Back Unlocking Renewable Natural Gas as a viable feedstock for SAF production

This article was first published by Biofuel Digest and is republished with their kind permission.

By David Minguez, Thomas S. Christensen and Sandra Winter-Madsen, Topsoe Special to The Digest

The aviation industry’s push toward net-zero emissions by 2050 has placed Sustainable Aviation Fuel (SAF) at the center of its decarbonization efforts. While feedstock availability remains a critical challenge, Renewable Natural Gas (RNG) or biomethane is emerging as a promising feedstock. 

RNG, a pipeline-quality biogas, offers the potential to scale SAF production by utilizing established gas-to-liquid (GTL) technologies, such as SynCOR™ autothermal reforming and Fischer-Tropsch (FT) synthesis. This approach combines proven processes with innovative adaptations, creating a high-carbon-efficiency pathway for SAF production. With demand for SAF growing globally and new mandates driving its adoption, RNG could unlock a more sustainable, scalable future for aviation.

Introducing RNG and its potential as a SAF feedstock

RNG (or biomethane) is a purified form of biogas derived from organic waste sources such as agricultural residue, food waste, or wastewater treatment. Upgraded to meet the quality standards of fossil natural gas, RNG is pipeline-compatible and can be used as a direct substitute for traditional natural gas. RNG has great potential to serve as a renewable feedstock for producing SAF due to its renewable origin and compatibility with existing infrastructure.

RNG can either be used locally on-site, primarily as a fuel or for power generation, or injected into natural gas distribution pipelines for broader regional export. For SAF production, pipeline distribution is the more practical option. It enables access to sufficient RNG volumes to supply large-scale production facilities, where economies of scale become viable for output exceeding 2,000 barrels per day of liquid fuels.

RNG can help meet demand

The potential for increased RNG production is significant in both the U.S. and Europe. According to the ICF – American Gas Foundation (2019), the U.S. could produce between 1,500 and 6,500 trillion Btu of RNG annually by 2040. In Europe, the European Commission aims to boost production from 3 billion cubic meters in 2022 to 35 billion cubic meters by 2030.

Meanwhile, SAF demand is expected to grow steadily to meet aviation’s net-zero carbon emissions commitments by 2050. SAF mandates are advancing in the EU and UK, and earlier this year, the U.S. introduced federal blenders tax credit guidance. Other countries, including Japan, Singapore, India, Brazil, Indonesia, and Malaysia, are also making strides to drive domestic SAF adoption. In short, RNG has the potential to play a critical role in supporting SAF production targets, enabling the scale-up needed to meet these ambitious goals.

Alternatives for SAF production from RNG

RNG is a biological feedstock with properties equivalent to fossil-based natural gas, so the first step in leveraging RNG for SAF production involves examining technologies previously used to convert natural gas into liquid fuels (GTL).

The Fischer-Tropsch (FT) process has been the primary technology used in GTL plants at a commercial scale. Two key GTL technology routes have been successfully deployed. Thes first is the integrated Sasol’s GTL technology, as applied among others in Oryx GTL Qatar and Uzbekistan GTL. This uses Topsoe’s low steam-to-carbon SynCOR™ autothermal reforming to convert natural gas and oxygen into syngas, followed by Sasol’s LTFT™ low-temperature FT process in the slurry reactor technology to produce synthetic hydrocarbons like diesel and kerosene (jet fuel).

The second is Shell’s GTL technology, as applied in Pearl GTL Qatar, which converts natural gas and oxygen into syngas through partial oxidation, steam methane reforming, and fixed-bed FT reactors with cobalt-based catalysts.

Current alternatives for SAF production from RNG could be grouped in two main routes. Fischer-Tropsch based process producing jet-fuel as per D7566 A1 and syngas to ethanol to jet based process producing jet-fuel as per D7566 A5.

Approaches

From previous experience, autothermal reforming, such as SynCOR™, is considered among the best technologies for gas-to-liquid (GTL) processes. Operating at a low steam-to-carbon ratio, it produces syngas with the optimal H₂/CO ratio for FT synthesis and supports large-scale single train capacities.

GTL plants have traditionally been developed to operate in remote locations in island mode while maximizing plant capacity. Considerations applied in the gas-to-liquid plants are not necessarily all applicable in an RNG based concept, where reduction of carbon emission and maximizing carbon efficiency is a key aspect of the design. By improving process efficiency and using renewable energy along with reducing carbon-rich off-gases for heating and utility needs, RNG concepts can achieve these goals.

Case study provides proof point

In a recent case study, several process adaptations were implemented to maximize the carbon efficiency of the fuels plant. These types of RNG based facilities are conceptualized at lower capacities and more integrated in existing power and utility networks. This eliminates the need for an island design approach.

A key step change is the ability to utilize carbon-rich off-gases within the fuels production process, which boosts carbon efficiency and reduces the overall carbon intensity of the fuel system.

The SynCOR™ design replicates the layout of a traditional GTL plant while incorporating necessary adjustments by modifying the unit’s operating conditions. As a result, the core design and equipment of an RNG-based fuels plant closely resemble those of a fossil-based GTL plant.

GTL process adaptation

To maximize the utilization of carbon-rich off-gases within the process, the operating steam-to-carbon ratio of the SynCOR™ unit is adjusted to produce syngas with a hydrogen-to-carbon monoxide ratio of 2.0 – adequate for FT processes. Additional external hydrogen could further enhance carbon utilization within the process.

Superheated steam requirements are minimized and generated solely within the fuels unit. All requirements for process steam and preheating purposes are met with saturated steam produced within the unit. This eliminates the requirement for external steam superheaters, thereby reducing demand for carbon-based fuels.

The process also includes the option to recycle carbon-containing streams, such as light-end and naphtha fractions from the upgrading section. Naphtha can be reprocessed in the SynCOR™ unit, boosting syngas production for SAF. Naphtha pre-reforming and reforming are well-established steps for several decades, making them an excellent choice for maximizing SAF output.

Conclusion

The case study demonstrated that adapting the well-established GTL process is highly suitable for producing SAF from RNG. It achieves high carbon efficiencies and offers flexibility in accommodating various RNG compositions. Additionally, the process allows for further carbon efficiency enhancements, such as naphtha recycling into the SynCOR™ unit or incorporating green hydrogen for extended carbon utilization.

The study also revealed that layout modifications for efficient use of renewable RNG feedstock are insignificant, with most adjustments addressed through changes in operating conditions.

Ultimately, autothermal reforming technology like SynCOR™, combined with the Sasol Fischer-Tropsch process, leverages decades of reliable operational experience. This robust foundation significantly de-risks SAF production from RNG by utilizing a well-established approach to fuel synthesis.