Study The Effect Of Additives On The Fusibility Of Coal Ash And Its Mechanism From A Mineralogical Point Of View

The coals with high fusion point temperature (FT>1400℃) are in a large proportion of total coals in China. That limits the coals using in gasification applications. Adjustment to the coal ash fusibility of high fusion point coal has attracted attentions of many researchers in coal gasification and combustion. The purpose of this paper is to find methods to adjust the fusibility of coal ash, for adapting to different slagging ways in combustion and gasification technologies and expanding the scope of application of the coals.MgO and Na2CO3were added into high fusion point temperature coal ash, to study the effect of Mg2+and Na+on the fusibility of coal ash at high temperature. The ash fusion temperatures were tested according to GB/T219-2008determination of fusibility of coal ash, and the fusibility mechanism at high temperature was investigated by XRD, SEM and Ternary phase diagram. Then, basing on the results, the effect of minerals (albite, dolomite and limestone) on coal ash fusibility and the mechanism were further studied by XRD, FT-IR, SEM and OM. In addition, muscovite, aluminum silicate fiber and zirconia aluminum silicate fiber were added into lower fusion point coal ash to study the effect of morphology and structure of the minerals on coal ash fusibility. The results show that: (1) The ash fusion temperatures monotonically decrease with increasing addition amount of MgO, while the ash fusion temperatures exhibit low valley and reach the minimum when the addition amount of Na2CO3is15%. Investigated by XRD, mullite and cristobalite are detected in the Yangquan Guzhuang coal ash, which results in the ash fusion temperature of the coal ash higher than1750℃. Additions reacting with silicate minerals can form more low-melting eutectic minerals, such as cordierite and nepheline, etc. Ternary phase diagram and SEM micrograph confirm that the local clustering of partial elements and reunited phenomenon of coal ash under high temperature condition result in the different effect of Mg2+and Na+on the coal ash fusibility behavior.(2) Albite, dolomite and limestone all can reduce the fusion temperatures of Tengzhou coal ash; however, their impacts are significantly different. To decrease the FT with100℃, the adding proportion of albite, limestone and domolite are0.5,0.2and0.05, respectively. The three minerals have a similar function on Shengli coal ash. From the results of XRD analysis, albite and dolomite respectively react with refractory minerals in coal ash at high temperature and form lower fusion minerals:nepheline, anorthite and diopside. The amount of anorthite which is already in Tengzhou coal ash increases because of adding domolite, and MgO also strengthens the melting. Those all lower the fusion temperature of coal ash. Albite having no obvious effect is because a lot of Al and Si in albite increasing the fusion temperature. From the results of FT-IR analysis, adding domolite and increasing temperature paly the same role in the fusibility of coal ash. They all can change the connection of [SiO4], reducing the extent of polymerization, and adjusting the fusibility of coal ash.(3) The four melting characteristic temperatures in weak reducing atmosphere are all lower than those in oxidizing atmosphere. It is uncertain whether the different valance of Fe is the cause of changement in fusion temperature with different atmosphere by detection of XRD. It is because that Tengzhou coal ash has a low amount of Fe2O3content, in addition, iron mineral exists at a lower temperature while that transforms into glass at high temperature. However, it can be inferred that the weak reducing atmosphere can accelerate the melting of the refractory minerals.(4) The fusion temperature of coal ash can be decreased by adding Mg2+in the carrier of dolomite. Compare with Ca2+, the corrosivity of Mg2+is slitht. So the dolomite is an optimal flux compare with limestone. The reason for appearing of holes at the ash particle surface is the release of gases in mineral reaction at high temperature.(5) The fusion temperatue of coal ash can be improved by adding muscovite, aluminosilicate fiber and zirconium aluminum silicate fiber. It is worth noting that, zirconia aluminum silicate fiber is the optimized additive. When the adding proportion is0.2, the FT increases with120℃. The fusion temperature of coal ash has little influence with the morphology and structure of adding minerals. In contrast, the composition and high-temperature reaction of minerals have an important influence to the fusion temperature of coal ash.