Pyrolysis Of Low-rank Coal And Its Tetrahydrofuran Extract

Coal, as the main primary energy source of China, plays an important role in the economic develoment; however, the extensive use of coal results in serious environmental problems. One of the methods to coordinate the economic development and environmental protection is to study and develop more efficient coal utilization technologies. Coal pyrolysis is the basis of coal gasification, liquefaction, combustion and carbonization, therefore, it’s helpful to develop new and efficient coal utilization technology by understanding the mechanism of pyrolysis. Considering the complexity of coal structure and composition, the solvent extracts of coal with simpler structure relatively were selected as sample to investigate the distribution and formation mechanism of its pyrolysis primary products by in-situ pyrolysis coupled with time of flight mass spectrometry (Py-MS). The results can provide the basic data for further studying on the mechanism of coal pyrolysis. The pyrolysis behavior of coal and extraction residue was investigated using a fixed bed reactor. The effect of extractable components on coal pyrolysis was discussed by analyzing the distribution, composition and structure of products. In addition, the distributions of pyrolysis primary and final products were studied by Py-MS and TG-MS. And the transformation of the primary products to final products during pyrolysis process was analyzed to recognize the role of secondary reaction on coal pyrolysis. The main research works and results of this thesis are mainly as follows:Xilinguole (XLGL), Huolinhe (HLH), Baiyinhua (BYH) lignite and Buliangou (BLG) sub-bituminous coal were extracted by tetrahydrofuran (THF), and the structure and composition of extracts were analyzed by multiple analytical technologies to provide basic data for studying its pyrolysis behavior. The average molecular weight of HLHE (Mw,798; Mn, 494) is less than that of coal tar. The distance of aromatic layer of crystallite of extract (HLHE, doo2=0.42 nm) is larger than that of coal (HLH, d002=0.357 nm). In addition, the La and La/Lc of extract is larger than that of coal which shows the crystallite of extract being flatter than that of coal. The extract molecule is rich in aliphatics, carbonyl, ether bond but lack of hydroxyl. Its average aliphatic carbon chain length is 2 and its aromatic structural unit is naphthyl ring with 3-4 substituent groups. These groups are mainly ether bond and phenolic hydroxyl.The self-developed in-situ pyrolysis coupled with vacuum ultraviolet photoionization and/or electron impact ionization mass spectrometry (Py-PI/EI-MS) was applied to diagnose pyrolysis process of HLHE and to investigate the distribution and formation of primary products. The El and PI sources were used to analyze primary gas and tar, respectively. The Py-EI-MS results show that H2O and CO2 come from the HLHE pyrolysis via ion reaction mechanism at 100-400 ℃ and radical reaction mechanism at 400-750 ℃. CH4 and H2O are from the decomposition of alcohols and ethers via radical reaction mechanism. The Py-PI-MS results show that the main primary alkanes, olefins and monocyclic aromatics from HLHE pyrolysis are C17H36-C19H40, C16H32-C19H38, C17H28-C18H30, respectively. However, the main alkanes, olefins and monocyclic aromatics in HLHE are C12H36-C15H40, C14H28-C15H30, C16H26-C17H28, respectively. The TG-MS analysis also shows that CH4 and C2H6 are main products of secondary reactions of aliphatic volatiles. All of the results above indicate aliphatic and monocyclic aromatic free radicals can combine with CH3 and C2H5 radicals easily. The main primary phenols are C7H8O and C8H10O owing to the easy cleavage of Cα-Cβ in aliphatic side chain of phenolic precursors.The fixed-bed pyrolysis experiments of three lignites and a sub-bituminous coal and their THF residue were carried out. The composition and structure of their products were analyzed to investigate the effect of extraction on coal pyrolysis. The results show that THF extraction of coal causes less H2O and tar yields but higher gas yield in pyrolysis. H2, CO and CO2 yields of residue pyrolysis are higher than those of coal, which is quite opposite to CH4 yield. GC was used to analyze the contents of 63 samples with high relative content. The results show that phenols and monocyclic aromatics yields of residue tar are increased but aliphatics and polycyclic aromatic hydrocarbons (PAHs) show a different tendency. The NMR analysis of tar show that the average length of aliphatic carbon chain of residue tar was shorter than that of coal tar. The aromaticity of residue tar is improved and the content of light components in residue tar is increased. The pyrolysis kinetic parameters of coal and residue were calculated based on TG data. All the activation energy and reaction rate constant of residue are lower than those of raw coal, suggesting that the extractable components in coal is beneficial to coal pyrolysis. The composition and distribution of primary products of coal and residue were further analyzed by Py-MS. THF extraction results in higher yields of CH4, H2, CO and CO2 but lower H2O yield. The total yield of phenols in primary residue tar decrease and total yield of diphenols, monocyclic aromatics and aliphatics increase. However, the yields of small molecule, such as C6H6O, C7H8O, C6H6O2, CeH6, C7H8 and C8H10 decrease obviously. The different distribution of primary products from that of final products illustrates that the secondary reaction of primary products changes the distribution of final products.In order to investigate the transformation of primary products to final products during pyrolysis, the Py-MS was employed to analyze the primary products of HLH pyrolysis and tar (final products) from fixed-bed pyrolysis of HLH. The final gaseous products of HLH pyrolysis analyzed by TG-MS show that the primary gaseous products are stable. The secondary reaction of volatiles is preferable to the generation of H2 (400-600 ℃), H2O (>600 ℃), CO2 (500-800 ℃), CH4 (>550℃), C2H6 (>550℃). The reaction pressure affects the formation of CO and CO2 at about 100-400 ℃. CH3 and C2H5 free radicals from the cleavage of aliphatic components increase the average molecular weights of monocyclic aromatics and phenols. Primary PAHs containing more than three rings always are prone to form char.