CCSEM Study On Mineral Evolution And Formation Behavior Of Ash Including PM₁₀ During Coal Combustion

Fossil fuel, especially the coal, will dominate our country’s energy consumption in the foreseeable future of China. During the combustion of pulverised coal, the inorganic minerals in coal undergo a variety of physical and chemical processes to be transferred to ash residues. The ash residues may not only cause the slagging onto the heat-transfer walls threatening the safe and economical operation of boiler in power plant, but also generate a great deal of PMk, and finer PM2.5(the aerodynamic diameter≤10or2.5μm), both of which give rise to severe environmental and health effect. Based on these risks, the advanced particle analytical technology, i.e. CCSEM (computer-controlled scanning electron microscopy) was adopted, and the coals with typical coal rank, i.e, lignite, bituminous coaJ and anthracite, were burned in drop tube furnace (DTF) for studying the evolution of minerals and the formation behaviors of ash including PM10. Main works of this thesis are summarized as follow:The mineral properties of the coals, chars and bulk ashes were characterized by CCSEM. including type, content, size distribution, the association with carbonaceous matter (included or excluded), and the mode of occurrence of typical elements in minerals. The chemical transformation of major included/excluded minerals during devolatilization and combustion of coals, together with the physical transformation including the minerals decompostion. fragmentation, coalescence, etc. were analyzed in depth. The work prompts our understanding about the evolution behavior of minerals during coal combustion, for instance, seldom quartz in various coals are transformed during the stage of coal devotilization. while during the stage of char combustion they react with other minerals to form Al-silicates; During coal dcvotilizalion. kaolinite would decompose or fragment to decrease the size, afterwards they coalesce to form coarser mullite-like phase after chars are burned out.The attempt for quantifying the mineral interactions during coal combustion was carried out by CCSEM. the intensity of mineral interactions illustrated by the transformation of Fe/Ca (also includes the intensitv derived from excluded minerals), as well as the types and contents of original minerals and target mineral species related to interactions were quantified.28.3-57.7%of Fe and51.3-68.1%of Ca interact with other minerals during coal combustion, the intensity of mineral interactions illustrated by Ca is higher than that by Fe for each coal. Less than half of Fe associated with exclude minerals occurs interactions while higher than half of Ca in excluded minerals interacts with others. The intensity of excluded mineral interactions illustrated by Fe or Ca accounts for60%or higher of total intensity, which implies a strong impact of excluded mineral interactions on the transformation of Fe and Ca.The circularity of mineral and ash particles can be measured by CCSEM, therefore for the ash components with different chemical composition, the original minerals in coal and the change of circularity can be analyzed by CCSEM to indicate the fusion behavior of ash components, further to supply a new method to identify the slagging potential of various ash components. Mullite-like phase is mainly formed by the direct transformation of kaolinite in coals, a slight increase of basic elements (Ca. Na) after burnout is the likely candidate for the fusion in ashes of YQ anthracite and CF lignite. K Al-silicate in ash of DT bituminous coal do not melt after combustion, it is generated from the high-melting point minerals, dominantly kaolinite and quartz, thus contributing little to ash melting. The high fraction of unclassified in CF ash is Al-silicates containing much Ca and Fe, abundant Ca is assimilated into ash to result in its obvious melting.For clarifying the roles of carbonaceous matter combustion and mineral properties in the emission of PMio. pulverized coals were separated to a narrow density range (1.4-1.6g/cm3) for holding much included minerals, then were burned in DTF. The burning behavior and structure of coal/char, the occurrence and distribution of inherent inorganic elements/minerals, and the yields and chemical compostions of PM10were analyzed by various instruments. The formation of PM0.6is affected obviously by the combustion of carbonaceous matter, rather than the properties of inorganic elements. The obvious local reducing condition (enhance the vaporization of refractory elements), rather than the higher particle burning temperature during lower-rank coal combustion, promotes the vaporization and PM0.1yield significantly. The vaporization/coagulation and heterogeneous condensation of most inorganic elements are evidently promoted to form more PM0.1-0.6during the lower-rank coal combustion. The formation of PM0.6-10is affected by both the behavior of carbonaceous matter and minerals. Char fragmentation at early stage of combustion, and the abundance of refractory included minerals result in the highest yield of PM0.6-10from HLH lignite. PDS bituminous coal is favorable to the fragmentation of char compared to CZ anthracite, more fine minerals are remained as fine PM. consequently more PM0.6-10is generated than anthracite.The formation of PM10during char combustion in both air-firing and oxy-firing were investigated. The new combustion pattern, i.e. oxy-fuel combustion, replaces the air combustion medium N2by CO2the formation and burning properties of CO2-char were different from those of the N2-char, which is likely due to gasification reactions coal particles experienced during devolatilization in CO; atmosphere. Regardless of the combustion modes. PM10formation in combustion of N2-char and CO2-char from the same coal was found to be significantly dependent on the devotilization way and coal type properties. The production ability of PM0.5and PM0.5-10during the combustion of N2-char and CO2-char from different coals is observed to be different adequately, thus showing a coal-type dependent.