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Carbon Capture Science & Technology 11 (2024) 100192 Contents lists available at ScienceDirect Carbon Capture Science & Technology journal homepage: www.elsevier.com/locate/ccst Air pollution impacts of amine scrubbing for CO2 capture Gary T. Rochelle Texas Carbon Management Program, McKetta Department of Chemical Engineering, The University of Texas at Austin, 10500 Exploration Way, Austin, TX 78758, United States a r t i c l e i n f o Keywords: Carbon capture and storage Amine scrubbing Air pollution a b s t r a c t The current political discussion in the United States around carbon capture and storage includes statements that suggest a need for a technical review to clarify the expected air pollution impacts of amine scrubbing. The Center for International Law and 50 other organizations published an open letter claiming that “CCS is not consistent with the principles of environmental justice… CCS makes dirty energy even more dangerous for frontline communities. Facilities equipped with carbon capture technology have to burn more fossil fuel to get the same energy output, resulting in increased emissions of toxic and hazardous pollutants, like fine particulates (PM2.5). ” This paper reviews air pollutants produced by the use of amine scrubbing on coal- and gas-fired power plants in the U.S. and the process features and mitigation strategies that will minimize their impact on air quality. Even with atmospheric reactions, emissions of amine, nitrosamine, and other air toxics are likely to have insignificant environmental and health impacts. Stacks will disperse emissions with a dilution factor greater than 8000. Water wash with or without acid will reduce emissions of amine and nitrosamine that is produced from atmospheric reactions. Nitrosamine emissions will be managed with selective catalytic reduction (SCR) to reduce the NO/NO2 and/or selection of primary or non-volatile amines. With coal-fired power plants, amine aerosols, Hg, SO3 , and fine particulate emissions will probably be managed by a fabric filter with alkali addition. Carbon capture by amine scrubbing will reduce significantly the effect of the power plant emissions on ambient levels of PM2.5. With coal-fired power plants, the application of amine scrubbing will significantly reduce SO2 emissions. NOx emissions will usually be minimized by selective catalytic reduction (SCR) in both gas- and coal-fired plants. Ammonia emissions will be minimized by managing amine oxidation, and if necessary, by adding an acid wash or other controls. 1 1 i i I c l h i l g t s t t a w w a a S S d t t o ( m t b u a t A h R 2 B . Introduction .1. Political considerations The current political discussion around carbon capture and storage ncludes statements that indicate the need for a technical review to clar- fy the expected air pollution impacts of amine scrubbing. The Center for nternational Law and 50 other organizations published an open letter laiming that “CCS makes dirty energy even more dangerous for front- ine communities. Facilities equipped with carbon capture technology ave to burn more fossil fuel to get the same energy output, resulting in ncreased emissions of toxic and hazardous pollutants, like fine particu- ates (PM2.5) ” ( CIEL, 2021 ). The Environmental Defense Fund has sug- ested that “impacts related to the presence and potential release of ni- rosamines (a toxic carcinogen associated with the breakdown of amine olvents) may pose serious hazards to workers and the public near cap- ure facilities utilizing certain amine solvents in post-combustion cap- ure processes. …This risk and its associated solutions are not well char- cterized in the literature ” ( EDF, 2022 ). E-mail address: gtr@che.utexas.edu ttps://doi.org/10.1016/j.ccst.2024.100192 eceived 10 October 2023; Received in revised form 5 January 2024; Accepted 9 Jan 772-6568/© 2024 The Author(s). Published by Elsevier Ltd on behalf of Institution Y-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Fig. 1 shows the unit operations of an existing coal-fired power plant ith retrofit of amine scrubbing for CO2 capture. The existing plant ill remove 70–90% of the NOx by selective catalytic reduction with mmonia ( U.S. EPA, 2020 ). A fabric filter with addition of activated C nd Ca(OH)2 will remove 99–99.9% of the fly ash ( U.S. EPA, 2023a ), O3 , and Hg. Flue gas desulfurization will remove 90 to 98% of the O2 ( U.S. EPA, 2023b ). The flue gas is discharged up a tall stack to isperse effectively any remaining air pollutants. The flue gas will pass hrough new prescrubbers that can be designed to remove 95 to 99% of he SO2 to less than 1 ppm ( U.S. EPA, 2022 , 2023c ). With the addition f Na2 S2 O3 , the new prescrubber can remove 80 to 95% of the NO2 Selinger, 2018 ). After passing through the absorber where CO2 is re- oved, most of any volatile amine and nitrosamine will be removed by he water wash. If an acid wash is used, most of the ammonia will also e removed. The clean, mostly decarbonized flue gas will be exhausted p a new stack to disperse effectively any remaining pollutants. The rich mine solvent loaded with CO2 is circulated through a heat exchanger o the stripper to produce practically pure CO2 and solvent lean in CO2 . mmonia, volatile amines, and other volatile products from amine oxi- uary 2024 of Chemical Engineers (IChemE). This is an open access article under the CC https://doi.org/10.1016/j.ccst.2024.100192 http://www.ScienceDirect.com/science/journal/27726568 http://www.elsevier.com/locate/ccst http://crossmark.crossref.org/dialog/?doi=10.1016/j.ccst.2024.100192&domain=pdf mailto:gtr@che.utexas.edu https://doi.org/10.1016/j.ccst.2024.100192 http://creativecommons.org/licenses/by-nc-nd/4.0/ G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Fig. 1. Retrofit of amine scrubbing on an existing coal-fired power plant. d C b t i s d d r r g a U 1 1 p p b l t 1 a T s u a m i t n l t 1 a a c p s m T s a 1 c d a e g t m c G t T t w ( 2 s p p b ( d p ation may be purged to waste treating with water condensed from the O2 . This review will establish that the expected designs of amine scrub- ing for CO2 capture on coal- and gas-fired power plants will reduce he emissions and ground level concentrations of PM2.5 without signif- cantly increasing impacts of toxic and hazardous pollutants. It will also how that nitrosamines should not be a significant air quality issue in the eployment of amine scrubbing. Nevertheless, additional research and evelopment is needed to quantify and minimize the risk of atmospheric eactions that produce PM2.5 and nitrosamines. Other important envi- onmental impacts of fossil fuel, including the effects of coal mining and reenhouse gas emissions in the supply chain for coal and natural gas, re not addressed here. The paper focuses on air quality impacts in the nited States and does not address conditions in other countries. .2. Previous reviews .2.1. EEA (2011) The European Environmental Agency prepared a comprehensive re- ort on the air pollution impacts of carbon capture ( EEA, 2011 ). The re- ort considered not only post-combustion (including amine scrubbing), ut also precombustion and oxycombustion technology. EEA included a ife cycle analysis that considered the effects of additional fuel consump- ion and the indirect effects of fuel preparation, transportation, etc. • EEA concluded that amine scrubbing on coal-fired power plants would practically eliminate direct SO2 emissions. However, the life cycle analysis for Europe in 2011 showed that total SO2 emissions decrease by only 20% when the effects45Q credit, if the plant was previously operated at maximum apacity, much of its lost electricity production would probably be re- laced by new renewable capacity and production on the grid. No ad- itional fuel would be burned at the power plant, so the indirect effect n air emissions will be negligible. On the other hand, in the long term, the existing plant may only be perated when wind and solar are not available. Therefore, its remain- ng useful capacity for this purpose will be reduced and additional fuel ill be required for the power that is produced. In the event that more apacity is required to back up the renewables, the capture system can e turned off, increasing its CO2 emissions, but releasing critical power o the grid. In this future scenario, because it will only be operated a mall fraction of the year, the plant will be producing less pollution and sing less fuel than its current status, but more than if the capture system ere not used. It is also possible