Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

All the fuel burns instantaneously producing high energy release
Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
Explosion in furnace
Fuel burns with less air

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Steam Boilers, Engines, Nozzles and Turbines
In fire tube boilers

Water passes through the tubes which are surrounded by flames and hot gases
Forced circulation takes place
The flames and hot gases pass through the tubes which are surrounded by water
None of these

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Steam Boilers, Engines, Nozzles and Turbines
Which is not the correct statement about moisture in coal?

It increases thermal efficiency
In some coals moisture may be present up to 40%
Some moisture in coal helps in better burning which is not possible with completely dry coal
Inherent moisture and surface moisture are different things

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