PhD Theses

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Browsing PhD Theses by Subject "Biomass energy"

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    Densification, relaxation, and combustion characteristics of densified Biomass fuels: a comparative study with wood combustion.
    (University of Dar es Salaam, 2001) Rajabu, Hassan M.
    Densification of loose biomass waste such as agricultural, forest and industrial residues to form Densified Biomass Fuels (DBF) is an attractive option in areas where these resources remain unutilized. The potential of DBF to substitute fuelwood or coal in industrial combustion systems depends on how closely its handling and combustion behaviour match the existing facilities. In this work, experiments were conducted to study densification, relaxation, and combustion characteristics of rice straw and sawdust DBF. Experiments on wood combustion were also conducted for comparison to DBF. Densification tests were performed in uniaxial closed-die mode. Die diameters of 2.5 and 3.8 cm were used with die pressure up to 134 MPA. Results showed that an exponential relationship in the form: P=Ae(Bp), seems to best represent the uniaxial densification process for the material tested. Where: P is the die pressure, and p is the density of DBF. The values for coefficients `A' and `B' depend on die size, material type, and storage relative humidity. The relaxation behavior of DBF was studied in low, medium, and high, relative humidity conditions. DBF made of rice straw showed 52-68% increase in length in the pressure range tested, which is 6-18% higher than for sawdust. DBF from the larger diameter die showed 3-9% higher expansion compared to the smaller die. DBF showed higher percentage increase in length when stored in high humidity conditions. The change of moisture content of DBF during storage delays the stabilization period and increases their expansion considerably. Combustion tests were carried out in an isothermal thermogravimetric analyser. Results demonstrate that, during pyrolysis the cylindrical particles of both DBF and wood loose mass in a thin reaction zone. DBF were found to expand during combustion by up to 50% of its original volume. The anisotropic properties in wood have an influence on the direction of volatile gases coming out of the particle during pyrolysis. Model predictions show that drying and pyrolysis overlaps. Model predictions on the advancement of pyrolysis zone in radial and axial directions, and burning rate of the particle are in good agreement with experiment results.
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    Investigation of the effect of the extent ofrefining jatropha and castor oils on the quality of biodiese
    (University of Dar es Salaam, 2013) Kombe, Godlisten Gladstone
    The influence of refining and pre-treating crude jatropha and castor Oil on biodiesel quality has been studied. Crude jatropha and castor oil were extracted and characterized before being refined and pre-treated into the following oil categories: degummed oil, neutralized oil, deodorized oil, full refined oil, acid esterified oil and re-esterified oil. Thereafter, all samples were subjected to homogeneous base catalyst transesterification. A central composite design using response surface methodology in design expert v7 software was employed in modelling and optimizing refining, pre-treatment and biodiesel production processes. The qualities of biodiesel produced from crude, refined, and pre-treated oil were analysed and compared with ASTM D6751, EN 14214 and drafted TBS CDC15 biodiesel standards. The study shows that the chemical characteristics and fuel properties of crude jatropha oil and castor oil present a superior quality and present them as a good feedstock for biodiesel production. The effect of refining and pre-treatment of crude jatropha and castor oil has been discussed in details. Neutralised oil, deodorized oil, full refined oil, acid-pre-treated oil and chemical re-esterified oil have good potential in producing biodiesel which meets most of the ASTM D6751, EN 14214 and TBS CDC15 standards quality requirements. The studies on the chemical kinetics of the newly developed low temperature glycerolysis, economic comparison of different pre-treatment and refining methods were recommended for a better decision making on economic biodiesel production route.
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    Modeling of fast pyrolysis of biomass for bio-oil production
    (University of Dar es Salaam, 2015) Said, Mahir Mohammed
    Biomass processing and use can meet the challenge of reducing fossil resources by producing a liquid feedstock that can lessen the fossil dependence and meet the increased demand via rapidly emerging thermochemical technologies such as pyrolysis. The main objective of this research work was to develop a mathematical model for fast pyrolysis of biomass for liquid fuel (bio-oil) production. Few tropical biomass types have been studied for pyrolysis and nothing has been previously studied on the effect of mixing of different varieties of biomass for production of bio-oil. This research studied the pyrolysis of six tropical biomass materials of eucalyptus, pine, mangrove, cashew-nut shells, coconut husk and rice husk. The chemical and physical properties were studied to enable to develop the model that predicts the yields of bio-oil. Good agreement of the developed model and existing models were observed for all biomass except rice husk and coconut husk whose deviations were above 10%. The validation of the model with experimental results gave a variation of less than 10% for all biomass materials. Cashew nut shells and eucalyptus produced about 0.7 wt/wt yields of bio-oil. Mangrove and pine produced about 0.6 wt/wt yields and the least bio-oil yield was observed from coconut husk and rice husk which was about 0.5 wt/wt yields. The maximum yield of mixed biomass was about 0.5 wt/wt, and the gas produced was 0.95 wt/wt. It is concluded that the model works in both forest and agricultural biomass. The model is also flexible enough to predict the yield even if the biomass is in the mixed form.
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    A study on the process of modifying plant oils for rural liquid biofuel in Tanzania
    (University of Dar es Salaam, 2015) Mlay, Happiness
    A study on the process of modifying plant oil (PO) to produce a liquid biofuel, that has similar specifications of diesel fuel grade 4-D for use in low- and medium speed diesel engines was carried out. These engines are commonly used in rural areas for many socio-economic activities. PO’s chemical composition is the major factor that affects diesel engine performance and was the basis for modification. PO samples under study were: Jatropha curcas L. (JO), Croton Megalocarpus (CROT), Ricinus communis (CST) and Excoecaria bussei (EIX) seed oils. PO Modification was carried out in the laboratory and involved degumming, neutralising and blending as major process steps. The reduction efficiencies in viscosity, free fatty acids (FFAs) and phospholipids (gums) were used as performance indicators. Main materials used for blending were fusel oil (FO) and natural gas condensate (NGC). Degumming and neutralisation had no impact on viscosity but they were effective (98%) in gums and FFAs removal respectively. The effect of blending was significant with 50-80% viscosity reduction. The optimum parameters for formulating blends were found to depend on the modifier characteristics and on the fatty acid (FA) composition of PO. The overall results indicated that degummed-neutralised-blended PO’s properties meet the specifications for diesel fuel grade 4-D. basing on laboratory results and other sources, techno-economic analysis of the process was carried out with an aid of SuperPro Designer software. A small-scale modified plant oil (MPO) production plant with an annual capacity of 15 t of MPO (batch process) was designed and simulated and found to be economically viable. It was recommended that engine performance tests to be done in future in order to analyse the short- and long-term effects of MPO on engine.