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The character of fireside ash deposits depend on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing furnace heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. Illustrative trends in these properties as deposits mature and grow are presented together with their effect on furnace exit temperature and efficiency. The reflective character of initial deposits from particular coals is then considered with predictions and measurements of the spectral character of such deposits, during the first three hours of growth, using on-line FTIR spectroscopy.

The evolution of fuel nitrogen during devolatilization and the formation of NOx during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20–30% of coal nitrogen was converted to NOx, Conversion increased markedly with increased fuel-lean conditions. The NOx formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH3. The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NOx depended primarily on char burnout. The contribution of char nitrogen to NOx formation was greater than that of volatile nitrogen under fuel-rich conditions.

Abstract The analysis of carbon oxidation data presented in a previous Fuel paper is shown to contain an error, as a result of which the intrinsic reactivity of carbon to oxygen is under-estimated by a factor of between one and four.

Abstract A one-dimensional heat transfer method was used to determine the thermal conductivity for a range of coal ash and synthetic ash samples at elevated temperatures. The effect of parameters such as temperature, porosity, and sintering time were investigated. The thermal conductivity of the samples was generally observed to increase with increasing temperature. During heating of the samples, softening of minerals and sintering reactions resulted in changes in the physical structure of the ash, which then altered the observed thermal conductivity. The thermal conductivity of sintered ash samples was found to be higher than that of unsintered samples. The sintering temperature and sintering time were found to increase the observed thermal conductivity irreversibly. A decrease in sample porosity was also observed to increase the thermal conductivity. Chemical composition was found to have little effect on the thermal conductivity, apart from influencing the extent of sintering. Predictions of the thermal conductivity of ash samples based on Rayleigh's model are also presented. The thermal conductivity of slag and particulate structures was modelled by considering spherical pores distributed in a continuous slag phase. A particulate layer structure was modelled by considering solid particles dispersed in a continuous gas phase. The Brailsford and Major model of random distribution for mixed phases gives results within 20% of the measured values for a partially sintered sample.

AbstractA comprehensive overview of coal oxidation methods under mild conditions is provided, including oxidation with oxygen, ozone, ruthenium ion catalysis, oxidizing acid, electrolysis, sodium hypochlorite, and hydrogen peroxide. The changes in chemical structure of coal during oxidation, the mechanism of different types of oxidation methods, and the possible applications of oxidation products are summarized. Comparison of these oxidation methods demonstrated that hydrogen peroxide and sodium hypochlorite oxidation of coal under mild conditions are the most suitable methods to study the molecular structure of coal and to obtain high‐value organic chemicals. Applications of different analytical techniques for determination and quantification of products and coal structures are also reviewed.

A theoretical-experimental study of the combustion of pulverized coal in the blowpipe and raceway of the blast furnace was undertaken. The objective of the study was to develop a mathematical model of combustion, and to evaluate it against experimental data. Both experimental and theoretical work involved three levels of complexity: (1) Laboratory-scale studies on coal devolatilization; (2) Pilot-scale combustion studies in a physical blowpipe-model; and (3) Combustion studies in a full-scale blast furnace. The predictions of the theoretical models compared favourably with the experimental data. Coal grind and devolatilization characteristics are shown to have a substantial effect on coal burnoff. Dispersion of the injected coal within the main blast is found to have a significant effect on coal burnoff. Gas recirculation and coke combustion within the raceway are found to influence the combustion of the injected coal.

Abstract The ash behaviour during combustion of a wheat straw sample was assessed using chemical fractionation and thermodynamic equilibrium calculations. Chemical fractionation has been used to distinguish the inorganics into reactive and non-reactive fractions. The reactive part of fuel comprises mainly of alkalis (K and Na), chlorine, sulphur and a part of alkaline earth metals (Ca, Mg), while the non-reactive fraction is dominated by silicon, aluminium and iron. The reactive fraction of inorganics is expected to reach equilibrium during combustion, while the non-reactive fraction simply passes through the combustion process unaffected. Literature on interaction of silica with sodium has indicated that, due to the high temperatures involved, a part of non-reactive fraction may react. The use of chemical fractionation alone does not take these reactions into account. In order to take these reactions into account a second model has been developed based on chemical fractionation and inclusion of a fraction of non-reactive fraction into the reactive fraction. This has been modelled to predict the interaction of reactive layer of larger ash particles with the combustion gases. Two models were developed, one based on only chemical fractionation and the second one based on chemical fractionation as well as secondary reactions involving ash (mainly silica). The results suggest that the amount of potassium sequestered in the condensed phase increased with the increase in the availability of non-reactive fraction (mainly silica) in the high-temperature section (1600-1300 °C) and hence less potassium is available for condensation in the fouling sections, in the case of K: Cl ratio greater than unity. It is observed that the ratio of chlorine to alkalis determines the importance of alkali/ash reactions at the high temperatures and thereby slagging and fouling propensities involving potassium.

Abstract The physical and chemical character of fireside ash deposits depend on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. The literature data for the properties are reviewed and are shown to depend principally on the physical ash character and whether the deposits have a particulate character or are sintered or fused. Bounds for the range of properties and expected variations with temperature and deposit porosity and other physical properties are suggested and compared with those for pure oxides and salts. For the radiative properties, theoretical predictions based on the optical properties are shown to predict trends with temperature and particle size for particulate ash and the observed transfer to higher emittance values for slags found at higher temperatures. The influence of the variation of these properties on radiative transfer is then quantified using mathematical models of furnaces.

Abstract Oxy-fuel combustion is an emerging technology to mitigate CO 2 emissions from power plants. Compared with other CO 2 capture technologies, gas impurities in oxy-fuel flue gas are highly concentrated, among which SO 2 is of concern. Dynamic transient experiments have been conducted in a semi-batch well stirred reactor (WSR) relevant to the conditions of the flue gas scrubbers operating at atmospheric pressure prior to CO 2 compression in oxyfuel technology for CCS. SO 2 in N 2 as well as in CO 2 were considered. The initial pH of the solutions considered was those of NaOH (with concentrations of 0.01 M and 1 M), Na 2 CO 3 (0.005 M) and NaHCO 3 (0.01 M). Analysis of the liquid solutions showed that the concentration of bicarbonate increases with pH at a pH larger than 5.5 indicating a loss of effectiveness of the sodium reagent. A practical discharge pH of the liquid discharge from the scrubber is also greater than 4 as at a lower pH the absorption rate of SO 2 reduced. This operating range is consistent with the reported operating range of the scrubber used in the Vattenfall oxyfuel pilot plant.