| Residential coal combustion(RCC)is the most important emission source of black carbon(BC)and polycyclic aromatic hydrocarbons(PAHs)in China.Their emission factors are obviously controlled by coals' geological maturity,therefore they may be related with the concentration and composition of soluble PAHs in coals.Seven bituminous coals with different maturity were selected in this study to determinate the soluble PAHs by GC/MS after Soxhlet extraction successively with hexane and dichloromethane solvent.Through analyzing normal alkane series,sixteen parent PAHs(p PAHs)and sixteen alkylated PAHs(aPAHs),the paper discussed the relationship between concentration and composition of PAHs and coal's maturity.Moreover the study may be helpful to explain the formation mechanism of PAHs and BC during RCC.Secondly,through Fourier transform infrared spectrometer(FTIR)and solid superconducting mri(13C-NMR)to test the raw coal chemical structure as well as to analysis the spectral,the study obtained seven kinds of different maturity of China coals chemical structure information.The main conclusions are as follows:The results showed that the different bituminous coal is alkanes content ranged from 25.12 mg/kg to 369.53 mg/kg.With the increase of the content of volatile matter content,the corresponding normal alkanes was presented to increase and then decreased.At the same time,compared with other bituminous coals,the total corresponding normal alkanes was two to fifteen times.Volatile and at the same time,the study found the highest levels of bituminous coal inherited the alkane content characteristics of plant,and with the increase of the content of volatile matter of raw coal alkane Cmax value decreases after increasing first.This study found that the characteristics of the biggest carbon number,carbon advantages index(CPI)and low/high molecular polymer(?C24)/(>C24)ratio of different maturity raw coals were in the same trends.These characteristic values can better instruction behavior characteristics of paraffin in the process of coal metamorphism.The results showed that:(1)the total PAH concentrations in the six tested coals ranged from 39.8 mg/kg to 289.0 mg/kg,which the coals with middle volatile material content in dry and ash-free basis(Vdaf)were higher PAHs than high and low Vdaf bituminous coals.Seven times of difference occurred among various coals,which is similar with the trend of emission factors of PAHs and BC of RCC;(2)pPAHs concentrations ranged from 8.3 mg/kg to 58.4 mg/kg in the seven coals,which the highest value occurred in coals with Vdaf from 25.6 mg/kg to 30.5 mg/kg.The content of aPAHs ranged from 31.4 mg/kg to 236.5 mg/kg;aPAHs concentration was higher than pPAHs by four times in average.The similar ranges of PAHs concentration in both raw coals and RCC smoke as well as higher alkylated PAHs than pPAHs may illustrate the succession of PAHs in coal smoke from soluble compounds;(3)as for the composition,low-ringed compounds(2-3 ringed)dominated pPAHs in coal with Vdaf less than 25.6%,while middle to high-ringed PAHs(4-6 ringed)dominated the concentration at Vdaf over 30%;aPAHs had this similarity,in which 2-3 ringed compounds is the main part in the bituminous coals with middle to high maturity while 4-ringed compounds is highest at Vdaf from 30.5% to 35.5%.This compositional trend of soluble PAHs reflected the evolution rule of geological maturity of coal chemistry,and to some extent also explains the phenomenon of highest emission factors of PAHs and BC for middle volatile bituminous coals during RCC.Secondly,The research results of raw coal chemical structure:Different content of oxygen functional groups the structure features of coal as follows:(1)the content of oxygen functional groups(hydroxy,conjugated C=O skeleton vibration,unsaturated carboxylic acid C=O skeleton)in the high volatile coal LK and low volatile coal LL were consistent;(2)with the increase of volatile content,three kinds of content of oxygen containing functional groups are changed,the hydroxyl and carbonyl content proportions increase,unsaturated carboxylic acid ratio decreased,when the volatile content increased to 29%,the hydroxyl and carbonyl content is the highest,unsaturated carboxylic acid content is the lowest;(3)with the increase of maturity again,carbonyl content and unsaturated carboxylic acid content in raw coal showed a similar trend,at the same time the hydroxyl content is lower.Above findings shows that the characteristics of three kinds of content of oxygen functional groups in coal with a high volatile and low volatile showed a similar trend,and oxygen containing functional groups in the volatile medium coal mainly hydroxyl,followed by carbonyl group,but the content of unsaturated carboxylic acid is low.The characteristics of different fat carbon content in the raw coal structure are as follows:(1)with the reduction of volatile content,the ratio of RCH3 content in raw coal decrease,but when the volatile content more than 30%,the RCH3 content reached the maximum,with the ratio of 7:1 RCH3 content;(2)With the increase of volatile matter content and the content is less than 35%,the ratio of RCH3 content in raw coal reduce to 3:1;(3)when the content of the volatile reached 48%,the ratio of RCH3 content increased,with the ratio of 4:1.The above transformation law and is consistent with the content of the free PAHs in the raw coal ratio,that free small molecular structure and the characteristics of content to a certain extent were on behalf of the coal chemical structure information.Raw coal structure of aromatic structure units characteristics as follows:(1)the nitrogen in the coal is the main form of pyrrole and pyridine;(2)1-ringed mainly exist in the higher coal volatile content in LK,and also was the major of coal macromolecular structure unit;(3)with the reduction of volatile matter of coal molecular structure unit was mainly in the form of 2 rings,and 30.0% of the volatile content in the raw coal YCW reached the maximum;(4)when the volatile content is less than 30%,the aromatic structure in the raw coal unit gradually from two rings into three rings and four rings.To sum up,the chemical structure of different coal rank,free of small molecules and coal macromolecular network structure characteristics are consistent,aromatic unit changes in performance for the transition from epipodium primarily to give priority to the trend of low ring;Raw coal in the form of fat carbon mainly from methyl carbon content is reduced,to other forms of fat carbon gradually;Oxygen in raw coal mainly from conjugated C=O and unsaturated carboxylic acid to the transition of the hydroxyl.The study has found that the models characteristics of seven kinds of bituminous coals chemical structure are as follows:(1)the models characteristics of low volatile bituminous coals had big molecular size,high aromaticity,less side chains,and the aromatic unit were mainly composed of 2-3 ringed;(2)the models characteristics of the medium volatile and the medium-high volatile bituminous coals were mainly based on 2-3 ringed,and more branched chains and fat,and the medium-high volatile bituminous coals with more methyl side chain and carbonyl content,in the medium volatile bituminous coals carbon mainly dominated by methylene fat;(3)the models characteristics of the high volatile bituminous coal were small molecular size and high aromaticity.The research result shows that maturity are the main factors influencing the coal macromolecular structure,the second is related to its sedimentary environment and the sediment types.With the increase of maturity,aromatic carbon content in the bituminous coals presents increasing trend after lower first,coal molecular chemical structure arranged regularity and had high thermal stability.At the same time,the midterm volatile bituminous coals and high volatile bituminous coals macromolecular structure consisted of more fat and side chains,such as the methyl,ethyl.Its thermal stability were poor,which lead to its particularity. |
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