Dryer for a combustible material

Field of the Invention

The invention relates to a dryer for use in drying materials such as wood bark, wood chips, mud, peat or the like.

Background of the invention

Dryers can be used to remove moisture from a variety of combustible materials. An example of such combustible materials is peat or peat balls that are destined to burn as fuel. Such products tend to have considerable moisture content because they are often stored in locations where they are exposed to the elements. When these products are used as fuel in a burner, a substantial part of the thermal energy generated during consumption tends to be lost in a burner cell as the moisture contained in the product evaporates and escapes. Fuel economy can be improved by reducing the moisture content of these products before combustion. Drying apparatuses have been used in which wood by-products have rotatably dried while being subjected to drying air. This way of drying tends to separate the fine and coarse materials by thus providing a dry product that has non-uniform combustion properties. This separation of fine materials from the coarse ones also tends to contribute to dust problems, fine particles that tend to creep with the drying air or otherwise disperse from the dryer. US 1 783 965 describes a carbon dryer according to the preamble of claim 1, as used in the pulverized fuel preparation and handling plants and includes an endless chain that carries shelves that supply the coal through the dryer.

Summary of the Invention

The invention provides a dryer for drying a material that is used as fuel according to claim 1. The dryer comprises means for transporting the material to be dried along a substantially vertical path that extends between an upper end of the means of transportation, where the material is received; and a lower end of the transportation means, where the material is discharged. The dryer also includes steering means to direct a hot drying gas through the vertical path to remove moisture from the material as it is transported. The steering means includes a feeding duct means for use in the supply of hot drying gas to the conveying means on one side of the vertical path, and an exhaust duct means for removing the charged drying gas from humidity of the means of transport on another side of the vertical path.

Brief description of the figures

The invention will be better understood with reference to the drawings illustrating a preferred embodiment of the

invention. In the drawings:

Fig. 1 schematically illustrates a steam generation system that employs a dryer that

it is incorporated in the invention;

Fig. 2 is a terminal view of the dryer;

Fig. 3 is a side view of the dryer showing the input and output conveyors and their

drive motors;

Fig. 4 is a plan view along lines 4-4 of Fig. 3 with superfluous details

omitted to illustrate the dryer ducts and their mounting brackets;

Fig. 5 is a view along lines 5-5 of Fig. 3 detailing the structure of the tapes

dryer conveyors;

Fig. 6 is a perspective view detailing the structure of the chains used to carry the

conveyor belts in the dryer;

Fig. 7 is a fragmented view illustrating a sensor switch that regulates operation of an input screw conveyor; Y, Fig. 8 schematically illustrates the control circuit system for use in regulating the dryer operation.

