Thesis (M.S.) -- Mississippi State University. Department of Civil Engineering. / Title from title screen. Includes bibliographical references.

Air entraining admixtures are surfactants, that are commonly used in Portland cement materials, because they form and stabilize small air bubbles distributed in the binder, thereby changing the pore size distribution. The aim of these additives is to increase the durability of the cured binder, especially in climatic conditions, where it must withstand low temperatures, and simultaneously improving the workability in the plastic state. The effect of air entraining admixtures in alkali-activated slag was studied for certain properties. Air entraining admixtures were able to entrain the air in the alkali-activated matrix, they changed the pore size distribution, which resulted in a reduction in capillary pressure in the porous complex, and there was a reduction of shrinkage under autogenous conditions, respectively. The improvement of workability is certainly also positive, but on the other hand there has been a bad effect on flexural strength and compressive strength. The morphology of the air entrained binder was observed by scanning electron microscopy, and simultaneously the effect of air entrained admixtures on hydration was observed by isothermal calorimetry.

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La gazéification de la biomasse a été étudiée dans les conditions d'un réacteur à flux entraîné, à savoir à vitesse de chauffage et à température élevées. Des expériences ont été réalisées dans un four à chute entre 800°C et 1400°C, à partir de particules de bois de taille 0,35 mm et 0,80 mm, dans une atmosphère inerte (100% molaire de N2), ou contenant de la vapeur d’eau (25% molaire). Les expériences ont également été simulées grâce à un modèle 1D avec des résultats positifs, ce qui a permis de mieux comprendre les phénomènes mis en jeu. Les solides obtenus (suies et char) ont été analysés et caractérisés. Des rendements élevés en gaz et goudrons, et un faible rendement en char ont été mesurés. Par conséquent, l'évolution de la phase volatile est déterminante pour les rendements des produits finaux. Au-dessus de 1000°C, la formation de suies devient importante. Les suies sont formées à partir de C2H2 et de HAP. En présence de vapeur d’eau, le rendement en suies est nettement moins élevé, ce qui s’explique essentiellement par le vaporeformage des précurseurs de suie, mais aussi par leur gazéification. La réaction de water-gas shift joue un rôle important dans la distribution des gaz majoritaires. La gazéification du char a été mise en évidence à 1200°C et 1400°C sous atmosphère humide. L'ensemble de ces réactions conduit à un gaz riche en H2, CO et CO2. L'équilibre thermodynamique est presque atteint à 1400°C avec une concentration de 25% molaire de H2O dans l’atmosphère. La graphitisation et la désactivation du char porté à haute température ont été mises en évidence expérimentalement. Néanmoins, ces phénomènes ont une influence négligeable sur l’évolution du rendement en char lors des expériences en four à chute. Enfin, la taille des particules n’a presque aucune influence sur les résultats expérimentaux. / Biomass gasification was studied in the conditions of an entrained flow reactor, namely at high heating rate and temperature. Experiments in a drop tube reactor were performed between 800°C and 1400°C, with wood particles of 0.35 mm and 0.80 mm size, under inert and steam containing - 25 mol% of H2O - atmospheres. These experiments were also simulated with a 1D model which gave good predictions. The collected solids, soot and char, were analyzed and characterized. This study highlights the importance of gas phase reactions on the yields of the final products, mainly gaseous compounds, in these conditions. These reactions are hydrocarbons cracking, reforming and polymerization, leading to soot formation, and water-gas shift. Char graphitization and deactivation were experimentally demonstrated. However, these phenomena have a negligible influence on char evolution in the drop tube reactor. Finally, the particle size was shown to have almost no influence on experimental results.

