Patent Publication Number: US-6220190-B1

Title: Water-cooled oscillating grate system

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to boiler systems, and more particularly, relates to a water-cooled oscillating grate system for a boiler, for example, used with biomass fuels. 
     DESCRIPTION OF THE RELATED ART 
     Various vibrating grate boiler arrangements are known. For example, U.S. Pat. No. 4,389,978 issued Jun. 28, 1983, discloses a grate having a fuel supporting and conveying surface including a plurality of elongate longitudinally-oriented, generally V-shaped channels. Water cooling pipes are provided for base and top part of each channel. Air feed openings are provided between both sidewalls of each channel. The grate is vibrated by a motor located outwardly of the boiler shell. 
     U.S. Pat. No. 3,126,846 entitled INCINERATOR GRATE issued Mar. 31, 1964 discloses a grate including multiple, alternate stationary and movable grate members. The movable grate members are reciprocated relative to the stationary grate members from front to rear relative to the incinerator. Space below the grate is divided into multiple chambers that communicate with a forced air supply duct. The amount of air for combustion supplied to various sections of the grate is controlled by adjusting dampers. 
     U.S. Pat. No. 4,987,834 entitled SIFTINGS REMOVAL DEVICE discloses a furnace having an ash discharge system which collects and receives siftings falling from portions of an incinerator grate. The ash discharge system includes multiple hoppers disposed under a grate. Each hopper forms an air plenum for directing and controlling the flow of combustion air to the furnace. 
     U.S. Pat. No. 4,437,452 entitled ROTARY CONTINUOUS ASH DISCHARGE STOKER discloses a rotary continuous ash discharge stoker having a circular grate for supporting burning fuel. The circular grate includes a central stationary section and an outer rotating section or ring. The rotating ring is supported and guided on rails and rollers which allow for the complete rotation of the grate section. Pressurized air is supplied into the housing below the circular grate via a plurality of air plenums. 
     Effective air distribution is not easily accomplished in the many known vibrating grate systems. A need exists for an improved water-cooled vibrating grate system that facilitates efficient combustion by effective combustion air distribution. 
     In one known design only the air permeated water-cooled grate is vibrated. Flat bar type springs are used to support the grate. The drive consisting of a number of eccentric crank arms spread along the length of a shaft, is directly attached to the vibrating grate. Usually one crank arm for each section of grate is utilized. The common shaft i s powered by pulleys connected to an electric motor. The conveying speed of the ash on the water-cooled grate is essentially fixed and not easily electrically adjusted while it is in operation. This stoker grate design often causes excess vibration to the boiler and the surrounding structure. 
     Typically the vibrating grate is essentially pushed and pulled by the crank arm located at one end, its conveying stroke is not always equal or the same along its full length. Thus, the conveying of the ash over the surfaces of the grate is not uniform. A relatively large amount of input horsepower is required to drive this vibrating grate because the single input or brute force kind of drive used is not energy efficient. To compensate for the non-uniform ash movement, this kind of vibrating grate is usually declined downhill instead of being mounted horizontally. This added slope require s more vertical height. The grate sections are typically 6 feet wide sections, so that the needed full grate width dimension had to be made up in multiple sections. Steep walled hoppers are required underneath to collect the ash siftings that fall down through openings between the grate sections. The overall combination of this type of vibrating grate combined with the needed ash siftings collecting hoppers below required an excessive amount of headroom. Multiple water pipes are projected from each end of the grate to stationary water headers. This arrangement adds cost, causes excess vibration transmission, and prompts metal fatigue in the water pipes. 
     In many vibrating grate systems often excessive vibration is coupled to the boiler and the surrounding structure. This occurs, particularly when the grate is not effectively counter-balanced. A need exists to provide an improved water-cooled vibrating grate system that minimizes the vibration coupled to the boiler and the surrounding structure. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an improved water-cooled, vibrating grate system. Other objects of the invention are to provide a water-cooled, vibrating grate system that provides effective, efficient and reliable operation, and that overcomes some disadvantages of prior art arrangements. 
