Human and Environmental Impact Assessment of Postcombustion

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Environ. Sci. Technol. 2010, 44, 1496–1502

Human and Environmental Impact Assessment of Postcombustion CO2 Capture Focusing on Emissions from Amine-Based Scrubbing Solvents to Air KARIN VELTMAN,* BHAWNA SINGH, AND EDGAR G. HERTWICH Industrial Ecology Programme and Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Høgskoleringen 5, NO-7491, Trondheim, Norway

Received July 15, 2009. Revised manuscript received October 5, 2009. Accepted December 18, 2009.

Carbon Capture and Storage (CCS) has become a key technology in climate change mitigation programs worldwide. CCS is well-studied in terms of greenhouse gas emission reduction potential and cost of implementation. Impacts on human health and the environment have, however, received considerably less attention. In this work, we present a first assessment of human health and environmental impacts of a postcombustion CO2 capture facility, focusing on emissions from amine-based scrubbing solvents and their degradation products to air. We develop characterization factors for human toxicity for monoethanolamine (MEA) as these were not yet available. On the basis of the limited information available, our assessment indicates that amine-based scrubbing results in a 10-fold increase in toxic impact on freshwater ecosystems and a minor increase in toxic impacts on terrestrial ecosystems. These increases are attributed to emissions of monoethanolamine. For all other impact categories, i.e., human toxicity, marine ecotoxicity, particulate matter formation, photochemical oxidant formation, and terrestrial acidification, the CO2 capture facility performs equally well to a conventional NGCC power plant, albeit substantial changes in flue gas composition. The oxidative degradation products of MEA, i.e., formaldehyde, acetaldehyde, and ammonia, do not contribute significantly to total environmental impacts.

Introduction Capture and storage of CO2 (CCS) has emerged as a promising technology to mitigate global warming by effectively reducing carbon dioxide emissions from large point sources, such as fossil fuel power plants. It is promoted as a “bridging”technology to smooth the transition from today’s fossilenergy based system to a future sustainable energy system (1-3). Carbon dioxide capture is relatively well-studied in terms of power generation efficiency (4, 5), CO2 emission reduction (3, 6, 7), and cost of implementation (4, 5, 8), but little is known about the potential impacts on human health and the environment. It is of utmost importance to quantify these impacts to identify potential trade-offs. In the present * Corresponding author phone: +47 -735 98955; e-mail: [email protected]. 1496

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work, we therefore provide a human and environmental impact assessment of a postcombustion CO2 capture facility, focusing on emissions from amine-based scrubbing solvents. Carbon capture and storage involves the separation of CO2 from industrial and energy-related sources and the subsequent sequestering of concentrated CO2 in secure storage locations, as onshore and offshore geological formations (oil and gas fields, saline aquifers), ocean storage, and industrial fixation into inorganic carbonates (3). In postcombustion capture, CO2 is removed from the flue gas at the end of a power plant cycle. Currently, amine-based scrubbing is the only commercially available technology to capture CO2 from dilute atmospheric pressure gas (9). The industrially most important and well-studied amine-based scrubbing solvent is monoethanolamine (MEA) (10). The advantage of MEA compared to other solvents is the high reaction rate and the capability of removing even traces of CO2. In an ideal MEA-CO2 absorption system, the solvent is continuously recycled and reused. However, monoethanolamine has a relatively high vapor pressure, which results in significant vaporization and solvent loss (10). Additionally, MEA has a high propensity to degrade and is known to react with flue gas impurities as SO2 and NO2, and with O2 (10, 11). The latter reaction may be particularly important for natural gas combined cycles (NGCC), as gas turbines burn natural gas with a high rate of excess air in order to control combustion temperatures (12). Solvent degradation is a major concern for three reasons: first, it results in a loss of CO2scrubbing capacity and fresh amine must be continually added to the process at a significant cost. Second, the formation of degradation products gives rise to a number of operating problems including equipment corrosion, foaming, and increased viscosity of the solvent (11, 13). Third, MEA degradation may result in increased environmental impacts, as volatile degradation products are emitted to air with flue gas exhaust. Also, nonvolatile degradation products must be removed from the solvent. These nonvolatile degradation products are commonly separated in an evaporative reclaimer and are treated as hazardous chemical waste (14). Although it is widely recognized that it is extremely important to assess impacts of MEA-related emissions on human health and the environment, including both emissions of the parent solvent and its degradation products (11, 15), such an analysis has not yet been performed. This is mainly due to a lack of quantitative information on emissions of MEA and degradation products (7, 15). Additionally, human toxicity characterization factors are not available for MEA, which hampers the performance of a life cycle impact assessment for CO2 capture (15). Therefore, the aim of the present study is two-fold: (1) To characterize and quantify emissions of MEA and degradation products; and (2) To estimate human and environmental impacts of solvent-related emissions at a CCS-facility. The methodology of life-cycle impact assessment (LCIA) is followed to assess human and environmental impacts of a postcombustion power plant compared to a conventional power plant. LCIA relies on chemical-specific characterization factors (CF) that combine exposure potential and toxicity to represent the relative contribution of the chemical to human health and environmental impacts. As human toxicity characterization factors were not available for monoethanolamine, these were developed for the present study according standardized, well-established LCIA methods (16-18). Environmental emissions were quantified for three scenarios: (1) a conventional 420 MW natural gas combined 10.1021/es902116r

