Decomposition of Nitrosamines in CO2 Capture by ... - ACS Publications

Apr 14, 2014 - Nathan A. Fine, Paul T. Nielsen, and Gary T. Rochelle*. The University of Texas at Austin, McKetta Department of Chemical Engineering, ...
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Decomposition of Nitrosamines in CO2 Capture by Aqueous Piperazine or Monoethanolamine Nathan A. Fine, Paul T. Nielsen, and Gary T. Rochelle* The University of Texas at Austin, McKetta Department of Chemical Engineering, 200 E Dean Keeton St. Stop C0400, Austin, Texas 78712-1589, United States S Supporting Information *

ABSTRACT: Amine scrubbing is an efficient method for carbon capture and sequestration, but secondary amines present in all amine solvents can form carcinogenic nitrosamines. Decomposition kinetics for n-nitrosopiperazine (MNPZ), nitrosodiethanolamine (NDELA), and nitroso-(2-hydroxyethyl) glycine (NHeGly) were measured over a range of temperature, base concentration, base strength, and CO2 loading pertinent to amine scrubbing. MNPZ and NDELA decomposition is first order in the nitrosamine, half order in base concentration, and base-catalyzed with a Brønsted slope of β = 0.5. The activation energy is 94, 106, and 112 kJ/mol for MNPZ, NDELA, and NHeGly, respectively. MNPZ readily decomposes at 150 °C in 5 M piperazine, making thermal decomposition an important mechanism for MNPZ control. However, NHeGly and NDELA are too stable at 120 °C in 7 M monoethanolamine (MEA) for thermal decomposition to be important. Base treatment during reclaiming could rapidly and selectively decompose NHeGly and NDELA to mitigate nitrosamine accumulation in MEA.

1. INTRODUCTION Carbon dioxide (CO2) emissions from fossil-fuel combustion represent the single largest source of greenhouse gases (GHG) in the US. Coal-fired power plants remain the largest contributor to these emissions, accounting for 43% of the 5.4 billion metric tons of CO2 emitted in 2009.1 Carbon capture and sequestration (CCS) from these point sources will be a major focus for mitigating climate change, making up 20% of total GHG reductions necessary to reach the target goal of a 2 °C global temperature rise.2 Amine scrubbing remains a frontrunner for CCS from existing coal-fired power plants due to its thermodynamic efficiency, retrofitting capability, and industrial maturity. One of the unresolved issues facing amine scrubbing is the formation and release of potentially hazardous byproducts from the scrubber. NO2 in the flue gas can react with the amine solvent to form nitrosamines (NNO), the most carcinogenic byproducts known to form in appreciable quantities during amine scrubbing.3,4 Once the nitrosamines form, they may then be released through either gaseous emissions or solvent spills.5,6 Nitrosamine formation is highly dependent on the solvent and operating conditions of the amine scrubber. Primary amines nitrosate to unstable nitrosamines that instantaneously decompose to nitrogen gas and carbocations.7 Tertiary and moderately hindered amines are unlikely to nitrosate because they do not form carbamates, an important precursor for rapid nitrosamine formation.8,9 Only secondary amines will form relatively stable nitrosamines that may accumulate in the solvent, representing a potential environmental hazard.7 Therefore, nitrosamines in solvents with concentrated secondary amines must be properly characterized before those solvents can be used. Furthermore, secondary amines from © 2014 American Chemical Society

solvent degradation and their respective nitrosamines must be properly managed in all amine solvents. Previous research has shown that nitrosamines can decompose in basic buffer solutions at high temperatures common to the desorber.4,10,11 Nitrosamine thermal decomposition is first order in nitrosamine, which leads to a steady state nitrosamine concentration in an amine scrubber where nitrosation from NO2 balances with thermal decomposition. This research focuses on further characterizing the kinetics and main products of n-nitrosopiperazine (MNPZ) thermal decomposition under desorber conditions. Decomposition kinetics at desorber conditions were also measured for nitrosodiethanolamine (NDELA) and nitroso-(2-hydroxyethyl) glycine (NHeGly), two nitrosamines found in monoethanolamine (MEA) pilot plant samples.12

