Migration of nitrogen and phosphorus during supercritical water gasification of sewage sludge: Effect of alkaline additives

The effect of alkaline additives (KOH, NaOH, Ca(OH)2, NaCO3 and K2CO3) on syngas yield, phosphorus (P) and nitrogen (N) migration during supercritical water gasification (SCWG) of sewage sludge were investigated. The alkaline additives elevated the syngas yield by affecting the steam reforming reaction, water-gas shift reaction and methanation reaction. In addition, alkaline additives enriched P and N into the aqueous phase by promoting the dissolution of phosphide (Al-P, Ca-P, and Fe-P) and the decomposition of nitrogenous organic matter and hydrolysis of nitrile in sludge. The results showed that K2CO3 exhibited the best catalytic effect in these five alkaline additives. In the presence of 0.5 g K2CO3 (5%), the syngas yield was up to 18.2 mol/kg, as well as, 78.2% of P and 80.2% of N were dissolved in the aqueous phase. Moreover, the effects of different K2CO3 additions (5%- 11%) on the syngas yield and the migration of N and P were discussed.