13C-NMR Study on Structure Evolution Characteristics of High-Organic-Sulfur Coals from Typical Chinese Areas

Qiang Wei,Yuegang Tang

doi:10.3390/min8020049

Abstract

The structure evolution characteristics of high-organic-sulfur (HOS) coals with a wide range of ranks from typical Chinese areas were investigated using 13C-CP/MAS NMR. The results indicate that the structure parameters that are relevant to coal rank include CH3 carbon (fal*), quaternary carbon, CH/CH2 carbon + quaternary carbon (falH), aliphatic carbon (falC), protonated aromatic carbon (faH), protonated aromatic carbon + aromatic bridgehead carbon (faH+B), aromaticity (faCP), and aromatic carbon (farC). The coal structure changed dramatically in the first two coalification jumps, especially the first one. A large number of aromatic structures condensed, and aliphatic structures rapidly developed at the initial stage of bituminous coal accompanied by remarkable decarboxylation. Compared to ordinary coals, the structure evolution characteristics of HOS coals manifest in three ways: First, the aromatic CH3 carbon, alkylated aromatic carbon (faS), aromatic bridgehead carbon (faB), and phenolic ether (faP) are barely relevant to rank, and abundant organic sulfur has an impact on the normal evolution process of coal. Second, the average aromatic cluster sizes of some super-high-organic-sulfur (SHOS) coals are not large, and the extensive development of cross bonds and/or bridged bonds form closer connections among the aromatic fringes. Moreover, sulfur-containing functional groups are probably significant components in these linkages. Third, a considerable portion of “oxygen-containing functional groups” in SHOS coals determined by 13C-NMR are actually sulfur-containing groups, which results in the anomaly that the oxygen-containing structures increase with coal rank.

Highlights

Sulfur is one of the most hazardous elements in coal, and organic sulfur is more difficult to remove than other forms of sulfur in the conventional coal washing process
The coal structure suffered immense alteration in the first coalification jump, and the aromatic structures condensed at the initial stage of bituminous coal resulting in a decrease in the quantity of bonding sites for aromatic methyl
The structure evolution of HOS coals exhibits no correlations between the aromatic CH3 carbon, aromatic bridgehead carbon (f a B ), phenolic ethers (f a P ), and alkylated aromatic carbon (f a S )

Summary

Introduction

Sulfur is one of the most hazardous elements in coal, and organic sulfur is more difficult to remove than other forms of sulfur in the conventional coal washing process. Organic sulfur released during coal utilization could have severe adverse effect on the environment and human health. The coal with >1% organic sulfur is called high-organic-sulfur coal, and the organic sulfur content in super-high-organic-sulfur (SHOS) coal exceeds 4% [1,2]. The distribution of SHOS coals in the world is fairly limited. Croatia Raša coal is the representative with an incredible organic sulfur content of 11.4 wt % [3]. Spain Mequinenza lignite [4], New Zealand

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Conclusion