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

Fuel burns with less air
All the fuel burns instantaneously producing high energy release
Explosion in furnace
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

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

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

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