| Coal pyrolysis,as the basis of coal conversion and utilization technology,is closely related to the structure and properties of coal sample.Low rank coal accounts for a large proportion of coal reserves and production in China,pyrolysis is an effective way for its efficient clean utilization.With attempts to achieve a further comprehension on the structural characteristics of coal,this thesis investigated the formation principle and reaction pathways of initial pyrolysis products or intermediates from the perspective of structure and mechanism.The obtained results are expected to guide the optimization of conversion reaction pathways of low-rank coal,upgrading and directional controlling of products,which provides important theoretical and practical significance in promoting the progress of coal direct conversion and developing new energy-saving technologies.Therefore,a homemade in-situ pyrolysis time-of-flight mass spectrometry(Py-TOF-MS)combined with non-destructive soft ionization technique was applied to perform pyrolytic studies of a variety of model compounds with coal-related structures and eight coal samples varying in rank.And the pyrolysis reactivity and mechanism of low rank coal was summaried with a common understanding on the structural characteristics,weak bond dissociation pathways and free radical reactions,as well as effect of functional groups and product controlling features of low rank coal.The main contents and results are as follows:In-situ Py-TOF-MS was used to investigate the pyrolysis behavior of five model compounds with C-C bridged bonds(polypropylene,polyvinylpyrrolidone,poly-4-vinylpyridine,polystyrene and poly-4-vinylphenol)and two heteroatomic bonded model compounds(polyphenylene oxide and polyphenylene sulfide),by which the information of free radicals,distribution of original products and evolution characteristics could be obtained.The resultes indicated that the pyrolysis processes of selected model compounds are mainly initiated by homolytic cleavage of covalent bridged bond to generate free radicals,and then the corresponding pyrolysis products are produced by free-radical reaction.Besides,the primary pyrolysis products mainly contain monomers,dimers,multimers,and their homologous series or alkyl derivatives.The structural differences among these model compounds caused the disparities in dissociation temperature and reaction selectivity.Combined with theoretical calculations,the steric effect,conjugation effect and hydrogen bonding interaction due to different chemical environments were found to reduce the bond dissociation enthalpies(BDE)of covalent bonds at β position,resulting in the increase of pyrolysis reaction selectivity and the shift towards lower reaction temperature.The thermal decomposition of coal structure is not only the dissociation of a single covalent bond,but also the interaction in a complex reaction system.Hence,the co-pyrolysis investigations of poly-4-vinylpyridine with poly-4-vinylphenol and polypropylene with polystyrene were performed to study reaction property and interaction mechanism in co-pyrolysis progress via the Py-TOF-MS.The results indicated the dissociation of different covalent bonds and interactions between intermediate species during the co-pyrolysis reaction,and the interaction was proved to be related with the temperature section of covalent bond dissociation.The co-pyrolysis process can induce the reaction to proceed,and the interaction between the model compounds with overlapping pyrolysis temperature section is more pronounced.Eight coal samples with different ranks were used for in-situ analyses on coal pyrolysis progress,focusing on the effects of coal structure characteristics such as maceral composition,mobile phase,and functional groups on the dissociation of bridged covalent bond.The volatile release temperature of different coal samples is basically in accordance with coal rank,while the product composition is obviously various.The peak evolution temperatures of alkenes,aromatic hydrocarbons,phenols and diphenol products show a decreasing trend with the increase of carbon number of substituent groups.The existence of mobile phase in low rank coal makes the emission of volatiles appear in two stages.At low temperature,the volatiles evolved by the thermal volatilization of the mobile phase,while at high temperature,it mainly corresponds to the thermal decomposition of the skeleton structure of coal.The increase of carbon number in the functional group and the oxygen-containing group can reduce the BDE value of bridged bonds between aromatic rings,of which the effect of the oxygen-containing group is more obvious,thereby accelerating the cleavage of weak covalent bonds and promoting the decomposition of coal structure.Meanwhile,the influential degree of functional groups is related to the intrinsic properties of coal,which was found to be negatively correlated with the carbon content and positively correlated with the volatile content,indicating that the substituted functional groups has a more significant influence on the pyrolysis process of low rank coal.Based on the researches on in-situ pyrolysis behavior of coal,Py-TOF-MS was utilized to further study the products controlling and mechanism of calcium-containing minerals on pyrolysis products of Shenmu coal.Results demonstrated that calcium nitrate can significantly change the distribution of primary pyrolysis products of Shenmu(SM)coal by catalyzing the cracking of macromolecular products and dehydroxylation of phenolic products.With the addition of calcium nitrate,the content of aromatic hydrocarbons in pyrolysis product increased significantly,while the relative content of phenols and macromolecules as well as the average relative molecular weight of volatile products decreased remarkably from 182 to 132,and the proportion of light products increased from 62.2%to 84.6%.Fe-based catalysts have higher catalytic activity than calcium-containing minerals,which can increase the yield of coal pyrolysis products and improve the quality of tar.Therefore,three Fe-based catalysts loaded with γ-Al2O3,nano-SiO2 and HZSM-5 were selected to carry out catalytic pyrolysis experiments of Shenmu(SM)coal in the Py-TOF-MS.In-situ analyses on the catalytic effect of the three Fe-based catalysts on the pyrolysis products of SM coal were operated with the TOF-MS analyser.The type of supporter significantly affects the product controlling of Fe-based catalysts on SM coal pyrolysis:The Fe/Al2O3 catalyst has prominent effect of dehydroxylation and aromatization,and the relative content of benzene,toluene and xylene(BTX)in the primary pyrolysis products of SM coal sample with Fe/Al2O3 was 4.68%,which was 3.67 times higher than that of raw coal,and the corresponding content of phenol and diphenol products decreased from 4.58%and 2.47%of raw coal to 2.27%and 1.23%,respectively;The effect of Fe/SiO2 catalyst was not that obvious as former,of which the product distribution is similar to that of raw coal,and the content of alkenes,aromatic hydrocarbons and phenols increased slightly,while the relative content of diphenols and macromolecules decreased;In the presence of Fe/HZSM-5,the alkene content of catalyzed pyrolysis products increased to 7.86%,which was 2.87 times to that of raw coal,and the relative content of BTX in the product also increased by 2.35 times to 3.00%,indicating that Fe/HZSM-5 catalyst has great ability of catalytic cracking,dehydroxylation and aromatization,resulting in a significant increase in the content of alkenes and aromatics in the pyrolysis products. |
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