The effect of densification with NaOH on brown coal thermal oxidation behaviour and structure

氢氧化钠增密对褐煤热氧化特性及结构的影响

Fuel, Volume 216, 15 March 2018, Pages 548-558

Mohammad Reza Parsa, Alan L. Chaffee

摘要：The thermal behaviour and chemical structure of densified products of two Victorian brown coals prepared with NaOH addition were analysed by differential gravimetric analysis/differential thermal analysis (DGA/DTA) and Fourier transform infrared spectroscopy (FTIR). Acid washing of coal led to lower tendency to spontaneous combustion through a decrease in the total enthalpy change and a shift of the main mass loss stages to higher temperatures due to strengthening coal hydrogen bonding network. The addition of a small amount of NaOH (such that pH < 7.5) shifted the main stages of the DTG/DTA curves to lower temperatures and had a catalytic effect on spontaneous combustion by disrupting the hydrogen bonds network and suppressing cross-linking reactions. The products densified with higher amount of NaOH (such that pH ≥ 7.5) showed lower proportions of mass loss at lower temperatures stages of the DGA/DTA curves during oxidation with air. The ion exchange of Na ions for hydrogen in acid functional groups, increases the polarity and strength of bonds leading to a more rigid coal structure, reduces the surface area and decreases the coal’s tendency to react with oxygen and consequently delays the onset of combustion. FTIR results showed that the order of thermal vulnerability of brown coal’s functional groups toward decomposition/oxidation was as follows: aliphatic groups > carboxylic groups > carboxylate and aromatic groups > substituted aromatic groups ≈ polymerized ethers and ketones.

Laboratory study on the inhibitory effect of free radical scavenger on coal spontaneous combustion

自由基清除剂对煤自燃抑制效果的实验室研究

Fuel Processing Technology, Volume 171, March 2018, Pages 350-360

Jinhu Li, Zenghua Li, Yongliang Yang, Biao Kong, Chaojie Wang

摘要：In this paper, the inhibition of free radical reaction in the coal oxidation process was taken as the starting point, and free radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) which combines with free radicals very easily to generate inactive substances was regarded as the inhibitor. The occurrence of coal spontaneous combustion was suppressed by inhibiting the chain reaction of free radicals. TEMPO was compared with common halide salt inhibitors through thermogravimetry and differential scanning calorimeter (TG-DSC) thermal analysis, crossing point temperature (CPT), CO production and other data obtained from experiments. The results show that the inhibitory effect of TEMPO is stronger than that of common halide salt inhibitors and the inhibitory effect increases with the rise of TEMPO concentration. The influence of TEMPO on active functional groups in coal at different temperatures was obtained through Fourier transform infrared spectroscopy (FTIR) analysis, suggesting that the inhibitory effect of TEMPO on active functional groups (CO, CH, CO and OH) in coal gets more significant with the rise of temperature and the increase of active free radicals in coal. On this basis, five common active free radical models were constructed, and their active sites and thermodynamic parameters were calculated according to density functional theory (DFT). The calculation reveals that compared with oxygen-containing free radicals, alkyl free radicals are more active in the thermodynamic reactivity with TEMPO. The experiments prove that TEMPO has a very strong inhibitory effect on coal spontaneous combustion, because the combination between TEMPO and alkyl free radicals during coal spontaneous combustion decreases the activity of free radicals and reduces the concentration of active free radicals, thus inhibiting the chain reaction of free radicals.