2. EXPERIMENTAL METHODS 2.1. Nitrosamine Synthesis. A list of chemicals and purities is included in Supporting Information A. MNPZ and NDELA were purchased at high purity for the purpose of calibrating the total nitrosamine method and the high performance liquid chromatography method. For MNPZ decomposition in piperazine (PZ), the MNPZ was made in situ by adding sodium nitrite (NaNO2) prior to heating. MNPZ decomposition analysis was performed only after all of the nitrite had reacted so that nitrosamine formation did not confound the results. Nitroso-(2-hydroxyethyl) glycine Received: Revised: Accepted: Published: 5996

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Table 1. Nitrosamine Decomposition Observed and Calculated Kinetics experiment #

temperature (°C)

base (M)

1a 2a 3a 4a 5a 6a 7a 8a 9a 10a 11 12 13 14 15 16 17 18

100 120 135 150 165 150 150 150 150 150 150 150 150 150 150 150 150 150

4.9 PZ 4.9 PZ 4.9 PZ 4.9 PZ 4.9 PZ 3.0 PZ 1.7 PZ 0.9 PZ 0.4 PZ 0.2 PZ 1.7 PZ 4.9 PZ 4.9 PZ 4.7 PZ 0.05 NaOH 0.1 NaOH 0.5 NaOH DDI water

19 20 21 22 23 24 25 26 27 28 29

150 150 150 120 120 120 150 150 150 150 150

6.6 MEA 2.9 MEA 1.0 MEA 6.6 MEA 2.9 MEA 1.0 MEA 6.6 MEA 0.1 NaOH 3 MDEA 4.9 PZ 3.5 AMP

30 31

120 150

6.7 M MEA 6.7 M MEA

α (mol CO2/mol N)

kobs (s−1 × 10−6)

kcalc (s−1 × 10−6)

MNPZ 0.31 0.31 0.31 0.31 0.31 0.30 0.30 0.30 0.30 0.30 0.6 H2SO4 0.01 0.1 0.39 ≈0 ≈0 ≈0 ≈0

0.72 3.7 11.1 26.9 65.3 22.6 15.4 10.7 6.8 5.0 19.0 31.3 28.6 17.9 223 350 810 0.58

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.03 0.1 0.5 1.6 0.7 0.6 0.3 0.2 0.3 0.2 0.8 1.0b 0.4b 0.8b 8 20 60 0.03

0.76 3.6 10.2 27.4 68.3 21.2 15.8 11.4 7.4 5.2 15.8 27.4 27.4 26.8 260 380 850 0.66

0.40 0.40 0.40 0.40 0.40 0.40 ≈0 ≈0 ≈0 0.38 ≈0

5.2 3.1 1.7 0.49 0.31 0.18 3.8 110 1.1 6.0 2.8

± ± ± ± ± ± ± ± ± ± ±

0.1 0.02 0.07 0.01 0.05 0.01 0.2b 6 0.04 0.1 0.03

5.0 3.3 1.7 0.49 0.32 0.18 5.0 110 1.0 6.3 2.6

NDELA

NHeGly

a

0.41 0.41

1.1 ± 0.04 17 ± 1.8

1.1 17

Previously reported in ref 10. bNot regressed into model.

nitrosamine decomposition. The prepared solution was spiked with a small quantity of nitrosamine or NaNO2, up to 50 mM, and pipetted into 3/8 in. O.D. high pressure stainless steel cylinders for thermal decomposition. 2.3. Thermal Decomposition. The closed cylinders were heated to 120−150 °C in convection ovens and kept at autogenous pressures similar to those in the desorber. The cylinders were removed periodically and immediately quenched in a water bath at room temperature. Typical experiment times ranged from 3 h to 3 weeks, which usually allowed for greater than 90% decomposition. The samples were stored at room temperature in amber vials to prevent artifactual UV nitrosamine decomposition. Since all of the cylinders for a given experiment are prepared with the same stock solution, the periodic removal of cylinders allows analysis for nitrosamine concentrations as a function of time at elevated temperatures. Previous work has shown that nitrosamine decomposition is first order in nitrosamine concentration.10 Experimental results were therefore regressed as a pseudo-first order decomposition using a linear regression according to eqs 1 and 2 with NNOi and kobs as regressed parameters. The observed pseudo-first order rate constants (kobs) were then transformed and linearly regressed as a function of experiment parameters (eq 3).

