Kinetics of N-nitrosopiperazine formation from nitrite and piperazine in CO2 capture

Role of various factors affecting the photochemical treatment of N-nitrosamines related to CO2 capture

Post-combustion CO₂ capture using amine solvents is the most feasible method of reducing anthropogenic CO₂ emissions, which are the largest contributor to global warming. The formation of carcinogenic N-nitrosamines (i.e. by-products) can hinder the industrial application of this technology. In this study, the effects of direct UV photolysis (N-nitrosamine concentration and amines) and advanced oxidation processes (UV/H₂O₂ and UV/O₃) on the three specific N-nitrosamines that are commonplace in amine-based CO₂ capture (i.e. N-nitrosodiethylamine (NDEA), N-nitrosodiethanolamine (NDELA), and N-nitrosomorpholine (NMOR)) were examined. A significant decrease in the photodegradation rate constants was observed for NDEA (1.02 × 10⁰ to 2.94 × 10^-1 min^-1), NDELA (1.52 × 10⁰ to 3.32 × 10^-1 min^-1), and NMOR (1.93 × 10⁰ to 2.20 × 10^-1 min^-1) as their concentrations increased within 1-50 mg/L. This is the first report of a significant increase in the degradation rate constants of N-nitrosamine with an increase in amine concentrations (i.e. monoethanolamine, diethanolamine, and morpholine) within 10-200 mM. The photodegradation rate constants increased as the molar ratio of H₂O₂ to N-nitrosamine increased to 20, but then decreased at molar ratios beyond this. O₃ had a negligible effect on the photodegradation of N-nitrosamines.

Role of absorber and desorber units and operational conditions for N-nitrosamine formation during amine-based carbon capture

The formation of carcinogenic N-nitrosamines from reactions between solvent amines and flue gas NO_x is an important concern for the application of amine-based processes to capture CO₂ post-combustion. Using an advanced test rig with interconnected absorber and desorber units, we evaluated the importance for N-nitrosamine formation of the desorber relative to the absorber, and any synergism between the two units. Variations in desorber temperature and in flue gas composition indicated that N-nitrosamine formation from fresh monoethanolamine (MEA) occurred predominantly in the absorber. N-nitrosamine formation was driven by high NO₂ and O₂ flue gas concentrations, although NO also contributed. In contrast, N-nitrosamine formation from piperazine (PZ) was driven by reactions with nitrite in the heated desorber, and accelerated concurrent with nitrite accumulation. A complementary experiment simulating aged MEA solvent (high nitrite, 1.5% sarcosine as a proxy of secondary amine degradation products) suggested the desorber becomes an order of magnitude more important than the absorber for N-nitrosamine formation. For fresh MEA solvent, increasing the desorber temperature from 110 °C to 130 °C promoted thermal decomposition of N-nitrosamines, reducing N-nitrosamine accumulation rates two-fold. Compared to the test rig, the prevailing practice of using separate absorber columns and autoclave-like treatments to mimic desorber units predicted the direction, but underestimated the magnitude of N-nitrosamine formation. Because N-nitrosamine accumulation rates are the net result of competing formation and thermal decomposition processes, use of continuously cycling test rigs may be necessary to understand the impacts of different operating conditions.

Computational investigation of the nitrosation mechanism of piperazine in CO2 capture

