National University of Science and Technology MISiS, Moscow, Russian Federation:

Epshtein S. A., Head of Laboratory, Doctor of Engineering Sciences, apshtein@yandex.ru
Sokolovskaya E. E., Leading Engineer
Dobryakova N. N., Researcher, Candidate of Engineering Sciences
Hao J., Engineer, Associate Professor

Oxidation of coal leads to some unwanted consequences, such as losses of products quality and increase in proneness to spontaneous combustion. One of the indicators of coals’ oxidation is changing in amount and composition of active functional groups in organic matter. On using a wide number of samples and adjusting the experimental procedure, a method was developed for coals’ oxidation degree evaluation by potentiometric titration based on evaluation of contents of total acidic groups (a sum of phenol and carboxyl groups). It was shown that potentiometric titration is not applicable for determination of phenol and carboxyl groups in coals separately. A phenomenon of appearance of two waves at titration curve was connected with absorption of CO₂ from air into the alcohol-alkaline solution. For coals with vitrinite reflectance varying in range of 0.5-0.72 %., total acidic groups amount decrease with rank growth, whereas for higher rank coals such an alteration is less pronounced. It is shown that it is expedient to use the developed method of potentiometric titration together with the petrographic method for determining the oxidation degree. On the basis of the proposed method of coals’ oxidation degree determination by potentiometric titration, a National standard of the Russian Federation GOST R 59012-2020 has been introduced. According to it, the oxidation degree is determined by the increase of the total amount of acidic groups – phenolic and carboxyl hydroxyls in coal as compared to the content of total acidic groups in the control sample, which is used as a seam sample of coal taken outside the oxidation zone, or coal taken when laying the pile, or coal received for benefication, sorting or moving.

The research has been carried out in the framework of strategic academic leadership program “Priority 2030”. The paper was written with the participation of E. L. Kossovich, Senior Researcher, Candidate of Physico-mathematical Sciences.

