Reactions between sodium and silicon minerals during gasification of low-rank coal

Abstract

Fluidised bed gasification (FBG) associated with integrated gasification combine cycle (IGCC) power generation is an important way of using vast low-rank coal resources in an economic and environmentally acceptable manner. A key factor in the successful operation of coal gasification systems is the ability to control and mitigate ash-related problems. Such problems are closely tied to the abundance and association of the inorganic components in coal and the gasification conditions. Of particular importance for low-rank coals is the presence of sodium, which has been found to cause FBG operational problems such as bed agglomeration and ash deposition. However, the critical fundamental mechanisms of sodium behaviour in coal gasification systems are not fully understood. The main objective of this study was to elucidate the role of sodium and silicon minerals in fo1mation of liquid phases potentially responsible for fluidised bed agglomeration during gasification of a high-sulphur low-rank coal in order to identify ways of preventing formation of those phases. Experimental investigations involved the preparation of synthetic coals, or separate mineral mixtures, with known quantities of organically-bound sodium or sodium chloride and silica or kaolin either separately or in combination. The mineral mixtures were used as an aid in the interpretation of the reactions of sodium with silica or kaolin in the coal char. In addition, thermodynamic predictions were made for the possible compositions and phase distribution of sodium and silicon species formed during gasification and pyrolysis of these synthetic coals. The synthetic coals were pyrolysed and gasified in a horizontal tubular reactor under conditions representative of a typical fluidised bed gasifier. Other than mineral composition parameters, the reaction temperatures (650°C, 750°C and 850°C), gas environment (pure atmospheres of either nitrogen, carbon dioxide or steam) and reaction times (45 seconds to 35 minutes) were varied. Mineral mixtures were exposed to the same experimental conditions. The collected coal char and post-reaction mineral mixture products were analysed by wet chemical methods, electron microscopy and mineralogical methods. The experimental program investigated sodium transformation and extent of vaporisation in each of the individual atmospheres. The organically-bound sodium was found to be transformed into sodium carbonate, contrary to thermodynamic predictions of the formation of sodium sulphide for pyrolysis conditions. Up to half of the sodium was vaporised from the char. Volatilisation of sodium increased with temperature and time, and was highest for gasification with carbon dioxide. Sodium chloride present in coal vaporised during pyrolysis and gasification and reacted with coal partly forming sodium carbonate. The release of sodium was disproportionate to that of chlorine. Almost all of the chlorine was released at 850°C, and its release was twice as high as sodium. The release of sodium and chlorine was dependent on temperature and time, but not on the particular gas atmosphere. Steam was found, both theoretically and experimentally, to be the most important component of the gasification environment. Steam substantially reduced the melting temperature of sodium carbonate and consequently gasification with steam resulted in the formation, in a liquid-solid state reaction, of liquid silicates at as low as 750°C, while gasification with carbon dioxide resulted in the same at 850°C. Sodium chloride and silica reacted only in steam and formed fused silicates at 750°C, with the rate of silicate fo1mation substantially slower than for reaction between silica and sodium carbonate. Formation of silicates around silica particles and fused silicate joints between individual silica grains inside the char was established to occur uniformly throughout char particles in gasification conditions. Liquid silicates would be a cause of bed agglomeration and defluidisation during fluidised bed gasification of coal. Qualitative agreement was found for gasification but not for pyrolysis conditions between experimental results and thermodynamic predictions for the formation of liquid silicates from organically-bound sodium and silica, but at higher than predicted temperatures. The results for mineral mixtures were in better agreement with thermodynamic calculations as the rate of formation of silicates was much higher in mixtures than in the synthetic coal. The prediction by equilibrium calculations for all of the silica to be fully dissolved in liquid silicates was not observed under any of the experimental conditions. However, partial silica dissolution was concluded for mineral mixture products. Silica solubility in the formed liquid silicates will increase the total mass of fused silicate glass formed in a FBG. Partial agreement has been established between theoretical predictions and experimental results for gasification and pyrolysis of coal with organically-bound sodium and kaolin. Experimental results showed that kaolin and sodium had reacted upon reaching 650°C to form a solid sodium aluminosilicate Na₂O.Al₂O₃.2SiO₂, principally nepheline with a melting point above 1250°C. The reaction rate was faster in steam than in carbon dioxide or nitrogen. Sodium chloride reacted with kaolin, but at a slower rate, also to form sodium aluminosilicate Na₂O.Al₂O₃.2SiO₂, with steam reaction rate much higher than in carbon dioxide. Increasing the process temperature increased the reaction rate. It is inferred that under FBG temperature conditions, as kaolin is transformed with the preservation of its hexagonal crystal structure into meta-kaolinite Al₂O₃.2SiO₂ it reacts with sodium into nepheline, with the further preservation of the hexagonal structure. Reactions of sodium with kaolin will prevent reactions of sodium with silica to form liquid silicates. No formation of sodium aluminosilicate albite Na₂O.Al₂O₃.6SiO₂ was established experimentally, contrary to thermodynamic predictions for both forms of sodium. The results from experiments showed that carbon conversion in steam was considerably higher than in carbon dioxide for coals containing either form of sodium. It was established that coal activation energy is associated with catalytic activity of sodium. For coal containing sodium chloride, activation energies are substantially higher than for coals containing organically-bound sodium. The presence of such minerals as silica and kaolin significantly increases the activation energies for coal gasification reactions with steam and carbon dioxide. However, the impact was lower for coal containing sodium in the form of sodium chloride. Recommendations made for future work include establishing efficient ways to introduce kaolin to low-rank coal during gasification to reduce the formation of liquid silicates and hence inhibit agglomeration and defluidisation.