1. Evaluating performance during start-up and shut down of the TCM CO₂ capture facility (2022)

Mai Bui,^a,b* Matthew Campbell,^c Anette Beate Knarvik,^c Alexander Tait,^d Xiaomian Baxter,^e James Bowers,^e Niall Mac Dowella,^b

^aCentre for Environmental Policy, Imperial College London, UK ^bCentre for Process Systems Engineering, Imperial College London, UK ^cTechnology Centre Mongstad (TCM), Mongstad, Norway

^d bp Net Zero Teesside, UK

^e SSE Thermal, UK

Abstract

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The value of flexible CO₂ capture and storage (CCS) becomes greater with higher penetrations of intermittent renewable power (e.g., solar PV, wind). Moreover, start-up and shut-down cycles of power plants with CCS will become increasingly frequent, which could potentially increase fugitive emissions. Whilst start-up and shut down of power plants have been characterized, the effect of start-up and shut down on CCS plants is still unclear. The objective of this study is to quantify the residual CO₂ emissions, energy requirements, and time requirements of start-up and shut down in amine-based CO₂ capture plants. We performed a series of start-up and shut down tests in March, June and November of 2020 at the Technology Centre Mongstad (TCM) CO₂ capture facility in Mongstad, Norway. These tests used CESAR-1 amine solvent (aqueous blend of AMP and PZ) to capture CO₂ from flue gas (~4 mol% CO₂) exiting from the natural gas-fired CCGT combined heat and power (CHP) plant. These tests provide valuable insights on key process parameters and operation strategies that could help minimise disruption (e.g., time and energy requirements) and CO₂ emissions during start-up and shut down of CO₂ capture plants. Performance improvement could be achieved through optimising the solvent inventory volume, the timing of stream availability, and key process parameters such as solvent CO₂ loading or amine concentration.

Keywords: CO₂ capture, dynamic operation, flexible operation, post-combustion capture, pilot plant, demonstration, CCGT, start-up, shut down

1.  Introduction

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As energy systems transition to net-zero emissions, the share of intermittent renewable energy (e.g., wind and solar power) will increase significantly. However, this presents major operational challenges in terms of balancing the electricity grid to ensure energy demands are met. Technologies that can provide flexibility to the energy system will be required, this includes technologies such as batteries, or pumped hydroelectricity storage. Thermal power plants with carbon capture and storage (CCS) will also likely have an important role in providing affordable dispatchable low carbon capacity, thus helping to maintain security of supply and enable the expansion of other low-carbon energy sources.

Whilst the value of flexible CCS has been demonstrated in a number of previous studies [1-5], work to-date focusing on start-up and shut down of CO₂ capture plants has been limited [6-8], particularly with respect to the dynamic effects, e.g., timing, energy requirements and residual CO₂ emissions. This is particularly important with increasing penetration of intermittent renewable energy as these start-up and shut-down cycles will become increasingly frequent, thus the potential for fugitive emissions must be quantified and minimised. Further work is therefore necessary to develop an understanding of the process dynamics during start-up and shut down in order to identify the key factors that will impact CO₂ capture performance and operability.

The objective of this study was to examine the potential for performance improvements during the start-up and shut down of a CO₂ capture process in a power plant. This involved conducting a series of start-up and shut down tests at the Technology Centre Mongstad (TCM) CO₂ capture facility in Norway.

This paper presents a summary of some key insights from this study. For the detailed analysis and discussion of the test data from this work, interested readers can refer to the technical report published by IEAGHG [9].

2.  Start-up and shut down tests at the Technology Centre Mongstad (TCM) CO₂ capture plant

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The TCM plant is an industrial-scale post-combustion absorption plant that is located in Mongstad, Norway, adjacent to the Equinor oil refinery. The plant uses amine-based solvents to capture CO₂ from a natural gas-fired combined heat and power (CHP) plant, and it can also be configured to process a gas slipstream from the refinery residue fluid catalytic cracker (RFCC). However, this study used CESAR-1 solvent to capture CO₂ from the CHP flue gas to develop an understanding around the start-up and shut down (SUSD) dynamic behaviour.

