Chemical Speciation of Nickel in Residual Oil Ash

Abstract

The speciation of Ni emissions from residual oil-fired utility boilers requires investigation because the possible presence of small respirable particles containing Ni3S2 is a health concern. An experimental approach was used to investigate the Ni speciation of residual oil combustion ash. Ash from a low- and high-S (0.33 and 1.80 wt %, respectively) residual oil was produced using a 42-MJ/h combustion system at excess O2 concentrations of ≤1 and 2 or 3 mol %. Ni speciation analyses were performed using X-ray absorption fine-structure (XAFS) spectroscopy and sequential extraction−anodic stripping voltammetry (ASV). XAFS measurements indicate that >95% of the total Ni (3−9 wt %) present in the ashes occurs as Ni2+ coordinated to O2-. Both methods indicate that NiSO4 is the dominant form, although significant proportions of NiO (5−24%) were measured by sequential extraction−ASV. The sequential extraction−ASV method also indicated the presence of very small proportions, <2%, of Ni sulfide (NixSy). Fuel S con...