     In brief, a water-cooled, vibrating grate system for a boiler for use with biomass and other fuels includes a grate unit having a top grate surface. The top grate surface includes air-flow apertures. A plurality of water-cooling pipes support the top grate surface. The plurality of water-cooling pipes are coupled to a water supply. An air plenum unit is positioned under and attached to the top grate surface. The air plenum unit is coupled to an air supply for providing combustion air through the top grate surface air-flow apertures. A vibration drive isolation assembly vibrates the grate unit. 
     In accordance with features of the invention, the vibration drive isolation assembly includes a longitudinally extending counterbalance member. A plurality of drive springs are supported by the counterbalance member. The drive springs are distributed across the width and the length of the enclosed grate unit. At least one vibratory motor or mechanism is installed on the counterbalance member. A plurality of isolation springs support the longitudinal counterbalance member. 
     In accordance with features of the invention, the air plenum unit includes multiple zones. Each zone has an associated air flow control damper for controllably providing combustion air flow. The air plenum unit receives grate ash siftings. Usually, a plurality of ash-siftings discharge openings are located at a defined discharge end of the air plenum unit. The ash siftings being directionally vibrated to the ash-siftings discharge openings. The air plenum unit is directly attached to the top grate surface to minimize under grate air leakage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawing, wherein: 
     FIG. 1 is a fragmentary side elevational view of a boiler including a water-cooled, vibrating grate system arranged in accordance with the present invention; 
     FIG. 2 is a top elevational view of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; 
     FIG. 2A is an isometric view of an alternative grate surface together with water cooling pipes of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; 
     FIG. 3 is an isometric view of a plenum chamber of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; 
     FIG. 4 is a top elevational view illustrating water-cooling components of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; 
     FIGS. 5A and 5B are side sectional views taken along line  5 - 6  of FIG. 1 illustrating a grate to boiler sealing arrangement of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; and 
     FIGS. 6A,  6 B, and  6 C are side sectional views taken along line  5 - 6  of FIG. 1 illustrating alternative sealing arrangements of the under grate air plenum chamber to the grate surface of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention; and 
     FIG. 7 is an end view of a pair of the preferred vibratory motors with shaft mounted, eccentric weights and the counter-balance of the water-cooled oscillating grate assembly of FIG. 1 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Having reference now to the drawing, FIG. 1 illustrates a water-cooled, air permeated, vibrating grate system generally designated by reference character  100  and arranged in accordance with the present invention in a boiler  102 . In accordance with features of the invention water-cooled oscillating grate system  100  includes a single grate unit generally designated  104 . Among its primary components, grate unit  104  includes a top grate surface  106 , an air plenum  108  and a plurality of water cooling tubes  110 . Grate unit  104  is an enclosed and air permeated, integral unit. Water-cooled, vibrating grate system  100  has a vibration isolated drive system generally designated by  112  arranged in accordance with the present invention. As shown in FIG. 1, the boiler  102  includes a fuel inlet  114  to permit fuel, such as biomass fuel, to be fed downwardly onto the grate surface  106 . Boiler includes multiple overfire air ports  116  for supplying overfire air within the boiler shell  118 . It should be understood that the utility of the present invention is not restricted to a particular boiler or furnace arrangement. 
     In accordance with features of the invention, the water-cooled oscillating grate system  100  is arranged for firing biomass fuels, which vary in moisture content and heating value. Each fuel requires its own proportion of combustion air quantity, combustion air temperature, degree of oscillation, and speed of fuel travel on the grate. Water-cooled, vibrating grate system  100  allows the use of high temperature undergrate air for high moisture fuels, with grate components being protected from overheating. The constant flow of cooling water through pipes  110  is also sufficient protection for the grate surface  106  when firing the boiler with auxiliary fuel burners properly located above the grate surface  106 . The grate surface  106  does not require a layer of insulating material for protection. To conserve heat and water, boiler feedwater (supply line  119  in FIG. 1) is generally used for grate cooling; however it should be understood that other water sources may also be used. 
