The transport of coal particles at low velocities by forming plugs was studied in a 0.0254-m vertical pipe. The pressure drop and the plug velocity were measured at different air flow rates and for different plug lengths. The conditions to produce stable plugs were studied. The pressure drop was found to be independent of the air flow rate and to vary linearly with the plug length. The plug velocity was independent of the plug length in the conditions studied. A model that predicts the pressure drop was tested and found to be in good agreement with the experimental data. It suggests that in the case of vertical plug flow at low velocities, gravitational forces are the most important ones. © 1987.