(135a) Fast-tracking SAF production by co-processing in kerosene hydrotreaters

Sustainable Aviation Fuels (SAF) have been at the forefront of the fuel industry's agenda for several years, with increasing attention driven largely by regulatory pressures. Across the globe, from the European Union and the United Kingdom to Japan, Brazil, India, and Singapore, SAF mandates are being implemented, sometimes accompanied by significant penalties for non-compliance, as exemplified by the ReFuelEU aviation initiatives. These mandates are set to significantly accelerate the deployment of SAF production, and projections indicate that, bolstered by such measures, SAF production could reach approximately 13 million tons by 2030, and potentially soar to around 120 million tons by 2050^[1].

Simultaneously, regulators are incentivizing SAF supply through attractive credit schemes. In the United States, SAF suppliers can benefit from Renewable Identification Number (RIN) credits, Low Carbon Fuel Standard (LCFS) credits, Inflation Reduction Act (IRA) production tax credits, and state-based credits from states like Washington, Minnesota, and Illinois.

However, there are significant challenges to overcome to increase the supply of SAF. Production of SAF in standalone plants is capital intensive, and the process of revamping an existing unit or constructing a new one can span several years.

This is where co-processing in a kerosene hydrotreater presents a compelling alternative. Not only is it a low-capital expenditure (CAPEX) approach, but it also allows for the rapid deployment of SAF production within a few months (given the right conditions).

With more than 50 catalyst cycles of co-processing renewable feedstocks in various hydroprocessing units worldwide and with a wide range of renewable feedstocks, Topsoe has accumulated extensive experience in this field.

Kerosene hydrotreater: the preferred choice for SAF production

Co-processing can be carried out in various refinery units, including Fluid Catalytic Cracking (FCC), diesel hydrotreaters, hydrocrackers, and kerosene hydrotreaters. However, the optimal choice often hinges on the principle of selectivity and optimal cost of production: which unit will yield the highest SAF output from the renewable feedstock at the most optimal cost of production?

In this context, a kerosene hydrotreater emerges as the most preferred choice. With the right catalysts, this unit can maximize the conversion of biogenic carbon into the jet fuel fraction, thereby optimizing the production of SAF. Low pressure reactor operation (~25-30 barg) makes co-processing in kerosene hydrotreater option unique with respect to the high recovery of biogenic carbon.

While co-processing in general comes up with some challenges, the limited catalyst volume, lower hydrogen partial pressure, and use of relatively simple metallurgy in a kerosene hydrotreater make this challenge even more interesting.

If your end-product is to obtain SAF certification, there are three main challenges to be aware of when producing SAF by means of co-processing in your kerosene unit. In this section, we will cover the three challenges and their solutions.

It’s crucial for certification to retain the biogenic carbon from the renewable feedstock in the liquid jet fuel product and meet the freezing point requirement. The key differentiator between co-processing in a diesel unit versus a kerosene hydrotreater is the requirement to achieve the product jet freeze point. Thus the first major challenge for kerosene co-processing is meeting the freeze point requirement and keeping the biogenic carbon in the jet fuel product. Jet A and Jet A1 fuels have stringent product specifications which include a requirement for the freezing point to be below -40°C for Jet A and -47°C for Jet A1 (as outlined in ASTM D1655). Even a modest addition of renewable feedstock can significantly increase the freezing point due to the biogenic n-paraffins formed during the oxygen removal. Adding just 1% renewable feedstock can result in an increase of the freezing point by more than 30°C, and adding 5% renewable feedstock can result in a deterioration of the freezing point close to 50°C. To meet the cold flow properties requirements of Jet A and Jet A1 fuels, deep dewaxing of these biogenic n-paraffins is essential.

Freezing point improvement can be obtained through hydrocracking, where the n-paraffins are cracked into smaller molecules. This reaction unfortunately results in high yield loss to gas and naphtha. A much more efficient pathway to lower the freezing point is through hydroisomerization. The n-paraffins are isomerized to iso-paraffins, which have much better cold flow properties without losing any biogenic carbons to the gas and naphtha.

