The transformation and release of phosphorus during conversion of chars produced from NH4H2PO4-doped biomass

Phosphorus (P) additives, such as NH 4 H 2 PO 4 , improve bio-oil quality during biomass pyrolysis, but the fate of the retained phosphorus during char conversion remains unclear. This study investigates the P transformation and release during secondary pyrolysis (800–1100 °C) and combustion (5% O 2 , 550–1100 °C) of chars prepared from pine wood and wheat straw doped with NH 4 H 2 PO 4 and pyrolyzed at 550 °C. Analytical characterization (XPS, XRD, ICP-OES, and SEM-EDS) revealed that P release increased significantly with temperature, reaching up to 53% during secondary pyrolysis and 79% during combustion. XPS identified C-O-P and C-P bonds in all secondary-pyrolyzed chars. XRD revealed that P remained amorphous in the secondary-pyrolyzed pine wood-NH 4 H 2 PO 4 chars, whereas KPO 3 formed in the wheat straw-NH 4 H 2 PO 4 chars. Ca 3 (PO 4 ) 2 , Ca(PO 3 ) 2 , and KCaPO 4 were more abundant in the wheat straw-NH 4 H 2 PO 4 ashes than in the pine wood-NH 4 H 2 PO 4 ashes. We identified four primary mechanisms for P release: (1) P 2 O 5 vaporization from phosphoric acid dehydration; (2) C-O-P and C-P bond cleavage during char oxidation; (3) elemental phosphorus release through carbothermic reduction (800–1100 °C); (4) KPO 3 vaporization (950–1100 °C). These findings provide new insights into P transformation mechanisms and shed light on the feasibility of process improvement towards better P recovery.