A furnace and a process for temperature control of a material stream, wherein the furnace has a first combustion chamber, at least one reactor tube for receiving the material stream to be heated, and at least one second combustion chamber. The at least one reactor tube extends through the first combustion chamber and through the at least one second combustion chamber. The furnace is designed to establish a first temperature in the first combustion chamber and a second temperature in the at least one second combustion chamber, wherein the first temperature and the second temperature are separately adjustable.