Grindability of torrefied wood chips and wood pellets.

Abstract Torrefaction of biomass improves the grindability of raw biomass by the transformation from a fibrous structure to a more brittle and friable, coal-like material. Torrefied biomass is a more desirable feedstock for co-firing applications, especially in existing coal-fired power plants. Therefore, measuring the grindability and the specific energy consumption are critical for understanding the comminution behavior of torrefied biomass, and thus selecting appropriate milling equipment. In this study, the effect of torrefaction temperature on the specific grinding energy consumption and the grindability of torrefied wood pellets and wood chips was investigated and compared with that of coal. The applicability of three well-known grinding equations (Kick, Rittinger and Bond) was studied for the torrefied biomass using a knife mill. The specific grinding energy of both torrefied wood chips and pellets linearly decreased with increased torrefaction temperature over a range of 250–290 °C. Rittinger's model was the best-fit for both wood chips (R2 = 0.72–0.90) and wood pellets (R2 = 0.67–0.76). When the intercepts were considered, the Rittinger's and Bond's equations fitted well with the experimental data (R2 = 0.79–0.99) for all torrefied biomass. The grindability of torrefied biomass was measured using the Hardgrove Grindability Index (HGI) and the International Organization for Standardization (ISO) grindability index. A relationship between the Bond Work Index (BWI), based on the Bond's theory and HGI was developed for wood chips and wood pellets. Contrary to HGI index, BWI decreased with an increase in torrefaction temperature. The grindability parameters developed for torrefied biomass can be used for modeling and selecting suitable milling equipment. Highlights • Specific grinding energy required for torrefied wood chips and pellets were determined • Grinding energy decreased linearly with increased torrefaction temperature (250–290 °C) • Rittinger's equation was fitted the best with grinding data for all torrefied biomass. • HGI and BWI for torrefied biomass produced at 290 °C were comparable to that of coal. [ABSTRACT FROM AUTHOR]