Coal is a porous medium. Oxygen molecules in the air penetrate through the pores of coal and are adsorbed on the coal surface. Low-temperature oxidation of coal then occurs, by which coal spontaneous combustion is promoted. Given this process, the authors analysed the physisorption characteristics of O₂ in pulverized coal from the perspective of nanopore structure. In this study, five different kinds of coal samples (two lignites, one bituminous coal, and two anthracites) were selected, and the surface morphology, pore structure parameters and oxygen physisorption capacity of the pulverized coals were determined by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and oxygen adsorption with chromatography (OAC), respectively. The experimental results of SEM and MIP show that with the development of coal, the surface folds increase, and the pores increase in number and shrink, which leads to the nanopores of anthracite and bituminous coal being smaller and more complex than those of lignite. The experimental results of OAC show that adsorbed oxygen is physisorbed by pulverized coal in the order lignite > bituminous coal > anthracite. Analysis of the oxygen desorption curves shows that the oxygen desorption rates of the anthracites and bituminous coal are slower than those of the lignites. The results show that the amount of oxygen physisorbed by pulverized coal is proportional to the fractal dimension of the coal pores, proportional to the pore volume of the nanoscale pores, and inversely proportional to the number of closed pores in the coal. Based on the results of the analyses mentioned above, it is important to analyse the process of coal-oxygen chemisorption and the mechanism for low-temperature oxidation of coal to prevent coal spontaneous combustion.

We propose a terahertz surface plasmon resonance sensor based on dielectric metagrating coupling to the spoof surface plasmon (SSP) mode on periodically grooved metal films. The well-designed silicon metagrating converts the normal incident to the necessary angle in the dielectric substrate exciting SSP with the transmission coupling between couplers and SSP metasurfaces. Using an all-dielectric metagrating as an external coupler, the tightly confined SSP mode can be excited within a small resonant cavity, causing the strong light–matter interaction. The proposed SSP dielectric meta-couplers will pave new routes for ultra-thin and compact sensing devices. The dielectric substrate thickness, the air gap distance between the substrate and the metal groove array, and metal groove gaps have remarkable influences on the characteristics of SSP sensing. The SSP sensing by metagrating coupling with the resonance frequency around 0.404 THz provides a high sensitivity of up to 335 GHz/RIU and a detection limit less than 0.0001 RIU with a frequency resolution of 33.5 MHz. Dielectric metagrating coupling SSP provides enormous potential for constructing ultra-sensitive and compact SSP sensors in the terahertz frequency region.

The adsorption mechanism of carbon dioxide, oxygen and nitrogen in coal is important for preventing and controlling coal spontaneous combustion and for understanding the technology of CO₂ storage in goaf. Adsorption amount and adsorption heat are key adsorption parameters that are required to understand the material and energy conversions during adsorption in coal. In this study, we studied the factors that influence the adsorption amounts and adsorption heat values of carbon dioxide, oxygen and nitrogen in coal by testing four different coal samples using conventional coal quality analysis, low-pressure nitrogen and carbon dioxide adsorption, Fourier transform infrared spectroscopy and three gas adsorption experiments at different temperatures. Then, we analyzed the relationships between the structural parameters of the coal samples and the adsorption amounts and the adsorption heat values of carbon dioxide, oxygen and nitrogen. The results show that the adsorption isotherms of carbon dioxide conform to the Langmuir equation, and the adsorption isotherms of oxygen and nitrogen conform to Henry’s law between 0 and 110 kPa. The adsorption amounts of carbon dioxide, oxygen and nitrogen decreased with an increase in temperature, and the change in the rate of the adsorption amount with temperature was determined by the adsorption heat. The results of the pore structure show that the pores of the coal samples are composed of mesopores and micropores; the micropores contribute to the main specific surface area. The micropore and mesopore structures are the main determinants of the adsorption amounts of carbon dioxide, oxygen and nitrogen in coal. The gas adsorption heat is affected by the pore structure and the chemical composition of coal. The adsorption heat of nitrogen correlates positively with the pore structure of the coal. The adsorption heat of oxygen correlates positively with the ash, elemental nitrogen, elemental sulfur and mineral contents of the coal. The adsorption heat of carbon dioxide correlates positively with the elemental sulfur content of the coal.

Carbon dioxide (CO₂) is widely used in the prevention and control of spontaneous coal combustion. In this manuscript, three low-rank coals with different metamorphic degrees were selected as the research objects. The temperature-programmed experiments, in situ infrared cooling experiments, simulation of the competitive adsorption of CO₂ and oxygen (O₂) in coal pores, and simulation study of the CO₂ inhibition of the coal oxygen composite reaction were used to obtain the role and effect of CO₂ in preventing oxygen adsorption in coal at the low-temperature oxidation stage. It was concluded that CO₂ can displace the O₂ near the pore wall to physically prevent the adsorption of O₂. Through the changing law of heating rate and a kinetics analysis, it was found that CO₂ can increase its activation energy by 5.3–108.3% during the slow heating stage of coal and reduce its heat rate. At around 120 °C, coal loses the protective effect of CO₂. From the changes in functional groups, it can be seen that when coal was cooled in the CO₂ atmosphere, mainly pyrolysis and condensation reactions occurred due to the lack of O₂. In addition, CO₂ can also inhibit the chain reaction of the chemical adsorption of oxygen in coal. This work provides a theoretical basis for CO₂ prevention and the control of spontaneous coal combustion.