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Experimental research on the process of compression and purification of CO2 in oxy-fuel combustion
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Experimental research on the process of compression and purification of CO2 in oxy-fuel combustion ⁎
Wu Hai-boa,b, , Xu Ming-xinc, Li Yan-binga, Wu Jin-huad, Shen Jian-chongd, Liao Haiyana a
Shenhua Guohua Electric Power Research Institute (Beijing) Co., Ltd., Beijing 100025, China College of Electromechanical Engineering, Qingdao University of Science & Technology, Qingdao 266061, China c National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing, China d Hangzhou Kuaikai HI-TECH CO., TD, Hangzhou 310051, China b
H I GH L IG H T S
optimal temperature and pressure for liquefaction were explored. • The overall average desulphurisation and denitration efficiencies exceeded 98%. • The existence of water and NO were beneficial to the oxidation of SO . • The • High CO concentration capture was realized. x
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A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 compression and purification Synergistic removal SO2 NO Full-flow process
Oxy-fuel combustion is a leading potential CO2 capture technology for power plants. Thus, oxy-fuel technology has the highest requirement for CO2 purification. Compression and purification systems are important for oxyfuel combustion. The tests in this study were conducted on a 50 kg/h CO2 compression and purification test platform, which is currently the largest oxy-fuel combustion platform in China. The CO2 compression and purification technology in oxy-fuel combustion was developed. The experiments verified the feasibility of the proposed technology and realised high concentration of CO2 capture. In addition, the optimal temperature and pressure for liquefaction were explored, and the optimal CO2 energy consumption was obtained. The overall average desulphurisation rate and denitration efficiencies were more than 98%, and the ratio change of n(SO2):n (NOx) exerted little effect on sulphur and nitrate purifications. The oxidation of SO2 was promoted with the increase in pressure. Moreover, the existence of water and NOx were beneficial to the oxidation of SO2, whereas NO had a slight promoting effect on the transformation of SO2. This study makes the replacement of traditional desulphurisation and denitration systems in oxy-fuel combustion power stations possible and might establish a technical foundation for large-scale oxy-fuel combustion demonstration projects.
1. Introduction Global warming is becoming increasingly severe in recent years, thereby leading to the urgent requirements for the development of CO2 capture, utilisation and storage (CCUS) [1,2]. Currently, three main technologies are available for the mitigation of CO2 emissions from coal-fired power plants, namely, pre-, post- and oxy-fuel combustion [3,4]. Compared with the other technologies, oxy-fuel is regarded as one of the most promising choices for CCUS commercialisation because of its advantages in manufacturing, operation and retrofitting [5–7]. During oxy-fuel combustion, the mixtures of pure oxygen and recycled
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flue gas are utilised for fuel combustion. As a result, the concentration of CO2 in the flue gas can be sufficiently high for direct compression and purification, thereby significantly simplifying the process of CCUS [8,9]. Moreover, the simultaneous removal of multiple pollutants, such as NOx and SOx, can be achieved in the process of CO2 compression and purification during oxy-fuel combustion, which is one of the most impressive features of the process [10–13]. According to the mechanism proposed by Allam et al., desulphurisation and denitration can be synergistically accomplished in the process of CO2 compression [14,15]. However, the removal efficiencies of NOx and SOx are significantly affected by multiple factors during the
Corresponding author at: Shenhua Guohua Electric Power Research Institute (Beijing) Co., Ltd., Beijing 100025, China. E-mail address: [email protected] (H.-b. Wu).
https://doi.org/10.1016/j.apenergy.2019.114123 Received 20 July 2019; Received in revised form 15 October 2019; Accepted 11 November 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wu Hai-bo, et al., Applied Energy, https://doi.org/10.1016/j.apenergy.2019.114123
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system design [44]. A comprehensive dynamic model with specified control system was established, which provided the possibility to integrate CPU with full-train oxy-combustion power plants. Pipitone et al. proposed several methodologies for the removal of a selection of impurities from the CO2 in the flue gas of two oxy-combustion power plants fired with either natural or pulverised fuel [45]. The energy consumption, CO2 product purity and CO2 recovery rate of different CPU configurations were systematically analysed. In addition, Yan and Ritter separately reviewed the latest process concepts and integrations for CO2 purification process, verifying that the integration of oxy-fuel combustion and CO2 compression and purification process is feasible [46,47]. At present, some institutions, such as Schwarze Pumpe, Air Products and Huazhong University of Science and Technology, are conducting pilot tests on CO2 compression and purification during oxy-fuel combustion and developed viable and unique solutions for CO2 capture, which can be readily integrated into retrofitted and new building power plants. Anheden et al. investigated the performance of CO2 compression and purification system on an oxy-fuel combustion pilot platform [48]. The results showed that operating pressure and residence time can affect the conversion efficiencies of NO to NO2. In addition, 40% of NOx was removed during the compression process. Air Products proposed a new method for CO2 purification that combined some of their oxy-fuel purification technologies, such as the use of a membrane to recover CO2 and O2 and the sour compression technology [49,50]. Some demonstrative tests were performed at a 1MWth pilot-scale test facility. Recently, Zheng et al. reported the latest research advances on a 3MWth oxy-fuel combustion test platform at Huazhong University of Science and Technology, which is a full-process oxy-fuel combustion pilot-scale system in China [51]. However, relevant reports on the behaviours of CO2 compression and purification units are insufficient. Most studies on the synergistic removal of pollutants during oxyfuel combustion mainly focus on theoretical research and processing simulations. Few studies reported the actual operating characteristics of CPU in the full-process of oxy-fuel combustion. The aim of this study is to investigate the key problems in optimising operation conditions, control methods and pollutant removal during CO2 compression and purification. This work has important practical significance. Moreover, because the efficiencies of desulphurisation and denitration are greatly affected by the operating parameters, additional systematic experimental investigations should be performed to establish the integration between typical factors, such as pressure, temperature, concentrations of pollutants and the synergistic removal of multiple pollutants during CO2 compression and purification. Therefore, to systematically reveal the detailed operating characteristics and synergistic removal of SOx and NOx in the process of CO2 compression and purification during oxyfuel combustion, some tests were conducted on a 50 kg/h CO2 compression and purification platform. The characteristics of the synergistic removal of SO2 and NOx were investigated. The detailed effects of the typical operating parameters on the purification of CO2 were systematically analysed for the first time. The result can provide fundamental design criteria for the integration of oxy-fuel combustion with CO2 compression and purification.
