Department of Physics

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    Characterization and computer simulation of corn stover / coal blends for co-gasification in a downdraft gasifier
    (University of Fort Hare, 2014) Mabizela, Polycarp Sbusiso
    The need for sustainable alternative energy technology is becoming more urgent as the demand for clean energy environment increases. For centuries, electricity in South Africa has been derived mostly from coal with results growing in multifold annually due to concerns about the impact of fossil fuel utilization related to emission of greenhouse gasses. It is practically impossible at the moment to replace coal with biomass resources because of the low energy value of biomass. However, the conversion of coal has experienced some challenges especially during its gasification which includes, but are not limited to a high reaction temperature exceeding 900°C which most gasifiers cannot achieve, and if achieved in most cases, combustion of the resulting syngas usually occur, leading to low conversion efficiency and the risk of reaching extremely high temperatures that may result in pressure build up and explosion may also occur. Therefore, this study sought to investigate the possibility of co-gasifying corn stover with coal with the ultimate aim establishing the best mixing ratio that would result in optimum co-gasification efficiency after computer simulation. Proximate and ultimate analysis, including energy values of corn stover and coal as well as their blends were undertaken and results showed significant differences between the two feedstocks and narrow range composition betwee their blends in terms of properties and energy value. Corn stover showed a higher fraction of volatile matter and lower ash content than coal, whereas those of their blends vary considerably in terms of physical properties. Differences in chemical composition also showed higher fraction of hydrogen and oxygen, and less carbon than coal while those of their blends vary according to the ratio of corn stover to coal and vice versa in the blends. The thermal stability of corn stover and coal as well as their blends were also established and the maximum temperature reached for thermal degradation of their blends was 900°C as depicted by TGA analysis. The SEM results revealed no changes in morphology of the pure samples of corn stover and coal which was due to the fact that a pre-treatment of the samples were not undertaken, whereas the blends showed significant changes in morphology as a result of blending. However, luminous and non-luminous features were noticed in both SEM images of the blends with the 10% coal/90% corn stover blend having higher percentages of luminosity as a result of higher quantities of coal in the blend. The energy density of the samples were also measured and found to be 16.1 MJ/kg and 22.8 MJ/kg for corn stover and coal respectively. Those of their blends varied from 16.9 to approximately 23.5 MJ/kg. These results were used to conduct computer simulation of the co-gasification process in order to establish the best blend that would result in maximum co-gasification efficiency. The blend 90% corn stover/10% coal was found to be the most suitable blend for co-gasification resulting in an efficiency of approximately 58% because its conversion was efficiently achieved at a temperature that is intermediate to that of coal and biomass independently. The simulation results were, however, compared with experimental data found in the literature and results showed only slight variation between them.
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    Selecting high-order modes in solid state laser resonators
    (2014) Iheanetu, Kelachukwu
    The first chapter considered the fundamental processes of laser operation: photon absorp-tion, spontaneous and stimulated emissions. These processes are considered when design-ing a laser gain medium. A four-level laser scheme was also illustrated. Then, the basic components and operating principle of a simple laser system was presented using a diode end-pumped Nd:YAG solid state laser resonator. The second chapter considered laser light as light rays propagating in the resonator and extensively discussed the oscillating field in the laser resonators. It examined the characteristics of the fundamental Gaussian mode and the same theory was applied to higher-order modes. Chapter three started with an introduction to beam shaping and proceeded to present a review of some intra-cavity beam shaping techniques, the use of; graded phase mirrors, diffractive elements – binary phase elements and spiral phase elements. Also, a brief dis-cussion was given on the concept of conventional holography and digital holography. The phase-only spatial light modulator (SLM) was presented, which by default is used to per-form (only) phase modulation of optical fields and how it can be use to perform amplitude modulation also. Finally, a detailed discussion of the digital laser which uses the intra-cavity SLM as a mode selection element was presented, since it was the technique used in the experiment. The elegance of dynamic on-demand mode selection that required only a change of the grey-scale hologram on the SLM was one quality that was exploited in using the digital laser. The next two chapters presented the experiments and results. The concept of the digital laser was first used in the experiment in chapter four, to assemble a stable diode end-pumped Nd:YAG solid state laser resonator. Basically, the cavity was of hemispherical configuration using an intra-cavity SLM (virtual concave mirror) as a back reflector and a flat mirror output coupler. A virtual concave mirror was achieve on the SLM by using phase modulation to generate the hologram of a lens, which when displayed on the SLM made it to mimic a concave mirror. Then the next phase was using symmetric Laguerre-Gaussian mode function, of zero azimuthal order to generate digital holograms that correspond to amplitude absorbing concentric rings. These holograms, combined with the hologram that mimics a concave mirror were used on the SLM to perform high-order Laguerre-Gaussian modes selection in the cavity. The fifth chapter presented the results of the mode selection and considered the purity of the beam at the output coupler by comparing measured modal properties with the theoretical prediction. The outcome confirmed that the modes were of high purity and quality which further implied that the cavity was indeed selecting single pure high-order modes. The results also demonstrated that forcing the cavity to oscillate at higher-order modes (p = 3) extracted ≈ 74% more power from the gain medium compared to the fundamental mode (p = 0), but this extra power is only accessible beyond a critical pump input power of ≈ 38.8 W. Laser brightness describes the potential of a laser beam to achieve high intensities while still maintaining a large Rayleigh range. It is a property that is dependent on beam power and its quality factor. To achieve high brightness one needs to generate a beam that ex-tracts maximum power from the gain with good beam quality. Building on the experiments demonstrated in this study, one can make the correct choices of output coupler’s reflectivity, the laser gain medium’s length and doping concentration and the pump mode overlap for a particular mode to further enhance energy extraction from the cavity, and then using well known extra-cavity techniques to improve the output beams quality factor by transforming the high-order mode back to the fundamental mode. This will effectively achieve higher laser brightness.
