Characterization of Combustion and NOx Emissions with Respect to Overfire Air Damper Opening in a Down-Fired Pulverized-Coal Furnace

Abstract

The application of deep-air-staging combustion technology to significantly reduce NOx emissions and simultaneously control combustible loss to acceptable levels is already well established in tangential-fired and wall-arranged furnaces. However, this technology has not yet been widely applied to down-fired furnaces, and thus there is a requirement to investigate the potential benefits. This work presents an experimental investigation of the combustion and NOx emission characteristics in a newly operated down-fired 600 MWe furnace equipped with a deep-air-staging combustion configuration. Full-load industrial-size measurements were performed by acquiring various data such as gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash. Overfire air (OFA) damper opening settings of 15%, 30%, 50%, and 70% were tested in turn so as to evaluate the OFA effect, following which the staged-air damper was opened from 30% to 50% to assess its ability to improve the poor burnout associated with low NOx operating conditions. It was found that, as OFA was opened, gas temperatures in the burner zone and along the flame travel path (prior to penetration of the flame into the hopper region) exhibited an initial increase and subsequently decreased. Similar to trends that have been seen in most tangential-fired and wall-arranged furnaces, opening OFA continuously increased the exhaust gas temperature, CO levels in flue gas, and carbon in fly ash, while simultaneously decreasing NOx emissions and boiler efficiency. When opening OFA to generate deep-air-staging conditions, only the 50% setting balanced NOx emissions and combustible loss, although it was still generating high levels of carbon in fly ash. With the OFA damper opening fixed at 50%, increasing the staged-air damper opening from 30% to 50% reduced combustible loss and had a slight influence on NOx emissions. On the basis of the observed high levels of carbon in fly ash, accompanied by reasonably low levels of NOx emissions [9.81% and 878 mg/Nm3 at 6% O2 (dry)], a combustion retrofit based on enlarging the shallow staged-air angle is recommended to improve furnace burnout under deep-air-staging conditions.