Category: biogas

Methane – an origin story

Emissions from fossil fuels or biological sources, a decrease in the atmosphere’s ability to break down methane or something else? The emissions work of the SGI relies on data that uses advanced chemical science to enable the identification of methane from different sources. In part one of a two-part blog, Dr Semra Bakkaloglu from our emissions team, explains the atomic-level forensics that scientists use to determine where methane came from.

The SGI’s work on methane emissions is important for understanding how governments and industries involved in energy supply chains can reduce emissions of this very significant greenhouse gas (GHG).  

Our research focuses on methane emitted from natural gas, biogas and hydrogen supply chains. These are anthropogenic sources of methane; neither would be a significant problem without the human ingenuity that created their supply chains. To do our research, we need to be sure the sampling and monitoring data we use has distinguished correctly between naturally occurring methane such as that from wetlands – and anthropogenically induced emissions such as from livestock farming and energy supply chains.

But surely methane is just methane, right? CH4 – an odourless, invisible, flammable gas made up of one carbon atom, bonded to four hydrogen atoms. Where do you start to tell the difference between methane that has bubbled up from the swampy depths of decomposing wetland and fossil methane that has propelled itself to freedom through a leaky flange in a 50-year-old natural gas pipeline?

Fig 1 A plausible CH4 emissions budget ca. 2015 based on Lan et al., 2021 that is in agreement with both observed atmospheric CH4 and 13CH4 (given in Fig 2). Atmospheric methane grew by 10 ppb in 2015 implying a global atmospheric chemical sink of 548 TgCH that year (Lan et al., 2021; NOAA, 2021 Picture taken from NOAA, 2022)

To calculate that – and arrive at a chart such as fig. 1 – the answer lies in the number of neutrons. Methane isotopes are variations of the methane molecule that have a different number of neutrons in their atomic nuclei. Tracing the sources and dynamics of methane in the environment involves using the specific characteristics of these isotopes to identify where the methane came from and how it is behaving. This is done by analysing the ratios of different isotopes in a sample and comparing them to known ratios from each specific source.

The most common methane stable isotope is carbon-12 (C12), which has 6 protons and 6 neutrons, and carbon-13, the heavier isotope; as well as hydrogen isotopes such as H-1 or H2 (deterium, denoted as D). Each CH4 source exhibits different partitioning of light (12C or H) and heavy (13C or D) isotopes of carbon and hydrogen, so 13C/12C or D/H. This provides the “DNA” for us to determine the source.

By carefully calculating the ratio of the most prevalent form of carbon, carbon-12, and its stable isotope, carbon-13 (13C), which has an extra neutron, we can determine the origin of the carbon in a methane sample.

13C is a tiny bit heavier than 12C.  Methane produced by microorganisms in environments such as wetlands, landfills, and agriculture has very little 13C (Bakkaloglu et al. 2022), so the signature of microbial methane is “lighter” than methane produced by fossil fuels, such as natural gas. Livestock methane is created as a by-product of digestion in the stomachs of ruminants, such as cattle, sheep, and goats. The isotopic composition of livestock methane is also dependent on the diet of the animals, but it is generally similar to that of biogas methane, with a lower ratio of 13C to 12C compared to fossil fuel methane.

By analysing the methane isotopic composition, scientists can determine the relative contribution of different sources to the overall methane levels in the atmosphere and track how methane is distributed and transformed over time, whether it is formed biogenically (agricultural, waste or wetlands), thermionically (natural gas, oil or coal mining) or pyrogenically (incomplete combustions, such as wildfires).

Scientific isotopic methane data provides an essential fingerprint without which we’d be unable to carry out our work on understanding emissions.  If we can understand the source of methane, we can develop better mitigation strategies to tackle it.

Fig 2. Globally averaged atmospheric CH4 (top) and the trend in the ratio of the isotope carbon-13(bottom) from samples collected by NOAA’s Global Greenhouse Gas Reference Network. The blue curves are derived from weekly data and the black curves are annual means. The data from 2020 are preliminary. Credit: Xin Lan, NOAA Global Monitoring Laboratory(Lan et al. 2021; NOAA,2021) 2021

But there’s still some controversy.  Fig 2 shows the trend in atmospheric methane concentration (top) and  Carbon-13 ratio.  It is clear that whilst overall methane levels in the atmosphere are rising +100ppb in 20 years, (NOAA, 2023) the ratio of C-13 is falling. Schaefer et al. 2016 suggested that the methane isotopic signature is shifting from fossil fuels to biogenic sources in the 21st century. So the question is – is this because we have reduced fossil fuel-related methane emissions (Schwietzke et al. 2016), or has there been a greater  increase in biogenic methane emissions or the changes in sink mechanism or both? According to the IEA (2022), methane emissions from fossil fuels increased by close to 5% in 2021, and Menoud et al (2022) found that fossil fuel-related sources could have more depleted values than previous estimates used in global models. But there is still large uncertainty coming from natural sources. We should take into account both each source methane emission and their isotopic signature to better understand the global methane budget.

We’ll look at that in Part 2.