(NHeGly) was not commercially available, so it was synthesized in two steps. N-(2-Hydroxyethyl) glycine (HeGly), the parent amine for NHeGly, was synthesized by adding 0.5 M sodium chloroacetate into 5 M MEA and reacting at 65 °C for 6 h. The product was diluted by a factor of 10 in 7 M MEA with a CO2 loading (α) of 0.4 mol CO2/mol N and nitrosated with 100 mM NaNO2 at 120 °C for 18 h. Nitrosamine yield from the reaction was approximately 5%; the rest of the nitrite was scavenged by MEA. The final product was analyzed using high resolution mass spectrometry, which confirmed the presence of both HeGly and NHeGly (Supporting Information B). 2.2. Sample Preparation. Nitrosamine decomposition kinetics was quantified using four amine solvents common to amine scrubbing (MEA, PZ, MDEA, and AMP). The solvents were prepared gravimetrically at concentrations ranging from 0.1 to 6 M. The solutions were then sparged with CO2, which absorbs into the amine solution either as carbamate or as bicarbonate; final CO2 loadings were α = 0−0.4. Different amine concentrations were made by diluting the concentrated loaded amine solution with water to ensure a consistent loading across a concentration range. Nitrosamine decomposition was also studied in distilled deionized water and dilute sodium hydroxide (NaOH) to examine the effect of base strength on 5997

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Table 2. Nitrosamine Decomposition Parameters

a

nitrosamine

ko (s−1M−γ)

β

γ

Ea (kJ/mol)

MNPZ NDELA NHeGly

4.6 ± 0.2 × 10−10 2.2 ± 0.2 × 10−10 8.2 ± NA × 10−10

0.52 ± 0.02 0.51 ± 0.02 0.50b

0.53 ± 0.02 0.54 ± 0.03 0.53b

94 ± 3a 105 ± 4 112 ± NA

Previously reported in ref 10. bAssumed parameters from MNPZ and NDELA results.

dNNO = −kobs NNO dt

(1)

ln(NNO) = ln(NNOi ) − kobst

(2)

log(kobs) = a × log(Par) + b

(3)

4. RESULTS AND DISCUSSION Table 1 summarizes all of the observed pseudo-first order decomposition rates for MNPZ, NDELA, and NHeGly. The error represents the standard error given by the linear regression from eq 2 and is almost always less than 5% of the observed rate. Across all experiments, the average error in estimating the observed pseudo-first order rate constant was approximately 3%. The calculated pseudo-first order rate constant is given by eq 5 with parameters from Table 2.

2.4. Nitrosamine Safety. Nitrosamines are highly carcinogenic, and the utmost precaution must be taken when handling them. Concentrated nitrosamine samples were kept either in a vented hood or in a refrigerator, and all experiments were performed using 50 mM or less nitrosamine.

dNNO = −kcalc[NNO] dt

(4) Ea

1

1

kcalc = koK aHB+ β[B]γ × e R ( 423K − T )

