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Biomethane

Displacing fossil fuel usage with clean energy production

Apsley Farm has been supplying biomethane to the UK gas grid since 2011, and we are now a leading provider of biomethane. We manufacture biomethane in our biodigester facility, which breaks down plant material obtained from our own and neighbouring farms. With a carbon footprint of around one-fifth of ‘natural gas’, Apsley Farm’s biomethane provides a sustainable fuel to help meet the UK’s energy needs and climate targets.

Biomethane vs natural gas

Biomethane is a greener, sustainable alternative to natural gas. Once properly treated, it can be fed into the gas grid to supply homes, industries, public buildings and so on. The difference between the two fuels is how they are obtained.

Natural gas

Obtained from underground reserves. Around half of the UK’s supply comes from those under the North Sea and East Irish Sea; the rest is imported by ship and long-distance pipelines. The energy costs (and therefore carbon footprint), of extracting, processing and transporting natural gas are considerable. Far worse, leaks of methane from these processes add huge quantities of this potent greenhouse gas to the atmosphere.

Biomethane

Produced by using bacteria to break down plant material. This is an inherently lower energy process than extracting fossil fuels. When derived from the right crops, using optimal farming and production techniques, biomethane can be produced with a much smaller carbon footprint, and with minimal leakage of methane. Gas can also be produced closer to end users, reducing transport.

By any measure, biomethane produced through efficient and environmentally responsible methods is a cleaner, more sustainable alternative.

You can read more about the sustainability of biomethane on our sustainability pages.

The need for gas

The UK is the second largest consumer of natural gas in Europe, and the 11th largest in the world. Despite ambitious plans to reduce consumption, it seems clear that gas will contribute to our national energy needs for many years.

This begs the question of how we can green our gas supply.

Digestion: eating up our greens

The process begins with feeding plant material into our biodigesters. Our three biodigesters can handle around 300 tonnes of plant material a day. Inside our biodigesters, nature gets to work, thanks to 37,000m3 of bacteria-rich material we have grown.

One of our biodigesters, Lagoon LS3, is the largest primary on-farm digester in the world.

How anaerobic digestion works

Anaerobic digestion is a series of chemical reactions carried out by bacteria in the absence of oxygen. The bacteria digest organic matter, such as plant material, to produce methane, CO2, and a range of other substances. There are four main stages to the reactions

Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis

At Apsley, the digestion process takes place within four biodigesters: BS1, BS2, Lagoon LG3 and our combibag. The largest is Lagoon LG3: measuring 52m in diameter, 6.5 m in depth and holding 9,500m3 of liquid digestate, it produces more than 50,000m3 of biogas a day.

For anaerobic digestion to work optimally, the conditions have to be just right. Small variations in temperature, acidity and amounts of feed can reduce gas production. The Apsley Farm team constantly monitors conditions within the biodigesters, making adjustments as required.

Keeping conditions optimal

Our onsite laboratory carefully monitors conditions within the digesters. Each one requires slightly different conditions. Some of the most important variables we monitor are:

  • temperatures
  • feed inputs
  • substrate levels
  • recurculation
  • retantion times

FOS/TAC

FOS/TAC analysis tells us the ratio of volatile organic acids and the total inorganic carbon in the system. This tells us if we are overfeeding or underfeeding the digester, so we can fine tune the daily feed requirements.

Acidity

The pH within the digester is crucial, since methane production only occurs in a small range — about 7.4 to 8.0 in our system.

Temperature

Keeping the bacteria happy means keeping them at optimal temperature. Our plant operates in the mesophilic temperature range which is between 37 and 46 C.

Dry matter content

The amount of dry material must also be kept under control. To keep this consistent, we use a separator to split the liquid from the solids and recirculate the liquid fraction back through our hydroliser.

Cleaning: taking out the impurities

The biogas produced in the digester is a complex mix of methane, carbon dioxide, water vapour, hydrogen sulphide and more. For this to be useful, it must first be cleaned and upgraded.

Cleaning involves removing the substances which can cause corrosion and/or those unsuitable for the natural gas grid. For example, by adding ferrous chloride to our system, we can reduce harmful hydrogen sulphide from 400ppm to 10-40ppm.

What’s in our gas?

The composition of the raw gas produced by our biodigesters is:

Compounds Biogas Natural gas Unit
Methane 45 – 70 mol %
Carbon dioxide 30 – 45 ≤ 2 mol %
Hydrogen sulfide ≤ 2000 ≤ 5 mg/m3 (STP)
Ammonia ≤ 1000 0 mg/m3 (STP)
Oxygen ≤ 2 ≤ 0.5 mol %
Nitrogen ≤ 8 ≤ 5 mol %
Water 37 at 1 bar ≤ 8 at 40 bar °C
Combustion heat 6.7 – 8.4 10.7-12.8 kWh/m3(STP)
Wobbe index 6.9 – 9.5 13.3 – 15.7 kWh/m3(STP)

The aim of the process is to remove most of the impurities from biogas to obtain biogas with a concentration of over 97% methane. Pure methane has a calorific value of 9100 kcal/m3 (15,5°C, 1 atm).

Re-using gas: running our plant on plants

Some of the biomethane is used to support Apsley’s farming operations, including heating the offices. Through our CHP generator, some of the methane is used to produce electricity. This produces significant amounts of electricity. A proportion is used to run the farm and biogas production, and the rest is exported to the national grid. We produce around 1.1MW of electricity, enough to meet the needs of around 3,800 houses.

Plant-based power

Apsley Farm harnesses the energy within our biogas using two CHP (Combined Heat and Power) generators. 61% of the energy output is in the form of heat, which keeps the digesters optimally warm. The remaining 39% is electricity, used to power our operations and export to the national grid.

The first CHP has been running since 2012, with the second added in 2015. Together, they utilise 505 m3 of biogas per hour and produce 1.1 MW of electricity.

Upgrading biogas: ‘sieving’ out the CO2

Upgrading the cleaned gas centres on removing the CO2 from the biogas. The untreated gas is between 30 and 45% CO2, and this can be removed by passing the biogas through a membrane system. As the CO2 and methane molecules are different sizes, the membranes act like a sieve, allowing the two gases to be separated.

Our recovered CO2 is cleaned, processed and stored for use within the food industry.

The membrane system is a highly energy-efficient alternative to traditional methods of removing CO2, helping us towards our goal of becoming a carbon-neutral business.

Analysis and supply

The excess biomethane is now almost ready to export to the national gas grid. In order to get the right calorific value (how much energy the gas can supply), small amounts of propane are injected  – it’s about 3% of the total mix. The mixture is analysed at the Network Entry Facility to check that it fits the required specification. A little bit of odouriser is added and the gas is on its way.

Apsley Farm produces 50,000m3 of sustainable biogas a day. We contribute 2% of the UK’s biomethane, an amount sufficient to heat 8,500 homes.