Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by browse.metadata.advisor "Collard, Francois-Xavier"

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    Catalytic pyrolysis conversion of lignin from different sources to phenols
    (Stellenbosch : Stellenbosch University, 2019-04) Naron, David Rangnaan; Gorgens, Johann F.; Tyhoda, Luvuyo; Collard, Francois-Xavier
    ENGLISH ABSTRACT: Lignin is a by-product of the paper and pulp industry and the emerging cellulosic ethanol production technologies. Both industries only considered lignin a source of energy to complement the energy needs of their processes. However, the phenolic nature of lignin makes it a valuable renewable resource for sustainable production of chemical products. In the current study, prior to lignin conversion to phenolic chemical products, physico-chemical characterisation of several lignins were conducted using conventional methods namely, wet chemical, gel permeation chromatography (GPC), and Fourier transform infra-red spectroscopy (FTIR). In addition to these methods, a novel analytical pyrolysis method was developed combining thermogravimetric analysis (TGA), thermal desorption (TD), and gas chromatography coupled to mass spectrometry (GC-MS), named as TGA-TD-GC-MS. It was used to analyse and estimate the monomeric phenolic products namely, syringol (S), guaiacol (G) and phenol (H) from lignins. The phenolic monomeric proportions (S/G/H), obtained using the TGA-TD-GC-MS was compared with the ones obtained by thioacidolysis (wet chemical method). The lignin monomeric products obtained by pyrolysis, based on internal calibration, was in the range of 5.5-12.9 wt.%. The ability of the TGA-TD-GC-MS to break several types of bonds gave it the advantage over thioacidolysis, resulting in the production of monomeric phenolic compositions that were more representative of the lignin. A comparison of phenols production from catalytic pyrolysis of lignins of different biomass origin, namely eucalyptus (hardwood) lignin, pine (softwood) lignin, and sugarcane bagasse (herbaceous) lignin was studied using the TGA-TD-GC-MS. The lignins were impregnated with two hydroxides (NaOH and KOH) and two metal oxides (ZnO, and Al2O3), with amounts equivalent to 1 wt.% of the lignin mass, and pyrolysed at the temperature of 600 °C using a heating rate of 10 °C/min. KOH produced the most catalytic effect on the yield of total phenols from sugarcane bagasse (S-S) lignin, leading to the highest increase of +26%, and likewise NaOH for eucalyptus (E-K) lignin (+40%). Syringol yield being the major syringol-type (S-type) phenols reached a record high of 1.8 wt.%, equivalent to 90% increase from E-K lignin, catalysed by NaOH. Additionally, NaOH increased the yield of 4-vinylguaicol-the guaiacol-type (G-type) phenol from E-K lignin up to 2.8 wt.%, equivalent to 39% increase, as compared to the non-catalytic yield. A catalyst screening study was conducted in which twelve catalysts, namely Al2O3, Fe2O3, MoO3, TiO2, Ni/Al2O3-SiO2, CaO, ZnO, MgO, NaOH, CuO, KOH and NiO were each impregnated on three different types of sugarcane bagasse lignins with amounts equivalent to 1 wt.% of the lignin mass. KOH, CaO, and Fe2O3 recorded the highest effects on the total yield of phenols from soda (SD), soda-anthraquinone (SAQ), and steam explosion combined with enzymatic hydrolysis lignins respectively. The increases were 11.2 wt.%, 8.2 wt.%, and 8.6 wt.%, equivalent to + 26%, + 60% and + 43% respectively. Syringol, guaiacol, and 4-vinylguaiacol were the most improved with yield increases ranging from 0.6 to 2.8 wt.%, equivalent to 32-121% from the lignins. Optimisation of phenols was investigated. Pyrolysis was first conducted at analytical scale and then applied in the second stage at bench scale. The results at analytical scale showed that the amounts of KOH required to maximise the yield of phenols (15.3-16.0 wt.%) were in the range of 5-7 wt.%. Analysis of the bio-oil showed that the yields of syringol and guaiacol had their maximum values at 450 °C and 4.5 wt.% KOH content with yield increases of 0.70 wt.% and 0.6 wt.%, which represent 106% and 83% respectively, compared with that of pyrolysis without catalyst. Phenol (the P-type phenol) achieved a maximum at 450 °C and 8.5 wt.% KOH content, with a yield of 0.96 wt.%, corresponding to 141% increase.
