The Nature of Adsorbed CO2 and Amine Sites on the Immobilized

Aug 3, 2013 - The nature of adsorbed CO2 on immobilized amine sorbents regenerated with utility boiler steam was studied by in-situ infrared spectrosc...
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The Nature of Adsorbed CO2 and Amine Sites on the Immobilized Amine Sorbents Regenerated by Industrial Boiler Steam Mathew Isenberg† and Steven S. C. Chuang*,‡ †

Department of Chemical and Biomedical Engineering and ‡Department of Polymer Science, The University of Akron, Akron, Ohio 44325 S Supporting Information *

ABSTRACT: The nature of adsorbed CO2 on immobilized amine sorbents regenerated with utility boiler steam was studied by in-situ infrared spectroscopy. The use of industrial boiler steam is the key factor lowering the overall operating cost of the immobilized amine CO2 capture process for megawatt-scale coal-fired power plants. The industrial steam is known to contain trace amounts of Cu, Fe, and Zn species. The present study was undertaken to understand the chemistry occurring between the Cu contaminants in the steam and the amine sites on the sorbent. Cu-containing steam from the utility boilers resulted in a 91% loss of CO2 capture capacity over 30 continuous CO2 capture and regeneration cycles. UV−vis and in-situ IR studies suggest that the degraded sorbent contained CuII ions coordinated with nitrogen atoms of an imine species. These imine species are incapable of adsorbing CO2. The remaining amine species could bind weakly with CO2, mainly in the form of carbamic acid. The formation of ammonium ion species was inhibited by adsorbed Cu species on the degraded sorbent. Cu-free steam is required for stripping adsorbed CO2 from the amine sorbent in the CO2 capture process. The findings presented here should be considered before implementing a pilot-scale CO2 capture process from coal-fired power plant flue gas.

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INTRODUCTION Coal-fired power plants are a major source of CO2 emissions. Recent data have revealed that the atmospheric CO 2 concentration has been continuously increasing at a rate of more than 2.1 ppm/year over the past decade.1 Immobilized amine solid sorbents have shown a great potential for capture of CO2 from coal-fired power plant flue gas. Immobilized amine processes possess a number of inherent advantages over the liquid amine processes including reductions in (i) equipment corrosion, (ii) the energy required for regeneration, and (iii) amine loss via degradation reactions and evaporation. A costeffective sorbent technology for the large-scale CO2 capture requires a high CO2 capture capacity at low partial pressure2 and resistance toward amine degradation over multiple heating and cooling cycles.3 The added cost of a CO2 capture technology to the coal-fired power plant has been set at less than a 35% increase in COE (i.e., cost of electricity).4 The basic principle of the sorbent and solvent processes for CO2 capture is binding CO2 with adsorption sites at temperatures between 40 and 60 °C in the adsorber and desorbing CO2 from binding sites at temperatures between 100 and 135 °C in the regenerator where high-purity CO2 can be collected using steam as the regeneration source and purge gas. Low-temperature steam at 135−150 °C is an ideal heat source for heating the sorbent for regeneration because steam which is readily available in the coal-fired power plants has been shown to slow down the degradation of the amine sorbent.5 Removal of CO2 from adsorber with steam resembles steam-stripping of CO2 from the CO2-containing solvents in the liquid amine process.6 We have found that steam from utility boilers contain Cu, Zn, and Fe species and may cause rapid degradation of the amine sorbents. These metal species are commonly found in © 2013 American Chemical Society

utility steam lines and equipment. In particular, the boilers used in coal-fired power plants may produce steam with elevated concentrations of Cu.7 The Electric Power Research Institute (EPRI) has conducted a thorough investigation of the presence and sources of copper ions and/or particulate oxides in the steam/water cycle of fossil-fuel-based plants. The initial findings state that copper corrosion, by dissolution and/or exfoliation, may occur at any location where steam or condensate is in contact with a copper-containing material. The objective of their study was to determine the influence of various parameters that would result in copper entering the water/ steam cycle. These parameters included pH, temperature, amount of dissolved oxygen, and actual oxidation−reduction potential of the environment. The parameters were tested at low temperatures (95 °C) and high temperatures (250 °C), using simulated feed water that matched actual plant conditions. These conditions were tested on three coppercontaining materials that are commonly found in fossil-fuelbased plants: admiralty brass C44300, 90/10 Cu−Ni alloy, and Alloy 400. Their results found that copper, along with iron and zinc, were transferred to the water/steam cycle under every condition. This study provided us with the basis of the present study to understand the effect of copper contaminants on the CO2 adsorption stability of immobilized amine sorbents being cycled in a metal ion environment. The boiler steam used to regenerate the sorbent in the present study is supplied directly from the University of Akron’s water/steam cycle and is Received: Revised: Accepted: Published: 12530

