The aim of this diploma thesis is to describe the gasification of biomass with a mixture of oxygen and water vapor. The theoretical part is focused on the gasification process, gasification reactors and the quality of the resulting gas. In the experimental part we look for the ideal ratio of oxygen to water vapor, which are used as gasification media. Subsequent measurements are carried out on the fluidized bed reactor, the results of which are further processed and evaluated.

Thermochemical technologies have been proven effective in treating municipal solid waste (MSW) for many years. China, with a rapid increase of MSW, plans to implement more environmental friendly ways to treat MSW than landfill, which treats about 79 % of total MSW currently. The aim of this master thesis was to find out a suitable thermochemical technology to treat MSW in Shanghai, China. Several different thermochemical technologies are compared in this thesis and plasma gasification was selected for a case study in Shanghai. A model of the plasma gasification plant was created and analysed. Other processes in the plant including MSW pre-treating and gas cleaning are also proposed. By calculating the energy balance, it is demonstrated that plasma treatment of 1000 ton/day MSW with 70 % moisture reaches an efficiency of 33.5 % when producing electricity, which is higher than an incineration WtE plant (27 % maximum) and a gasification WtE plant (30 % maximum). Besides of the efficiency comparison, costs and environmental impacts of different technologies are also compared in this paper. The result indicated that given the characteristics and management situation of MSW in Shanghai, plasma gasification is a better choice to treat MSW in Shanghai.

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In this study, efforts ensued to increase our knowledge of fluidization and gasification behavior of Geldart A particles using CFD. An extensive Eulerian-Eulerian numerical study was executed and simulations were compared and validated with experiments conducted at Utah State University. In order to improve numerical predictions using an Eulerian-Eulerian model, drag models were assessed to determine if they were suitable for fine particles classified as Geldart A. The results proved that if static regions of mass in fluidized beds are neglected, most drag models work well with Geldart A particles. The most reliable drag model for both single and binary mixtures was proved to be the Gidaspow-blend model. In order to capture the overshoot of pressure in homogeneous fluidization regions, a new modeling technique was proposed that modified the definition of the critical velocity in the Ergun correlation. The new modeling technique showed promising results for predicting fluidization behavior of fine particles. The fluidization behavior of three different mixtures of coal and poplar wood were studied. Although results indicated good mixing characteristics for all mixtures, there was a tendency for better mixing with higher percentages of poplar wood. In this study, efforts continued to model co-gasification of coal and biomass. Comparing the coal gasification of large (Geldart B) and fine (Geldart A) particles showed that using finer particles had a pronounced effect on gas yields where CO mass fraction increased, although H2 and CH4 mass fraction slightly decreased. The gas yields of coal gasification with fine particles were also compared using three different gasification agents. Modeling the co-gasification of coal-switchgrass of both fine particles of Geldart A and larger particles of Geldart B showed that there is not a synergetic effect in terms of gas yields of H2 and CH4. The gas yields of CO, however, showed a significant increase during co-gasification. The effects of gasification temperature on gas yields were also investigated. / Ph. D.

Yes / In this study, air, steam and CO2-enhanced gasification of rice straw was simulated using Aspen PlusTM simulator and compared in terms of their energy, exergy and environmental impacts. It was found that the addition of CO2 had less impact on syngas yield compared with gasification temperature. At lower CO2/Biomass ratios (below 0.25), gasification system efficiency (GSE) for both conventional and CO2-enhanced gasification was below 22.1%, and CO2-enhanced gasification showed a lower GSE than conventional gasification. However at higher CO2/Biomass ratios, CO2-enhanced gasification demonstrated higher GSE than conventional gasification. For CO2-enhanced gasification, GSE continued to increase to 58.8% when CO2/Biomass was raised to 0.87. In addition, it was found that syngas exergy increases with CO2 addition, which was mainly due to the increase in physical exergy. Chemical exergy was 2.05 to 4.85 times higher than physical exergy. The maximum exergy efficiency occurred within the temperature range of 800 oC to 900 oC because syngas exergy peaked in this range. For CO2-enhanced gasification, exergy efficiency was found to be more sensitive to temperature than CO2/Biomass ratios. In addition, the preliminary environmental analysis showed that CO2-enhanced gasification resulted in significant environmental benefits compared with stream gasification. However improved assessment methodologies are still needed to better evaluate the advantages of CO2 utilization.

