What Fits the Process

April 11, 2024

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(BDI Bioenergy/Biofuels International) ... When biodiesel producers shift to waste-based material as feedstock, the first choice is often used cooking oil (UCO). The main reason is that UCO’s feedstock parameters are close enough to that of vegetable oil. The result is that there are no, or only minimal changes, to the processes involved. To incorporate low-quality waste oils as feedstock, significant alterations to the current process are necessary, leading to increased investment requirements. In certain instances, a completely fresh process design will be essential, which could involve retrofit units such as additional washing steps or distillation, the establishment of a parallel production line, or the creation of an entirely new standalone production process.

Technology

Yet, for producers of renewable diesel and sustainable aviation fuel (SAF), progress in utilising waste-based feedstock is advancing at a slower pace due to the diverse array of impurities present in the feedstock, posing challenges for downstream processing. Typically, processes involving hydration (such as HVO and SAF) necessitate higher feedstock purity requirements because the catalysts utilised in these processes are sensitive to common catalyst poisons like phosphorus and metals. Therefore, the typical input specification for HVO or SAF process plants are quite restrictive when compared with biodiesel production facilities. Well-known technologies for HVO have limits in phosphors in the single digit ppm range as well as for metals group one and two. In addition, existing facilities for co-processing might not fulfil corrosion protection necessities, which also strictly limits chlorides and acid values in the feedstock.

For biodiesel production, these parameters in wastebased feedstock are not that crucial because most of the production plants are equipped with a final purification step – a biodiesel distillation unit. In this unit, the problematic impurities end up in the distillation bottom and not in the distilled product.

A general overview of the different kinds of feedstock are listed here. Apart from UCO, the most common feedstock is animal fat or waste and residue streams from palm oil production. Feedstock types, which are rarer on the market include trap greases from oil separation systems and acids oils from soap stock splitting processes from large-scale vegetable oil refining. Generally, these types of feedstock can be categorised into different qualities, listed below from typically favourable to typically inferior.

Each feedstock is accompanied by its most crucial and characteristic parameter, which is discussed briefly, regardless of its intended use in biodiesel or SAF production.

Feedstock types

UCO is a common advanced feedstock for renewable fuel producers, as already discussed.

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Since UCO can be a complex mix of vegetable oils and animal fats, typical contents like phosphorous can be up to 50 ppm. Metal content, sodium chloride and other cooking ingredients in UCO can be on the high side. Animal fat has a range of between 5 to 45% FFA depending on the source and time of storage.

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Greases are known by various names and classifications, including yellow, brown, black, and trap grease.
For instance, yellow grease is often synonymous with UCO.

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Due to their diverse sources, they tend to have high levels of moisture, impurities, and insoluble substances (MIU), and the triglycerides within the fats and oils can undergo significant chemical degradation.

These characteristics collectively render greases as low-quality feedstock, often lacking reliable availability in substantial quantities.

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Despite the daunting parameters associated with even the high-quality waste oils mentioned earlier, numerous state-of-theart technologies exist for converting such waste oils into valuable biofuel products. Among these technologies are the BDI RepCAT process, which produces biodiesel, and the BDI Advanced PreTreatment, which prepares the feedstock for subsequent use in SAF production through co-processing or hydration.

Purification

As described above, the prerequirements for feedstock used in SAF and HVO production are very strict.

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From the producer’s perspective, the preferred feedstock will be of lower quality, which implies a lower asking price for the feedstock and, thus, higher margins when selling biodiesel as a product.

The technology was specifically developed to process materials with a high proportion of free fatty acids (up to 99% FFA) and is insensitive to typical impurities. The design of the RepCAT technology makes it a robust choice when producers aim for the lowest quality of feedstock, like acids oils.

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The products are biodiesel according the required norm (ASTM or EN14214) and is an absolutely salt-free glycerin of distilled quality.

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Due to the disproportionate demand for SAF and HVO, coupled with the increased effort required for thorough feedstock purification, higherquality feedstock such as virgin vegetable oil, UCO, and others will be largely absorbed by this market. Consequently, biodiesel producers will primarily have access to lower-quality feedstock types that are generally unsuitable for SAF/ HVO production. This evolving landscape in the feedstock market places added pressure on biodiesel producers to adapt their processes, integrate new lines with appropriate technologies, and maintain profitability while striving to regain a competitive edge. READ MORE