Size reduction is the processing of solid material to break it into smaller pieces. Size reduction processes relevant to CDR include chipping, grinding, and milling of biomass and crushing and pulverizing rock and charcoal. Generally, the energy for size reduction in rock-like substances is estimated to be proportional to either the diameter reduction that is achieved or the area increase that is achieved (and these two measures are NOT generally equal), known as Rittinger’s Law and Kick’s Law, respectively. (Rock Crushing Theory and Formula using Kick & Rittinger's Law)
Size reduction in biomass can be by shearing or splitting away pieces as with a chipper; or crushing or pounding as in hammermills. Wood size reduction is important in the pulp-and-paper industry, for making various engineered wood products (particle boards), and now for feeding pyrolysis and making fuel pellets. Pyrolysis has commonly been conducted with wood of 3mm particle size or less (Grinding Woody Biomass – Wood Energy)