Yes / In this study, a novel single-step synthesis of dimethyl ether (DME) based on CO2-enhanced biomass gasification was proposed and simulated using ASPEN PlusTM modelling. The exergetic and environmental evaluation was performed in comparison with a conventional system. It was found that the fuel energy efficiency, plant energy efficiency and plant exergetic efficiency of the CO2-enhanced system were better than those of the conventional system. The novel process produced 0.59 kg of DME per kg of gumwood with an overall plant energy efficiency of 65%, which were 28% and 5% higher than those of conventional systems, respectively. The overall exergetic efficiency of the CO2-enhanced system was also 7% higher. Exergetic analysis of each individual process unit in both the CO2-enhanced system and conventional systems showed that the largest loss occurred at gasification unit. However, the use of CO2 as gasifying agent resulted in a reduced loss at gasifier by 15%, indicating another advantage of the proposed system. In addition, the LCA analysis showed that the use of CO2 as gasifying agent could also result in less 21 environmental impacts compared with conventional systems, which subsequently made the CO2-22 enhanced system a promising option for a more environmental friendly synthesis of bio-DME. / Part of this work is sponsored by Ningbo Bureau of Science and Technology under its Innovation Team Scheme (2012B82011) and Major R&D Programme (2012B10042).

The thesis, through experimental and numerical investigations reports on the work related to packed bed reactors in co-current configuration for biomass gasification. This study has extensively focused on the gasification operating regimes and addressing the issues of presence of tar, an undesirable component for engine application. Systematically, the influence of fuel properties on the gasification process has been studied using single particle analysis and also in packed bed reactors. Studies related to the effect of fuel properties - size, surface area volume ratio and density on the reactor performance are addressed. The influence of these parameters on the propagation rate which indirectly influences the residence time, tar generation, gas compositions is explicitly elucidated. Most of the reported work in literature primarily focuses on counter-current configurations and analysis on propagation flame front/ignition mass flux and temperature profiles mostly under the combustion regime. In this work, flame propagation front movement, bed movement and effective movement for a co-current packed bed reactor of different reactor capacities and a generalized approach towards establishing ‘effective propagation rate’ has been proposed. The work also reports on the importance of particle size and sharing of air from the top and through nozzles on tar generation in the open top down draft reactor configuration. Firstly, pyrolysis, an important component of the thermochemical conversion process has been studied using the flaming time for different biomass samples having varying size, shape and density. The elaborate experiments on the single particle study provides an insight into the reasons for high tar generation for wood flakes/coconut shells and also identifies the importance of the fuel particle geometry related to surface area and volume ratio. Effect of density by comparing the flaming rate of wood flakes and coconut shells with the wood sphere for an equivalent diameter is highlighted. It is observed that the tar level in the raw gas is about 80% higher in the case of wood flakes and similar values for coconut shells compared with wood pieces. The analysis suggests that the time for pyrolysis is lower with a higher surface area particle and is subjected to nearly fast pyrolysis process resulting in higher tar fraction with low char yield. Similarly, time for pyrolysis increases with density as observed from the experimental measurements by using coconut shells and wood flakes and concludes the influence on the performance of packed bed reactors. Studies on co-current reactor under various operating conditions from closed top reactor to open top reburn configuration suggests improved residence time reduces tar generation. This study establishes, increased residence time with staged air flow has a better control on residence time and yields lower tar in the raw gas. Studies on the influence of air mass flux on the propagation rate, peak temperature, and gas quality, establishes the need to consider bed movement in the case of co-current packed bed reactor. It is also observed that flame front propagation rate initially increases as the air mass flux is increased, reaches a peak and subsequently decreases. With increase in air mass flux, fuel consumption increases and thereby the bed movement. The importance of bed movement and its effect on the propagation front movement has been established. To account for variation in the fuel density, normalized propagation rate or the ignition mass flux is a better way to present the result. The peak flame front propagation rates are 0.089 mm/s for 10 % moist wood at an air mas flux of 0.130 kg/m2-s and while 0.095 mm/s for bone-dry wood at an air mass flux of 0.134 kg/m2-s. These peak propagation rates occur with the air mass flux in the range of 0.130 to 0.134 kg/m2-s. The present results compare well with those available in the literature on the effective propagation rate with the variation of air mass flux, and deviations are linked to fuel properties. The propagation rate correlates with mass flux as ̇ . during the increasing regime of the front movement. The extinction of flame propagation or the front receding has been established both experimentally supported from the model analysis and is found to be at an air mass flux of 0.235 kg/m2-s. The volume fraction of various gaseous species at the reactor exits obtained from the experiment is 14.89±0.28 % CO2, 15.75±0.43 % CO and 11.09±1.99 % H2 respectively with the balance being CH4 and N2. The model analysis using an in-house program developed for packed bed reactor provide a comprehensive understanding with respect to the performance of packed bed reactor under gasification conditions. The model addresses the dependence on air mass flux on gas composition and propagation rate and is used to validate the experimental results. Based on the energy balance in the reaction front, the analysis clearly identifies the reasons for stable propagation front and receding front in a co-current reactor. From the experiments and modelling studies, it is evident that turn-down ratio of a downdraft gasification system is scientifically established. Both the experimental and the numerical studies presented in the current work establishes that the physical properties of the fuel have an impact on the performance of the co-current reactor and for the first time, the importance of bed movement on the propagation rate is identified.

