The green gas coming to a pipe near you

Have you ever wondered where your food waste goes? If you’re lucky enough to live somewhere with an organised food waste collection, you may have. Any sneaking suspicions that it might just end up in a landfill site anyway?

If you live in London, the chances are that the contents of your food waste bin is shipped to a collection site in a neighbouring county and used to produce biogas. At Bio Collectors, the largest food waste site within the M25, all the waste actually comes from businesses in the capital.

We visited the site recently to see how the business works and understand the technical process behind it.

Located in an industrial hinterland near Mitcham in Surrey, Bio Collectors processes food waste from big name clients like Sainsbury’s and the Shard. Other customers include schools, hospitals, hotels and food factories across London which are attracted by the relatively lower cost of ‘recycling’ food waste as compared to sending it to landfill.

The Bio Collectors fleet of trucks picks up around 50, 000 tonnes of food every year to be processed into a thick ‘soup’ that meets animal by-product standards. Around a third is then used to make biomethane which is injected in to the gas grid, with the rest shipped to other sites or used in their small 0.5 MWe Combined Heat and Power (CHP) plant.

So how does it all work? First up, the science. Biogas is produced through anaerobic digestion, which is when food or other organic matter decomposes in a chamber with no oxygen and produces methane. This can then be used as a fuel (commonly in CHP plants) or is otherwise refined to bring it up to a standard where it can be injected into the gas grid (which we call ‘biomethane’).

Why bother? The benefits are twofold. Firstly, the process reduces the volume of methane emissions released into the atmosphere and, secondly, it produces low-carbon or ‘green’ gas which can be used as a substitute for gas across the economy.

This can be particularly valuable for heavy industry where there are special requirements on the quality of process heat (for example, the high temperatures or flame control required for fine ceramics, which rule out the use of biomass heat). You might also look to keep gas boilers in homes in certain areas and switch the gas supply over to biomethane, although this option is limited by the amount of bio feedstock available.

Currently, feedstocks largely come from agriculture, followed by food waste, while a smaller but growing share comes from sewage sludge. EU targets to reduce the amount of food waste going to landfill, along with the landfill tax, are helping to increase the available volumes.

Biogas also counts towards the UK’s Renewable Energy target, which seeks to ensure 15% of all energy comes from renewables by 2020. Because of this, it receives government support in the form of Renewable Heat Incentive (RHI) subsidies, which pay a fixed price per unit of heat produced. A higher tariff is paid for biomethane which can be injected into the grid. This support has seen the biomethane industry expand rapidly, increasing from two to 29 plants in 2014 according to figures from the Green Gas certification scheme – which could theoretically supply over 100,000 homes.

Ensuring quality is important and the biogas production process requires highly technical equipment to monitor the gas produced. Bio Collectors work in partnership with the local gas grid operator, Southern Gas Networks (SGN), which operates all the monitoring equipment. This is locked away in a large chamber onsite, keeping a close watch on everything from moisture content to the amount of propane that needs to be mixed with the biomethane to increase its energy content (calorific value). SGN can switch off the injection of gas into the grid at any point, depending on demand for gas – though hopefully not too often from Bio Collectors’ perspective.

So are we going to see natural gas replaced with biomethane in the same way that natural gas supplanted coal-based town gas in the 1960s and ’70s? The short answer is probably not, unless substantial shares of global feedstocks are diverted to the UK.

The CCC decarbonisation pathways, which set out how the UK can meet its target to reduce emissions by 80% by 2050, do feature a prominent role for biomethane though. Our central scenario includes 27 TWh of biomethane by 2030 – enough to heat nearly two million homes. However, longer-term the challenge will be to ensure that this scarce resource is used where it is most needed, including in areas like industrial process heat.

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