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 O2 concentrations of  1 and 2 or 3mo1%. High-S ashes are distinguished from low-S ashes by an abundance of (Na, K)x v4 x+ V5 6x+O15 (0.90 > x > 0.54) and lack of (Na, K)2,SO4 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 NlSO4 and Cr2(SO4)3 and not the more toxic Ni3S2 and Cr6 forms predominate in the ashes. Thermodynamic modeling results support the empirical results in that NiSO4 XH20 and Cr2(SO4)3 are predicted to be stable low-temperature species in both low an high-S residual oil ashes produced at  3 mol% excess O2.