Scholarly article on topic 'The Advances of Post-combustion CO2 Capture with Chemical Solvents: Review and Guidelines'

The Advances of Post-combustion CO2 Capture with Chemical Solvents: Review and Guidelines Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Xiaomei Wu, Yunsong Yu, Zhen Qin, Zaoxiao Zhang

Abstract Carbon dioxide is the major greenhouse gas that contributes to the global warming more than 60%. Carbon dioxide from power plants is one of the main sources. Therefore it is essential to reduce the CO2 emission from power plant flue gas. Post- combustion carbon capture is the optimal choice in near-to meddle-term, since it does not need to change the configuration of the power plants essentially. Chemical absorption with amine-based solvents is currently the state-of-the-art technology for post- combustion carbon capture. This paper introduces the typical CO2 amine absorption process and analyses the main problems during the absorption process. To make the process more efficient, several improvements were investigated: increasing gas-liquid contacting area, searching for new type of reactant, and dilution of the aqueous fraction with organic liquids. The present study aims to summarize the improvements in amine scrubbing process for CO2 capture and forecast the promising research directions in the future.

Academic research paper on topic "The Advances of Post-combustion CO2 Capture with Chemical Solvents: Review and Guidelines"

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Energy Procedia 63 (2014) 1339 - 1346

GHGT-12

The Advances of Post-Combustion CO2 Capture with Chemical Solvents: Review and Guidelines

Xiaomei Wua, Yunsong Yua, Zhen Qina, Zaoxiao Zhanga,b*

aSchool of Chemical Engineering and Technology, Xi 'an Jiaotong University, No.28 Xianning West Road, Xi 'an 710049, P.R. China bState Key Laboratory of Multiphase Flow in Power Engineering, Xi 'an Jiaotong University, No.28 Xianning West Road, 710049, P.R. China

Abstract

Carbon dioxide is the major greenhouse gas that contributes to the global warming more than 60%. Carbon dioxide from power plants is one of the main sources. Therefore it is essential to reduce the CO2 emission from power plant flue gas. Postcombustion carbon capture is the optimal choice in near-to meddle-term, since it does not need to change the configuration of the power plants essentially. Chemical absorption with amine-based solvents is currently the state-of-the-art technology for postcombustion carbon capture. This paper introduces the typical CO2 amine absorption process and analyses the main problems during the absorption process. To make the process more efficient, several improvements were investigated: increasing gas-liquid contacting area, searching for new type of reactant, and dilution of the aqueous fraction with organic liquids. The present study aims to summarize the improvements in amine scrubbing process for CO2 capture and forecast the promising research directions in the future.

© 2014TheAuthors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: CO2 emission, post-combustion capture, chemical absorption, absorbents

1. Introduction

1.1. Background

Fossil fuels provide more than 80% of the world's total energy demands and the emission of carbon dioxide (CO2) from the burning of fossil fuels has been identified as the major contributor to global warming [1]. However, it is

* Corresponding author. Tel.: +86-29-82660689; fax: +86-29-82668566. E-mail address: zhangzx@mail.xjtu.edu.cn

1876-6102 © 2014 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/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi:10.1016/j.egypro.2014.11.143

hard to reduce the dependency on fossil fuels and switch to non-fossil fuels such as hydrogen and renewable energy at the current state of development. Moreover, the conversion efficiency of other non-fossil fuels is mostly not as high as that of fossil fuels [2]. Thus to meet mid-to long-term targets, it is necessary to find some cost-effective solutions to reduce CO2 emission into the atmosphere [3, 4].

