The difficulty in reducing biomass to a small size compared to coal makes it a more difficult fuel to combust. It is impractical to reduce most biomass to sizes less than 3-6 mm (1/8 to 1/4 inch). Such particles are very large compared to the largest coal particles introduced into pc boilers.
|This particle represents the largest coal particle that will generally be consumed in a typical pulverized coal boiler (about 100 microns)|
|This particle represents a relatively small (1x3 mm) biomass particle on the same scale as the coal particle above. It, too, has to burnout within the boiler.|
However, significantly more biomass is consumed by devolatilization that is coal (typically 90% for biomass and 60% for coal). Biomass is also much less dense, which leads to more rapid burnout. Finally, biomass particles have slightly more residence time in the boiler because they rise through the boiler at somewhat less than the gas velocity. These effects combine to allow larger biomass particles to be consumed in the boiler than would be possible for coal.