Abstract:
A biomass energy system utilizes an automated biomass distribution system for evenly distributing biomass within a furnace of the biomass energy system. The even distribution of biomass dramatically increases efficiency of the biomass energy system. The automated biomass distribution system includes a control unit, a set of UP control boxes, and a set of valve assemblies. Each valve assembly includes a pneumatic actuator, a plug and a discharge duct matching the shape of the plug.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit and priority of U.S. patent application Ser. No. 61/931,873, entitled “AUTOMATED BIOMASS AIR SWEEPING SYSTEM,” filed Jan. 27, 2014, assigned to Valvexport, Inc. of Miami, Fla., and which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure generally relates to an energy production system. More particularly, the present disclosure relates to a biomass stoker boiler. More particularly still, the present disclosure relates to a biomass air spreading system for evenly distributing biomass over a stoker boiler furnace grate. 
       DESCRIPTION OF BACKGROUND 
       [0003]    Biomass is biological material, such as plants or plant-derived materials. Biomass is a renewable energy source when burned to produce heat, or converted to various forms of bio-fuel. The thermal method to generate energy or electricity from biomass usually involves a stoker boiler with a furnace for burning the biomass that is fed into it. For many years, since the first biomass boilers where designed and manufactured, biomass was seen as a waste material that needed to be incinerated. During the last 20 years, with the escalating cost of fuels used to generate electricity, a new vision of biomass as a renewable fuel is changing the design conception of these boilers. Higher thermal efficiencies with lower particulate emissions are driving many boiler design changes. Controlled biomass deposition on the furnace grate using improved air spreading systems is one of the major goals encountered in the new designs. Trying to avoid biomass piling on the grate, many boilers are operated with excess air as well as high carryover of unburned particulate. 
         [0004]    Some studies on sugar cane bagasse fired boilers have found that maintaining a uniform thin bed of bagasse, between 1″ and 3″ inches (25 to 75 mm) deep, over the complete area of the grate, assures a continuously burning grate bed which rapidly dries and heats the bagasse fibers in suspension, acting as pilot flames for the incoming fuel stream. When the bed is partially uncovered or has very thin beds, less than 1″ inch deep (about 25 mm), the ignition zone, immediately above contains an unstable and highly fluctuating flame of low luminosity that induces combustion cycling which becomes evident with furnace puffing or cycled pressurization. When the bagasse accumulates in piles above 6″ (meaning six inches) deep, it reduces the grate heat release. Accordingly, optimizing partial biomass distribution on the grate, while burning the rest in suspension, with minimum excess air, is ideal for stable combustion and efficient steam generation. 
         [0005]      FIG. 1  depicts a prior art biomass spreading system  120 , coupled to a furnace  104  of a typical boiler  100  including a grate  106 . The grate  106  can be fixed or travelling in a horizontal or inclined fashion. The grate  106  illustrated in  FIG. 1  is a horizontal stationary pinhole grate. Various Biomass distributors  108  are attached to a front wall  109  of the furnace  104 . Through the biomass distributors  108 , biomass material  132  is fed into the furnace  104 . Under grate air  142  is fed into a furnace chamber  133  by a forced fan  135 . Air passes through many small holes on the grate  106  to provide oxygen for burning the biomass material  132 . To distribute the biomass material  132  over the furnace grate  106 , a biomass distribution system  120  is operatively coupled to the biomass distributors  108 . 
         [0006]      FIG. 2  presents a zoomed view of  FIG. 1 , and details of the biomass distribution system  120  that is operatively coupled to the distributor  108 . The biomass material  132  is spread into the furnace  104  by the sweeping action of air passing through a narrowly slotted passage  131  which is a part of the biomass distributor  108 . The air is supplied by the fan  110 . The distribution system  120  includes a main header  122 , which feeds various secondary ducts  123  that in turn feed various valve housings  124 . Each valve housing  124  contains one or two dampers. One of the dampers is a rotary damper  126 , while the other, if it exists, is a manual damper  127 . As air flows from the valve housing inlet  121  through the passages left open by the dampers  126  and  127 , it loses pressure depending on the variable open area of these passages. The valve housing outlet  150  discharges into a header  151  after a 90° (meaning 90 degrees) air flow turn from the valve housing  124 . Another 90° flow turn is required to exit the header  151  and enter a rectangular duct  152  which connects to the distributor  108  with a flange  153 . 
