Chemical compositional and structural characteristics of Late Permian bark coals from Southern China

Fuel Volume 126, 15 June 2014, Pages 116–121.

Shaoqing Wang1,2, Yuegang Tang1, Harold H. Schobert3, Di Jiang1, Xin Guo1, Fang Huang1, Yanan Guo1, Yufei Su1

  1.  College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), D11, Xueyuan Road, Beijing 100083, PR China and
  2. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology (Beijing), D11, Xueyuan Road, Beijing 100083, PR China and
  3. The EMS Energy Institute, The Pennsylvania State University, University Park, PA 16802, USA.

 

Abstract

The characteristics of an unusual liptinitic component, known in Chinese petrographic nomenclature as barkinite, have been studied over the past 80 years. The chemical characteristics of barkinite have been studied for many years, but there has been little progress in establishing the chemical structural differences between barkinite and other liptinitic macerals of the same rank and age. In this paper, aspects of the chemical composition and structural characteristics of bark coal/barkinite are discussed. Bark coal is high volatile bituminous coal and has high volatile matter yield (>37%, daf, dry-ash free). The hydrogen content and H/C atomic ratio of barkinite are higher than those of vitrinite. The classification of barkinite would be type I–II kerogen. In addition, the chemical structure of barkinite is characterized by rich aliphatic structure concentration although dominated by aromatic structure. Chemical structural characteristics of barkinite seem close to vitrinite.

 

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Significance Statement

This study focused on barkinite, an unusual component of coals from southern China. Barkinite derives its name from its visual appearance of being composed mainly of coalified tree bark. Coals having a high concentration of the barkinite component are known as bark coals. In most coals, the dominant maceral, vitrinite, derives from coalified wood. Coals also contain macerals from the coalification of other plant parts, such as spores (sporinite) and resins (resinite). Some material not derived from woody plants might be incorporated in coals, e.g. alginate from algae. Bark coals are characterized not only by containing relatively large amounts of barkinite, but also have high hydrogen contents of about 7%, high volatile matter, and high hydrogen-to-carbon atomic ratios, >1%,  relative to vitrinite-rich coals. However, the values of these parameters for barkinite tend to be lower than those of macerals such as resinite, sporinite, or alginate. The present paper is one of a series dealing with the characterization and rational utilization of these special coals. Bark coals and the barkinite separated from them were studied by various techniques, primarily Fourier-transform infrared spectroscopy and carbon-13 nuclear magnetic resonance spectroscopy. Consistent with its relatively high amount of hydrogen, barkinite contains abundant aliphatic carbon structures, many of which appear to be present as long aliphatic chains. Although the structure is much more notably aliphatic than vitrinites, aromatic structures still have a significant presence in barkinite. The kind of structure characterized by long aliphatic chains differentiates barkinite from such macerals as resinite. The structural characteristics of barkinite are consistent with the composition of bark from modern woody plants. As such plants grow, their epidermal cells (bark) become enriched in highly aliphatic materials derived from fatty acids, compounds such as suberin and cutin. These compounds help make the bark somewhat impervious and water-resistant, in keeping with its role as the exterior protective layer of the plant. The bark also incorporates lignin, an important component providing structural rigidity to the plant, and sometimes tannins, which help regulate growth and provide protection from predators. Overall, the composition and major structural features of barkinite are what would be reasonably expected from coalification of lignin impregnated with cutin, suberin, and tannins. That is, the chemical make-up of barkinite supports the notion that it is indeed coalified bark.

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