A reactor and its internals for thermal processing of a mixture, said reactor being a cylinder and comprising: a. a rotating indirectly fired kiln rotating around its center axis, said axis being horizontal or slanted; b. heating means comprising at least one heating system external to the walls of the reactor; c. at least one shelf (3, 20) on the reactor wall, one or more shelves (3, 20) being attached to the internal walls of said reactor, the form of the shelves being selected in the group constituted by flat, concave, convex, spiral and slanted, the at least one shelf (3, 20) being either parallel to said center axis of the reactor, when the reactor is horizontal, or slanted with respect to the center axis when the reactor is horizontal, or slanted with respect to the center axis when the reactor is slanted or not slanted; d. a charge of flat metal plates of consistent thickness and shapes so the plates (2, 23) can stay on the shelves (3, 20) and/or each other, while protecting the reactor wall from direct contact with the relatively cold feed; e. means for bringing the mixture to be thermally processed on the surface of at least part of the plates; f. means for removing the fine solids from the reactor, preferably either through entrainment with the exiting vapours, or through a separate solids exit, or both; g. means for recovering the reaction and straight run products and h. means for venting the gas obtained by the thermal processing outside the reactor zone. and wherein the heat coming through the reactor walls heats the plates (2, 23) as they are dragged and lifted against the reactor wall by one or more shelves (3, 20). a. a rotating indirectly fired kiln rotating around its center axis, said axis being horizontal or slanted; b. heating means comprising at least one heating system external to the walls of the reactor; c. at least one shelf (3, 20) on the reactor wall, one or more shelves (3, 20) being attached to the internal walls of said reactor, the form of the shelves being selected in the group constituted by flat, concave, convex, spiral and slanted, the at least one shelf (3, 20) being either parallel to said center axis of the reactor, when the reactor is horizontal, or slanted with respect to the center axis when the reactor is horizontal, or slanted with respect to the center axis when the reactor is slanted or not slanted; d. a charge of flat metal plates of consistent thickness and shapes so the plates (2, 23) can stay on the shelves (3, 20) and/or each other, while protecting the reactor wall from direct contact with the relatively cold feed; e. means for bringing the mixture to be thermally processed on the surface of at least part of the plates; f. means for removing the fine solids from the reactor, preferably either through entrainment with the exiting vapours, or through a separate solids exit, or both; g. means for recovering the reaction and straight run products and h. means for venting the gas obtained by the thermal processing outside the reactor zone. and wherein the heat coming through the reactor walls heats the plates (2, 23) as they are dragged and lifted against the reactor wall by one or more shelves (3, 20).