Article pubs.acs.org/est
Effects of Flue Gas Compositions on Nitrosamine and Nitramine Formation in Postcombustion CO2 Capture Systems Ning Dai† and William A. Mitch*,‡ †
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020, United States
‡
S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION
Fossil-fuel-fired power plants generated 68% of the electricity in the United States in 20121 and contributed to 40% of anthropogenic CO2 emissions.2 Capturing and sequestering CO2 from these power plants is an important strategy to ameliorate global climate change.3 Amine-based CO2 absorption, widely used for CO2 removal from natural gas precombustion, is considered the most mature technology for postcombustion CO2 capture.4 In an amine-based CO2 capture system, power plant flue gas is passed through a countercurrent stream of a concentrated amine solution (“solvent”) within an absorber column. CO2 is absorbed into the liquid phase by reacting with unhindered primary and secondary amines to form a carbamate (e.g., eq 1 for morpholine (MOR)) or with tertiary amines and hindered primary and secondary amines to form bicarbonate (e.g., eq 2 for triethanolamine).5 The CO2loaded solvent is then routed to a desorber where high temperatures reverse these reactions, releasing high purity CO2 for compression and underground storage. The exhaust from the absorber is passed through a washwater unit to remove residual amines and other contaminants prior to atmospheric release. © 2014 American Chemical Society
The detection of potentially carcinogenic nitrosamines and nitramines in lab-scale and pilot-scale amine-based postcombustion CO2 capture systems has raised widespread concern due to their potential to contaminate downwind airsheds and drinking water supplies if emitted to the atmosphere.6−8 Nitrosamines and nitramines form from the reactions between NOx in the flue gas and amines in the absorber7,9 or between nitrite and amines under the high temperature conditions within desorber units.10 Nitrosamine and nitramine emissions at the level of 2−50 ng/Nm3 have been observed in a pilot Received: Revised: Accepted: Published: 7519
April 14, 2014 June 6, 2014 June 11, 2014 June 11, 2014 dx.doi.org/10.1021/es501864a | Environ. Sci. Technol. 2014, 48, 7519−7526
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system using monoethanolamine, the benchmark CO 2 absorption solvent.11 Total nitrosamine levels of 0.73 μM and 59 μM were detected in the washwater of a pilot system operated with 35% monoethanolamine or a mixture of 25% amino-2-methyl-1-propanol (AMP)/15% piperazine, respectively.7 Concerns over the release of nitrosamines and nitramines have hindered the full-scale implementation of amine technologies for postcombustion CO2 capture. Evaluation of the washwater unit is of particular importance when considering the environmental impacts of nitrosamine and nitramine formation from amine-based CO2 capture systems, as the exhaust from washwater units is directly emitted to the atmosphere. Previously, we reported that nitrosamine formation could occur within the washwater units from the reactions between the accumulated amines slipped from the absorber and residual NOx in the absorber exhaust.12 Nitrosamine and nitramine accumulation in the washwater unit could be controlled by applying to the washwater recycle line ultraviolet light and ozone, which destroys nitrosamines and nitramines, and accumulated amines, respectively.12 In addition to remediating washwater, preventing nitrosamine and nitramine formation in the absorber and desorber is also of critical importance. Previously, we studied the influence of solvent amine structure on nitrosamine formation by evaluating 16 amines featuring systematic structural variations within three amine categories being considered for CO2 capture (alkanolamines, diamines, and amino acids).13 Our results indicated that the order of the amine was the dominant determinant of their nitrosamine formation reactivity under conditions relevant to the absorber or desorber, with the following order: secondary > tertiary ≫ primary. Both the volatility of the amine and the amine order were important for nitrosamine accumulation within the washwater unit. The low volatility of secondary and tertiary alkanolamines reduced nitrosamine formation in the washwater by reducing the accumulation of