The Need for Accurate Primary and Secondary Air Measurement and Control in a Pulverized Coal Fired Utility

Abstract

Air-fuel balance is the key to clean combustion. Coal-fired public utilities are under constant pressure to meet environmental air quality standards in reducing NOx emissions. Reduction of NOx is critically dependent, on controlled combustion conditions. Accurate measurement and control of the Primary and Secondary Air is an important factor in attaining this goal. Once the Primary Air can be measured accurately, the next variable to consider in the chain is the Secondary air. The actual “Burner Zone Stoichiometry” is the sum of the two air flows to any one burner plus the fuel flow. As shown in Figure 1 below, the Primary air conveys the fuel to the burner and supplies some of the combustion air. The Secondary air provides the oxygen to complete the combustion in the “Burner Zone” to the extent the burner is designed. On a typical burner, there is more combustion air delivered to the burner by the Secondary air than the Primary air generally by a factor of 2, depending on burner design. The additional air or Secondary air completes the combustion in the “Burner Zone”, based on optimizing a set of conditions such as NOx, CO, and boiler tube flame impingement. Maintaining desired stoichiometric ratio and Primary/Secondary air ratio to minimize emissions requires accurate measurement/control of the Primary and Secondary air.