Kevin C. Galbreath

Mercury transformations in coal combustion flue gas

Abstract Mercury chlorination [i.e., formation of HgCl 2 (g)] is generally assumed to be the dominant mercury-transformation mechanism in coal combustion flue gas. Other potential mechanisms involve mercury interactions with ash particle surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg 0 (g) to Hg 2+ X(g) (e.g., where X is Cl 2 or O) as well as Hg 0 (g) and HgCl 2 (g) to particulate mercury, Hg(p). Results from an investigation of Hg 0 (g)–O 2 (g)–HCl(g) and Hg 0,2+ (g)–HCl(g)–CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consistent with the following mechanisms: mercury chlorination, catalysis of mercury oxidation by Al 2 O 3 (s) and/or TiO 2 (s), and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additions of 50 and 100 ppmv of HCl(g) and ≈12.6 wt.% of CaO(s) to the subbituminous coal combustion environment inhibited Hg(p) formation, primarily via a change in ash surface chemistry and a decrease in particle surface area, respectively.

Collaborative study of quantitative coal mineral analysis using computer-controlled scanning electron microscopy

Six laboratories collaborated in an international study of the computer-controlled scanning electron microscopy (CCSEM) method of quantitative coal mineral analysis. A total of five analyses were performed by most of the laboratories on three bituminous coal samples: Pittsburgh No. 8, Illinois No. 6 and Prince. Repeatability relative standard deviation (RSDr) was <20% for the four minerals analysed: calcite, kaolinite, pyrite and quartz. Reproducibility relative standard deviations (RSDR) ranged from 21 to 83%. Reproducibility of the kaolinite results was the poorest, with an average RSDR of 60%, and pyrite was the best, with an average RSDR of 22%. The reproducibility of calcite and quartz analysis results was similar, with an average RSDR of 38 and 36% respectively. Although pyrite content was determined the most precisely, normative mineral calculations indicate that the results are overbalanced. Improvement in the interlaboratory agreement of CCSEM results will require the development of a standardized calibration procedure.

Evaluating mercury transformation mechanisms in a laboratory - scale combustion system

Mercury speciation measurements during injections of 10 microg/m3 Hg0(g) into a 42-MJ/h combustion system containing gaseous O2-Ar- and O2-N2-rich mixtures indicate that 43 and 55% of the Hg (g) spike was transformed rapidly (< 0.1 s) to Hg2+X(g) within a refractory-lined heat exchanger where gas temperatures decrease from approximately 620 to 200 degrees C. O2(g) is the probable Hg0(g) oxidant (i.e. X = O2-). The apparent formation of HgO(g) involves a heterogeneous reaction with adsorbed Hg0 or O2 on refractory surfaces or a Hg0(g)-O2(g) reaction catalyzed by corundum (Al2O3) and/or rutile (TiO2) components of the refractory. The potential catalytic effects of Al2O3 and TiO2 on Hg0(g) oxidation were investigated by injecting Al2O3 and TiO2 powders into approximately 650 degrees C subbituminous coal (Powder River Basin, Montana, USA) combustion flue gas. On-line Hg0(g) and total mercury measurements indicate, however, that Al2O3 and TiO2 injections were ineffective in promoting the formation of additional Hg2+X(g). Apparently, either the chemically complex flue gas hindered the catalytic effects of Al2O3 and TiO2, or these compounds are simply not Hg0(g) oxidation catalysts.

Nickel Speciation of Residual Oil Fly Ash and Ambient Particulate Matter Using X-ray Absorption Spectroscopy

ABSTRACT The chemical speciation of Ni in fly ash produced from ~0.85 wt % S residual (no. 6 fuel) oils in laboratory (7 kW)- and utility (400 MW)-scale combustion systems was investigated using X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD), and acetate extraction [1 M NaOAc-0.5 M HOAc (pH 5) at 25 °C]-anodic stripping voltammetry (ASV). XAFS was also used to determine the Ni speciation of ambient particulate matter (PM) sampled near the 400-MW system. Based on XAFS analyses of bulk fly ash and their corresponding acetate extraction residue, it is estimated that >99% of the total Ni (0.38 wt %) in the experimentally produced fly ash occurs as NiSO4-xH2O, whereas >95% of the total Ni (1.70 and 2.25 wt %) in two fly ash samples from the 400-MW system occurs as NiSO4-xH2O and Ni-bearing spinel, possibly NiFe2O4. Spinel was also detected using XRD. Acetate extracts most of the NiSO4-xH2O and concentrates insoluble NiFe2O4 in extraction residue. Similar to fly ash, ambient PM contains NiSO4-xH2O and NiFe2O4; however, the proportion of NiSO4-xH2O relative to NiFe2O4 is much greater in the PM. Results from this and previous investigations indicate that residual oil ash produced in the 7-kW combustion system lack insoluble Ni (e.g., NiFe2O4) but are enriched in soluble NiSO4-xH2O relative to fly ash from utility-scale systems. This difference in Ni speciation is most likely related to the lack of additive [e.g., Mg(OH)2] injection and residence time in the 7-kW combustion system.

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.