CO2 compression process [16]. Normann et al. studied the chemical reaction process of desulphurisation and denitration under elevated pressures [17]. Their results established that the oxidation of NO into NO2 governed the absorption of NOx. In addition, the complex chemistry of the liquid phase, including reactions between HNO2, H2SO3 and H2SO4, was critical for the rate of absorption of NOx and SOx. Kühnemuth et al. simulated the process of desulphurisation and denitration in oxy-fuel combustion [18]. The simulation results showed that (1) SO2 can be completely removed and (2) the removal rates of NOx can reach 80–85% in the compression and purification unit (CPU) equipped in the process of oxy-fuel combustion. Ting et al. conducted a special research on the removal of NOx in the process of flue gas compression and discussed the conversion from NO to NO2 in dry and wet conditions [19]. The results showed that NO can be readily oxidised to water-soluble NO2. In addition, the reaction was kinetically controlled. Moreover, the overall mass balances across the gas–liquid system were complicated by the stability of the absorbed NOx species in the liquid. Luo et al. performed an experimental study on the process of NO removal and NO/ SO2 co-removal in various conditions [20]. The operating temperatures, initial NO concentration and other typical factors can affect the removal of NOx and SOx. White et al. studied the behaviour of SO2 removal in the compression process where NO, SO2 and O2 coexisted [21]. They discovered that the removal efficiency of SO2 decreased with the increase in the ratio of SO2/NOx. Murciano et al. investigated the conversion from gaseous agents to liquid acid in the process of CO2 compression [22,23]. The results confirmed that operating factors such as pressure, temperature and residence time can greatly affect the conversion from NO and SO2 to nitric and sulphuric acids, respectively. Lu et al. studied the synergistic removals of SOx and NOx during oxy-fuel combustion [24]. The results verified that NO in the flue gas can be effectively converted to HNO3 under high pressure and normal temperature. Hui found that increasing the pressure can enhance the conversion from SO2 to SO42-, thereby increasing the SO2 removal efficiency [25]. In conclusion, the synergistic removal of NOx and SOx during oxy-fuel combustion was practicable and the removal efficiencies were significantly influenced by the operating conditions [26,27]. Several reports presented thermodynamic models [28], process analysis [29], techno-economic analysis [30,31] and optimisation method [32] to provide references and analytical analysis of the relevant CO2 capture system. Cheng et al. presented gas-phase oxidation of NO at high pressure relevant to the sour gas compression purification process for oxy-fuel combustion flue gas [33]. Wu and Li et al. studied the compression of CO2 related to oxy-fuel combustion systems. The influence of CO2 concentration, temperature or pressure on liquefaction ratio and energy consumption was also investigated [34,35]. Zhou et al. studied the simultaneous removal of NOx and SO2 under high pressure on an experiment bench, as well as the removal of NOx and SO2 at different pressures in dry/wet N2 atmosphere and the simulated oxyfuel combustion flue gas [36,37]. In addition to the above mechanism, the technical feasibility of denitration and desulphurisation in the process of CO2 compression during oxy-fuel combustion was investigated. Many schemes for CO2 CPU have been proposed [38–40]. White et al. presented an integrated process for the compression and simultaneous purification of CO2 to produce CO2 product streams [41]. The purities of CO2 after the treatment in the presented process reached 95–98%, which satisfied the required purity of CO2 for enhanced oil recovery. Alam et al. proposed a method for the purification of CO2 with oxygen and CO without the consideration of NOx and SOx removal [42]. Posch et al. designed two different purification processes for CPU optimisation for oxy-fuel combustion power plants [43]. The results indicated that double flash separation had low power cost and cooling requirement, whereas separation by rectification achieved high purity at nearly the same separation efficiency. Jin et al. investigated the methods for CPU optimisation and control in oxy-combustion power plant via single variable analysis, multivariable optimisation, dynamic simulation and control
2. Experimental 2.1. Test platform To completely investigate the operational feasibility and reliability of CPU in the process of oxy-fuel combustion, a 50 kg/h CO2 compression and purification test platform, which is currently the largest and most complete pilot-scale facility in China, was constructed in Hangzhou. The synergistic removal of SOx and NOx was achieved by combining the modified tower-type sulphuric acid production and pressurised condensation distillation purification. The platform consists of gas distribution, compression, low-temperature washing, purification 2
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Fig. 1. Flowchart of the CO2 compression and purification process.
and condensing liquid units and a low-temperature storage tank. The detailed schematic of the facility is shown in Fig. 1. The feeding gases were firstly compressed to the specific pressures and temperatures to synergistically liquidate NOx, SOx and CO2. Then, the mixtures were washed in the gas scrubbing tower to separate NOx and SOx from the liquified CO2 flow. The purified CO2 was stored in the CO2 tank after further liquefaction. The separated NOx and SOx was then heated to 200 °C to evaporate NOx from the liquid mixtures in the denitration tower, where high-purity sulphuric acid could be recovered simultaneously. The gaseous NOx was then purified and oxidised to NO2 in the oxidation tower. Lastly, the flow of gaseous NO2 was completely absorbed by water in the diluted nitric tower, where the dilute nitric acid was recovered. SOx and NOx were removed simultaneously through the process described above, and sulphuric and nitric acids were recovered as by-products. Some instruments used to measure gaseous species were also equipped in the platform. The concentrations of CO2, O2 and N2 were detected through gas chromatography, and an infrared flue gas analyser was adopted to measure the concentrations of NO, NO2 SO2 in the flue gas. The contents of the sulphuric and nitric acids were analysed through acid-base titration (GB/T 337.1-2014). During long-term tests, experimental data were obtained under stable operating stages, and repeated tests were carried out to confirm the uncertainty quantification of the experimental results. The gas distribution system does not include water vapor. Hence, additional water vapor must be removed before compression and purification. Although the gas distribution system does not include water vapor in this test, a certain amount of water is present in the compression process, and the corrosion of the compressor occurred.
Fig. 2. Conversion of N-containing compounds in the system.
3. Results and discussion 3.1. Effect of temperature and pressure on CO2 liquefaction The characteristics of CO2 liquefaction and energy consumption in different operating temperatures and pressures were investigated. The results (Figs. 3 and 4) indicate that the effects of operating temperatures and pressures varied with the volume fractions of CO2. The liquefication rate is an important control index in the production process. This rate represents the ratio of the amount of liquefied CO2 to the total amount of CO2 in the raw flue gas, whereas the liquefied energy consumption refers to the energy consumed in the production of liquid CO2 per unit mass. As shown in Fig. 3, the liquefication rate was much higher in lowtemperature condition (−30 °C) regardless of the operating pressures. In addition, the pressure exerts a positive effect on the liquefication rate; as the operating pressure increased, the liquefication rate of CO2 increased remarkably. Moreover, the energy consumption (kJ/kg) significantly decreased as the operating pressure increased from 2.5 MPa to 3.4 MPa (Fig. 4). For example, when the operating temperature was −20 °C, the energy consumption at a pressure of 2.5 MPa was about 11000 kJ/kg, which decreased to 2050 kJ/kg when the pressure increased to 3 MPa. The result also indicates that the energy consumption was significantly reduced when the operating temperature decreased, especially under low-pressure conditions. A comparison of the tendencies in Figs. 3 and 4, shows that operating temperature and pressure had opposite effects on the rates and energy consumption of CO2 liquefaction. For example, although the energy consumption decreased with the increase in operating pressures
2.2. Mechanism research The majority of the NOx formed during oxy-fuel combustion is in the form of NO, which is difficult to absorb directly. As a result, NOx should be firstly oxidised to NO2, N2O3 and N2O4 during the denitration process because of their high solubility in water. However, the conversion of these components in liquid reactions is complex [52] due to the mutual transformation amongst NO2, N2O3, N2O4, HNO2 and HNO3 in liquid reactions (Fig. 2). In addition, NO2 can be reduced to NO by SO3, following the lead chamber process for the production of sulphuric acid [53,54]. Moreover, the equilibria of the above mentioned reactions introduced above are significantly affected by the operating temperatures and pressures [55]. To achieve the high efficiencies of the synergistic removal of SOx and NOx, the optimisation of the typical parameters should be verified through tests in the actual CO2 compression and purification facility. 3
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Fig. 3. Effects of liquefaction temperature and pressure under 75% CO2 volume fraction.
