Experimental Investigation On The Deposition And Fouling Characteristics Of High Alkali Coal Under Oxy-fuel Combustion Conditions

Zhundong coal is widely used in pulverized coal-fired boilers in China due to a large amount of coal reserves,but it is easy to cause serious ash deposition and slagging on the heating surface of the boiler during the combustion process because it contains higher alkali metal and alkaline earth metal elements.Oxy-fuel combustion technology is considered to be one of CO₂ capture technologies with large-scale application and commercialization.However,the CO₂ volumetric concentration in flue gas is more than 80%under oxy-fuel combustion conditions.And it could change the combustion characteristics of pulverized coal in the furnace and the release characteristics of alkali metals and alkaline earth metals,which may cause ash deposition on the heating surface.In this paper,the drop tube furnace pulverized coal combustion system was used to study the ash deposition characteristics of Xiheishan high alkali coal under air combustion and oxy-fuel combustion conditions,and the influence of wall temperature and ash deposition time on the ash deposition characteristics was further analyzed.Firstly,the drop tube furnace pulverized coal combustion system was set up to investigate the ash deposition and fouling characteristics of high-alkali coal combustion under air combustion and oxy-fuel combustion conditions.A camera visualization system of the sampling probe was designed to capture the ash deposition on the heating surface under air combustion and 20%O₂/80%CO₂ environments.The dynamic process of ash deposition for high-alkali coal under oxy-fuel combustion conditions was studied.The area of the ash deposition section and the ash deposition boundary were analysed by the Open CV Python program.Combining with the deposition efficiency obtained by the ash weight,the ash deposition growth with the time was obtained.The results show that under air combustion and oxy-fuel combustion environments,the surface area of the ash deposition of high-alkali coal first increases with a rapid rate,then increases with a slow rate,and finally it does not increase.Under the oxy-fuel combustion conditions,the area of the ash deposition is larger,but the ash deposition layer is looser.Under the air combustion conditions,the ash deposition layer is denser.The amount of the ash deposition under the air combustion conditions is greater than that under the oxy-fuel combustion conditions under the same other conditions.Secondly,the micro structure and composition of the ash deposition layer of Xiheishan high-alkaline coal under the air combustion conditions was analyzed by SEM-EDS and XRD.The result shows that a large amount of milky white viscous bottom layer for Xiheishan high-alkaline coal exists on the side wall of the ash deposition probe under the air combustion condition.According to SEM-EDS and XRD analysis,it is found that the cohesive bottom layer contains more Na₂SO₄,which may cause high alkali coal a higher deposition tendency.According to the adhesion ability of the ash deposition,the ash deposition layer is divided into three layers:a viscous bottom layer,a bottom floating ash and a surface floating ash.The influence of the wall temperature on the ash deposition characteristics shows that the ash deposition mass on the ash deposition probe increases with the increase of the wall surface temperature under the air combustion condition,and it reaches the maximum at 550℃.The ash deposit mass slightly decreases when the wall temperature further rises to 650℃.This may be because the higher wall temperature is beneficial to the melting of the low melting point ash on the surface of the probe,which promotes the deposition of large particles on the wall of the ash deposit.However,the excessive wall temperature is not conducive to the adhesion of micro particles on the probe surface.Finally,the ash deposition of Xiheishan high alkali coal was investigated under 20%O₂/80%CO₂ oxy-fuel combustion conditions and compared with that under air combustion conditions.The results shows that the ash deposition under oxy-fuel combustion conditions also is divided into three layers,but its viscous bottom layer is relatively thin.The XRD analysis shows that Fe element in the ash deposition layer is more divalent under the oxy-fuel combustion conditions.This is because the gasification reaction of high concentration CO₂ in the flue gas with char to produce more CO,and part of Fe is reduced in the form of Fe O.Fe O is easy to react with Ca-Mg-Al to form new crystals,resulting in the irregular surface of the ash deposition and relatively loose ash layer under the oxy-fuel combustion condition.In addition,the mass amount of the ash deposition under the oxy-fuel combustion decreases first and then increases with the increasing wall temperature.The ash amount will increase again when the ash surface temperature increases to 650℃,which results in the adhesion of larger ash particles on the ash surface.