Comparison between a primary amine (MEA) and a secondary amine (CESAR1) solvent for flue gas CO2 capture

This article examines one of the key trade-offs that must be made when selecting a solvent for amine post-combustion capture of CO2 from a flue gas, which is whether to use primary amines that won’t themselves form stable nitrosamines in service or to use secondary amines, usually in a blend, that will form stable nitrosamines but which have slightly lower energy requirements for regeneration.? Other properties of the solvents, in particular reclaimability but also toxicity of the amines themselves and their other degradation products, their volatility (which may also represent a tradeoff with thermal reclaimability) and their oxidative and thermal stability, are also important.? And, finally, for the overall cost of operation, the prices for purchasing solvent to replace losses during operation are likely to be a major factor, plus the ability of the global supply chain to ramp up to match an increasing demand as CCS takes off.

The US National Energy Technology Laboratory (NETL) in their 2015 Carbon Dioxide Capture Handbook compared amine solvent properties relevant for commercial use between classes of amines as follows:

Primary and secondary amines typically have higher rates of reaction with CO2 compared to tertiary amines. Among various primary amines, MEA has the highest reaction rate, and blends of MEA and other tertiary or hindered amines are typically used to exploit this feature while maintaining relatively-low reboiler loads. Primary/secondary (mono)amines with a 1:2 stoichiometry have lower CO2 carrying capacity compared to tertiary amines which bind 1:1 with CO2. Further, polyamines such as piperazine have a higher carrying capacity because they have two amine groups per molecule. Tertiary amines have higher CO2 capacities but the reaction kinetics with CO2 are significantly slower than primary and secondary amines. Because the CO2 carrying capacity is expressed in wt% CO2 in the solvent, or the quantity of solvent circulated to capture a unit quantity of CO2, the molecular weight and density of the solvent also play a role in determining its volumetric or weight-based CO2 carrying capacity.

From a health and environmental safety perspective, MEA is highly biodegradable, and has no direct adverse effects on human health, animals, and vegetation. Other amine solvents such as AMP, MDEA and PZ are toxic and are not easily biodegraded compared with MEA.? The reaction of amines with NOx in the flue gas leads to the formation of nitrosamines, which are carcinogenic.? The reactivity with NOx varies with the amine structure.

This article contrasts two non-proprietary solvents for which published data is available to illustrate possible tradeoffs in their potential suitability for commercial CO2 capture.? The primary amine is MEA (monoethanolamine) used at ~35% w/w in water and the secondary amine blend is PZ (piperazine)/AMP (AminoMethyl Propanol) blend, known as CESAR-1 and containing 12.9% w/w PZ and 26.7% w/w AMP in water.? Differentiating characteristics fall into three categories: potential showstoppers, marginal improvements (scope for cost reduction), plus some ‘in between’.

But an essential caveat, that cannot be repeated too many times, is that all of these characteristics can only be reliably quantified after long-duration operational experience in full-scale plants and even then only for the specific applications for which multiple examples exist.? Pilot testing can also give an indication of likely characteristics but needs to be on the specific flue gas, to incorporate all the relevant features of the proposed commercial plant, including actual solvent management and emissions control techniques, and to be conducted for at least a year to give time for solvent composition to stabilise and factors such as climatic variations and normal plant ‘upsets’ to come into play.

Key characteristics for commercial CO2 capture using amines include:

a) Potential showstoppers:

• Emissions to atmosphere
• Emissions to water
• Operator health and safety
• Uneconomic project – higher-than-expected running costs that are in excess of revenues
• Uneconomic project – unexpected maintenance costs making capital recovery or even operation unviable

?b) Marginal improvements (scope for cost reduction):

• Reduced equipment sizing / lower-cost materials
• Reduced solvent make-up and waste disposal costs
• Improved equipment performance – e.g. better capture rates, reduced energy requirements
• Reduced financing costs since lower project risk premium

?c) In-between issues:

• Process upsets triggering closure – could exceed annual emission allowances in a short period
• Corrosion – reduced equipment lifetime or increased waste disposal costs – catalytic effect on degradation
• Solvent inventory or waste streams may be hazardous material
• Biological fouling

?COMPARISON OF MEA AND CESAR1 ON SHOWSTOPPER ISSUES

?Emissions to atmosphere from both solvents can be reduced to low levels by the addition of an acid wash and, for CESAR1, possibly also a ‘dry bed’ at the top of the absorber.? Relatively high acetone emissions reported from CESAR1 cannot, however, be stopped by an acid wash; conversely reported acetaldehyde emissions from MEA appear to be relatively high.? PCC project environmental performance assessments that show impacts below maximum environment assessment levels (EALs) for toxic conversion products such as nitrosamines are in the public domain for MEA (see End Note #1) but no similar studies for CESAR1 appear to have been undertaken.

