Dissertations / Theses on the topic 'Low rank coals'

The kinetics of gasification of two Alberta sub-bituminous coal chars with CO₂ have been investigated in the temperature range of 800-950°C. The reactor utilized in the experimental work was a laboratory-size batch fluidized bed. The overall gasification kinetics were followed by measurements of gas composition and flow rates. Chars in the particle size -841 + 420 μm were gasified with gas mixtures involving CO, CO₂ and He. Initially it was determined that beds containing 20.0 g of char (L/D ≈ 0.25), and a total inlet flow rate of 10 l/min were able to provide an adequate mixing of the reactants and near isothermal conditions in most of the experiments. Moreover these operating conditions allowed the measurement of reaction rates with minimum influence of CO₂ starvation, elutriation of char particles and fluidization hydrodynamics. Chars prepared with longer soak time and lower heating rate were less reactive due to their lower surface area and more closed pore structure. For the conditions studied, Highvale chars were more reactive than similarly prepared Smoky Tower chars primarily due to differences in surface area. Increasing PCO₂ in the inlet gas caused a non-linear increase in the reaction rate, and CO strongly retarded the reaction, mainly at low temperatures, suggesting that Langmuir-Hinshelwood kinetics are followed. The reaction also was heavily affected by temperature, especially with increasing concentration of CO due to the poisoning effect of this gas. The external appearance of char particles and the changes in pore structure and ash characteristics with the extent of reaction were examined by SEM. The initial rate of reaction follows the LH equation. The apparent activation energies for the rate constants k₁ and k₂ of this equation are respectively 176 and 286 kJ/mole. However the LH equation was not the most suitable equation to represent the kinetics of the reaction under the conditions investigated. Therefore a power-law rate equation that accounted for the variation in surface area of the chars was employed. The orders of reaction obtained for Highvale and Smoky Tower chars are 0.4 and 0.7 respectively. The initial apparent activation energies for Highvale chars are 143,210 and 255 kJ/mole for CO/CO₂ ratios of 0, 0.25 and 0.50 respectively. For Smoky Tower chars the initial apparent activation energy is 202 kJ/mole. The equations of Bhatia and Perlmutter and Dutta et al. were used to correlate the reactivity of both chars with the extent of reaction. The equation of Bhatia and Perlmutter was able to represent the data better at any temperature for Smoky Tower chars and up to 900°C for Highvale chars. For Highvale chars, the equation of Dutta et al. fitted the data better at 950°C. These results and the values of apparent activation energy obtained suggest that, for the conditions investigated, the gasification reaction was mainly chemically controlled with pore diffusion effects increasing for Highvale chars at 950°C. In addition, the increase in apparent activation energy when the carbon conversion and the CO/CO₂ratio increased, supports the contention that the reaction was under chemical control.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Bibliography: leaves 279-293. Temperature-programmed Pyrolysis experiments employing Bowmans and Lochiel low-rank coal and treated Bowmans coals, were conducted to investigate the sulphur transformations during pyrolysis.

