The fundamental principle of a BFB furnace is that the fuel is dropped down a chute from above into the combustion chamber where a bed, usually of silica sand, sits on top of a nozzle distributor plate, through which air is fed into the chamber with a velocity of between 1 and 2.5 m/s (http://www. esru. strath. ac. uk/EandE/Web_sites/06-07/Biomass/HTML/ combustion_technology. htm). The bed normally has a temperature of between 800 and 900 °C and the sand accounts for about 98% of the mixture, with the fuel then making up a small fraction of the fuel and bed material. BFBs have two main advantages in terms of fuel size and type over more traditional fixed-bed systems. First, they can cope with fuel of varying particle size and moisture content with little problem, and second, they can burn mixtures of different fuel types such as wood and straw. BFBs are only a practical option with larger plants with a nominal boiler capacity greater than 10 MWth.