Comparing the effect of rotor sizing versus rotor speed adjustment on centrifugal compressor energy requirements for CO₂ pipeline transportation

Okezue, Chimaoge

CCS transport; Compressor performance curve; Energy requirement; Working fluid; Impurities; CO₂ pipeline; Quasi-dimensional model; Pump; Superficial phase; Critical point; Equation of state; Laws of conservation


Kuvshinov, Dmitriy
© Chimaoge Okezue, All rights reserved. No part of this publication may be reproduced without a written permission of the copyright holder.

At the design stage of pipeline networks, it is common for engineers to include a margin of safety in sizing compressors to account for future expansions in system capacity. In the context of carbon capture and storage (CCS), this margin of safety is extended to account for changes in the purity of the supercritical carbon dioxide working fluid because of its effect on compressor energy requirement. However, when deciding on a margin of safety, it is not always possible to anticipate all the variations that will occur in the chemical composition of the working fluid over a long period of time. So it may be inevitable that over a long period of time, the energy requirement of a compressor will rise beyond what was originally anticipated due to significant changes in the composition of the impure CO₂ stream caused by alteration in capture technology or type of fuel burned in a power plant connected to the transport pipeline. Nevertheless, savings can be made in operating costs, if compressor performance can be optimized.
When the composition of the CO₂ stream changes, the minimum operating pressure required to maintain that fluid in supercritical phase inside the pipeline changes as well. In order to meet the required minimum operating pressure, the compressor rotor speed will either have to be increased or the machine can be re-sized while rotor speed remains constant. Either way, an increase in energy requirement is unavoidable. Increasing the rotor speed while machine size remains unchanged will incur higher energy losses than a proportional increment in machine size while keeping rotor speed constant. Despite this fact, in pipeline engineering, it is common practice to adjust compressor’s rotor speed to maintain the minimum operating pressure because it is easier than re-sizing the compressor’s rotor.
As a prerequisite for developing a procedure for optimizing centrifugal compressor performance, it is vital that the effect of increasing the machine’s speed be analyzed and compared against the effect of increasing its size. To this end, a quasi-dimensional model based on the laws of conservation was developed in MATLAB® to compare the effect of re-sizing the rotor against the effect of adjusting its speed on the energy requirement of a compressor handling supercritical carbon dioxide of varying chemical purity. Results of the study demonstrate that the performance of a compressor is affected by the impurities in the CO₂ stream. Moreover, the study confirms that it is more cost-effective to increase compressor’s rotor size while the rotor speed remains constant than vice-versa.

School of Engineering and Computer Science, University of Hull
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