Characterization of coal-based humic acids in relation to their preparation methods

ABSTRACT

Lignite resource is abundant in China. Its high moisture and high self-ignition tendency have limited its utilization as a fuel. Preparing humic acids is considered to be an economic choice for the utilization of lignite. Herein regenerated humic acids (RHAs) have been obtained by means of hydrolysis of Chinese-aged lignite with NaOH as the catalyst. It has been verified that the hydrothermal process has enabled 67% increase in the extraction yield of humic acids (HAs). Moreover, RHAs, original humic acids (OHAs) and commercial humic acids (CHAs) were characterized by means of UV-Vis, acid functional groups analysis, FT-IR spectroscopy, XPS and thermogravimetric analysis in addition to basic elemental analyses in relation to their preparation methods. Comparison of the studied HAs shows that they are apparently different from each other in structural features regarding the degree of aromaticity, the oxygen content and the distribution of functional groups. CHAs contain the most aromatic structure and largest oxygen content than RHAs and OHAs, and the carboxyl groups are the main oxygen-containing functional groups. These structural characteristics are probably responsible for the lowest pyrolysis temperature and maximum mass loss of CHAs during the pyrolysis process. Although there are some distinct differences between the structure OHAs and RHAs, but the thermogravimetric curves of them are almost the same in the temperature range from ambient temperature to 1100°C.

KEYWORDS:

• Humic acids
• lignite
• hydrolysis
• characterization
• low rank coal

Disclosure statement

The authors declare that there is no conflict of interest regarding the publication of this paper.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [21646006, U1704255, 41472127, 41372161]; the Natural Science Foundation of Henan province [162300410115]; and the Research Fund of Henan Key Laboratory for Green and Efficient Mining & Comprehensive Utilization of Mineral Resources [S201618].

Notes on contributors

Jianbo Jia

Jia Jianbo received his Ph. D. from Taiyuan University of Technology, Taiyuan, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and carbon materials science.

Yaojie Zhang

Zhang Yaojie obtained his M.S. degree from Henan Polytechnic University, Jiaozuo, China. At present, he is a doctor candidate in the fields of carbon materials and clean coal utilization.

Quanrun Liu

Liu Quanrun obtained Ph. D. from Dalian University of Technology, Dalian, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and carbon materials science.

Guangxu Huang

Huang Guangxu obtained Ph. D. from HenanPolytechnic University, Jiaozuo, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and carbon materials science.

Baolin Xing

Xing Baolin obtained Ph. D. from Henan Polytechnic University, Jiaozuo, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and carbon materials science.

Chuanxiang Zhang

Zhang Chuanxiang obtained Ph. D. from East China University of Science and Technology, Shanghai, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and carbon materials science.

Hongyu Guo

Guo Hongyu obtained Ph. D. from Henan Polytechnic University, Jiaozuo, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and coalbed methane.

Jienan Pan

Pan Jienan obtained Ph. D. from China University of Mining and Technology-Beijing, Beijing, China. At present, he is employed at Henan Polytechnic University. His fields of interests include coal conversion technology and coalbed methane.

Yijun Cao

Cao Yijun obtained Ph. D. from Kunming University of Science and Technology, Kunming, China. At present, he is employed at Zhengzhou University. His fields of interests include coal processing technology and mineral materials.