that the loss of production at the retrofit plant ould be replaced by constructing a new gas-fired combined cycle with ntegrated carbon capture. The new plant would not be fired by coal so here will be no emissions of Hg, SO3, or other hazardous pollutants. The ew gas plant would use SCR for NOx reduction, but the incremental ombustion to replace the power lost from the capture facility would esult in a small incremental release of NOx . This new plant could be ited at a distance from the existing plant so it need not make it “even ore dangerous for frontline communities ” ( CIEL, 2021 ). .3. Role of 45Q in increasing load factor With the expected incentives of the 45Q tax credit, when carbon cap- ure is applied to an existing power plant, it will incentivize the plant to G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 o t b i d f p m 8 t P w m s b t w t a a t e a s t g 9 f t t n p m a q d e p p t n m p d p D t c b a A U g G t p c p o c d t T s t t r R A A A B B C C C C C D D D D D E E F F perate at a greater load factor to generate the maximum cash flow. To he extent that this occurs, the air pollution (primarily NOx ) contributed y this plant will increase. However, in a reasonably dispatched grid, the ncreased power production of this plant will not reduce the power pro- uced by renewables, but will instead decrease the load factor of other ossil power plants that do not have capture. Since this single plant will robably have SCR and other modern pollution controls and the others ay not, the net emissions will probably decrease. . Conclusions Carbon capture by amine scrubbing will result in significant reduc- ion of the effect of the power plant emissions on ambient levels of M2.5. For coal-fired power plants, the application of amine scrubbing ill significantly reduce SO2 emissions. NOx emissions will usually by inimized by SCR in both gas- and coal-fired plants. Ammonia emis- ions will be minimized by managing amine oxidation, and if necessary, y adding an acid wash. Even with atmospheric reactions, air quality impacts of amine, ni- rosamine, and other air toxics will probably be insignificant. Water ash with or without acid wash will minimize emissions of amine and herefore it will also reduce nitrosamine produced by atmospheric re- ctions. Nitrosamine emissions will primarily be managed with SCR nd/or by amine selection. to reduce the NO/NO2 On a typical applica- ion on a coal-fired plant, amine aerosols, Hg, SO3 , and fine particulate missions will be managed by a fabric filter with activated carbon and lkali addition. The indirect effect of additional energy use for capture should be in- ignificant but will depend on the specific source of energy for the cap- ure system and the incremental operation and expansion of the power rid. . Recommendations The U.S. EPA should recommend reasonable emission requirements or amine scrubbing to allow for planning and to avoid emission expec- ations that are not science-based. These requirements should address he specific principal amines, nitrosamines, acetaldehyde, and ammo- ia and could include the listing of selected chemicals as hazardous air ollutants. This effort will probably require more quantification of the itigation methods and the toxicology of the specific pollutants. Amine scrubbing should prepare to use a two-stage water wash or water wash plus an acid wash in the event emission standards re- uire this additional control for ammonia and residual amine. Process evelopers should explore purge of condenser water to reduce ammonia missions without an acid wash. Sources with potential for amine aerosol emissions should use pilot lant testing at the site to demonstrate aerosol control. Coal-fired power lants without fabric filters should be avoided. Additional research should be performed to reduce uncertainties in he effects of atmospheric reactions on the formation of and exposure to itrosamines, nitramines, and PM2.5. More extensive gas sampling and analysis is needed to confirm the inimal emissions of potential pollutants at pilot and demonstration lants of amine scrubbing. High sensitivity methods should be used to emonstrate amine, nitrosamine, and aldehyde emissions less than 100 pb. eclaration of competing interest The authors declare the following financial interests/personal rela- ionships which may be considered as potential competing interests: Gary Rochelle reports a relationship with Honeywell UOP that in- ludes: consulting or advisory. 12 Gary Rochelle has financial interests in intellectual property owned y the University of Texas that includes scrubbing technology and mines reported in this paper. cknowledgements This report was prepared as an account of work supported by niversity of Texas at Austin and the Texas Carbon Management Pro- ram and sponsored in its initial phase by an agency of the United States overnment. Neither the United States Government nor any agency hereof, nor any of their employees, makes any warranty, express or im- lied, or assumes any legal liability or responsibility for the accuracy, ompleteness or usefulness of any information, apparatus, product, or rocess disclosed, or represents that its use would not infringe privately wned rights. Reference herein to any specific commercial product, pro- ess, or service by trade name, trademark, manufacturer, or otherwise oes not necessarily constitute or imply its endorsement, recommenda- ion, or favoring by the United States Government or any agency thereof. he views and opinions of authors expressed herein do not necessarily tate or reflect those of the United States Government or any agency hereof. The author has financial interests in intellectual property owned by he University of Texas that includes scrubbing technology and amines eported in this paper. eferences ECOM, 2021. The Keadby 3 Low Carbon Gas Power Station Project. 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Rochelle Carbon Capture Science & Technology 11 (2024) 100192 V W W Z oice, A.K., 2013. Amine oxidation in carbon dioxide capture by aqueous scrubbing, Ph.D. dissertation, The University of Texas at Austin. HO (World Health Organization), 2021. Global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. ISBN 978-92-4-003422-8. https://www.who.int/publications/who-guidelines . 14 u, Y., 2022. Mitigation methods for piperazine oxidation in post-combustion carbon capture, Ph.D. dissertation, The University of Texas at Austin. hang, Y., 2018. Absorber and aerosol modeling in amine scrubbing for carbon capture, Ph.D. dissertation, The University of Texas at Austin. https://www.who.int/publications/who-guidelines Air pollution impacts of amine scrubbing for CO2 capture 1 Introduction 1.1 Political considerations 1.2 Previous reviews 1.2.1 EEA (2011) 1.2.2 SEPA (2020) 1.2.3 Fang et al. (2020) 1.2.4 Buvik et al. (2021) 2 Expected air pollutants from power plants with amine scrubbing 2.1 Emissions regulated directly by NAAQS 2.2 PM2.5 (particulate matter less than 2.5 µm) and ozone 2.3 Emissions designated as hazardous air pollutants 2.4 Currently unregulated emissions 3 Sources of additional emissions from amine scrubbing 3.1 Amine volatility 3.2 Oxidation of amine to volatile and more impactful species 3.2.1 Ammonia and alkyl amines 3.2.2 Aldehydes 3.3 Nitrosamines from amine scrubbing 3.3.1 Nitrosamine chemistry in the amine scrubbing process 3.3.2 Health effects of nitrosamines 3.3.3 Determinants of nitrosamine emissions 3.4 Amine aerosols 4 Expected controls in plants that use amine scrubbing 4.1 Site selection 4.2 Fabric filter with carbon and alkali addition 4.3 Tall stack 5 Features of amine scrubbing that mitigate gaseous emissions 5.1 Solvent selection 5.2 Process features to mitigate degradation 5.3 Prescrubbing 5.4 Water wash 5.4.1 Single stage 5.4.2 Two stages of water wash 5.4.3 One stage water, one stage dilute acid 5.5 Condensate purge 6 Measured emissions from demonstrations and pilot plants 6.1 Amine emissions 6.2 Acetaldehyde 6.3 Ammonia 6.4 Nitrosamine 6.5 Petra Nova 7 Indirect effects of energy used by CCS 7.1 Heat and power provided by a natural gas turbine 7.2 Heat and power extracted from the existing power plant 7.3 Role of 45Q in increasing load factor 8 Conclusions 9 Recommendations Declaration of competing interest Acknowledgements Referencesof ocean transport of coal and additional coal consumption required to provide the energy for capture are included. • EEA concluded that ammonia emissions would increase significantly because of MEA degradation, but that this increase if applied to all coal plants in Europe would be only 6% of the