?Emissions to water are not expected directly but may occur via deposition of stack emissions, possibly after solution in water droplets in the stack plume, or as a result of upsets in solvent handling, including in the supply chain.? The assessment by NETL suggests that CESAR1 would present a higher risk.

?Operator health and safety appears to present more challenges with CESAR1 than MEA, again based on the NETL assessment, and in particular the management of nitrosamines, potent cancer-causing chemicals, needs to be resolved for CESAR1.? Nitrosamine management for MEA would be achieved by a high level of thermal reclaiming to remove possible degradation product nitrosamine precursors and probably to remove or thermally destroy any nitrosamines present (although this approach does not appear to have been verified by publicly-available data).? CESAR1 readily forms stable nitrosamines with the NO2 that will be present in flue gases.? It has been proposed that nitrosamine levels in circulating CESAR1 can be controlled by periodically increasing the temperature in the reboiler to 130?C for 5-12 days at a time but, although measured, the absolute concentrations of nitrosamines that were achieved using this approach were not reported; it was just shown that total nitrosamines varied between 100% and about 50% of an unidentified peak value and the piperazine-specific nitrosamine, MNPZ, between 100% and about 30%.

Uneconomic project – higher-than-expected running costs that are in excess of revenues – Based on limited full-scale operational experience this is probably most likely to occur because of unexpectedly-high solvent management costs.? As noted, for meaningful comparisons between MEA and CESAR1 fully-representative test data is required. The scope for high solvent management costs on commercial projects using ‘low energy’ amine blends is illustrated in the figure above that shows a Global CCS Institute analysis of PCC costs for Boundary Dam Unit 3 (BD3) and Petra Nova. The ‘Variable O&M’ values (expected to be mainly solvent management costs), particularly for BD3, are very high and, as confirmed by evidence to a Saskatchewan parliamentary committee , were, at least for several years, perhaps up to four times the amount per tonne of CO2 captured than was expected, at around $20/tCO2 captured (and much higher that the extra fuel costs for meeting the additional energy requirements of the PCC plant).? Commercial PCC projects with mixed amines may now be using specialized reclaiming facilities to clean up their solvent inventories, and possibly stored dirty solvent, on an annual, or more frequent, basis, although some level of impurities may build up in the intervals between these reclaiming campaigns.? But no information is in the public domain.

Uneconomic project – unexpected maintenance costs making capital recovery or even operation unviable.? No examples of this appear to exist in the very limited number of projects to date.? Experience suggests that both MEA and CESAR1 can be operated in PCC pilot units with stainless steel or similar construction without undue maintenance requirements.

COMPARISON OF MEA AND CESAR1 ON MARGINAL IMPROVEMENTS (SCOPE FOR COST REDUCTION)

Reduced equipment sizing / lower-cost materials – Based on pilot testing (i.e. no FEED studies using CESAR1 have been identified) both equipment sizing (e.g. main vessel sizes) and construction materials (stainless steels or similar) appear to be similar for MEA and CESAR1.? Minor differences may exist, e.g. possibly an extra dry bed in the absorber for CESAR1, slightly smaller reboiler units for CESAR1, but given the relatively small differences in specific reboiler duties indicated by the test data presented in this report (i.e. a range 0-15% less in specific reboiler duty (SRD) for CESAR1 vs MEA in conventional PCC systems) these equipment differences must be limited in magnitude.

Reduced solvent make-up and waste disposal costs – as already noted, fully-representative long-term tests are needed to determine the actual performance of a solvent in a particular application.? Based on experience in the RWE Niederaussem pilot a recent paper suggested potentially higher costs for managing the more expensive CESAR1 solvent, but it must be emphasised that no meaningful data for either solvent has been identified for commercial PCC operation.

The ease with which the solvent can be reclaimed or cleaned (these are not the same thing, as a previous article discusses) is a key factor in determining solvent management costs and overall performance.? Tests at Technology Centre Mongstad (TCM) have demonstrated that MEA can be thermally reclaimed with very high rejection of all impurities.? In these tests the reclaimer was operated intermittently but in commercial plants (e.g. Pentair , Bechtel ) it is likely that MEA reclaiming will take place continuously.? The reclaimer, or at least the first stage of reclaiming, may be vented into the stripper for full energy recovery, allowing reclaiming at very high rates (e.g. up one inventory volume per week) with limited penalties on plant output.? Ion exchange cleaning has also been used for MEA operation in pilot testing but, given the efficacy of thermal reclaiming for MEA, seems unlikely to be the preferred commercial option.

