Development of an energy-efficient and cost-effective catalytic regeneration system in the post-combustion CO2 capture process (Flexible Funding 2021)

Eni Oko at the University of Hull was awarded funding in the UKCCSRC’s Flexible Funding 2021 Call to look at the “Development of an energy-efficient and cost-effective catalytic regeneration system in the post-combustion CO2 capture process”.

Current studies in the literature show that catalyst-assisted regeneration in the post-combustion CO2 capture process, illustrated in Fig 1, not only reduces the thermal energy requirement but also enables solvent regeneration at a lower temperature under 100oC, in contrast to over 120oC without catalyst-assisted regeneration. This project developed a computational model to screen and analyse different catalysts for catalyst-assisted regeneration in solvent-based post-combustion CO2 capture (PCC) process.

Figure 1: Schematic illustration of catalyst-assisted regeneration in the post-combustion CO2 capture system

A computational approach was adopted to screen different catalyst options to identify the potentially most energy-efficient in assisting the regeneration of CO2 loaded 30 wt% monoethanolamine solvent. The catalyst options evaluated included zeolite (HZSM-5), MoO3, Ce(SO4)2/ZrO2, V2O5 and Al2O3. The computational calculation was performed using Density Functional based Tight-Binding methods, an approximate DFT model in the ADF Suite (Computational Material Chemistry software). The results showed that HZSM-5 with an energy barrier of 0.009546 Hartree and activation energy of 29.84 kJ/mol has the highest potential to achieve reduced regeneration energy requirement (Figure 2).

Figure 2: Energy barrier for catalyst-assisted MEA-CO2 desorption

A process model of the PCC process with catalyst-assisted regeneration for 30 wt% MEA solvent and HZSM-5 catalyst was thereafter developed in Aspen Plus® with the solid phase catalytic reaction kinetics implemented as user-defined via FORTRAN subroutine. The integrated absorber-regenerator model was validated using pilot data from the Clean Energy Technologies Research Institute (CETRI), University of Regina, Canada with average deviations for rich loading and CO2 product mass flow rate within <10% for different conditions. The obtained thermal energy requirement was 6.5 GJ/ton CO2 but with a regeneration temperature below 100oC which confirms that the regeneration can be obtained using hot water rather than steam. The available data from CETRI is limited with several data unavailable for different operating parameters. It is therefore important to conduct extensive experiments to obtain a more robust validation.

This project, led by the University of Hull, UK was supported under the UK Carbon Capture and Storage Research Centre (UKCCSRC) Flexible Funding 2021 (Ref: EP/P026214/1).

Anna Lichtschlag at the National Oceanography Center Southampton was awarded funding in the UKCCSRC’s Flexible Funding 2021 Call to look at “Sensor Enabled Seabed Landing AUV nodes for improved offshore Carbon Capture and Storage (CCS) monitoring”.

Within Europe, much of the geological capacity for storing CO₂ lies in reservoirs that are located deep below the seafloor, such as depleted oil and gas reservoirs. These offshore storage reservoirs might play an important role in CCS as they are large and the infrastructure for transport and storage often already exists. Although these reservoirs are likely to safely contain the CO₂ over geological times, national and international regulations require that these offshore storage complexes are monitored to assure that no CO₂ escapes into the seawater or the atmosphere.

Currently, one of the most efficient ways to monitor these offshore storage complexes is with sensors that either measure the CO₂ directly in the seawater above the complex or related parameters such as pH (as the water becomes more acidic when CO₂ dissolves). These sensors can either be mounted on autonomous underwater vehicles (AUVs) that survey the area or on stationary platforms, deployed on the seafloor. Both of these approaches have some limitations, e.g. they are either very costly or only cover a small area.

In our UKCCSRC funded project, we assessed the possibility of a new approach that would combine the advantages of both monitoring platforms, i.e. novel seabed-landing AUVs (so called ‘Flying Nodes’) that are developed by Autonomous Robotics Ltd. (ARL). In particular, we tested if we can integrate the long-term deployable Lab-on-Chip chemical pH sensors, developed at the National Oceanography Centre Southampton, on these seabed-landing AUVs and how these sensor-enabled Flying Nodes then could be used to monitor potential CO­₂ leakage.