The largest CO2 potential sources today are fossil fuel power plants which emit more than one-third of the CO2 emissions worldwide [5-7]. Therefore it is essential to reduce the CO2 emission from power plant flue gas. The CO2 capture technologies studied in this paper can be classified as Fig. 1. There are three major approaches for carbon capture and storage (CCS): pre-combustion capture, oxyfuel combustion capture and post-combustion capture [8]. Post-combustion carbon capture is the suitable choice in the near-to middle-term, since it can be retrofitted to the existing power plants compared to the other two approaches [9-11]. As the final carbon capture step of whole combustion process, post-combustion CO2 capture can be roughly divided into three categories: biological, physical and chemical methods. As to the biological method, CO2 is fixed by photosynthesis of plants, alga and photo synthetic bacteria without energy consumption. The biological method includes the biological fixation by terrestrial vegetation [12] and marine or freshwater microalgae [13]. But the low absorption capacity and efficiency obstruct its application. Physical method is a way to absorb and remove CO2 from flue gas with organic solutions. There is no chemical reaction in the CO2 capture process and the key point of this method is to determine the characteristics of absorbents. The physical method contains physical absorption [14], cryogenic condensation and membrane separation technology [15]. Compared with the biological method, the physical method has higher CO2 absorption efficiency, but the cost of absorbent is higher because of some special requirements. As to chemical method, CO2 is removed from power plant flue gas by reacting with chemical absorbents. The chemical method includes chemical adsorption [16], absorption [17] and chemical looping combustion [18]. By comparison, the chemical absorption method has a lot of obvious advantages, e.g., high efficiency, mature technology and low cost [19]. In chemical absorption methods, the most representative one is CO2 capture by amine solution. Its basic principle is neutralization reaction in which amine solution is used as absorbent to react with CO2 in the flue gas and then form carbonate and/or bicarbonate. Among all these techniques, chemical absorption into an aqueous amine solution is the most mature technology for post-combustion CO2 capture [20].

Fig. 1. Technology options for CO2 capture.

Amine scrubbing is a promising post-combustion technology for carbon capture from coal-fired power plants. A good amine solvent should have fast CO2 absorption/desorption rate, high CO2 capacity, low degradation rate, low viscosity, and low amine volatility [21]. The energy consumption in the whole process is mainly depend on the desorption process, so it is essential to reduce the energy penalty of CO2 desorption process [22, 23].

1.2. Outline of the paper

This paper aims to summary the prevailing research work carried out so far in post-combustion capture with chemical absorption. In section 2, the typical CO2 amine absorption technology is introduced. Then the main problems in the absorption process are summarized. To make the process more efficient, in section 3, several improvements were investigated: increasing gas-liquid contacting area, searching for new type of reactant, and dilution of the aqueous fraction with organic liquids. Conclusions are drawn in section 4. The future research directions based on the available achievements are also predicted in the end.

2. Post-combustion CO2 capture with chemical absorption

2.1. Typical CO2 amine absorption process

Post-combustion CO2 capture means that CO2 is removed after combustion of fossil fuel. In other words, CO2 is captured from power plant flue gas or other large point sources. The most commonly used solvent is monoethanolamine (MEA) because it has a faster rate of reaction with CO2 than other secondary or tertiary amine, which allows absorption to take place in a smaller column. The process flow diagram is as follows.

Fig. 2. The schematic overview of CO2 removal process by MEA.

2.2. Process description

The continuous CO2 capture system has been widely studied by many researchers [24, 25], as shown in Fig. 2. MEA is the most used solvent to absorb CO2 in the capture system. Absorber and stripper are the main components where absorption and desorption process take place, respectively [26]. With the advantage of stability and high efficiency, packed column is usually chosen as the absorber and stripper in CO2 capture system. During the chemical absorption process, flue gas enters a packed bed absorber from the bottom and counter-currently contacts with CO2 lean solvent for absorption, then the CO2 rich solvent flows into a stripper for thermal regeneration [27]. After solvent regeneration, the produced CO2 lean solvent is pumped back to the absorber to absorb CO2 again. And the pure CO2 released from the top of stripper is then compressed to about 15MPa for the subsequent transportation and storage. The operation pressure of the whole system is approximate atmospheric pressure and the temperatures in the absorber and stripper are generally in the ranges of 40-50°C and 100-140°C, respectively [8, 28]. The theoretically minimum energy requirement of CO2 recovery from power plants flue gas and compression of CO2 to 150 bar is 0.396 GJ/tonne CO2. For operation in a practical power plant, 0.72 GJ/tonne CO2 is hopefully to be achieved [20].

