J. Demonte and S. GRILLS, Emerson, Marshalltown, Iowa
Greenhouse gas (GHG) reduction projects have become a focus for many nations around the world—carbon capture technologies are a way to reduce the release of carbon into the atmosphere and help contribute to meeting emissions targets. For example, the U.S. Department of Energy (DOE) recently announced $3.7 B in funding to back projects that remove carbon dioxide (CO2) from the atmosphere. This program now opens the door to the use of taxpayer dollars to fund carbon capture projects that produce fossil fuels through a process known as enhanced oil recovery (EOR).
One technique offering promise is geological carbon capture and storage (CCS), which captures CO2 from a variety of sources and sequesters it deep underground. While conceptually easy to understand, the logistics of handling CO2 for these applications is not so straightforward.
A changing environment. CO2 buildup in the atmosphere has been increasing rapidly, creating dramatic changes in the Earth’s climate, including temperature extremes, glacial melt and weather events (FIG. 1). Most of this CO2 (about 87%) comes from fossil fuels combustion, with the remaining produced by industrial processes and land use changes.
Initial GHG reduction efforts have focused on methane, which is 22 times more potent than CO2. However, as those efforts have progressed, the world has begun tackling the challenge of reducing CO2 emissions. A host of carbon reduction technologies are being pursued, with many focusing on electrification and energy efficiency to reduce fossil fuels consumption. Another promising method for immediate and meaningful atmospheric CO2 reduction is carbon sequestration.
The carbon sequestration process. Carbon sequestration is a technique to capture CO2 and safely store it in a location where it remains out of the atmosphere. Some carbon sequestration methods seek to sequester CO2 using biological methods by encouraging plant and tree growth and by avoiding carbon releases due to fire or poor farm management. These efforts have had some success, but the potential volume of CO2 elimination is rather limited.
Much more promising is geologic carbon sequestration, which seeks to capture and store CO2 in underground porous rock formations. The worldwide potential for storage is enormous, with an estimated 3,000 gigatons of potential storage capacity in the U.S. alone. This injected CO2 can also be used as part of EOR methods (FIG. 2), which use the CO2 to reduce the viscosity of hydrocarbons and drive them towards existing wellheads.
The geologic carbon sequestration process is composed of three major tasks: CO2 capture, CO2 compression and CO2 injection. Energy efficiency is a primary goal for any carbon sequestration project since every watt of power used generates more CO2. Therefore, each of these processes must be controlled tightly and efficiently to minimize energy loss.
Carbon capture. Carbon sequestration starts with carbon capture, where CO2 is separated and recovered. Historically, industrial CO2 separation has been performed using amine solvents to absorb CO2 from the gas stream, with heat and pressure reduction then used to separate the gas from the solvent. This method remains the most common means of CO2 separation; however, other technologies such as membrane, adsorption and a host of new experimental technologies are being pursued. Some of the newer technologies are using electrolysis to efficiently pull CO2 out of the air.
Amine separation remains the most widely used method for carbon capture, and control valves used in these applications face significant challenges. These valves are subjected to high pressure drops and outgassing conditions as the entrained CO2 separates from the amine solution while passing through the valves (FIG. 3).
The amine solution used in this process tends to be corrosive, so valve internals must employ hardened trim materials, as well as some type of severe service trim to minimize damage and lengthen maintenance cycles. An environmental seal packing is required, and a diagnostic positioner is a wise addition to detect and alarm, as the valve eventually develops the unavoidable trim damage that occurs in this very difficult service.
There is no international control valve sizing standard for outgassing applications; therefore, it is important to partner with an experienced vendor to properly size and select these valves to avoid issues.
CO2 compression. Low-pressure CO2 is the usual product of the CO2 separation process, and it must be compressed for efficient transmission to injection sites. Centrifugal compressors are typically used to boost low gas pressures to medium pressures, which then feed larger multi-stage compressors better suited to pressurize the gas to a supercritical state for pipeline transport.
