Steam Boilers, Engines, Nozzles and Turbines
The efficiency of a boiler is defined as

Ratio of the heat liberated in the furnace to the heat actually used in producing steam
Ratio of heat actually used in producing steam to the heat liberated in the furnace
None of these
Ratio of the mass of steam produced to the mass of total water supplied in a given time

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

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
Fuel burns with less air

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