We present results from three different test campaigns carried out in March, June and November 2020. These tests

have provided valuable insights around key factors and operation strategies that could help minimise disruption and CO₂ emissions during start-up and shut down of CO₂ capture in a power plant.

The key factors that were investigated in this study include:

• Comparing cold start-up performance with hot start-ups to demonstrate the effect of the starting temperature.
• Combined effect of start-up and shut down on overall performance.
• Effect of using different solvent inventory volumes, 53 m³ versus 42 m³, which is significant enough to illustrate a difference in process dynamics and identify any potential benefits.
• Timing of steam availability on start-up time, different tests were conducted to show steam introduced

(i) before, i.e., preheating with an auxiliary boiler, (ii) at the same time, and (iii) after the start of flue gas flow, i.e., delayed steam supply when steam extraction is unavailable.

• Effect of solvent CO₂ loading on the start-up performance.

The results demonstrate the influence of these factors on the capture performance in the context of conventional hot/cold start-ups and shut down protocols, also showing the results for strategies that improve start-up and shut down performance, e.g., the effect of preheating before start-up (shown in Figure 4), lower CO₂ loading and higher amine flow on hot and cold start-ups. The learnings from this study are important as they helps identify potential protocol measures which can provide substantial reductions in CO₂ emissions, time requirements and energy demand for start- up and shut down protocols.

3.  Results & Discussion

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In this contribution, we have focused on analysing start-up and shut down separately, e.g., the yellow shaded regions in Figure 2. The tests typically begin with a cold/hot start-up, followed by a period of steady state operation before ending with a shut down. A summary of the time it takes to reach these different periods of operation is summarised in Table 1 for tests conducted in November 2020, which show the effect of solvent inventory volume and a hot start- up with delayed steam supply.

Readers can refer to the detailed report [9] for analysis of the combined start-up and shut down performance of the TCM CO₂ capture plant. The report [9] also shows the effect of analysing over different start-up time periods (i.e., 82-minutes compared with 200-minutes) on the cumulative CO₂ capture performance – illustrates whether start-up shut down has any impact once you consider the steady state phase of operation.

Table 1: Duration of time for start-up and shut down with different solvent inventory volumes. tSU = start-up time is the period when CO₂ product flow starts; tSS = time when CO₂ product flow reaches steady state; tSD = shut down time is the period between the time flue gas is turned off until steam is turned off.

3.1 Effect of solvent CO₂ loading on the start-up performance

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To ensure the plant starts with solvent that is sufficiently low in CO₂ loading, shut down continues to supply steam to the reboiler even after the flue gas was turned off to “lean out” the solvent (Figure 2). Steam is turned off once all of the solvent reaches the required target CO₂ loading, which represents the end of shut down. By the end of shut down, all of the solvent will have lean loading of ≤ 0.2 mol CO₂/mol amine for two reasons: (1) to prevent precipitation during plant downtime, which can occur if CESAR-1 is rich and low temperature [10-13], and (2) having lean solvent loading for the next start-up will maximise the CO₂ capture capacity. Lean out for non-precipitating solvents such as 30 wt% MEA would also be beneficial for the latter reason. Due to the use of solvent with low CO₂ loading (0.05– 0.11 mol CO₂/mol amine) for the cold and hot start-up tests, the online CO₂ capture rate is very high at 98–99% upon start-up, and is observed as soon as flue gas is introduced.

3.2 Cold start-up vs hot start-up

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Cold start-ups typically take longer to reach steady state compared to hot start-ups. As the starting temperatures of

cold start-up (25–30°C) is much lower than hot start-up (~90°C), cold start-ups require more time to heat the stripper/reboiler to the target temperatures. Compared to hot start-ups, cold start-ups have lower cumulative CO₂ capture rates and higher specific reboiler duty. For a start-up period of 82-minutes with a 53 m³ solvent inventory, the cold start-up achieved a cumulative CO₂ capture rate of 78% on an absorbed CO₂ basis, whereas a hot start-up cumulatively captured 90% (Table 2).

