1. Introduction
The “30–60” dual-carbon goal is China’s major strategy since Chinese President Xi’s announcement to reach peak carbon emissions by 2030 and achieve carbon neutrality by 2060 came out in September 2020. At present, the focus of realizing the dual-carbon goal involves the transformation of the energy structure and the low carbonization of fossil energy. When compared with coal and petroleum, natural gas has clean and low-carbon characteristics, so it plays an irreplaceable role in the process of energy structure transformation and carbon neutrality.
LNG is the main form of international natural gas trading. A standard vessel (125,000 m
3) of LNG emits 220~300 thousand tons of CO
2 equivalent from production to final usage, of which storage and regasification stages account for 0–3% [
1]. The LNG receiving terminal can emit about 100 thousand tons of carbon annually [
2]. In 2020, direct carbon emissions from SCV at an LNG receiving terminal reached 56 thousand tons. SCV uses the heat generated by the combustion of natural gas to gasify LNG [
3,
4]. In June 2019, Royal Dutch Shell announced that it had reached the world’s first carbon-neutral LNG trade with Tokyo Gas Co., Ltd. and South Korea’s GS Energy. As of September 2021, 24 ships running on carbon-neutral LNG have been traded globally [
5]. Therefore, it is crucial to capture the carbon of SCV flue gases to achieve low-carbon operation to be carbon-neutral in the LNG receiving terminal. The CC (carbon capture) technologies include the chemical adsorption method, the physical absorption method, the membrane separation method, and the low-temperature separation method, etc. [
6,
7]. The cryogenic separation method uses the difference in bubble & dew points of each component in the flue gas, allowing CO
2 to be condensed from the gas phase and transferred to the liquid phase or solid phase by compressing and cooling flue gas several times, to achieve CC. The disadvantage of the cryogenic separation method is that the conventional refrigeration process causes extensive energy consumption. However, at the LNG receiving terminal, the LNG cold energy can be used for the CCC (cryogenic carbon capture) process [
8,
9].
Domestic and foreign scholars have done a lot of research on using LNG cold energy for CC. Depending on the variance of composition and the CO
2 concentration of the flue gas, the temperature and pressure of the CC vary. For the pure CO
2 flue gas, the conditions of flue gas CC are that the pressure and temperature are below the triple point of CO
2. Shi [
10] established a three-stage system for LNG cold energy utilization, including air separation, CO
2 liquefaction, and cold energy power generation. The CO
2 liquefaction rate of the system was 0.223 kg/kg LNG.
For the flue gas with simple composition and high CO
2 concentration, in the process of flue gas CC, the flue gas will be dehydrated and dried, and its carbon capture pressure or temperature becomes higher than the CO
2 triple point. Huang [
11] proposed a transcritical RC (Rankine cycle) waste heat power generation and CO
2 liquefaction process, with CO
2 as the working fluid. The exergy efficiency and CO
2 liquefaction rate of the system were 36.33% and 0.273 kg/kg LNG, respectively. On this basis, Wang [
12] improved the process. After the flue gas released heat in the RC, it was separated, compressed, and condensed in two stages, and finally exchanged heat with the RC working fluid and LNG to achieve CO
2 liquefaction. The system performance has improved. Xiong [
13] and Gomez [
14] proposed a zero-carbon emission gas-fired power generation system using LNG cold energy, which combined the RC and BC (Brayton cycle). Liu [
15] proposed an innovative CCHP (combined cooling heating and power) system based on the LNG cold energy and the ORC + BC + gas turbine, which realized energy cascade utilization and flue gas CC. The exergy efficiency and CO
2 capture capacity of the system were 62.29% and 79.2 kg/h, respectively. Yu [
16] proposed a method to use LNG cold energy to reduce CC energy consumption, which combined the ORC. The system exergy efficiency and CO
2 capture capacity could reach 70% and 15 t/h, respectively. In terms of the Allam cycle using LNG as fuel, Liang [
17] proposed a near-zero carbon emission solid oxide fuel cell (SOFC) system based on LNG cold energy and the ORC + RC process. The exergy efficiency and CO
2 capture capacity of the system were 53.07% and 5219.21 t/a, respectively. Ouyang [
18] proposed a new LNG cold energy utilization system that combined the ORC and carbon capture cycle. After the optimization of the system process parameters and the circulation of working fluids by genetic algorithm, the system exergy efficiency and CO
2 liquefaction rate were 47.2% and 0.78 t/t LNG, respectively.