3. ANALYTICAL METHODS 3.1. Analysis for MNPZ, NDELA, and NHeGly. Analysis for individual nitrosamines was done using reverse-phase high performance liquid chromatography with UV detection at 240 nm. The eluents used were 10 mM ammonium carbonate (NH4)2CO3 (pH = 9.1) polar phase and acetonitrile (ACN) nonpolar phase. The analytical chromatography column must be able to handle the basic buffer, so a Dionex Polar Advantage II 4 × 250 mm column was used. MNPZ eluted between 4.8 and 6.4 min, NDELA eluted between 3.5 and 5.5 min, and NHeGly eluted between 2.0 and 3.0 min. The nitrite concentration was kept to less than 1% of the total NHeGly since nitrite also absorbs UV at 240 nm and elutes between 2.0 and 3.0 min. Concentrated amine samples were diluted gravimetrically in water by a factor between 20 and 50 before analysis. Calibration curves are linear up to 2 mM nitrosamine with limits of detection of approximately 5 μM in the diluted sample. 3.2. Nitrosamine Group Analysis. Analysis for total nitrosamines was done by reacting the nitrosamine sample with hydrobromic acid in a mixture of ethyl acetate, acetic acid, and acetic anhydride at ambient conditions.13−15 The reaction selectively forms one mole of nitric oxide (NO) gas for every mole of nitrosamine; the NO gas is sparged from the reactor with 1.5 SLPM of N2 carrier gas and then analyzed using a chemiluminescent NOx analyzer. Sample injections were compared against an MNPZ calibration curve made daily. Nitrite, a possible signal interference for total nitrosamine analysis, was always less than 1% of the total nitrosamine (Supporting Information C). 3.3. Nitrosamine Product Analysis. Liquid samples of decomposed MNPZ in PZ were analyzed for total aldehyde concentration by reacting the sample with 2,4-dintrophenylhydrazine (DNPH) and analyzing the product on the HPLC (Supporting Information D). Gaseous products from MNPZ decomposition were indirectly sampled using N15 NMR. A solution of loaded PZ was spiked with 600 mM NaN15O2, heated to 150 °C, and analyzed using N15 NMR to study the loss of nitrogen from the aqueous phase.

(5)

4.1. MNPZ Decomposition Dependence on Base Concentration. Concentrated PZ has been proposed as a new standard for amine scrubbing due to its fast CO2 absorption rates, high capacity, and chemical stability.16 However, PZ is a secondary diamine so it readily nitrosates to form MNPZ. To study MNPZ thermal stability, MNPZ was decomposed at 150 °C in loaded aqueous PZ solutions with PZ concentration ranging from 1 to 5 M. Decomposition is first order in MNPZ and has a roughly half-order dependence on total PZ concentration. Approximately 1−5 mM of MNPZ standard was then added to 0.05−0.5 M NaOH and decomposed at 150 °C. MNPZ decomposition was half-order in NaOH and 100 times faster than in PZ (Figure 1).

Figure 1. MNPZ decomposition dependence on total PZ, α = 0.3 (squares) and NaOH (diamonds) at 150 °C (Experiments 4, 6−10, 15−17).

4.2. MNPZ Decomposition Dependence on Loading and pH. Nitrosamine decomposition has been reported to have a strong dependence on pH.11 To study this dependence, MNPZ was decomposed in 5 M PZ with 0−0.4 mol CO2/mol N. Since CO2 is an acidic gas, it acidifies the PZ solution as it chemically absorbs (eqs 6 and 7). MNPZ was also decomposed in 2 M PZ acidified with 0.6 M H2SO4 in order to remove the effect of carbamates. 5998

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PZ + CO2 → PZCOO− + PZH+

(6)

PZCOO− + CO2 → PZ(COO−)2 + H+PZCOO−

(7)

The pH change due to CO2 loading did not have a dramatic impact on decomposition rates. Even though pH varied by more than one as PZ loading increased to 0.4, the rates only decreased by 75%. Loading might also affect decomposition by reducing the free amine in solution. The rate decreases as PZ is loaded with CO2, but decomposition is not half-order in free nitrogen estimated from eq 8 (Figure 2). The difference in

Figure 3. MNPZ decomposition dependence on base strength (Experiments 12, 16, and 18).

