Ludovic Chahen

Thierry Huard

Lorena Cuccia

Vincent Cuzuel

José Dugay

Valérie Pichon

Jérôme Vial

Camille Gouédard

Leslie Bonnard

Pierre-Louis Carrette

Comprehensive monitoring of MEA degradation in a post-combustion CO 2 capture pilot plant with identification of novel degradation products in gaseous effluents

Keywords: Ethanolamine degradation products in gas effluents, pilot plant monitoring, novel analytical methods, solid phase extraction, nitrosamines, stripper gas effluents analysis

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Introduction

Most mature post-combustion CO 2 capture technology is based on CO 2 absorption by aqueous amine solutions. 2-ethanolamine (MEA) remains the benchmark solvent. When a new formulation is developed, it is always compared to MEA in terms of energy requirement of the process, in terms of corrosion and also in terms of degradation of the solvent. Indeed, a main problem associated with chemical absorption using amines is degradation through irreversible side reactions mainly with CO 2 and O 2 but also with NO x [START_REF] Fostas | Effects of NOx in the flue gas degradation of MEA[END_REF] and SO x [START_REF] Zhou | Thermal Degradation of Monoethanolamine in CO 2 Capture with Acidic Impurities in Flue Gas[END_REF]. These reactions can lead to different consequences for the process: solvent loss causing cost [START_REF] Carrette | How CO 2 capture cost could be reduced in short term? IFPEN solutions[END_REF]Rao and Rubin, 2012), formation of volatile compounds potentially unfriendly for environment [START_REF] Thitakamol | Environmental impacts of absorption-based CO 2 capture unit for post-combustion treatment of flue gas from coal-fired power plant[END_REF], corrosion [START_REF] Lepaumier | CO 2 capture, why and how? What constraints?[END_REF][START_REF] Martin | New Amines for CO 2 Capture. IV. Degradation, Corrosion, and Quantitative Structure Property Relationship Model[END_REF], foaming and fouling (Islam et al., 2011). It is important to list formed compounds and their concentration because some of them could be emitted to atmosphere and could be potentially a concern for human health like nitrosamines for instance [START_REF] Iarc | Monographs on the Evaluation of Carcinogenic Risks to Humans. Some N-Nitroso Compounds[END_REF]NTP, 2012;[START_REF] Thitakamol | Environmental impacts of absorption-based CO 2 capture unit for post-combustion treatment of flue gas from coal-fired power plant[END_REF]. Above all, the knowledge of their presence enables the development of adapted counter measures to avoid any risk for environment or human health. Amine degradation is mainly due to oxidative degradation (Gouédard et al., 2012;Lepaumier et al., 2009b;[START_REF] Rooney | Oxygen's role in alkanolamine degradation[END_REF][START_REF] Sexton | Amine oxidation in carbon dioxide capture processes[END_REF] but also to thermal degradation, which occurs at high temperature and high CO 2 partial pressure in the stripper [START_REF] Davis | Thermal degradation of aqueous amines used for carbon dioxide capture[END_REF][START_REF] Holub | Amine degradation chemistry in CO 2 service[END_REF][START_REF] Lepaumier | Étude des mécanismes de dégradation des amines utilisées pour le captage du CO 2 dans les fumées[END_REF]Lepaumier et al., , 2009a)). Oxidative degradation occurs because of a large amount of O 2 in flue gases. These two mechanisms of degradation have been studied mainly under laboratory conditions, sometimes on pilot plant [START_REF] Reynolds | Monoethanolamine Degradation during Pilot-Scale Post-combustion Capture of CO 2 from a Brown Coal-Fired Power Station[END_REF][START_REF] Da Silva | Understanding 2-Ethanolamine Degradation in Postcombustion CO 2 Capture[END_REF][START_REF] Lepaumier | Comparison of MEA degradation in pilot-scale with lab-scale experiments[END_REF][START_REF] Strazisar | Degradation pathways for monoethanolamine in a CO 2 capture facility[END_REF] and rarely on real CO 2 capture process. Most of the studies were focused on solvent analysis. This approach is certainly the most adapted and the simplest method to study the degradation of the solvent. Nevertheless, a lot of degradation products are volatile and it is necessary to consider also gas effluents. Indeed, gas pollutions may have negative impacts on operator but also on the environment (nature or closed population). Environmental acceptance cannot be achieved without a thorough knowledge of all process effluents, gas or liquid, and eventually, adapted cleaning processes. Thus, in this work a complete study of all the effluents of a pilot plant was done during a dedicated campaign. This work exposes a global analytical methodology which can be adapted to the monitoring of any process with any solvent. Especially, a new sampling method based on solid sorbents was developed to identify trace compounds present in gaseous effluents.

