A study of the design and operation of centrifugal compressors for CO2 pipeline transportation

Okezue, Chimaoge N.

Abstract

Carbon Capture and Storage (CCS) is one of the technological options recommended by the United Nations Inter-Governmental Panel on Climate Change (IPCC) for achieving a net reduction in CO2 emissions to the atmosphere. CCS process is made up of three aspects, namely: carbon capture, transport and storage. Historically, researchers tended to focus heavily on the capture and storage aspects of the CCS process chain with the CO2 transport aspect receiving less attention. However, in recent years, the level of research in CO2 transport has increased sharply. Research shows that pipelines are the most viable means of transporting large volumes of anthropogenic carbon dioxide from offshore and onshore sources of emission to the place of permanent storage. In pipelines, CO2 can be transported in gaseous, liquid or supercritical state. Supercritical CO2 occurs when its pressure and temperature are both above the critical point (73.76 bar; 30.97°C). Carbon dioxide in supercritical state has high density close to that of a liquid and low viscosity comparable to that of a gas. This means that a larger amount of CO2 per unit time can be transported in supercritical state than in gaseous or liquid state with low pipeline frictional pressure drop per unit mass and less energy costs. Therefore, for long distance pipeline transportation, it is economically sound to convey CO2 in supercritical state rather than in gaseous or liquid states.
Pipeline infrastructure required in the CCS context is on a scale that vastly outsizes similar infrastructure commonly used for transportation of air, natural gas, petroleum, etc. CCS pipeline networks operate on an industrial scale, transporting several metric megatons of anthropogenic CO2 per annum captured from multiple power stations and other process plants to designated places of storage. A large amount of energy is consumed by compressors and booster pumps in building up and maintaining the high pressure required to ensure anthropogenic CO2 mixtures are in a supercritical state within the pipeline. Furthermore, pure and impure supercritical CO2 exhibit erratic internal flow behaviour, a consequence of large, abrupt and barely controllable changes to its fluid properties provoked by minor shifts in pressure and temperature. All these make design and operation of supercritical CO2 pipeline networks more challenging and costlier than conventional natural gas pipelines where compression pressures and volumetric delivery rates are relatively lower and neither phase change nor radical variations in thermophysical properties are
expected.
A review of published literature on supercritical CO2 pipeline transportation shows a lot of effort has been made by other researchers to address design issues such as pipeline sizing, corrosion and fracture propagation and operational issues such as start-up, shut-down, rapid depressurization, valve blockage, etc. All these studies have reduced several of the technical challenges in the design and operation of the pipeline. Yet there is still a wide scope for further improvements and new developments. Compressors and booster pumps are responsible for most of the high amount of energy consumed in the operation of the pipeline. Therefore, the long-term economic feasibility of running a supercritical CO2 pipeline networks is achievable only if energy consumption and associated operating costs of both machines can be kept as low as possible. One way of reducing energy consumption is by sizing compressors and booster pumps optimally to ensure that power losses in both machines are minimized.
In this PhD thesis, a new mathematical model was developed from first principles. This new mathematical model uniquely combines the geometry and working processes of a centrifugal machine with anomalous and erratic non-linear real fluid flow behavior peculiar to supercritical CO2 and its mixtures. Successfully validated with available experimental data, the model was used to carry out a detailed investigation of the performance of centrifugal compressor handling supercritical carbon dioxide of varying purity under different operating conditions. Results of the investigation showed that parameters that characterize compressor performance such as power requirement, isentropic efficiency and pressure ratio are strongly dependent on impeller size, shaft speed, mass flow rate and the chemical composition of the supercritical CO2 and its mixtures.
More importantly, the work carried out in this thesis also demonstrated that the quasi-dimensional model can be used as a robust tool for optimal sizing of centrifugal compressors and booster pumps installed on a supercritical carbon dioxide transport pipeline.

Citation

Okezue, C. N. (2019). A study of the design and operation of centrifugal compressors for CO2 pipeline transportation. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4224617

• Thesis Type: Thesis
• Deposit Date: Jan 10, 2023
• Publicly Available Date: Feb 24, 2023
• Keywords: Chemical engineering
• Public URL: https://hull-repository.worktribe.com/output/4224617
• Additional Information: Department of Chemical Engineering, The University of Hull
• Award Date: Aug 1, 2019