Patent Application
20050229824
The invention relates generally to two-stage wet waste gasifier burner design and specifically to a design for a two-stage wet waste gasifier burner including a self-cleaning ash removal system, a novel profile for grate support and grate layout, a novel grate conformation and ash plate mechanism, and a novel fuel feeder system.
Wood-waste fuel burners, sometimes known as hog fuel burners, have generally been inefficient in combustion, discharging undesirable amounts of gaseous and particulate pollution. In addition, when the gaseous effluent from such burners is used to heat a boiler, the gases emitted to the boiler have typically been dirty, causing depositions on the heat transfer tubes of the boiler, which, as a consequence, require frequent and expensive cleaning. Some of the disadvantages of wood-waste fuel burners have been addressed in the development of previously known two-stage wood-waste gasifier burners, such as described in U.S. Pat. No. 4,177,740 to Lamb, et al. Such two-stage gasifier burners combust wood waste that is heaped on a grate in the first stage of the burner to form a conical pile of fuel, commonly fed into the burner from below, with preheated underfire air percolating up through the pile in controlled amounts, drying and gasifying the waste fuel in the pile. The volatile gases driven off the pile are then partially oxidized by additional combustion air introduced into the first stage with the total amount of combustion air admitted to the first stage being maintained at less than stoichiometric proportions so that the temperature in the first stage remains lower than that necessary to melt the natural ash, dirt or other inorganic substances in the fuel. The volatile gases are discharged from the throat at the top of the first stage of the burner and enter the second stage, generally located directly above the first stage. In the second stage, secondary combustion air is introduced in an amount sufficient to allow complete combustion of the remaining volatile gases before the products of combustion exit the second stage of the burner to be used for process heat or directly exhausted to the environment.
However, even though prior combustion chamber designs have addressed the problem of gaseous combustion and particulate removal from the gaseous effluent, two-stage wood-waste gasifier burners known in the prior art, as described above, suffer from inefficiencies in operation due in large part to inefficient or impractical fuel supply and ash removal systems. Such inefficiencies in the material handling systems of known gasifier burners tend to cause less efficient gasification of fuel due to uneven distribution of wood-waste on grate surfaces, potentially lengthy and frequent shutdowns for removal of ash and residues due to inefficient ash collection and removal equipment, and overall reduction in wood-waste throughput and thermal generation.
Wet waste other than wood waste can also be burned, but biomass waste of any sort does not burn efficiently at moisture content levels above about 60%. Furthermore, some types of biomass waste, such as sewage sludge, are not easily conveyed if their moisture content falls appreciably below 60%. Below the 60% moisture content level, the sludge tends to become cohesive, requiring unacceptably high consumption of energy for its conveyance. The reason for this is understood to be the tendency of water to combine hygroscopically with fiber in the sludge. At higher moisture content, the sludge moves relatively freely in auger-type conveyors, and any binding within the sludge that occurs tends to be tolerable. These two characteristics of wet biomass waste create the conundrum that if the waste can be efficiently burned, it cannot be efficiently conveyed, and vice versa. A technique is needed that will overcome or at least mitigate the foregoing problem.
The present inventive two-stage wet-waste gasifier burner addresses some of the problems and inefficiencies inherent in the design of prior gasifier burners described above, and in particular, includes relatively simple and efficient inventive designs for the wet-wood-waste feed apparatus and feed control system, gasification grate geometry and underfire air supply, and ash plate and ash removal actuation apparatus and control systems. Such inventive designs provide for relatively high efficiency and reliability of operation of the present inventive two-stage wet-waste gasifier burner.
The gasifier to be described in detail in this specification is particularly suitable for the combustion of wet wood waste. However, with suitable adaptation, the gasifier may also be used for burning other types of wet biomass waste, including sewage sludge.