         [0007]    The sudden changes in direction of the air flow as well as the sudden contractions described above create high turbulence and high pressure drops, and thereby reducing the effectiveness of the air jet  130  in sweeping the biomass material  132  into the boiler  100 . An electric motor (not shown) provides rotation to a shaft  125 , common to all the rotary dampers  126 , inside the valve housings  124 . The valve housings  124  feed sweeping air to all the biomass distributors  108  in a stoker boiler. The rotary damper blade  126  of each valve housing  124  is set in a position different from the rest, so that they will create different pressure drops as the blades  126  rotate simultaneously. In other words, when one damper  126  is in the open position, the other dampers  126  are closed to various degrees. Accordingly, each blade  126  is at a different rotation position from the other blades  126 . The manual dampers  127  are set individually, based on the boiler operators&#39; experience, to establish a minimum sweeping flow to help distribute the biomass evenly over the grate  104 . 
         [0008]    When any rotary damper  126  is at the closed position, it partially or substantially blocks the air flow from the secondary duct  123  to the discharge duct  152 . In such a case, the biomass distribution system  120  provides the lowest air pressure in the discharge duct  152 , minimizing the air sweeping action for biomass spreading. After the rotary damper  126  rotates 90° from the closed position, it is in the open position. At the open position, the rotary valve  126  provides the least resistance to the air flow from the secondary duct  123  to the discharge duct  152 . In other words, when the rotary valve  126  is at the open position, the biomass distribution system  120  provides the highest air pressure in the discharge duct  152 , maximizing the air sweeping action for biomass spreading. 
         [0009]    Air flows from the discharge duct  152  into distributor  108  and through the air sweeping nozzle  131 , thereby creating the air jet  130 . The biomass material  132  is fed vertically down into the distributor  108  by a biomass feeder (not shown). The air jet  130  velocity (meaning the velocity of the air jet  130 ) is the result of the air flow contraction as it passes through the air sweeping nozzle  131 , and encounters the biomass material  132  falling through the distributor  108 . The air jet  130  momentum (meaning air mass multiplied by air velocity of the air jet  130 ), created by the air jet  130  passing through the air sweeping nozzle  131 , pushes the biomass  132  into the furnace  104 . When the air pressure in the discharge duct  152  is at the highest point, the air jet momentum is expected to be the highest level and the biomass material  132  moves furthest into the furnace  104 . In such a case, the biomass material  132  falls onto an area of the grate  106  that is close to a back wall  107  (see  FIG. 1 ) of the furnace  104 . In contrast, when the air pressure in the discharge duct  152  is at the lowest level, the biomass material  132  travels a shortest distance into the furnace  104  and falls on the area of the grate  106  that is closest to the front wall  109 . 
         [0010]    Even distribution of the biomass material  132  over the grate  106  is very important for the reasons described above and other reasons described below. For example, an even distribution allows for higher biomass burning capacities as well as higher and more stable heat release rates, which in turn provide higher boiler steam generation at stable pressure and temperature. As an additional example, the thermal efficiency of a biomass stoker boiler is reduced when the biomass covers the grate unevenly, meaning that some areas have a thick bed while other areas have a thin bed. The uneven distribution of biomass  132  on the grate  106  forces the operators to work with more excess air, an unnecessarily high quantity of unburned fibers and incombustibles carried over by the flue gases. 
