Myth: Carbon dioxide is always pumped

With the onset of the hydrogen energy economy, and the significant cost and logistical advantages of blue hydrogen (hydrogen derived from fossil fuels with carbon sequestration) over green hydrogen, comes the need for added new compression of carbon dioxide (CO2) from hydrogen production for transport and sequestration injection.

Every pound of blue hydrogen produced through steam reforming or partial oxidation gasification creates about 10 pounds of CO2. This CO2 must be compressed from near atmospheric conditions to pipeline operating pressure. It then must be transported in the pipeline, and finally compressed to geological formation storage pressures for long-term sequestration. Compression is also required for CO2 derived from power plant post-combustion flue gas separation. The pressure of CO2 gas from this process depends on the type of separation process used and can vary from slightly above atmospheric to several hundred psi.

In addition, there is significant uncertainty about the injection pressure required by the geological formation since it is contingent on the type of formation and its drilled depth of injection. A generally accepted rule for storage in geological formations is that for every mile of depth of injection, about 1800 psi of gas pressure is required. Since many of the geological formations considered are relatively shallow, injection pressures well below 2000 psi are often sufficient.

Thus, carbon capture and sequestration in most power plant applications require these new compression duties:

1. Compression from the separation processes (membrane, thermal or chemical) to pipeline pressure.
2. Pipeline boost compression.
3. Injection for sequestration storage in a geological reservoir.

SUPERCRITICAL STATE

Further, new power plant cycles such as sCO2 cycles, or oxy combustion, are gaining acceptance. The generally accepted industry convention for CO2 transport is that CO2 should be transported as a supercritical fluid in pipelines at 2000-2200 psi operating pressure. At 2000 psi, CO2 is well above its critical point in a supercritical (dense phase) state for all ambient temperatures. Fluids in a dense phase share both some physical properties of liquids, such as very high density, and of gases, such as compressibility (albeit very low) and the fact that they expand in space to fill voids.

Every pound of blue hydrogen produced through steam reforming or partial oxidation gasification creates about 10 pounds of CO2.

The advantage of transporting CO2 at supercritical pressures is its very high density and low viscosity, allowing for efficient transport. This significantly reduces the power demand for the pumping or compression stations along a CO2 pipeline. The disadvantage of operating at these high pressures is the added compression ratio required at the pipeline header station and the significantly higher costs for materials to build a pipeline designed for maximum allowable operating pressure well above 2100 psi. This also means that very few natural gas pipelines can be converted for CO2 transport.

However, transport at 2000 psi is not required for all applications. The actual transport pressure of CO2 depends on the separation process outlet pressure, the distance the CO2 must be transported and the geological sequestration injection pressure (often well below 2000 psi). Since the CO2 available from separation is usually at low, near atmospheric pressures (<100 psia), the pipeline header station must always use a compressor.

Specifically, a 2000 psi CO2 pipeline requires a high-pressure ratio header compressor with many intercooled stages that can handle the significant volume reduction. But for this case, in the pipeline beyond the header station, the gas is transported in dense phase, either by pump or by compressor. On the other hand, if a lower pressure CO2 pipeline is used, conventional compressors are preferred for the header station and re-compression along the line. Clearly, selection of transport pressure depends on the carbon sequestration application, but it is not always advantageous to go to the pressure needed for a supercritical CO2 pipeline.

In general, CO2 is a heavy gas that is relatively easy to compress with centrifugal compressors from a compressor stage thermodynamic perspective. But it presents some other technical challenges that need to be addressed to make the overall compression or pumping process efficient and reliable:

• Most equations of state for CO2 and CO2 mixtures are inaccurate at high pressures and temperatures.
• Even in dense phase, CO2 is compressible.
• CO2 is a heavy gas that results in amplified rotor dynamic and impeller dynamic forces, and it has strong thermodynamic path dependence and multi-phase behavior.
• Carbonic acid is formed in the presence of water, driving corrosion, and CO2 is soluble in elastomeric materials, which can lead to rapid decompression failures. It selectively leeches certain elements from common metals.
• When rapidly expanded, CO2 quickly forms liquids and dry ice, which can become a potential problem for shaft seals.
• Aerodynamic concerns come from its low sonic speed, which can result in high Mach numbers, causing shock losses and a reduced operating range.

Proven machinery solutions for compression and pumping of CO2 are commercially available. Nonetheless, it is still important to be aware of these issues and to recognize that CO2 compression and pumping comes with technical challenges that must be individually addressed for all new carbon sequestration technologies.