 2010 American Chemical Society

Published on Web 01/22/2010

dustrial) reports (9, 12, 21-26) and literature (7, 10, 11, 14, 20) and selected relevant data that refer to MEA-based scrubbing systems and natural gas fired power plants. Second, we setup a mass-balance of MEA to evaluate the consistency of this data. Third, we quantified emissions of MEA, degradation products, and traditional flue gas components, as SO2, based on the mass balance and known chemical reaction mechanisms. MEA-Mass Balance. As a starting point for the MEA-mass balance, we use the framework presented by Rao et al. (25). This framework is expanded to include O2-based degradation, MEA loss in the water-wash section, and MEA loss with reclaimer waste (eq 1)

FIGURE 1. Process flow diagram for CO2 capture with aminebased absorption/stripping for a natural gas combined cycle (12, 19). cycle (NGCC), representing the reference case; (2) a conventional 420 MW coal-fired power plant; and (3) a 420 MW NGCC with capture. Amine-based scrubbing is a relatively new technology for the abatement of CO2 from fossil fuelfired power plants. Inherent to new technologies, reliable data on process design, environmental emissions, and toxicity was observed to be scarce or nonexistent. We therefore performed a sensitivity analysis to check the robustness of our conclusions. Additionally, we identified key topics associated with MEA impact assessment that require further research.

Methods CO2 Capture: Process Description. Here, a brief description is provided of postcombustion CO2 capture with amine-based solvents for a natural gas combined cycle (NGCC). More detailed descriptions can be found in Goff (19) and Peeters et al. (5). In a typical absorber/stripper system (Figure 1), a flue gas stream with ∼4% of CO2 and ∼13% of O2 enters the bottom of the absorber and is counter-currently contacted with 15 to 30 wt % lean MEA (12, 19). The MEA solution absorbs CO2 from the flue gas and runs down to the bottom of the tower, while “clean” flue gas (containing 10-15% of original CO2-concentration) escapes at the top. The CO2MEA solution is pumped through a cross heat exchanger and subsequently introduced into the top of the stripper, where it counter-currently contacts steam at 120 °C and 2 bar. The steam, produced in a reboiler, provides the necessary energy to reverse the reaction of the amine with CO2 and strips the acid gas from the amine solution. The gas leaving the stripper contains CO2 and water and can be dehydrated and compressed before being sequestered (11). After the CO2 is stripped, the lean MEA is cooled down in the heat exchanger before it is recycled back to the absorber. A reclaimer takes a slip stream from the stripper bottoms in order to avoid accumulation of nonvolatile degradation products in the sorbent stream. The reclaimer waste is disposed of as hazardous chemical waste (14). More volatile degradation products can be emitted to air with flue gas exhaust. To reduce emissions of these volatile organic compounds and of water vapor, a water wash and/or mist eliminator is commonly installed at the top section of the absorber (20). Quantification of Environmental Emissions. At present, MEA-based scrubbing technologies have not been applied on large scale to modern power plants. Publicly available emission-data of MEA and its degradation products is scarce and often inconsistent. We therefore took the following approach to quantify environmental emissions of MEA and MEA-based degradation products: first, we reviewed (in-