Further, in accordance with the invention, an apparatus and method for burning wet waste having a relatively high moisture content comprises, in its method aspect, drying a selected portion of the waste to a relatively dry solid consistency such that water binding of the waste is substantially reduced, mixing dried waste with wet waste in proportions selected to support two-stage combustion of the mixed product and to support ease of conveyance of the mixed product, and burning the combustible portion of the mixed product preferably in a two-stage combustion process that preferably but not necessarily includes the two-stage wet-waste combustion apparatus according to the invention. The combustion effluent may be used to generate electricity and may be treated using a selective catalytic reaction to reduce the content of noxious gases in the ultimate effluent delivered to the atmosphere.
In its apparatus aspect, the foregoing inventive concept may be expressed as the combination of apparatus comprising, in a preferred embodiment a) a divider for dividing the waste proportionally into a direct-feed component and a component to be dried;
The divider can be omitted if the overall process is a batch process instead of a continual flow process. The critical criterion is that the mixer mix the processed dried waste with the wet waste component in proportions selected to support two-stage combustion of the mixed product and to support ease of conveyance of the mixed product.
Additional apparatus components can be provided for use with the combination, such as an electric power generator driven by steam generated using the heat of the combustion gases, and a selective catalytic reactor for treating the effluent gases before they are released to the atmosphere to reduce the quantity of noxious gases in the ultimate effluent.
As noted above, wet-wood-waste burners of the type described in U.S. Pat. No. 4,177,740 to Lamb, et al. have a lower combustion chamber into which the wood waste is introduced for burning and an upper combustion chamber that burns gases received from the lower chamber, the gases being generated in the first stages of combustion of the waste. The waste is delivered into the lower chamber by a suitable feed mechanism, and falls onto and reposes on a grate array. The grate arrays of some prior systems are symmetrically arranged on either side of an A-frame support.
The present invention is not concerned with the upper combustion chamber in which gases are burned but rather with a number of the structural and operational aspects of the lower combustion chamber, and particularly the fuel input feed mechanism, the grate array structure, the underfire air supply arrangement, the ash removal mechanism including the drive for such mechanism, and suitable control arrangements for the feed and ash removal apparatus. Symmetry of the grate arrays is preserved in the preferred embodiment of the invention, wherein two symmetrically disposed grate arrays are installed in the burner, one on either side of the central A-frame support structure. The grate arrays are of inventive design, each having a concavely curved upper profile that approximates or at least is generally compatible with the expected natural angle of repose of the fuel material.
The wet-wood-waste feed system in a preferred embodiment of the present inventive gasifier burner incorporates a twin-auger conveyor including two separate single-helix augers rotating within separate parallel partial enclosures and driven by separate rotary motors, such as hydraulic motors. In a preferred embodiment, the two auger conveyors are installed along the central vertical plane of symmetry of the burner above the level of the grate arrays, one on either side of the plane of symmetry, such that they span the entire length of the burner chamber, and such that the auger on each side of the central vertical plane of symmetry supplies fuel material to the grate array on the same side of the plane. The augers are supported for rotation in open troughs within the gasification chamber, said troughs having sidewalls which slope downwardly from the input end to the output end of the troughs. In the preferred embodiment of this twin-auger fuel feed apparatus, each auger is mounted on a slight incline to the horizontal, so that the the sidewalls of the troughs, which slope downwardly from the input end to the output ends of the troughs, are horizontal. In other words, the input proximal end of the auger is lower than the distal end distant from the proximal end. The distal ends of the troughs are preferably fitted with a reverse auger flight in order to prevent binding of the augers by undischarged fuel material at the distal end of the auger.
As a result of the inventive combination of the proximal-to-distal upward slope of the auger and the proximal-to-distal downward slope of the trough sidewall, the fuel material transported by the auger tends to be discharged and fall onto the underlying grate surface in a relatively uniform distribution over the entire length of the augers, and therefore over the entire length of the grate arrays. Such uniform distribution of the fuel results in greater efficiency of gasification and consequently decreased combustive residues including particulate and ash. The uniform distribution of fuel over the grate surface also allows for accurate monitoring of the fuel pile height using conventional nuclear sensors (such as those using Cesium 137 radionuclides) mounted on the walls of the first stage of the burner, or some other suitable location. Signals from such nuclear sensors can be used to control the speed of the rotary motor powering the fuel feed augers, and thereby control the depth of the fuel pile on the grates to maintain a fuel pile height conducive to optimum gasification efficiency.