Thermogravimetric analysis studies were performed on three low rank coals and a lignite (average size 41um), as well as the dense media separated fractions of these in nitrogen and air. All experiments were carried out at 20°C/min and over the temperature range of 25 to 850°C. Such studies have been used to examine the effect of the different minerology of the coals upon the devolatilization and combustion processes, to investigate the effect of mineral matter on coal reactivity, and to provide data for mathematical models of pulverized coal combustion and gasification in entrained systems. The first-order kinetic model, used to characterise the devolatilization and combustion processes, seemed to correlate the experimental data reasonably well. The activation energy values corresponding to devolatilization, and ranging from 22 to 38 KJ/mole, were similar for all coals, while those corresponding to combustion of the devolatilized coal varied between 41 and 96 KJ/mole and were significantly higher. The presence of the mineral matter slightly increased the reactivity of the coals in nitrogen, whereas it affected greatly the temperature sensitivity of the reaction in air, A mathematical model, incorporating thermogravimetric analysis data of Whitwick coal, was developed in order to predict the burning history of a single entrained coal particle, and to study the effect of ambient gas temperature and boundarylayer thickness on the final conversion, This model included a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes, as well as a partial differential equation, for computing the temperature profile within the particle. The system of equations was solved numerically, The location of the reaction zone on the solid surface, where gas-phase and heterogeneous combustion could occur simultaneously, appeared to describe successfully the combustion mechanism of the particle. The combustion process was chemical reaction rate controlled. The particle behaved essentially isothermally and its lifetime was estimated to be approximately 1.23s. A higher ambient gas temperature or boundary-layer thickness resulted in shorter burn-out times. Finally, a one-dimensional, steady-state model, for an entrained flow coal gasifier, was developed, by using combustion data from thermogravimetric analysis of Whitwick coal. The model was based on mass and energy balances, heterogeneous reaction rates and homogeneous gas-phase equilibrium. The resulting set of nonlinear mixed ordinary differential-implicit algebraic equations was solved numerically, by using modified Euler's method in conjunction with a nonlinear algebraic equation solver. Parametric studies were made, in order to provide a better understanding of the reactor performance, in terms of coal conversion, product gas composition and temperature profiles along the reactor, under various operating conditions, such as feed flow rates and gasifier pressure. High conversion of carbon could be predicted only if the devolatilization reaction proceeded in parallel with the heterogeneous reaction at the coal surface, with oxygen and steam. The model suggested that a two-stage gasification with precombustion, followed by reaction with steam would be possible. The critical parameters in gasification were the steam-to-coal and oxygen-to-coal feed ratios. Data is presented showing their effect on total conversion, synthesis gas composition and calorific value, as a function of reactor pressure. No experimental data for the verification of the simulation was performed, but comparison of the results with those of previous investigations showed consistency.

The solvent-induced swelling behaviour of Victorian brown coals was examined in detail to probe the bonding mechanisms in very low rank coals (in this case Victorian brown coal). Correlation of solvent properties with differences in observed swelling behaviour were interpreted in terms of the coal structure, and means of predicting the observed behaviour were considered. Modification of the coal structure via physical compression (briquetting), chemical digestion, thermal modification, and functional group alkylation was used to further elucidate those structural features which govern the swelling behaviour of Victorian brown coals. Briquette weathering (i.e. swelling and disintegration of briquettes when exposed to variations in humidity and temperature) was examined by making alterations to briquette feed material and observing the effects on swelling in water. The application of solubility parameter alone to prediction of coal swelling was rejected due to the many exceptions to any proposed trend. Brown coal swelling showed a minimum when the solvent electron-donor number (DN) minus its electron-acceptor number (AN) was closest to zero, i.e. when DN and AN were of similar magnitude. The degree of swelling increased either side of this point, as predicted by theory. In contrast to the solubility parameter approach (which suffers from the uncertainty caused by specific interaction between coal and solvent), the electron donor/acceptor approach is about specific interactions. It was concluded that a combination of total and three-dimensional solubility parameters and solvent electron donor/acceptor numbers may be used to predict solvent swelling of unextracted brown coals with some success. Solvent access to chemically densified coal was found to be insensitive to a reduction in pore volume, and chemical effects were dominant. Thermal modification of the digested coal resulted in reduced swelling for all solvents, indicating that the structure had adopted a minimum energy configuration due to decarboxylation and replacement of hydrogen bonds with additional covalent bonds. Swelling of oxygen-alkylated coals demonstrated that the more polar solvents are able to break relatively weak hydrogen bonded crosslinks. The large difference between the rate and extent of swelling in water (and hence weathering) of Yallourn and Morwell briquettes was shown to be almost entirely attributable to exchanged magnesium. Magnesium exchange significantly increases the rate and extent of swelling of Yallourn coal. It was also shown that the swelling of briquettes due to uptake of water by magnesium-exchanged coals is reduced significantly with controlled ageing of the briquettes. The solvent swelling behaviour of Victorian brown coals is consistent with the notion that coal is a both covalently and non-covalently crosslinked and entangled macromolecular network comprising extractable species, which are held within the network by a wide range of non-covalent, polar, electron donor/acceptor interactions. Solvents capable of significant extraction of whole brown coals are also capable of significant swelling, but not dissolution, of the macromolecular coal network, which supports the view that the network is comprised of both covalent and ionic bonding. Victorian brown coals have also been shown to exhibit polyelectrolytic behaviour due to a high concentration of ionisable surface functionalities.

