Milling Characteristics of Coal and Torrefied Biomass Blends in a Roller Mill

Abstract

CO2 emissions from coal power generation could be reduced by maximizing the amount of torrefied biomass that can be used in pulverized-coal-fired power plants. In this study, to evaluate the milling performance of coal and torrefied wood pellets (TPs), various blend ratios were tested using a bench-scale roller mill. Neutral sugar analysis was performed to evaluate the biomass-derived part of the milled products. Under the test conditions, mill power consumption and differential pressure increased with the TP content. As the TP content increased, the particle size of the milled products also increased. Furthermore, the biomass-derived neutral sugar content and the xylose/glucose (X/G) ratio were higher in the larger particles of the milled product and in the samples collected inside the roller mill than those in the input feedstock. The biomass-derived part with the highest X/G ratio accumulated inside the roller mill, which is probably why the mill power and differential pressure increased with the TP content. The TP, with poor grindability, was discharged from the mill with a larger particle size than that of coal. Although torrefaction treatment pyrolyzed the biomass, the degree of torrefaction can vary within the pellets depending on the torrefaction conditions. To ensure stable operation of the roller mill and an effective reduction in the size of the coal–TP blend, the selection and use of uniformly and sufficiently torrefied wood pellets are important.