CO2 transportation research: Liquefaction of gas mixtures and recovering liquefaction cost (Flexible Funding 2020)

Prof. Kumar Patchigolla at Cranfield University was awarded funding in the UKCCSRC’s Flexible Funding 2020 call to look at the “CO₂ transportation research: Liquefaction of gas mixtures and recovering liquefaction cost”.

Increase in CO₂ emissions has led to global warming driving humanity to adopt Carbon Capture Utilisation and Storage (CCUS) processes more extensively. In this research project, work is carried out to lower the cost of CCUS deployment, in particularly for the marine sector by using a cold energy utilisation technology. Captured CO₂ is liquified and transported in a ship container at optimal conditions of -28.5°C and 15 bar. After the ship container has unloaded the liquid CO₂, the empty container tends to warm up due to heat transfer from ambient temperature. A Cold Thermal Energy Storage (CTES) unit is designed to utilise the cold energy of liquefied CO₂ to precool the ship container before loading, saving operational cost.

Fig. 1. Schematic of the (a) cold thermal energy storage 3D design and (b) cold energy utilisation technology

The designed CTES system consisted of heat exchanger, control centre, inlet and outlet valves as shown in the Fig. 1a. Heat exchanger thermodynamic calculations were done for the required flow of cold energy using liquefied CO₂. The material of the ship container was identified, and a process flowchart was created accordingly using Aspen HYSYS providing the flow rate of the liquefied CO₂ into the CTES system. It is concluded that the CTES unit of volume 15m³ utilising phase change material (PCM) can store 309 kWh of cold energy required to cool down a typical 11,500m³ ship container. Commercial PCM E-26 were chosen as the material costing £1.95/Kg and Carbon steel grade A517 was selected for manufacturing storage tank costing £0.97/kg. The cost estimates were excluding the packaging and transport costs. It has also been estimated that a typical ship taking around 200 trips per year, saving 309 kWh of energy per trip and with current UK electricity cost of 1 MWh being £1000, the overall cost saving by the proposed CTES system is £200,000 per year. This concept has the potential to its return within several years of implementation.

Additional research has been secured to explore its potential implementation with Oman shipping company.

Furthermore, as part of this project, we were able to arrange a site visit to Uniper Grian Power Plant in Kent, UK, where they are having significant long-term regasification capacity at the Grain LNG terminal, to convert LNG back to natural gas. We would like to extend our thanks to Uniper colleagues for their support in showing our future “Heat Engineers” around the power plant.

Fig.2. Uniper power plant site visit for Prof. Patchigolla’s group along with MSc Advanced Heat Engineering students at Cranfield University

An open-technology and open-access post-combustion capture initiative – Professor Jon Gibbins, Director, UK CCS Research Centre and Professor of CCS, University of Sheffield; and Dr Stavros Michailos, Researcher Co-Investigator, University of Sheffield (now Lecturer, University of Hull)

Post-combustion CO₂ capture (PCC) plants are the basis for the majority of projects in the current wave of UK CCS deployment.  Applications actively being considered include gas-fired and biomass power plants, energy-from-waste plants, refineries and steam-methane reformer retrofits.  PCC plants are also widely applicable globally, particularly for retrofit applications.

This proposal addressed two issues:

1. Knowledge-sharing for cost reduction
2. Residual CO₂ emissions from PCC plants in the context of net-zero

The delivery of the objectives of this project was enhanced by a number of subsequent open-access projects that it facilitated.  The following non-experimental projects are completed:

• UPCC: Ultra-high Post-Combustion CO₂ Capture – a University of Sheffield QR SPF project (Dec 2020 to March 2021) – £18k funding for additional modelling work
• AECOM for BEIS, Start-up and Shut-down times of Power CCUS Facilities – showing solutions for 95% CO₂ capture at all times (Feb-July 2020), Gibbins acted as an academic reviewer. https://www.gov.uk/government/publications/start-up-and-shut-down-times-of-power-carbon-capture-usage-and-storage-ccus-facilities
• BAT Review for New-Build and Retrofit Post-Combustion Carbon Dioxide Capture Using Amine-Based Technologies for Power and CHP Plants Fuelled by Gas and Biomass as an Emerging Technology under the IED for the UK – UKCCSRC review by Gibbins and Lucquiaud https://ukccsrc.ac.uk/best-available-techniques-bat-information-for-ccs/ – 95% capture including start and stop are recommended in the associated guidance https://www.gov.uk/guidance/post-combustion-carbon-dioxide-capture-best-available-techniques-bat