when the volume fraction of CO2 increased to 90% continuously. Moreover, when the volume concentration of CO2 in the feeding gases was 80%, the liquefaction rates of CO2 could be higher than 80%, and the electric energy consumption of the unit was only 1273 kJ/kg. Meanwhile, the purity of liquid CO2 in the purification tower exceeded 99%. Therefore, we can conclude that the optimal operating parameters in the CO2 compression and purification process are a CO2 volume fraction of 80%, a pressure of 3.0 MPa, and a temperature of −30 °C.
and decrease in temperatures, the decreasing extent was gradually reduced (Fig. 3(b)), which can be attributed to the increasing loads of the cooler and compressor in conditions with lower operating temperatures and pressures. Similar tendencies can be found in the other figures. The results indicate that an optimal operating pressure and temperature should be selected for CO2 compression and purification. As can be seen in Figs. 3(a) and 4(a), the liquefication rates of CO2 can be as high as 85.92% at −30 °C. Moreover, energy consumption can decrease to 1441 kJ/kg when the pressure is 3.5 MPa. Meanwhile, the energy consumption when the pressure is 3.0 and 3.5 MPa was similar (Figs. 3(b) and 4(b)). Therefore, a pressure of 3.0 MPa and temperature of −30 °C could be regarded as the optimal operating parameters in this study.
3.3. Conversion of NOx during CO2 compression and purification As previously mentioned, the concentration of NOx and SOx significantly affects the operation of CPU during oxy-fuel combustion. To investigate the conversion of NOx during CO2 compression and purification, several tests on the behaviours of desulphurisation and denitration in CPUs were performed using the pilot-scale platform. The test parameters are listed in Table 1. The flow rates of CO2 and O2 were 4.4 m3/h and 5.72 L/min, respectively, the pressure was 2.0 MPa and the temperature was 220 °C. During the tests, the concentrations of NO in the flue gas were measured online in various sampling locations, such as the inlet of the compressor, the outlet of the compressor and the outlet of the precooler. The behaviours of the NO concentration are displayed in Fig. 6. The conversion rate of NO was about 83.3% when the pressure and temperature of the compressor are 3.0 MPa and −30 °C, respectively. The conversation rate of NO can reach 99.75% when the pressure and temperature of the precooler are 3.0 MPa and −11 °C, respectively. The conversion from NO to NO2 was significantly enhanced by the increase in pressure and decrease in temperature. As a result, almost all the NO
3.2. Effect of CO2 concentration on liquefaction To reveal the effects of CO2 concentration in the feeding gases on the characteristics of liquefaction, tests were conducted under an operating temperature and pressure of −30 °C and 3.0 MPa, respectively. The results are summarised in Fig. 5. The concentration of CO2 had positive effects on the liquefaction rates and energy consumption of the unit, which can be attributed to the increasing partial pressure of CO2 in the feeding gases. As the concentration of CO2 in the feeding gases increased, the partial pressure of CO2 in the compressor increased, which was beneficial to the further liquefaction of CO2. When the volume fraction of CO2 in the feeding gases increased from 60% to 75%, the energy consumption of the compression unit significantly decreased from 5876 kJ/kg to 1487 kJ/ kg. However, the reduction in the energy consumption was not obvious
Fig. 4. Effects of liquefaction temperature and pressure under 80% CO2 volume fraction. 4
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Fig. 5. Effect of CO2 content in feed gas on CO2 liquefaction and energy consumption. Table 1 Arrangement of the test conditions. No.1
1 2 3 4 5 6 7
NO inlet flow rates
SO2 inlet flow rates
Volume flow (mL/min)
Concentration (ppm)
Volume flow (mL/min)
Concentration (ppm)
452.58 184.38 92.19 184.38 184.38 92.19 92.19
5400 2200 1100 2200 2200 1100 1100
0.11 0.11 0.11 0.42 0.84 0.84 0.42
1266 1266 1266 5000 10,000 10,000 5000
N(SO2):N (NOx)
0.50 1.23 2.46 4.85 9.70 19.39 9.70
Fig. 7. Total desulphurisation and denitration efficiency during sulphur and nitrate purification process.
completed in the denitration tower. The SO2 separated in the denitration tower was absorbed in the sulphuric acid tower, wherein H2SO4 can be recovered as a by-product. The flow of the NO separated in the denitration tower should be treated further. After low-temperature compression, parts of NO were basically converted to NO2, whereas the rest was oxidised to NO2 in the oxidation tower. Subsequently, all NO2 was removed in the nitric acid tower, in which HNO2 and HNO3 were the by-products. The desulphurisation and denitration reactions are actually interrelated with CO2 compression and purification. To explore the fundamental characteristics of the synergistic removal of SO2 and NOx, the effects of the ratios of SO2 and NOx in the feeding gases were systematically investigated. The results are shown in Fig. 8. The ratios between SO2 and NO had little effects on the desulphurisation and denitration efficiencies. In fact, the total removal efficiencies of NO and SO2 were extremely high, in which the effects of the ratios between SO2 and NO were not apparent. Therefore, the integration between the tower type sulphuric acid process and pressurised condensation distillation process was proven feasible. In addition, the average of the desulphurisation efficiency was more than 98%, whereas the overall denitration efficiency exceeded 98%. The ratio changes of n(SO2):n(NOx) had little effect on sulphur and nitrate purifications. SO2 can be partially oxidised into SO3 under the pressing and existence of O2 and NOx. Furthermore, the existence of H2O and NOx was beneficial to the oxidation of SO2 [56].
