Theses and dissertations (Engineering and Built Environment)
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Item Techno-economic analysis and life cycle assessment for production of biofuels from spent coffee grounds(2024-05) Kisiga, Wilberforce; Chetty, Manimagalay; Rathilal, SudeshSpent Coffee Grounds (SCGs) are one of the most abundant agro-industrial residues generated from the coffee brewing industry and coffee espresso machines in restaurants, cafeterias, cafes and homes. It is believed that for every ton of coffee beans processed, 650 kg of SCG is left as solid residues. Coffee being the second traded commodity after petroleum, means that a lot of SCGs are generated annually and end up into landfills. Efforts are being made to turn this valuable waste into biofuels, however, most of these efforts end up at laboratory benches and few studies have focused on industrial scale production of biofuels from SCG. Six biomass-to-energy conversion technologies were compared from technical, economic and environmental perspectives: Fast pyrolysis, Hydrothermal Liquefaction (HTL), gasification, Anaerobic Digestion (AD), fermentation and biodiesel production. The processing technologies were selected because they are the most researched biomass-to-fuel conversion routes. Each of the processing routes was simulated in Aspen plus V11 using input data from literature. The mass and energy balances obtained from simulations were used to conduct Techno-Economic Analyses (TEAs) and Life Cycle Assessments (LCAs). TEA was conducted with help of Aspen Process Economic Analyzer (APEA) and Microsoft Excel spreadsheets whereas OpenLCA V1.11.0 software was employed for LCA. After the processing routes were successfully simulated, APEA was used to estimate the installed Cost of all Equipment (COE). The Capital Expenditure (CAPEX) required to build the biorefineries was then estimated basing on COE for each biorefinery. Then the Operating Expenses (OPEX) required for running the day-to-day operations of the plant were estimated as the sum of Variable Operating Expenses (VOC) and Fixed Operating Expenses (FOC). The revenues from the sales of finished products were estimated and used to calculate the gross profit. For the plant life of 25 years; using straight-line depreciation of 10% per year, discount rate of 12% and tax rate of 28%, the Discounted Cash Flow Analysis (DCFA) was used to calculate the economic indicators i.e. the Net Present Value (NPV), Profitability Index (PI), Internal Rate of Return (IRR) and Discounted Payback Period (DPBP). For LCA, the methodology outlined by the ISO 14040/44 framework was used. The method outlines four steps followed to conduct LCA i.e. goal and cope definition, Life Cycle Inventory (LCI), Life Cycle Impact Assessment (LCIA) and interpretation of results. The goal of this study was to identify the processing route with least environmental impacts and the cradle-to-gate system boundary was selected. LCI was conducted using the mass and energy balances obtained from Aspen plus simulation and the flows present in the Agribalyse Version 3 database, downloaded from OpenLCA nexus. LCIA was conducted using the ReCiPe 2016 Midpoint (H) and was also downloaded from OpenLCA nexus. Eight impact categories namely, global warming, fossil resource scarcity, particulate matter formation, terrestrial acidification, freshwater eutrophication, marine eutrophication, mineral resource scarcity and water consumption were selected. The results were analysed to identify the conversion route with less environmental effects. Results from the economic analysis showed that fast pyrolysis was the most economically profitable processing route with a NPV, PI, DPBP and IRR of 6.3 million USD, 1.85, 5.4 years and 37%, respectively. In the second position was biogas production with a NPV, PI, DPBP and IRR of 3.4 million USD, 1.65, 5.7 years and 34%, respectively. Gasification was in the third position with a NPV, PI, DPBP and IRR of 5.4 million USD, 1.48, 6.0 years and 32%, respectively. In the fourth position was biodiesel production with a NPV, PI, DPBP and IRR of 3.9 million USD, 0.86, 8.0 years and 24%, respectively. HTL was in the fifth position with a NPV, PI, DPBP and IRR of 0.68 million USD, 0.29, 13.0 years and 16%, respectively. Bioethanol production was not economically profitable as the