A mathematical model for entrained flow coal gasification was developed with the objective of predicting the influence of coal properties and gasification conditions on the performance of entrained flow gasifiers operating at pressures up to 21 atmospheres (2.1MPa). The model represents gasifiers as plug flow reactors and therefore neglects any mixing or turbulence effects. Coal properties were predicted through use of correlations from a variety of literature sources and others that were developed from experimental data in the literature. A sensitivity analysis of the model indicated that errors in the calculated values of coal volatile yield, carbon dioxide gasification reactivity and steam gasification may significantly affect the model predictions. Similarly errors in the input values for gasifier wall temperatures and gasifier diameter, when affected by slagging, can cause model prediction errors. Model predictions were compared with experimental gasification results for a range of atmospheric and high pressure gasifiers, the majority of the results being obtained by CSIRO at atmospheric pressure for a range of coals. Predictions were accurate for the majority of atmospheric pressure results over a large range of gas feed mixtures. Due to the limited range of experimental data available for high pressure gasification the capability of the model is somewhat uncertain, although the model provided accurate predictions for the majority of the available results. The model was also used to predict the trends in particle reactions with gasification and the influence of pressure, gasifier diameter and feed coal on gasifier performance. Further research on coal volatile yields, gasification reactivities and gas properties at high temperatures and pressures was recommended to improve the accuracy of model inputs. Additional predictions and model accuracy improvements could be made by extending the model to include fluid dynamics and slag layer modelling. / PhD Doctorate

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Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

It is common practice to purposely trap small air-voids in concrete in order to give it frost resistance. A large number of factors have been recognized to impact the ability to entrain and stabilize these microscopic air-voids in concrete. This dissertation investigates a number of these variables. However, the primary focus of this work is on investigating problems entraining and stabilizing air in concrete utilizing fly ash. These investigations include: evaluation of existing and newly created test methods to measure the impact of fly ash on the ability to air-entrain concrete, and the fresh and hardened properties of air-entrained fly ash concrete is investigated. Additional work is presented concerning some of the fundamental physical and chemical properties of air-void shells separated from cement paste and how they change with time.

This work has examined the thermochemical conversion of sugar cane processing wastes (bagasse and cane harvesting trash) for use in the design of pressurised entrained flow gasification power cycles (~20 Barg, 600-900oC). The two key parameters of interest were the residual char yield from initial pyrolysis and the heterogeneous reactivity of the char with respect to carbon dioxide. Char yield and gasification rates were measured by a conventional wire mesh reactor and thermogravimetric (TGA) technique, an in-situ sample charring TGA technique and with an entrained flow reactor specifically designed for this work. The new experimental reactor concept is one of the major contributions of the work. Chars from the entrained flow experiments were characterised by optical microscope, SEM/EDS, TEM/EDS and XPS techniques, to help elucidate the processes occurring during pyrolysis and gasification. The key findings and conclusions of the work were as follows: 1. Initial (pyrolysis) char yields were consistent with the data reported in literature for similar materials. Char yields varied with reaction conditions, from 6 to 49 wt% daf for cane trash and 4 to 40 wt% daf for bagasse. Ash content also had a significant effect on char yield. The char yield for both cane trash and bagasse increased in proportion to the logarithm of system pressure. 2. A relatively simple empirical model for char yield under pressurised entrained flow conditions was formulated. This could predict char yields for both the experimental data in this work and those reported in literature for similar biomass materials. While temperature, pressure and ash content were all significant parameters in the model, the primary fitting parameter was a measure of the contribution of secondary char forming reactions and ongoing pyrolysis to char yield. The identification of this parameter is one of the contributions of this work. 3. The measured initial rate of char gasification by carbon dioxide was 0.06 to 1.2 mg per gram of initial char, over the temperature range 750 to 900oC. The rate of gasification was so low as to not contribute significantly to overall fuel conversion in the reaction residence times iv expected of a commercial gasifier. In essence almost all of the experimentally measured fuel conversion could be attributed to pyrolysis, which resulted in 85-95% fuel conversion. 4. Both the raw materials and the residual chars had low surface areas and negligible microporosity. The majority of the measured surface area may have been associated with the ash component rather than the carbonaceous component, which supported the finding of low reactivity. 5. The silica component of the chars exhibited crystalline silicate formation by migration of metal species over time periods of minutes. These silicates displayed signs of sintering, but otherwise remained physically intact; leaving a characteristic skeleton that corresponded to the original structure in the raw materials. 6. The gasification rate showed a time dependent decrease in the entrained flow experiments. This was attributed to coke formation on the char surface, followed by carbon trapping in the ash component at high levels of conversion. Both findings are significant contributions from this work, because they highlight key mechanisms that hinder fuel conversion in the proposed gasification concept. The broad coverage achieved in this work has provided an overall picture of how fuel conversion progresses during the pressurised entrained flow gasification of sugar cane wastes. It is recommended that many of the aspects highlighted in this work be examined further, to confirm the findings and to investigate the means to avoid the factors identified in this work as hindering fuel conversion.

