The Influence of Water Immersion on the Physical and Chemical Structure of Coal

ABSTRACT

In order to reveal the reaction mechanism responsible for the spontaneous combustion of water-immersed coal, this study researched the changes in the physical and chemical structures of coal samples before and after water immersion. The results of the specific surface and pore size analyzer test showed that compared with the original coal sample, the microporous (<10 nm) ratio decreased, but the proportion of macropores (>50 nm) increased obviously in the water-immersed coal sample. Also, as the water immersion time increased, the proportion of micropores decreased from 78.5% to 74.2%, while the proportion of macropores increased from 8.5% to 13.4%. The increase in the number of macropores increased the exposure of coal to oxygen. The scanning electron microscopy (SEM) results showed that the surface pore diameter of the water-immersed coal sample increased obviously, the surface of the coal sample was rougher, and the adsorption and diffusion of oxygen were enhanced. The Fourier transform infrared (FTIR) experiment showed that the contents of hydroxyl and aliphatic groups with higher activity in water-immersed coal samples increased significantly. The content of hydroxyl groups increased in lignite, bituminite, and anthracite by 2.7%, 2.27%, and 0.11% respectively, while the contents of aromatic hydrocarbons and oxygen-containing functional groups showed relative decreases. Compared with the original coal, the physical and chemical structure of water-immersed coal changed obviously. Therefore, it was easier for the water-immersed coal to react with oxygen, and it was also prone to spontaneously ignite.

KEYWORDS:

• Coal spontaneous combustion
• water-immersed coal
• physical chemistry
• microstructure

Highlights

1. After water immersion, increase of lignite macropores increased the chance of coal–oxygen contact.

2. After water immersion, pore volume expanding, number of pores decreasing, the specific surface area decreased.

3. After water immersion, the surface folds and expanded pores increased the risk of spontaneous combustion.

4. The active groups combined with water hydrogen bonds to form hydroxyl groups, making spontaneous combustion possible.

Acknowledgments

The authors appreciate the financial support provided by the Fundamental Research Funds for the Central Universities under project No. FRF-IC-20-01 and No. FRF-IC-19-013, the National Natural Science Foundation of China under project No. 51974015, No. 51904292, and No. 51474017, the National Key Research and Development Program of China under project No. 2018YFC0810600, the Fundamental Research Funds for the Central Universities (China University of Mining and Technology) under project No. 2017CXNL02, the Natural Science Foundation of Jiangsu Province under project No. BK20180655, the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) under project No. WS2018B03, and the Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines of China (Hunan University of Science and Technology) under project No. E21724.

Additional information

Funding

This work was supported by the Central Universities in China [No. FRF-IC-19-013,No. FRF-IC-20-01]; China University of Mining and Technology [No. 2017CXNL02]; Henan Polytechnic University [No. WS2018B03]; Hunan University of Science and Technology [No. E21724]; National Natural Science Foundation of China [No. 51474017,No. 51904292,No. 51974015]; Natural Science Foundation of Jiangsu Province [No. BK20180655]; National Key Research and Development Program of China [2018YFC0810600].