The Influence of Material Properties on Equipment Selection and Design

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Chute Design and Flow Diversions

The effect of high total moisture is to make the lignite clog the chutes, build on the surfaces in contact, and generally cause flow problems during handling.

Chutes should be straight as far as possible and where it is necessary to provide a change of direction, the side plate angle in the area of change of direction should be 70°. Furthermore, long chutes should be avoided as far as possible. Gates should be an chutes be provided with stainless steel liners to aid smooth flow. Skirt board liners should be stainless steel liners or polymer liners. Although lignite is always transported through conveyors from mines, due to clogging of chutes and other flow problems, the efficiency of unloading will get reduced and an efficiency factor of 0.75 to 0.8 will have to be considered while fixing the system capacity.

For flow diversion two way chutes should be avoided and instead travelling shuttle or belt feeders can be provided. For feeding bunkers, travelling trippers should be avoided and shuttle feeders should be used instead. At times the travelling shuttle feeders have to be provided with tail end shifting arrangement which will stretch the tail pulley to a certain position through the take-up, at which location feed from the same source can be diverted to a different conveyor. The same arrangement is provided on the shuttle feeders feeding the bunkers from common feeding conveyors. However, compared to chutes for diverting the feed, the provision of shuttle feeders increases the power consumption.

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Short feeders also generally are reported to pose operational problems such as belt sway, etc. and distribution of lignite to different bunkers should be through shuttle feeders. For splitting the feeds, flow dividers shall be avoided and the scheme shall be such that it would be possible to convey the entire feed at the feed rate desired without splitting. Surge hoppers with multiple outlets shall be totally avoided as this restricts free flow of lignite and can cause chocking of the material in the outlets. When two streams (one stream operating +one stream standby) are to be installed and interconnection is required, the interconnection has to be through shuttle feeders. Stream interconnection at all locations is necessary since lignite handling system is more prone to break downs due to flow problems.

Lignite Crushing and Screening Systems

High moisture content causes reduction of throughput of screens and crushers. In Neyveli Lignite Corporation’s Neyveli thermal stations, such problems are being faced. To obviate such problems, flushing of the equipment by water is being adopted. Also the screens have to be heated to eliminate the moisture in the lignite. Crushers are required to be provided with self cleaning equipment to remove the clogged material. These problems have also made procurement of crushers and screens difficult due to two reasons:

• the high capacities of the equipment involved, and

• the high moisture content.

In case of PF boilers, the system can be made simple since the final size required at the boiler bunkers is -25 mm. In case of CFBC boilers, the size required at the boiler is about -10 mm for which three-stage crushing would be required considering the reduction of size required and also to control the fines generation. For good performance of CFBC boilers, two requirements are very important:

• Generally the fines content should not be in excess of 50 % from all stages of crushing including the fines present in the “as received” lignite.

• The top size should not exceed -10 mm.

The friable nature of lignite increases the fines content in the as fired lignite. Hence to obviate these problems and to achieve the above objectives, the following measures are adopted in screening and crushing system design.

Crushing and Screening Design Measures

The large size lignite and stones have to be reduced to about -300 mm in a feeder breaker. Then the -300 mm size lignite has to be reduced to -50 or -100 mm in a roll crusher or roll sizer to minimise the generation of fines. For further size reduction a hammer mill would be required. Hammer mills are known to generate high quantities of fines which has to be considered while designing the system.

As far as screening is required, pre-screening before the tertiary crusher has to be used to prevent -10 mm size lignite going directly into the crusher which increases the fines content. The “as received” lignite also will contain fines and hence screening of -10 mm size lignite before crushing is important. The -10 mm size lignite thus separated before the last stage crusher has to be conveyed to the bunkers. The +10 mm size lignite will be crushed and passed through second stage screens.

The first stage screen or the pre-screen will either be a roller screen, a grizzly screen, or a banana screen which will have to separate -10 mm size lignite. The availability of such screens is not yet established especially for handling lignite. But this constraint must not come in the way of the implementation of such projects. The reason why such screens have not been established is that there are only very few lignite based CFBC boilers in operation the world over and hence not much development has taken place. The author feels that development in this area is very much essential since in future many lignite based CFBC boilers are expected to be installed.

The second stage screens are of the flip flow type whose performance has been very well established and is considered as the most effective for screening lignite of -10 mm size. There are quite a few manufacturers of this type of screens. However, the main problem encountered with this type of screens is their limitation in capacity. The maximum capacity actually possible is 150 t/h for lignite of -10 mm with a moisture content of 50 %. Therefore a bank of screens would be required to meet the system capacity which can be very high as compared to coal for the same station capacity.

Because of such complex crushing and screening arrangement the whole system design becomes very complicated. There are two schemes possible:

• flip flow screens before the final stage of crushing; there will be no pre screen or first stage screening, and

• flip flow screens to be located after the crushers.

In the second scheme a pre screen as discussed above will be required which will have to have a capacity which is equal to the total steam capacity and separate -10 mm. This is the main difficulty in this scheme. However, there are many who feel that this is a better scheme than the first one. These designs are in engineering stage and which of the two is better will be known after successful implementation.

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