Control Of Air To Fuel Ratio

The fuel/air ratio is the lowest if excess air level (measure in percent of oxygen 02) is present in the flue gas at a set firing rate without the carbon monoxide (CO) being produced. At low firing rates; the burner design requires more excess air to ensure the mixing of air with the fuel in proper ratio. At higher firing rates there is enough differential pressure drop (burner wind box to combustion chamber area) for the air to mix with the fuel. All boilers have a fuel/air ratio curve and it is extremely important that the plant has this documented information. In other words ratio control is adopted to maintain the air fuel ratio. The air to fuel ratio is the proportion of air to fuel supplied during combustion process. The optimal ratio (the stoichiometric ratio) occurs when all the fuel and oxygen in the reaction chamber balance each other perfectly. Rich burning occurs when there is more fuel than air in the combustion chamber while lean burning occurs when there is more air and less fuel in the combustion chamber. Measurement of the flue gas and oxygen content often provides a good indication of combustion performance, but it must be appreciated that the presence of 'tramp air' due to leakages into the combustion chamber can lead to anomalous readings. In the presence of significant leakage, reducing the air/fuel ratio to minimize the flue gas and oxygen content can result in the burners being starved of air.