Article pubs.acs.org/EF
Role of SO2 in Non-aqueous Solution of Blended Monoethylethanolamine and Diethylethanolamine for CO2 Capture from Power Plant Flue Gas Streams Siming Chen, Shaoyun Chen, Yongchun Zhang,* Huanhuan Chai, and Mengxing Cui State Key Laboratory of Fine Chemistry, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China ABSTRACT: The aim of this work was to study the role of SO2 in non-aqueous solution of blended monoethylethanolamine (EMEA) and diethylethanolamine (DEEA) for CO2 capture. The CO2-loaded EMEA + DEEA solution (0.4 mol of CO2/mol of EMEA) was degraded with 0.015 m3 h−1 mixed gas (200 ppm of SO2 balanced with N2) at 333.15 K for 10 days. The reaction of SO2 with the CO2-loaded EMEA + DEEA solution was explored using the 1H and 13C nuclear magnetic resonance, electrospray ionization mass spectrometry, and elemental analysis methods. The results revealed that SO2 can be absorbed immediately and accumulated in the solution. SO2 can react with amino and hydroxyl in EMEA and hydroxyl in DEEA and carbamate, directly forming organic sulfide, which can be further degraded. Additionally, the CO2−SO2-loaded EMEA + DEEA solution can be purified using ion-exchange resins to remove heat-stable salts. In effect, the flue gas from coal-fired power plants contains SO2, NOx, and fly ash, which is quite different from natural gas.27,28 In addition, SO2 in the flue gas is one of the main factors causing free amine loss. Rao et al. reported that the presence of SO2 in the flue gases can substantially increase the reagent costs.28 Beyad et al. studied the effect of SO2 on MEA solution during CO2 capture and concluded that SO2 could be immediately absorbed by the MEA solution and transformed initially into sulfurous acid.29 Zhou et al. suggested that the existence of 60 ppm of SO2 could inhibit MEA oxidative degradation, while a higher concentration of SO2 (150 ppm) could weaken the inhibitory effect, which is the same as the report of Sun et al. that SO2 is a potent inhibitor of MEA oxidative degradation.30,31 Uyanga and Idem found that the strong basicity of primary amine resulted in an irreversible reaction with SO2.32 Supap et al. discovered that SO2 has a higher propensity to cause MEA degradation than O2.33 Thompson et al. found that heat-stable salts (such as sulfate, thiosulfate, formate, nitrate, nitrite, and chloride) from acidic flue gas components, including SO2, NOx, and HCl, can accumulate in amine solutions and lead to corrosion and extra energy requirements.34 Jeon et al. concluded that the SO2 absorption into 2-amino-2-methyl-1-propanol (AMP)/NH3 solution can cause AMP degradation and reduce CO2 absorption efficiency, while the addition of NH3 solution can increase the selective CO2 absorption.35 Bonenfant et al. revealed that the presence of SO2 in the gas decreased the CO2 absorption rate and capacity in the 2-(2-aminoethylamino)ethanol (AEEA) solution as a function of the SO2 concentration.36 Gao et al. concluded that there would be more serious amine degradation and more heat-stable salts after the addition of SO2 to CO2.27,37 However, the reaction media reported in this literature is limited to aqueous solution,
1. INTRODUCTION It is well-known that greenhouse gas emissions, mainly carbon dioxide (CO2), have caused global warming.1,2 There are three main technologies for CO2 capture: pre-combustion, postcombustion, and oxy-combustion. The most widely used route is post-combustion, already being used in the chemical sector.1,3−6 Alkanolamines are commonly used as the solvents in CO2 chemical absorption, including monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA).7,8 Particularly, mildly steric hindered alkanolamine of monoethylethanolamine (EMEA) has been of interest to people as a result of the advantages of high CO2 loading and reaction rate, low regeneration cost, and good corrosion resistance.9−11 Thorough foundation and application studies on these aqueous alkanolamine solutions have been made. However, it has been well-understood that energy consumption was the main factor affecting the efficiency of power plants when employing the CO2 capture