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Title: Mercury retention by fly ashes from coal combustion: Influence of the unburned carbon content

Abstract

The objective of this study was to evaluate the effect of unburned carbon particles present in fly ashes produced by coal combustion on mercury retention. To achieve this objective, the work was divided into two parts. The aim of the first part of the study was to estimate the amount of mercury captured by the fly ashes during combustion in power stations and the relationship of this retention to the unburned carbon content. The second part was a laboratory-scale study aimed at evaluating the retention of mercury concentrations greater than those produced in power stations by fly ashes of different characteristics and by unburned carbon particles. From the results obtained it can be inferred that the unburned carbon content is not the only variable that controls mercury capture in fly ashes. The textural characteristics of these unburned particles and of other components of fly ashes also influence retention.

Authors:
; ;  [1]
  1. CSIC, Oviedo (Spain)
Publication Date:
OSTI Identifier:
20862315
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research; Journal Volume: 46; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; MERCURY; FLY ASH; RETENTION; CARBON; COAL; CAPTURE; FOSSIL-FUEL POWER PLANTS; TEXTURE

Citation Formats

Lopez-Anton, M.A., Diaz-Somoano, M., and Martinez-Tarazona, M.R. Mercury retention by fly ashes from coal combustion: Influence of the unburned carbon content. United States: N. p., 2007. Web. doi:10.1021/ie060772p.
Lopez-Anton, M.A., Diaz-Somoano, M., & Martinez-Tarazona, M.R. Mercury retention by fly ashes from coal combustion: Influence of the unburned carbon content. United States. doi:10.1021/ie060772p.
Lopez-Anton, M.A., Diaz-Somoano, M., and Martinez-Tarazona, M.R. Wed . "Mercury retention by fly ashes from coal combustion: Influence of the unburned carbon content". United States. doi:10.1021/ie060772p.
@article{osti_20862315,
title = {Mercury retention by fly ashes from coal combustion: Influence of the unburned carbon content},
author = {Lopez-Anton, M.A. and Diaz-Somoano, M. and Martinez-Tarazona, M.R.},
abstractNote = {The objective of this study was to evaluate the effect of unburned carbon particles present in fly ashes produced by coal combustion on mercury retention. To achieve this objective, the work was divided into two parts. The aim of the first part of the study was to estimate the amount of mercury captured by the fly ashes during combustion in power stations and the relationship of this retention to the unburned carbon content. The second part was a laboratory-scale study aimed at evaluating the retention of mercury concentrations greater than those produced in power stations by fly ashes of different characteristics and by unburned carbon particles. From the results obtained it can be inferred that the unburned carbon content is not the only variable that controls mercury capture in fly ashes. The textural characteristics of these unburned particles and of other components of fly ashes also influence retention.},
doi = {10.1021/ie060772p},
journal = {Industrial and Engineering Chemistry Research},
number = 3,
volume = 46,
place = {United States},
year = {Wed Jan 31 00:00:00 EST 2007},
month = {Wed Jan 31 00:00:00 EST 2007}
}
  • Unburned carbon concentrates with different mineral matter contents were obtained from coal combustion fly ashes by an oil agglomeration procedure. The concentrates were then heated in the temperature interval 1800-2700{sup o}C for the purpose of exploring their ability to graphitize. The influence of the treatment temperature and mineral matter of the unburned carbon on the structural characteristics of the materials prepared was studied. The interlayer spacing, d{sub 002}, and crystallite sizes along the c-axis, L{sub c}, and the {alpha}-axis, L{sub a}, calculated from X-ray diffractometry (XRD) as well as the relative intensity of the Raman D-band, I{sub D}/I{sub t}, weremore » used to assess the degree of structural order of the materials. Graphite materials with structural characteristics comparable to those of other oil-derived synthetic graphites were prepared from the unburned carbon concentrates at temperatures {>=}2400{sup o}C. It was also observed that more-ordered materials were obtained from the unburned carbon concentrates with higher mineral matter content. The influence of the mineral matter on the graphitization of the unburned carbon concentrates is the result of two countereffects, thus limiting its extent. On the one hand, the lateral coalescence of the crystallites is preferentially promoted. Reasonably good linear correlations were attained between the mineral matter of the unburned carbon concentrate and the XRD parameter L{sub a} of the materials. However, on the other, this coalescence also facilitates the flattening of the pores, thus decreasing the temperature at which their breakage occurs. As a consequence, from this point on, the structural evolution of the materials with increasing mineral matter is only noticeable by the slow vegetative growth of the crystallites along the a-axis. 40 refs., 2 figs., 3 tabs.« less
