 1B . 
         FIG. 3  is a top view of the prior art vertical spindle mill as in  FIGS. 1-2 . 
         FIG. 4  depicts computational fluid dynamics (CFD) simulation results for the particulate concentration distribution in a vertical spindle mill with contour legend shown at left. 
         FIG. 5  depicts CFD simulation results for the velocity vector field of the air flow with velocity vector legend shown at left. 
         FIG. 6  is a side section view (at A) and top view (B) illustrating an array of individually adjustable flow control elements  200  (one being shown at A) positioned inside the funnel-shaped discharge turret  108  of a vertical spindle mill. 
         FIG. 7  is a side section view (at A), top view (at B) and orthogonal side section view illustrating flow control element  200  utilizing a turret top mounting seat. 
         FIG. 8  is a partial cutaway perspective of view of a positioning rod having an internal pinion gear for controlling vane orientation. 
         FIG. 9  is a side view illustrating the shape and relative dimensions the presently-preferred flow control element  200  with adjustment rod  210 . 
         FIG. 10  is a front view of the flow control element  200  with adjustment rod  210  as in  FIG. 9 . 
         FIG. 11  illustrates the percentage of pulverized coal flow imbalance between the outlet pipes with and without the flow control elements  200 . 
         FIG. 12  is a comparative graph showing the effect on primary air flow distribution both with and without the flow control elements  200 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is imperative for good combustion that any flow control mechanism incorporated in a vertical spindle mill as described above have little or no effect on the distribution of primary air. However, most coal boilers use baffles or orifice-type flow restrictors in individual pipes which have precisely this direct effect. Specifically (and referring back to  FIG. 2 ), the air and coal particle flow structures within the discharge turret  108  determine the coal and air flow distributions between the pulverized coal outlet pipes  111 . The present inventors have undertaken computational fluid dynamics (CFD) simulations to understand the coal and air flow structures within the discharge turret  108  of such a vertical spindle mill. 
       FIG. 4  depicts CFD results for the coal flow concentration distribution within the vertical spindle mill with particle concentration mapped and indexed at left. The CFD simulation results showed a complex, 3-dimensional flow with very high radial and tangential velocity components of the air and particle flows within the discharge turret  108 . The coal and air mixture makes several turns before it reaches the inlet of the outlet pipes  111 . The flow mixture first makes a U-turn in the z-axis plane as it gains tangential velocity while going through classifier vanes  109  in the horizontal plane. Immediately before the discharge turret  108  inlet, the mixture makes another U-turn in the z-axis plane just before it enters the discharge turret  108 . Immediately after particles enter the funnel-shaped discharge turret  108 , they are forced toward the outer wall by the tangential and radial velocity components of the air flow. In a very short axial distance in the discharge turret  108  the majority of the particles accumulate in the vicinity of the discharge turret  108  outer wall. The drag force in the radial direction due to the flow expansion and the centrifugal force created by the tangential velocity within the discharge turret  108  are the major parameters that determine the particle trajectories and consequently the particle flow distribution between the outlet pipes. 
       FIG. 5  depicts CFD results for the velocity vector field of the air flow. Similar to the coal flow, stratification in air flow is also observed as the air flow makes U-turns. A gradually decreasing air velocity profile from the inner to the outer wall of the discharge turret  108  is established at the inlet plane of the discharge turret  108 . Phase segregation within the discharge turret  108  is initiated at the entrance of the discharge turret  108  and propagates as the mixture advances in the axial direction. 
     The flow in the pulverized coal outlet pipes  111  is categorized as dilute phase pneumatic conveyance in which air and micron size particles flow together. The density of the coal particles is almost 1,400 times higher than that of the air. The particulate and air flows show significant differences when they flow together in a pipe due to this enormous density difference. The air flow can quickly respond to the geometrical changes in the pipe layout while it takes longer times for particles. 