Um novo laboratório experimental desenvolvido para investigar a combustão de carvão em atmosferas de oxicombustão é descrito em detalhes na tese. Um reator ciclônico é utilizado para a combustão de carvões de baixo rank em condições de escoamento turbulento e com swirl, e operando em temperaturas similares às encontradas em câmaras de combustão industriais. Um sensor potenciométrico de oxigênio, instalado dentro do reator ciclônico, é utilizado para medir o consumo de oxigênio durante a combustão das amostras de carvão. Amostras de carvão com alto teor de cinzas, das minas do Leão e Bonito localizadas no sul do Brasil, e amostras de carvão linhito pré-seco (LTBK), da região da Lusácia, na Alemanha, foram submetidos à combustão em atmosferas de ar e de oxicombustão (O2/CO2/H2O). Os experimentos foram realizados em três temperaturas médias do gás de combustão: 1073, 1173 e 1273 K. Para as amostras do carvão LTBK, a oxicombustão foi composta com duas atmosferas de O2/CO2 sem vapor d’água (21/79 e 30/70) e três atmosferas de O2/CO2/H2O (30/60/10, 30/50/20 e 30/40/30) em base molar, enquanto para as amostras dos carvões Leão e Bonito, a oxicombustão foi composta com duas atmosferas de O2/CO2 (21/79 e 30/70). As amostras de carvão foram peneiradas para uma faixa de tamanho de partículas de 1250 a 2000 μm e 125 a 500 μm, e com massas de 1g e 3g. Em adição aos testes, amostras de char dos carvões Leão e LTBK foram preparadas para investigar o comportamento da combustão de suas matrizes carbonosas com diferentes níveis de matéria volátil. Primeiramente, a investigação da combustão do carvão é feita diretamente a partir das curvas de concentração de oxigênio medidas para a combustão das amostras de carvão LTBK submetidos às atmosferas de ar e oxicombustão com vapor d’água, conforme explicado no Capítulo 2 desta tese. Entretanto, devido ao grande número de experimentos realizados e a necessidade de entender a influência dos fatores, como temperatura e as composições das atmosferas oxidantes, e as interações entre esses fatores na combustão do carvão, a análise pela metodologia do Projeto de Experimentos (DoE) é aplicada nos experimentos, conforme detalhado no Capítulo 3. A reatividade dos carvões de baixo rank é investigada por meio do cálculo dos parâmetros cinéticos globais e dos coeficientes da taxa de reação de combustão do char, considerando a hipótese de um reator bem misturado, a ser verificada ao longo da investigação, conforme descrito no Capítulo 4. A partir da análise das curvas de concentração de oxigênio (Capítulos 2 e 3), os resultados mostram que a atmosfera de oxicombustão com 79% de CO2 (21/79, O2/CO2) aumenta o consumo de oxigênio para os carvões de alto teor de cinzas, Bonito e Leão, e para o carvão linhito com alto teor de voláteis (LTBK), devido à influência da reação de gaseificação do CO2 a partir de temperaturas acima de 1073 K. A partir da análise dos parâmetros cinéticos globais calculados para avaliar a reatividade dos carvões, os resultados mostram que a hipótese do reator bem misturado não é o suficiente para capturar a cinética da combustão do carvão por batelada no interior do reator ciclônico. A reatividade dos carvões investigados, através das constantes efetivas da taxa de reação, variando-se a composição da atmosfera oxidante, temperatura do gás de combustão, tamanho de partículas, massa da amostra e posição do sensor de oxigênio, têm influência do escoamento com swirl e seus termos advectivos e difusivos.
