Fuels and Combustion
Fuel gases containing hydrocarbons (ie.g. coke oven gas) are not preheated before burning, mainly because

It reduces its calorific value tremendously
It reduces its flame temperature tremendously
The hydrocarbons crack thereby choking and fouling the heat transfer surface by carbon soot
There are chances of explosion during preheating

ANSWER DOWNLOAD EXAMIANS APP

Fuels and Combustion
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
All of these
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

ANSWER DOWNLOAD EXAMIANS APP