NZ232752A - Cylindrical furnace for wood drying with coaxial inner tube extending down from horizontal partitioning wall for efficient combustion of wood flour - Google Patents

Description

232 i •- Patents Form 5 N.Z. PATENT OPS-'SC.K Mj6 FEB 193? BECEtvrn N.Z. No. 232752 Dated 1 March 1990 NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION "IMPROVEMENTS IN OR RELATING TO DRYING SYSTEMS" w We, PATRICK POTTER & ADDOCIATES, a New Zealand Company, of -7-4—La-ko— Oe C& #TO(ii:i^?dciefTCi"d^^M.tn^^F^an4^©«T"HamvHjonT^few-'55,ea!|J?afidv do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement 23 2752 This invention relates to improvements in, or relating to, drying systems.

Known drying systems typically include a fan to force circulate hot air or gas through a drying chamber, and an air or gas heating arrangement. To maintain an even temperature distribution throughout the drying chamber the fan circulation must be periodically reversed. Often, relatively extensive lag times can be experienced as the fan reverses direction. Accordingly, it will be appreciated that precise control of temperature variations within the drying chamber is difficult to achieve.

As a part of the drying process it is commonly necessary to "condition" a product. Typically the conditioning agent used is low pressure steam. In the drying process the moisture content of the product is usually lowered to a level below that actually required. The low pressure steam is then introduced into the drying chamber to "condition" the product, and thereby bring the moisture content of the product back up to the level desired. The low pressure steam is usually produced by a separate boiler plant, which can be an expensive piece of capital equipment.

Further, the furnaces of many known direct fired drying systems are limited in the types of fuel that may be suitably burnt in them to provide a supply of clean hot gas for use in the drying chamber. Thus, the ability to take advantage of a cheap fuel source when it is available, such as waste biomass material, particularly sawdust, does not exist in many systems.

In a broad aspect of this invention there is provided a furnace for use in a drying system, said furnace 2 >/-\ * ^ ,.'"23 A'JC1?93 comprising a hollow, closed cylinder having a sidewall, a base, and a top, wherein an open ended cylindrical tube is suspended inside said cylinder by a wall dividing the hollow interior of the cylinder into lower and upper chambers, such that communication between the two chambers is possible by passage through the cylindrical tube, the cylindrical tube extending part way into the lower chamber and forming an annular gap between the periphery of the cylindrical tube and the inner surface of the sidewall of the cylinder, the cylinder further having a burner inlet in the lower chamber adjacent the annular gap, through which inlet combustible material can be admitted substantially tangentially into the annular gap, a secondary air inlet opening into the upper chamber, a hot gas outlet from the upper chamber, and means for controlling the pressure inside the furnace.

Preferably, the furnace further includes a burner combination comprising a primary burner and a secondary, startup, burner. Conveniently, the primary burner is adapted to burn a micro particulate fuel such as powered sawdust, and the secondary burner is adapted to burn fuel oil.

In a further aspect of this invention there is provided a direct fired drying system including apparatus according to at least one of the two immediately preceding paragraphs.

In order that the invention may be better understood, and so that further features thereof will be appreciated, a presently preferred embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which:- 2*275t Figure 1 illustrates schematically a direct fired high temperature timber drying kiln in accordance with the present invention; Figure 2 illustrates a side elevation of a gas flow directing valve; Figure 3 illustrates a front elevation of a vapour generating apparatus; Figure 4 illustrates a plan view of a cyclone furnace in accordance with the present invention; Figure 5 illustrates a side elevation of the cyclone furnace of Figure 4; Figure 6 illustrates a sectional view of the cyclone furnace of Figure 4 showing detail of the burner inlet region; and, Figure 7 illustrates a sectional elevation of a burner nozzle suitable for use with the furnace of Figure 4.

As shown in Figure 1, there is provided a direct fired high temperature timber drying kiln, as generally indicated at 1, comprising a cyclone furnace 2 connected into a ducting arrangement 3, which in turn is connected to a drying chamber 4.

