David J. Hassett

Mercury capture on coal combustion fly ash

Abstract A study was performed at the Energy and Environmental Research Center (EERC) to test the hypotheses that (1) different carbon types contained in coal combustion fly ash have variable sorption capabilities relative to mercury and (2) the inorganic fraction of coal combustion fly ash may sorb mercury through mechanisms distinct from sorption by carbon in the ash. The purpose of this study was to conduct laboratory experiments to better understand the phenomenon of mercury sorption on coal combustion fly ash. Tests were conducted on the laboratory scale using samples generated from both commercial-scale utility boilers and pilot-scale combustion equipment at the EERC. Selected samples represented ash from various coal sources, of varying loss-on-ignition (LOI) content, and exhibiting different combinations of carbon types. Results indicate a direct correlation between carbon content and mercury partitioning among individual ash samples. The direct relationship between carbon content and mercury-sorbing capacity of the bulk ash samples demonstrated in the loading experiments is not reflected by the low- and high-carbon fraction data. The mercury-sorbing capacity of the inorganic fraction is extremely low with respect to carbon present in the ash. There are likely to be significant differences between the mercury-sorbing capacities of these various carbon forms. The mercury-sorbing capacity of ash studied in this research was highly temperature dependent. Additional work is needed on experimental design to evaluate the loading of high-carbon ash samples with particular attention to the phenomenon of sorptive capacity regeneration, which should be further investigated.

Oxyanion Substituted Ettringites: Synthesis and Characterization; and their Potential Role In Immobilization of As, B, Cr, Se and V

Long-term batch leaching studies of composites of lignite combustion and gasification ashes with a calcium-based scrubber waste have shown the prominent formation of an ettringite structure phase, accompanied by reductions in solution concentrations of potentially hazardous elements such as boron and selenium. The possibility that oxyanions such as arsenate, borate, chromate, molybdate, selenate and vanadate might substitute for sulfate in the ettringite structure has been explored. There are literature reports of fully substituted borate, chromate and selenate [ettringites]*, and of two minerals with partial borate for sulfate substitution. X-ray diffraction phase pure products with chromate and selenate, substituted completely for the sulfate in ettringite, and with arsenate, borate and vanadate partially substituted for sulfate, have been synthesized at room temperature by mixing soluble Ca, Al and sulfate sources and maintaining a pH> 12 with NaOH additions. Attempts to substitute substantial amount of molybdate for sulfate were unsuccessful. The resulting phases were characterized chemically by ICAP spectrometry, ion chromatography and thermal analysis, and for phase purity and unit cell size by XRD. The speciation of the oxyanions in the substituted [ettringite] were confirmed by FTIR spectrometry. In solid solutions, the sulfate/oxyanion ratio was greater in the precipitated solid than in the synthesis solution. Chemical analyses of the [ettringites] did not give simple stoichiometries analogous to Ca 6 Al 2 (SO 4 )(OH) 12 ·26H 2 O. Because nonstoichiometry can be attributed to numerous possibilities for charge balance and defects, structural formulae of the oxyanion substituted [ettringites] could not be established.

Scientifically valid leaching of coal conversion solid residues to predict environmental impact

Abstract The disposal of wastes from energy production, particularly solid wastes from coal conversion processes, requires a thorough understanding of both the waste and the environment. Environmentally responsible disposal requires an appreciation and understanding of all of the complex interactions that can occur between the waste and the environment. Additionally, the importance of site characterization cannot be overemphasized, since local geology plays such an important part in the transport and fate of potentially problematic trace elements and other contaminants. Waste characterization techniques must be scientifically valid, comprehensive, and thorough. The focus of this paper is on the importance of the selection and application of scientifically valid methods for characterizing wastes. Selection criteria for scientifically valid evaluation techniques for the characterization of coal conversion solid waste are discussed, and guidelines for selection are provided. The importance of understanding all possible environmental consequences must be considered, including water-to-waste and waste-to-waste interactions, as well as leachate-to-sediment interactions. The importance of the philosophical integration of all aspects of disposal, site characterization, and waste characterization are discussed. The application of scientifically invalid characterization methods or incomplete site characterization is poor science at best and can lead to false conclusions.

Synthesis and characterization of selenate and sulfate-selenate ettringite structure phases

Crystalline ettringite structure phases with selenate substituted partially and completely for sulfate have been synthesized and characterized by chemical analysis, XRD, SEM and FTIR spectrometry. There is complete miscibility between sulfate and selenate ettringite. According to a proposed structural formula model, the phases were deficient in sulfate/selenate, and all but the selenate end member were slightly deficient in aluminum, compared to the nominal stoichiometry Ca6Al2(XO4)3(OH)12·26H2O, X = S, Se. Solid solution with a hydroxide ettringite is proposed. Selenate-substituted ettringite could be a host phase for selenium in solid waste forms, especially those based on aluminosulfate cements.

Use of a Database of Chemical, Mineralogical and Physical Properties of North American Fly Ash to Study the Nature of Fly Ashand Its Utilization as a Mineral Admixture in Concrete

A database of chemical, mineralogical and physical characteristics of North American fly ashes has been assembled by the Western Fly Ash Research, Development and Data Center. One-hundred and seventy-eight representative ashes were divided into three groups according to their analytical CaO content: low-calcium, <10%; intermediate-calcium, 10–20%; high-calcium, 20+%. Statistical analyses were performed within each of the three groups. Thirty-two plots relating chemical composition, mineralogy and physical test results are presented. Extensive discussions relating the chemistry and mineralogy of the ash to the source coal, the distribution of the chemical constituents among crystalline and glassy phases, and the reactions of these phases in concrete are given. The consistency of high-calcium fly ash generated at a Midwestern U.S. power station fired with Wyoming bituminous coal was studied using ninety-three ashes collected over a two year period. The availability of mineralogy for each ash leads to a more thorough understanding of the bulk chemical and physical test results used in evaluating fly ashes for utilization, and in modeling their behavior after disposal

