Assessing carbon capture opportunities for oil refineries is unique compared to other heavy emitting industries. This is because refinery emissions are distributed across multiple sources rather than dominated by a single unit (to learn more about CO₂ emission sources in refineries, click here). To achieve meaningful emissions reductions, carbon capture efforts need to focus on the larger CO₂ emitting processes within refineries, such as fluid catalytic cracking (FCC) units. 

A typical FCC unit produces around 20-30% of a refinery’s total emissions¹. Despite this, FCC units have not yet seen widespread carbon capture deployment. This is largely due to the unique challenges to integrate carbon capture plants into existing refineries:

• Limited space: FCC technology has been in use since the early 1940s. As refineries have expanded over the decades, many older FCC units become surrounded by newer processing units. This results in highly congested layouts with little available space nearby for additional equipment. Traditional amine-based capture systems require a large amount of space, making it challenging to integrate into congested refinery layouts.
• Construction and operations risks: Major construction projects in refineries typically take place during scheduled outages or turnarounds. As the typical turnaround frequency for older FFC units is 2-3 years, opportunities for large-scale construction are infrequent. Construction of a carbon capture plant would need to occur while the refinery is online and operating, introducing other risks for the host facility.

To address these challenges, oxy-firing is a promising technology to support FCC decarbonization. Commercially known as Synthesized Air FCC (i.e., Honeywell UOP’s proprietary system), this technology is an operational upgrade that improves refinery margins by either increasing hydrocarbon throughput or enabling the processing of lower-quality feedstock. Oxy-firing technology can also improve a carbon capture project’s feasibility by enabling the use of smaller, non-solvent based capture units. This significantly reduces a carbon capture system’s spatial footprint, making integration into existing refineries more practical.  

To convert heavy, low-value hydrocarbon feedstock into lighter, high-value hydrocarbons such as gasoline, the traditional air-fired FCC process uses a cracking catalyst. This process works through a fluidized circulating bed mechanism, where the cracking catalyst moves continuously between the reactor vessel and the regenerator vessel.

Traditional Air-Fired FCC Process:

1. Reactor: The hydrocarbon feedstock is sent to a reactor where it reacts with the cracking catalyst at high temperatures. This produces a cracked, lighter gas stream containing gas oils, gasoline, and liquefied petroleum gas, which are then sent to a fractionator for further processing. As the catalyst reacts with the feedstock within the reactor in a high temperature “cracking process”, coke, a carbon-rich byproduct, is deposited on the catalyst.
2. Regenerator: The spent catalyst is transferred to the regenerator, where coke is burned off using air as the oxidizing media. This reaction releases heat and forms CO₂. The flue gas leaving the regenerator is primarily diluted by nitrogen (N₂), with CO₂ concentrations of approximately 10-20%. Because the FCC unit is thermally balanced, the heat released by burning the coke in the regenerator must be sufficient to supply energy for the endothermic cracking reactions occurring in the reactor. Once regenerated, the catalyst is returned to the reactor.
3. Catalyst Fines Separation and Heat Recovery: The flue gas leaving the regenerator is sent through treatment systems to remove catalyst fines. As flue gas temperatures leaving the regenerator can exceed 700°C, it is also sent through heat recovery units (e.g., steam generators) to utilize this heat and reduce its temperature before being sent to a carbon capture system. 

Oxy-firing technology replaces the combustion air entering the regenerator with a custom oxidant mixture made of oxygen (O₂) and recycled CO₂-rich flue gas. 

Oxy-Firing FCC Process:

1. Air Separation Unit: Oxygen is supplied to the regenerator through an air separation unit, where the N₂ is removed from the air. 
2. Regenerator: Oxygen and recycled CO₂-rich flue gas are used as the oxidizing media in the regenerator to regenerate the spent catalyst from the reactor. This reaction produces flue gas with higher CO₂ concentrations than a traditional air-fired FCC. Once regenerated, the catalyst is returned to the reactor.
3. Catalyst Fines Separation, Heat Recovery, and Pre-Treatment: The flue gas leaving the regenerator is sent through treatment systems to remove catalyst fines and through heat recovery units to utilize the heat and decrease its temperature. In addition to this, the flue gas is also sent through a pre-treatment system to remove impurities (i.e., water, nitrogen, sulfur oxides, nitrogen oxides, particulates, etc.), resulting in a flue gas stream with high CO₂ concentrations (i.e., greater than 90%). A large portion of the total flue gas flow (i.e., 70-80%) is recycled back to the regenerator to sustain catalyst regeneration and maintain thermal balance within the system, with the remainder being sent to a carbon capture plant. 

By changing the oxidizing media in the regenerator from air to oxygen and recycled flue gas, the resulting flue gas is highly concentrated in CO₂ (i.e., >90%). With these higher concentrations, traditional methods such as amine-based carbon capture are not economical. Alternative non-solvent based capture methods (such as compression and purification processes using bulk water removal or cryogenic carbon capture) may be more suitable and cost-effective. 