June 17, 2013 July 28, 2013 August 3, 2013 August 3, 2013 dx.doi.org/10.1021/ie401892u | Ind. Eng. Chem. Res. 2013, 52, 12530−12539

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Long-Term CO2 Capture Cycling Studies of Amine Sorbent. CO2 adsorption and regeneration cycles were performed in a 5 g adsorber for which inlet flow and adsorber temperature were monitored and controlled by Labview 8.6. The adsorber was loaded with 5 g of fresh pellets with a thermocouple located directly in the center of the sorbent bed. The bed density of the sorbent was approximately 550−610 kg m−3. The volumetric flow rates and pressure of all streams entering the adsorber were maintained at 1.5 L/min and 40 psi; with the exception of the cooling water pressure which was not monitored. Adsorption was performed with a 15 vol % CO2/air mixture for approximately 5−6 min until the CO2 concentration at the outlet was equal to the inlet concentration. Following adsorption, air was used to evacuate the gas-phase CO2 from the adsorber, leaving strongly adsorbed CO2 species on the sorbent. Cooling water was used to maintain the bed temperature at 40 °C during adsorption and adsorber evacuation. Sorbent regeneration was performed through a series of consecutive steps. First, the adsorber was heated to 100 °C, via steam through a heating jacket, having a closed inlet and outlet, and maintained for 5 min. Steam from (i) an inhouse generator or (ii) utility boilers was then directly pulsed through the sorbent bed, sweeping the desorbed CO2 gas into the condenser/sensor manifold. The utility boiler steam, containing the CuII contaminants, was provided from The University of Akron’s water/steam boiler supply. An outlet of an existing The University of Akron steam line was routed to the CO2 capture system for the steam regeneration step. The CO2 concentration of the reactor effluent was continuously recorded in real time with a temperature-compensated infrared sensor (FDG10A; Status Scientific Controls) throughout the entirety of the experiment. XRF (Shimadzu) analysis was performed on the amine sorbent before and after the long-term cycling to determine the mole percent of steam contaminants (i.e., Cu) adsorbed on the sorbent. FTIR CO2 Capture Cycling Studies of Amine Sorbent. FTIR spectroscopy was used to evaluate the evolution of adsorbed species and the surface characteristics of the cycled particles. The evolution of adsorbed species on the fresh and cycled particles was determined by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell in a Nicolet 6700 bench. The DRIFTS cell was loaded with 60 mg of sorbent particles (i.e., contaminated steam and steam regenerated particles). All volumetric gas flow rates were maintained at 200 cm3 min−1. Pretreatment was carried out under a continuous Ar flow by increasing the bed temperature to 110 °C to remove water and CO2 adsorbed while in storage. The temperature was held constant for 5 min before cooling back to 30 °C. Adsorption was performed by flowing a 15% CO2/air mixture for 15 min. The residual gas-phase CO2 was evacuated from the cell by switching the flow, via a four-port valve, to Ar. Evacuation was continued for 20 min to gain insight into how the presence of CuII affects the binding strength between CO2 and the amine site of the sorbent, such as a decrease in IR intensity, evaluated through analysis of pseudo-decay isotherms. Temperature programmed desorption (TPD) was performed under flowing Ar with a heating rate of 10 °C min−1 from 30 to 110 °C. The temperature was held at 110 °C for 5 min to complete the CO2 desorption. Following CO2 desorption, the DRIFTS cell was cooled to 30 °C in preparation of another cycle.

thought to accurately simulate the real environment of an industrial boiler. The objective of this study is to determine the effect of copper contamination in steam on the CO2 capture capacity as well as the nature of adsorbed CO2 and its binding sites of amine-based solid sorbents. We found that sorbent degradation was accelerated by the adsorption of Cu from steam as copper(II) ions, CuII, which coordinated with the N-atoms on the amine sorbents. The results of this study show that steam for amine sorbent must be free of Cu and Zn metal ions.