Material and Methods

Chemicals N,N'-(Bishydroxyethyl)oxalamide (BHEOX), N-(2-hydroxyethyl)acetamide (HEA) (90 %), N-(2hydroxyethyl)formamide (HEF) (97 %), , 2-methyloxazoline (≥ 99 %) and 3-methypyridine (99 %) were purchased from Alfa Aesar (Schiltigheim, France). Glycolic acid, potassium acetate, sodium formate, sodium propionate, sodium nitrite, magnesium sulphate, oxalic acid, lithium nitrate, potassium hydroxide, formic acid, formaldehyde (37%), dimethylamine solution (40 wt%), acetamide (99%), acetic acid (>99.7%), ethyleneglycol (EG) (99.8%), 1H pyrrole (98%), N,N-dimethylformamide (DMF) (99.8%), pyrazine (>99%), 2-oxazolidinone (OZD) (98%), 2-methylpyrazine (2MP) (>99%), diethyleneglycol (DEG) (>99%), triethyleneglycol (TEG) (>99%), 2-ethylpyrazine (>98%), 2,3dimethylpyrazine (>95%), N-hydroxyethylimidazole (HEI) (97%), bicine, N-(2hydroxyethyl)ethylenediamine (HEEDA), N,N-bis-2-(hydroxyethyl)ethylenediamine (BHEEDA) (97%), N-(2-hydroxyethyl)imidazolidin-2-one (HEIA), diethanolamine (DEA) (>98%), 2,5-dimethylpyrazine (>98%), ethanolamine (MEA) (98 %), N-nitrosodimethylamine (NDMA), N-nitrosodiethanolamine (NDELA), N-Nitroso-N,N-diethylamine (NDEA) and N-nitrosomorpholine (NMOR) were purchased from Sigma-Aldrich (Saint Quentin Fallavier, France). Oxazoline (96 %) was purchased from Interchim (France). 4-(2-hydroxyethyl)piperazin-2-one (4HEPO) and N-(2-hydroxyethyl)pyrrole (HEPyr) (99 %) were purchased from Tygersci (Hopkinsville, USA). N-(2-hydroxyethyl)glycine (HEGly) (95 %) was purchased from Enamine (Kiev, Ukraine). Glycine (99.7 %) was purchased from Merck (Lyon, France). Oxazolidine was purchased from Selectlab (Münster, Germany). 2,6-dimethylpyrazine, 2-ethyl-6methylpyrazine, 2-ethyl-5-methylpyrazine and 2-ethyl-3-methylpyrazine were purchased from SAFC (Sigma-Aldrich, Saint Quentin Fallavier, France). N,N'-Bis-(2-hydroxyethyl)urea (BHEU) was purchased from Chemos GMBH (Regenstauf, Germany). 1-(2-Hydroxyethyl)piperazinone hydrochloride (1HEPO) was purchased from Ukrorgsyntez (Kiev, Ukraine). Methanol, acetonitrile and ethylacetate were purchased from Carlo-Erba (Val de Reuil, France). Ultra-pure water was produced using a Direct-Q UV 3 system (18.2 MΩ.cm) from Millipore (Molsheim, France). N-nitroso-2-methyl-oxazolidine was synthesized according to the protocol of Saavedra [START_REF] Saavedra | Deamination of Primary Aminoalkanols. Formation of Substituted N-Nitroso-1,3oxazolidines and N-Nitroso-1,3-tetrahydrooxazine[END_REF]. N,N'-bis(2-hydroxyethyl)piperazin-2,5-dione (2,5-BHEPDO) and 2-methyloxazolidine were synthesized according to Gouedard et Al. [START_REF] Gouedard | Amine degradation in CO 2 capture. 3. New degradation products of MEA in liquid phase: amides and nitrogenous heterocycles[END_REF].

Pilot plant description

IFPEN pilot plant uses two columns of around 1 meter each with a diameter of 5 cm. The first one is used as an absorber (C10) where the solvent (30 % wt of MEA in water) is loaded in CO 2 (Figure 1). The second column is used as a stripper (C20) where the solvent is regenerated and CO 2 is released. Each column is equipped with high performance packings for ensuring good gas/liquid mass transfer: The absorber contains DX packing and the stripper contains BX packing.

Figure 1. Scheme of IFPEN pilot plant

The campaign lasted around 1700 hours. Synthetic flue gas contained 80.7% of N 2 , 14.1% of CO 2 , 5.1 % of O 2 , 97 ppm of NO, 9 ppm of SO 2 and 5 ppm of NO 2 . Total absolute pressure of the absorber was 1,2 bar. The gas flow rate was 1000 NL/h at 40°C and the solvent flow rate was 2 L/h. The total volume of solvent was 20 liters. Absorber outlet gas was directed to a heat exchanger (E10) to condensate water. Condensates was separated from gas phase in separator V10. Heat exchanger E10 followed by separator V10 may be considered as a low efficiency water wash section: E10 cooling liquid temperature is 4°C and E10 outlet gas temperature is 15°C enabling large amount of water condensation and therefore partial MEA and degradation products dissolution. V10 condensates was mixed with CO 2 lean amine in tank T-01. Once loaded with CO 2 , CO 2 rich amine was pumped with P02 and heated at 100°C in charge/effluent exchanger E21 before entering at the top of the stripper. The stripper was heated at 125°C which enabled the regeneration of the amine and the release of CO 2 .

Released CO 2 was directed to a heat exchanger (E20) to condensate water. The condensate was separated in separator V20 and was reintroduced at the top of the stripper with the CO 2 rich amine.

At the bottom of the stripper, the CO 2 lean amine was cooled in charge/effluent exchanger E21 then in exchanger E40 to be introduced in tank T-01 at a temperature close to room temperature. CO 2 lean amine is pumped from T-01 with P01 and heated at 40°C to be introduced at the top of absorber. MEA concentration was maintained between 30 and 35 % wt during the campaign by water addition in tank T-01. At the end of the campaign, concentrations of Iron, Nickel and Chromium, measured by Inductively Coupled Plasma with Optical Emission Spectroscopy (ICP-OES) were less than 5 mg/kg. Thus, corrosion was assumed negligible.