         [0011]    Accordingly, the prior art biomass distribution system  120  fails to spread the biomass material  132  evenly over the furnace grate  106 . The main reason for the failure is that the system  120  cannot control the momentum variation of the air jet flow  130 , with respect to time or observed biomass bed deposition depth over the grate  106 . Such limitation of the system  120  is caused by a number of reasons. First, the system  120  does not provide a controlled air jet  130  momentum variation with respect to time, because it does not provide a controlled variation of pressure behind the air sweeping nozzle  131  during the damper rotating cycle. Second, the system  120  does not allow for individual adjustment of air pressure to a distributor  108  independently from the other distributors  108 , because the system  120  is operated by a single motor through a common shaft. Third, the system  120  creates high air pressure losses and turbulence that reduce the sweeping effectiveness of the air jet  130 , thereby requiring higher fan pressures and causing higher energy cost and less sweeping control. 
         [0012]      FIG. 3  illustrates a graph depicting the typical air pressure behind the prior art air sweeping nozzle  131  (10 to 20 inches of water column (“inWC”)) during a cycle of ten (10) seconds corresponding to a 90° rotation of the damper  126 . As shown by the graph, during the latter 35% of the cycle (about three and a half seconds), the air pressure behind the sweeping nozzle  131  stays almost constant at 18 inWC. Beyond the first six seconds of the cycle, the air pressure decays almost linearly from 17 to 7 inWC. Accordingly, the graph indicates that most of the biomass  132  is spread towards the rear zone of the furnace grate  106 . In other words, piles of the biomass  132  are formed in the rear zone of the grate  106  and are not burned efficiently. In contrast, the section of the grate  106  near the front wall  109  tends to remain uncovered, thereby lowering heat release rates. In fact most prior art biomass boilers depend on frequent manual spreading of the piled biomass in order to maintain desired steam production levels. The manual spreading is accomplished by opening manhole doors (not shown) located at the front wall  109  and below the distributor  108  openings, manually introducing long spreading rakes, and dragging the piled biomass so as to spread it evenly over the depth and width of the grate  106 . 
         [0013]    To correct the uneven distribution of the biomass material  132  over the grate  106 , operators of the system  120  usually try to throttle the air pressure. However, the reduction in the air pressure fails to solve the problem of uneven distribution of the biomass material  132  over the grate  106 . Rather, the reduction in the air pressure shifts the uneven deposition of the biomass  132  towards the front section of the grate  106 . In addition to the problem of uneven distribution along the depth of furnace grate  106 , there is the problem of uneven distribution across the width of the furnace grate  106  due to variations in feeder discharge. The system  120  does not allow individual adjustments of each air jet  130  to each distributor  108  over the complete cycle, it can only effect de minimis adjustments in air flow passing through the manually adjustable damper  127 . 
         [0014]    Neither does the prior art system  120  allow for individual adjustments to each jet flow  130  in response to higher bagasse density and/or friction as it moves through the distributor  108 . Higher bagasse density is caused by, for example, higher moisture content. Another disadvantage of the prior art system  120  is that it creates very high turbulence and pressure losses for numerous reasons, such as inefficient flow throttling through single blade butterfly dampers, sudden changes in direction and flow contractions as air flows through the valve housing  124  and into the lateral exit port  150 , and sudden change in flow direction as air flows out of the header  151  into the lateral rectangular duct  152 . The air flow is highly irregular and thus creates high turbulence when it exits the duct  152 . The momentum of air jet  130  is thus reduced. In other words, the current state of the art distribution system  120  fails to provide even biomass distribution. Such shortcomings of the prior art system become even worse when there is higher moisture content or uneven biomass feeding from one feeder to another. Furthermore, the system  120  consumes more fan power than necessary. 
         [0015]    Accordingly, there is a need for a new biomass distribution system that evenly distributes biomass over a grate surface. 
       OBJECTS OF THE DISCLOSED SYSTEM, METHOD, AND APPARATUS 
       [0016]    Accordingly, it is an object of this disclosure to provide an improved biomass air spreading system for use with stoker boilers. 
         [0017]    Another object of this disclosure is to provide an improved biomass air spreading system for evenly distributing biomass over the width and depth of a stoker boiler grate surface. 
         [0018]    Another object of this disclosure is to provide an improved biomass air spreading system requiring lower energy consumption for fan operation. 
