Abstract: A low temperature conversion apparatus comprising tri-tubes that simultaneously carry out the functions of vessel and heat exchanger is provided. The apparatus is capable of thermally decomposing any kind of organic material to obtain coal, oil, water and no-condensable gasses, and also of decontaminating soils and residues contaminated with organochlorides and dioxins. The apparatus specifically comprises an outer box with a hermetic lid; a thermal insulation layer; and a structure with three concentric tubes disposed internally, positioned substantially vertically and with a wall width suitable for heating by means of gases from an inner side and outer side of the structure. A process for thermal decomposition is also provided using the apparatus.