net loss of MEA ) (flue gas exhaust) + (oxidative degradation) + (heat stable salt formation gain in reclaimer) + (reclaimer waste) + (water-wash) + (polymerization) (1) The reported total solvent loss ranges from 1.6 to 3.1 kgsolvent per tonne CO2 captured (Table 1), with the lower value representing a well-managed power plant (22). The average capture efficiency of CO2 is 90% (22), which results in capture of 1.3 × 106 tonne CO2/year for a 420 MW NGCC (SI) and a (geometric mean) solvent loss of 2.8 × 106 kg · yr-1 (1.6 mol · s-1) (Table 1). A part of the MEA loss is caused by emission to air with flue gas exhaust. A typical emission rate is 8.0 × 104 kg · yr-1 for a NGCC with water-wash (12). Reaction of MEA with O2 and acid gas impurities as SO2 and NO2 presents another pathway of solvent loss (11, 25, 27). The loss due to reaction with SO2 and NO2 has been estimated according Rao et al. (21): [MEA]deg ) 2 · fSO2 · [SO2] + 2 · fNO2 · [NO2]

(2)

where [MEA]deg is the amount of MEA degraded (in mol · s-1), fSO2 is the reaction efficiency of SO2 with MEA (99,5%) (21), fNO2 is the reaction efficiency of NO2 with MEA (25%) (21), [SO2] is the amount of SO2 in flue gas (mol · s-1), and [NO2] is the amount of NO2 in flue gas (mol · s-1). The MEA degradation rate due to reaction with oxygen is obtained from a large scale laboratory study by Goff and Rochelle (11). Goff and Rochelle (11) obtained an MEA degradation rate of 0.29-0.73 kg/tonne of CO2 captured based on flue gas containing 3% CO2 and 5% O2. Using a geometric mean degradation rate of 0.46 kg of MEA/tonne of CO2, this results in an O2-induced degradation rate of 3.4 × 10-1 mol · s-1. The total MEA loss due to reaction with SO2, NO2 and O2 is collectively termed “oxidative degradation” and sums up to 3.9 × 10-1 mol · s-1. The organic acids formed due to oxidative degradation react further with a new MEA-molecule to form heat-stable salts (HSS) (11). The exact nature of these salts is unknown. The most conservative estimate is that each mole of organic acid takes up one mole of fresh MEA (25). HSS-formation is a reversible reaction and caustic soda (NaOH) is added to the reclaimer to regenerate part of the MEA from the heatstable salts. Each mole of NaOH regenerates 1 mol of MEA, and adds the corresponding sodium salt of organic acid to the reclaimer bottoms (25). The net loss of MEA due to HSSformation can be estimated as follows: [MEA]netloss,HSS ) [organic acids] - [Na]

(3)

where [MEA]net loss, HSS is the net MEA loss due to heat-stable salt (HSS) formation (mol · s-1), [organic acids] is the amount of organic acids formed due to MEA reaction with SO2, NO2, and O2 (mol · s-1), and [Na] is the amount of sodium added to reclaimer to regenerate MEA (mol · s-1). A typical value of caustic added to the reclaimer is 0.13 kg NaOH /tonne CO2 (25). This corresponds to a sodium VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Monoethanolamine Loss-Mass-Balance parameter

value

unit

value -1

kg · yr

unit

reference -1

total

2.8 × 10

oxidative degradation (O2, NO2 and SO2) polymerization emission to air with flue gas exhaust (with water-wash) water-wash heat stable salts (net loss) reclaimer waste

6.9 × 105 kg · yr-1 3.9 × 10-1 mol · s-1 this study

6

1.2

mol · s

literature (range)

1.7 × 105 kg · yr-1 9.7 × 10-2 mol · s-1 this study 8.0 × 104 kg · yr-1 4.5 × 10-2 mol · s-1 (12)

reference

-1

1.8 × 10 -3.1 × 10 kg · yr (24) (22)(14)b (23) 2.1 × 106 kg · yr-1 2.8 × 106 kg · yr-1 2.1 × 106 - 3.9 × 106 kg · yr-1 5.8 × 105 kg · yr-1 (33) 6