To meet the cold flow properties requirements of Jet A and Jet A1 fuels, deep dewaxing of these biogenic paraffins is essential. Fortunately, this process is made possible by Topsoe’s highly selective dewaxing catalyst, TK-930 D-wax™, which effectively facilitates the necessary deep dewaxing and at the same time retains the biogenic carbon in the jet fuel product. This catalyst has been designed to operate even at reactor pressure as low as 25 barg. To this extent, a highly selective dewaxing catalyst and HDO selective grading serve as a drop-in replacement for existing kerosene hydrotreaters.

The second challenge is removing oxygen atoms from lipidic feedstocks converting free fatty acids and triglycerides into n-paraffins. Two possible reaction routes are available: Hydrodeoxygenation (HDO), leading to water formation, or decarboxylation (DCO), leading to CO₂ formation. The preferred route is HDO, as it maximizes yield of the renewable fraction and retains the biogenic carbon in the jet fuel fraction rather than converting it to CO₂. Topsoe's selective HDO catalyst can increase HDO selectivity to 97%, resulting in a renewable yield that is more than 2 vol% higher than when using traditional fossil catalysts.

The third challenge is the contaminants of the renewable fuel. High level of impurities in renewable feeds i.e., metals, phosphorous, unsaponifiable can be reduced to acceptable levels by a feed pre-treatment step which, among others, includes a degumming and bleaching process. The feed pre-treatment step can be avoided if pretreated renewable feeds can be procured. The feed impurities can be further efficiently handled by a specially developed grading catalyst to minimize deactivation of bulk catalyst, pressure drop related issues to achieve higher unit reliability and availability.

In conclusion, co-processing 5% renewable feedstocks in your kerosene unit can, as far as process compatibility is concerned, be done easily without modifications (aside from changing the catalyst) or with minor modifications depending on the existing unit configuration and design conditions.

Extensive pilot plant testing

By combining Topsoe’s grading, HDO and dewaxing catalysts it is possible to produce SAF by co-processing in a kerosene hydrotreater. This has been extensively investigated in Topsoe’s hydrotreating pilot plant facilities using various renewable feedstocks and fossil kerosenes. The tests were conducted in a once-through, fixed-bed, high-pressure pilot plant setup. At each condition, gas and liquid samples were drawn and analyzed after reaching line-out. Gas and liquid analyses were carried out in order to close mass balance calculations and giving the basis for calculating yields and hydrogen consumption. Furthermore, the feed and liquid products were subjected to a range of analytical methods to determine properties such as freezing point, simulated distillation, biogenic carbon, and density.

Conclusion

Demand for SAF is increasing, driven by regulatory pressures across the world. However, SAF production faces a crucial obstacle: standalone SAF production plants are capital-intensive, and revamps of existing units require a timeline of several years. Under these conditions, it will take years before the supply of SAF can meet the expected demand currently fostered by the mandates.

Co-processing is a fast-track to SAF production that can function as a short-term solution to comply with the upcoming SAF mandates in the next few years. Among various options, co-processing renewable feedstocks in your kerosene hydrotreater is the best choice for SAF production with respect to lower operating cost, lower capital investment, higher recovery of biogenic carbon in the SAF product, short implementation time, and low payback period. This method can save up to two years of construction time compared to building new facilities.

The main operational challenges of using a kerosene hydrotreater as a co-processing unit to produce SAF are the limited catalyst volume, operating temperature limitations, and low hydrogen availability. These challenges can be overcome in most cases, without major modifications in the units, by limiting the amount of renewable feedstock to less than 5 vol%.

Conclusively, producing SAF by co-processing 5% renewable feedstocks in your kerosene hydrotreater will enable the acceleration of SAF production and enable a fast-track to an increase in the current supply.

[1] Sustainable aviation fuel market outlook 2023, May 2023, SkyNRG, SkyNRG's Sustainable Aviation Fuel Market Outlook May 2023 – SkyNRG