Are Ionic Liquids Suitable for Suppressing Coal Spontaneous Combustion?

Xuanxuan Huang; Yongliang Xu; Yan Wang; Yao Li; Lanyun Wang; Zhengyan Wu

doi:10.1021/acsomega.0c05504

Are Ionic Liquids Suitable for Suppressing Coal Spontaneous Combustion?

ACS Omega. 2021 Mar 4;6(10):6681-6690. doi: 10.1021/acsomega.0c05504. eCollection 2021 Mar 16.

Authors

Xuanxuan Huang¹, Yongliang Xu^{1

2

3}, Yan Wang^{1

2

3}, Yao Li^{1

2

3}, Lanyun Wang^{1

2

3}, Zhengyan Wu⁴

Affiliations

¹ College of Safety Science and Engineering, Henan Polytechnic University, 2001, Century Avenue, Jiaozuo, Henan 454000, China.
² Collaborative Innovation Center for Coal Safety Production & High-Efficient-Clean Utilization by Provincial and Ministerial Co-construction, 2001, Century Avenue, Jiaozuo, Henan 454000, China.
³ State Key Laboratory Cultivation Base for Gas Geology and Gas Control in Henan Polytechnic University, 2001, Century Avenue, Jiaozuo, Henan 454000, China.
⁴ Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, China.

Abstract

Due to the reported fact that the active functional groups in coal can be dissolved and destroyed by ionic liquids, it is expected that the spontaneous combustion of coal can be affected from a thermodynamic perspective. However, ionic liquids with different thermal stabilities have distinct influences on coal combustion. Here, the thermal stability of long-flame coal in the presence of five pure ionic liquids ([Bmim][BF₄], [Bmim][Ac], [Bmim][NO₃], [Hoemim][BF₄], and [Pmim][BF₄]) was analyzed by thermogravimetric analysis, and the flammability of the raw coal, pure ionic liquids, and coal-IL mixtures (mass ratio of 1:1) were tested using a cone calorimeter according to the indexes of the time to ignition (TTI), mass loss rate (MLR), heat release rate (HRR), total heat release rate (THR), specific extinction area (SEA), and CO production. It is shown that the TTIs of mixtures containing coal-[Bmim][BF₄], coal-[Hoemim][BF₄], and coal-[Pmim][BF₄] are relatively long, and the MLR, HRR, THR, and SEA values are relatively low, indicating that these fluorine-containing ionic liquids have a better flame-retardant effect than the other two fluorine-free ones, which may be ascribed to their similar role to halogen inhibitors. In addition, the endothermic process of [Bmim][BF₄], [Hoemim][BF₄], and [Pmim][BF₄] can reduce the temperature of the coal surface and delay the ignition time of coal. In contrast, the TTI of coal-[Bmim][NO₃] and coal-[Bmim][Ac] mixtures is much shorter than that of coal alone, and the MLR, HRR, and THR values are larger. This may be caused by the poor thermal stability of the two nonfluorine ionic liquids that began to decompose and release heat prior to coal, providing a large amount of heat for the low-temperature oxidation of coal and thus accelerating coal oxidation and combustion. Although the F-containing ionic liquids show the ability to inhibit spontaneous combustion of coal to some extent, their organic cations are potentially combustible and release large amounts of heat, smoke, and CO under high temperatures.