Hydrothermal carbonization of sawmill shavings for fuel pellet production: A robust strategy for sustainable energy

Hydrothermal carbonization (HTC) is a pre-treatment technology enhancing fuel characteristics of wood wastes. However, HTC aqueous phase (HTC-AP) is the main source of environmental pollution associated with this technology. In this study, a sequential HTC and pelletization was performed to convert Spruce wastes (SP) into fuel pellets. For the first time, HTC-AP was utilized in pellet formulation as a moisture source. HTC at 260 °C for 1 h obtained a SP hydrochar with fixed carbon and higher heating value of 45.2 wt% and 28.7 MJ/kg, respectively. Carboxymethyl cellulose (CMC) was used as a binder for SP hydrochar pellets. Effects of HTC-AP and CMC mass fraction on the relaxed density, compressive strength, and durability were investigated by a 3-level full factorial design. SP7 pellet (15.0 wt% CMC and 17.5 wt% HTC-AP) showed the relaxed density of 1042.7 kg/m3, durability of 99.5%, higher heating value of 24.6 MJ/kg, with the highest compressive strength of 6.0 MPa. The quality of SP7 pellet was beyond the international standards for ENplus A1 (non-industrial) and I3 (industrial) pellets set by European Pellet Council and International Organization for standardization in terms of moisture content, durability, density, and calorific value. Additionally, Micro-computed tomography (Micro-CT) was employed for visualization of porosity and internal structure of SP pellets before and after moisture exposure. SP7 pellet exhibited the lowest moisture adsorption rate constant (0.0008 min−1) and porosity (1.7%) compared with other formulations. Quantification of micro-structural properties of SP pellets also confirmed the contribution of micron-scale pores to the moisture-induced crack formation and growth as well as porosity expansion, deteriorating pellet quality during a long-term storage of fuel pellets. These findings suggested the recovery of HTC-AP in SP hydrochar pellet formulation to achieve circular economy. [Display omitted] •Sequential HTC and pelletization was used for development of coal-like solid fuels.•HTC aqueous phase was used as the moisture source in pellet formulation.•Micro-structural properties of pellets were quantified by Micro-CT.•Micron-scale pores contributed to moisture-induced crack formation and propagation.•SP7 pellet reached the HHV of Bituminous coal with a substantially lower ash content.