A refractory tunnel assembly for a steam reformer furnace, said tunnel assembly comprising: a plurality of hollow base components, each said base component comprising a plurality of corresponding mechanical mating members; a plurality of hollow wall blocks (1, 10, 100), each said wall block comprising a plurality of corresponding mechanical mating members that further correspond to said mechanical mating members of said base components; and a plurality of hollow lid components, each said lid component comprising a plurality of mechanical mating members that further correspond to said mechanical mating members of said base components and said wall blocks (1, 10, 100); wherein said base components are arranged to extend in a horizontal arrangement direction defining a width of said tunnel assembly and a longitudinal arrangement direction defining a length of the tunnel assembly; wherein said wall blocks (1, 10, 100) are stacked upon and mechanically interconnected to said base components via said corresponding mechanical mating members, without the use of mortar, in a vertical arrangement direction and along said longitudinal arrangement direction, and are stacked upon one and mechanically interconnected to another via said corresponding mechanical mating members, without the use of mortar, in both said vertical and longitudinal arrangement directions, to define two parallel tunnel walls, spaced a distance apart from one another in the horizontal arrangement direction, wherein said tunnel walls extend upwardly from said base components in said vertical arrangement direction and along the length of said tunnel assembly on said base components; and wherein said plurality of lid components are stacked upon and mechanically interconnected to said wall blocks (1, 10, 100) via said mechanical mating members, without the use of mortar, in the vertical arrangement direction and along said longitudinal arrangement direction, so that said lids extend along the longitudinal arrangement direction and the horizontal arrangement direction in order to cover said distance between said tunnel walls along at least a portion of said length of said tunnel assembly. a plurality of hollow base components, each said base component comprising a plurality of corresponding mechanical mating members; a plurality of hollow wall blocks (1, 10, 100), each said wall block comprising a plurality of corresponding mechanical mating members that further correspond to said mechanical mating members of said base components; and a plurality of hollow lid components, each said lid component comprising a plurality of mechanical mating members that further correspond to said mechanical mating members of said base components and said wall blocks (1, 10, 100); wherein said base components are arranged to extend in a horizontal arrangement direction defining a width of said tunnel assembly and a longitudinal arrangement direction defining a length of the tunnel assembly; wherein said wall blocks (1, 10, 100) are stacked upon and mechanically interconnected to said base components via said corresponding mechanical mating members, without the use of mortar, in a vertical arrangement direction and along said longitudinal arrangement direction, and are stacked upon one and mechanically interconnected to another via said corresponding mechanical mating members, without the use of mortar, in both said vertical and longitudinal arrangement directions, to define two parallel tunnel walls, spaced a distance apart from one another in the horizontal arrangement direction, wherein said tunnel walls extend upwardly from said base components in said vertical arrangement direction and along the length of said tunnel assembly on said base components; and wherein said plurality of lid components are stacked upon and mechanically interconnected to said wall blocks (1, 10, 100) via said mechanical mating members, without the use of mortar, in the vertical arrangement direction and along said longitudinal arrangement direction, so that said lids extend along the longitudinal arrangement direction and the horizontal arrangement direction in order to cover said distance between said tunnel walls along at least a portion of said length of said tunnel assembly.