Nickel and Chromium Speciation of Residual Oil Combustion Ash

Ash from a low- and high-S (0.33wt% and 1.80wt% S, respectively) residual oil was produced using a laboratory-scale combustion system at excess O 2 concentrations of ⩽ 1 and 2 or 3mo1%. High-S ashes are distinguished from low-S ashes by an abundance of (Na, K) x v 4 x + V 5 6x +O 15 (0.90 > x > 0.54) and lack of (Na, K) 2 ,SO 4 crystals. Discrete phases of Ni or Cr were not detected using SEM and XRD, even though these metals are relatively abundant - 1.5 to 5.5w1% and 0.08 to 0.1 wt%, respectively. Ni and Cr K-edge XAFS spectroscopy analyses indicate that NlSO 4 and Cr 2 (SO 4 )3 and not the more toxic Ni 3 S 2 and Cr 6 forms predominate in the ashes. Thermodynamic modeling results support the empirical results in that NiSO 4 XH 2 0 and Cr 2 (SO 4 ) 3 are predicted to be stable low-temperature species in both low an high-S residual oil ashes produced at ⩽ 3 mol% excess O 2 .