A combustor apparatus is provided, comprising a mixing arrangement for mixing a carbonaceous fuel with enriched oxygen and a working fluid to form a fuel mixture. A combustion chamber is at least partially defined by a porous perimetric transpiration member, at least partially surrounded by a pressure containment member. The combustion chamber has longitudinally spaced apart inlet and outlet portions. The fuel mixture is received by the inlet portion for combustion within the combustion chamber at a combustion temperature to form a combustion product. The combustion chamber directs the combustion product longitudinally toward the outlet portion. The transpiration member is configured to substantially uniformly direct a transpiration substance therethrough toward the combustion chamber, such that the transpiration substance is directed to flow helically about the perimeter and longitudinally between the inlet and outlet portions, for buffering interaction between the combustion product and the transpiration member. Associated systems are also provided.