Article pubs.acs.org/EF
Vanadium and Nickel Speciation in Pulverized Coal and Petroleum Coke Co-combustion Luis F. O. Silva,†,‡ Marcos L. S. Oliveira,§ Carlos H. Sampaio,∥ Irineu A. S. de Brum,∥ and James C. Hower*,⊥ †
Environmental Science and Nanotechnology Department, Catarinense Institute of Environmental Research and Human Development (IPADHC), 88745-000, Capivari de Baixo, Santa Catarina (SC), Brazil ‡ Mestrado em Avaliaçaõ de Impactos Ambientais em Mineraçaõ , Centro Universitário La Salle, Avenida Victor Barreto, 2288 Centro 92010-000, Canoas, Rio Grande do Sul (RS), Brazil § Development Department of Touristic Opportunities, Catarinense Institute of Environmental Research and Human Development (IPADHC), 88745-000, Capivari de Baixo, Santa Catarina (SC), Brazil ∥ Escola de Engenharia, Departamento de Metallurgia, Centro de Tecnologia, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500 Bairro Agronomia 91501-970, Porto Alegre, Rio Grande do Sul (RS), Brazil ⊥ Center for Applied Energy Research (CAER), University of Kentucky, Lexington, Kentucky 40511, United States ABSTRACT: Vanadium and nickel in emissions from fossil-fuel combustion and in the fly ash can be an environmental concern. The fly ash from the combustion of a 70% coal/30% petroleum coke blend in a 500 MW pulverized-fuel utility boiler was studied by a variety of X-ray, optical microscopy, and electron beam methods. The fly ash V and Ni are present in heterogeneous silicates, glass, sulfates, oxides and oxyhydroxides, and crystalline and/or amorphous mixed clay minerals, and also in Ni, detrital ferromagnesian silicates. Vanadium- and Ni-bearing spinels are incorporated into magnetite structures. Multiwalled nanotubes encapsulate V and Ni, and C60, C70, and C80 fullerenes and their derivatives are present. investigated.8−10 However, fly ashes that result from co-firing with fuel oil tend to produce more acidic pH values under leaching conditions.6 Start-up combustion procedures with fuel oil, for example, promote the condensation of SO3 on the ash particles,11 which forms H2SO4 in the presence of humidity and gives rise to very low (2−3) pH values.7 Thus, to evaluate ultrafine/nanoparticles occurrence, fate, and behavior in the fly ashes and to define real scenarios for environmental risk assessment studies, sensitive and robust analytical methods are required. To the authors knowledge, this is the first detailed analytical method developed and applied to assess the content of ultrafine/nanoparticle contamination in fly ashes from pulverized coal and petroleum coke co-combustion. In this study, the first on the detection and complex characterization of ultrafine/nanoparticle assemblages in fly ash from a power plant burning mixtures of coal and petroleum coke, we investigate the forms of pure and/or mixed nanocarbons, crystalline and/or amorphous nanominerals, and other compounds containing hazardous trace elements (in particular, V and Ni).
1. INTRODUCTION Petroleum coke (commonly referred to as pet coke), a byproduct of the refining industry, is an inexpensive fuel that typically has low ash, low moisture, and high heating value. It can also be a challenging fuel in terms of its low volatile content and high trace element, carbon, sulfur, and nitrogen contents, all of which give rise to undesirable emission characteristics.1,2 However, the availability of petroleum coke and the volatile but, generally, low price provides a powerful economic stimulus to use it for steam and power generation, as either the primary fuel or a coal/pet coke blend.3 Forecasting of the environmental contamination because of emissions of hazardous volatile elements (HVEs) from the thermal coal power industry is only possible in terms of the objective knowledge of their speciation, concentrations, distribution, and accumulation conditions in the original fuel and the combustion byproducts. In the co-combustion of petroleum coke and coal, hazardous elements, including V and Ni, concentrate in the combustion byproducts, in particular in the fly ash.4,5 V and Ni, however, do not exhibit the volatility and larger magnitude partitioning of elements, such as Zn and As.4,5 The combustion and pollution control technologies used in the plant, including any use of oil co-firing,6 may also play important roles, for example, a significant impact on fly ash chemical characteristics.7 Silva et al.6 noted that some elements, namely, As, Bi, Cd, Ga, Ge, Hg, Mo, Pb, Sb, Se, Sn, Tl, W, and Zn, may have a higher concentration in fly ash derived from coal/oil co-combustion. In addition, the impact of petroleum coke on the leaching of As, Mo, Ni, S, and V from coalcombustion fly ashes under alkaline conditions has been © 2013 American Chemical Society
2. MATERIALS AND METHODS 2.1. Sampling. Coal and fly ash were sampled at a western Kentucky 500 MW pulverized-fuel utility power plant in the course of the pentannual sampling of Kentucky power plants by the University of Kentucky Center for Applied Energy Research (CAER),12 with a Received: August 29, 2012 Revised: February 15, 2013 Published: February 18, 2013 1194
dx.doi.org/10.1021/ef4000038 | Energy Fuels 2013, 27, 1194−1203
Energy & Fuels
Article
previous study.5 The pulverized coal + petroleum coke fuel feed (sample 93501) was sampled at one point along the path from the pulverizer to the boiler. The fly ash was sampled in one hopper for each of the second and third electrostatic precipitator (ESP) rows (samples 93468 and 93469, respectively). We were not able to sample the first ESP row, closer to the boiler and, therefore, with a hotter flue gas T than the rows sampled. Specific information related to the flue gas temperature at the point of fly ash collection for either row was not made available to us. 2.2. Petrology. Fly ash petrology was analyzed on Sudan Blacklaced epoxy-bound pellets prepared to a final 0.05 μm polish with 50× oil-immersion optics and polarized white light, following procedures initially defined by Hower et al.13 and refined and expanded since that publication. 2.3. Elemental Chemistry. Basic fly ash chemistry was conducted following ASTM procedures. Major and minor elements were determined on a Philips PW2404 X-ray fluorescence unit at the CAER following procedures outlined by Hower and Bland14 and by a variety of methods at the U.S. Geological Survey’s Denver Laboratories following procedures after Meier et al.15 Mercury was analyzed at the CAER on a LECO AMA 254 advanced mercury analyzer absorption spectrometer. 2.4. X-ray Photoelectron Spectroscopy (XPS). Surface composition for trace element speciation was determined by XPS. The XPS spectra of fly ash samples were obtained using a X-ray photoelectron spectrometer at the University of Vigo, Spain, with a monochromatic Al Kα (1486.6 eV) source, analytical chamber pressure of