1. Desna N. A., Miroshnichenko D. V. Oxidized coal in coking: A review. Coke and Chemistry. 2011. Vol. 54, No. 5. pp. 139–146.
2. Zhou C., Zhang Y., Wang J. et al. Study on the relationship between microscopic functional group and coal mass changes during low-temperature oxidation of coal. International Journal of Coal Geology. 2017. Vol. 171. pp. 212–222.
3. Beamish B. B., Barakat M. A., St. George J. D. Spontaneous-combustion propensity of New Zealand coals under adiabatic conditions. International Journal of Coal Geology. 2001. Vol. 45, No. 2–3. pp. 217–224.
4. Hao S., Wen J., Yu X., Chu W. Effect of the surface oxygen groups on methane adsorption on coals. Applied Surface Science. 2013. Vol. 264. pp. 433–442.
5. Chen Y., Mastalerz M., Schimmelmann A. Characterization of chemical functional groups in macerals across different coal ranks via micro-FTIR spectroscopy. International Journal of Coal Geology. 2012. Vol. 104. pp. 22–33.
6. Semenova S. A., Patrakov Y. F., Majorov A. E. Assessment of the likelihood of underground coal oxidation and self-ignition: a review. Coke and Chemistry. 2020. Vol. 63, No. 5. pp. 223–231.
7. van Krevelen D. W. Coal: typology, physics, chemistry, constitution. Amsterdam ; New York : Elsevier, 1993. 1002 p.
8. He X., Liu X., Nie B., Song D. FTIR and Raman spectroscopy characterization of functional groups in various rank coals. Fuel. 2017. Vol. 206. pp. 555–563.
9. Semenova S. A., Patrakov Yu. F., Yarkova A. V. et al. Changes in the Properties of Native Low-Metamorphozed Coal in Contact with Air. Khimiya tverdogo topliva. 2022. No. 3. pp. 3–12.
10. Malysheva V. YU., Fedorova N. I., Ismagilov Z. R.. Study of brown coals by method of infrared spectroscopy. Chemistry for sustainable development. 2020. Vol. 28, No. 6. pp. 560–565.
11. Suggate R. P., Dickinson W. W. Carbon NMR of coals: the effects of coal type and rank. International Journal of Coal Geology. 2004. Vol. 57, Iss. 1. pp. 1–22.
12. Okolo G. N., Neomagus H. W. J. P., Everson R. C. et al. Chemical–structural properties of South African bituminous coals: Insights from wide angle XRD–carbon fraction analysis, ATR–FTIR, solid state 13 C NMR, and HRTEM techniques. Fuel. 2015. Vol. 158. pp. 779–792.
13. Kus J., Misz-Kennan M. Coal weathering and laboratory (artificial) coal oxidation. International Journal of Coal Geology. 2017. Vol. 171. pp. 12–36.
14. Yudina N. V., Savel’eva A. V., Linkevich E. V. Changes in the composition and properties of humic substances upon the mechanical treatment of coals with mineral salts. Solid fuel chemistry. 2021. Vol. 55, No. 4. pp. 229–235.
15. Khil’ko S. L., Rogatko M. I., Makarova R. A. Antiradical and potentiometric characteristics of humic substances from different natural sources. Vestnik Donetsk National Technical University. 2019. No. 1(15). pp. 73–79.
16. ASTM D5263-15 – Standard Test Method for Determining the Relative Degree of Oxidation in Bituminous Coal by Alkali Extraction. 2015. Available at: https://www.astm.org/d5263-15.html (accessed: 18.08.2022).
17. Marinov V. N. Self-ignition and mechanisms of interaction of coal with oxygen at low temperatures. 3. Changes in the composition of coal heated in air at 60 °C. Fuel. 1977. Vol. 56, Iss. 2. pp. 165–170.
18. Pringle W. J. S. Potentiometric Titration of Coal in a Non-Aqueous Medium. Nature. 1959. Vol. 183. pp. 815–816.
19. Murata S., Hosokawa M., Kidena K., Nomura M. Analysis of oxygenfunctional groups in brown coals. Fuel Processing Technology. 2000. Vol. 67, Iss. 3. pp. 231–243.
20. Kolesnikova S. M., Kamenskii E. S., Kuznetsov P. N. et al. Activity of coals from mongolian deposits in the process of thermochemical degradation. Solid Fuel Chemistry. 2012. Vol. 46, No. 5. pp. 305–309.
21. Sakurovs R., Lewis C., Wibberley L. Effect of heat and moisture on surface titratability and pore size distribution of Victorian brown coals. Fuel. 2016. Vol. 172. pp. 124–129.
22. Clemow L. M., Jackson W. R. Non-aqueous titration of the acidic groups in coals: a warning! Fuel. 2002. Vol. 81, Iss. 7. pp. 959–961.
23. Saeed K., Ishaq M., Ahmad I., Shakirullah M., Park S. -Y. Investigation of surface acidity of coal by aqueous potentiometric titration. Journal of chemical society of Pakistan. 2007. Vol. 29, No. 2. pp. 111–115.
24. Khil’ko S. L., Kovtun A. I., Rybachenko V. I. Potentiometric titration of humic acids. Solid Fuel Chemistry. 2011. Vol. 45, No. 5. pp. 337–348.
25. Maroto-Valer M. M., Love G. D., Snape C. E. Relationship between carbon aromaticities and HC ratios for bituminous coals. Fuel. 1994. Vol. 73, Iss. 12. pp. 1926–1928.
26. Maroto-Valer M. M., Andrésen J. M., Snape C. E. Verification of the linear relationship between carbon aromaticities and H/C ratios for bituminous coals. Fuel. 1998. Vol. 77, Iss. 7. pp. 783–785.
27. Syskov K. I., Kukharenko T. A. Determination of constitutional groups in coals and their constituents by the sorption method. Zavodskaya Laboratoriya. 1947. Vol. 13, No. 1. pp. 25–28.
28. Golovin G. S., Lesnikova E. B., Artemova N. I., Dementeva O. A. Ion-exchange properties of lignites and products of their chemical modification. Solid Fuel Chemistry. 1996. Vol. 30, No. 5. pp. 24–28.
29. Golovin G. S., Lesnikova E. B., Artemova N. I., Lukicheva V. P. Ion-exchange properties of cation exchangers produced on the basis of brown coal from the Kansk-Achinsk basin. Solid Fuel Chemistry. 2000. Vol. 34, No. 4. pp. 32–38.
30. GOST 8930-2015 Hard coals. Method for determination of oxidation. Introduced: 2017–04–01. Moscow : Standartinform, 2017.
31. GOST R 59012-2020 Hard coals. Determination of oxidation by potentiometric titration. Introduced: 2020–12–01. Moscow : Standartinform, 2020.