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METHODS AND MATERIALS Sorbent Preparation. The sorbent particles were prepared by mixing an ethanol solution containing 22.5 g of TEPA (tetraethylenepentamine, Sigma-Aldrich), 15.0 g of PEG (poly(ethylene glycol), H(OCH2CH2)nOH, where n = 4.2; MWav = 200 ± 10 g/mol; 86/14 tetra-/pentaethylene glycol, Sigma-Aldrich), 8.4 g of PL (polymeric linker), and 1.625 g of AO (antioxidant) with 25.0 g of SiO2 (Tixosil 68B, Rhodia). The names of the polymeric linker and antioxidant have not been revealed due to the filing of a patent. The addition of the PL and AO does not have an effect on the adsorption chemistry between CuII and amine functional groups, which is the focus of this work. The mixture was dried in a convection oven at 100 °C. The nominal amine loading of the resulting sorbent is 31 wt %. The sorbent is a white powder with an average particle diameter of 20 μm as determined by scanning electron microscopy (SEM). The powder was then combined with a polymeric binder solution, developed in-house, at a weight ratio of powder to binder solution of 1:1. The formulation of the polymeric binder solution will not be revealed due to the filing of a patent. The composition of the polymeric binder solution has no effect on the adsorption chemistry between CuII and the amine functional groups present on the sorbent. The appearance of the mixture resembled that of a typical bread dough. The dough was cured at room temperature for 45 min, followed by forming into 2 mm diameter rods by a manual screw extruder. The rods were cut and then dried in a convection oven at 100 °C resulting in an average pellet diameter of 2 mm and length of 1 cm. The final amine loading of the pellet and particle was 18 mol %, and the number of NHx (x = 1 or 2) functional groups was 8.20 mmol/gsorb. Figure S.1 of the Supporting Information illustrates the overall scheme of the experimental studies of the sorbent pellets: (i) long-term CO2 capture cycling studies, (ii) in-situ Fourier transform infrared (FTIR) characterization, (iii) UV− vis characterization, and (iv) X-ray fluorescence (XRF) characterization. Condensed samples from two sources, uncontaminated and copper-contaminated steam, were taken at the entry port of the 5 g adsorber. The uncontaminated condensed steam sample is referred to as steam (Stm., in figures), and the condensed steam containing CuII as contaminated steam (Ct.S., in figures). Following the CO2 adsorption/regeneration cycles the fresh pellets are referenced by the steam source in which they were regenerated. Pellets regenerated with steam are given the name “steam regenerated pellets” (Stm.Reg). Similarly, pellets regenerated with the contaminated steam are referred to as “contaminated steam regenerated pellets” (Ct.S.Reg). The spectroscopic analysis was performed on pulverized pellets. The pulverized form of the sorbent is denoted by the name “particle” rather than “pellet”, i.e., steam regenerated particles. 12531

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Figure 1. (a) CO2 gas concentration (vol %) and temperature profile of several CO2 adsorption/regeneration cycles generated during the long-term cycling of the steam regenerated pellets in the 5 g adsorber. (b) Details of gas/steam segments and calculated working CO2 capture capacity in one adsorption/desorption cycle, taken from cycle 15.

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RESULTS AND DISCUSSION Long-Term CO 2 Capture Cycling. Long-term CO 2 capture cycling studies were carried out for 30 CO 2 adsorption/desorption cycles. Each cycle consists of the steps illustrated in Figure 1a: flowing air over the sorbent bed for cooling the sorbent bed to 40 °C, CO2 adsorption, CO2 evacuation by flowing air, and purging CO2 out of the adsorber with steam at temperatures above 80 °C. Adsorption of CO2 caused more than a 20 °C rise in the sorbent bed temperature. The working capacity for each individual cycle was calculated by integrating the area under the CO2 volume percent profile, shown as the shaded area in Figure 1b. This CO2 profile was produced by desorbed CO2 which was purged from adsorber using steam. The area is then converted to millimoles of CO2 by multiplying by the total gas flow rate divided by the molar volume of CO2 (22.3 L mol−1). The calculated millimoles of CO2 is further divided by the total weight of sorbent loaded into the adsorber yielding the working CO2 capture capacity of that cycle. The working CO2 capture capacity was reduced by 91%, from 2.75 to 0.25 mmolCO2 gsorb−1, over the course of long-term CO2 capture cycling with Cu-contaminated utility boiler steam. In contrast, Cu-free steam resulted in a 46% reduction in working CO2 capture capacity. The CuII contaminated steam nearly doubled the rate of degradation, suggesting that metal− amine interactions have a more significant impact on a sorbents working CO2 capture capacity than thermal and oxidative

degradation. The change in the working CO2 capture capacity as a function of each cycle has been provided in Table S.1 of the Supporting Information. Figure 2 shows the physical appearance of the pellets and condensed steam samples used in the long-term CO2 capture

