MEA/SG capture CO2 in thermal electrochemical co-drive system
Introduction
The massive emissions of carbon dioxide (CO2) have caused a series of global problems, such as global warming and rising sea levels, which threaten the global environment and human health. According to the International Energy Agency, the average annual global atmospheric CO2 concentration in 2018 is 407.4 ppm, an increase of 2.4 ppm from 2017. This is far beyond the pre-industrial level, which is only between 180 ppm and 280 ppm. Therefore, CO2 capture has received great attentions, and carbon capture and storage (CCS) technology has also developed rapidly in recent years [1].
CO2 is produced during the burning of fossil fuels [2]. The most recognized methods for CO2 capture include chemical absorption, physical and chemical adsorption, membrane separation and mineralization. The most commonly applied technique is chemical absorption by solvents.
The general chemical absorption method is that the flue gas in the absorber contacts with amine solution counter-currently to proceed the absorption reaction. After absorbing CO2, the rich solution is sent to the stripper. In the stripper, the rich solution contacts with the high-temperature steam counter-currently for desorption. The desorbed CO2 is dehydrated and compressed to about 11 MPa before being sent out for storage. The lean solution flows out from the bottom of the stripper and is recycled and fed to the top of the absorber for recycling absorption.
Although chemical absorption is considered as the most economical Post Combustion Capture(PCC) technology [3], energy consumption and deployment costs are still too high [4].
Electrochemical separation is an emerging CO2 capture technology. This technique was firstly proposed to promote the transport of NO across the liquid membrane using Fe(II) [5]. Following the footprint, the researchers proposed several electrochemical separation techniques, mainly focusing on process, reactor and solvent.
For the electrochemical separation processes, researchers did a lot of work. Stern et al. [6] studied the performance of copper and ethylenediamine (EDA) complexes in the CO2 desorption process. EDA has a strong absorption capacity as a chemical solvent [7], and has been studied by substantial scholars. Singh et al. [8] have demonstrated a completely new type of chemically reversible, electrochemical process for capture and release of CO2 based on an organic disulfide/thiocarbonate redox couple. In this new process, the binding constant was calculated to be −66 kJ·mol−1. Carbon dioxide separation can also be achieved by using electrochemically controlled pH swings as the primary driver for CO2 capture and release [[9], [10], [11]]. Additionally, Ranjan et al. [12] recently studied the processof CO2 capture by the electrochemically generated Bipy − anion radical and the subsequent release of CO2 by the oxidation of the bipyridinyl radical − N-carboxylate adduct. In this case, both the oxidative electron transfer step and the decarboxylation are extremely fast, which is an attractive feature for any CO2 separation scheme. However, more additional experiments are needed to explore CO2 separations based on these processes.
For the electrochemical separation reactor, Rexed et al. [13] retrofitted Molten Carbonate Fuel Cells (MCFC) for CO2 separation from post-combustion flue gases. Using the MCFC cycle principle, they designed a plate-and-frame electrochemical reactor to retrofit the stripper. The effects of different current intensities and different flow rates on carbon dioxide desorption were studied. The stability of the system and the performance of the porous electrode were tested. The feasibility of the reactor provides a useful reference for the follow-up research. However, the research lacks in-depth discussion of the principle, which produces the subsequent optimization problems.
For the solvents used for electrochemical separation, Gurkan et al. [14] have used ionic liquids as electrolytes and ruthenium as a carbon dioxide carrier for electrochemical separation. It is suggested that the electrochemical separation process in the ionic liquid requires a lower reaction potential. They estimated that the energy consumption is 70 kJ·mol−1 CO2, which is much lower than the traditional process of 145−200 kJ·mol−1 CO2. However, the cost is still too high for the engineering application since the solvent is quite expensive.
The researches above suggest that electrochemical separation is feasible to separate CO2. However, the main problem is still the energy consumption. In order to reduce the energy consumption further, the Thermal Electrochemical Co-drive System (TECS) [15] has been developed, which has the advantages of high desorption rate and low energy consumption by our previously theoretical analysis. However, the fluid flow and capture performance is still unknown in the stripper in TECS. In order to conduct an in-depth study, a pilot-scaleTECS was set up and a series of experiments and simulations were conducted. Since sodium glycinate(SG)solution has the advantages of low volatility and fast absorption rate [16,17], and the reaction enthalpy between SG and CO2 is lower than that of monoethanolamine(MEA) [18,19], the MEA-SG mixed solution is adopted in the experiment. Meanwhile, the CO2 capture performance of MEA-SG solution was studied in the TECS.