3. Main problems and improving methods

MEA is always chosen as the absorbent in CO2 chemical absorption process because it has a faster reaction rate, which allows absorption to take place in a shorter column [29]. But the typical MEA process suffers the following disadvantages for CO2 separation from flue gas [30]: (1) low carbon dioxide capture capacity (g CO2/g absorbent); (2) amine degradation by SO2, NOX and oxygen in flue gas which induces a high absorbent makeup rate; (3) high equipment corrosion rate; and (4) high energy consumption for solvent regeneration.

To make the process more efficient, several promising improvements were investigated: increasing gas-liquid contacting area, searching for promising new type of absorbent, and dilution of the aqueous fraction with organic liquids. The present study aims to summarize the improvements in amine scrubbing process for CO2 capture and forecasts the promising research directions in the future.

3.1. Gas-liquid contacting area

As has been proved, increased contacting area per unit volume will enhance gas-liquid interactions and thus mass transfer. Structured packing is widely used for post-combustion CO2 capture by amine scrubbing because of its low pressure drop, good mass transfer efficiency, and ease installation. In the CO2 capture process, absorber and stripper performance are highly depended on the effective mass transfer area of the packing [31].

Structured packing involves a certain structure (not specified) to packing the material in the column and random packing involves a random structure to the packing material. They both rely on the principle that packing will increase the contacting area of the solvent and entering flue gas, and decrease the residence time. In an experiment, CO2 entering an absorber which was packed with different materials such as BX gauze, Flexipac, and intalox saddle (made by Norton Chemical Process Products). The experiment obtained that structured packing showed the greatest solvent utilization of 43.9% (fraction of solvent that absorbed CO2), while the random packing had only 28.6% solvent utilization [32]. Absorption experiments also obtained that for a particular amine, structured packing improves the absorber efficiency and absorption rate compared to the traditional random packing [33]. So the development of new structured packing is a feasible way to improve the CO2 absorption process.

3.2. Promising new type of absorbent

In recent years, a lot of efforts have been put to develop new solvents in order to enhance CO2 absorption performance. The most researched and promising solvents are mentioned as follows.

The development of sterically hindered amine is considered to be a breakthrough because of its significant number of advantages. Possibly the most promising absorption process is based on the KS-1, KS-2, and KS-3 solvents being developed by Mitsubishi Heavy Industries (MHI) [34]. This family of solvents shows higher CO2 loading, lower regeneration conditions, and almost no corrosion, degradation, and amine loss [35]. A novel packing material, KP-1, has also been developed which aims to further improve this process. Development has reached the pilot plant test stage at MHI [36].

Piperazine (PZ) is a cyclic amine with two secondary amine nitrogens and it is a promising solvent for CO2 absorption in post-combustion carbon capture process [37]. It has a high absorption rate [38], good capacity [39], and good resistance to thermal and oxidative degradation [40]. Concentrated 8 molal (m) (40 wt %) PZ with two-stage flash regeneration is an advanced second generation amine-based process for CO2 capture which has been developed by the University of Texas [41]. 8 m (40 wt %) PZ has doubled the CO2 absorption rate and capacity of 7 m MEA. PZ also has a moderately high heat of absorption (70 kJ/mol) and is oxidatively and thermally stable, which should provide 10 to 20% better energy performance than 7 m MEA with thermal swing regeneration at 150°C and 0.7 to 1.5 MPa [42]. In addition, a new high temperature two-stage flash stripping process at 150°C has been developed that utilizes the high thermal stability of PZ [43]. The two-stage flash stripping process offers a smaller footprint and lower capital costs than a conventional packed stripper column. The main disadvantage of 8 m PZ is for its narrow solid solubility window.

