Recent furnace draft control considerations

Control of the air-gas parameters in a fossilfuel boiler is critical to the safe, efficient operation of a power plant. The air-gas dynamics have been generally excluded from rigorous analysis of generating station controls because of the short time constants involved in the process and because of a lack of concern due to the acceptable performance of traditional air-gas control structures. Recent utility experiences involving furnace damage caused by excessively low furnace-draft have now created significant interest in gas dynamics (1-8). Induced draft fans are now being installed which have significant excess capacities to allow for the future addition of air pollution equipment Rapid fuel shut-off times under trip conditions can contribute to furnace pressure upsets which occur so quickly that conventional automatic controls are powerless to take corrective action. Manual intervention is also impossible. This paper discusses three models which have been developed for furnace dynamics analysis, their suitability for various studies, and some of the significant conclusions drawn from their use about furnace pressure control during normal and emergency conditions.