Facebook LinkedIn

Thursday September 9 2021

How Biogas is Produced – Anaerobic Digestion Explained

Biogas has become an integral part of the UK’s energy mix over the past few years, generating localised heat and electricity that reduces our reliance on less sustainable forms of power and feeding environmentally friendly electricity directly into the grid.

It also forms the basis of biomethane, a green gas with similar properties to natural gas that can be used as a direct source of fuel in domestic and commercial settings.

But what is biogas and how is it produced? In this article, we take a deep dive into the topic exploring all aspects of this renewable energy source.

What is biogas?

Biogas is the product of the breaking down of organic waste such as municipal waste, manure, wastewater biosolids, food waste, animal manures, or purposely grown crops, in the absence of oxygen. The composition of biogas is principally methane and carbon dioxide, but it also contains water vapour and other gases such as hydrogen sulphide in far smaller concentrations.

The usable part of biogas is the methane and the concentration of this tends to vary from around 50 per cent to 75 per cent depending on a number of factors, the most important being the nature of the organic material that has been broken down. The type and efficiency of the process used to break down the matter also has a bearing on the relative concentrations of methane and carbon dioxide. The higher the methane concentration, the better quality the biogas as it contains a greater volume of useable fuel.

Biogas can be used as a direct source of fuel to power generators that produce electricity or to create heat for localised use such as sustaining the temperature of a biogas reactor or providing heat and hot water for a commercial premises.

It can also be cleaned by removing the CO2, creating renewable natural gas or biomethane, which can then be exported to the national gas network. The CO2 produced is captured during this process to ensure it is not released into the atmosphere.

How is biogas produced?

Biogas is produced via the decomposition of organic matter in the absence of oxygen. As a result, it depends on anaerobic bacteria – microorganisms that are capable of living without oxygen – to break down the organic matter. It occurs naturally in soils, watery environments such as swamps and lakes, as well as in the human gut.

Biogas is also produced by man made processes such as disposing of waste in landfill and whereas once this was just left to escape into the atmosphere, it can now be captured and used as a fuel. However, most usefully, it can be created in controlled environments via a process called anaerobic digestion where it can be harvested for us as a renewable fuel.

Anaerobic digestion takes place in an anaerobic digester plant, also called a biogas plant, AD plant or biogas reactor. An AD plant has large tanks known as digesters where organic feedstocks such as food waste, animal manure, energy crops, or wastewater biosolids are housed and the conditions needed for biogas production created.

The biochemistry of biogas production is broken down in to four key microbial phases. These include:

  • Hydrolysis – complex organic matter such as proteins, carbohydrates and fats are broken down by bacterial enzymes into their constituent parts, such as sugars, fatty acids, and amino acids.
  • Acidogenesis – various fermentation reactions convert larger molecules into organic acids, alcohols, ammonia, carbon dioxide, hydrogen and hydrogen sulphide.
  • Acetogenesis – the fermented products are then oxidised into simpler forms such as acetate and carbon dioxide.
  • Methanogenesis – finally, Archaea (single cell organisms) convert hydrogen and acetic acid into methane and carbon dioxide.

Of these four phases, the final two have the most influence on the quality of the biogas produced because these are responsible for the ratio of methane to carbon dioxide. Different feedstocks also significantly affect the concentration of methane created.

Biogas advantages

There are many advantages of biogas production. The main ones include:

  • The creation of renewable energy from waste materials

Clearly, this is what biogas production is all about, turning waste that cannot be effectively used in other industries into clean, green energy. However, it is still worth stating this as the primary advantage of biogas.

  • It reduces waste to landfill

Burying waste in landfill is not a sustainable solution. However, waste streams such as wastewater sludge and municipal waste have little value to other industries and cannot be repurposed for other uses. Therefore, they would traditionally go to landfill. Using them as feedstocks for anaerobic digestion means they don’t end up there.