For this, we first designed a specification and tested how NOC’s pH sensors can be electronically and mechanically integrated into a Flying Node, and also established how the pH sensor and the Flying Node software can be integrated. This was followed by a first successful communication test between the two technologies. We concluded that the integration will be possible and ascertained that the Flying Node has the required capacity to fly properly whilst carrying all components of the sensor. As a next step, we used the commercial Comsol Mulitphysics software to identify the requirements of a potential deployment of the sensors-equipped Flying Nodes. Assuming a typical North Sea scenario, our model results showed that, for a small CO₂ leakage rates (below 500 kg/d), the monitoring should occur as close as possible to the seafloor as this increases the detectable area of the plume (i.e. ca 100 m longer at 0.5 m height compared to 3 m height). The CO₂-enriched plumes in the modelled setting are long (hundreds of metres), but thin and close to the seafloor, suggesting that a monitoring pattern of transverse sampling across the potential plume location, some tens of metres downstream of the likeliest release sites, would be the most successful strategy.

As part of this project we closely worked together with Autonomous Robotics Ltd. (ARL) who develop the Flying Nodes and we are very grateful for their contributions and the productive discussions during this project.

Work on this Flexible Funding 2021 project so far has involved two main components:

1. Pilot scale testing on the TERC amine capture plant (ACP) to explore plant parameters required to obtain very high capture levels, 95% and above
2. Development of a lab-scale reclaiming capability

In the first ACP trials, for elevated temperature solvent reclaiming, the ACP experienced a heater failure, although within its rated operating envelope.  The lost time while this was repaired caused severe pressure on the overall ACP testing schedule, covering a number of projects, which was already tight because of a flood in 2019 and delays in reinstating the plant due to COVID.  As a result, subsequent tests to investigate absorber performance were delayed until November 2022, and access to install and test temporary solvent storage has not been feasible to date.

Nonetheless, work on the ACP has shown that high levels of CO₂ capture can indeed be achieved even with the modest amount of absorber packing (12m) available and that the ACP reclaimer can deliver solvent with very low lean loadings (~0.1 molCO₂/molMEA), although not at high enough pressures and temperatures to test modelling predictions of low specific reboiler duties.  Techniques to operate at high capture levels have been explored and the limits on achievable gas and solvent flow rates for operation under gas turbine conditions have been defined – the plant was originally designed for operation on coal flue gases.

Figure 1: TERC amine capture plant (ACP) – the two absorber columns are on the left

Two 0.25 m diameter absorber vessels are installed in series to increase residence time and contact between liquid and gas.  Each of the absorbers is equipped with two beds of Flexipac 350X structured packing, 3m each. In further discussions they are viewed as a single absorber with a total of 12m of packing.

The objective of the main test campaign undertaken was to investigate whether high capture levels, 95% and above, and high rich loadings (~0.45 molCO₂/molMEA) could be obtained with the 12m of packing in the ACP absorber, using a combination of low lean loadings and low liquid-to-gas (L/G) ratios.  Artificial flue gas mixtures of CO₂ in air were used.

Results for the nine tests undertaken are shown in the table below.  High capture levels (>95-99%) were measured using gas concentration measurements, giving a good confidence, at lean loadings up to around 0.15 molCO₂/molMEA.  Where L/G ratios allowed, rich loadings of 0.45 molCO₂/molMEA or higher were measured.  Based on previous modelling work (see UKCCSRC Co-Cap project) lean loadings in the range shown below are expected to require little or no additional reboiler heat per tonne of CO₂ captured, provided a sufficiently-high stripper pressure is used to suppress excessive steam production.

Table 1: Steady-state test points from high-capture test campaign

Measured absorber temperature probe readings (an uncontrollable combination of gas and liquid temperatures) and flue gas CO₂ concentrations at the midpoint (see Figure 2 below) show the upper absorber has little to do; by the time the flue gas reaches it, CO₂ concentrations are already very low.