process.12−14 The water evaporation in the conventional aqueous amine solutions is the major reason for the high energy consumption.15−17 Recently, non-aqueous amine solutions for CO2 capture have been widely studied as a result of their reduced decomposition and low energy consumption at high temperatures.16,18,19 However, all of the non-aqueous solvents that have been reported in the literature were alcohols, glycols, esters, or ether.12,16−25 Meanwhile, the CO2 loading of these solutions is relatively low. In our previous work, a tertiary amine of diethylethanolamine (DEEA), which has a high boiling point, a low viscosity, and a high stability, was used as the non-aqueous solvent. EMEA was chosen as the chemical reactant. The non-aqueous solution of EMEA + DEEA has shown some advantages, such as a higher CO2 loading than other non-aqueous solutions using alcohols or glycols as solvents, a higher CO2 desorption efficiency than aqueous solution, and a high CO2 cyclic loading.26 It has potential for industrial application. © XXXX American Chemical Society
Received: June 28, 2016 Revised: September 6, 2016
A
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels and few studies available introduce the specific products from SO2 degradation. Therefore, it is mandatory to study the role of SO2 in EMEA + DEEA solution for CO2 capture. Amine reclamation can prevent potential secondary pollution by heat-stable salt removal. Ion-exchange resins can be used to remove the heat-stable salts.38−41 This method was first widely used in water purification in the 1950s.42−44 Rassi and Horvath used it as a way to separate acidic and basic proteins in a single chromatographic run in the mid-1980s.38 Recently, Gao et al. reported that ion-exchange resins can also be used in coal-fired power plants to treat polluted alkanolamine solutions.45 In this work, the CO2-loaded 30 wt % EMEA + 70 wt % DEEA solution (0.4 mol of CO2/mol of EMEA) was degraded with 0.015 m3 h−1 mixed gas (200 ppm of SO2 balanced with N2) at 333.15 K for 10 days. The reaction of SO2 with the CO2-loaded EMEA + DEEA solution was explored using the 1H and 13C nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI−MS), and elemental analysis methods. The CO2−SO2-loaded EMEA + DEEA solution was purified using ion-exchange resins.
A total of 200.0 g (±0.1 g) of 30 wt % EMEA + 70 wt % DEEA solution was contacted with pure CO2 gas at a rate of 0.015 m3 h−1 (V(dot)) in the absorption apparatus (Figure 2) until the desired CO2 loading (0.4 mol of CO2/mol of EMEA). The range and accuracy of the equipment in Figure 2 are listed in Table 1. During this process, the
2. EXPERIMENTAL SECTION
where Vm is the standard molar volume of CO2 and nEMEA is the molarity of EMEA. The CO2 loading was ascertained by the titration method.47,48 The degradation apparatus used in this work was similar to the absorption apparatus. The temperature of the degradation experiment was controlled by the oil bath, and the uncertainty of the temperature was ±0.10 K. Prior to the introduction of the CO2-loaded EMEA + DEEA solution, the oil bath was first heated to 333.15 K and the whole apparatus was purged with N2 to remove air. Then, the mixed gas of 200.0 ppm of SO2 + N2 continuously flowed into the solution at a rate of 0.015 m3 h−1 for 10 days (usually more than 200 h according to the literature).27,33 During this process, the SO2 concentration of the outlet gas was analyzed using GC-9790 every 12 h. After the degradation experiment, the solution was sampled for 1H and 13C NMR, ESI−MS, and elemental analysis. 2.3. Purification Procedure. The CO2−SO2-loaded EMEA + DEEA solution can be purified using ion-exchange resins to remove heat-stable salts. The ion-exchange column consisted of a glass column with the length of 35 cm and the diameter of 2.0 cm. The column was filled with 100 mL of anion-exchange resins (D201). The constant flow rate of the solution through the anion-exchange resins was 5 mL min−1, which was controlled using a high-pressure constant flow pump (BKH-JL 10). The experiment was performed at room temperature and atmosphere pressure. The apparatus is shown in Figure 3. The range and accuracy of the high-pressure constant flow pump in Figure 3 are listed in Table 1. D201 strong basic anion-exchange resins were used to adsorb the charged polar components in the CO2−SO2-loaded EMEA + DEEA solution after the degradation experiment. D201 anion-exchange resins
