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Biomethane (renewable natural gas)

Displacing fossil fuel usage with clean energy production

Apsley Farms is the UK’s third largest biomethane plant, supplying renewal natural gas to the UK gas grid since 2014. We manufacture the gas 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 production 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 as a direct replacement to supply homes, industries and public buildings.

The difference between the two gases is the original source:

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

Biomethane (renewable natural gas) is created by refining the biogas produced by the bacteria which breaks down plant material which has absorbed atmospheric CO2 in order to grow and create biomass.

This is an inherently lower energy process than extracting fossil fuels. When derived from the right crops, and using optimal farming and production techniques, biomethane can be produced with a much smaller carbon footprint, and with minimal leakage of methane. Our green gas is also produced closer to end-users, reducing transport emissions and carbon footprint.

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

You can read more about the sustainability of biomethane natural renewable gas on our sustainable farming page here

The need for gas

The UK is the second largest consumer of natural gas in Europe, and the 11 th 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.

So how we can make the UK’s gas supply greener?

Digestion: eating up our greens

The process begins with feeding plant material into our biodigesters. Our three primary 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 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 trace gases, collectively described as biogas.  There are four main stages to the reactions.

Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis

At Apsley, the digestion process takes place within our five biodigesters; our smallest two biodigesters are cylindrical above ground tanks and our biggest three are lagoon type digesters which are bespoke and designed in-house.  Our biggest digester is 84m long and 8.5m deep, holding 14,800m³ of digestate.

For anaerobic digestion to work optimally, the conditions have to be just right. Small variations in feed mix, feed quantity, temperature, pH and trace elements can affect gas production. The Apsley Farms team constantly monitors conditions within the biodigesters, making adjustments as required.

Keeping conditions optimal

Our on-site 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
  • recirculation
  • retention times

FOS/TAC

FOS/TAC is 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.

pH

The pH within the digester is crucial, since methane production only occurs in a small range — about 7.4 to 8.3 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 degrees centigrade.

Dry matter content

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

Cleaning: taking out the impurities

The biogas produced in the digester is a 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 amounts of oxygen to our system and using sophisticated carbon filters our system, we can reduce harmful hydrogen sulphide from 400 parts per million to 10-40 ppm.

Dutch manufacturer DMT has developed a membrane system that does just that, separating the carbon dioxide from the methane. Because the CO2 and CH4 molecules are different sizes, the membrane system acts like a sieve, allowing them to be separated.

This separation system has massive advantages compared to traditional methods of removing CO2. Its low cost, high energy efficiency, easy operation, modular construction and flexibility all add up to a more efficient and sustainable approach.

  • The CO2 from the membrane is 98.7% pure – pretty good, but not a high enough standard for the food industry. We start the cleaning process by compressing the gas up to 17 bars. For comparison, pressure inside the tyres of passenger cars is typically 2 – 2.2 bar.
  • The CO2 is ‘dried’, turned into liquid, re-boiled and turned back into liquid again. Each process helps to remove further impurities. moving gases that can be condensed into liquid.
  • Liquid CO2 at -43 degrees centigrade is passed into our storage tanks, with a total capacity of 150 m3.
  • The final stage is to check the quality of the CO2 once certified, the CO2 is collected and distributed.

Sustainable biomethane

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

Compounds Biogas Natural gas Unit
Methane 50 – 60 mol %
Carbon dioxide 40 – 50 ≤ 2 mol %
Hydrogen sulfide ≤200 ≤ 5 mg/m3 (STP)
Ammonia ≤50 0.5 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 impurities from biogas to obtain a renewable natural gas alternative with a concentration of over 97% methane.

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 Combined Heat and Power (CHP) generator, biomethane is used to produce around 1.1 Mega Watt of green electricity. which we use to run the biogas plant and also export to the national grid.

Plant-based power

Apsley Farms harnesses the energy within our biomethane using two CHP generators. 60% of the energy output is in the form of heat, which keeps our digesters optimally warm. also heating our offices, the maintenance workshop, and domestic houses.  The remaining 40% is electricity, which partly powers our operations, whilst the rest is exported to the national grid.

The first CHP has been running since 2012, with the second added in 2015. Together, they utilise 500 m3 of biomethane per hour and produce 1.1 MW of electricity.

Upgrading biogas: ‘sieving’ out the CO2

The untreated gas contains around 45% CO2 and this can be removed by passing the biogas through a triple 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.

 

Analysis and supply

Once the CO2 has been removed, the 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 UK specification. A little bit of odouriser is added and the gas is on its way.

Apsley Farm produces 28,800m3 of sustainable biomethane a day, approximately. We contribute 2% of the UK’s biomethane, an amount sufficient to heat 8,500 homes continuously.