Integrated optimization of coal-fired power plant and CO2 capture system coupled with membrane condenser for recovering flue gas hydrothermal energy

https://doi.org/10.1016/j.enconman.2023.116689Get rights and content

Highlights

  • An MEA-based CO2 capture system coupled with membrane condenser is proposed.

  • The regeneration duty of CO2 capture system can be reduced through the membrane condenser.

  • An improved coal-fired decarbonization system is proposed for recovering the waste heat from flue gas and CO2 capture system.

  • The performance of energy and water saving of the improved coal-fired decarbonization system are detailly presented.

Abstract

CCS can effectively solve the problem of large-scale carbon emissions from coal-fired units, but at a considerable expense in terms of high energy and water consumption. In this paper, the conventional MEA-based CO2 capture system process is improved and integrated optimization with coal-fired power plant. An MEA-based CO2 capture system coupled with membrane condenser and an improved integrated decarbonization system are proposed based on the recovery potential of waste heat from flue gas and CO2 capture system. Besides, an MEA-based CO2 capture system coupled with membrane condenser is proposed. The membrane condenser is added to exchange heat with the rich solvent flowing from the bottom of the absorber before the flue gas into the absorber for recovering the flue gas hydrothermal energy. The regeneration duty of the CO2 capture system is reduced from 4.341 MJ/kg CO2 to 4.275 MJ/kg CO2. The extraction steam is further utilized by a small steam turbine before being sent to the reboiler. And the heat released by CO2 condensation at the top of stripper and CO2 cooling in the middle of multi-stage compression is used to heat the condensate of coal-fired system. Compared with the simple integration scheme which adopts the same CO2 capture system, the output power of the integrated decarbonization system increases from 510.719 MW to 593.850 MW. Correspondingly, the thermal efficiency increases by 6 %, the exergy efficiency increases by 5.86 % and the cooling water saves 1497.12 kg/s. Moreover, the variation of output power and saving cooling water with the membrane condenser parameters of the improved integrated system is analyzed.

Introduction

The coal-fired power generation accounts for almost 36 % of the total electricity generation, accounting for 9.67 Gt CO2 of the 13 Gt CO2 emitted by the power system [1]. To limit further global warming, the pressure of carbon emission had driven many nations to create their own carbon-neutral strategies. The Chinese government announced the vision of “Carbon Neutral” before 2060 [2]. The United States has proposed a pathway to reduce carbon emissions by 50 % by 2030 and by 80 % by 2050 [3]. Wherein, reducing CO2 emissions of the existing energy system has a grant influence.

The post-combustion CO2 capture technology can reduce CO2 emissions of coal-fired power plants effectively. However, the reboiler of the CO2 capture system requires massive energy to desorb CO2, which is usually provided by the extracted steam from the coal-fired power plant, causing an energy penalty of the coal-fired power plant. In addition, the CO2 capture system will consume a lot of cooling water. Recent researches are focused on how to reduce the energy consumption of CO2 capture system, which is the main obstacle to its popularization in coal-fired power plants. The research of CO2 capture technology usually focuses on three aspects:(1) Development of new CO2 chemical absorbent [4], [5], [6], [7], [8]; (2) Optimization of CO2 capture system [9], [10], [11]; (3) Integrated optimization of post-combustion CO2 capture system and coal-fired power plant [12], [13]. Researchers found that a lot of new absorbents can reduce the regeneration duty. For example, the regeneration energy consumption of ammonia water absorbent can be as low as 2.2 GJ/t CO2 [4], and that of potassium carbonate absorbent are 2.6 GJ/t CO2 [5]. However, MEA is still the most widely used solvent because of its maturity, high capture rate, and is usually used as the benchmark solvent. The optimization of CO2 capture system is also an effective way to reduce the regeneration duty. Process optimization is usually studied in terms of CO2 absorption, desorption and heat transfer. The results show that the technology of rich split and intermediate cooling can effectively reduce the energy consumption of regeneration.

In recent years, membrane technology has provided a new idea for the optimization of CO2 capture system. As a new type of heat exchanger, the membrane condenser has the following advantages compared with the conventional heat exchanger. (1) Both mass and heat transfer can occur in the membrane condenser, while only heat transfer occurs in the conventional heat exchanger. (2) Conventional heat exchangers are usually used for high-grade heat recovery under certain temperature requirements and material constraints, but membrane condensers are not limited. (3) Membrane condenser with extremely high contact area, compact structure, suitable for engineering reconstruction. For the coal-fired power plant, the flue gas temperature is generally below 130 °C, and the flue gas contains 10–16 vol% of water vapor, which has considerable latent heat. If the flue gas can be reused, it can save both energy and water. Membrane technology is suitable for the recovery of water and heat in the flue gas.

