NOVELTY - An on-site device comprises tangential inlet (1), reverse heat exchanger (2), tangential outlet (3), porous medium (4), electrical heating silicon carbon rod (5), reaction coil (6), reaction coil inlet end (7) and reaction coil outlet end (8). The tangential inlet and outlet are welded at outmost layer of heat exchanger. The porous medium is provided in center of the heat exchanger. The carbon rod is provided in center of porous medium. The reaction coil is provided in the channel of heat exchanger and porous medium. The reaction coil inlet end is provided in tangential outlet.    USE - On-site device for producing hydrogen (claimed).    ADVANTAGE - The device is convenient to operate, has high ignition reliability and simple structure, and is self-preheating, self-insulating, and self-cooling. The method has low consumption, and is stable and can produce pure hydrogen.    DETAILED DESCRIPTION - The on-site device comprises tangential inlet (1), reverse heat exchanger (2), tangential outlet (3), porous medium (4), electrical heating silicon carbon rod (5), reaction coil (6), reaction coil inlet end (7) and reaction coil outlet end (8). The tangential inlet and tangential outlet are welded at outmost layer of reverse heat exchanger. The porous medium is provided in the center of the reverse heat exchanger. The electrical heating silicon carbon rod is provided in center of porous medium. The reaction coil is provided in the channel of reverse heat exchanger and porous medium. The reaction coil inlet end is provided in the tangential outlet, where reaction coil outlet end is provided in tangential inlet. The reaction coil is provided in reverse heat exchanger, where and natural gas flows into reaction coil from reaction coil inlet end. The natural gas is pyrolyzed at a high temperature at porous medium in central combustion chamber of reverse heat exchanger. The product is charged from reaction coil from reaction coil outlet end. An INDEPENDENT CLAIM is included for a method for producing hydrogen comprising:    (A) starting electric heating silicon carbon rod in the center of reverse heat exchanger, preheating porous medium to 600-700 degrees C;    (B) filling premixed gas of natural gas and air from tangential inlet, igniting premixed gas in porous medium, and turning off power supply;    (C) discharging flue from tangential outlet of reverse heat exchanger and exchanging heat from flue to the premixed gas in the reverse heat exchanger;    (D) heating up central porous medium to 1200 degrees C, filling natural gas from reaction coil inlet end, exchanging heat from natural gas with flue in the channel of reverse heat exchanger through reaction coil;    (E) exchanging heat with porous medium in central region of reverse heat exchanger to pyrolysis temperature to generate carbon and hydrogen; and    (F) exchanging heat from product with premixed gas in channel of reverse heat exchanger through reaction coil until the reaction coil outlet end is cooled off to the temperature for collection.    DESCRIPTION OF DRAWING(S) - The drawing is a cross-sectional view illustrating an on-site device.    Tangential inlet (1)    Reverse heat exchanger (2)    Porous medium (4)    Silicon carbon rod (5)    Reaction coil (6)