An interesting subject and one that is filled with subtleties.
Pretty much its difficult to apply the ruler because coal is essentially a very complex product.
Moisture is a key element because you will transport less product per tonne and it is a double whammy because you need to use some of the energy in the coal to dry it out in the power station.
It is also very difficult to predict how a coal will actually burn until it undergoes a trial burn. Small amounts of impurities like Calcium and Iron can cause clinker to build up in the boiler and make the coal essentially worthless without blending. Ash fusion temperatures can give you an idea of likelihood of this occurring but are by no means definitive.
The hardness of the coal and its ability to be ground fine enough to enter the furnace also has a factor on price. It also depends on how well the coal burns through as some hard coals like those from the Surat burn through very well and dont necessarily need to be ground as fine as other coals.
At the end of the day if you have a large enough deposit of coal it is possible to tailor a power station to handle your coal. It is more desirable though to have a coal that will meet a specification that is required by a large number of buyers.
