Fossil Fuel Supply for India's Coal-fired Power Plants

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Selection of Ship or Barge Unloader

For selection of the capacity of the ship unloader, the required unloading rate is again central to selection of the unloader. The number of unloaders can also be fixed based on the required unloading rate. Next step is to decide on the type of unloader which is governed by the cost and the lump size of coal to be handled viz whether the unloader is berth mounted or the ships are geared and therefore are of self unloading type. This is to be decided based on various factors such as chartering rate of ships, availability of geared ships capable of giving the desired unloading rate, etc.

In India, self unloading vessels with conveying systems for unloading are not very popular. In India, when geared ships are to be adopted, generally these are ships fitted with grab bucket cranes, the number of such cranes depending upon the required unloading arte. Such geared vessels are adopted in the TNEB’s Tuticorin coal unloading terminals and in the Ennore satellite port. When it is not possible to charter such ships for varying reasons, berth mounted unloaders are to be adopted and generally it is so.

An intermittent unloader (either berth mounted or mounted inside the ship for self unloading), can handle higher size lumps of up to 500 millimetres as in the case of Tuticorin project but has a lower efficiency and lower average to peak ratio compared to continuous unloaders which will result in higher peak evacuation rate compared to the continuous unloaders and this peak capacity will be the rated capacity of the ship unloader.

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A continuous unloader can be fitted with a screw unloader or with bucket unloader with facility for vertical and horizontal travel. These types of unloaders can handle coal of generally 50 to 75 millimetres and higher size lumps are difficult to be handled. Continuous unloaders have higher efficiency and higher average to peak ratio compared to grab bucket type unloaders which will result in lower peak evacuation rate compared to the grab bucket unloaders and this peak capacity will be the rated capacity of the ship unloader.

Unloading Berth Details

An unloading berth would have rail tracks for movement of the unloaders as in case of berth mounted unloaders or travelling hoppers as in the case of Tuticorin terminals where geared ships are adopted, evacuating berth conveyor, water supply and dust suppression systems, fire fighting system facilities etc. The berth would also be fitted with mooring dolphins on either side for mooring and anchoring of ships, fenders and other miscellaneous facilities.

The length of the berth is decided by the maximum length of highest capacity ships to be received, single banking or double banking in case of an unloading of barges, length of highest capacity barges to be received etc. the width of the berth would be decided considering the rail centers for the unloader or the unloading hoppers, evacuating conveyor requirement etc. Typically, the length of an unloading berth for receiving 50 000 dwt ships would be of order of about 250 metres and the width would be suitable for the selected unloading arrangements.

The evacuation system would comprise of feeders at the outlet of the unloading hoppers with facility for speed variation and evacuating conveying system. The feeders can of belt type, reciprocating type etc. The capacity of the feeders and the evacuating conveyors would be equal to the peak unloading rate of the unloaders.

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