Fig. 6. NO conversion rate during compression and precooling.
could be converted into NO2 during flue gas compression and condensation. 3.4. Synergistic removal of SO2 and NOx during CO2 compression and purification To further study the removal efficiencies of NOx and SO2 during CO2 compression and purification, some relevant tests were conducted. The test conditions are presented in Table 1. The removal efficiencies of NOx and SO2 are displayed in Fig. 7. The results show that most of NO and SO2 can be removed simultaneously. The NO removal efficiency ranged from 97.70% to 99.30%, with an average of 98.30%. For the desulphurisation, the SO2 removal efficiency was also extremely high, ranging from 96.4% to 99.6%. The separation of NOx and SO2 was mainly 5
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Table 2 Detailed testing parameters. Serial No. Test (without NO/with NO)
Test conditions Ingredients
CO2 N2 O2 SO2 NO Operational pressure
Flow
Concentration 3
50.68 m /h 80 %vt. 150 L/min 14.8 %vt. 50 L/min 5.2 %vt. 1.185 L/min 3237 mg/m3 100 mL/min 119.5 mg/m3 1.6/2.5/3.5 MPa
Fig. 8. Relationship between n(SO2):n(NOx) and the desulphurisation and denitration efficiencies.
3.5. Full-flow process removal tests The concentrations of SO2, NO2, and NO in different sampling points, such as raw gases, compressor outlet, precooler outlet, the top of denitration tower, the top of the sulphuric acid tower, the top of oxidation tower and the top of nitric acid, are shown in Fig. 9. Most of the SO2 was removed through oxidation and nitric acid towers. The concentration of SO2 at the top of the nitric acid tower was only 25 ppm, thereby fulfilling the requirement of the ultra-low emission of atmospheric pollutants from coal-fired power plants in China. The NO concentration of raw gas significantly decreased after compression and precooling, whereas the NO2 concentration increased significantly, which could be attributed to the oxidation of NO under low temperature and high pressure. Afterwards, all NO2 was absorbed by the sulphuric acid in the denitration tower and reacted with the SO2 in liquid phase, in which 80% of the SO2 was converted to SO3 and then subsequently absorbed by water. Meanwhile, parts of NO2 were reduced to NO, leading to the increased concentration of NO in the denitration and sulphuric acid towers. In the oxidation tower, the remaining NO was oxidised into NO2 and then absorbed by the water in the denitration tower. To explore the effect of water, pressure, O2 and NO on SO2 oxidation, additional tests were conducted. The test conditions are listed in Table 2. The ranges of the temperature were selected according to [57].
Fig. 10. SO2 content of the compressor inlet and outlet (no NO).
The pressure in the second section of the compressor was maintained at about 2.0 MPa. A bubbling kettle was added to vaporise the water into droplets, which shifted to saturation state during the compression and oxidation tests. From Fig. 10, the conversion rate of SO2 increased with the increase in operating pressure during the compression process. Under the same pressure, the conversion rate of SO2 with water addition was significantly higher than that without water addition. Meanwhile, the conversion rates of SO2 were enhanced by NO addition. The effect of water on SO2 conversion was more obvious than that of NO. Because some water vapor was condensed into liquid water (small droplets) during the compression. The removal of SO2 can be attributed to the direct reaction of liquid water with SO2. However, NO had an indirect effect on the removal of SO2; NO cannot react with SO2 directly, but NO2, which is formed via NO oxidation under high pressure, can react with SO2. In conclusion, SO2 can be partially oxidised into SO3 under the pressing and existence of O2 and NOx. In addition, the oxidation of SO2 was promoted with the increase in pressure, and the existence of water and NO was beneficial to the oxidation of SO2. 4. Conclusions A 50 kg/h CO2 compression and purification test platform was designed and constructed. Tests on the behaviours of CO2 purification were conducted on the platform. The key problems in the optimisation of the operation conditions, control methods and pollutant removal during CO2 compression and purification were investigated. Through this study, the traditional desulphurisation and denitration system in an oxy-fuel combustion power station can be replaced. This study is an important link and basis for the large-scale development of oxy-fuel combustion technology. The amplification effect of CO2 compression and purification is not evident, but the process in this study is feasible, which can be referred to as an actual project with a design that can be applied in practical engineering. Furthermore, the compression and purification test results are basically consistent with the calculation results, thereby showing that the optimised results can be used for amplification design using Aspen and other software.
Fig. 9. SO2, NO2 and NO contents in different stages. 6
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The conclusions of this study are as follows: [18]
(1) The technology of CO2 compression and purification in oxy-fuel combustion was developed. On the basis of the experimental results, the technology process presented in this study was determined to be feasible, and high concentration of CO2 capture was realised. (2) The removal of NOx and SO2 at different pressures and temperatures and their interaction in the removal process were intensively studied. The optimal operation temperature and pressure for liquefaction are −30 °C and 3.0 MPa, respectively, when the volume content of CO2 in the feed gas was 80%. Moreover, the optimal value of CO2 energy consumption was 1273 kJ/kg. (3) The control method of the synergistic removal of SO2 and NOx was investigated. Adjusting the operating conditions of sulphur and nitrate purification enabled the overall average desulphurisation and denitration efficiency to exceed 98%. The ratio changes of n (SO2):n(NOx) had little effect on sulphur and nitrate purifications. (4) The effect of water, pressure, O2 and NO on SO2 oxidation was explored. The rule of NO converting to NO2 under different conditions was investigated. The oxidation of SO2 was promoted with the increase in pressure. Furthermore, the existence of water and NOx is beneficial to the oxidation of SO2, and NO had a slight promoting effect on the transformation of SO2.
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[29]
Declaration of Competing Interest [30]
All authors of this manuscript have directly participated in planning, execution, and analysis of this study. The contents of this manuscript are not now under consideration for publication elsewhere. The contents of this manuscript have not been copyrighted or published previously.
[31]
[32]
[33]
Acknowledgements [34]
This work is supported by National Key R&D Program of China (2018YFB0605305) and Shenhua Group Program (SHGF-11-74).
