Phosphorous in Ash From Co-Combustion of Municipal Sewage Sludge With Wood in a CFB Boiler: A Comparison of Experimental Data With Predictions by a Thermodynamic Equilibrium Model

Combustion tests have been carried out in a circulating fluidised bed (CFB) boiler co-firing wood and municipal, digested and mechanical dewatered sewage sludge from two Swedish wastewater treatment plants, using either iron sulphate or aluminum sulphate as precipitating chemicals. Experimental data on the concentration of various phosphorous compounds in the ashes are compared with theoretical equilibrium predictions. A clear relationship is not found between the employed precipitation chemical and the formation of phosphorous compounds in the ash. Hematite (Fe₂O₃(s)) is predicted to be the stable iron compound, and even in the sludge precipitated by iron sulphate, the aluminium content is sufficient to form aluminium phosphate (AlPO₄(s)) in similar amounts as in the sludge precipitated by aluminium sulphate. Lime addition to the bed gives a decreased formation of AlPO₄(s) in favour of calcium orthophosphate (Ca₃(PO₄)₂(s)), which interferes with the sulphur capture efficiency of lime.