Determination of Nickel Species in Stack Emissions from Eight Residual Oil-Fired Utility Steam-Generating Units

XAFS spectroscopy has been used to determine the Ni species in particulate matter collected on quartz thimble filters in the stacks of eight residual (No. 6 fuel) oil-burning electric utility steam-generating units. Proper speciation of nickel in emitted particulate matter is necessary to correctly anticipate potential health risks. Analysis of the spectroscopic data using least-squares linear combination methods and a newly developed method specific for small quantities of Ni sulfide compounds in such emissions show that potentially carcinogenic Ni sulfide compounds are absent within the detection limits of the method (≤ 3% of the total Ni) in the particulate matter samples investigated. In addition to the major nickel sulfate phase (NiSO(4)·6H(2)O), lesser amounts of (Ni,Mg)O and/or NiFe(2)O(4) were also identified in most emission samples. On the basis of the results from these emission characterization studies, the appropriateness of the U.S. Environmental Protection Agencys assumption that the Ni compound mixture emitted from residual oil-fired power plants is 50% as carcinogenic as nickel subsulfide (Ni(3)S(2)) should be re-evaluated.

Feasibility of combined wavelength/energy-dispersive computer-controlled scanning electron microscopy for determining trace metal distribution

Abstract The utility of wavelength-dispersive and energy-dispersive electron probe microanalysis (WD-ED EPMA) for quantifying the distribution of seven trace metals (Cr, Ni, As, Se, Cd, Hg, and Pb) among coal minerals and ash particles was investigated. The technique is limited to analyzing particles greater than about 7 μm in diameter because of the relatively intense beam conditions (25 kV, 100 nA) required to attain reasonable detection sensitivity; statistical limits of detection are generally

Nickel and Sulfur Speciation of Residual Oil Fly Ashes from Two Electric Utility Steam-Generating Units

Abstract Representative duplicate fly ash samples were obtained from the stacks of 400- and 385-MW utility boilers (Unit A and Unit B, respectively) using a modified U.S. Environmental Protection Agency (EPA) Method 17 sampling train assembly as they burned 0.9 and 0.3 wt % S residual (No. 6 fuel) oils, respectively, during routine power plant operations. Residual oil fly ash (ROFA) samples were analyzed for Ni concentrations and speciation using inductively coupled plasma-atomic emission spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, and X-ray diffraction (XRD). ROFA deionized H2O extraction residues were also analyzed for Ni speciation using XAFS and XRD. Total Ni concentrations in the ROFAs were similar, ranging from 1.3–1.5 wt %; however, stack gas Ni concentrations in the Unit A were 0.990 μg/Nm3 compared with 0.620 μg/Nm3 for Unit B because of the greater residual oil feed rates employed at Unit A to attain higher 400-MW load conditions with a lower heating value oil. Ni speciation analysis results indicated that ROFAs from Unit A contain ∼3 wt % NiSO4 ∼ xH2O (where x is assumed to be 6 for calculation purposes) and ∼4.5 wt % of a Ni-containing spinel compound, similar in composition to (Mg,Ni)(Al,Fe)2O4. ROFAs from Unit B contain on average 2 wt % NiSO4 ∼ 6 H2O and 1.1 wt % NiO. XAFS and XRD analyses did not detect any nickel sulfide compounds, including carcinogenic nickel subsulfide (Ni3S2) (XAFS detection limit is 5% of the total Ni concentration). In addition, XAFS measurements indicated that inorganic sulfate and organic thiophene species accounted for >97% of the total S in the ROFAs. Unit A ROFAs contained much lower thiophene proportions because cyclone-separated ROFA reinjection is employed on this unit to collect and reburn the larger carbonaceous particles.

Development of Fireside Performance Indices for Coal-Fired Utility Boilers

A series of eight fireside performance indices have been developed for pulverized coal-fired utility boiler systems. The indices are calculated to predict slagging, high-temperature fouling, low-temperature fouling, slag tapping ability, opacity, tube erosion, coal grindability, and sootblower necessity. The indices are most useful for screening new fuels or fuel blends for pulverized coal-fired systems. Coal data input required to calculate the indices includes proximate/ultimate analysis results, elemental oxide coal ash chemistry, coal mineral quantities, and juxtaposition as derived using computer-controlled scanning electron microscopy and quantities, of organically bound or submicron mineral components as determined by chemical fractionation analysis. Limited boiler inputs are also required, such as boiler type, power-generating capacity, percentage of load, gas velocities, type of particulate control, and basic furnace dimensions. The indices were formulated primarily by combining inorganic transformation and ash deposition theory with empirical correlations derived from bench-, pilot-, and full-scale combustion tests. The ash deposition and opacity indices have been validated at several full-scale utility boilers.

LONG-TERM DEMONSTRATION OF SORBENT ENHANCEMENT ADDITIVE TECHNOLOGY FOR MERCURY CONTROL

Long-term demonstration tests of advanced sorbent enhancement additive (SEA) technologies have been completed at five coal-fired power plants. The targeted removal rate was 90% from baseline conditions at all five stations. The plants included Hawthorn Unit 5, Mill Creek Unit 4, San Miguel Unit 1, Centralia Unit 2, and Hoot Lake Unit 2. The materials tested included powdered activated carbon, treated carbon, scrubber additives, and SEAs. In only one case (San Miguel) was >90% removal not attainable. The reemission of mercury from the scrubber at this facility prevented >90% capture.