Introduction: The Strategic Role of Co‑processing in Decarbonizing Refineries

The global refining industry faces mounting pressure to reduce its carbon footprint while maintaining profitability. Co-processing biofuels with petroleum products offers a pragmatic bridge between existing fossil‑based infrastructure and a lower‑carbon future. By blending renewable feedstocks such as vegetable oils, animal fats, and waste greases directly into crude oil or intermediate streams, refineries can produce drop‑in fuels that meet current specifications without building dedicated biorefineries. This approach leverages capital already in place, reduces greenhouse gas (GHG) emissions, and helps refiners comply with tightening environmental regulations. Recent innovations in catalysts, process control, and feedstock pretreatment have significantly improved the efficiency and scalability of co‑processing, making it one of the most promising near‑term strategies for integrating renewables into the fuel supply chain.

Understanding Co‑processing: Principles and Integration

Co‑processing is the simultaneous conversion of bio‑based oils and petroleum fractions in existing refinery units. The core principle is that bio‑feedstocks contain oxygen, nitrogen, and other heteroatoms that must be removed to produce hydrocarbon molecules compatible with conventional fuels. This removal occurs through hydrogen‑addition reactions (hydrodeoxygenation, hydrodenitrogenation) or carbon‑rejection reactions (cracking, coking). The choice of unit and operating conditions determines the yield, product slate, and catalyst life.

Feedstock Types and Pretreatment

Common co‑processing feedstocks include virgin vegetable oils (soybean, rapeseed, palm), used cooking oils, animal tallow, and algae oil. Each feedstock brings a unique fatty acid profile, oxygen content, and impurity level. Pretreatment steps such as degumming, bleaching, and esterification are often necessary to reduce contaminants like phosphorus, alkali metals, and free fatty acids that can poison catalysts. The US Department of Energy’s Bioenergy Technologies Office provides guidelines on acceptable feedstock quality for refinery integration. Recent research also explores lignocellulosic biomass pyrolysis oils, though their high oxygen content and acidity pose additional challenges.

Refinery Unit Operations for Co‑processing

Two primary refinery units are used for co‑processing:

  • Fluid Catalytic Cracking (FCC): Bio‑oils can be co‑fed with vacuum gas oil into the FCC riser. The catalyst’s acidity promotes cracking of triglycerides and fatty acids, producing gasoline‑range hydrocarbons. However, oxygenated intermediates can lead to increased coke formation and catalyst deactivation.
  • Hydrotreating: Vegetable oils are co‑processed with diesel‑range streams in hydrotreaters. Hydrogen removes oxygen as water, yielding straight‑chain alkanes. Operating at moderate temperatures (300–400°C) and pressures (30–80 bar) with standard sulfide catalysts produces a diesel‑like product with excellent cetane number.

Each unit requires careful control of feed ratio (typically 5–20% bio‑content by volume) to avoid yield loss, catalyst deactivation, or equipment corrosion. The IEA Bioenergy technology collaboration programme publishes case studies on successful industrial co‑processing trials across Europe and North America.

Innovative Techniques in Co‑processing

Recent advances have moved co‑processing from proof‑of‑concept to commercial deployment. Below are the most innovative techniques driving adoption.

Hydroprocessing with Bio‑Oils

Conventional hydrotreating uses sulfided NiMo or CoMo catalysts. Innovations include non‑sulfide catalysts (e.g., transition‑metal phosphides, nitrides, carbides) that offer higher resistance to sulfur poisoning and allow operation at lower hydrogen pressures. Researchers have demonstrated that using hydrogen‑rich bio‑oils from catalytic pyrolysis can reduce overall hydrogen consumption by 15–30% compared to raw vegetable oils. The oil’s partial deoxygenation upstream also extends catalyst lifetime. Commercial references include Neste’s Porvoo refinery, where hydrotreated vegetable oil (HVO) is produced via dedicated units; newer designs integrate these hydrogen‑rich bio‑oils as a co‑feed in existing diesel hydrotreaters.

Catalytic Co‑processing

Specialized catalysts that tolerate high oxygen and water content are being developed. Zeolite‑based catalysts with hierarchical pore structures improve mass transfer of bulky triglyceride molecules. Adding a small amount of noble metal (e.g., Pt, Pd) to the formulation enhances hydrogenation activity and reduces coke formation. A breakthrough from the Pacific Northwest National Laboratory involves a “bifunctional” catalyst that simultaneously cracks and hydrogenates bio‑oils, achieving near‑complete deoxygenation in a single reactor. These catalysts are now being scaled in pilot units at several US refineries.

Thermal Co‑processing

High‑temperature processes such as delayed coking and visbreaking can also accept bio‑feedstocks. In delayed coking, bio‑oils mixed with heavy petroleum residues produce coke and liquid products. The oxygen in the bio‑oil generates CO and CO₂, which modify coke morphology and reduce its market value. However, thermal co‑processing is attractive for waste‑derived feedstocks that would otherwise be landfilled. Recent work at the University of Saskatchewan showed that adding 5% pyrolysis bio‑oil to vacuum residue in a coker increased liquid yield by 4% while reducing sulfur content in the coke.

Emerging Technologies

Beyond conventional thermal and catalytic routes, several emerging technologies hold promise:

  • Electrochemical co‑processing: Using renewable electricity to drive hydrogenation reactions at ambient pressure. Lab‑scale systems have achieved near‑complete deoxygenation of fatty acids with low energy input.
  • Biological co‑processing: Engineered microbes that convert CO₂ and light hydrocarbons into lipids within the refinery environment. This “hybrid” approach could integrate gas fermentation with conventional refining.
  • Plasma‑assisted cracking: Non‑thermal plasma reactors can break carbon‑carbon bonds in bio‑oils at low temperature, reducing coking and allowing higher bio‑feed rates in FCC units.

