A coal with high ash content is undesirable, as

• it is abrasive to the coal pulveriser (ie.g. ball mill) and the combustion chamber.
• the ash in molten condition gets absorbed in the pores of the refractory lining of the furnace and causes its spalling due to different co-efficient of expansion/contraction of the refractory and the ash.
• the ash retains the sulphur & phosphorus and thus affects the quality of products in metallurgical furnace apart from increasing the slag volume. Besides, it may fuse and stick to the boiler tubes thereby reducing the heat transfer.
• all (a), (b) and (c).