An outlet 5 of the furnace 2 is connected to a side inlet 6 of a venturi mixing section 7 which in turn is connected to an inlet 8 of a circulating fan 9. An outlet 10 of the fan 9 is connected to a primary hot gas supply duct 11 which terminates at an inlet 12 of a flow directing valve " 23 A!JCD23r 13. In use, the flow directing valve 13 diverts the flow of hot gas from the primary supply duct 11 into either of two secondary ducts 14,15. The secondary ducts 14 and 15 connect into and are open to the drying chamber 4. In use, a flow of hot gas passes down the primary supply duct 11 and into either of the two secondary ducts 14,15. The hot gas then flows through the secondary duct 14 (or 15) and into the drying chamber 4. After circulating through the drying chamber 4 the hot gas exhausts into the other secondary duct 15 (or 14), back to the flow directing valve 13 wherein the hot gas is diverted to a primary exhaust duct 16. The downstream end 17 of the primary exhaust duct 16 is connected to a steam generating apparatus 18, which in turn is connected to a main inlet 19 of the venturi mixer 7. Thus, the ducting arrangement 3 of the drying kiln 1 provides for a fully looped gas flow.

The furnace 2 may be fired by any fuel, for example, oil, gas, coal, or biomass material. Preferably however, wet sawdust residue resulting from the milling of green timber is utilised. In order to utilise wet sawdust having an initial moisture content of 160% as a fuel, a two stage sawdust drying system must first be employed.

The first stage of the sawdust drying system may typically comprise a drying apparatus 20, as described in New Zealand Patent Specification No 224218. In the second stage 21 the sawdust is fed into a secondary stream of hot gas taken from the furnace outlet 6 and is conveyed in suspension to a grinding mill 22 where drying is completed, and the sawdust reduced in particle size to form wood flour capable of being rapidly combusted in suspension. From the mill 22 the ground fuel is blown through a pulverised fuel burner 2 3 into the cyclone 2 3 A' *3*73g furnace 2.

Referring now also to Figure 2, the flow directing valve 13 comprises a housing 2 4 within which there is provided a central chamber 25. Typically four passages, 12,2 6,27 and 28 are provided in the housing 24 allowing access to the central chamber 25. When in use, the passages 12,26,27 and 28 may be connected to the primary supply duct 11, the primary exhaust duct 16, and the secondary ducts 14 and 15 of the drying kiln 1 respectively. The passages 12,2 6,27, and 28 lie in a common plane, and are oriented at 90° to each other. It will readily be appreciated, however, that any number of secondary ducts may be used, that the relative passage orientation may be changed, and that the passages connected to the secondary ducts need not all lie in the same plane as the passages connected to the primary ducts. However, it has been found that when two passages are provided for the secondary ducts, and when these passages are oriented at 90° and coplanar with the passages connecting to the primary ducts it is more convenient to later divide the second ducts into tertiary ducts to provide a satisfactory distribution of hot gas in the drying chamber 4. A flow directing vane 29 is pivotably mounted about its central axis, perpendicular to the plane of the passages 12,26,27, and 28 so that, in use, by pivoting between a first position and a second position the flow directing vane 29 can cause the flow from the primary duct 11 into, for example the secondary duct 14 to shift to the secondary duct 15, and conversely, to cause the flow of exhaust gas to change from the secondary duct 15 to the secondary duct 14.

For ease of construction the flow directing vane 29 may be planar. However, to minimize pressure losses, or to accommodate a different set of passage orientations it may ' 23 AUGD93 2?5 be advantageous to utilise a curved vane form.

Incorporating a flow directing valve 13 into a drying system has a primary benefit of allowing for continuous operation of the circulating fan 9 in the same direction. Operation of the valve 13 may be carried out by means of a mechanical actuator programmed to operate automatically at preset timed intervals.

The steam generating apparatus 18, as illustrated in Figure 3, comprises a circular duct section 30 through the wall of which extend five spray nozzles 31 directed radially inwards. The nozzles 31 are displaced around the duct section 3 0 in a semicircular configuration at 60° intervals. A supply pipe 3 2 interconnects all of the nozzles 31 and supplies each with high pressure water.