Quantum chemistry calculations and kinetic modeling were performed to investigate the nitrosation mechanism and kinetics of diamine piperazine (PZ), an alternative solvent for widely used monoethanolamine in postcombustion CO₂ capture (PCCC), by two typical nitrosating agents, NO₂^- and N₂O₃, in the presence of CO₂. Various PZ species and nitrosating agents formed by the reactions of PZ, NO₂^-, and N₂O₃ with CO₂ were considered. The results indicated that the reactions of PZ species having NH group with N₂O₃ contribute the most to the formation of nitrosamines in the absorber unit of PCCC and follow a novel three-step nitrosation mechanism, which is initiated by the formation of a charge-transfer complex. The reactions of all PZ species with NO₂^- proceed more slowly than the reactions of PZ species with ONOCO₂^-, formed by the reaction of NO₂^- with CO₂. Therefore, the reactions of PZ species with ONOCO₂^- contribute more to the formation of nitrosamines in the desorber unit of PCCC. In view of CO₂ effect on the nitrosation reaction of PZ, the effect through the reaction of PZ with CO₂ shows a completely different tendency for different nitrosating agents. More importantly, CO₂ can greatly accelerate the nitrosation reactions of PZ by NO₂^- through the formation of ONOCO₂^- in the reaction of CO₂ with NO₂^-. This work can help to better understand the nitrosation mechanism of diamines and in the search for efficient methods to prevent the formation of carcinogenic nitrosamines in CO₂ capture unit.

Nitrosamines and Nitramines in Amine-Based Carbon Dioxide Capture Systems: Fundamentals, Engineering Implications, and Knowledge Gaps

Amine-based absorption is the primary contender for postcombustion CO₂ capture from fossil fuel-fired power plants. However, significant concerns have arisen regarding the formation and emission of toxic nitrosamine and nitramine byproducts from amine-based systems. This paper reviews the current knowledge regarding these byproducts in CO₂ capture systems. In the absorber, flue gas NO_x drives nitrosamine and nitramine formation after its dissolution into the amine solvent. The reaction mechanisms are reviewed based on CO₂ capture literature as well as biological and atmospheric chemistry studies. In the desorber, nitrosamines are formed under high temperatures by amines reacting with nitrite (a hydrolysis product of NO_x), but they can also thermally decompose following pseudo-first order kinetics. The effects of amine structure, primarily amine order, on nitrosamine formation and the corresponding mechanisms are discussed. Washwater units, although intended to control emissions from the absorber, can contribute to additional nitrosamine formation when accumulated amines react with residual NO_x. Nitramines are much less studied than nitrosamines in CO₂ capture systems. Mitigation strategies based on the reaction mechanisms in each unit of the CO₂ capture systems are reviewed. Lastly, we highlight research needs in clarifying reaction mechanisms, developing analytical methods for both liquid and gas phases, and integrating different units to quantitatively predict the accumulation and emission of nitrosamines and nitramines.

Selective Removal of Nitrosamines from a Model Amine Carbon-Capture Waterwash Using Low-Cost Activated-Carbon Sorbents

Nitrosamines generated in the amine solvent loop of postcombustion carbon capture systems are potent carcinogens, and their emission could pose a serious threat to the environment or human health. Nitrosamine emission control strategies are critical for the success of amine-based carbon capture as the technology approaches industrial-scale deployment. Waterwash systems have been used to control volatile and aerosol emissions, including nitrosamines, from carbon-capture plants, but it is still necessary to remove or destroy nitrosamines in the circulating waterwash to prevent their subsequent emission into the environment. In this study, a cost-effective method for selectively removing nitrosamines from the absorber waterwash effluent with activated-carbon sorbents was developed to reduce the environmental impact associated with amine-based carbon capture. The results show that the commercial activated-carbon sorbents tested have a high capacity and selectivity for nitrosamines over the parent solvent amines, with capacities up to 190 mg/g carbon, under simulated amine waterwash conditions. To further reduce costs, an aerobic thermal sorbent regeneration step was also examined due to the low thermal stability of nitrosamines. To model the effect of oxidation on the sorbent performance, thermal- and acid-oxidized sorbents were also prepared from the commercial sorbents and analyzed. The chemical and physical properties of nitrosamines, the parent amine, and the influence of the physical properties of the carbon sorbents on nitrosamine adsorption was examined. Key sorbent properties included the sorbent hydrophilicity and hydrophobicity, surface pK_a of the sorbent, and chemical structure of the parent amine and nitrosamine.

