Rotary kiln off-gas vent system

Abstract: A rotary kiln has a rotatable cylinder through which a product passes in counter-current to a hot gas stream which dries the product. At the product entrance end, a vent annulus enters into the kiln for a length sufficient to allow entrained particles in the exhaust gas to either fall out within the rotating cylinder or pass through the annulus at a velocity sufficient to maintain their entrainment until discharge at a downstream location. Utilizing a vent annulus which enters into the kiln prevents entrained particles from becoming disentrained in the feed breech of the kiln which causes accumulation requiring periodic shut-downs for clean-outs which are disruptive of continuous operation. Utilizing the present invention incorporating a vent annulus in the rotary kiln minimizes down time and increases the efficiency and utilization of the rotary kiln. Additionally, the Separate vent annulus substantially simplifies the feed breech structure of the kiln simplifying construction.

Description: TECHNICAL FIELD This invention relates to rotary kilns and more particularly to an off-gas vent system for a rotary kiln which limits product accumulation in the feed breech. BACKGROUND A rotary kiln is an apparatus primarily used for drying which consists of a slowly rotating cylinder which is slightly inclined relative to a horizontal plane. The length of the cylinder may range from 4 to more than 10 times its diameter. A wet solid product is fed into the higher end of the cylinder. The solids progress through the cylinder by virtue of the rotation and slope of the cylinder for discharge as a dry or semi dry product at the cylinder lower end. Hot gas flows through the cylinder and interacts with the solids to cause drying. The rate of solids flow may increase or decrease depending on whether the gas flow is co-current or counter-current to the solids flow. Rotary kilns operate at fairly high temperatures and are typically lined with insulating block and/or refractory brick. The means for supplying the hot gas may vary. Some kilns utilize direct firing of a combustible such as natural gas with the combustion products in direct contact with the solid product. If contamination of the product with combustion gases is undesirable, indirect gas or oil fired air heaters may be employed to achieve temperatures in excess of steam temperatures. Of course, gasses other than air may also be used. The method of feeding the kiln depends upon the solid product characteristics and on the location and type of upstream processing equipment. When the feed comes from above, a chute extending into the cylinder may be employed. For sealing purposes, or if gravity feed is not convenient, some other feeding means like a screw feeder are normally used. Typically, auxiliary equipment may be associated with the kiln. For example, the exiting gas may pass through an apparatus for removing entrained dust prior to discharge and/or through apparatus for recovering heat. When utilizing a counter-current rotary kiln, where gases flow in a direction opposite to the direction in which the product is preceding, it is common for dust or solid particles to become entrained with the gas flow. With such rotary kilns, as the gas is redirected at the kiln feed end, commonly known as the "feed breech", to exit the kiln through a vent duct, the entrained particles strike the kiln wall, become disentrained and accumulate. Prior art of this kind is shown in Fig. 1 and Fig. 2 where particle trap and outlet pipe for cleaning the trap is used. Such accumulations in the feed breech require periodic kiln shut-downs for clean-out through access holes. Such shut-downs require time for the kiln to cool down, removal of insulation, removal of the accumulated material, etc., which is quite disruptive of continous operations. A rotary kiln having no particle trap but the features of the introductory part of claim 1 is shown in FR-A-1,208,309. The vent duct extends into a rotating cylinder for a considerable length, which means that said cylinder has a dead end where no gas flow exists and the disentrained particles accumulate. The product is directly introduced into said dead end by a conveying duct which extends into the cylinder for a length shorter than the length of the vent duct. A cloud of dust particles created inside the conveying duct and released by the product is therefore not sucked away by the hot gas but may only be removed together with product itself. SUMMARY OF THE INVENTION It is an object of the present invention to provide a rotary kiln utilizing counter-current flow which eliminates particle accumulation in the feed breech by using the counter-current flow for removing dust particles which are released by a conveying duct for the product. It is a further object of the present invention to provide an apparatus for preventing the accumulation of solids in the feed breech and thus eliminate the requirement for shut-down and clean-out of the feed breech. It is another object of the present invention to provide a rotary kiln utilizing counter-current flow which has a simplified construction of the feed breech to reduce product accumulation. These and other objects of the present invention are achieved by providing a rotary kiln according to claim 1. This kiln has a rotatable cylinder, a gas inlet end, and a gas outlet end, the gas outlet end being a product feed inlet, and the gas inlet end being the product feed outlet end, the kiln having means for feeding a product therein and outlet means for exhausting a gas at the gas outlet end and further comprising vent means including a vent duct which extend partially into the cylinder, the vent duct having one or more openings within the cylinder and being connected to the outlet means such that gas flowing through the kiln exits the kiln by passing at least an opening at the end of the vent duct. A stationary feed breech is used for conveying the product through a conveying duct and releasing it to the cylinder at a location more towards the product outlet end of the cylinder than the end of the vent duct. Therefore, all dusty particles are removed from the cylinder, but nevertheless no particle