Figure 2: Absorber temperature profiles and measured CO₂ concentrations for selected runs with liquid and gas flows approximately matched

A key lesson for commercial plant operation at high capture levels is that they may benefit from using lean and rich solvent storage all the time (i.e. not just for start/stop), to give independent operation of absorber and stripper and thus allow precise solvent flow control to meet time-specific requirements of each unit, for the reasons noted below:

Absorber:

• important to have the L/G ratio in the absorber neither too low, to get the capture level required,
• nor too high, to get the highest possible rich loading

Stripper/reboiler:

• liquid flow to the stripper no higher than the energy available can strip to the required lean loading (otherwise high capture impossible)
• and rich flow not too low either if the lean loading then goes beyond the specific reboiler duty (SRD)/loading inflection point at that pressure (see UKCCSRC Co-Cap modelling for a more detailed explanation of the inflection point in SRD vs lean loading)
• Some flexibility in lean loading if above the inflection point and higher than required for the capture level, but at the expense of higher reboiler temperatures and extra packing in the absorber (for periods when lean loading is lower than the inflection value).

Approximate quantification of the consequences of solvent flow (based on UKCCSRC Co-Cap modelling) are as follows:

1. Being short on solvent by 1% will decrease capture by about 1 percentage point, although the rich loading may increase slightly (but depends on packing height) and hence SRD may decrease slightly.
2. Being high on solvent flow by 1% for e.g. a lean of 0.15 and a rich of 0.45 molCO₂/molMEA will decrease rich loading by roughly 0.35/100 = 0.0035, which would correspond to 0.035 GJ/tCO2 or 1% of 3.5 GJ/tCO₂ total SRD.

Possible control options that commercial plants using solvent storage might use to match solvent flows precisely to absorber and stripper conditions are summarized below:

Absorber:

• Rapid assessment of capture level possible based on gas measurements
• If capture level is too low then can increase solvent flow up to limit set by lean loading and packing height
• Max lean flow needs to be limited to achieve high rich loading, ideally would have rapid rich loading assessment and not go below limiting rich loading value; could also have upper lean solvent flow limit based on inlet CO₂ and total flue gas flow, with matrix of values set by experience
• The value of marginal increase/decrease in capture level will change with effective electricity and carbon price – but may be market distortions due to DPA terms

Stripper/reboiler:

• In principle could use reboiler temperature at a given stripper pressure to indicate lean loading achieved, but may not be precise enough; otherwise need to measure lean loading directly or estimate from solvent flow and CO₂ flow
• If lean loading is too high, increase steam or decrease liquid flow to the stripper and vice versa
• Could also make heat input a function of solvent flow and/or CO₂ flow, or limit solvent flow based on heat available, with ratios based on experience

Overall, absorber and stripper must match flows but with buffering from storage to avoid disturbances propagating and long delays in adjusting to changed flue gas inlet conditions.  There ought to be economic incentives to time-shift the capture energy penalty as well (i.e. deliberately delay in replenishing the lean solvent store to a period when the value of the electricity output penalty is as low as possible), within the hardware limits of the plant.  This is an obvious job for an ‘Efficiency Engineer’ on a large power/PCC plant!

The proposed future PCC-CARER work using industry funding is an investigation of absorber performance under conditions where high capture levels can be achieved, with comprehensive data collection to give a more detailed understanding of ACP performance and to inform future testing and commercial plant operating principles.

The reclaimer work is being taken forward by a new PhD student, Marcin Pokora, supervised by co-investigator Abby Samson.  Starting in a new laboratory, an atmospheric, or reduced, pressure reclaimer system has been set up and is nearly ready to use (see Figure 3).  Dr Samson is also supervising an external student at the University of Lincoln who is building a stainless steel reclaimer rig capable of operating at elevated pressures. Since both of these students are at a relatively early stage in their studies, further results are not available at this time, but scope exists for a sustained research programme on the relationship between reclaimer operation and effectiveness.