Nitrosamine Formation in Amine-Based CO2 Capture in the Absence of NO2: Molecular Modeling and Experimental Validation

A computational chemistry approach was used to elucidate and verify the different nitrosamine formation mechanisms and pathways. These included nitrosamine formation under acid or basic environments in the presence of NO, O₂, SO₂ and CO₂ without NO₂. The results clearly showed that nitrosamine could be formed without NO₂ via 2 different types of mechanisms, namely, addition and elimination forming N-N bond before proton transfer and proton transfer before N-N bond formation, respectively. The essence of these mechanisms identified in this work was that two reaction steps were required to complete both reaction mechanisms with different nitrosating agents. Two steps were both necessary neither of which could be neglected, if the nitrosamine formation reaction was to be completed. Computational simulation performed on the reactant, intermediate, transition state, and product for each set of reactions also validated the proposed mechanisms. Experiment also detected nitrosamine from the reaction of diethylamine and NO, SO₂, O₂, and CO₂ in both liquid and gas phase. Thus, NO₂ is not necessary for nitrosamine formation to occur in the CO₂ capture system.

Influence of Dissolved Metals on N-Nitrosamine Formation under Amine-based CO2 Capture Conditions

As the prime contender for postcombustion CO2 capture technology, amine-based scrubbing has to address the concerns over the formation of potentially carcinogenic N-nitrosamine byproducts from reactions between flue gas NOx and amine solvents. This bench-scale study evaluated the influence of dissolved metals on the potential to form total N-nitrosamines in the solvent within the absorber unit and upon a pressure-cooker treatment that mimics desorber conditions. Among six transition metals tested for the benchmark solvent monoethanolamine (MEA), dissolved Cu promoted total N-nitrosamine formation in the absorber unit at concentrations permitted in drinking water, but not the desorber unit. The Cu effect increased with oxygen concentration. Variation of the amine structural characteristics (amine order, steric hindrance, -OH group substitution and alkyl chain length) indicated that Cu promotes N-nitrosamine formation from primary amines with hydroxyl or carboxyl groups (amino acids), but not from secondary amines, tertiary amines, sterically hindered primary amines, or amines without oxygenated groups. Ethylenediaminetetraacetate (EDTA) suppressed the Cu effect. The results suggested that the catalytic effect of Cu may be associated with the oxidative degradation of primary amines in the absorber unit, a process known to produce a wide spectrum of secondary amine products that are more readily nitrosatable than the pristine primary amines, and that can form stable N-nitrosamines. This study highlighted an intriguing linkage between amine degradation (operational cost) and N-nitrosamine formation (health hazards), all of which are challenges for commercial-scale CO2 capture technology.

Controlling Nitrosamines, Nitramines, and Amines in Amine-Based CO₂ Capture Systems with Continuous Ultraviolet and Ozone Treatment of Washwater

Formation of nitrosamines and nitramines from reactions between flue gas NOx and the amines used in CO2 capture units has arisen as a significant concern. Washwater scrubbers can capture nitrosamines and nitramines. They can also capture amines, preventing formation of nitrosamines and nitramines downwind by amine reactions with ambient NOx. The continuous application of UV alone, or a combination of UV and ozone to the return line of a washwater treatment unit was evaluated to control the accumulation of nitrosamines, nitramines and amines in a laboratory-scale washwater unit. With model secondary amine solvents ranging from nonvolatile diethanolamine to volatile morpholine, application of 272-537 mJ/cm(2) UV incident fluence alone reduced the accumulation of nitrosamines and nitramines by approximately an order of magnitude. Modeling indicated that the gains achieved by UV treatment should increase over time, because UV treatment converts the time dependence of nitrosamine accumulation from a quadratic to a linear function. Ozone (21 mg/L) maintained low steady-state concentrations of amines in the washwater. While modeling indicated that more than 80% of nitrosamine accumulation in the washwater was associated with reaction of washwater amines with residual NOx, a reduction in nitrosamine accumulation rates due to ozone oxidation of amines was not fully realized because the ozonation products of amines reduced nitrosamine photolysis rates by competing for photons.

