Effect of Phosphorus on the Behavior of Potassium during the Co-combustion of Wheat Straw with Municipal Sewage Sludge

The effect of phosphorus on the behavior of potassium and the reaction mechanism between potassium and phosphorus during the co-combustion of wheat straw (WS) and municipal sewage sludge (MSS) are investigated in a bench-scale reactor. The elemental composition of ash is analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES), and the morphology and crystalline phase are analyzed by scanning electron microscopy (SEM), and powder X-ray diffraction (XRD). The results indicate that the addition of MSS to WS enhances potassium retention in ash due to the formation of high-melting-point compounds such as potassium aluminosilicates (e.g., KAlSi 2 O 6 and KAlSi 3 O 8 ) and alkali phosphates (e.g., Ca 9 MgK(PO 4 ) 7 ). Calcium phosphate (Ca 3 (PO 4 ) 2 ) is selected as a model phosphorus compound in municipal sewage sludge ash to study the phosphorus chemistry. The addition of Ca 3 (PO 4 ) 2 is effective in preventing sintering and fusion of wheat straw ash. Ca 3 (PO 4 ) 2 not only dilutes the ash but also absorbs and reacts with potassium compounds to form Ca 10 K(PO 4 ) 7 . Further reaction mechanism of the phosphorus and potassium is investigated through the reaction between Ca 3 (PO 4 ) 2 and KCl, which produces Ca 10 K(PO 4 ) 7 and Ca 5 (PO 4 ) 3 Cl at ∼800 °C and higher temperatures, revealing the mechanism of potassium phosphate formation during the co-combustion of MSS and WS.