NH3 emissions from other sources. • EEA did not address the indirect effect of emissions on PM2.5 and other pollutants such as nitrosamine. .2.2. SEPA (2020) The Scottish Environmental Protection Agency reviewed the avail- ble results for emissions from amine scrubbing systems ( SEPA, 2015 ). hey concluded that “whilst emissions to air of the amine solvents them- elves are unlikely to be of significant concern there is a higher degree of ncertainty associated with emissions of amine reaction products such s nitrosamines. ” Amine emissions (mostly monoethanolamine [MEA]) 2 easured at 13 pilot plants varied from 0.02 to 11 mg/m3 . The max- mum nitrosamine emissions measured at 6 pilot plants varied from 3 o 9 𝜇g/m3 . They also concluded that “there is not a standardized tech- ique for monitoring nitrosamine in stack emissions and there has been imited stack emission monitoring of amine compounds at carbon cap- ure pilot plants. ” .2.3. Fang et al. (2020) Fang et al. reviewed measured emissions and mitigation methods for mine scrubbing pilot plants that have significant emissions of amine erosol ( Fang et al., 2020 ). Amine aerosol emissions will be a criti- al economic problem for deployment of amine scrubbing on coal-fired ower plants and other applications such as biomass combustion with ources of aerosol nuclei ( Fujita et al., 2014 ) unless suitable mitigation ethods are developed. Fang discusses 10 proposed control measures. he review does not discuss the important strategy of only using amine crubbing on coal-fired boilers that already have fabric filters with alkali nd carbon injection for control of fine particulate, SO3 , and Hg. .2.4. Buvik et al. (2021) Buvik et al. (2021) reviewed degradation and emissions in post- ombustion CO2 capture pilot plants. The primary emphasis was on egradation, but they provide a comprehensive table of ammonia and mine emissions from pilot plants that have mitigated amine aerosol missions or have avoided those emissions with a clean source of flue as. “Based on the pilot results it is clear that for MEA, the wash wa- er section can limit the MEA emission to a few hundred ppb, am- onia in the low ppm range, and methylamine in low ppb range in ase no mist is present ( Morken et al., 2017 ; Lombardo et al., 2017 ; jernes et al., 2017 ). Furthermore, in these cases, there is no observa- ion of nitrosamine and nitramine emissions over the detection limit. he solvent emissions of the tested proprietary solvents can be con- rolled to similar levels as seen with 30 wt% MEA (aq.). This is in line ith reported numbers for commercially available proprietary solvents Singh and Stéphenne, 2014 ; Feron et al., 2020 ).” . Expected air pollutants from power plants with amine crubbing Coal- and gas-fired power plants without CO2 capture may emit air ollutants in varying quantities and significance. Table 1 summarizes ossible air pollutants from coal- and gas-fired plants with amine scrub- ing and their quantitative health effects. The Threshold Limit Values TLV) recommended by the American Conference of Governmental In- ustrial Hygienists (ACGIH) for workplace exposure to hazardous gases rovide a means to quantify the relative impacts of specific pollutants. G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Table 1 Potentially significant air pollutants from power plants with amine scrubbing. 2019 TLV a (ppmv) [(ACGIH] NAAQS b annual avg (ppbv) On HAP List LD50 a for Rodents (mg/kg) TD50 c for Rodents (mg/kg) PM2.5 particlesSO2 emissions that are xidized to H2 SO4 , from NOx emissions that are oxidized to nitric acid HNO3 ), and from ammonia emissions that react with the acids to make ne particulate solids. Ozone results from atmospheric reactions of hydrocarbons and Ox , so an important secondary effect of NOx emissions from power lants may be the formation of ozone in downwind metropolitan areas here there are hydrocarbon emissions from transportation and indus- ry. There are several metropolitan areas that are not in compliance ith the AAQS for ozone ( US EPA, 2023e ). However, the major fac- ors resulting in ozone are emissions of NOx and hydrocarbons from ransportation sources and point sources in the immediate urban area Soleimanian et al., 2023 ), not the NOx emissions from a rural power lant that may be some distance from the urban area. Facilities that emit more than 50 tons per year (tpy) (25 tpy in nonat- ainment areas) of VOCs or NOx must apply for a Title V Air Permit. mine scrubbing may emit VOCs such as amines, acetaldehydes, and ther partially oxidized hydrocarbons that may contribute to ozone for- ation and trigger a requirement for a Title V permit. As little as 0.5 to ppmv of these combined emissions from the power plant and capture ystem will result in 25 to 50 tpy from a 500 MW power plant. Sections that follow will address in detail how amine scrubbing may mpact PM2.5 and how these impacts may be mitigated. .3. Emissions designated as hazardous air pollutants EPA has designated a list of Hazardous Air Pollutants (HAPs) that ust be regulated under section 112 of the Clean Air Act. Any source hat emits more than 100 tpy of a combination of HAPs is required to G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 o P o g p c t m a d u l t h n r d p D n i E t f e 2 b t t d l c i w e a i t t l 3 3 t w E t fl a o f w s l o C P p l i m s A t t d w s 3 3 a a o R d d p a t r m r m g d R a w m 1 l t t a t $ o s b o s t r n o b s a btain a Title V Air Permit to operate. “Major Sources ” of Hazardous Air ollutants are designated as those with more than 10 tpy of a single HAP r 25 tpy of any combination of HAPs. This designation could be trig- ered by as little as 0.1 to 1 ppm of the specified HAP on a large power lant. 0.5 ppm acetaldehyde would give 10 tpy on a 460 MW gas-fired ombined cycle. “Major Sources ” may require maximum achievable con- rol technology (MACT) for the HAPs. Some coal-fired plants emit regulated hazardous air pollutants, com- only including mercury and SO3 . Many of the modern plants have dded fabric filters with injection of alkali and activated carbon to re- uce these emissions to the level required by the regulations. These reg- lations have practically limited the negative health effects of these pol- utants. As shown in Table 1 , the current list of HAPs includes five molecules hat might be emitted by amine scrubbing. Formaldehyde and acetalde- yde may be produced by the oxidation of parent amines. The specific itrosamines, N-nitrosodimethylamine and N-nitrosomorpholine may esult from the reactions of NO2 with the amine degradation products imethylamine and morpholine. Specific parent amines that degrade rimarily to dimethylamine or morpholine will probably be avoided. iethanolamine is a potential amine solvent component, but there are o published solvent compositions that include it. As amine scrubbing s widely deployed, the HAP list should evolve to provide the means for PA to limit emissions of other specific, hazardous molecules from cap- ure technology, such as the amines themselves, nitrosamines derived rom parent amines, and other volatile degradation products of the par- nt amine. .4. Currently unregulated emissions Amine scrubbing will emit low levels of gaseous amine that may e significant. LD50 for rodents is the oral dosage that will kill half of he exposed rats or mice (see Table 1 ). It sets a relative scale for the oxicity of liquid or solid chemicals. The relative toxicity of gases (TLV) oes not vary in the same way as the oral toxicity (LD50 ) of the same iquid. Piperazine appears to be the most toxic of the gaseous amines onsidered in Table 1 , because it causes an allergic sensitivation when nhaled ( OSHA, 2023 ). Methylamine is the most toxic of the amines hen ingested ( OSHA, 2023 ). Table 1 includes examples of carcinogenic chemicals that may be mitted from power plant stacks or formed from amine emissions by re- ctions in the atmosphere. TD50 is the oral dosage that will cause tumors n 50% of exposed rats or mice. The nitrosamine from dimethylamine is he most potent of these ( CPDB, 2023 ). Its nitramine is 5.5 times less po- ent ( CPDB, 2023 ). The nitrosamine formed from piperazine is 90 times ess potent than nitrosodimethylamine ( CPDB, 2023 ). . Sources of additional emissions from amine scrubbing .1. Amine volatility The flue gas leaving the absorber will usually be in equilibrium with he lean solvent at the top of the absorber. Commercial amine solvents ill frequently have a volatility of 5 to 50 ppm at these lean conditions. missions are mitigated by an additional water wash after the absorber o remove the vapor amine from treated flue gas, producing a clean ue gas with less than 1 ppm amine. Amines with greater volatility are voided because of the cost of the water wash and the potential cost f lost amine. Amines with lower volatility are usually larger, less ef- ective molecules that increase solvent viscosity. Spent amine solvents ith low volatility are not easily reclaimed (purified for recycling) by imple boiling of the solvent as with MEA or PZ. Process developers have an economic incentive to reduce amine osses in the vapor. With a reasonably inexpensive amine at $2/lb, 1 ppm f amine loss in the flue gas is equivalent to a makeup cost of $1/tonne O . 