When considering decarbonization options in refineries and other industrial facilities, there are two categories of CO₂ sources that can be captured: the low-concentration CO₂ streams generated by combustion of fuel, and the high-concentration CO₂ streams produced as part of a chemical reaction or treatment process.

Dilute low-pressure CO₂ streams: Commonly referred to as post-combustion sources, dilute CO₂ streams are a product of fuel combustion. These normally high-volume streams have low concentration of CO₂, typically 15% to 22% by volume. Depending on the fuel used, these streams may also contain nitrogen, oxygen, sulfur dioxide, nitrogen oxides or other components. Typical sources of dilute low-pressure CO₂ within a refinery or industrial facility include fired heaters that generate heat for a process unit, utility boilers that provide steam for use throughout a facility, or combustion turbines used for either power generation or rotating equipment drivers.

High-pressure CO₂ streams: Pressurized CO₂ streams also exist in refineries and industrial facilities. These streams may be a byproduct of a chemical reaction, or they may be a waste stream generated when CO₂ is removed from a process stream. Concentrations of CO₂ in these streams have a much wider range of CO₂ percentage by volume. Typical examples of high-pressure process streams are those generated in the steam methane reforming (SMR) process used in hydrogen and ammonia production, or generated in the CO2 stream resulting from syngas cleanup.

Concentrated CO₂ streams: These streams are the most accessible, as they typically contain CO₂ and water and require very little cleanup. The classic example is a concentrated stream of CO₂ produced at an ethanol plant as a byproduct of the fermentation process. Other concentrated streams are those that result from the purification of natural gas in a gas processing or liquefied natural gas (LNG) facility.

The treatment required for CO₂ streams varies according to the stream’s composition and the product specifications for the intended end use or disposal location of the CO₂.

For post-combustion dilute CO₂ streams, treatment typically consists of a multistep process that begins with gas cooling, followed by absorption with an amine-based or physical solvent that selectively absorbs the CO₂ in a liquid phase. The rich solvent containing the CO₂ is then regenerated using heat to drive off the CO₂ as a gas stream. Next, the CO₂ stream goes through a dehydration process to remove any water, and then it is compressed to the required pipeline disposal pressure. Given the lower concentration of CO₂ in these high-volume flue gas streams, as well as the low pressure of the post-combustion streams, treatment equipment is large and compression costs can be significant.

By comparison, concentrated CO₂ streams typically only require dehydration and compression prior to disposal or use. These streams tend to have much smaller flow rates than post-combustion streams.

Decisions about whether or how to take advantage of carbon capture opportunities depend on a variety of factors, beginning with an evaluation of financial feasibility. The levelized cost of CO₂ capture is a common way to evaluate carbon capture alternatives. This method includes an evaluation of both the capital cost of the treatment process and the operating cost of treating and handling the CO₂ product, levelized over a 20-year period. To determine if the installation cost is justified, the cost of CO₂ capture on a cost-per-ton basis can then be compared to any financial incentives or penalties.

To take advantage of economies of scale, it may make economic sense for the flue gas from two, three or more process heaters to be captured, combined and treated together in a single unit. This combined approach must be balanced with the cost to combine the streams that may not be adjacent as well as the steam demand for larger units, as a refinery may need to add boilers to generate the required amount. New boilers compound the CO₂ emissions with additional flue gas.

In addition to financial incentives, an increasing number of eyes are focused on reducing carbon emissions across the globe. The goals set by the Paris Climate Agreement, as well as national, state and local emission regulations, all point to the adoption of CCUS solutions. Investors who evaluate public companies using ESG criteria will increasingly screen for corporate participation in carbon capture projects.

Carbon capture technologies and utilization and storage solutions will continue to evolve as more innovation occurs and more projects take shape.

For those interested in participating, it is time to start planning and utilizing available funding for CCUS projects. As our industry learned when implementing selective catalytic reduction technologies and reducing sulfur dioxide emissions, the process before us will not be easy. But the benefits to the energy sector, the environment and the future of our planet make it worth the effort.