Related experimental projects are still ongoing:

• PCC-CARER – UKCCSRC/industry funding https://ukccsrc.ac.uk/research/flexible-funding/flexible-funding-2021/prof-jon-gibbins-university-of-sheffield/
• FOCUSS – BEIS/industry funding https://www.ssethermal.com/news-and-views/2022/06/carbon-capture-rates-in-focuss-as-sse-thermal-secures-grant-from-beis/

Delivery of the project objectives was started by early workshops, as envisaged in the proposal, and then continued through stakeholder engagement on the BAT review, a number of conference presentations, an open-access journal peer-reviewed paper and updates on the project web page.  A list is given at the end of this blog.  This amounted to more material and engagement than anticipated in the proposal, due to assistance by the unforeseen related projects, and activity was extended over a considerably longer period than originally envisaged.  The project also helped to get a new detailed engineering study for an amine capture plant made available on the UKCCSRC web site https://ukccsrc.ac.uk/open-access-sherman-feed/ and publicised to potential users.

In addition to high levels of CO₂ capture from combustion flue gases, the research outcomes also suggested a way to reduce the regeneration energy penalty for CO₂ capture from air using amines.  This has resulted in a new UKCCSRC project with co-investigator Stavros Michailos as PI, Co-DAC: Low-energy Direct Air Capture potential when combined with a Post Combustion Capture plant, with the concept described in a GHGT-16 presentation/paper https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4283821.  Stavros Michailos has also progressed to a new position as Lecturer at the University of Hull.

As a result of computer modelling in this project, a clear theoretical understanding has been gained of how PCC plant operating parameters can be adjusted to achieve high levels of CO₂ capture and the factors that determine the amount of heat required when doing so.  In general, previous studies have not undertaken an holistic analysis and have therefore concluded that there are greater barriers to achieving high capture levels than, in fact, exist – the problems they encountered being due to pre-selection of operating parameters such as stripper pressure or absorber liquid/gas ratio.

An outline illustration of a PCC plant is shown below.  This project modelled the performance of such a plant using a non-proprietary capture solvent, a solution of 35% w/w monoethanolamine (MEA) in water.

One key finding was that lean solvent loadings of ~0.1 mol CO₂/mol MEA are required to achieve high capture levels (95-99%) in the absorber.  This is not a surprising result as the leaner a solvent is, the more aggressively it will remove CO₂ from a gas mixture.  The problem is how to produce this low lean loading without requiring excessive energy.  Previous studies, using a fixed stripper pressure, have concluded that a lot of energy would be required but we were able to show that, provided the pressure in the stripper/reboiler was high enough, the amount of heat required per tonne of CO₂ captured would not go up.  The high pressure is important to suppress wasteful water vapour production in the reboiler; some water vapour is essential to ‘strip’ the CO₂ from the solvent when it is heated, but a lot of heat is required to produce it.

It was also found that a high rich loading in the solvent leaving the absorber was essential to minimise heat demand in the stripper/reboiler; not doing so means that ‘unused’ solvent is being circulated and heated and also involves more water vapour being emitted from the stripper with the CO₂.  High solvent rich loadings require that enough packing is available in the absorber to transfer enough CO₂ from the flue gas to the solvent.  Up to 24 m of packing height was found necessary to capture the 99% of the CO₂ in the flue gas from a gas turbine that comes from the burnt fuel, leaving only the 1% that came from the air going into it.  But this study emphasised that the solvent flow through the absorber is also critical: too low and it is insufficient to capture the CO₂, too high and the CO₂ is captured but the rich loading will be lower than the optimum value.

Work is now ongoing to test these theoretical findings on the amine capture pilot plant at the University of Sheffield’s Translational Energy Research Centre (www.terc.ac.uk), shown below, at a scale of around 1 tonne of CO₂ capture per day.