revenues generated from sales of finished products were smaller than the operating expenses, thus no profit could be generated. Results from environmental impact assessment showed that fast pyrolysis was the most environmentally friendly processing route, followed by biogas production, biodiesel production, gasification, and bioethanol production, whereas HTL had the highest environmental impacts. Electricity consumption was the biggest contributor to the environmental impacts, making HTL, which was the highest electricity consuming processing route, to be the worst environmentally. However, biogas production was the least electricity consuming processing route but not the best environmentally due to large production of carbon dioxide and methane (biogas) from anaerobic digestion. The large production of carbon dioxide can be mitigated through using it to grow algae or in supercritical carbon dioxide extraction of lipids. However, the cost associated with additional unit processes can escalate the biogas production costs. These greenhouse gases were the biggest contributors of global warming, pushing biogas production to the second position after pyrolysis.Fast pyrolysis was proposed to be the best environmentally and economically feasible processing route for the production of biofuels from SCG.Item Thermal conversion of algal biomass and its derivatives to fuels and petrochemicals(2021-04) Mustapha, Sherif Ishola; Isa, Yusuf Makarfi; Bux, FaizalThermal conversion processes have gained increased attention since they can be applied to whole microalgae (not lipids alone) resulting in higher biofuel yield with potential for production of other high-value products. The major challenges of microalgal thermal conversion are the high level of nitrogen and oxygen content present in the product stream, as well as high acidity which makes the bio-oil unstable and unfit for use as transportation fuels directly. Transportation fuels are expected to be low in oxygen and acid content for stability and also have low nitrogen content to meet environmental emission standards for combustion. Nutrient stress as a tool for enhancement of yields and quality of bio-oils produced from thermal conversion of microalgae has not received sufficient attention. This study investigated the conversion of Scenedesmus obliquus microalgae via three different thermal conversion processes which include pyrolysis, hydrothermal liquefaction and hydrothermal gasification. Scenedesmus obliquus microalgae were grown under nutrient stressed and unstressed conditions. To better understand the effect of nutrient stressing on the process, pyrolysis experiments were conducted on unstressed S. obliquus microalgae biomass (N3), nutrient- stressed S. obliquus microalgae biomass (N1) and its residual algae biomass after lipid extraction (R-N1) at different temperatures (400 °C to 700 °C) and the results compared. Detailed biomass characterization which includes proximate analysis, ultimate analysis, biochemical analysis, Fourier-transform infrared spectroscopy (FTIR) analysis, and thermogravimetric analysis (TGA/DSC) were carried out on the microalgae biomass (N1, R-N1 and N3) to provide useful information about the combustion behaviour of the biomass during pyrolysis. The biomass characterization results indicated that nutrient-stressed condition altered the microalgae biomass composition and empirical formula for N1, R- N1, and N3 microalgae biomass were CH2.00N0.07O0.71, CH2.36N0.08O0.75, and CH2.35N0.14O0.71, respectively. The maximum bio-oil yield for N1 (46.37 wt%) and R-N1 (34.85 wt%) were obtained at 500 °C, while the highest yield of bio-oil for N3 (41.94 wt%) was obtained at 600 °C. Also, the proportion of nitrogen compounds in N3 bio-oil (47.4 %) was significantly higher than that obtained in the nutrient stressed microalgae biomass (N1) bio-oil (5.92%) at pyrolysis temperature of 500 °C. Thus, nutrient stressed approach is considered more promising to produce a higher yield and good-quality pyrolytic bio-oil from microalgae biomass. A predictive model was developed based on artificial neural network (ANN) and can serve as a framework for the prediction of bio-oil yield from the pyrolysis of microalgae biomass. Finding better heterogeneous catalysts that can enhance the quality of microalgal bio-oils to meet transportation fuels