Better understanding of CO2 liquefaction (Towards identifying optimal transport conditions for ship-based CCS)  

Authors: Han Deng, Simon Roussanaly, Geir Skaugen

Ship-based transport of CO₂ is an attractive technology for early deployment of CCS. It has several advantages like cost-effectiveness for long distances and small volumes, low capital investment, flexibility, and opportunities for co-utilisation of infrastructure. As CCS chains based on ship transport are moving closer to implementation, the question of optimal transport conditions (pressure, temperature and composition) is being raised. To date, no openly public study has satisfactorily concluded on optimal transport conditions for CCS. A first step towards identifying these conditions is better understanding of the CO₂ liquefaction process.

The components of the CO₂ shipping chain

The CO₂ shipping chain starts after the CO₂ capture and lasts until storage. The chain involves liquefaction, buffer storage, loading/unloading, shipping transport and reconditioning. In practice, the CO₂ could be transported under different transport conditions (temperature and pressure).

What did we previously know about optimal CO₂ shipping conditions?

In the past decade, the low pressure-based transport (7 bar and -50℃) and medium pressure-based transport (15 bar and -30℃) have been the two most discussed options. For the Norwegian full-scale CCS project, initial work pointed at the 15 bar option as the best solution from the cost, safety and maturity perspective. Recently, it was indicated that in the future, lower pressure-based transport could be a better solution.

NCCS – The value of bringing the CCS world together

In Norwegian CCS Research Centre (NCCS), we therefore do research to identify the optimal transport conditions and provide recommendations on the best measures to cut CCS costs. As a first step toward this ambitious target, we started by focusing on the liquefaction, especially on trying to understand the impact of the targeted transport pressure and impurities on the design and cost of the process. The blog presents the results of this work performed in NCCS (Task 1) and presented in the following publication.

• Deng H., Roussanaly S., Skaugen G. (2019) Techno-economic analyses of CO₂ liquefaction: Impact of product pressure and impurities. International Journal of Refrigeration, 103, 301-315.

The CO₂ liquefaction process

The CO₂ liquefaction consists of a series of compression stages and cooling, through which the CO₂ stream is liquefied to reach the conditions for temporary storage or transport. We considered a liquefaction process including a compression train, an impurity removal unit, a refrigeration cycle, flash tanks and other minor components.

In the compression train, the CO₂ stream is compressed to the desired liquefaction pressure and water is removed by condensation to prevent hydration. The impurity removal unit will be used to remove other impurities when the delivered CO₂ needs to meet a high purity requirement. The refrigeration cycle is to condense the CO₂ stream.

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How the delivery pressure impacts the CO₂ liquefaction cost

In our study, the impact of the targeted delivery pressure on the design and cost of the liquefaction process has been investigated with an integrated techno-economic optimisation model. We looked at four cases; a case based on pure CO₂ after the capture process, which corresponds to the ideal situation, and three cases considering CO₂ composition obtained from the following combination of industrial application and capture technologies:

1) Amine-based post-combustion CO₂ capture from a cement plant

2) Membrane-based post-combustion CO₂ capture from a refinery

3) Rectisol-based pre-combustion CO₂ capture from a coal power plant.

While the first case helps us understand the performances of the CO₂ liquefaction process under “ideal” conditions (i.e. no impurities), the three last cases enable a good understanding of the impact of type and level of impurities on the liquefaction process design and cost.

After evaluating the technical and cost performances of the liquefaction process for 10 targeted transport pressures (ranging from 7 bar to 70 bar) for the four scenarios, we found that:

• For pure CO₂, the highest liquefaction cost is obtained at 7 bar among the range considered, while a minimum lies around 40-50 bar (13% cheaper than at 7 bar). The decrease in cost is due to the significant reduction in the power requirement associated with cooling of the CO₂
• For impurity cases considered, the liquefaction cost is increased significantly compared to pure CO₂ (up to 34% in some cases). This increase varies from case to case, with the membrane-based case being the most affected and the amine-based case being the least. Overall, delivery pressure under 30 bar is the most heavily impacted, while a more moderate increase is observed beyond. The rise in the cost is partially caused by the loss of CO₂ through purge to remove impurities that are not soluble in the CO₂ This results in a reduction in the amount of CO₂ delivered by the liquefaction process. Besides, the rise in cost is also linked to the higher investment cost of the liquefaction process, due to higher compression and cooling requirement.
• In the comparison of the most discussed 7 and 15 bar options, the cost is reduced by 1.05 €.tCO₂^-1 at the 15 bar option for pure CO₂ case, while the reduction varies between 1.3 and 1.7 €.tCO₂^-1 depending of the impurity scenarios considered.

The effect of purity requirements

For a given impurity case, the different delivery pressures may result in different purities of the delivered CO₂ after the liquefaction process. In the above analysis for the impurity scenarios, no requirement is imposed on the purity of delivered CO₂. However, strong purity constraints may be imposed to the liquefaction process due to requirement down the chain (transport and/or storage). This could have a significant impact on the comparison of the different possible transport pressures. We selected one impurity case to investigate the impact of the purity requirement, with considering industrial-grade and food-grade purity requirements (≥99% and ≥99.9% purity respectively).

It turns out that potential purity requirements also have a significant impact on comparisons of the delivery pressure. For example, the cost difference between the 7 and 70 bar pressure options is significantly reduced when 99% purity requirement is met. The difference ranges between 3.2 and 4.5 €.tCO₂^-1 depending on the cost of impurity removal, while this difference is 6 €.tCO₂^-1 in the case without a purity constraint. For further details and findings, please read the full-length paper.

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Next step – integrating with the rest of the chain

We are continuing the work to integrate results from CO₂ liquefaction with the shipping part of the chain (buffer storage, ship transport and reconditioning), to reach our goal of identifying the optimal transport conditions for the whole CO₂ shipping chain.