Figure 3: Glassware laboratory reclaimer – atmospheric and sub-atmospheric testing

Jon Gibbins, Muhammad Akram, Daniel Mullen, Marcin Pokora, Mohamed Pourkashanian, Abby Samson (University of Sheffield)

If you’re reading this, you’re probably interested in learning about this project or got lost somewhere on the internet. This was one of those projects that came about from a random idea to a problem presented at the UKCCSRC “what are industries’ needs?” events. I spoke with one of the panel speakers from Petrofac (Chet Biliyok) and started discussing the idea, and a few months later, here I am writing about the outcomes of the project that arose. (Thank you UKCCSRC for funding the idea and turning it into this project)

Amines are compounds that are commonly used in the carbon capture processes to strip CO₂ out of the flue gas. But this CO₂ capture process isn’t perfect as some of the amines can leave the top of the tower as aerosols or vapours – this is called amine slip. When they escape into the atmosphere, they can have negative effects on the environment and human health. In this blog post, I’m going to talk about some ways to prevent fugitive amine emissions.

First and foremost, it’s important to understand where fugitive amines come from. In carbon capture processes it’s often caused by the stripper columns operating conditions not being controlled accurately or it can be caused by a fluctuation upstream of the CO₂ capture step. So, there is potential to reduce amine slip by controlling processes more accurately.

Another way to prevent fugitive amine emissions is to use engineering controls. These are measures that are put in place to prevent or reduce the release of fugitive amines into the atmosphere. For example, some facilities may use a water wash step, where water is sprayed into the top of the CO₂ stripping column to reduce the temperature and coalesce/condensate some of the amine slip. To improve the effectiveness of this water wash step, it’s also possible to add an aqueous acid to the water to produce an acid wash and even a UV light step after that to help further.

Location of the amine electrostatic precipitator relative to the CO2 capture plant

These water wash steps are currently the best available technology, but maybe there’s something better, as even these technologies don’t work under transient conditions. There’s another issue too – environmental limits for amines and their degradation products (nitrosamines) are getting tighter and harder to reach. We are also likely to see regulations get more stringent as carbon capture technology is more widely deployed. This is the reason that new technology is required and why the technology I developed in this project is important.

The aim of this project was to mathematically validate and scrutinise the potential of a new amine slip recovery technology based on wet electrostatic precipitation. The Cranfield University team (Siqi Wang – PhD Candidate, Alek Gonciaruk – Research Associate) were fantastic and jumped straight in, building a MATLAB model to simulate an electrostatic precipitator and setting up the system. We had regular meetings with the Petrofac team (Chet Biliyok and Duncan Harrison) to get input into our boundary and initial conditions, to ensure our model was representative. Petrofac were also heavily involved in critically evaluating the technology, checking facts and helping with the engineering – thank you Duncan and Chet.

The outputs of the modelling and work on the project was generally good news. It definitely threw up some surprises that we weren’t expecting and meant the engineering of the system needed careful thought but, crucially, no showstoppers.

Modelling results of amine ESP showing aerosols moving towards collection plate

Building on from this project, I ran an individual thesis project conducting further modelling at Cranfield University – thank you Potsawee Yiengvanichchakul. This modelling went into even more depth and built on the knowledge gained in this UKCCSRC project. Going further, I was fortunate to secure funding in the UKCCSRC’s Flexible Funding 2022 call to build a prototype system – you’ll be hearing about this soon!

This project was a fantastic start to developing the next generation of amine slip prevention technology and UKCCSRC must be thanked (again) for enabling it. Since starting the project and learning of the issue in detail, I’ve noted the need for other supplementary technology to be developed including actually measuring the amines and their degradation products in the flue gas – no mean feat at the limits the Environment Agency has set.

Bioenergy with Carbon Capture and Storage (BEECS) is a net-negative technology that is believed to be a promising tool in the uphill battle against climate change. However, combustion of biomass results in production of large quantities of problematic waste, Biomass Combustion Ash (BCA). Our previous work has revealed the potential for utilising this waste as the CO₂ capturing media, thus facilitating in-situ decarbonisation of the power sector whilst simultaneously avoiding the “classical” pathway for such waste – landfilling. As such, biomass combustion bottom ash (BA) was used as the precursor for a carbonaceous adsorbent, whereas biomass combustion fly ash (FA) was processed to become a zeolitic sorbent.

With the promising and positive outcome of “Phase 1” of this idea, “Phase 2” had the objective of improving the performance of the waste-derived sorbents. As such, our aim was to enhance the capture capacity of the produced materials by thoroughly investigating the synthesis pathways.

