Flame acceleration and deflagration-to-detonation transition (DDT) are investigated in a 24-m-long and 80-mm by 80-mm-square cross-section steel duct with arrayed obstacles. Blockage ratio and spacing are varied to determine their effect on flame acceleration. The experiments were conducted with stoichiometric coal gas-air mixture, and the effect of mixture reactivity on flame propagation was also investigated by varying the concentration of coal gas. The experimental results indicate that the turbulence induced by obstacles can lead to a continuously accelerated flame and, under appropriate conditions, this can lead to transition to detonation. Also, it was found that, in the presence of obstacles, the flame propagation may experience an overall augmented acceleration due to the interaction of three main mechanisms: (1) the flow pattern distortion under the effect of obstacle-induced turbulence; (2) the flame acceleration resulting from a compression wave generated in front of the combustion wave; and (3) the unburned gaseous mixture being heated under the effect of that compression wave. Three different propagation regimes (quasi-detonation, high-speed turbulent flame deflagration, and slow subsonic turbulent deflagration) reported by previous investigations for other combustible gaseous mixtures are also observed in the present study for the coal gas and air mixture, and the flame propagation characteristics in these regimes for different coal gas concentrations as well as with different obstacle settings in the duct are analyzed in detail, providing useful insights for reducing the risk of flame propagation disaster.