Bio-oil stability through stepwise pyrolysis of groundnut shells: Role of chemical composition, alkali and alkaline earth metals, and storage conditions

Bio-oil obtained from crop residues is unstable because of multiple reactive oxygenated compounds and alkali and alkaline earth metals (AAEMs), which hinder its use as a chemical feedstock. In the current study, stepwise pyrolysis of groundnut shells was performed in three-step (160/200–320/340−600 °C), two-step (320/340−600 °C), and continuous (600 °C) heating regimes to obtain a more stable bio-oil. The stability of bio-oils was compared in terms of changes to their pH, water content, AAEM concentration, and chemical composition over a fixed period. During three-step pyrolysis, 94.85–97.38 % of chemicals (anhydrosugars, organic acids, aldehydes, and ketones) were concentrated in step 2. In contrast, for two-step pyrolysis, 93.14–94.78 % were concentrated in step 1. The AAEMs transferred from groundnut shells to bio-oil were 7.04–9.63 % for three-step, 6.68–11.26 % for two-step, and 11.87 % for continuous pyrolysis. An accelerated aging test (80 °C for 24 h) showed that two-step pyrolysis at 340 °C and 600 °C, produced the most stable bio-oil despite a higher concentration of AAEMs. Further improvement in this bio-oil's stability was explored by altering storage temperature and adding solvents to the bio-oil. The decrease in concentrations of anhydrosugars and phenolics was <1% after four weeks of storage at 4 °C with the addition of methanol. The obtained results contribute to implementing stepwise pyrolysis units for crop residue management.