scrubbing, it can form secondary amines that can nitrosate to stable nitrosamines. Two secondary amines, diethanolamine (DEA) and HeGly, have been quantified in degraded MEA samples from pilot plants and on the bench scale.22−24 Furthermore, both NDELA and NHeGly have been quantified in degraded MEA with the latter accounting for over half of the total nitrosamine concentration.12 NDELA decomposition was measured in loaded 1−7 M MEA at 120 and 150 °C. Like MNPZ, NDELA decomposition was first order in NDELA and roughly half-order in amine concentration. Decomposition had an activation energy of 105 ± 2 kJ/mol with a rate of 1.7 × 10−6 s−1 in 1 M MEA at 150 °C. Unloaded MEA actually decomposed more slowly than loaded MEA, possibly due to ionic strength effects (Figure 4). NDELA kinetics was also measured in MDEA, PZ, AMP, and NaOH; decomposition was base catalyzed with a Brønsted slope of 0.51 (Figure 5). It was difficult to synthesize NHeGly safely at high concentrations, so its decomposition was measured in MEA where it was made in situ. NHeGly decomposition was 1.1 × 10−6 s−1 in 7 M MEA at 120 °C with an activation energy of 112 kJ/mol. Assuming that NHeGly has similar base and

Figure 2. MNPZ decomposition in 5 M PZ at 150 °C as a function of loading (Experiments 4 and 12−14).

ionic strength at different loadings is the most probable explanation for the low dependence on loading. The higher ionic strength in loaded solutions could stabilize any charged intermediates in nitrosamine decomposition and counteract the loss of free amine. Since CO2 loading only has a significant impact at very high loadings where the concentration of free amine plummets, it was not regressed into the final model. 2N 1 free nitrogen = [PZ] × × PZ 1 − 2(α) (8) 4.3. MNPZ Decomposition Dependence on Base Strength. The observed MNPZ decomposition constant was transformed according to eq 9 and compared across three different bases of varying pKa (H2O, PZ, and NaOH). The pKa values for the bases were extrapolated to 150 °C from available literature.17−19 Since there is very little pKa data at these high temperatures and pressures, base strength estimation represents the largest source of error for these experiments. MNPZ decomposition kinetics were found to be base catalyzed with a Brønsted slope of β = 0.52 across the entire range (Figure 3). This suggests that MNPZ decomposition is a concerted mechanism with the abstraction of a hydrogen in the rate limiting step.20,21 k2 =

kobs [base]0.53

(9)

4.4. NDELA and NHeGly Decomposition Kinetics. MEA is the most well characterized and widely used amine for amine scrubbing. Since MEA is a primary amine, it nitrosates to an unstable nitrosamine that immediately decomposes into N2 and a carbocation. However, as MEA degrades during amine

Figure 4. NDELA decomposition in MEA, α = 0.4 at 150 °C (squares) and 120 °C (triangles) and in unloaded MEA at 150 °C (circle) (Experiments 19−25). 5999

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Figure 5. NDELA decomposition dependence on base strength (Experiments 25−29).

Figure 7. MNPZ decomposition and 2-piperazinol formation in 7.5 M PZ at 175 °C.

concentration dependencies to the other nitrosamines, NHeGly decomposition is fastest (ko = 8.2 × 10−10 s−1 M−0.5), followed by MNPZ decomposition (ko = 4.6 × 10−10 s−1 M−0.53) and NDELA (ko = 2.2 × 10−10 s−1 M−0.54). 4.5. MNPZ Aqueous Decomposition Products. Nitrosamines are good nitrosating agents themselves, so it is possible that nitrosamines transnitrosate into other nitrosamines.25 Since nitrosamines as a family are carcinogenic, transnitrosation would appear to decompose the nitrosamine but would not necessarily reduce the carcinogenicity of the solvent. To test for the possibility of transnitrosation, samples of decomposed MNPZ were analyzed for both MNPZ and total nitrosamine. Total nitrosamine decomposed at the same rate as MNPZ, disproving transnitrosation (Figure 6). In a separate experi-

analyzed using N15 NMR. The unheated sample showed a spike at a 610 ppm shift from ammonia, agreeing well with literature values for nitrite shift.26 After heating for 1 h at 150 °C, all the nitrite had reacted to form nitrosamine, which had a 540 ppm shift from ammonia. The sample after 1 day of heating showed much less nitrosamine without any other major peaks, while the sample after 2 days showed no signs of N15 above the naturally occurring N15 in the PZ solution (Figure 8). The disappearance