     Referring also to FIGS. 2,  2 A,  3 ,  4 ,  6 A,  6 B, and  6 C, in accordance with features of the invention, the top grate surface  106  of grate unit  104  includes a plurality of air-receiving openings  120  for receiving combustion air from the air plenum  108 . In FIG. 2A, there is shown an alternative, water jacketed air-permeation flat deck  106 A forming the grate top surface of the grate unit  104 . The flat deck  106 A similarly includes a plurality of air-receiving openings  120 A for receiving combustion air from the air plenum  108 . 
     As can be seen in FIGS. 2,  6 A,  6 B, and  6 C, the grate surface  106  is formed by a plurality of low-maintenance grate clips  122  made of high temperature cast material, seated on the water cooling tubes  110  with high conductivity grout. Grate clips  122  provide a high pressure drop grate surface  106  for better air distribution through the grate unit  104 . 
     In accordance with features of the invention, air plenum unit  108  includes multiple air flow zones  130  beneath the grate surface  106  to allow for balancing the air flow across the front, middle and rear grate sections. Siftings fall down into the plenum  108  and are simultaneously conveyed to discharge openings  140  in the plenum  108  by directional vibratory motion provided by assembly  112 . 
     Incoming air plenum  108  is installed directly under the water-cooled grate surface  106  and is an integral part of the unit  104 . This plenum  108  receives the incoming air and properly distributes this air to predefined sections of the grate. The vibratory drive assembly  112  is located underneath the enclosed air plenum  108 . 
     As shown in FIGS. 1 and 3, the grate air flow is controlled to three air plenum zones  130  consisting of front, middle and rear zones labeled ZONE  1 , ZONE  2  and ZONE  3  in FIGS. 1 and 3. Each zone  130  has an associated air flow control damper  132  located upstream of an expansion joint  134  in a respective zone air supply line  136 . The result is air flow can be biasing to improve the air to fuel mixing. When needed, and in addition to the multiple zones, air distribution in either the longitudinal or transverse direction can be controlled with added sleeves constructed of tubular type perforated plate (not shown). A flat bottom conveying pan  138  forms the lower section of the air plenum  108 . The bottom  138  of the air plenum  108  acts as an ash siftings collector for any passed particles being burned on top of the grate unit  104 . By accomplishing this, the ash collecting hoppers previously utilized could be omitted or eliminated. The ash siftings are collected and simultaneously conveyed to the discharge end of the grate unit  104 . The grate ash siftings to the air plenum  108  are directionally vibrated to a plurality of front siftings discharge openings  140  at a discharge end  142  of the air plenum unit  108 . An air plenum ash siftings receiving hopper  144  can be cleaned on-line. Since the grate unit  104  carries the conveyed ash and the cooling water load, the lower enclosure portion of  146  of grate unit  104  must provide adequate structural strength to enable grate unit  104  to be driven by the vibratory drive configuration  112 . The lower enclosure portion of  146  is a structural grid frame. Transverse and longitudinal structural beams supporting the frame  146  are connected to the vertical sidewalls  146  of the air plenum  108 . The vertical walls  150  between the air plenum zones  130  are structurally reinforced with added columns appropriately spaced internally and externally. 
     The top ash conveying grate surface  106  is air permeated and water-cooled via multiple water cooling pipes  110 . As shown in FIG. 1, top ash conveying grate surface  106  is installed generally horizontally. The top ash conveying grate surface  106  could be installed slightly declined or inclined, if preferred. A pair of water headers  160  and  162  are included as an integral part of the grate unit  104  and vibrate with unit  104 . 
     Referring to FIG. 1, an inlet water header  160  and an outlet water header  162  installed on one end of the grate unit  104  are respectively connected to inlet and outlet water lines  164  and  166 . Since the inlet header  160  and outlet water header  162  are an integral part of the grate unit  104 , the headers  160  and  162  vibrate with the unit  104 . The water lines  164  and  166  are flexibly connected to the two headers  160  and  162 . 
