Ash Fusion Behaviors Of Coal And Biomass During Pressurized Fluidized Bed Gasification Process

The ash fusibility is an important factor in the coal and biomass combustion and gasification system. The effect of ash fusion characteristics on the bed agglomeration, deposition on gas circuits and heat exchanger tubes is significant during the combustion and gasification process. The ash fusibility determines how to remove slags and influences the combustion and gasification process of coal and biomass. In order to investigate the effect of ash fusion behavior on the pressurized fluidized bed combustion and gasification system, we used the pressurized pressure-drop measuring device, high pressure TGA and ash fusion determinator, together with thermodynamic software FactSage, X-ray diffraction analyzer (XRD) and scanning electron microscopy fitted with X-ray energy dispersive spectroscopy (SEM-EDS) and studied the ash fusion behavior of coal and biomass systematically. The effects of ash composition, temperatures, reaction atmospheres and pressures on the ash fusibility, as well as the mineral transformation characteristics, were investigated. Finally, we employed the phase theory to investigate the ash fusibility mechanism during the pressurized fluidized bed combustion and gasification, based on the experimental results. The main contents are listed as follows:(1) The effects of different atmospheres, pressures and ash composition on the sintering temperature were studied using the pressurized pressure-drop device, together with XRD analyzer. The results showed that the sintering temperatures under the reducing atmosphere were lower than those of oxidizing atmosphere. Under a reducing atmosphere, some Fe3+would be converted to Fe2+at higher temperature, and Fe2+reacted with other ash constituents to form fayalite, hercynite, etc., which can form the low temperature eutectics and decreased the sintering temperature. Pressure had big influences on the sintering temperatures. This was because the pressure affects the transformaions of minerals during heating and facilitates the form of fluxing minerals such as anhydrite, microcline and hematite etc. and minerals like anorthite which can react with other minerals to form low-temperature eutectics, leading to the reduction of sintering temperatures. The additives of CaO, Fe2O3and Na2O reduced the sintering temperatures under the combustion and gasification atmospheres during atmospheric and high pressure. Moreovver, the influences of CaO and Na2O on the sintering temperature were more significant than that of Fe2O3. With the increases in CaO, Fe2O3and Na2O, some fluxing minerals and low temperature eutectics were formed in the coal ash, such Na2SO4, anorthite, albite and iron-bearing minerals like hercynite and fayalite etc (in the gasification atmosphere). These minerals reacted with other minerals to form the low temperature eutectics, leading to the reduction of the sintering temperature.Secondly, the ash sintering behavior of the coal and biomass was studied using the pressurized pressure-drop device, together with SEM-EDS and XRD analyzer. The results showed that the sintering temperatures of ash samples decreased with the additive of biomass under the combustion and gasification atmospheres, the higher the proportion of biomass addition in the blend, the lower the ash sintering temperature. SEM-EDS results showed that agglomeration and sintering were detected in the ash samples and the content of K and Cl increased obviously with the addition of straw. High contents of alkali metal like K and Na facilitated the form of K and Na-containing minerals, which have low melting points. This is the main reason resulting in the decrease of sintering temperature. At the same blending ratio, the ash sintering temperature of the blend with the straw addition was lower than that with the sawdust addition, because the straw had a higher K and Cl content but lower Ca than the sawdust. The EDS and XRD analysis confirmed that the ash samples with straw addition contained more K-bearing minerals, while the ash samples with sawdust contained more Ca and K-bearing minerals. Both Ca and K-bearing minerals reacted with other minerals in the coal to form low temperature eutectics, leading to the reduction of sintering temperatures of the coal and biomass blends. However, the melting point of Ca-containing is higher than the K-containing minerals, therefore, the sintering temperatures of ash with straw addition were lower than those with sawdust addition. The sintering temperatures decreased with the increase in pressure. As the pressure increased, the ash with the addition of biomass showed denser in the texture. This indicated that more fluxing minerals melted with increasing the pressure, and thereby leading to the agglomeration singnificantly. XRD analysis showed that the pressure facilitated the transformations of diopside and gelhcnite into augite and anorthite. The iron-bearing mineral augite and feldspar mineral anorthite can reacted with other minerals to form the low temperature eutectics, leading to the decrease of the sintering temperature.Finally, blends of Jincheng coal, and a wheat straw and a pine sawdust, respectively, were subjected to three different ash preparation procedures, namely, a low temperature oxygen plasma ashing device, muffle furnace and drop tube furnace. The resulting ash samples were then subjected to the sintering temperature measurement using a pressure-drop sintering device, morphological and mineralogical characterisation using SEM-EDS and XRD, respectively. For the same coal/biomass blends but different ash preparation methods, the sintering temperatures were always the lowest for the ash samples from the plasma ashing device and the highest for the drop tube furnace, with the muffle furnace ash samples being in between. The SEM imaging showed that the texture of ash samples from plasma ashing device was irregular, loose and more fibrous but the muffle furnace and drop tube furnace ashes were denser and more uniform in shape. In addition, the drop tube furnace ash particles were mostly in spherical-shape, indicating ash melting had occurred in the drop tube furnace. The XRD analysis revealed that different minerals were present in the ash samples due to different ash preparation temperatures. The minerals of the ash from the plasma ashing device were the low-temperature minerals, like aphthitalite. The fluxing minerals, such as KCl and muscovite, were present in the Muffle furnace ash and the high temperature minerals, such as anorthite and mullite were present in the drop tube furnace ash.