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Shah M, Degenstein N, Zanfir M, Kumar R, Bugayong J, Burgers K. Near zero emissions oxy-combustion CO2 purification technology. Energy Procedia 2011;4:988–95. White V, Allam R, Miller E. Purification of oxyfuel-derived CO2 for sequestration or EOR. In: Eighth International Conference on Greenhouse Gas Control Technologies (GHGT-8), Trondheim, Norway; 2006. White V, Allam RJ. Purification of carbon dioxide. Google Patents; 2012. Posch S, Haider M. Optimization of CO2 compression and purification units (CO2 CPU) for CCS power plants. Fuel 2012;101:254–63. Jin B, Zhao H, Zheng C. Optimization and control for CO2 compression and purification unit in oxy-combustion power plants. Energy 2015;83:416–30. Pipitone G, Bolland O. Power generation with CO2 capture: technology for CO2 purification. Int J Greenhouse Gas Control 2009;3:528–34. Jinyue Y. Carbon capture and storage (CCS). Appl Energy 2015;148:A1–6. Ritter R, Kutzschbach A, Stoffregen I. Energetic Evaluation of a CO2 purification and compression plant for the Oxyfuel process; 2009. Anheden M, Burchhardt U, Ecke H, Faber R, Jidinger O, Giering R, et al. Overview of operational experience and results from test activities in Vattenfall’s 30 MWth oxyfuel pilot plant in Schwarze Pumpe. Energy Procedia 2011;4:941–50. Li B, Duan Y, Luebke D, Morreale B. Advances in CO2 capture technology: a patent review. Appl Energy 2013;102:1439–47. White V, Torrente-Murciano L, Sturgeon D, Chadwick D. Purification of oxyfuelderived CO2. Int J Greenhouse Gas Control 2010;4:137–42. Zhou N. Design of 3MWth burner and numerical simulation on oxy-fuel
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental research on the process of compression and purification of CO2 in oxy-fuel combustion ⁎
Wu Hai-boa,b, , Xu Ming-xinc, Li Yan-binga, Wu Jin-huad, Shen Jian-chongd, Liao Haiyana a
Shenhua Guohua Electric Power Research Institute (Beijing) Co., Ltd., Beijing 100025, China College of Electromechanical Engineering, Qingdao University of Science & Technology, Qingdao 266061, China c National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing, China d Hangzhou Kuaikai HI-TECH CO., TD, Hangzhou 310051, China b
H I GH L IG H T S
optimal temperature and pressure for liquefaction were explored. • The overall average desulphurisation and denitration efficiencies exceeded 98%. • The existence of water and NO were beneficial to the oxidation of SO . • The • High CO concentration capture was realized. x
2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 compression and purification Synergistic removal SO2 NO Full-flow process
Oxy-fuel combustion is a leading potential CO2 capture technology for power plants. Thus, oxy-fuel technology has the highest requirement for CO2 purification. Compression and purification systems are important for oxyfuel combustion. The tests in this study were conducted on a 50 kg/h CO2 compression and purification test platform, which is currently the largest oxy-fuel combustion platform in China. The CO2 compression and purification technology in oxy-fuel combustion was developed. The experiments verified the feasibility of the proposed technology and realised high concentration of CO2 capture. In addition, the optimal temperature and pressure for liquefaction were explored, and the optimal CO2 energy consumption was obtained. The overall average desulphurisation rate and denitration efficiencies were more than 98%, and the ratio change of n(SO2):n (NOx) exerted little effect on sulphur and nitrate purifications. The oxidation of SO2 was promoted with the increase in pressure. Moreover, the existence of water and NOx were beneficial to the oxidation of SO2, whereas NO had a slight promoting effect on the transformation of SO2. This study makes the replacement of traditional desulphurisation and denitration systems in oxy-fuel combustion power stations possible and might establish a technical foundation for large-scale oxy-fuel combustion demonstration projects.
1. Introduction Global warming is becoming increasingly severe in recent years, thereby leading to the urgent requirements for the development of CO2 capture, utilisation and storage (CCUS) [1,2]. Currently, three main technologies are available for the mitigation of CO2 emissions from coal-fired power plants, namely, pre-, post- and oxy-fuel combustion [3,4]. Compared with the other technologies, oxy-fuel is regarded as one of the most promising choices for CCUS commercialisation because of its advantages in manufacturing, operation and retrofitting [5–7]. During oxy-fuel combustion, the mixtures of pure oxygen and recycled
⁎
flue gas are utilised for fuel combustion. As a result, the concentration of CO2 in the flue gas can be sufficiently high for direct compression and purification, thereby significantly simplifying the process of CCUS [8,9]. Moreover, the simultaneous removal of multiple pollutants, such as NOx and SOx, can be achieved in the process of CO2 compression and purification during oxy-fuel combustion, which is one of the most impressive features of the process [10–13]. According to the mechanism proposed by Allam et al., desulphurisation and denitration can be synergistically accomplished in the process of CO2 compression [14,15]. However, the removal efficiencies of NOx and SOx are significantly affected by multiple factors during the
Corresponding author at: Shenhua Guohua Electric Power Research Institute (Beijing) Co., Ltd., Beijing 100025, China. E-mail address: [email protected] (H.-b. Wu).
https://doi.org/10.1016/j.apenergy.2019.114123 Received 20 July 2019; Received in revised form 15 October 2019; Accepted 11 November 2019 0306-2619/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wu Hai-bo, et al., Applied Energy, https://doi.org/10.1016/j.apenergy.2019.114123
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system design [44]. A comprehensive dynamic model with specified control system was established, which provided the possibility to integrate CPU with full-train oxy-combustion power plants. Pipitone et al. proposed several methodologies for the removal of a selection of impurities from the CO2 in the flue gas of two oxy-combustion power plants fired with either natural or pulverised fuel [45]. The energy consumption, CO2 product purity and CO2 recovery rate of different CPU configurations were systematically analysed. In addition, Yan and Ritter separately reviewed the latest process concepts and integrations for CO2 purification process, verifying that the integration of oxy-fuel combustion and CO2 compression and purification process is feasible [46,47]. At present, some institutions, such as Schwarze Pumpe, Air Products and Huazhong University of Science and Technology, are conducting pilot tests on CO2 compression and purification during oxy-fuel combustion and developed viable and unique solutions for CO2 capture, which can be readily integrated into retrofitted and new building power plants. Anheden et al. investigated the performance of CO2 compression and purification system on an oxy-fuel combustion pilot platform [48]. The results showed that operating pressure and residence time can affect the conversion efficiencies of NO to NO2. In addition, 40% of NOx was removed during the compression process. Air Products proposed a new method for CO2 purification that combined some of their oxy-fuel purification technologies, such as the use of a membrane to recover CO2 and O2 and the sour compression technology [49,50]. Some demonstrative tests were performed at a 1MWth pilot-scale test facility. Recently, Zheng et al. reported the latest research advances on a 3MWth oxy-fuel combustion test platform at Huazhong University of Science and Technology, which is a full-process oxy-fuel combustion pilot-scale system in China [51]. However, relevant reports on the behaviours of CO2 compression and purification units are insufficient. Most studies on the synergistic removal of pollutants during oxyfuel combustion mainly focus on theoretical research and processing simulations. Few studies reported the actual operating characteristics of CPU in the full-process of oxy-fuel combustion. The aim of this study is to investigate the key problems in optimising operation conditions, control methods and pollutant removal during CO2 compression and purification. This work has important practical significance. Moreover, because the efficiencies of desulphurisation and denitration are greatly affected by the operating parameters, additional systematic experimental investigations should be performed to establish the integration between typical factors, such as pressure, temperature, concentrations of pollutants and the synergistic removal of multiple pollutants during CO2 compression and purification. Therefore, to systematically reveal the detailed operating characteristics and synergistic removal of SOx and NOx in the process of CO2 compression and purification during oxyfuel combustion, some tests were conducted on a 50 kg/h CO2 compression and purification platform. The characteristics of the synergistic removal of SO2 and NOx were investigated. The detailed effects of the typical operating parameters on the purification of CO2 were systematically analysed for the first time. The result can provide fundamental design criteria for the integration of oxy-fuel combustion with CO2 compression and purification.
