Gasification of Victorian lignite in a laboratory scale fluidised bed gasifier

Posted with permission of the Organising Committee, 5th Asia Pacific Conference on Combustion, The University of Adelaide, ASPACC05.A 200-mm diameter, laboratory-scale atmospheric-pressure fluidised-bed reactor was designed and constructed by the Cooperative Research Centre (CRC) for Clean Power from Lignite. The purpose of this facility is to obt ain experimental data for the air/steam gasification of Australian lignite in order to validate the Centre’s mathematical model of a bubbling fluidised bed gasifier. An air-dried mixture of low-ash Victorian lignite has been used in air-steam and air-only gasification tests. The product syngas composition demonstrated successful gasification of coal with carbon monoxide and hydrogen concentrations each in the range 16-20 vol%. More carbon monoxide was measured in the syngas during coal gasification with air only. The gas composition of major species was observed to be relatively constant within the freeboard of the gasifier

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

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.Thesis (Ph.D.) -- University of Adelaide, Dept. of Chemical Engineering, 2001

Professor Toynbee philosophy of history

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Control of agglomeration and defluidization during fluidized-bed combustion of South Australian low-rank coals

South Australian low-rank coals have high sulfur and high sodium contents, which cause operational problems when the coals are combusted. Fluidized-bed combustion (FBC) of these coals allows for efficient combustion and for convenient sulfur removal by the addition of in-bed sorbents, such as limestone or dolomite. However, the presence of sodium may result in operational problems for FBC because sodium compounds, such as sodium sulfate, which is present in the coal ash, may cause the bed particles to become "sticky" and lead to a loss of bed fluidization. Combustion experiments have been performed in a laboratory-scale fluidized-bed combustor for two South Australian low-rank coals: Kingston and Lochiel. This work was undertaken to compare the behavior of these two coals and to allow for comparisons to previous experience gained using Lochiel coal in both laboratory experiments and pilot-plant operation. Kaolinite-rich clay additives were used in these experiments in an attempt to alleviate the problems associated with sodium present in coals. The effect of refreshing and removing the bed material without interruption of the combustion process was also studied experimentally. Kingston coal showed better performance in the FBC process than Lochiel coal. The ash layer formed from FBC of Kingston coal was found to be less sticky than that formed by Lochiel coal, resulting in longer defluidization times for Kingston coal than for Lochiel coal when no clay additives were used. Kingston coal was able to be combusted at 850 °C with the addition of clay at 5% of the total feed rate and with the addition/removal of bed material at 5% of the total feed rate. Analysis showed that the sodium from the coal had reacted with the kaolinite in the clay to form nepheline, a high-melting-point solid compound, which thus restricted the formation of liquid sodium sulfates in the bed. The results of this study show good agreement with the results of previous studies that showed that the addition of kaolinite-rich clays led to problem-free combustion of Lochiel in both small- and pilot-scale operations at 800 °C. © 2011 American Chemical Society.Philip J. van Eyk, Adam Kosminski, and Peter J. Ashma

Modelling the performance of a laboratory-scale fluidised-bed gasifier

The long term supply of relatively low-cost electricity within South-eastern Australia is favoured by the efficient utilisation of low-rank coal reserves within this region. A number of advanced technologies have been considered by the Cooperative Research Centre (CRC) for Clean Power from Lignite, and its predecessor, with the most promising of these options all relying on the high-pressure, gasification of lignite. An ongoing focus of the Centre’s Advanced Gasification program is its mathematical model of a bubbling fluidised bed gasifier (FBG). This model has been developed previously, and partially validated, using literature data obtained for mainly high-rank coals. While the model has been successfully applied to gain further insight into experimental observations in some limited instances, it has not yet been rigorously validated for Australian brown coals. The Centre has recently commissioned, and is now operating, a 200-mm laboratory-scale, atmospheric-pressure bubbling fluidised bed gasifier with the specific intention of collecting in-bed and process data for validation of the FBG model. An air-dried mixture of low-ash Victorian lignite is gasified using air and steam. Gases are sampled using water-cooled probes at various locations within the bubbling bed and in the freeboard. Gas samples are analysed for major products (CO, H2 and CO2), minor products (CH4 and higher hydrocarbons) and pollutant species (e.g. H2S, SO2, COS, etc) using a suite of analysers that includes a Fourier transform infrared (FTIR) spectrometer and a micro-gas chromatograph (GC). The product syngas is also sampled and analysed after exiting the gasifier. Other data necessary for the model validation, such as pressure, temperature, and feed flow rates, are also collected. This paper reports on the first phase of this validation program and discusses deficiencies of the existing theoretical model when describing the performance of a real bubbling, fluidised-bed gasifier.http://www.icms.com.au/chemeca2005/abstract/226.ht