(2) A high pressure thennogravimetric analyzer, aided with X-ray diffraction (XRD) analyses and SEM-EDS analyses, were used to investigate the effects of reaction atmospheres, temperatures, pressures and ash composition on the mineralogical transformation of coal ash during pressurized fluidized bed combustion and gasification. The results showed that the pressure had obvious influences on the transformation of mineral under different reaction atmosphere. The pressure facilitated the transformations of oldhamite into anhydrite and the fusion of hematite and suppressed the decomposition of the fluxing minerals like muscovite and anhydrite under the gasification atmosphere. The influence of pressure on the mineral types was not obvious but only facilitated the transformations of the muscovite and microcline into mullite and the albite under the inertia and combustion atmospheres. The effect of temperature on the ash fusion characteristics was strongly dependent on the atmospheres. In gasification atmospheres, the iron-bearing minerals and feldspar minerals, such as hercynite and anorthite, became abundant and reacted with other minerals to form low-temperature eutectics, thus decreasing the fusion temperatures. In combustion atmospheres, more high temperature minerals such as sanidine and mullite were formed and dominate, leading to increases in the fusion temperatures. With the increase in Fe2O3, there were muscovite, magnetite and anhydrite, etc. in the combustion atmosphere, resulting in the decrease of the fusion temperature. In the gasification atmosphere, hercynite and albite were detected, which can react with other minerals to form low temperature eutectics. With increasing the CaO, there were the fluxing minerals (anhydrite, and hematite) and some feldspar minerals, such as calcite and gehlenite, easy to produce low-temperature eutectics with other minerals, and hereby declining the fusion temperature in the combustion atmosperes. In the gasification atmosphere, the anorthite was present along with anhydrite, hematite, calcite and gehlenite. With the increase in Na2O, there were fluxing minerals, such as muscovite, magnetite, anhydrite and nepheline in the combustion atmosphere; there were the fluxing minerals like nepheline and the minerals, like gehlenite, easy to produce low-temperature eutectics with other minerals, leading to the reduction of the fusion temperature.(3) AFTs were examined under different atmospheres and ash composition using ash fusion determinator. The effects of chemical components of ash and reaction atmospheres on the ash fusion behaviors have been analyzed under typical gasification and combustion atmospheres, aided with the FactSage software. The results indicated that with increasing the Fe2O3,CaO and Na20contents under the combustion and gasification atmospheres, the four temperatures DT, ST, HT and FT declined dramatically. With the increase in Fe2O3, CaO and Na2O contents, the generation and transformation of minerals occurred. The iron-containing minerals, such as hercynite and fayalite, were formed as increased the content of Fe2O3; the Ca-bearing feldspar minerals, like anorthite and gehlenite, were present with increasing the CaO content; and the Na-containing feldspar minerals, like carnegiete etc., were detected as the Na2O was increased. These three minerals can form low temperature eutectics, decreasing the fusion temperature.(4) Based on the experimental results, the mineral distributions under different temperatures and pressures were displayed using the phase theory. The mechanism of ash fusion was analyzed using the phase theory. The effect of temperature on the mineral transformations was significant while the effect of pressure was not obvious. Meanwhile the atmospheres which the minerals were in were related to the effects of temperature and pressure. The transformations of minerals under gasification atmosphere were more than under the combustion atmosphere. This indicated that more phase transformations occurred under the gasification atmosphere than the combustion atmosphere. With increasing the temperature, the Fe2+-bearing minerals, like hercynite, reacted with SiO2, Al2O3in ash to form iron-containing minerals like hercynite and fayalite etc. under the reducing atmosphere. The feldspar minerals, such as anorthite and microcline, were formed by the reactions between the alkali and alkali earth oxides and the SiO2Al2O3in the oxidizing atmosphere. In general, the feldspar minerals and the iron-bearing minerals can react with other minerals, such as Na2O·2SiO2、K2O·4SiO2, to form low temperature eutectics like Na2O·2SiO2+SiO2+Na2O·3CaO·6SiO2. With the increase in pressure, the mineral transformations are not obvious in the oxidizing and inert atmospheres, however, the pressure affects the crystallization of minerals. Moreover, the fluxing minerals like muscovite and anhydrite coexisted and produced low temperature melting. Pressure facilitated the form of the feldspar minerals, like anorthite, sanidine etc. in the gasification atmosphere. The feldspar minerals can form the low temperature eutectics.