Analysis Ionic chromatography

Ion chromatography was commonly used to quantify acids in their anionic form: glycolate, formate, acetate, oxalate, sulfate, nitrite, and nitrate ions. The concentration of CO 2 -species (carbamate, carbonate and bicarbonate ions and carbonic acid) was also determined by Ionic Chromatography to determine CO 2 loading of amine given as mol CO 2 /mol MEA. Samples were diluted in ultrapure water. 25 μL of solution were injected twice on a Dionex ICS-3000 ion chromatograph (ThermoFisher Scientific, Villebon-sur-Yvette, France). The column was an AS-15 (4 × 250 mm). Eluent was aqueous KOH solution with a concentration gradient (from 8 to 60 mM) at a flow rate of 1 mL.min -1 . Detector was a conductimeter. A run of 60 minutes enabled an optimal separation. Some samples were pretreated with HNO 3 69 % to reach a pH below 4 to eliminate MEA carbamate peak which overlaps acetate and glycolate peaks. Quantification of anions was obtained with an uncertainty of ± 10%. Quantification of CO 2 loading was obtained with an uncertainty of ± 15%.

LC-MS/MS

Analyses were performed on a LC Thermo Scientific Dionex Ultimate 3000 (Analytical Autosampler WPS-3000SL, Quaternary Analytical Pump LPG-3400SD) coupled with a MS Thermo Scientific TSQ Quantum Access MAX with HESI-II source (ThermoFisher Scientific, Villebon-sur-Yvette, France). It was used in positive mode, probe in position C, electrospray voltage of 2500V and capillary temperature of 200°C. The sheath gas was nitrogen at a flow rate of 40mL/min and the auxiliary gas at 8 mL/min. Chromatographic separations were conducted on a Thermo HyperCarb column (PGC) 150 mm x 3 mm, 5 µm-particles (ThermoFisher Scientific, Villebon-sur-Yvette, France). The mobile phase was a mixture of (A) water with 0.1% formic acid and (B) methanol with 0.1% formic acid at a flow rate of 350µL/min. 5 µL of sample were injected. A pre-run rinse of 100% A for 8 min was performed, then the solvent gradient started at 100% of A for 10 min before reaching a ratio of 80:20 (A:B v:v) in 8 min. This ratio was maintained for 12 min. Samples had to be at least 1000-fold diluted before injection to prevent the mass spectrometer from being polluted by MEA. The MRM transitions of the targeted compounds used are given in supplementary material. They are reported from the literature or determined with standards. Quantification was obtained with an uncertainty of ±20%. Two internal standards were used: Diethanolamine-d8 for DEA, HEGly and BHEU, and Glycine-d5 for Glygly and Gly. Compounds quantification method and performances are detailed in our previous work [START_REF] Cuzuel | Validation of a liquid chromatography tandem Mass spectrometry method for targeted degradation compounds of ethanolamine used in CO 2 capture: application to real samples[END_REF].

GC-FID

MEA concentration was determined by GC-FID performed on Agilent HP6890 chromatograph (Agilent Technologies, Massy, France) with a carbowax amines column (15 m x 0.53 mm, 1 µm) made by Supelco (Sigma-Aldrich, Saint Quentin Fallavier, France) and with triethyleneglycol as internal standard. The oven program started at 100 °C then the temperature was raised at 8°C/min to 200 °C and held for 5 min. 1 µL of the sample was injected in the inlet at 250°C and the split flow was 100 mL/min. The Helium gas flow was 8mL/min with a top column pressure of 4.2 psi. Quantification was obtained with an uncertainty of ±5%.

GC-MS

Analyses were performed on an Agilent 7890A gas chromatograph coupled with an Agilent 5975C inert XL MSD mass spectrometer (Agilent Technologies, Massy, France). The device was equipped with a MPS (MultiPurpose Sampler) auto sampler from Gerstel (RIC, Saint-Priest, France) that enabled fully automated HS-SPME analyses. Two columns (Chromoptic, Villejust, France) were used to separate all the target compounds, a non-polar fused silica capillary column CP-SIL8 CB-MS (30 m x 0.25 mm, 1µm)) and a polar fused silica capillary column DB-WAX (30 m x 0.25 mm, 0.5µm). For the non-polar column, initial temperature was 40°C held for 2 min then raised to 130°C at 7°C/min, increased to 280°C at 13°C/min and held for 10 min. For the polar column, oven temperature program started at 40°C, held for 2 min then raised to 130°C at 7°C/min, then increased to 200°C at 10°C/min and held for 7 min. In both cases, helium was used as carrier gas in constant flow mode at 1 mL/min. The transfer line temperature to the MS detector was set at 280°C. For liquid injection procedures, real samples were diluted 10 times in methanol before injecting 1µL in split mode (1:5) at 250°C. Quantification of a selection of products was obtained with an uncertainty of ± 15%. For thermodesorption of tubes, gas flow rate of helium was 40 mL/min in splitless mode. Initial temperature of desorption was 35°C held for 2 min then raised to 300°C at 120°C/min and held for 6 min. Desorbed molecules were cryofocused in the injector at -40°C with liquid CO 2 . Then temperature increased from -40°C to 300°C at 12°C/s and the molecules were injected in the column in splitless mode. The same GC/MS method as for liquid samples was used. Detection was performed with a mass spectrometer using electronic ionization (EI) or chemical ionization (CI) source. The latter was both used in positive (PCI) and negative (NCI) mode. The EI source (70 eV) was heated to 250°C, the scan range was 25 to 250 amu. As for the CI source, it was heated to 300°C for positive mode and to 150°C for negative mode, CH 4 was used as reactant gas and the scan range was 50 to 250 amu. Some complementary analyses were also performed on a Thermo Finnigan Tempus (GC-TOF (Time of flight)/MS) (Thermo Finnigan, Villebon-sur-Yvette, France), which was used with CP-SIL8 CB-MS (30 m x 0.25 mm, 1 µm ) (Chromoptic, Villejust, France). Initial temperature was 35ºC then raised to 230ºC at 3°C/min and held for 50 min. Helium was used as carrier gas in constant flow mode at 0.9 mL/min. The transfer line temperature to the MS detector was set at 250°C. Mass spectrometer was used with the EI source (70 eV) heated at 200°C. The scan range was 10 to 350 amu.