The quantitative nondestructive evaluation (NDE) of cement-based materials is a critical area of research that is leading to advances in the health monitoring and condition assessment of the civil infrastructure. Ultrasonic NDE has been implemented with varying levels of success to characterize cement-based materials with complex microstructure and damage. A major issue with the application of ultrasonic techniques to characterize cement-based materials is their inherent inhomogeneity at multiple length scales. Ultrasonic waves propagating in these materials exhibit a high degree of attenuation losses, making quantitative interpretations difficult. Physically, these attenuation losses are a combination of internal friction in a viscoelastic material (ultrasonic absorption), and the scattering losses due to the material heterogeneity. The objective of this research is to use ultrasonic attenuation to characterize the microstructure of heterogeneous cement-based materials. The study considers a real, but simplified cement-based material, cement paste - a common bonding matrix of all cement-based composites. Cement paste consists of Portland cement and water but does not include aggregates. First, this research presents the findings of a theoretical study that uses a set of existing acoustics models to quantify the scattered ultrasonic wavefield from a known distribution of entrained air voids. These attenuation results are then coupled with experimental measurements to develop an inversion procedure that directly predicts the size and volume fraction of entrained air voids in a cement paste specimen. Optical studies verify the accuracy of the proposed inversion scheme. These results demonstrate the effectiveness of using attenuation to measure the average size, volume fraction of entrained air voids and the existence of additional larger entrapped air voids in hardened cement paste. Finally, coherent and diffuse ultrasonic waves are used to develop a direct relationship between attenuation and water to cement (w/c) ratio. A phenomenological model based on the existence of fluid-filled capillary voids is used to help explain the experimentally observed behavior. Overall this research shows the potential of using ultrasonic attenuation to quantitatively characterize cement paste. The absorption and scattering losses can be related to the individual microstructural elements of hardened cement paste. By taking a fundamental, mechanics-based approach, it should be possible to add additional components such as scattering by aggregates or even microcracks in a systematic fashion and eventually build a realistic model for ultrasonic wave propagation study for concrete.

Thesis (M.S.)--University of Alabama at Birmingham, 2007. / Description based on contents viewed July 8, 2009; title from PDF t.p. Includes bibliographical references (p. 115-116).

The last couple of decades have seen entrained-flow gasification of black liquor (BL) undergo an incremental process of technical development as an alternative to combustion in a recovery boiler. The ability of the technology to combine chemical recovery with the production of clean syngas renders it a promising candidate for the transformation of chemical pulp mills into integrated forest biorefineries. However, techno-economic assessments have shown that blending BL with the more easily transportable pyrolysis oil (PO) can not only increase the system efficiency for methanol production but remove a significant roadblock to development by partially decoupling production capacity from limitations on black liquor availability. The verification and study of catalytic co-gasification in an industrially-relative scale can yield both scientifically interesting and practically useful results, yet it is a costly and time-consuming enterprise. The expense and time involved can be significantly reduced by performing thermodynamic equilibrium calculations using a model that has been validated with relevant experimental data. The main objective of this thesis was to study, understand, quantify and compare the gasification behaviour and process performance of black liquor and pyrolysis oil blends in pilot-scale. A secondary objective of this work was to demonstrate and assess the usefulness and accuracy of unconstrained thermodynamic equilibrium modelling as a tool for studying and predicting the characteristics of alkali-impregnated biomass entrained-flow gasification. The co-gasification of BL/PO blends was studied at the 3 MWth LTU Green Fuels pilot plant in a series of experimental studies between June 2015 and April 2016. The results of the studies showed that the blending of black liquor with the more energy rich pyrolysis oil increased the energetic efficiency of the BLG process without adversely affecting carbon conversion. The effect of blend ratio and reactor temperature on the gasification performance of PO and BL blends with up to 20 wt% PO was studied in order to assess the impact of alkali-dilution in fuel on the conversion characteristics. In addition to unblended BL, three blends with PO/BL ratios of 10/90, 15/85 and 20/80 wt% were gasified at a constant load of 2.75 MWth. The decrease in fuel inorganic content with increasing PO fraction resulted in more dilute green liquor (GL) and a greater portion of the feedstock carbon ended up in syngas as CO. As a consequence, the cold gas efficiency increased by about 5%-units. Carbon conversion was in the range 98.8-99.5% and did not vary systematically with either fuel composition or temperature. The validation of thermodynamic equilibrium simulation of black liquor and pyrolysis co-gasifications with experimental data revealed the need to be mindful of errors and uncertainities in fuel composition that can influence predictions of equilibrium temperature. However, provided due care is to taken to ensure the use of accurate fuel composition data, unconstrained TEMs can serve as a robust and useful tool for simulating catalytic entrained-flow gasification of biomass-based feedstocks. / LTU Biosyngas (Catalytic Gasification)