         [0019]    Another object of this disclosure is to provide an improved biomass air spreading system utilizing multiple high efficiency air valve assemblies. 
         [0020]    Another object of this disclosure is to provide an improved biomass air spreading system utilizing multiple high efficiency valve assemblies, each one of which includes an actuator and an actuator control box. 
         [0021]    Another object of this disclosure is to provide a programmable automated biomass air spreading system for use with stoker boilers. 
         [0022]    Another object of this disclosure is to provide an improved biomass air spreading system which can be tuned online through a computer interface, in such a way as to maintain, at all times, an optimum biomass distribution on the furnace grate. 
         [0023]    Other advantages of this disclosure will be clear to a person of ordinary skill in the art. It should be understood, however, that a system or method could practice the disclosure while not achieving all of the enumerated advantages, and that the protected disclosure is defined by the claims. 
       SUMMARY OF THE DISCLOSURE 
       [0024]    Generally speaking, pursuant to the various embodiments, the present disclosure provides a programmable and automated biomass air spreading system for multiple distributors in a stoker boiler. In accordance with the present teachings. The air spreading system includes a central control unit which holds various operational programs. These programs can be modified during boiler operation. The central control unit delivers preprogrammed pneumatic signals to actuators, operatively coupled to a set of high efficiency valve assemblies, which in turn are coupled to a set of biomass distributors on the boiler. 
         [0025]    Further in accordance with the present teachings is a biomass distribution system that includes a central control unit adapted to generate a set of control signals, and a set of converters connected to the central control unit. Each converter within the set of converters is adapted to receive a subset of control signals of the set of control signals and convert the received subset of control signals into a set of air pressure signals. The system also includes a set of actuators connected to the set of converters respectively. Each actuator within the set of actuators receives the set of air pressure signals from a corresponding converter within the set of converters. In addition, the system includes a set of valve plugs operatively coupled to the set of actuators through a set of spindles respectively. Each valve plug within the set of valve plugs is actuated by a corresponding actuator within the set of actuators through a spindle within the set of spindles in response to each air pressure signal within the set of air pressure signals. The system further includes a set of discharge ducts operatively coupled to a set of biomass distributors. The set of biomass distributors are attached to a furnace of a boiler stoker and adapted to receive biomass. The furnace includes a grate for burning the biomass. Each discharge duct within the set of discharge ducts receives a portion of a corresponding valve plug within the set of valve plugs to form a throttling passage to regulate airflow moving into a corresponding biomass distributor through the throttling passage. The airflow moves biomass over the grate. A nozzle pressure of the airflow corresponds to an air pressure signal within the set of air pressure signals. The airflow is provided by an air supplier through a main duct. 
         [0026]    Further in accordance with the present teachings is a method for regulating airflow provided to a furnace of a boiler stoker. The method includes a central control unit generating a set of control signals, and each valve plug within the set of valve plugs is partially received by a corresponding discharge duct that is operatively coupled to a corresponding biomass distributor. In addition, the method includes each converter within a set of converters converting the subset of control signals into a set of air pressure signals, and each actuator within the set of actuators receiving the set of air pressure signals from a corresponding converter within the set of converters. The method further includes, based on the set of air pressure signals, each actuator within the set of actuators actuating a corresponding valve plug within a set of valve plugs. The set of actuators is operatively coupled to the set of valve plugs through a set of spindles respectively. Each valve plug within the set of valve plugs is partially received by a corresponding discharge duct that is operatively coupled to a corresponding biomass distributor. Each biomass distributor is attached to a furnace of a boiler stoker. Each discharge duct and a corresponding valve plug within the set of valve plugs form a throttling passage to regulate airflow moving into a corresponding biomass distributor through the throttling passage. The airflow moves biomass over a grate inside the furnace. A nozzle pressure of the airflow corresponds to an air pressure signal within the set of air pressure signals. The airflow is provided by an air supplier through a main duct. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Although the characteristic features of this disclosure will be particularly pointed out in the claims, the invention itself, and the manner in which it may be made and used, may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part hereof, wherein like reference numerals refer to like parts throughout the several views and in which: 
           [0028]      FIG. 1  is a system diagram depicting a prior art biomass boiler spreading system. 
