To investigate the formation mechanism of slag and prevent its occurrence during lignite fluid-bed gasification, mixed samples of graphite and 450 °C Xiaolongtan (XLT) ashes (1:19, mass ratio, MA) treated with different temperatures were prepared. The samples and the block slag formed during the tests of XLT ash agglomeration fluid-bed (AFB) pilot-scale gasification were examined by chemical composition, scanning electron microscope with energy dispersive X-ray detector (SEM-EDX), and X-ray diffraction (XRD) analyses. The results show the elements of iron and silicon are enriched in the slag due to the adherence of low-melting-point ferro-silicate. The interactions of the mineral matter in MA formed the same kinds of mineral crystals as those in the slag under reducing atmosphere (CO2/H2 = 1:1) at 950 °C. The formation mechanism of slag is proposed by the generation of adhesive glassy material, the formation of partial melting entities, the substitution of the iron in liquid aluminate or aluminosilicate by calcium and the mergence of individual particles. Pilot-scale AFB tests indicate that the reduction of operating temperature to 850−930 °C by increasing steam/oxygen ratio can successfully prevent slag problems during XLT AFB gasification.

High aluminum coal (HAC, Al 2 O 3 in its ash content ranges 38−51%) is abundant and plays an important role in energy and chemical product in China. The ash fusion characteristics of HAC and its modification behaviors by coal blending or flux were investigated by ash fusion temperature (AFT) analyzer, X-ray fluorescence spectrometry, X-ray diffractometry combined with normalized reference intensity ratio method, and scanning electron microscopy. The AFT variation is manly dependent on mineral composition and its transformation. A large amount of high melting point (MP) mullite formation results in HAC (Sunjiahao coal, SJH) AFT high. Compare with the coal blending of Shenmu (SM) and Xiangyuan (XY), CaO addition provides a better way to adjust ash fusion characteristics of SJH. For coal blending, that calcium and iron element in low AFT coal react with high MP mineral and then evolve into eutectic and amorphous matter makes the AFT of SJH mixtures decrease. The formations of low MP anorthite, and gehlenite, and their eutectic resulting from the reaction of CaO and mullite lead to a decrease in the AFT.