New insights into the carbon neutrality of microalgae from culture to utilization: A critical review on the algae-based solid biofuels

Microalgae-based solid biofuels offer a promising option for efficient and low-cost utilizations of microalgae. This work investigates the key aspects of microalgae-based solid fuels, from production to combustion, based on the perspective of microalgae's complete carbon cycle, from culture to utilization. Additionally, recent advances in research on the physical properties, energy consumption during production, and combustion characteristics of microalgae-based solid fuels were evaluated. The results show that microalgae is viewed as a promising feedstock for the production of solid fuels and constitutes a widely followed topic of research. Nowadays, co-pelletization and co-combustion of microalgae with other biomass is considered the most competitive way to utilize microalgae-based solid fuels. The addition of about 10–30% microalgae can reduce the energy consumption of the original pelletizing process by about 24–28%, while the density, durability and calorific value of the solid fuel can be significantly increased. More importantly, thanks to the high lipid content of microalgae, its addition can effectively improve the combustion performance of coal, sludge and typical biomass fuels, as well as positively influence the combustion process. Finally, the opportunities and challenges for microalgae-based solid biomass feedstock was discussed, including the impact of species diversity on algae-based solid fuels, an exploration of further industrial applications, and possible integration pathways with the existing microalgae industry chain.