Study On Breakage Of Covalent-bonds And Mechanism Of Pyrolysate Generation During Oil Shale Pyrolysis

Studying the chemical structure and pyrolysis mechanism of oil shale is helpful to comprehensive utilization of oil shale,which is of significance to alleviating the scarce problem of energy.The essence of chemical reactions is the cleavage and regrouping of covalent linkages.Therefore,analyzing pyrolysis process of oil shale through studying covalent bonds is a good way to understand oil shale's chemical structure.As one of the by-products of paper industry,alkaline lignin has not been made good use of and discharged arbitrarily every year resulting in significant environmental pollution.But alkaline lignin plays a role of hydrogen donor during co-pyrolysis of alkaline lignin and oil shale,and promotes the synergistic effect.Thus,analyzing the mechanism of co-pyrolysis of oil shale and alkaline lignin based on covalent bonds is significant to the utilization of synergy.During pyrolysis process,functional groups crack and covalent bonds break,generating numbers of free radicals.Then rapid regrouping reactions happened among free radicals leading to the generation of pyrolytic vapor.Methane?CH₄?and carbon monoxide?CO?are main pyrolysates during oil shale pyrolysis,related to methyls of different locations and different oxygen-containing functional groups,respectively.In the present study,based on thermogravimetry coupled with fourier transform infrared spectroscopy?TG-FTIR?analysis and deconvolution method through numerical analysis,the breakage of covalent bonds during individual pyrolysis and co-pyrolysis was studied by analyzing the mechanism of CH₄ generation.In addition,the mechanism of CO generation during oil shale pyrolysis was investigated with the help of ¹³C-nuclear magnetic resonance(¹³C-NMR)and X-ray photoelectron spectroscopy?XPS?.The influences of heating rates and metamorphic grade on the CO generation were also studied.Individual pyrolysis processes of oil shale from Tongchuan and Maoming were studied.Compared with Maoming oil shale,Tongchuan oil shale has a more complicated chemical structure.CH₄ FTIR lines and DTG lines of oil shale pyrolysis were fitted into sub-curves through deconvolution method.It is found that the generation of methane is related to three kinds of covalent bonds,and thermal weight loss of oil shale is related to the breakage of three groups of covalent bonds.With the heating rate increases,the thermal weight loss of oil shale is more determined by the generation of methane.The mechanism of CH₄ generation and the breakage of covalent bonds of alkaline lignin were also investigated.It is found that during alkaline pyrolysis,the generation of methane is related to two kinds of covalent bonds,and thermal weight loss of alkaline lignin is related to two groups of covalent bonds.Theory lines of methane generation during co-pyrolysis of oil shale and alkaline lignin was made by superposing lines of methane generation during individual pyrolysis in corresponding ratios.Experimental lines of methane generation during co-pyrolysis of oil shale and alkaline lignin was compared with theory lines.Results showed that synergy of co-pyrolysis of oil shale and alkaline lignin promotes the generation of methane.CH₄ FTIR lines and DTG lines of co-pyrolysis of oil shale and alkaline lignin were fitted into sub-curves through deconvolution method.It is found that co-pyrolysis of oil shale and alkaline lignin can promote the breakage of three groups of covalent bonds in different ways.The lines of CO generation during three different kinds of oil shale pyrolysis was analyzed.Because of different metamorphic grades,categories and quantities of oxygen-containing functional groups of oil shale also vary.Organic carbon and oxygen-containing functional groups of oil shale were investigated through conducting experiments of¹³C-NMR and XPS on kerogen of oil shale from three different places.From lines of CO generation during oil shale pyrolysis and the fitting results of XPS,the cracking of oxygen-containing functional groups at different temperatures and condensation reactions of tar molecules lead to the generation of carbon monoxide.The generation of CO during pyrolysis of oil shale from different places at different heating rates was analyzed.With the heating rate increases,the generating rate of CO increases significantly,and the FTIR lines of CO move to a higher temperature with a decreasing line width.Compared to Fushun oil shale,Huadian oil shale with a lower metamorphic grade has a larger number and a higher variety of CO precursors,which leads to a higher generating rate of CO.In addition,constituent reactions of CO precursors during Huadian oil shale pyrolysis tend to overlap each other easily,resulting in a smaller line width.Also,increasing heating rates promotes various constituent reactions simultaneously,so the increasing amplitude of CO generating rate of Huadian oil shale is higher than that of Fushun oil shale.