trap is necessary. Utilizing the present invention, all gas exits the rotary kiln through the vent means rather than through an outlet disposed in the feed breech such that any entrained particles are disentrained within the body of the cylinder or are carried through the tube for removal in downstream processing equipment. Utilizing the vent means prevents material accumulation at the kiln feed entrance thus avoiding periodic clean-out with insulation removal. Additionally, the feed breech for the rotary kiln can be modified to avoid the provision of particle traps and clean-out ports, thus reducing capital costs. BRIEF DESCRIPTIONS OF THE DRAWINGS Fig. 1 is a prior art rotary kiln having a conventional counter-current product feed section. Fig. 2 is a enlarged view of the product feed section of the prior art rotary kiln. Fig. 3 is a view of the inventive rotary kiln product feed section. Fig. 4 is a view taken along line 4-4 of figure 3. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, a prior art rotary kiln is shown. The rotary kiln 1 has a rotatable cylinder 2 actuated through gear 3 by a motor 4 for continuous rotation. The cylinder is rotatably supported on bearings 5 and 6 disposed on supports 7 and 8 with the cylinder at an incline to promote the flow of product through the kiln from a product entrance end 9 to a product exit end 10. The ends 9 and 10 are sealed to stationary structures, with the end 9 being sealed by seal 11 to a feed breech structure 12 and the end 10 sealed by seal 13 to a firing chamber 14 within which direct combustion occurs of a fuel 15 for providing the hot gas which passes through the rotating cylinder. A product 16 enters the kiln through a hopper 17 which has a screw conveyer 18 for depositing the product within the cylinder 2. A discharge chute 19 directs the product exiting the end 10 into a second kiln 20. Air 21 enters the second kiln to flow counter-currently with the product to cool the product prior to discharge. When the air enters the firing chamber 14, it mixes with the fuel 15 and is burned to produce the hot combustion gas which then flows through the cylinder 2. The hot gas exits the cylinder, passing by the conveyer 18, as shown by arrow A, through the feed breech structure, ultimately exiting through an outlet pipe 22. Fig. 2 shows an enlarged view of the feed breech structure 12 for the rotary kiln. The feed breech 12 includes a trap section 23 and a clean-out port 24, where entrained particles are captured and accumulated as the gas exiting the kiln turns for discharge through the pipe 22. Periodic shut-down and removal of this material is required to prevent upset in the operation of the kiln. Referring to Fig. 3, the inventive product feed section for a rotary kiln is shown. A kiln 25 has a rotating cylinder 26, a feed hopper 27 and feed conveyer 28 for deposit of a product 29 within the cylinder 26. However, a feed breech structure 30, unlike the structure 12, has no outlet pipe, no trap section and no clean-out port. Instead, a separate vent annulus 31 passes through the structure 30, having a length less than the length of the conveyer preferably being about 25 to 75% of the length of conveyer. The annulus has a first opening 32 within the rotating cylinder and a second opening 33 adjacent the hopper. The first opening has an open area corresponding to the open area of the second opening to maintain a high velocity in the pipe to prevent product accumulation and plugging. The first opening is substantially inward of the end of the rotating cylinder such that the end of the cylinder is essentially dead headed, i.e., there is substantially no gas flow and thus any entrained particles will be deposited in the cylinder not in the feed breech structure. When particles entering the low velocity area, they simply fall to the bottom of the cylinder and are blended with the product as it moves through the cylinder. Consequently, these materials are recovered rather than accumulated. Any particles which enter the first opening 32 are maintained at a high enough velocity such that they remain entrained for separation in a downstream unit such as a dust collector or other apparatus. Referring to Fig. 4, a front view of the kiln end is shown where the annulus 31 has an approximate half pipe shape, partially covering and being supported by the conveyer which enters the rotary kiln. The feed breech structure, without the trap, clean-out port and outlet pipe, is substantially simplified reducing the cost of construction. The elimination of the clean-out port eliminates the need for periodic shut-downs. While a direct fired rotary kiln is discussed for illustrative purposes, indirect heated kilns would similarly benefit by the present invention. Also, while a half pipe annulus is shown and described, it will be understood that any shaped vent annulus can be used with the present invention.

Claims: A rotary kiln comprising: an inclined rotating cylinder (26), a stationary feed breech (27) disposed outside of said cylinder (26) and adjacent a first end (30) of said cylinder (26), means (15) for providing a hot gas at the second end of the cylinder, vent means (31, 32, 33) at the first end for removing the hot gas from the kiln, said vent means including a vent duct (31) extending into said cylinder (26) for a first length, feed means including a conveying duct (28) extending into said cylinder for a second length and conveying a product from said stationary feed breech (27) into said cylinder (26), and means (19) for discharging the product at the second end (10) of said cylinder, characterised in that said vent duct (31) being adjacent to said conveying duct (28) and the first length being about 25 to 75 % of the second length. The rotary kiln of claim 1 wherein the vent duct is a vent annulus (31). The rotary kiln of claim 1 wherein the vent duct has an opening (32) at its first end inside the cylinder, which has about the same open area as a second opening (33) of said vent duct at the second end outside the cylinder.

Inventors: URZA INAKY JOE, URZA, INAKY JOE
Applicants: SIEMENS POWER CORP, SIEMENS POWER CORPORATION
Application year: 1993
Main classification: F27B7
IPC codes: F27D3, F27B7