Figures 4,5, and 6 show details of the cyclone furnace 2. The furnace 2 comprises an upright cylindrical shell 3 3 having sidewalls 34, a base 35, and a top 36. An open ended cylindrical tube 3 7 is suspended inside the cylindrical shell 33 by a wall 38 such that the longitudinal axis of the cylindrical shell 3 3 is colinear with the cylindrical tube 37, and the wall 38 divides the interior of the cylindrical shell 3 3 into an upper chamber 39 and lower chamber 40. The cylindrical tube 37 extends part way into the lower chamber 40 so that an annular gap 41 is formed between the outer periphery of the cylindrical tube 3 7 and the inner surface of the sidewalls 34. A burner inlet 42 is provided through the sidewall 34 of the cylindrical shell 3 3 into the lower chamber 40 adjacent the annular gap 41. The burner 2 3 and a startup burner 43 are positioned so as to emit combustible material substantially tangentially into the annular gap 41. It will be appreciated that the startup burner 41 may 23 A"JCS333 232?5Z be dispensed with, and, for example, combustion may be initiated by simply lighting a fire in the lower chamber 41, with the smoke etc resulting being vented to the atmosphere. As the total air supplied at the burner 2 3 is less than that required to complete combustion, secondary air inlets 44 arranged to introduce air tangentially into the chamber, are provided in the upper chamber 3 9 through the sidewall 34 of the cylindrical shell 33, adjacent wall 38. The outlet 5 for hot gas produced in the furnace 2 is located in the sidewall 34 of the cylindrical shell 3 3 adjacent the top 36. Finally, an excess pressure relief valve 45 and furnace pressure control damper 46 are located in the top of the cylindrical shell 33 to maintain the desired pressure in the furnace 2.

Figure 7 illustrates a burner blast tube 47 suitable for use with the burner 23. Because the relatively large mass of air in which the wood flour is entrained would retard ignition at the burner 23, the burner blast tube 47 is divided into a central core 48 surrounded by an outer annulus 49.. The air fuel mixture from the grinding mill 22 is conveyed through a first portion 50 of the burner blast tube 47 incorporating a spinner 51 arranged to centrifuge the wood flour to the outer wall of the portion 50 concentrating it into some 25% of the total air flow. This concentrated air fuel mixture passes through the outer annulus 49 of the burner blast tube 47 and is slowed to the velocity of flame propagation of the fuel and at the same time is reduced in spin velocity. The remaining 75% of the air is relatively devoid of wood flour and passes through the central core 48 of the burner blast tube 47. While the outer air fuel envelope ignites at slower velocity the inner core air at higher velocity spins outwards into the air fuel stream to supply combustion air as it scrubs the air fuel envelope and is O'v "\\ of, 2 "3 A'J C t y13' J *3 27 52' slowed to the velocity of the air fuel mixture in the outer envelope.

In use, the drying chamber 4 of the direct fired high temperature timber drying kiln 1 is charged with wet timber stacked in a manner which permits air to flow through the stack and contact all surfaces to be dried. A baffle arrangement is set up inside the drying chamber 4 to direct the air flow as shown in, and described with respect to Figure 1.

The furnace 2 is then brought up to temperature by either lighting the startup burner 43, or by simply lighting a wood fire in the base of the lower chamber 4 0 of the furnace 2. At this stage all gas produced by the furnace is vented to atmosphere. When the furnace 2 is at the correct temperature the sawdust drying process is initiated and suitable wood flour fuel produced. Full scale combustion in the furnace 2 is begun. In the lower chamber 40 of the furnace 2 any oversized particles that have escaped through the mill 22 are centrifuged to the wall of the chamber 4 0 and drop out of the conveying gas stream to complete combustion on the base 3 5 of the furnace 2. Thus, the furnace design ensures that no particle that is incompletely combusted is allowed to pass from the lower chamber 40 of the cyclone furnace 2. This ensures that a clean atmosphere inside the drying chamber 4 is maintained. As the fuel is burnt completely at the rate at which it is metered to the furnace 2 the switching off of the fuel will immediately extinguish the flame and there will be no combustible residue.

It will be appreciated that it is essential that the cyclone furnace 2 produce clean hot gas so that this gas does not damage the timber to be dried. .. r -> : A. ^ "\\ • CV.i 23 AUG393 j *3 2? Hot gas from the upper chamber 3 9 of the furnace 2 enters the venturi mixing section 7 to mix intimately with gas returning from the drying chamber 4 to maintain the preset temperature of the gas entering the furnace 2 for optimum drying conditions. The venturi mixing section 7 performs a number of further functions, including the maintenance of a vacuum inside the furnace 2. The outlet of the venturi mixing section 7 slows the velocity of gases and allows static regain to occur to provide an even flow of gas to the inlet 8 of the circulating fan 9 without shock.

Periodically, the flow directing valve 13 reverses the air flow in the drying chamber 4 in order to maintain as even a temperature as possible throughout the drying chamber 4.

