The Influence of Material Properties on Equipment Selection and Design

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Effect of the Calorific Value of Coal

The capacity of the coal handling system will be generally lower than lignite handling system for the same station capacity unless the coal selected is of low rank due to its lower calorific value. Hence, the capacity of the crushers, screens, conveyors, etc., will be lower compared to those of lignite handling system.

For lower station capacities say of 2 × 250 MW and when higher calorific value coal is used the capacity of coal handling will be about 1000 t/h. In such cases, the coal handling system will be quite simple with the capacities of crushers and screens being well within the available range. If the capacity of the system is higher than 1400 t/h, the system will have to be split in to two streams which is not a serious problem while handling coal.

In coal fired power plants, coal can be received by rail, or by ships or through conveyors. Even if the incoming feed rate is higher than the plant requirements, splitting of the feed rate is quite simple compared to lignite. Flow dividers work quite efficiently provided proper care is taken such as keeping divider out of the flow stream. For the design capacity of the system the inefficiency of the unloading systems has to be considered which can vary from 0.5 to 0.8, depending upon whether the unloading system is for ship or rail unloading.

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Chute Design and Flow Diversion

The effect of total moisture is to make the coal clog the chutes, build up on the surfaces in contact, and generally cause flow problems during handling just similar to lignite although not as severe. Chutes shall be straight as far as possible and where it is necessary to provide change of direction, the side plate angle in the area of change of direction should be 65°.

Two way chutes with gates work quite effectively and hence short belt and shuttle feeders can be avoided along with their associated inherent problems when coal from the same conveyor has to be fed to two conveyors. However, in a few locations such feeders may be required. The chutes shall be provided with stainless steel liners to aid smooth flow. Skirt board liners shall be stainless steel liners or polymer liners. Long chutes shall be avoided as far as possible as in the case of lignite handling systems. However, hoppers shall be avoided for flow diversions as well.

Coal Unloading System

Coal is generally received either by rail, ships or via belt conveyors. The capacity of the unloading system may be higher than the minimum plant requirements. In case of rail receipt, the coal will be transported either through BOX-N wagons, for which wagon tipplers need to be adopted, or through Bottom Opening Broad Gauge Rail (BOBR) wagons, for which track hoppers are required. The unloading rate governs the total system capacity. Presently as per the guide lines of the Railways Design and Standards Organisation (RDSO) norms the number of wagons to be unloaded per hour is 25. This aspect has to be considered for deciding the unloading rate.

The coal rakes will consist of 59 wagons with a capacity of 67 t each. The coal unloading systems such as wagon tipplers are to be designed for achieving the required unloading rates. It can be difficult to achieve the unloading rate due to presence of stones and boulders or large lumps. The rail unloading system is cyclic which also reduces the throughput. Large lumps cause heavy wear and tear and hence the liners should be stainless steel. Stones and large lumps can also damage the belt of the evacuating belt conveyor. Hence, apron feeders are provided to take the impact of the falling lumps and boulders. In rainy season in addition to the problems caused by large lumps, wet coal that tends to stick to the walls causes flow problems. Track hopper walls have a side wall angle of 65° and the wagon tippler hopper have a side plate angle of 65°. All such problems reduce the unloading rate.

If ship unloading systems are adopted, two types of ship unloaders are generally adopted: intermittent unloaders or continuous unloaders. In case of the intermittent unloaders the efficiency of the unloaders is 50 % while in case of the continuous unloader the efficiency can be 75%. These efficiency factors have to be considered while calculating the system capacity.

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