Figure 8. N15 NMR scan of decomposing MNPZ in 5 M PZ, α = 0.4, T = 150 °C. Figure 6. MNPZ and total nitrosamine decomposition in 5 M PZ, α = 0.4 at 150 °C (Experiment 14).

of spiked N15 from the aqueous phase proves the formation of nitrogenous gaseous products from nitrosamine decomposition. The most likely gas product is N2O; however, neither NO nor N2 have been ruled out. 4.7. MNPZ Decomposition Mechanism. The mechanism for MNPZ decomposition can now be hypothesized on the basis of the empirical rate law and decomposition products. The mechanism starts with the abstraction of the hydrogen on the carbon alpha to the nitroso group and the breaking of the N−N bond to form a nitrosyl anion via an E2 elimination. The nitrosyl anion goes on to form hyponitrous acid and then nitrous oxide (N2O)27 while the charged PZ species forms 2PZOH (Figure 9). First order dependence on nitrosamine follows directly from the mechanism. However, dependence on amine concentration should also be first order; variance in ionic strength of the solution and the activity of the amine are the most likely reasons for the discrepancy.

ment, decomposed MNPZ was analyzed using the total aldehyde method. The total aldehyde accumulated as MNPZ decomposed with a yield of 0.51 ± 0.09 (Figure 7). The final sample was analyzed using high resolution mass spectrometry, and 2-piperazinol (2-PZOH) was positively identified in the solution matrix. PZ is also hypothesized to form during MNPZ decomposition, but the high concentration of PZ in the original solution prohibited analysis of formed PZ. 4.6. MNPZ Gaseous Decomposition Products. A solution of 5 M PZ at α = 0.4 was spiked with 600 mM NaN15O2 and heated to 150 °C in the thermal cylinders. The cylinders were removed periodically and cooled at room temperature. Once cooled, the solution was removed from the cylinders, allowing nitrogenous gases to escape, and then 6000

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ASSOCIATED CONTENT

S Supporting Information *

Chemical list; mass spectrometry for HeGly and NHeGly synthesis; total nitrosamine analysis; total aldehyde analysis; list of uncommon abbreviations and symbols. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: 512 471 7230; fax: 512 471 7060. Notes

The authors declare the following competing financial interest(s): One author of this publication consults for Southern Company and for Neumann Systems Group on the development of amine scrubbing technology. The terms of this arrangement have been reviewed and approved by the University of Texas at Austin in accordance with its policy on objectivity in research. The authors have financial interests in intellectual property owned by the University of Texas that includes ideas reported in this paper.

Figure 9. MNPZ decomposition mechanism.

5. ENVIRONMENTAL IMPACT 5.1. Importance of Nitrosamine Decomposition in MEA and PZ Solvents. Stable secondary nitrosamines will build up in the amine scrubber until formation from NO2 absorption balances with thermal decomposition. In a simplified model (eqs 10 and 11), the nitrosamine concentration and the time to reach steady state are both inversely proportional to the decomposition rate in the desorber. A scrubber using concentrated PZ will frequently operate at 150 °C. Under these conditions, the MNPZ decomposition rate constant is approximately 2.6 days−1 and steady state can be reached in a week as demonstrated at the pilot plant scale.28 For comparison, a scrubber using an MEA solvent usually operates at 120 °C to keep the MEA from decomposing. At these conditions, NHeGly and NDELA have decomposition constants of 0.08 days−1 and 0.04 days−1, respectively, so these nitrosamines will accumulate over a period of months. Base treating as a reclaiming technique can dramatically increase the decomposition constant of NHeGly and NDELA to help control their accumulation. Thermal decomposition in tandem with base treating can bring the steady state nitrosamine concentration in amine scrubbing to acceptable levels set by the hazards of solvent spillage and gaseous emissions. Future environmental work must be done to determine these maximum nitrosamine levels for amine scrubbing regulations. δyNO G 2 NNOss = × kDesτDes L (10) tss ≈ 3 ×

τtot 15 ≈ kDesτDes kDes

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ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge financial support from the Texas Carbon Management Program in the preparation of this work.