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    Impact of translucent water-based acrylic paint on the thermal performance of a low cost house.
    (University of Fort Hare, 2014) Overen, Ochuko Kelvin
    Insulation materials are selected based on their R-value, which is a measure of the thermal resistance of a material. Therefore, the higher the R-value of a material, the better its thermal insulation performance. There are two major groups of insulation materials: bulk and reflective insulation (or combine bulk and reflective). Bulk insulation is design to resist heat transfer due to conduction and convection. Reflective insulation resists radiant heat flow due to its high reflectivity and low emissivity. Insulation materials are not restricted to these materials only. Other low thermal conductive materials can be used as long as the primary aim of thermal insulation, which is increasing thermal resistance, is achieved. Hence, the aim of the project is to investigate the insulation ability of Translucent Water-based Acrylic Paint (TWAP) on the thermal performance of Low Cost Housing (LCH). To achieve the aim of the study, the inner surfaces of the external walls of LCH was coated with TWAP. Before the inner surfaces of the external walls were coated, the following techniques were used to characterised the paint; Scanning Electron Microscopy/ Energy Dispersive X-ray spectroscopy (SEM/EDX), Fourier Transform Infra-Red (FTIR) and IR thermography. SEM/EDX was adapted to view the surface morphology and to detect the elemental composition responsible for the thermal resistance of the TWAP. FTIR spectroscopy was used to determine the functional group and organic molecular composition of the paint. The heat resistance of TWAP was analyzed using IR thermography technique. A low cost house located in the Golf Course settlement in Alice, Eastern Cape, South Africa under the Nkonkobe Municipality Eastern Cape was used as a case study in this research. The house is facing geographical N16°E, It comprises a bedroom, toilet and an open plan living room and kitchen. The house has a floor dimension of 7.20 m x 5.70 m, giving an approximate area of 41 m2. The roof is made of galvanized corrugated iron sheets with no ceiling or any form of roof insulation. The walls of the buildings are made of the M6 (0.39 m x 0.19 m x 0.14 m) hollow concrete blocks, with no plaster or insulation. The following meteorological parameters were measured: temperature, relative humidity, solar irradiance, wind speed and wind direction. Eleven type-K thermocouples were used to measure the indoor temperature, inner and outer surfaces temperature of the building walls. Two sets of HMP50 humidity sensors were used to measure the indoor and outdoor relative humidity as well as the ambient temperature. The indoor temperature and relative humidity were measured at a height of 1.80 m so as to have good indoor parameter variation patterns that are not influenced by the roof temperature. The outdoor relative humidity sensor together with a 03001 wind sentry anemometer/vane and Li-Cor pyranometer were installed at a height of 0.44 m above the roof of the building. Wind speed and direction were measured by the 03001 wind sentry anemometer/vane, while solar radiation was measured by the Li-Cor pyranometer. The entire set of sensors was connected to a CR1000 data logger from which data are stored and retrieved following a setup program. The SEM image shows that TWAP is transparent in its dry state. EDX spectrum reveals the presence of Al in the paint, which is present as Al2O3. Due to the refractive index (1.73) of Al2O3, it is used as IR reflective pigment in reflective paints. Other elements like Si were also identified as SiO2 that contributes to the thermal resistance of the paint. The functional groups that made up the different molecular bonding of the paint were clearly shown by the FTIR spectrum. These include O–H, CH2, Si–H, C═C and others. From the IR thermography, average decrement factor of 0.54 and 1.04 were found for the coated and uncoated, respectively. This implies that the coated surface has more heat reduction than the uncoated surface. The indoor temperature was observed to be influenced by the temperature of the building envelope in both summer and winter seasons. In summer, it was observed that the indoor temperature variation closely follow the external wall temperature. On a typical summer hot day, the average maximum heating rate of the North, West and South walls were 5.93 W, 5,07 W and 2.24 W, respectively. It was found that the indoor temperature was mostly higher than the outdoor temperature. Also, that the indoor temperature was within the comfort zone for only 46% of the time. The indoor relative humidity was within the comfort zone throughout the day. A time lag of 2.5 hours was observed between the time the solar radiation and indoor temperature were at their maximum value. In the winter season, it was found that the indoor temperature is influenced more by the middle and external walls temperature. On a typical cold winter day, the average cooling rate for the North, West and South walls were found to be 0.89 W, 8.27 W and 0.30 W, respectively. The indoor and outdoor temperatures were seen to be completely below the comfort zone. On the other hand, the indoor relative humidity was found at the upper region of the comfort zone, ranging from 55% to 65%. It was found that the wind speed experienced in winter was 12% higher than the maximum wind speed in the summer season. After coating, the thermal performance of the building showed significant improvement. The amount of cooling and heating degree hour required by the inner space of the house to maintain thermal comfort, decreased by an average of 50% and 41%, respectively, in both winter and summer periods. Finally, the house was found to save an average of 37% of cooling and cooling energy demand after coating. The model developed shows that all components of the building envelope contributed positively to the indoor temperature, in the summer. Indoor, outdoor relative humidity and winter speed contribute negatively to the indoor temperature. In the winter, it was observed that the floor and South walls contributed negatively to the indoor temperature, as well as the wind speed and indoor relative humidity. In both seasons, the measured and predicted indoor temperatures showed a best fit with approximately 99% of the response data set perfectly on the predicted temperature. This proves that the predictors are the parameter that can serve as the basis for indoor temperature.
