Mercury occurs naturally and as a result of human activities. One such activity is the combustion of mineral-enriched, sub-bituminous coal to produce electricityan industry that has existed for over 100 years. Although coal is absolutely necessary to supply the power that South Africa and its neighbouring countries requires, the emitted gases, especially mercury, impact the environment and present a complex array of health-related problems. Controlling the impact of mercury present in the environment depends on the efforts of governments, scientists, business and industry, agriculture, environmental organizations and individuals. Mercury is emitted from the point sources in different forms. Accurate determination of the emitted forms or species of mercury has become a global interest. Determination of the various mercury species requires several well-understood analytical techniques for the confident assessment of potentially contaminated samples. This study focuses on the development, validation and application of analytical methodologies that are capable of differentiating between the different forms of mercury in environmental samples (air, liquids and solids) from coal-fired power plants. Capillary electrophoresis with amperometric detection, high performance liquid chromatography with amperometric detection, and atomic fluorescence spectrometry methods were developed for mercury speciation. Very low detection limits observed using the methods. For capillary electrophoresis with amperometric detection, the detection limits were 0.005±0.002 μg/l for Hg2+ and 0.4±0.05 μg/l for MeHg+. Detection limits of 2±0.04 ppt and 0.01±0.02 μg/l for Hg2+ were observed for high performance liquid chromatography with amperometric detection and atomic fluorescence spectrometry respectively. These detection limits are attractive for the monitoring of mercury in the environment. Total mercury in solids (coal and ash) was measured by direct mercury measurement using a well-established method, involving the use of the mercury analyzer LECO AMA-254. Total gaseous mercury was measured using the Tekran 2537B system. On application to environmental samples, very good correlations in results were observed between the different methods. Mercury speciation in South African coal after acid extraction showed that only Hg2+ species was detected from the extracts and that 96% of total Hg in acid extracts is in the Hg2+ species form. Different trends in Hg speciation results at the Elandsfontein Air Quality Monitoring (AQM) station were observed over the sampling period. During winter sampling, Hg2+ was the predominant species, while Hg0 was predominant the species during summer sampling. Mercury speciation carried out at Duvha Power Station (units 1 and 2), equipped with fabric filter devices, revealed that the predominant form of Hg after the fabric filter devices was Hg2+, due to oxidation of Hg0 to Hg2+ as the flue gas temperature decreases. Mercury speciation at the Majuba Underground Coal Gasification flare revealed that although mercury is emitted from power plants in the form of different chemical species, with each species have a different fate in the atmosphere, the climate, wind direction and terrain also play roles in the transport of mercury emissions. Therefore, it is difficult to predict the transport patterns of emissions. Nonetheless, with correct measuring equipment and modelling, the patterns of emissions should be able to be predicted. The patterns observed and data recorded at the Elandsfontein AQM station and Duvha Power Station, respectively, were however, insufficient to permit accurate modelling. This study raised a number of other questions which are too comprehensive for this study to address. Therefore, more comprehensive atmospheric and combustion studies should be done.

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No description available.

The usage of the coal is very extensive. For each particular process, there is always a better method to burn some kinds of coals than the others. Each application of the combustion of coal is an individual problem. This problem involves not only the single factor of burning of coal but also the factors of the economy, materials, and manufacture. An efficient burning process is not always the suitable one to be used. On some practical field of coal combustion the testing, or trial method serves very satisfactorily for finding the best firing method. The boiler and furnace testing method applied in the steam power plant is an excellent example. However, there is an important thing in common in all the coal firing processes which is the burning of coal. An understanding of the fundamental characteristics of the coal burning is obviously very helpful to the practical usages, especially to the design of a new process. / Master of Science

Atmospheric pollution resulting from an open-cast coal mine situated 10 km southwest of Witbank (Mpumalanga, South Africa) was investigated during summer and winter 2004. Industrial and urban activities in and around Witbank release large amounts of toxic and criteria pollutants into the atmosphere. Spontaneous combustion from the many collieries in the Witbank area contributes to this problem. Direct, automated, and continuous in situ measurements of trace gas concentrations and prevailing meteorological parameters were carried out by a mobile monitoring unit and an automatic weather station. The data collected show that spontaneous combustion is a source of CO, NO, SO2 and H2S. Summer daily averages of SO2, NO, NO2 and O3 concentrations ranged between 1 and 18 ppb, 0.3 and 40 ppb, 12 and 75 ppb and 0.9 and 19 ppb respectively. Winter daily concentrations of SO2 and O3 were much higher, ranging between 15 and 180 ppb and 14 and 30 ppb respectively. NO and NO2, in contrast, were lower in winter (0.8 to 15 ppb and 2 to 28 ppb for daily means). Winter daily average concentrations of H2S, CO and CO2 ranged between 16 and 217 ppb, 2100 and 5100 ppb and 322 and 436 ppm). Synoptic circulations over the Highveld were found to affect pollutant concentrations. During winter, temperature inversions played a significant role in increasing the pollutant concentrations in the early morning hours until about 10:00. Although considerable amounts of NO, NO2 and O3 were captured; their concentrations were within the South African Department of Environmental Affairs and Tourism’s permissible levels as contained in the National Environmental Management: Air Quality Act (2004). SO2 concentrations during winter 2004 exceeded the allowed standards. Elevated concentrations of pollutants were mostly observed when the wind blew from the SE, SSE, S and WSW directions, implicating the 2A south pits of the open-cast mine investigated as the major source of the emissions.

