An Experimental Study of the Effect of Dynamic N₂ Injection on the Emission and Diffusion Dynamics of Coalbed Methane

N₂ displacement technology is widely regarded as a promising approach for enhancing coalbed methane recovery. However, the broader application of conventional N₂-ECBM has been constrained by low N₂ utilization efficiency and high operating costs. Dynamic N₂-ECBM can significantly improve both N₂ utilization efficiency and displacement performance, yet it still lacks sufficient theoretical support. Therefore, to gain a deeper understanding of the mechanism by which dynamic N₂-ECBM enhances coalbed methane recovery, a self-developed experimental platform for N₂ displacement of CH₄ was employed, and dynamic injection modes were, for the first time, introduced to analyze the diffusion behavior of CH₄ under the influence of N₂. At an average injection rate of 100 mL/min, two dynamic injection modes, namely sinusoidal and stepped modes, were designed with three flow-rate variations and compared with the constant-flow injection mode. The results showed that, compared with the Constant flow-100 mode, the final CH₄ production under the Sin-150-50, Sin-175-25, Laddered-150-50, and Laddered-175-25 modes increased by approximately onefold. The average effective diffusion coefficients of CH₄ under the Sin-150-50 and Sin-175-25 modes increased by factors of 1.15 and 1.10, respectively. Before N₂ breakthrough, dynamic N₂ injection was more effective in enhancing the average effective diffusion coefficient of CH₄. Although the Sin-200-0 mode resulted in relatively low CH₄ production, the times required to reach T _50%, T _30%, and T _10% were shortened by 183, 263, and 558 s, respectively, while the cumulative N₂ injection volumes were reduced by 233 mL, 393 mL, and 668 mL, respectively, thereby substantially lowering both the process duration and the N₂ injection cost of the N₂-ECBM process. The Sin-150-50 mode effectively shortened the time required to reach the concentration threshold limit value (TLV) and reduced the residual gas content at the TLV, which is beneficial for methane hazard control in coal mines. Under dynamic N₂ injection, smaller amplitudes led to higher CH₄ production and larger average effective diffusion coefficients. When the CH₄ concentration dropped below 10%, the stepped injection mode yielded higher CH₄ production. Assuming that the effects of different injection modes on gas residence time and adsorption-retention behavior are broadly universal, dynamic injection may also be more favorable for CO₂ adsorption and retention in coal. This experimental study demonstrates that different injection modes exhibit distinct advantages and provides theoretical guidance for enhancing coalbed methane recovery, improving N₂ utilization efficiency, controlling methane hazards in coal mines, and promoting CO₂ sequestration. Further field-scale validation is still needed in future work.