amines in the washwater. The tendency of the secondary amino acid sarcosine to undergo decarboxylation to produce volatile dimethylamine resulted in a higher than expected accumulation of N-nitrosodimethylamine within the washwater. This study systematically investigated the effects of varying NOx, O2, and CO2 concentrations on the formation of nitrosamines and nitramines in a lab-scale CO2 capture reactor composed of absorber and washwater units. NO and NO2 are sources of nitrosating and nitrating agents. NO2 is particularly important, because it may directly initiate nitrosation reactions14 or dimerize to form N2O4 (eq 3), which has both nitrosating and nitrating capabilities.9 Alternatively, NO2 reacts with NO to form N2O3 (eq 4), a potent nitrosating agent.9 O2 also plays an important role. Biomedical research has indicated that, even in the absence of NO2, NO can nitrosate amines in the presence of O2 at neutral pH, presumably due to partial formation of NO2 or other NO oxidation products.14−16 Nitrite, the hydrolysis product of NOx (eqs 5 and 6), did not contribute to significant nitrosamine formation in concentrated morpholine solutions at temperatures relevant to absorbers (e.g., 45 °C)17 but formed nitrosamines in piperazine solutions at almost 100% yield at temperatures relevant to desorbers (e.g., 120 °C).10 2 •NO2 ↔ N2O4
NO + •NO2 ↔ N2O3
(4)
N2O3 + H 2O → 2 NO−2 + 2H+
(5)
N2O4 + H 2O → NO−2 + NO−3 + 2H+
(6)
Carbon dioxide, the main species of interest in CO2 capture systems, may also affect nitrosamine and nitramine formation by interacting with nitrosating/nitrating agents or amines, respectively. Theoretical calculations suggested that CO2 catalyzes nitrosation reactions by nitrite,18,19 the nitrosating agent in the desorber. These calculations agree with experimental data showing that the nitrosation rate of piperazine increased with CO2 loading under simulated desorber conditions.10 Although CO2 does not directly react with strong nitrosating agents, such as N2O3, present in the absorber,19 its hydration product bicarbonate (eq 2) scavenges N2O3, as shown by both experimental results20 and theoretical calculations,19 and may thereby reduce nitrosation. In addition, in primary and secondary amine-based solvents, CO2 forms a carbamate with the amines (eq 1),5 lowering the reactivity of the amine group toward nitrosating and nitrating agents by reducing the electron density on the nitrogen and increasing steric hindrance.15 The inhibitory effects of CO2 on Nnitrosomorpholine (NMOR) formation was observed with CO2 loadings up to 0.36 C/N when 100 ppm of NO2 was purged through 100 mL of 5 M morpholine solution at 30 °C with a flow rate of 0.94 L/min.17
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MATERIALS AND METHODS Materials and Sample Analysis. Morpholine (MOR) was used as a model CO2 absorption solvent because MOR resists degradation to other nitrosatable or nitratable products under absorber-relevant conditions7 and, as a secondary amine, features nitrosamine formation potential higher than that of tertiary and primary amines.13 Sources and purities of chemicals and analytical methods for sample analysis are provided in the Supporting Information. Briefly, nitrite was measured by the N(1-naphthyl)ethylenediamine dihydrochloride colorimetric method.21 NMOR in both the absorber solvent and washwater and N-nitromorpholine (NO2-MOR) in the absorber solvent were measured by high performance liquid chromatography with UV detection. NO2-MOR in the washwater was measured by gas chromatography with tandem mass spectrometry. Laboratory-Scale CO2 Capture System. The system consists of an absorber column and a washwater column. Both columns are 5 cm diameter × 40 cm length borosilicate glass and contain one-stage 6 mm borosilicate glass Raschig rings. A 2.5 M MOR solution (200 mL, “solvent”) was circulated through the absorber unit at 15 mL/min in a countercurrent fashion. The absorber column and solvent reservoir were maintained at 40 °C during reactor operation. The amine solutions were self-buffered without additional pH adjustment. Deionized water (500 mL) without added buffer was circulated through the washwater unit at 30 mL/min at room temperature in a countercurrent fashion. The synthetic flue gas was constituted by N2, O2, CO2, NO, NO2, and water vapor to make up a total flow of 6 L/min. The 6.5% humidity was set by purging the N2, O2, and CO2 mixture through a prewetting column containing deionized water at 40 °C prior to the addition of NO and NO2. Each gas species was varied individually while others were maintained at the following baseline levels: 15 ppmv NO, 3 ppmv NO2, 14% O2, 3.3% CO2,
