Carbon capture options for refiners
An outline methodology identifies the most appropriate way to achieve a refinery’s carbon capture goal
SUZANNE FERGUSON and MIKE STOCKLE
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Regardless of how well designed and operated a refinery is, there are always some unavoidable energy demands and carbon emissions. As concern over the impacts of carbon emissions grows and, around the world, government legislation controlling or putting a cost on carbon emissions comes into force, refineries are becoming increasingly concerned with cost-effectively minimising their carbon footprint.
Greenfield development projects have the advantage of being able to design their processes for reduced CO2 emissions through process selection and choice of primary energy supply. However, both new and existing plants can consider the following options to reduce their carbon footprint:1
• Efficiency improvements
• Fuel substitution
• Feedstock substitution
• Configuration modifications
• Carbon capture and storage.
In this article, we will consider the application of carbon capture to the refinery for carbon footprint reduction. It should be noted that a potentially profitable market for captured CO2, such as enhanced oil recovery, may also lead to the consideration of carbon capture and storage.
Assuming that fugitive carbon losses are negligible, most of the carbon that enters the refinery leaves again in the form of hydrocarbon products, most of which will be combusted as fuels, emitting that carbon to the atmosphere. The processing of these products also contributes to their carbon footprint, since carbon is emitted due to the energy demand, or chemistry, of the refinery processes. Typically, around 5-10% of the crude feed to a refinery is lost as fuel in the refinery.
The carbon emitted from the refinery is much easier to reduce than that emitted from, say, the exhaust pipe of a car. Hence, carbon captured from a refinery not only reduces the refinery’s overall emissions and carbon footprint, but also the embedded carbon of its products.
Introduction to carbon capture and storage
Carbon capture and storage (CCS) is the process of removing or reducing the CO2 content of streams normally released to the atmosphere and transporting that captured CO2 to a location for permanent storage. CCS can be applied to a wide range of large single-point sources, such as process streams, heater and boiler exhausts, and vents from a range of high CO2 footprint industries, including power generation, refining, natural gas treating, chemicals, cement production and steel production. There are three main classifications of technologies employed:
• Pre-combustion capture
• Post-combustion capture
• Oxyfuel combustion capture.
Once captured, the CO2 is compressed, dried and transported to a suitable storage location such as a saline aquifer, depleted oil field (where enhanced oil recovery could be employed) or depleted gas field.
Each CCS route described below comprises a group of technologies based on similar process flow schemes. For each method, an example process description and flow scheme are given and the key variations highlighted.
Post-combustion CO2 capture
Combustion flue gas is cooled by direct water contact before entering a blower designed to overcome the absorption system pressure drop. The flue gas enters the absorption column, where it is washed with a physical solvent such as monoethanolamine (MEA). The flue gas is scrubbed of up to 90% of its CO2 content and is returned to the combustor stack and released to atmosphere. The CO2-rich solvent is then heated against lean solvent and regenerated in a stripping column. The solvent then returns to the absorption column, while the released CO2 is dried and compressed for export. The highlight of the post-combustion process is that it is suited not only to new installations but may also be retrofitted to existing plants (see Figure 1).
There are variations on this post-combustion scheme:
• A range of processes exists utilising different solvents: MEA, ammonia, sterically hindered MEA and even seawater
• For high-sulphur feeds, the process may be coupled with a flue gas desulphurisation unit, allowing the direct contact cooler to be eliminated.
Post-combustion CO2 capture is a simpler system than pre-combustion, described below, and can be combined with almost any type of combustion system. Very large single-point sources such as utility-scale power plants present a challenge in terms of maximum scale-up in a single leap. However, the scheme shown above has already been demonstrated for many years for the production of CO2 for use in the food and chemicals industries. Some of these plants are already at an appropriate size to capture CO2 from point sources the size of refinery-fired heaters and boilers.2,3
Depending on the specific site, post-combustion carbon capture could be applied to a number of refinery flue gas sources at the same time (such as fired heaters, FCC, hydrogen production units), with the cooler, blower and absorber located as close as possible to each source (or group of sources), with the rich solvent then pumped to one or multiple solvent regeneration units and one or multiple compression units. This offers flexibility to fit in the large capture equipment around the existing, often crowded plot.
Pre-combustion CO2 capture
A hydrocarbon feedstock is fed to an oxygen- or air-blown pressurised gasifier or reformer, where it is converted to syngas. The syngas is then passed through a shift reactor, which increases the hydrogen and CO2 content of the syngas. This high-pressure, high-temperature syngas is then cooled before being washed with a solvent to absorb the CO2, leaving an essentially pure hydrogen stream and a CO2-rich solvent stream.
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