Influence of tree species and machine settings on chip quality and specific energy consumption of a stationary drum chipper

Abstract

Chipper fine tuning, i. e. the selection of suitable feedstocks and machine settings for comminution, might help to optimize wood chip production in terms of chip quality and energy efficiency. A total of 46 wood chip samples were produced with an electrically powered stationary drum chipper. Feedstocks derived from small diameter stem wood (Ø < 12 cm) of six different coniferous and broad-leaf tree species. Chipper settings varied in knife sharpness, screen mesh size and infeed-roller speed. Wood chip properties were determined following ISO standards for solid biofuels. Furthermore, image analysis allowed for highly detailed information on particle size and particle shape. Classification of wood chips followed ISO 17225-4. Throughput rate and energy consumption during chipping were related to volume and dry matter of the bulk material. Results indicate that chipping with narrow chipper screens reduced the share of coarse particles while chipping with blunt knives significantly increased the share of fine particles, the maximal particle length, the length to width ratio of particles and their particle shape factor. Specific electrical energy consumption during chipping strongly increased from approx. 4.2 kWh t−1 (d. b.) up to 12.8 kWh t−1 (d. b.) when blunt knives were applied. Overall, approx. 96% of all samples could be classified as “graded wood chips” according to ISO 17225-4 but only chipping with sharp knives and a narrow chipper screen (40 × 40 mm) allowed for the consequent production of high-quality wood chips of the particle size class P31s.