A process of improving overall quality of coals or other solid fuels comprising heat treating those coals in a controlled, spontaneous, fluidized bed reactor system. The bed system is of such a geometry that in combination with the control system a higher velocity region exists where the fluidizing gas and fuel are fed than where the treated solids exit. The offgas exiting the top of the reactor has a velocity of the average of the inlet gas velocities. Treated coal is withdrawn intermediate to top and bottom. Residence time is controlled by the amount of the solid fuel feed, the location of the treated solid fuel overflow, and the temperature is maintained by oxygen to flue gas ratio, feedstock size and fluidizing velocity. The unit can be operated in a drying mode or a devolatilization mode, to control volatiles, moisture and fines.