Anaerobic digestion (AD) plants are built to produce renewable energy, which is either used as biogas to power a combined heat and power (CHP) engine or upgraded to biomethane, to be used in domestic boilers. They also produce digestate biofertiliser and carbon dioxide (CO2).
These outputs can all be used, either domestically or in industry, which contributes to the development of a circular economy, in which natural resources are valued appropriately and nothing is wasted.
Each of the outputs from the AD process can benefit people or the planet in a different way.
When organic materials are broken down in an oxygen-free environment, such as inside an AD tank, biogas is produced. This is a mixture of gases which includes biomethane, CO2 and small quantities other gases such as hydrogen sulphide and water vapour.
Biogas is a useful output which can be used to power a CHP engine, to generate electricity. Using biogas to generate electricity typically emits less than 50 g CO2 /kWh, which easily meets the Committee on Climate Change’s target to reduce the carbon intensity of the UK’s electricity grid. CHP engines also produce heat, which can be used by homes or businesses in the vicinity, for example to heat commercial greenhouses to produce food.
Biomethane can be extracted from biogas using an upgrader, leaving pure biomethane. This can be used in domestic boilers to heat homes and businesses, and for cooking on gas-powered appliances.
A molecule of biomethane is identical to a molecule of methane, otherwise known as natural gas, meaning it is an easily substituted renewable alternative to fossil fuel derived gas. In the UK, biomethane can be injected into the national gas grid, without requiring any change of infrastructure, other than the injection point itself.
Biomethane can also be compressed to produce fuel to power vehicles, providing a renewable alternative to fossil fuels to support the decarbonisation of transport.
Not only does AD produce biogas and biomethane, it also produces digestate biofertiliser, which is the remaining solid and liquid material following the AD process.
Digestate biofertiliser can be used by farmers as an alternative to chemically produced ‘bagged’ fertiliser. It provides a source of nitrogen, phosphorus and potassium, the key nutrients needed for crops to grow, which boosts the organic matter content in soils, improving the overall soil quality.
The production of chemically produced ammonium nitrate fertiliser is a carbon intensive process, so using digestate biofertiliser instead can contribute towards reduced emissions from the agricultural supply industry. Digestate biofertiliser can also be used on farms near to the AD plant where it’s produced, unlike manufactured fertiliser which often travels long distances from where it’s made to farm, adding to its carbon footprint.
CO2 is a byproduct of upgrading biogas to produce biomethane. Carbon capture and storage (CCS) technology removes CO2 at the point where biogas is being upgraded to biomethane, which can then be stored long-term, preventing its release into the atmosphere where it can contribute towards heating the global climate.
Captured carbon can be used in industries which require CO2 for the production or packaging of their products. For example, the food industry uses CO2 for transporting meat and fresh produce to prevent deterioration of quality or safety.
Using carbon capture technologies at an AD plant can capture 1.9kg CO2 per cubic metre of CO2 which would normally be released.
Feedstock and a circular economy solution
The inputs into an AD plant are referred to as feedstock. These inputs are organic materials which can be broken down to produce biogas, digestate biofertiliser and CO2. Feedstock often includes waste materials, such as slurry or food waste, or by-products with high yield and low value, like straw.
By using organic materials which would otherwise be wasted, to produce renewable gas, a circular economy is created. This can contribute towards reducing carbon emissions and supporting global net zero targets, making AD a key source of renewable energy.