The fuel is burned with more air case of (b) 70 percent excess air. This is because 70 percent excess air = 170 percent theoretical air which is more than in the first case.
Excess air is commonly defined as the percentage of total air supplied that is higher than what is needed for perfect combustion. A good rule of thumb for natural gas is to have about 10 cubic feet of air for every one cubic foot of fuel gas for perfect combustion.
The theoretical air is the lowest amount of air required for complete combustion of a fuel. If all of the carbon in the fuel burns to CO2, all of the hydrogen burns to H2O, and all of the Sulphur (if any) burns to SO2, the combustion process is complete.
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