Potential of stepwise pyrolysis for on-site treatment of agro-residues and enrichment of value-added chemicals.

This paper compares the yield and composition of bio-oil derived through stepwise and continuous pyrolysis of agro-residues. The temperature steps were selected using thermogravimetric analysis. Groundnut shell, empty fruit bunch pellets, and rice straw were pyrolyzed at 305 °C in the first step and 600 °C in the second step and pinewood chips were pyrolyzed at 340 °C and 600 °C. The cumulated yields of bio-oil were lower in both continuous and stepwise pyrolysis for agro-residues in comparison with woody biomass, with a corresponding increase in biochar and pyrolysis gas yields. Approximately 50% of the pinewood and 31-40% of the agro-residues were converted to bio-oil through both methods. This trend is explained by the differing biochemical composition of biomasses, and concentrations of inorganic components. Bio-oil fractions were separated into water-soluble and water-insoluble compounds before characterization. Bio-oil obtained in the first step was richer in organic acids, anhydrosugars, and carbonyl compounds, while the later fraction of bio-oil was richer in unbranched phenolic compounds. Thus, compounds known to promote bio-oil aging were separated into different fractions. Further, stepwise pyrolysis proved to be most effective for groundnut shells in isolating chemicals with a minimum loss of yield compared to continuous pyrolysis. A preliminary economic assessment of bio-oil showed that the value of bio-oil for crop residues was improved by 2-2.5 times during stepwise heating while for pinewood it reduced by half. This study provides a good starting point for further research in optimizing the temperature steps for pyrolysis and separation of chemicals from bio-oil.