  • High-resolution transmission electron microscopy (HRTEM) has been used in this work to study the microstructural (structure and microtexture) changes occurring during the high-temperature treatment of the unburned carbon concentrates from coal combustion fly ashes. Emphasis was placed on two aspects: (i) the development of graphitic carbon structures and (ii) the disordered carbon forms remaining in the graphitized samples. In addition, by coupling HRTEM with energy-dispersive spectroscopy, the transformations with the temperature of the inorganic matter (mainly iron- and silicon-based phases) of the unburned carbon concentrates were evidenced. The HRTEM results were compared to the averaged structural order of the materialsmore » as evaluated by X-ray diffraction (XRD) and Raman spectroscopy. As indicated by XRD and Raman parameters, more-ordered materials were obtained from the unburned carbon concentrates with higher mineral/inorganic matter, thus inferring the catalytic effect of some of their components. However, the average character of the information provided by these instrumental techniques seems to be inconclusive in discriminating between carbon structures with different degrees of order (stricto sensu graphite, graphitic, turbostratic, etc.) in a given graphitized unburned carbon. Unlike XRD and Raman, HRTEM is a useful tool for imaging directly the profile of the polyaromatic layers (graphene planes), thus allowing the sample heterogeneity to be looked at, specifically the presence of disordered carbon phases. 49 refs., 9 figs., 3 tabs.« less
  • The combustion of pulverized coal in a drop tube furnace has been studied optically. The coal passes through an excess air flame to ensure ignition and then burns as it passes down a segmented ceramic tube electrically heated to 1,100 C. The method permits the optical measurement of particle size distribution as a function of residence time. The results of this demonstrate the relative reactivity of the coals used and also indicate that fragmentation may occur. Samples of the particles are taken at each furnace stage for Carbon-Hydrogen-Nitrogen (CHN) analysis. Comparison of the carbon content by mass with the cross-polarizationmore » in the backscattered light shows that a linear correlation can be established for particles up to at least 100 {mu}m diameter. This implies the possibility of a simple optical tool for the measurement of unburned carbon content, and, thus, for monitoring burnout and combustion efficiency.« less
  • Nine fly ash samples were collected from the particulate collection devices of four full-scale pulverized coal utility boilers burning eastern bituminous coals and three cyclone utility boilers burning either Powder River Basin coals or PRB blends. As-received fly ash samples were mechanically sieved to obtain six size fractions. Unburned carbon content, mercury content, and Brunauer-Emmett-Teller surface areas of as-received fly ashes and their size fractions were measured. In addition, UBC particles were examined by scanning electron microscopy, high-resolution transmission microscopy, and thermogravimetry to obtain information on their surface morphology, structure, and oxidation reactivity. It was found that the UBC particlesmore » contained amorphous carbon, ribbon-shaped graphitic carbon, and highly ordered graphite structures. The mercury contents of the UBCs in raw ash samples were comparable to those of the UBC-enriched samples, indicating that mercury was mainly adsorbed on the UBC in fly ash. The UBC content decreased with a decreasing particle size range for all nine ashes. The mercury content of the UBCs in each size fraction, however, generally increased with a decreasing particle size for the nine ashes. The mercury contents and surface areas of the UBCs in the PRB-CYC ashes were about 8 and 3 times higher than UBCs in the EB-PC ashes, respectively. It appeared that both the particle size and surface area of UBC could contribute to mercury capture. The particle size of the UBC in PRB-CYC ash and thus the external mass transfer was found to be the major factor impacting the mercury adsorption. Both the particle size and surface reactivity of the UBC in EB-PC ash, which generally had a lower carbon oxidation reactivity than the PRB-PC ashes, appeared to be important for the mercury adsorption. 26 refs., 16 figs., 2 tabs.« less
  • Mercury capture by coal-combustion fly ash is a function of the amount of Hg in the feed coal, the amount of carbon in the fly ash, the type of carbon in the fly ash (including variables introduced by the rank of the feed coal), and the flue gas temperature at the point of ash collection. In their discussion of fly ash and Hg adsorption, Lu et al. (Energy Fuels 2007, 21, 2112-2120) had some fundamental flaws in their techniques, which, in turn, impact the validity of analyzed parameters. First, they used mechanical sieving to segregate fly ash size fractions. Mechanicalmore » sieving does not produce representative size fractions, particularly for the finest sizes. If the study samples were not obtained correctly, the subsequent analyses of fly ash carbon and Hg cannot accurately represent the size fractions. In the analysis of carbon forms, it is not possible to accurately determine the forms with scanning electron microscopy. The complexity of the whole particles is overlooked when just examining the outer particle surface. Examination of elements such as Hg, present in very trace quantities in most fly ashes, requires careful attention to the analytical techniques. 36 refs., 3 figs., 1 tab.« less