The Sustainable Gas Institute, an international research hub based at Imperial College London, recently launched a new white paper entitled ‘A Greener Gas Grid: What Are The Options?’. The paper examines the gas grid and explores the ‘low-carbon’ alternatives for these extensive networks. Erin Johnson, a research postgraduate in the Department of Chemical Engineering and an expert in ‘green gas’, discusses the paper’s findings and how research can help green the gas grid.
The existing natural gas grid delivers flexible and affordable energy to consumers, and is especially important for heating. This is important in the UK, which has one of world’s best-connected gas networks; and where gas demand doubles in winter to heat four out of five homes. Gas is much cheaper than electricity, which provides only half as much energy at three times the price. However, the natural gas that flows through the grid today is a fossil fuel, and its supply chain and combustion contribute to global greenhouse gas emissions (estimates range from 230-318 gCO2eq./kWh).
But, the gas grid could deliver low-carbon biomethane or hydrogen gasses to meet the fluctuating heat demands of millions of consumers, and therefore play an important role in the transition to a low-carbon energy system. Here are five ways to bring about a low-carbon gas grid:
1. Use less, waste less
Methane, the main component of natural gas, has a global warming potential 34 times higher than carbon dioxide (over a 100-year period), so even small leaks from pipework have a large impact. The UK is undergoing a programme to replace old iron pipework with new plastic pipes to reduce leaks, thereby improving safety and environmental performance. The new pipes are also suitable for transporting hydrogen, which could be used in future.
The amount of gas used can be further reduced by improving efficiency and building insulation. Typically, modern condensing boilers use 8.3% less gas than 10-year-old models. Alternatively, hybrid systems can provide most heat by an electric heat-pump, only using gas to top-up the temperature on very cold days.
2. Switch from natural gas to biomethane
Natural gas is a fossil fuel which, upon burning, releases carbon into the atmosphere that had been locked underground for millions of years. Biomethane is made from biological matter, or biomass, containing carbon that was recently circulating in the atmosphere; burning it can be considered carbon-neutral because of the short time-scales involved before that carbon returns to the atmosphere. However, there are environmental implications for growing biomass, and estimates of the emissions associated with the biomethane supply chain vary widely (ranging from -50 to 450gCO2eq./kWh, although most are under 100gCO2eq./kWh).
Biomethane can be used and transported in the same way as natural gas. It is produced by anaerobic digestion of biomass, such as sewage or maize. In the UK, there are 85 biomethane plants connected to the gas grid, while another 466 plants produce and burn unrefined biomethane for heat and power, providing energy to power one million homes. The industry is growing quickly – production has grown 18% since 2016. The first UK commercial-scale demonstration plant producing biomethane by ‘gasification’ of household refuse is due to start operating next year; this technology can make biomethane from more types of biomass including wood.
3. Blend natural gas with hydrogen
When hydrogen burns it releases only water vapour, so using it emits no greenhouse gases. However, estimates for supply chain emissions vary widely depending on the production method (from -371 to 642 gCO2eq./kWh). One low-carbon method uses renewable electricity to produce hydrogen by electrolysing water (25 to 178 gCO2eq./kWh).
Blending small quantities of hydrogen with natural gas or biomethane would reduce the carbon intensity of the gas grid, but is not yet allowed in the UK as gas quality regulations were designed before it was considered. However, the HyDeploy project will be demonstrating how much hydrogen could be safely added, without having to make changes to the network or home appliances.
4. Convert the grid to use pure hydrogen
There is potential for some parts of the gas network to use 100% pure hydrogen gas, as shown by the H21 Leeds City Gate project. This could provide extremely low-carbon gas at much larger scales than could be achieved with biomethane and hydrogen blending. However, to achieve this, home appliances would have to be replaced with ones that run on pure hydrogen.
The current technology used to manufacture hydrogen on a large scale involves converting methane and steam into hydrogen and carbon dioxide. To be low-carbon, these plants would have to be developed with carbon capture and storage (CCS) or carbon capture and utilisation (CCU) infrastructure to prevent carbon dioxide being released into the atmosphere (net emissions range from 23 to 150 gCO2eq./kWh).
5. Combine bio-hydrogen with carbon capture
Bio-hydrogen can be created from gasified (partially combusted) biomass, with the resulting carbon dioxide captured and stored or used. This solution is a type of ‘bioenergy with carbon capture and storage’ (BECCS), and has the potential to remove carbon dioxide from the atmosphere – atmospheric carbon is absorbed by growing biomass, then captured and stored permanently (one estimate predicts net negative emissions of -371 gCO2eq./kWh). Although it remains to be seen whether this is viable at the scales required, it is one of very few options with the potential to reverse the upward trends in atmospheric carbon dioxide levels.
These green gas options use existing infrastructure to decarbonise heat and do not depend on individual consumer decisions. As such, they could be implemented quickly and have a substantial impact on carbon emissions. Our new white paper, A greener gas grid: what are the options?, was launched at an event last week. It focuses on the technical potential, carbon intensity and costs of using biomethane and hydrogen. Carbon intensity and costs vary widely for all the options and there may be no single ‘best’ option for decarbonising the gas grid. Despite this, gas networks have the potential to play an important role in decarbonising the future energy system.
 gCO2eq./kWh – grams of carbon dioxide equivalent greenhouse gas emissions per kilowatt-hour