Kinetic characterization of solvents for CO2 capture under partial oxy-combustion conditions

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi: 10.1016/j.egypro.2017.03.1340 13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland Kinetic characterization of solvents for CO2 capture under partial oxy-combustion conditions Fernando Vega*, Mercedes Cano, Esmeralda Portillo, Sara Camino, José Antonio Camino, Benito Navarrete Chemical and Environmental Engineering Department, School of Engineering, University of Seville, C/ Camino de los Descubrimientos s/n, Sevilla 41092, Spain Abstract Partial oxy-combustion is proposed as a new carbon capture concept based on the use of oxygen-enriched air as an oxidizer instead of air during the combustion stage followed by a CO2 chemical absorption. This novel approach has showed potential advantages compared with other CCS technologies that may lead to a further reductions of the energy penalties related to the CO2 separation process. In this work, a lab-scale apparatus operating under semi-batch conditions was used for the kinetic evaluation of potassium carbonate (K2CO3), aminoethylpiperazine (AEP) and methyldiethanolamine (MDEA) and monoethanolamine (MEA). Four experiments were set for each solvent where the flue gas composition varied from 15%v/v (post-combustion) to 60%v/v. In general, the use of a more CO2 concentrated flue gas enhanced the CO2 absorption process and the kinetic was significantly improved under partial oxy-combustion conditions. The CO2 loading also increased in the presence of a more CO2 concentrated flue gas for each solvent tested. Those results strengthened partial oxy-combustion as a potential CCS approach in comparison with more mature technologies such as post-combustion and oxy-combustion. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: CCS; CO2 capture; solvent; kinetic; absorption * Corresponding author. Tel.: +34-954481391; fax: +34-954487385. E-mail address: [email protected] © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. 2056 Fernando Vega et al. / Energy Procedia 114 ( 2017 ) 2055 – 2060 1. Introduction In recent years, much effort are focused in the field of CCS technologies in order to achieve further improvements on energy efficiency and novel solvent formulation in comparison with conventional (non-CCS) power plants. In this sense, new capture concepts are currently under research that can improve performance of the CO2 separation process in terms of lower efficiency penalty, capture costs and, therefore, electricity costs related to those from actual state-of art of CCS and low-carbon technologies in comparison with conventional CCS technologies. It is estimated that CO2 capture accounts for 70–90% of the total operating costs of an entire CCS unit operates in an energy-intensive process [1,2]. In respect to the most feasible options on CCS technologies, concerns about the energy consumption required for solvent regeneration and high purity oxygen production are the major drawbacks for the commercial deployment of post-combustion and oxyǦcombustion technologies, respectively. Partial oxy-combustion is proposed as a new carbon capture concept consisting of a hybrid CO2 capture process between post-combustion and oxy-combustion. It is based on the use of oxygen-enriched air as an oxidizer instead of air for fossil-fuel combustion stage followed by a CO2 separation process based on chemical absorption [3, 4, 5]. This novel approach has showed potential advantages compared with other CCS technologies that may lead to a further reductions in terms of both CO2 capture and electricity costs [6]. Improvements on the CO2 separation process should be identified to balance the energy requirements associated to oxygen-enriched air production for fossil fuel combustion stage and, therefore, make partial oxy-combustion competitive in comparison with post-combustion and oxy-combustion capture. The aim of this work is to evaluate the kinetic behavior of conventional solvents proposed for post-combustion under partial oxycombustion conditions. Partial oxy-combustion development requires support from the above-mentioned advantages to be considered as a real CCS capture option. In this respect, improvements on the absorption kinetics areconsidered the first step towards significant reductions of the energy consumption associated with the overall CO2 capture process. 