Effect of Struvite Addition on Relieving the Slagging Tendency during Zhundong Coal Combustion

Abstract

Zhundong coal is a kind of high-quality power coal, but serious fouling and slagging may occur due to the rich content of alkali and alkali earth metals (AAEMs). Combustion experiments were carried out on an electrically heated tube furnace at different temperatures in air atmosphere. The raw coal and its mixtures with struvite additive (2−8 wt % of coal mass) were respectively burned in air atmosphere. Combustion-generated ashes were collected and analyzed by inductively coupled plasma−optical emission spectroscopy, X-ray diffractometry, and scanning electron microscopy−energy dispersive spectroscopy. Results show that the mixed coal ash at high temperatures has a looser and more brittle structure than the raw coal ash and is more easily removed. With the introduction of struvite, whitlockite and chladniite appear, which improves the melting temperature of the mixed coal ash, while gehlenite (Ca 2 Al 2 SiO 7 ) and akermanite (Ca 2 MgSiO 7 ) disappear. The high content of calcium in Zhundong coal leads to the formation of a low-temperature eutectic with some minerals such as Al and Si at temperatures higher than 1000 °C. Ca separates out after these minerals melt, flows within the flue gas, and congeals on the heating surfaces, causing serious slagging. The fixation rate of Ca in the mixed coal ash blended with struvite increases obviously. The optimum addition of struvite is 4%.

Experimental investigation of Zhundong high‐calcium coal ash adhesion characteristics on metal surfaces at high temperatures

“…Gehlenite will undergo low-temperature eutectic reaction with pseudowollastonite and anorthite at 1150-1250 C, forming a low melting point, amorphous eutectic. 21 Combined with the thermodynamic calculation results of Figure 5, the liquid slag is mainly composed of silicate (SiO 2 Al 2 O 3 CaO), so it is considered that the liquid slag is mainly produced by the melting of gehlenite.…”

“…Conventional ash fusion temperatures (AFTs) successfully predict the slagging behavior of steam coals. According to AFTs, the softening temperature of ZDc exceeds 1250 C, 21 and it belongs to medium slag coal. However, the operation practice shows that the ZDc-fired boiler have serious slagging phenomenon.…”