TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Components: The portion of the closed volume inside which the gas-liquid mixture self-circulates protrudes inside the boiler section as a single elongated element or as elongated elements to form circumferentially and radially spaced elongated elements, and in which the elongated elements are in fluid communication with one another. The boiler section of the reactor is provided with a number of vertically spaced or horizontally spaced baffles in which the baffles are arranged transverse to the flow direction of the flue gas generated in the combustion section. The elongated components, which are preferably formed as tubes, are provided with a sleeve to allow for the passage of flue gas through the annular region defined by the elongated component and the sleeve. The process feed tube(s) carrying the process fluid to be converted enters the reactor through a conduct arranged in the outer wall of the reactor and the process fluid is preheated by indirect contact with exiting converted gas from the reforming section of the reactor. It extends vertically into a transition compartment from which process tube(s) carrying process gas to be converted extends vertically inside the closed volume of the reactor and the process tube(s) carrying the process gas is formed as a coil. The closed volume containing the reforming section and boiler section are surrounded by an insulated housing encased by a first annular region carrying flue gas and a second annular region carrying combustion air. The process fluid entering the reactor is a mixture of methanol and water and the gas-liquid mixture is a saturated steam-water system circulating at a pressure of 55-110 bar g and at 270-320 degrees C. The combustion section is provided with a single catalytic burner, which is provided as wire mesh layers arranged in series which are coated with ceramic and impregnated with an oxidation catalyst, where the heat generated in combustion is transferred by a convection mechanism to the self-circulating gas-liquid mixture via the generated flue gas.