Detailed description of the preferred mode

Reference is made to Fig. 1 illustrating a steam generation system 10 that includes a dryer 12 constructed in accordance with a preferred embodiment of the invention. The temperatures indicated at or adjacent to the components of the steam generation system 10 are temperatures of the inlet or outlet air flows, as the case may be. These temperatures are intended to be indicative of the typical system and applications may vary particularly. The steam generation system 10 includes a solid fuel burner 14 that receives peat, wood bark or other similar products at a fuel inlet 16, and the combustion air at the air inlets 18 and air outlet 20 which is coupled to an air pump 22. The solid fuel burner 14 has an outlet of the burner 23 from which hot air is released at a temperature of approximately 982 degrees Celsius (1,800 degrees Fahrenheit). The hot air generated at the burner outlet 23 is received by a steam generator 24. The steam generator 24 uses the heat received with the air at the burner outlet 23 to generate the steam, which is then made available in a steam outlet 28. The air originally received by the steam generator 24 is then allowed to escape to an air outlet port 30, where it is at a temperature in the order of 454 degrees Celsius (850 degrees Fahrenheit). The air that is allowed to escape from the steam generator 24 to the outlet port 30 is received by a heat exchanger 32. The heat exchanger 32 also receives air at room temperature (approximately 21 degrees Celsius (70 degrees Fahrenheit)) from a air pump 34. The air thus received from the air pump 34 is heated by the air that is allowed to escape from the steam generator 24 at a temperature of approximately 232 degrees Celsius (450 degrees Fahrenheit) and is left in a port of outlet 36. Hot air through heat exchanger 32 is received at an inlet port 38 of dryer 12, and is used to dry wet peat or other product received in a wet fuel inlet 40. (Alternatively, dryer 12 it can be manufactured to receive hot air directly from the outlet port 30 of the steam generator 24). The peat or other product, once dry, is supplied by a conveyor (not shown) to the fuel inlet 16 of the solid fuel burner 14. Water vapor (at a temperature of approximately 104 degrees Celsius (220 degrees Fahrenheit )) is removed from the dryer 12 in an exhaust port 42 and supplied to an exhaust battery 44, together with the exhaust air (at a temperature of approximately 288 degrees Celsius (550 degrees Fahrenheit)) of the heat exchanger 32. The average temperature of battery 44 is in the order of 177 degrees Celsius (350 degrees Fahrenheit). The preferred embodiment of the steam generation system 10 is intended to be illustrative of a particular use of the dryer 12, and should not be construed as limiting the types of application for which a dryer constructed in accordance with the invention is intended. The dryer 12 according to a preferred embodiment of the invention is best illustrated in the views of Figs. 2-3. The dryer 12 has a support frame 50 (constructed of steel beams I) that supports a double conveyor 52 suitable for transporting wood bark, peat, mud, or the like. The conveyor 52 comprises the first and second endless steel belts 54, 56. The belts 54, 56 are carried by the sprockets 58, and are driven by a 3/4 horsepower electric motor 60 mechanically coupled to one of the wheels toothed 58 by means of a reduction gear assembly 62. The movement and speed of the belts 54, 56 are synchronized by means of a synchronization chain 64 that moves around the synchronization gears 66 (best illustrated in the view of Fig. 2) two of which are mounted on the axles shown on each of the sprockets 58. Due to this arrangement, the second belt 56 is effectively driven by the first belt 54. The tapes 54 , 56 have two substantially parallel strokes that define under the center of the conveyor 52 a substantially vertical path (not specifically indicated) having a depth of approximately 7.6 cm (three inches), and an approximate width te 2.74 m (9 feet). The material that is transported is dried along this vertical path. The belts 54, 56 carry (in conventional manner for endless conveyor belts) a plurality of rectangular steel pallets 68 (two are specifically indicated in the terminal view in Fig. 5) which serve to propel the material through the conveyor 52 in a controlled way. The movement of the tapes 54,

56 is synchronized so that the vanes 68 advance along the vertical path evenly (in Fig. 8) effectively closing the vertical path and avoiding the free fall of material through the conveyor 52.

The arrangement described above has three main advantages. First, as the material to be dried moves vertically through the conveyor 52, the movement is stimulated by gravity and therefore a relatively small horse electric motor can be used to drive the conveyor 52. Second, the vertical arrangement allows the conservation of the floor space in a plant where the dryer is to be used 12. Third, the fine material is suspended along with the thick material during drying, and therefore a relatively dry product is made available homogeneous, and dust problems are reduced. The belts 54.56 are constructed of a plurality of flat steel plates that are articulated with respect to each other for movement around the sprockets 58. The plates are perforated to allow the passage of the drying gas into or out of the vertical path during the transport of a material to dry. A plate 70 is typical of those found in tapes 54, 56, and is illustrated in the terminal view in Fig.

