Minimal thermal treatment of cellulosic biomass to produce torrefied pellets

The production of solid biofuels from torrefied biomass holds significant potential for renewable energy applications. Pelletizing torrefied biomass offers advantages over its ground or loose form, though it often requires a binding agent to enhance pellet durability, thus increasing costs and energy use. This research aimed to produce durable pellets from torrefied loblolly pine (Pinus taeda) residues without using external binders or adjusting pelletization parameters, leveraging the benefits of torrefaction such as greater heating value and hydrophobicity. Experiments that involve various combinations of mild torrefaction temperature (230°C and 250°C) and shorter residence time (10, 15, and 30 minutes) across four particle size ranges (as-received, <0.42mm, 0.42-1mm, and >1mm) were conducted to determine the effects of these factors on the physicochemical properties of the biomass. The torrefied materials were then subjected to pelletization, and the influence of torrefaction on the mechanical durability and fuel characteristics of pellets was examined. Thermogravimetric analysis of the pyrolysis and combustion behaviour of torrefied pellets indicated the improvement of fuel characteristics in terms of a higher comprehensive pyrolysis index and greater thermal stability with reduced risk of spontaneous combustion. The Response Surface Methodology (RSM) was employed for optimizing these factors to produce durable pellets from biomass, with an aim to minimize dry matter loss while maximizing the benefits of torrefaction. Results of RSM along with a predicted desirability of 0.674 indicated that the optimal treatment conditions are 250°C torrefaction temperature and 10 min residence time regardless of the particle size range. Under these conditions, the pellets can be produced with less mass loss at 14.5 wt% and pellet quality indicators which include single pellet durability of 63.4% that corresponds to tumbler durability of 87.1%, HHV (higher heating value) of 20.5 MJ/kg db, and EMC (equilibrium moisture content) of 12.5%. Overall, the study demonstrated that mild torrefaction temperature and shorter residence time preserved wood polymers, enhancing the physicochemical properties of the pellets, and enabling pellets to be produced with improved moisture resistance and fuel characteristics compared to untreated pellets. Besides, shorter torrefaction time would not only reduce energy consumption and operational costs but also minimize mass loss.