FBC Underfeed stoker has been popularly used in many European countries for small size boiler s of below 10 ton/hr. capacity. Coal is conveyed by the screw unit forcibly into the screen chamber with many air holes installed like in the left picture and coal will be pushed upward by overflowing movement. Fire stay burning above the screw. For this method, again the size of coal plays an important factor. Usually coal for this system must be selected strictly between 10-30 mm.