Today, 2-amino-2-methyl-1-Propanol (AMP) and methyldiethanolamine (MDEA) are drawing a great deal of attention because their relatively low energy consumption for solvent regeneration, leading to significant saving in process costs, but they are suffered from low reaction rate [44, 45]. Then solvent formulation achieved by blending a variety of single alkanolamines attracts a lot of interest, because it combines the favorable characteristics of different solvents while suppressing their unfavorable characteristics [46]. Quite a large amount of scientific papers on blended alkanolamines especially AMP-based and MDEA-based solvents have been published in recent years [47, 48]. Among these papers, two major criteria have been considered to choose the appropriate amine blends: the absorption performance (higher with primary and secondary amines) and the energy consumption for the solvent regeneration (lower with tertiary and sterically hindered amines). Therefore blends of primary/secondary and tertiary amines (such as mixtures of MEA/DEA and MDEA) or primary/secondary and sterically hindered amines (such as mixtures of MEA/DEA and AMP), which combine the higher equilibrium capacity of the tertiary amine or sterically hindered amine with the higher reaction rate of the primary or secondary amine, lead to significant saving in process costs and have been suggested for industrial CO2 absorption processes. The blended PZ solvents are also discussed a lot in recent years and achieved some good capture performance.

3.3. Dilution of the aqueous fraction with organic liquids

Another improvement can be made by diluting the MEA solution with organic solvents, such as alcohols, instead of water. In the MEA process, the aqueous fraction is about 80% of the solution, which should be heated and will cause large amount of energy consumption during the regeneration step [49]. As we know, certain organic diluents have specific heats about 50% lower than water, and thus the possibility exists that replacing a portion of the water in the MEA solution with organic diluent may reduce the energy penalty during the regeneration process [50, 51]. The concept is that if the organic solutions have lower heat capacities, the energy penalty for regeneration will decrease. However, adding methanol to MEA (instead of water) did not significantly decrease the absorption rate, because a large amount of methanol evaporated while heating the solution. Thus, methanol is not suitable for regeneration at atmospheric pressure but could be used in high-pressure systems such as the Integrated Gasification

Combined Cycle (IGCC). Searching for other organic solvents for MEA would be a challenge for regeneration at atmospheric pressure [52].

As has mentioned above, the development of new structured packing is a feasible way to improve the CO2 absorption process by increasing contacting area per unit volume to enhance gas-liquid interactions. Diluting the amine solution with organic solvents, such as methanol or ethylene glycol, is a possible way to greatly reduce energy penalty during the regeneration process. However, how to choose the appropriate organic solvents which can significantly reduce the energy penalty of regeneration but do not inhibit the CO2 absorption process greatly, is a problem that needs a lot of attention. Meanwhile, exploring new type of absorbent is also an effective measure to enhance the heat and mass transfer between the absorbent and CO2, and improve the absorption efficiency. Today, KS-1, KS-2, KS-3 solvents, and PZ solvents are recognized as promising solvents for CO2 absorption. Blended alkanolamines solvents also attract a lot of interest, because they combine the favorable characteristics of different solvents while suppressing their unfavorable characteristics. There are many other improvements like process improvement and structure improvement, which can also be applied to improve the CO2 capture process need a lot of research.

4. Conclusions and prospects

Post-combustion capture with chemical absorption has become the primary means of CO2 separation in the combustion flue gas, owing to its advantages of economy and mature technology. This paper summarizes a lot of research work carried out so far in post-combustion capture with chemical absorption. Including introducing the typical amine absorption process, analyzing the main problems in the absorption process, and summarizing several improvements can be applied to improve the capture process. Improvements can be done in many areas: increasing gas-liquid contacting area by developing new structured packing, searching for new type of reactant with better absorption performance, and dilution of the aqueous fraction with organic liquids to reduce the energy penalty of regeneration process.

Based on these analyses, we tried to predict future challenges and potential breakthroughs. To make chemical absorption into practical application, the author thinks more efforts in the future should be directed to the following approaches:

• Combine removal of other pollutants in the flue gas system: SOX, NOX, HCI and other acid gases;

• Enhancing high gas-to-liquid mass and heat transfer rates in absorber and stripper;

• Ionic liquid and other alkaline absorbents as well as their mixtures also need a lot of research.

In conclusion, deeper explore alkaline solution to capture and further storage CO2 in the post-combustion flue gas has a certain significance to control greenhouse effect.

Acknowledgements

Financial supports of the National Natural Science Foundation of China (nos. 51276141) are gratefully acknowledged. This work is also supported by the China Postdoctoral Science Foundation funded project (no. 2013M530422) and "Fundamental Research Funds for the Central Universities".

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