Acacia Tortilis Encroacher Bush as a Bioenergy Source

Abstract

Invasive and encroaching (IE) species have posed a threat to biodiversity in ecosystems, rangeland productivity, groundwater generation, the environment and economy in savannah ecosystems like Botswana and Namibia. While de-bushing of large diameter stocks have yielded huge economic returns for countries like Namibia through a lucrative charcoal and fuelwood or chips, smaller diameter ones have little economic value, especially the shrubs that encroach space in cities and towns. The goal of this study is to explore possible means of deriving economic value from the encroachers, which are currently disadvantaging Botswana through reduced rangeland productivity and de-bushing costs. Pyrolysis has received attention as a potentially low cost thermochemical method that can efficiently utilize such lignocellulosic residues. The primary goal is to use the bio-oil in stationary engine applications for power generation, especially in remote areas of sparsely populated Botswana, where it has been deemed uneconomic to connect them to the national grid. Characterization of encroacher bush indicated it could be a good feedstock for pyrolytic conversion due to the relatively high volatile matter (76.51 %), moderately low ash content (3.90%) and high gross calorific value (17.3 MJ/kg). The A. tortilis was then pyrolyzed under varying overall and condenser temperatures to establish the optimum operating parameters. The obtained bio-oil was characterized, comparing its properties to conventional fuels. The optimum pyrolysis temperature was found at 550°C, while the optimum primary condenser temperature, with the best quality oil (36.809MJ/kg), was at 125°C. The viscosity of this oil was estimated to be 24.024 mPa.s using the other characterized samples. Gas chromatography was also carried out on the oil samples and the compounds with the highest mass presence were catalogued. It is concluded that the bio-oil could be used in moderate-slow engines with moderate upgrading, while it needs to be diluted with a solvent then blended with a diesel for use in fast diesel engines. It is also important to establish the particulate composition and flash point of the bio-oil, though these tests were not carried out in this study.