Development of chemical absorption system for biogas upgrading

Abstract

Biogas has been given a great attention recently as a fossil fuel supplement. Produced via anaerobic digestion, biogas is usually composed of 40 – 60 % methane, 40 – 60 % carbon dioxide (CO2), and 100 – 3,000 ppm of hydrogen sulfide (H2S). The presence of CO2 and H2S decreases the quality of biogas as an efficient and safe fuel. Several methods have been employed for the purpose of upgrading and purifying biogas. Chemical absorption is believed to be among the most potential for the removal of the two main impurities present in biogas H2S and CO2. The presence of large fraction of CO2 in biogas limits its utilization to power generation at low energy-conversion efficiency. This thesis aims at investigating and extending the knowledge required for the implementation of chemical absorption of CO2 for biogas upgrading. A laboratory-scale packed-column apparatus containing efficient and cheap packing material (plastic bioball) was designed and fabricated to perform the experimental work in this thesis. Initial absorption runs were performed to select the best solvent type and concentration between 10 – 50 % monoethanolamine (MEA), 4 – 12 % aqueous sodium hydroxide, and 5 – 25 % aqueous ammonia. 20 – 30 % MEA has shown the highest ability for producing methane-rich biogas using the absorber-column apparatus. The fabricated absorber apparatus was used to optimize the biogas upgrading process using MEA as a scrubbing solvent. The statistical analysis and optimization process were carried out varying MEA concentration in the range 20 – 30 % and at ranges of gas flow rate 4 – 8 kg/h, solvent flow rate 40 – 100 kg/h and column active height 100 – 200 cm. Box-Behnken design of experiment was selected, and the statistical analysis and the optimization of the chemical absorption process were performed using Design Expert (v10) software. Statistical solutions of optimum sets of conditions were obtained and verified experimentally. The optimization process have resulted in producing upgraded biogas with up to 95.7 % (98.7 % after drying) methane content and with enhanced CO2 loading capacity up to 4.8 mole-CO2/kg-MEA. Calculations were performed to scale up the laboratory scale absorber apparatus to an industrial scale apparatus. The Acid Gas property package of Aspen Hysys (v8.8) simulation software was verified using experimental data and used to analyze the performance of the industrial scale version of the absorber apparatus. Integrated designs were built for three biogas upgrading systems using MEA and aqueous ammonia as scrubbing solvents. The simulation results have demonstrated the ability of producing upgraded biogas containing less than 0.5 % CO2 and more than 99.5 % methane after drying. Among the three suggested designs, MEA-based system with complete solvent regeneration and recycle has shown highest energy demand with 3.8 MW. However, the specific energy demand of the reboiler of the system was calculated to be 4.93 MJ per kg-CO2 removed which is lower than the value of 5.57 recently reported in literature. Aqueous ammonia-based system and hybrid ammonia-amine system have shown to require less energy, estimated with 240 and 302 kW, respectively. However, the production of a considerable amount of ammonia-rich solvent through these systems is considered a major drawback. The future studies performed in the field should verify the possible strategies for converting theses ammonia-rich liquids to valuable products such as ammonia-based fertilizers.