Reaction Behavior And Conversion Mechanism Of Typical Oxygen-containing Functional Groups In Coal Chemical Looping Combustion Process

Chemical looping combustion is a new technology and an important development direction for clean and efficient utilization of coal.Ningxia Ningdong coal reserves are abundant,with low metamorphism,relatively shallow burial and relatively high oxygen content.The study of the reaction behavior and microscopic mechanism of oxygen-containing functional groups in Ningxia coal during the thermochemical conversion is crucial to realize the clean and efficient conversion of coal resources.In this paper,based on chemical looping experiments,material characterization,mathematical modeling and mechanism analysis,we investigate the intrinsic link between the conversion behavior and thermochemistry of the coal chemical looping combustion process in this region,which is of great significance for the conversion and utilization of coal in Ningdong coal chemical base.In view of the complex system of coal macromolecules,this thesis firstly investigates the conversion process and mechanism of typical oxygen-containing functional groups such as ether bond,carbonyl group,hydroxyl group and carboxyl group during chemical looping combustion by using four typical model compounds such as polyformaldehyde,poly(methyl acrylate),poly(vinyl alcohol)and poly(acrylic acid)as the research objects.The thermal weight loss behavior of polymers in chemical looping combustion was investigated using a thermogravimetric analyzer to probe the effect of different heating rates on the thermal weight loss behavior.The model-free method was further used to obtain the activation energy of the reactions by comparative analysis,based on which the kinetic mechanism of the reactions was analyzed with 14 reaction models.In situ Fourier transform infrared spectroscopy was used to monitor the dynamic changes of oxygen-containing functional groups during the reaction process in real time.On the basis of the above single oxygen-containing functional group study,the reaction behaviors and mechanisms of oxygen-containing functional groups in the chemical looping combustion process of LW,ZQ,and YCW from Ningxia Ningdong region were selected and analyzed.The specific conclusions obtained are as follows:(1)The weight loss rate of the four oxygenated polymers was significantly higher than that of pyrolysis under the action of oxygen carriers,and the chemical looping combustion resulted in sufficient degradation of the polymers.The rate of temperature increase has a significant effect on the pyrolysis characterization parameters of polymer chemical looping combustion,as the rate of temperature increase the starting weight loss temperature is advanced,the maximum weight loss temperature is shifted to the right and the faster the rate of temperature increase the smaller the weight loss rate.The apparent activation energies of the chemical looping combustion process calculated by model-free methods(GB,FWO,KAS,Friedman)were in the order of PMMA>PAA>PVA>POM,and the reaction fitting models of PAA,PVA,and POM were calculated from the solid-solid reaction model of chemical looping combustion as secondary reaction models,while the reaction fitting model of PMMA was a diffusion reaction model.C-O-C group in chemical looping combustion process tends to decrease slowly,and its transformation path can be divided into oxidation to C=O by lattice oxygen and dissociation of C-O-C group.The-OH group shows a trend of decreasing and then increasing during the combustion of the chemical looping,and completely disappears at 350℃.This is because the hydroxyl group is mainly removed from the water molecule,and the mid-term increase is because the C-H group connected with the hydroxyl group will be further oxidized to-OH when the oxidation reaction occurs,and the dehydroxylation reaction will continue with the increase of temperature.The trend of the COOH group in the chemical looping combustion is a slow decrease and then a gentle increase,which is due to the decarboxylation reaction in the early stage and the formation of carboxylate and carbonyl oxidation in the later stage.(2)The weight loss rate of coal chemical looping combustion varies with different degrees of metamorphism,and the higher the degree of coalification,the lower the weight loss rate.The main gaseous products of coal chemical looping combustion process are CO2 and H2O.C-O-C group is a strong thermally stable non-reactive oxygen group,which first decreases slowly during chemical looping combustion,and then increases slightly due to the conversion of groups such as C=O.C=O group increases first and then decreases continuously under chemical looping combustion conditions,and the increase is due to the conversion of transition states of other groups such as hydroxyl oxidation.The-OH group is the first to be removed in the form of absorbed water and gradually decreases with increasing temperature,but overall it does not disappear completely,but only decreases in quantity.The-COOH group,as the most unstable reactive functional group,undergoes decarboxylation reaction at the early stage to produce the main gaseous product CO2,and the carboxyl group disappears almost completely at the later stage of the reaction.