| Zhundong coal is a kind of low-rank coal abundantly reserved in east Junggar Basin,Xinjiang,China,having high fusinite and semifusinite contents in its macerals and high AAEM content in its ash.These two aspects will necessarily bring significant influence on the pyrolysis of raw coal and gasification of char,and will also affect the industrial application of Zhundong coal.For the low rank coal,AAEM has obvious catalytic effect on pyrolysis process,and the residual AAEM in char after pyrolysis can be a good catalyst on gasification reaction of char.The situation of AAEM and the chemical structure of char are the two most important factors determining the reactivity of char.Therefore,in this paper,the research is focused on the pyrolysis characteristics of Zhundong coal,especially the catalytic effect and migration behavior of AAEM in the pyrolysis process.The conclusions obtained will not only be conducive to in-depth understanding of influencing mechanism of AAEM in pyrolysis reaction of coal,having a positive impact on the further enrichment and development of basic theoretical system of coal chemistry,but also can provide a beneficial reference for the large-scale,high-efficiency and clean utilization of Zhundong coal.The selective enrichment of different components in the process of coal grinding directly affects the chemical structure and pyrolysis characteristic of samples with different particle sizes.Therefore,in order to determine the sampling standard for the whole issue,taking TGA and FT-IR as the test instruments,the difference among the chemical structure and pyrolysis reactivity of Zhundong coal samples with different particle sizes was explored firstly.By means of semi-quantitative characterization with a series of infrared structure parameters established by the ratio between specific peak areas,the results show that in the range less than 150 μm,with the decreasing particle size,clay mineral,carboxylate,aliphatic hydrogen,aromatic hydrogen,and oxygen containing functional group show vastly different enrichment behavior,in addition,the length of aliphatic side chains and bridge bonds and the substitution degree of the aromatic ring system also change.Finally,the relationship between the chemical structure and pyrolysis reactivity of various samples was established,and the sampling standard for the whole issue was determined.In order to provide a reliable basis for determining the kinetic evaluation method,taking TGA as the test instrument,the limitation of the most representative ones of single and multiple scanning methods in global pyrolysis kinetics of Zhundong coal was discussed detailedly.The results show that the different viewpoints on the segmentation and different treatments of the weight loss in the dehydration and degassing stage can both affect the results of single scanning method,and due to the existence of multiple fitting limitation,the comprehensive application of the integral and differential methodsas well as the three heating rates cannot provide an accurate and unique kinetic triplet.Due to the effect of heating rate on the pyrolysis process and the change in reaction mechanism,the applicability of multiple scanning method on the data of weight loss in the secondary degassing stage is relatively poor.However,the activation energy distribution function is well independent of the inclusion or exclusion of the weight loss in the dehydration and degassing stage or the secondary degassing stage or both of them,and is reliable within a wide temperature range.Finally,the multiple scanning method is used as the kinetic evaluation method in this paper.After determining the sampling standard and kinetic evaluation method,taking TGA and FT-IR as the test instruments,on the basis of a detailed investigation on the impacts of chemical fractionation on Zhundong coal’s chemical structure,the effect of inherent ion-exchangeable AAEM in Zhundong coal on the global generation characteristics and kinetics of char was discussed.The results show that,compared with other low-rank coals,Zhundong raw coal’s pyrolysis reactivity is low;a high proportion of ion-exchangeable AAEM integrates with carboxyls among Zhundong coal’s inorganic minerals,and its organic matter has low aliphatic hydrogen content,high aromatic hydrogen content,long aliphatic side chains and bridge bonds,and an aromatic ring system with low-substitution degree;deionized water and dilute sulfuric acid washings both have insignificant impacts on the chemical structure of organic matter in Zhundong coal,but concentrated HCl/HF washing introduces a series of obvious changes;ion-exchangeable AAEM can increase the equilibrium moisture content of Zhundong coal,reduce the volatile release rate in the main