An embodiment of the invention consists of a process heater with a radiant section and an overhead convection section, the radiant section having a vertical cylindrical, contiguous, helical, tubular coil, having a bottom process fluid inlet and top process fluid outlet. The process fluid coil is contained in a refractory lined enclosure of square cross section and is exposed to an up flowing stream of combustion products, generated by burning coal on a continuous, traveling, chain grate stoker. The stoker is located at the base of the radiant section, receives a continuous deposit of coarse coal, with a minimum of fines, from a hopper located at one end of the stoker chain grate and discharges essentially coal free ash at the opposite end of the chain grate, thru a nozzle provided with a water seal. One or more centrifugal solids—gas separators are provided at the radiant section flue gas outlets for the purpose of removing ash particulates from the flue gas streams, before the latter enter the overhead convection section. Since the flue gas enters the convection section very nearly devoid of particulates, the convection section may be of conventional design, utilizing bare or extended surface tubes, on closely spaced equilateral or quadrilateral centers.