Interaction Of Organic Structures During Slow Pyrolysis Of NMHcoal

Low-rank coals,such as lignite coal and subbituminous coal,have attracted increasing attention in the field of coal utilization due to their advantages,for example,high volatile content and large reserves.Coal pyrolysis is the fundamental of coal combustion,gasification,liquefaction,and other thermal conversion processes,which would affect the entire coal conversion process.The pyrolysis of coal was mainly composed of the cracking reaction of non-aromatic ring functional groups and the crosslinking reaction of the free radicals or the interaction between functional groups generated by the cracking reaction.The interaction between functional groups and free radicals changed the connection state of the aromatic cluster network in coal,and the reaction rate and coupling relationship affect the yield and composition of tar,char pore structure,and reactivity.Therefore,the study of interactions between organic structures was conductive to understanding the pyrolysis mechanism of coal and improving the efficient utilization of coal.Thus,the research on the structural transformation of functional groups and free radicals and the interaction between volatiles in the pyrolysis process was still the important focus of coal pyrolysis research.In this study,using a drop tube furnace reactor to prepare rapid pyrolysis char（NRPchar）whose functional group composition was quite different from raw coal,but still had some reactivity and relatively simple product composition under the condition of slow pyrolysis.Mixing NRPchar and Naomaohu coal（NMHcoal）to regulate the composition of functional groups.Through analyzing the influence of blending ratios and pyrolysis temperature on the products of co-pyrolysis.And using in-situ pyrolysis vacuum ultraviolet single-photon ionization time of flight mass spectrometry（Py-VUVPI-TOFMS）to investigate the escaping behavior of main primary volatiles.Furtherly exploring the interaction between organic structures during co-pyrolysis.Thermogravimetric（TG） analyzer was used to analyze the pyrolysis behaviors of the separate pyrolysis of NMHcoal and NRPchar and the co-pyrolysis of their mixtures with different proportions.The results showed that the pyrolysis activation energy was high when m_coal/m_char was equal to 5/5,and the functional group interaction was obvious at 500℃.The experiment results showed that volatiles were the main interaction intermediary.The volatiles generated from the pyrolysis of NMHcoal partially diffused into the NRPchar,and·CH₃ had more binding probability and time with aromatic carbon radicals and·O.As a result,the structure of naphthalene and phenols containing methyl groups in tar,and also the alkylated oxyaromatic carbon structure and ether structure in the char increased.The increased release of phenols reduced the oxyaromatic carbon structure in the char.The precursors of polycyclic aromatic hydrocarbons generated during rapid pyrolysis in NRPchar promoted polycondensation reactions to generate H₂ and reacted with phenolic substances in the co-pyrolysis to generate polycyclic aromatic hydrocarbons and CO,which increased the 5-6 ring compounds in the co-pyrolytic tar.A few of the 5-6 ring compounds remained in the char reduced the specific surface area of the co-pyrolysis char.The tar yield of NMHcoal was the highest at 600°C.The co-pyrolysis of mixtures with different mixing ratios was carried out in the fixed bed reactor at 600°C,and the in-situ Py-VUVPI-TOFMS was used to analyze the behavior of in-situ primary product escaping during the pyrolysis of NMHcoal,NRPchar and their mixture.The results of fixed-bed experiments showed that the alkyl benzenes in the volatiles broke bonds to form benzyl radicals,which can cross link the active sites of char to increase the CH₂ structure and the 1-2 small aromatic ring structure in the copyrolytic char.·CH₃ can be captured in time,which promoted the formation of naphthalene and phenols containing methyl groups in copyrolysis tar.When the addition amount of fast pyrolysis char is higher than 40%,the ratio surface area of copyrolysis char changes from decrease to increase,and the pyrolysis changes from inhibition to promotion.The results of in situ Py-VUVPI-TOFMS showed that the types and contents of volatiles detected in the pyrolysis process of NRPchar were very few,mainly including benzene,naphthalene,phenanthrene,pyrene and some aromatic radicals,which were partly come from aromatic compounds remaining in the pore and escape when heated.The release of polycyclic aromatic compounds in the NRPchar at lower temperature and the larger percent surface area promoted the coupling of CH₃ with aromatic compounds and formed aromatic compounds with alkyl side chains finally.At the same time,the alkyl substituent reduced the dissociation energy of the bonds during pyrolysis,so that the volatiles such as benzene,naphthalene,and phenols with alkyl substituents were preferentially escaped.With the increase in pyrolysis temperature,the addition of NRPchar had significant effects on the species and concentration of oxygen-centered and carbon-centered radicals in char.The concentration of carbon-centered free radicals increased significantly at 400℃,while the higher reaction temperature（600℃）promoted the participation of aromatic carbon radicals with one oxygen atom in the reaction,and the content of alkyl-substituted phenols in the volatiles increased.At higher temperatures（400-600℃）,the coupling effect of oxygen-centered radicals was obvious.