This paper highlights the care to be taken by an Engineer while specifying and selecting a centrifugal compressor for CO2 compression.
With the increase in the levels of CO2 in the atmosphere, there is an increase in the popularity of capturing CO2 emitted from the large source points such as fossil fuel power plants, steel mills, cement plants, etc. before its release to the atmosphere and storing it under the geological formations (also used for Enhanced Oil Recovery (EOR) where possible). The compressors used to transport and store the CO2 at such depths need to compress the gas from atmospheric pressure to the pressures of the order of 200 bars or more.
The critical temperature of CO2 is only 31.1 deg. C, so the CO2 is generally transported and stored in supercritical state. The thermodynamic properties of supercritical CO2 are considerably different from the other real gases generally compressed. Further, to achieve this supercritical state, the critical point of CO2 is crossed somewhere in the compression stage. Near critical point, the ideal gas laws will not hold good for CO2. Moreover there is a reduction in the choke margin of the compressor due to the reduction of the sound speed in CO2, particularly near the thermodynamic critical point. Also the CO2 compressibility and specific heats are not linear near the critical pressure and temperature. The impurities in the CO2 will further affect the thermodynamic characteristics of the working fluid. Also the water content in the CO2 makes it extremely corrosive.
Considering all the aspects mentioned above, specifying the CO2 compressor correctly in terms of the equation of state to be used, the interstage pressures and temperatures to be maintained, suggesting the number of impellers per stage to maintain the desired flow coefficient, metallurgy to be selected and scheme of compressor dry gas seals, etc becomes all the more important and are described in the paper.