Figure 2. Picture of (a) fresh pellets, (b) contaminated steam regenerated particles, (c) steam regenerated pellets, (d) condensed uncontaminated steam sample, and (e) condensed contaminated steam sample.

cycling. The steam (i.e., Cu-free steam) regenerated pellets exhibited a light yellow color, a sign of oxidative degradation via hydrogen abstraction, which is further elaborated by type I in Scheme 1. This oxidative degradation is a result of the air purging at elevated temperatures.8 The contaminated steam regenerated pellets showed a pale blue/green color. The condensed steam samples were collected from process lines after they had cooled and the steam had condensed. The steam and pellet samples underwent XRF analysis to determine the 12532

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Scheme 1. Proposed Structures of Degraded Species on a TEPA Molecule

Figure 3. UV−vis absorbance spectra of Ct.S.Reg, steam regenerated Stm.Reg, fresh, CuII−amine (ref), and SiO2 particles.

a result of CuII coordination with N-groups of an amine.12,13 The UV−vis features for the Ct.S.Reg particle show similar absorbance features as the reference sample with less absorption between 500 and 800 nm. It is clear that amines can be an effective agent for CuII adsorption at various initial copper concentrations, Co, with high capture capacities. Current amine based adsorbents, listed in Table S.3 of the Supporting Information, have been reported to have maximum CuII adsorption capacities, qmax, up to 116.28 mgCuII gsorb−1.14 The main focus of these reports was regarding CuII capture capacity from aqueous solution systems and lacked information about amine coordination and stoichiometry of Cu to N ratios. To our knowledge, the present work is the first to discuss adsorption of copper by amines from a steam phase. Adsorption of CuII from a gas phase introduces several new considerations relevant to a large-scale CO2 capture process from flue gas. Among these considerations are oxidative degradation of the amine through coordination with CuII and the competitive adsorption between CuII and CO2. The competitive adsorption between CuII and CO2 by amines is in favor of CuII because it acts as a stronger Lewis acid than CO2 and will have a higher affinity toward the amine, a Lewis base. In-Situ FTIR CO2 Capture Cycling. Figure 4 illustrates the method of data analysis for the in-situ FTIR coupled with mass spectrometry (MS) for a typical CO2 capture cycle. The MS intensity for CO2 (m/e = 44) generated from cycling of the steam regenerated particles is provided in Figure 4a. The increase in CO2 MS intensity in the CO2 adsorption step, between 1.5 and 7 min, represents the breakthrough of CO2 exiting the sorbent bed. The analysis of the CO2 breakthrough curve provides information regarding the adsorption capacity of the sorbent and flow characteristics through the bed. The second increase in CO2 MS intensity between 17 and 22 min is the result of sorbent regeneration via TPD. The area under the CO2 MS profile during the TPD is correlated with the volume of CO2 released from the sorbent through a calibration curve. The sorbent capture capacity is then determined by dividing the volume of CO2 released by the sorbent weight. The MS intensity was scaled by a factor of 20 in the TPD portion of the figure. The MS CO2 breakthrough profile was plotted against the IR CO2 profile (gas phase, 2350 cm−1) in the inset of Figure 4a to elucidate the adsorption and flow characteristics through the sorbent bed. The IR CO2 profile shows a steplike increase in

molar composition of trace elements. UV−vis spectroscopy of the particles was performed to further elucidate the cause of the color change after cycling. Table S.2 of the Supporting Information lists the results of the XRF analysis performed on the particles and steam samples. The XRF quantifies the elements with an atomic number greater than 11 (Na) and is not capable of detecting elements such as hydrogen and oxygen. The mole percent provided in the table are based on the content of those elements detectable by XRF. The results show copper represents 97.07 mol % of the trace elements in the utility boiler steam. This value serves as a reference for comparison with the other contaminants. Low levels of zinc and iron were also detected by the XRF. The contaminated steam regenerated particles were found to contain 2.51 mol % of copper, 0.93 mol % zinc, and 0.25 mol % iron. A sample, CuII−amine (ref), prepared by soaking the fresh pellets in an aqueous solution of 35 wt % CuIICl2 for 30 min, was included in the analysis. This high copper concentration is in excess of the 1:4 CuII:N stoichiometric molar ratio needed for a complete reaction between CuII and the amine of the sorbent. This sample provides the reference UV−vis and IR spectra. Figure 3 shows the UV−vis absorbance spectra of SiO2, fresh, steam regenerated, contaminated steam regenerated, and CuII− amine (ref) particles. The absorbance spectrum of SiO2 does not show significant light absorption. The formation of a strong absorption below 284 nm has been reported for unprotonated amine groups in a poly(amidoamine) dendrimer structure and for N-[3-(trimethoxysilyl)propyl]ethylenediamine.9,10 The presence of copper in the CuII−amine (ref) sample resulted in a suppression of the amine absorbance, accompanied by an increase in absorption at 410 nm and a subtle broadening in the range of 500−800 nm. The increase of the 410 nm absorption is likely due to the formation of coordinated imine species.11 The broadening between 500 and 800 nm has been reported as 12533

