Research articleRelation between char structures and formation of volatiles during the pyrolysis of Shenfu coal: Further understanding on the effects of mobile phase and fixed phase
Graphical abstract
Proposed reaction pathways of volatiles responsible for the evolution of chemical structures of coal: Firstly, the primary devolatilization process would produce large amounts of free radicals. Then, the alkane radicals would participate in methylation and dehydrogenation reaction (secondary reaction), leading to the attendant phenomenon of alkanes and alkenes in light tar (or gas product). The decomposition of asphaltene in mobile phase and thermal cracking of polar radicals and decomposition of solvent extracted metaplast materials made predominant contribution to tar formation. The cracking of peripheral aromatic structures of coal skeleton structure, through which secondary reaction took place, resulted in tar formation.
Introduction
Pyrolysis plays an important role in almost all thermal conversions process of coal such as upgrading, gasification, liquefaction and combustion [1, 2]. Generally, pyrolysis consists of two sets of reactions: primary devolatilization reactions with radical formation, polymerization-condensation, radical recombination, etc. and subsequent secondary gas phase reactions with decomposition of the volatile products produced through the primary reactions [[3], [4], [5]]. In the past decades, different pyrolysis models such as FG-DVC (Functional Group-Depolymerization, Vaporization, Cross-Linking) model [6], CPD (Chemical Percolation Devolatilization) model [7, 8], etc., have been proposed, attempting to predict coal pyrolysis characteristics. In these traditional models, researchers generally considered coal as single macromolecular structure to investigate the cleavage of certain chemical bonds at macro level. They can hardly clarify the detailed evolution characteristics of pyrolysis products, especially for the reaction pathways of different chemical structures of coal/char during devolatilization process.
Based on the two-phase model [9, 10], the chemical structures of coal/char can be divided into two types, that are mobile phase and fixed phase, which represent the solvent-extracted matter and residues, respectively. Previous study [11] showed that only the mobile phase and the peripheral structure of the fixed phase (comprising mainly of the polycyclic aliphatic substances) can be decomposed during the initial stage of pyrolysis, which would produce abundant free radicals to influence the whole thermal conversion process [12].
Recent years, many attempts have focused on the complex evolution characteristics of mobile phase and fixed phase during pyrolysis and found that the pyrolysis behaviors of these two parts are very different [[12], [13], [14], [15]]. The extractable mobile phase can be directly converted to gas-phase products, or cracked into smaller molecules during pyrolysis, serving as mobile hydrogen donors to immobilize the fragments radical that dissociated from coal macrostructure [13, 14]. While, the pyrolysis of extraction residue can accompany with polycondensation reactions and free radical reaction processes [12]. Our recently study [16] gave insights into the chemical structural evolution of different parts in the semi-chars and found that the cracking of the peripheral aromatic structures of the fixed phase would inducing the increase of aromaticity of semi-char. From above studies, it is clear that different parts of coal/char act as different contributors to the formation of pyrolysis product via specific reactions. However, few studies have taken simultaneously investigation on the relation between mobile phase and fixed phase of coal/char and the formation of pyrolysis product, especially that of the heavy tar during pyrolysis, and thus the reaction pathways responsible for the evolution of chemical structure of coal/char remains to be elucidated to gain further insight into the pyrolysis behaviors of coal.
In this study, all-components, that are semi-char, tar and gas product obtained by low-rank Shenfu coal pyrolyzed under various residence time at 435 °C were comprehensively analyzed at molecular level. Combined the findings obtained in our previous relevant work [[16], [17], [18], [19]], this paper aims to associate the fundamental evolution pathways of volatiles during low-temperature pyrolysis process with different chemical structure of coal/char.
Section snippets
Materials
Shenfu coal from Shaanxi province, China, was used in this work as coal samples and pulverized to pass through the 74 μm mesh sieve followed by drying at 80 °C to constant weight. The results of its ultimate and proximate analyses are shown in Table 1.
Solvents used for light tar and heavy tar separation were n-hexane and tetrahydrofuran (THF), respectively, purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). They were purified before use.
Pyrolysis process
The pyrolysis experiment was conducted
Evolution of semi-char free radicals
It is generally known that primary devolatilization process usually accompanies with radical formation. Early studies have revealed the presence of stable free radicals in coal with ESR method [23]. The variation of free radical concentration (Ng) in semi-chars with different residence time are shown in Fig. 2a. It can be seen that Ng firstly increased drastically at the early stage of pyrolysis, and then declined with prolonged residence time. It is likely that the early drastic rise in Ng in
Conclusion
At the initial stage of pyrolysis, cracking of bridged bonds, especially heteroatom bridged bonds, such as O, N and S, generated relatively stable radicals in semi-char. Phase change of light components and decomposition of asphaltene in mobile phase can be strongly related with light tar aliphatic compounds formation. At the middle and later stage of pyrolysis, thermal cracking of heteroatom radicals and decomposition of solvent extracted metaplast materials made predominant contribution to
Acknowledgement
This work was supported by the National Natural Science Foundation of China (NSFC) (grant numbers, 51576072, 51576081), and the Foundation of State Key Laboratory of Coal Conversion (grant number J17-18-904).
References (35)
- et al.
Relationship between pyrolysis products and organic aerosols formed during coal combustion
Proc. Combust. Inst.
(2015) Coal pyrolysis I. Pore evolution theory
Combust. Flame
(1983)Mild conversion of coal for producing valuable chemicals
Fuel Process. Technol.
(2000)- et al.
On the rank-dependence of coal tar secondary reactions
Proc. Combust. Inst.
(2011) - et al.
A comparison of different methods for predicting coal devolatilisation kinetics
J. Anal. Appl. Pyrolysis
(2001) - et al.
The concept of a mobile or molecular phase within the macromolecular network of coals: a debate
Fuel
(1986) - et al.
Role and composition of the mobile phase in coal
Fuel
(1985) - et al.
Lack of synergetic effects in the pyrolytic characteristics of woody biomass/coal blends under low and high heating rate regimes
Biomass Bioenergy
(2002) - et al.
Pyrolysis of Huolinhe lignite extract by in-situ pyrolysis-time of flight mass spectrometry
Fuel Process. Technol.
(2015) - et al.
Separation and analysis of high range extractable molecules formed during coal pyrolysis using coupled thin layer chromatography-imaging mass spectrometry
Fuel
(2017)
Effect of tetrahydrofuran extraction on lignite pyrolysis under nitrogen
J. Anal. Appl. Pyrolysis
Study on the structural evolution of semi-chars and their solvent extracted materials during pyrolysis process of a Chinese low-rank coal
Fuel
Mechanistic influences of different solvents on microwave-assisted extraction of Shenfu low-rank coal
Fuel Process. Technol.
Molecular structure characterization of the tetrahydrofuran-microwave-extracted portions from three Chinese low-rank coals
Fuel
Identification of the structural characteristics of the asphaltenes in the tetrahydrofuran-microwave-extracted portions from two Chinese coals
Fuel Process. Technol.
Experimental investigation of high-temperature coal tar upgrading in supercritical water
Fuel Process. Technol.
Pyrolysis of coal-tar asphaltene in supercritical water
J. Anal. Appl. Pyrolysis
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