Hydrogen-free carbon in the form of coke is burned
(a) with complete combustion using theoretical air,
(b) with complete combustion using 50% excess air, or
(c) using 50% excess air but with 10% of the carbon burning to CO only.
In each case calculate the gas analysis that will be found by testing the flue gases on a dry basis.
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The combustion reaction of carbon (C) with oxygen (O2) can be written as:
C + O2 -> CO2
(a) In the case of complete combustion with theoretical air, all the carbon is converted to CO2. Theoretical air contains 21% O2 and 79% N2. Therefore, the flue gas will contain CO2 and N2. The ratio of CO2 to N2 will be the same as the ratio of C to O2 in the combustion reaction, which is 1:1. Therefore, the flue gas will contain 21% CO2 and 79% N2.
(b) In the case of complete combustion with 50% excess air, the amount of O2 available for the combustion is 1.5 times the stoichiometric amount. Therefore, the flue gas will contain CO2, O2, and N2. The amount of CO2 will be the same as in case (a), i.e., 21%. The amount of O2 will be 50% of the stoichiometric amount, i.e., 10.5%. The rest of the flue gas will be N2, i.e., 68.5%.
(c) In the case of combustion with 50% excess air but with 10% of the carbon burning to CO only, the flue gas will contain CO2, CO, O2, and N2. The amount of CO2 will be 90% of the amount in case (a), i.e., 18.9%. The amount of CO will be 10% of the amount in case (a), i.e., 2.1%. The amount of O2 will be the same as in case (b), i.e., 10.5%. The rest of the flue gas will be N2, i.e., 68.5%.
Please note that these calculations are based on the assumption that all the nitrogen in the air remains unreacted and is present in the flue gas. In reality, some of the nitrogen may react with oxygen to form NOx compounds, especially at high combustion temperatures.
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