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South Africa has large coal reserves and produces approximately 74% of its primary energy from coal. Coal gasification using moving bed gasifiers is one of the most important coal utilisation technologies, consuming ± 17.5% of locally produced coal. This study was motivated by the need to investigate alternative coal gasification technologies for the utilisation of fine, high-ash and caking coals for future Integrated Gasification Combined Cycle (IGCC) and coal to liquids (CTL) plants. These coals are estimated to form a large percentage of the remaining coal reserves in South Africa and could be difficult to utilise efficiently in moving bed gasifiers. Fluidised bed gasification was identified as a technology that could potentially utilise these coals. Coals from the New Vaal and Grootegeluk collieries were selected as being suitable for this investigation. The coals were subjected to detailed characterisation, bench-scale and pilot-scale fluidised bed gasification tests. The results of the pilot-scale atmospheric bubbling fluidised bed gasification tests show that stable gasification is possible at temperatures between 880 °C and 980 °C. The maximum fixed carbon conversion achievable in the pilot plant is, however, limited to ± 88% due to the low reactivity of the coals tested and to thermal fragmentation and attrition of the coal in the gasifier. It was found that oxygen enrichment of the gasification air from 21% to 36% by means of oxygen addition produces a significant increase in the calorific value of the gas (3.0 MJ/Nm3 to 5.5 MJ/Nm3). This observation has not previously been reported at pilot-plant scale. A mathematical model for a bubbling fluidised bed coal gasifier was developed based on sub-models for fluidised bed hydrodynamics, coal devolatilisation, chemical reactions, transfer processes and fines generation. A coal devolatilisation sub-model to predict the products of coal devolatilisation in a fluidised bed gasifier was developed and incorporated into the model. Parameters associated with the rates of the gasification reactions and the devoltilisation process were obtained by means of bench-scale tests. The heat loss parameter (Q) in the model was estimated by means of a heat loss calculation. The results from the pilot-scale gasification tests were used to evaluate the predictive capability of the model. It was found that for temperature, fixed carbon conversion and calorific value of the gas the difference between measured and predicted values was less than 10%. Recommendations are made for further refinement of the model to improve its predictive capability and range of application. The model was used to study the effect of major operating variables on gasifier performance. It was found that increasing the reactant gas (air, oxygen and steam) temperature from 250 °C to 550 °C increases the calorific value of the gas by ± 9.3% and the gasification efficiency by ± 6.0%. Increasing the fluidised bed height has a positive effect on fixed carbon conversion; however, at higher bed heights the benefit of increasing the bed height is less due to the inhibiting effects of H2 and CO on the rates of char gasification. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2014