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Show storage . At smaller industrial sites, the reception bunker itself forms a significant part of the on-site storage. Boiler feed hoppers are usually positioned above the boiler so that the coal can flow by gravity. The hopper capacity must be sufficient to ensure continuity of boiler operation in the event of a conveyor system breakdown. This was traditionally at least 24 hours supply with mechanical coal conveying but can now be substantially less with more readily repaired pneumatic systems. Medium term coal storage, for up to a week's operation, can also be stored in high level bunkers. Longer term storage is conveniently in open stockpiles: various stock reclaimation systems are available, depending on the scale of operations. However, coal storage space is often no longer available at many industrial sites and silos are being used increasingly for medium and long-term storage. Capacities are typically 100 to 500 tonnes, although units up to 1000 tonnes are available. A new development in coal reception for industrial sites is for the lorry to tip the coal on to a platform, which is then tipped to feed the conveying system. This avoids the expense of excavating a reception hopper below ground. Correct design of bunkers, hoppers and silos is essential to ensure that difficulties are not encountered due to blockage, inconsistent flow or segregation of size components. The minimum angle of inclination of the cone base must be specified, depending on the flow properties of the particular coal, typically 70 to the horizontal for smalls coal, less for singles. To ensure the flow of smalls coals, bunkers and hoppers are preferably lined with non-corroding smooth-surface materials, such as glass or high density polyethylene. Also proprietary vibrating devices are available to promote coal flow. In all situations where coal is stored, care must be taken to avoid spontaneous combustion. If air is present, coal will oxidise slowly at ambient temperature, giving out heat, and if the heat is not dissipated then the temperature will gradually rise and the rate of combustion will increase. There are two approaches to preventing this. In the case of singles coal in a hopper or silo, it is preferable to ensure good 20-22 |