A novel laboratory facility designed to investigate coal oxy-fuel combustion is described in the present work. A cyclone chamber allows for the combustion of low-rank coal under turbulent conditions and swirling flows, covering a temperature range similar to those found on practical furnaces. A potentiometric oxygen sensor with oxide-ion conducting solid electrolytes, as stabilized zirconia, installed within the cyclone reactor, is used to measure the oxygen consumption during the combustion of coal samples. High ash coals samples, from Leão and Bonito mining sites located in South Brazil, and pre-dried lignite coal samples (LTBK), from the Lusatian region, in Germany, were burned under air and oxy-fuel (O2/CO2/H2O) atmospheres. Experiments were carried out at three average gas combustion temperatures: 1073, 1173 and 1273 K. For LTBK coal, oxy-fuel combustion was composed with two O2/CO2 atmospheres (21/79 and 30/70) and three O2/CO2/H2O atmospheres (30/60/10, 30/50/20 and 30/40/30) in molar basis, whereas for Leão and Bonito coals, the oxy-fuel combustion was composed with two O2/CO2 atmospheres (21/79 and 30/70). Coal samples were sieved to a size range of 1250 to 2000 μm and 125 to 500 μm and with 1g and 3g. In addition, char samples from Leão and lignite coals were prepared in order to investigate the combustion behavior of its carbon matrix for different levels of volatile matter. The investigation of coal combustion behavior is made firstly directly on the oxygen concentration curves measured for LTBK coal samples burned under air and oxy-fuel atmospheres with water vapor, as detailed in Chapter 2 of this thesis. However, due to the great number of experiments performed and the need to understand the influence of the factors, as temperature and atmosphere compositions, and the interactions between them on the coal combustion, the Design of Experiments (DoE) analysis is applied in the laboratory test facility, as developed in Chapter 3. The reactivity of low rank coals is assessed by means of global kinetic parameters and of char combustion reaction coefficients, under assumption of a well stirred reactor to be tested along the investigation, as described in Chapter 4. From the analysis of oxygen concentration curves (Chapter 2 and 3), results show that oxidizer oxy-fuel atmosphere with 79% CO2 (21/79, O2/CO2) increases the oxygen consumption for high ash coals, Bonito and Leão, and for pre-dried lignite coal, LTBK, due to the influence of CO2 gasification reaction on their coal combustion reactions from gas combustion temperatures higher than 1073 K. From the analysis of global kinetic parameters calculated to assess the coal reactivity, the results show that the hypothesis of a well stirred reactor is not enough to capture the kinetic involved in coal combustion burned in batch mode within the cyclone reactor. The coal reactivity investigated with the effective reaction rate constants for different oxidizer atmospheres, gas combustion temperatures, particle diameters, sample masses and oxygen sensor position, is influenced by the swirling flow with its advective and diffusive terms.

Bibliography: leaves 234-252. The devolution times of seven coals were determined by measuring the centre temperature response for single particles held stationary in a bench scale atmospheric fluidised-bed reactor.

Includes bibliographical references (leaves 127-136).
