Forecast of coal spontaneous combustion based on the variations of functional groups and microcrystalline structure during low-temperature oxidation
Fuel, Volume 253, 1 October 2019, Pages 339-348
Jiawen Cai, Shengqiang Yang, Xincheng Hu, Wanxin Song, Yawei Song
摘要：Variations of gas products and reaction characteristics of main functional groups during low-temperature oxidation of coals with different metamorphic degrees were analyzed based on the gas chromatographic analysis of gas products and the FTIR determination of oxidized coal samples. Meanwhile, changes in the microcrystalline structure parameters during the low-temperature oxidation were determined with the aid of the XRD technology. Since gas product contents of coal with different metamorphic degrees grow continuously with the rise of oxidation temperature, a forecast system that adopted CO and C2H4 as indicator gases in the low-temperature oxidation process was established. The content of CH3/CH2, the primary participant in the oxidation reaction, gradually falls with the rise of oxidation temperature. During the low-temperature oxidation of coal, the stacking height Lc, ductility La, stacking layer N and graphitization degree P of aromatic layers all increase with the rise of oxidation temperature. Finally, the critical temperature ranges (80–110 °C for RL coal, 110–140 °C for YL coal and 130–170 °C for WY coal) for drastic changes during low-temperature oxidation were proposed by comparatively analyzing the variation laws of gas products, main reaction functional groups and aromatic layers, as well as the corresponding temperature variations.
Early detection system for coal spontaneous combustion by laser dual-species sensor of CO and CH4
通过CO 与 CH4激光二元传感器实现煤自燃早期探测系统
Optics & Laser Technology, Volume 121, January 2020, Article 105832
Xuanbing Qiu, Yongbo Wei, Jie Li, Enhua Zhang, Jilin Wei
摘要：As a common hazard, coal spontaneous combustion (CSC) causes the loss of coal in piles and mines. To rapidly and reliably forecast CSC, a portable and powerful detection system based on a laser spectroscopic technique is proposed, to precisely measure gaseous inhibitors; the technique uses a single, distributed-feedback diode laser emitting at 2.33 μm as a sensing light source for dual species. The detection system carries out wavelength-modulation spectroscopy to suppress detection noise by using a two-channel analogue lock-in amplifier (ALIA) and a complex programmable logic device (CPLD) controller. To improve detection sensitivity, a Herriott cell is adopted to increase the interaction length between the laser and the target species. The embedded system on a chip (SoC) of STM32F103 is the charge of the current driver and temperature control of the laser. Deployment of the SoC of STM32H743 acquires and digitally filters the original demodulated absorption signal, after which communication with a personal computer takes place. The second demodulation component, normalized by the first demodulation component, is used to improve the signal-to-noise ratio. The sensor-calibration experiment reveals the square of the correlation coefficients R2 to be 0.99718 and 0.99905 for carbon monoxide (CO) and methane (CH4), respectively, over the concentration range of 5–200 ppm. Using Allan-Werle deviation analysis, the detection sensitivity for CO and CH4 are 0.27 ppm and 0.20 ppm with a 2.5 s sampling time, and 0.05 ppm and 0.03 ppm with ~125 and 95 s integration times, respectively. Finally, the detection system is used for the online measurement and analysis of the growth pattern of gaseous products which are released during the CSC process, the temperature varying from 30 to 200 °C. Two critical temperatures were correctly determined to further forecast CSC status. On beginning a vigorous oxidation process, the exponential-evolution trends of the CH4 and CO gases agree with each other. Compared with CH4, CO may be more suitable for online, early detection of CSC.
Experimental research on gel-stabilized foam designed to prevent and control spontaneous combustion of coal
Fuel, Volume 254, 15 October 2019, Article 115558
Quanlin Shi, Botao Qin
摘要：The gel-stabilized foam was prepared successfully by forming gel structure in the bubble film using thickening agent (TA) and crosslinking agent (CLA). The viscosity characteristic, foamability, stability, and microstructure of gel-stabilized foam were investigated systematically. The formation of gel structure reduced the foamability of foaming solution due to the increased viscosity, whereas it increased the foam stability significantly. By comprehensively considering foam expansion ratio >5 and half-life >120 h, the effective component range of gel-stabilized foam was determined as 3.4–5.5 g/L for TA and 2.1–4.0 g/L for CLA. Microstructure and liquid drainage kinetics analysis indicated that gel-stabilized foam demonstrated the smaller average bubble size, lower growth rate of bubble size and drainage rate compared to the traditional aqueous foam, mainly ascribing to the thickened bubble film and the formation of a viscoelastic shell on the bubble surface. The simulation experiment of spontaneous combustion of coal indicated the better inhibition efficiency of gel-stabilized foam to spontaneous combustion of coal than that of traditional foam, as evidenced by higher crossing point temperature, lower emission of carbon monoxide and ethylene for treated coal by gel-stabilized foam, which revealed its better water-retaining property to retard the oxidation of coal. Moreover, gel-stabilized foam will form a complete film on the surface of coal particles to isolate coal from the oxygen to inhibit spontaneous combustion.