           [0029]      FIG. 2  is a zoomed view of a prior art biomass boiler spreading system. 
           [0030]      FIG. 3  illustrates a graph depicting the relationship between static pressure behind the air sweeping nozzle versus cycle time in a prior art sweeping system. 
           [0031]      FIG. 4  illustrates a system diagram depicting a boiler furnace with a biomass spreading system in accordance with this disclosure. 
           [0032]      FIG. 5  is a cross sectional view of a high efficiency valve assembly, with its pneumatic actuator, local control box, main header and cables connecting to a main or local control panel in accordance with this disclosure. 
           [0033]      FIG. 6  is a block diagram illustrating a boiler furnace with an improved biomass spreading system in accordance with this disclosure. 
           [0034]      FIGS. 7A and 7B  are schematic drawings of the high efficiency valve with the plug in fully closed and fully open positions as constructed in accordance with this disclosure. 
           [0035]      FIGS. 7C ,  7 D and  7 E depict graphs of the operational parameters of the high efficiency valves constructed in accordance with this disclosure. 
           [0036]      FIG. 8  depicts the program selector from a local control panel constructed in accordance with this disclosure. 
           [0037]      FIGS. 9A ,  9 B,  9 C and  9 D depict graphs of nozzle pressure versus cycle time for a system constructed in accordance with this disclosure. 
           [0038]      FIGS. 10A and 10B  depict graphs of nozzle pressure versus cycle time for a system constructed in accordance with this disclosure. 
           [0039]      FIG. 11A  depicts a schematic drawing of a stoker boiler constructed in accordance with this disclosure. 
           [0040]      FIG. 11B  is a zoomed view of  FIG. 11A . 
       
    
    
       [0041]    A person of ordinary skills in the art will appreciate that elements of the figures above are illustrated for simplicity and clarity, and are not necessarily drawn to scale. The dimensions of some elements in the figures may have been exaggerated relative to other elements to help understanding of the present teachings. Furthermore, a particular order in which certain elements, parts, components, modules, steps, actions, events and/or processes are described or illustrated may not be actually required. A person of ordinary skills in the art will appreciate that, for the purpose of simplicity and clarity of illustration, some commonly known and well-understood elements that are useful and/or necessary in a commercially feasible embodiment may not be depicted in order to provide a clear view of various embodiments in accordance with the present teachings. 
       DETAILED DESCRIPTION 
       [0042]    Turning to the Figures and to  FIG. 4  in particular, a boiler stoker  300  with an improved biomass spreading system  302  is shown. The boiler stoker  300  includes a furnace  332  having a grate  334  and various distributors  108  through which biomass material  352  enters into the furnace  332  and falls on the grate  334 . The biomass material  352  is fed into the distributors  108  by a feeder (not shown). The biomass material  352  is distributed based on the momentum of an air jet  130 , which is controlled as described herein. In one implementation, the grate  334  is a pinhole grate. Alternatively, the grate  334  is a vibrating grate, or any other type of grate known to a person of ordinary skills in the art. Under grate air  338  is provided by an air supplier  310 . Air  338  further flows through many holes evenly distributed in the grate  334  and mixes with the biomass material  352 . When the biomass material  352  is burned, flames  354  are created inside the furnace  332 . When the biomass material  352  is evenly distributed over the grate  334  by the system  302 , the flames  354  are usually short flames. Furthermore, short flames cover the entire area of the grate  334 , and thus create stable combustion inside an interior chamber of the furnace  332 . 