For flue gas with multi-component and low CO
2 concentration, the temperature and pressure of flue gas carbon capture are much higher than the triple point of CO
2. Xu [
19] and Rifka [
20] proposed utilizing LNG cold energy to capture CO
2 in the flue gas. After the flue gas was first compressed and dried, the flue gas was cooled by the LNG cold energy to obtain solid-phase or liquid-phase CO
2. The decarbonized flue gas was discharged to the atmosphere after expansion. Liu [
21] proposed a CCHP system based on LNG cold energy and flue gas waste heat combined with CO
2 capture. After the multi-objective optimization of the system, the exergy efficiency and CO
2 capture capacity were 41.38% and 7.9236 t/h, respectively. Zhao [
22] proposed a CC system based on the LNG cold energy and industrial flue gas, which also combined the horizontal two-stage ORC. The exergy efficiency and CO
2 liquefaction rate of the system were 57% and 0.75 kg/kg LNG, respectively. Some studies have also combined CCC technology with chemical adsorption [
23,
24] and membrane separation [
25,
26] to increase the concentration of CO
2 in the flue gas entering the CCC process.
In this paper, a carbon capture system for SCV flue gas using LNG cold energy is proposed. The system includes an ORC power generation subsystem that uses LNG and SCV flue gas as cold and heat sources. The thermodynamic mathematical model and carbon emissions mathematical model of the system were established, and the thermodynamic parameters of the system were calculated. The sensitivity analysis of some key process parameters was carried out, and Aspen HYSYS was used to optimize the parameters.
2. System Description
This paper builds an SCV flue gas carbon capture process using LNG cold energy, defined as System A, which consists of the SCV flue gas CC subsystem, an ORC subsystem, an exhausted gas direct expansion cycle (DEC) subsystem, and an LNG regasification subsystem. The HYSYS simulation model of System A is depicted in
Figure 1, and the HYSYS simulation model of the conventional LNG regasification system, define as System B, is shown in
Figure 2.
The SCV flue gas CC subsystem consists of a dehydrator, two compressors, four heat exchangers, and a separator. SCV flue gas goes through dehydration, cooling, compression, cooling, compression, precooling, condensation, and separation to achieve SCV flue gas CC. The process takes place through F1–F9, and F9 contains high CO2 concentration. The ORC subsystem is composed of a working fluid pump, two turbines, and three heat exchangers. The working fluid, composed of methane and ethane, successively goes through the cyclic process of condensation liquefaction, pressurization, evaporation, expansion, reheat, secondary expansion, and condensation liquefaction, which is the R1–R6 route. The exhausted gas DEC subsystem consists of a heat exchanger, an air temperature heater, and a turbine, and the process is takes place through E1–E4. The LNG regasification subsystem consists of a tee, a mixer, and an SCV that consists of the heat exchanger, burner, and air compressor, and the LNG process takes place through L1–L8. In System A and System B, A1–A4 is air, W1–A3 is softening water, F is fuel gas, and the isentropic efficiency of the turbine, pump, and compressor is 0.85.
4. Results and Discussion
In this paper, HYSYS software was used to simulate the SCV flue gas carbon capture process using LNG cold energy, and the P–R equation was used to calculate the thermodynamic properties of each logistics. In this section, the thermodynamic performance of System A was studied based on the first and second laws of thermodynamics, and the energy consumption and carbon emissions of System A and System B are comparatively analyzed. In addition, the sensitivity analysis of key parameters to the performance of System A was studied. Finally, the parameters were optimized using Aspen HYSYS software. Initial data of the process simulation for System A and System B are listed in
Table 3.
