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Energy & Fuels 1998, 12, 818-822
Chemical Speciation of Nickel in Residual Oil Ash Kevin C. Galbreath* and Christopher J. Zygarlicke Energy & Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9018
Frank E. Huggins and Gerald P. Huffman Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506
John L. Wong Chemistry Department, University of Louisville, Louisville, Kentucky 40292 Received February 13, 1998. Revised Manuscript Received May 8, 1998
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 e1 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, 95% of the Ni in the four samples occurs as Ni2+ in coordination with O2- ligands. The spectral features in Figure 4 are inconsistent with those for NiCO3, NiO, C4H6NiO4‚4H2O, Ni(OH)2, NiCr2O4, KNiPO4, Ni2SiO4, CaNiSi2O6, and Ni-bearing Na2Si2O5 glass and most closely resemble those of NiSO4‚7H2O, NiSO4(aq), and NiSO4. Compared in Figure 5 are RSF spectra for the four oil combustion ashes. The major peak at about 1.65 Å in the RSF spectra for all the ashes is also compatible with Ni2+ in coordination with O2-. Ash RSF spectra most closely resemble the spectra for NiSO4 in Figure 3, thus confirming the XANES evidence that NiSO4 is the dominant species in the four ash samples. Each ash sample was analyzed in duplicate using the sequential extraction-ASV method. Average total Ni concentrations and speciation results, reported as a proportion (%) of the total Ni, for each ash sample are (15) Brown, G. E., Jr.; Farges, F.; Calas, G. In Structure, Dynamics and Properties of Silicate Melts; Stebbins, J. F., McMillan, P. F., Dingwell, D. B., Eds.; Mineralogical Society of America Reviews in Mineralogy: 1995; Vol. 32, Chapter 9, pp 317-410.
provided in Figures 6 and 7. Agreement between the duplicate analysis results for a given ash was very good: relative percent differences from the average values for a given species determination were e18%. On the basis of the total Ni measurements, Ni recoveries from the five extracts were excellent, ranging from 96 to 98%. Consistent with the Ni contents of the low- and high-S residual oils (40 and 86 ppm, respectively), the sequential extraction-ASV analysis results of total Ni in Figures 6 and 7 indicate that the high-S oil ashes are enriched in Ni by a factor of about 2 relative to the low-S oil ashes. The sequential extraction-ASV speciation results indicate that a soluble Ni species is dominant in the four ash samples. This soluble species is NiSO4, based on Ni K-edge XAFS spectroscopy measurements. The results in Figures 6 and 7 also indicate that for a given excess O2 concentration, the Ni speciation of the low- and high-S oil ashes are generally similar, except for the much greater proportions of Ni silicate(s) in the low-S oil ashes. Apparently, fuel S content did not significantly affect the formation of NiSO4 or Ni sulfides (NixSy). NiO is relatively abundant, especially in the ashes produced at e1 mol % excess O2. The inverse relationship between the proportions of NiO and NiSO4 with varying excess O2 implies that the effect of increas-
822 Energy & Fuels, Vol. 12, No. 4, 1998
Galbreath et al.
Table 2. Comparison of Nickel Speciation Investigations ash source(s) sampling location sampling method speciation results
Goldstein8 and Bell et al.9
this study
full-scale utility boilers duct or stack isokinetic sampling on a heated quartz thimble filter (EPA Method 5 train) 5-26% NixSy, 40-68% NiO, 25-44% NiX (e.g., X ) SO4, CO3, Cl2), and x > 0.54), crystals and lack of alkali sulfate, (Na, K)2SO4, crystals. Crystalline (Na, K)xVxV6-xO15 occurs naturally as the condensate mineral bannermanite near fumaroles in volcanically active areas.16 Analogous to its natural occurrence, (Na, K)xVxV6-xO15 condensed on collected particulate and the glass filter substrate during sampling and was not entrained as particulate in the high-S flue gas stream. The condensation of (Na, K)xVx+4V6-x+5O15 implies that V2O5(g) was present in the high-S flue gas. It is well-known that V2O5(g) is a catalyst for sulfation reactions that should promote the formation of NiSO4.17 Despite its abundance, no discrete crystals of NiSO4 were detected in the oil ashes using SEM and XRD, thus indicating that the NiSO4 occurs as minute crystals or is amorphous. The sequential extraction-ASV method indicated the presence of significant proportions of NiO (5-24%) that were not detected using XAFS spectroscopy, even though the XANES and RSF spectra of NiO are very distinct from those for the dominant NiSO4 species (Figures 2 and 3). In future research, residue from the first step of the extraction procedure in Table 1 will be analyzed using XAFS spectroscopy to corroborate the existence of NiO in residual oil ash. The inverse relationship between the proportions of NiSO4 and NiO with varying excess O2 concentrations, as indicated in Figures 6 and 7, is generally consistent with thermodynamic modeling predictions of Ni speciation for residual oil ash.13 These predictions indicate that increasing excess O2 from e1 to 3 mol % excess O2 extends the high-temperature stability limit of NiSO4 at the expense of NiO. (16) Hughes, J. M.; Finger, L. W. Am. Mineral. 1983, 68, 634-641. (17) Clark, R. J. H. In Comprehensive Inorganic Chemistry; Bailar, J. C., Jr., Emele´us, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.; Pergamon Press Ltd.: Oxford, England, 1973; Chapter 34, pp 491551.
1-2% NixSy, 5-24% NiO, 79-92% NiSO4, 1-3% Ni silicate(s), and
