In carbon capture, utilization, and storage (CCUS), compression and dehydration are two steps often used before transporting and storing captured carbon dioxide (CO2) underground. Compressing and dehydrating the CO2 is done to improve the efficiency of transportation and to meet the transport and storage operators’ specifications. Compression and dehydration systems are typically single train systems, meaning they consist of a single, non-redundant processing line. Since these systems lack redundancy, they need to be reliable.
There are multiple compression and dehydration technology options available. Explore the differences between technologies.
For post combustion amine-based carbon capture, the CO2 captured from flue gas produces a low-pressure CO2 product stream. Compression increases the pressure of this stream, reducing its volume and converting the CO2 into dense phase. Dehydration reduces the water content within the CO2 product stream to meet the moisture limits set by the transportation and storage operators. Dehydration is crucial for preventing pipeline corrosion and hydrate formation.
CO2 compression is typically achieved through transcritical compression, where low pressure gaseous CO2 is compressed in multiple stages to reach supercritical conditions, and then cooled to dense phase conditions suitable for pipeline transportation. Compression is divided into two sections -wet and dry- separated by a dehydration unit.
Six stages of compression (four wet stages and two dry stages) are shown; however, it is common to see a total of eight stages (four wet and four dry) in industry. The number of stages is dependent on the inlet pressure of the compressor and the final transportation pressure required.
Compare the various compressor technologies for CCUS applications below.
How it works: Features multiple impellers arranged on a single rotating shaft, increasing gas pressure by accelerating the fluid through each stage in series.
Technology Application: This design allows for higher pressure ratios to be achieved within a compact unit.
How it works: Utilizes a large single bull gear to drive multiple pinions, each connected to an individual impeller operating at its optimal speed.
Technology Application: This design allows for independent speed control of different compression stages, leading to higher efficiency and specialized pressure ratios in a compact footprint.
How it works: Employs multiple cylinders in series, where pistons move back and forth to sequentially compress gas to increasingly higher pressures.
Technology Application: This design is highly effective for achieving very high-pressure ratios and handling a wide range of gas types, making it robust for demanding industrial applications.
Capacity is key when selecting a compressor technology. Larger scale capture facilities (over ~1 million tonnes of CO2 per year) can’t typically use reciprocating compressors as multiple would be required which increases capital costs. Selecting a single shaft or an integrally geared compressor isn’t as straight-forward. There are many factors that can influence the project owner’s decision such as:
Integrally geared centrifugal compressors are selected the most often for CCUS applications. This is due to their early and proven adoption in CCUS applications, their relatively small footprint, and highest overall efficiency when compared to the other compressors.
| Maturity |
TRL 9 – Commercially proven. Well established technology used in several industries such as oil production, refineries, fertilizer plants, etc. |
TRL 9 – Commercially proven. Mature and reliable technology with notable applications in CCUS. |
TRL 9 – Commercially proven. Mature and reliable technology used in hydrogen production and Enhanced Oil Recovery (EOR) using CO2. |
|---|---|---|---|
| Configuration |
Typically includes two units (low-pressure and high-pressure section) which are interconnected by a speed increaser (gearbox) |
Single unit can incorporate all compression |
Typically includes multiple units (which are volume dependent) with separate drivers |
| Capacity, m3/h |
Up to 800,000 |
Up to 500,000 |
Up to 100,000 |
| Overall Efficiency, % |
Higher |
Highest |
High |
| Pressure ratio per stage |
1.1-1.7 |
2.5 |
1.3-3 |
| Maximum Discharge Pressure, bar |
Up to 500 |
Up to 250 |
Up to 800 |
| Reliability, % |
> 99 |
> 99 |
95 – 97 |
| Turndown Ratio |
1.25:1 |
1.25:1 |
5:1 |
| Footprint (assuming equivalent capacities between compressors) |
Larger footprint than integrally geared centrifugal compressors, but allows for a compact layout. |
Smaller footprint in comparison to single-shaft centrifugal compressor and reciproctating compressors. Intercooler is arranged below the compressor and no separate speed increaser is necessary. |
Larger footprint than integrally geared centrifugal compressors and single-shaft centrifugal compressors, as a multiple compressor train is required and does not allow a compact layout |
| Interval Between Overhaul, yrs |
~ 5 |
~ 5 |
~ 3 |
| Maintenance Duration |
Medium |
High |
Low |
| Maintenance Accessibility |
Relatively easy to maintain |
Complex due to multiple stages and gearing |
Easier to repair |
| Risk for CCUS application |
Technology is widely adopted in other industries, but has minimal applications in operating CCUS facilities |
Technology is widely adopted, with many successful installations in operating CCUS facilities |
Technology is widely adopted, with proven CO2 applications at smaller scales. Increased risk when compared to single shaft centrifugal compressor due to a larger amounts of moving parts. |
| Power Consumption |
10-20% higher than integrally geared centrifugal compressors |
10-15 MW |
5-10% higher than integrally geared centrifugal compressors |
Dehydration systems remove water from a CO2 product stream to achieve the desired levels of dryness to meet pipeline and storage specifications. Factors such as the phase in which CO2 is being transported, and the concentration of other impurities present in the stream influence the product’s water limitations.
Compare the various dehydration technologies with a proven track record in CCUS applications below.