Table 1. Range and Accuracy of the Equipment equipment mass flow meter (m3 h−1) wet gas flow meter (m3 h−1) oil bath (°C) thermometer (°C) high-pressure constant pump (mL min−1)
range
accuracy
0−0.030 0−0.2 room temperature−400 0−150 0.001−10.000
±1.5% ±1.0% ±0.10 °C ±1 °C ±0.2%
absorption time (t) was recorded using a stopwatch and the outlet gas volume (V) was directly recorded by the wet gas flow meter. The CO2 loading (β1, mol of CO2/mol of EMEA) was defined as follows: β1 =
2.1. Materials. EMEA (>98%) and DEEA (>99%) were purchased from Sahn Chemical Technology (Shanghai) Co., Ltd. and used without further purification. The structures of EMEA (secondary alkanolamine) and DEEA (tertiary alkanolamine) are shown in Figure 1. The anion-
Figure 1. Structures of the reagents used in this work. exchange resins (D201) were purchased from Langfang Sonnert Chemical Co., Ltd. Sodium hydroxide (NaOH, 97.0%) was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Pure CO2 gas (99.995%) and the mixed gas of 200 ppm of SO2 + N2 were purchased from Dalian Institute of Chemical Physics, Chinese Academy of Science. 2.2. Degradation Procedure. We attempted to use operating conditions that are close to industrial operation. The EMEA concentration is 30 wt % (common concentration in the industrial operation); the SO2 concentration is 200.0 ppm (as high as 196 ppm normally found after the desulfurization process);46 the CO2 loading is 0.4 mol of CO2/mol of EMEA (within the range of lean and rich loadings); and the temperature is 333.15 K (328−393 K representing typical temperatures of absorber and regenerator, respectively).
V(dot)t − V VmnEMEA
(1)
Figure 2. Flow diagram of the experimental system: (1) mass flow meter, (2) condenser, (3) drying bottle, (4) wet gas flow meter, (5) oil bath, (6) thermometer, (7) three-neck flask, (8) gas chromatograph, (9) computer, and (10) rotor. B
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
ESI−MS (LTQ Orbitrap) was used to verify the results obtained using the 1H and 13C NMR spectra through the analysis of the molecular mass. The specifications of the ESI−MS system were listed in Table 3.
Table 3. Specifications of the ESI−MS System ESI−MS ion source heater temperature (K) sheath gas flow rate aux gas flow rate spray voltage (kV) capillary temperature (K) capillary voltage (V) tube lens (V)
ESI 313 N2, 30 He, 10 4.00 593 39.00 0.00
The EMEA and DEEA concentrations before and after anionexchange purification were analyzed using gas chromatography (GC) Agilent 7890A. The specifications of the GC system are listed in Table 4.
Figure 3. Ion-exchange experimental setup: (A) CO2−SO2-loaded EMEA + DEEA solution, (B) ion-exchange column, (C) purified solution, and (D) high-pressure constant pump.
Table 4. Specification of the GC System are used in the chloride form when the following reaction takes place, with R representing the charged species:39 +
−
−
+ −
−
resin N(CH3)3 Cl + R ↔ resin N(CH3)3 R + Cl
(2)
Therefore, the anion-exchange resins D201 were first pretreated using 10 wt % NaOH solution, so that the resins can be used in the hydroxide form. resin N(CH3)3+ Cl− + OH− ↔ resin N(CH3)3+ OH− + Cl− (3) Before the anion-exchange process of the CO2−SO2-loaded EMEA + DEEA solution, the pH of the anion-exchange resins D201 was adjusted to about 7 through rinses with deionized water. The solution after the anion-exchange process was sampled for elemental analysis and 13C and 1 H NMR spectroscopic analyses. 2.4. Analysis Methods. The oxygen bomb combustion ion chromatography (IC) method was used to measure the sulfur elemental content.49−53 Combustion with oxygen in a sealed Parr bomb is to convert liquid samples into soluble form ready for chemical analysis. The IC instrument Dionex 500 was used to analyze the soluble form of sulfur. The specifications of the IC system were listed in Table 2.