The secondary benefit is the reduced flue gas flow rate to the capture plant (as the majority of the CO₂ is recycled back to the regenerator), reduces the size of carbon capture equipment required. The use of smaller capture units provides greater flexibility for managing layout constraints within a refinery.

Pilot Plant Overview 

In 2012, the pilot CO₂ Capture Project at the Petrobras Refinery in Parana, Brazil completed oxy-firing field testing (to learn more about the demonstration project, click here). This pilot was funded through a partnership of major energy leaders including BP, Chevron, Shell, and Petrobras. The pilot’s objective was to demonstrate the technical feasibility of retrofitting an existing FCC unit processing 33 barrels per day with oxy-firing technology.

For the pilot, two key systems were installed:

• Oxygen Supply System & O₂ Injection: This system, comprised of a liquid tank, vaporizer system, control skid, gaseous injector, and piping, supplied the pure O₂ to the regenerator and was able to achieve O₂ concentrations of 99.7%. 
• CO₂ Recycle System & Gas Cleaning: This system treated the flue gas leaving the regenerator. The flue gas was cleaned to remove catalyst fines and impurities like sulfur oxides (SO_x) using an alkaline washing tower and a chilled water washing tower. This treatment resulted in a clean, CO₂-rich flue gas that was then recycled back to the regenerator. 

To form the oxidant stream for the regenerator, the recycled CO₂-rich flue gas was compressed and mixed with pure O₂ (99.7 mol %). This resulting oxidant stream supplied to the regenerator contained O₂ concentrations ranging from 23 to 29 mol %.

Pilot Results

This pilot confirmed that an existing FCC unit can be retrofitted with oxy-firing technology. The result of this addition was increased concentrations of CO₂ in the flue gas stream. The pilot results comparing traditional air-firing FCC and oxy-firing FCC are outlined below. 

*dry basis

Operational Key Learnings

The operational differences between traditional air-firing and oxy-firing stem from the higher heat capacity and specific gravity of the CO₂ when compared to N₂. To demonstrate the operational flexibility of the FCC unit with oxy-firing technology, the pilot tested two extreme operating conditions, with the results provided below.

This pilot demonstrated that the oxy-firing method increased the FCC unit’s operational flexibility. This flexibility allowed for either increased hydrocarbon throughput, improving the refinery margins, or decreasing the catalyst entrainment in the recycled flue gas, reducing risk on regenerator performance and reliability. Operating at an intermediate oxy-firing condition, where catalyst entrainment matches that of traditional air firing, may offer a balanced approach that increases catalyst circulation without compromising performance and equipment integrity.

UOP Case Study

In 2023, Honeywell UOP evaluated the economic and environmental impact of converting a standard 37,500 barrels per day FCC unit from a European Refinery to Synthesized-Air operation (to learn more about Honeywell UOP’s technology and case study, click here). The study aimed to demonstrate that the technology could serve as a profitable investment rather than just a carbon capture expense.

The retrofit involved revamping existing FCC equipment and installing a new recycle blower, an nViro FCC section (treatment equipment for pollutant removal), and a carbon capture section. The total capital expenditure (capex) of these additions was approximately $100 million. The air separation unit was not included in the capex as the supply of oxygen would be handled through an “over-the-fence” model with an industrial gas supplier.

Results

This study achieved a 95% capture efficiency, reducing CO₂ emissions from 1,213 tonnes per day (t/d) to 61 t/d. The system increased CO₂ concentration to greater than 90%, while reducing flue gas flow sent to the capture facility by 70-80%. 

In addition to emissions reductions, the study identified two primary strategies for generating a return on investment. The oxidant mixture (i.e., CO₂-rich flue gas and O₂) acts as a large heat sink in the regenerator, unlocking an additional coke-burning capacity of 20%. Two scenarios were evaluated to use this additional capacity, with the results provided in the table below.

Oxy-firing technology is an operational upgrade for FCC units that improves refinery margins by either increasing processing flexibility (i.e., by enabling processing of lower-quality feedstock) or increasing the hydrocarbon throughput. These economic benefits can significantly offset the cost of a carbon capture project. Compared to traditional air-fired operation, oxy-firing increases the CO₂ concentration in the flue gas from approximately 10-20% to greater than 90%, while also reducing flue gas volumes sent to the capture plant by 70-80%.  This enables the use of smaller, non-solvent based capture units, drastically reducing the physical footprint and cost of a capture system compared to traditional amine-based systems. 

Broader adoption of oxy-firing technology is primarily constrained by the strategic and operational risks associated with long-term oxygen procurement. To mitigate upfront capital costs and the risks of operating new equipment such as air separation units, refiners can enter agreements with third-party oxygen suppliers. These agreements often require refiners to enter a 15-to-25-year ‘take-or-pay’ contract. Despite additional revenues that can be achieved with oxy-firing, in an era where fuel demand is shifting, these agreements are a massive, fixed annual liability for refineries. Additional barriers include the technical challenges of modifying a regenerator’s combustion chemistry, as well as the construction logistics risks of complex FCC revamps within an existing refinery’s footprint.