Fuels and Combustion
For which pair of the fuel gases, calorific value (C.V.) of one fuel is almost double that of the other on volume basis (i.e., kcal/Nm³ ), while the C.V. is same on weight basis (i.e., kcal/kg) ?

Sewage gas and gobar gas
Propane and LPG
F. gas and coke oven gas
Propane and acetylene

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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
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
All of these

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Fuels and Combustion
Rate of low temperature oxidation of coal due to bad storage conditions

Decreases with increase in surface area
Does not vary with increase in surface area
Is more for low volatile coal compared to high volatile coal
Is accelerated by storage in large heaps with small surface to volume ratio

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