Additionally, inspired by cryogenic CO2 capture [20] and considering the redundant LNG cold energy, the LNG cold energy is proposed to intensify the CO2 desorption. By the literature review, Hirakawa et al. [21]has concluded that the LNG regasification process can be integrated into the low temperature CO2 capture process. Zhao et al. [22] combined the LNG regasification system with CO2 capture system by using two parallel Rankine cycles, and the combined cycle was analyzed according to the conditions of Xianrendao port and Dashiqiao area in China. In terms of engineering applications, in 2019, China has built 19 large LNG receiving stations along the coast and numerous small LNG satellite stations in the interior. The distance between LNG receiving stations and thermal power plants is very short, and quite a number of receiving stations have carried out LNG cold energy utilization projects. Among them, LNG cold energy utilization in cold energy storage technology has become one of the main ways in LNG satellite stations. Thus, it is possible to integrate the LNG cold energy with the TECS in this work. The idea of this work is to use the LNG cold energy to separate the substances in the anode and cathode. It will reduce the energy consumption for TECS further. The CO2 desorption performance after the LNG cold energy integration is discussed in the anode and cathode side in TECS.
Section snippets
TECS system
A Thermal Electrochemical Co-drive System (TECS) experimental device was established in this work. The reactor is 60 cm high and 6 cm in diameter. As shown in Fig. 1, the reactor consists of the main column, and copper electrodes. The jacket is arranged outside the column to heat the liquid. The total 10 sets of electrodes are used. The order of anode and cathode is set as follows: the top of each set of electrodes is the anode, and the bottom is the cathode. The electrode is plate electrode.
Gas and liquid two-phase flow model
The CO2 desorption model is developed by considering the CO2 concentration Y in the reactor, which is developed by including the electrochemical effects.
The A term above represents the drive force for the CO2 desorption, which is developed by the reaction kinetics model in Section 2.4.
The continuity equation for the liquid phase is developed as
The parameter is determined by the chemical reaction and molecular weight of the reactant and product.
Similarly, the
Temperature distribution
The temperature distribution is obtained in Fig.7. As shown in Fig.7, the temperature becomes low at the electrode zone. The temperature in the center of the reactor is between 353 K and 349 K. The temperature away from the electrode is higher than the zone around the electrode. This is due to the fact that the electrochemical effect for CO2 desorption is strong around the electrode zone. It is found that the temperature difference is only about 5 K, which is much lower than that at the
Energy consumption
The energy consumption is calculated by considering the consumption of electrical energy and heat. Considering the different CO2 desorption temperature of 343 K,353 K,363 K and 373 K, the energy consumption is calculated and the result is provided in Table 4. As shown in Table 4, the energy consumption amount is determined at 1.3 G J/t to 2.1 G J/t, which is much lower than the conventional process in the literature [33,34]. This energy consumption amount is calculated by considering the
Conclusions
Experiment, reaction kinetics model and developed gas-liquid two-phase flow model were used to understand the flow and capture performance in TECS by MEA-SG solution. The temperature distribution, CO2 concentration distribution, electrical potential distribution and fluid flow field results in the stripper were obtained. It was found that current density and CO2 loading have great influence on CO2 desorption. In TECS, the desorption temperatures reduced to below 363 K. The energy consumption
CRediT authorship contribution statement
Wancheng Ding: Methodology, Writing - original draft, Software, Conceptualization, Writing - review & editing. Yunsong Yu: Supervision. Zaoxiao Zhang: Supervision. Geoff Wang: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Financial support of the National Natural Science Foundation of China (nos. 51506165 and 21736008) is gratefully acknowledged. This work is also supported by the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2019JM-052), “Fundamental Research Funds for the Central Universities”, The Fundamental Research Funds for the Central Universities Creative Team Plan (No.cxtd2017004 in Xi’an Jiaotong University), and Shaanxi Creative Talents Promotion Plan-Technological Innovation
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