Welcome to the second part of our series on natural gas production. This article will focus on biogas, which is a renewable natural gas. If you missed the first one, you can read it HERE!
Having looked at how natural gas is produced traditionally; it is time to look at the way that biogas is produced. But first, what is biogas? According to the EU legislation, biogas means gaseous fuels produced from biodegradable waste and residues from biological origin from agriculture, forestry and related industries, and waste. This is quite a broad description, but simply put, biogas is a mixture of gases produced from organic matter.
Well, how is it produced then?
The above definition gives us a lot of information about the origins of biogas – organic matter from waste and residues from biological origins, from different industries. But how does waste get transformed into a climate-friendly energy source? The key process by which biogas is produced is called anaerobic digestion. It is a natural process, where microorganisms break down organic matter in an environment that lacks oxygen. For example, in nature it occurs in marshes and swamps, producing marsh gas. An interesting fact is that methane was discovered by Alessandro Volta in 1778, exactly in the muddy waters of Lake Maggiore (Italy) from marsh gas.
Nowadays, in order to produce gas, we do not rely on swamps, but anaerobic digesters! These are sealed constructions where the process takes place under controlled conditions. They are specifically constructed for the chosen production site and take into account, among other things, the location and expected feedstock. Simply put, the digesters contain microbial communities which “eat” the waste and produce biogas and digestate.
The process from start to finish…
The process begins with the feedstock that is fermentable matter, which is placed in the anaerobic digester. This organic matter can be livestock waste, crop residues, household organic waste, industrial organic waste (from paper, chemistry and agro-industries), green waste and sewage sludge. You can find examples of plants that use different feedstocks for biogas production in our Best Practices section!
After the feedstock is placed in the digester, bacteria begin the anaerobic digestion. As mentioned before, this digestion takes place at controlled temperature and without oxygen present in the digester. The temperature depends on the type of the digestion, but generally there are two main types. The first one is mesophilic digestion, which optimally takes place between 30ºC to 38ºC. The second is thermophilic digestion, that takes place optimally between 49ºC to 57ºC.
Top of the digester and bottom of digester – what is left behind?
Anaerobic digestion results in two products, the main one is biogas and the secondary is digestate. Biogas represents a mixture of gases where the main one is methane (CH4), which is the primary component of natural gas. The percentage of methane in the mixture is anywhere between 50% to 75%, depending on the digester, feedstock and bacteria. Other gases in the mixture are carbon dioxide (CO2) at around 30% to 40% and trace amounts of gases like hydrogen sulphate (H2S), water vapour and nitrogen (N2) and others. This gas mixture rises to the top of the digester, where it is extracted for further treatment.
On the bottom of the digester is the secondary product of biogas production – digestate. Digestate is a nutrient-rich solid or liquid material, left remaining after the process. It contains the recycled nutrients, which were originally in the feedstock, but in a readily available form. Again, as with biogas, the composition and nutrients depend on the feedstock and digestion method, but this digestate is a valuable product which can be used as fertilizer, for soil amendment or as livestock bedding.
Coming back to biogas, at the top of the digester the biogas mixture is compressed and then it can be sent for direct use or for further processing. Direct use consists of transporting biogas to Combined Heat and Power Units, located on site, where the gas is used to generate electricity and heat. From there electricity is used to power the operation of the biogas plant or is fed into the local electricity grid. Heat can be used to keep the temperature of the digester at the appropriate level.
If not used directly, biogas takes the second pathway. This involves purification and turning it into biomethane. Each local or national gas grid has technical specifications for the quality and composition of gas that can be injected. These specifications exist to guarantee the safety and technical integrity of the grid, but also that gas combusts properly either at your stove or car engine. So, in order to comply with these specifications, biogas has to be purified into biomethane. At this stage, carbon dioxide can also be purified from the biogas mixture, so that it becomes a product of value for the food and beverage industry. Lastly, since methane is odourless, an odorant must be added so that natural gas leaks can be detected easily.
After the purification process, biomethane can be used as natural gas, but with the big advantage that it is renewable and prevents greenhouse gas emissions. From here, it can be injected in the existing gas grid for domestic or industrial use. It can be compressed as Bio-CNG or cooled down as Bio-LNG for use as fuel in cars, trucks or ships. Lastly, it can be used as feedstock in the chemical industry for production of plastics, methanol, ammonia, formaldehyde, renewable hydrogen, or fertilizers.
Biogas production starts with feedstock, which is simply organic waste from different sources. This feedstock is placed in a digester where microorganisms “eat” the feedstock and produce biogas and digestate. The process is called anaerobic digestion because it takes place in the absence of oxygen. The result of this natural process is a biogas mixture. This biogas mixture is a renewable natural gas, which can be either used directly or it can be further processed and purified into biomethane.
Now you know the basics of how biogas is produced. Below you can find our sources and if you would like us to explain some part of what you read in further detail, leave us a comment below!
- TPA Climate Change and Energy Transition Coursebook
- Song, Young-Chae et al., 2004, “Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge”, Water Research 38(7), pp. 1653-1662
- United States Environmental Protection Agency – How does Anaerobic Digestion work?
- United States Environmental Protection Agency – Basic Information about Anaerobic Digestion (AD)
- Environmental and Energy Study Institute – Fact Sheet Biogas: Converting Waste to Energy
- Museo Galileo – Methane
- Business Insider – How Rotting Vegetables Make Electricity (Video)