Pumps for Carbon Capture

Carbon capture and storage (CCS) is expected to be an important tool to meet reduction targets on CO₂ emissions. CCS by aid of scrubbers or the so-called Amine Process is the most mature end of pipe solution today but intensive research is going on to find cheaper and less energy-demanding processes. Process simulations are an important tool for researchers and process engineers as they can optimize and benchmark many different solutions against each other before the actual construction of the facilities.

In this article, the result from a process simulation of an amine process is disclosed together with various experiences aligning with selecting the most appropriate pumps in respect to their hydraulic performance, corrosion resistance, and mechanical sealing system for this application.

By Jens Peter Hansen – DESMI

For nearly three decades, the UN has brought together almost every country on earth for global climate summits. The final text of the latest summit, the COP27 in Sharm El-Sheikh, Egypt, in­cludes a provision to boost low-emis­sions energy. In practice, this means that many different technologies must be used to help cut emissions – includ­ing carbon capture and storage (CCS) systems.

The International Energy Association (IEA) has provided an excellent over­view of the potential of capturing, storing, and utilizing carbon dioxide. An estimated 45 Metric Tonne per year (Mt/year) is currently being captured and this must increase to about 1300 Mt/year in 2030 to meet a net zero emission scenario (NZE). Around 35 facilities are already in operation, and over 200 new facilities are planned to be in operation by 2030. However, this corresponds to ‘only’ 220 Mt/year of CO₂ being captured, so facilities for at least an additional 1000 Mt/year will be required to soon meet the Net Zero Emissions (NZE) target.

Today, most facilities are installed to capture CO₂ emissions from natural gas and hydrogen processing plants. Somewhat ironically, the captured CO₂ is mainly used for enhanced oil recov­ery (EOR). However, as oil production must decrease in the future, most of the new planned facilities are aimed for power plants and the cement and steel industries, as these sectors are impossible to electrify completely due to the nature of their raw materials. Furthermore, the captured CO₂ should not be used for EOR but stored perma­nently underground.

CCS By Aid of Scrubbers

Capturing the CO2 in a scrubber or absorption tower is the most com­mon and well-known method for carbon capture. As shown in Figure 1, a liquid mixture of water and an or­ganic amine, for example Mono-eth­anol-amine (MEA), is circulated be­tween an absorber and desorber unit.

The inlet flue gas is brought in direct contact with the liquid in the absorp­tion or scrubber tower. The scrubber is usually a packed-bed type and typically 20-40 meters in total height (only the height of the bed is indicated on the figure). A relatively high tower is required if the concentration of CO₂ is low in the flue gas or if high removal efficiencies are required.

The lean amine is fed into the top of the scrubber and is enriched with the CO₂ by flowing downwards in count­er-current with the flue gas. A first pump is used to force the solution through a plate heat exchanger and to the desorption or stripping tower. The CO₂ is released in pure form by heating the liquid in the reboiler section. A second pump is used to force the liquid through the hot side of the plate heat exchanger and back to the absorption tower again. An additional cooler is often required to reduce the evaporation of water in the absorption tower.

Pump Operating Expenses

The simulation results shown on Figure 1 are based on many preliminary assumptions which must be further val­idated and adjusted as more experience with the process is gained. However, it is interesting to try to relate the calculat­ed pump power to the amount of CO2 captured, and users can arrive at an initial estimate of the operating expenses.

As can be seen, 1061 kW of electrical power (496 kW + 565 kW) is used by the two pumps for catching 20 kg/s of pure CO₂. This is equivalent to 52 kJ/kg of CO2 or 14 kWh/ton of CO₂. If it was assumed an electricity cost of €0.15/ kWh, the operating expense for the two pumps will be €2.1/ton of CO₂.

As CCS plants become more widespread and operate with increased capacity. Selecting the most appropriate pumps for a certain scrubber installation is not a trivial task; par­ticularly considering that two or three pumps are often in­stalled in parallel to provide some redundancy in the scrub­ber system. At a CCS plant capable of processing 1 Mt/year, for example, annual operating expenses with the pumps used in the example above would be €2.1 million. At this scale, even small power consumption reductions make a significant difference to the operating economy. Selecting pumps that are sized correctly and operate at or near its best efficiency point (BEP) is an effective way of achieving these cost reductions.

Sizing and Selecting Pumps for CCS

Often times users see that too much safety margin is added in the design phase and so, the pump will run too far from its BEP (the blue line on the Q-H plot in Figure 2). In the worst cas­es, the pump may even run outside the recommended operating area, which is between 70% and 120% of its BEP (the area between the green dotted lines on the Q-H plot).

It is usually better to add a safety mar­gin in the frequency converter so the pump can run at higher revolutions per minute (RPM) in extreme cases where max flow and head might be required. This will save both CAPEX and OPEX as smaller pumps can be installed, and it will be easier to op­erate them at its best efficiency point. The pumps will also run with fewer vibrations and generate less heat, meaning that there will be less excess energy that could cause damage.

A low NPSHr value is often required. For the rich amine (pump 1 in Figure 1), the solution is almost saturated with CO₂ and therefore has a relative­ly high vapour pressure. For the lean amine (pump 2 in Figure 1), the solu­tion will only contain little CO2, but the temperature will be higher and therefore will also cause a relatively high vapour pressure.

Pumps in high-grade stainless steel or super duplex steel (SAF2507) are usually selected for MEA solutions based on the latest scientific findings. In addition, it is recommend equip­ping pumps with a Dual Cartex seal due to the toxicity of the MEA. The sealing system can also be connected to an external barrier fluid to ensure that the gaskets will not exceed the max limit temperatures.

For projects that use a less toxic solu­tion of water and potassium carbon­ate instead of an amine (for example the Swedish BECCS @ STHLM proj­ect), cheaper pump materials and a standard balanced sealing system can be used; the potassium will act as a buffer to ensure an almost neu­tral pH-value.

Conclusion

If CCS is to make a significant con­tribution to reaching the world’s cli­mate targets, the technology needs to be developed further. Researchers worldwide are intensively searching for more efficient liquids that require less energy for regeneration. How­ever, this will usually be at the cost of reduced reactivity so that larger scrubbers must be used, or more liquid must be circulated to catch the same amount of CO₂.

References:

1. What are the key outcomes of Cop27 climate summit? | Cop27 | The Guardian
2. Carbon Capture, Utilisation and Storage – Analysis – IEA
3. Kittel, R. Idem, et. Al., Corrosion in MEA units for CO2 capture: Pilot plant studies; Energy Procedia. Volume 1, Issue 1, February 2009, Pages 791-797, November 2016, Lausanne, Switzerland. https:// www.sciencedirect. com/science/article/pii/ S1876610209001064
4. Silje Hjelmaasa, Erlend Storheim et al, Results from MEA amine plant corrosion processes at the CO2 Technology Centre Mongstad; 13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18
5. Monoethanolamine (MEA) (dow.com)
6. How it works – Beccs Stockholm