Biogas and biomethane emissions – a quick win for decarbonising future energy systems?

In our latest blog, Dr Semra Bakkaloglu reflects on our newly published research into biogas and biomethane emissions.

Over 100 years, methane has 27.2 times the global warming potential of carbon dioxide. So when it comes to climate change, it’s pretty potent stuff. Methane concentrations in the atmosphere are increasing, which makes it a quick-win target in the drive to decarbonise our global energy system.

It’s no coincidence that over 100 countries signed the Global Methane Pledge at COP26 in Glasgow. The pledge is a commitment to reduce methane emissions by 30% (from 2020 levels) by 2030. But a 30% reduction is ambitious, not to mention a complete reversal of the current trajectory of increasing methane emissions. Achieving that will require a combination of measures, which include reducing emissions from the existing supply chain whilst also reducing reliance on natural gas as a fuel by switching to cleaner energy alternatives, such as electrification or hydrogen.

But those new energy alternatives are at wildly different stages of technological readiness and require major infrastructure changes and investment. They are unlikely to happen overnight – and many are unlikely to become mainstream within the next 8 years.

One sector that is expected to grow and contribute to decarbonisation in that transition period is biogas and biomethane – a mixture of gases (mostly methane (CH4 and carbon dioxide (CO2)) produced from biodegradable materials. It’s a technology with a lot of factors in its favour: the volume of organic waste – known as feedstock – generated by modern societies is increasing, it provides a beneficial alternative disposal method for that waste, and conversion of energy from waste to biogas can begin to replace fossil fuel gas – which in turn reduces overall greenhouse gas (GHG) emissions. Ultimately it contributes to meeting those government commitments.

But biogas and biomethane production can also emit methane and it’s an area that’s been lacking in research. No study has assessed methane emissions from the biogas and biomethane supply chain – until now.

For our recent study, we synthesised methane emissions from the biogas and biomethane supply chains by breaking down stages and identifying key elements from direct measurements studies. We used a statistical model Monte Carlo approach to estimate aggregate methane emissions with uncertainty assessment, which can account for up to 343 g CO2-eq. We observed that biogas and biomethane supply chains exhibit similar emission characteristics to oil and natural gas with super-emitters present at all stages. In our study, 62% of the emissions come from just 5% of the point sources – these super-emitters waste a disproportionately large amount of methane

“..62% of the emissions come from just 5% of the point sources – these super-emitters waste a disproportionately large amount of methane.”

The International Energy Agency’s (IEA) inventory, (the only other benchmark data currently available) estimated total methane emissions from bioenergy to be 9.1 Tg in 2021.

Our study, which only looked at one aspect of bioenergy (biomethane), discovered that methane emissions are more likely to be in the range of 6.4 – 7.8 Tg per year (95th percentile), but the average methane emissions are around 2.8 Tg according to the IEA’s global biogas and biomethane generation rate 1.47 EJ in 2018. If biogas and biomethane production are expanded to the same scale as the oil and natural gas industries and no action is taken, they could emit almost 4 times as much methane as oil and gas supply chains (82.5 Tg in 2021). At present, our results indicate they are high – higher even than natural gas, which is clearly a worry.

Considering the latest IPCC-AR6-WG3 climate change mitigation scenarios that achieve the Paris Agreement 1.5°C temperature rise mitigation target with low or no overshoot, we can see that biogas and methane could have up to 28 EJ of production by 2050. Using our mean emissions rate (52.3 g CO2-eq per MJHHV) from this study, this would result in 53.8 Tg methane emissions. This drives home the point that it is extremely important to reduce biogas and biomethane emissions for these energy sources to play a constructive role in our future energy system.

Finding and removing these large emitters is a critical step toward significantly reducing overall emissions from biomethane and natural gas supply chains. It’s not just about controlling greenhouse gases either. There’s a significant economic argument for addressing emissions – all that lost gas has a commercial value.

According to the European Biogas Association (EBA), biomethane can be produced for as little as €55 per MWh, while natural gas costs around €80 per MWh. When our findings are combined with the cost of biomethane, we can calculate that emitting 2.8 Tg of methane per year in average (based on the IEA’s global biomethane and biogas generation rate for 2018) can result in an average a global economic loss of 2.4 billion euros in average.

“… emitting 2.8 Tg of methane per year can result in a global economic loss of €2.4billion in average.”

Through improved design, detection, measurement, and repair techniques, much of the observed emissions can be avoided. If we focus on super-emitters, there are some potential quick wins too. We found that the digestate stage and upgrading units need the most attention in this regard. There’s a lot of overlap with oil and natural gas supply chains too – preventing gas venting, reducing flaring activities and designing a closed unit with a vapour recovery system can all contribute to reducing emissions.

Additionally, we need better regulations, continuous emission measurements, and close collaboration with biogas plant operators in order to address methane emissions and meet the Paris Agreement temperature target.

We know what we need to do to tackle those emissions; the important thing is to get started right away. Biomethane is an important renewable energy source, but it could be even better! Combating biomethane emissions is not only significant for meeting Paris Agreement’s target but also boosting the global economy.