Transformation of aromatic structure of vitrinite with different coal ranks by HRTEM in situ heating

Significance 

Coal is well known as fossil fuel and raw material for feedstocks. Coal has generally complex structure but can be classified differently into its constituent components. Vitrinite is the most common maceral in coal. With an increase in rank, vitrinite experiences a decrease in its aliphatic characteristics and an increase in its aromaticity. Several techniques have been proposed to investigate their aromatic structures. Among these techniques, high resolution transmission electron microscope (HRTEM) has been identified as a promising method for investigating the aromatic structures and associated features. Unfortunately, the study of the aromatic structure of vitrinite is not fully explored in literature.

To this note, Dr. Shaoqing Wang, Dr. Hao Chen, and Dr. Xiaomei Zhang from China University of Mining and Technology (Beijing) investigated the transformation of the aromatic structure of vitrinite, a common maceral in coal, for different coal ranks through high resolution transmission electron microscope heating. The authors explored the relationship between aromatic structural changes and evolution characteristics of gaseous generations. Three vitrinite samples of different coal rank were investigated. The aromatic structural transformations were investigated at a temperature range of 200 – 800°C. Besides, the gaseous release was measured using thermogravimetry coupled with mass spectrometry. The work is published in the research journal, Fuel.

Results showed that the changes in aromatic layers were influenced by the rank of coal. Low-rank vitrinite was characterized by high naphthalene content while high-rank vitrinite was characterized by low naphthalene content. Furthermore, the transformation behaviors were also influenced by the coal rank. For instance, for low-rank coal, naphthalene change with temperature rise exhibited four stages while for high-rank coal, it exhibited three or two stages. Additionally, it was worth noting that naphthalene content increase took place at the same temperature for which both water and carbon dioxide was released. This indicated the probable dependence of naphthalene on the decomposition of the oxygen-containing functional groups. A remarkable change in the 4 by 4 aromatic layers accompanied by methane release was observed for a temperature range of 650 – 750 °C.

The length distributions of the tested samples were evaluated and compared. Despite having low naphthalene content at high temperatures, the length distributions of 3 by 3 and 4 by 4 aromatic layers were observed to be higher as compared to 2 by 2 aromatic layers. Moreover, the rings and length of all the three samples increased with an increase with temperature. Generally, the combination and decomposition simultaneously occur in in-situ heated coal. With decomposition dominating at low temperatures, small-size aromatic structures were formed. On the other hand, dominant combination at high temperature large aromatic structures like the 4 by 4 aromatic layer were formed.

In a nutshell, the study investigates the aromatic structural transformation of three vitrine samples with different ranks heated in high resolution transmission electron microscope. Decomposition resulted in the formation of small-size aromatic structures at low temperatures while combination formed large-size aromatic structures at high temperatures. Also, the transformation behaviors were noted to highly depend on the coal rank. Altogether, the study provides useful insights that will advance clean and effective utilization of coal by taking into consideration their unique structural characteristics.

Reference

Wang, S., Chen, H., & Zhang, X. (2020). Transformation of aromatic structure of vitrinite with different coal ranks by HRTEM in situ heating. Fuel, 260, 116309.

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