6

a

4 × 104 - 1.6 × 105 kg · yr-1 1.6 × 104 kg · yr-1

(12) (21)

4.2 × 105 kg · yr-1c

(14)

1.5 × 106 kg · yr-1 8.6 × 10-1 mol · s-1 This study 1.0 × 105 kg · yr-1 5.8 × 10-2 mol · s-1 (25) 1.8 × 105 kg · yr-1 1.0 × 10-1 mol · s-1 (21)

a

b

400 MW pulverized bituminous coal fired power plant. Coal fired power plant. c The amount of MEA in reclaimer waste was estimated from GC-chromatography results from Strazisar et al. (14) assuming that the area under the peak is indicative for the mass of chemical present (Supporting Information). The total reclaimer waste is assumed to be 1.1 kg waste/tonne CO2 captured () 1.4 × 106 kg · yr-1) (9). Strazisar et al. (14) identified chemical compounds in the reclaimer bottom of a coal-fired power plant using MEA as a scrubbing solvent.

amount of 0.93 mol · s-1 and an equal amount of reclaimed MEA. The total amount of organic acids formed is estimated to be 2.7 × 10-1 mol · s-1 (see “solvent degradation”). This results in a net MEA loss due to HSS formation of 2.7 × 10-1 mol · s-1. Additionally, some MEA will be lost with the slip stream in reclaimer waste (14, 21). According to a report on CO2 capture by Fluor/Statoil (21) the amount of MEA lost with reclaimer waste is 1.0 × 10-1 mol · s-1. This value is in close correspondence with the calculated value based on Strazisar et al. (14) for a coal-fired power plant. A water-wash section is commonly included at the top of the absorber to reduce emissions of MEA and volatile degradation products (12, 20). This presents a pathway of loss, if MEA in the water-wash section is not recovered. The typical removal efficiency of wet scrubbers for volatile organic compounds ranges between 70 and 99% (28, 29). We used a 95% efficiency to estimate MEA loss in the water-wash (Table 1). Polymerization forms the last route of MEA loss. It is initiated by reaction of monoethanolamine with CO2 and results in the formation of various high molecular weight products (11, 13, 14). The extent of reaction occurrence has not been quantified under “real” power plant conditions, although it is thought that oxidative degradation is a more significant source of solvent degradation than carbamate polymerization (14, 27). To close the MEA mass-balance, we here estimate a MEA-loss due to polymerization of 9.7 × 10-2 mol · s-1. Solvent Degradation. Different degradation products result from three degradation pathways of amine solvents (SI) (11): (1) Thermal degradation; (2) Carbamate polymerization; and (3) Oxidative degradation due to reaction with O2, SO2 and NO2 Thermal degradation only occurs at temperatures in excess of 200 °C and is assumed to be negligible in flue gas applications (11). Carbamate polymerization occurs at stripper conditions in the presence of CO2 and results in the formation of nonvolatile high molecular weight products (11, 13). It is initiated by the formation of 2-oxazolidone, which can react further with another MEA-molecule to N-(2-hydroxyethyl)ethylenediamine, via intermediates of N,N′-di(hydroxyethyl)urea and 1-(2-(hydroxyethyl)-2-imidazolidinone (14) (Supporting Information). These nonvolatile degradation products are removed from the MEA-sorbent by taking a slip stream 1498

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from the stripper to the reclaimer. The degradation rate due to polymerization is estimated to be 5.0 × 10-1 mol · s-1 (Table 1). Oxidative degradation occurs by reaction with flue gas impurities such as SO2, NO2, and O2 and results in fragmentation of the amine solvent (11, 30). Oxidative degradation is a liquid phase reaction, i.e., O2, SO2, and NO2 have to diffuse into the solvent before reaction can occur (27, 19). Volatile degradation products can evaporate from the MEAsolution and escape with exhaust flue gas to air, whereas less volatile degradation products will be removed by the reclaimer. The oxidative degradation rate, the degradation pathways and formation of degradation products, depend on several operating conditions as the concentration of acid gases in the flue gas, the temperature and pressure, and the presence of corrosion inhibitors (10, 27). Oxidative degradation by O2 is catalyzed by the presence of dissolved metals such as iron or copper (11). The primary degradation product is ammonia (NH3) and each mole of MEA degraded results in formation of 1 mol of ammonia (eq 4). Aldehydes, including formaldehyde and acetaldehyde, are formed as reaction intermediates (11, 19) (eq 4) (Table 1). MEA + RO2 f NH3 + β[aldehyde]

