OR WAIT null SECS
Part one of this two-part myth on reciprocating versus centrifugal compressors addresses design and application differences.
With the transition toward a more sustainable energy economy, the old debate between reciprocating versus centrifugal compressors —and which one is better—has reared its ugly head again. For conventional hydrocarbon compression applications, this discussion has been raging for at least 50 years. But over the last 10 years or so most industry experts have arrived at some kind of peace agreement and a common acceptance that there are some applications where one or the other makes more sense. This has not yet happened in the new energy economy where this debate has just started. Since this is a complex subject, we will discuss it in a two-part myth with part one focusing primarily on applications and design, and part two will outline the fundamental challenges of each compressor type.
When talking about alternative energy sources, the primary gases of concern are green or blue hydrogen and CO2 for carbon sequestration. Both hydrogen and CO2 are physically very different from each other and from natural gas—which is still the most widely used energy source. Whereas the widespread use of hydrogen for major energy production applications is forecast to occur in maybe 10-20 years, the implementation of carbon capture and sequestration in power plants is happening right now. Therefore, we will focus this article on what type of machine is best for CO2 compression. We can have a similar debate about hydrogen later … maybe in a few years from now.
Reciprocating and centrifugal compressors are fundamentally very different machines. They use different compression principles and function mechanically differently. A reciprocating compressor uses a piston inside of a cylinder with suction and discharge valves for a positive displacement volumetric gas compression process. These compressors are well-suited for lowflow capacity requirements, high-pressure ratio applications, and can handle varying process gas compositions. However, reciprocating compressors are relatively bulky, less reliable, require more maintenance, and can produce higher levels of pulsations, vibrations, and noise.
A centrifugal compressor uses rotating impellers and stationary diffusers in a fixed-volume gas compression process. Compression is accomplished by dynamically adding work to the gas. These compressors are ideal for medium-to-high flow capacity applications, are more compact, require less maintenance, and provide a smoother operation with reduced vibration and noise levels. Centrifugal compressors excel in handling large volumes of gas, making them ideally suitable for large-scale industrial processes.
So, what are the applications of a typical carbon capture and sequestration application? In the United States, over 70% of the CO2 from stationary sources is emitted by fossil-fueled electric power plants. In many developing industrial nations, this percentage is much higher. Thus, the primary and initial target—the low-hanging fruits—for industrial-scale decarbonization using capture and sequestration are large fossil-fueled power plants. These power plants are mostly fueled by natural gas or coal. There are different options for carbon separation in fossil-fueled plants, including pre-combustion hydrogen production or post-combustion flue gas cleanup. In both cases, the CO2 will be produced at relatively low pressures, usually below 30 psia. After separation, the CO2 needs to be compressed to supercritical pipeline transport pressures to around 2,000-2,200 psia, and then injected into a geological formation for permanent storage at similarly high pressures. The advantage of transporting CO2 at supercritical pressures is that it has a high density (similar to that of liquid water) and very low viscosity, allowing for efficient pipeline transport. Operating in the dense phase also significantly reduces the power demand for the pumping or compression stations along a CO2 pipeline, although applications where existing natural gas pipelines are to be used will operate at much lower, typically subcritical, pressures. Thus, for the new sustainable energy economy, the most common compression applications are:
(1a) pipeline injection from near atmospheric to 2,000+ psia
(1b) pipeline injection from near atmospheric to 600 psia
(2) pipeline recompression or re-pumping with typical pressure ratios around 1.5, and
(3) geological injection compression or pumping from the pipeline at pressures that depend on the final geological storage pressure, which can vary widely but can be well above 2500 psia.
As a side note, from an aerodynamic design perspective, there is not much of a difference between a compressor and a pump when they operate at high pressures above 1080 psia in the dense phase. Here the fluid is not very compressible, and any work input does not result in significant density and temperature changes. The primary difference is that pumps are usually designed to operate at the synchronous speed of an electric motor while compressors often run at much higher speeds using a gearbox of high-speed drivers. Consequently, to achieve a similar head, pumps usually require more stages than compressors, but because of their construction, they are usually less expensive.
The reality is that centrifugal compressors can also produce high-pressure ratios for fluids that have physical properties associated with low specific heat and a high molecular weight, i.e., most heavy gases.
Back to the discussion of what is the best compressor for a sustainable energy economy: Reciprocating compressor manufacturers (and their supporters) claim that their machines are good for hydrogen compression. There is some truth to that for low-flow applications since reciprocating compressors using piston compression can achieve high-pressure ratios with relatively few stages. Similarly, they claim that, since most CO2 capture applications require very high-pressure ratios, reciprocating compressors are equally superior in this area. However, this argument is false and based on a fundamental misunderstanding of compression thermodynamics.
The reality is that centrifugal compressors can also produce high-pressure ratios for fluids that have physical properties associated with low specific heat and a high molecular weight, i.e., most heavy gases. This is especially true for CO2, which has a molecular weight of 44 and a specific heat of 0.8 kJ/kg K. A basic reading and simple analysis of Euler’s turbomachinery equation (which relates fluid angular momentum to head) and the isentropic relationships (which relate temperature differential - and thus head rise - to pressure ratio) demonstrates this. Here the physical relationship between pressure rise and head for centrifugal compressors shows that each rotating impeller stage can produce very high-pressure rise even at moderate impeller tip speeds. For example, a single-casing centrifugal compressor can easily compress CO2 fluid from 20 psia to 1,200 psia (well above its critical point) using only seven impellers running at normal tip speeds of 1,200 fps. This is a staggering pressure ratio of 60:1. Clearly, these impeller stages must have some intercooling to avoid excessive temperatures and to maintain high efficiency, but this is no different than what is required from all types of compressors, including reciprocating compressors.
On the other hand, centrifugal compressors have major operational advantages over reciprocating compressors in that they can handle much higher flows, are more reliable, require less maintenance, have nearly zero gas leakage, do not contaminate the process gas with lubrication oil, do not require pulsation control, are significantly quieter, have lower vibrations, have a wider operating range, are easier to scale, and provide for more concise and stable flow control. In the past, for lower-flow carbon sequestration applications, reciprocating compressors rather than centrifugal compressors were the traditional option. These were mostly electric motor-driven. However, as the industry is moving rapidly toward much higher CO2 capture flow rates, the limits of the practical use of reciprocating compressors are being reached.
In part two of this Myth Busters article, we will discuss compression challenges and some of the lesser-understood disadvantages of reciprocating compressors for CO2 applications.
The authors want to thank Karl Wygant and Rob Pelton, both at Elliott Turbo, for their contributions to this article.
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.
Any views or opinions presented in this article are solely those of the authors and do not necessarily represent those of Solar Turbines Incorporated, Elliott Group, or any of their affiliates.