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Second and third-generation renewable natural gas, produced using advanced technologies such as biomass gasification, CO2 capture and power-to-gas, is part of the renewable energy mix of the future. In 2018, the European Union decided to limit the use of first-generation biofuels in transport, such as ethanol, and increase the amount of 2nd and 3rd generation biofuels required. The United States could follow suit in 2019. The potential of this renewable energy is great. For example, the biomass used, such as algae and straw, is available in larger quantities and allows large-scale production of RNG able to meet the energy needs of transport sector, for example. In addition, it helps producers avoid “food versus fuel” debate, which happens because of energy crops. What is 2nd and 3rd generation RNG? What technologies are available?
The 2nd and 3rd generation RNG differs from 1st generation RNG by the kind of biomass and production technologies used. However, in all cases, injection into the gas network is possible if the RNG produced meets the requirements.
Here is a summary table:
2nd generation renewable natural gas is made with dry biomass. This kind of biomass includes lignocellulosic materials such as wood, straw or paper products. In contrast, resources used to produce 1st generation RNG are waste, agricultural materials or sludge from wastewater treatment plants.
Second-generation RNG is produced by methanation, a synthesis reaction obtained by combining a catalyst, methane (or CH4) and dihydrogen (H2). It can be either chemical methanation with power-to-gas system or biological methanation that can be added as a polishing step after an anaerobic digestion.
Third-generation renewable natural gas is made with algae biomass. Producers can transform cultivated microalgae into RNG through high yielding photosynthetic reactors, natural light, water and minerals.
Microalgae cultivation is seen as a sustainable solution for long-term RNG production because of its high growth potential and its ability to capture CO2.
Note, however, that these technologies are still in the development and research stage. In particular, efforts are being made to make large-scale production more profitable and to reduce seasonal constraints.
Read this recent article from GRT Gaz (In French) to find out more about it.
Already, many advanced technologies are being tested and improved to enable the production of 2nd and 3rd generation RNG.
Biomass gasification is a particularly valuable technology to treat dry waste materials, such as forest waste wood or municipal construction waste.
This process involves heating the biomass at temperatures ranging from 850 to 1300 ° C in various fluids. It produces a mix of gas made up of carbon monoxide, methane and nitrogen, which then undergo a methanation process. This last step aims to produce a synthetic natural gas, which is subsequently purified to remove the tar from the mixture.
Biomass gasification can produce 210 m3 of RNG per tonne of wood. The marketing of this type of energy could be carried out as early as 2025.
Several ongoing projects aim to produce RNG by biomass gasification, including:
It is the largest semi-commercial plant that mostly uses wood pellets. This project has a capacity of 20 MW, but it was shut down and is currently looking for a buyer.
This project aims to develop the best approaches and practices for biomass gasification and methanation by testing different feedstocks such as straw and forest waste, all mixed with 50% wood chips. Investments amounted to € 60 million. Visit their website for more information.
RNG production through CO2 capture is a form of power-to-gas. Through an electrochemical process, the captured CO2 is separated into carbon monoxide and dioxygen.
Subsequently, carbon monoxide is combined with dihydrogen, created from electrolysis of water. This allows water to be separated into hydrogen and oxygen. These gases react to this methanation process and produce synthetic natural gas.
While this technology is promising, professionals are still looking for ways to make it more competitive in comparison with other energy sources. Indeed, several factors should be taken into account to produce synthetic natural gas at a reasonable cost: the low price for the acquisition of renewable electricity and for the capture of CO2.
The production of biomethane by CO2 capture is, however, only one of the possible applications of this technology, which increases the competition between them.
This power-to-gas process is also based on the transformation of electricity into hydrogen through the electrolysis of water. Subsequently, hydrogen undergoes a methanation process to convert into synthetic natural gas.
The difference? The CO2 used is captured, this time, through the biogas upgrading process. In addition, surpluses of renewable electricity make it possible to produce the necessary hydrogen.
Take a look at this UniPer diagram, which illustrates the available possibilities for producing RNG using power-to-gas combined with methanation and biomass gasification.
Several projects demonstrate and test the potential of this kind of power-to-gas. You can consult the list of all current projects in Europe on the European Power to Gas website.
However, here are some examples of promising projects:
Managed by the Viessmann group, the plant produces methane through a biological process using wind and solar renewable electricity surpluses. The goal is to inject the product gas into the network. For more information, visit the organization’s website.
This project aims to demonstrate the potential of an efficient power-to-gas process that also serves as a means of energy storage. It is co-financed by the European Union. For more information, see the project website.
Launched in 2018, the GRHYD project is the first power-to-gas demonstration plant in France. It aims to test injection of hydrogen produced from renewable electricity into the natural gas network. Its goal is also to produce hytane, a mixture of natural gas and hydrogen. Take a look at Engie’s website for more information.
The 2nd and 3rd generation RNG stands out as a sustainable green energy with high potential, able to meet the energy needs of the population, especially in the transport sector.
Many benefits derive from the use of this RNG:
The circular economy favors loop consumption, which is more sustainable. The production of 2nd and 3rd generation RNG and the use of technologies presented above are anchored to this approach.
For example, cultivated microalgae could act as a biogas upgrading system by absorbing CO2. We could then convert it into RNG. Moreover, as mentioned, the surplus of renewable electricity can be converted into hydrogen during a power-to-gas process.
Some technologies, including power-to-gas, are seen as ways to store renewable electricity in the long run. In fact, electricity collected can be converted into RNG and, then, injected into the gas network.
This is an issue that will become crucial: renewable energy tends to be weather dependent and intermittent. Energy storage by converting into RNG could be one of the solutions to this problem.
Some experts argue that companies should favor RNG produced from non-biological sources, such as power-to-gas, or algae. They want to limit the land allocated to energy production and avoid competition between energy crops and food farming.
Industry needs to keep an eye on the latest advanced technologies available in RNG production, including 2nd and 3rd generation. Indeed, energy transition is moving forward, but the energy needs of countries and cities are far from diminishing. We must, more than ever, open the door to new options for green energies, or even seek to multiply them.
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