Amine-based CO2 capture technology development from the beginning of 2013-a review

It is generally accepted by the scientific community that anthropogenic CO2 emissions are leading to global climate change, notably an increase in global temperatures commonly referred to as global warming. The primary source of anthropogenic CO2 emissions is the combustion of fossil fuels for energy. As society's demand for energy increases and more CO2 is produced, it becomes imperative to decrease the amount emitted to the atmosphere. One promising approach to do this is to capture CO2 at the effluent of the combustion site, namely, power plants, in a process called postcombustion CO2 capture. Technologies to achieve this are heavily researched due in large part to the intuitive nature of removing CO2 from the stack gas and the ease in retrofitting existing CO2 sources with these technologies. As such, several reviews have been written on postcombustion CO2 capture. However, it is a fast-developing field, and the most recent review papers already do not include the state-of-the-art research. Notable among CO2 capture technologies are amine-based technologies. Amines are well-known for their reversible reactions with CO2, which make them ideal for the separation of CO2 from many CO2-containing gases, including flue gas. For this reason, this review will cover amine-based technology developed and published in and after the year 2013.

Amine reclaiming technologies in post-combustion carbon dioxide capture

Amine scrubbing is the most developed technology for carbon dioxide (CO2) capture. Degradation of amine solvents due to the presence of high levels of oxygen and other impurities in flue gas causes increasing costs and deterioration in long term performance, and therefore purification of the solvents is needed to overcome these problems. This review presents the reclaiming of amine solvents used for post combustion CO2 capture (PCC). Thermal reclaiming, ion exchange, and electrodialysis, although principally developed for sour gas sweetening, have also been tested for CO2 capture from flue gas. The three technologies all have their strengths and weaknesses, and further development is needed to reduce energy usage and costs. An expected future trend for amine reclamation is to focus on process integration of the current reclaiming technologies into the PCC process in order to drive down costs.

Comparative in vitro toxicity of nitrosamines and nitramines associated with amine-based carbon capture and storage

Amine-based CO2 capture is a prime contender for the first full-scale implementation of CO2 capture at fossil fuel-fired power plants postcombustion. However, the formation of potentially carcinogenic N-nitrosamines and N-nitramines from reactions of flue gas NOx with the amines presents a potential risk for contaminating airsheds and drinking water supplies. Setting regulatory emission limits is hampered by the dearth of toxicity information for the N-nitramines. This study employed quantitative in vitro bioassays for mutagenicity in Salmonella typhimurium, and chronic cytotoxicity and acute genotoxicity in Chinese hamster ovary (CHO) cells to compare the toxicity of analogous N-nitrosamines and N-nitramines relevant to CO2 capture. Although the rank order was similar for genotoxicity in CHO cells and mutagenicity in S. typhimurium, the Salmonella assay was far more sensitive. In general, mutagenicity was higher with S9 hepatic microsomal activation. The rank order of mutagenicity was N-nitrosodimethylamine (NDMA)>N-nitrosomorpholine>N-nitrodimethylamine>1,4-dinitrosopiperazine>N-nitromorpholine>1,4-dinitropiperazine>N-nitromonoethanolamine>N-nitrosodiethanolamine>N-nitrodiethanolamine. 1-Nitrosopiperazine and 1-nitropiperazine were not mutagenic. Overall, N-nitrosamines were ∼15-fold more mutagenic than their N-nitramine analogues.

