Characterization and densification of carbonized lignocellulosic biomass

This thesis focuses on developing two main areas: characterization and densification of carbonized lignocellulosic biomass. \n \nThermally treated biomass undergoes changes that enrich the content of carbon in the remaining solid fraction. The carbon content is correlated to the temperature and residence time of the treatment and affects the properties of the material as a fuel e.g. gross calorific value. Near infrared (NIR) spectroscopy was used to predict a wide range of variables from forest- and agro-based biomass thermally treated at 240 to 850 ˚C. The result showed that NIR provided excellent predictions e.g. for energy, carbon, oxygen and hydrogen contents. \n \nThe changes of the biomass properties after thermal treatment, such as torrefaction, change also the pelletizing properties. A parametric study was conducted at bench scale in a single pellet press tool where four parameters were examined with respect to pellet quality responses. The study showed a narrow process window for pelletizing at around 5% moisture content. Further pelletizing studies in pilot scale demonstrated that higher moisture contents were needed for satisfying pellet quality. This indicated that there is a discrepancy between the material’s moisture content before pelletizing and at the actual moment of feed layer formation and pelletizing. By drying both torrefied and untreated material it was shown that torrefied materials dried at a significantly higher rate. Thus, observed uneven pellet production caused by feed layer breakage was related to the drying rate due to heat from friction in the pellet press channels. This was demonstrated by developing two methods for cooling the pelletizing process: one with direct cooling by water injection and one with indirect cooling by coils in the die, and hereby reduce drying and keep the moisture content at a level where pelletizing was possible. This showed that cooling of the pelletizing process can be beneficial for the pellet quality. \n \nThe overall result for successful densification of thermally treated lignocellulosic biomass into a standardized commodity with high energy and bulk density stresses the need: (1) to find tools that characterize biomass facilitating suitable settings in the densification step; (2) to apply new innovative steps in sub-processes like cooling of the feed layer; and, finally, (3) to find matching combinations of torrefaction and pelletization.