5. Plate 70 is provided with upper and lower tabs 72, 74, respectively. A downwardly inclined baffle 76 is integrally formed with the lower flange 74, and has a function that will be described more fully below. The plate 70 has a plurality of baffles 78 perforated on its surface (only one specifically indicated in Fig. 5). The baffles 78 are tilted down when the plate 70 moves along the defined vertical path between the tapes 54.56. As is evident in Fig. 3 (in which the outwardly oriented surface of the endless belt 54 is visible) the baffles 78 are placed in a staggered manner, which is preferred in order to avoid the formation of relatively stagnant pockets. or air dead on the vertical path. It will be appreciated that all the plates of the tape 54 are formed with such baffles (which have not been fully illustrated due to excessive detail). The baffles 78 and the openings provided thereunder allow a drying gas (typically hot air) to be delivered to the material being transported and thereafter escaped substantially without obstructions. Because the baffles 78 are tilted downward (when moving through the vertical path) they tend to prevent the material being transported from obstructing the openings below the baffles 78. Furthermore, due to their downward orientation, the Baffles 78 divert the drying gas downward as it enters the vertical path, and then divert the moisture-laden drying gas upwardly as it is removed. Because the baffles 78 force the drying gas to move in such a way, there is less tendency for the dust particles to be entrained with the drying gas and thus removed from the conveyor 52. Additionally, it will be appreciated that the baffles 78 function as pallets, which are sufficient to transport thick materials such as peat balls or bark, but that pallets 68 that extend more completely through the vertical path are better suited to transport materials such as mud so controlled. A plate 80 just above the plate 70 has a bottom flange 82 (similar to the tab 74 of the plate 70). A baffle 84 hangs down from the flange 82 (when the plate 80 moves along the vertical path), and covers the space between adjacent tabs 72, 82 of the plates 70, 80. The baffle 84 serves at both to avoid the housing of the material that is transported between the plates 70, 80, and reduces the escape of dust between the tabs 72, 82. The plates are secured to the endless chains 88, 90 that are constructed of flat links (such as illustrated in Fig. 6) suitable for traveling along the teeth of the sprockets 58. Fig. 6 shows the connection structure of the chain links that is used in conventional manner to secure the plates to the chain links. A supply conveyor 92, located at an upper end of the conveyor 52, and secured to the support frame 50 in any suitable manner serves to distribute the material to be dried through the vertical path between the belts 54.56. The supply conveyor 92 comprises a hopper 94 with an open top face where the material to be dried can be received, from a conventional conveyor. A helical gear 98 contained within a steel casing 100 serves to distribute the material received in the hopper 94 through the vertical path. The housing 100 is illustrated in the views of Figs. 2, 3 and 7. The housing 100 comprises a trough 102 of generally U-shaped cross section (as in Fig. 2) a covered plate 104, and an end plate 106, which can be screwed together in any manner suitable for provide an enclosure along which the helical gear 98 can move the material to dry. The trough 102 has a longitudinally directed opening 108 through which the material to be dried can escape into the conveyor 52 (in a substantially controlled manner) while moving horizontally by the helical gear 98. The opening 108 has a corresponding length substantially to the width of the tapes 54, 56 so that the material can be distributed across the entire width of the vertical path.

A pair of guide plates 114 extend downwardly from the trough 102, one on each side of the opening 108, substantially parallel to each other, to direct the material to be dried towards the conveyor 52. The guide plates 114 lean slightly toward each other. another, and the lower edge portions are separated so that the guide plates 114 can in practice extend substantially toward the conveyor 52 (as will be apparent from the view of Fig. 2). Preferably, a certain amount of space is provided between the tapes 54, 56 and the guide plates 114 to avoid contact between the guide plates 114 and the vanes 68 during operation. In practice, the trough 102 need not be provided with a U-shaped cross section, and a generally rectangular shape may be preferred for ease of construction. If desired, the longitudinal opening provided at the bottom of such a trough can be constructed as several aligned openings, each of which is provided with a sliding gate to regulate the size of the opening. If the bottom of the trough is flat (as with a rectangular trough), each gate can be constructed of a steel plate with a flange bent from an end portion thereof (for use in the sliding of the steel plate through one of the openings), and two protruding flanges can be provided at the bottom of the housing to receive the side edge portions disposed opposite to the steel plate to retain the plate and also to guide its movement of glide. The gates thus constructed can be used to restrict the speed at which the material is supplied to the conveyor 52, and to vary the distribution of material supplied to the conveyor 52. The operation of the supply conveyor 92 is preferably regulated by an end switch. of the feed sensor 116 detailed in the view of Fig. 7. The function of the feed sensor end switch 116 is to ensure that an excessive amount of material is not delivered to the conveyor 52. For this purpose, the power switch power sensor end 116 is electrically coupled to and controls the operation of an electric motor 118 (shown in Fig. 3) that drives the helical gear 98. The power sensor end switch 116 is mounted on the power plate. end 106 of the housing