And so, we did! The produced activated carbon from BA and the zeolite from FA both feature a doubled CO₂ adsorption capacity compared to their respective predecessors from “Phase 1”. These results have been presented and published as part of the IEEE 2022 International Conference on Nanotechnology (Development of Nanoporosity on a Biomass Combustion Ash-derived Carbon for CO2 Adsorption and Synthesis of Nanoporous Type A and X Zeolite Mixtures from Biomass Combustion Fly Ash for Post-Combustion Carbon Capture). Furthermore, a poster created by the RA has been featured at the UKCCSRC Autumn 2022 Conference at The University of Edinburgh, receiving the best poster award!

IEEE International Conference on Nanotechnology 2022 – Mike Gorbounov, RA (left) and Ben Petrovic, RA (right)

We’re pleased to share the blog report as an output of ‘Advancements in mixed amine atmospheric kinetic models‘, one of our Flexible Funding 2021 projects from Dr Kevin Hughes and Prof Mohammed Pourkashanian, and their Research Associate Dr Christopher Parks, at the University of Sheffield.

Context

Modern civilisation is built on the exploitation of the world’s natural resources, chief amongst these being the burning of fossil fuels – coal, oil and gas to provide energy. A byproduct is the release into the atmosphere of carbon dioxide, thus pushing up the atmospheric carbon dioxide concentration approximately 50% since the beginning of the industrial age. It has been known for many years that carbon dioxide acts as a “greenhouse gas” leading to a warming of the lower atmosphere that could impact the world’s climate, and climate change in the context of global warming arising from these carbon dioxide emissions and the problems it leads to – ever more extreme weather events, sea level rise and potential disruption to world food production is an issue taking on ever more urgency in the world’s consciousness, as exemplified by the ongoing series of the “conference of the parties” (COP) meetings under the auspices of the United Nations,  first held in the 1990s and most recently culminating in COP26 held in Glasgow in 2021 leading to a range of commitments designed to limit the average rise in world temperature to within 1.5 °C of pre-industrial levels. In the context of the UK, this has led to legislation being introduced to require the country to reach a “net zero” level of greenhouse gas emissions by 2050.

Implications – why is this research needed?

A net-zero target of greenhouse gas emissions implies that the major sources of carbon dioxide emissions will need to be eliminated. While efforts are underway to increase renewable power sources such as wind and solar, there is still a role for gas-fired power generation for the foreseeable future as a flexible backup to when wind or solar is insufficient. The carbon dioxide emitted will thus need to be captured, and similarly from large single point industrial sources such as steel and cement production. The capture technology most likely to be deployed involves retrofitting existing plant to capture the carbon dioxide from the flue gas using an amine-based solvent. This solvent is continually recirculated through a stage of carbon dioxide capture, then stripping of the captured carbon dioxide to storage, regenerating the solvent to be reused in the capture stage. It is unavoidable that some of this solvent escapes the capture plant and is emitted into the atmosphere where the question arises at to what happens to it, and especially in terms of any potentially harmful products it may produce. This project aims to address that question by developing models to predict what happens to this solvent, based on a fundamental understanding of the chemistry of the processes that occur, and thus increase our understanding of both existing solvents used in this process, and possible alternatives, or even mixtures of solvents that might be used in the future.

Modelling Details

A commercial software package, Gaussian 09, was used to predict the properties of the chemical species that compose the solvent or solvent mixture, and to predict the outcome of their reaction with other chemical species that might be released in the flue gas or already present in the atmosphere. This gives details on the structure of both the reacting species, their products, and any intermediate species that connect the reacting species to the products allowing a ”potential energy surface” for the system. Two specific amine solvents that may in future be used in carbon dioxide capture were investigated, namely AMP and piperazine. Reaction networks for the atmospheric degradation of each are illustrated in figures 1 and 2.

Figure 1: Schematic reaction network for atmospheric degradation of AMP

Figure 2: Schematic reaction network for atmospheric degradation of piperazine

Analysis of the species properties calculated by the Gaussian 09 software allows the prediction of how fast the individual reactions represented in figures 1 and 2 occur. In ongoing work these will be used to adapt the atmospheric dispersion model, known as ADMS, to be able to predict the real-world outcome of the release of these solvents into the atmospheric conditions prevalent in the locations where these capture plants are likely to be constructed in future.

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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