pyrolysis stage,improve the volatile release rate in the secondary degassing stage,and reduce the final weight loss of pyrolysis;the activation energy of Zhundong coal’s pyrolysis increases with the increasing conversion degree,ion-exchangeable AAEM does not change the position of the main interval of the activation energy distribution significantly,but will make the activation energy value higher and the distribution more concentrated,and decrease the pyrolysis activity;ion-exchangeable AAEM changes the slope and intercept of the linear form of kinetic compensation effect between the frequency factor and activation energy of Zhundong coal’s pyrolysis.Then,taking TGA as reactor,and SEM-EDS,FT-IR,Raman spectroscopy and TG-FTIR as test instruments,the effects of inherent AAEM in different states in Zhundong coal on the chemical structure of char during pyrolysis were studied in detail.The results show that the water soluble AAEM inhibits the decomposition of carboxylic acid groups in char before 250 ℃,but has little influence on the thermal stability of phenolic hydroxyl in the whole pyrolysis process;above 250 ℃,ion-exchangeable AAEM in char of Zhundong coal hinders the migration of —COO-and Ar—O-groups to macromolecular volatile matter;above 600 ℃,insoluble AAEM has catalytic effect on the decomposition of ether in char,but this effect of water soluble and ion-exchangeable AAEM is small;in the range of 400~600 ℃,water soluble and ion-exchangeable AAEM in Zhundong coal both inhibit the release of aliphatic C—H structure in char,especially the latter,which delays the escape of aliphatic CH2 structure in char more obviously;above 550 ℃,water soluble AAEM in Zhundong coal has a catalytic cracking effect on the aromatic clusters in char,and above 700 ℃,ion-exchangeable AAEM hinders the dehydrogenation and polycondensation reaction of these structures.Afterwards TGA and horizontal tube furnace are taken as reactors,and ICP-OES is taken as test method,the volatilization of different states of inherent AAEM in Zhundong coal during pyrolysis process in different reactors was explored.The study finds that the AAEM species in Zhundong coal are mainly Na,Ca and Mg,and the content of K is very low;the ion-exchangeable state is the main existence form of AAEM in Zhundong coal;the main existence form of Na is water soluble state,while these of Ca and Mg are ion-exchangeable state;the water soluble AAEM is mainly Na,while the ion-exchangeable and insoluble AAEM are dominated by Ca;the thermal stability of insoluble AAEM is very high,during the pyrolysis process below 1000 ℃,the volatilization amount of this form of AAEM is very small,while the volatilization of water soluble and ion-exchangeable AAEM increases with the increasing final pyrolysis temperature;for ion-exchangeable AAEM,the volatility of Na is stronger than that of Ca and Mg,while the situation is just opposite for water soluble AAEM;the volatilization of Na and Ca is mainly from the ion-exchangeable state,while for the volatilization of Mg,in addition to the most important contribution from ion-exchangeable state,the contribution from water soluble state is also significant;the increase of heating rate promotes the volatilization of water soluble and ion-exchangeable Na,but the volatilization of Ca and Mg is less influenced.Finally,taking TGA and horizontal tube furnace as reactors,the combined effect mechanism of chemical structure and AAEM situation of char of Zhundong coal from pyrolysis in different reactors on char reactivity is discussed deeply.The results show that the effect of insoluble AAEM on char reactivity is negligible,while ion-exchangeable AAEM increases the char reactivity greatly;the water soluble AAEM which is mainly Na in Zhundong coal reduces the char reactivity,and this effect is derived from the catalysis of this form of AAEM on the chemical structure of char during pyrolysis;during the pyrolysis of 400~1000 ℃,the increasing final temperature causes the ordering of the chemical structure of char and the enrichment of ion-exchangeable Ca and Mg,the former reduces the char reactivity,while the latter has the opposite effect;in the pyrolysis temperature range of 400~600 ℃,the dominant factor controlling the char reactivity is the enrichment of ion-exchangeable Ca and Mg,while in the pyrolysis temperature range of 700~1000 ℃,the ordering of the chemical structure of char dominates the char reactivity;at the pyrolysis temperature above 600 ℃,the increase of heating rate enhances the volatile-char interaction,and then promotes the ordering of the chemical structure of char and the volatilization of ion-exchangeable AAEM,eventually reduces the char reactivity. |
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