The simulation results of flue gas and exhausted gas in System A and System B are shown in
Table 4, and the simulation results of LNG and the working fluid in System A are shown in
Table 5. According to
Table 4, solid CO
2 will appear at state points F7, F8, and F9. This is due to the low concentration of CO
2 in the flue gas, resulting in the freeze point temperature of the flue gas being greater than the dew point temperature [
29]. Therefore, in the process of cooling the flue gas, the freezing point of the flue gas will first be reached, and solid phase CO
2 will be formed, which may lead to blockage of the heat exchanger. To solve the problem of blockage, dynamically operated packed beds [
30] and condensate separators [
29] can be used to replace heat exchangers in the actual project. The hybrid membrane cryogenic process [
25,
26] can also be used to increase the concentration of CO
2 in the flue gas, so that the dew point temperature of the flue gas is greater than the freezing point, so as to prevent the formation of solid CO
2.
4.1. Analysis of System Carbon Emission and Energy Consumption
Equations (2)–(9) and Equations (12)–(14) were used to calculate the energy consumption and carbon emissions of System A and System B, respectively. The calculation results are shown in
Table 6. When the LNG cold energy was used for SCV flue gas carbon capture, the fuel gas consumption, energy consumption, and carbon emissions of the system were reduced by 7.16%, 6.3%, and 89.6%, respectively. The CO
2 capture rate and CO
2 capture capacity of System A were 95.6% and 6663.5 kg/h, respectively, and the direct carbon emissions generated during SCV operation were significantly reduced.
Therefore, utilizing LNG cold energy to capture the carbon of SCV flue gas can provide a feasible scheme for the low-carbon operation of the LNG receiving terminal, as shown in
Figure 3. After the LNG from the tank is pressurized, it first enters the CCC unit to release cold energy and then enters the SCV gasification unit. The flue gas generated by the SCV operation enters the CCC unit for carbon capture, which ultimately reduces the direct carbon emissions of the LNG receiving terminal.
4.2. Exergy Analysis
The reference states for exergy calculations were set as 274.15 K and 101.325 kPa. The calculation results of exergy input, output, and loss of System A (without LNG regasification subsystem) under the initial conditions are shown in
Table 7. It can be seen that the exergy input contained the physical exergy of the flue gas and LNG, as well as the electric power input through compressors and pumps. The exergy output included the physical exergy of CO
2 and the mechanical work produced by the ORC subsystem and the DEC subsystem. As calculated by Equations (10) and (11), the exergy efficiency and LNG cold exergy utilization rate of System A (excluding the LNG regasification subsystem) were 13.32% and 77.61%, respectively. When compared with the literature [
21,
22], the exergy efficiency of the system is low.
The exergy loss percentages of the System A components are shown in
Figure 4, including heat exchangers, pumps, compressors, and turbines. Because of the temperature difference in the heat transfer process, the main exergy losses within the heat exchangers were the main exergy losses of the system (more than 72.4%). The exergy loss of other equipment accounted for 27.6% of system exergy loss, and the exergy loss of E-3 even reached 14.6%. For analyzing the exergy losses in heat exchangers, the temperature differences under the initial conditions in heat exchangers were calculated as shown in
Figure 5. The LMTD (log mean temperature difference) of H-1, H-3, H-4, and H-5 was 12.21 K, 10.76 K, 12.32 K, and 20.41 K, respectively. In the heat exchanger (H-1, H-4, H-5), the matching degree of the temperature–heat flow curve of the cold and hot logistics was not high, resulting in a higher LMTD of the heat exchanger, which increased the exergy loss of the heat exchanger and reduced the exergy efficiency of the system.