The coal biosolubilisation processes may be used to convert low rank coal to either a clean, cost-effective energy source or to value-added products. This can lead to increased utilisation of low-rank coal. Low-rank coal is currently under-utilised because of its low calorific value, high moisture and sulphur content. Most research on coal biosolubilisation has centred on pre-treated coal. Little work is reported on naive coal. Low yields of solubilised coal products are currently reported in the literature. This may be due to further degradation of the soluble processes or to limitation of solubilisation step. These products have potential as starting materials for biotransformation to value-added products. However, to date, small volumes of solubilised coal products are available to assess their potential for further biotransformation owing to current biosolubilisation of low-rank coal being widely carried out as a small scale Petri dishes or Erlenmeyer flask of volume. This dissertation presents the results of the investigation of biosolubilisation of low-rank coal in slurry systems using Trichoderma alroviride. Its main objectives were to investigate key operating variables influencing untreated low rank coal biosolubilisation and degradation of soluble products, and to study different reactor configurations for coal biosolubilisation

This thesis pertains to the processing of ultra-fine mineral particles and low rank coal using the hydrophobic--hydrophilic separation (HHS) method. Several explorative experimental tests have been carried out to study the effect of the various physical and chemical parameters on the HHS process. In this study, the HHS process has been employed to upgrade a chalcopyrite ore. A systematic experimental study on the effects of various physical and chemical parameters such as particle size, reagent dosage and reaction time on the separation efficiencies have been performed. For this, a copper rougher concentrate (assaying 15.9 %Cu) was wet ground and treated with a reagent to selectively hydrophobize the copper-bearing mineral (chalcopyrite), leaving the siliceous gangue minerals hydrophilic. The slurry was subjected to a high-shear agitation to selectively agglomerate the chalcopyrite and to leave the siliceous gangue dispersed in aqueous phase. The agglomerates were then separated from dispersed gangue minerals by screening and the agglomerates dispersed in a hydrophobic liquid (n-pentane) to liberate the water trapped in the agglomerates. The chalcopyrite dispersed in the hydrophobic liquid was separated from the medium to obtain a concentrate substantially free of gangue minerals and moisture. The copper recoveries were substantially higher than those obtained by flotation. The HHS process was also tested on ultrafine mono-sized silica beads. The results were superior to those obtained by flotation, particularly with ultrafine particles. The HHS process has also been tested successfully for upgrading subbituminous coals. Low-rank coals are not as hydrophobic as high-rank coals such as bituminous and anthracite coals. In the present work, a low-rank coal from Wyoming was hydrophobized with appropriate reagents and subjected to the HHS in a similar manner as described for processing copper. The results showed that the HHS process reduced the moisture substantially and increased the heating value up to 50% without heating the coal. Laboratory-scale tests conducted under different conditions, e.g., particle size, reagent type, reaction time, and pretreatments, showed promising results. Implementation for the HHS process for upgrading low-rank coals should help reduce CO2 emissions by improving combustion efficiencies.
Master of Science

京都大学
0048
新制・課程博士
博士(エネルギー科学)
甲第20731号
エネ博第359号
新制||エネ||70(附属図書館)
京都大学大学院エネルギー科学研究科エネルギー応用科学専攻
(主査)教授 大垣 英明, 教授 松田 一成, 教授 河瀬 元明
学位規則第4条第1項該当
Doctor of Energy Science
Kyoto University
DFAM

1v.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Thesis (Ph.D.)--University of Adelaide, Dept. of Chemical Engineering, 1990

Title page, summary and table of contents only. The complete thesis in print form is available from the University of Adelaide Library.
Ash deposition is a problem in power generation when coal with high ash and alkali contents are utilised. The problem is more severe in fluidised bed combustion where the ash deposition can causes agglomeration of the bed material, may lead to defluidisation of the unit. The successful operation of fluid bed combustion with coal high in ash and alkali content will depend on the ability to control ash deposition. The rheological properties of coal ash under furnace conditions are important in controlling the stickiness and mobility of the molten ash deposition. Therefore, a good knowledge of the rheological properties of coal will improve the understanding of the mechanisms associated with ash deposition, and may assist in controlling the deposition and agglomeration of fluid bed material. At present, a good deal of information about coal ash rheology under conditions similar to those found in fluidised bed combustion is not known, and greater understanding is required. This is primarily due to a lack of reliable instruments and measurement techniques. In this work, a new high temperature rheometer has been developed