The Influence of Material Properties on Equipment Selection and Design

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Stacking and Reclaiming Systems for Lignite

This system is quite standard except that the height of the pile has to be restricted to 9 m to prevent fires. The size of the lignite to be stored is primarily crushed lignite if the run-of-mine lignite is more than 300 mm. In case the top size of the “as received” lignite is 300 mm and below it is preferable to store the “as received” lignite. The reason for this is that if crushed lignite is stored, the fines along with the crushed lignite can cause fires in the piles.

In rainy seasons the total moisture will increase and hence the lignite will have to be covered at least partially. This structure is expensive and adds to cost. The shed should allow operation of the stacker/reclaimer inside it. The pile height should be restricted to 9 m to contain fires since the volatile content of lignite is very high.

Additional Design Measures

In case of lignite, bunker feeding will have to be through shuttle feeders and not trippers. Another important problem when handling lignite is that dust extraction systems do not operate satisfactorily due to high moisture content. Hence, fog spray dust suppression systems which consume less water should be adopted.

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Due to the flow problems encountered lignite bunkers have a rather low capacity if compared to coal bunkers. Also, lignite bunkers are kept open to ensure proper ventilation and as such no separate bunker ventilation system is provided.

As already indicated, the impurities in “as received” lignite are stones, marcosite and sulphur pyrites. Stones are generally picked up manually from the lignite stream. Removal of marcosite has not been tried so far and requires to be explored further. Sulphur pyrites are removed using gamma-ray sensors which detect the presence of pyrites by the density difference between lignite without sulphur pyrites and with sulphur pyrites.

Characteristics of Coal

Coal is at the highest end of the natural process of metamorphosis of fossil material and hence has higher calorific value compared to lignite. However, the calorific value of coal supplied for power stations range from 12 500 kJ/kg up to 33 500 kJ/kg. Coal has less inherent moisture of about 15 % which can increase to 25 % in rainy season as compared to lignite. Coal has low hard groove index of about 50, and a bulk density varying between 0.8 and 1 t/m³.

Coal is not friable unlike lignite, however, during crushing a considerable amount of fines is generated which results in lot of flow problems and dust nuisance. At times “as received” coal contains stones also. Coal received from countries other than India, may contain high volatiles which will aid spontaneous combustion similar to lignite.

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