Nitrosamine formation in amine scrubbing at desorber temperatures

Amine scrubbing is a thermodynamically efficient and industrially proven method for carbon capture, but amine solvents can nitrosate in the desorber, forming potentially carcinogenic nitrosamines. The kinetics of reactions involving nitrite and monoethanolamine (MEA), diethanolamine (DEA), methylethanolamine (MMEA), and methyldiethanolamine (MDEA) were determined under desorber conditions. The nitrosations of MEA, DEA, and MMEA are first order in nitrite, carbamate species, and hydronium ion. Nitrosation of MDEA, a tertiary amine, is not catalyzed by the addition of CO2 since it cannot form a stable carbamate. Concentrated and CO2 loaded MEA was blended with low concentrations of N-(2-hydroxyethyl) glycine (HeGly), hydroxyethyl-ethylenediamine (HEEDA), and DEA, secondary amines common in MEA degradation. Nitrosamine yield was proportional to the concentration of secondary amine and was a function of CO2 loading and temperature. Blends of tertiary amines with piperazine (PZ) showed n-nitrosopiperazine (MNPZ) yields close to unity, validating the slow nitrosation rates hypothesized for tertiary amines. These results provide a useful tool for estimating nitrosamine accumulation over a range of amine solvents.

Effects of flue gas compositions on nitrosamine and nitramine formation in postcombustion CO2 capture systems

Amine-based technologies are emerging as the prime contender for postcombustion CO2 capture. However, concerns have arisen over the health impacts of amine-based CO2 capture associated with the release of nitrosamines and nitramines, which are byproducts from the reactions between flue gas NOx and solvent amines. In this study, flue gas compositions were systematically varied to evaluate their effects on the formation of nitrosamines and nitramines in a lab-scale CO2 capture reactor with morpholine as a model solvent amine. The accumulation of N-nitrosomorpholine in both the absorber and washwater increased linearly with both NO and NO2 for concentrations up to ∼20 ppmv. These correlations could be extrapolated to estimate N-nitrosomorpholine accumulation at extremely low NOx levels (0.3 ppmv NO2 and 1.5 ppmv NO). NO played a particularly important role in driving N-nitrosomorpholine formation in the washwater, likely following partial oxidation to NO2 by O2. The accumulation of N-nitromorpholine in both the absorber and washwater positively correlated with flue gas NO2 concentration, but not with NO concentration. Both N-nitrosomorpholine and N-nitromorpholine accumulated fastest in the absence of CO2. Flue gas humidity did not affect nitrosamine accumulation in either the absorber or the washwater unit. These results provide a basis for estimating the effects of flue gas composition on nitrosamine and nitramine accumulation in postcombustion CO2 capture systems.

Decomposition of nitrosamines in CO2 capture by aqueous piperazine or monoethanolamine

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.

Influence of amine structural characteristics on N-nitrosamine formation potential relevant to postcombustion CO2 capture systems

Concerns have arisen for the possible contamination of air or drinking water supplies downwind of amine-based CO2 capture facilities by potentially carcinogenic N-nitrosamines formed from reactions between flue gas NOx and amine solvents. This study evaluated the influence of amine structure on the potential to form total N-nitrosamines within the absorber and washwater units of a laboratory-scale CO2 capture reactor, and in the solvent after a pressure-cooker treatment as a mimic of desorber conditions. Among 16 amines representing 3 amine classes (alkanolamines, straight-chain and cyclic diamines, and amino acids), the order of the amine was the primary determinant of total N-nitrosamine formation in the absorber unit, with total N-nitrosamine formation in the order: secondary amines ≈ tertiary amines ≫ primary amines. Similar results were observed upon pressure-cooker treatment, due to reactions between nitrite and amines at high temperature. For secondary and tertiary amines, total N-nitrosamine formation under these desorber-like conditions appeared to be more important than in the absorber, but for primary amines, significant formation of total N-nitrosamines was only observed in the absorber. For diamines and amino acids, total N-nitrosamine accumulation rates in washwaters were lowest for primary amines. For alkanolamines, however, total N-nitrosamine accumulation in the washwater was similar regardless of alkanolamine order, due to the combined effects of amine reactivity toward nitrosation and amine volatility. While total N-nitrosamine accumulation rates in washwaters were generally 1-2 orders of magnitude lower than in the absorber, they were comparable to absorber rates for several primary amines. Decarboxylation of the amino acid sarcosine resulted in the accumulation of significant concentrations of N-nitrosodimethylamine and N-nitrodimethylamine in the washwater.