based on the principle concepts of visco metric flow. The developed rheometer allows fundamental rheological properties, such as shear stress and shear rate, to be obtained without relying on calibrations with materials of known properties. With this instrument the flow characteristics of the tested samples can be determined directly, without assuming a particular fluid model. The new rheometer has the capability to measure the rheological properties of materials at temperatures ranging from 500°C to 1300°C and under different processing conditions. Rheological characteristics and properties of a range of low rank Australian coal ashes have been carried out using the newly developed high temperature rheometer, equipped with a cone and plate measuring geometry. It has been found that coal ash samples exhibit thixotropic and visco plastic flow behaviours. SEM and XRD analyses have revealed that during high temperature rheological measurements the coal ash sample is basically a suspension of colloidal mineral solids in a molten eutectic liquid. The solid phase is mainly silica (SiO₂,), and the liquid phase is a mixture of alkali sulphates mainly CaS0₄, MgS0₄ and Na₂S0₄ compounds. The equilibrium visco metric data of coal ash samples is found to be satisfactorily described using the Herschel-Bulkley model. The equilibrium rheological properties are strongly affected by the concentration levels of CaS0₄, MgS0₄ and Na₂S0₄ . The operating temperature and chemical composition of the surrounding gas phase were also found to affect the rheological properties of the coal ash samples. In order to obtain a better understanding and to model the rheological properties of the coal ashes, a series of synthetic ash mixtures were examined. The synthetic mixtures contained the key chemical components that represent the solid and the liquid phases. The solid phase is represented by silica (SiO₂), while a mixture of CaS0₄, MgS0₄ and Na₂S0₄ compounds represented the liquid phase. In this work, the rheological characteristics of mixtures of synthetic ash were investigated using a factorial experimental design. Using the synthetic ash mixtures together with the statistical design experiment, the effect of key chemical compounds on the rheological properties could be systematically investigated. The rheological results showed that the synthetic mixtures exhibited thixotropic and viscoplastic behaviours. It was also found that mixtures predominantly high in CaS0₄and MgS0₄ had a high degree of thixotropy behaviour, while those mixtures predominantly high in Na₂S0₄ showed a lower degree of thixotropy behaviour. The statistical analysis also showed that Na₂S0₄ is the most significant chemical compound causes a high yield stress and high viscosity. In contrast, CaS0₄and MgS0₄ were found to decrease the value of the yield stress and the viscosity. The rheological behaviour of the synthetic ash mixtures can be used to describe rheological behaviour of the coal ash samples. Relationships between equilibrium flow properties and chemical compounds, and temperatures are developed using a linear regression method. The statistical analysis has shown that CaS0₄, MgS0₄ and Na₂S0₄ , and their interactions are all significant compounds that have effects on the yield stress and viscosity of the synthetic mixtures. It was also found that the yield stress and viscosity decreased with increasing concentration level of either CaS0₄or MgS0₄ . Yield stress and viscosity are increased with increases in the concentration of Na₂S0₄ . The statistical models can successfully predict rheological properties of ash with high concentrations of CaS0₄, MgS0₄ and Na₂S0₄ , but it fails to predict the rheological properties of ashes that also high concentrations of either Fe₂0₃ or Al₂0₃, or a combination of both. The relationship between ash rheology and fluidised bed agglomeration has been established. The yield stress of a coal ash may be used to describe the tendency of the molten ash to deposit on surface of the fluid bed particles. Yield stress also determines the tendency of stickiness of the molten ash deposit to adhere the fluid bed particles during fluidised bed combustion process. The viscosity of the molten ash describes the ability of the molten ash layer to adhere the fluid bed particles after a collision. High viscosity ash tend to hold the colliding particles together longer than a low viscosity ash. Shear thinning behaviour of the ash samples (decreasing viscosity with increasing shear rate) suggests that the operating conditions could be arranged so as to minimise the chance of agglomeration. For example, in order to avoid agglomeration a high viscosity coal ash would benefit from operating the fluidised bed combustion at a high velocity, this is because a high velocity means a higher shear rate and this causes a reduction in the viscosity of the molten ash. Thus, particles agglomerated by a low viscosity ash would be easily broken by the hydrodynamic forces present during the fluidised bed process. Finally, information about ash rheology has formed a basic knowledge for estimating tendency of fluid bed agglomeration when coal obtained from different source is being used.
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Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2004