Article pubs.acs.org/est
Measurement of Nitrosamine and Nitramine Formation from NOx Reactions with Amines during Amine-Based Carbon Dioxide Capture for Postcombustion Carbon Sequestration Ning Dai,† Amisha D. Shah,† Lanhua Hu,† Michael J. Plewa,‡ Bruce McKague,§ and William A. Mitch*,† †
Department of Chemical and Environmental Engineering, Yale University, Mason Lab 313b, 9 Hillhouse Avenue, New Haven, Connecticut 06520, United States ‡ Department of Crop Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States § CanSyn Chemical Corporation, Toronto, Canada M5S 3E5 S Supporting Information *
ABSTRACT: With years of full-scale experience for precombustion CO2 capture, amine-based technologies are emerging as the prime contender for postcombustion CO2 capture. However, concerns for postcombustion applications have focused on the possible contamination of air or drinking water supplies downwind by potentially carcinogenic N-nitrosamines and N-nitramines released following their formation by NOx reactions with amines within the capture unit. Analytical methods for N-nitrosamines in drinking waters were adapted to measure specific Nnitrosamines and N-nitramines and total N-nitrosamines in solvent and washwater samples. The high levels of amines, aldehydes, and nitrite in these samples presented a risk for the artifactual formation of N-nitrosamines during sample storage or analysis. Application of a 30-fold molar excess of sulfamic acid to nitrite at pH 2 destroyed nitrite with no significant risk of artifactual nitrosation of amines. Analysis of aqueous morpholine solutions purged with different gas-phase NO and NO2 concentrations indicated that N-nitrosamine formation generally exceeds N-nitramine formation. The total N-nitrosamine formation rate was at least an order of magnitude higher for the secondary amine piperazine (PZ) than for the primary amines 2amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA) and the tertiary amine methyldiethanolamine (MDEA). Analysis of pilot washwater samples indicated a 59 μM total N-nitrosamine concentration for a system operated with a 25% AMP/15% PZ solvent, but only 0.73 μM for a 35% MEA solvent. Unfortunately, a greater fraction of the total N-nitrosamine signal was uncharacterized for the MEA-associated washwater. At a 0.73 μM total N-nitrosamine concentration, a ∼25000-fold reduction in concentration is needed between washwater units and downwind drinking water supplies to meet proposed permit limits.
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INTRODUCTION Generation of electricity and steam from fossil fuel-fired power plants accounted for 41% of global carbon dioxide (CO2) emissions in 2008.1 With fossil fuels providing ∼75% of U.S. power generation capacity, conversion to alternative energy supplies is a long-term venture. Postcombustion carbon sequestration, the removal and underground storage of CO2 from the exhaust gases of fossil fuel-fired power plants, likely will play a vital role in reducing greenhouse gas emissions over the next several decades. The Norwegian government and Statoil are planning the development of one of the first industrial-scale postcombustion CO2 capture and storage projects (CO2 Capture Mongstad project, CCM). The CCM project would at full capacity capture ∼1.2 million metric tons of CO2/year from the exhaust gases of a combined heat and power plant at the Mongstad refinery in Norway. Preeminent among CO 2 capture technologies are amine-based technologies.2 Briefly, flue gases are passed through an “absorber” column with a countercurrent © 2012 American Chemical Society
amine-based solvent (e.g., 30% by weight monoethanolamine (MEA) in water). Following solvent partitioning, the CO2 forms a carbamate complex with the amine (Scheme 1). Because the amine’s lone electron pair is involved in carbamate formation, the solvent pH generally is greater than the amine pKa (e.g., 9.5 for MEA). The amine- and carbamate-containing solution proceeds to a second “desorber” column, where high Scheme 1
Received: Revised: Accepted: Published: 9793
May 9, 2012 July 18, 2012 July 25, 2012 July 25, 2012 dx.doi.org/10.1021/es301867b | Environ. Sci. Technol. 2012, 46, 9793−9801
Environmental Science & Technology
Article
First, nitrosamines and nitramines formed within the aminebased solvent could be stripped into the washwater and thence transferred back into the exhaust gas. Second, nitrosamines and nitramines could form downwind if amines released in the exhaust gas react with ambient NOx.15−17 Nitrosamines are potentially potent carcinogens.18−20 The U.S. EPA Integrated Risk Information System database indicates for several nitrosamines that drinking water concentrations in the low nanogram per liter level are associated with a 10−6 lifetime excess cancer risk.21 Several nitrosamines are listed in the U.S. EPA’s Contaminant Candidate List 3 for possible regulation in drinking water,22 and the U.S. EPA is considering applying a group regulation to nitrosamines according to its new Drinking Water Strategy.23 For the Mongstad project, the Norwegian Climate and Pollution Agency has issued a permit prohibiting the sum of all N-nitrosamine and N-nitramine concentrations from exceeding 0.3 ng/m3 in air or 4 ng/L in waters downwind of the plant.24 In previous research, 2.9 mM total nitrosamines were detected in the MEA-based solvent of a small-scale postcombustion capture facility at a coal-fired power plant.7 Up to 5.3 μM N-nitrosodiethanolamine was detected within an MEA-based solvent at a pilot facility lacking a desorber unit after 5 d of operation, but the concentration increased to 190 μM when the solvent was autoclaved to mimic desorber conditions.14 The first objective of this study was to develop methods to quantify specific N-nitrosamines and N-nitramines, as well as total N-nitrosamines in aqueous solutions associated with amine-based carbon capture. Analytical methods for specific and total N-nitrosamines were adapted from existing drinking water methods. However, CO2 capture sample matrixes are challenging. Pilot washwater samples can exhibit up to ∼10 mM amines and ∼100 μM nitrite and formaldehyde (see below), while drinking waters generally contain
