Microalgal torrefaction is conducive to the utilization of biomass waste to achieve carbon neutrality. However, the use of solid biofuels usually faces the challenge of the instability of fuel properties due to long-term storage and transportation. This research investigates the variation and transformation of fuel properties in a humid environment. The main focus is the growth and reproduction of microorganisms on the surface of microalgal biomass/biochar, and therefore, to explore the mechanism of the influence of microorganisms on the performance of fuel and summarize the impact of humidity on the HHV, elemental composition, and hydrophobicity of microalgal solid biofuels. The results indicate that humidity treatment weakened the HHV, elemental C percentage, and contact angle of the microalgal biomass/biochar, leading to poor fuel properties. The HHV and contact angle of microalgal biomass/biochar declined by 3.40–20.60% and 15.21–98.11%, respectively. Humidity benefited the growth of microorganisms, which can consume the organic components in microalgal biomass, especially protein. The main microbial species that decomposed and metabolized the microalgal biomass were Proteobacteria, Bacteroidota, Firmicutes, and Actinobacterita at the phylum level, and Alcaligenes, Pseudomonas, Methylophaga, Halomonas, and Psychrobacter at the genus level. This research uncovers the variations of fuel properties of torrefied biomass affected by microorganisms.
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