Interaction mechanism of typical phosphates with potassium salts and the inhibitory effect on K release during co-combustion of sludge and biomass

Co-combusting biomass with sewage sludge can effectively inhibit the release of alkali metals. In addition to calcium phosphates, AlPO 4 and FePO 4 are typical phosphates in fresh sludge, resulting from phosphorus removal process compliant discharge. However, the inhibitory effect of AlPO 4 and FePO 4 on potassium release have rarely been studied. This oversight may stem from the absence of AlPO 4 and FePO 4 in high-temperature ash, which instead indicates their preferred reactivity. Thus, we proposed the hypothesis regarding whether the fixation effect of AlPO 4 and FePO 4 on potassium occurs earlier than that of calcium phosphate. This research investigated the interactions and inhibitory effects of these phosphates on potassium release during the co-combustion. In this simplified system using model compounds, the conversion of volatile potassium salts was quantitively assessed by measuring the content of produced water-soluble K-phosphates. The interaction mechanism has been discussed with multiple characterization methods. The results demonstrate a significant inhibitory effect of phosphates on KOH, KNO 3 and K 2 CO 3 , with the effectiveness ranking as follows: AlPO 4  > FePO 4  > Ca 3 (PO 4 ) 2 . Over 80 % of potassium remained as soluble K-phosphates in samples treated with AlPO 4 or FePO 4 at 1000 ℃. In contrast, none of the three additives exhibited a significant inhibitory effect on KCl and K 2 SO 4 . However, the priority of the reactions converting KOH, KNO 3 and K 2 CO 3 into K-phosphates by using AlPO 4 and FePO 4 , along with the stability of the produced phosphates, suggests a promising approach to enhance the formation of more potassium phosphate while reducing KCl—the most problematic volatile component—thereby effectively mitigating potassium release.