In this work the differences between and characteristics of water and high solids, heated black liquor sprays from air-assist atomizers are examined. Sprays were imaged with a high speed camera and the images analyzed with computer code to produce droplet size data and macroscopic spray characteristics such as mass distribution. Fluid flow rates were measured to allow relevant dimensionless groups for the spray to be calculated. A 1000 degree C tubular furnace was placed around the spray to determine the effect of industrially relevant temperatures on the droplet formation process, relative to room-temperature conditions. It was found that high solids black liquor forms long, thin ligaments rather than droplets. In high-temperature surroundings the size of these ligaments increases, which from a comparison with theory in the literature was attributed to enhanced skin-formation driven by heat transfer. The data suggest that this skin formation may prevent secondary breakup. All sprays for both fluids produced droplet size mass distributions that were well described by the square-root normal distribution. The normalized width (s*) of these distributions was similar for all sprays and consistent with literature data for other nozzle designs (0.24 < s* < 0.38). The image analysis method assumed droplets were spheres with the same projected area. When this assumption was changed for black liquor sprays to a cylindrical droplet assumption, the shape and normalized width of the resulting mass distributions remained the same, but the representative diameter (calculated from surface area to volume ratios) decreased. Based on the agreement between the normalized distribution width in this work and that in the black liquor spraying literature it was concluded that the addition of atomizing air cannot be considered a means to narrow a droplet size distribution independent of droplet size. The results also indicate the importance of including the effects of skin formation and temperature- and time-dependent fluid properties in spray modeling. It is intended that these results contribute to increased understanding of the black liquor atomization process and lead to improved computational modeling of the same.

