Purification of Biogas from
Anaerobic Digestion
Ajin Sunny and Kofi Afriyie
University of Kentucky and Morgan State University
Biogas Digester
Background
As 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.
Overview
Anaerobic 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 Biogas
Methane 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 Oxide
Iron 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 Filtration
Injection 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
Goal
Our 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.
References
Zafar, 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
