You generally need excess air for combustion, and the requirements differ for every medium. For natural gases, the range is between 5 - 10%, 5 - 20% for fuel oil, and the range varies from 15 - 60% for coal. So, the ideal CO2 content of flue gas would be between 10 and 18%. Carbon dioxide is one of the most vital gases in the combustion process in the combustion process. The amount of carbon dioxide in a flue gas indicates the combustion process's efficiency. Just like the quantity of air varies for combusting different mediums, the required percentage of carbon dioxide also varies. Such as, when it comes to natural gases, the required amount is only 10%, whereas, for other lighter oils or fuel, the range is generally between 13 - 18%. Hence, the carbon dioxide concentration in flue gas is critical, and the flue gas analyser helps to provide an accurate reading to ensure efficient combustion.