Properties And Control Of Fine Particulate Matter During The Combustion Of High Ash Fusion Temperature Coal

Particulate matter (PM) having an aerodynamic diameter of smaller than2.5μm and coming from the coal combustion is generally considered to contribute to air pollution and a threat to human health. Huainan city is one of the important energy power base in China. Huainan coal is a typical representative of the high ash fusion temperature coal, it is essential and significantly important to know the mineralogical properties in raw coal. To reduce the PM2.5emissions from coal combustion, a good understanding of the transformation of minerals in the combustion process is required. The mineral composition, size and their association with organic materials in the raw coal samples were analyzed by Computer-controlled scanning electron microscope (CCSEM). Singel Coal、coal blending and coal with additives was burnt in a laboratory-scale drop tube furnace(DTF) respectively. The particulate matter were collected by a Low Pressure Impactor (LPI). PM were analyzed by High-resolution transmission electron microscopy (HRTEM)、CCSEM、Scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) and X-ray florescence (XRF). Properties of fine particulate matter generated from single coal were investigated, firstly. Then, effect of coal blending and additives on fine PM emissions were studied. The particle size distributions and chemical composition of PM2.5were also compared to that predicted by an advanced coalescence and fragmentation model, finally. The main conclusions are shown as below.Huainan coal is rich in kaolinite, quartz, montmorillonite and unknown (a complicated compound containing Si, Al, Ca/Fe, P, and O together). The mineral matter was dominated by clay minerals with quartz(more than85%of the total mineral), The ash fusion temperature(AFT) of Huainan coal ash samples is higher than1500℃, while in the HT coal, only about60%of clay minerals with quartz is found, and there are about21%of calcite, dolomite and pyrite in HT coal,which are beneficial to ash melting (FT=1278℃). The contents of excluded kaolinite are comparable with that of included kaolinite in the Huainan coal, so is quartz. The size distribution of minerals in coal is one of the most important factors in determining ash size during combustion. The content of excluded minerals (<1.0μm) in Huainan ZJ、 XQ coal and HT Coal is approximately5.95%、2.00%、1.86%, respectively, and the content of excluded minerals (1.0μm-2.5μm) in Huainan ZJ、XQ coal and HT Coal is approximately15.09%,5.52%,4.44%, respectively. The content of included minerals (<1.0μm) in Huainan ZJ, XQ coal and HT Coal is approximately3.67%,5.79%,6.20%, respectively, and the content of included minerals (1.0μm-2.5μm) in Huainan ZJ, XQ coal and HT Coal is approximately13.86%,14.52%,17.29%, respectively.It further indicates the different distributions of the minerals in three coals.A bimodal distribution was obtained for the emission of PM10from the combustion of the Huainan coals and HT coal. The large mode is around2.5um and the small one around0.05μm.The amount of PM1and PM2.5produced from Huainan coal is much higher than that from coal HT. The amount of PM1and PM25generated from Huainan ZJ coal combustion is highest among three coals, which is due to the Huainan ZJ coal is riched in fine excluded minerals (<1.0μm and1.0μm-2.5μm) And a small amount of fine excluded minerals might contribute to the formation of fine PM. With regard to the morphology of PM1, it is composed of two typical structures. A portion of particulates, having a diameter<0.1μm, is formed as fractal aggregates consisting of a primary particle around20nm. EDS analysis proves the abundance of volatile metals including S, P, and alkali elements and a small amount of Fe as well. They should be caused by the vaporization-condensation pathway. Moreover, the molten single particles around0.1-0.2μm were also observed, which mainly consist of refractory elements such as Si,Al, and Fe as detected by EDS. Clearly, liberation of the fine minerals and their liquid droplets contributes to the formation of this kind of particles in PM1. PM1.2.5is also the case, liberation of the fine Al-Si minerals(1.0μm-2.5μm) and their liquid droplets contributes to the formation of this kind of particles in PM1.2.5. during the combusting of Huainan coal. For the HT coal, the coalescence among mineral particles reduce the emission of PM1and PM2.5, due to its low ash melting point.HT coal with low ash melting point is mixed with Huaian coal to investigate effect of coal blending on reduction of PM emissions during Huainan coal combustion. The results indicate that emissions of PM1and PM25are reduced compared to their calculated linear results during combustion. The transformation of S, P, Si, and Al from submicron particles to PM1+reduces PM1emissions. The transformation of Ca, Fe, Al, and Si from PM2.5to PM2.5+reduce PM1-2.5emissions. The high concentration of Ca and Fe in coal blends enhances the liquid phase percentage produced during combustion, and as a result, improves both the adhesion of volatilized S, P on the sticky surface of large particles to be transformed to PM1+, and the probability of collision and coalescence of particles to form larger particles of Ca-Fe-Al-Si, Ca-Al-Si, or Fe-Al-Si. Thus, as Ca, Fe, Al, and Si are transformed into PM2.5+.PM1and PM1-2.5emissions are reduced accordingly. PM1and PM25are reduced by two pathways:vapors adhesion to the larger liquid particles and the coalescence among submicrometer mineral particles.The addition of the Ca-or Mg-or Fe-based additives can affect the mineral transformation process, and thus, control the emissions of PM1and PM2.5during combustion. Because the additive is able to reduce the ash melting point via the formation of low-melting eutectic compounds. Several fine Al-Si particles adhering to large Ca Al-Si particles or Fe Al-Si particles or Mg Al-Si particles. These particles derive from the liquid or partial liquid phase according to the shape-related structure. Therefore, the collision of primary particles and their subsequent coalescence is a reasonable agglomeration route for particle growth. The addition of the Ca-or Mg-or Fe-based additive increased the coarse ash fraction and substantially reduced the amount of ash particles smaller than2.5μm (PM2.5). The additive has a pronounced impact on particle size distribution of PM. CaO, SiO2, Al2O3have a single mode distribution, which is prevalent in PM1+, and its low content in PM1implies negligible vaporization of calcium. The transformation of CaO, SiO2,Al2O3was likely caused by the direct transformation of inherent refractory metals within the mixtures. A solid-to-particles pathway governs their transformation. MgO and Fe2O3have a bimodal distribution. The transformation of Fe2O3should be governed by two pathways. A portion of it undergo direct transformation whereas the remaining portion vaporizes and condenses into ultrafine particles. For the MgO, its presence in PM1is likely caused by adsorption of its vapors on the submicrometer particles.The particle size distributions and chemical composition of PM10were also compared to that predicted by an advanced coalescence and fragmentation model during the combustion Huainan coal added with additives. The comparisons indicate that the model can satisfactorily predict ash formation and properties, taking into account both coalescence of included minerals and fragmentation of excluded minerals at high temperature. With the liquidus amount increasing, the average coalescence number of minerals in raw coal and coal mixed with additives increases.