Thesis (MScEng)--Stellenbosch University, 2013. / Cellular concrete is a type of lightweight concrete that consists only of cement, water and sand with 20 per cent air by volume or more air entrained into the concrete. The two methods used for air entrainment in cellular concrete are (1) the use of an air entraining agent (AEA), and (2) the use of pre-formed foam. If pre-formed foam is used to entrain air into the concrete the concrete is named foamed concrete and if an AEA is used the concrete is termed aerated concrete. Depending on the type of application, structural or nonstructural, cellular concrete can be designed to have a density in the range of range of 400 to 1800 kg/m3. Non-structural applications of cellular concrete include void and trench filling, thermal and acoustic insulation. Structural applications of cellular concrete include pre-cast units such as concrete bricks, partitions, roof slabs etc. Due to the high levels of air in cellular concrete it is challenging to produce compressive strengths that are sufficient to classify the concrete as structurally useful when non-autoclaving curing conditions are used. The autoclaving process combines high temperature and pressure in the forming process, which causes higher strength and reduced shrinkage. This process is also limited to prefabricated units. Non-autoclave curing conditions include moist curing, dry curing, wrapping the concrete in plastic, etc. However, now that the world is moving in an energy efficient direction, ways to exclude energy-intensive autoclaving are sought. It has for instance been found that the utilisation of high volumes of fly-ash in cellular concrete leads to higher strengths which make it possible to classify the concrete as structurally useful. Now, that there is renewed interest in the structural applications of the concrete a design methodology using an arbitrary air entraining agent needs to be found. The research reported in this thesis therefore attempts to find such a methodology and to produce aerated concrete with a given density and strength that can be classified as structurally useful. For the mix design methodology, the following factors are investigated: water demand of the mix, water demand of the mix constituents, and the amount of AEA needed to produce aerated concrete with a certain density. The water demand of the mix depends on the mix constituents and therefore a method is proposed to calculate the water demand of the mix constituents based on the ASTM flow turn table. Due to the complex nature of air entrainment in concrete, the amount of air entrained into the concrete mix is not known beforehand, and a trial and error method therefore had to be developed. The trial mixes were conducted in a small bakery mixer. From the trial mixes estimated dosages of AEA were found and concrete mixes were designed based on these mixes. The factors that influence the mix design and strength of aerated concrete include filler/cement ratio (f/c), fly-ash/cement ratio (a/c) and design target density. Additional factors that influence the strength of aerated concrete are specimen size and shape, curing, and concrete age. It was found that the sand type and f/c ratio influence the water demand of the concrete mix. Sand type and f/c ratio also influence compressive strength, with higher strength for a finer sand type and lower f/c ratios. However, the concrete density is the factor that influences the strength the most.

Increasing the share of biomass, preferably by replacing fossil fuels, is one way to mitigate the present climate change. Fossil coal can be directly replaced by co-combustion of coal and biomass and fossil engine fuels (gasoline and diesel) could potentially partly be replaced by synthetic renewable fuels produced via entrained flow gasification of biomass. The use of biomass in these processes is so far limited, partly because of the fibrous and hygroscopic nature of biomass which leads to problem in storing, transportation, handling and feeding. This thesis demonstrates how the challenging characteristics of raw biomass are mitigated by the pretreatment method torrefaction. Torrefaction is a process where biomass is heated in an oxygen deficient atmosphere to typically between 240 and 350°C for a time period of 2 minutes to 1 hour. Most of the torrefaction R&amp;D in the literature have so far been performed with bench-scale batch reactors. For the purpose of carefully studying continuous torrefaction, a 20 kg/h torrefaction pilot plant was therefore designed, constructed and evaluated. The overall conclusion from this thesis is that the many benefits of torrefied biomass are valid also when produced with a continuous pilot plant and for typically Swedish forest biomasses. Some of the documented improved biomass properties are increased heating value, increased energy density, higher friability (lower milling energy) and less hydrophilic biomass (less moisture uptake). Most of the improvements can be attributed to the decomposition of hemicellulose and cellulose during torrefaction. The most common variables for describing the torrefaction degree are mass yield or anhydrous weight loss but both are challenging to determine for continuous processes. We therefore evaluated three different methods (one existing and two new suggestions) to determine degree of torrefaction that not require measurement of mass loss. The degree of torrefaction based on analyzed higher heating value of the raw and torrefied biomass (DTFHHV) predicted mass yield most accurate and had lowest combined uncertainty. Pelletizing biomass enhance transportation and handling but results from pelletization of torrefied biomass is still very limited in the literature and mainly reported from single pellet presses. A pelletization study of torrefied spruce with a ring die in pilot scale was therefore performed. The bulk energy density was found to be 14.6 GJ/m3 for pelletized torrefied spruce (mass yield 75%), a 40% increase compared to regular white pellets and therefore are torrefied pellets more favorable for long distance transports. More optimization of the torrefied biomass and the pelletization process is though needed for acquiring industrial quality pellets with lower amount of fines and higher pellet durability than attained in the present study. Powders from milled raw biomass are generally problematic for feeding and handling and torrefied biomass has been proposed to mitigate these issues. The influence of torrefaction and pelletization on powder and particle properties after milling was therefore studied. The results show that powder from torrefied biomass were enhanced with higher bulk densities, lower angle of repose as well as smaller less elongated particles with less surface roughness. Even higher powder qualities were achieved by pelletizing the torrefied biomass before milling, i.e. another reason for commercial torrefied biomass to be pelletized. Entrained flow gasification (EFG) is a promising option for conversion of biomass to other more convenient renewable energy carriers such as electricity, liquid biofuels and green petrochemicals. Also for EFGs are torrefied fuels very limited studied. Raw and torrefied logging residues were successfully gasified in a pilot scale pressurized entrained flow biomass gasifier at 2 bar(a) with a fuel feed corresponding to 270 kWth. Significantly lower methane content (50% decrease) in the syngas was also demonstrated for the torrefied fuel with mass yield 49%. The low milling energy consumption for the torrefied fuels compared to the raw fuel was beneficial for the gasification plant efficiency.

