The composition, energy, and carbon stability characteristics of biochars derived from thermo-conversion of biomass in air-limitation, CO 2 , and N 2 at different temperatures.

This study systemically investigated the characteristics of biochars derived from thermo-conversion of pine sawdust and wheat straw in air-limitation, CO ₂ , and N ₂ atmospheres at the temperatures of 300-750 °C. Meanwhile, their energy and C stability parameters were also evaluated here. The results showed that biochar produced in air-limitation had less yield, fixed C and bulk C, as well as more volatile matter and inorganic elements than that produced in CO ₂ and N ₂ . Biochars derived from thermo-conversion of pine sawdust in CO ₂ and N ₂ at 450-750 °C had the greatest energy densification ratios (EDR) (range: 1.40-1.61), because pine sawdust contained more lignin than wheat straw, and the thermo-conversion of lignin in N ₂ and CO ₂ at 450-750 °C benefited for the formation of fixed C. Recalcitrance potential (R ₅₀ ) results showed that the biochars produced in CO ₂ and N ₂ at 600-750 °C had the highest carbon stability (R ₅₀ : 0.54-0.64) for given biomass, owing to the thermo-conversion of biomass in CO ₂ and N ₂ at 600-750 °C preferring to form the organic C with high aromaticity and low polarity. Nonetheless, thermo-conversion of biomass in CO ₂ and N ₂ at 300 °C presented the greatest C sequestration potential, owing to high biochar yields under these conditions. Generally, the temperature-variability for the composition, EDR, and C sequestration potential followed the order: air-limitation > CO ₂  > N ₂ , whereas carbon stability presented an opposite order. Our results contributed to selecting the appropriate atmosphere to optimize the properties and performances of biochars.
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