A flash pyrolysis reactor to thermally decompose a feedstock provided in the form of an organic substance, comprising - a reactor body enclosing a closed reactor space, said reactor body extending longitudinally and having a cylindrical inner wall sealed air-tightly by a closing lid at each of its opposing ends; - an inlet for introducing feedstock into the reactor space; - a first outlet for withdrawing vapor/gas fraction from the reactor space (1 0a); - a second outlet for withdrawing solid fraction from the reactor space; wherein the inlet and the outlets (22, 23) are in the form of sealed apertures of the reactor body, the inlet is located at one end of the reactor body, the second outlet is located at the opposite end of the reactor body; - a conveying element for transporting feedstock from the inlet toward the outlet, said conveying element is supported by bearings in the reactor space coaxially to the longitudinal axis of the reactor body, is arranged with certain spacing apart from the inner wall and extends along full length of the reactor space, said conveying element is equipped with electric driving means and has a region extending along the inner wall which is configured to perform mechanical film formation, characterized in that the conveying element is a shaftless spiral and said region of the conveying element is formed by blades extending along the full length of the shaftless spiral and attached to the circumference of the shaftless spiral at attachment points, wherein each blade extends towards the inner wall of the reactor body into a spacing of a few millimeters, at most 5 mm, which separates the shaftless spiral and the inner wall, thereby providing a film forming gap for the mechanical film formation smaller than the size of said spacing, wherein the conveying element is characterized by a transmission coefficient with a value of at least 0.6, said transmission coefficient measures the ability of the conveying element in affecting unobstructed vapor/gas flow within the reactor, in use, said transmission coefficient being defined as a ratio of the area of said conveying element unobstructed in a cross-section perpendicular to the longitudinal axis of the reactor body and the total cross-sectional area of said reactor space. - a reactor body enclosing a closed reactor space, said reactor body extending longitudinally and having a cylindrical inner wall sealed air-tightly by a closing lid at each of its opposing ends; - an inlet for introducing feedstock into the reactor space; - a first outlet for withdrawing vapor/gas fraction from the reactor space (1 0a); - a second outlet for withdrawing solid fraction from the reactor space; wherein the inlet and the outlets (22, 23) are in the form of sealed apertures of the reactor body, the inlet is located at one end of the reactor body, the second outlet is located at the opposite end of the reactor body; - a conveying element for transporting feedstock from the inlet toward the outlet, said conveying element is supported by bearings in the reactor space coaxially to the longitudinal axis of the reactor body, is arranged with certain spacing apart from the inner wall and extends along full length of the reactor space, said conveying element is equipped with electric driving means and has a region extending along the inner wall which is configured to perform mechanical film formation, characterized in that the conveying element is a shaftless spiral and said region of the conveying element is formed by blades extending along the full length of the shaftless spiral and attached to the circumference of the shaftless spiral at attachment points, wherein each blade extends towards the inner wall of the reactor body into a spacing of a few millimeters, at most 5 mm, which separates the shaftless spiral and the inner wall, thereby providing a film forming gap for the mechanical film formation smaller than the size of said spacing, wherein the conveying element is characterized by a transmission coefficient with a value of at least 0.6, said transmission coefficient measures the ability of the conveying element in affecting unobstructed vapor/gas flow within the reactor, in use, said transmission coefficient being defined as a ratio of the area of said conveying element unobstructed in a cross-section perpendicular to the longitudinal axis of the reactor body and the total cross-sectional area of said reactor space.