     The present invention relies on the fact that a phase separation between air and coal flows occurs within the discharge turret as shown in the CFD simulation results ( FIGS. 4 ,  5 ). Highly concentrated particle flow and high primary air velocity regions are established in the outer and inner walls of the discharge turret  108 , respectively. This separation in the flow is due to the drag force in the radial direction caused by the flow expansion and the centrifugal force created by the tangential velocity within the discharge turret  108 , which is a generally funnel-shaped conduit. In accordance with the present invention, individually-adjustable flow control elements are positioned in the region where highly concentrated particle flow exists proximate the discharge turret  108  outer wall. This allows control of the coal flow distribution ( FIG. 4 ) without affecting the distribution of primary air ( FIG. 5 ). 
       FIG. 6  is a side section view (at A) and top view (B) illustrating one embodiment of the present invention comprising an array of individually adjustable flow control elements  200  (one exemplary one being shown at A) positioned inside the funnel-shaped discharge turret  108  of a vertical spindle mill. It should be noted that while the depicted embodiment implies a one to one relationship between flow control elements  200  and coal outlet pipes  111 , no such correlation is required and optimized coal flow balance may be achieved with a greater or lesser number of flow control elements as compared to outlet pipes. As will be described, the geometry, position and orientation of the flow control elements  200  are optimized in such a way that the coal flow rate adjustments between the outlet pipes  111  has negligible effect on the pre-existing primary air flow distribution in the pulverized coal outlet pipes  111 . 
     Each individual flow control element  200  is adjustably mounted for independent linear positioning up and down along the walls of the discharge turret  108 , radially in and out from the walls of the discharge turret  108 , and rotationally. In the illustrated embodiment this is accomplished by mounting each individual flow control element  200  on an articulated positioning rod  210  which is pivotally and slidably retained in a rod seat  214  inside the wall of the discharge turret  108 . The rods  210  pass through an aperture in the wall of the turret and are rigidly affixed to the corresponding flow control element  200 . Each independently adjustable positioning rod  210  is retained in a substantially horizontal position in the rod seat  214  which may be one or more sealed bushings or bearings. The rod seat permits the rod to slide horizontally in and out of the turret wall in a radial direction relative to the vertical axis of the turret in order to permit radial adjustment of the flow control element  200  position. Once adjusted to the desired horizontal (radial) position the rod may be locked in place. The rod seat  214  further permits rotation of the positioning rod  210  about its primary axis by +/−90 degrees thereby adjusting the orientation of the rigidly attached flow control element within the coal flow stream of the turret. Once rotationally adjusted to the desired orientation the rod may also be locked in place. Locking of the rod  210  horizontal (radial) position is independent of rotational movement/locking of the rod  210 . 
     Rod Seat  214  is further slidably retained to the wall of the turret so as to be slidable in a vertical (up and down) plane. Sliding of the rod seat  214  is independent of and does not affect the rotational orientation of the positioning rod  210  within the seat, but may affect the radial position of the flow control element  200  within the wall of the turret inasmuch as the walls of the turret  108  may be inclined (funnel shaped) as shown. Sliding of the rod seat  214  in a vertical (up and down) plane may be accomplished as shown by journaling the rod seat  214  bushing or bearing into a linear motion guide track  218  for slidable translation there along, the track  218  is in turn being mounted along an outside surface of the outer wall of the turret  108 . Lateral translation of the rod seat  214  vertically in the track  218  necessarily translates the attached positioning rod  210  and flow control element  200  in the vertical thereby adjusting its position upstream relative to the inlets of the outlet pipes. Once positioned vertically as desired the rod seat  214  is preferably locked in position, and this locking of the rod seat  214  position is independent of movement/locking of the rod  210  in any other degree of freedom. 
     The aperture through which the positioning rod enters the turret wall  108  may be appropriately elongated in a slot configuration to accommodate vertical movement due to sliding of the rod seat  214 . An overlapping gasket or other suitable means of sealing portions of the slot not occupied by the position rod  210  may be used to prevent pressure loss in the turret or dust expulsion at the aperture. In an alternate embodiment (not pictured) the aperture may be eliminated by mounting the track  218 , rod seat  214  and rod  200  strictly on an inside surface of the outer wall of the turret  108 . However, inside mounting of the rod seat  214  sacrifices the independent radial and vertical translation of the flow control element  200  in favor of correlated lateral and vertical translation of the flow control element  200  as the rod seat  214  is moved up or down the sloped outer wall of the turret  108 . 