(4)

where MEA is the amount of monoethanolamine (mol); R is the 0 for acetaldehyde, and 1 for formaldehyde; β is the 1 for acetaldehyde, and 2 for formaldehyde; and [aldehyde] is the amount of acetaldehyde and formaldehyde formed (mol). These aldehydes can react further with oxygen to form carboxylic acids, as formic acid, glycolic acid, acetic acid, and oxalic acid (11). These carboxylic acids are nonvolatile, i.e., they have a low vapor pressure, and have been detected in the liquid phase of the solvent (28). These degradation products will end up in the reclaimer and emission to air is negligible. In contrast, NH3, formaldehyde, acetaldehyde, and MEA have a high vapor pressure and have been detected in the vapor phase of the absorber (31) and can potentially be emitted to air with flue gas exhaust. Carboxylic acids are not final degradation products; they can react further with MEA to form heat-stable salts, or with other degradation products to form a wide variety of high molecular weight products (10). The MEA degradation rate induced by O2 is 3.4 × 10-1 mol · s-1 (11). Each mole of MEA degraded results in one mole of NH3 formed and therefore the NH3 formation rate equals 3.4 × 10-1 mol · s-1 (eq 4). It is not known which aldehyde,

TABLE 2. Emissions to Air for a 420 MW NGCC with Post-Combustion Capture of CO2 chemical

value

unit

reference -1

CO2 SO2 NO2 MEA

1.4 × 10 1.9 2.9 × 104 7.9 × 104

NH3

4.4 × 104 kg · yr-1

8

kg · yr kg · yr-1 kg · yr-1 kg · yr-1

formaldehyde 3.3 × 102 kg · yr-1 acetaldehyde 2.1 × 102 kg · yr-1

literature (range)d

reference

this study this study this study (12) 3.9 × 104 - 1.6 × 105 kg · yr-1 w1.6 × 104 kg · yr-1 (12) (21)(20)a(20)a pw 1.4 × 105 - 9.3 × 105 kg · yr-1 w/o3.9 × 104 kg · yr-1 w (12) 2.2 × 104- 8.7 × 104 kg · yr-1 w9.6 × 103w5.2 × 105 kg · yr-1 (12) This studyb (21)(7)c(26) w/ o2.7 × 105 kg · yr-1 w/o7.9 × 103w this study this study max. 1.6 × 105 kg · yr-1 pw (12)

a Coal-fired power plant with MEA-based capture. b Calculated based on the total amount of NH3 formed due to oxidative degradation and a water-wash efficiency of 95% c Estimated value for a coal-fired power plant with MEA-based capture. d w ) with water-wash; w/o ) without water-wash; pw ) probably with water-wash.