These technologies are at TRL 3–5 and require further R&D to reach commercial readiness.

Benefits of Co‑processing

Environmental Benefits

Life‑cycle assessments consistently show that co‑processing reduces GHG emissions by 40–80% compared to fossil‑only refining, depending on feedstock type and hydrogen source. For example, co‑processing used cooking oil in a diesel hydrotreater emits about 50% less CO₂eq per MJ of fuel than conventional diesel. The reduction stems from the biogenic carbon content and avoided land‑use change when waste feedstocks are used. Additionally, co‑processing reduces sulfur and aromatic content in fuels, improving air quality. The US EPA Renewable Fuel Standard grants D4 (biomass‑based diesel) and D5 (advanced biofuel) RINs for qualified co‑processed volumes, providing a clear regulatory driver.

Economic and Operational Benefits

From an operator’s perspective, co‑processing avoids the $1–3 billion capital cost of building a standalone biorefinery. It also enables refiners to diversify feedstocks and hedge against crude oil price volatility. Co‑processing can increase refinery margins by 2–5 $/bbl when bio‑feedstock prices are competitive. Operational flexibility is enhanced: during periods of low renewable fuel credit prices, the bio‑feed rate can be reduced without shutting down a dedicated unit. Furthermore, co‑produced fuels often command a premium due to their drop‑in compatibility and higher cetane number (diesel) or octane number (gasoline).

Challenges and Limitations

Feedstock Variability and Supply

Bio‑feedstock quality varies by season, origin, and collection method. High free fatty acid content (above 5%) can cause soap formation in hydrotreaters, leading to fouling. Inconsistent supply volumes also complicate blending logistics. For a typical 100,000 bpd refinery, securing a steady 5% bio‑feed (5,000 bpd) requires sourcing from multiple suppliers. Supply chain disruptions, such as those seen during the COVID‑19 pandemic, highlighted the fragility of waste‑oil collection networks. Developing robust pretreatment and storage infrastructure is critical to mitigating these risks.

Catalyst Deactivation and Process Optimization

Oxygenates and trace metals (Na, K, Ca, Mg) from bio‑oils accelerate catalyst deactivation by forming deposits or poisoning active sites. Sulfide catalysts suffer from sulfur stripping when oxygen reacts with the sulfide phase. Regeneration costs can rise 20–30% compared to conventional units. Researchers are addressing this through guard beds, optimized temperature profiles, and periodic catalyst rejuvenation. Real‑time monitoring of catalyst activity using advanced sensors and machine learning models is also being deployed to predict deactivation and adjust feed composition dynamically.

Economic Viability and Scale

The profitability of co‑processing depends on the spread between bio‑feedstock and crude oil prices, hydrogen costs, and renewable fuel credit values. In many regions, government subsidies (e.g., California’s Low Carbon Fuel Standard credits) are necessary to make co‑processing economically attractive at scale. Without these incentives, the cost per ton of avoided CO₂ can be $100–200. As carbon pricing regimes expand, the business case strengthens. However, some refineries experience “blending wall” constraints: beyond 15–20% bio‑feed, product quality (e.g., oxidation stability, cold‑flow properties) degrades, limiting potential GHG reductions. Overcoming the blending wall requires new chemical routes and advanced hydrotreating.

Future Directions and Research

Advanced Biotechnologies

Genetically engineered algae and yeast that produce tailor‑made oils with low oxygen content and high energy density are under development. Such feedstocks could reduce hydrogen demand by 30–50% and minimize catalyst poisoning. Synthetic biology companies like LanzaTech and Amyris are already piloting fermentation processes that convert industrial waste gases into lipids that can be directly co‑processed.

Digitalization and Process Control

Digital twins of co‑processing units allow operators to simulate different feed blends, catalyst states, and operating conditions in real time. Machine learning models trained on historical data can predict optimal feed ratios to maximize product yield while minimizing catalyst degradation. The Integrated Refinery Digital Twin (IRDT) project led by the National Renewable Energy Laboratory (NREL) demonstrates how sensor fusion and AI can reduce operational costs by 10–15% and improve GHG savings by 5%.

Policy Support and Collaboration

Long‑term success will depend on consistent policy frameworks. The EU’s Renewable Energy Directive (RED III) and the US Inflation Reduction Act provide tax credits and mandates that support co‑processing investments. Industry consortia such as the Co‑Processing Alliance (an initiative by Shell, BP, and TotalEnergies) are collaborating on open‑source data sharing and best practices. Future R&D should focus on scaling innovative catalysts to TRL 8, developing circular feedstock supply chains, and harmonizing life‑cycle accounting rules across jurisdictions.

Conclusion

Co‑processing biofuels with petroleum products represents a tangible, low‑risk pathway for refiners to decarbonize without waiting for breakthrough technologies. The combination of innovative hydroprocessing, tailored catalysts, and emerging electrochemical and biological methods continues to push the boundaries of what is possible. While challenges such as feedstock variability, catalyst deactivation, and economic dependence on policy support remain, the trajectory is clear: co‑processing will play a central role in the multi‑fuel refining system of the coming decades. For operators, the most prudent next step is to invest in feedstock pretreatment, catalyst optimization, and digital monitoring tools that build a foundation for higher bio‑feed rates and deeper GHG reductions. Collaboration across the value chain—from farmers and waste collectors to catalyst suppliers and regulators—will determine how quickly these innovative approaches can be deployed at the scale needed to make a meaningful impact on global emissions.