Browsing by Author "Chitawo, Maxon Lexon"

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    Systems approach in developing a model for sustainable production of bioenergy in Malawi
    (Stellenbosch : Stellenbosch University, 2018-03) Chitawo, Maxon Lexon; Chimphango, Annie F. A.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Bioenergy production from primary forest and rice residues can contribute to modern energy supply, such as electricity, liquid biofuels and gas, to rural communities in Malawi. These bioresources can be utilised for bioenergy production without alienating land from cultivation of other crops. However, sustainability of forest and rice residues-based bioenergy systems is complex owing to the dependency of availability of the residues on timber and rice production. Alterations in process operations in timber and rice production systems can cause variations in production and supply of the residues to a bioenergy conversion plant over time. For instance, forest management systems have evolved from sustainable yield management, which promotes clear cutting of mature forest stand to maximise the yield of wood products to sustainable forest management that promotes partial harvesting of mature forest stand to allow for ecosystem balance. Switching the harvesting regimes from clear cut to partial harvesting of mature forest stand, can influence variation in yield of forest residues in forest plantations over time. Variations are also evident in rice residues production and supply chains, emanating from seasonal production of rice and demand of the rice residues for competing uses. Stability in production and supply of the residues over a long time horizon can promote availability of the residues-based bioenergy systems and reliability of bioenergy supply to end use processes over time. Systems approach modelling based on systems thinking and system dynamics modelling methodology, was used in this study to develop a model for sustainable production of bioenergy (SAS-Biopro model). The model demonstrates state limiting processes to resilience of primary forest and rice residues supply chains for bioenergy production. Simulation results of the model show that variations in primary forest residues value chain over time result from variations in stocks of mature stand caused by over-exploitation for timber production, delayed replanting, high death (mortality) rate of replanted trees and underinvestment in plantations management. Results from scenario testing show that an integrated framework for forest plantations management and forest residues-based bioenergy production, can promote synchronised operation and management of the forest plantations and bioenergy production as a unit (whole) system. The framework entails setting an annual allowable cut for harvesting mature forest stand, synchronizing harvesting and replanting 100% of the annual allowable cut immediately after harvesting, reducing tree mortality fraction to less than 0.1, and sizing the scale of operation of bioenergy conversion plants based on the amount of residues generated from the annual allowable cut. The framework can promote stability of residues production and supply to bioenergy conversion plants. Similarly, modelling sustainability of rice residues-based bioenergy production has shown that a synergetic integration of bioenergy and rice production can simultaneously increase bioenergy and rice production over time. Thus, synergetic integration of bioenergy and rice production can promote stability, availability and reliability of rice (food) and rice residues supply for bioenergy production. This research has filled a significant gap in strategic information such as dynamics in residues-based bioresource flow and consumption rates that create a transient state, which can guide formulation of strategies for synchronising the scale of operation of the residues-based bioenergy conversion plants and operation processes in the primary systems that generate the residues. The research outputs provide innovative whole systems and synergetic integration, for production and deployment of residues-based bioenergy, to promote resilience of the residues supply chains to bioenergy conversion plants. These concepts can promote uptake and diffusion of small-scale bioenergy conversion technologies in primary systems that generate the residues. Matching the scale and rate of operation of the bioenergy conversion plants with the annual rate of production of the residues can provide opportunity for incremental uptake of small-scale residues-based bioenergy systems. Therefore, the concepts, although approached from the technology and process point of view, are flexible to respond to policy and societal changes in the value chain of bioenergy production. The concepts can be adopted in forest plantations and rice farms management systems to promote sustainability of bioenergy production from forest and rice residues.