1 ppm is acceptable, but > 5 ppm would start to be prohibitive. 2 4 rocess developers avoid selecting amines with excessive volatility. The rimary criterion for suitable volatility is a solvent molecule with at east 2 hydrophilic groups. Emission of amine in the clean flue gas will only be an environmental ssue in a poorly developed amine scrubbing process. The amine itself ay constitute a health and safety risk. It may also participate in atmo- pheric reactions that produce ozone, nitrosamine, nitramine, or PM2.5. useful target for amine emissions, achieved in many pilot plants, is 1 o 2 ppm. At this level, the toxicity risk of the amine itself is less than hat of SO2 (200 ppm) in a typical coal-fired power plant with flue gas esulfurization, and the risk of PM2.5 is also less than that associated ith the SO2 . However, the potential for contributing to PM2.5 in sen- itive areas may require lower total emissions of amine. .2. Oxidation of amine to volatile and more impactful species .2.1. Ammonia and alkyl amines Ammonia is one of the initial products of the oxidation of primary mines such as monoethanolamine. Solvents that start with a primary mine adjacent to a primary or secondary carbon oxidize with cleavage f the C–N bond to give ammonia and an aldehyde or ketone. -CRH–NH2 + 0.5 O2 → R-CR = NH + H2 O → R-CR = O + NH3 Secondary or tertiary amines such as piperazine do not initially pro- uce ammonia, but as primary amine degradation products are pro- uced, those in turn will oxidize to produce ammonia. Therefore, if ilot plant campaigns have not been run long enough to accumulate ny primary amine, the measured ammonia emissions may be less than hose expected from a long-term commercial operation and may not be epresentative of the amine degradation. With secondary amines, am- onia emissions can be minimized by continuousthermal reclaiming to emove the less volatile primary amine degradation products. Amines that oxidize or thermally degrade to a volatile amine such as ethylamine should and will be avoided. Secondary amines with alkyl roups such as methylamine or ethylamine will initially oxidize to pro- uce the corresponding volatile amines. -CRH–NRH + 0.5 O2 → R-CR = NRH + H2 O → R-CR = O + RNH2 Like ammonia, unless removed by some other means, these volatile mines will accumulate in the solvent until they pass through the water ash and are emitted in the clean flue gas. Amine oxidation can result in emissions in the cleaned flue gas. Am- onia is the primary volatile product of oxidation. It may be present at to 10 ppm in the clean flue gas after the water wash. As with amine osses, each mol of ammonia represents an equivalent loss of amine, but he soluble degradation products must be removed from the solvent and he waste solvent must be disposed of. This results in a cost per mol of mmonia of 2 to 4 times the makeup cost of the amine. Therefore, as lit- le as 2 ppm ammonia can represent a financial burden to the process of 5–10/tonne CO2 . This motivates process developers to minimize amine xidation and ammonia emissions. Ammonia emissions are otherwise less significant as ammonia is con- iderably less toxic that SO2 and other pollutants. Ammonia will already e present at 1 to 10 ppm as slip from selective catalytic reduction (SCR) f NOx by ammonia. However, ammonia can react with the NOx emis- ions to produce PM2.5 as ammonium nitrate. If necessary, ammonia can be removed by adding an acid wash on he exiting flue gas ( Drewry and Rochelle, 2023 ). This acid wash will emove practically all of the ammonia and any volatile amine that is ot present as amine aerosol. Acid wash will produce a waste solution f ammonium sulfate and amine sulfate. Ammonia may also be managed y bleeding condensate from the stripper overhead to water treating or ome other external system ( Hatcher and Weiland, 2012 ). Developers will take steps to reduce amine oxidation and minimize mine makeup costs that will also reduce emissions. G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 3 a a h u m h fl a 3 n b t n a t a a 3 a f ( ( N a m m f g a R P P P P P P w t P t P o c 2 t m 3 o T o t t a ( c ( m r s t t a 0 g c s 2 c 3 fl a n t F t b I t m e s p l a a ( o l a r a w r • Select an amine that is resistant to oxidation ( Voice, 2013 ). • Sparge the rich solution with nitrogen to remove dissolved oxygen ( Wu, 2022 ). • Use various reclaiming methods to remove degradation products that chelate dissolved iron and the dissolved iron itself ( Wu, 2022 ). • Minimize NO2 in the inlet flue gas as in nitrosamine management. If necessary, retrofit SCR to reduce the total NOx ( Chen, 2022 ). .2.2. Aldehydes As shown in the reactions above, formaldehyde and other aldehydes re expected products of amine oxidation. Aldehydes interact strongly nd reversibly with primary and secondary amines to make aminal, emiaminal, carbinolamine, imine, enamine, and other related molec- lar structures ( Kondo et al., 2015 ). These tend to be hydrated, polar olecules that have low volatility in aqueous solution. Formaldehyde as not been measured at significant concentration in cleaned, washed ue gas. Acetaldehyde is less polar and has sometimes been observed as significant air pollutant with MEA and other amine systems. .3. Nitrosamines from amine scrubbing Gaseous emissions of nitrosamine have been identified and recog- ized as a potential health and safety risk of CO2 capture by amine scrub- ing. NO2 in the flue gas and secondary amine in the solvent are required o produce nitrosamine; primary and tertiary amines do not make stable itrosamines. Therefore, the selection and use of primary and tertiary mines for the solvent will greatly reduce the risk of nitrosamine forma- ion. However, primary and tertiary amines may degrade to secondary mines, so there will still be some risk if these degradation products are llowed to accumulate in the solvent. .3.1. Nitrosamine chemistry in the amine scrubbing process All amines will react with NO2 in the absorber to remove the NO2 nd produce nitrite. Piperazine (PZH) is a secondary amine that reacts ast enough with NO2 to remove 40–60% of it in a typical absorber Closmann et al., 2022 ; Fine, 2015 ). The initial reaction produces nitrite NO2 − ) and a free radical of piperazine (PZ. ). O2 + PZH → NO2 − + PZ. + H + Fine (2015) showed that the rate of this reaction and the mount of NO2 removed is in the order: tertiary amines (e.g., ethyldiethanolamine [MDEA]) > secondary amines (e.g., PZH) > pri- ary amines (e.g., monoethanolamine [MEA]). With all amines, the ree radical will probably initiate a chain reaction with dissolved oxy- en in the absorber, resulting in the oxidation of 2 to 40 mols of mine. For example, with piperazine ( Closmann et al., 2022 ; Chen and ochelle, 2022 ): Z. + O2 →PZOO. ZOO. + PZH →PZOOH + PZ. ZOOH → PZO. + OH. ZH + PZO. →PZ. + PZOH ZH + OH. → PZ. + H2 O Z. + PZ. + H2 O → PZH + PZOH At the elevated temperature of the stripper, the nitrite will react ith secondary amines such as PZH to produce one mole of mononi- rosopiperazine (PZNO) ( Fine, 2015 ). ZH + NO2 − → PZNO + OH− 5 Primary and tertiary amines react with nitrite at stripper temperature o produce oxidation products of the amines, but not nitrosamine. As the ZNO accumulates in the solvent it will degrade at the high temperature f the stripper to piperazinol, so that the PZNO will reach a steady-state oncentration ( Fine, 2015 ). PZNO + H2 O → 2PZOH + N2 O With 1 ppm of NO2 in the flue gas, the steady-state PZNO concen- ration in the solvent with stripping at 150 °C has been observed and odeled at about 1 millimol/kg ( Chen and Rochelle, 2022 ). .3.2. Health effects of nitrosamines The tendency of a nitrosamine to cause tumors in rats is given by the ral dose TD50 , (mg/kg/day), which causes tumors in 50% of the rats. able 1 gives measured values of the TD50 for some nitrosamines and ther common chemicals. The TD50 for N-nitrosodimethylamine