Study On Chlorine Migration During The Co-Hydrothermal Carbonization Of Pine Sawdust And PVC And Hydrochar Pyrolysis Characteristics

In view of the alkali corrosion,easy slagging and chlorine corrosion of high-alkali solid fuel and chlorine-containing waste in the process of thermochemical utilization,In order to improve the properties of hydrochar,the high-alkali solid fuel and polyvinyl chloride(PVC)were combined to remove chlorine and alkaline and alkaline earth metals(AAEMs)simultaneously.In this thesis,the pine sawdust(high-alkali solid fuel)and PVC were co-hydrothermal carbonized,focusing on the migration characteristics of chlorine elements in the process of co-hydrothermal carbonization.On this basis,the pyrolysis characteristics of hydrochar prepared by this method were further studied,and the influence law of related factors on the pyrolysis characteristics of hydrochar was explored,in order to realize the resource utilization of organic solid waste through coupled hydrothermal carbonization-pyrolysis process.The main conclusions were as follows:(1)Pine sawdust was hydrothermal carbonized at 220 ~oC,260 ~oC and 300 ~oC,and the obtained hydrochar was denoted as C-220,C-260 and C-300.The pine sawdust and PVC(mass ratio 9:1)were co-hydrothermal carbonized at the hydrothermal pretreatment temperatures of 220 ~oC,260~oCand 300 ~oC,and the hydrochar obtained was denoted as Cp-220,Cp-260 and Cp-300.The pine sawdust and hydrochar were characterized by elemental analysis,industrial analysis,ICS-1100 ion chromatograp,fourier infrared spectrometer(FTIR)and nitrogen physical adsorption desorption instrument.The results showed that the H/C atomic ratio,O/C atomic ratio,volatile matter content,and the number of oxygen-containing functional groups of hydrochar decreased,while the fixed carbon content and high calorific value increased.The H/C atomic ratio and O/C atomic ratio of the hydrochar obtained from the co-hydrothermal carbonization of pine sawdust and PVC at the same hydrothermal pretreatment temperature are more obvious than those obtained from the hydrothermal carbonization of pine sawdust alone.In addition,the removal rate of chlorine in the raw material can reach more than 80%by the co-hydrothermal carbonization of pine sawdust and PVC.The specific surface area of Pine sawdust and hydrochar follows the rule of Cp-260>Cp-300>C-300≈pine sawdust>Cp-220>C-220≈C-260,the change trend of total pore volume and specific surface area is consistent.Based on the variation of specific surface area of hydrochar,the pore structure evolution mechanism of hydrochar was proposed,which was affected by hydrothermal pretreatment temperature and pressure.(2)In order to study the migration properties of chlorine in the co-hydrothermal carbonization process of pine sawdust and PVC,a monomolecular model of PVC was constructed in combination with the possible nucleophilic substitution reaction of hydroxyl(-OH)and intramolecular elimination reaction to generate HCl in the co-hydrothermal carbonization process of PVC.The geometric shape of PVC monomer molecular model was optimized by M062X/6-311G(d,p)base group level in Gauss 16 program.The optimized PVC monomer molecular model was labeled with four C-Cl site(1,2,3,4).Multiwfn 3.7 program was used to perform Mayer bond order(MBO),Laplacian bond order(Laplacian bond order,LBO),Bond dissociation enthalpy(BDE),and charge and Electrostatic potential(ESP)of C atoms on C-Cl bonds.Based on the results of density functional theory calculations,it was fond that the sequences of LBO and BDE values was the same as 4>2>3>1,and the sequence of elimination reaction at four sites in the process of co-hydrothermal carbonization of PVC is 1-3-2-4.The carbon atomic charge of the four sites is 2>1>3>4,and then the four sites are replaced by-OH in the process of co-hydrothermal carbonization of PVC in the order of 2-1-3-4.(3)The pyrolysis characteristics of pine sawdust and hydrochar were studied by thermogravimetric analysis.The pyrolysis kinetics and pyrolysis characteristics of pine sawdust and hydrochar were solved using the Arrhenius expression and the Coats-Redfern approximate integral method.The results showed that the pyrolysis process of each sample was mainly the volatilization stage with only one obvious weight loss peak.The higher the heating rate,the greater the temperature corresponding to the maximum weight loss rate and the maximum weight loss rate.At the same heating rate,the activation energy of each sample in the main thermal weight loss stage can satisfy the Pine sawdust≈Cp-220>C-220>Cp-260>C-260>C-300>Cp-300trend.At the same heating rate,the pyrolysis characteristics of Pine sawdust and hydrochar are consistent with the trend of maximum weight loss rate,which satisfies the law of C-220>Cp-220>Pine Sawdust>Cp-260>C-260>C-300>Cp-300.(4)The fast pyrolysis characteristics of pine sawdust and hydrochar in nitrogen atmosphere were investigated by a sinking tube furnace with a fixed bed.The influence of hydrothermal pretreatment temperature,whether PVC is added for hydrothermal carbonization and pyrolysis temperature on the distribution and characteristics of three phase products of raw material pyrolysis was studied.The pyrolysis temperatures studied in this thesis are 700 ~oC,800 ~oC and 900~oC.The results show that the yield of solid products at the same pyrolysis temperature meets the law of Cp-300>C-260>C-300>Cp-260>C-220>Cp-220>Pine sawdust,which is consistent with the change trend of fixed carbon content in industrial analysis.The variation trend of gas product yield is opposite to that of solid product.At the same pyrolysis temperature,the yield of hydrochar tar obtained from hydrothermal carbonization of pine sawdust gradually decreased with the increase of hydrothermal pretreatment temperature,while the yield of hydrochar tar obtained from co-hydrothermal carbonization of pine sawdust and PVC first increased and then decreased with the increase of hydrothermal pretreatment temperature.When the pyrolysis temperature increases,the yield of CH4,CO and H2 increases.FTIR and electron scanning microscope(SEM)were used to characterize the sample and its pyrolytic solid products.It was found that after pyrolysis,the number of functional groups decreased,and C=O double bond and ether group(C-O-C)almost disappeared,while the number of hydroxy groups of Cp-220 and Cp-260 pyrolytic char increased.According to the SEM spectrum,the sample will split into many massive fragments after the hydrothermal carbonization process.After fast pyrolysis,these massive fragments were further broken into small fragments,showing a slight honeycomb shape.In particular,the SEM spectra of Cp-220 and Cp-260 showed broken spheroidal particles.