Fuel/Air 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 of the Primary and Tempering Air provides the capability for an improved control scheme. Measuring the Primary and Tempering Air separately avoids the need to measure the mass air flow in the mixed duct where large temperature variations exist. Such variations cause inaccurate, non-repeatable measurements. Very often, when measuring total air flow in a mixed air duct, a change in the Tempering Air damper will cause more of a temperature distribution change than a mass rate change. Erroneous mass flow rate indications and even wrong air flow direction indications are possible. In addition to the Fuel/Air ratio errors that result, unstable control can lead to an operational change from automatic to manual control.

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