A refractory block for a steam reformer furnace tunnel includes a hollow main body, at least one first mechanical mating member defining a protruded portion extending from an upper surface of the main body, at least one second corresponding mechanical mating member defining an opening corresponding to the protruded portion formed in a portion of a lower surface of the main body, at least one third mechanical mating member defining a tab provided in a portion one of a first end and an opposed second end or a first side and an opposed second side of the main body, at least one fourth mechanical mating member comprising a groove formed in the other of the first end and the second end or the first side and the opposed second side of the main body, and at least one cavity formed in the lower surface of the main body.