| Popis: |
Water hyacinth (Eichhornia crassipes, (Mart) Solms), a floating aquatic plant, has long been recognised as a potential commercial resource but, despite many attempts, its conversion from a nuisance into an asset has not been achieved on a significant scale. The thesis is an analysis and assessment of the options for overcoming the many difficulties encountered in the use of water hyacinth. Following a literature survey, from which a process flow path for optimum use of water hyacinth is devised, the thesis leads to an evaluation of the key components of the proposed system for use of water hyacinth as a large-scale energy resource. The principle component of a system to produce energy from water hyacinth is the anaerobic digester. Trials with high-rate anaerobic digesters were conducted in Bangladesh and Thailand. In Bangladesh, with the assistance of senior personnel from the Department of Chemistry of Dhaka University, an 8.3 cubic metre, multi-stage, upflow anaerobic digester was built within the grounds of the Housing and Building Research Institute in Dhaka. Trials with this unit, and associated laboratory work, demonstrated and quantified both the need and the scope for pre-treatment of raw water hyacinth prior to anaerobic digestion. Initial experimentation in Bangladesh laid down the foundations for an understanding of water hyacinth and led to the experimental programme performed in Thailand. In Thailand, following an extensive search and selection of suitable juicing apparatus, a series of batch reactors were run with juice made from separate parts of the whole plant. These results were compared with each other and with a reactor running on juice made from whole plant. The conclusion drawn from this experimentation was that, when mechanically pre-treated, the root section of the plant will contribute more to gas production than will the stem portion. In many previous trials the root has been discarded because of its resistance to anaerobic digestion in a raw form. A multi-stage upflow anaerobic digester was conceived with inclined weir plates, intended to resist blocking of the flow paths by insoluble solids in water hyacinth juice. A series of four of these units were built on a laboratory scale and trials carried out over a period of one month. These trials demonstrated that the proposal to juice water hyacinth prior to low-solids, high-rate anaerobic digestion is one that is technically feasible. The final sections of the thesis use an economic model of the proposed system to conclude that small-scale (3 m3 biogas per day) and medium- scale (1,000 m3 biogas per day) utilisation of water hyacinth will be difficult to achieve in a commercial setting. Large-scale (above 100,000 m3 biogas per day) utilisation of water hyacinth, however, is concluded to be of significant commercial potential. |