(1) Energy Information Administration. Emissions of Greenhouse Gases in the United States 2009; http://www.eia.gov/environment/ emissions/ghg_report/pdf/tbl6.pdf. (2) Price, I.; Smith, B. Carbon Capture and Storage, Meeting the Challenge of Climate Change; IEA Greenhouse Gas R&D Programme: Cheltenham, U.K., 2008. (3) Dai, N.; Shah, A. D.; Hu, L.; Plewa, M. J.; Mckague, B.; Mitch, W. A. Measurement of nitrosamine and nitramine formation from NOx reactions with amines during amine-based carbon dioxide capture for postcombustion carbon sequestration. Environ. Sci. Technol. 2012, 46, 9793−9801. (4) Chandan, P. A.; Remias, J. E.; Neathery, J. K.; Liu, K. Morpholine nitrosation to better understand potential solvent based CO2 capture process reactions. Environ. Sci. Technol. 2013, 47, 5481−5487. (5) Da Silva, E. F.; Anders, K.; Booth, A. Emissions from CO2 capture plants; an overview. Energy Proc. 2013, 37, 784−790. (6) Mertens, J.; Lepaumier, H.; Desagher, D.; Thielens, M.-L. Understanding ethanolamine (MEA) and ammonia emissions from amine based post combustion carbon capture: Lessons learned from field tests. Int. J. Greenhouse Gas Control 2013, 13, 72−77. (7) Mitch, W. Critical Literature Review of Nitrosation/Nitration Pathways; http://www.gassnova.no/frontend/files/CONTENT/ Rapporter/NitrosamineandNitramineformationchemistry_YALE.pdf. (8) Goldman, M. J.; Fine, N. A.; Rochelle, G. T. Kinetics of Nnitrosopiperazine formation from nitrite and piperazine in CO2 capture. Environ. Sci. Technol. 2013, 47, 3528−3534. (9) Sartori, G.; Savage, D. W. Sterically hindered amines for carbon dioxide removal from gases. Ind. Eng. Chem. Fundam. 1983, 22, 239− 249. (10) Fine, N. A.; Rochelle, G. T. Thermal decomposition of nnitrosopiperazine. Energy Procedia 2013, 37, 1678−1686. (11) Fan, T. Y.; Tannenbaum, S. R. Stability of N-nitroso compounds. J. Food Sci. 1972, 37, 274−276. (12) Einbu, A.; Dasilva, E.; Haugen, G.; Grimstvedt, A.; Lauritsen, K. G.; Zahlsen, K.; Vassbotn, T. A new test rig for studies of degradation of CO2 absorption solvents at process conditions; comparison of test

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5.2. Importance of Nitrosamine Decomposition Products. The main aqueous product for nitrosamine decomposition is hypothesized to be a hemiaminal which is generally nontoxic. The main gaseous product is N2O, an extremely potent GHG with 180 times the global warming potential of CO2.29 Most likely, the trace levels of N2O will exit the desorber with the CO2 for compression and sequestration. In the worst case, the N2O is emitted from the absorber along with the scrubbed flue gas. The maximum N2O emission is half of the 1−5 ppm of NO2 in the inlet flue gas, which is conservatively equivalent to 500 ppm of CO2. A scrubbed flue gas usually emits over 1% CO2, so the maximum N2O is too dilute to add significantly to overall GHG emissions. 6001

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dx.doi.org/10.1021/es404949v | Environ. Sci. Technol. 2014, 48, 5996−6002