The Transformation Behavior Of Carbonates Under Oxy-coal Combustion Conditions

Oxy-fuel combustion technology is one of the promising ways to capturing CO2from power plant, which can simultaneously remove NOx, SOx and particulate matters (PM) from coal combustion. The difference atmosphere in oxy-fuel combustion makes the mass and heat transfer of the boiler, behaviors of coal combustion and the emission of the pollutants different from the traditional power plant. Further, these differences may have an obvious impact on the transformation of minerals in the coal, and then the ash deposition and the generation of PM will be changed. However, the researchers still haven’t reached an agreement on ash deposition tendency under oxy-fuel combustion. Carbonates like siderite. calcite and dolomite are ubiquitous in coal, which have a big impact on the ash deposition and PM formation during coal combustion. The research of carbonates transformation in oxy-fuel combustion is critical to the understanding of ash deposition behavior and PM generation in this advanced combustion technology. This dissertation aims to explore the effect of oxy-fuel combustion on the transformation of carbonates under coal combustion conditions. The main content of this thesis is as follow:The thermal decomposition of siderite under oxy-fuel atmosphere was studied by thermogravimetric analyzer. For temperature programming, when O2is existed in the atmosphere, the decomposition behavior of siderite under O2/CO2and O2/N2conditions is almost the same, while the concentration of O2and CO2has little effect on the decomposition of siderite. Further, the influence mechanism of O2and CO2on the transformation of siderite is different, as siderite mainly reacts with O2under O2/CO2conditions, so the effect of CO2on the transformation of siderite becomes little. When H2O is added, the decomposition temperature of the siderite becomes lower. When same concentration of H2O was added to the atmosphere, the decomposition temperature of O2/CO2/H2O is higher than that in O2/N2/H2O. As O2, CO2and H2O exist in the atmosphere, they have a combined effect on the decomposition of siderite, and in turn the effect of CO2becomes obvious. For isothermal process, the effect of different gases on the decomposition of siderite is similar to that in temperature programming.The excluded siderite was simulated by adding siderite into a demineralized coal sample. Pure siderite and the excluded siderite were combusted in a drop tube furnace to explore the transformation of excluded siderite under oxy-fuel combustion. Five combustion conditions, i.e. O2/N2=7/93, O2/N2=21/79, O2/CO2=7/93, O2/CO2=21/79and C2/CO2=32/68were tested for elucidating the effect of CO2both in reducing and oxidizing conditions. For pure siderite, the products generated in those five conditions have similar morphology and mineralogy, suggesting negligible effects of CO2. The products of those five conditions experienced neither melting nor fragmentation, only magnetite is detected in the products except for O2/CO2=32/68condition, which also has a small amount of hematite. For two reducing conditions, the products generated from excluded siderite are distinctly different in morphology and mineralogy, which indicates significant effect of CO2on the transformation of excluded siderite. The product generated in O2/CO2=7/93experiences a milder melting than that generated in O2/N2=7/93. Wustite and magnetite are detected in both products, but the production of magnetite is much higher in O2/CO2=7/93than that in O2/N2=7/93, which is attributed to the presence of a high concentration of CO2. For three oxidizing conditions, the products have similar morphology and mineralogy. Most of the products are dominated by spherical particles, suggesting a strong melting happened. Only magnetite is detected under these three conditions, it appears that the high concentration of CO2under oxy-fuel makes no difference on the mineral transformation of excluded siderite. Particles fragmentation of excluded siderite is found under oxidizing conditions, the fragmentation degree increases with increasing the temperatures of siderite particles. The highest particles temperature of O2/CO2=32/68condition is, the fiercest fragmentation it is. Under oxidizing conditions, the oxy-fuel combustion affects the fragmentation of excluded siderite by influencing the coal combustion behavior.Synthesis char doped with siderite was prepared to study the behavior of included siderite under oxy-fuel combustion. For reducing conditions, the products generated in O2/N2=7/93and C2/CO2=7/93conditions are distinctly different in morphology and mineralogy, which indicates that CO2also has significant effects on the transformation of included siderite. For O2/N2=7/93condition, most of siderite residues still remain in the synthesis char, a small amount of residues are separated from the char, and α-Fe is the most abundant mineral in the residues. However, under O2/CO2=7/93condition, most of the products are separated from the synthesis char. Most of the char disappears due to the combustion and the severer gasification under oxy-fuel combustion. As no more combination with carbonaceous matter, much of the wustite is kept to the end. Under reducing condition, the severe gasification between CO2and synthesis char has a significant effect on morphology and mineralogy of included siderite. While, the products generated in O2/N2=21/79, O2/CO2=21/79and O2/CO2=32/68conditions have similar morphology and mineralogy, indicating the oxy-fuel combustion has little effect on the transformation of included siderite under oxidizing conditions. Most of the products are in spherical particles, magnetite and Fe3+-glass are detected in all these three products. The lowest char temperature of O2/CO2=21/79condition results in more Fe3+-glass generation and largest particles size of the included siderite. When both siderite and quartz were added to the synthesis char, testing the interaction between siderite and quartz, more Fe-glass is generated in the products of all conditions. As a lower melting point of Fe-glass, the particles generated in O2/CO2=7/93become smoother than that generated in only siderite addition. Furthermore, the differences between oxidizing conditions become smaller because of the good fluidity of Fe-glass.The behavior of calcite and dolomite under oxy-fuel combustion was investigated by the experiments in drop tube furnace. The results show that fragmentation of calcite samples takes place under oxy-fuel combustion. Particle fragmentation decreases with increasing CO2concentration and decreasing the particle size. The fraction of CaCO3that is transformed decreases when the concentration of CO2increases. When SO2is added into the atmosphere, the content of CaSO4in the residues under oxy-fuel conditions is higher than air conditions. Moreover, the fraction of transformed CaCO3under all conditions increases when SO2is added. The S/Ca ratio in dolomite residues under oxy-fuel condition is three times of that in air condition, while temperature has little effect on the sulfation behavior. The higher concentration of CO2under O2/CO2=21/79condition result in a higher S/Ca ratio than that generated in O2/CO2=32/68. This phenomenon is as ascribed to the high concentration of CO2under oxy-fuel combustion, which promotes the direct sulfation of CaCO3, and more S is retained in the residues.