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    Optimisation of thermal treatment of invasive alien plants (IAPs) for char production for use in combustion applications
    (Stellenbosch : Stellenbosch University, 2018-03) Mundike, Jhonnah; Gorgens, Johann F.; Collard, Francois-Xavier; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Due to the popular worldwide demand for need to use cleaner fuels, lignocellulosic-derived char is gaining importance as a possible component in co-firing with coal. In order to avoid deforestation of indigenous forests in Zambia for char production, possibilities of using alternative feedstocks from invasive alien plants (IAPs) were investigated. In the present study, torrefaction and slow pyrolysis were used for char production from IAPs for energy applications. Both processes were optimised individually at milligram-scale in a thermogravimetric analyser (TGA) for char yield and higher heating value (HHV), through manipulation of the temperature, heating rate and holding time. Two IAPs, namely Lantana camara (LC) and Mimosa pigra (MP), from Zambia were used as feedstock materials. The feedstock particle size distribution (PSD) used was from 425 to 600 μm. The optimisation results for torrefaction and slow pyrolysis showed that temperature majorly influenced char yield and HHV. In case of torrefaction, operating at temperatures ≤ 300 ˚C, heating rate and hold time also influenced char HHV, while neither parameters had a statistically-significant influence on char yield and HHV during slow pyrolysis. During torrefaction at 300 ˚C, LC recorded a higher char yield of 43 wt.%, and a corresponding HHV of 27.0 MJ kg-1, mainly due to increased hemicelluloses content, compared with MP that had a char yield of 52 wt.% with HHV of 24.4 MJ kg-1. In case of slow pyrolysis, MP recorded the highest char HHV of 31.0 MJ kg-1 at 580 ˚C, due to increased lignin, in comparison with LC that had a highest char HHV of 30.0 MJ kg-1 at 525 ˚C. Based on optimised conditions from milligram-scale, LC and MP samples of PSD from 850 to 2800 μm were used for char production at gram-scale in a bench-scale reactor. Scaling-up promoted secondary char formation due to mass and heat transfer limitations in larger particles and increased sample size, thereby increasing char yields for both biomasses. Char yields were increased by 4 and 2 wt.% for MP and LC, respectively, due to scale-up. The highest HHVs at bench-scale were 30.8 MJ kg-1 (614 ˚C) and 31.6 MJ kg-1 (698 ˚C) for LC and MP, respectively. For the purposes of coal substitution and co-firing, a combustion study was conducted in a TGA reactor using LC and MP chars (torrefied and pyrolysed) from gram-scale of PSD from 850 to 2800 μm. LC and MP chars were blended with three South African coals between 5 to 90 wt.% (biomass char). The combustion characteristic results showed that LC chars were more reactive than MP chars, with significantly lower combustibility temperatures than the coals. During co-combustion, the combustion indices for blends < 30% were similar to those of the individual coals, showing that partial coal substitution could be done without significant modifications to existing equipment. There was better combustion performance through increased combustion indices for blends > 60%, though probably with a likelihood of modifications to existing reactors that were initially designed for coal combustion, as the conversion was much faster. In summary, this study has shown that LC and MP IAPs could be valorised through torrefaction and slow pyrolysis to produce char for direct energy applications and co-firing with coal. LC samples torrefied at 300 ˚C were found to be equivalent to high volatile bituminous C coal, while pyrolysed chars for LC and MP were equivalent to high volatile bituminous B coal. To confirm the practicality of co-firing possibilities, it is recommended that scale-up studies to pilot-scale be conducted in order to assess overall energy efficiency, techno-economics, operating conditions of industrial reactors and a life cycle assessment.
