Heterogeneous Reactions Mechanism During Coal Combustion With Desulfurization And Solid Residues Utilization

Coal is the most abundant and widely used fossil energy resource in china. During coal combustion, large amounts of solid residues (coal combustion by-products: CCBs) are generated besides gaseous pollutants, which may cause serious environmental pollution and need well disposal. In this paper, coal combustion, pollutants formation and control as well as high efficiency utilization of the inorganic constituents have been systematicly integrated, and the chemical reaction process of different compound additives during coal combustion and desulfurization, especially the transformation and reaction mechanisms of the inorganic fractions were well investigated in the presence of many advanced testing methods, based on the knowledge of combustion science, mineralogy, physical chemistry of silicate, environment science, material science etc. SOx emission as well as mineral composition and usability of solid resiudes were maily studied before and after additives added into coal, simultaneously the desulfurization mechanisms of different additives with the rule of desulfurization products decomposition and transformation were discussed, furthermore, the heterogeneous reaction mechanisms as well as thermodynamics and kinetics of reactions in inorganic fractions were carefully analyzed when injecting compound additives into coal.To investigate the heterogeneous reaction mechanisms of inorganic constituents, combustion (calcination) experiments of pulverized coal, coal ash, additives and pure mineral reagent as well as the mixture blended with each others were conducted on testing facilities such as high temperature reactor, drop tube furnace and single burner furnace respectively. The mineral composition and micro appearance of the combustion (calcination) solid products were analyzed by XRD and FESEM, then the influences of additive composition and quantity, temperature, particle size, residence time, combustion style and contacting extent, CaO content, and mineralizer on heterogeneous reaction process and products composition of inorganic fractions in the coal-additive system were studied. Based on experimental results, according to heterogeneous equilibrium principles, calculation model of the products about complex mineral system CaO-SiO2-Al2O3-Fe2O3-CaSO4 was established and the equilibrium product compositions under different conditions were simulated, in order to investigate the heterogeneous reaction mechanisms between additives and the inorganic components in coal. The solid products were predicted when coal mixing with additives, moreover, the activation and potential utilization of modified ash were discussed according to the mineral species, concentration and hydration capacity.Researches on desulfurization and the phase characterization of desulfurization residues were conducted on high-temperature electric resistance furnace and drop tube furnace. High sulfur coals mixed with different additives were combusted at the temperature of 800～1400℃, with automated sulfur determinator and integrated flue gas analyzer to determine the sulfur retention efficiency and the emission characteristics of SOx under different combustion conditions, also the mineral composition and microstructure of desulfurization residues were dectected by XRD and FESEM. The effects of temperature, Ca/S ratio, composition of additives, residence time, and particle size on desulfurization efficiency and desulfurization residues composition were investigated to discover the desulfurization mechanisms of different additives and the formation and decomposition fundamentals of the products resulted from the reactions among inorganic constituents as well as reations between inorganic fractions and the sulfur-contained species.For exploring the formation, decomposition and transformation of desulfurization products, integrated TG-DTG-DSC-FTIR was used to study the interactions of CaO,SiO2,Al2O3,Fe2O3,BaCO3 and CaSO4. With the facilities and methods such as high temperature reactor, XRD and SEM, the composition and micro appearance of products generated from calcinations experiments of different minerals blended with CaSO4 were analyzed. Combined with the theories of multiphase equilibrium, thermoanalysis dynamics and silicate solid reaction kinetics, the influences of minerals species on the stability of desulfurization product CaSO4 were studied, and the CaSO4 decomposition kinetic model in different mineral systems was developed to disclosure the formation, decomposition and transformation behaviors of sulfur-contained species in various mineral systems.Combustion and fusion behavior were also investigated with the research of desulfurization and ash utilization. Combustion test of coals doped with different ratio of additives was finished using TG analysis and single burner furnace, and the influences of additives on coal ignition, stable combustion, burn-off as well as exothermic were discussed. Basing on thermodynamic calculation, the change of ash fusion temperature after coal doped with additives was analyzed. The influences of additives on coal combustion and ash fusion were exposed from macroscopic view.Through detail experimental research and theoretic analysis, heterogeneous reaction mechanism in complicated mineral system was explored, and the optimal conditions were obtained to realize coal desulfurization and ash utilization. The whole research work provided scientific basis for effective pollution control and byproducts utilization during coal combustion, as well as harmonious development of energy-resource-environment.