CESAR1 has been operated satisfactorily for limited periods (total 3800 hours reported) with intermittent thermal reclaiming but of the order of 10% of impurities seem unable to be removed by this method and the build-up of these compounds over the longer term is therefore a possibility.? Somewhat better levels of impurity removal, but still not complete, appear to be feasible with ion exchange cleaning and longer periods of satisfactory operation (>3 years), with appreciable levels of impurities present, have been demonstrated using this method (with also some level of solvent withdrawal for sampling purposes as well as losses in cleaning and associated make-up for, effectively, bleed and feed).? There is a caveat here, though, in that this is at a single plant (RWE Niederaussem) and the fly ash at this plant has been demonstrated to have beneficial effects with respect to amine solvent degradation.

Improved equipment performance – e.g. better capture rates, reduced energy requirements.?

Comparison of capture rate and regeneration energy data

This is covered at some length below as improved capture performance is the perceived main advantage of CESAR1. ?Based on the pilot plant performance data below, MEA and CESAR1 appear very similar in respect of achievable capture rates and the packing height required.? Energy requirements for CESAR1 can be up to 15% lower than 30% w/w MEA, probably ~10% lower than 35% w/w MEA, but in some runs no advantage was seen for CESAR1.? Some poorer examples of CESAR1 energy performance have been attributed to solvent ‘damage’ after a period of operation without thermal reclaiming.

A. Tests on TCM CHP flue gas, ~3.5-3.7% v/v CO2

The minimum flue gas temperature was 40°C with CESAR1 solvent because of precipitation at lower temperature in the absorber, while it was 30°C in MEA case. It was easy to reverse the precipitation by flushing the absorber with hot solvent at high flow rate, but this might be not optimal for a full-scale plant. The flue gas temperature strongly influences the steam consumption.

TCM run series?????????? %w/w MEA??? Capture rate?? Packing (m)???? SRD (GJ/tCO2)

MEA ???????????????????????????????????????

MEA-3 ??????????????????????? 43%???? ??????????? 86%???? ??????????? 18??????? ??????????? 3.6

F2??????? ??????????????????????? 36%???? ??????????? 90%???? ??????????? 18??????? ??????????? 3.8

B3-rep ??????????????????????? 37%???? ??????????? 91%???? ??????????? 18??????? ??????????? 3.6

D3-rep ??????????????????????? 36%???? ??????????? 97%???? ??????????? 24??????? ??????????? 3.7

????????????????????????????????????????

CESAR1 ??????????????????????????????????????????????

K????????????????????? ??????????????????????????????????? 85%???? ??????????? 18??????? ??????????? 3.5

C????????????????????? ??????????????????????????????????? 90%???? ??????????? 18??????? ??????????? 3.4

D????????????????????? ??????????????????????????????????? 98%???? ??????????? 18??????? ??????????? 3.9

K????????????????????? ??????????????????????????????????? 85%???? ??????????? 24??????? ??????????? 3.3

AA?????????????????? ??????????????????????????????????? 90%???? ??????????? 24??????? ??????????? 3.5

BB??????????????????? ??????????????????????????????????? 98%???? ??????????? 24??????? ??????????? 3.75

Hume (2021) ?????????????? ??????????????????????? ~96%?? ??????????? 24??????? ~3.45 minimum*

?* Higher values SRD are ascribed to foaming in the stripper.

B. Tests on TCM RFCC flue gas, 13-14% v/v CO2

TCM run series?????????? %w/w MEA ??? Capture rate?? Packing (m)????SRD (GJ/tCO2)

MEA

1A-1??? ??????????????????????? 30%???? ??????????? 90.5%? ??????????? 18??????? ??????????? 3.5

1A-2??? ??????????????????????? 30%???? ??????????? 89.4%? ??????????? 18??????? ??????????? 3.54

CESAR1

EPRI Baseline????????????? ??????????????????????? 91%???? ??????????? 18??????? ??????????? 3.23

B4REP2??????????????????????? ??????????????????????? 89.6%? ??????????? 18??????? ??????????? 3.06

B1??????????????????? ??????????????????????????????????? 89.5%? ??????????? 18??????? ??????????? 3.13

C. RWE Niederaussem tests, ~15% v/v CO2

Tests results at Niederaussem were summarised into expected plant performance as follows:

Solvent?????????? %w/w MEA ??? Capture rate?? Packing (m)???? SRD (GJ/tCO2)

MEA ?????????????? 30%???? ??????????? 90%???? ??????????? 18??????? ??????????? 3.6

CESAR1?????????????????????????????????? 90%???? ??????????? 28*????? ??????????? 3.0

CESAR1?????????? ??????????????????????? 95%???? ??????????? 28*????? ??????????? 3.0

CESAR1?????????? ??????????????????????? 98%???? ??????????? 28*????? ??????????? 3.24

?* Including dry bed for amine emission reduction

D. National Carbon Capture Center tests – ~30%w/w MEA, ~10.3% v/v CO2

These tests were undertaken as part of generated a pre-determined matrix of conditions for model calibration, i.e. they were not optimised high capture rate tests, as suggested by the relatively high lean loadings and low rich loadings.? Packing bed height is 6 m, i.e. 18 m total.