Study on the oxidation and release of gases in spontaneous coal combustion using a dual-species sensor employing laser absorption spectroscopy
Infrared Physics & Technology, Volume 102, November 2019, Article 103042
Xuanbing Qiu, Jie Li, Yongbo Wei, Enhua Zhang, Zhenzhong Zang
摘要：Coal spontaneous combustion (CSC) is a common hazard in coal mines and a significant reason for the loss of coal in stockpiles and mines. To investigate the oxidation and release of gas due to of CSC, a miniature purpose-built laser-based sensor system capable of precise gas measurement and data processing was configured. A distributed feedback (DFB) diode laser with a central wavelength of 2.33 μm, in conjunction with a Herriot-type cell and wavelength modulation spectroscopy (WMS), was used for the simultaneous detection of methane (CH4) and carbon monoxide (CO) during coal combustion. The second-harmonic of WMS was applied to ensure sensor robustness and sensitivity with long-term performance for the system being evaluated by calculating the Allan-Werle deviations for the concentrations of CH4 and CO. The limits of detection (LoDs) for CH4 and CO were 0.05 ppm based on integration times of 120 s and 70 s, respectively. The dual-species sensor was employed for real-time and in situ investigations of coal samples for temperatures ranging from 85 °C to 200 °C and was considered suitable for integration into field monitoring equipment, especially for online early warning forecasting applications in CSC.
Study on the methane explosion regions induced by spontaneous combustion of coal in longwall gobs using a scaled-down experiment set-up
Fuel, Volume 254, 15 October 2019, Article 115547
Dong Ma, Botao Qin, Lin Li, Ang Gao, Yuan Gao
摘要：In order to effectively control and prevent the disasters of methane explosions induced by spontaneous combustion of coal in longwall gobs, the formation condition of methane explosions induced by spontaneous combustion of coal was proposed. Furthermore, the effect of the area of spontaneous combustion of coal on the distribution of oxygen and methane concentration was studied using a scaled-down experiment set-up of mine gob. The results indicated that the area of spontaneous combustion of coal had little effect on the oxygen distribution, but played a key role in the methane distribution. With a ventilation velocity of 0.2 m/s, 0.6 m/s and 1.0 m/s, the methane concentration showed a conspicuous accumulation distribution near the area of spontaneous combustion of coal. In addition, based on the formation condition of methane explosions proposed, the explosion range induced by spontaneous combustion of coal was determined quantitatively. The results showed that compared with the experimental conditions of no spontaneous combustion of coal in the gob, the range of methane explosions with the occurrence of spontaneous combustion of coal increased by 16.45%, 13.36%, 11.87% and 9.30% with a ventilation velocity of 0.2 m/s, 0.6 m/s, 1.0 m/s and 1.4 m/s, respectively. The results revealed that the entrainment effect created by hot gases in the spontaneous combustion area had expanded the explosion region. Finally, the relationship between the area of methane explosion and the ventilation velocity was developed. The experimental results may provide some instructions on how to control and prevent the disaster of methane explosions induced by spontaneous combustion of coal in longwall mining gobs.
Experimental study on foaming properties of anion-cation compound foaming agent to prevent coal spontaneous combustion
Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 581, 20 November 2019, Article 123847
Min Li, Deming Wang, Shan He, Zhenlu Shao, Yiding Shen
摘要：In this study, an anion-cation compound foaming agent (ACFA) was proposed based on the synergistic mechanism, so as to realize efficient production of low-concentration foam for preventing coal spontaneous combustion. The foaming and coal spontaneous combustion prevention properties of ACFA and anion compound foaming agent (AFA) were investigated by using Foamscan and the coal spontaneous combustion testing system, respectively. The results show that foaming speeds of the two agents both increase with the rise of concentration. The critical micelle concentrations of ACFA and AFA are 1‰ and 4‰, respectively. Compared with AFA, ACFA can reach the required foam volume faster. Besides, the foam produced by ACFA can stabilize 115% longer and maintain its effect longer, because ACFA contains more liquid than AFA at the critical micelle concentration. Moreover, the densely and uniformly distributed foam produced by ACFA can more effectively prevent oxygen from feeding the coal. Since coal samples treated with ACFA and AFA both boast high inhibition rates, they can serve as foam fire-extinguishing agents to prevent coal spontaneous combustion in underground mines, but ACFA is more efficient and economical.