         [0043]    The improved biomass distribution system  302  includes a central control unit  304 , such as a Programmable Logic Controller (“PLC”), Distributed Control System (“DCS”) or Supervisory Control And Data Acquisition (“SCADA”) system. The central control unit  304  generates current or voltage control signals. In one implementation, the control unit  304  is a PLC connected to an engineering workstation (not shown) and an application server (not shown), which sends the programmed control signals to individual control boxes  380 . In another implementation, a local control panel  380  holds all the I/P transducers and a PLC, which contains various programs. A selector switch or a touch screen monitor allow the boiler operator to choose from various programs. The interface screen or front panel clearly indicates the application for each selector position, as depicted in  FIG. 8 . View port (or ports)  337  allows an operator to observe the distribution of biomass  352  over the grate  334 . 
         [0044]    Referring to  FIG. 5 , a cross sectional view of the system  302  is shown. The current or voltage signals  601  are sent by the central control unit  304  and received by local control device  380 , which can be a local control panel or local control box (or boxes). I/P (meaning current to pressure) or V/P (meaning voltage to pressure) transducers  604  within the local control device  380 , convert the signals  601  into air pressure signals  602 . The air pressure signals  602  are used to operate a pneumatic actuators  312 . When the air pressure signal  602  is increased, an actuator spindle  315  of the actuator  312  extends forward. As the actuator spindle of the actuator  312  extends, the actuator  312  displaces a valve plug  316  within a valve housing  314  towards a contracting discharge duct  318 . A spring  313 , inside the pneumatic actuator  312 , retracts the plug  316  when the air pressure signal  602  is decreased. As used herein, each local control device  380  and the transducer  604  within it is said to be connected and operatively coupled to a corresponding actuator  312  and the central control unit  304 ; and each valve plug  316  is said be to operatively coupled to a corresponding actuator  312  through a spindle  315 . 
         [0045]    In other words, as the plug  316 , displaces forward or retracts, it efficiently converts part of the static pressure of the air behind the plug  316 , into dynamic pressure in the throttling passages  504 , between the plug  316  and the contracting duct  318 , and back into static pressure at the discharge duct  318 . To evenly distribute the biomass material  352  over the grate  334  (see  FIG. 4 ), the distribution system  302  provides airflow at variable pressure through the contracting discharge duct  318  which is operatively coupled to the distributor  108 . As the biomass material  352  falls into the distributor  108 , the airflow from the discharge duct  318  blows the biomass  352  into the furnace  332 . The sweeping nozzle  131  and flange  153  operate as described in the background. In certain embodiments, an intermediate duct  154  is used to connect the distribution system  302  to the distributor  108 , thereby allowing control of 
         [0046]    The air flow at a higher air pressure in the discharge duct  318  moves the biomass material  352  along a longer trajectory  340  (see  FIG. 4 ) and delivers it to the far side of the grate  334  away from the distributor  108 . In contrast, when the air pressure at the contracting discharge duct  318  is lower, the biomass material  352  travels a shorter trajectory  342  (see  FIG. 4 ) and falls on the near side of the grate  334  that is closer to the distributor  108 . The air pressure at the contracting discharge duct  318  is controlled by the valve plug  316  position, which in turn is programmed and controlled by the control unit  304  through the VP or V/P transducers  604  inside the local control device  380 . 
         [0047]    Air flows through a main duct  306  receiving air from an air supplier  311 , to the valve housings  314 , through openings  320  that match the valve housing inlet. The discharge duct  318  is connected to the biomass distributor  108 . Each valve housing  314  incorporates a local control device  380 . The biomass material  352  enters the furnace  332 , while air flows into the distributor  108  from the duct  318 . 
         [0048]    In one embodiment of the present teachings, each local control device  380  contains a controller or transducer which converts the control signals  601  from the central control unit  304 , to pneumatic control signals  602  fed to the actuators  312 . The air supplied to the converter or transducer  604 , is known as instrumentation air, at a pressure higher than the air sweeping pressure. The instrumentation air pressure is usually between 60 to 100 PSI (meaning pounds per square inch). For example, the signal from the central control unit is 4-20 mA (meaning milliamps) and the pneumatic signal to the actuator  312  is 6-30 PSI. The air sweeping pressures are usually between 0.5 to 1 PSI. In another implementation a local control panel  380  contains the transducers for the valves. 
