Purification of Biogas from
Anaerobic Digestion
Ajin Sunny and Kofi AfriyieUniversity of Kentucky and Morgan State University
Biogas Digester
BackgroundAs energy sources are sought in place of the non-renewable sources it has become imperative to make use of more environmentally friendly methods. Anaerobic digestion has been the topic of discussion for most environmentalists due to its efficiency and cost effectiveness.
Anaerobic digestion is the biological breakdown of organic matter by bacteria in the absence of oxygen.
OverviewAnaerobic digestion produces biogas which is essentially made up of CO2, CH4, and trace amounts of other gases. The other gases are scrubbed while methane is used in the production of energy.
The biogas is separated via fractional distillation and methane is controlled through a compressor which increases the temperature of methane for running through a turbine. The turbine makes use of the high temperature gas and converts it to energy which can be used to power the entire process or sold profit.
Methods of Purification of BiogasMethane gas is the most useful and valuable gas produced for future applications. Although, there are impurities present in the biogas besides CO2
These impurities can be removed by using methods such as:
• High Pressure Membrane Separation• Gas Liquid Adsorption• Adsorption by Iron Oxide• Biological Filter
Membrane Separation• Transportation of raw gases through the membrane• Permeability of membrane is the direct component
of the solubility of the gases• Membranes are constructed with hollow fibre
modules.• Shape of the membrane determines the
permeability of the different molecules• Purity of methane could always be increased by
increasing the number of membranes but the quantity of methane would be lost.
High Pressure Membrane Separation• High pressure gas feed
from compressor
• Membrane separator will separate the gases based on solubility, shape or size of the molecules.
• Three stage membrane producing 90% CH4
Gas Liquid Adsorption• Uses micro-porous
hydrophobic membrane as an interface between gas and liquid.
• CO2 and H2S are dissolved in the liquid while CH4 is collected for use.
• Molecules of gas flow in one direction and diffuse through the membrane.
Adsorption by Iron OxideIron oxide reacts with hydrogen sulfide to form iron sulfide and water.
The reaction is endothermic and temperature of 12 0C is required for necessary energy. However optimal temperature is between 250C - 500C.
Iron oxide regenerated by oxidation after the above process is highly exothermic and may lead to self ignition if not properly controlled.
Biological FiltrationInjection of O2 directly into the digester tank.
Thiobacillus bacteria growing on the filter bed. Before entering filter bed 4-6% of air is added to the biogas.
H2S is oxidized by bacteria growing the filter bed. 350C is the temperature at which H2S is converted to biologically converted to sulfur. Sulfur is retained as yellow clusters from the biological filter bed.
Conclusion and Inferential Reasoning
Biological Filtration • Does not adapt to the capability of fluctuating
amounts of H2S. • Frequent replacement of filter.
Adsorption by Iron Oxide• Endothermic reaction as well as Exothermic
regenerative reaction• Careful control of temperature and airflow required
to prevent self ignition. Membrane Separation• High yield of methane produced. Process is
compact. Light in weight. Low energy and maintenance requirement.
Motivation
GoalOur model and research process was able to show a direct relationship between the mass of methane produced, hydraulic residence time, volume of organic waste and the profit assumed.
Ultimately we wish to pitch this information to farmers to help in the production process.
We hope to highlight the project benefits and positive externalities involved maximizing the use of waste to save costs and protect the environment.
Acknowledgements
We would like to thank the many graduate students and volunteers that helped us navigate through our thinking processes and understanding the material.
Also, the many donors whose donations made it possible for us to acquire the materials necessary for this process.
ReferencesZafar, Salman. "Biological Cleanup of Biogas." BioEnergy Consult. N.p., 14 Nov. 2015. Web. 25 May 2016.
Wang, De Ming. Breakthrough Behavior of H2S Removal with an Iron Oxide Based CG-4 Adsorbent in a Fixed-Bed Reactor. Thesis. University of Saskatchewan, 2008. Saskatoon: ECommons, 2008. Print.
Q, Zhao, Leonhardt E, MacConnell C, Frear C, and Chen S. Purification Technologies for Biogas Generated by Anaerobic Digestion. Rep. no. 2010 - 001. N.p., n.d. Web