GC-HRMS

Analyses were performed on an Agilent 7890A gas chromatograph (Agilent Technologies, Massy, France) coupled with Autospec Premier mass spectrometer from Waters (Saint-Quentin-en-Yvelines, France). A capillary column DB5-MS (30m x 0.25 mm, 0.50 µm) (Chromoptic, Villejust, France) was used to separate target compounds. Initial temperature was 35°C held for 4 min then raised to 150°C at 10°C/min, increased to 250°C at 20°C/min. Helium was used as carrier gas in constant flow mode at 1.5 mL/min. Inlet temperature was 230°C, the split ratio was 1: 5 and the volume injected was 0.8 µL. The transfer line temperature to the MS detector was set at 260°C. Detection was performed with a magnetic sector mass spectrometer using electronic ionization (EI) source in positive mode.

The EI source (35 eV with trap 450µA) was heated to 250°C. The mass spectrometer was used in a Single Ion Recording mode (SIR) with a scan rate of 0.5 s, an interscan of 0.2 s and a resolution comprised between 5000 and 6000. The recording ion mass were m/z = 74.0480 Th for NDMA and m/z = 116.0590 Th for NMOR.

Head Space -SPME

For Head Space -SPME (HS-SPME) procedures, the volume of sample introduced in the 20 mL HS vial was 5 mL. The fully automated HS-SPME procedure was as follows. First, the vial was equilibrated at 70°C during 5 min then the 75 μm Carboxen/PDMS fibre SPME, obtained from Supelco (Sigma-Aldrich, Saint Quentin Fallavier, France) was placed into the head-space of the sample for the extraction, still maintained at 70°C for 30 min. At the end of the extraction, the fibre was desorbed directly in the injector set of the GC/MS at 250°C in split mode (1:5). This method was initially developed for identification and quantification of pyrazines [START_REF] Rey | Amine degradation in CO 2 capture. 2. New degradation products of MEA. Pyrazine and alkylpyrazines: Analysis, mechanism of formation and toxicity[END_REF]. It is adapted here to the identification of other degradation products present in liquid phase.

FT-IR

A FT-IR disposal provided by GASMET (SISTEC, Vienne, France) was used to monitor different gas effluents (see the computer symbols on Figure 1). It enabled the monitoring of several products in gas phase, especially ammonia, CO 2 , NO, NO 2 and SO 2 . Limit of Detection (LOD) and uncertainties were determined by the equipment provider. The uncertainty is 2 % of the full scale. Used range and LOD are given in Table 1.

Gas sampling

Gas sampling was performed to analyze degradation products which were not observed by FT-IR. Solid phase extraction (SPE) of gaseous compounds was performed during 1 or 19 hours in function of solid adsorbent type. During gas sampling, the gas flow in the absorber was reduced to 500 NL/h and the solvent flow was reduced to 1 L/h. Gas samplings have been done after the separator V10 every two weeks and once after the separator V20 at the end of the campaign.

Assisted Liquid-Liquid Extraction (ALLE)

An original sample handling approach using assisted liquid-liquid extraction on diatomaceous earth cartridges (ChemElut) (Agilent Technologies, Massy, France) allowed to extract selectively nitrosamines from samples. This extraction on solid matrix allowed to eliminate a large amount of MEA and thus enhanced the extraction of nitrosamines (improvement in sensitivity of an order 5 to 10). 2 mL of phosphate buffer (pH = 7.4) were mixed with 1 mL of sample. This mixture was adsorbed during 5 min on ChemElut (3mL). Then, elution was performed with 9 mL of ethyl acetate. Solution was smoothly evaporated with the help of a nitrogen flux gas to 0.25 mL and redissolved in 100 µL of ultrapure water. Sample were then directly analyzed with LC-MS/MS.

Solid phase extractions (SPE)

A device enabling simultaneous adsorption on four different solid phase extraction tubes was implemented on outlet gas lines after V10 and V20. After V10 and V20, gas temperature (15°C) was below room temperature avoiding any water condensation in tubes. Gas flow rate was regulated with a flow meter in order to respect the maximum flow rate specified by tubes suppliers. For each adsorbent type, three tubes were used in series to enable quantification of the retained products in case of saturation or piercing of the two first tubes. The sampling conditions are given in Table 2. Excepted for formaldehyde, all aldehydes were absorbed at more than 99% in the first tube.

Quantification was obtained with an uncertainty of ±30%. Thermosorb/N are the best sorbents to analyze nitrosamines because they are free of artifacts formation [START_REF] Roundbehler | Nitrosamine air sampling sorbents compared for quantitative collection and artifact formation[END_REF]. However, these sorbents were not available for our study. Therefore ORBO 60 tubes were used. They contain a Florisil phase (Sigma-Aldrich, Saint Quentin Fallavier, France) which is particularly efficient to adsorb nitrosamines and especially NDMA which is one of the most volatile and one of the most carcinogenic nitrosamine potentially present in CCS gas effluents. Florisil sorbent are known to form slightly NDMA artifacts [START_REF] Roundbehler | Nitrosamine air sampling sorbents compared for quantitative collection and artifact formation[END_REF]. In this study, only NDMA was quantified. Orbo 60 tubes were chemically desorbed twice with 2 mL of ethylacetate. Results showed that 2mL were sufficient to elute more than 95% of NDMA. NDMA was quantified by GC/HRMS. NDMA was adsorbed at more than 90% in the first tube.