In a preferred embodiment of the fuel feed control system for the inventive gasifier burner, nuclear pile height sensors positioned to detect the height of the fuel pile on each gasifier grate array are connected to at least one control unit, such that the control unit is responsive to signals received from such pile height sensors. Such a control unit is also connected to the rotary motors powering the fuel feed augers such that the motors are responsive to signals received from the control ullit. The control unit functions such that when the fuel pile height drops below a suitable empirically determined lower limit for efficient gasification, as detected by signals received from the fuel pile height detectors installed above one of the grate arrays, a signal is sent to the motor controlling the fuel feed auger for that grate array such that the rate of fuel feed is increased by a suitable amount. Similarly, when the fuel pile height exceeds a suitable empirically determined upper limit for efficient gasification, as detected by signals received from the fuel pile height detectors mounted above a grate array, a signal is sent to the applicable auger motor such that the rate of fuel feed to that grate array is suitably decreased. In this way, the rate of fuel feed to each of the two grate arrays is controllable to maintain a preferred optimum fuel pile height, with relatively uniform fuel distribution across the width of the grate array, resulting in improved gasification efficiency.
It has been found through empirical methods that the optimum fuel pile height for effecting optimum fuel gasification depends upon the type of fuel used, and the moisture content of the fuel. In a preferred embodiment of the invention, shredded (to nominal particle sizes of less than about 1-5 inches) wet wood waste is commonly used as the fuel, with moisture contents typically ranging from about 30% to about 65% on a wet basis. Alternatively, other types of biomass reduced to suitable particle sizes for handling by the fuel feed system can also be used as fuel in the present inventive gasifier burner, albeit with potentially reduced efficiency. Through empirical testing, it has been found that the optimum fuel pile height increases as the moisture content of the fuel increases, due to the increased amount of water that must be evaporated, which in turn requires a longer residence time of the fuel in the fuel pile and thus a greater fuel pile height. In order to vary the fuel pile height required to optimize gasification of fuels having significantly different moisture contents, in accordance with another aspect of the invention, a preferred embodiment of the inventive twin auger fuel feeding system can be vertically raised or lowered with respect to the underlying grate surface to increase or decrease the fuel pile height as required. In a preferred embodiment of the inventive burner, the twin auger feeder system can be vertically movably mounted to the burner support structure (including the outer walls of the burner and the central A-frame support structure) by any appropriate means in order to allow the raising or lowering of the fuel feeding system to adjust the height of the fuel pile to correspond to changes in moisture content of the fuel. Alternatively, the fuel feeding system can be fixed to the burner support structure, and the grate arrays and accompanying ash removal system can be vertically movably mounted to the support structure to allow the raising and lowering of the grate and ash removal system to effect changes in the fuel pile height.
A preferred embodiment of the gasifier burner of the present invention incorporates a relatively simple and efficient inventive combustion grate configuration and associated underfire air supply system. In a preferred embodiment, each of the two symmetrical grate array substructures located on either side of the central vertical plane of symmetry of the burner are composed of a series of discrete grate boxes placed laterally adjacent to one another along the central vertical plane, wherein the rear of the grate boxes are nearest the central vertical plane. The grate boxes include a multiplicity of individual grate support plates installed at varying vertical heights spanning two laterally spaced side plates, the innermost grate support plates nearest the vertical plane of symmetry being highest, and the height of the grate support plates progressively decreasing with distance outward from the plane of symmetry, toward the front of the grate box, in an irregular stair step-like arrangement. For convenience of ash removal, the depths and installed heights of the grate support plates are substantially identical in each adjoining grate box. Ash removal plates are slidingly mounted for reciprocating forward and rearward horizontal motion across the top stationary surface of each grate support plate.
