The device comprises: an inner chamber (56) which has an upper aperture or mouth (36) and is delimited by a lateral wall (20) and by a base wall (19), and which is intended to contain a primary fuel to be subjected to gasification and/or pyrolysis or vaporization to generate combustible gases; an outer chamber (58) delimited by an outer lateral wall (22) positioned coaxially with the outside of the lateral wall (20) of the inner chamber (56), by a base wall (14) which is located under the base wall (19) of the combustion chamber (56) and which has at least one aperture (28) communicating with the external environment, and by a top wall which unites the outer (22) and inner (20) lateral walls, in such a way that the outer chamber (58) comprises a lower portion positioned under the inner chamber (56) and an upper annular portion (59) which surrounds the inner chamber (56); a plurality of circumferentially staggered lower passages (50) through which the base of the inner chamber (56) communicates with the surrounding annular portion (59) of the outer chamber (58); and - a plurality of circumferentially staggered upper passages (39) through which the top of the annular portion (59) of the outer chamber (58) communicates with the top of the inner chamber (56).