Study On Chemical Percolation Devolatilization Model For Slow Pyrolysis Process Of NMH Coal

During coal pyrolysis,the crosslinking reaction will add connections between aromatic clusters,and then affect the product distribution.However,the cross-linking reaction is mostly accompanied by the cleavage of the side chain structure,and the relationship between free radicals is complicated.In the CPD model,a complete description method of cross-linking reaction kinetics has not been established,which leads to a large deviation between the predicted value of the product yield and the experimental value under the slow pyrolysis condition.By taking advantage of the characteristics of unstable bridge chain cracking earlier than side chain cracking,char with relatively complete bridge chain cracking and more side chain content can be prepared.Then the cross-linking reaction mechanism of side chain cracking can be specifically studied with the secondary pyrolysis of char.In this paper,coal was rapidly pyrolyzed at different temperatures to prepare char with different bridge chain and side chain content.Then the char was subjected to second pyrolysis to analyze the kinetic characteristics of the cross-linking reaction.Finally,the influence of the cross-linking reaction path was considered in the CPD model,and the input parameters of the model were revised to improve the accuracy of the product yield prediction under the conditions of slow pyrolysis.First,NMH coal with more aliphatic chain content was selected to conduct rapid pyrolysis experiments at different temperatures by drop tube furnace reactor to collect char,then the relationship between FT-IR,Raman,XRD,HRTEM and N₂ adsorption/desorption on the structural characteristics of char was analyzed.When the pyrolysis temperature is less than800℃,depolymerization is the main reaction during coal pyrolysis.With the increase of temperature,the breakage of ether bridge chains increases,and the depolymerization of large aromatic clusters generates small aromatic cluster fragments.At 800℃,the content of small aromatic cluster fragments（1-5 aromatic rings）is the largest,which proves that the bridge chain fracture is the most complete and the relative content of side chains is the largest.When the pyrolysis temperature exceeds 800℃,the side chains begin to crack and undergo cross-linking and polycondensation reactions.The size of aromatic nucleic microcrystals increases,and the structure of cross-linking bridge chains and the content of larger aromatic clusters with more than 6 aromatic rings increase.With the increase of pyrolysis temperature,the decomposition of aliphatic structures such as methyl group and methylene in the coal increased,but the amount of aromatic cluster fragments precipitating from the coal into tar also increased,resulting in a relative increase in the content of aliphatic structure.As a result,the absorption peak of aliphatic structure in the FT-IR spectrum was firstly increased and then decreased with the increase of temperature,and reached the maximum at 900℃.The specific surface area of char begins to increase at 800℃when the bridge chain fracture is relatively complete,reaches the maximum at 900℃when the aliphatic structure content reaches the maximum,and gradually decreases at higher temperature.This phenomenon proved that the assumption in the CPD-PS model that the specific surface area of char during coal pyrolysis increases with the increase of the number of open pores generated by side chain fracture and decreases with the decrease of the number of adsorption pores attached to open pores caused by aliphatic structure cracking is correct.The change of specific surface area of NMH coal during pyrolysis process predicted by CPD-PS model is consistent with the experiment.According to the thermogravimetric experiment of char,the kinetic parameters in the secondary pyrolysis process of char were calculated.The results showed that the activation energy of the secondary pyrolysis of char rapid pyrolysis at 800℃is the smallest,combined with the structure characterization results of the char,it is believed that the secondary pyrolysis of the 800℃char is mainly the cleavage of the carboxyl groups and methyl groups on the side chains,which need a lower activation energy.There are more unbroken bridge chains in the fast pyrolysis char obtained at lower temperature.The volatiles are formed through two-step reaction of bridge chain and side chain scission,and the pyrolysis activation energy is higher than 800℃.The side chain functional group in the fast pyrolysis char obtained at higher temperature has been cracked relatively completely,and the activation energy of the secondary pyrolysis is higher.On the basis of the laws obtained in the experiment and considering the influence of the cross-linking reaction,the hypothesis of the reaction path of the CPD model was revised and the kinetic parameters in the CPD model were improved.After comparison with experiments,the revised model maintains the accuracy of the CPD model for predicting the yield of fast pyrolysis products,and improves the accuracy of the model for predicting the yield of slow pyrolysis products.