standards is seen as a major advance toward developing efficient and sustainable thermal conversion processes. In this study, pyrolysis of nutrient- stressed Scenedesmus obliquus microalgae over various supported metal M/Fe3O4-HZSM- 5 catalysts (M = Zr, W, Co and Mo) was investigated. The synthesized catalysts were characterized by X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), high-resolution scanning electron microscopy and energy dispersive spectroscopy (HRSEM/EDS). The catalyst: biomass ratio and temperature influence on pyrolysis product yield was also investigated. Between these, Co/Fe3O4-HZSM-5 catalyst showed better activity in enhancing the bio-oil quality and yield; it had the lowest nitrogen content (4.77 wt%) and highest bio-oil yield (17.73 wt %) as well as highest HHV (40.78 MJ/kg) which is almost similar to that of crude petroleum. The results showed that all the supported metal catalysts during pyrolysis promote aromatization and acid ketonization of bio-oils. The total amounts of acids present in pyrolytic bio-oil significantly decreased from 26.68% (non-catalytic) to between 0.58 – 9.68% (catalytic). Also, production of 2-pentanone was observed to increase from ~10% (non-catalytic) to 27.36 – 53.90% (catalytic). In terms of energy recovery, Co/Fe3O4-HZSM-5 had about 40% energy recovery, which was the highest while the least performing catalyst was W/Fe3O4-HZSM-5 with 24.18% energy recovery in bio-oil. Overall, Co/Fe3O4-HZSM-5 was the most effective catalyst in enhancing the quality of pyrolytic bio-oil produced from nutrient stressed Scenedesmus obliquus microalgae with properties close to that of petroleum crude. Hydrothermal liquefaction (HTL) of nutrient-stressed microalgae (Scenedesmus obliquus) (N1) with and without the use of Zr/HZSM-5 catalyst was investigated under temperature conditions ranging from 250 – 350 °C. The Zr/HZSM-5 catalyst was synthesized using wet impregnation technique and characterization was conducted on the synthesized catalyst for its crystalline nature, morphology and thermal stability using X- ray diffractometer (XRD), High-resolution scanning electron microscopy (HRSEM) and thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). The HTL experiments were also conducted on the unstressed microalgae (N3) for comparison. Under the stressed condition, the protein content of the microalgae was reduced from 42.35% to 22.08% while the carbohydrate and lipid contents were increased from 25.36% to 42.55% and 17.16% to 21.62% respectively. The maximum HTL bio-oil yield of 52.80 wt% and 24.27 wt% were found for N1 and N3 respectively at 350 °C with addition of Zr/HZSM-5 catalyst. Higher denitrogenation and deoxygenation was achieved with N1 compared to N3. At high temperature of 350 °C, the most abundant fatty acid in N1 was found to be cis- vaccenic acid (omega-7- fatty acid), and this could be explored for possibility of extracting products of great value from the bio-oil for applications other than biofuels. Mainly, the use of Zr/HZSM-5 catalyst on nutrient-stressed S. obliquus microalgae resulted in enhanced bio-oil yield and characteristics which compared well with petroleum crude. The potential of using whole algae, lipid and residual algae of S. obliquus microalgae as feedstocks for production of high-quality hydrogen and methane-rich gas via hydrothermal gasification technique was also examined. The effect of operating parameters such as temperature, pressure and biomass concentration on the yield and composition of gaseous products using whole algae, lipid, and lipid extracted algae (LEA) as feedstocks was examined. The results showed that reaction pressure had minimal impact while temperature, biomass concentration and feedstock composition had significant effects on the composition of gaseous products. It was also found that low temperature (400 oC) and biomass concentration of 40 wt% favoured the production of methane-rich gas. In contrast, high temperature (700 oC) and low biomass concentration (10 wt%) favoured hydrogen- rich gas production in all the three feedstock considered. The highest mole fraction achieved for CH4 was 53.45 mole%, 61.70 mole% and 52.20 mole% which corresponded to CH4 yield of 31.14 mmol/g, 56.90 mmol/g and 30.15 mmol/g for whole algae, lipid and LEA respectively. For H2 rich gas production, the highest mole