CO2 compression process [16]. Normann et al. studied the chemical reaction process of desulphurisation and denitration under elevated pressures [17]. Their results established that the oxidation of NO into NO2 governed the absorption of NOx. In addition, the complex chemistry of the liquid phase, including reactions between HNO2, H2SO3 and H2SO4, was critical for the rate of absorption of NOx and SOx. Kühnemuth et al. simulated the process of desulphurisation and denitration in oxy-fuel combustion [18]. The simulation results showed that (1) SO2 can be completely removed and (2) the removal rates of NOx can reach 80–85% in the compression and purification unit (CPU) equipped in the process of oxy-fuel combustion. Ting et al. conducted a special research on the removal of NOx in the process of flue gas compression and discussed the conversion from NO to NO2 in dry and wet conditions [19]. The results showed that NO can be readily oxidised to water-soluble NO2. In addition, the reaction was kinetically controlled. Moreover, the overall mass balances across the gas–liquid system were complicated by the stability of the absorbed NOx species in the liquid. Luo et al. performed an experimental study on the process of NO removal and NO/ SO2 co-removal in various conditions [20]. The operating temperatures, initial NO concentration and other typical factors can affect the removal of NOx and SOx. White et al. studied the behaviour of SO2 removal in the compression process where NO, SO2 and O2 coexisted [21]. They discovered that the removal efficiency of SO2 decreased with the increase in the ratio of SO2/NOx. Murciano et al. investigated the conversion from gaseous agents to liquid acid in the process of CO2 compression [22,23]. The results confirmed that operating factors such as pressure, temperature and residence time can greatly affect the conversion from NO and SO2 to nitric and sulphuric acids, respectively. Lu et al. studied the synergistic removals of SOx and NOx during oxy-fuel combustion [24]. The results verified that NO in the flue gas can be effectively converted to HNO3 under high pressure and normal temperature. Hui found that increasing the pressure can enhance the conversion from SO2 to SO42-, thereby increasing the SO2 removal efficiency [25]. In conclusion, the synergistic removal of NOx and SOx during oxy-fuel combustion was practicable and the removal efficiencies were significantly influenced by the operating conditions [26,27]. Several reports presented thermodynamic models [28], process analysis [29], techno-economic analysis [30,31] and optimisation method [32] to provide references and analytical analysis of the relevant CO2 capture system. Cheng et al. presented gas-phase oxidation of NO at high pressure relevant to the sour gas compression purification process for oxy-fuel combustion flue gas [33]. Wu and Li et al. studied the compression of CO2 related to oxy-fuel combustion systems. The influence of CO2 concentration, temperature or pressure on liquefaction ratio and energy consumption was also investigated [34,35]. Zhou et al. studied the simultaneous removal of NOx and SO2 under high pressure on an experiment bench, as well as the removal of NOx and SO2 at different pressures in dry/wet N2 atmosphere and the simulated oxyfuel combustion flue gas [36,37]. In addition to the above mechanism, the technical feasibility of denitration and desulphurisation in the process of CO2 compression during oxy-fuel combustion was investigated. Many schemes for CO2 CPU have been proposed [38–40]. White et al. presented an integrated process for the compression and simultaneous purification of CO2 to produce CO2 product streams [41]. The purities of CO2 after the treatment in the presented process reached 95–98%, which satisfied the required purity of CO2 for enhanced oil recovery. Alam et al. proposed a method for the purification of CO2 with oxygen and CO without the consideration of NOx and SOx removal [42]. Posch et al. designed two different purification processes for CPU optimisation for oxy-fuel combustion power plants [43]. The results indicated that double flash separation had low power cost and cooling requirement, whereas separation by rectification achieved high purity at nearly the same separation efficiency. Jin et al. investigated the methods for CPU optimisation and control in oxy-combustion power plant via single variable analysis, multivariable optimisation, dynamic simulation and control
2. Experimental 2.1. Test platform To completely investigate the operational feasibility and reliability of CPU in the process of oxy-fuel combustion, a 50 kg/h CO2 compression and purification test platform, which is currently the largest and most complete pilot-scale facility in China, was constructed in Hangzhou. The synergistic removal of SOx and NOx was achieved by combining the modified tower-type sulphuric acid production and pressurised condensation distillation purification. The platform consists of gas distribution, compression, low-temperature washing, purification 2
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Fig. 1. Flowchart of the CO2 compression and purification process.
and condensing liquid units and a low-temperature storage tank. The detailed schematic of the facility is shown in Fig. 1. The feeding gases were firstly compressed to the specific pressures and temperatures to synergistically liquidate NOx, SOx and CO2. Then, the mixtures were washed in the gas scrubbing tower to separate NOx and SOx from the liquified CO2 flow. The purified CO2 was stored in the CO2 tank after further liquefaction. The separated NOx and SOx was then heated to 200 °C to evaporate NOx from the liquid mixtures in the denitration tower, where high-purity sulphuric acid could be recovered simultaneously. The gaseous NOx was then purified and oxidised to NO2 in the oxidation tower. Lastly, the flow of gaseous NO2 was completely absorbed by water in the diluted nitric tower, where the dilute nitric acid was recovered. SOx and NOx were removed simultaneously through the process described above, and sulphuric and nitric acids were recovered as by-products. Some instruments used to measure gaseous species were also equipped in the platform. The concentrations of CO2, O2 and N2 were detected through gas chromatography, and an infrared flue gas analyser was adopted to measure the concentrations of NO, NO2 SO2 in the flue gas. The contents of the sulphuric and nitric acids were analysed through acid-base titration (GB/T 337.1-2014). During long-term tests, experimental data were obtained under stable operating stages, and repeated tests were carried out to confirm the uncertainty quantification of the experimental results. The gas distribution system does not include water vapor. Hence, additional water vapor must be removed before compression and purification. Although the gas distribution system does not include water vapor in this test, a certain amount of water is present in the compression process, and the corrosion of the compressor occurred.
Fig. 2. Conversion of N-containing compounds in the system.
