Achieving Uniform Combustion Using Burner Line Orifices to Balance Coal Flow Distribution

The combination of more stringent NO_x regulations and utility heat rate improvement programs has led to an increased interest in balancing coal flow distribution to burners to improve combustion. Uniform low O₂ combustion is essential to minimizing NO_x emissions without adversely impacting combustible losses (CO and LOI). Many retrofit low-NO_x burner installations require balanced coal flows from each pulverizer to the burner pipes to within ± 10% of the mean in order to meet these goals. Numerous questions have arisen concerning the best methods to measure, achieve, and maintain a balanced coal flow distribution from the pulverizer to the burners. These questions concern: 1) the validity of clean air vs. “dirty” air primary air velocity measurements, 2) ASME vs. ISO “RotorProbe™” pulverized coal sampling methods, 3) fixed vs. adjustable burner line orifices, and 4) the performance of continuous on-line coal flow instrumentation. Some have questioned the need for or effectiveness of using orifices to balance coal flow distribution, while others have argued that deviations between burner line coal flows should not exceed as little as ±5% of the mean (pipe-to-pipe) within a pulverizer. This paper addresses many of the questions concerning the effectiveness of balancing coal flows with orifices. Extensive test data are presented documenting the improvement in coal flow balance using orifices on over 25 boilers. This experience covers units ranging in size from 110 to 750 MWe, equipped with roller, ball tube and exhauster type pulverizers, and for both wall-fired and T-fired burner configurations. The improvement in combustion uniformity achieved by balancing burner line coal flow distribution is illustrated by a comparison of economizer exit duct emission profiles before and after orifice application. This paper also addresses questions concerning: 1) maintaining a coal flow balance over time, 2) variations in coal flow balance with pulverizer load, and 3) recent developments in burner line continuous coal flow monitoring. Test data are included which compare coal flow measurements made with a RotorProbe™ and a microwave real-time coal flow measurement system.