Influence of Phosphorus on Potassium during Co-Combustion of Wheat Straw with Municipal Sewage Sludge

Paper from the AFRC 2013 conference titled Influence of Phosphorus on Potassium during Co-Combustion of Wheat Straw with Municipal Sewage Sludge by Qiangqiang Ren

Alkali metal-related problems are the major barrier for large scale utilization of biomass, and municipal sewage sludge which is rich in sulfur, nitrogen and phosphorus is a potential choice to inhibit the alkali metal related problems during biomass combustion. The effect of phosphorus on the behavior of potassium and the reaction mechanism between potassium and phosphorus during co-combustion of wheat straw and municipal sewage sludge 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 X-ray powder diffraction (XRD). The results indicate that the addition of sewage sludge to wheat straw enhances potassium retention in ash due to the formation of high melting point compounds such as potassium aluminosilicates (KAlSi2O6 and KAlSi3O8) and alkali phosphates Ca9MgK(PO4)7. Calcium phosphate (Ca3(PO4)2) is selected as a model compound of phosphorus in municipal sewage sludge ash to study the phosphorus chemistry. The addition of Ca3(PO4)2 is effective in preventing sintering and fusion of wheat straw ash. Ca3(PO4)2 not only dilutes the ash but also absorbs and reacts with potassium compounds to form Ca10K(PO4)7. Further reaction mechanism of phosphorus and potassium is investigated through the reaction between Ca3(PO4)2 and KCl, which produces Ca10K(PO4)7 and Ca5(PO4)3Cl at about 800ºC and higher temperatures, revealing the mechanism of potassium phosphate formation during the co-combustion of sewage sludge and wheat straw.