Biogas is a renewable bio-fuel that is produced from the decomposition of organic wastes in the absence of oxygen (anaerobic digestion). Organic matter, such as food scraps, agricultural wastes, crop residues and animal wastes are broken down by bacteria producing a mixture of gases. Primarily methane and carbon dioxide and trace amounts of hydrogen sulfide , moisture and siloxanes.
Methane can be combusted or oxidised with oxygen; the energy released during combustion allows biogas to be used as fuel for heating, cooking and power generation. It can also be used in gas engines to convert the energy in the gas into mechanical energy, electricity and heat. A methane concentration ranges from 50 – 75 per cent (vol/vol) depending on the type of feed material used. Methane burns with a clean deep blue flame.
Economic benefits of biogas
Saves money: With biogas, cooking fuel expenses can be reduced by as much as 80per cent. Biogas replaces firewood, charcoal and kerosene.
Cooking on biogas is faster and easier and cleaner than cooking on charcoal or firewood.
Reduces household waste: Biogas systems convert organic household waste or manure into gas for cooking and lighting. Biogas helps you to manage your waste and contributes to improved hygiene.
Produces high quality organic fertilizer: The biogas-slurry that comes out of biogas systems is rich in nutrients and can be applied directly to plants and vegetables to help them grow. This saves money and is better for the environment.
Biogas is an environmentally-friendly energy source.
Biogas is produced in bioreactors commonly referred to as biogas digesters. These are anaerobic reactor systems that process waste into biogas, and then channel it to a point of collection or use. Biogas production systems range from small domestic digesters, medium size, industrial and large electricity generating plants.
There are several types of biogas digester systems and plants; each model differs depending on input, output, size, and type. However, the biological process that converts organic waste into biogas is uniform. Some of the most common biogas digesters include; tubular or flexible types, fixed dome reactors, floating drum type etc.
Biogas production occurs at various stages; each stage requiring different types of microbes and temperature regimes. However, the most important condition in biogasproduction is to ensure that anaerobic conditions are maintained throughout the process. Once the gas is produced it is channeled to a purification plant, a storage facility or end point of use.
Purification and Upgrading
Biogas contains combustible and non-combustible gases which are undesirable. Most applications require high methane content and removal of corrosive gases such as hydrogen sulphide, ammonia and water vapor. To achieve high heat content in the flame or gas fuel in gas engines, biogas requires purification and upgrading to remove the undesirable constituents of biogas namely; carbon dioxide, hydrogen sulphide, ammonia, siloxanes and water vapor. Wet scrubbing, adsorption on solid adsorbents, high pressure swing systems, condensation and absorption in appropriate solvents are some of the methods used for biogas purification and upgrading to Biomethane.
[The writer is an expert in development of Waste to Energy Conversion Technologies and Consultant in Environmental Management and a Senior Researcher at the Institute of Energy and Environmental Technology. [email protected]]