The Application of Combustion Diagnostic Techniques in Boiler Tuning for NOx Control

Field test programs to characterize the NOx emissions from coal-fired utility boilers have become an increasingly important effort as utilities comply with the Clean Air Act Amendment requirements. Since NOx emissions are so strongly dependent on furnace combustion conditions, diagnostic techniques and test protocols have been developed to aid in evaluating the boiler combustion process. The evaluation of combustion uniformity in the burner zone is one of the most critical, but difficult and time consuming, tasks in a NOx emissions test program. The use of diagnostic techniques has become increasingly important in all phases of NOx emissions compliance testing including: • obtaining representative baseline NOx emissions (e.g., for Phase II units); • making a preliminary assessment of the NOx reduction potential of combustion modifications (e.g., simulated OFA); • identifying equipment related operating constraints that impact combustion uniformity and NOx emissions (applicable to post-retrofit tuning and warranty tests). The paper presents a very practical, problem-solving approach to identifying regions of non-uniform combustion and NOx emissions. A checklist of common operational problems and associated diagnostic test procedures is provided based on experience from a variety of boiler field test programs. The paper emphasizes the benefits of using advanced diagnostic instrumentation such as a multi-point O2 analyzer, a HOT FOIL LOI analyzer, and an acoustic pyrometry gas temperature measurement system to identify problems and tune combustion conditions. The discussion of combustion diagnostic techniques is illustrated with field test data from several wall-fired and tangentially-fired units where "as-found" non-uniform combustion conditions in the furnace were reflected in significant NOx and/or LOI gradients. The causes of these gradients are discussed along with the maintenance and/or changes in firing practice changes that were implemented to achieve uniform combustion and NOx emissions.