Ash Fusion Behaviors And Transformation Of Coal Minerals During Coal Conversion Process

Coals from different formation conditions, coalification degree and mining process are likely to have different compositions of mineral matter and inorganic component and the modes of occurrence of the mineral matter and inorganic component are various and complicated. Different kinds of coals and methods of coal conversion and utilization can result in different ash fusion characteristics of coal. During coal combustion and gasification technology, ash fusion characteristics have decisive impacts on series of phenomena, such as the ash particle aggregation, fouling on heat exchanger and slagging in the pipe wall and so on, and directly determine the selection of ash discharging mode. Ash fusion characteristics are an important factor affecting the process of coal combustion and gasification.Four Chinese lignites of distinctly different mineralogical features, namely high-silica WANG, high-sulfur LLI, high-alumina BLG and high-alkaline ERDOS, were chosen for this study. The ash samples were prepared and analyzed under different coal combustion and gasification conditions. The influence of process conditions was investigated, accompanied by characterization analysis of elements, ash compositions, ash morphology and the inherent laws in the ashing process. Based on this, ash fusion characteristics and mineral matter transformations in coal during coal combustion and gasification process, especially the migration mechanism of potassium element, were investigated and analyzed. The main contents are listed as follows:1) Ash sintering characteristics and mineral matter transformations during coal combustion were revealed. The effects of different process conditions(ashing temperature, potassium carbonate and ashing atmosphere, aging time and so on) under combustion condition on the ash sintering characteristics were studied using a pressure-drop sintering device. For the ash preparation method, a combination procedure of low temperature oxygen plasma ash sample preparation method and traditional ash sample preparation method was employed to systematically reveal the mineral transformations from intrinsic minerals in coal to high temperature minerals after ashing.The results showed that potassium carbonate can reduce the ash sintering temperature in a certain range of concentration, but the effect of potassium carbonate on ash sintering temperature mainly depends on the concentration of potassium carbonate, the nature of coal and ash preparation method. With the increase of ashing temperature, ash sintering temperature of coal sample was gradually increasing and the low temperature or fluxing minerals turned into high temperature minerals. The fluxing minerals existing in low ashing temperature of 150°C mainly are arcanite; iron(Ⅲ) oxide and feldspar minerals mainly presented in 600°C and albite, gehlenite and potassium bearing minerals(such as potassium feldspar and kaliophilite) mainly presented in 1500°C as fluxing minerals. K, Ca, Fe elements can contribute to the generation of fluxing minerals, thus triggering the formation of low temperature eutectics and the occurrence of sintering. Ash produced under air atmosphere had the highest sintering temperature than that under steam and CO2 atmospheres, ash produced under steam atmosphere had the lowest sintering temperature. Besides, the ash sintering temperature in the air sintering atmosphere was higher than that in CO2 atmosphere. The influence of size of coal particle on the ash sintering temperature was relatively large but no obvious trend was found between them. At last, with the increase of potassium carbonate, ashing temperature and ash aging time, ash sintering degree was gradually deepening.2) The effects of potassium carbonate, additives of kaolin, bauxite and calcium carbonate on the ash sintering characteristics during K2CO3-catalyzed steam gasification of coal were studied, and the interaction reactions among coal minerals, potassium carbonate and additives and the influences of additives on the catalytic gasification efficiency were also revealed. The results showed that with the concentration of K2CO3 increasing to 10 wt%, no matter what refractory additive of kaolin and bauxite or fluxing additive of calcium carbonate within coal, the ash sintering temperature decreased gradually. Molten surfaces of ash particles from coal with kaolin additive were mainly comprised of potassium bearing minerals(such as arcanite and kaliophilite). Beside those potassium bearing minerals, iron oxide was detected on the molten surfaces of ash particles from coal with bauxite. In addition, molten surfaces of ash particles from coal with calcium carbonate were mainly comprised of kaliophilite, lime and calcium bearing feldspars. Those minerals related with molten surfaces were the main cause to facilitate the formation of liquid phases and trigger the occurrence of sintering. With a same concentration of K2CO3 catalyst and the concentration of bauxite or kaolin raised from 0 to 10 wt%, the sintering temperatures of ash samples got increased; however, with the concentration of calcium carbonate raised from 0 to 20 wt%, the sintering temperatures of ash samples got reduced. From the view of mineralogy, calcium element from calcium carbonate had the priority over potassium to react with other minerals in ash, thus the catalytic gasification activity of potassium element was kept in ash. However,the addition of bauxite or kaolin can decrease the gasification rate of lignite by reacting with potassium to generate water-insoluble kaliophilite and deactivating the potassium catalyst. The influences of the addition of bauxite and kaolin on ash melting characteristics have a lot of similarity.3) A high pressure thermo-gravimetric analyzer, aided with X-ray diffraction(XRD) analyses and SEM-EDS analyses, was used to investigate the effects of temperature, potassium carbonate, pressures and reaction atmosphere on the ash sintering characteristics and mineralogical transformation during pressurized K2CO3-catalyzed gasification of coal. The results showed that with the increase of temperature from 700°C to 850°C, the ash surface melting and sintering phenomena got more and more serious no matter what steam or CO2 atmosphere. In addition, with the increase of the concentration of potassium carbonate, ash surface morphology changed from irregular and unmelted into melted, and ash sintering became more obvious. Within 3.5MPa, the higher reaction atmosphere pressure, the more serious molten degree of ash particles, which explained by the more interaction reactions between minerals in ash under high pressure. Molten degree of ash particles from coal produced under steam atmosphere was more serious than that under CO2 atmosphere,because stronger reducibility of steam could lead to a low rate of Fe3+/ΣFe in K2O-Al2O3-Si O2-Fe-O eutectics.4) Ash melting and sintering mechanism during coal combustion and gasification process was proposed, and the existence and migration mechanism of some main mineral elements were systematically revealed, especially the migration mechanism of potassium element. The formation of low temperature eutectics and the ash melting and sintering mechanism were discussed using ternary phase theory, and the effects of minerals of ash produced from experiments in this study on the sintering characteristics were verified. The generation of low temperature eutectics and fluxing minerals is the main mechanism of ash melting and sintering. The working mechanism of the potassium ion and K2CO3 addition in coal ash sintering process is that K2CO3 dissociates when it was exposed to heat to release potassium ion, which subsequently reacts with mineral matters in ash to form K-containing aluminosilicate and some K-bearing sulfate minerals such as potassium sulfate. The nature of the aforementioned K-containing aluminosilicate is determined by the amount of K2CO3 addition and the interactions between K2CO3 and the ash, which in turn affects the sintering behaviour of lignite ashes. When potassium was added, most of silicon element exists in the way of Si-Al. Sulfur prefers to exist in the form of Ca-S and Fe-S phases, and Al atom prefers to combine with potassium atom rather than the silicon atom in ash and exists in K-bearing aluminosilicates.This thesis systematically studied ash fusion characteristics and ash mineral transformations during coal combustion and gasification process. The formation mechanism of ash fusion and sintering in the engineering field of fluidized bed catalytic gasification and combustion technology was investigated. This study can provide theoric support and reference for finding efficient methods and process conditions to avoid slagging and for safely operating the catalytic gasification and combustion technology.