A. Fundamentals

Reduction in physical size is often required before biomass is used as a fuel or feedstock. Size-reduction techniques are employed to prepare biomass for direct fuel use, fabrication into fuel pellets, cubes, and briquettes, or conversion. Smaller particles and pieces of biomass reduce its storage volume, facilitate handling of the material in the solid state and transport of the material as a slurry or pneumatically, and sometimes permit ready separation of components such as bark and whitewood. The size of the pieces or particles can be critical when drying is used because the exposed surface area, which is a function of physical size, can determine drying time and the methods and conditions needed to remove moisture. There are a few exceptions where size reduction is not needed, such as in whole-tree burning.

The physical dimensions of the feedstock are also related to the conversion method that is used. Particle size should satisfy the requirements of supplying feedstock to the conversion reactor and of the conversion process itself. For combustion systems, the combustion chamber and heat exchanger designs, the operating conditions, and the methods of delivering solid fuel and removing the ash determine the optimum size characteristics of the fuel. For thermal gasification and liquefaction processes, particle size and size distribution can influence the rate of conversion, the operating conditions of the process, and product yields and distributions. Biological processes are also affected by the physical size of the feedstock. In general, the smaller the substrate particles, the higher the reaction rate because more surface area is exposed to the enzymes and microorganisms that promote the process.

If the particle size of the biomass fuel or feedstock is not predetermined by its history, as is the case for sawdusts, nutshells, and a few other waste biomass materials, size reduction is usually carried out with one or more units that make up the “front end” of the total processing system. Many different kinds of machines are employed. Generally, the size of the feed is reduced by grinding, cutting, or impact mechanisms. Not all of the designs are suitable for biomass energy applications because the equipment is customized for certain uses or the cost of size reduction is excessive. Agricultural crops and woody biomass are also usually processed by different types of machines. A brief review of the basic types of machines that are or have been used for biomass follows to illustrate the variety of size-reduction equipment and their biomass applications.