Full Length ArticleInfluence of carbon type on carbon isotopic composition of coal from the perspective of solid-state 13C NMR
Introduction
Carbon isotopic analysis techniques have been widely applied to estimate palaeo-atmospheric carbon dioxide levels [1], [2] and determine changes in modern-atmospheric δ13C values, such as atmospheric carbon dioxide [3], atmospheric organic aerosol [4], atmospheric methane [5] and atmospheric chloromethane [6], and trace their different emissions sources. For example, Yan et al. [4] used carbon isotope ratios as a tracer to identify the relative contributions of specific source to organic aerosol. By testing the δ13C values of Chinese residential coals, they provided the direct evidence that coal combustion could primarily contribute to increasing brown carbon in Beijing during winter [4]. As more studies require data of carbon isotopic signatures of natural organic materials, such as coal, the need to understand and evaluate factors of influence on isotopic compositions becomes more significant.
Numerous studies have focused on carbon isotopic ratios of whole coals [7], [8], [9], [10], [11], and it is thought that δ13C values of coals range from −20‰ to −29‰ in general [12], within the range for modern C3 plants (−23‰ to −34‰) [13]. Coal is mainly composed of complex organic components. It is well known that there are important differences in carbon isotopic ratios within organic components [14], [15], [16], [17], and thus the distribution of these components in coal could significantly influence δ13C value of the whole coal. Some detailed studies considering the variations of maceral compositions have been published [18], [19], [20]. Whiticar [20] found a shift towards higher δ13C value for the more refractory maceral, i.e., liptinite < vitrinite < inertinite based on handpicked concentrates. Rimmer et al. [18] also reported a similar relationship between carbon isotopic compositions and relatively pure macerals by density-gradient centrifugation in a study of a high volatile A bituminous coal from eastern Kentucky. Nevertheless, Elswick et al. [19] noted a contradicting trend with more depleted δ13C values in the inertinites. Valentim et al. [20] observed that the vitrinite-rich coal (∼92%) and semifusinite + fusinite rich coal (∼25%, with ∼61.4% vitrinites) were similar with δ13C values of −24.8‰ and −24.61‰, whereas the macrinite-rich coal (∼63%, with ∼14.9% vitrinites) shifted to a lower δ13C value of −26.80‰ from the Peach Orchard coals in America. In addition, Erdenetsogt et al. [22] showed that the carbon isotopic compositions of the Mongolian lignites were governed by the O-containing functional groups to some extent. However, knowledge of the control of organic components on carbon isotopic compositions of coal is still limited.
There are some indirect methods relying on solvent extracts or pyrolysates [23], [24], [25] that can be explored to investigate organic macromolecular components of coal. Unfortunately, there exists a major uncertainty of the results of these chemical methods due to sample degradation and destruction in the process of measurement [26], [27]. However, several alternative, direct physical non-destructive measuring techniques can be applied to study organic components in solid materials [26], [27], [28]. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy [29], [30], [31] has been effectively used to gain quantitative information about organic components in bulk carbonaceous materials including coal. For example, the percentage of aromatic carbons can be precisely determined in coal [28].
More detailed investigations of the influence of the distribution of organic macromolecules on the carbon isotope composition of coal are needed. The purpose of the present study was to examine the influence of the distribution of organic carbon types on the carbon isotopic composition of coal. Samples from China were chosen, and we determined δ13C values and assigned organic carbon types using isotope ratio mass spectrometry analysis and solid-state 13C NMR spectroscopic analysis techniques.
Section snippets
Samples and coal quality analyses
The coal samples utilized in this study were obtained from the Huainan coalfield of Anhui Province, which is a main coal production location in China [32] (Fig. 1). A total of twelve coals were sampled following Chinese National Standard GB/T 482-2008, which are from the No.20 minable coal seams in the Permian Upper Shihezi formation from twelve coal mines (Fig. 1). Hence it is expected that coal-forming plant assemblages, burial depth and thermal maturation for the studied coals from the
Coal quality
The results of proximate and ultimate analyses for twelve coal samples are listed in Table 1. The moisture of the studied coals varies from 1.24% to 1.65%. According to Chinese National Standards for moisture (MT/850-2000, ≤6.00% for ultra-low moisture coal), the studied coals are classified as ultra-low moisture coal. The ash yields of the studied coals range from 22.40% to 27.74%, indicating that the studied coals belong to middle-ash coal based on Chinese National Standards (GB/T
Conclusions
The coal quality, carbon isotopic compositions and carbon types of twelve Chinese bituminous coals were examined. The δ13C values ranged from −22.9‰ to −25.1‰, and significantly correlated with atomic H/C ratios and the ratios of aromatic carbons and aliphatic carbons, but did not correlate with atomic O/C ratios. As H/C ratios and aliphatic carbon content decreased, as well as aromatic carbon content increased, the δ13C values became greater. This could result from preferential removal of 12C
Acknowledgments
This work was supported the National Natural Science Foundation of China (41672144). We acknowledge editors and reviewers for improving the language of the paper and for in-depth discussion.
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