An improved spectral analysis procedure to online estimate gaseous sodium in a large-scale Zhundong coal-fired boiler

• Na self-reversal spectra are observed in coal-fired boilers • An improved spectral analysis procedure to online estimate Na • Adjustment of the coal-air ratio was found to inhibit Na release • Translate experimental results in engineering combustion Zhundong coal is valued for its abundant reserves and high flammability but poses operational challenges due to sodium (Na) release. The condensation of volatilized Na on heating surfaces during combustion causes slagging. In contrast to offline measurement, online measurement offers the advantages of speed and real-time monitoring. Therefore, online measurement of gaseous Na concentration in the furnace is of great significance for guiding the adjustment of Zhundong coal combustion and mitigating slagging caused by Na release. This work proposes an improved flame emission spectroscopy (FES) method for estimating the gas-phase alkali metal concentration in Zhundong coal-fired boilers. It combines the FES with the internal standard method, utilizing the lithium element in coal as the internal standard element to estimate gas-phase Na concentration. This improved method addresses the limitations of existing FES methods in dealing with strong self-absorption and self-reversal phenomena occurring in large furnaces. Calibration results based on the Scheibe-Lomakin formula are obtained for the integrated intensity of alkali metal characteristic spectral lines, flame temperature, and gas-phase alkali metal concentration. The experimental results demonstrate that the gas-phase Na concentration is approximately 159 times higher than that of lithium and shows a proportional relationship with the load. However, adjusting the coal-to-air ratio can modulate the release of alkali metals under the same load. This study can serve as a quantitative indicator of fouling and slagging intensity in reactors of high-alkali coal-fired boilers, thereby providing a theoretical foundation for quantitative analysis via uncalibrated emission spectroscopy.