100. The power sensor end switch 116 includes a microswitch 120 activated by a plunger 122, and a plate 124 that rotates around a hinge 126 attached to the end plate 106. The plate 124 is deflected by the material supplied to through the opening 108 by the helical gear 98, and when it deviates thus presses the plunger 122 of the microswitch 120. A lever arm 128 extends through an opening 130 in the end plate 106 and supports a counterweight 132. Counterweight 132 ensures that the piston 122 is not pressed by the plate 124 until some predetermined accumulation of material occurs at the upper end of the conveyor 52. In practice, the proper choice of a weight for the counterweight 132 will depend primarily on the type of material that dries, which generally increases with the density of the material. Alternatively, a spring can be mounted between the plate 124 and the end plate 106 to press the plate 124 away from the microswitch 120. When the piston 122 is pressed, the movement of the electric motor 118 is stopped. Therefore no additional material is supplied to the conveyor 52 until any delay that has occurred at the upper end of the conveyor 52 is cleared. The feed sensor end switch 116 is preferably coupled also to the conveyor that feeds the supply conveyor 92 so that no additional material is supplied. to hopper 94. A discharge conveyor 134 (shown in Figs. 2 and 3) is attached to the support frame 50 at a lower end of the conveyor 52. The discharge conveyor 134 is positioned directly below the path vertical to receive and carry the dry material through the dryer 12. The discharge conveyor 134 has a structure similar to that of the the supply conveyor 92. The discharge conveyor 134 comprises a helical gear 136 arranged in a casing similar to a trough 138 (an upper face which opens to receive the material from the dryer 12). An electric motor 140 (indicated in Fig. 3) rotates the helical gear 136 to advance the dry material into a discharge hopper where it can be carried by any of a variety of means. The operation of the discharge conveyor 134 does not need to be regulated by any type of power sensor switch; helical gear 136 simply needs to be rotated at a sufficient speed to ensure that all possible material supplied to the casing similar to a trough is carried

138. The construction, assembly and operation of the dryer ducts will now be described with reference mainly to Figs. 2, 3 and 4. As will be apparent from Fig. 2, the dryer 12 comprises four substantially identical inlet ducts 144, 146, 148, 150, and four substantially identical exhaust ducts 152, 154, 156, 158, in pairs as shown. These ducts are mounted inside the endless belts, as is evident in Fig. 2, with only substantially the inlet and exhaust ports extending from within the belts. The movement of drying air in and out of two typical ducts is indicated by the arrows in the view of the

Fig. 4. Preferably, the particular arrangement of the ducts is such that two pairs of the inlet-exhaust ducts (par 144,158 and par 148,154) direct drying air in a first direction through the vertical path, and the two Remaining pairs (pair 146,156 and pair 150,152) direct the drying air in an opposite direction, thus ensuring that the transported material tends to dry equally on both sides of the path.