2. Methodology A lab-scale apparatus operating under semi-batch conditions was used for the kinetic evaluation of a number of selected solvents. A detailed description of the lab-scale apparatus can be found in a previous work [3]. Three different solvents were studied - potassium carbonate (K2CO3), aminoethylpiperazine (AEP) and methyldiethanolamine (MDEA), all of them with analysis grade 99% of purity. Aqueous solutions of these solvents were compared with the benchmark solvent – MEA 30%p/p – which was tested under the same conditions. A preliminary evaluation of the solvent behavior in aqueous solution was performed before choosing the optimal composition of aqueous solution that ensure the stability of the blend. Solid formation during the preparation of the different blends must be avoided, among others considerations such as low volatility and viscosity, high CO2 capacity and solvent degradation. Several experiments for each solvent were performed in which the CO2 concentration of the flue gas varies between 15%v/v - 60%v/v. The kinetics of the CO2 absorption process were evaluated from both the gas phase and the liquid phase. To evaluate the kinetic behavior from the gas phase, the CO2 absorption rate was determined using a FTIR analyzer installed in the exhaust gas leaving the reactor. The FTIR analyzed allowed to determine the evolution of the CO2 absorption rates expressed as the CO2 flow-rate (mL/min) transferred from the bulk gas phase to the liquid phase by means of the Eq. 1, obtained from Li et al. [7]: Fernando Vega et al. / Energy Procedia 114 ( 2017 ) 2055 – 2060 2057 (1) Where, Rabs, represents the CO2 absorption rate expressed in mL CO2/min. Qfg, denotes the synthetic flue gas flowǦrate expressed in mL/min. ܻ ஼ைమ ூே and ܻ ஼ைమ ை௎், denote the molar CO2 flue gas composition at the inlet and the outlet of the rig. In respect to the evaluation of the liquid phase, aliquots of loaded solvents were withdraw at different intervals during the experiments to determine the CO2 loading evolution, referred as mole CO2 per mole solvent. Therefore, the evolution of the CO2 loading of the solvents is evaluated for each experiment. According to Lin et at. [8], the system response to a step input change in terms of CO2 composition of the flue gas could be treated as a general first order dynamics system. The system response can be defined using a characteristic time (IJ) and an equilibrium CO2 loading (Į*) as Eq. 2 shows: (2) In which, Į(t), denotes the CO2 loading in the liquid phase as a function of time. Į*, denotes the CO2 saturated loading at equilibrium. t, denotes experimental time expressed in min. IJ, denotes the characteristic time of the absorption process expressed in min. The experiments were extended until the equilibrium was reached. In this case, the CO2 concentration of the exhaust gas leaving the reactor is compared with the CO2 concentration at the inlet to ensure a completely saturation of the solvent in aqueous solution. 3. Results The preliminary test regarding the solvent stability in aqueous solution determined the appropriate compositions of the solvents were K2CO3 30%p/p, AEP 30%p/p and MDEA 50%p/p that were further studied in the lab-scale semibatch apparatus. In general, the CO2 composition of the flue gas had a relevant impact on the absorption kinetics, as it described in the following sections. 3.1. Gas phase evaluation From the gas phase evaluation, the Fig.1 summarizes the evolution of the CO2 absorption rates for the experiments using AEP 30%p/p and 20%v/v, 40%v/v and 60%v/v CO2 concentration. This solvent exhibited a significant increase of the CO2 absorption rate from CO2 concentrations above 20%v/v, which meant that equilibrium was reached faster than in the 15%v/v (post-combustion test). Similar behavior was obtained in MEA experiments [3]. The presence of CO2 concentrations higher than 20%v/v speeded up the CO2 absorption process and a substantial change of the solvent kinetic behavior was also observed. 