The Influence of Material Properties on Equipment Selection and Design

Related Vendors

Elementar Analysensysteme GmbH

Glatt Ingenieurtechnik GmbH

TRM Filter d.o.o.

Tata Consulting Engineers

Design Measures for Coal Handling Systems

Since the total moisture content in coal is less than that in lignite, the achievable capacity per flip flow screen will be higher while handling coal than while handling lignite. As per the information available flip flow screens handling coal of 300 t/h capacity are available. Thus the number of screens can be reduced and the system design can be simplified.

The system design features are generally same as in the case of lignite handling systems. Covering sheds are generally avoided since the total moisture is not as high as in the case of lignite. However crushed coal can be stacked unlike in the case of lignite. Pile height has to be below 9 m. Bunker feeding is through trippers and feeding through shuttle feeders can be avoided making the system more simple and also avoiding additional equipment.

Dust extraction systems do operate satisfactorily and unlike in the case of lignite need not be avoided. Dust suppression system if provided would be required as only a stndby system to dust extraction system as mandated by certain state pollution control boards. When required to be provided, fog spray systems can be avoided. For control of dust in the stock piles, plain spray systems are required to be adopted.

Gallery

Gallery with 5 images

Coal bunkers have higher capacity as opposed to lignite bunkers in view of the lesser intensity of flow problems as opposed to lignite bunkers. Also, coal bunkers are kept closed unlike lignite bunkers. To ensure proper bunker ventilation separate bunker ventilation system is provided.

Impurities in the “as received’ coal being mainly stones, manual removal of stones is adopted.

From the above, it is seen that lignite and coal handling requires care and certain special considerations. To the credit of the system designers it must be said that most of the features described above have been implemented satisfactorily and the systems are operating successfully all over the world. However, improvements are still required and with time these would be ironed out.

Conclusions Common to Lignite and Coal

Both lignite and coal are fossil fuels but have varying characteristics especially with respect to calorific value and moisture content. The calorific value of lignite is lower, whereas its moisture content is higher than that of coal. Lignite is friable and coal is not. Lignite as mined may contain stones, marcosites and sulphur pyrites which varies from place to place of the deposits, whereas coal only contains stones. These differences in characteristics are the corner stones on which robust design concepts have to be built. Generally it can be said that coal is an easier-to-handle material compared to lignite.

Conclusions for Lignite Handling Systems

The system capacity of lignite handling systems is higher compared to coal for the same station capacity. An efficiency factor of 0.75 to 0.8 shall be considered to cater for sluggish flow.

The side plate angle of conveyor chutes should be 70°, and stainless steel liners should be provided in chutes to improve flow. For diversion of flow, maybe shuttle and belt feeders ought to be provided instead of two way chutes. Same capacity system cannot be split into several streams. Hoppers should not be used for diversion of flow or for distribution to different conveyors, similarly flow dividers should be avoided.

Because of the higher capacity of lignite handling systems procurement of equipment can be a difficult process considering the availability of high capacity equipment.

The high moisture content of lignite reduces the throughput of crushers and screens, and the friable nature of lignite causes generation of high quantity of fines.

Compared to PF boilers, CFBC boilers call for more complicated screening and crushing systems. If uncrushed lignite or primarily crushed lignite needs to be stocked the height of the pile should not exceed 9 m. Sheds have to be provided to protect the lignite during rainy seasons. Feeding of bunkers shall be through shuttle feeders and not through trippers. Bunkers are left open and hence there is no need for bunker ventilation system.

Dust extraction systems are to be avoided and fog spray systems should be considered at lignite transfer points and for stockpile dust suppression.

Impurities in lignite such as stones and pyrites can be separated, however marcosites cannot be removed.

Conclusions for Coal Handling Systems

Coal is received either through rail or ships. When received through rail wagon tipplers or track hopper systems are adopted. Apron feeders are used in wagon tipplers to absorb the impact of large stones. Stainless steel liners are provided in wagon tippler hoppers to provide for smooth flow. Similarly, track hoppers are also lined.

The system capacity of coal handling systems is lower compared to that of lignite for the same station capacity. The capacity of the unloading system is governing. For rail unloading systems, an efficiency factor of 0.75 to 0.8 shall be considered to cater for sluggish flow in rainy seasons, problems due to large lumps, cyclic nature of unloading, whereas for ship unloading systems, an efficiency factor of 0.5 to 0.75 shall be considered. Because of the lower capacity of coal handling systems for the station capacity procurement of equipment is not as difficult as for lignite handling systems.

Although the moisture content is lower than that of lignite, coal also causes flow problems by clogging chutes and hoppers during rainy seasons. The side plate angle of conveyor chutes should be 65°. For diversion of flow, two way chutes and flow dividers can be used instead of shuttle and belt feeders at all locations except in a few locations where called for.

Compared to PF boilers, CFBC boilers call for more complicated screening and crushing systems. Although the moisture content of coal is not as high as that of lignite, the throughput of crushers and screens gets reduced in rainy seasons. The crushed coal needs to be stocked, and the height of the stockpile should be less than 9 m to prevent self-ignition. Sheds are not required even during rainy seasons.

Feeding of bunkers should be through trippers and not shuttle feeders. Furthermore, the bunkers should be closed and an effective bunker ventilation system should be provided.

Whereas dust extraction systems can be avoided at coal transfer points, fog spray systems shall be considered as standby systems if called for in the state pollution control board mandate. For stock pile dust suppression, plain water spray systems should be provided.

References:

[1] Kölling, G, and Schnur, F.: Chemierohstoffe aus Kohle. Thieme, Stuttgart 1977.

* B.V. Sambashivan is Managing Director at Tata Consulting Engineers, India

(ID:38717020)