Explosion suppression characteristics and molecular dynamics of NH4H2PO4 in explosions of CH4–coal dust mixtures

Abstract

Explosions of CH₄–coal dust mixtures are associated with severe casualties and economic losses. Therefore, controlling and preventing such explosions are essential. Based on the complexity of gas-solid two-phase explosions, three conditions involving CH₄–coal dust (condition A: 8 vol% CH₄ + 500 g/m³ coal dust; condition B: 10 vol% CH₄ + 130 g/m³ coal dust; and condition C: 12 vol% CH₄ + 130 g/m³ coal dust) were examined to determine the effect of NH₄H₂PO₄ on their explosive behavior and to identify their explosion suppression mechanism through molecular dynamics simulations. Comparison of flame transmission and gas product release patterns revealed that NH₄H₂PO₄ effectively inhibited explosions of CH₄–coal dust mixtures. When the concentration of NH₄H₂PO₄ was increased from 200 to 1000 g/m³, the explosion flame propagation velocity, explosion pressure, and explosion flame temperature exhibited a linear decreasing trend. The inhibitory effect of NH₄H₂PO₄ powder on explosions of CH₄–coal dust mixtures reached its strongest level at a NH₄H₂PO₄ concentration of 1000 g/m³. Specifically, under conditions A, B, and C, the maximum reduction observed in flame propagation speed was 31.49 %, 21.62 %, and 31.65 %, respectively; the maximum reduction observed in explosion pressure was 20.21 %, 14.52 %, and 12.50 %, respectively; the maximum reduction observed in the temperature of the upper thermocouple was 38.02 %, 26.68 %, and 21.73 %, respectively; and the maximum reduction observed in the temperature of the lower thermocouple was 25.90 %, 17.16 %, and 22.90 %, respectively. Under condition B, the addition of a small quantity of inhibitory dust increased the pressure generated by explosions of CH₄–coal dust mixtures. Molecular dynamics simulations revealed that CH₃•, CH₂O, H•, and OH• were the main intermediate substances in the combustion process. NH₄⁺ inhibited the chain reaction of CH₄–coal dust by inhibiting the formation of CH₃• and accelerating the consumption of free radicals through OH• and O• free radical trapping in the detached H• system. H• was easily captured by H₃PO₄ and its decomposition products H₂PO₄•, HPO₃, PO₂•, HOPO, and PO•, which in turn inhibited the combustion of CH₄, CH₃•, and CH₂• by reacting with H• instead of O₂. The characteristics of explosion suppression in complex working conditions were explored, and the mechanism of explosion suppression was described from the atomic point of view. It provided a theoretical basis for further exploration of better inhibitor based on NH₄H₂PO₄ modification and compounding.