An experimental study of the transformation of phosphorus additives during biomass pyrolysis

• Transformation of P additives in pine wood and wheat straw pyrolysis studied. • Char yield from pine wood pyrolysis increased with NH 4 H 2 PO 4 and Ca(H 2 PO 4 ) 2 addition. • P from NH 4 H 2 PO 4 distributed homogenously on the char as H 3 PO 4 and H 4 P 2 O 7 . • P additives reduced char oxidation reactivity. The transformation of three phosphorus additives, NH 4 H 2 PO 4 , Ca(H 2 PO 4 ) 2 , and NaH 2 PO 4 , and their effect on the yield and properties of char during the pyrolysis of pine wood and wheat straw were investigated experimentally. The biomasses were physically mixed with the additives at a PO 4 3- to biomass ratio of 5 wt%, and pyrolyzed in a lab-scale fixed-bed reactor at 550 °C. The collected char was examined for its yield and characterized using ICP-OES, SEM-EDS, XRD, solid-state 31 P NMR, and TGA. The addition of NH 4 H 2 PO 4 and Ca(H 2 PO 4 ) 2 increased char yield in pine wood pyrolysis, while NaH 2 PO 4 had no effect. The phosphorus additives barely influenced char yield in wheat straw pyrolysis. ICP-OES analysis revealed that the ash-forming elements, including phosphorus, potassium, calcium, and sodium, all remained in the char. SEM-EDS analysis showed that the phosphorus from NH 4 H 2 PO 4 distributed on the char homogenously. XRD analysis indicated that the phosphorus from NH 4 H 2 PO 4 remained amorphous in pine wood char, whereas in wheat straw char, it reacted with potassium species to form KPO 3 . Both Ca(H 2 PO 4 ) 2 and NaH 2 PO 4 dehydrated and reacted with potassium species in pine wood char and wheat straw char to form KPO 3 . Solid-state 31 P NMR analysis indicated that the phosphorus from NH 4 H 2 PO 4 existed as H 3 PO 4 and H 4 P 2 O 7 in the char, while Ca(H 2 PO 4 ) 2 converted to Ca 3 (PO 4 ) 2 , Ca 2 P 2 O 7 , and Ca(PO 3 ) 2 . TGA experiments revealed that the phosphorus additives reduced the char oxidation reactivity, with the effect being most pronounced for NH 4 H 2 PO 4 , followed by Ca(H 2 PO 4 ) 2 and then NaH 2 PO 4 .