Diffusion of Surface CO2 in Coalfield Fire Areas by Surface Temperature and Wind

In the early stages of a coalfield fire, CO₂ emissions are relatively low, and it is challenging to detect CO₂ concentrations in the soil surface due to the impact of surface temperature and wind. Investigating CO₂ concentration changes under surface temperature and wind conditions can provide experimental evidence and theoretical foundation for selecting optimal sampling locations and time. Using an automated monitoring platform for shallow soil CO₂, this study analyzed how surface wind speed and temperature affect the diffusion of CO₂ gas of surface sands. The effects of surface wind and temperature on CO₂ concentrations growth at different depths of the shallow surface were studied experimentally. When the surface temperature was 40 ℃ higher than the ambient temperature, the decrease of CO₂ concentrations for coarse sands with permeability of 2.13 × 10^-9 m² was most significant under high surface temperature conditions. However, the effect of high surface temperature on fine sands with permeability of 1.1 × 10^-12 m² was insignificant. Coarse sand with high medium permeability was most vulnerable to the fluctuation of surface wind speed. The surface CO₂ concentrations decreased by 93% at a depth of 22 cm in the coarse sands on the downwind side of the surface compared to natural convection conditions. In comparison, the CO₂ concentrations decreased by 37.5% on the upwind sides under small wind speeds. The coupling effect of high temperature and wind speed on the surface had a greater disturbance depth on fine and medium sands than low windy conditions. In addition, detecting shallow surface concentrations of CO₂ for the localization of fire sources was more advantageous during low temperature detection periods. In order to describe gas diffusion at the surface, mathematical and physical equations were developed. A combination of experimental and simulation theory was used to predict the depth of penetration of shallow surface gas by wind speed and temperature. The critical Darcy–Rayleigh number for temperature disturbance to shallow surface gas was approximately 6.3 when using medium and coarse sands with high permeability. Simulations show that the wind-induced penetration depth was 40.8 cm for coarse sand and 23.5 cm for medium sand at a surface wind speed of about 0.4 m/s combined with the experiments. It is necessary to detect CO₂ concentrations at least at depth of 23.5 cm in conditions of low surface wind speed, particularly in the overlying medium with high porosity.