Manufacturing-Engineering

Transcription

1 Manufacturing-Engineering BAGASSE STORAGE AND HANDLING IN QUEENSLAND D. J. Muir The Australian Estates Co. Ltd. Kalamia Estate Ayr, North Queensland ABSTRACT Previous methods of bagasse handling in Queensland, involving storage in a loft adjacent to the boiler firing floor and manual feeding of bagasse to the fires during mill stops, created fire risks and unsuitable working conditions for operators. With a requirement for better working environment, and also because of increasing costs of labour, the sugar industry has generally changed to some form of mechanical storage of bagasse and automatic control of bagasse feed to the boiler feeders. A system generally adopted in Queensland of shed storage and boiler feeding is described and discussed in light of more modern developments. Finally, an improved system to be installed at Kalamia Sugar Mill for the 1971 season is described together with a discussion of the main points of design. INTRODUCTION Apart from bagasse storage requirements to meet normal mill stops of short duration, some considerable storage capacity is required to meet fuel requirements during weekly boil out of equipment and mill startup, prior to, and following the weekend shut down period in Queensland. The fibre content of the cane supply changes from hour to hour, depending on the type of cane being crushed. During periods of the crushing season, when the bagasse supply is barelysufficient for boiler fuel requirements, the use of bagasse storage can offset fuel oil usage by providing reclaimed bagasse fuel during periods of low fibre cane grinding and storing excess bagasse while high fibre cane is being crushed. The storage systems are not designed to meet the protracted periods of some weeks of low fibre cane as these conditions usually occur at the commencement of a crushing season, andit has been found that the bagasse stored over a 5-month off season has little fuel value and proves difficult to burn in the furnace. Traditionally, bagasse had been stored in lofts adjacent to the fires and reclaimed as required by manual forking to the furnace. Some forms of portable conveyors were introduced to reduce the labour requirements; however, workmen were in a constant cloud of bagacillo dust, and fires were not uncommon. Investigation revealed that prolonged exposure to such a dust laden atmosphere could possibly produce some irritation of the lungs; therefore, to improve working conditions thought was given to a form of mechanical handling whereby operators would not be in contact with the bagacillo dust. The designs adopted in Queensland have overcome the dust hazard to operators and reduced fire risk. The replacement of drag scraper conveyors by conveyor belts has resulted 1316

2 ..,} I I i, '.' [..... D. J. MUIR 1317 in maintenance savings, while at the same time both the bagasse storage system and the automatic feeding of bagasse to boiler feed chutes have reduced operator labour costs. The control design is suitable for extension towards more complete automation as this trend develops in the remainder of the factory. RECTANGULAR SHED STORAGE The original machine of this type was installed at Pleystowe Mill, Mackay, in The design was revised for the next installation at Kalamia Mill in 1960, and this design, with some modification, has generally been adopted by Queensland mills. The machine is illustrated in Fig. I and consists of an over- Fig. 1. The Kalamia bagasse storage and reclaim machine is shown receiving bagasse from the storage belt in the!1pper right hand corner and discharging bagasse to the reclaim boiler supply belt in the lower right hand corner of the photograph. head traversing gantry which supports a scraper conveyor capable of being raised or lowered on winch ropes from the gantry above. Bagasse from the mill is ploughed from the conveyor belt in the upper right hand corner onto the tray underthe top flight on the scraper conveyorvhence is moved out and dumped at the left hand side of the shed. The scraper is raised during storage operations and lowered to reclaim bagasse to the boiler supply belt in the lower right hand side of the photograph.

3 1318 MANUFACTURING--ENGINEERING The knee-action supports on each side of the scraper conveyor provide lateral stability to the conveyor as it traverses along the shed, The plough to take bagasse from the incoming belt is mounted on a trolley which traverses with the gantry, placing no restriction on the position of the machine along the shed during the operation of reclaiming or storage of bagasse,, Some disadvantages of the design, found in practice, can be listed. The scraper, in traversing over the stored head while reclaiming, moves a small' quantity of bagasse along with the leading edge of the unit which, over a period,. accumulates at each end of the shed and prevents use of the full length of the shed, unless time is taken to raise the machine to a higher level at each end of the traverse. Depending on the nature of the bagasse, there is a tendency for the bagasse on the inclined face of the storage heap to avalanche down the slope and cause blockages on the return belt system. Some installations have mdinted a.trirn-' ming device on the gantry which maintains the inclined face at a safe angle of repose. During the winter nights, condensation of moisture within the shed will result in wet gantry track rails. At times traction of the driving wheels is insufficient to maintain a high reclaim rate from the machine. A considerable volume of the storage shed is not utilised, as the inclined face extends the full length of the shed. Due to the above difficulties and the reversing of the machine at each end of the shed, the machine requires the attention of a full time operator. An exception is at Farleigh Mill, Mackay, where, with the addition of considerable electrical controls and the provision of trimming devices on the inclined, face, the machine has been operated without an attendant. CIRCULAR SHED STORAGE For some years the advantages of bagasse storage within a circular shed have been considered at Kalamia. As crushing rate increased and it became necessary to increase the capacity of the bagasse handling system, the decision was made to erect the new system on the basis of a circular shed design. Initially the design envisioned a circular shed containing the conventional storage and reclaim scraper conveyor, with a centre pivot supported from the roof and driving wheels on a circumferential track. The bagasse to storage would enter the shed through the hollow centre pivot and fall onto the scraper conveyor to be distributed in the usual manner. Bagasse would be reclaimed by lowering the scaper conveyor onto the heap and moving bagasse down the inclined face to feed out of a centre well in the floor. Such a machine, with the inclined face of the heap forming an inverted cone, would be very susceptible to problems of an avalanche and would requireto be fitted with a trimmer to limit the angle of repose. Consideration of the mechanical arrangement of such a design led to the Cranitch concept of a pair of scraper conveyors fulfilling the combined functions of storage, reclaim and avalanche prevention by hinging the conveyor at the centre of the shed slightly above the centre well, whereby they would traverse an inverted conical bagasse storage or reclaim surface. The arrangement is shown in Fig. 2, where it can be seen storage is accomplished by increasing the angle of repose or reclaim by decreasing the angle; however,the angle can be limited