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and 6.5% H2O. This baseline gas composition was selected to represent typical flue gas from natural gas power plants with NOx removal prior to CO2 capture. N2 was used to make up the volumetric flow rate when the O2 or CO2 content was varied. Twenty different flue gas conditions were tested in total. To evaluate experimental reproducibility, three of the conditions (15%) were tested in duplicate. The average percent difference between replicate results for the accumulation rates of NMOR and nitrite in the absorber solvent and washwater was 11%, and the maximum percentage difference was 19%, observed for the accumulation rate of nitrite in the washwater in the absence of oxygen. Because of the lack of desorber unit, the system was preequilibrated with CO2 without NOx for 1 h, after which the NOx was turned on. The CO2 remained on throughout the experiment. Solvent and washwater samples were collected at various time points after NOx addition and analyzed for nitrite and NMOR. The absorber solvent and washwater samples collected at the end of the experimental runs were also analyzed for NO2-MOR. Because the volume of the solvent and washwater could change over the course of the experiment (e.g., due to volatilization or accumulation of amines and water vapor), the volume changes in solvent and washwater reservoirs were monitored to allow the calculation of mass accumulation rates for the analytes.
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RESULTS Effects of NO2. Four NO2 concentrations were tested (0, 3, 6, 15 ppmv), while the other gas species remained at their baseline levels (i.e., 15 ppmv NO, 14% O2, 3.3% CO2, and 6.5% H2O). Within the experimental periods, NMOR concentration increased linearly in the absorber solvent with time (Supporting Figure SI-1A), but increased at slightly accelerated rates in the washwater (Supporting Figure SI-1B) as the accumulation of amines promoted NMOR formation within the washwater unit.12 Accumulation rates were derived by linear regression of the time series data. In turn, the dependencies of NMOR accumulation rates in the solvent and washwater on NO2 concentration were determined by linear regression of the accumulation rates on the four NO2 concentrations (Figure 1A); because the y-intercepts of the linear regressions were not significantly different from zero (p > 0.1; Supporting Table SI1), the equations were derived by forcing the y-intercepts through the origin. In both the solvent and the washwater, NMOR accumulation rates increased linearly with flue gas NO2 concentration. NMOR accumulation rates in the solvent were approximately 1 order of magnitude greater than in the washwater. N-Nitromorpholine (NO2-MOR) formed at 2 orders of magnitude lower levels than NMOR and therefore often was detected only in the final sample collected at the end of the experimental runs. Figure 1B provides the accumulation rates assuming linear accumulation of NO2-MOR. Similar to NMOR, NO2-MOR accumulation rates in both the solvent and the washwater increased with increasing flue gas NO2 concentration, and accumulation in the solvent was nearly an order of magnitude higher than in the washwater. With 0 ppmv NO2, NO2-MOR concentrations in the final solvent and washwater sample were below the detection limit, corresponding to accumulation rates lower than 4 × 10−3 and 2 × 10−5 nmol/s, respectively. Nitrite was the main hydrolysis product of NOx (eqs 5 and 6) under the flue gas conditions tested in this study. For
Figure 1. Effect of NO2 concentrations in the synthetic flue gas on the accumulation rates of (A) N-nitrosomorpholine, (B) N-nitromorpholine, and (C) nitrite in the absorber solvent and the washwater with 15 ppmv NO, 14% O2, 3.3% CO2, 6.5% H2O. Error bars are derived from the linear regression of time series data, except for those of the baseline condition (i.e., 3 ppmv NO2), which represent 1 SD of duplicate runs. Linear regressions for NMOR accumulation rates were derived without using data for 0.3 ppmv NO2.
