If the air velocity is increased to 5-10 m/s then a CFB system can be achieved, where the sand is carried upward by the flue gases and a more thorough mixing of the bed material and fuel takes place. The sand is then separated from the gas in a hot cyclone or U beam separator at the top of the furnace and fed back into the combustion chamber where the whole process begins again. CFBs deliver very stable combustion conditions, but it involves higher cost. CFB systems exhibit several advantages, such as the adaptation to various fuels with different properties, sizes, shapes, and high moisture (up to 60%), and ash contents up to 50% (http://www. esru. strath. ac. uk/EandE/Web_sites/06-07/Biomass/HTML/combustion_ technology. htm).