3.3 Effect of solvent inventory volume

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For cold start-ups, using the smaller solvent inventory (42 m³) reduced the time needed to reach steady state and slightly decreased energy requirements (Table 1). On the other hand, using a larger solvent inventory (53 m³) increased cumulative CO₂ capture rates, hence reducing residual CO₂ emissions. Cold start-ups benefit from using a greater solvent inventory of 53 m³ as more lean solvent is available to capture CO₂ as the plant heats up. Hot start-up for the two solvent inventory cases (42 m³ vs 53 m³) achieved the same cumulative CO₂ capture rates of 90% (absorbed CO₂ basis) over an 82-minute time period, shown in Table 2. The cumulative specific reboiler duty based on MJ/kg absorbed CO₂ is marginally greater when using the smaller inventory.

In addition to potentially decreasing solvent consumption costs when using smaller inventory systems, the findings indicate that hot start-up performance could potentially benefit from smaller solvent inventory. The smaller inventory volume reduces the solvent circulation time; thus, the effects of solvent regeneration occur on a faster time scale. Therefore, the smaller inventory generates more product CO₂ (absolute terms) for a given start-up time period, resulting in a higher product basis CO₂ capture rate, which in turn, reduced specific reboiler duty on a product basis (MJ/kg product CO₂).

3.3 Effect of delayed steam supply

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During the start-up of a power plant with CO₂ capture, power generation is typically prioritised, and steam extraction is not available for a period of time. Consequently, there will be a delay in the supply of steam to the CO₂ capture plant. To illustrate this effect, a hot start-up using a 42 m³ inventory with 20 minutes delayed steam supply was demonstrated. The delay in steam availability increased the time required to heat the reboiler, which results in a start- up time similar to cold start-ups (Table 1). Compared to the normal hot start-up, delaying steam supply decreased the specific reboiler duty, however, the cumulative CO₂ capture rate (absorbed and product CO₂ basis) also significantly reduced. To maximise the CO₂ capture rates and minimise residual CO₂ emissions during start-up, it is essential to make steam available to the CO₂ capture plant as soon as possible (Figure 3).

Table 2: Cumulative CO₂ capture performance of 82 min start-up (SU) and shut down (SD) scenarios. Refer to Appendix A for equations.

1. Effect of preheating on the start-up performance

This study compared start-ups with low preheating of steam flow rate 2500 kg/h and high preheating of steam flow 5000 kg/h. The effect of preheating is illustrated in Figure 3 for hot start-up and Figure 4 for cold start-up. Hot start- up with high preheating had the highest cumulative capture rate compared to the other tests using lower preheating steam flow. The two cold start-up tests had the same preheating steam flow, consequently, the cumulative CO₂ capture rates based on absorbed CO₂ were similar (97–98%). Higher preheating results in a higher cumulative capture rate on a product CO₂ basis (i.e., hot start-up cases). This indicates the use of higher preheating rates before start-up allows the capture plant to reach the steady state flow of product CO₂ in a shorter timeframe, making the capture plant more responsive to the requirements of the CO₂ compression and transport system.

3.6 Shut down performance

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The time required to shut down the CO₂ capture plant is a function of the solvent inventory volume, which impacts the absolute steam energy required. Compared to the 42 m³ case (using the same amine flow rate), shut down with 53 m³ of solvent inventory required more time and steam energy on a MJ basis to lean out the larger solvent volume in the stripper section (Table 3). Moreover, shut down with 53 m³ of solvent inventory recovers more CO₂ product (2274 kg CO₂) compared to 42 m³ (1476–1507 kg CO₂ product). As a result, the specific reboiler duty of shut down with 53 m³ of solvent is lower (5.63 MJ/kg CO₂) compared to the 42 m³ case (6.40–6.49 MJ/kg CO₂). As shown in Table 3, the cumulative amount of supply CO₂ is very small as the flue gas flow is turned off during shut down. Subsequently, calculating the cumulative CO₂ capture rate on a product CO₂ basis will result in very high percentages (Table 2). Importantly, the cumulative CO₂ capture rate on an absorbed CO₂ basis is relatively high at 84–89%, which indicates that the residual CO₂ emissions during shut down is low.