Project outputs

We’re pleased to share a blog report below from Stavros Michailos, Mohammed S. Ismail and Lin Ma of Energy 2050 at the University of Sheffield, on Lin’s Flexible Funding 2020 project, Evaluation of different CCUS systems based on the MCFC technology for decarbonising the power generation sector:

Carbon capture utilisation and storage (CCUS) is pivotal for delivering net-zero GHG emissions and strategically significant to the UK economy. The study details the performance of three CCUS systems based on MCFC via multiscale modelling, i.e. multiphysics modelling of the MCFC unit and process modelling. A distinct advantage of the MCFC over other CO₂ capture technologies is that, instead of absorbing energy (electricity), it generates it. Mass and energy balances have been established in COMSOL Multiphysics® and Aspen Plus software. Based on the simulations, a levelised cost analysis has been undertaken to appraise the capture cost of the investigated technologies. The MCFC concentrates CO₂ that can be stored or further utilised. Figure 1 is a visual representation of the scenarios examined in the study by means of simplified block flow diagrams (BFD).

Figure 1. Block flow diagrams of the investigated CCUS scenarios

Multiphysics model of MCFC

A two-dimensional MCFC multiphysics model has been created to inform the process model of the entire system. Figures 2 displays the profiles at a typical cell potential (0.7 V) for the temperature, CO₂ concentration along the cathode channel and H₂ concentration along the anode channel. The temperature is almost uniform across the various components of the fuel cell with slightly increased temperatures at the anode side of the fuel cell (Figure 1a). Figure 1b shows that the concentration of CO₂ (mol/m³) expectedly decreases from the inlet to the outlet, with very low amount of CO₂ at the cathode electrodes where the cathodic half reaction takes place. This observation also applies to H₂ concentration in the anode side of the fuel cell (Figure 1c).

Figure 3 shows that the cell voltage and power density increase with increasing current density after 3500 A/m² (Figure 3(a-b)); this is attributed to the availability of amount of CO₂ that is higher than that what is needed for the half reaction at the anode electrode. However, before 3500 A/m², the fuel cell performance appears to slightly improve with decreasing CO₂ concentration; this seems to be due to availability of amount of oxygen that is substantially higher than what is needed to complete the half reaction at the cathode electrode. Evidently, the carbon capture factor, which is the ratio between the flow rate of CO₂ concentrated at the anode and the flow rate of CO₂ available in the flue gas, substantially decreases with increasing CO₂ concentration in the flue gas.

Process modelling

Aspen Plus has been utilised to model the electrolyser and the Fischer-Tropsch (FT) plant. In particular, for the FT plant, CO₂ and H₂ are fed to a reverse water gas shift reactor to convert CO₂ to CO. The produced syngas is sent to the FT reactor, which generates a mixture of light gases, syncrude oil and water. After proper separation a part of the unreacted gases and light hydrocarbons are sent to a CHP unit modelled as combined cycle turbine and the rest is recycled to the FT reactor. The syncrude oil is fractionated to naphtha, kerosene and diesel cuts. The CO₂ that is not utilised is sent for compression and permanent storage. Figure 4 presents the basic mass balances of the Scenarios 2 and 3.

Figure 4. Mass balances of the investigated scenarios

Conclusions and future work

The study revealed that MCFC is an interesting CO₂ capture technology with prospects to play a role in the mid-term for decarbonising scenarios. The stand-alone case, i.e. scenario 1, with certain technology improvements can raise the competitiveness of the MCFC. The multiphysics model of the MCFC has been built and used to inform the process model on the amount of CO₂ concentrated at the anode of the MCFC at a typical operating cell potential. The parametric study shows that the MCFC performance in terms of power density improves with decreasing temperature, increasing CO₂ concentration in the flue gas and increasing H₂ concentration in the fuel mixture. Conversely, the CO₂ capture factor was found to increase with increasing operating temperature, decreasing CO₂ concentration in the flue gas and decreasing H₂ concentration in the fuel mixture. In the future, experimental testing of the MCFC in the Translational Energy Research Centre (TERC) will enable the improvement of the models developed in the current project.

The UK Carbon Capture and Storage Research Community (UKCCSRC) is supported by the Engineering and Physical Sciences Research Council (EPSRC) as part of the UK Research and Innovation (UKRI) Energy Programme.

The mission of the UKCCSRC is to ensure that carbon capture and storage (CCS) plays an effective role in helping the UK achieve net-zero greenhouse gas emissions by 2050.

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