4 J). J. MUIR 1319!I l,-.: ::1. Fig. 2. The drawing illustrates the Cranitch concept of a rotating pair of scrapers operating in a circular shed, storing or reclaiming bagasse while maintaining a safe angle of repose on the inclined face of the stored bagasse. so that the natural angle of repose of the bagasse is not reached and, therefore, an avalanche is prevented. Subsequent refinement of the design by the fabricators has resulted in an upper level central support of the machine from the roof by means of a crane slewing bearing of sufficient diameter to contain the hollow bagasse feed shaft. The section of the feed shaft below the roof trusses rotates at constant speed and carries the pair of scraper conveyors hinged at its lower end. The conveyors are raised or lowered by means of a winch located on the platform surrounding the rotating section of the feed shaft. The scraper conveyors are driven by hydraulic motors powered from pump units mounted on the winch platform, while the lower section of the feed shaft is rotated by'a hydraulic motor and ring gear mounted at the lower truss chord level. The bagasse handling conveyor belt system has been arranged with a minimum of transfer points and consists of a storage conveyor belt leading directly from the milling train to the discharge point over the centre of the bagasse storage shed roof. The boiler supply belt conveyor receives bagasse from below the centre well in the storage shed floor and discharges onto the distributing belt above the bagasse feeders on the boiler station. Surplus bagasse passing the boiler feeders is returned by chute to the outgoing storage belt for return to the shed. The new design is considered to overcome' all the disadvantages of the rectangular shed design and is to be operated by an automatic control system located at the boiler house control console.

5 1320 MANUFACTURING-ENGINEERING A feeler plate located on the boiler supply belt from the shed measures the rate of bagasse supply to the boiler station. The automatic control system compares this signal to a set point adjustable by the boiler house operator, and either lowers or raises the scraper conveyors to increase or decrease the bagasse supply and so maintain the set rate to the boiler station. The set point is adjusted to maintain some surplus bagasse passing the boilers at maximum steam load. The purpose ofpassing all mill bagasse through the shed and mixing with stored and surplus bagasse is to reduce the effects of wet portions of bagasse in the furnace. Small portions of wet bagasse do occur temporarily during periods of abnormal mill operation and cause uneven firing, or complete blackout, if fed directly to a suspension fired boiler. Also, by placing the storage system in the main bagasse flowpath, delays at the Commencement of reclamation are avoided and a continuous supply of bagasse to the furnaces is assured. AUTOMATIC CONTROL OF BAGASSE FEED TO BOILER CHUTES A system of automatically controlling the level of bagasse in boiler feed chutes was installed at Kalamia in The bagasse is ploughed from a conveyor belt into the hoppers of the feed chutes, the troughed belt being flattened at the plough operating position. Conveyor belts are designed on conventional methods, using a bagasse density of 81bjft 3 and limiting the belt speed to no greater than 400 ftjmin. Two types of ploughs are illustrated in Fig. 3. The front plough is pivoted Fig. 3. Two types of bagasse ploughs are illustrated, the front plough being of the swinging type to push bagasse off the belt while the rear plough has a chopper or guillotine action moving down into the bagasse stream.

6 D. J. MUIR 1321 on the far side of the belt and swings over the belt to push the bagasse into the feed chute hopper. This was the original type of plough installed and has operated with marked success; however, as wider belts are used to meet increased capacities, the operating length along the belt becomes excessive and is not suitable to match the normal spacing of feed chutes on suspension fired boilers. The plough to the rear in Fig. 3 is a chopper type plough which is supported from a post on the far side of the belt and moves down into the bagasse stream to divert bagasse to the feed chute hopper. This plough has proved to be reliable, and a similar type is to be installed on all feed chutes for the rearrangement at Kalamia Mill in Other designs, which move into the bagasse stream from the side, have been successful at other Queensland mills. As Queensland bagasse now consist of relatively fine particles, the bridging of bagasse which might be expected on the leading edge of such ploughs is not encountered. Each plough is operated by a level detection device located at the desired level to be maintained in the feed chute. The normal device is a micro-switch mounted against a plastic diaphragm in the chute wall, which operates the plough on an on-off basis. Older type boilers with feed chutes leading directly into the furnace can utilise thesedevices by the installation of some metering device in the chute. ACKNOWLEDGEMENTS The author wishes to thank the Australian Estates Co. Ltd. for their kind permission to publish this information and to thank fellow engineers at Kalamia who have contributed so much effort towards the successful design, installation and operation of the systems described.