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Thermal power plants in Zhundong area of Xinjiang equip Π‐typed boiler widely in recent years. With the increasing ratio of Zhundong coal in blended fuels, the slagging is strengthened. And the boiler shutdown was caused by ash deposition on the convective heating surface frequently. In order to study the mechanism of this problem, a 0.4‐MWth pilot‐scale facility was built and expected to study ash deposition and heat transfer characteristics on the convective heating surface of 100% burning calcium‐rich Zhundong coal. Thermodynamic model calculation was implemented on the heat transfer characteristics of two groups of tube panels, on which ash samples were collected after experiments. Then, simulation calculation was applied by software for verification. The ash samples were characterized by X‐ray diffraction spectroscopy (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive X‐ray detector (EDX). FactSage 7.3 thermochemical software package and a high‐temperature viscometer were also used to figure out the viscosity‐temperature characteristics. Results showed that a "step change" of heat transfer characteristics occurred when the flue gas temperature of inlet tube panel #2 approached to 1000°C. After the "step change," the heat transfer characteristics became stable. The ash deposition mainly consisted of silicon‐calcium compounds of tube panel #1, while in contrast, that of tube panel #2 consisted of gehlenite, CaSO4, and Na2SO4. The ratio of liquid phase in raw coal ash samples increased sharply when the flue gas temperature exceeded 1030°C. With the increase of surface temperature of fouling tube panel, the CaSO4 would be decomposed to release CaO, which reacted with gehlenite or other compounds to form low temperature eutectics. The fouling of ash would transfer to slagging and lead to the deterioration of ash deposition, which induced the formation of "step change" during the experiment. [ABSTRACT FROM AUTHOR]

• Over 20 units of 660MW grade are operating in Zhundong district, Xinjiang, China. Kaolin is commonly co-combustion with the Wucaiwan Zhundong coal (WCW), but the issue of abrasion caused by kaolin is becoming increasingly severe. • A previously unreported Fe-rich mediocre ash coal (MAC) has been discovered in the Jiang Jun Temple area of Zhundong district. Co-combustion of MAC with WCW results in higher slag viscosity compared to sole combustion of WCW. • The interaction of Fe with Ca/Na during medium to high flue gas temperatures leads to a decrease in Si/ Al consumption, thereby increasing their availability for reaction with Ca/Na at medium to low flue gas temperatures. As a result, the formation of Na 2 SO 4 and CaSO 4 is inhibited. A type of coal with low calorific value and high ash content, locally known as mediocre ash coal (MAC), has been discovered in the interlayer of stratified coals in Jiang Jun Temple area Zhundong district, Xinjiang, China. In order to investigate slag deposition on the convective heating surface during co-combustion of MAC and Wucaiwan Ca-rich Zhundong Coal (WCW), a 0.4 MW pilot-scale facility, with tube panels #1 and #2 located in horizontal flue gas ducts, was constructed. The tube panels were designed to simulate the superheater and reheater of the Π-type boiler. FactSage was utilized to simulate the material balance, whereas the classic models were employed to calculate the slag viscosity. Combustion tests were conducted while maintaining flue gas temperatures (FGT) of 1050∼1100 °C and 900∼950 °C at tube panels #1 and #2, respectively, which were consistent with those observed during sole combustion of WCW. The slags collected from tube panels were analyzed by X-ray fluorescence, X-ray diffraction and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Detector. Furthermore, Computer-Controlled Scanning Electron Microscopy analysis was used to obtain a quantitative characterization of the symbiotic relationship among mineral elements. The study of co-combustion MAC led to the following conclusions: The liquid phase ratio of slag decreased by 44% compared to sole combustion of WCW at 1100 °C. The viscosity was higher than that of sole combustion of WCW from 900 to 1650 °C. Compared to the sole combustion of WCW, the average S, Ca, and Na contents in the slag obtained on the two tube panels decreased by more than 85%, 40%, and 43.5% respectively. Sulfate formation in the slag on the tube panel was significantly reduced at medium to low FGT, and no Na 2 SO 4 and CaSO 4 compounds could be detected in the slag. The interaction between Fe-bearing minerals and Si- and Al-bearing minerals during combustion was evidently stronger than that between Ca-bearing minerals and Si- and Al-bearing minerals during combustion. The interaction of Fe with Ca/Na during medium to high FGTs led to a decrease in Si/ Al consumption, thereby increasing their availability for reacting with Ca/Na at medium to low FGTs. As a result, the formation of Na 2 SO 4 and CaSO 4 was inhibited. [ABSTRACT FROM AUTHOR]