i.e., formaldehyde or acetaldehyde, is preferentially formed. It is therefore assumed that there is an equal probability to form acetaldehyde and formaldehyde, i.e. 50% of 3.4 × 10-1 mol MEA · s-1 is used to form formaldehyde and 50% of 3.4 × 10-1 mol MEA · s-1 is used to form acetaldehyde. This leads to the formation of 1.0 × 10-1 mol formaldehyde · s-1 and 5.0 × 10-2 mol acetaldehyde · s-1. Both aldehydes react further with oxygen to form carboxylic acids (11). We are aware of only one study that has determined oxidative degradation products in both the gas-phase and the liquid-phase. Sexton (31) determined a 1.6% and 2.8% occurrence of aldehydes in the gas-phase, for formaldehyde and acetaldehyde, respectively. On the basis of this limited information, we assume that 98% of the formed formaldehyde and 98% of the formed acetaldehyde will react further with oxygen to organic acids. The remaining 2% is assumed to be emitted to air with flue gas exhaust. Similar to O2 induced degradation, the primary degradation product for reaction with SO2 and NO2 is ammonia (25). Each mole of MEA degraded produces one mole of ammonia. On the basis of eq 2, this results in degradation of 5.4 × 10-2 mol MEA · s-1 and formation of 5.4 × 10-2 mol NH3 · s-1. The degradation pathways for reaction of MEA with SO2 and NO2 are less well studied. SO2 induced degradation results in formation of inorganic acids of thiocyanate and thiosulfate, which are nonvolatile. Presently, it is unknown if other volatile degradation products than ammonia are formed due to reaction with SO2 and NO2, and degradation products could therefore not be included. Quantification of Emissions. We calculated emissions for three scenarios (Table 2): (1) A conventional 420 MW NGCC without CO2 capture; (2) A conventional 420 MW coal fired power plant without CO2 capture; and (3) A 420 MW NGCC with CO2 capture. MW NGCC without Capture. Environmental emissions for a conventional NGCC were obtained from the Ecoinvent database using NORDEL natural gas (32) (Supporting Information). All emissions were calculated for a 420 MW power plant, which is a characteristic size of a natural gas fired power plant. The net cycle efficiency of the power plant is assumed to be 56.9% (5). The plant is in operation for 8000 h per year (12). MW Coal-Fired Power Plant without Capture. Environmental emissions for a conventional coal-fired power plant, situated in Germany, were obtained from the Ecoinvent database (32) (Supporting Information). All emissions were calculated for a 420 MW power plant. The net cycle efficiency of the power plant is assumed to be 46% (7). The plant is in operation for 8000 h per year (12). MW NGCC with Capture. The emissions of a power plant with capture are dependent on several performance parameters, including the energy efficiency, CO2 capture efficiency and flue gas flow, and characteristics. The Eco-

Invent database (32) was used to obtain flue gas characteristics for a NGCC without CO2 capture. Capture of CO2 results in a reduced energy efficiency of the power plant, as a significant amount of heat is needed to strip CO2 from the amine-based solvent. On the basis of the assumptions of Peeters et al. (5), the additional energy (Er) required to produce one kWh is 0,116 kWh. As a consequence flue gas emissions increase with (1 + Er) times the flue gas emission for a plant without capture (Supporting Information). The emissions of monoethanolamine and ammonia for a plant with water-wash are obtained from NVE (12). These emissions are 8.0 × 104 and 4.8 × 104 kg · yr-1, for MEA and NH3, respectively. These values are in line with emissions reported by others (Table 2). Consistently higher emissions rates have been reported for CO2 capture facilities without water-wash (Table 2).The typical removal efficiency for volatile organic compounds (VOCs) ranges from 70% to greater than 99% (28, 29). High efficiencies can be achieved for readily water-soluble compounds, as formaldehyde, and relatively high pollutant concentrations (>2000 ppmv) (28, 29). A NGCC has a relatively low pollutant concentration (approximately 200 ppmv) and we therefore assume a waterwash efficiency of 95% (29). We estimated the emissions of formaldehyde and acetaldehyde, based on the total amount of aldehydes present in the vapor phase and a water-wash efficiency of 95%. This results in an emission of 3.3 × 102 and 2.2 × 102 kg · yr-1 for formaldehyde and acetaldehyde, respectively. The water-wash section also reduces emissions of SO2 and emission of other VOCs present in the flue gas. A typical reduction of 97% is obtained for SO2 (28). For watersoluble VOCs, we used a removal efficiency of 95% and for less water-soluble VOCs, such as PAHs and dioxins, we used a removal efficiency of 70% (28) (Supporting Information). Human and Environmental Impact Assessment. Human and environmental impacts for each impact category are calculated as follows: Ii )