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Figure 4. (a) CO2 MS gas profile and isolated MS/IR CO2 gas breakthrough and decay curve (inset), (b) IR absorbance during a typical CO2 capture cycle, and (c) difference spectra during a typical CO2 capture cycle. Absorbance is obtained by abs = log(1/I), where I is the single beam spectra taken at different points along the capture cycle. Difference is obtained by diff = abs(I) − abs(Io) where abs(Io) is the absorbance spectra after pretreatment.

intensity at the moment CO2 is switched into the reactor compared to the lagging trend generated in the MS intensity. The lag in the MS profile resulted from the adsorption of CO2 by the sorbent. The lag was not observed in the IR intensity because the IR detects the gas-phase CO2 concentration above the sorbent bed which remains constant under continuous flow conditions. The profiles show the opposite trend during CO2 evacuation with Ar. The MS CO2 profile shows a steplike decrease back to the baseline when the gas flow is switched from 15% CO2/air to Ar, and the IR CO2 profile shows significant trailing. The trailing resulted from the retention of gas-phase CO2 in the interparticle spaces of the sorbent. The retention occurs from the continuous adsorption/desorption of CO2 from the particle amine sites as the partial pressure of CO2 above the sorbent bed is reduced. This point will be further discussed for the steam regenerated and contaminated steam regenerated particles in the context of the surface adsorbed species. The changes in surface characteristics of the sorbent by CO2 capture cycling were observed in-situ by FTIR. The singlebeam spectra were collected at various points throughout the CO2 capture cycle using OMNIC 9 software (Thermo Scientific). The single-beam spectra taken in DRIFTS are equivalent to transmittance in the transmission IR, which

represents the raw detector response versus wavenumber. The intensities were converted to absorbance spectra, shown in Figure 4b, by logarithmic transformation (abs = log (1/I)). The spectra clearly show the characteristic N−H stretching vibrations of TEPA at 3358 and 3300 cm−1, and C−H stretching at 2931 and 2818 cm−1.15−17 The formation and removal of surface adsorbed CO2 species as a result of CO2 adsorption/regeneration is interpreted through the difference spectra shown in Figure 4c. The absorbance spectrum generated by the presence of CO2 (II − I) reveals strong adsorptions at 1324, 1411, and 1567 cm−1, corresponding to the formation of carbamate (−NHCOO−) species.15 The formation of −NHCOO− species was accompanied by the formation of NH2+/NH3+ indicated by sharp bands at 1495 and 1633 cm−1 and the broad absorptions in the 3000−3200 and 1800−2800 cm−1 regions.15,18 Evacuation of gas-phase CO2 from the cell resulted in the partial removal of adsorbed species evidenced by the low absorbance intensities for difference spectrum III − I. This was the result of a decrease in CO2 partial pressure and points out that a large fraction of adsorbed species are weakly bound to the amine sites of the sorbent. The removal of the remaining strongly adsorbed species was accomplished by performing TPD. The nearly flat absorbance shown by difference spectrum IV − I confirms the removal of 12534

