Patrick,

Coal size is rarely the problem for large-scale coal handling systems. The main difficulties that usually arise with this material are flow problems due to fines and/or moisture content. The Universities of Wollongong and Newcastle in New South Wales, Australia have undertaken considerable work on the flow properties of coal.

Extensive tests on material from various sources shows considerable differences in the arching potential, of coal according to its origin. Well-established core practices for coal handling installations are the use of mass flow hoppers and fitting Ultra High Density Polyethylene lines in hoppers that have to hold damp coal.

Notes on the probability of flow blockages due to the interaction of lumps are available from tech@ajax.co.uk if of interest but, in general, the scale of bulk coal handling is such that mechanical arching is rarely a consideration.

Lyn Bates ■

Dear Patrick,

Coal size received at power station does not pose any difficulty. The coal size being received depends upon source and location. However, coal size do affect quantum of equipment / system at power station. Following are general observation:

1. If coal is received from steamers (ocean ships), then most probably it will be less than 100mm size. This is due to reason that handling of such coal at port of export as well as port of receipt is convenient. Therefore, coal crushing system to create this size will be in country of export (either at mine or at port).

2. If the coal is from the same country and arriving by rail, it can have size less than say 400mm to less than 100mm. This depends upon mining method and coal handling system there. Coal subjected to in-land handling is often kept as less than 300mm size, such size reduces dusting and loss of fine coal during handling while it is not inconvenient in handling. (-)300mm size implies 90% less than (-)150mm.

3. If the power station is at mine head it may receive coal as per sr. no. 2 or even more in size.

After mining, the coal needs to passes through primary crushing and secondary crushing before feeding into boiler bunkers. So, its location could be at mine or at port or at power station, according to conveniency.

Regards

Ishwar G Mulani - Author book: Engineering Science and Application Design for Belt Conveyor

Advisor (consultant) for bulk material handling systems

EmailID: parimul@pn2.vsnl.net.in ■

I am aware of at least one power station in Australia that sets top size of 50mm for incoming coal from nearby mines. This specification is applied tightly.

Other sources reveal that 50 mm is a commonly specified top size, due to the requirements of pulverisers.

The definition of what constitutes a 50 mm lump in a material that contains many acicular particles is an issue that can be argued from various perspectives. ■

Maximum 50 mm size implies that it is 'unsized' material composition, where 100% material is less than 50mm size, 90% material is less than half that size i.e. less than 25mm, etc. ■