| Coal pyrolysis polygeneration is an effective method for clean and efficient utilization of coal. The method can achieve the coal hierarchical transformation to low carbon fuel and chemicals. It can improve the quality and added value of pyrolysis tar and gas from pyrolysis source using hydrothermal treatment. Then, the fundamental research of low rank coal structure and pyrolysis, the research on effect of hydrothermal treatment on pyrolysis product distribution based on carbon structure and the process of coal tar have a certain theoretical significance and economic value.Xiaolongtan lignite (XLT), Xianfeng lignite (XF) in Yunnan and Shengli lignite in Inner Mongolia (IM) were used as raw coal. Hydrothermal treatment was carried out in a lab autoclave to study the effect of hydrothermal treatment on pyrolysis product distribution. Free radical regulation theoretic was applied to explicate the pyrolysis mechanism. The raw and treated lignite structure were monitored by 13C Nuclear Magnetic Resonance (13C-NMR) and Fourier Transform Infrared Spectroscopy (FTIR). The results show that hydrothermal treatment disrupts the weak bond in lignite. The oxygen contents in lignite decrease. When the lignite was treated at 220-260 ℃, the oxygen functional group hydrolysis during hydrothermal treatment. The hydrogen in water is transferred into lignite by the synergistic effect of ionic and radical reaction. The CH2/CH3 ratio increases. The coal tar yield increases and the pyrolysis gas decreases. Thereinto, the aliphatic components in tar and the hydrocarbon gas in gaseous products increase after hydrothermal treatment.Free radical concentration in raw and treated lignite was detected by Electron Paramagnetic Resonance (EPR). The decrease of oxygen functional group results in the increment of free radical concentrations. However, the hydrogen donated from water was transferred to coal to stabilize the radicals. The increment ratio of free radical concentration has a minimum treated at 220-260 ℃. The equilibrium of free radical formation and stabilization is obtained in this condition so that the components with medium molecular weight were easily formed during pyrolysis after hydrothermal treatment and the pyrolysis tar yield is maximum.In order to elaborate the effect of carbon structure in coal on pyrolysis product distribution. The research of three other bituminous coal shows that the pyrolysis product distribution is correlated with carbon structure and the mechanism is similar to that on lignite. The pyrolysis tar yield of coal with Cdaf% ranged 73% to 84% has a certain numerical relationship to the aliphatic substituted aromatic carbon, CH2/CH3 ratio and oxygen-linked carbon in coal structure when pyrolysis at 600 ℃. The pyrolysis gas yield is associated with O/C atomic ratio and aliphatic carbon. The composition of pyrolysis tar and gas is also related to carbon structure in coal.Furthermore, an integrated method was utilized for process of coal tar in this study. The results show that light components in coal tar pitch can regulate and control the formation of quinoline-insoluble (QI). In one hand, the light components were adhered on the surface of toluene insolubles (TI) and formed steric stabilization or depletion stabilization during the initial thermal polymerization, the formation of QI cluster is hindered. In the other hand, the small groups were adhered to QI clusters by cross-linking bond which is broken easily with the increase of temperature to form drag force. The middle molecular components are unbound from the molecular network and dispersed in a free region during thermal treatment, so the QI in modified pitch decreases. The modified pitch with QI<0.2%, β resin>23% and SP=90-100℃ is obtained. The modified pitch can provide test and theory data for preparation of high quality carbon materials. |
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