Experimental technique and modeling for evaluating heat of rewetting effect on coals' propensity of spontaneous combustion based on adiabatic oxidation method

基于绝热氧化法的评估再湿润发热对煤自燃倾向性影响的实验技术与模拟

International Journal of Coal Geology, Volume 187, 15 February 2018, Pages 1-10

Xinyang Wang, Yi Luo, Brunno Vieira

摘要：In order to test and quantify the effect of heat of rewetting on the propensities for spontaneous combustion of coals, especially low rank coals, a new experimental procedure and a mathematical model were developed based on the standard R70 adiabatic oxidation method. Dry and moist tests were performed on coal samples collected from Powder River Basin and from South America coal fields. Based on the linear and moving average parts of the self-heating curves, self-heating indices in terms of Moist R70 and R100 were proposed. The developed analytical model, calibrated with the experimental data, was used to simulate self-heating of coal with different amount of water condensation (2%, 5%, 10% and 20%) and relative humidity (40%, 60% and 90%). It was shown that an increase of water condensation and relative humidity significantly promotes the self-heating rate of coal. A practical consequence of this finding is the heat of rewetting mechanism should not be neglected in any precautionary or preventive measures to avoid potential spontaneous combustion in PRB and Colombia coal mines.

Effects of igneous intrusions on the structure and spontaneous combustion propensity of coal: A case study of bituminous coal in Daxing Mine, China

岩浆侵入对煤体结构与自燃倾向性的影响：中国大兴矿烟煤实例研究

Fuel, Volume 216, 15 March 2018, Pages 181-189

Quanlin Shi, Botao Qin, Hongjun Liang, Yuan Gao, Bao Qu

摘要：An investigation of the effects of igneous intrusions on the physicochemical structure and spontaneous combustion characteristics of coal is important for safe production in coal mines. In this study, unaltered coal and thermally-altered coal affected by igneous intrusions were collected from Daxing Mine. The mineral and ash composition, surface functional groups, surface morphology and pore structure were characterized by XRD, XPS, SEM, N2 adsorption and mercury intrusion porosimetry. The spontaneous combustion propensity of coal samples was investigated using oxidation kinetics experiment. Mineral and ash composition analysis revealed that igneous intrusions increased the relative contents of calcite and CaO in the thermally-altered coal. The results of XPS tests indicated that the content of CC/CH slightly increased and an obvious π − π∗ shake-up satellite peak appeared in the thermally-altered coal, but that the contents of CO, CO, and COO showed a decreasing trend. Compared with the pore structure of the unaltered coal, higher porosity, larger pore volume, and larger average pore diameter were found in the thermally-altered coal. Moreover, more pores with diameter >20 nm were formed due to igneous intrusions, which would improve the transport capacity of O2 in pores to react with active sites on pore surfaces in the thermally-altered coal. Moreover, igneous intrusions increased the O2 consumption rate of the thermally-altered coal, and decreased its crossing point temperature and spontaneous combustion propensity evaluating index I. This study concluded that igneous intrusions improved the spontaneous combustion risk of Daxing bituminous coal.

Unique spatial methane distribution caused by spontaneous coal combustion in coal mine goafs: An experimental study

煤矿采空区煤自燃引发的甲烷空间分布特征

Process Safety and Environmental Protection, In press, accepted manuscript, Available online 1 February 2018

Lin Li, Botao Qin, Dong Ma, Hui Zhuo, Ang Gao

摘要：A new type of incident in the form of a methane explosion caused by spontaneous combustion of coal is receiving more and more attention due to the high ground temperature and high methane content in coal mines. To investigate the formation process of this type of incidents and advance the research on methane explosion risk caused by spontaneous coal combustion, a self-designed experimental platform was used to determine the influence of spontaneous coal combustion on methane migration in the goaf. Additionally, the effect of different ventilation velocities at the mining face on methane migration in the goaf was studied. The results reveal that air leakage plays a key role in methane migration in the goaf. The bigger the ventilation velocity, the larger the area affected by air leakage. In addition, the superposition of the air leakage and the horizontal and vertical movement of hot gases creates the entrainment effect in the spontaneous combustion area. Eventually, the methane concentration at the location of spontaneous combustion is the lowest and it gradually increases outside the spontaneous combustion area because of the superposition of air leakage, ‘fire pressure’ and the entrainment effect.