Abstract: In order to make clearly the process and features of gas emission by injecting N₂， N₂ displacement coal seam CH₄ tests were performed by using coal bed gas injection displacement experiment device. The results show that N₂ displacement coal bed CH₄ is a time-dependent process with a gradual decay of drained CH₄ concentration， and CH₄ flow increases at first and then decreases. Gas injection pressure has significant impact on the displacement effect， the higher the injection pressure， the faster the drained CH₄ concentration decay， the larger injection gas flow， the more significant the flow rate increases. During the same time， the higher gas injection displacement pressure is， the larger the drained CH₄ volume. According to the theoretical analysis， the mechanisms of injecting N₂ displacement coal bed CH₄ include the gas displacement， the air carrying and the dilution and diffusion of gas flow.

Key words: N₂-ECBM, promoting gas drainage, timeliness, displacement, carrying, diffusion