is 100 imes less than for N–Nitrosopiperazine, so dimethylamine and solvents hat might produce dimethylamine as a degradation product should be voided, but PZ should be acceptable if the PZ emissions are minimized. Norway has carefully evaluated the risks of N–Nitrosodimethylamine NDMA) and has recommended a tolerable ambient, ground-level con- entration of 0.0003 𝜇g/m3 based on the potency (TD50 ) of NDMA NIPH, 2016 ). The UK recommends a similar Environmental Assess- ent Level (EAL) of 0.0002 𝜇g/m3 for NDMA ( Gov.uk, 2021 ). It is easonable that the tolerable air concentration of other carcinogens hould vary inversely with their TD50 compared to NDMA, although he authorities may choose the more conservative approach of set- ing targets for all nitrosamines based on NDMA. For example, the cceptable air concentration of N-nitrosopiperazine (PZNO) should be .0003 ×8.78/0.0959 = 0.027 𝜇g/m3 . Power plant stacks routinely get round level concentrations that are 1000 to 10,000 times less than stack oncentrations. If PZ is emitted at 4 mg/m3 (1.0 ppmv) at the top of the tack, 1% of it is oxidized to nitrosamine in the ambient air ( Tan et al., 021 ), and it is diluted a factor of 2000 by dispersion, the ground level oncentration of nitrosamine would be acceptable at 0.027 𝜇g/m3 . .3.3. Determinants of nitrosamine emissions The resulting nitrosamine must be volatile to escape with the clean ue gas from the water wash provided in the design of practically all mine scrubbing processes. Since the parent amine must be reasonably onvolatile to avoidcostly losses of amine in the exhaust gas, any ni- rosamine from the parent amine will also be reasonably nonvolatile. or example, PZNO has been estimated to be significantly less volatile han PZ ( Thompson et al., 2018 ), so if PZ emissions are managed well y the water wash, then there should be low emissions, if any, of PZNO. f needed, nitrosamines with a free amine group can be removed from he exhaust gas with the additional cost of an acid wash. To constitute a risk of gaseous emissions, nitrosamine must accu- ulate in the solvent until it has a significant vapor pressure. How- ver, nitrosamine thermally degrades in the stripper, limiting its steady- tate concentration in the solvent. Thermally stable amines such as PZ ermit stripper operation at higher T, which minimizes any accumu- ation of nitrosamine. In pilot plant testing of CESAR-1 (a blend of PZ nd aminomethylpropanol (AMP)), the stripper was occasionally oper- ted at 130 °C to reduce the steady-state concentration of nitrosamine Moser et al., 2023a ). Dispersion in daytime with stacks results in rapid UV decomposition f the nitrosamine, greatly reducing any adverse health effects at ground evel ( Tan et al., 2021a ; Tan et al., 2021b ). Adequate dispersion of parent mines should reduce any nitrosamine that is produced by atmospheric eaction to an insignificant ground level concentration. Because significant direct emissions of nitrosamine are unlikely, the tmospheric reactions of amine emissions to produce nitrosamine may arrant more attention. In typical ozone chemistry ( Fig. 2 ) hydroxyl adical reacts rapidly with PZ at the alpha or beta position to produce G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Fig. 2. Daytime atmospheric chemistry of piperazine with NO, OH (and ozone) ( Tan et al., 2021 ). t t 1 w t b p P d u n a e d c N o r P g n u r w n e N o o t s ( 3 t μ s s o i t g w c ( 4 4 f m s fi w m t b n w b b s t 9 r he respective radicals ( Tan et al., 2021a ). 82% of the reaction is with he beta position ultimately producing oxidation products of the PZ. 8% of the reaction is with the alpha position and ultimately reacts ith NO to produce nitrosamine, NO2 to produce nitramine, or oxygen o produce oxidation products. The reaction with NO can be reversed y sunlight. In sustained sunlight all of the PZ ends up as PZ oxidation roducts. NO2 reaction with ozone produces nitric acid. Nitric acid reacts with Z to produce ammonium nitrate aerosol, which picks up water and hy- rophilic oxidation products. As a result, most of the degradation prod- cts and some PZ will end up in PM2.5 aerosol. Sunlight with little ozone and low OH will rapidly decompose any itrosamine and convert it to oxidation products. In the plume with relatively high ratio of NO to OH, the alpha radical (18%) mostly nds up as nitrosamine. Steady-state, first-order reverse reaction will rive continuous conversion to oxidation products that will later be onverted to oxidation products with exposure to sunlight at lower O. The conversion to nitramine will depend on the ratio of NO2 to xygen. With substantially greater NO2 in the plume there will be elatively more nitramine, maybe even mostly nitramine. Fortunately, liss et al. (1982) concluded that nitramines were much less carcino- enic than the corresponding nitrosamine. Therefore, nitramines should ot be an air quality issue. Reactions in the plume will be limited because of low ozone and OH, nless oxidation products such as aldehydes photolyze and initiate free adicals that react with the PZ. High humidity in the near stack plume ill stabilize aerosols formed in the plume. Addition to or formation of itric acid in the plume will make PM2.5 of all the available amine. Faster nighttime reactions will be driven by NO3 with the in- vitable formation of PM2.5 from the available amine ( Tan et al., 2021a ; eilsen et al., 2012 ). NO3 is formed by the nighttime reaction of daytime zone with NO/NO2 in the plume. Little is known about the formation f nitrosamine by reactions in PM2.5 containing secondary amine. A box model at average summer conditions in Oslo demonstrated hat the maximum concentration of PZNO occurs 1 h downwind of the tack with a value that is 1% of the PZ concentration at that point Tan et al., 2021a ). f 6 .4. Amine aerosols Flue gas from coal-fired power plants and other dirty sources some- imes includes very fine particulate ( 10 ppm) ( Fang et al., 2020 ; Akinpelumi, 2021 ). . Expected controls in plants that use amine scrubbing .1. Site selection The gas- and coal-fired power plants that make the best candidates or amine scrubbing will probably also be the best choices for mini- izing air quality impacts. Even in the most optimistic scenario, amine crubbing will not be applied to all of the existing fleet of coal- and gas- red power plants in the U.S. The best candidates for amine scrubbing ill be selected first. The oldest units will be retired. Amine scrubbing ay or may not be used in the rest of the existing units depending on he incentives and specific circumstances. However, meeting net zero y 2050 will probably require that amine scrubbing be applied to any ew gas-fired combined cycles, especially if they are base-loaded. There ill probably be no new coal-fired power plants in the U.S. The ideal existing coal-fired power plant to retrofit with amine scrub- ing will be a large plant providing economies of scale with two or more oilers producing greater than 1000 MW with uncontrolled CO2 emis- ions exceeding 10 million tonnes/yr. To minimize the need for addi- ional prescrubbing, existing flue gas desulfurization will remove 90 to 8% of the SO2 . Selective catalytic reduction (SCR) by ammonia will educe NOx to less than 20 ppm to minimize amine oxidation and the ormation of nitrosamine. Fine particulate will be removed by a high ef- G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Table 2 Coal-fired power plants that might be considered for amine scrubbing ( arcgis.com, 2022 ). Power Stations with DOE FEEDs Population within 3 m Demo index (%ile) MW Utilization (%) Emissions to air (lbs/MWh) PM2.5 Fabric filter CO2 NOx SO2 Gerald Gentleman (NE) 20 24 1363 70 2078 1.8 5.6 0.03 Yes Milton R. Young (ND) 29 29 734 73 2384 3.7 1.1 0.49 No San Juan (NM) 111 71 1848 29 2555 2.8(SNCR) 0.6 0.01 Yes Prairie State (IL) 311 4 1766 78 2300 0.7 1.8 0.22 Yes Large Plants in Texas J. K. Spruce (TX) 2644 71 1444 53 2233 0.9 0.2 0.03 Yes Oak Grove (TX) 132 31 3397 48 1902 0.6 1.0 0.10 Yes W. A. Parish (TX) 1098 52 4008 41 1899 0.6 4.0 0.16 Yes Martin Lake (TX) 545 58 3180 43.6 2289 1.6 7.7 0.15 No fi a a m s p f c l w m o r P h g c t A a b t p a T e f fi t y 4 t fi o ( r c fi c s f g e r d a b T h g t w o t t w i w t r a d ( 4 m p o d D c s l t ( 8 t t d t p fl b a T 2 g c a ciencyfabric filter to minimize amine aerosol. Hydrated lime or other lkali will be injected before the filter to remove SO3 resulting in less mine aerosol. Activated carbon will be injected before the filter to re- ove mercury and it may also remove NO2 . The plant will have