Thesis (Ph.D.)(Chemistry)--University of Pretoria, 2007. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

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Gasification is a promising technique for transforming solid biomass into a gas that can be used to produce renewable heat, power, fuels or chemicals. Biomass materials, such as forestry residues, can be high moisture, heterogeneous mixtures with low bulk density - properties that make them difficult to handle and convert. Consequently, this means that feedstock pre-treatment is usually necessary in order to facilitate its conversion by gasification. Pre-treatments methods, which include comminution, drying, pelletization, torrefaction, or carbonization will affect the properties of the biomass which will affect their gasification in a fluidized bed. The objective of this thesis was to determine how biomass pre-treatment can influence gasification in a fluidized bed. A single forestry residue was processed using five pre-treatment process levels: sieving (as a surrogate for comminution), drying (moisture content), pelletization, torrefaction, and carbonization. The fractions derived from these processes were gasified in a small pilot-scale air blown bubbling fluidized bed gasifier (feed rate 8 – 25 kg/h). The particle size and form had an impact on the gas composition, tar content, and cold gas efficiency of the gasification. Over the conditions tested, the finest fraction produced a gas with a H2/CO ratio of 0.36 – 0.47 containing 7 – 59 g/m3 tar (gravimetric) at a cold gas efficiency of 30 - 41%. The pellets on the other hand yielded a gas with a H2/CO ratio of 0.89 - 1.14, containing 3 – 37 g/m3 tar (gravimetric) at a cold gas efficiency of 41 – 60%. Drying, torrefaction and carbonization also had an impact on the gasification performance. Carbonization was able to reduce the yield of tar (as measured by gas chromatography) by more than 95% relative to the parent material. Finally, four different forestry residues were gasified in a large pilot-scale bubbling fluidized bed with air and steam-oxygen mixtures (feed rate 200 – 245 kg/h) in order to assess whether the comminution effect could be observed at the large scale. One feedstock with a significant portion of small particles showed the expected effects compared to the feed materials with large feed particles: lower H2/CO ratio, greater tar yield, lower cold gas efficiency while the other feed material containing a substantial amount of small particles did not show these effects.

Energy based on non-renewable resources such as gas, oil, coal and nuclear fission, even with their serious problems of pollution, contributes to 86% of the global energy consumption. Oil will remain the dominant transport fuel: about 87% of transport fuel in 2030 will still be petroleum-based. Discoveries of conventional sources of light easy-to-access crude oil are becoming less common and current oil production levels are struggling to match demand, it is necessary to develop new non-conventional sources of oil in order to supplement conventional oil supply, whose demand is increasing continuously. A possible clue to solve this situation could be to take advantage of the extensive reserves of heavy crude oils existing in different places around the world, which could be an excellent source of more valuable hydrocarbons. In this context, some facilities called upgraders are used to process theses heavy crude oils to both increase the hydrogen-carbon ratio and improve their quality, reducing their density and decreasing their viscosity, sulphur, nitrogen and metals. The main objective in this work is to study the heavy crude oil upgrading processes in order to identify new operation schemes which explore different opportunities of integration between the upgraders and other processes or new schemes for upgraders that can sustain on its own through the production of a wide range of products. Each design alternative has been modelled with state-of-the-art commercial software packages. The crude oil dilution process was evaluated using naphtha and a light crude oil as diluents. Sensitivity analyses were done with the purpose of selecting the type and flow rate of diluent. Once the best diluent was selected, the integration of an upgrader to a refinery was studied. Heavy ends from both the upgrader and the refinery were taken as feedstocks to an integrated gasification combined cycle (IGCC). The best operation schemes for IGCC, in order to achieve the requirements of power and hydrogen for the upgrader and the refinery was determined. Different schemes for heavy crude oil processing to produce transportation fuel instead of syncrude were proposed, too. Finally, economic evaluation of all the schemes was performed to find the best solution for heavy crude oils. The best results for the dilution process of heavy crude oils were obtained when naphtha was used as diluent. The configuration proposed for the upgrader allows producing a synthetic crude oil with 35.5 °API. The integration of the upgrader to a refinery allows the treatment of the heavy streams of the refinery and transforms them into products of higher qualities. The integration of the IGCC to the upgrader and the refinery permits a complete elimination of the heavy residues produced in these units and produces hydrogen and power to be used in the site or to export. Economic evaluation shows that all the proposed processing schemes studied are economically attractive. The proposed processing schemes chosen include the integration between upgrader refinery and IGCC unit with CCS.