fraction achieved were 55.77 mole%, 52.29 mole% and 55.34 mole% which corresponded to H2 yield of 75.44 mmol/g, 105.51 mmol/g and 73.49 mmol/g for whole algae, lipid and LEA respectively. The ranking order for the yield and lower heating value (LHV) of the product gas from the HTG process was lipid > whole algae > LEA. This study has shown that hydrogen-rich and methane-rich gas can be produced from the hydrothermal gasification of microalgae as a function of the reaction conditions and feedstock composition. Also, the suitability of nutrient stressed approach and use of catalysts to enhance the quality of bio-oil produced from thermal conversion of microalgae biomass was established.Item Anaerobic co-digestion of agricultural biomass with industrial wastewater for biogas production(2021-03-26) Armah, Edward Kwaku; Chetty, Maggie; Deenadayalu, NirmalaWith the increasing demand for clean and affordable energy which is environmentally friendly, the use of renewable energy sources is a way for future energy generation. South Africa, like most countries in the world are over-dependent on the use of fossil fuels, prompting most current researchers to seek an affordable and reliable source of energy which is also,a focal point of the United Nations Sustainable Development Goal 7. In past decades, the process of anaerobic digestion (AD) also referred to as monodigestion, has proven to be efficient with positive environmental benefits for biogas production for the purpose of generating electricity, combined heat and power. However, due to regional shortages, process instability and lower biogas yield, the concept of anaerobic co-digestion (AcoD) emerged to account for these drawbacks. Given the considerable impact that industrial wastewater (WW) could provide nutrients in anaerobic biodigesters, the results of this study could apprise decisionmakers and the government to further implement biogas installations as an alternative energy source. The study aims at optimising the biogas production through AcoD of the agricultural biomasses: sugarcane bagasse (SCB) and corn silage (CS) with industrial WW sourced from Durban, KwaZulu-Natal, South Africa. The study commenced with the characterisation of the biomasses under this study with proximate and ultimate analysis using the Fourier transform infrared spectroscopy (FTIR), the thermo gravimetric analysis (TGA), the scanning electron microscopy (SEM) and the differential scanning calorimetry (DSC). The untreated biomass was subjected to biochemical methane potential (BMP) tests to optimise and predict the biogas potential for the selected biomass. A preliminary run was carried out with the agricultural biomass to determine which of the WW streams would yield the most biogas. Among the four WW streams sourced at this stage, two WW streams; sugar WW (SWW) and dairy WW (DWW) produced the highest volume of biogas in the increasing order; SWW ˃ DWW ˃ brewery WW > municipal WW. Therefore, both SWW and DWW were selected for further process optimisation with each biomass. Using the response surface methodology (RSM), the factors considered were temperature (25-55 °C) and organic loading rate (0.5-1.5 gVS/100mL); and the response was the biogas yield (m3 /kgVS). Maximum biogas yield and methane (CH4) content were found to be 5.0 m3 /kgVS and 79%, respectively, for the AcoD of CS with SWW. This established the association that existed among the set temperatures of the digestion process and the corresponding organic loading rate (OLR) of the AcoD process operating in batch mode. Both CS and SCB have been classified as lignocellulosic and thus, ionic liquid (IL) pretreatment was adapted in this study to ascertain their potential on the biogas yield. Results showed that the maximum biogas yield and CH4 content were found to be 3.9 m3 /kgVS and 87%, respectively, after IL pretreatment using 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) for CS with DWW at 55°C and 1.0 gVS/100mL. The IL pretreatment yielded lower biogas but of higher purity of CH4 than the untreated biomass. Data obtained from the BMP tests for the untreated and pretreated biomasses were tested with the existing kinetic models; first order, dual pooled first order, Chen and Hashimoto and the modified Gompertz. The results showed that for both untreated and pretreated biomass, the modified Gompertz