Thermal Behavior and Characteristics of Functional Groups on Lignite Secondary Oxidation

ABSTRACT

At present, the thermal behavior of lignite and the reaction process of functional groups during secondary oxidation have not been accurately explained. Therefore, the macro thermal behavior and micro characteristics of spontaneous combustion of lignite during secondary oxidation were studied. Coal samples were pre-oxidized to 80 °C, 120 °C, 160 °C, and 200 °C (labeled as raw coal, C-80, C-120, C-160, and C-200, respectively). A thermogravimetric (TG) analyzer and Fourier-Transform Infrared (FTIR) spectrometer were used. The results showed that the high adsorption temperature T₁ and critical temperature T₂ of C-80 were higher than those of raw coal by 4.41 °C and 11.07 °C, respectively. They indicated that pre-oxidation inhibited the low-temperature oxidation process. The active temperature T₄ gradually decreased from C-80 to lower than that of raw coal. The T₄ of C-200 was about 20 °C lower than that of raw coal, proving that pre-oxidation accelerated oxygen absorption and the weight-gaining process. The activation energy of pre-oxidized coal samples was significantly lower than that of raw coal samples. The C-80 and C-120 samples had the lowest activation energies: 114.542 kJ/mol and 114.539 kJ/mol, respectively. The content of free hydroxyl groups in pre-oxidized coal samples decreased in general. At C-120, the aliphatic hydrocarbon generation rate significantly increased, and the percentage of peak area continued to increase. During secondary oxidation of C-120 coal samples, various absorption peaks gradually enhanced. Consequently, C-120 was more prone to spontaneous combustion during secondary oxidation.

KEYWORDS:

• Coal spontaneous combustion
• activation energy
• functional groups
• FTIR
• thermogravimetric curve

Acknowledgments

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

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [No. 51474017,No. 51904292,No. 51974015]; Natural Science Foundation of Jiangsu Province [No. BK20180655]; Fundamental Research Funds for the Central Universities (China University of Mining and Technology) [No. 2017CXNL02]; National Key Research and Development Program of China [No. 2018YFC0810600]; State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) [WS2018B03]; Fundamental Research Funds for the Central Universities [No. FRF-IC-19-013]; 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) [E21724].