| Coal is still the basic energy of China in the next few decades. Taking energy security and national security into account, it is necessary to develop coal chemical industry to reduce imports of oil and natural gas. Coal staged conversion poly-generation technology that based on coal pyrolysis can achieve the efficient use of low-and-mid rank coals that are abundant in our country, and the technology can be co-produced oil and many kinds of chemicals. Based on the technology, a number of processes have been formedat home and abroad, such as "coal pyrolysis + char combustion","coal pyrolysis + char gasification","coal pyrolysis + calcium carbide production by char". The main problems in these processes include low rank coal dehydration, low yield of tar during the pyrolysis of coal, difficulty in dust separation, high temperature in char combustion and gasification, high power consumption in calcium carbide production and so on. "Oil generation from pyrolysis of low-and-mid rank coal + calcium carbide production by oxygen-fuel method" is an efficient process that proposed by the team led by professor Zhenyu Liu (Beijing University of Chemical Technology).Oxygen-fuel method uses the heat produced by char in situ oxidation to replace energy generated from coal combustion which is adopted in the traditional electric arc method, so the total energy consumption is reduced by 56% in oxygen-fuel method.In order to solve the problems, high dust content in tar, difficulties in dust/volatiles separation, frequent clogging of volatile product lines and devices, effects of inorganic components on mass transfer and reaction,unclear cognition about spontaneous combustion in the process of lignite drying, that existed in the process of "oil generation from pyrolysis of low-and-mid rank coal + calcium carbide production by oxygen-fuel method",we studied coking behavior and mechanism of volatiles, morphological changes of inorganic components in the process of calcium carbide production by oxygen-fuel method, influences of oxygen and moisture on the concentration of radicals during lignite drying, respectively. In this thesis, we studied the reaction of tars generated .from pyrolysis of a subbituminous coal in a fixed-bed reactor in the process of up to 600 ? to simulate the reaction of volatiles in a large fast pyrolysis reactor. It includes the changes in tar components, radical concentration and coking behavior. The properties of coke formed in a coal tar under the conditions of typical coal pyrolysis are also studied. The coke is characterized by its composition, morphology,structure and reactivity. The powder of CaO and char are well mixed and then react at high temperature which is low than that of CaC2 formation. Inorganic components are obtained by low temperature ashing and then its morphology,composition and morphological are characterized. The changes of radicals concentration during the process of lignite drying in different methods are measured. The main results are described in the following:(1) An increase in the volatiles' residence time in the pyrolysis reactor reduces the yield and increases the radical concentration of tars. The higher pitch content in tar, the more coke can be formed in thermal reaction.The pitch fraction of the tars is mainly responsible for coke formation which occurs significantly at temperatures higher than 420 ?. The tars contain radicals,mainly in their coke and pitch fractions. The coke concentration in the tars increases with increasing heating time, linearly at 420 and 450 ? but in a sigmoidal pattern at higher temperatures. The coking behavior within 10 min can be expressed by the zero-order or zero-order+autocatalytic kinetics with activation energies of 128-138 kJ/mol. The coke formation in the tars is accompanied with radical formation, and their relation is approximately linear in a coke concentration range of less than 40 wt.%.(2) The residence time of volatiles has great effects on the composition and amount of pyrolysis gas. The yield of gas decreases when the residence time are shortened. The oil content in tar generated at different final pyrolysis temperature is about 60-70%. The higher the final pyrolysis temperature, the lower of H/C and the more of heavy components, also, the more coke would be formed after heated to same temperature. With increasing final pyrolysis temperature, the radicals concentration of tar increase and radicals in coke accounts for 16-27% of total free radicals. Coke formation is significant at 300? in semi batch experiments and indicates an autocatalytic characteristic. The light fractions in tar restrain the condensation of heavy fraction in tar unless the temperature is high enough to cause condensation of the light fractions,such as at 500 ?. The radicals concentration of coke formed in semi batch experiment is 14 times as much as that of coke formed in batch experiment.(3) The coke is categorized into two types, the one suspended in THF,Coke-S and the one deposited on the wall of tube reactor, Coke-D. With increasing tar reaction temperature and time the particle size of Coke-S increases from a most probable size of approximately 0.1 ?m to 700-800 ?m.This change is accompanied with reduction in alkyl side chains and heteroatoms (O, N and S), as well as the enrichment in aromatic Car-Car bond,which lead to a decrease in H/C ratio from 0.9 to 0.6 and increase in aromaticity fa from 0.70 to 0.86. The carbon distribution in Coke-S is similar to that in bituminous coals, and is composed of 3-7 fused aromatic rings. The changes in Coke-S also include increase in radical concentration and decreases in the radicals' g value and linewidth, indicating continued pyrolysis and condensation of the coke due to the removal of oxygen atoms and side chains on the aromatic structure. Comparing with Coke-S, Coke-D formed under the same conditions is more condensed. The morphological change in Coke-D includes transformation of small irregular particles to spherical-like particles and to coke film.(4) Inorganic components in char agglomerate in an inert atmosphere at 1500 ?, which is accompanied by both physical and chemical changes.Inorganic components in char which size is blow 10 ? m are more easily to agglomerate to large particles. CaO can increase the degree of agglomeration,and its reactivity is stronger than that of Si-containing inorganic components.CaO can react with Si-containing or Al-containing inorganic components to form Ca-Si-O or Ca-Al-O compound, respectively. The composition of inorganic components affect agglomeration, and Si-containing component has a great influence on agglomeration, especially.(5) While drying under air atmosphere at 280 ?, radicals in lignite can be oxidized by oxygen, at the same time, the heat released by the oxidation promotes the escape of moisture. Drying method affects the structure of coal and the species and number of functional groups on the surface of coal affect the state of re-absorption moisture. Oxygen can react with radicals to reduce the concentration of radicals or react with functional groups in coal to produce peroxy radicals. |
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