Theses and Dissertations

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Browsing Theses and Dissertations by Subject "maize stubble"

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    Effect of Anaerobic Co-digestion of Cow Dung and Maize Stubble on Biogas Production: Optimization and Economic Viability Analysis
    (GULU UNIVERSITY, 2026-04-09) Kiir, Peter Bol Kuol
    ABSTRACT The production of biogas depends on factors like temperature and substrate mixing ratios. This study involved the characterization of cow dung and maize stubble, as well as their anaerobic co-digestion to optimize biogas volume and methane yield while minimizing carbon dioxide concentration. An economic assessment was also conducted to evaluate the viability of anaerobic co-digestion. Feedstock characterization included proximate and ultimate analyses, along with determination of higher heating values. Process conditions for anaerobic co-digestion were optimized using Response Surface Methodology based on Central Composite Design (CCD). The independent variables were mixing ratio and temperature while the dependent variables were biogas volume, methane yield, and carbon dioxide concentration. Thirteen experimental conditions were generated using the Design Expert software version 13. 1.5 L plastic bottles with a working volume of 1.2 L, were loaded with a mixture of cow dung (CD) and maize stubble (MS) in ratios of 100:0 (CD:MS), 0:100 (CD:MS), 40:60 (CD:MS), and 70:30 (CD:MS), and submerged in a water bath operated at temperatures ranging from 25–45 °C. Daily readings were taken to measure the biogas volume using the water displacement technique for 26 days, and a 3-day gas composition analysis was performed using a portable biogas analyzer (Geotech, 5000). The carbon content of CD and MS were found to be 32.87% db and 31.55% db, respectively. The C/N ratios of CD and MS were found to be 21.54 and 29.84, respectively, which lay within the range of 20–30 ideal for anaerobic digestion. pH levels for CD and MS were found to be 8.6 and 5.46, respectively. Higher heating values of CD and MS were found to be 13.63 and 16.05 MJ/kg, respectively, suggesting a higher energy potential of MS compared to CD. Low sulfur levels of 0.09% in CD and 0.11% in MS suggest minimal emissions due to sulphur oxides (Sox). The optimal conditions were found to be a mixture ratio of 40:60 (CD:MS), and temperature 35 °C, resulting in a maximum biogas volume and methane yield of 5.35 L/kgVS and 95.20 %/kgVS, respectively, with a minimum CO2 concentration of 15.24 kgVS. An economic analysis revealed a positive net present value (NPV), confirming the project's economic viability. The payback period was found to be 4 years. Based on accounting rate of return (AAR), the project results in a 27% annual return, which is economically attractive. Additionally, a profitability index (PI) greater than 1 confirms that the future value exceeds the initial investment, highlighting strong economic viability of anaerobic co-digestion of cow dung with maize stubble.