With the increasing need for renewable energy technologies in the world, biomass fuel transformation technology is growing towards meeting that need, among others. Challenges remain certain and new innovations are being tested in bid to overcome them with the application of biomass as energy source. This report presents some studies carried out into understanding the potential and challenges associated with utilization of biomass fuel, especially for technologies that are appropriate for rural applications.  Utilization of biomass gasification technologies is in the focus of this study.   The great potential biomass fuel provide to Uganda for possible energy production in small scale application is presented. This study was carried out to understand the possibility of using biomass as fuel in electricity power generation in Uganda. It takes into consideration the use of biomass gasification technology in energy production. Challenges related to the application of biomass fuels are discussed, mostly with tar and alkali metal compounds in the gas stream.   Suggested methods to combat some of the challenges with biomass fuels are pointed out in this study. Application of externally fired gas turbine (EFGT) system is a particular approach discussed and its technical performance analyzed.  The analysis revealed that efficiency of the EFGT system is greatly dependent on the heat exchanger effectiveness and on turbine inlet temperature. Optimum performance can be realized with air compression ratio of 3.4. It is also noted that fouling and deposition in the heat exchanger can affect its performance.   A related study carried out was on the retention of alkali metals in an updraft gasifier. The gasifier was chosen for possible integration with the EFGT system. Finding was that about 99% of the alkali metals are retained in the gasifier. It is anticipated that this would reduce on the deposition in the heat exchanger, reducing on maintenance time. A need is identified to determine the level of deposition mentioned. A simple thermodynamic modeling of the alkali metals condensation reaction on a high temperature heat exchanger surface was conducted. The results showed that with appropriate quantity of S in the raw fuel, alkali metals bounding with Cl are greatly reduced. Cl is passed out as gaseous HCl, leaving deposition of none corrosive sulphates. Recommendation is made to study this phenomenon in an experimental setting.   Biomass gasification technology integration with an internal combustion (IC) engine is also studied. Here requirements for the producer gas quality have been discussed. Some tests carried out with wood pellets and wood cylinders compared the yield of tar from the two physically different fuels.  Wood pellets were found to yield more tar than wood cylinders.   Economic analysis of biomass gasifier integrated with an IC engine running a generator of 100 kWe was carried out. Comparison with a diesel electricity generator of similar capacity was made for a scenario in Uganda over a project life of 20 years. Different Plant Capacity Factors (PCF) and fuel costs including subsidies were considered. The analysis showed that over long period of time biomass power plant was more beneficial than the diesel power plant at PCF over 40%. This is more pronounced with unsubsidized diesel fuel. / QC 20111206

Thesis. 1978. Sc.D.--Massachusetts Institute of Technology. Dept. of Chemical Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 382-396. / by Eric M. Suuberg. / Sc.D.

Tars have been identified as one of the major impurities associated with the utilization of biomass gasification fuel gas. Tars may result in blockages, plugging, corrosion and catalyst deactivation, leading to serious operational and maintenance problems during biomass gasification. Therefore, tar removal is essential to insure economic and effective fuel gas utilization. This study investigates the catalytic activity of zeolites, and nickel-supported zeolites for tar removal. Tests were conducted using a bench scale reactor and naphthalene as a model tar compound. Zeolites with varying pore sizes and acidity were tested to evaluate the effect of pore size and acidity on tar removal. Test results suggested that the catalytic activity increased with an increase in pore size and the number of acidic sites on the zeolite. The steam reforming ability of nickel towards naphthalene removal was evaluated by impregnating nickel on zeolites; this impregnation improved the activity of the catalysts significantly. Long term catalytic activity tests were performed, which showed that nickel supported ZY-30 and ZY-80 had the best naphthalene conversion, with naphthalene conversions of greater than 99%, followed by nickel- supported ZY-5.2, SiO2/Al2O3, and chabazite, respectively. This study also evaluated the catalytic activity of montmorillonite and nickel- supported montmorillonite as tar removal catalysts. Montmorillonite, and Ni-montmorillonite were tested for their efficiency in reforming tars. Also, the efficacy of nickel-supported montmorillonite catalyst was tested as a function of nickel content, reaction temperature, naphthalene loading, and the steam to carbon ratio. The results demonstrated that montmorillonite was catalytically active in removing naphthalene. In addition, Ni-montmorillonite had very high activity towards naphthalene removal via steam reforming, with removal efficiencies of greater than 99%. The activation energy was calculated for Ni-montmorillonite assuming first order kinetics and was found to be 84.5 kJ/mole. Long-term activity tests showed that the catalyst was active with naphthalene removal efficiencies of greater than 95%, which were maintained over a 97-hour test period. Very little loss of activity was observed with a drop in removal from 97 to 95%. The drop in catalytic activity was attributed primarily to a decrease in catalyst surface area, nickel sintering, and coke formation.

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The emergence of biomass based energy warrants the evaluation of syngas from biomass gasification as a fuel for personal power systems. The objectives of this study were to determine the performance and exhaust emissions of a commercial 5.5 kW generator modified for operation with 100% syngas at different syngas flows and to compare the results with those obtained for gasoline operation at same electrical power. Maximum power output for gasoline operation was 2451 W and maximum power output for syngas operation was 1392 W. Overall efficiencies of the generator were same at maximum electrical power outputs for operation with both the fuels. At four different electrical power output categories, the exhaust concentrations of carbon monoxide and oxides of nitrogen were significantly lower while the carbon dioxide emissions were significantly higher for the syngas operation. The unit cost of electricity generation was $6.38/kWh for syngas operation and $0.56/kWh for gasoline operation.

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