1) A system for burning solid carbonaceous fuel efficiently and at a relatively low temperature, thus minimizing the emission of pollutants. The fuel is crushed to very small particles or a powder and mixed with particles of an inert material. The mixture is fed into a bed through which air is pumped vertically upwards, agitating the particles so they behave like a fluid. The forced circulation of air and the small size and separation of fuel particles ensures efficient burning. 2) A bed of finely divided solid through which air or a gas is blown in a controlled manner so that it behaves as a liquid.