Case No.????????? Capture rate?? L/G????? Lean / Rich loading?? SRD???? No. of beds (gas data)? (w/w)? ??(mole CO2/MolMEA) (GJ/tCO2) ? (Inter-coolers)

K15????? ??????????? 99.4%? ??????????? 3.042?? 0.224?? ????0.413?? ??????????? 3.81???? ??????????? 3 (2)

K14????? ??????????? 98.3%? ??????????? 3.055?? 0.224?? ??? 0.42???? ??????????? 3.86???? ??????????? 3 (2)

COMPARISON OF MEA AND CESAR1 ON IN-BETWEEN ISSUES

Process upsets triggering closure – could exceed annual emission allowances in short period.? For both solvents problems in the water or acid washes could lead to much higher amine losses than normal.? This highlights the need for both continuous emission monitoring and condition monitoring on key equipment.

Corrosion – reduced equipment lifetime or increased waste disposal costs – catalytic effect on degradation.? Corrosion is expected to be a greater potential problem with MEA then CESAR1, but this is on the basis of tests where continuous reclaiming has not been used.? MEA degradation products appear to enhance corrosion rates and corrosion products appear to enhance degradation rates.? Thermal reclaiming on MEA has been shown, however, to give complete removal of both corrosion products and degradation products.? So the expectation is, although yet to be verified by appropriate testing, that continuous thermal reclaiming at a high enough rate will maintain both corrosion and degradation of MEA at relatively low rates.

Solvent inventory or waste streams may be hazardous material.? As already discussed, if the concentration of inherently-stable nitrosamines in CESAR1 cannot be continuously maintained at a low level then this introduces a particular hazard.? For both solvents, given the range of chemical species, metals etc. that can be concentrated in reclaiming/cleaning residues, these are highly likely to be needed to be treated as hazardous materials unless extensive testing in actual commercial operation were to indicate otherwise.

Biological fouling.? No evidence has been identified for biological fouling in either MEA or CESAR1, although the former has seen extensive flue gas service at a range of concentrations and scales over a number of decades.? But the existence of biological fouling at Boundary Dam , where a low-energy solvent is in use, and the biodegradability of MEA and other amines, suggests this may be risk under certain circumstances.

SOME BOTTOM LINES

Based on the limited testing and publishing of results to date the main differences between MEA and CESAR1 for the purposes of commercial amine post-combustion amine capture appear to be:

a)???? At most, 15% lower specific reboiler duty (SRD) than MEA for CESAR1, which for power projects translates into a few percent lower electricity output for the same fuel input – see the figure at the top.?

b)???? Unproven differences in solvent management costs but those for CESAR1 are expected to be somewhat higher, influenced by higher costs for the solvent ingredients; as noted in a recent paper “We found that for lignite fired power plants, CESAR1 is not per se a cheaper solvent system than MEA when actual solvent losses (or their mitigation measures) are considered, contrary to what earlier studies have suggested”.?

c)???? Other showstopper, marginal and in-between issues appear to be close, but proper testing over a year or longer – or actual commercial use – is needed to come to detailed conclusions, except that ….

d)???? The piperazine in CESAR1 and the stable nitrosamines that it forms present higher toxicity risks, particularly for operators.

Acknowledgement and disclaimer

Support for this work from the DESNZ-funded project 'UK BECCS-MCFC: Next Generation CCUS Technology for Net-Zero 2050' is gratefully acknowledged, but the content is entirely the responsibility of the author. A KKD report covering this topic in more detail will appear in due course via https://www.gov.uk/government/collections/carbon-capture-and-storage-knowledge-sharing

End Note #1

For an example of UK ongoing experience of air quality impact assessments for amine releases from capture plants see the Keadby CCGT+PCC project permitting page: https://consult.environment-agency.gov.uk/psc/dn17-3ef-keadby-generation-limited-v011/ and specifically, for routine operation stack emissions: Environmental Statement Volume II - Appendix 8C: Air Quality Assessment of Amine Degradation Products and for major unplanned emissions: Document 6.2.18 - ES Chapter 18 - Major Accidents and Disasters 070921

Also for the Net Zero Teesside CCGT+PCC project, here: https://consult.environment-agency.gov.uk/psc/ts10-5qw-net-zero-teesside-power-north-sea-storage/ Environmental Risk Assessment and Supporting Info - Appendix F - Air Impact Assessment Model input data files for noise and air quality modelling

For a smaller Energy-from-Waste (EfW) plant with PCC, for Suez on Teesside see: https://www.developmentmanagement.stockton.gov.uk/online-applications/applicationDetails.do?activeTab=documents&keyVal=ROQF1UPK03400 ES VOLUME 2_APPENDICES PART4

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