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In furnaces and other types of fluidized bed reactors one can easily observe gas bubbles flow and fluid jet preferable rise zones where solid particles in form of fountains are carried away from the layer. Let us call these layers as zones of local flowing. We assume that in these zones of local flowing occurs the removal of unburned fuel particles from fluidized bed. It results in heat leakage up to 20–40% due to mechanical imperfection of fuel combustion in types of vessels with fluidized bed furnaces. We investigated particles with diameter of 2,5–3,0 mm motion and local flowing zones formation in a model column of 172 mm in diameter at room temperature. Experimental application of air-separating grid enabled to form at the layer input “flat” and “convex” air velocity line. At the “flat” line the air distributed uniformly on the input section of the device. When the line was “convex” the air velocity at the layer input in the device center was twice higher than at the device periphery. It was found out that at “flat” line when air velocity increases there appeared zones of local flowing in the layer. To eliminate these zones one should increase the height of stationary particles layer. At “convex” line of input air velocity elimination of local flowing zones is possible by stable increase of air velocity or by lowering the height of stationary particles layer. It was concluded that for small fluidized bed fireboxes it is better to apply air-separating grids forming “convex” input air velocity line. The grid under discussion was manufactured to be applied in experimental fluidized bed firebox. There was carried out a detailed study of ordinary anthracite combustion in a firebox. It was established that at “convex” input air velocity line fuel loss due to entrainment amounted 2,0–2,5%. According to experimental results there were designed water boilers with heating efficiency from 300 up to 800 kilowatt. In these water boilers there was applied the grid forming “convex” input air velocity line. During tests there was burned culm (particle size from 0 up to 6 mm, heating efficiency of 20,6 MJ/ kg, output of volatile matter up to 6%, zoning 26,2–31,1%). Efficiency factor of these water boilers is 83–85% (without entrainment return to the firebox).Copyright © 2005 by ASME