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published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

The United States holds the world's largest estimated reserves of coal and is also a net exporter of it. Coal gasification provides a cleaner way to utilize coal than directly burning it. Gasification is an incomplete oxidation process that converts various carbon-based feedstocks into clean synthetic gas (syngas), which can be used to produce electricity and mechanical power with significantly reduced emissions. Syngas can also be used as feedstock for making chemicals and various materials. A Computational Fluid Dynamics (CFD) scheme has been used to simulate the gasification process for many years. However, many sub-models still need to be developed and improved. The objective of this study is to use the improved CFD modeling to understand the thermal-flow behavior and the gasification process and to provide guidance in the design of more efficient and cheaper gasifiers. Fundamental research has been conducted to improve the gasification sub-models associated with the volatile thermal cracking, water-gas-shift (WGS) reaction, radiation effect, low-rank-coal gasification, coal to synthetic-natural-gas (SNG), and ash deposition mechanisms. The improved volatile thermal cracking model includes H2S and COS contents. A new empirical WGS reaction model is developed by matching the result with experimental data. A new coal demoisturization model is developed for evaporating the inherent moisture inside the coal particles during low-rank-coal gasification. An ash deposition model has also been developed. Moreover, the effect of different radiation models on the simulated result has been investigated, and the appropriate models are recommended. Some improved model tests are performed to help modify an industrial entrained-flow gasifier. A two-stage oxygen feeding scheme and a unique water quench design are investigated. For the two-stage oxygen feeding design, both experimental data and CFD predictions verify that it is feasible to reduce the peak temperature and achieve a more uniform temperature distribution in the gasifier by controlling the injection scheme without changing the composition and production rate of the syngas. Furthermore, the CFD simulation can acceptably approximate the thermal-flow and reaction behaviors in the coal gasification process, which can then be used as a preliminary screening tool for improving existing gasifiers’ performance and designing new gasifiers.

Anlagen zur Herstellung von Synthesegas (CO·H2) aus kohlenstoffhaltigen Rohstoffen werden durch hohe Temperaturen bis zu 1600 °C und Drücken bis zu 50 bar beansprucht und benötigen daher Schutz durch eine feuerfeste Ausmauerung. Zur Steigerung der Effizienz und Lebensdauer solcher Vergasungsanlagen ist die Entwicklung neuer keramischer Hochtemperaturwerkstoffe erforderlich. Solch ein Material stellt eine Al2O3-reiche Gießmasse dar, welche durch den gezielten Einsatz verschiedener ZrO2- und TiO2-Gehalte optimiert wurde. Es hat sich gezeigt, dass bereits durch die Zugabe geringer Mengen an ZrO2 bzw. TiO2 sowohl die Temperaturwechselbeständigkeit als auch die Korrosionsbeständigkeit von Al2O3 gegenüber Kohleschlacken erheblich verbessert werden kann, was auf die Ausbildung einer Spinell-Schutzschicht während des Korrosionsvorgangs zurückzuführen ist.

Gasification of black liquor is an alternative to the combustion of black liquor, which is currently the dominant form of chemical recovery in the paper industry. Gasification of black liquor offers the possibility of higher thermal efficiencies than combustion, reducing manufacturing costs and creating new revenue streams through a forest biorefinery. Pressurizing the gasification reactor further enhances the efficiency advantage of gasification over combustion. This study uses a pressurized entrained flow reactor (PEFR) to study black liquor gasification behavior under pressures, temperatures, and heating rates similar to those of next-generation high-temperature black liquor gasifiers. The effects of pressure on black liquor char morphology, gasification rates, pyrolysis carbon yields, and sulfur phase distribution were studied. These characteristics were investigated in three main groups of experiments at 900oC: pyrolysis (100% N2), gasification with constant partial pressure (0.25 bar H2O and 0.50 bar CO2), and gasification with constant mole fraction (10% CO2, 2% H2O, 1.7% CO, 0.3% H2), under five, ten, and fifteen bar total pressure. It was found that pressure had an impact on the char physical characteristics immediately after the char entered the reactor. Increasing pressure had the effect of decreasing the porosity of the chars. Pressure also affected particle destruction and reagglomeration mechanisms. Surface areas of gasification chars decreased with increasing pressures, but only at low carbon conversions. The rate of carbon conversion in gasification was shown to be a function of the gas composition near the particle, with higher levels of inhibiting gases slowing carbon conversion. The same phenomenon of product gas inhibition observed in gasification was used to explain carbon conversions in pyrolysis reactions. Sulfur distribution between condensed and gas phases was unaffected by increasing total pressure in the residence times investigated. Significant amounts of sulfur are lost during initial devolatilization. With water present this gas phase sulfur forms H2S and did not return to the condensed phase.

