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Simulation and Optimization of Biogas Production Via Anaerobic Digestion of Biomass

Zamanian Howach, Hadi | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57766 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Rashtchian, Davoud; Askaripour, Hossein
  7. Abstract:
  8. Biogas, a mixture of methane and carbon dioxide, is produced through the anaerobic digestion of organic materials where microorganisms, under oxygen-free conditions, convert organic materials through four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. This renewable and sustainable energy source significantly contributes to reducing dependence on fossil fuels, decreasing greenhouse gas emissions, managing organic waste effectively, and preventing soil and water pollution. In this study, the ADM1 model was used to simulate the conversion of organic materials into biogas, with simulations conducted using Aspen Plus and MATLAB software. The system simulated consists of two reactors in series—one for hydrolysis and the other for subsequent anaerobic digestion stages—along with two heat exchangers, a compressor, and two membranes for separating hydrogen and methane. Validation results show an error of less than 5% for methane production and purity in biogas, and an error of less than 15% for biogas production. The highest methane content production was observed in animal manure and wheat straw, while the highest methane purity in biogas, at 65%, was associated with animal manure. Additionally, food waste and microalgae exhibited higher hydrogen content production compared to other feedstocks. When sunflower shell were co-digested with other feedstocks, methane content production increased by 150% (by volume), and the methane purity in biogas improved by 60%. The highest methane production in co-digestion occurred with a mix of 73.7:14.2:12.1 (by mass) of sunflower shell, animal manure and microalgae respectively, yielding 306.31 L/kg dry feed. Sensitivity analysis identified a temperature of 53°C as the optimal condition for maximum methane content, yielding 12% more than mesophilic conditions. Various parameters, such as temperature, organic loading rate, and hydraulic retention time, influence these processes. The byproduct of biogas, organic fertilizer, serves as a nutrient-rich resource for soil, enhancing soil quality and agricultural productivity. The use of energy jackets around the reactors was incorporated to capture heat from the exothermic reactions, supplying part of the process's energy demand. The energy jackets stabilized reactor temperatures and prevented equipment damage caused by temperature fluctuations. Optimization was conducted by linking MATLAB software with Aspen Plus, employing a genetic algorithm as an efficient optimization method. This algorithm simulated natural selection processes to find optimal parameter combinations and feedstock ratios for biogas production. The co-digestion of feedstocks showed that wheat straw was unsuitable for co-digestion with other materials, while the combination of two different feedstocks led to a noticeable increase in biogas and methane production. Adequate hydrogen injection was assumed for each feedstock, significantly enhancing methane production by 95% and purity by 100% compared to non-hydrogen-injected conditions. Utilizing hot water from the energy jacket reduced energy consumption by 40–51% for different feedstocks. Economic optimization suggested a 63.4:36.6 mass ratio of sunflower shell to animal manure as the most cost-effective feedstock mix. This feedstock mixture reduced operational costs by 22% compared to animal manure alone, 33% compared to sunflower shell and 15% compared to the simulation stage feedstock. Capital, operational, and annual costs were calculated, revealing that while capital costs were relatively low and economical, the current technological limitations rendered the project economically unfeasible. However, when accounting for environmental benefits, reduced pollution, lower waste management costs, and diminished pipeline infrastructure expenses, the project demonstrated significant economic and environmental advantages
  9. Keywords:
  10. Methane Production ; Biocompatibility ; Simulation ; Acclimated Activated Sludge ; Anaerobic Digestion ; Aspen 11.1 Simulator ; Biomass ; Biogas Production ; Co-Digestion

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