  • It can be used to create heat and electricity simultaneously

By incorporating a combined heat and power (CHP) engine into an AD plant, the biogas produced can simultaneously create electricity for local use or to be exported to the National Grid, and heat which can be used to keep the biogas reactors at the correct temperature and provide heat and hot water for local buildings.

  • It can be cleaned to create biomethane

Biogas doesn’t meet the cleanliness requirements to be used in the national gas network. However, scrubbing can produce biomethane of more than 98 per cent methane concentrate, which can then be exported to the National Gas Grid.

This, along with exporting electricity generated by biogas to the national network, enables energy diversification, reducing the UK’s dependence on fossil fuels and helping to decarbonise energy production.

  • The by-product of producing biogas is a nutritious bio-digestate

As well as biogas, anaerobic digestion creates digestate which is rich in nutrients and can be used as a bio-fertiliser. This helps promote a green and circular economy in which energy crops are raised, used to create bioenergy and the digestate fertiliser is then created to help grow the next energy crop.

Are there any disadvantages to biogas?

Well, nothing is perfect and there are a few disadvantages of biogas. The main ones are:

  • Odour

Wastewater biosolids (aka sewage) and animal manures do have a bit of an odour, so if these are going to be used as a feedstock, it is best located away from heavily populated areas. In fact, you’re unlikely to get planning permission for a plant using these feedstocks near residential areas.

  • Transport

Although biogas generation is environmentally friendly, feedstocks need delivering to site and this might be from many miles away. The products of anaerobic digestion may also need transporting off site too. Both these involve road transportation which leads to increased carbon emissions.

However, this is still far lower than traditional, fossil fuel-based methods of generating power.

  • Certain types of organic matter cannot be used to create biogas

AD plants are capable of utilising a large range of feedstocks to generate biogas, but they must be organic, biodegradable and non-woody. This is because the anaerobic microorganisms used in a biogas plant are not capable of breaking down lignin, the substance that gives plants their strength. Therefore, materials such as wood and paper cannot be used in a digester plant.

Biogas production process flow

As outlined above, there are four phases of producing biogas – hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

But what does this look like in practice, inside an AD plant?

Phase one of the biogas production process flow is preparing the feedstock for digestion. This involves segregating mixed waste to ensure a more efficient operation and removing any inorganic materials. Once this is done, the feedstock is ready to enter the digester.

Once in the digester, the bacteria begin to break the feedstock down into biogas and digestate and the four processes in biogas production take place.

There are two types of biogas reactor – mesophilic and thermophilic. Thermophilic reactors run at temperatures of between 40 – 60oC, whereas mesophilic is between 30 – 40oC. Thermophilic has the potential to create larger biogas yields, but mesophilic anaerobic digestion tends to be more stable and able to accommodate different organic loading rates (OLRs) more easily so is more widely used.

The complete process of breaking down the feedstock takes anywhere from 14 days to 40 days, depending on the material used, the micro-organisms in the digester, temperature, and pH*. Once this has happened, the biogas can be collected and the digestate removed.

Anaerobic digestion can be carried out as a continuous process or a batch process. In a batch process, the feedstock is added to the digester at the start and then the tank is sealed for duration of the process. Once that is complete, the operation is repeated.

Anaerobic digestion and the environment

It may seem ironic that anaerobic digestion is considered a renewable process when it creates methane, one of the most potent greenhouse gases.

However, it is sustainable because the methane is created in a control environment and can be harvested for use in generating electricity and heat. When combusted to create electricity and heat, the carbon released is essentially recycled atmospheric carbon – it was once plant material formed via photosynthesis and it is being returned to the atmosphere for plants to reuse.

The electricity and heat created in this way replace power that would otherwise have been generated by burning fossil fuels and so anaerobic digestion and biogas play a significant role in lowering the carbon emissions the UK’s power generating network**.

To find out more about biogas and if anaerobic digestion is right for you, get in touch.

* https://envirocare.org/anaerobic-digestion-process-from-waste-to-energy/ ** https://farm-energy.extension.org/environmental-benefits-of-anaerobic-digestion/