Sodium/ash reactions during pulverised coal combustion - treatments and additives to control deposition

A theoretical and experimental investigation is undertaken to examine treatment methodologies to modify the ash chemistry and to reduce the slagging and fouling of furnace walls associated with the firing of Victorian brown coals.The experimental program involved detailed studies with a high slagging and fouling coal from the Loy Yang open cut mine. The ash forming elements namely (Na, Mg, Ca, Fe, Al, Cl and S) in Loy Yang coal are usually bound within the coal matrix to oxygenated functional groups. This coal also contains discrete 8 to 80 μm mineral particles of silica. The coal was treated to adjust levels of Na, Cl and AI by (a) washing with water to reduce the levels of Na and Cl, (b) washing with an aqueous solution of aluminium lactate to reduce the levels of Na and Cl and to increase the levels of soluble aluminium, (c) adding aluminium lactate to the coal without reducing Na and Cl levels, (d) adding sodium acetate to the coal without reducing Na and Cl and (e) adding sodium acetate and aluminium lactate without reducing Na and Cl. Closely controlled combustion experiments utilising a drop-tube furnace at furnace temperatures of 1000°C, 1200°C and 1400°C have been conducted. The reaction products were quenched and collected using sampling systems. The experiments with coal also included the investigation of deposition behaviour of untreated coal and the treated coals on a deposition probe maintained at different probe temperatures (500°C, 600°C, 700°C and 800°C) and at one furnace temperature of 1250°C. Additional experiments involving nine mixtures of oxides and sodium salts investigated the reactions of sodium with kaolin, silica and alumina and the dependence of these reactions on the form of the sodium in coal. Thermodynamic calculations have also been performed using representative coal systems as well as mixtures of oxides and sodium salts. In general, qualitative agreement was found between the experimental results and thermodynamic equilibrium predictions for the influence of sodium, aluminium, chloride and sulphate concentrations on the extent of silicate/aluminate formation. Ash deposition experiments indicated that the treatment of Loy Yang coal with soluble aluminium would result in less adhered ash on deposition probe, suggesting a marked reduction in fouling of heat transfer surfaces in furnaces. Ash formation experiments indicated that the treatment of Loy Yang coal with soluble aluminium and with washing would result in substantial reduction in the amount of condensable salts and an increase in aluminium containing phases with aluminium treated coals, leading to substantial increase in the viscosity of the exterior layers of silicate ash. Calculations using a deposition model indicated that aluminium containing coals would result in less sticky ash and a subsequent reduction in the retention of ash on furnace walls. A descriptive model for ash formation has been developed using thermodynamic calculations which is relevant to the Victorian brown coal. The predictions from thermodynamic calculations were obtained with two models, namely a mixed model and an unmixed model. The mixed model situation allows silica particles to interact with other inorganics, whereas the unmixed model restricts silica interactions with sodium only. The model calculations performed on treated and untreated coals suggest that the ash formation is closer to the unmixed model. Experiments with mixtures of oxides and sodium salts indicated that the sodium in acetate form would result in increased silicate formation when compared to other sodium salts. From the experimental results it was found that the presence of chlorine in the reaction system causes a substantial reduction in the formation of silicates. The results of present study are of practical significance and have increased the knowledge on various treatment methodologies to reduce ash related problems by modifying the ash chemistry of brown coals. The results also showed that the fouling characteristics of a coal can be controlled by employing external additives. Estimates of additive requirements suggested that kaolin or alumina (2-3 wt% of coal) with a particle size ranging 10-20μm would control fouling/slagging problems associated with the combustion of Victorian brown coals.