Myth Busters: Part II: Reciprocating vs Centrifugal Compressors for CO2 Capture and Storage: Challenges and Disadvantages

In part one of our two-part column on compressors for carbon capture and storage, we discussed the applications and design of reciprocating versus centrifugal compressors for a sustainable energy economy. For lower-flow carbon sequestration applications, reciprocating compressors have been the traditional base option over centrifugal compressors. In many cases, electric motor-driven reciprocating compressors are utilized. However, as the industry is rapidly moving toward much higher CO₂ volume flow rates for large-scale carbon capture, the limits of the practical use of reciprocating compressors are being reached. We conclude our discussion on this topic with a look at compression challenges and the often overlooked, lesser-known disadvantages of reciprocating compressors for CO₂ capture and storage applications.

CO₂ is generally considered a heavy gas with a molecular weight twice that of natural gas and 22 times that of hydrogen. As noted, this makes CO₂ from a simple thermodynamic perspective, relatively easy to compress, but it presents other technical challenges that need to be addressed to make the overall compression or pumping process efficient and reliable.

• For most carbon-capture applications, the compression process must cross the critical point of CO₂ from the gas phase to either the liquid or supercritical dense phase. Therefore, it has strong thermodynamic path dependence and multi-phase behavior. Its physical properties vary considerably during the phase transitions, which makes aerodynamic design challenging.
• CO₂ is a heavy gas, which results in amplified dynamic forces on any moving components. It has significant volume reduction and temperature increase with work input that results in the need for a wide range of flow coefficient stages (for centrifugal compressors), cylinder sizes (for reciprocating compressors), and multiple intercoolers.
• Carbonic acid is formed in the presence of water, which corrodes metal. At high pressures, CO₂ also selectively leeches certain elements from common metals.
• CO₂ is soluble in elastomeric materials, which can lead to rapid decompression failures. Also, when rapidly expanded, CO₂ quickly forms liquids and dry ice, which can become a problem by plugging dynamic and static seals.
• Aerodynamic concerns come from its low sonic speed, which can result in high Mach numbers, causing shock losses and a reduced operating range. It also has a very low viscosity, which, while being beneficial from a pressure-drop perspective, can result in earlier boundary layer separation and increased secondary flows.

These issues apply to both centrifugal and reciprocating compressors, but reciprocating compressors have additional issues when operating with CO₂. Some of the top challenges are pulsation control, leakage, flow control, and mechanical reliability.

PULSATION CONTROL

Reciprocating compressors produce pressure pulsations that are induced into the piping system. These pulsations can cause acoustic resonance and mechanical vibration if they are not properly attenuated. This is especially true for dense gases, such as CO₂, that amplify dynamic forces and are capable of transmitting pulsations over very long distances. Complex attenuation systems, including bottles, choke tubes, side branch absorbers, and orifices, are usually required to control these pulsations. Due to the low sonic speed and the high density of CO₂, the reciprocating compression process injects strong periodic pulsations that result in high-amplitude acoustic resonances at low frequencies in the piping systems of the compressor station.

Not only can these pulsations induce strong structural vibrations resulting in piping failures at these stations, but they can also travel miles down the pipeline causing noise and structural issues. It is therefore imperative to design the compressor stations with appropriate pulsation attenuation devices and resonance avoidance to eliminate the possibility of developing strong pulsations in the flow.

Compressing high-pressure CO₂ creates several challenges, including a higher risk of pulsation and vibration problems than typical natural gas transmission stations. Current existing CO₂ pipelines operate above the critical point between 1,200 - 2,200 psia to maintain single-phase operations in the dense-phase region. With a typical pipeline pressure of 2,100 psia at ambient temperatures, the associated density is around 60 lb/ft³, which is nearly equal to that of liquid water. Since density directly correlates to pulsation forces, this can cause significant issues with pulsations and shaking forces if the reciprocating compressor pulsations are not adequately attenuated.

