A combustor (2) for the combustion of fuel gas is described. The fuel gas entering the combustor (2) by an inlet pipeline (20) is a compressed admixture with air of a concentration below the lower explosive limit. The fuel gas/air admixture passes within a tube (6) to pass to the combustion zone (8) where combustion occurs only by reaction/self-combustion. The combusted gas passes by a tortuous path provided by an arrangement of baffles (10) to the outlet duct (22). The exhausting combusted gas passes over the outer surface of the tube (6) and gives up preheating heat to the inlet gas therein. A gas turbine system (100) for utilising waste fuel gas also is described that includes a combustor (2, 114) as described that provides combusted fuel to the expansion stage (104) of the turbine (100).