TurboTime Podcast: The ‘Critical’ Points of CO2

In this episode of the TurboTime Podcast, the Myth Busters Klaus Brun and Rainer Kurz engage in a comprehensive, wholesale discussion regarding CO₂, including the turbomachinery and technology used to capture, compress, transport, and sequester the gas; its applications across the decarbonizing power industry; and some of the technical challenges CO₂ presents. Also, Brun and Kurz thoroughly explain the gas’ supercritical phase and forecast the future of CO₂-based turbomachinery.

Kurz: For the last 4 – 5 years, there have been questions as to whether people will invest in creating better CO₂ compressors or hydrogen compressors. The reality is, where we stand right now, you’d need a lot of hydrogen to make a centrifugal compressor worthwhile and there’s nothing in the near future to provide large amounts of hydrogen. In the decarbonizing world, we’re now accepting the fact that CO₂ will be produced, and fossil fuels won’t go away, so what do we do with the CO₂?

In this context, there’s a lot of compression required; for example, if we capture CO₂ from the exhaust of fossil fuel or natural gas power plants and want to transport it to a sequestration site, then we’d have to compress the CO₂. Sequestration sites could sound a bit abstract, but they’re essentially old, abandoned gas fields—or things like that—which allow you to store a gas permanently. It turns out that the most economical way of transporting CO₂ is to go supercritical, which means relatively high pressures.

Brun: When you’re looking at CO₂ compression, what really makes it interesting is that CO₂ is not difficult to compress—it’s a relatively heavy gas with a molecular weight around 42 g/mol and this makes it, from a thermodynamic perspective, easy to compress. With a low head or low energy input, you get a very high compression ratio for the compression stage. But you also get a high temperature ratio across each stage. In most applications we’re examining for CO₂—not just carbon capture and sequestration as there’s also energy storage, power cycles, and heat pumps—the pressure ratio starts at a relatively low pressure around atmospheric and rises to supercritical.

This means you must cross the critical point, in which phases of the fluid change from gas to liquid to supercritical. At this point, all physical properties are changing drastically and makes it very challenging from a compressor design perspective. When you design a compressor, you try to compress away from that point due to these severe physical changes: the volume flow, pressure ratio, density ratio, and more will be off the closer you get to the critical point.