Comprehensive comparison of microalgae-derived biochar from different feedstocks: A prospective study for future environmental applications

Biochar obtained from pyrolysis of biomass presents high potential for industrial applications owing to its beneficial physicochemical properties, such as high surface area. Microalgae are promising biomass feedstocks, with arising interest in the scientific community for fuel production (due to their high productivity and the absence of soil consumption), but the potential applications of biochar obtained from microalgae are poorly explored yet. This work aims at shedding light on the possible applications of biochar derived from the slow pyrolysis of three different microalgae species (Chlorella vulgaris, Spirulina sp. and Nannochloropsis sp.) and compare them with a lignocellulosic biomass feedstock (nutshells). The pyrolysis process was investigated through thermogravimetric analysis and the resulting biochar was characterized through elemental analysis, infrared spectroscopy and scanning electron microscopy. The characterization revealed the intrinsic presence of doping heteroatoms in microalgae-derived biochar (i.e., N and P, which present tenfold values in concentration compared with lignocellulosic biomass), as well as a higher presence of oxygen containing surface functional groups, suggesting their high potential for environmental applications. Finally, the low temperature (350 °C) slow pyrolysis of microalgae allows achieving noticeable yields of oil (up to 15% more than lignocellulosic biomass), a valuable fuel product for a more sustainable production process, avoiding waste of feedstock.