Co-processing of bio-based feedstocks in the FCC unit
Using bio-based feedstocks as a decarbonisation lever is one of the most effective methods to address Scope 3 downstream emissions.
Bob Riley, Stefan Brandt and Kenneth Bryden
W. R. Grace & Co.
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While the landscape varies around the world, factors including societal, regulatory, and investor pressures have created a driving force for the decarbonisation of the refining system. When the potential for systematic changes to refining was recognised decades ago, W. R. Grace & Co. began research activities to support refiners as the energy transition unfolds. One of the areas of particular interest for many refiners has been the processing of bio-based feedstocks in existing operating units, including the fluid catalytic cracker (FCC).
Processing bio-based feedstock is a particularly interesting option for many refiners, as it is one of the few viable options to address the largest part of the refining system’s emissions — Scope 3 downstream emissions. For refiners, Scope 3 downstream emissions are those created from the products in use — which in many cases are combustion emissions associated with various forms of transportation. By far, this emissions scope represents the largest share of emissions in a life cycle assessment of petroleum refining.
What are bio-based feedstocks?
While many often refer to bio-based feedstocks in tandem or in conjunction with other renewable resources, there are important differences to note. The carbon in bio-based feedstocks is sourced from the environment as part of the biogenic carbon cycle (International Energy Association, 2022). Compared to carbon from traditional fossil-based sources, carbon from biogenic feedstocks is not accretive to global atmospheric carbon inventories. This way, when fuels based on biogenic carbon feedstocks are produced and consumed, global CO2 inventories are not increased. Figure 1 illustrates the difference between biogenic carbon cycle sources and fossil-based sources.
A large variety of materials can be used to create sustainable and renewable feedstocks for refinery application. To support our refining customers, Grace has evaluated many possible feedstocks for their suitability in existing refining processes as either a feed blend component (co-processing) or, in some cases, a pure feedstock stream. Although the most popular feedstocks for refinery use include lipid-based seed oils and animal fats, many alternate feedstocks sourced from materials listed in Table 1, and converted through a variety of processes, have been considered by refiners.
Motivation for processing bio-based feedstocks is largely driven by ambitions to control global CO2 inventories. These ambitions can be embodied in regulatory frameworks (such as the European Renewable Energy Directive [EU RED-II], US Renewable Fuel Standard [US RFS], or California’s Low Carbon Fuel Standard [CA LCFS]), driven by corporate strategic directives, or the result of stakeholder engagement initiatives with refiners. Some of these frameworks can provide a financial return (often in the form of credits, which are used to demonstrate compliance and can be traded to other market participants).
Recently, there has been an increase in the use of recycled carbon fuels. The best example of these types of feedstocks is oil derived from hard-to-recycle plastics or other hydrocarbon-based materials that cannot be recycled. The US EPA’s Waste Hierarchy dictates that avoidance of waste, by first reuse/recycling and then energy recovery, is preferred to disposal, and care must be taken to ensure recycled carbon feedstocks for refinery use are sourced from materials that cannot be easily recycled via other means (Sustainable Materials Management: Non-Hazardous Materials and Waste Management Hierarchy, 2022).
Recycled carbon feedstocks serve important societal purposes: to increase recycled content in the plastics value chain and reduce plastic waste, while at the same time reducing the amount of fossil fuels required. However, they do not carry the same decarbonisation benefit that bio-based feedstocks carry in the fuels value chain. Instead, these feedstocks act more like fossil resources with respect to their contribution to global emission inventories. So, while processing recycled materials is an important future role for refining, it is important to differentiate recycled carbon feedstocks from bio-based feedstocks with respect to the impact on global emissions inventories.
What is necessary to co-process bio-based feedstocks?
Generally, bio-based feedstocks introduce new challenges in the refining system. There are several possible unit operations in a refinery where bio-based feedstocks can be introduced, but the FCC unit has received significant attention. Table 2 presents representative properties of a selection of renewable feedstock types that Grace has received from refiners, along with a comparison to typical FCC feedstock. The primary difference to note between the renewable feedstocks and traditional FCC feedstock is the high level of oxygen content found in many of the renewable feeds. However, beyond the challenges that come with high oxygen content, many of the renewable feedstocks contain elevated and highly variable levels of metals. Both the overall contaminant level and the lot-to-lot variability can create challenges in refining operations.
The FCC is especially well suited to handle this level of variation in properties due to the flexible nature of the process. The continuous replacement of catalyst offers a method to manage metals contaminants that is not available in other refining processes, and the ability to adjust operating conditions continuously to optimise against constraints makes the FCC an attractive place to consider co-processing of renewable feedstocks. Finally, the FCC process is known for its high degree of deoxygenation without the necessity for separate hydrogen addition, which means that the renewable fuels produced in the FCC have the potential for very low carbon intensity.
However, not all feedstocks are easy to co-process in the FCC, and the FCC is not the only unit in the refinery used to co-process bio-based feedstocks. To help refiners determine the optimum processing scheme, Grace has developed a multi-stage advanced evaluation protocol to determine the suitability of new feedstocks, and we have applied this protocol across a wide variety of challenging bio-based feedstocks. Our protocol evaluates the physical and chemical compatibility of new bio-based feedstocks with existing FCC feedstock streams and determines the FCC yield potential of the new feedstocks. Grace achieves the latter using pilot-scale testing units, up to and including our Davison Circulating Riser (DCR) Pilot Plant (see Figure 2).
The DCR Pilot Plant offers an excellent set of capabilities for the evaluation of bio-based and alternative feedstocks. (Bryden, Weatherbee, & Habib, 2013). The DCR Pilot Plant is a continuously operated pilot plant with riser cracking, stripping, and regeneration capabilities, which closely simulates commercial FCC unit yields (Bryden, Predicting FCC unit performance with laboratory testing, 2013). Grace has licensed the DCR Pilot Plant technology to over 30 licensees, making it the leading commercially available technology for detailed evaluation of FCC dynamics on the pilot scale. Additionally, Grace has undertaken specific modifications to both the DCR and other technologies in our portfolio to enable these evaluations.
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