     Movement of the rod seat  214  and/or positioning rod  210  is accomplished by a positioning actuator  240  which may be any suitable positioning actuator providing precision 2-axis translation and 1-axis rotation adjustment for independent linear positioning of the rod  210  and rod seat  214  up and down along the walls of the discharge turret  108 , radially in and out from the walls of the discharge turret  108 , and rotationally. Positioning actuator  240  may be a combination of a track positioner for positioning of the rod  210  and rod seat  214  up and down along the track  218 , a linear actuator for pushing/pulling the rod  210  radially in and out from the walls of the discharge turret  108 , and a rotary actuator for rotating the rod  210 . Positioning actuator  240  may include one or a combination of hydraulic actuators, hydraulic motors, electric motors, or manual adjustment knobs, or other means capable of opposing the forces applied to the flow control elements by the coal, and to a lesser extent the air, moving through the turret. 
     Coal mass flow sensors  252  and air flow sensors  254  may be placed within individual coal pipes to monitor coal distribution and air flow, respectively, and to automatically and individually adjust the positions of the flow control elements  200  to maintain the desired distribution between the various outlet pipes  111 . In this case the positioning actuators  240  slave to a control device  260  which implements automatic control and positioning logic. The control device  260  may be tied to, or part of, the vertical spindle mill central control system. This control device  260  may comprise a suitable programmable logic controller (PLC), a distributed control system (DCS), a central computer, a series of interconnected discrete control components, or any combination thereof. 
     One skilled in the art should recognize that downstream conditions may further comprise or incorporate monitoring of burner and/or exhaust gas performance and conditions (such as temperature, NOx emissions, CO emissions, and exhaust particulate content) in order to optimize coal distribution to the burners. Monitoring of downstream conditions by any of a variety of sensors and corresponding automatic adjustment of the coal flow control elements  200  may be accomplished using control device  260 . The control device  260  receives sensor monitoring information as input from the downstream sensors  252 ,  254  or others, and determines the optimum position of the flow control elements  200  in real time. The control device  260  then actuates the positioning actuator  240  to move the flow control elements  200  into the position necessary to achieve the determined optimum conditions. 
     As illustrated, the presently-preferred shape of the flow control elements  200  is a substantially flat plate having an oblique trapezoidal shape, the oblique angle conforming to the slope of the discharge turret outer wall  108 . The upper-outer edge of each flow control element  200  is truncated (such as rounded) to allow at least +/−90 degree rotation without obstruction when fully retracted against the discharge turret  108  outer wall. The flow control element  200  position is considered to be 0 degrees when it is positioned vertically (inline parallel to the outlet pipes  111 ). 
       FIG. 6  illustrates the flow control elements  200  in their +/−90 degree position (substantially horizontal). 
     With reference to  FIGS. 7A ,  7 B and  7 C, an alternate embodiment of the present invention is disclosed in which the rod seat  1214  is position at the top of the turret (best seen in  FIG. 7A  or  7 C. As above, positioning rod  1210  is slidably retained in the rod seat and affixed to the flow control element  1200  via an aperture in the turret wall (top). Sliding of the positioning rod  1200  into/out of the seat adjusts the vertical positioning of the flow control element within the turret and thereby adjusts the upstream position of the flow control element with respect to the outlet pipe  111 . The rod seat  1210  is further slidably affixed to the top of the turret so as to be slidable radially in the horizontal plane thereby adjusting the horizontal position of the flow control element  1200  radially within the turret and with respect to the outlet pipe  111 . 
