pump for CCS Selecting the best

Pumps play an important role in Carbon Capture and Storage (CCS). Selecting the most effective pump is the key to achieving low operating costs, cutting CO2 emissions, and limiting the global temperature increase. Senior Product Development Manager Jens Peter Hansen and Key Account Manager Marius Berg Rygvold from DESMI outlined the options.

For nearly three decades, the UN has organised 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. This requires the combination of many different technologies to help cut emissions – including Carbon Capture and Storage (CCS) systems.

What is the potential of CCS?

The International Energy Association (IEA) provided an excellent overview of the potential of capturing, storing and utilising CO2. It estimates that 45 Mt/year is currently being captured, but this must increase to 1,300 Mt/year in 2030 to meet the net zero emission scenario (NZE). Around 35 facilities are already in operation, and over 200 new facilities are planned for 2030. However, this corresponds ‘only’ to 220 Mt/year of CO2 being captured, so facilities for at least another 1,000 Mt/year will be required soon to meet the NZE target.

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

The CO2 scrubbing process

Capturing the CO2 in a scrubber or absorption tower is the most common and well-known method for Carbon Capture. A liquid mixture of water and an organic amine 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 metres. A relatively high tower is needed 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.

Calculating pump expenses

The simulation results 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 relate the calculated pump power to the amount of CO2 captured, thus reaching an initial estimate of the operating expenses.

1,061kW of electrical power (496kW + 565kW) is used by the two pumps catching 20kg/s of pure CO2. This is equivalent to

52kJ/kg of CO2 or 14kWh/ton of CO2. At an electricity cost of €0.15/kWh, the operating expense for the two pumps will be €2.1/tonne of CO2.

As CCS plants become more widespread and operate with increased capacity, selecting the most appropriate pump(s) for a certain scrubber installation is a major task; particularly considering that two or three pumps are often installed in parallel to provide redundancy in the scrubber system. At a CCS plant capable of processing 5Mt/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 differ ence to the costs. Selecting pumps that are sized correctly and operate at, or near, their best efficiency point (BEP) is an effective way of reducing expenses.

Tips for selecting pumps

Too much of a safety margin is often added in the design phase, meaning that the pump will typically operate far from its BEP. In the worst cases, the pump may even run outside the recommended operating area, which is between 70 and 120% of its BEP.

It is usually better to add a safety margin in the frequency converter so the pump can

Click here to see the range of centrifugal pumps from DESMI for CCS and other applications run at higher 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 efficiently. The pumps will also run with fewer vibrations and generate less heat, meaning there will be less potentially damaging excess energy.

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

Full range of pumps & tools

DESMI has developed more than 60 centrifugal pumps for the scrubbing industry, generally designed for low NPSHr values. Most of the pumps are available in highgrade stainless steel or super duplex steel. The Dual Cartex seal is recommended 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 their maximum temperatures.

For projects using a less toxic solution of water and potassium carbonate instead of an amine, cheaper pump materials and standard balanced sealing systems can be used; the potassium acts 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 further development. Researchers worldwide are intensively searching for more efficient liquids that require less energy for regeneration. However, this may be at the cost of reduced reactivity, meaning larger scrubbers, or more liquid must be circulated to catch the same amount of CO2. These challenges will be overcome, and future CCS solutions will become more effective, capturing ever larger amounts of CO2 using minimal energy.

Pumps are not the only important component of a CCS plant. Components such as compressors and reboilers consume far more electricity. However, given the number and scale of future CCS plants, and the fact that succeeding at the green transformation will require us to lower energy consumption in all industrial sectors, selecting energy-efficient pumps for CCS applications makes good financial sense. This is also an effective way of contributing to reaching current and future climate targets. n

Latest innovations from the engineering industry