Release of mercury vapor from coal combustion ash

Abstract The long-term stability of Hg in coal combustion byproducts (CCBs) was evaluated at ambient and near-ambient temperatures. Six CCB samples with atypically high levels of total Hg were selected for study assuming a greater potential for release of measurable amounts of Hg vapor. The samples selected included two fly ash samples from U.S. eastern bituminous coal, two fly ash samples from South African low-rank coal, one fly ash from Powder River Basin (PRB) subbituminous coal blended with petroleum coke, and one PRB subbituminous coal fly ash incorporated with flue gas desulfurization material. Air scrubbed of Hg was passed through compacted 100-g aliquots of each sample at 1 mL/min and vented to a gold-coated quartz trap to collect released Hg vapor. The samples were maintained at ambient and near-ambient (37 °C) temperatures. All samples released low-picogram levels of Hg after 90 days. No pattern was evident to link the total Hg content to the rate of release of Hg vapor. An average of 0.030 pg Hg/g CCB/day was released from the samples, which equates to 2.2 x 10-8 lb Hg/ton CCB/year. If this were applied to a coal-fired power plant production of 200,000 tons of fly ash per year, there would be a maximum potential release of 0.0044 lb, or 2.00 g, of Hg per year. Experiments are continuing to determine long-term vapor release of Hg from CCBs. All samples have been set up in duplicate at ambient temperature with an improved apparatus to reevalu-ate results reported in this article.

Heat of hydration of fly ash as a predictive tool

A test procedure based on heat of hydration was developed to assess more accurately the reactivity and behaviour of fly ash for utilization. The technique uses a Dewar flask or a modified oxygen bomb calorimeter to determine the temperature change and rate of change after the addition of water to fly ash. X-ray diffraction is performed on the hydrated material after the hydration test to follow mineralogical changes due to the hydration process. A study of coal ash samples is under way to determine whether correlations exist between the temperature change and results from empirical tests, such as those of the ASTM, used to determine properties of ash for engineering applications. A protocol has been developed for an improved classification scheme for coal ash.

ENVIRONMENTAL EVALUATION FOR UTILIZATION OF ASH IN SOIL STABILIZATION

The Minnesota Pollution Control Agency (MPCA) approved the use of coal ash in soil stabilization, indicating that environmental data needed to be generated. The overall project goal is to evaluate the potential for release of constituents into the environment from ash used in soil stabilization projects. Supporting objectives are: (1) To ensure sample integrity through implementation of a sample collection, preservation, and storage protocol to avoid analyte concentration or loss. (2) To evaluate the potential of each component (ash, soil, water) of the stabilized soil to contribute to environmental release of analytes of interest. (3) To use laboratory leaching methods to evaluate the potential for release of constituents to the environment. (4) To facilitate collection of and to evaluate samples from a field runoff demonstration effort. The results of this study indicated limited mobility of the coal combustion fly ash constituents in laboratory tests and the field runoff samples. The results presented support previous work showing little to negligible impact on water quality. This and past work indicates that soil stabilization is an environmentally beneficial CCB utilization application as encouraged by the U.S. Environmental Protection Agency. This project addressed the regulatory-driven environmental aspect of fly ash use for soil stabilization, but the demonstrated engineering performance and economic advantages also indicate that the use of CCBs in soil stabilization can and should become an accepted engineering option.

Leaching Behavior of Fixed-Bed Gasification Ash Derived from North Dakota Lignite

three different fixed-bed gasifiers have been leached using: (1) the EPA-EP leaching test; (2) a similar test that starts with a synthetic North Dakota groundwater; (3) the ASTM D3987–81 method; (4) a long-term (120 day) leaching experiment. The gasification ashes were highly alkaline and produced pHs in the range 10–13 during tests 2 through 4. Compositions of major, minor and trace elements were determined by AAS and ICAP analyses of leachates. None of the EPA-EP test leachates from any of the ashes exceeded the EP Trigger values that define a hazardous waste according to the RCRA criteria. The long-term leaching experiments provided insights into the rates of extraction of elements from ash and could be useful in modeling selected failure scenarios in an ash disposal pit. At various times during the long-term leaching experiments, substantial fractions of the Na, K, Al, S, As, Mo, Se and V in the ash were extracted. Liquid to solid (l:s) ratios were important in determining the quantities of elements extracted and in controlling solution pH; during most of the 120 day experiments, pHs were near 13 in a 2:1 ratio experiment and just below 10 in a 20:1 ratio experiment. X-ray diffraction analysis of the leaching residue indicated that only a portion of the ash reacted with the leaching solutions. The reaction products were largely noncrystalline, but minor amounts of zeolites, calcite and gibbsite were identified. The formation of such secondary solid phases appears to be important in controlling leachant concentrations.

An Assessment of Residues from Duct Injection Demonstration Sites

The duct injection retrofit process for flue gas cleanup of sulfur dioxide emissions has been demonstrated at several sites in the United States through a series of U.S. Department of Energy research projects. As a subcontractor on one of the projects, the University of North Dakota Energy & Environmental Research Center performed a comprehensive characterization of four residues from three demonstration sites. The characterization task objective was to facilitate understanding of materials handling and to identify potential disposal and utilization options for these high-volume coal utilization solid residues. Characterization included evaluation of the physical and engineering properties, chemical composition, mineralogy, and leaching potential. Chemical characterization results provided interesting and valuable information from the standpoint of the bulk chemistry and phase assemblages, as well as potential environmental issues associated with the use or disposal of these materials. Results of the chem...