Gas-liquid annular two phase flow in pipes is important in the oil and gas, nuclear and the process industries. It has been identified as one of the most frequently encountered flow regimes and many models (empirical and theoretical) for the film flow and droplet behaviour for example have been developed since the 1950s. However, the behaviour in large pipes (those with diameter greater than 100 mm) has not been fully explored. As a result, the two- phase flow characteristics, data, and models specifically for such pipes are scarce or non-existent such that those from smaller pipes are extrapolated for use in design and operation. Many authors have cautioned against this approach since multiphase pipe flow behaviour is different between small and large pipes. For instance the typical slug flows seem not to occur in vertical upwards flows when the pipe diameter exceeds 100 mm. It is therefore imperative that theoretical models and empirical correlations for such large diameter pipes are specifically developed.

A criação de programas federais voltados para o desenvolvimento da infraestrutura brasileira no final da década passada resultou no aquecimento do setor de construção civil. Nesse contexto, o sistema de paredes de concreto moldadas no local se destacou devido à sua alta produtividade e repetitividade. Os requisitos e procedimentos para o dimensionamento de paredes de concreto moldadas no local são contemplados na NBR 16055:2012. Dentre as abordagens consideradas nessa norma, ressalta-se a recomendação do uso de concreto autoadensável. No entanto, alguns pontos de interesse não são contemplados na regulamentação vigente como, por exemplo, a incorporação de ar na composição do concreto, bem como os limites de teores de ar incorporado. Assim, a presente pesquisa busca avaliar a influência da incorporação de ar no comportamento de concretos autoadensáveis destinados à execução de paredes de concreto moldadas no local. Para isso, foram produzidos dois traços de concreto autoadensável de classes de resistência C25 e C40 e, a partir de cada um deles, produzidos outros dois traços modificados com aditivo incorporador de ar, variando o teor de ar incorporado, em um total de seis misturas cujas propriedades foram avaliadas tanto no estado fresco quanto no estado endurecido. Além disso, foram moldadas paredes de concreto a fim de analisar a estrutura interna do material com aparelho de ultrassom e o acabamento superficial do elemento por meio de análise de imagens. No caso da classe C25, os resultados demonstram que a influência do teor de ar para os traços modificados foi tão significativa que os concretos produzidos se tornaram inadequados para uso estrutural. Por outro lado, apesar da elevada perda de resistência à compressão aos 28 dias, os dois traços produzidos com incorporação de ar no concreto de classe C40 se adequam às recomendações da NBR 16055:2012. Alia-se a isso o fato de que, quando da execução das paredes em laboratório, os concretos utilizados demonstraram grande capacidade de preenchimento, resultando em elementos com bom acabamento superficial e estrutura interna homogênea, mesmo em regiões de difícil acesso. Sendo assim, o presente trabalho propõe traços muito promissores para utilização em paredes de concreto moldadas no local. / The creation of federal programs aimed at the development of Brazilian infrastructure at the end of the last decade resulted in the heating of civil construction sector. In this context, the system of concrete walls cast in place stood out due to its high productivity and repetitiveness. The requirements and procedures for the design of concrete walls cast in place are contemplated in the NBR 16055:2012. Among the several approaches considered in this code, the recommendation for the use of self-compacting concrete is emphasized. However, some points of interest are not considered in the current regulations, such as the incorporation of air in the concrete composition, as well as the limits for entrained air contents. Thus, the present research seeks to evaluate the influence of air entrainment in the behaviour of self-compacting concrete for the construction of concrete walls cast in place. For this, two self-compacting concrete mixes design of strength classes C25 and C40 were produced and, from each of them, two other mixes modified with air entraining admixture were produced, varying the entrained air content, in a total of six mixtures whose properties were evaluated in both fresh and hardened state. In addition, concrete walls were cast to analyse the internal structure with ultrasonic equipment and the element surface finish by image analysis. In the case of class C25, the results demonstrate that the influence of the air content for the modified mixes was so great that the concretes produced became inappropriate for structural use. On the other hand, despite the high loss of compressive strength at 28 days, the two mixes produced with entrained air in the concrete of class C40 respect the recommendations of the NBR 16055:2012. In addition, when the walls were cast in laboratory, the concretes used showed great filling capacity, resulting in structures with good surface finish and homogeneous internal structure, even in difficult access regions. Therefore, the present work proposes very promising mixes for use in concrete walls cast in place.