Chemical properties of fluidised bed boiler ash relevant to its use as a liming material and fertiliser

A by-product from the New Zealand Dairy Corporation (NZDC) factory at Te Awamutu, fluidised bed boiler ash (FBA), contains agro-nomically useful amounts of lime and sulphur (S). The FBA is produced from injection of limestone into the combustion chamber to absorb and reduce sulphur dioxide (SO2) emission from burning high S coal. NZDC FBA, an alkaline material (pH in water = 12.4, 51.8% calcium carbonate (CaCO₃equivalent), contained 6.2% S, 25.4% calcium (Ca), and negligible amounts of phosphorus (P), potassium (K), and magnesium (Mg). Approximately 50% of the S in the slaked FBA was water-soluble (gypsum, CaSO₄.2H₂O) with the remainder being ettringite [Ca6Al₂SO₄)₃OH)₁₂.26H₂O] which has low solubility in water. The soluble fraction had release characteristics similar to the gypsum in single superphosphate. The “lime” in FBA is mainly in hydroxide form and it is a quicker-acting, more caustic liming material than limestone. The concentrations of heavy metals were low in NZDC FBA (< 6.5 mg/kg) and are unlikely to cause toxic problems at normal rates of S fertiliser application and lime requirement.