Effect of acid treatment on the characteristics and structures of high-sulfur bituminous coal

In this paper, a series of modern advanced analysis and measurement technologies such as X-ray Fluorescence (XRF), X-ray Diffraction (XRD), 13C Nuclear Magnetic Resonance (13C NMR), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) have been used to analyze the sample after acid treatment. The result shows that after hydrochloric acid treatment and combined treatment of hydrochloric acid/nitric acid, the water content of coal sample and fixed carbon increase to some extent while the volatile component and ash content decrease. Acid treatment has reduced the content of carbon, hydrogen and sulfur but increased the oxygen content. In addition, hydrochloric acid treatment has resulted in the increase of chlorine content in coal and nitric acid treatment increase the nitrogen content. Acid treatment will influence the microcrystalline structure of the coal and improve the aromaticity which finally makes the carbon atom arrangement more orderly. After hydrochloric acid treatment, the ratio of methyl carbon, aliphatic carbon attached to oxygen, carboxyl carbon and carbonyl carbon decreases slightly, while that of other carbon functional groups increases correspondingly. The raw coal shows large differences from coal sample after the beneficiation of hydrochloric acid and nitric acid. The content of methyl carbon and the aliphatic carbon attached to oxygen is reduced obviously, while that of methylene and carbonyl carbon is raised dramatically. After acid treatment, the ratio of CC bond and CH bond on the coal surface increases gradually, but carbon in oxidation state decreases step by step which means acid treatment will destroy the active functional groups on coal surface and reduce the ratio of oxygen-containing functional groups such as ether bond and hydroxyl group. Meanwhile, acid treatment will influence the organic sulfur such as mercaptan, thioether, sulfoxide, sulfone, and sulfonate, which will change the component of organic sulfur on coal surface and decrease its content. The coal particles have no obvious change after the beneficiation of hydrochloric acid and nitric acid but their surface morphology has changed. The edges of coal particles turn obscure and their surface becomes blurred. The combined treatment of hydrochloric acid/nitric acid has increased the specific surface area of coal sample, leading to more micropores on coal surface, and thus oxygen molecules are more easily absorbed by the coal. In general, hydrochloric acid treatment has little influence on coal characteristics, chemical construction of the organic matter, inorganic minerals, functional groups on coal surface, microstructure, etc., while nitric acid treatment has significant effect on coal quality. Therefore, the influence must be taken into consideration when using the nitric acid to pretreat the coal sample.