Effects of crude oil on the microstructure and spontaneous combustion characteristics of coal: a case study of weakly caking coal in the Huangling No. 2 mine, China

The effects of crude oil on the microstructure and spontaneous combustion characteristics of coal were assessed, using raw coal sample and coal samples containing 5%, 10% and 15% oil. The pore structures, oxygen adsorption capacity, surface functional groups and spontaneous combustion characteristics were characterized by the LP-N2A, TG, FTIR and temperature-programmed experiments. The LP-N2A and TG tests revealed that the presence of crude oil reduced the porosity, specific surface area and pore volume in the coal sample containing crude oil, which would suppress the transport capacity of oxygen in mesopores and micropores not to adsorb on the specific active sites of coal surfaces. FTIR analysis indicated that the amount of oxygen-containing functional groups and –OH groups decreased tremendously in the coals containing crude oil, but the content of aliphatic C–H groups showed an increasing trend. Moreover, the values of A(CH2)/A(CH3), Ahy/Aar, Aoxygen/Aar all decrease gradually with the rising crude oil contents, indicating that the crude oil reduced the average alkyl chain length and compressed the space between aromatic clusters. The results of temperature-programmed experiments showed that O2 consumption and CO production rates decreased gradually with the increase in crude oil content. The results were in agreement with the varying trend of characterization of microstructure and function groups obtained by LP-N2A, TG and FTIR tests. Furthermore, the Pearson correlation coefficient cleared that the breaking of long aliphatic chains of aromatic rings and reduction of hydrogen bonds content have a greater contribution to the suppression of spontaneous combustion than the attenuation of physical adsorption. This study concluded that crude oil lowers down the spontaneous combustion properties of coal, which could provide a reference for the prevention of spontaneous combustion of coal seam containing crude oil.