Particle fragmentation of raw and torrefied biomass during combustion in a drop tube furnace

Particle fragmentation corresponds to the particle disintegration during combustion, yielding two or more particles of smaller size. Torrefaction consists in exposing biomass to an inert atmosphere between 260 °C and 300 °C. In such conditions, hemicellulose is partially decomposed allowing some low calorific volatiles to be released from the raw biomass, thus enhancing the biomass energetic potential. From this process, it is possible to obtain a new material whose properties range between biomass and coal. However, one of the torrefied biomass characteristics consists in its brittle nature, and, thus, more prone and easily breakable in milling processes. This study aims to understand if and how torrefaction promotes particle fragmentation during biomass combustion. Initially, pine shells, olive stones and wheat straw were torrefied at 280 °C. Subsequently, all biomass fuels, both raw and torrefied, were burnt in a drop tube furnace (DTF) at 1100 °C. The results reported include profiles of temperature, particle burnout and PM concentrations and size distributions measured along the DTF. In addition, particles of the raw biomass fuels and PM collected for all biomass fuels from two DTF axial positions were examined in a scanning electron microscope. The data reveal that (i) pine shells present the lowest PM concentrations due to its lower ash content and highest burnout values; (ii) particle fragmentation does not occur during the combustion of raw and torrefied olive stones; (iii) particle fragmentation occurs during the combustion of raw and torrefied pine shells and wheat straw, but torrefaction promotes particle fragmentation only in the case of the straw.