The pair of the supply and exhaust ducts 150, 152 (whose construction and relative orientation are typical of all ducts) are best illustrated in the plan view of Fig. 4. The ducts 150,152 can be constructed primarily of sheet metal, and are preferably substantially identical in structure. Preferably, the inlet port 160 of the feed duct 150 is approximately 50% larger than the exhaust port 162 of the exhaust duct 152 (with related changes in the dimensioning of the duct body) to reflect the fact that The hot air supplied to the conveyor 52 will be cooled and come into contact considerably before the dryer 12 escapes. Only the exhaust duct 152 will be described in detail, since the remaining ducts preferably have substantially identical structure. The exhaust duct 152 has two openings. One such opening is in the exhaust port 162, and the second is an open face (not specifically indicated) that extends substantially from the top to the bottom of the exhaust duct 152. When the dryer 12 is assembled, The open face is preferably positioned immediately adjacent to one side of the vertical path, that is, substantially parallel and adjacent to the vertical stroke of the endless belt 54 which defines one side of the vertical path. A corresponding face of the feed duct 150 is positioned similarly adjacent to a vertical stroke of the endless belt 56, opposite the feed duct 150. In this way the feed duct 150 can supply hot drying air to one side of the vertical path, and the exhaust duct 152 can let the moisture-laden drying air escape on the opposite side. The open face of the exhaust duct 152 is placed in substantially sealed coupling against the vertical stroke of the endless belt 54. For this purpose, a sealing strip 166 (which can be constructed in four lengths) is secured by means of a strip of metal retention (together with trigger rivet or bolts) to the inner surfaces of the exhaust duct 152. The sealing strip 166 is circumscribed to the open face, and comes into contact with an inner surface of the endless belt 52, such as It is illustrated in the view of Fig. 5. In Fig. 5, the end walls of the duct have been separated to expose the chains supporting the endless belts 54, 56, and consequently only an upper stroke of sealing strip 166 is illustrated therein. It will be appreciated that in the context of a mechanical device such as dryer 12 the perfect sealing coupling will be difficult, if not impossible, and that where the sealing coupling is mentioned in this specification the air leakage can be tolerated as long as a greater part of the drying air supplied by a feed duct to the vertical path is allowed to escape through a corresponding exhaust duct. The way of mounting the feed and exhaust ducts 150, 152 is typical of all dryer ducts 12. The ducts 150, 152 are supported from the frame 50 by means of oppositely arranged mounting assemblies generally indicated by the numbers reference 172, 174. The assembly assemblies 172,174 are substantially identical in structure, and therefore only assembly assembly 172 will be described in detail. Assembly assembly 172 comprises an elongated rectangular support plate 176 which is secured by bolts to the support frame 50. The support plate 176 is arranged substantially vertically in the support frame 50, shown (fragmented) in the view of Fig. 2. A channeled guide member 178 is screwed to the support plate 176. Guide member 178 has a substantially uniform cross-section (shown in the plane of Fig. 4) that defines two channels 180 that serve to guide ar the chains that carry the endless belts 52, 54. A number of connection tabs are welded to the guide member, and the corresponding connection tabs are secured to the supply and exhaust ducts 150, 152. The connection tabs in couples have holes that can be placed in the register and through which a bolt can be passed in order to secure conduits 150,152 to guide member 178 and support plate 176. Three pairs of connection tabs support each conduit, a localized pair towards the top of each duct, a pair, towards the bottom of each duct, and a pair arranged substantially midway between the other two pairs. The basic operation of the dryer 12 according to a preferred embodiment of the present invention is as follows. The material to be dried is distributed by the supply conveyor 92 through the vertical path defined through the conveyor by the endless belt 54, 56. The material is then transported through the conveyor 52 by the vanes 68 of the belts 54 , 56 (whose vanes prevent the free fall of material through the conveyor 52 under gravity). With thick materials, it will be clear that