Existing membrane processes are mainly based on the use of hydrophobic porous membranes [14], [15] on the feed side or hydrophilic nanoporous membranes on the permeate side, which can recover 20–60 % of heat and 30–80 % of water from flue gas at 50–90 °C [16]. Researchers [17], [18], [19], [20] used a membrane condenser to recover water and heat from flue gas of power station under laboratory conditions, and studied the effects of operating parameters on the performance of membrane mass transfer and heat transfer process. In addition to testing the flue gas hydrothermal recovery performance of membrane elements, researchers also coupled the membrane condenser with the CO2 capture system to reduce the capture energy consumption. Zhaoet al. [21] put forward a new type of membrane-assisted liquid absorption and regeneration (MALAR) process earlier. The system has two gas-liquid membrane contactors, which are membrane condenser at the top of the stripper and membrane evaporator at the bottom of the stripper. Combining MALAR concept with improved liquid adsorbent can reduce the energy consumption of regeneration by 50 %. Yan et al. [22] further studied the energy-saving potential of membrane condenser for recovering latent heat of steam discharged from the stripper. The results show that the heat recovery increases significantly with the increase of inlet temperature and the decrease of outlet temperature.

The coal-fired power plant integrated with CO2 capture system extracts steam from the intermediate pressure cylinder of the steam turbine to provide energy for the reboiler, but the extraction of steam leads to the obvious reduction of the power generation of the power plant. At the same time, the temperature difference between extraction steam and absorbent is large, resulting in energy loss. To further reduce the energy penalty, the integration of different systems is put forward to achieve energy level matching and waste heat utilization. Researchers [23], [24], [25] proposed that before entering the reboiler, the extracted steam should be used to heat the condensed water, or the extracted steam should be mixed with the condensed water in the reboiler. This method can make full use of the waste heat of extraction steam. Xu et al. [26] used the heat of the cooler and stripper condenser in CO2 multistage compression to heat condensed water, recovered about 180 MW of heat from CO2 capture system and reduced the loss by about 67 %. Pfaff et al. [27] simulated the integration of an advanced ultra-supercritical power plant with a CO2 capture system after combustion, the preheating part of feedwater, and the preheating combustion-supporting air system with the waste heat of CO2 capture and stripper products. Compared with the basic situation, the net efficiency increased by 1.02 %.

Most of the studies on the integration of membrane condensers and CO2 capture systems have focused on using membrane condensers to replace the traditional condensers at the top of the stripper. However, there is little relevant research on the use of membrane condenser to recover and utilize the flue gas hydrothermal to the rich solvent at the bottom of the absorber in the CO2 capture system. Based on the recovery potential of membrane condenser for waste heat in flue gas, a CO2 capture system coupled with membrane condenser is proposed, and improved integrated with a 660 MW coal-fired power plant in this paper. The innovation of this scheme is that the membrane condenser can recover part of the flue gas hydrothermal into the rich solvent before the flue gas enters the absorber in the CO2 capture system, which is conducive to reducing the regeneration duty of the system. The improved integration scheme of coal-fired system and CO2 capture system is put forward to recover flue gas hydrothermal energy in the coal-fired system to reduce energy penalty and save cooling water.

Section snippets

Coal-fired power plant system

Fig. 1 is a schematic diagram of a 660 MW supercritical coal-fired power plant. The coal-fired power generation system includes a boiler, steam turbine, power generator, condenser, deaerator and feedwater heaters. The main steam and reheat steam are generated in the boiler, which parameters are both 24.2 MPa and 566 °C. The steam turbine is divided into three cylinders with high-pressure (HP) cylinder, intermediate-pressure (IP) cylinder and low-pressure (LP) cylinder. Three high-pressure

Basic data

The main parameters of the 660 MW supercritical coal-fired unit are shown in Table 1.

Table 2 shows the test conditions of Tarong CO2 capture pilot plant [34] which is used as the baseline MEA-based CO2 capture system data.,

Table 3 lists the parameters of CO2 absorber and CO2 stripper, as well as the main relationships and settings of absorber and stripper models based on rate [34]. The reaction formula shown in Table 4 is used in the absorber and stripper. The equilibrium constants of reactions

Thermodynamic performance of improved MEA-based CO2 capture system

The following are reasonable assumptions for the modeling process [30], [36], [37].

  • (1)

    There is no heat loss in the membrane condenser and the rich-poor liquid heat exchanger;

  • (2)

    There is no CO2 transmembrane transfer in the membrane condenser;

  • (3)

    The fluid along the membrane condenser flows counter-currently;

  • (4)

    The latent heat released by the condensation of water vapor can be quickly absorbed by the rich liquid.

The simulation of the improved CO2 capture system is carried out on the premise that the flow

Discussion

The flue gas temperature and the water recovery rate will affect the regeneration duty, cooling load and condensation load, thus affecting the cooling water amount and the output of the integrated system. This section will explore the influence of changing the parameters of the membrane condenser on the water saving and output of the improved integrated system. The influence on saving cooling water is simulated under the conditions that the inlet temperature of cooling water is 15 °C, the

Conclusions

To deal with the high energy consumption of decarbonization in coal-fired power plant, this contribution proposed an MEA-based CO2 capture system coupled with membrane condenser by recovering the waste heat from flue gas and integrated optimization with the coal-fired power plant. Based on the simulation model of a 660 MW coal-fired power plant integrated with conventional MEA-based CO2 capture system, the thermodynamic performance of the proposed system is comparatively presented to show the

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.

Acknowledgments

This work was supported by the National Science and Technology Major Project [grant number J2019-I-0009-0009]. This support is highly appreciated.

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