Partition Mechanism And Interaction Of Minerals And Trace Elements During Coal Combustion

Coal is the mainly energy source in China, huge amount of coal utilization cause serious pollution to the environment. Especially for the emission of mercury and particulate matter which have caused damage to human health. Mineral matter is the mainly composition of the coal, the partition and transformation of minerals during coal combustion are responsible to the safe utilization and pollutant emission. The transformation process of mineral during coal combustion is still not well understood because of the complex mineral composition in coal. More and more novel methods and techniques are using to study the mineral partition.The object of the thesis is to clarify the transformation mechanism of minerals and trace elements during coal combustion. At the first, magnetic separation, seizing, and float-sinking procedures were used to separate the fly ash. X-ray diffraction, field scanning emission microscopy combine energy dispersive spectroscopy and X-ray fluorescence were used to analyze the physical-chemical characteristics and microstructures evolution mechanism of high aluminum fly ash, high-calcium fly ash, and ferrospheres. A mineral melting thermodynamic simulation method was developed to analyzer the influence of mineral transformation on ash melting and particle formation. The interactions of trace elements arsenic, chromium and mineral elements were studied, the capture and oxidation of mercury by fly ash was investigated, a mercury adsorption dynamic model was developed, which provide basement for pollution control in coal-fired power plants.The transformation of typical aluminum minerals in high temperature was investigated by systematic drop tube furnace (DTF) experiments and thermo gravimetric analysis. The phase transformation of boehmite in coal during high temperature treatment is undergone four stages include: boehimte dehydroxylation, transition phaseθ-Al₂O₃ formation, crystal nucleation andα-Al₂O₃ formation, and growth ofα-Al₂O₃ crystal. The DTF experimental results indicated that the growth ofα-Al₂O₃ crystal has significant impact on PM₁ emission. Mineralogy, crystallography and other multi-disciplinary theories were combined to reveal the influence of mineral lattice changes on the formation of ultra-fine particles, and establish the relationship of micro crystal structure and macro PM emission. Besides, lower temperature ashing, high temperature thermal gravimetric analysis was conducted to describe the formation mechanism of different calcium-bearing compounds. Calcium oxide phase is mainly derived from the decomposition of excluded calcium-bearing mineral, while calcium aluminosilicate phase is formed by the fusion of included calcium-bearing minerals. And both of calcium sulphate phase and Ca-S-X phase are the self-desulphurization production of calcium-bearing mineral, calcium sulphate phase is formed by the excluded calcium-bearing minerals easily; while Ca-S-X phase may derive from the fusion of included calcium-bearing minerals' self-desulphurization production and other minerals in coal. Then, thermodynamic calculation software HSC was used to calculate and predict the transformation of different mineral speciation during coal combustion. A kinetic model for describing single pyrite partition during coal combustion was developed. The partition process of iron-bearing mineral was studied and the influence factors on pyrite oxidation and sulfur release were discussed. The results show that pyrite particle can rise to the eutectic point in a short time in furnace, excluded iron-bearing minerals oxidized to form ferro-oxides, included iron-bearing minerals mixed with other minerals to form complicated Fe-Al-Si solid solutions, the transformation processes of Fe-bearing minerals were related to temperature, atmosphere, and the occurrence of Fe-bearing minerals. The formation of Fe²⁺ intermediate products and Fe-S-0 eutectic ash particles were the important sources of the initial layer which occur in deposits formed in coal burning systems. Based on the X-ray diffraction mineral quantitative analysis of low temperature ash and classical thermal analysis theory, a mineral melting dynamic method (MQRLSTA method) was developed to calculate mineral melting curve. Compared to the ash melting points measured by conventional methods, the mineral melting curve calculated using MQRLSTA method can reflect the melting process better. Mineral melting characteristic curves indicated mineral melting is multi-stage reaction process. The influences of mineral evolutions on ash melting and particle formation were described.With the aim of better understanding partition of semi-volatile trace element arsenic and non-volatile element chromium, combustion of two kinds of high-arsenic coals and high chromium coals was studied in a bench-scale drop tube furnace (DTF). The occurrences of arsenic and chromium in coal have significant effects on their enrichments on particles. The distribution of mainly mineral elements calcium and iron in the particles is also an important influence factors for the emission of trace elements during coal combustion. Petrography classification standard was applied to distinguish fly ash carbon, systematic experiments of fly ash capture mercury were conducted on a fixed-bed reactor to investigated the interaction between fly ash and mercury, the results imply that the carbon content is not the only variable that controls mercury capture in fly ashes, there are likely to be significant differences between the mercury-sorbing capacities of these various carbon forms. Hg capture capacity mainly depends on the content of anisotropy carbon particles with porous network structure. Compared to the organic carbon, the inorganic composition has less influence on Hg capture capacity of fly ash. Temperature, flow and Hg concentration in flue gas and other conditions has significant effect on Hg capture and oxidation. Three dynamic models were used to calculate mercury adsorption on fly ash, the oxidation mechanism was clarified. The reaction mechanism of mercury oxidation by fly ash is mainly Mars-Maessen reaction, the lattice oxygen in inorganic component of fly ash is the mainly oxidant.In summary, the evolution mechanisms of typical minerals in coal have been revealed; a mineral melting dynamic analysis method has been established, which would provide a theoretical basis to study mineral ash deposition in-depth. The interaction of mineral, fly ash, and trace elements is helpful for cheap pollution emission control technology development.