a tall tack that effectively disperses any residual pollutants and will also dis- erse pollutants from amine scrubbing. It will have long-term contracts or low-cost coal from a nearby dedicated mine or Powder River Basin oal shipped from Wyoming that will provide long-term operation at ow cost. The ideal existing or new gas-fired power plant for amine scrubbing ill be a combined cycle producing more than 500 MW with two or ore gas turbines and a steam turbine. It will have SCR for NOx control r the ability to readily add SCR. The ideal gas- or coal-fired power plants will have been installed in ural areas since 2000. The surrounding area will be in compliance with M2.5 and Ozone National Ambient Air Quality Standards. The site will ave ample space to add the equipment for CO2 capture. There will be ood opportunities for sub-surface storage of CO2 . Because of the additional costs associated with failing to meet these riteria, it is unrealistic to expect that amine scrubbing will be retrofitted o the majority of power plants that do not meet most of the criteria. large number of plants do come close to this scenario and will be ttractive sites to decarbonize baseload power generation and provide ackup for renewable energy when or where it is not available. Table 2 shows characteristics of several large coal-fired power plants hat might be considered for retrofit of amine scrubbing. Four of the lants have completed Front End Engineering Designs for a variety of mine scrubbing processes. The other four plants are target sites in exas. These plants meet most of the criteria for an ideal site. How- ver, it appears that Milton R. Young and San Juan do not yet have SCR or NOx control. Milton R. Young and Martin Lake do not have fabric lters. W.A. Parish and J.K. Spruce are on the outskirts of the popula- ion centers of Houston and San Antonio. Gerald Gentleman does not et have flue gas desulfurization. .2. Fabric filter with carbon and alkali addition An effective means of avoiding amine aerosol emissions is to remove he nuclei and practically all of the PM2.5 from the flue gas using a fabric lter with the addition of hydrated lime or sodium bicarbonate upstream f the fabric filter to remove H2 SO4 aerosol nuclei and its precursor, SO3 Beaudry, 2016 ). Fabric filters rather than electrostatic precipitators are outinely specified as the primary device for fly ash removal in modern oal-fired power plants. With the addition of activated carbon, fabric lters are also used to remove mercury from the flue gas. Therefore, oal-fired power plants that already have fabric filters will be preferred ites for the application of carbon capture by amine scrubbing. When a abric filter was retrofitted on the coal-fired power plant providing flue as for the NCCC pilot plants, along with greatly reduced amine aerosol missions ( Beaudry, 2016 ), the NO in the flue gas was also greatly 2 7 educed ( Selinger, 2018 ). Fabric filters are not inexpensive, so if a plant oes not already have this system it is improbable that it will be added. Two other options have been tested to pretreat flue gas to manage mine aerosol emissions. A candle filter/Brownian diffusion filter may e installed before the prescrubber to remove sulfuric acid aerosol. The echnical centre Mongstad (TCM) has successfully used this option and as greatly reduced sulfuric acid aerosol from a catalytic cracker flue as ( Shah et al., 2018 ). It also appears that the use of a heat exchanger o cool the hot flue gas by exchanging it with clean, treated flue gas ill minimize aerosol emissions, probably by controlled condensation f sulfuric acid on the exchanger surface ( Fang et al., 2020 ). Amine aerosol may also be managed in situ using operating condi- ions and post treating ( Akinpelumi, 2021 ). A high temperature at the op of the absorber, typically achieved by not using a lean solvent cooler, ill induce greater growth of the aerosol, which facilitates its collection n the water wash. A high efficiency mist eliminator at the exit of the ater wash can remove aerosol as small as 3 μm. A Brownian demis- er filter can be installed after the water wash ( Fang et al., 2020 ) to emoval submicron nuclei and aerosol. Adding a trickle bed of packing bove the absorber solvent feed and before the water wash provides ad- itional residence time for the growth of aerosol to facilitate collection Moser et al., 2014 ). .3. Tall stack Tall stacks with resulting flue gas dispersion play a critical role in inimizing the impact of emissions from power plants. Coal-fired power lants routinely emit 100–500 ppm SO2 at the top of the stacks with- ut exceeding the ground level SO2 standard of 0.14 ppm, effectively iluting their emissions by a factor of 1000 or more ( Dunlap, 1975 ). ispersion modeling with a FEED study on a 460 MW combined cy- le gas power plant in west Texas ( Rochelle et al., 2022b ) showed that a tack height of only 45 m reduced the maximum annual average ground evel concentration by a factor of 8000 from the NO2 concentration at he stack exit. Dispersion modeling for a large coal-fired power plant Ferrybridge) with a 198 m stack resulted in a dilution factor of about 000 for MEA emissions ( Manzoor et al., 2014 ). Most detailed designs of capture systems put a new stack on top of he amine absorber/water wash rather than running ductwork back to he existing tall stack. With total absorber heights of 130–250 ft, the ad- ition of a 70 ft steel stack will give an effective total stack height of 200 o 320 feet. If specific conditions require a taller stack for adequate dis- ersion, the stack on top of the absorber can be made taller, or the clean ue gas can be directed back to the existing tall stack. Tall stacks will e more effective at mitigating the effects of pollutants such as amines nd nitrosamines that have finite life because of atmospheric chemistry. ypical atmospheric lifetimes for these molecules vary from 13 min to 4 h ( Tan et al., 2021a ; Tan et al., 2021b ; Nielsen et al., 2012 ). For any iven stack height and site conditions, careful modeling of atmospheric hemistry and dispersion will be necessary to determine the impact of ny amine or nitrosamine emissions ( AECOM, 2021 ). G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 5 5 c s e T a a e o s ( t t p S n p t f 5 s o t u m s t o c c t o 5 g t S n a a r h s r a a a o t u t t T r t h t ( t s a 4 o p t t t c p a w a t o c i 5 n p 2 5 t w o b m a i t . Features of amine scrubbing that mitigate gaseous emissions .1. Solvent selection Amine selection plays a critical role in avoiding emissions of the prin- ipal amine and amine degradation products. Process developers avoid electing amines with excess volatility. Most commercial amines have an quilibrium vapor pressure over the lean amine less than 20–50 ppm. he primary criterion for suitable volatility is a solvent molecule with t least 2 hydrophilic groups ( Nguyen, 2013 ; Du, 2016 ). Nonvolatile mines such as amino acids will greatly reduce the probability of amine missions, but nonvolatile amines cannot be easily reclaimed by evap- ration. Amines that oxidize or thermally degrade to a volatile amine uch as methylamine should also be avoided. Economic performance dictates that competitive second generation 2 G) amine scrubbing processes will use amines that have been selected o be resistant to oxidative and thermal degradation.Thermal degrada- ion is well understood and characterized by a simple bench-scale ex- eriment ( Rochelle, 2012 ). Oxidative degradation is more complicated. creening experiments for oxidation at absorber conditions can elimi- ate unsuitable amines ( Voice, 2013 ). Amine selection can also avoid emissions of problematic degradation roducts. Primary amines can be selected to minimize issues with ni- rosamines and nitramines. Amine structures that may produce volatile ragments by oxidative and thermal degradation can also be avoided. .2. Process features to mitigate degradation Excessive thermal degradation of amine is avoided by operating the tripper at a maximum temperature that results in an acceptable rate f thermal degradation ( Shah et al., 2018 ). MEA is limited to a stripper emperature of about 120 °C. PZ can be used up to 150 °C. Oxidation of amine is more difficult to mitigate ( Wu, 2022 ). Besides sing SCR of NOx with ammonia, NO2 in the inlet flue gas can be re- oved by prescrubbing with sulfite ( Selinger, 2018 ). Sparging of rich olution with nitrogen will remove dissolved oxygen that may react with he amine in the hot rich line before the oxygen is desorbed at the top f the stripper. Keeping the