     Referring to FIGS. 5A and 5B, where the water-cooled grate unit  104  engages the boiler shell  118 , an appropriate flexible connection is provided. Perimeter sealing connections between the boiler  102  and grate unit  104  are provided by a labyrinth type seal  170  and a flexible fabric expansion joint connection  174  as shown in FIG. 5A and 5B. The perimeter bladed labyrinth type seal connection  170  is provided in-line with the vibratory stroke angle of the vibration drive isolation assembly  112 . The perimeter flexible fabric expansion joint  174  provides sealing for the boiler  102  thermal expansion movement. 
     Referring to FIGS. 6A,  6 B and  6 C, all four walls of air plenum chamber  108  are directly attached to the grate surface  106  with all four walls to provide a tight air seal. In FIGS. 6A,  6 B, and  6 C, there are shown alternative sealing arrangements of the under grate air plenum chamber  108  to the grate surface  106  respectively generally designated by  600 A,  600 B and  600 C of the water-cooled oscillating grate assembly  100 . As shown in FIG. 6A, a sealing member  602  attached by a bolt  604  to the side walls  148  of air plenum unit  108 , provides sealing to the grate surface  106  which reduces under grate air seal leakage rates. The result is improved air to fuel mixing and reduced maintenance requirements. In FIGS. 6B and 6C, a respective alternative sealing member  606  and  608  is attached by bolt  604  to the side walls  148  of air plenum unit  108 , provides sealing to the grate surface  106 . 
     Referring now to FIGS. 1 and 7, the vibration drive isolation system or assembly  112  is arranged to minimize vibration to exterior plant equipment. Vibration drive isolation system  112  includes a longitudinal counterbalance member  180 , a plurality of drive springs  182  supported by counterbalance member  180  and a plurality of isolation springs  182  supporting the counterbalance member  180 . A structural steel base  188  supports the isolation springs  184  and is isolated from the boiler  102 . The vibration unit has the following capabilities. Variable speed motor control capable for adjusting the vibration intensity. Control capability of ramping up and ramping down the vibration intensity during a timed cycle. The result is vibration system can easily be tuned and emissions can be controlled during a vibrating cycle. 
     At least one small variable speed drive motor  190 , such as two or four horsepower motor(s), is included in assembly  112 . The motor(s)  190 , drive springs  182 , and isolation springs  184  are mounted on the support steel under the grate unit  104  and are totally open and accessible even while the grate unit is in operation. An adjustable rate controller  192  operatively controls the variable speed drive motor(s)  190 . 
     U.S. Pat. No. 3,251,457 entitled METHOD AND APPARATUS FOR DRIVING VIBRATORY DEVICES issued May 17, 1966 to George Dumbaugh, one of the present inventors, discloses a drive for vibratory devices where both frequency and stroke can be varied simultaneously. The subject matter of the above identified U.S. Pat. No. 3,251,457 is incorporated herein by reference. 
     Both the time between oscillations and the intensity of the oscillation can be controlled with an easy control panel adjustment of controller  192 . They require no mechanical adjustment of eccentrics. Typically, oscillation cycles are approximately five minutes apart with oscillation five to ten seconds long. The times will vary depending on the fuel characteristics and the moisture content. Actual motion of grate unit  104  is about a quarter of an inch, and the entire grate surface  106  oscillates at once. Grate surface  106  do not have to be broken into separate oscillating zones. Variable oscillation control also allows the five to ten second oscillating cycles to start slowly and build up to full intensity. 
     The electric motors  191  of the vibratory drive assembly  112  are not attached to the grate unit as conventionally done. The dynamic counter-balance  180  is longitudinal and positioned under the combination of the steel coil drive springs  184  and multiple flat bar type of stabilizers  196 . The assembly  112  is supported from the longitudinal counter-balance  180  by the appropriately spaced isolating springs  184  mounted in compression and appropriately spaced along its length. The vibratory motors with shaft mounted eccentric weights  190  are either installed on each side of the counter-balance  180  as shown in FIG. 7, or combined together, and placed underneath the counter-balance, or if one motor  190  is used, it is preferably put on top of the counter-balance  180  near the mid-point of the counter-balance  180 . 