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around a centralized CuII. Although the orientation of the amine coordination, either axial or planar, is still undetermined, a number of studies suggested that a square-planar orientation is most likely in the case of the amine molecule having less than four coordinated N-atoms.22−24 N-atom coordination is a wellestablished phenomenon in various biological and wastewater treatment systems.14,25−30 In particular, CuII has been reported to readily coordinate with four N-atoms of the amine molecule27,29 and in some cases five N-atoms.25 Such coordination would render the amine molecule incapable for CO2 adsorption resulting from the higher binding energy of CuII with TEPA of 136.87 kJ mol−125 compared to CO2 with TEPA of 52 kJ mol−1. It must be pointed out that the value reported for the binding energy of CuII with TEPA is for absorption in the liquid phase, and that reported for CO2 with TEPA was calculated by TPD for the fresh particles in a previous experiment. Although the binding energy is affected by the surrounding environment of the molecules, the values provided here are considered to be within reason. Figure 6a shows the absorbance spectra of the steam and contaminated steam regenerated particles at selected temperatures during pretreatment from room temperature to 110 °C. The subtle variations in absorbance intensities are highlighted in the difference spectra, presented in Figure 6b. The contaminated steam regenerated particles exhibit a stronger intensity than the fresh particles at 3600 cm−1 for surface adsorbed water. This suggests that more of the SiO2 surface is exposed on the contaminated steam regenerated particles than on the steam regenerated particles. The exposed SiO2 surface increased the hydrophilicity resulting in more surface adsorbed water compared to the steam regenerated particles. The increased affinity toward adsorbing water may also contribute to the reduction in CO2 capture capacity of the contaminated steam regenerated particles. This would result from adsorbed water blocking the remaining available amine sites from adsorbing CO2. Figure 7 shows the absorbance spectra of the steam and contaminated steam regenerated particles produced from insitu CO2 capture cycling. The comparison of CO2 adsorbed species in the presence of gas-phase CO2, Figure 7a, shows that the relative intensity of adsorbed CO2 species on the fresh particles is higher than those of the other three particle samples, demonstrating that degradation had occurred in all cycled samples. The degradation observed in the steam regenerated particles can be attributed to (i) oxidative degradation through the conversion of C−H to CO and (ii) depletion of amine functional groups through its coordination with CuII ions.8 Such a coordination would result from an electron abstraction from the N-atoms of the amine to the Cu; converting the amine to an imine species. It is clear that the degree of degradation increased with an increasing amount of CuII. The degradation observed in the contaminated steam regenerated particles is likely the combination of oxidative degradation and amine coordination with CuII. The highest CuII-content particles, CuII−amine (ref), are completely incapable of adsorbing CO2, suggesting all of the amine functional groups are consumed by coordinating with CuII. These particles represent sorbent degradation resulting exclusively from amine coordination with CuII. The inset in Figure 7 shows the absorbed species on the steam and contaminated steam regenerated particles. The spectrum generated by the contaminated steam regenerated particles was scaled by a factor of 3.8 to better evaluate the nature of adsorbed species. The spectra indicate that the

the majority of CO2 adsorbed species and that the sorbent has been regenerated. Figure 5 shows the IR absorbance spectra of the particles regenerated after pretreatment at 110 °C. Pretreatment was

Figure 5. Absorbance spectra of pretreated fresh powder-1, fresh powder-2, fresh particle, Stm.Reg, Ct.S.Reg, and CuII−amine (ref) sorbents. Absorbance is obtained by abs = log(1/I), where I is the single beam spectra of interest. Fresh powder-1 is the immobilized amine sorbent before addition of the antioxidant and polymeric linker. Fresh powder-2 is the immobilized amine sorbent before addition of the polymer binder solution.

performed to better resolve the bonding vibrations on the particle surface by removing water and CO2 adsorbed from ambient. Adsorption of CuII led to the formation of imine species on the particle surface, producing a new band at 1655 cm−1 for CN stretching.19 The formation of the CN band was accompanied by a suppression of CH stretching and deformation vibrations and a broadening of the NH2 stretching vibrations. The inclusion of the CuII−amine (ref) spectrum for comparison further supports this analysis. The high CuII content produced an increased concentration of imine species, concluded from the growth in the CN band intensity and CH suppression. The suppression observed in the CH stretching vibrations for the steam regenerated particles is likely the result of oxidative degradation next to the secondary amines.8 The CuII−amine (ref) spectrum exhibits a broadening of the NH2 bending vibrations and disappearance of the asymmetric NH2 stretching vibrations. The formation of imine species may be explained by an electron abstraction from the N-atoms of the amine to the CuII.20,21 Figure 5 also shows the absorbance spectra of AO, PL, and binder-free immobilized amine sorbent (fresh powder-1) and the binder-free immobilized amine sorbent which contains the AO and PL (fresh powder-2). These two absorbance spectra exhibit similar NH and CH stretching/bending intensities as the fresh particle, providing evidence that the addition of these components does not affect the chemistry occurring between the amine functional groups and CuII ions. The perturbation in NH2/CH vibrations associated with the amine coincide with the formation of a new band which can be assigned as a Cu/NHx-coordination structure at 3450 cm−1.22 The development of the Cu/NHx-coordination band provides evidence that the N-atom of the amine may be coordinated 12535

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Figure 6. (a) Infrared absorbance spectra of the regenerated particles at 60, 80, and 100 °C during the pretreatment, where abs = log (1/I). (b) Absorbance (difference) spectra of regenerated particles at 60, 80, and 100 °C, where diff = abs(a) − log(1/(60 °C)), where abs(a) is the absorbance spectra in panel a and log(1/(60 °C)) is the background absorbance spectrum after pretreatment at 60 °C.