         [0049]    In one implementation, the actuator  312  is attached to the inlet housing  314  through a cover plate  317  which also provides access for inserting the valve plug  316  into the valve housing  314 . The spring return pneumatic actuator  312  provides forces to displace the plug  316  with a plug spindle  315 . In other words, the plug spindle  315  transfers force from the actuator  312  to the plug  316 . Depending on the air pressure signal  602  that the actuator  312  receives from the local control device  380 , the actuator  312  drives the plug  316  towards or away from the discharge duct  318 . When lower sweeping air pressure is desired for the airflow, the plug  316  is pushed toward the discharge duct  318 . Accordingly, the space between the plug  316  and the duct  318  becomes smaller, and less air flows around the plug  316  and into the duct  318 . On the contrary, when higher air pressure is desired for the airflow, the plug  316  is pulled away from the discharge duct  318 . Accordingly, the space between the plug  316  and the duct  318  becomes bigger, and more air flows around the plug  316  and into the duct  318 . In other words, the position of the plug  316  determines the air pressure of the airflow (also referred to herein as nozzle pressure). 
         [0050]    The contoured plug  316  and the contoured discharge duct  318  are designed to embody matching physical shapes to allow precise control of the nozzle pressure while minimizing pressure losses when the highest flows are required. In one implementation, the contoured plug  316  is substantially in the shape of a diamond. Accordingly, the front end of the contoured plug  316  incorporates surfaces that are substantially parallel to the surfaces of the rear end of the duct  318 . In other words, the top surface of the front end of the plug  316  is substantially parallel to the inner top surface of the rear end of the duct  318 ; and the bottom surface of the front end of the plug  316  is substantially parallel to the inner bottom surface of the rear end of the duct  318 . Accordingly, it can be said that the front end of the plug  316  and the rear end of the duct  318  have substantially the same geometric shape. Other plug shapes may be designed in order to obtain certain flow characterizations with respect to plug positioning as it approaches the discharge duct. 
         [0051]    Referring to  FIG. 6 , a block diagram illustrating a boiler furnace with an improved biomass spreading system in accordance with this disclosure is depicted. The boiler furnace includes a typical furnace  332 , with an automatic, programmable biomass spreading system  302  coupled to the biomass distributors  108 , a video camera  702  installed on a furnace wall, a video monitor  404  receiving the video signals  401  from video camera  702  and a central control unit  304  sending control signals  601  to the local control panel  380 . A boiler control room operator  403 , observes the video image sent by the camera  702  and displayed on the monitor  404 , identifies the position where uneven biomass distribution problems exist and the corresponding location over the grate surface. The boiler operator  403  uses a mouse  406 , a keyboard  407  or a touch screen  405  to input the bed depth changes observed on the camera monitor  404  to the central control unit  304 . The central control unit  304 , the monitor  404 , the keyboard  407 , the mouse  406  and the touch screen  405  can be disposed within a central control room  309 . 
         [0052]    In a separate embodiment, when a video image is not available to the central control unit  304 , the local operator  408 , observes the biomass distribution on the grate through view ports  337  on the furnace walls, changing the programs manually on the local control panel  380 . 
         [0053]    The programs, stored in the central control unit  304  or in the local control panel  380 , define the current or voltage signals sent to each high efficiency valve assembly as well as the duration of each signal. A current or voltage value held during a preprogrammed time period is referred to herein as a programmed pulse. Turning now to  FIGS. 9A ,  9 B,  9 C, and  9 D, graphs of air pressure versus elapsed cycle time are shown. It can be observed that the pressure pulses can vary according to any desired relationship. These programmed air pulses  602  are sequentially emitted based on control signals  601 , one after the other, to the valve actuator  312  until completing a predetermined total time. The predetermined total time is referred to herein as a valve program cycle. Each programmed pulse corresponds to a plug position of the plug  316  within the contracting discharge duct  318 . Accordingly, the central control unit  304  provides for a precise control of the valve throttling passages  504  and controls the discharge duct pressure. 
