Carbon Intensity in Biogas: counting molecules, creating certificates, and maybe not growing Maize after all.

As you will recall from previous blogs, biogas is made by putting organic material into big tanks and in the absence of oxygen (hence anaerobic) tiny bacteria break down the long chain hydrocarbons to release methane (CH4) and CO2. Your stomach does pretty much the same thing which is why you burp and fart. This biogas can be burnt in engines to make electricity or upgraded to biomethane by separating the CH4 from the CO2 using membranes.  

However, as with George’s cows and the pigs, although it is the same gas, not all biogas is of equal merit. Following our farmyard theme, the biogas that comes from manure is good because it avoids uncontrolled methane emissions and displaces fossil fuels. The same gas that comes from growing millions of acres of maize is not as good. The thing that differentiates the two is of course the carbon intensity of the production process, but you probably already knew that.  

The Basics: What Is Carbon Intensity — and why should we care? 

To get everyone on the same page, Carbon Intensity (CI) is the kg of CO2 equivalent  grams of CO₂ equivalent per megajoule of energy (gCO2e/MJ)  produced. For biogas, it means calculating the CO2 emissions from: 

• Sourcing the feedstock 

• Transporting it to the plant 

• Running the Anaerobic digestion and gas upgrading units 

• Distributing the digestate 

• And crucially, any avoided emissions, such as stopping uncontrolled methane leaks from untreated manure or food waste. 

This number, your CI score, now decides whether your project: 

• Gets a subsidy. 

• Sells premium green gas certificates. 

• Is welcome at corporate environmental committees (or left outside with the single-use plastics) 

Maize-Based Biogas: Renewable? Technically. Green? Not So Much. 

For maize based biogas, the CI score is the big snorty elephant in the corner of the room. Maize is the biogas world’s favourite feedstock because its reliable, high gas yielding per ton and massively scalable. But is it climate-friendly? Well, no not really. 

Growing huge fields of maize for digestion comes with all the accompanying trimmings: fertiliser, irrigation, chemicals for pest control, diesel-fuelled tractors and combine harvesters all pumping out tons of CO2 throughout the growing season just to produce a supposedly green alternative to natural gas. At an absolute push, the CI score of biogas maize is maybe +25 gCO2e/MJ which just about limbos under regulatory thresholds, but only if you squint and use generous default assumptions. 

It’s a bit like claiming health benefits from a deep-fried salad. It’s sort of green but not really if we are brutally honest.  

Meanwhile, projects digesting animal manure, food waste, or landfill gas are delivering actual emissions reductions and often net-negative carbon intensities because left untreated, the original feedstock would produce methane in an uncontrolled way which as we know is much more harmful. So, the very act of capturing that methane means you start from a place of negative carbon emissions. This is the Holy Grail of renewable energy, not just reducing the impact of climate change but actually putting it into reverse. Biogas made from manure has a CI score of negative -150 to 300, so every unit of gas reduces the equivalent amount of CO2 in the atmosphere.  

To put this into context, burning fossil fuel gas has a score of +90 and as above, biogas from maize is plus 25 gCO2e/MJ so you can see how much better it is to make gas from manure.  

 RGGOs: Good Start, Needs Work 

This leads us on to the use (or lack of) CI scores in Renewable Gas Guarantees of Origin (RGGOs). As you will recall, RGGOs, or their European cousins, GGOs are certificates awarded to biogas producers for every unit to certify their gas is renewable. These certificates can then be sold to gas wholesalers and industrial corporates to offset their CO2 emissions from burning fossil fuel gas.  Think of them as name tags for molecules on your party guest list that say “Hello, although I’m a gas, I’m renewable!” (as a brief aside, I once went to a posh law firm’s corporate event where my lanyard said “Michael Ware, B List invite” – true story). 

But there’s a problem: GGOs don’t really care how renewable the molecule actually is, as CI scores are not a primary consideration. As long as you breathe in hard and sneak below the threshold, they give the same status to a m3 of biogas from maize as a m3 from food waste. Even though the former produces a lot of net CO2 via fertiliser, burning diesel etc., and the latter doesn’t and actually reduces emissions overall.

Both Infrastructure funds and corporate buyers are starting to notice this and feel a bit uncomfortable. It doesn’t sit too well with your pension fund investors if their £100m flagship green biogas plant is built on a feedstock foundation of hundreds acres of monoculture maize. This maize is regularly drenched with chemicals and tended to by dozens of diesel burning tractors and combine harvesters just so you can claim it’s good for the planet. With a positive CI score, the subsequent RGGOs are just little bits of green paper literally papering over the cracks of your plant’s net CO2 emissions.  

Enter: The Voluntary Carbon Market – Where Carbon Intensity = Cash 

Thankfully, things are a bit better in the Voluntary Carbon Market (VCM). This is where companies buy certificates to offset their emissions and stay within their emissions trading allowance. This is where certificates with negative CI scores become very valuable because they reduce emissions on two fronts. As a result, they are very much in demand and certificates with negative CI scores are much more valuable than their CI indifferent RGGO rivals. This fact is starting to filter down to both plant operators and their investors and we increasingly speak to lots of plant managers who live and breathe their daily negative CI score (obviously metaphorically, not literally) 

In a few short years, the bio methane industry has gone from virtually nothing to over 1,600 gas-to-grid plants and maybe 7,000 gas-to-engine ones. This astonishing growth has been fuelled originally by electricity subsidies (and hence the huge number of gas-to-engine plants) and more recently by certificates. That said, we have to some extent taken our collective eye of the ball and by allowing subsidy chasing CO2 emitting maize based plants to proliferate we have given a lot of metaphorical ammunition to the anti-green gas lobby.  

Subsidy-based schemes for new gas to engine schemes have largely ended although surprisingly the UK and Italy are implementing new gas-based tariff schemes. However, most commentators expect existing tariff schemes to be extended, and this is an opportunity for Governments to bring in CI measurement across the board. 

So, What Needs to Change? Glad You Asked. 

1. RGGOs should have tougher carbon intensity thresholds. Green gas molecules should be genuinely green and not just claim that on their lanyard. 

1. Corporates should be more transparent about carbon intensity – if you claim you are using green gas in your production, be upfront and tell us your CI score. 

1. Carbon intensity should drive subsidies If you save more CO2, you should earn more. 

Final Thought: Biogas – Is it a tool in the energy revolution or just greenwashing?  

Spoiler alert but ultimately Animal Farm is a depressing book. The revolutionary little pigs have big ideas but end up being as bad as the capitalist farmers they displaced. Biogas was for a period in danger of going the same way. Creating CO2 to claim green subsidies doesn’t look much like the original plan and has given the sector a bad reputation with some policymakers and investors. The introduction of certificates with accompanying CI scores is a real game changer and puts biogas at the forefront of the energy revolution. But this will only work if we stop pretending that all feedstocks are equal and start using and publicising carbon intensity as the one metric to assess all feedstocks by.  

In the end, it’s not about what your CH4 molecule is made of, it’s about what it avoids. If we make biogas with a negative CI score and this displaces fossil fuel gas, then we start to actually reverse climate change by a tiny amount with every M3 of gas we produce. The little pigs didn’t succeed in revolutionising agriculture, but we may have more of a chance when it comes to gas.