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

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

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Fuels and Combustion
A sample of natural gas containing 80% methane (CH₄) and rest nitrogen (N₂) is burnt with 20% excess air. With 80% of the combustibles producing CO₂ and the reminder going to CO, the Orsat analysis in volume percent is

CO₂ : 6.39, CO : 1.60, O₂ : 3.99, H₂O:25.96, N₂:72.06
CO₂ : 7.42, CO : 1.86, O₂ : 4.64, N₂:86.02
CO₂ : 7.60, CO : 1.90, O₂ : 4.75, N₂ : 85.74
CO₂ : 6.26, CO : 1.56, O₂ : 3.91, H₂O :15.66, N₂ : 72.60

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