Tenax TA TDU tubes (Gerstel, Saint-Priest, France) were thermally desorbed. Tenax is a porous polymer widely used as an adsorbent in both air collection and purge and trap applications. Its structure provides alternate adsorption/desorption of a wide variety of compounds. Quantification of a selection of products was obtained with an uncertainty of ±30%. Activated charcoal tubes (Gerstel, Saint-Priest, France) were used as non-specific adsorbent such as Tenax TA TDU tubes. The large surface area of this carbon form enables excellent adsorption/desorption of a wide variety of volatile compounds. The activated charcoal tubes were also thermally desorbed. Tenax TA TDU tubes enabled to adsorb larger amounts of products but activated charcoal tubes were useful to confirm (or infirm) punctually the presence of some volatile compounds observed with Tenax tubes. Indeed, some products detected on Tenax might be due only to the degradation of the Tenax phase in gas effluent conditions and it was necessary to be able to cross the results between the two different adsorbent phases.

Results and Discussion

Pilot plant monitoring

Gas online FTIR analyses enabled to study the composition of the gas, in terms of major compounds, at the different point of the pilot plantTable 3 gives CO 2 , SO 2 , NO and NO 2 concentrations at the inlet and outlet of absorber and stripper. NH 3 concentration was between 10 and 20 ppm vol at C10 exit during the whole campaign. Based on the total inlet flow rate and assuming that nitrogen did not absorb and that O 2 consumption was negligible, CO 2 capture yield was around 60 % which corresponds to 282 kg of CO 2 treated in around 1700 hours. Solvent sampling was done three times a week at the bottom of the absorber (C10) for the CO 2 rich amine and in the tank T-01 for the CO 2 lean amine. All samples were stored in brown flaks between 4 and 8°C. The campaign monitoring consisted in analyzing the concentrations of MEA, CO 2 -species and anions. The concentration of MEA was determined by GC-FID enabling also an estimation of water loss (Figure 2). The pilot lost approximately 1,5 L of water per week in V10 and V20 outlet gases. Water loss was compensated by water addition (Figure 2). 14,4 kg of water were added during the campaign. Some MEA was lost also because of sampling.

Initially 20 kg of 30% wt MEA in water was introduced in the pilot plant. 1,3 kg of 30 % wt MEA solution were therefore added during the campaign. MEA concentration was then maintained between 30% and 35% wt during the whole campaign. The concentration of CO 2 -species (carbamate, carbonate and bicarbonate ions and carbonic acid) was determined by Ionic Chromatography to determine CO 2 loading of amine given as mol CO 2 / mol MEA. CO 2 loading was close to 0,3 for CO 2 lean amine and 0,5 for CO 2 rich amine in stationary conditions (Figure 3). The concentration of formate, acetate, oxalate, glycolate, propionate, nitrite, nitrate and sulfate (heat stable salts) was determined by ionic chromatography.

Figure 2. Evolution of MEA concentration in T01 (CO 2 lean amine) and added water quantities

Figure 3 shows that CO 2 loading in the solvent at the exit of absorber and stripper was quite stable at roughly 0,5 and 0,3 mol/mol respectively. Two kinds of heat stable salts (HSS) were formed: the ones due to flue gas impurities such as NO 2 or SO 2 and the organic ones due to solvent oxidative degradation. Figure 4 shows the evolution of nitrates, nitrites and sulfates during the campaign.

Figure 4. Evolution of nitrates, nitrites and sulfates

The evolution of sulfates was almost linear from the beginning to the end of the campaign as expected. The concentration of sulfates reached 2416 mg/kg in 1701 hours. It means that 94 % of the total amount of injected SO 2 gave sulfates if SO 2 concentration at absorber exit is taken equal to 2 ppm mol which is the detection limit of FTIR. Nitrites concentration was quite constant at around 19 ppm from 69 to 1701 hours which may indicate that nitrites are some kind of intermediates. They were formed and consumed throughout the campaign, mainly due to their oxidation into nitrates which reached 383 mg/kg at 1701 hours. Nitrates and nitrites are mainly due to the presence of NO 2 and NO in flue gas but they may also be a resultant of MEA degradation. Table 4 gives the mass balance between NO 2 /NO absorbed in solvent based on FTIR results and nitrates/nitrites accumulation in the solvent at the middle and at the end of the campaign. In the middle of the campaign measured nitrites/nitrates are close to NO 2 /NO absorption. But at the end of the campaign, the amount of nitrites/nitrates exceeds NO 2 /NO absorption because the proportion due to MEA degradation increased. On the other hand, they may be also consumed to form other degradation products like nitrosamines but this contribution is less important.

A quantitative monitoring of organic HSS was focused on formates, glycolates and oxalates (Figure 5). Evolution of formates was almost linear and the concentration reached 195 mg/kg at 1701 hours. It is the main organic acid formed by MEA degradation as expected [START_REF] Vevelstad | Oxidative degradation of 2-ethanolamine: the effect of oxygen concentration and temperature on product formation[END_REF]. Glycolates were already present in supplied MEA and their concentration fluctuated between 60 and 100 mg/kg during the campaign without significant evolution. Organic acids are known to be starting material of many degradation products [START_REF] Gouedard | Amine degradation in CO 2 capture. I. A review[END_REF] which may explain such a constant concentration. On the other hand, oxalates concentration was lower than the limit of quantification of the analytical method until 812 hours and reached 39 mg/kg at 1701 hours. Expected acetates and propionates [START_REF] Cuzuel | Amine degradation in CO 2 capture. 4. Development of complementary analytical strategies for a comprehensive identification of degradation compounds of MEA[END_REF] have not been detected at all during this campaign. Acetates are assumed to have mainly reacted with MEA to form N-(2-hydroxyethylacetamid (HEA) (see table 4).

