May-2025

SAF production via the HEFA route: chemistry and catalysis

The crucial role of the catalyst system in producing sustainable aviation fuel from waste and residue oils and fats via the HEFA route.

Jaap Bergwerff
Ketjen

Article Summary

Global mandates and incentives are driving efforts to increase the production of sustainable aviation fuel (SAF). Although alternative methods, such as alcohol-to-jet processes and the production of e-fuels, are being developed, hydrotreated esters and fatty acids (HEFA) are expected to remain the primary feedstock for commercial-scale SAF production in the coming years (S&P Global, 2025). Operators of HEFA units are increasingly inclined to switch production from renewable diesel to SAF.

Several approaches can be employed to convert oils and fat streams into products that meet the specifications for the SAF component, which can be blended with conventional kerosene. These approaches are illustrated in the schematic process schemes in Figure 1. One approach involves mixing the renewable feedstock with a conventional kerosene fraction to be co-processed in an existing kerosene hydrotreater, producing aviation fuel with a certain renewable content that meets JET A-1 specifications. This approach requires (i) hydroprocessing of the kerosene fraction to remove nitrogen and sulphur compounds, (ii) hydrodeoxygenation (HDO) of the triglycerides, and (iii) dewaxing to reduce the freezing point of the final product, all within a single reactor under the same operating conditions. This constraint typically limits the amount of renewable feedstock that can be co-processed to no more than a few per cent.

Alternatively, standalone processes can be used to convert oils and fats into a SAF stream that can be blended with conventional kerosene. In any case, HDO and subsequent hydroisomerisation (HI) of the n-paraffins generated are required. Two types of processes can be distinguished to produce 100% hydrogenated vegetable oils (HVO) based on the composition of the gas stream in the HI reactor. In sour mode or single-stage operation, the conversion of triglycerides to linear paraffins and their isomerisation are carried out without intermediate gas purification. Alternatively, in sweet mode or two-stage operation, HI is essentially carried out in the absence of NH3 and H2S in a separate reactor, allowing the use of highly selective and active noble metal catalysts. This configuration enables a high degree of isomerisation, making it ideally suited for on-purpose SAF production with high SAF yields.

Regardless of the process used, the challenge of producing SAF from waste fat and oil streams remains the same, as illustrated in Table 1. This table compares the typical properties of a hydrogenated triglyceride stream produced after the HDO step with the specifications for SAF as a blending component and the final aviation fuel. It is evident that a significant shift in both boiling point and freezing point must be achieved in the HI step to produce in-spec SAF.

Hydrodeoxygenation
With waste oil and fat streams, such as used cooking oil and animal fats, replacing relatively pure vegetable oils as feedstock for the HEFA process, the management of heteroatoms during the HDO step has become increasingly important. Regardless of the mode of operation, inorganic impurities, such as phosphorus (P) and metals, which can be present in significant concentrations in waste feedstocks, can severely deactivate the HDO catalysts, resulting in unstable operation and short cycles. Therefore, it is critical that these components are effectively removed. Dedicated guard catalysts provide the delicate balance between (i) activity to decompose the organic compounds containing P and metals, and (ii) accessibility and pore volume to allow for optimal uptake capacity for these contaminants throughout the cycle.

Figure 2 illustrates the evolution of the ReNewFine catalyst portfolio for P-trapping guard-bed catalysts. As shown by the P-profile over the catalyst extrudates obtained via scanning electron microscopy (SEM), each generation of guard bed catalysts has effectively utilised a larger fraction of the extrudate for trapping P, thereby drastically improving P-uptake capacity. Currently, ReNewFine 102 represents Ketjen’s latest generation of guard catalysts especially developed for HEFA applications. Notably, the core of the catalyst extrudate, where only a low concentration of P is deposited, represents just 24% of the catalyst volume, indicating that the effective use of the catalyst is nearly 76%.

Another important aspect of an HDO catalyst is its ability to selectively remove oxygen via the hydrodeoxygenation pathway, splitting off oxygen as H2O instead of CO or CO2 during the HDO step. Besides reaction conditions, such as temperature (T) and pressure (P), the HDO selectivity is determined by the catalyst applied. The use of a selective HDO catalyst minimises the formation of CO, which can cause issues in off-gas handling and results in maximum carbon yield as valuable products. This HDO selectivity can be derived from the ratio of C17 and C18 n-paraffins in the HDO product since fatty acid chains with an uneven C-number do not naturally occur in biogenic triglycerides.

Ketjen’s ReNewFine 204 catalyst was designed to deliver optimal HDO selectivity. As illustrated in Figure 3, compared to previous-generation HDO catalysts, a 15% higher selectivity can be achieved using ReNewFine 204 over a broad temperature range, which translates to almost 1% higher carbon yield in the final product. This catalyst provides supreme stability towards the deposition of phosphorus (P), ensuring that the high HDO selectivity can be maintained throughout the HDO cycle. By targeting the use of this catalyst grade in the reactor zone where triglycerides are converted, a small layer of this catalyst is required to improve the HDO selectivity of the entire reactor loading.

N-compounds, such as fatty amines and amides, form another class of heteroatom compounds that require special attention in the HDO reactor. If N-compounds are not effectively removed by the HDO catalyst system, this may significantly lower the performance of the downstream HI catalyst. This is particularly critical when the goal is to produce SAF, which requires deep isomerisation. N-slip from the HDO reactor can lead to sub-optimal performance. While deep hydrodenitrogenation (HDN) is important, care must be taken to prevent the oligomerisation of n-paraffins in the final zone of the HDO reactor, as this would result in the formation of high molecular weight compounds that can severely impact the properties of the final product.

Considering the range of different reactions that need to be catalysed in the HDO reactor, it is inevitable that an optimal HDO reactor loading consists of several different HDO catalysts, each with a specific function. For this reason, Ketjen has developed its ReNewFine portfolio, which includes guard bed catalysts (ReNewFine 100 series), HDO catalysts (ReNewFine 200 series), and HDN catalysts (ReNewFine 300 series). By combining the catalysts in this portfolio, an optimal ReNewSTAX reactor loading can be designed for each HDO unit, tailored to the unique operational requirements to ensure minimum N-slip and oligomerisation, as well as maximum cycle length.