Selectively retaining nutrients in biochar in magnesium added two-zone staged copyrolysis of blue algae and corn gluten wastes

The disposal of blue algae (BA) and corn gluten (CG) wastes and the simultaneous recovery of abundant phosphorus (P) and nitrogen (N) by pyrolysis to obtain biochars with high fertility is a promising strategy. However, pyrolysis of BA or CG alone by a conventional reactor cannot reach the target. Herein, we propose a novel MgO-enhanced N and P recovery method by designing a two-zone staged pyrolysis reactor to highly efficiently recover N and P with easily available plant forms in BA and CG. The results show that a 94.58% total phosphorus (TP) retention rate was achieved by means of the special two-zone staged pyrolysis method, in which the effective P (Mg₂PO₄(OH) and R-NH-P) accounted for 52.9% of TP, while the total nitrogen (TN) reached 4.1 wt%. In this process, stable P was formed first at 400 °C to avoid rapid volatilization and then to form hydroxyl P at 800 °C. Meanwhile, Mg-BA char in the lower zone can efficiently absorb N-containing gas generated by the upper CG, forming dispersible N. This work is of great significance for improving the green utilization value of P and N in BA and CG.