had the best fit amongst the four models tested with coefficient of correlation, R 2 values of 0.997 and 0.979, respectively. Comparatively, the modified Gompertz model could be the preferred model for the study of industrial WW when used as co-substrate during AcoD for biogas production. The study showed that higher biogas production and CH4 contents were observed when CS was employed as a reliable feedstock with maximum volume of the untreated and pretreated feedstock reported at 31 L and 20 L respectively.Item Catalytic conversion of alcohol-waste vegetable oil mixtures over aluminosilicate catalysts(2018) Ganda, Elvis Tinashe; Isa, Yusuf MakarfiThermochemical catalytic conversion of ethanol-waste cooking oil (eth-WCO) mixtures was studied over synthesised aluminosilicate catalysts HZSM-5, FeHZSM-5 and NiHZSM-5. The thermochemical reactions were carried out at temperatures of 400° and 450°C at a fixed weight hourly space velocity of 2.5 h-1 in a fixed bed reactor system. Successful conversion of the eth-WCO mixtures was carried out over the synthesised catalyst systems and in order to fully understand the influence of the catalysts, several techniques were used to characterise the synthesised materials which include XRD, SEM, EDS, BET techniques. Results of the catalyst characterisation showed that highly crystalline solid material had been formed as evidenced by the high relative crystallinity in comparison with the commercial HZSM-5 catalyst at 2θ peak values of 7°- 9° and 23°- 24°. The introduction of metals decreased the intensity of the peaks leading to lower values of relative crystallinity of 88% and 90% for FeHZSM-5 and NiHZSM-5, respectively. However this was even slightly higher than the commercial sample which had a value of 86% with respect to HZSM-5 synthesised catalyst taken as reference material. There was no significant change in XRD patterns due to the introduction of metal. Elemental analysis done with energy dispersive spectroscopy showed the presence of the metal promoters (Fe, Ni) and the Si/Al ratio obtained from this technique was 38 compared to the target ratio of 50 set out initially in the synthesis. From the SEM micrographs the morphology of the crystals could be described as regular agglomerated sheet like material. Surface area analysis showed that highly microporous crystals had been synthesised with lower external surface area values ranging from 57.23 m2/g - 100.82 m2/g compared to the microporous surface area values ranging from 195.96 m2/g to 212.51 m2/g. For all catalyst employed in this study high conversions were observed with values of over 93 %, almost total conversion was achieved for some samples with values as high as 99.6 % with FeHZSM-5 catalysts. Despite the high level of conversion the extent of deoxygenation varied with lower values recorded for FeHZSM-5 (25%WCO) at 400°C and NiHZSM-5 (75%WCO) at 450°C with oxygenated hydrocarbons of 19.5% and 19.33% respectively. The organic liquid product yield comprised mostly of aromatic hydrocarbon (toluene, p-xylene and naphthalene) decreased with the introduction of metal promoters with NiHZSM-5 producing higher yields than FeHZSM-5. For the pure waste cooking oil (WCO) feedstock the parent catalyst HZSM-5 had a liquid yield of 50% followed by NiHZSM-5 with 44% and lastly FeHZSM-5 had 40% at 400°C which may be seen to follow the pattern of loss of relative crystallinity. An increase in operating temperature to 450°C lowered the quantity of organic liquid product obtained in the same manner with the HZSM-5 parent catalyst still having the highest yield of 38% followed by Ni-HZSM-5 with 36% and Fe-HZSM-5 having a value of 30% for pure waste cooking oil feedstock which may be attributed to thermally induced secondary cracking reactions. For all catalyst systems with an increase in the content of waste cooking oil from 25% to 100% in the feed mixture there was a linearly increasing trend of the liquid product yield. HZSM-5 catalyst increased from 14% to 50% while FeHZSM-5 increased from 16% to 40% and NiHZSM-5 increased from 12% to 44% at a temperature setting of 400°C with lower values observed at 450°C.Results obtained in this study show the potential of producing aromatics for fuel and chemical use with highly microporous zeolite from waste material such as waste cooking oil forming part of the feedstock.