Browsing by Author "Chireshe, Farai"

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    Production of an upgraded bio-oil by catalytic pyrolysis of forest residues
    (Stellenbosch : Stellenbosch University, 2019-04) Chireshe, Farai; Gorgens, Johann F.; Collard, Francois-Xavier; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Forest plantations generate solid residues which are usually disposed of by combustion. Sometimes these residues are simply left in the forest, where they contribute little to soil fertility, and yet pose a significant fire hazard. Unlike combustion, which produces heat and is limited to onsite use only, pyrolysis can be used to convert the forestry residues to produce a bio-oil that is easier to transport and use offsite. However, bio-oil has limitations in its use as a liquid fuel because its high oxygen content (about 40 wt. %), gives it undesirable qualities such as high acidity, oxidative instability and low energy content. The aim of the project was to catalytically upgrade the bio-oil by lowering its oxygen content so that it can be co-processed in a conventional crude oil refinery to produce transportation fuels. Eucalyptus grandis was chosen as the feedstock based on it being the most abundant species used in the regions with the most forestry residue in South Africa. From literature, 3 catalysts: CaO, MgO and Al2O3 were selected based on their ability to promote deoxygenation. Screening tests were carried out in a batch reactor, under intermediate pyrolysis conditions at 550 °C and 30 wt.% catalyst concentration. The improvement in the bio-oil HHV in the catalyst screening was similar for all the catalysts; it increased from 21.8 MJ/kg without catalyst to between 26.3 – 26.8 MJ/kg for all 3 catalysts. Optimisation experimental designs (CCD) were then carried out for each of CaO and MgO to maximise bio-oil quality in terms of HHV at an acceptable yield. Temperature was varied from 444 – 656 °C and the catalyst concentration from 1.7 – 58.3 wt.%. For MgO deoxygenation occurred mainly via decarboxylation reactions and the maximum HHV was at 26.9 MJ/kg at 560.0 °C and 33.8 wt.% catalyst concentration, at a yield of 19.4 wt.%. For CaO, dehydration reactions were dominant and the maximum HHV was 27.5 MJ/kg at 490.0 °C and 59.0 wt.% CaO concentration. The bio-oil yield was 13.4 wt.% which was low to achieve the target bio-oil blending ratio. A statistical desirability function was then used, and the desirable optimum conditions were found to be those of the catalyst screening. The best performing catalyst was found to be CaO based on energy conversion assessment and the better applicability of char derived from CaO catalytic pyrolysis in soil amendment. A pilot reactor with a 1 kg/hr capacity was then used to scale up the process from bench scale. Better contact between the solid catalyst and organic volatiles in the pilot reactor meant that the optimum reaction at bench (550 °C) had to be reduced to 500 °C to limit the effect of severe catalytic cracking. The resultant bio-oil had an oxygen content of 12.6 wt.%, a water content of 19.7 wt.% at a yield of 15.6 wt.%, which meet the specifications required for successful co-processing in a crude oil refinery at a 10 wt.% blending ratio. However, it is recommended that the bio-oil be tested for co-processing in a Fluid Catalytic Cracking unit (FCC).