22 2 2 () min 1 IN OUT CO CO abs fg OUT CO YY mLCO RQ Y  ªº  ¬¼  ªº  ¬¼ * () (1 – ) t te W DD   2058 Fernando Vega et al. / Energy Procedia 114 ( 2017 ) 2055 – 2060 Fig. 1. CO2 absorption rates obtained from the experiments using AEP run in the semi-batch lab-scale rig. In K2CO3 30%p/p and MDEA 50%p/p experiments, the presence of more CO2 content in the flue gas had no influence on the kinetics and the absorption process occurred following the same pattern in all the cases. However, the CO2 absorption rate obtained in MDEA 50%p/p experiments was globally increased at higher CO2 concentrations as occurred in MEA and AEP experiments, as Fig. 2 shows. Fig. 2. CO2 absorption rates obtained from the experiments using MDEA run in the semi-batch lab-scale rig. 3.2. Liquid phase evaluation The evolution of the CO2 loading also exhibited higher values in the experiments using higher CO2 concentrated flue gas for all the solvent tested. In MEA 30%p/p cases, a flue gas composed of 60%v/v CO2 reached a CO2 loading up to 0.60 mole CO2/mole solvent at equilibrium, whereas the experiment performed using 15%v/v CO2 provided 0.48 mole CO2/mole. Therefore, an increase of 25% in the CO2 loading at equilibrium was accomplished with MEA 30%p/p using a four times higher CO2Ǧconcentrated flue gas than the conventional flue gas composition derived from airǦfired coal combustion. The rest of solvent studied in this work follow the same trend as it is shown in Fig. 3. Fernando Vega et al. / Energy Procedia 114 ( 2017 ) 2055 – 2060 2059 Fig. 3. CO2 loading at several CO2 concentrations of the flue gas for each solvent studied in this work. The higher increase of the CO2 loading was achieved for K2CO3 30%p/p and MDEA 50% p/p which the CO2 loading at 60%v/v CO2 were four times and two times higher respect to 15%v/v CO2 experiments, respectively (Table 1). Table 1. Į* and Į enhancement (%) at several CO2 concentrations of the flue gas for each solvent studied in this work. MEA MDEA K2CO3 AEP Į* ǻĮ (%) Į* ǻĮ (%) Į* ǻĮ (%) Į* ǻĮ (%) 15% CO2 0.535 --- 0.216 --- 0.217 --- 1.221 --- 20% CO2 0.573 7.1 0.265 22.5 0.390 79.4 1.272 4.2 40% CO2 0.584 9.2 0.376 73.6 0.630 189.8 1.320 8.1 60% CO2 0.599 12.0 0.448 107.0 0.835 283.8 1.329 8.8 Fig. 4 represents the characteristic time for all the experiments carried out in this work. A favored kinetic by means of a reduction of the characteristic time was found for each solvent as CO2 concentration was increased. AEP 30%p/p and MEA 30%p/p showed the fastest absorption kinetics, providing characteristic time below 2000 s in most of the experiments. K2CO3 30%p/p significantly reduced the characteristic time in this conditions but both solvents – including MDEA – showed slow kinetics even though at elevated CO2 concentrations. Fig. 4. Characteristic time at several CO2 concentrations of the flue gas for each solvent studied in this work. 2060 Fernando Vega et al. / Energy Procedia 114 ( 2017 ) 2055 – 2060 4. Conclusions A lab-scale apparatus operating under semi-batch conditions was used for studying the absorption kinetic of K2CO3 30%p/p, AEP 30%p/p and MDEA 50%p/p under partial oxy-combustion conditions. MEA 30%p/p was also tested as benchmark. Several experiments for each solvent were performed in which the CO2 concentration of the flue gas varies between 15%v/v - 60%v/v. In particular, an increase of the CO2 content in the flue gas significantly enhanced the CO2 absorption process in both terms, CO2 absorption rate and CO2 loading. The use of a more CO2 concentrated flue gas enhanced the CO2 absorption process for any solvent tested in this work. The kinetic significantly improved under those conditions, showing a further reduction of the characteristic time of the system response. The CO2 loading achieved higher values – up to four times for K2CO3 30%p/p– as CO2 concentration was shifted from 15%v/v to 60%v/v. Those results strengthened partial oxy-combustion as a potential CCS approach. The favored kinetics could lead to large reductions of the absorption equipment size. The feasibility of this novel technology could also have impact on the global performance of the separation process, particularly on the energy requirements associated to the solvent regeneration. 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