∑E

x,air

· CFx,air,i

i

where I is the impact category I, E is the emissions of substance x to air (in kg · yr-1), and CF is the substance (x) and emission-media (air) specific characterization factor for impact category i. The following impact categories were included: human toxicity (HT), freshwater ecotoxicity (FET), marine ecotoxicity (MET), terrestrial ecotoxicity (TET), photochemical oxidant formation (POF), particulate matter formation (PMF) and terrestrial acidification (TA). Midpoint characterization factors were obtained from ReCiPe (18), which is a standardized methodology for LCIA. Consensus (hierarchist) characterization factors were used (SI). For monoethanolamine characterization factors for human toxicity potential are not available. Therefore, the human toxicity potential for MEA VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Human and environmental impact assessment comparison of three scenarios: (1) a conventional 420 MW NGCC, (2) a conventional 420 MW coal-fired power plant, (3) a 420 MW NGCC with CO2 capture, with a water-wash section in the absorber Abbreviations of included impact categories: POFP ) photochemical oxidant formation potential, PMFP ) particulate matter formation potential, TETP ) terrestrial ecotoxicity potential, FETP ) freshwater ecotoxicity potential, METP ) marine ecotoxicity potential, HTP ) human toxicity potential, and TAP ) terrestrial acidification potential. was calculated following ReCiPe procedures (Supporting Information) (16-18). Only limited chronic toxicity testing of MEA has been conducted, and the CF is based on a no observed effect level for dogs (34). The impact assessment was performed for the three scenarios as described above. The complete emission inventory is used in calculating the impact assessment, including emissions of substances that are not affected by CO2 capture (Supporting Information).

Results The calculated human toxicity characterization factor for MEA is 0.24 kg1.4-DCB to urban air /kgMEA. for a generic location (Supporting Information). The intake fraction (iFx) and toxic effect factor (EFx) are 5.4 × 10-2 and 4.0 × 10-3, respectively. In Figure 2 it is shown that CO2 scrubbing results in a 10-fold increase in toxic impacts on freshwaters for a facility with capture compared to a conventional power plant (Figure 2). This increase is predominantly due to volatilization of monoethanolamine (Figure 3b). Additionally, there is a minor increase (factor of 4) in terrestrial toxicity due to MEA emission (Figure 3b). It should be noted, however, that the total impact on freshwater and terrestrial ecosystems is still

lower for a 420 MW NGCC with capture than a conventional 420 MW coal-fired power (Figure 2). For all other impact categories, impacts are comparable to a plant without capture (Figure 2). This is a surprising result as flue gas composition changes considerably due to CO2 capture (Supporting Information). For example, SO2 emissions are reduced by 5 orders of magnitude, whereas new substances, as ammonia and MEA, are introduced to the flue gas in relatively large amounts. Nevertheless, impacts on photochemical oxidant formation, particulate matter formation, and terrestrial acidification do not change considerably, due to a dominant contribution of NO to these categories (Figure 3b). NOx emissions are not affected by CO2 capture. Analogously, the toxic impact of CO2 capture facilities on humans and marine ecosystems is comparable to a plant without capture, due to a dominant contribution of mercury to these impact categories. Oxidative degradation of MEA results in release of ammonia, acetaldehyde, and formaldehyde. Although NH3 release contributes to the particulate matter formation (5%) and acidification potential (14%) of a CO2 capture facility, total impacts in these categories do not increase, due to the dominant contribution of NO emissions.(Figure 3b). Emissions of the other degradation products, formaldehyde and acetaldehyde, do not contribute significantly to the total impacts in all impact categories (Figure 3b).

Discussion Sensitivity Analysis and Data Quality. In this work, a first assessment of human health and environmental impacts of a postcombustion CO2 capture facility is presented, based on estimates of emissions of MEA and degradation products to air and limited toxicity tests. Emissions of MEA to air (Table 2), and aquatic toxicity of MEA (36) are relatively well-characterized, providing confidence in the estimation of the freshwater impact score. In contrast, other required data for an impact assessment of CO2 capture by amine-based scrubbing solvents was found to be scarce or nonexistent. This refers in particular to: the removal efficiency of the water-wash, emission data of MEAdegradation products, and nonaquatic toxicity data for MEA. It is therefore important to evaluate our results by performing a sensitivity analysis and to identify key uncertainties. Two (potentially) critical assumptions needed to be made that define the magnitude of emissions and consequentially the potential impact of the capture plant. These are the 95% water-wash efficiency for water-soluble volatile compounds, as ammonia and MEA, and the 2% aldehyde volatilization rate. To analyze the sensitivity of the final impact assessment

FIGURE 3. Relative contribution of individual chemicals to each impact score. “Other substances” represent all chemicals that are already present in flue gas due to natural gas combustion and that are not affected by CO2 capture (32) (SI). 1500