solvent clean by carbon treating, ion ex- hange, and/or thermal reclaiming should minimize dissolved iron that atalyzes the oxidation. Minimizing the residence time of solvent at high emperature in the stripper sump may minimize oxidation by Fe+ 3 and ther reactants. .3. Prescrubbing When amine scrubbing is designed for a coal-fired power plant or a as source containing SO2 , it will usually include a prescrubber system o remove residual SO2 and to cool the flue gas and remove excess water. O2 removal is required because it is a reasonably strong acid that will eutralize and deactivate the amine. If SO2 is not removed, it will be bsorbed as sulfite and then oxidized to sulfate (SO4 = ). The sulfate will ccumulate as the protonated amine salt in solution. The sulfate can be emoved by reclaiming the solvent, usually with the addition of sodium ydroxide to produce sodium sulfate. It is easier and less costly to pre- crub the flue gas to achieve less than 1–2 ppm SO2 . In any case, any esidual SO2 will be removed by the amine solvent in the main absorber. The flue gas must be cooled and excess water removed so that the bsorber can be operated at lower temperature. The amine solvent will chieve a higher rich loading of CO2 at a lower temperature. Typically, single-stage prescrubber system can be designed with 10 to 20 feet f structured packing to reduce the SO2 to 1–2 ppm and to reduce the emperature to 40 °C with 7–8 mol% water left in the flue gas. The prod- ct solution of dilute sodium sulfate is recycled through a cooler to wet he packing and provide for the cooling heat transfer. The pH is main- ained at 7–9 by the addition of sodium hydroxide or sodium carbonate. he water produced has a low concentration of sodium sulfate and is 8 eturned for reuse in the limestone slurry scrubber or other systems. In esting at the National Carbon Capture Center (NCCC), the prescrubber as reduced inlet SO2 in the coal-fired flue gas slipstream from 25 ppm o less than 1 ppm with 20 feet of structured packing ( U.S. EPA, 2022 ). In a two-stage prescrubber, SO2 is removed in an adiabatic stage polishing scrubber) by concentrated (5–20 wt%) sodium sulfate solu- ion controlled at pH 7–9. This concentrated solution is a more suitable, maller waste stream. In the second stage direct contact cooler, usually fter the polishing scrubber, the flue gas is cooled from 50 to 60 °C to 0 °C to condense an almost pure stream of water that can be used in ther parts of the power plant. These prescrubbing systems also remove particulate and other im- urities from the flue gas. They will usually not remove submicron par- iculate and aerosol nuclei such as H2 SO4 aerosol. The TCM facility demonstrated a typical polishing scrubber before he main absorber ( Shah et al., 2018 ). At TCM this prescrubber is used o remove SO2 from the incoming flue gas by contacting it with sodium arbonate/sulfite solution at pH 7–9. With 6 m of structured packing the rescrubber will remove more than 99% of the incoming SO2 . The amine bsorber will remove the rest, so that the SO2 leaving the water wash ill be below the detection limit of a source level analyzer ( 8 by the addition of 10 wt% NaOH. .4. Water wash The water wash is an important feature of all amine scrubbing tech- ologies. It serves three major functions: 1. The water wash recovers volatile and aerosol amine from the clean flue gas and recycles it to the main solvent to avoid economically sig- nificant losses of the principal amine in the solvent. It also reduces the emission of the principal amine to a level that will have insignif- icant impact on the PM2.5 and nitrosamine that may be formed by atmospheric reactions. 2. As the water wash is cooled to condense water from the flue gas, it serves to maintain the water balance of the solvent loop, so that the amount of water entering the main absorber in the flue gas is equal to the water leaving with the clean flue gas. This typically requires that the exit flue gas temperature is 1–4 °C higher than the saturated temperature of the entering flue gas. 3. The water wash may also be used to reduce the emissions of am- monia and highly volatile amines that are products of the principal amine degradation. If the wash water bleed is returned to the sol- vent loop, the ammonia and volatile amines must be removed by sequential reactions or other special operations. If the water wash contains a final acid wash stage, the ammonia and volatile amines will be purged to a treatment facility rather than the solvent loop. Fig. 3 shows three water wash configurations that have been pro- osed and tested ( Morton et al., 2018 ; Mimura et al., 2004 ; Graff et al., 015 ; Lemaire, 2013 ; Khakharia et al., 2014 ). .4.1. Single stage The single-stage water wash ( Fig. 3 ) is the most common configura- ion that has been tested in pilot plants and prototype facilities. Water ith a low concentration of amine and ammonia is recycled to the top of ne section of structured packing. The water is cooled with water or air efore it is returned to the water wash. The optimum circulation rate inimizes the amount of packing required for flue gas cooling while lso minimizing the size of the cooling exchanger. This circulation rate s usually more than adequate to wet the packing and provide for effec- ive removal of amine. G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Fig. 3. Water wash configurations. p a l m M a w d t t fl t v S i u a a ( r T w t s 𝑁 r a p o d w w r u w s b w s g g c i s s i 5 s a g s Typically, there are 3 m of a conventional packing such as Mella- ak 250Y. Because cooling of the flue gas and the absorption of water, mmonia, and amine is controlled by resistance in the gas boundary ayer, this packing should provide a high value of the overall gas film ass transfer coefficient. Therefore, low pressure drop packing such as ellapak Plus may not be as effective. Cooling the saturated flue gas produces water to dilute the flue gas nd facilitate operation of the water wash. There is no need for fresh ater makeup to make the water wash function. Nor is wastewater pro- uced by the absorber or water wash. The removal of volatile amine by the water wash will depend on he height andefficiency of the packing and on the accumulation of he amine in the wash water. If most of the amine is removed from the ue gas, the amine concentration in the wash water (Xam ) is given by he ratio of amine in the flue gas (Yam ) to the difference in the water apor entering and leaving the water wash ( ΔYH2O = YH2Oin –YH2Oout ). o Xam = Yam / ΔYH2O . For example, with a lean solvent at 0.2 mol CO2 /equiv PZ, YPZ leav- ng the top of the absorber at 50 °C could be 10 ppmv. YH2Oin (gas sat- rated with water at 50 °C) would be 12%. If the gas entering the main bsorber contains 8% H2 O, the gas leaving the water wash must contain bout 8.3% H2 O to maintain water balance. The PZ in the wash water Xam ) will be 0.015 mol PZ/kg water. At this concentration the equilib- ium concentration of PZ over the water wash will be about 0.03 ppm. herefore, this is the lowest concentration that can be achieved with the ater wash with a large amount of packing. The mass transfer performance of the packing can be quantified as ransfer units (NG ). The number of transfer units required for a given eparation is: 𝐺 = 𝑌𝐴𝑚 𝑖𝑛 − 𝑌𝐴𝑚 𝑜𝑢𝑡 𝑎𝑣𝑒 ( 𝑌𝑎𝑚 − 𝑌 𝑒𝑞 ) Λ𝑚 𝑖𝑛 9 In this example a design for 0.06 ppm in the cleaned flue gas would equire 5.8 gas phase transfer units. Typical structured packing gives bout 3 transfer units/m, so this performance would require 2 m of acking. Pilot plants such as those at TCM and NCCC have used 3 m r 6 m of packing in a stage of water wash. Note that this performance epends on the amount of water removed from the flue gas by the water ash and on the volatility of the amine at a given concentration in the ater wash. A more accurate analysis would require an estimate of the equilib- ium vapor pressure of amine and CO2 over dilute loaded amine. Resid- al CO2 in the flue gas will absorb to an equilibrium loading in the ater wash, so the amine that is absorbed will have a lower vapor pres- ure than the amine in water without the CO2 . This effect of CO2 will e less with reduced CO2 in the clean gas. Therefore, amine emissions ill be greater with greater CO2 removal and with sources of lower CO2 uch as gas turbines. If the principal amine is more volatile, its concentration in the flue as entering the water wash will increase proportionately, resulting in reater concentration in the wash water. Because the amine is intrinsi- ally more volatile and because there will be