Fixed below each grate support plate in a preferred embodiment of the grate apparatus is an associated underfire air supply tube that extends laterally across the width of the grate box, and serves to supply heated underfire air to the fuel pile through a plurality of air exit ports drilled in the sidewall of the tube. In the aforementioned preferred embodiment of the present inventive gasifier burner, the underfire air supply tubes are rectangular in cross-sectional configuration, with a multiplicity of underfire air exit ports spaced along the outer surface of the forward vertical face of the air supply tube, relative to the rear of the grate box. This location of the air exit ports in a preferred embodiment of the invention advantageously provides for a relatively reduced likelihood of clogging of the air ports by particles of fuel, due at least in part to the fact that air is admitted through the vertical “riser” of each grate “step”. The air supply tubes in a single grate box are each connected to a vertical air supply riser duct that extends vertically downwards from the center of the bottom wall of each air supply tube to connect to a common underfire air supply plenum, which extends centrally from the back to the front of the grate box underneath all of the air supply tubes. In such a way, the single air supply plenum supplies each of the air supply tubes in a particular grate box with heated underfire air. The underfire air supply plenums extending centrally from the back to the front of each grate box are in turn connected to a suitable source of heated air, typically located external to the gasifier burner.
The underfire air supply tubes are spaced from one another transversely in a horizontal sense, and are also staggered from one another in a vertical sense so that those air supply tubes nearest the central vertical plane of symmetry of the gasifier chamber are highest, and those nearest the peripheral walls are lowest. The preferred slope or curvature of the grate (in a profile end view of the grate) achieved by the spacing of the air supply tubes and associated grate support plates is determined empirically to generally correspond to the angle of repose of the fuel material being burned, to improve the uniformity of the fuel pile lying on the upper surface of the grate. Further, the length of the individual grate support plates vary from the top to the bottom of the grate box, with the higher plates being shorter than those lower down on the grate box. As an additional feature of a preferred embodiment of the inventive grate design, the vertical dimension of individual rectangular air supply tubes in the grate box is selected so as to effect the required vertical spacing of the grate support plates which in combination with the variations in length of the grate support plates define the preferred slope or curvature of the grate surface, while at the same time permitting and simplifying the mechanism used to effect a reciprocal sliding motion of associated ash removal plates, as described below, for which the longer length of the lower grate support plates requires a longer reciprocal sliding motion of the associated ash plates, relative to support plates installed higher in the grate box.
Slidingly fitting between the upper surface of each grate support plate and the air supply tube directly above it are two associated ash removal plates that lie on top of the grate support plate and are in sliding contact with both the grate support plate below them and the air supply tube above them. The two ash removal plates sliding on top of each grate support plate are laterally spaced apart from each other; one ash plate is located on either side of the central vertical air supply riser duct connected to the air supply tube above the ash plates.
Each ash removal plate is connected to a horizontally reciprocating connecting arm element that extends generally horizontally from the rear edge of the ash plate towards the central plane of symmetry of the burner chamber, terminating in a connection with a suitable drive device. In a preferred embodiment, the drive devices that drive the ash removal plates are each a generally vertically oriented pendular driving element. As there are two laterally separated sets of ash removal plates in each grate box, one set located on one side of the central vertical air supply riser duct, and one set on the opposite side of the duct, there are two corresponding vertical pendular driving elements for each grate box, each connected by means of multiple horizontal connecting arm elements to one set of ash removal plates. The pendular driving elements are suspended from pivoting joints located above the highest grate support plate at the rear (nearest to the centre of the gasifier chamber) of each grate box. The pivoting joint at the top of the pendular driving element, and the portion of the pendular element extending above the highest grate support plate are enclosed within a cylinder attached to the top of the uppermost grate support plate, such that the pendular element and associated pivoting joint are protected from falling fuel material from the fuel feed system above. The bottom of each pendular driving element is vertically slidably pivotally connected to a horizontally sliding linear mechanism having one or more linear bearings in such a way as to allow a horizontally reciprocating motion of the said linear mechanism, oriented in a direction parallel to the side plates of the grate box, to produce a swinging motion of the pendular element about its pivot, said swinging motion also parallel to the sides of the grate box. The slidable pivotal connection at between the bottom of the preferred pendular driving element and the linear mechanism allows for the relatively small vertical movement of the bottom of the preferred pendular element as it swings forward and rearward of a central vertical position. Alternatively, the connection at the bottom of the preferred pendular element can be purely pivotable and the pivot at the top of the preferred pendular element can allow vertically slidably pivotal movement to accomodate relatively small vertical movements of the pendular element during its arcuate swinging path. By means of the foregoing linkage, the frontward-and-rearward horizontal reciprocation of the linear mechanism located at the bottom rear of the grate box imparts a frontward-and-rearward swinging motion to the pendular drive that in turn imparts a frontward-and-rearward horizontally reciprocating sliding motion to the sliding ash removal plates. Each of the linear mechanisms located at the bottom rear of a particular grate box is connected to a suitable controllable actuator mechanism located outside the gasifier chamber by means of an actuator connecting element, such that the actuator mechanism is capable of causing a horizontal reciprocation of the linear mechanism, which causes swinging of the pendular drive, which in turn generates the sliding reciprocation of the ash removal plates.
