Minimum explosion concentration of coal dusts in air with small amount of CH4/H2/CO under 10-kJ ignition energy conditions

•The presence of small amount of CH4/H2/CO poses a high explosion risk for coal dusts below their MEC.•Both Pex and (dp/dt)ex increase with increasing flammable gas concentration for all the hybrid mixtures.•CH4 has a higher promotion effect on the MEC and Pex of coal dusts than H2 and CO.•MEC of hybrid mixture in the synergic explosion region can be determined by combustion duration time method.•The empirical formulas are insufficient to prevent explosion accidents from safety considerations.