The “Integrated Coal Gasification Combined Cycle” (IGCC) represents an advanced approach for green field projects for power generation. This process requires separation of carbon dioxide from the shifted-synthesis gas mixture (fuel gas). Treated fuel gas consists of approximately 40% CO2 and rest H2. Gas hydrate based separation technology for hydrate forming gas mixtures is one of the novel approaches for gas separation. The present study illustrates the gas hydrate-based separation process for the recovery of CO2 and H2 from the fuel gas mixture and discusses relevant issues from macro and molecular level perspectives. Propane (C3H8) is used as an additive to reduce the operating pressure for hydrate formation and hence the compression costs. Based on gas uptake measurement during hydrate formation, a hybrid conceptual process for pre-combustion capture of CO2 is presented. The result shows that it is possible to separate CO2 from hydrogen and obtain a hydrate phase with 98% CO2 in two stages starting from a mixture of 39.2% CO2. Molecular level work has also been performed on CO2/H2 and CO2/H2/C3H8 systems to understand the mechanism by which propane reduces the operating pressure without compromising the separation efficiency.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

In der vorliegenden Arbeit werden verschiedene Methoden zur Bestimmung von Sintertemperaturen für Brennstoffaschen vorgestellt und verglichen, mit dem Ziel die Agglomerationsneigung von Aschen zu charakterisieren. Es wurden Untersuchungen an drei ausgewählten Kohleaschen unter inerten, oxidierenden und reduzierenden Bedingungen durchgeführt. Die Methoden Erhitzungsmikroskopie (ASV), Hochtemperatur-Röntgendiffraktometrie (HT-RDA), Thermogravimetrische Differenz-kalorimetrie (TG-DSC), Thermodynamische Gleichgewichtsberechnungen (GGW), Elektrochemische Impedanzspektroskopie (EIS), Untersuchung der Schereigenschaften (SV) und die Bestimmung der Kaltdruckfestigkeit (KDF) wurden angewendet. Die Kombination der Untersuchungen ließ eine umfangreiche analytische Charakterisierung der Sintervorgänge zu. Unter der Berücksichtigung einer guten Vergleichbarkeit hinsichtlich der ermittelten Sintertemperaturen, stellt die EIS eine Alternative zur etablierten aber zeitaufwändigen Bestimmung der KDF dar. In Abhängigkeit von der Aschezusammensetzung, der Korngröße und der Gasatmosphäre, ist bereits ab einer Temperatur von 650 °C eine Agglomeration von Aschepartikeln möglich.

Fossil fuels constitute a significant fraction of the world’s energy demand. The burning of fossil fuels emits huge amounts of carbon dioxide into the atmosphere. Therefore, the limited availability of fossil fuel resources and the environmental impact of their use require a change to alternative energy sources or carriers (such as hydrogen) in the foreseeable future. The development of methods to mitigate carbon dioxide emission into the atmosphere is equally important. Hence, extensive research has been carried out on the development of cost-effective technologies for carbon dioxide capture and techniques to establish hydrogen economy. Hydrogen is a clean energy fuel with a very high specific energy content of about 120MJ/kg and an energy density of 10Wh/kg. However, its potential is limited by the lack of environment-friendly production methods and a suitable storage medium. Conventional hydrogen production methods such as Steam-methane-reformation and Coal-gasification were modified by the inclusion of NaOH. The modified methods are thermodynamically more favorable and can be regarded as near-zero emission production routes. Further, suitable catalysts were employed to accelerate the proposed NaOH-assisted reactions and a relation between reaction yield and catalyst size has been established. A 1:1:1 molar mixture of LiAlH4, NaNH2 and MgH2 were investigated as a potential hydrogen storage medium. The hydrogen desorption mechanism was explored using in-situ XRD and Raman Spectroscopy. Mesoporous metal oxides were assessed for CO2 capture at both power and non-power sectors. A 96.96% of mesoporous MgO (325 mesh size, surface area = 95.08 ± 1.5 m2/g) was converted to MgCO3 at 350°C and 10 bars CO2. But the absorption capacity of 1h ball milled zinc oxide was low, 0.198 gCO2 /gZnO at 75°C and 10 bars CO2. Interestingly, 57% mass conversion of Fe and Fe3O4 mixture to FeCO3 was observed at 200°C and 10 bars CO2. MgO, ZnO and Fe3O4 could be completely regenerated at 550°C, 250°C and 350°C respectively. Furthermore, the possible retrofit of MgO and a mixture of Fe and Fe3O4 to a 300 MWe coal-fired power plant and iron making industry were also evaluated.