The state of the art in woody biomass comminution and sorting in Northern Europe

This review provides a survey of the literature on methods for comminuting and sorting woody biomass. The reported methods were classified according to their basic operating principles: methods of comminution were separated based on whether they use sharp, semi-sharp or blunt tools; methods of discrimination were classified according to the parameters they use. The identified methods of comminution differ significantly in terms of productivity and energy use. The differences between individual biocomponents with respect to the parameters used for discrimination (such as solid density, or particle size and shape) are often rather small and species dependent, which makes it difficult to efficiently sort woody biomass from mixed species. Other parameters (e.g. optical and near-infrared properties) can be used as inputs for active sorting on larger scales. Several methods for tree processing and chip sorting that were originally developed for pulp-chip production now see little use because they are not considered cost-effective. However, the economic importance of fuel products has increased substantially in recent years. Fuel can be produced using material with a higher bark content than is tolerated in pulp chips, so it may be worth re-evaluating some of the methods that were previously discarded. Examples include tree-processing machines such as drums and cradles for partial foliage removal, processes such as delimbing and debarking (which facilitate downstream sorting of the comminuted material), and screens and wind classifiers for processes such as separating fines (often ash-rich) from comminuted material. The available data on the performance, required investment, and operating costs for these methods are incomplete and often not comparable. This issue must be addressed to facilitate the development of optimized fuel supply chains.