In a preferred embodiment of the inventive burner, the length of the pendular driving element, the vertical dimensions and therefore vertical spacing of the individual underfire air supply tubes, the length of the grate support plates, and therefore the horizontal spacing of the air supply tubes, and the stroke length of the actuator and therefore also the stroke of the linear mechanism, are determined empirically to produce the preferred sloping profile of the grate surface which generally approximates the angle of repose of the fuel material, as described above. The design of the pendular driving element, air supply tubes, grate support plates and stroke of the actuator is also preferably optimized to result in the efficient synchronous horizontal reciprocation of the ash removal plates such that in their fully extended position, corresponding to maximum forward travel of the linear mechanism and ash removal plates towards the front of the grate box, the sliding ash plates cover most or all of the upper surface of the grate support plates, and in fully retracted position, corresponding to maximum rearward travel of the linear mechanism and ash plates, the ash plates retract as far as the outside surface of the air supply tube above, leaving the upper surfaces of the grate support plates exposed to the fuel pile. Ash resulting from combustion of the fuel nearest the grate that accumulates on these upper surfaces of the grate support plates is swept off the grate support plates by the ash plates as the ash plates extend towards the front of the grate box under the motion of the preferred pendular driving element which is in turn driven by the actuator mechanism. During subsequent reciprocating cycles of extension and retraction of the ash plates, the ash moves progressively downwardly also from the innermost higher grate support plates to the outermost lower plates, and thence to an ash collection trough located below the outermost grate support plate from whence it can be removed by conventional means. In a preferred embodiment of the invention, such conventional ash removal means include an auger ash conveyor powered by a controllable rotary motor that functions to remove ash from the ash collection trough to ash disposal means installed outside of the gasifier burner structure. In this way, the relatively simple design of the present inventive ash plate reciprocation and ash removal systems provide for increased operating efficiency of the inventive gasifier burner by reducing the need for frequent and lengthy shutdowns to manually remove ash buildup and maintain complex ash plate actuation systems as is required by many of the gasifier burners known in the prior art.
In another inventive aspect of the present gasifier burner design, at least one control unit is implemented to control the operation of the reciprocating ash removal plates and associated ash removal system. Conventional thermal sensors are installed near the surface of at least one of the grate support plates in at least one grate box such that the sensors are responsive to temperature changes at the surface of the grate support plates, and are also connected to the control unit such that the control unit is responsive to signals sent from the thermal sensors. The control unit is also connected to the motor powering the ash removal systems such that the motor is controlled by signals received from the control unit. The control unit is further connected to the controllable actuator mechanisms that drive the ash removal plates such that the actuator mechanisms are controlled by signals received from the control unit. The control unit functions such that when the temperature at the surface of the grate support plates decreases below a suitable empirically determined lower limit indicating a buildup of ash on the surface of the support plate, signals are sent to activate the actuator mechanisms and the ash removal system motor such that the ash removal plates reciprocate and the ash removal system operates to remove ash from the ash collection trough at the bottom of the gasifier burner. Similarly, when the temperature at the surface of the grate support plates increases above a suitable empirically determined upper limit indicating the removal of ash and the proximity of burning fuel to the surface of the support plates, signals are sent to deactivate the actuator mechanisms and the ash removal system motor such that the ash removal plates stop moving and the ash removal system stops operating. In this way the design of the present inventive gasifier burner provides for increased gasification efficiency during extended operation by controlling the buildup of ash on the gasifier grate, and the removal of accumulated ash from the bottom of the gasifier burner.