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    Thermochemical biomass upgrading for co-gasification with coal
    (Stellenbosch : Stellenbosch University, 2018-03) Nsaful, Frank; Gorgens, Johann F.; Collard, Francois-Xavier; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Lignocellulosic biomass is considered as a sustainable and renewable fuel source with the potential to substitute or partially replace coal in applications such as gasification for energy generation due to its sustainable carbon as well as its potential to reduce greenhouse gas emissions. However, raw biomass differs significantly from coal in terms of several important fuel properties, such as low energy density, high moisture, oxygen and volatile matter contents. Due to this the co-utilization of raw biomass with coal in gasification systems has been shown to result in the increase in the production of oxygenated volatile compounds (tar precursors) which impacts negatively on the quality of the gasification products and causes critical operational problems. This challenge has limited the development of biomass-based gasification processes. Hence to ensure the effective and efficient utilization of lignocellulosic biomass with coal, an upgrading process is required to improve some biomass properties to make them more similar to that of coal. The approach of this work consists in a thermal pretreatment in order to generate char products with reduced oxygen and volatile matter contents. The overall aim of the study therefore was to use thermochemical technologies (torrefaction and slow pyrolysis) as methods to pretreat lignocellulosic biomass feedstocks; pine (PN), bamboo (BB), corn cob (CC) and corn stover (CS) to produce upgraded biomass feedstocks (char), with reduced oxygen content as well as improved fuel properties, comparable to coal for use in co-gasification. For this task the study was divided into several objectives. The initial part focused on the characterization of the lignocellulosic chemical composition of the various biomass feedstocks. Next the types and quantities of oxygenated volatile products produced during the devolatilization of raw biomass feedstocks were studied. For this objective a novel analytical method incorporating the use of Thermogravimetric Analysis, thermal desorption and Gas Chromatography–Mass Spectrometry (herein referred as (TGA-TD/GC-MS) was developed and used to analyse and quantify the oxygenated volatile products. The analysis of the volatile composition data by means of principal components analysis (PCA) showed a clear correlation between lignocellulose chemical composition and the type and quantities of oxygenated volatile compounds produced during biomass devolatilization. The influence of thermal pretreatment conditions (temperature and time) on the structural transformation of raw biomass and on the volatile evolution mechanism of the resulting char during subsequent char devolatilization was also studied. Thermal pretreatment was done within the temperature range of 250-400 oC and hold time at pretreatment temperature of 30 and 60 min. It was observed that the temperature had a more profound effect than hold time during thermal pretreatment. The distribution of char devolatilization products was shown to be consistent with the extent of biomass transformation during thermal pretreatment. The biomass composition, particularly cellulose crystallinity, had an impact during thermal pretreatment. It was shown that for biomass feedstock with high degree of crystallinity such as PN a higher temperature (>300 oC) was required to achieve significant cellulose degradation. Hence char produced from such feedstock at temperature ≤300 oC generated high amount of anhydrosugar and furan volatiles during the char devolatilization. With the aim of using pretreated biomass with coal for co-gasification, biomass chars produced at different pretreatment temperatures were compared to coal in terms of fuel properties (proximate and elemental composition and Higher Heating Value), with particular attention given to the composition of oxygenated volatile compounds generated during the devolatilization stage. The result of the study showed that chars produced at temperature ≥350 oC had fuel properties comparable to that of coal. In addition these chars produced mainly aromatic hydrocarbons and phenolics during devolatilization which were similar to the volatiles generated from coal under identical conditions. Hence the pretreatment temperature of at least 350 oC is recommended when considering coal substitution, while 400 °C could be considered in the case of samples with high lignin (softwood) or high inorganic contents. Finally, the reactivity of the biomass chars under gasification condition was investigated. The devolatilization characteristics and CO2 gasification kinetics of biomass/char (produced at 350 oC) and coal at different blend ratios were studied. The devolatilization characteristics of char were found to follow the profile of coal especially at blend ratios of 10 wt% and 20 wt% with no particular synergy detected, while the kinetic parameters were also comparable. This work confirmed the potential of the use of thermally pretreated biomass chars for coal substitution in gasification process and brought decisive insights for the implementation of future tests at pilot scale.