On the other hand propionic acid was not present neither as propionate nor as N-(2hydroxyethylpropanamide). In our previous work [START_REF] Cuzuel | Amine degradation in CO 2 capture. 4. Development of complementary analytical strategies for a comprehensive identification of degradation compounds of MEA[END_REF] both were observed but MEA was more concentrated than in the present work: 40% wt instead of 30% wt. Therefore, in present work, degradation is slower and they might be detectable after a longer running time.

Degradation products in the solvent

Organic products identified in the solvent throughout the campaign are given in Table 5. The main degradation products described in our previous paper [START_REF] Cuzuel | Amine degradation in CO 2 capture. 4. Development of complementary analytical strategies for a comprehensive identification of degradation compounds of MEA[END_REF] were observed here. Nevertheless, our previous pilot plant campaign was done with 40% wt MEA enabling higher concentrations of degradation products. That is why less degradation products were observed in this work but all should be observed after a longer running time. Each degradation product identification was confirmed by corresponding standard analysis and a mechanism of formation was proposed for each one (Gouedard et al., 2014a;[START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF][START_REF] Rey | Amine degradation in CO 2 capture. 2. New degradation products of MEA. Pyrazine and alkylpyrazines: Analysis, mechanism of formation and toxicity[END_REF]. For each identified molecule, the corresponding standard was purchased or synthesized. Identification was confirmed by obtaining the same mass or NMR spectrum. hydroxyethyl)formamide (HEF), N-(2-hydroxyethyl)acetamide (HEA) and oxazolidinone (OZD) were quantified by GC/MS (Figure 7). Their proportions were globally in agreement with state of art [START_REF] Da Silva | Understanding 2-Ethanolamine Degradation in Postcombustion CO 2 Capture[END_REF]Vevelstadt et al., 2013). Nevertheless, there were some differences. HEgly was not the main degradation product in our case. The first reason is that this compound is an intermediate to some other degradation products [START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF] whose formation may be conditions dependent. The second reason is that the main product in our case was BHEU which was never quantified by other authors to the best of our knowledge. Its formation may be explained by a reaction between MEA carbamate and MEA: nucleophilic attack of MEA amine function on carbamate carbonyl group as in the case of imidazolidinones formation which are cyclic ureas (Lepaumier et al., 2009a). The high concentration of both reagents may explain the high concentration of BHEU. OZD concentration increased rapidly during the first 500 hours before reaching a kind of maximum at 0.9 g/L. OZD is an intermediate easily formed by carbamate cyclisation but it is also easily consumed by reaction with nucleophilic compounds (Lepaumier et al., 2009c): it reacts especially with amine functions of MEA and its degradation products. HEF was already present in supplied MEA at 0.2g/l and kept quite constant during the campaign: it is formed from MEA and formic acid [START_REF] Gouedard | Amine degradation in CO 2 capture. I. A review[END_REF] and is also an intermediate to other degradation products as oxazoline for example (Gouedard et al., 2014a). On the contrary, 4HEPO started relatively slowly but seemed to increase more rapidly after 1000 hours. Several mechanisms are proposed for 4HEPO formation [START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF]. In any case, starting material is a degradation product of MEA: HEEDA or HEGly. Therefore their accumulation during the campaign may explain 4HEPO evolution. In this work 4HEPO concentration was much higher than HEGly contrary to Da Silva et al. results [START_REF] Da Silva | Understanding 2-Ethanolamine Degradation in Postcombustion CO 2 Capture[END_REF] suggesting that our conditions promoted HEGly consumption to form 4HEPO. 1HEPO was less concentrated than 4HEPO because its formation is less favorable [START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF].

Nitrosamines are the subject of a growing interest in the CCS literature due to their potential carcinogenic properties. Three nitrosamines were identified in liquid phase in this work: Nnitrosodiethanolamine (NDELA), N-nitrosomorpholine (NMOR) and N-nitrosodimethylamine (NDMA). N-nitroso-HEGly observed by Morken et al. couldn't be confirmed here because the corresponding standard couldn't be provided [START_REF] Morken | Emission results of amine plant operations from MEA testing at the CO 2 Technology Centre Mongstad[END_REF]. Some other nitrosamines were only identified in the gas phase as shown below. NDELA could not be quantified accurately by our method but its concentration was definitely superior to our limit of detection 50 µg/kg. The NMOR was detected but its concentration was inferior to our limit of quantification of 4 µg/kg. Finally, the concentration of NDMA was determined in both CO 2 rich and CO 2 lean amine (Table 6). Concentrations of NDMA in CO 2 rich and CO 2 lean amine are equivalent if higher loading of CO 2 in rich amine compared to lean amine (dilution of nitrosamine) and uncertainty of the method are taken into account. These concentrations are quite high comparing to other results in the literature [START_REF] Einbu | A new test rig for studies of degradation of CO 2 absorption solvents at process conditions; comparison of test rig results and pilot plant data for degradation of MEA[END_REF] and that could be due to the IFPEN pilot specifically. However, NDMA is known to be carcinogenic and volatile. Thus, the monitoring of the degradation products in the gas effluent in CCS plant is definitely needed.

