Experimental Study on Morphology and Chemical Composition of Ash Deposition during Oxy-fuel Combustion of High-Alkali Coal

The Zhundong coalfield has attracted ever-increasing attention as a result of its super-large reserve of coal resources. However, the power plants encounter great challenges in the utilization of Zhundong coal as a result of its high-alkali feature, which deteriorates the phenomenon of ash deposition and slagging. Oxy-fuel combustion of Zhundong coal benefits the near-zero emission of pollutants in coal-fired power plants, while previous studies mainly focused on the behaviors of alkali and alkaline earth metals and ash deposition during air combustion. The present study focused on the characteristics of ash deposition during oxy-fuel combustion of high-alkali coal, especially the differences in morphologies and chemical compositions of ash deposits using a drop-tube furnace, with the ash deposition mechanisms being further elucidated. Experimental results showed that the contents of sulfur (S), chlorine (Cl), and calcium (Ca) in ash deposits from oxy-fuel combustion were lower at the identical O-2 content but the contents of aluminum (Al) and silicon (Si) were increased in comparison to air conditions. The average particle size of ash deposits under oxy-fuel conditions was smaller with less calcium sulfate and mullite than the case in air combustion of Zhundong coal. The adhesion phenomenon between ash particles was aggravated in the ash deposits of high-alkali coal, with the oxygen content elevated. The proportion and diameters of spherical particles declined with an increasing oxygen concentration in the ash deposits of Zhundong coal. The contents of iron (Fe) and calcium in ash deposits of high-alkali coal were closely bound with the decrease of the melting point of ash and the formation of spherical ash particles. The caking phenomenon became more serious as the flue gas temperature was increased in the ash deposits of Zhundong coal. The deposition propensity of Zhundong coal was inclined to the minimum when the probe surface temperature was 550 degrees C, while the deposition propensity of Lu'an coal was different possibly as a result of the differences in coal properties and ash compositions. During the combustion of high-alkali coals, some components deteriorated ash deposition, including the low-temperature eutectics formed by the reactions of Fe and Ca with Si-Al compounds and sulfates and chlorides of sodium or calcium. This paper is beneficial for the improved understanding of ash deposition during oxy-fuel combustion of high-alkali coal.