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to the water-wash efficiency, we incrementally decreased the removal efficiency from 95% to 0% for water-soluble VOCs. Concurrently, the water-wash efficiency for SO2 was incrementally decreased from 97.5% to 0%. This analysis shows that there is only a minor increase (factor 1.1-3.6) in most impact categories. This results from the dominant contribution of mercury to human toxicity impacts and of NO to the terrestrial acidification potential and the formation of photochemical oxidants and particulate matter. The removal efficiency has a stronger impact on the toxic score on freshwater and terrestrial ecosystems and a maximum 200-fold increase is observed for a power plant without waterwash. This increase is, however, unlikely as CO2 capture power plants will be operated with a water-wash (12). More realistically, a relatively low water-wash efficiency of 90% results in a 20-fold increase in toxic impacts compared to a conventional power plant, due to MEA emissions (1.6 × 105 kg · yr-1 ≈ 4 ppmv). MEA emissions have to be reduced to 1.6 × 104 kg · yr-1 (0.4 ppmv) to obtain comparable freshwater toxicity impacts to a conventional power plant. One aim of this work was to investigate if MEA degradation results in substantially increased impacts on human health and the environment. Our results indicate that emissions of MEA-degradation products, in particular ammonia, formaldehyde, and acetaldehyde, do not contribute significantly to total impacts. Although emissions of ammonia are relatively well characterized, quantitative emission data for other volatile degradation products, including formaldehyde and acetaldehyde, is largely absent. To include these compounds in our impact assessment, we estimated emission rates of aldehydes based on limited laboratory experiments of monoethanolamine degradation (31). To analyze the sensitivity of the final impact assessment to formaldehyde and acetaldehyde emissions, we incrementally increase the aldehyde volatilization rate. This evaluation provides useful information on the potential importance of aldehyde emissions on the total impact score. It is shown that the magnitude of aldehyde emissions has no impact on three categories: particulate matter formation potential, photochemical oxidant formation potential, and terrestrial acidification potential, and only a minor impact on ecotoxicity potential (Supporting Information). If the water-wash operates at a 90% efficiency, a maximum 4-fold increase is observed in human health impacts. This increase is relatively low, due to the dominance of mercury emissions to human health impacts. This sensitivity analysis suggests that our assumptions on the aldehyde volatilization rate do not have a considerable impact on the final result. In contrast to the wide availability of aquatic toxicity data for MEA, there is a paucity of reliable nonaquatic toxicity data. At present, there are no (chronic) toxicity studies available that allow for unambiguous establishment of a noobserved-effect-level (NOEL). The human toxicity characterization factor was based on the highest dose (22 mg × kg-1 · d-1) that did not result in observed effects in dogs (34). This value represents the highest concentration tested (34) and is therefore not a “true” NOEL. Implications and Recommendations for CCS. The control of greenhouse gas emissions is arguably one of the most challenging issues facing environmental policy makers and the scientific community. Emerging sustainable energy technologies, including carbon capture and storage, are therefore commonly evaluated on their greenhouse gas reduction potential. Impacts on human health and the environment are thereby often overlooked. Carbon capture is a relatively new technology and considerable research is dedicated to development of new scrubbing solvents and alternative processes (3, 35). In the present work, it is shown that emissions of the amine-based scrubbing solvent, mo-

noethanolamine, result in a 10-fold increase in freshwater toxicity impacts. We therefore recommend including human health and environmental impacts in the evaluation of scrubbing technologies, in addition to costs and greenhouse gas reduction potential. For a better understanding, it is important to conduct experiments to determine degradation products, pathways and consequential emissions of volatile compounds. Also, for a complete integrated assessment of environmental impacts, it is essential to characterize emissions to other compartments, as water and soil. This is an important direction for future research, as a large amount of MEA is captured by the water-wash, which has to be treated in an environmentally responsible manner. Given the large-scale releases expected, better toxicity tests on MEA are also advisible.

Supporting Information Available Calculations of the human toxicity characterization factor and a specification of the power plant and its emissions to air, as well as a specification of reclaimer waste, as well as results of the sensitivity analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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