When the average moisture content in the drying chamber charge has been reduced to approximately 5%, a pre-timed conditioning period is carried out by subjecting the dried charge to an atmosphere of low pressure steam at approximately 100'C. Pressurised water is injected through the nozzles 31 of the steam generating apparatus 18 into the return air from the drying chamber 4. Minute particles of water from the nozzles 31 collide with the high velocity air, and are rapidly dispersed through the hot gas existing the furnace 2 by way of the outlet 5. The temperature of the air/gas mixture leaving the circulating fan 9 is reduced from approximately 120'C for the drying mode to 100°C for the conditioning mode. A slight surplus of water is added through the nozzles 31 to ensure that 100% saturation is maintained during the conditioning cycle. Excess water may be removed by a sealed trap positioned at the outlet of the venturi mixing section 7. During the conditioning cycle extra heat is provided by the burner 23 to compensate for the latent heat demand of the evaporated water, and vapour is ■'23 A13S1/ ?5g continuously emitted from a ventilating door provided in the drying chamber 4.

It is to be understood that all ducting for the direct fired high temperature timber drying kiln 1 as described above is to be designed and constructed for high velocity circulation. Conveniently, all bends incorporate turning vanes for low system pressure losses, and equalisers for equal flow to and from the drying chamber 4/ this is necessary with the air circulation inside the chamber 4 being at a very high rate, up to 2 2 changes per minute depending on the nature of the charge.

Additional advantages of the present invention will become apparent to those skilled in the art after considering the principles in particular form as discussed and illustrated.

Accordingly, it will be appreciated that changes may be made to the above described embodiment of the invention without departing from the principles taught herein.

Finally, it will be understood that this invention is not limited to the particular embodiment described or illustrated, but is intended to cover all alterations, additions or modifications which are within the scope of the appended claims. 11 , i-i T ■ '23 AUG j993 /

Claims (11)

^3 5y WHAT I CLAIM IS:

1. A furnace for use in a drying system, said furnace comprising a hollow, closed cylinder having a sidewall, a base, and a top, wherein an open ended cylindrical tube is suspended inside said cylinder by a wall dividing the hollow interior of the cylinder into lower and upper chambers, such that communication between the two chambers is possible by passage through the cylindrical tube, the cylindrical tube extending part way into the lower chamber and forming an annular gap between the periphery of the cylindrical tube and the inner surface of the sidewall of the cylinder, the cylinder further having a burner inlet in the lower chamber adjacent the annular gap, through which inlet combustible material can be admitted substantially tangentially into the annular gap, a secondary air inlet opening into the upper chamber, a hot gas outlet from the upper chamber, and means for controlling the pressure inside the furnace.

2. A furnace according to claim 1, wherein the secondary air inlet is adjacent the dividing wall.

3. A furnace according to claim 1 or 2, further including a burner combination comprising a primary burner and a secondary startup burner.

4. A furnace according to claim 3, wherein the primary burner is adapted to burn a micro particulate fuel carried in a suspension of air.

5. A furnace according to claim 4, wherein the micro particulate fuel is powdered sawdust. 12 N-Z.PATEI'MT OFFICE -9 SEP 1993 23 2 7 52

6. A furnace according to any one of the preceding claims wherein the longitudinal axis of the cylindrical tube is substantially co-linear with the longitudinal axis of the cylinder.

7. A furnace substantially as herein described with reference to the accompanying drawings.

8. A drying system including a furnace which is in accordance with any one of the preceding claims.

9. A drying system according to claim 8, in which first and second spaced apart ducts open into the drying chamber, and a gas flow directing valve connected to the hot gas outlet and to the first and second ducts, the valve being operable between a first condition in which the hot gas outlet is connected to the first duct and the second duct is connected to an exhaust duct, and a second condition in which the hot gas outlet is connected to the second duct and the first duct is connected to the exhaust duct, whereby the direction of flow of the hot gas through the drying chamber may be reversed.

10. A drying system according to claim 8 or claim 9, including vapour generating apparatus comprising a duct section having a peripheral wall through which one or more spray nozzles extend, each said nozzle being arranged to direct, in use, a fog like spray of a liquid conditioning agent into a fast moving flow of hot gas passing through the duct section, the duct section being connected to feed the vapour through the drying chamber.

11. A drying system substantially as herein described with reference to the accompanying drawings. 13 PATRICK POTTER By his attorneys