Call number: LD2668 .T4 1979 P63 / Master of Science

published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

The high temperature interactions of alkali metal compounds with solids present in coal conversion processes are investigated. A temperature and concentration programmed reaction method is used to investigate the mechanism by which organically bound alkali is released from carbonaceous substrates. Vaporization of the alkali is preceded by reduction of oxygen-bearing groups during which CO is generated. A residual amount of alkali remains after complete reduction. This residual level is greater for potassium, indicating that potassium has stronger interactions with graphitic substrates than sodium. Other mineral substrates were exposed to high temperature alkali chloride vapors under both nitrogen and simulated flue gas atmospheres to investigate their potential application as sorbents for the removal of alkali from coal conversion flue gases. The compounds containing alumina and silica are found to readily adsorb alkali vapors and the minerals kaolinite, bauxite and emathlite are identified as promising alkali sorbents. The fundamentals of alkali adsorption on kaolinite, bauxite and emathlite are compared and analyzed both experimentally and through theoretical modeling. The experiments were performed in a microgravimetric reactor system; the sorbents were characterized before and after alkali adsorption using scanning Auger microscopy, X-ray diffraction analysis, mercury porosimetry and atomic emission spectrophotometry. The results show that the process is not a simple physical condensation, but a complex combination of several diffusion steps and reactions. There are some common features among these sorbents in their interactions with alkali vapors: In all cases the process is diffusion influenced, the rate of adsorption decreases with time and there is a final saturation limit. However, there are differences in reaction mechanisms leading to potentially different applications for each sorbent. Bauxite and kaolinite react with NaCl and water vapor to form nephelite and carnegieite and release HCl to the gas phase. However, emathlite reacts to form albite and HCl vapor. Albite has a melting point significantly lower than nephelite and carnegieite; therefore, emathlite is more suitable for lower temperature sorption systems downstream of the combustors/gasifiers, while kaolinite and bauxite are suitable as in-situ additives.

Chemical kinetic mechanisms governing the fate of coal nitrogen in the fuel-rich stage of a pulverized-coal staged combustion process were investigated. Emphasis was on determination of the effects of coal rank, temperature and stoichiometric ratios on the speciation and rates of destruction of nitrogenous species and correlation of coal data by a unif1ed mechanism. The relative importance of homogeneous and heterogeneous mechanisms during post-flame interconversion reactions of the fuel nitrogen pool was quantified. Experiments with doped propane gas and a high- and low-grade coals, burned under a variety of conditions in a 2 Kg/h downflow combustor, yielded timeresolved profiles of temperature, major (H₂, CO, CO₂, O₂ and N₂), nitrogenous (NO, HeN and NH₃) and hydrocarbon (CH₄ and C₂H₂) species. These profiles allowed global mechanisms describing the speciation and destruction of fuel nitrogen species to be explored, using predictive models of increasing levels of sophistication. Fuel nitrogen speciation varied significantly from coal to coal and depended on stoichiometric ratio and temperature, which were varied independently. A general correlation describing the destruction rate of NO was derived from data. This rate, which was first-order in both NO and NH₃, was generally valid for all coals and all conditions examined. Fuel nitrogen interconversion reactions, especially destruction of NO and HeN, was predominantly homogeneous, but no single elementary reaction was controlling. Temperature quench down the combustor is the origin of OH equilibrium overshoot. Expressions for estimating the OH equilibrium overshoot as a function of the axial temperature decay along the combustor were derived both empirically and kinetically from fundamental considerations using data from doped propane gas runs. These expressions, together with available literature values of gas phase rate coefficients, could adequately describe the post-flame NO and HeN profiles of coal and gas runs. HeN profiles in the far postflame zone of the coal flames are strongly influenced by the slow release of nitrogen from the coal residue. This devolatilization plays a critical role in supplying the HeN that drives the multistep process converting fuel N into molecular nitrogen.

No description available.