example, the nitrate concentration measured in the solvent under the baseline flue gas concentration was less than 15% of the nitrite concentration. Nitrite accumulated linearly with time in both the solvent and the washwater (Supporting Figure SI1C and 1D), and its accumulation rates increased linearly with flue gas NO2 concentration (Figure 1C). Effects of NO. Four NO concentrations were tested (0, 5, 15, 22.5 ppmv), while the other gas species remained at their baseline levels (i.e., 3 ppmv NO2, 14% O2, 3.3% CO2, and 6.5% H2O). Within the experimental periods, NMOR concentration increased linearly in the absorber solvent with time (Supporting Figure SI-2A) but increased at slightly accelerated rates in the washwater (Supporting Figure SI-2B) as the accumulation of amines promoted NMOR formation within the washwater unit.12 Accumulation rates were derived by linear regression of the time series data. For 0 ppmv NO, NMOR was detectable only in the last sample, and the accumulation rates were derived assuming linear accumulation. The dependencies of NMOR 7521
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understand whether the NOx dependencies observed at higher NOx concentrations remain applicable under lower NOx conditions. In separate experiments, NO or NO2 was reduced to 1.5 or 0.3 ppmv, 10% of their baseline concentrations, respectively, while maintaining the other gas constituents at their baseline concentrations. The NMOR accumulation rates measured in the absorber solvent were within 7% and 22% of the values predicted from the NO2 and NO correlations in Figures 1A and 2A (Supporting Table SI-2). The observed NMOR accumulation rates in the washwater under low NO2 or NO conditions were higher than predicted values by 41% and 20%, respectively (Supporting Table SI-2). These greater differences likely result from the greater deviations from linear accumulation of NMOR over time in the washwater under these low NOx conditions (Supporting Figure SI-3). Next, both NO and NO2 were reduced to 1.5 and 0.3 ppmv within the same experiment. The NMOR accumulation rates in the solvent and washwater were predicted for this experiment in the following manner. First, the NMOR accumulation rates for 1.5 ppmv NO and 3 ppmv NO2 were predicted using the correlations developed in Figure 2A to capture the effect of the reduction in NO concentration. These predicted accumulation rates were then divided by 10 to estimate the effect of reducing NO2 concentrations from 3 to 0.3 ppmv, because the correlations for varying NO2 concentrations were linear and featured no significant y-intercepts. These predicted values were roughly 50% lower than the NMOR accumulation rates measured experimentally in the absorber solvent and washwater (Supporting Table SI-2). Although there were greater discrepancies between observed and predicted NMOR accumulation rates at extremely low NOx conditions, the results suggest that the correlations can be used to estimate rates over wide ranges of NOx concentrations. When NO was maintained at 15 ppmv while NO2 was completely removed from the flue gas, the NMOR accumulation rate in the solvent dropped by a factor of 30 and in the washwater by a factor of 16 (Table 1). In contrast, when NO2
accumulation rates in the solvent and washwater on NO concentration were determined by linear regression of the accumulation rates on the four NO concentrations (Figure 2A).
Table 1. Effects of NO and NO2 on NMOR Accumulation Rates
Figure 2. Effect of NO concentrations in the synthetic flue gas on the accumulation rates of (A) N-nitrosomorpholine, (B) N-nitromorpholine, and (C) nitrite in the absorber solvent and the washwater for 3 ppmv NO2, 14% O2, 3.3% CO2, 6.5% H2O. Error bars are derived from the linear regression of time series data, except for those of the baseline condition (i.e., 15 ppmv NO), which represent 1 SD of duplicate runs. Linear regressions for NMOR accumulation rates were derived without using data for 1.5 ppmv NO.
NOx (ppmv)
NMOR accumulation rates (nmol/s)
NO
NO2
O2 (%)
solvent
0 15 15 15
3 3 0 0
14 14 14 0
2.0 × 10−1 1.1 3.7 × 10−2 9.2 × 10−3
washwater 9.2 1.4 8.7 5.8
× × × ×
10−4 10−1 10−3 10−4
was maintained at 3 ppmv while NO was removed from the flue gas, the NMOR accumulation rate in the absorber solvent decreased by a factor of 5, but the accumulation rate in the washwater dropped by more than 2 orders of magnitude. These results suggest that NO2 is important for driving nitrosamine formation in both the solvent and the washwater, while NO has a more significant contribution to NMOR formation within the washwater than within the solvent. NO2-MOR was not detected (
