A study of sodium-ash reactions during the combustion of pulverised coal

Research Doctorate - Doctor of Philosophy (PhD)Sodium in coal has been associated with the fouling of heat transfer surfaces of boilers for many years even though its role has not been understood in detail. Currently, a theoretical and experimental study has been undertaken to investigate the reactions and reaction mechanisms of sodium with sulphur species and with inherent silica and kaolin during coal combustion and the cooling of the combustion gases. The dependence of these reactions on the form of the sodium in the coal and the combustion temperature has also been assessed. The experimental investigations involved combusting synthetic coals in which the concentrations, the size and the distribution of the inorganic constituents were controlled. These coals were prepared by adding into a slurry of demineralised Loy Yang brown coal, known quantities of inorganic constituents - sodium as a solution of sodium chloride or sodium acetate and as required, size graded silica or kaolin. The slurry was dried, pulverised and sieved to obtain the 63-90μm size fraction used in the experiments. The coals were combusted in a drop tube furnace and the supra micron product ash was collected in a heated cyclone, upstream of a filter to collect the submicron condensed sodium salts. As well as coal composition, other experimental variables examined, were the furnace wall temperatures of 1000°C, 1200°C and 1400°C, coal feed rates of 5.5 g/h and 2.7 g/h, position and temperature at which the reaction products were quenched and collected, and the addition of gaseous sulphur dioxide with the secondary air equivalent to 3% sulphur in the dry coal. The collected ash was analysed by wet chemical methods, examined by electron microscope and sized by a Malvern laser diffraction particle size analyser. The experimental program included the determination of coal particle temperatures by combusting a synthetic coal with inherent ash which was stable under combustion conditions and had a sharp melting point. The data from these experiments was used as input to a combustion model to establish the char reactivity which was then used in the same model to predict combustion parameters for the range of conditions employed in the other experiments. The experiments to investigate the reactions of sodium chloride with sulphur oxides in combustion gases, showed that below 1000°C, the sulphation of sodium chloride was kinetically limited for times which are normally available in p.c. fired boilers. For temperatures above 1100°C, the extent of sulkphation was found to be greater than that predicted at the thermodynamic equilibrium for the overall system, possibly due to the formation of the radicals NaSO₂ and NaSO₃ during combustion. From the experiments to study the reactions of sodium with silica, it was found that sodium silicates formed almost exclusively during coal combustion, probably due to the progressive agglomeration and coalescence of the ash reducing the surface of silica available for reaction. The rate of silicate formation increased with temperature but this was largely offset by a parallel reduction in combustion time, resulting in only small changes in the extent of formation with temperature. Generally, the extent of silicate formation was greatest at 1200°C and least at 1000°C. The presence of chlorine and to a much lesser extent sulphur, reduced the formation of sodium silicate. By firing coals which contained a mixture of two separately prepared coals, one with a sodium salt and the other with only silica, it was shown that at gas temperatures of 1200°C-1400°C, sodium was completely released from the coal prior to the start of silicate formation. At 1000°C, both organic sodium and sodium chloride were probably not fully released before silicate formation commenced. Exploratory experiments with kaolin evidenced that sodium, as sodium chloride, reacted to a greater extent with kaolin than with silica. Qualitative agreement was found between the experimental results and the thermodynamic equilibrium predictions of the influence of chlorine, sulphur, alumina and sodium concentration on the extent of sodium silicate or sodium aluminium silicate formation. Equilibrium calculations indicated that calcium or magnesium would reduce the extent of combination of sodium with silica. Theoretical predictions of the release rate of sodium from coal, during combustion, also agreed in general with experimental results. A theoretical model was developed to study the reaction of organic sodium with silica during coal combustion. From this model it was concluded that the reaction rate was controlled initially by the diffusion of sodium hydroxide in the char boundary layer but as combustion proceeded, the diffusion of sodium oxide in the product silicate melt combined with the diffusion of sodium hydroxide through the porous char increasing influenced the reaction rate. The results of the study show that the influence of sodium on the severity of fouling in boilers is dependent on the form and concentration of the sodium in the coal, the composition of the coal's other inorganic constituents and the combustion and furnace conditions