4.  Conclusion

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Dispatching low-carbon power into a rapidly evolving grid with increasing demands for system flexibility is of paramount importance in decarbonising electricity systems. The ability of an abated CCGT to start-up at short notice and at low cost will define its position in the merit order and therefore the overall value the asset can provide – this applies to the day ahead and intra-day (balancing mechanism) power markets along with ancillary services such as frequency response. The whole system should be designed for turn down, shut down and solvent lean out where the recovered CO₂ from the solvent inventory is sent to the downstream system post power plant shutdown.

The optimal CO₂ capture performance during start-up and shut down of the plant needs to balance several factors: solvent inventory volume, initial temperature (cold vs hot) and timing of steam availability (preheating vs steam & flue gas at the same time vs delayed steam). This study demonstrates that these are key factors that influence the time requirements and capture performance during start-up and shut down of a CO₂ capture plant.

Using a larger solvent inventory provides the benefit of increased cumulative CO₂ capture rate (i.e., Table 2). The improvement in cumulative CO₂ capture is most significant for cold start-ups, whereas the solvent inventory volume has a negligible effect on hot start-ups. Compared to cold start-ups, hot start-up achieved a much higher cumulative

CO₂ capture rate and lower specific reboiler duty. Due to the use of a low solvent loading (0.05–0.11 mol CO₂/mol amine) for the cold and hot start-up tests, high online CO₂ capture rates of 98–99% were achieved initially upon start- up, which occur immediately once flue gas is introduced to the system. The ability to sustain high CO₂ capture rates during start-up depends on the volume of the solvent inventory, starting solvent CO₂ loading, amine concentration and the timing of steam supply to the reboiler.

The results indicate the importance of timely steam supply upon start-up of the CO₂ capture plant. Any delay in steam availability can significantly decrease the cumulative CO₂ capture rate, which will increase the residual CO₂ emissions. These results show that start-up with preheating is a potentially valuable approach. Under specific operating conditions, preheating can increase the cumulative CO₂ capture rates during start-up of the capture plant, thereby minimising the residual CO₂ emissions. To maximise the value of preheating, a higher flow rate of steam supply can heat the system much faster, but the duration of preheating needs to be optimised to minimise the steam energy demand. The CO₂ emissions associated with supplying steam for preheating will depend on the source of energy used, which will have an impact on the overall CO₂ capture rate. For interested readers, the effect of the CO₂ intensity of steam supply is demonstrated in the technical report published by IEAGHG [9].

Appendix A: Calculating online CO₂ capture rate

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The proportion of CO₂ absorbed from the feed flue gas is equal to one minus the ratio of the depleted flue gas CO₂ concentration to the flue gas supply CO₂ concentration:

Where

𝑂_𝐶𝑂2 = CO₂ concentration of depleted flue gas, dry basis

𝐼_𝐶𝑂2 = CO₂ concentration of supply flue gas, dry basis

The main cumulative metrics include: (i) cumulative CO₂ capture rate on a product basis, and (ii) cumulative CO₂ capture rate, absorber side, (iii) cumulative specific reboiler duty (SRD) on a product basis, and (iv) cumulative specific reboiler duty on an absorbed CO₂ basis.

Equation 3 is the indicator for the amount of CO₂ cumulatively absorbed by the solvent represented as a percentage of the total supply CO2. The cumulative absorbed CO2 is the online CO2 capture rate (Equation 1) multiplied by the supply CO₂ flow rate, and is calculated cumulatively over the specified time period. The percentage of the supply CO₂ that is emitted as residual emissions is Equation 4:

5.  Appendix B: Shut down results from November 2020 tests

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Table 3: Calculations of cumulative amounts of CO₂ over the shut down period. *This shut down included an inventory volume adjustment from 53 m³ to 42 m³, consequently, less product CO₂ is generated compared to SD 53 m³ (1). This case cannot be used for comparison of performance.

Acknowledgements

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The authors Mai Bui and Niall Mac Dowell would like to acknowledge funding from the Research Councils UK (RCUK) under grants EP/M001369/1 (MESMERISE-CCS), EP/M015351/1 (Opening New Fuels for UK Generation), EP/N024567/1 (CCSInSupply), NE/P019900/1 (GGR Opt) and EP/T033940/1 (Multiphysics and multiscale modelling for safe and feasible CO2 capture and storage).

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