3. Results and discussion 3.1. Effect of temperature and pressure on CO2 liquefaction The characteristics of CO2 liquefaction and energy consumption in different operating temperatures and pressures were investigated. The results (Figs. 3 and 4) indicate that the effects of operating temperatures and pressures varied with the volume fractions of CO2. The liquefication rate is an important control index in the production process. This rate represents the ratio of the amount of liquefied CO2 to the total amount of CO2 in the raw flue gas, whereas the liquefied energy consumption refers to the energy consumed in the production of liquid CO2 per unit mass. As shown in Fig. 3, the liquefication rate was much higher in lowtemperature condition (−30 °C) regardless of the operating pressures. In addition, the pressure exerts a positive effect on the liquefication rate; as the operating pressure increased, the liquefication rate of CO2 increased remarkably. Moreover, the energy consumption (kJ/kg) significantly decreased as the operating pressure increased from 2.5 MPa to 3.4 MPa (Fig. 4). For example, when the operating temperature was −20 °C, the energy consumption at a pressure of 2.5 MPa was about 11000 kJ/kg, which decreased to 2050 kJ/kg when the pressure increased to 3 MPa. The result also indicates that the energy consumption was significantly reduced when the operating temperature decreased, especially under low-pressure conditions. A comparison of the tendencies in Figs. 3 and 4, shows that operating temperature and pressure had opposite effects on the rates and energy consumption of CO2 liquefaction. For example, although the energy consumption decreased with the increase in operating pressures
2.2. Mechanism research The majority of the NOx formed during oxy-fuel combustion is in the form of NO, which is difficult to absorb directly. As a result, NOx should be firstly oxidised to NO2, N2O3 and N2O4 during the denitration process because of their high solubility in water. However, the conversion of these components in liquid reactions is complex [52] due to the mutual transformation amongst NO2, N2O3, N2O4, HNO2 and HNO3 in liquid reactions (Fig. 2). In addition, NO2 can be reduced to NO by SO3, following the lead chamber process for the production of sulphuric acid [53,54]. Moreover, the equilibria of the above mentioned reactions introduced above are significantly affected by the operating temperatures and pressures [55]. To achieve the high efficiencies of the synergistic removal of SOx and NOx, the optimisation of the typical parameters should be verified through tests in the actual CO2 compression and purification facility. 3
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Fig. 3. Effects of liquefaction temperature and pressure under 75% CO2 volume fraction.
when the volume fraction of CO2 increased to 90% continuously. Moreover, when the volume concentration of CO2 in the feeding gases was 80%, the liquefaction rates of CO2 could be higher than 80%, and the electric energy consumption of the unit was only 1273 kJ/kg. Meanwhile, the purity of liquid CO2 in the purification tower exceeded 99%. Therefore, we can conclude that the optimal operating parameters in the CO2 compression and purification process are a CO2 volume fraction of 80%, a pressure of 3.0 MPa, and a temperature of −30 °C.
and decrease in temperatures, the decreasing extent was gradually reduced (Fig. 3(b)), which can be attributed to the increasing loads of the cooler and compressor in conditions with lower operating temperatures and pressures. Similar tendencies can be found in the other figures. The results indicate that an optimal operating pressure and temperature should be selected for CO2 compression and purification. As can be seen in Figs. 3(a) and 4(a), the liquefication rates of CO2 can be as high as 85.92% at −30 °C. Moreover, energy consumption can decrease to 1441 kJ/kg when the pressure is 3.5 MPa. Meanwhile, the energy consumption when the pressure is 3.0 and 3.5 MPa was similar (Figs. 3(b) and 4(b)). Therefore, a pressure of 3.0 MPa and temperature of −30 °C could be regarded as the optimal operating parameters in this study.
3.3. Conversion of NOx during CO2 compression and purification As previously mentioned, the concentration of NOx and SOx significantly affects the operation of CPU during oxy-fuel combustion. To investigate the conversion of NOx during CO2 compression and purification, several tests on the behaviours of desulphurisation and denitration in CPUs were performed using the pilot-scale platform. The test parameters are listed in Table 1. The flow rates of CO2 and O2 were 4.4 m3/h and 5.72 L/min, respectively, the pressure was 2.0 MPa and the temperature was 220 °C. During the tests, the concentrations of NO in the flue gas were measured online in various sampling locations, such as the inlet of the compressor, the outlet of the compressor and the outlet of the precooler. The behaviours of the NO concentration are displayed in Fig. 6. The conversion rate of NO was about 83.3% when the pressure and temperature of the compressor are 3.0 MPa and −30 °C, respectively. The conversation rate of NO can reach 99.75% when the pressure and temperature of the precooler are 3.0 MPa and −11 °C, respectively. The conversion from NO to NO2 was significantly enhanced by the increase in pressure and decrease in temperature. As a result, almost all the NO
3.2. Effect of CO2 concentration on liquefaction To reveal the effects of CO2 concentration in the feeding gases on the characteristics of liquefaction, tests were conducted under an operating temperature and pressure of −30 °C and 3.0 MPa, respectively. The results are summarised in Fig. 5. The concentration of CO2 had positive effects on the liquefaction rates and energy consumption of the unit, which can be attributed to the increasing partial pressure of CO2 in the feeding gases. As the concentration of CO2 in the feeding gases increased, the partial pressure of CO2 in the compressor increased, which was beneficial to the further liquefaction of CO2. When the volume fraction of CO2 in the feeding gases increased from 60% to 75%, the energy consumption of the compression unit significantly decreased from 5876 kJ/kg to 1487 kJ/ kg. However, the reduction in the energy consumption was not obvious
Fig. 4. Effects of liquefaction temperature and pressure under 80% CO2 volume fraction. 4
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Fig. 5. Effect of CO2 content in feed gas on CO2 liquefaction and energy consumption. Table 1 Arrangement of the test conditions. No.1
1 2 3 4 5 6 7
NO inlet flow rates
SO2 inlet flow rates
Volume flow (mL/min)
Concentration (ppm)
Volume flow (mL/min)
Concentration (ppm)
452.58 184.38 92.19 184.38 184.38 92.19 92.19
5400 2200 1100 2200 2200 1100 1100
0.11 0.11 0.11 0.42 0.84 0.84 0.42
1266 1266 1266 5000 10,000 10,000 5000
N(SO2):N (NOx)
0.50 1.23 2.46 4.85 9.70 19.39 9.70
Fig. 7. Total desulphurisation and denitration efficiency during sulphur and nitrate purification process.
completed in the denitration tower. The SO2 separated in the denitration tower was absorbed in the sulphuric acid tower, wherein H2SO4 can be recovered as a by-product. The flow of the NO separated in the denitration tower should be treated further. After low-temperature compression, parts of NO were basically converted to NO2, whereas the rest was oxidised to NO2 in the oxidation tower. Subsequently, all NO2 was removed in the nitric acid tower, in which HNO2 and HNO3 were the by-products. The desulphurisation and denitration reactions are actually interrelated with CO2 compression and purification. To explore the fundamental characteristics of the synergistic removal of SO2 and NOx, the effects of the ratios of SO2 and NOx in the feeding gases were systematically investigated. The results are shown in Fig. 8. The ratios between SO2 and NO had little effects on the desulphurisation and denitration efficiencies. In fact, the total removal efficiencies of NO and SO2 were extremely high, in which the effects of the ratios between SO2 and NO were not apparent. Therefore, the integration between the tower type sulphuric acid process and pressurised condensation distillation process was proven feasible. In addition, the average of the desulphurisation efficiency was more than 98%, whereas the overall denitration efficiency exceeded 98%. The ratio changes of n(SO2):n(NOx) had little effect on sulphur and nitrate purifications. SO2 can be partially oxidised into SO3 under the pressing and existence of O2 and NOx. Furthermore, the existence of H2O and NOx was beneficial to the oxidation of SO2 [56].