A front elevation view of a two stage gasifier burner according to a preferred embodiment of the present invention is shown in
As shown in the cross-sectional elevation view of a preferred embodiment of the inventive burner presented in
The structure and arrangement of the elements of a preferred embodiment of the inventive twin-auger fuel feed system are shown in
In a preferred embodiment of the invention, each auger 6 is supplied with fuel from an associated fuel supply metering bin 27 located above the auger trough 10 near the end of the auger which is outside the outer wall of the burner chamber 3, entering the auger trough 10 through an opening 17 in the top of the trough 10. (Only the exit chute of bin 27 is illustrated in
In a preferred embodiment of the invention, the uniform distribution of fuel over the surface of the grate arrays 12 as described above also allows for accurate monitoring of the fuel pile height using nuclear sensors (not shown) of conventional design mounted on the outer wall 7 of the burner, or some other suitable location. A control unit (not shown) suitably connected to both the pile height sensors (not shown) and the auger motors 8 can be used to detect signals from such pile height sensors (not shown) and to control the speed of the rotary motor 8 powering each fuel feed auger 6, and thereby control the depth of the fuel pile on the surface of the grate arrays 12 to maintain a fuel pile height conducive to optimum gasification efficiency.
The structure and arrangement of the elements of each of the substantially identical inventive grate boxes 11 making up each grate array 12 of a preferred embodiment of the present invention are shown in cross-sectional view in
Fixed below each grate support plate 34 in a preferred embodiment of the inventive grate box 11 is an associated underfire air supply tube 36 that extends laterally across the entire width of the grate box 11, and serves to supply heated underfire air to the fuel pile which rests on the upper surfaces of the grate support plates 34 through multiple air exit ports 40 drilled in the sidewall of each air supply tube 36. In the preferred embodiment of the present invention shown in
In the preferred embodiment of the present invention shown in
In the preferred embodiment of the present invention shown in
In another preferred embodiment of the present invention, multiple thermal sensors (not shown) of conventional design control are installed near the surface of at least one of the grate support plates 34 in at least one grate box 11 such that the sensors (not shown) are responsive to temperature changes at the surface of the grate support plates 34, and are suitably connected to at least one control unit (not shown). Such control unit (not shown) is additionally connected to the controllable motor 18 that powers the ash removal conveyor 16 that removes the ash from the ash trough 50 in front of the grate boxes 11 and is also connected to actuators 24 that drive the reciprocation of the ash plates 38. The control unit (not shown) operates in such a way as to activate the ash conveyor motor 18 and the ash plate actuators 24 when thermal sensors (not shown) respond to a temperature below a suitable empirically selected lower temperature limit, and to deactivate the ash conveyor motor 18 and the ash plate actuators 24 when the thermal sensors (not shown) respond to a temperature above a suitable empirically selected upper temperature limit.
Other variations and adaptations to the inventive subject matter above described will occur to one skilled in the art without departing from the scope of the invention which is defined by the claims.
This application is a continuation of U.S. patent application Ser. No. 10/383,991, filed 7 Mar. 2003, which claims priority from U.S. Provisional Patent Application Ser. No. 60/362,831 filed on 8 Mar. 2002.
| Number | Date | Country | |
|---|---|---|---|
| 60362831 | Mar 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10383991 | Mar 2003 | US |
| Child | 11142293 | Jun 2005 | US |
