PhD Theses

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Browsing PhD Theses by Subject "Biomass gasification"

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    Modeling thermodynamics of high temperature gasification process of biomass
    (University of Dar es Salaam, 2013) Kasembe, Ethel Donald
    The motive of this research endeavour was to model biomass thermal-chemical gasification process to identify inefficiencies in the high temperature environment. A downdraft gasifier was the focal research area since it is the least efficient unit in the gasification chain of components with losses of about 55% and require re-dress by use of mathematical modeling. The model evokes the first law of thermodynamics (quantification of energy) and the second laws of thermodynamics (qualification energy processes) to account for exergy. Gasification temperature was varied from 800 K to 1400 K at equivalence ratios (ER) from 0.3 to 0.4. The model results of syngas heating values, syngas composition, cold and second law thermodynamic efficiencies for five biomass materials; palm stem, rice husks, sugar bagasse, jatropha cake and jatropha husks in a downdraft gasifier are reported. Predictions from the current model show a reasonable degree of accuracy with those from gasification experiments and other models. The maximum cold efficiency values depended on the biomass materials and occurred at a temperature of 1400 K and ER of 0.3, and efficiencies varied from 64.6% to 81.6%. The corresponding maximum second law thermodynamic efficiency based on chemical and physical exergy at a temperature 1400K and ER of 0.4 varied from 70.9% to 79.9% while the efficiency which depends on chemical exergy ranged from 76.5% to 86.6%. In all these cases the degree of irreversibility (exergy losses) was less than 30%. Use of high temperature gasification process is recommended due to the enhancement of gasification efficiency when compared with low temperature gasification process. In this way, biomass resources can be used in a sustainable way in energy generation in place of fossil fuel resources, thus mitigating environmental degradation.