Abstract
Torrefaction used in combination with pelletization is a promising technology to upgrade solid biofuels and has been demonstrated worldwide. In comparison with normal biomass pellets, which disintegrate under wet conditions, one of the advantages of torrefied biomass pellets is better water resistance. An understanding of the differences in water proof properties for torrefied biomass pellets by different production schemes can promote their further application. In the communication, various torrefied pellets were exposed to indoor and outdoor conditions, and changes in moisture content and diameter were examined. Two production schemes for the torrefied pellets were used for comparison: the torrefaction of wood chips followed by pelletization (pre-torrefaction) and the pelletization of wood chips followed by torrefaction (post-torrefaction). It was found that the post-torrefied pellets had much lower moisture levels than the pre-torrefied pellets in both indoor and outdoor tests. In the outdoor test with no-roof condition, the rate of increase in moisture content for the pre-torrefied pellets was more than double that for the post-torrefied pellets, and the post-torrefied pellets exhibited almost no diameter change. The results on the superior water resistance of post-torrefied pellets were nearly consistent with those reported in previous literature. Torrefied pellets have been considered for industrial use, such as in co-combustion and gasification on a large scale. Taking advantage of the different water resistances, torrefied pellets could also be used by personal and community consumers on a small scale for long-term indoor and outdoor storages as advanced solid biofuels with high waterproof performance, energy density, and lower biodegradation.
Highlights
Biomass is an organic renewable energy resource with properties distinct from solar and wind energy
Two methods were used in Energies 2021, 14, 1618 the production process of the torrefied pellets: torrefaction of wood chips followed by pelletization and pelletization of wood chips followed by torrefaction
The post-torrefied pellets, prevealed that,that, inthat, comparison withwith normal pellets, the torrefied pellets had had lower moisture vealed vealed in in comparison comparison with normal normal pellets, pellets, thethe torrefied torrefied pellets pellets had lower lower moisture moisture pared pelletization followed torrefaction, had a lower much lower moisture content than pared byby pelletization followed byby torrefaction, a much lower moisture content pared by pelletization followed by torrefaction, had had a much moisture content thanthan wood-based bioenergy source in residential areas because of their suitability for longterm storage, high energy density, and small size, helping to establish a resilient society
Summary
Biomass is an organic renewable energy resource with properties distinct from solar and wind energy. Fossil fuels are organic resources originating from biomass; they are nonrenewable. One disadvantage of using biomass for energy is its lower energy density compared with fossil fuels, resulting in lower thermal output per volume during combustion and inefficient storage and transportation. To increase its energy density, the densification of biomass into cylindrical pellets using minimizing variations in density and moisture content has been commercialized [1]. The benefits of such pellets have been reported with regard to upgrading energy density, and for combustion [2,3,4]
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