Baffles of the plates that constitute the endless belts 54, 56 also serve as pallets that transport the materials. The hot-drying air is supplied from a suitable source (for example, the heat exchanger 32 of Fig. 1) to the feed lines, then supplied through the feed lines to the material being transported, and then removed through the exhaust ducts. The exhaust ducts are preferably coupled by the duct to an air pump that serves to extract the moisture-laden drying air in the exhaust ducts; and the powder dispersion of the dryer 12 can be significantly reduced by the use of suction as the means by which drying air is drawn from the feed lines in the vertical path. The particular arrangement of the feed and exhaust ducts illustrated, that is, one that allows the flow of drying gas in opposite directions through the vertical path, is preferable because it causes the material being transported to dry so more uniform on both sides of the conveyor 52, as mentioned above. The loss of dust from the dryer 12 can be reduced in several ways. First, the drying air is preferably extracted through the dryer 12 by means of the suction applied to the exhaust ducts, instead of being forced under positive pressure in the inlet ducts. The tendency of the powder that is dispersed from the conveyor 52 is reduced in this way significantly. In practice, the volume and speed at which air is drawn from the exhaust ducts (by an air pump or the like) will be determined primarily by the moisture content of the material being dried, the speed at which the material is transported, and the temperature of the incoming drying air. Secondly, the channeled guide member 178 can be provided with an elongated surface 192 (indicated in Fig. 4) that is positioned immediately adjacent to the side edge of the chains carrying the endless belts 54, 56 to close a side of the vertical path, by reducing the dust dispersion in this way. (A similar surface will be found in the corresponding guide member on the opposite side of the dryer 12). Consequently, the surface 192 is preferably positioned as close to the chains of the endless belts 54, 56 as possible without interfering with its movement. For this purpose the support plate 176 that supports the guide member 182 is preferably screwed to the support frame such that the separation between the surface 192 and the endless belts 54, 56 can be adjusted by proper insertion or removal of washers or pimples. As mentioned earlier, the dragging of dust particles with the drying air is reduced by the provision of deflectors that deflect the air in the panels that constitute the endless belts 54,56. By directing the air flow out of the conveyor 52 upward, the baffles stimulate the fine particles to remain in the material being transported, instead of exiting the exhaust ducts of the dryer. A dryer control system 194 according to the preferred embodiment is schematically illustrated in Fig. 8. The control system 194 comprises two control circuits 196, 198 that provide drive signals respectively to the motor 60 that operates the conveyor 52 and to the engine 118 that operates the supply conveyor 92. The control circuit 196 receives a steam demand signal from the boiler (from the steam generator 35 in Fig. 1, for example) in a terminal 200. The control circuit 196 generates from it a conveyor drive signal that is directly proportional to the steam demand signal of the boiler and that directly varies the speed of the engine 60. The speed of the conveyor 52 therefore varies directly with the steam boiler demand signal. Additionally, the control circuit 196 receives a temperature signal from a temperature sensor 202 located in the exhaust duct 158. The conveyor drive signal is then reduced in magnitude by a signal proportional to the excess of the temperature signal over a predetermined reference temperature signal generated by the control circuit 196. Therefore, if the material transported is excessively humid, the temperature of the moisture-laden drying gas in the exhaust conduit 158 will tend to be reduced from a certain temperature of predetermined reference (for example 210 ° F when the material dries is wood bark), and the conveyor 52 will decelerate through the control circuit 106 to allow for more thorough drying. If desired, a second temperature sensor 204 may be disposed in the supply line 144 to sense the temperature of the incoming drying air. The control circuit 196 can then generate a differential temperature signal indicative of the temperature drop that occurs in the drying air, and therefore more accurately reflect the moisture content of the material being transported and the point at which The heat causes moisture to be lost. The conveyor drive signal can then be reduced in magnitude by a signal proportional to the excess of the differential temperature signal over a certain predetermined reference differential temperature signal. The conveyor 52 will therefore be decelerated by the control circuit 196 to increase the point at which the transported material dries until the predetermined differential temperature signal between the supply and exhaust ducts 144, 158 is established. control 198 receives from the control circuit 196 the conveyor drive signal, and balances that signal to produce a control signal from the supply conveyor that varies the speed

of operation of the engine 118. The control circuit 198 also receives the pressure signals from a high pressure sensor 206 located in the supply conduit 144 and a low pressure 208 in the exhaust conduit 158. The control circuit 198 generates a from it a differential pressure signal indicative of the pressure difference between the supply and exhaust ducts 144,158. The control circuit 198 then reduces the drive signal of the supply conveyor by an amount proportional to the excess of the differential pressure signal over a certain predetermined differential pressure signal. Since the differential pressure signal will be indicative of the density of the packing of the material to be dried on the conveyor 52, the operation of the supply conveyor 92 will slow down when excessive amounts of material are supplied to the conveyor, quantities that cannot

10 dry properly. The operation of the power sensor switch 116 has been described above. When the power sensor end switch 116 is activated, it is indicated that the material is supported on the top of the conveyor 52, preferably the control circuit 198 simply turns off the operation of the engine 118 and the supply conveyor 92.