A box style steam methane reformer (15) has plural sections (37), with each section having walls (27-29-31, 33) forming an interior cavity (35) and open ends (43) that communicate with the interior cavity. Each section has a feedstock supply pipe (71) and a fuel supply pipe (63) located along the top wall, as well as a syngas collection pipe (79) and a flue gas collection duct (75) located outside of the bottom wall. The pipes and ducts have ends that are aligned with each other to allow the sections to be assembled together. Burners (67) are in the interior cavity and are connected to the fuel supply pipe. Reactor tubes (59) extend through the interior cavity. Refractory members (81) are located in the interior cavity and across a slot. The spacing between the refractory members varies to control the flow of flue gas.