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Laboratory scale evaluation of solar-driven supercritical water gasification of biomass




Kingsland, Michael

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The combination of concentrated solar power (CSP) generation and biomass gasification for energy storage can greatly increase the versatility of CSP and open new pathways for energy distribution. As shown in this thesis, CSP increases the energy content of the endothermic gasification reaction product in comparison to conventional gasification approaches by around 20 percent. The aim of this project is to gain understanding in what kind of biomass gasification technology as an energy storage solution could be integrated with existing CSP technology with focus especially on dish concentrators. Another major part is the development, building and running of a laboratory scale setup for evaluating the previously proposed technology. A review of different biomass gasification technologies examines their compatibility and integrateability with CSP. It appears that compared to conventional gas phase technologies hydrothermal gasification allows for the widest range of feedstocks - especially but not limited to very wet biomass sources like sewage waste streams or aquatic energy crops like algae. Furthermore, with hydrothermal gasification the biomass feedstock can be completely gasified and the formation of unwanted waste products like char and tar suppressed. This leaves only water and some ash mostly in the form of inorganic salts besides the product gas and a cleaning process to remove tars and ash particles from the gas stream is not required. Hydrothermal gasification technology also ties in well with readily available CSP infrastructure. At conditions just above the critical point around 400 C the reaction yields a product gas containing mainly methane (up to 60 percent), CO2 and other contaminants while above 600 C a hydrogen rich gas is produced. As ANU's ammonia dissociation dish concentrator setup was already well suited for high temperature supercritical gasification and unwanted byproducts like char and tar are more easily suppressed at these conditions, the high temperature approach was chosen for this thesis. Hydrogen from this product gas stream can be extracted at high pressures of around 25 MPa and in nearly pure form negating the need for subsequent energy intensive gas processing and compression for storage. One contribution of this project was to develop and build a laboratory scale setup for evaluating supercritical biomass gasification for solar thermal energy storage. Gasification was performed successfully with various feedstocks. However, a number of limitations were also identified, most notably sudden and large gas flow fluctuations during otherwise stable operating conditions and problems with pumping solid particle containing slurries. Gas flow fluctuations also caused the gas product composition to vary inconsistently and a stoichiometric model was developed to pinpoint the actual gas composition for one set of conditions and thereby overcome this limitation. An extension to this stoichiometric model can be used for scaling predictions and economic evaluation for larger CSP plants depending on their location. Causes for these and other minor limitations were investigated and solutions suggested. Also, a novel receiver design suitable for supercritical water gasification and a range of other uses was proposed.






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