Degradation products in gas effluents

The analysis of degradation products in gas effluent was based on different sampling methods using solid phase extraction. Tenax and activated charcoal adsorbents were used as non-selective adsorbents. Orbo 60 adsorbents were dedicated to the analysis of nitrosamines. Sep-Pak  tubes containing 2,4-Dinitrophénylhydrazine (DNPH) were used to adsorb aldehydes and ketones. These analytical campaigns were achieved on gas effluents after the condenser V10 of the absorber and after the condenser V20 of the stripper. Table 7 shows degradation products identified with Tenax, activated charcoal and Sep-Pak  tubes. Compounds written in bold are novel compounds which have never been detected in CCS literature so far to the best of our knowledge. Some of them were also identified in our previous work dedicated to liquid phase analysis [START_REF] Cuzuel | Amine degradation in CO 2 capture. 4. Development of complementary analytical strategies for a comprehensive identification of degradation compounds of MEA[END_REF]. Some volatile products observed in liquid phase (some pyrazines for example) were not detected in gas phase because their concentration was likely too low. Nevertheless they must be considered as potentially present in gas effluent of the pilot plant. On the other hand, several products were only observed in gas phase: 2-butanone, DMF, oxazolidine, 2-methyloxazolidine, 2-methylpyridine, 3methylpyridine, N-nitrosodiethylamine, N-nitroso-2-methyl-oxazolidine and also acetamide, butyraldehyde, 2-ethylpyridine, 2,3-dimethylpyridine, 3-ethylpyridine, 5-ethyl-2-methylpyridine, 2ethyl-2-methyloxazolidine and 2-propyloxazolidine only observed in stripper outlet gas. Two explanations may be considered: matrix effect can prevent their observation in liquid phase and/or they may be formed in gaseous phase. The latter may explain specific products formed in specific conditions of stripper outlet gas (high CO 2 concentration and high temperature) unless their stripping would be promoted by the higher temperature of stripper. Therefore, 38 degradation products were identified in gas phase, 17 of which for the first time, especially derivatives of pyridine and oxazolidine, 1H-pyrrole and a new nitrosamine, the N-nitroso-2-methyl-oxazolidine. 11 degradation products present in gas effluents were quantified in this work. Some aldehydes were quantified in gas effluents with the help of DNPH Sep-Pak  tubes. 2 tubes were used in series. Concentration profiles of these compounds in absorber outlet gas are given in Figure 8. The second tube contained some formaldehyde. Therefore, two consecutive tubes of Sep-Pak  with DNPH were not sufficient to retain all the formaldehyde with a flow of 6 NL/h during 1 hour. Thus, the quantification gave a minimum concentration of formaldehyde in gas effluent. Acetaldehyde was the main aldehyde detected maybe because of the even higher reactivity of formaldehyde. Propionaldehyde, likely formed from formaldehyde and acetaldehyde [START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF] increased slowly and remained at a low level. In stripper outlet gas, formaldehyde, acetaldehyde and propionaldehyde were roughly one order of magnitude more concentrated than in absorber outlet gas (Table 8). Two explanations may be considered: their formation is promoted by stripper conditions and/or their stripping is promoted by the higher temperature of stripper. With the help of the Sep-Pak  DNPH tubes, it was also possible to detect the presence of butanone and butyraldehyde in the stripper outlet gas exclusively. The concentrations of butanone and butyraldehyde were both 40 µg/m 3 . However, neither traces of benzaldehyde nor linear carbonyl compound (from five to eight carbons) were detected.

At the end of the campaign, three consecutive Tenax tubes were used to make a quantitative analysis of ethyleneglycol (EG), pyrazine, 1H-pyrrole, dimethylformamide (DMF), 2-methylpyrazine (2MP), Nnitrosodimethylamine (NDMA) and diethylenglycol (DEG) in absorber outlet gas (Table 9). For EG, pyrazine, pyrrole, NDMA and DMF, the third tube contained also these products which means that three consecutive tubes of Tenax were not sufficient to adsorb all compounds with a flow of 6 NL/h during 1 hour. However, the amount adsorbed on the third tube never exceeded 12% of the total amount except in the case of DMF. The cases of DMF and NDMA are suspicious because both of them are formed from dimethylamine. It is likely that dimethylamine reactions were still going on in the gas phase. Dettmer and Engewald reported the formation of NDMA from dimethylamine in the presence of nitrogen oxides on such tubes [START_REF] Dettmer | Adsorbent materials commonly used in air analysis for adsorptive enrichment and thermal desorption of volatile organic compounds[END_REF]. Therefore concentrations of DMF and NDMA might be over evaluated, especially NDMA which concentration was determined at much lower values by other techniques during the same pilot plant campaign.

Orbo 60 tubes were used to quantify NDMA. At the end of the campaign, the analysis of the gas effluent was done with two consecutive Orbo 60 tubes in absorber and stripper outlet gases: 75 µg/Nm 3 and 60 µg/Nm 3 respectively. On one hand the value for absorber outlet gas confirms that value obtained with Tenax Tubes was over evaluated. On the other hand these values are one order of magnitude higher than expected values considering NDMA in equilibrium with the liquid phase (Table 10). Indeed, the concentration of NDMA measured in the solvent (Table 5) and the Henry's law constant H of NDMA in water at 25°C (2.67 X 10 -7 atm m 3 /mol (EPA, 2008)) enable to calculate approximately NDMA concentration in gas phase (Table 9) by using the following equation: Therefore, NDMA concentration values in gas phase should be considered with caution. The Tenax tubes enabled to detect in gas phase a novel nitrosamine: the N-nitroso-2-methyl-oxazolidine (NMOx). NMOx was detected by GC/MS on three different Tenax tubes and was certainly formed by nitrosation of 2-methyloxazolidine also detected on Tenax tubes. NMOx has been synthesized and used as a standard to confirm its presence in gas effluent [START_REF] Gouedard | Novel Degradation Products of Ethanolamine (MEA) in CO 2 Capture Conditions: Identification, Mechanisms Proposal and Transposition to Other Amines[END_REF].