How it works: In the absorber, the water from the wet CO2 product stream is dissolved into a liquid solvent (i.e., liquid triethylene glycol (TEG)), removing it from the CO2 product stream and yielding dry CO2. The rich TEG (water and TEG) is heated and sent to a regenerator, where the water is boiled off and the lean TEG is recirculated back to the absorber for reuse.
Technology Application: Removal of water from gas streams to approximately 50 ppm.
How it works: This uses a cyclic process known as pressure swing adsorption (PSA). The wet CO2 product stream flows through one vessel containing a solid desiccant (i.e., molecular sieve, silica gel, or activated alumina) where the water molecules physically bind to the desiccant surface (adsorption), removing it from the CO2 product stream and yielding dry CO2. Once saturated, the wet desiccant in the same vessel is regenerated (desorption) using recycled dry CO2 that is heated, while the other vessel initiates the adsorption process. Typical PSA units have multiple two-vessel arrangements to accommodate the required flowrates.
The different desiccants used in the adsorption process have unique properties which dictate their ability to remove water.
Technology Application: Removal of water from gas streams to 10 ppm and below.
How it works: This uses a cyclic process known as pressure swing adsorption (PSA). The wet CO2 product stream flows through one vessel containing a solid desiccant (i.e., molecular sieve, silica gel, or activated alumina) where the water molecules physically bind to the desiccant surface (adsorption), removing it from the CO2 product stream and yielding dry CO2. Once saturated, the wet desiccant in the same vessel is regenerated (desorption) using recycled dry CO2 that is heated, while the other vessel initiates the adsorption process. Typical PSA units have multiple two-vessel arrangements to accommodate the required flowrates.
The different desiccants used in the adsorption process have unique properties which dictate their ability to remove water.
Technology Application: Removal of water from gas streams to 10 ppm and below.
How it works: This uses a cyclic process known as pressure swing adsorption (PSA). The wet CO2 product stream flows through one vessel containing a solid desiccant (i.e., molecular sieve, silica gel, or activated alumina) where the water molecules physically bind to the desiccant surface (adsorption), removing it from the CO2 product stream and yielding dry CO2. Once saturated, the wet desiccant in the same vessel is regenerated (desorption) using recycled dry CO2 that is heated, while the other vessel initiates the adsorption process. Typical PSA units have multiple two-vessel arrangements to accommodate the required flowrates.
The different desiccants used in the adsorption process have unique properties which dictate their ability to remove water.
Technology Application: Removal of water from gas streams to 10 ppm and below.
The main factors to consider when selecting a dehydration technology are the desired moisture level and operating costs. As the desired moisture level is primarily dictated by the transportation and storage operator, the project developer may be limited on their options. With higher allowances for moisture levels, absorption is typically selected and with stringent moisture levels, adsorption technologies are typically selected.
The most commonly selected dehydration technology for CCUS applications is absorption, using triethylene glycol (TEG) as the liquid solvent. Current CCUS projects are using TEG dehydration mainly because it is a proven technology that meets or exceeds pipeline moisture specifications, and it has a lower capital and operating cost when compared to adsorption.
| Maturity |
TRL 9 – Commercially proven |
TRL 9 – Commercially proven |
TRL 9 – Commercially proven |
TRL 9 – Commercially proven |
|---|---|---|---|---|
| Operability |
Continuous operation, requiring TEG makeup. |
The desiccant has a two-year lifespan. |
The desiccant has a five-year lifespan. |
The desiccant has a three to four year lifespan. |
| Desiccant Shape & Sizes |
N/A |
Spherical Sizes available include: 14 mesh, 3mm, 5mm, 6mm |
Spherical Sizes available include: 8 mesh, 2mm |
Pellets (extruded cylinders or spherical) Sizes available include: 1.6mm, 3.2mm, 6mm dia. cylinders |
| Desiccant Surface Area, m2/g |
N/A |
325-360 |
650-750 |
600-800 |
| Regeneration Temperature, °C |
190-204 |
150-200 |
100-120 |
200-300 |
| Regeneration Frequency |
Continuous |
High |
Moderate |
Low |
| Operating Pressure, barg |
Up to 64.5 |
Capable of withstanding higher pressures than adsoprtion dehydration using TEG |
Capable of withstanding higher pressures than adsoprtion dehydration using TEG |
Capable of withstanding higher pressures than adsoprtion dehydration using TEG |
| Turndown Ratio |
10:1 |
10:3 |
10:3 |
10:3 |
| CAPEX |
$ |
$$ |
$$ |
$$ |
| OPEX |
$ |
$$ |
$$$ |
$$$$ |
| Achievable Water Content, ppm mol |
50 |
10 |
10 |
1 |
| Footprint |
Smaller footprint compared to adsorption dehydration technologies |
Larger footprint than absorption dehydration technology using TEG |
Larger footprint than absorption dehydration technology using TEG |
Larger footprint than absorption dehydration technology using TEG |
| Risks |
– TEG carryover with the CO2 product |
– Sensitive to acidic impurities (SOx, NOx, H2S) so at risk of deactivation and increased replacement rate. |
‘- Risk of clogging due to sulphur deposition in the presence of H2S. H2S content should be kept below 5%. |
– Risk of molecular sieve binder breakdown in the presence of caustic and acidic impurities, forming dust. |
The CCUS Insight Accelerator (CCUSIA) is a partnership between the Government of Alberta and the International CCS Knowledge Centre to accelerate and de-risk CCUS by sharing knowledge and developing insights from projects.