         [0054]    Referring to  FIGS. 7A and 7B , these figures represent two extreme positions of the valve plug—fully closed and fully opened respectively. Plug displacement is represented by dimension ‘X’ in both drawings. 
         [0055]      FIG. 7C  depicts a graph of plug displacement versus actuator pressure. As 
         [0056]    As is apparent, actuator pressure gradually increases with plug displacement ‘X’. 
         [0057]      FIG. 7D  depicts a graph of nozzle pressure versus plug displacement. As is apparent, nozzle pressure generally decreases with plug displacement ‘X’. 
         [0058]      FIG. 7E  depicts a graph of nozzle pressure versus control signal current as measured in milliamps (mA). 
         [0059]    The aforementioned graphs have proven to be consistent from valve to valve, allowing precise repetitive pressure steps, which in turn provides predictable nozzle pressures at any time within the pre-programmed cycles. 
         [0060]    Turning to  FIG. 8 , in one embodiment of this disclosure, a control panel  380  incorporates an operator interface consisting of a program selector knob  700 , which can be a mechanic selector switch or part of a touch screen display. In one version of this interface, the operator may choose from various programs corresponding to different flow ranges.  FIG. 8  depicts four ranges: low flow  701 , medium flow  702 , medium high flow  703 , and high flow  704 . By operating the depicted knob  700 , the operator (not shown) can select the desired flow range. 
         [0061]    After observation of the biomass distribution on the grate for a period of, for example, a few seconds, the operator identifies whether the biomass is depositing evenly across the depth or it is accumulating the back or front of the grate. The operator can then adjust the control as required for the proper flow range to achieve even deposition of biomass on the grate. 
         [0062]    Turning to  FIGS. 9A ,  9 B,  9 C and  9 D, these figures depict equal time pressure steps generated by various positions of the interface program selecting knob. In particular, after changing the flow setting by, for example, turning knob  700  of  FIG. 8 , the operator can observe the impact on biomass distribution. 
         [0063]      FIG. 9A  depicts nozzle pressure (as a percentage of maximum nozzle pressure) versus the percentage of cycle time for the low flow range setting  701  of  FIG. 8 .  FIG. 9B  depicts nozzle pressure (as a percentage of maximum nozzle pressure) versus the percentage of cycle time for the medium flow range setting  702  of  FIG. 8 .  FIG. 9C  depicts nozzle pressure (as a percentage of maximum nozzle pressure) versus the percentage of cycle time for the medium high flow range setting  703  of  FIG. 8 .  FIG. 9D  depicts nozzle pressure (as a percentage of maximum nozzle pressure) versus the percentage of cycle time for the high flow range setting  704  of  FIG. 8 . 
         [0064]      FIGS. 10A and 10B  depict graphs that correspond to programs that target the medium flow range. These graphs depict nozzle pressure (as a percentage of maximum nozzle pressure) against percent of cycle time. 
         [0065]      FIG. 11  depicts a stoker boiler  500  constructed in accordance with this disclosure. As illustrated, a first observer  501  and a second observer  502  can view the operation of the boiler  500 . Turning to  FIG. 11B , the second observer  502  can be disposed near to the control device  380 . 
         [0066]    The foregoing description of the disclosure has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. The description was selected to best explain the principles of the present teachings and practical application of these principles to enable others skilled in the art to best utilize the disclosure in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure not be limited by the specification, but be defined by the claims set forth below. For example, while various specific dimensions were disclosed to better enable a person of skill in the art to easily reproduce the disclosed device without undue experimentation, different dimensions could be used and still fall within the coverage of the claims set forth below. In addition, although narrow claims may be presented below, it should be recognized that the scope of this invention is much broader than presented by the claim(s). It is intended that broader claims will be submitted in one or more applications that claim the benefit of priority from this application. Insofar as the description above and the accompanying drawings disclose additional subject matter that is not within the scope of the claim or claims below, the additional inventions are not dedicated to the public and the right to file one or more applications to claim such additional inventions is reserved.