     Rotation of the flow control element  1200  with respect to the horizontal radial axis of the turret may be accomplished by an electronically controlled stepper motor or hydraulic motor within the flow control element  1200 . In an alternate embodiment, opposing parallel thrust arms  99  may be inserted into the positioning rod  1210  which is hollow in this embodiment, as depicted in  FIG. 8 . The thrust arms are provided with opposing racks of teeth with a captured pinion gear  98  between them. Hydraulic actuators at the rod seat drive the thrust arms  99  in opposing directions thereby rotating the pinion  98  which is affixed at its center to the flow control element  1200  causing it to rotate and assume the desired position. 
     The preferred shape, size, and geometrical details of the flow control elements  200  (and  1200 ) as well as the preferred distance from the entrance to the pulverized coal outlet pipes  111  to the flow control elements  200  were quantitatively determined by laboratory tests using a laboratory scale vertical spindle mill type pulverizer having four outlet pipes  111  and configured with four flow control elements  200 . During the experiments both the distribution of pulverized coal into the individual pulverized coal outlet pipes and primary air flow were monitored. The results indicated that the positioning the flow control elements  200  within the discharge turret  108  upstream of the entrance to the pulverized coal outlet pipes  111  provides the most efficient method for controlling the distribution of pulverized coal flows among the outlet pipes while having a negligible effect on air flow distribution. 
       FIG. 9  is a side view illustrating the shape and relative dimensions the presently-preferred flow control element  200  with adjustment rod  210 , and  FIG. 10  is a front view. As stated above, the presently-preferred flow-control element  200  is an oblique trapezoid. The top-right corner of the flow control element is rounded to make the flow control element fit inside the discharge turret  108 . Of course, other flow control element  200  shapes are possible such as contoured instead of flat plate and with shapes other than trapezoidal, including triangular, rectangular, squared and ellipsoid shapes. The flow control elements  200  are positioned in the region where highly concentrated particle flow exists at the discharge turret  108  outer wall. 
     In all cases the shape, size, and distance of the flow control elements from the outlet pipes (both laterally and upstream) may be predetermined by testing and cataloging the results for a particular pulverizer in light of the different dimensions and internal configuration of the particular pulverizer. Test results confirm the effectiveness of the present invention in controlling the coal flow distribution, without affecting the pre-existing air flow distribution. 
       FIG. 11  illustrates the percentage of pulverized coal flow imbalance between the outlet pipes with and without the flow control elements. A number of trials were completed to balance the coal flows between the pulverized coal outlet pipes by adjusting the flow control elements  200  individually. 
       FIG. 12  is a comparative graph of the results of the laboratory experiments showing the effect on primary air flow distribution when the pulverizer was configured both with and without the flow control elements  200 . During the coal flow balancing process, the maximum primary air flow imbalance was within +/−4.0 percent (trial #1). For the case where there was no flow control element installed, the imbalance in the primary air flow between the pulverized coal outlet pipes was less than +/−3.0 percent (trial #0). There was no measurable effect of coal flow balancing on the primary air flow distributions between the coal outlet pipes  111  (trial #6). 
     With combined reference to  FIGS. 11 and 12 , more than twenty percent change in coal flow rate was achieved with the flow control elements  200  ( FIG. 11 ) while the maximum change in the primary air flow was less than 5 percent ( FIG. 12 ). 
     Laboratory experiments were also performed to investigate the effect of coal flow loading on the effectiveness of the present invention. The experiments were performed for a coal flow loading range of +/−30 percent at a constant primary air flow rate. Coal flow loading variations within +/−30 percent were found to have a negligible effect on the existing coal and primary air flow distributions once the coal flow rates between the pulverized coal outlet pipes were balanced. The coal and the primary air flow imbalances between the outlet pipes remained within +/−5.0 percent. This is a very useful feature of the present invention since it eliminates the need for re-adjusting the flow control element positions as the mill coal loading changes. In addition, no noticeable increase in pressure drop due to the flow control elements and their adjustments was measured during the experiments. 
     It is also noteworthy that in some vertical spindle mills, there are two, three, or more outlet streams. It should be understood that the present invention encompasses system configurations in addition to those described above (for 2 or more outlet pipes  111 ). 
     Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth in the appended claims.