Analysis of the Mechanical Properties of Biomass Pellets during Pelletization and Post-Pelletization Processes
Abdullah Sadeq, M.Sc.
Motivation
The rising global energy demand necessitates sustainable alternatives like biomass, which can be converted to syngas via thermochemical processes such as pyrolysis and gasification. In pyrolysis, biomass decomposes in an oxygen-free environment, producing volatiles and char. The char can then undergo gasification to generate more syngas, a key raw material in the chemical industry. Structural changes during pyrolysis vary based on whether the biomass is raw or densified into pellets, with factors like heating rate and residence time affecting the integrity of pellet char. While general structural changes are studied, detailed insights into radial porosity distribution are crucial for understanding heat and mass transfer effects.
Methodology
This study focuses on establishing a clear correlation between the properties of the wood pellets and the characteristics of the resulting pellet char after pyrolysis. To investigate this, spruce wood pellets of varying densities (i.e., different qualities) were produced using a flat die press with dies of press channel diameter-to-length ratios of 1:3, 1:4, and 1:5 (figure 1, left). The respective wood pellets were then pyrolyzed, and changes in the microstructure and the mechanical stability were quantified by micro-computed tomography (µCT) analysis.
Results
The results indicate that both the pellet production process and pyrolysis conditions significantly influence the structural characteristics and stability of the pellets and their corresponding pellet chars. Wood pellets produced with the shortest press channel exhibited the highest partial porosity, which contributed to the lowest mechanical strength. In contrast, wood pellets produced with the longest press channel exhibited a smooth and glossy surface with minimal cracks, reduced porosity, and higher mechanical strength. A better pellet quality can be attributed to a higher level of lignin plasticization at high die temperatures. The resulting compacted structure of the pellets, as assessed by µCT, also showed that a longer press channel resulted in a more uniform porosity over the pellet radius (figure, top right).
After pyrolysis, major changes were observed in the pellet char structure. Pellet chars exhibited structural deformation, including a pine cone-like morphology and a greater number of large cavities (figure, bottom right). These changes were due to the high gas flow through the pellet bulk and intense heat transfer during the fluidized bed pyrolysis process. Additionally, the central position of large cavities is assumed to be promoted by the presence of locally smaller particles within the wood pellets. Interestingly, high-quality wood pellets resulted in pellet chars with consistently higher mechanical strength compared to low-quality wood pellets. This could be attributed to their initial lower porosity and stronger structural integrity, which contributed to better preservation of mechanical properties during pyrolysis. Moreover, it was demonstrated that all pellet chars consisted mainly of fixed carbons, which led to brittle behavior under compression.
Funding
Deutsche Forschungsgemeinschaft (DFG), DFG Project GRK 2462 PintPFS
