The role of natural gas in our everyday lives is evident from examining the world’s energy balance, presented in a helpful format on IEA’s website (https://www.iea.org/sankey/). The total energy production and imports in 2020 was 19.5 billion tonnes oil equivalent, of which 21.7% was natural gas. Through the IEA’s Sankey diagram, one can get a good understanding of how countries use their energy. For instance, the Netherlands, Russia, and the United States use a fair share of natural gas (25%–35%), whereas Sweden and China use a mere 2.5% and 8.5%, respectively (data from 2019). Globally, the final natural gas consumption is 16.4%
Synthetic natural gas, also known as biomethane, renewable natural gas, or green gas, is an alternative to fossil natural gas produced from biogenic carbon. This molecule can currently be produced via three pathways: (1) digestion of wet feedstock (e.g., manure) with upgrading of the biogas to produce biomethane; (2) gasification of a variety of feedstocks, followed by gas cleaning, conditioning, and catalysis to maximize the SNG output; and (3) synthetic production from CO2 absorbed from the air coupled with renewable hydrogen. Each of these pathways has its own level of maturity and typical operational scale.
The Task 33 home page graphic depicts the different pathways from feedstock to end products. As can be seen, the route to SNG can be developed for virtually all feedstocks and involves gas cleaning and conditioning steps, typically followed by catalysis. The steps that need to be taken to reach SNG quality depend heavily on the gasification technology chosen.
In 2018, IEA Bioenergy Task 33 produced a gas analysis guideline report.1 The report deals what analysis is needed when developing gasification technology and highlights the typical impurities that need to be removed from the gas. These impurities, which are also relevant for developing catalysis processes after gas cleaning and conditioning, include:
- Benzene, toluene, and xylene (BTX)
- Sulphur components
- Nitrogen components
Off-the-shelf cleaning technologies are available for all these components. Some combine the removal of more than one of these components at the same time, requiring a thorough understanding of the cleaning process needed to connect to the gasifier and often best supported by engineering, procurement and construction companies. In this field, Wood PLC and Topsoe are two players that have developed their own pathways toward the production of SNG behind a gasifier.
Figure 1 depicts a simplified scheme of the various steps toward the production of SNG. In general, two pathways can be identified:
- Pure syngas platform (CO and H2) after cleaning to be converted into methane, typical for high-temperature gasification approaches. This is part of the commercial approach of SunGas Renewables and Torrgas.
- Product gas platform (syngas and larger hydrocarbons) after cleaning and upgrading to be converted into methane. This is a standard approach for low- to medium- temperature gasification routes because the product gas has a large share of ENGIE, Repotec, and Synova all offer this platform solution.
Starting with the gas obtained after gasification of the residues, the product gas can be upgraded to methane, explained in the following subsections on Wood PLC’s VESTA process and Topsoe’s TREMP process. The homepage graphic depicts the different pathways toward green gas production. It is a generic flow diagram, but it shows that virtually any feedstock can be valorised for downstream catalytic processes.
Figure 3 depicts the VESTA technology developed by Wood PLC.2 It can operate on a wide range of gasification gases, and the upfront cleaning requires a sulphur removal step combined with a clean shift reaction to meet the methanation requirements with the right ratio of CO:H2.
The CO2 removal (to meet grid specifications) takes place at the end of the process. The philosophy is that CO2 and H2O in the gas help minimize the chances of runaway in the exothermic methanation reactors.
The process was developed in close collaboration with Clariant. The first demonstration of VESTA took place in China behind a coal gasifier, and at a later stage Wood PLC developed a project in the UK on a biomass gasifier.
Figure 4 depicts a feasible lineup for Topsoe’s TREMP process.3
The TREMP process includes the upfront sour shift and subsequent acid gas removal system to tune the gas composition for the actual methanation. This means that within the TREMP process there is no thermal buffer in the form of CO2 surplus. This is distinct from the previous technology, where a recycle is employed to mitigate the possibility of thermal runaway.
Figure 5 depicts a possible TREMP layout, which shows the inlet syngas ratio and the recycle over the first methanation section. The second and third reactors are needed to reach full conversion of the feedstock, but the exothermicity is mostly related to the first reactor. By cooling the gas between R1–R2 and R3, the methanation reaction is pushed to the right, resulting in sufficiently low CO and H2 concentration to allow grid injection.
Opportunities to produce SNG via gasification lie in different fields of interest. In short, they can be summed up as:
- Security of supply
- Negative carbon footprint
- Buffering renewable energy
Security of Supply
Gasification technology can convert a broad range of feedstocks into gas, ultimately valorised as SNG. This allows countries to first look into locally grown feedstocks (e.g., forest residues, agricultural waste, sludge, demolition wood, MSW). For some countries, this can already generate a large supply of renewable natural gas. It also allows nations to obtain feedstocks from different regions than fossil feedstocks, creating more freedom in trade. Certain countries use little natural gas, allowing them to trade their feedstock to countries that need more renewable feedstock.
Negative Carbon Footprint
In the process of converting feedstock into methane, the composition of the starting material will lead to the coproduction of carbon. This can be in the form of biochar or CO2. Depending on the technology choice, this storage-ready carbon will be available during the production of SNG.
The potential of this carbon is twofold. First, it can act as a source to be sequestrated, thereby producing negative emissions. This helps various countries reach their CO2 reduction targets. Secondly, the carbon can also be used in the industry—and may be required to in the future. Many fuels and chemicals produced are carbon based and not easily decarbonized. However, through this process they can be defossilized by supplying renewable carbon to be used in “power-to-X” applications.
Buffering Renewable Energy
In line with negative emissions is the option to use renewable energy in the production process. The fact that there is a carbon surplus in the feedstock (or an oxygen presence) means that an SNG plant has a tremendous capacity to incorporate hydrogen obtained from renewables. These two markets are not co-developing at the same rate, but based on renewable energy’s latest trends, a direct or indirect use of the electricity can be included in SNG production.
When comparing the gasification platform to SNG, hydrogen, fuels, and/or chemicals, the similarities become clear. These other outlets are generated through small adjustments in the types of unit operations—primarily at the back end of the value chain. This means that a platform becomes versatile in the future when the core of the equipment can be utilized but the products vary.
Current Commercial Developments
There are several technology developers that offer technology for the production of SNG. Most of them can be found in the IEA Task 33 database.4 The current developments in Europe are such that there is a target for 35 billion m3 of SNG by 2030 in the European grid. In addition, some countries have feed-in obligations, such as the Netherlands with a 2-billion-m3 blend in by 2030. Given these changes, new business cases are being developed based on SNG production. Engie is one of the frontrunners in this field with several developments in the pipeline.
1 IEA Bioenergy. 2018. Gas analysis in gasification of biomass and waste: Guidance report. Document 1. https://task33.ieabioenergy.com/app/webroot/files/file/publications/Gas%20analysis%20report/IEA%20Bioenergy%20Task%2033%20gas%20analysis%20report%20-%20Document%201.pdf
2 Wood PLC. 2023. “VESTA methanation for renewable natural gas production.” Accessed April 26, 2023. https://www.woodplc.com/solutions/expertise/a-z-list-of-our-expertise/vesta-methanation.
3 Haldor Topsøe. 2009. From solid fuels to substitute natural gas (SNG) using TREMP™. https://www.netl.doe.gov/sites/default/files/netl-file/tremp-2009.pdf.