4.3. Sensitivity Analysis of Key Parameters on the System Performance
4.3.1. Working Fluid Evaporation Pressure
The evaporation pressure of the working fluid will affect the net work and efficiency of the ORC. The effect of the working fluid evaporation pressure on system performance is shown in
Figure 6. The electricity consumption and energy consumption of the system increased with the increment of the evaporation pressure of the ORC working fluid. According to the P–H phase diagram (
Figure 7) of the working fluid, with the increase in the ORC working fluid evaporation pressure, the cold energy required for the condensing working fluid in the H-5 heat exchanger decreased, and the cold energy released in the heat exchanger H-1 and H-2 decreased at the same time. This then led to a decrease in the SCV inlet LNG temperature and an increase in compressor inlet temperature for compressors C-1 and C-2, which increased SCV fuel gas consumption, power consumption, and system energy consumption. As the evaporation pressure of the working fluid increased, the direct carbon emission of the system increases slightly, which coupled with the growth of the electricity consumption of the system, so the carbon emission of the system also increased.
4.3.2. CO2 Capture Pressure
The CO
2 capture pressure affects the SCV flue gas CC amount, the system energy consumption, and the system exergy efficiency. The effect of CO
2 capture pressure on system performance is shown in
Figure 8. The energy consumption and electricity consumption of the system increased as the CO
2 capture pressure increased. With the increase in CO
2 capture pressure, the energy consumption of compressors C-1 and C-2 increased sharply, and the heat energy carried by the compressor outlet increased, resulting in a higher ORC working fluid flow, an augment in the cold energy needed by the heat exchanger H-5, and the LNG temperature rise at the SCV inlet. This resulted in a small reduction in SCV fuel gas consumption, a substantial increase in system electricity consumption, and an increase in system energy consumption. With the increase in CO
2 capture pressure, the irreversibility of compressor C-1, compressor C-2, and turbine E-3 enhanced. The system exergy loss increased while the system exergy efficiency decreased. Besides, the carbon capture amount of the system increased, and the direct carbon emissions of the system were reduced. As the electricity consumption of the system increased, the carbon emission of the system first decreased and then increased. When the CO
2 capture pressure was 530 kPa, the system had the lowest carbon emissions.
4.3.3. CO2 Capture Temperature
The CO
2 capture temperature influences the SCV flue gas CC amount, the system energy consumption, and the system exergy efficiency, and the effects on the system performance are shown in
Figure 9. The system energy consumption increased, the electricity consumption and exergy efficiency of the system decreased, and the CC amount of the system greatly reduced as the CO
2 capture temperature increased, so the direct carbon emission of the system increased significantly. Even when the electricity consumption of the system was reduced, the carbon emissions of the system still increased significantly. The CO
2 capture temperature increased, the cold energy needed by the heat exchanger H-3 decreased, and the LNG temperature at the SCV inlet decreased, thus increasing SCV fuel gas consumption. In addition, the inlet temperature of turbine E-3 increased, and its net work increased. Equation (10) can explain that the decrease in system exergy efficiency was due to the reduction in the CC amount of the system.
4.4. Key Parameter Optimization
Through the sensitivity analysis of the above important process parameters, it was found that the evaporation pressure of the working fluid was sensitive to the system energy consumption and less sensitive to the system direct carbon emission, while the CO
2 capture pressure and CO
2 capture temperature were sensitive to the system energy consumption and the direct carbon emissions. HYSYS software was used to optimize the key parameters of the system to achieve the lowest operating cost of the system, and the optimization variables were the evaporation pressure of the working fluid
, CO
2 capture pressure
, and CO
2 capture temperature
. The objective function is shown in Equation (15).
where,
a is fuel gas price, 8.3 ¥/kg;
b is electricity price, 0.62 ¥/(kW·h); and
d is carbon trading price, 0.07 ¥/kg.
The constraint conditions are Equations (16)–(18).
The optimization results were = 1300 kPa, = 750 kPa, and = 143.15 K. After optimization, the fuel gas consumption of the system was 2524 kg/h, the electric energy consumption of the system was 473.1 kW, the CO2 capture capacity of the system was 6620.4 kg/h, the carbon capture rate of the system was 94.9%, the carbon emissions of the system were 777.4 kg/h, the exergy efficiency of the system was 13.63%, and the utilization rate of the LNG cold exergy was 77.49%.