Numerical simulations of coal gasification process inside a generic 2-stage entrainedflow gasifier are carried out using the commercial CFD solver ANSYS/FLUENT. The 3-D Navier-Stokes equations and eight species transport equations are solved with three heterogeneous global reactions, three homogeneous reactions, and one thermal cracking equation of volatiles. Finite rates are used for the heterogeneous solid-gas reactions. Both finite rate and eddy-breakup combustion models are calculated for each homogeneous gas-gas reaction, and the smaller of the two rates is used. Lagrangian-Eulerian method is employed. The Eulerian method calculates the continuous phase while the Lagrangian method tracks each coal particle. Fundamental study is carried out to investigate effects of five turbulence models (standard k-ε, k-ω, RSM, k-ω SST, and k-ε RNG) and four devolatilization models (Kobayashi, single rate, constant rate, and CPD) on gasification simulation. A study is also conducted to investigate the effects of different operation parameters on gasification process including coal mixture (dry vs. slurry), oxidant (oxygen-blown vs. air-blown), and different coal distributions between two stages. Finite-rate model and instantaneous gasification model are compared. It is revealed that the instantaneous gasification approach can provide an overall evaluation of relative changes of gasifier performance in terms of temperature, heating value, and gasification efficiency corresponding to parametric variations, but not adequately capture the local gasification process predicted by the finite rate model in most part of the gasifier. Simulations are performed to help with design modifications of a small industrial demonstration entrained-flow gasifier. It is discovered that the benefit of opening the slag tap on the quench-type gasifier wider by allowing slag to move successfully without clogging is compromised by increased heat losses, reduced gasification performance, downgraded syngas heating value, and increased unburned volatiles. The investigation of heat transfer on fuel injectors shows that blunt tip fuel injector is less likely to fail compared to conical tip fuel injector because the maximum high temperature on the injector is scattered. Two concentric fuel/oxidant injections provide better fuel-oxidant mixing and higher syngas heating value than four separate fuel and oxidant injections.

Concrete marine structures are typically exposed to harsh marine environments where the ingress of chloride ions can lead to corrosion of steel reinforcing bars, reducing both strength and service life; therefore, concrete must be proportioned to resist these environments. Current recommendations for concrete mixtures and plastic shrinkage cracking both reduce the resistance to chloride ingress. The main objective of this thesis was to understand the benefits of fiber addition to concrete exposed to chlorides and quantify those benefits, which would lead to a concrete mixture suitable for marine structures in freeze thaw environments. The research program tested two different fibers in a total of nine concrete mixtures. The results demonstrated that fiber addition at dosages up to 0.33 % by volume resulted in significant reduction or elimination of plastic shrinkage cracking and the chloride tests determined that the ternary FRC mixtures had the best resistance to chloride diffusion.