Turbomachinery for Carbon Dioxide Transport and Storage
Authors: Brun Klaus 
Published: April 2023

Abstract
The trend towards a carbon free energy economy requires significant investment in technologies to efficiently produce, transport, store, and utilize hydrogen. Most hydrogen entering the market over the next 15-20 years will be natural gas derived blue hydrogen. Consequently, there will be a need for added new compression of carbon dioxide (CO2) for separation, pipeline, and storage injection. About 10 kg of CO2 is produced for every kg of blue hydrogen that is produced through steam reforming or partial oxidation gasification. This CO2 needs to be compressed from near atmospheric conditions to pipeline operating transport and then to geological formation storage pressures for long-term sequestration. The industry's generally accepted convention is that CO2 should be transported as a supercritical fluid above 2100 psi in pipelines. At 2100 psi, CO2 is well above its critical point in a supercritical or liquid (dense phase) state for almost all ambient temperatures. Fluids (in a dense phase) share some physical properties of liquids, such that they have a very low compressibility and also some of gases in that they will expand in space to fill voids. The advantage of transporting CO2 at supercritical pressures is that its density does not change much with pressure, and from a thermodynamic perspective, it is basically pumped rather than compressed.
Reciprocating, screw, multi-stage-centrifugal barrel, integrally-geared centrifugal, and diaphragm are the types of compressors that are usually considered for high-pressure ratio carbon sequestration CO2 applications. Since reciprocating, diaphragm, and screw compressors are severely flow limited, they cannot be practically used for large-scale carbon sequestration applications. However, centrifugal compressors tend to be non-ideal for applications when the process fluid is either a liquid or in the dense phase. Having both advantages and disadvantages, these could either be barrel-style compressors or integrally-geared compressors based on the application. One promising technology for large-scale carbon storage compression applications is a hybrid combination of a centrifugal compressor to compress the gas to slightly above its critical point in series with a dense phase pump to reach the desired process discharge pressure. This combination of turbomachinery can be packaged with a driver with a single gearbox for the compressor and a direct drive for the pump. Both low-pressure CO2 compressors and dense-phase pumps are proven technologies, but their hybrid combination has not seen significant service in the industry.
This paper discusses the background, applications, and challenges of CO2 compression including physical properties, thermodynamic optimization, safety, and regulator aspects. Barrel and integrally-geared compressors are covered in some detail, but reciprocating compressors are also briefly addressed. Discussed in more detail are the function, design, and operational limitations of a hybrid compressor and a pump package design for CO2 sequestration and transport applications. Machine, package design, layout options, footprint, and performance parameters are provided.