Conclusion

A pilot plant campaign was performed to study MEA degradation in CO 2 capture conditions. Specific methods were developed for sampling, sample conditioning and analysis of degradation products. This analytical methodology can be adapted to the monitoring of any process with any solvent, especially the new gaseous sampling method for trace compounds identification. The analysis of the liquid phase confirmed degradation products identified in our previous works. On the other hand, gas phase analysis went further than state of art enabling identification of 17 novel degradation products. In addition to pyrazines, highlighted in our previous work, several derivatives of pyridine and oxazolidine, 1H-pyrrole and a new nitrosamine, the N-nitroso-2-methyl-oxazolidine were identified here among some others. This is a very important point because these products may be emitted to the atmosphere. Nevertheless, the knowledge of their presence enables the development of adapted counter measures to avoid any risk for environment or human health.

Figure 3 .

 3 Figure 3. Evolution of CO 2 loading in CO 2 lean amine and in CO 2 rich amine

Figure 5 .

 5 Figure 5. Evolution of formates, glycolates and oxalates

Figure 6 .Figure 7 .

 67 Figure 6. LC-MS-MS monitoring of DEA, HEGly and BHEU during the campaign.

Figure 8 .

 8 Figure 8. Quantification of aldehydes in absorber outlet gas.

Table 1 : Range and LOD values of FTIR analysis

 1

	Gas Range (molar)	LOD
	NH 3 0-4000 ppm	0,3 ppm
	CO 2	0-100 %	200 ppm
	SO 2 0-10000 ppm	1 ppm
	NO	0-2500 ppm	4 ppm
	NO 2	0-200 ppm	1 ppm

Table 2 : Sampling conditions and SPE tubes according to target compounds

 2

	Target Compounds	Tubes References	Gas flow (NL/h)	Sampling time (h)
	Aldehydes and ketones	Sep-Pak  DNPH	6	1
	Nitrosamines	ORBO 60	10	19
	Volatile compounds	Tenax TA TDU	6	1
	Volatile compounds	Activated charcoal

6 1 Sep-Pak  tubes (Sigma-Aldrich, Saint Quentin Fallavier, France) containing 2,4-Dinitrophénylhydrazine (DNPH) were analyzed by the French institute INERIS (Verneuil-en-Halatte, France). Sep-Pak  tubes were chemically desorbed with 2 mL of acetonitrile and products were analyzed by High Performance Liquid Chromtography (HPLC) with diode array detector (DAD). Aldehydes were quantified by external calibration according to the method INRS Métropol 001.

Table 3 : Gas composition in C10 and C20 outlet gases after condensation step (LOD: Limit Of Detection).

 3

	Gas	Inlet C10	Exit C10 dry gas	Exit C20 dry gas
	CO 2	14,1 %	6,5 %	99,99 %
	SO 2	9 ppm	2 ppm	< LOD
	NO 2	5 ppm	< LOD	< LOD
	NO	97 ppm	83 ppm	50 ppm
	O 2	5,1 %	5,1 %	Not analyzed
	N 2	80,7%	88 % (calculated)	-

Table 4 : Comparison of concentration of products formed from introduced NOx with nitrates and nitrites concentration Time (h) Calculated concentration of products formed from introduced NOx (mol/kg) Total concentration of analyzed nitrates and nitrites (mol/kg) Ratio

 4

	836	4,12E-02	4,23 ± 0,4 E-02	1,0
	1701	8,39E-02	1,45 ± 0,15 E-01	1,7

Table 6 : Concentration of NDMA in CO 2 lean and CO 2 rich amine determined by GC-HRMS

 6

	Concentration of NDMA in µg/kg 400 hours 1400 hours
	lean amine (T01)	317 ± 63	541 ± 108
	rich amine( C10)	265 ± 53	446 ± 89

Table 8 : Concentrations of some aldehydes after 1600 hours in absorber and stripper outlet gases Aldehydes concentrations in g/L at 1600 h

 8

		Absorber	Stripper
	Formaldehyde	8	38
	Acetaldehyde	4970	48000
	Propionaldehyde	88	567

Table 9 : Concentration of a selection of degradation products in absorber gas effluent measured on Tenax tubes

 9

	Adsorbed quantity (ng)	Molecular weight (g/mol)	Tube 1	Tube 2	Tube 3	Total	Concentration in gas (µg/m3)
	EG	62	1469	851	193	2513	419
	Pyrrole	67	60	16	8	84	14
	DMF	73	69	17	23	109	18
	NDMA	74	1000	397	187	1584	264
	Pyrazine	80	850	203	53	1106	184
	2MP	94	149	0	0	149	25
	DEG	106	114	57	0	171	29

Table 10 : Estimation of NDMA concentration in gas phase in equilibrium with liquid phase

 10

	Total pressure Pt (bara)	1,2	400 hours	1400 hours
	y NDMA	absorber stripper	9,38E-10 7,84E-10	1,60E-09 1,32E-09
	C NDMA in gas phase (µg/m 3 )	absorber stripper	3 2	5 4

Acknowledgments

We would like to acknowledge financial support from French ANR (Research Project DALMATIEN: Degradation of liquid amines and methods of analysis: toxicity or innocuousness for the environment?).

Analysis methods GC-MS TOF

GC-MS LC-MS -MS

GC-HRMS

Supplementary Material

Click here to download Supplementary Material: Supplementary material.docx