Fuels and Combustion
Ratio of primary air to secondary air increases with increase in the rank of coal, because the

Calorific value of the coal increases
Ratio of fixed carbon to volatile matter increases
High rank coals have higher amount of volatile matter
Oxygen content progressively decreases

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Fuels and Combustion
Beehive coke oven

Takes 2-3 days for coking of coal but requires no external fuel for heating
Gives larger yield of coke (around 85%) as compared to by-product ovens
Facilitates by-products recovery
Produces coke with very poor strength

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