The other challenge to consider in compressing CO₂ is its relatively low speed of sound. Acoustic natural frequencies in the piping system are determined by the speed of sound of the medium. Low speed of sound results in low acoustic natural frequencies in the system. When a resonance condition is created at low frequencies, the resulting pulsations can travel for long distances in the piping with lower attenuation than what is typically seen in higher-frequency pulsations and with lighter fluids. Pulsations traveling over hundreds of miles upstream or downstream have been observed in dense gases. Low-frequency excitations can also result in a strong structural interaction that has the potential to cause severe damage to the piping system, even at piping locations far away from the pulsation source.

LEAKAGE

Leakage control in reciprocating compressors is very difficult, especially over time as the sealing rings and packings degrade. Current packing technology uses a series of specific cut-dry ring seals held in place with springs and cups to seal the pressurized gas in the cylinder against the air at atmospheric pressure in the distance piece. Abradable packing seals made from polymers are commonly used to minimize leakage. These tend to wear over time, so leakage can become excessive unless they are frequently changed. Once the piston moves, the pressure differential across the packing seals creates a twisting effect on the seal, allowing significant amounts of process gas to leak into the casing and then out into the atmosphere. The leakage through this pathway increases even more with wear and increases (sometimes exponentially) with static hold conditions since the seals are primarily designed to activate with a moving piston rod. This is obviously very problematic for carbon-capture applications since the primary objective of the system is to capture and sequester as much CO₂ as possible...not to leak it. As noted, when CO₂ is leaked and/or rapidly expanded, it can form dry ice due to the Joule-Thomson effect, which can cause significant operational problems. Reciprocating compressors could easily become a major source of CO₂ emissions from leakage. From a design perspective, it is difficult to completely eliminate a gas leakage path from the process gas cylinder to the piston rod.

FLOW CONTROL, RELIABILITY, NOISE, AND LUBRICATION

Some other concerns with reciprocating compressor operation are flow control, reliability, noise, and lubrication. Flow control is challenging in reciprocating compressors since they are fixed-volume displacement machines that typically only operate over a limited speed range because of pulsation and vibration limits and exclusions. Flow control is usually handled using variable cylinder clearance volume, external pockets, and recycling. This not only means added operations personnel for mostly manual control, but also that with a reciprocating the operation of the pipeline is usually transient in that one is constantly packing and unpacking the line.

Reciprocating compressors are mechanically less reliable and require more maintenance than most other compressor types. This is due to their many moving parts and high-speed alternating linear motion that puts inertial cyclic stresses on most reciprocating parts. In a power plant, where operational uptime, high start-up reliability, and low maintenance costs are imperative to remain profitable, this is especially problematic.

Reciprocating compressors create high levels of sound in a frequency range that is a major noise nuisance and very uncomfortable to human hearing. Unfortunately, because of the bulkiness and mechanical arrangement of these compressors with external moving parts, effective ambient noise control using sound-insulating blankets or enclosures is difficult.

Finally, most reciprocating compressors operate using wet seals, which means that they continuously leak lubricating oil into the process gas stream from the lubricating film between the cylinder and pistons. Even with a well-controlled lubrication make-up system and an efficient downstream separator, some of this oil will carry over into the pipeline. This can become a significant issue for CO₂ geological storage applications since oil contamination in the formation would cause an environmental and safety hazard (and when coke-hardened, may even plug injection lines over time). Alternatively, dry-operating reciprocating compressors could be used, but these are even less mechanically reliable than lubricated compressors.

This discussion is designed to present our perspective on the ongoing debate of reciprocating versus centrifugal compressors within sustainable energy economy gas compression systems. This is an ongoing industry debate that will obviously continue for a long time. We simply gave our opinion based on our engineering experience, and we are sure many will disagree with our assessment, but, regardless of the perspective, it is always important to clearly understand all the features, limits, advantages, and disadvantages of the different types of compressors for any given application when making critically important, long-term equipment and infrastructure decisions. 

ABOUT THE AUTHORS

Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas Applications Committee.

Rainer Kurz is the Manager of Gas Compressor Engineering at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil & Gas Applications Committee.