HP5-MS 30 250 0.25 323 3 3 240 15.6 1 250 280
3. RESULTS AND DISCUSSION 3.1. SO2 Removal Efficiency. The SO2 removal efficiency (ηSO2) can be calculated as follows: c − cout ηSO = in × 100% 2 c in (4)
Table 2. Specifications of the IC System
where cin and cout are the SO2 concentrations in the inlet and outlet gases, respectively. The mixed gas of 200.0 ppm of SO2 + N2 continuously flowed into the CO2-loaded EMEA + DEEA solution at a rate of 0.015 m3 h−1 for 10 days. During this process, the SO2
IC column column temperature (K) detector suppressor type suppressor current (mA) eluent solution eluent concentration (mM) flow rate (mL/min)
column length (m) internal diameter (μm) thickness (μm) initial temperature (K) initial hold time (min) oven ramp (1) (K min−1) final temperature (°C) final hold time (min) flow rate (mL/min) injector temperature (°C) detector (FID) temperature (°C)
IonPacAS22-AG22 303 CD ASRS-4 mm 25 NaHCO3 + Na2CO3 1 + 3.5 1.0
1 H NMR spectroscopic analysis, with the parameters of 90° pulse angle, 3.744 s acquisition time, 1 s delay time, and 16 number of scans, was obtained at 298 K with a Varian INOVA spectrometer operated at 500 MHz. Deuterated dimethyl sulfoxide (DMSO) was introduced into the NMR tube blending with the solution for deuterium lock. 13 C NMR spectroscopic analysis can be used as an efficient tool to identify different carbon-containing species during a chemical reaction.54−59 In this work, the 13C NMR spectroscopy, with the parameters of 90° pulse angle, 1.199 s acquisition time, 20 s delay time, and 256−512 number of scans, was obtained at 298 K with a Varian INOVA spectrometer operated at 100 MHz. DMSO was introduced into the NMR tube blending with the solution for deuterium lock.
Figure 4. SO2 concentration of the inlet and outlet gases during the degradation process: (■) inlet gas and (□) outlet gas. C
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 5. 1H NMR spectra: (a) 1H NMR spectrum of the CO2-loaded EMEA + DEEA solution, (b) 1H NMR spectrum of the CO2−SO2-loaded EMEA + DEEA solution, and (c) 1H NMR spectrum of the solution after purification.
3.2. 1H NMR Investigation of the Reaction. Figure 5b shows the 1H NMR spectrum of the CO2−SO2-loaded EMEA + DEEA solution after degradation treatment, along with the 1 H NMR spectrum of the CO2-loaded EMEA + DEEA solution in Figure 5a for comparison. The peak at 6.22 ppm was assigned to proton hydrogen of protonated EMEA and DEEA. The four peaks at 1.08, 2.79, 3.12, and 3.56 ppm were assigned to hydrogen atoms in carbamate. It can be seen that no new peaks appeared for the CO2−SO2-loaded EMEA + DEEA solution compared to the CO2-loaded EMEA + DEEA solution. There may be two reasons for this phenomenon: (1) SO2 may not react with the CO2-loaded
concentration of the inlet and outlet gases was recorded using GC-9790 every 12 h and is shown in Figure 4. It can be seen that the outlet SO2 concentration increased slightly with time increasing; however, the maximum value was still very small compared to the inlet SO2 concentration, which indicated that SO2 can be absorbed immediately and accumulated in the solution. After 10 days of degradation treatment, the SO2 removal efficiency has fallen from 100 to 86%. The SO2 loading of the solution after degradation treatment for 10 days was determined using elemental analysis, which can amount to 0.06 mol of SO2/mol of EMEA. D
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 6. 13C NMR spectra: (a) 13C NMR spectrum of the CO2-loaded EMEA + DEEA solution, (b) 13C NMR spectrum of the CO2−SO2-loaded EMEA + DEEA solution, and (c) 13C NMR spectrum of the solution after purification.