Centrifugal compressors are prone to a catastrophic condition known as “surge” at low flowrates. If flow is not restored through the compressor immediately, the internals of the compressor can be destroyed in a matter of seconds. To prevent this condition from occurring, anti-surge valves (FIG. 4) are installed between the discharge and suction of the compressor, and they open very quickly to establish forward flow the moment compressor surge is detected.
These valves are typically very large and have specialized actuators and positioner components to provide extremely fast and accurate response. The trims are specially designed to handle the very high pressure drop and high flow conditions common in this service.
Large multi-stage centrifugal compressors increase the CO2 pressure above 1,200 psi (83 bar) to a maximum of 2,800 psi (193 bar). At these pressures, CO2 becomes a compressible supercritical fluid, with a density like a liquid but viscosities similar to a gas (FIG. 5). These conditions allow for very efficient transport in pipelines but pose several challenges for control valve selection.
Supercritical or dense-phase CO2 does not behave as either a liquid or a gas, so the typical equations used for mass flow and physical properties do not apply. This makes control valve sizing difficult, requiring specialized knowledge and software to accurately predict the fluid properties and size the valve components.
Dense-phase CO2 is also a very strong solvent that readily penetrates certain elastomers at these pressures. When the pressure is released, these elastomer seals can literally blow apart in a process called rapid gas decompression. Therefore, packing and body seal materials for dense-phase CO2 applications must be carefully considered to avoid this condition and provide extended life. Potential solutions include changing valve plug seals with elastomeric backup rings to spring-loaded seal rings, or using thermoplastic rather than elastomeric O-rings.
CO2 injection. The last part of the CO2 geologic sequestration process is injection. Potential injection sites include abandoned aquifers, salt domes, or depleted gas and oil fields. CO2 can also be utilized for EOR where very low viscosity dense-phase CO2 is injected around an active oil site. The CO2 readily enters the hydrocarbon in the target zone, lowering its viscosity and increasing its volume to drive it toward surface wells. EOR has the dual potential of sequestering CO2 while increasing oil production, so the economics are very favorable. For this reason, the bulk of carbon sequestration injection projects were initially focused in this area.
Recently, several very large-scale CO2 injection projects have been undertaken around the world to prove the viability and economics of geologic carbon sequestration. Most of these projects are storing CO2 in geologically stable injection zones thousands of feet below the Earth’s surface.
The injection pressures depend greatly upon the depth and type of geologic formation. Automated injection valves can be subjected to thousands of pounds of pressure, high vibration and high noise, as well as corrosive conditions created when trace amounts of water or hydrogen sulfide (H2S) remain in the CO2. Body, trim and seal material selection are critical aspects of valve sizing and selection.
Supercritical dense-phase CO2 is also common in injection applications, so specialized valve sizing and proper elastomer selection are required. Two-phase flow is often possible since high pressure drops and the resulting cooling can cause dry ice to form inside the valve. These conditions may necessitate carefully designed valve trims to stage pressure drops and pass solids without plugging.
Seek advice. When faced with the task of specifying automated valves for a carbon sequestration project, users should consult with a knowledgeable valve vendor experienced with these types of applications. The wide range of potential conditions—as well as challenges associated with body and trim material selection, elastomer seal problems and difficulties with sizing supercritical valve applications—warrants careful consideration.
Fortunately, a host of control valve designs have proven track records of success in a wide range of CO2 applications. However, the best option for a particular service can only be obtained by careful study and analysis of the operating conditions, as well as a solid understanding of the available valve design options. With proper selection, control valves can perform well in these demanding applications, improving the overall operation of the entire carbon sequestration process. HP
NOTES
a Fisher™ DST-G valve
b Fisher™ easy-e™ EWT control valve
JOE DEMONTE is the Director of Hydrocarbon Industries for Emerson, focused on Fisher control valves. He has been with Emerson for more than 10 yr and is a subject matter expert in compressor anti-surge applications. DeMonte earned Bch and MS degrees in biomedical engineering from the University of Iowa.
SUZANNA GRILLS is a Sustainable Sales Engineer within Global Industry Sales at Emerson. She earned a chemical engineering degree with an emphasis in environmental from the University of Missouri-Columbia.