Search

To study the effect of erosion on coal microstructure and its oxidation characteristics, eroded coals were prepared by eroding coal samples in an acidic aqueous solution for 30 days, 90 days and 180 days, respectively. The microcrystalline structure, pore structure and active groups distribution of coal samples were studied using X-ray diffractometer, low-temperature N 2 adsorption and infrared spectra. Temperature-programmed experiment was used to analyze the oxidation characteristics of raw coal and eroded coals. The results reflected that the number of aromatic layers and coalification degree of eroded coal are reduced compared with raw coal. The average pore diameter increases, which promotes oxygen transportation in coal pores. Under the erosion effects, the content of active groups such as hydroxyl (–OH), methylene (–CH 2), and methyl (–CH 3) in coal increases. Eroded coals, especially the coal eroded for 180-day demonstrate more oxygen consumption and CO production than raw coal during the oxidation process, and their apparent activation energies decrease. Besides, the enhancement mechanism of eroded coal's oxidation activity is expounded from the aspects of the increase of active sites and the improvement of pore structure. In brief, erosion effect increases the spontaneous combustion tendency of coal. • The effect of weak acid erosion with different durations on coal oxidation behavior was studied. • Changes of coal physicochemical structure after erosion effect were studied. • The apparent activation energy of eroded coal decreases during low-temperature oxidation. • Mechanism for easier oxidation of dried eroded coal was expounded. [ABSTRACT FROM AUTHOR]

• The effect of aqueous solutions with different pH values on coal oxidation behavior was studied. • Erosion process promotes the evolution of fine-mesopores to mesopores and macropores. • The content of active functional groups in eroded coal increases, especially under effect of acid erosion. In order to study the effect of aqueous solutions with different pH values on coal oxidation behavior, coal samples were eroded in aqueous solution of various pH values (4, 5, 7 and 8) for 30 days and to study its microstructure change and oxidation behaviors. The pore structure and functional groups were characterized by low-temperature N 2 adsorption and Fourier transform infrared spectroscopy. The oxidation characteristics of coal were investigated by temperature-programmed experiments. Compared with raw coal, larger average pore diameter and pore volume, more active functional groups, such as methyl, methylene, hydroxy and ether, were found in the eroded coal. Among them, the microstructure of coal eroded in acid water changes most obviously. Eroded coal samples have more pores with diameter > 20 nm and an obvious increase in particle surface roughness, which facilitates the adsorption and transportation of O 2 in coal body. The O 2 consumption and CO production of dried eroded coal, especially eroded in aqueous solution with pH = 4, are more than those of raw coal, and its apparent activation energy decreases both in the slow oxidation stage and the fast oxidation stage, which indicates that the eroded coal is more prone to spontaneous combustion. [ABSTRACT FROM AUTHOR]

• Release of CO and CO 2 at constant temperature with time follows two different rules. • C–H in side chains is oxidized into precursors of carbon oxides, C = O and –COO. • CO and CO 2 can be produced by stepwise decomposition at ambient temperature. • Production of CO and CO 2 by direct decomposition occurs only at high temperatures. Precursors of carbon oxides in coals are considered to be related to aldehyde group(–CHO) and carboxyl group (–COOH), but the way of decomposition of precursors to produce carbon oxides is not clear. Through theoretical analysis, experimental research and quantum chemical calculation at molecular scale, this paper explores the decomposition mode and products of carbon oxide precursors during coal spontaneous combustion (CSC). A programmed heating device was used to investigate the change rule of gas product concentration during the constant temperature oxidation of coals, and the microstructure of the coals was characterized by low-temperature nitrogen adsorption experiments (BET), X-ray Diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The results of the study show that during coal oxidation, the inhomogeneous energy exchange between the coals and the external heat source results in the change of gas product concentration with time following two rules. BET experiments reveal that oxidation leads to the evolution of fine mesopores to macropores and an increase in the number of pores with diameters larger than 20 nm in the coals, which facilitates the transport of oxygen molecules through the pore structure. Within 200 °C, the microcrystalline structure of coals is basically unchanged, and the active structure C–C/C–H on the side chain is converted to –CHO and –COOH and then decomposed to produce carbon oxides. Quantum chemical calculation shows that the reaction energy barrier of carbonyl group and carboxyl group decomposed step by step to form carbon oxides is much smaller than that of direct decomposition, and carbon oxides and alkyl radicals can be produced even at room temperature by step decomposition. The research results provide theoretical guidance for the formation mechanism of carbon oxides during low temperature oxidation of coals. [ABSTRACT FROM AUTHOR]