• Energy Research

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.

A new technique has been developed to determine the sintering rate of coal ash based on the measurement of the pressure-drop across a pellet of ash. The technique monitors the changes in porosity as a function of time, which is an indication of the degree of strength development due to sintering. The technique developed in this study shows a good repeatability of the rate of sintering and confirms that viscous flow is the dominant mechanism for sintering of coal ash. The activation energies of the sintering process studied in this investigation were found to be in the range of 200−315 kJ/mol.

A model for the prediction of iron-based slagging precursors from the combustion of iron-containing coals is detailed. The model accounts for the form of iron (pyrite or siderite), the distribution of iron within the pulverized coal, temperature, and oxidizing or reducing conditions. The input required for the model is a CCSEM analysis of the pulverized coal. For oxidizing conditions, the index predicts similar behavior for pyrite, and siderite-containing coals, with iron alumino-silicate ash particles becoming sticky at temperatures greater than 1400 °C. This suggests that for oxidizing conditions, the extent of included iron minerals is the most important factor. For reducing conditions, the index predicts sticky ash particles are formed at lower temperatures, as low as 1000 °C for pyrite-containing coals as a result of the decomposition and partial oxidation of pyrite-forming sticky particles, and 1100 °C for siderite-containing coals. For reducing conditions, the level of excluded pyrite mineral for pyrite-containing coals and the level of included iron-containing minerals associated with clays for siderite- and pyrite-containing coals are the most important factors determining slagging.

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.

The effect of the operating temperature on the behavior of coal ash on ceramic filters in a hot gas filtration system was studied. It was found that when the operating temperature exceeds the sintering temperature of the ash generated during combustion, the chance of forming ash-bridges between these filters is high. Calcium and potassium aluminosilicate were found to initiate the sintering of these samples, which results in an increase in the compression strength of these samples by an order of magnitude.

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.

Abstract Oxyfuel combustion is a CO2 capture technology which is approaching commercial demonstration. Of practical interest is the use of the compression circuit to allow low-cost cleaning options for various flue gas impurities. This work has focussed on three species – NOx SOx and Hg – and their removal during compression of “real” oxyfuel flue gas sampled as a slip stream from the demonstration Callide Oxyfuel Project. The flue gas slip stream was compressed using a bench-scale piston compressor developed to allow measurements of impurity concentrations after each compression stage using adjustable pressures. Several operating configurations were investigated including variable pressures from 5 to 30 bar, interstage temperature changes and flow rate. Slip streams taken before and after SOx removal allowed the impact of mixed NOx/SOx gases to also be investigated. The results from the “real” oxyfuel flue gas experiments for the three species were similar to those performed in the laboratory using synthetic flue gas and reported previously. The capture of SO2 was found at be greater at low pressures than NOx capture, with 90% removal of SO2 by a pressure of 10 bar, with NOx capture extending to higher pressures. The effect of residence time during compression had the greatest influence at higher pressures (>10 bar) where the kinetic rate of NO oxidation to NO2 increases less with pressure increase. Capture of NOx was increased from 55% to 75% by doubling the residence time in the compressor and could be further extended to 83% by increasing back end pressure from 24 bar to 30 bar. Lowering the temperature during compression produced the greatest NOx and Hg capture. Overall, the results indicate that capture of mercury during compression occurred as a consequence of high pressure, longer residence time and concentration of NO2.

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.