Figure 7. IR absorbance for adsorbed species (a) in presence of CO2, (b) in absence of CO2, and (c) after TPD. Absorbance is obtained by abs = log(Io/I) = − log(I) − log(Io), where I is the single beam spectra taken at different points along the capture cycle and Io is the single beam taken after pretreatment.

presence of CuII had the greatest impact on preventing the formation of NH3+ species. This phenomenon was observed by the disappearance of the NH3+ band at 1633 cm−1 and suppression of the band at 1495 cm−1. It was interesting to observe an increase in the formation of carbamic acid

(-NHCOOH) species on the contaminated steam regenerated particles by the absorbance at 1720 cm−1.31 The suppression of NH3+ species in conjunction with an increase in -NHCOOH species provides important information about the nature of TEPA on the particle surface. Our recent work has shown that 12536

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species to weakly CO2 adsorbed species. The high percentage of weakly CO2 adsorbed species on the steam regenerated particles is a combination of oxidative degradation of the amines via hydrogen abstraction (type I) and the effect of the binder solution. The effect of the binder solution is highlighted by comparing the percentages of weakly and strongly CO2 adsorbed species with the PEG/TEPA/SiO2 sorbent. The pelletization process resulted in an increase of the weakly adsorbed CO2 species by 1 order of magnitude. Figure 8 shows the normalized IR intensity profiles of various NH3+ and -NHCOO− bands and a gas-phase CO2 band. These profiles are plotted as the formation and decay curves of adsorbed species for (a) the steam regenerated and (b) the contaminated steam regenerated particles in Figure 8. The gaseous CO2 intensity profile serves as a reference for evaluating the flow pattern in the DRIFT cell. The formation of NH3+ and -NHCOO− adsorbed species were observed to occur simultaneously on both tested sorbents. A similar observation was made for the decay curves. These results suggest that the two adsorbed species are formed as carbamate−ammonium ion pairs on both sorbents. There was not a significant difference in the formation rate of adsorbed species between the steam regenerated and contaminated steam regenerated particles, indicating that the normalized rate of adsorption on available amine sites was unaffected by the presence of CuII ions. Conversely, the decay curves show an increased normalized rate of removal of adsorbed species on the contaminated steam regenerated particles due to the high fraction of weakly adsorbed CO2 species. Note that the actual rate of the removal can be obtained by the normalized rate multiplied by the concentration of adsorbed CO2. The decay curve for the steam regenerated particles shows that a significant amount of adsorbed species remains on the surface even after complete evacuation of gas-phase CO2. Figure 9 shows the pseudo-decay isotherms as a function of the CO2 relative pressure. The pseudo-decay isotherms were constructed from the IR decay curves provided in Figure 8. The nonlinearity of the pseudoisotherms at low relative pressures has been reported for the formation of carbamate−ammonium ion pairs and the linearity at high relative pressures for the formation of physisorbed (weakly bound) species.33 The high slope generated by the steam regenerated particles at low relative pressures suggests a high binding energy with adsorbed CO2 (strongly adsorbed CO2).34 The weakly adsorbed CO2, represented by the low slope at low relative pressures on the contaminated steam regenerated particles is a direct result of amine site isolation resulting from the CuII−amine coordination. Weakly adsorbed CO2 species can be removed from the particle surface by evacuation of CO2 by flowing Ar and may reside on the isolated amine sites.32 In contrast, the adsorbed species on the steam regenerated particles can be removed through TPD at temperatures around 100 °C.