     The steel coil type drive springs  182  are distributed across the width and along the length of the underside of the enclosed vibrating grate unit  104 . The drive springs  182  are combined with flat bar type stabilizers  194  to assure a uniform stroking action. The flat bar type stabilizers  194  are used to guide the movement of the stiff drive springs  182 . 
     The drive springs  182  are sub-resonant tuned to cause them to inherently work harder under load, where sub means under and Resonant means natural frequency. Therefore, “Sub-resonant” means the maximum running speed of the vibratory motors  190  is always under the natural frequency of the combined drive springs. For example, if the top motor speed is 570 RPM, which in this instance is the same as CPM, then the natural frequency of all the drive springs  182  would be, for example, 620 CPM. While 570 CPM is preferred, other frequencies such as 720 CPM, 900 CPM or 1200 CPM, might be useful for various applications. 
     The axial centerline of the steel coil drive springs  182  is provided in line with the wanted stroke angle, but the axial centerline of the stabilizer  194  is perpendicular to the stroke angle. A stroke angle is illustrated with the plenum unit  108  in FIG.  1  and labeled STROKE ANGLE. By utilizing paralleled counter-balance or structural beams  180  as a longitudinal configuration, the enclosed vibrating grate unit  104  is dynamically counter-balanced. The structural Natural Frequency of the counter-balance assembly will be at least 1.4 times the maximum speed of the motors, but preferably will exceed it. In this instance, the RPM of the motor  190  is the same as the vibrating CPM of the enclosed grate unit  104 . 
     Relatively soft steel coil type isolation springs  184  preferable are used to support the longitudinal counter-balance  180  which in turn supports the enclosed vibrating grate unit  104  above it. Preferable needed input power is proved by two, three phase, A-C squirrel cage vibratory motors  190  by either installing motors  190  on each side of the dynamic counter-balancing member  180  (FIG.  7 ). 
     Electrical adjustment of conveying speed is provided by the controller implements either as a variable voltage or an adjustable frequency type of electrical control. The conveying speed of the ash over the vibrating grate unit  104  can be electrically adjusted. 
     In operation, the vibratory motor(s)  190  are energized and the shaft mounted eccentric weights are accelerated to full speed. The force output of the rotating eccentric weights excites or induces all the stiff steel coil drive springs  182  and flat bar stabilizers  194  to vibrate back and forth in a straight line. The speed (RPM) of the vibratory motors  190  is the same as the vibrating frequency (CPM) of the drive springs  182 . This happens even though the natural frequency of the drive springs  182  is above the motor speed. Consequently, the enclosed grate unit  104  vibrates at a prescribed amount of linear stroke at the wanted angle, which is usually 45°. As an equal reaction to the vibratory movement of enclosed grate unit  104 , the counter-balance member  180  inherently moves in an opposite direction. Thus, the opposing dynamic forces cancel one another. The counter-balance  180  freely moves or floats on top the soft isolation springs  184  supporting it. 
     A resulting directional, straight line stroke on the enclosed grate unit  104  induces the ash particles to unidirectionally move forward simultaneously over the top grate surface  106  and the bottom surface  138  of air plenum  108 . This ash movement is the result of a series of hops or pitches and catches by the applied vibration. Normally, the ash first settles on the grate. Then, it is gradually moved forward by repetitive on and off cycles of applied vibration. For example, the ash is moved  3  feet every  6  minutes. Alternatively, the ash movement over the grate surfaces could be electrically adjusted via adjustment of motor operation by controller  192  to provide, for example, a conveying speed of 0.5 FPM. The ash conveyed on the air permeated grate top  106  discharges into vertical chutes (not shown). The ash siftings that fall through any openings  120  in the grate surface  106  drop onto the bottom conveying pan  138  of the air plenum. When the vibratory conveying action is applied, these ash siftings move forward. Eventually, these particles fall down through outlets  140  located near the discharge end of the grate unit  104 . 
     While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.