Complementary effects of torrefaction and pelletization for the production of fuel pellets from agricultural residues: A comparative study

Torrefaction is an effective technique to produce high quality coal-like material, which can be used as a sub-stitute of coal in combustion or gasification processes. Combination of torrefaction and pelletization can be a promising technique for production of fuel pellet for long term storage and overseas transportation without deterioration of its quality. In the current study, two production pathways (I: co-pelletization of torrefied biomass, II: torrefaction of biomass pellet) have been compared in terms of fuel pellet quality (physical, me-chanical, thermal, and hydrophobicity). Characteristics of pellets produced via co-densification of canola hull, oat hull with mustard meal were investigated. Although up to 75 wt% of oat hull/canola hull was successfully pelletized with mustard meal, 50:50 ratio of oat hull/ canola hull: mustard meal was optimum ratio based on pellet properties. Co-pelletization of torrefied biomass with raw biomass makes competitive quality pellet without use of external additives, high temperature and high pressure. Additionally, the higher heating value, energy density, hydrophobicity increased, and atomic ratios (H/C and O/C) decreased after torrefaction in both cases. Synchrotron-based computed tomography showed that porosity and diameter of cavity enlarged signifi-cantly for torrefied pellet than that for pellet produced from torrefied biomass. However, the moisture uptake rate for torrefied pellet is lower than that for torrefied biomass pellet. Higher heating values of pellet were more or less similar for pellets obtained from both pathways. In terms of energy consumption throughout the whole procedure, scheme II is slightly more intense as it requires more energy during grinding while scheme I requires more energy for pelleting. Therefore, co-pelletization of torrefied biomass was more effective considering overall energy consumption and fuel pellet product quality.

Automated summary

Torrefaction is an effective technique for producing high quality coal-like material. This study compared two production pathways for fuel pellets and found that co-pelletization of torrefied biomass resulted in competitive quality pellets without the need for external additives, high temperature, and high pressure.