Experimental studies on premixed combustion of ultra low calorific gas (LCG) in an axial and radial gradually-varied porous media burner with annular for heat recirculation were conducted. The effect of firing rate for different heating value of low calorific gas on temperature profiles, flames stability and CO emission was studied. The flame stability limits increased while the CO emission decreased with the increase of heating value of LCG. The flame located near the front of the inner tube at the upper flame stability limit and the corresponding temperature profiles decreased along the flow direction. The flame location moved from the upstream to the downstream with the increase of firing rate, and the corresponding temperature profiles of upper flame stability limits were much uniform than that of lower flame stability limit. The combustion and flow characteristics could be improved by using an annular heat recirculation, and an ultra-low calorific gas of 1.4 MJ/m(3) can burn in the present porous burner. (C) 2016 Elsevier Ltd. All rights reserved.