-NHCOOH forms on isolated amine groups, having limited interactions with neighboring amine groups. The formation of -NHCOOH does not require the formation of NH3+ for stabilization.32 Therefore, it is inferred that one CuII may interact with all of the N-atoms of a single TEPA molecule rather than N-atoms of adjacent TEPA molecules because of the close proximity of N-atoms within a single TEPA molecule.25,27,29 The interaction and coordination with a single TEPA molecule will result in isolation of uncoordinated amine groups of the TEPA molecules, which do not bind with CuII. Those isolated molecules would then adsorb CO2 most likely in the form of -NHCOOH. These results indicate that a second oxidative degradation mechanism is occurring on the sorbent surface.19,33 The proposed structure of degraded species on TEPA by oxidation in the presence of CuII is given as type II in Scheme 1.8,21,27,33 Parts b and c of Figure 7 show that the nature of CO2 adsorption (i.e., fraction of weakly/strongly adsorbed) was altered through coordination of the amine with CuII. The CO2 adsorbed species on the contaminated steam regenerated particles were easily removed by evacuating the reactor with Ar. This is shown by a nearly flat absorbance spectrum. Strong absorbance for the CO2 adsorbed species on the steam regenerated and fresh particles remain after evacuation. Complete regeneration of the fresh and steam regenerated particles was accomplished by TPD at 110 °C. This suggests that CO2 adsorption on the isolated TEPA molecules remaining on the contaminated steam regenerated particles can bind CO2 weakly in the form of -NHCOOH and -NHCOO−. Table 1 provides the contributions of weakly and strongly CO2 adsorbed species to the total CO2 capture for the fresh, Table 1. Contribution of Weakly and Strongly CO2 Adsorbed Species to the Total CO2 Capture Capacity for the Sorbents in the Present Sudy and in a Past Publication from Our Group sorbent TEPA/SiO2 PEG/TEPA/ SiO2 Ct.S. Reg. particlea Stm. Reg. particleb fresh particle

total, mmolCO2 gsorb−1

weakly, % strongly, %

2.54 1.16

18 4

82 96

0.25

99

1

1.49

71

29

2.75

40

60

a Pulverized contaminated steam regenerated pellets. steam regenerated pellets.

ref 17 17 present work present work present work b

Pulverized

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steam, and contaminated steam regenerated particles. The contributions of weakly and strongly CO2 adsorbed species were calculated by evaluating the IR spectra during CO2 adsorption (total adsorbed) and prior to TPD (strongly adsorbed). The weakly adsorbed CO2 species were calculated by subtracting the amount of CO2 adsorbed species prior to TPD from the amount of CO2 adsorbed species during CO2 adsorption. The inclusion of the TEPA/SiO2 and PEG/TEPA/ SiO2 sorbents from a past publication of our group provides a comparison for the effect of the binder solution used for pelletization.17 The presence of the CuII contamination resulted in almost a complete conversion of strongly CO2 adsorbed

CONCLUSIONS Regeneration of an amine-immobilized solid sorbent using copper-contaminated steam from the utility boilers resulted in a 91% decrease in the CO2 capture capacity compared to a reduction of 46% using uncontaminated steam from an inhouse-built steam generator. The source of the degradation was determined by XRF to be contamination by copper, found as the most abundant trace element in the utility boiler steam. UV−vis and in-situ DRIFTS studies indicate the amine functional groups of the degraded sorbent coordinate their 12537

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Figure 8. Adsorbed species formation and decay curves for (a) steam regenerated and (b) contaminated steam regenerated particles during CO2 adsorption and CO2 evacuation. The adsorbed species are presented as -NHCOO− (black lines with squares) and NH3+ (red lines with dots). Gasphase CO2 (green lines with triangles) is plotted as a reference. Normalized IR = (Io − Imin)/(Imax − Imin), where Io is the IR profile of interest, Imin is the lowest intensity value within Io, and Imax is the highest intensity value within Io.

regeneration must be free of Cu-containing equipment and vessels.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figure and tables as stated in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 330.972.6993. Fax: +1 330.972.5856. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the U.S. Department of Energy under Grant DE-FE0001780, FirstEnergy Corp, and Babcock & Wilcox Co.

Figure 9. Pseudo-decay isotherm of adsorbed species on the steam regenerated and contaminated steam regenerated particles. Relative pressure calculated by P/Po, where P is the partial pressure of CO2 and Po is 0.15 psig of CO2.

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N-atom around a centralized CuII ion. This coordination of amine with the CuII ion resulted in a reduction of total amine sites available for capturing CO2. The remaining amine sites were isolated from neighboring amine sites. The isolated amine sites generated weakly adsorbed CO2 species in the form of carbamic acid (-NHCOOH) and carbamate (-NHCOO−) in the presence of CO2. The formation of ammonium ion (NH3+) species was inhibited by the coordination and subsequent isolation of amine sites. The reduction in CO2 capture capacity was accompanied with an increase in the sorbents hydrophilicity which allowed the contaminated steam regeneration sorbents to retain more surface adsorbed water than Cu-free amine sorbents. Boilers for generating steam for amine sorbent

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