Browsing by Author "Ridout, Angelo Mark Christopher Juan Johan"

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    Valorisation of paper waste sludge using pyrolysis processing
    (Stellenbosch : Stellenbosch University, 2016-03) Ridout, Angelo Mark Christopher Juan Johan; Gorgens, Johann F.; Carrier, Marion; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Due to depleting fossil fuel reserves and environmental concerns over global warming, alternate sources such as renewable energy are required. One such renewable energy source is biomass which includes plant matter, agricultural residues and industrial wastes. Of interest in this study is the industrial waste paper waste sludge (PWS) which is generated in large quantities by the pulp and paper industry. PWS is mainly landfilled which is costly and environmentally unfriendly, and thus alternative methods of valorisation such as thermochemical and/or biochemical conversion needs to be considered. The thermochemical process of pyrolysis thermally decomposes biomass, in the absence of oxygen, into products of bio-oil, char and non-condensable gas which have various beneficial applications. Alternatively, biochemical conversion of PWS into bioethanol using fermentation can be used as an initial step, followed by pyrolytic conversion of its fermentation residues (FR). The global objective of this PhD project was to assess the full potential of alternative pyrolysis processes, at varying key operating conditions, as part of a biorefinery to maximise the conversion of PWS and its FR, containing variable amount of organic material, into energy, chemical and biomaterial resources. In addition, statistical analysis of the product yields and quality were performed to reveal new mechanistic insights. The first part of the study considered the maximisation of the bio-oil yield from low and high ash PWS (8.5 and 46.7 wt.%) using fast pyrolysis (FP) processing. To do this, both reactor temperature and pellet size were optimised using a 2-way linear and quadratic model. Maximum bio-oil yields of 44.5 and 50.0 daf, wt.% were obtained at an intermediate pellet size of ~5 mm and optimum reactor temperatures of 400 and 340 oC for the low and high ash PWS, respectively. In addition to the above, a thermogravimetric study was implemented to gain insights in the thermodynamic mechanisms behind the increase in bio-oil yield with larger pellet sizes. Results indicated that fewer secondary tar cracking reactions were prevalent due to lower mass transfer limitations leading to greater yields of bio-oil. Vacuum, slow and fast pyrolysis processes were assessed and compared, at varying reactor temperatures and pellet sizes, for their ability to maximize the gross energy conversion (EC) from the raw PWS to the liquid and solid products. A 2-way linear and quadratic model was used for the statistical approach. Comparison of the overall EC, as a combination of the solid and liquid products, revealed that FP was between 18.5 and 20.1 % higher for low ash PWS (LAPWS), and 18.4 to 36.5 % higher for high ash PWS (HAPWS) when compared to slow and vacuum pyrolysis. This finding was mainly attributed to the higher production of organic condensable compounds during FP for both PWS. The calorific values displayed by the vacuum pyrolysis (VP) tarry phase and FP bio-oil for both PWSs, as well as the LAPWS char, were high (~18 to 23 MJ.kg-1) highlighting their potential for industrial energy applications. The capability of vacuum, slow and fast pyrolysis to selectively drive the conversion of raw PWS into chemicals (primarily glycolaldehyde and levoglucosan) and biomaterials (sorption medium or biochar) was assessed. Product yields were optimised according to reactor temperature and pellet size (2-way linear and quadratic model) and their variability quantified using principal component analysis (PCA). Results indicated that the high heating applied by FP significantly promoted depolymerisation and/or fragmentation reactions leading to higher yields of most organic compounds, particularly levoglucosan for both LAPWS (1.5 daf, wt.%) and HAPWS (3.7 daf, wt.%). The char biomaterial displayed by both PWSs were ultra-microporous, and the application of VP significantly enhanced the sorptive properties of the LAPWS char. Sequential PWS fermentation for bioethanol production (separate study), followed by pyrolytic conversion of the FR using alternative processes at varying reactor temperatures, was performed to maximise the recovery of energy, of which the performance was compared to stand-alone pyrolysis. The recovery of energy was maximised by coupling PWS fermentation and FR fast pyrolysis, resulting in gross ECs of between ~75 and 88% for the LAPWS, and ~41 and 48 % for the HAPWS. These gross ECs were up to ~10 % higher in comparison to those attained for stand-alone pyrolysis of PWS. The greater availability of lignin in FR, after fermentation, led to bio-oil products that were phenols-rich. In summary, the present study pointed out the promising potential of pyrolysis processing of PWS/FR as part of a biorefinery for production of fuels, chemicals and biomaterials resources. FP maximised the organic liquid and levoglucosan yields as well as the gross EC from PWS. Sequential fermentation of PWS coupled with FP of FR maximised the gross ECs, which were higher in comparison to stand-alone PWS pyrolysis ECs. To confirm which process option is best in terms of overall energy efficiency and economics additional modelling and economic feasibility studies are recommended.