Mechanisms for NOx emission control and ash deposition mitigation in sludge-coal blend combustion

Abstract

This paper introduces the coupled treatment of alkali and alkaline earth metals (AAEMs)-rich coal with ferric sludge as a method to efficiently and economically control sludge NOx emissions and mitigate coal ash deposition through fuel interactions. We utilized a pilot-scale drop tube furnace to quantify NOx emissions and ash deposition, supported by spectral and energy dispersive analyses. The results demonstrated co-combustion controls NOx emissions by catalyzing the reaction between char and NO and the transformation into more stable N-containing functional groups. It also facilitates the formation of harmless N2 instead of NOx by enabling Ca/Fe-based minerals to react with protein and pyrrolic nitrogen. At 950 °C, blending 10% ferric sludge mitigated ash deposition, thanks to the capture and dilution effect of Si/Al/Fe-based minerals in ferric sludge. However, blending ferric sludge aggravated ash deposition of AAEMs-rich coal at 1250 °C, due to the formation of eutectics by Fe-based minerals and Ca/Na-based feldspars. Besides, the control of NOx was reduced at 1250 °C. Therefore, the co-combustion of AAEMs-rich coal with 10% ferric sludge at 950 °C exhibited excellent performance in both emission control and ash deposition mitigation. This study, grounded in the characteristics of minerals, provides a theoretical foundation for the efficient and economical in-situ control of NOx emissions and ash deposition. It advances the development of low-carbon, environmentally friendly technologies for coupled sludge and coal combustion.