PRESSURISED ENTRAINED FLOW GASIFICATION OF SUGAR CANE WASTES FOR COGENERATION.

Joyce, James Alexander (2006). PRESSURISED ENTRAINED FLOW GASIFICATION OF SUGAR CANE WASTES FOR COGENERATION. PhD Thesis, School of Engineering, University of Queensland.

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Author(s) Joyce, James Alexander
Thesis Title PRESSURISED ENTRAINED FLOW GASIFICATION OF SUGAR CANE WASTES FOR COGENERATION.
School, Centre or Institute School of Engineering
Institution University of Queensland
Publication date 2006
Thesis type PhD Thesis
Supervisor(s) Dr. Terry Dixon, Dr. Joe da Costa and Prof. John Pohl
Abstract/Summary 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.
Keyword(s) Gasification
Bagasse
Cane Trash
Combined Cycle
Entrained Flow
Pyrolysis
 
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