proportionately more of it n the wash water, amine emission may vary as the square of the intrin- ic amine volatility ( Drewry and Rochelle, 2023b ). More volatile amines uch as AMP (amino-methyl-propanol) may require more effective mit- gation than a single-stage water wash ( Languillea et al., 2021 ). .4.2. Two stages of water wash Fig. 3 shows a configuration with two stages of water wash. The top tage is similar to a single-stage water wash with pump-around through n exchanger to cool the wash water and condense water from the flue as. The water produced in the top stage would purge to the bottom tage to pick up more amine and ammonia. G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 t t u a b s “ a r l a 5 u w i ( l m o c a T a t n e t w w a m r a I i 5 h w t o g a s c m 6 i p d a w 6 b p p b g w a R s w a d d s r ( e t r a t o 6 D t m t p V l r s The two-stage configuration will also use a pump-around in the bot- om section, but with no cooling. The pump-around is desirable to wet he packing and provide effective mass transfer. The test facility at TCM ses a two-stage water wash with 3 m of packing in each stage, but it llows for optional pump-around cooling in both stages. The bottom section could also be designed without pump-around, ut with wettable packing such as a structured packing made from creen that would be effective with a very low specific liquid rate. This trickle bed ” (or “dry bed ”) water wash has been used in the pilot plant t Niederaussem ( Shah et al., 2018 ). The two-stage configuration effectively acts as two-stage countercur- ent contactor. As a consequence, the water in the top stage has a much ower amine concentration. This means that the equilibrium limit on mine removal is much lower and better performance can be expected. .4.3. One stage water, one stage dilute acid Even more effective removal of amine and ammonia can be obtained sing one (or two) stages of water wash and a final stage of the acid ash as shown in Fig. 3 . Acid wash, usually with dilute sulfuric acid, s commonly used to remove amines and ammonia from exhaust gases CRcleanair.com, 2022 ). Acid (such as sulfuric acid) is fed to a recycle oop around the packing to maintain a pH of 3–5 as the residual am- onia and amine are absorbed from the clean flue gas. A bleed stream f salt solution is removed and processed as waste water or by a more omplicated system to recover ammonia and/or amine. Graff et al. (2015) patented the use of an acid wash to remove amine nd ammonia emissions from the flue gas leaving an amine scrubber. heir basic configuration treats the flue gas with a conventional, pump- round water wash with cooling followed by an acid wash with op- ional cooling. Graff tested this configuration in a system using mo- oethanolamine (MEA). There was 80–100 ppm MEA in the flue gas ntering the water wash stage. This was reduced to 0.7 ppm by the wa- er wash and further reduced at pH 6 to less than 0.05 ppm in the acid ash. Khakharia et al. (2014) also tested a water wash followed by an acid ash with 1.26 m of Mellapak 250 structured packing. With an inlet mmonia concentration of 140 to 155 mg NH3 /Nm3 , the acid wash re- oval of ammonia varied from 95% at pH 6 to 99% at pH 3. The water wash will be less effective or not effective at all in the emoval of amine aerosol. In the presence of the nearly pure water, mine aerosol drops in the bulk gas will pick up water and grow in size. f the aerosol drops are larger than 3–10 μm, they should be collected n the water wash packing and mist eliminator ( Zhang, 2018 ). .5. Condensate purge Ammonia and other volatile degradation products such as acetalde- yde or amines should be concentrated in the condensate produced hen cooling the CO2 product. A purge of that condensate will selec- ively remove ammonia from the solvent loop, reducing the emissions f ammonia from the water wash. This has been practiced with acid as treating at greater absorber pressure and may not be effective with n absorber at atmospheric pressure ( Hatcher and Weiland, 2012 ). The pecific effectiveness of this strategy will also depend on the stripper onfiguration. The purge can be further concentrated to produce an am- onia product or processed as waste water. . Measured emissions from demonstrations and pilot plants Measured gaseous emissions from several pilot plants are presented n Table 3 . These results are a small fraction of the total number of pilot lants. The following general methods have been used to obtain these ata. 1. Some data have been measured with hot gas FTIR. This is a contin- uous measurement that provides a sensitivity of about 1 ppmv for 10 ammonia, specific amines, and specific aldehydes. It is not sensitive enough to detect expected levels of nitrosamines. It is sometimes less reliable than other more specific methods in determining higher concentrations of some species, such as acetaldehyde. 2. Most pilot plants include batch sampling of treated flue gas to pro- vide condensate samples or samples in specific adsorbent tubes that can be further analyzed for specific components. This method can provide specific results for formaldehyde, acetaldehyde, and ammo- nia when other measurements are not available.However, the ad- sorbent tubes are usually analyzed by standard methods for specific amines and nitrosamines that may not be the compounds expected from a given operation. 3. The most sensitive measurements have been performed using the methods of Wisthaler ( Languillea et al., 2021 ) with proton-transfer- reaction time-of-flight mass spectrometry (ptr-tof-ms). This method is more sensitive than the hot gas FTIR, which has been used on more pilot plants that are not reported here. More measurements of amine emissions and nitrosamine emissions t different sites and with a broader range of second-generation amines ould be helpful to further quantify the expected performance. .1. Amine emissions Amine emissions in the pilot plants reported in Table 3 are managed y a one- or two-stage water wash and are generally less than 1000 pbv. Data have not been excluded at conditions that were clearly ex- eriencing amine aerosol emissions. Detailed measurements of the amine emissions for DC-103 are given y Fagerland et al. (2020) in pilot plant testing with an incinerator flue as. DC-103 is probably a less volatile amine. With a single-stage water ash the median amine emission was 5 ppb and 90% of the time the mine emission was less than 30 ppb. Emissions are reported with piperazine by Drewry and ochelle (2023a) with variable water wash configurations. With a ingle-stage water wash using 20 ft of packing, the piperazine emission as 100 ppb. With the addition of a second stage, a trickle bed, or an cid wash, the piperazine emission was less than 0.1 ppb, the nominal etection limit of the ptr-tof-ms. Expected amine emissions at less than 1 ppm should pose no risk of irect health effects. With a stack of moderate height (100 m), disper- ion should dilute by a factor of at least 1000. Even with piperazine, the esulting ground level concentration of 1 ppb will be well below the TLV 30 ppb) reported in Table 1 . There is a greater risk that 1 ppm of amine missions might create health effects with downwind atmospheric reac- ions to produce nitrosamine. Amine emissions will contribute as a volatile base to atmospheric eactions that produce PM2.5. However, they will usually be less than mmonia emissions and significantly less than NOx and SO2 emissions hat also contribute to PM2.5, so the relative effect of amine emissions n PM2.5 should be small. .2. Acetaldehyde Reported acetaldehyde emissions vary from 30 to 1500 ppb. rewry and Rochelle (2023a) showed that the wash water configura- ion, including one with acid wash, did not affect the acetaldehyde. The easured values of these emissions may include a contribution from he combustion source. Because acetaldehyde is listed as a hazardous ollutant, this level of emission from a large plant could trigger a Title permit. However, with a moderately high stack the expected ground evel concentration of acetaldehyde may not pose a significant health isk. Acetaldehyde may also be significant as a total hydrocarbon emis- ion in an area out of compliance with the ozone NAAQS. G.T. Rochelle Carbon Capture Science & Technology 11 (2024) 100192 Table 3 Measured emissions from pilot plants in the absence of amine aerosol. Solvent Amines (ppb) Nitrosamine (ppbv) Ammonia (ppm) Acetaldehyde (ppbv) Water Wash Refs. MEA 200–800 ND 18–30 200–500 2-stage Morken et al., 2017 MEA 1–30
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