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    Chemical kinetics of biomass and sorbent blends for gasification purposes
    (University of Fort Hare, 2015) Mabuda, Azwihangwisi Iren
    The main aim of this research was to determine the chemical kinetics of biomass and biomass/sorbent mixtures, which is crucial in assessing parameters including the feasibility, design, and scaling of industrial biomass conversion applications such as pyrolysis and gasification. Many studies have been conducted for co-gasification of coal and biomass, however little has been done for the biomass with sorbent mixtures. Biomass is one of the main renewable energy sources and coupled with carbon dioxide absorbent material such as calcium oxide (CaO) and/or magnesium oxide (MgO) sorbent it increases the biomass conversion efficiency during gasification. The thermogravimetric analyzer (TGA) was conducted in order to establish the material thermal behavior under temperatures for which gasification takes place with specific reference to the pyrolysis stage of gasification. Kinetics of thermal decomposition of biomass with sorbent mixtures of pine wood (wood), CaO and MgO, were investigated using the TGA technique. The different ratios of these materials, which ultimately determines the gasification characteristics of the blends, were investigated. The measurements were carried out in a nitrogen atmosphere at different heating rates of 10, 15 and 20 ºC/min. It was found that the material fully degraded in the devolatilazation zone, which is in the temperature range of 200-800 ºC. At higher temperature some samples were more stable compared to others. The significant mean difference in the rate of degradation between the samples was compared using one way analysis of variance (ANOVA). The kinetic parameters (activation energy, pre-exponential factor and the reaction order) were obtained from both model free (Kissinger and Flynn-Wall-Ozawa (FWO)) and the regression models. A computer simulation was performed to establish the ratio that resulted in higher conversion efficiency during gasification. A mixture of 80% pine wood and 12,5% CaO and 12.5%MgO to make 25%CaO.MgO has resulted in the highest thermal stability compared to other samples. In other words, at higher temperatures these samples degrade at a slower rate compared to others. The activation energy of this sample, 139.63 kJ/mol and the average value of 143.74 kJ/mol was obtained from the Kissinger and FWO method, respectively. The values obtained from the regression models were higher compared to model free methods. Both the model free and model fitting methods were effective in determining the kinetic parameters. However, for accurate results, it is crucial to develop your own model which can fit the data perfectly. Compared to other blends the 25%CaO.MgO sample was found to be suitable to perform better during gasification.
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    Design, manufacture, and performance evaluation of waste heat recovery unit in a gasification plant
    (University of Fort Hare, 2016) Nwokolo, Nwabunwanne Lilian
    Johansson biomass gasification system is a standalone power generation system as it utilizes the syngas produced from the downdraft gasifier in an internal combustion gas engine for power generation. The syngas exiting the gasifier and entering the cyclone dissipates heat on the body of the cyclone due to the high temperature at which it exit. In addition this same syngas undergoes some cooling process at the gas scrubber before reaching the gas engine. As the gas engine drives the synchronous generator for power generation, some of the un-combusted gases exit through the exhaust pipe at high temperatures. All these add-up as waste heat within the gasification system, hence there is a significant opportunity for waste heat recovery in Johansson biomass gasification system. Syngas cleanup prior to its utilization in downstream application is a necessity due to the presence of contaminants such as carbon particles. The cleanup of syngas depends on the composition of syngas, further application requirement and economic consideration. Hence the choice and the design of a syngas cleaning process should be influenced by economic and energetic cost. Cold gas cleanup which is usually carried out at low temperature is considered as a conventional approach in syngas cleanup due to proven reliability and efficient removal of contaminants. However, cold gas cleanup result in significant loss of heat in the form of waste heat.