Improving bioenergy feedstock quality of high moisture short rotation woody crops using air classification

Abstract

Short rotation woody crops have many advantages as perennial bioenergy feedstocks, including high biomass yields, high carbohydrate and low ash contents, and marginal land utilization. Through short rotation coppicing management, these biomass resources can be harvested year round. The challenge of year round harvesting is feedstock quality variability due to leaf content during periods of non-senescence. The low quality leaf fraction results in higher ash and moisture contents and lower carbohydrate content. Mechanical techniques, such as air classification, provide an economically feasible process to separate heterogeneous biomass samples based on particle density, size, and shape. In this work high moisture (>45%) hybrid poplar and shrub willow short rotation crops were air classified using a series of fan speeds for anatomical fractionation of the material. Air classification using an air velocity of ∼4.7 m/s removed a majority of the leaf material while retaining 88% and 87% of the hybrid poplar and shrub willow, respectively. At this velocity, the ash content was reduced from 2.34% to 1.67% for hybrid poplar and 2.60% to 2.14% for shrub willow. Concurrently, the carbohydrate content increased from 56.32% to 60.62% and from 54.03% to 55.99% for these same materials. As drying is a cost intensive step for processing high moisture biomass materials, the cost benefits (∼$3/Mg dry biomass) for removing low quality, high moisture materials prior to drying were also demonstrated.©Battelle Energy Alliance, LLC, contract manager for Idaho National Laboratory.