Fig. 6. NO conversion rate during compression and precooling.
could be converted into NO2 during flue gas compression and condensation. 3.4. Synergistic removal of SO2 and NOx during CO2 compression and purification To further study the removal efficiencies of NOx and SO2 during CO2 compression and purification, some relevant tests were conducted. The test conditions are presented in Table 1. The removal efficiencies of NOx and SO2 are displayed in Fig. 7. The results show that most of NO and SO2 can be removed simultaneously. The NO removal efficiency ranged from 97.70% to 99.30%, with an average of 98.30%. For the desulphurisation, the SO2 removal efficiency was also extremely high, ranging from 96.4% to 99.6%. The separation of NOx and SO2 was mainly 5
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Table 2 Detailed testing parameters. Serial No. Test (without NO/with NO)
Test conditions Ingredients
CO2 N2 O2 SO2 NO Operational pressure
Flow
Concentration 3
50.68 m /h 80 %vt. 150 L/min 14.8 %vt. 50 L/min 5.2 %vt. 1.185 L/min 3237 mg/m3 100 mL/min 119.5 mg/m3 1.6/2.5/3.5 MPa
Fig. 8. Relationship between n(SO2):n(NOx) and the desulphurisation and denitration efficiencies.
3.5. Full-flow process removal tests The concentrations of SO2, NO2, and NO in different sampling points, such as raw gases, compressor outlet, precooler outlet, the top of denitration tower, the top of the sulphuric acid tower, the top of oxidation tower and the top of nitric acid, are shown in Fig. 9. Most of the SO2 was removed through oxidation and nitric acid towers. The concentration of SO2 at the top of the nitric acid tower was only 25 ppm, thereby fulfilling the requirement of the ultra-low emission of atmospheric pollutants from coal-fired power plants in China. The NO concentration of raw gas significantly decreased after compression and precooling, whereas the NO2 concentration increased significantly, which could be attributed to the oxidation of NO under low temperature and high pressure. Afterwards, all NO2 was absorbed by the sulphuric acid in the denitration tower and reacted with the SO2 in liquid phase, in which 80% of the SO2 was converted to SO3 and then subsequently absorbed by water. Meanwhile, parts of NO2 were reduced to NO, leading to the increased concentration of NO in the denitration and sulphuric acid towers. In the oxidation tower, the remaining NO was oxidised into NO2 and then absorbed by the water in the denitration tower. To explore the effect of water, pressure, O2 and NO on SO2 oxidation, additional tests were conducted. The test conditions are listed in Table 2. The ranges of the temperature were selected according to [57].
Fig. 10. SO2 content of the compressor inlet and outlet (no NO).
The pressure in the second section of the compressor was maintained at about 2.0 MPa. A bubbling kettle was added to vaporise the water into droplets, which shifted to saturation state during the compression and oxidation tests. From Fig. 10, the conversion rate of SO2 increased with the increase in operating pressure during the compression process. Under the same pressure, the conversion rate of SO2 with water addition was significantly higher than that without water addition. Meanwhile, the conversion rates of SO2 were enhanced by NO addition. The effect of water on SO2 conversion was more obvious than that of NO. Because some water vapor was condensed into liquid water (small droplets) during the compression. The removal of SO2 can be attributed to the direct reaction of liquid water with SO2. However, NO had an indirect effect on the removal of SO2; NO cannot react with SO2 directly, but NO2, which is formed via NO oxidation under high pressure, can react with SO2. In conclusion, SO2 can be partially oxidised into SO3 under the pressing and existence of O2 and NOx. In addition, the oxidation of SO2 was promoted with the increase in pressure, and the existence of water and NO was beneficial to the oxidation of SO2. 4. Conclusions A 50 kg/h CO2 compression and purification test platform was designed and constructed. Tests on the behaviours of CO2 purification were conducted on the platform. The key problems in the optimisation of the operation conditions, control methods and pollutant removal during CO2 compression and purification were investigated. Through this study, the traditional desulphurisation and denitration system in an oxy-fuel combustion power station can be replaced. This study is an important link and basis for the large-scale development of oxy-fuel combustion technology. The amplification effect of CO2 compression and purification is not evident, but the process in this study is feasible, which can be referred to as an actual project with a design that can be applied in practical engineering. Furthermore, the compression and purification test results are basically consistent with the calculation results, thereby showing that the optimised results can be used for amplification design using Aspen and other software.
Fig. 9. SO2, NO2 and NO contents in different stages. 6
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The conclusions of this study are as follows: [18]
(1) The technology of CO2 compression and purification in oxy-fuel combustion was developed. On the basis of the experimental results, the technology process presented in this study was determined to be feasible, and high concentration of CO2 capture was realised. (2) The removal of NOx and SO2 at different pressures and temperatures and their interaction in the removal process were intensively studied. The optimal operation temperature and pressure for liquefaction are −30 °C and 3.0 MPa, respectively, when the volume content of CO2 in the feed gas was 80%. Moreover, the optimal value of CO2 energy consumption was 1273 kJ/kg. (3) The control method of the synergistic removal of SO2 and NOx was investigated. Adjusting the operating conditions of sulphur and nitrate purification enabled the overall average desulphurisation and denitration efficiency to exceed 98%. The ratio changes of n (SO2):n(NOx) had little effect on sulphur and nitrate purifications. (4) The effect of water, pressure, O2 and NO on SO2 oxidation was explored. The rule of NO converting to NO2 under different conditions was investigated. The oxidation of SO2 was promoted with the increase in pressure. Furthermore, the existence of water and NOx is beneficial to the oxidation of SO2, and NO had a slight promoting effect on the transformation of SO2.
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[20] [21]
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[23]
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Declaration of Competing Interest [30]
All authors of this manuscript have directly participated in planning, execution, and analysis of this study. The contents of this manuscript are not now under consideration for publication elsewhere. The contents of this manuscript have not been copyrighted or published previously.
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[32]
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Acknowledgements [34]
This work is supported by National Key R&D Program of China (2018YFB0605305) and Shenhua Group Program (SHGF-11-74).
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