EMEA + DEEA solution directly. (2) The structure of the products from SO2 degradation was similar to the existing matter in the solution, which led to the overlap of the peaks. 13C NMR and ESI−MS methods were further used to verify the hypothesis. 3.3. 13C NMR Investigation of the Reaction. Figure 6b shows the 13C NMR spectrum of the CO2−SO2-loaded
EMEA + DEEA solution after degradation treatment, along with the 13C NMR spectrum of the CO2-loaded EMEA + DEEA solution in Figure 6a for comparison. The original solution displays two groups of peaks: a, b, c, and d are assigned to EMEA, and 1, 2, 3, and 4 are assigned to DEEA. With regard to the 13 C NMR spectrum of the CO2-loaded EMEA + DEEA solution, E
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. ESI−MS spectroscopy (negative-ion mode) of the CO2−SO2-loaded EMEA + DEEA solution after degradation treatment.
Table 5. Some Experimental m/z Values and Their Probable Attribution
F
DOI: 10.1021/acs.energyfuels.6b01562 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels a new signal that appeared at 162.58 ppm was assigned to carbamate. Meanwhile, four other signals (a′, b′, c′, and d′) that appeared near the signals of pure EMEA corresponded to the same carbon atoms whose chemical shifts were changed as a result of the formation of carbamate. Only four common peaks for EMEAH+/EMEA and four common peaks for DEEAH+/ DEEA in the 13C NMR spectroscopy were observed as a result of the fast proton transfer between alkanolamine and protonated alkanolamine.60,61 For the 13C NMR spectrum of the CO2−SO2loaded EMEA + DEEA solution, two small and new peaks at 16.73 ppm (a″) and 44.12 ppm (b″) appeared near the signals of EMEA carbamate; they were assigned to the carbon atoms of the new product. According to the CO2 reaction mechanism with EMEA + DEEA solution62 and the acidic property of SO2, it is assumed that (1) SO2 may react with amino in EMEA and (2) SO2 may react with hydroxyl in EMEA, DEEA, or carbamate. The ESI−MS method was further used to verify the hypothesis. 3.4. ESI−MS Investigation of the Reaction. Figure 7 shows the ESI−MS spectroscopy in negative-ion mode of the CO2−SO2-loaded EMEA + DEEA solution, which was used to verify the results obtained using the 1H and 13C NMR spectra through the analysis of the molecular mass. Preliminary experiments were run without a chromatographic separation on the sample of the CO2−SO2-loaded EMEA + DEEA solution. The MS full-scan acquisition was run in the m/z range of 50−300. Table 5 reports some experimental m/z values and their probable attributions. Three peaks with the m/z values of 152.0386, 180.0698, and 196.0647 were assigned to organic sulfide forming from the direct reaction of SO2 with EMEA, DEEA, and carbamate, respectively, which verified the assumption. Some other peaks may be assigned to the degradation products generated by oxidation, such as the peaks with the m/z values of 59.0140, 75.0090, 168.0335, and 212.0418. Therefore, SO2 can react with amino and hydroxyl in EMEA and hydroxyl in DEEA and carbamate directly forming organic sulfide, which can be further degraded through oxidation. The proposed reaction pathways of the amine degradation after SO2 treatment for 10 days are shown in Scheme 1. 3.5. Purification of the CO2−SO2-Loaded EMEA + DEEA Solution. The CO2−SO2-loaded EMEA + DEEA solution after degradation treatment was purified using anion-exchange resins, and the purified solution was sampled for elemental analysis and 1 H and 13C NMR spectroscopic analyses. Figure 5c shows the 1 H NMR spectroscopy of the purified solution. It can be seen that the peaks of protonated amines and carbamate did not appear, which indicated that protonated amines were neutralized and amine carbamate was adsorbed by the anion-exchange resins. Figure 6c shows the 13C NMR spectroscopy of the purified solution. It can be seen that there were no peaks of amine carbamate and organic sulfide, which indicated that carbamate and organic sulfide were adsorbed by the anion-exchange resins. The sulfur elemental content of the solution after purification by the anion-exchange resins was underdetection (
