Article pubs.acs.org/EF
Abatement of Fine Particle Emissions from a Coal-Fired Power Plant Based on the Condensation of SO3 and Water Vapor Hao Wu,† Danping Pan,† Rui Zhang,† Linjun Yang,*,† Ziming Peng,‡ and Bing Yang‡ †
Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ‡ Jiangsu Ruifan Environmental Equipment Company, Limited, Nantong, Jiangsu 226200, People’s Republic of China ABSTRACT: A new process was proposed to reduce fine particle emissions in coal-fired flue gases via the condensation of SO3 and water vapor. In this new process, hot atomized water was sprayed before the electrostatic precipitator (ESP) system to decrease the flue gas temperature and increase the flue gas humidity, causing the flue gas temperature to drop below the acid dew point; thus, SO3 would condense on the particle surfaces ahead of the ESP, which would enhance the fine particle removal efficiency of the ESP. Then, a fluoroplastic heat exchanger was located between the ESP and the wet flue gas desulfurization (WFGD) system to make the WFGD inlet flue gas temperature decrease further at a high humidity, leading to the condensation of water vapor on the particle surfaces in the desulfurization scrubber, which would improve the removal efficiency of the WFGD. The feasibility of this new process was analyzed by numerical calculation, and the results showed that the new process was feasible for the original flue gas with high SO3 concentrations, high temperature, and high humidity. The effectiveness of this new process under typical operating conditions was also presented. Furthermore, the influence of several main parameters, such as the SO3 concentration of original flue gas, the temperature drop of flue gas before the ESP, and the temperature of flue gas at the WFGD inlet, were also investigated. The results indicated that the fine particle emissions in the final exhausted flue gas would be reduced by 50−70% with this new process.
1. INTRODUCTION As a result of the health risks and environmental issues related to the emission of fine particles in coal-fired power plants, intensive research has recently been carried out.1,2 Conventional techniques are reportedly far less efficient in collecting fine particles because of their small diameters.3,4 Therefore, to further reduce the fine particle emissions in flue gases, many methods have been put forward to enlarge the sizes of fine particles before entering the conventional dust removal devices. Heterogeneous vapor condensation is one of the most efficient methods to enlarge fine particle diameters before conventional separators, and it has been investigated for many years.5−8 When the supersaturated degree exceeds a critical value, heterogeneous nucleation will occur and the vapor around the fine particles will begin to condense on their surfaces, causing larger condensation-grown droplets, with fine particles acting as the condensation nuclei to be formed.9,10 Then, the removal performance of the subsequent conventional separator will be improved for these larger condensation-grown droplets. Because the majority of coal-fired power plants have installed wet flue gas desulfurization (WFGD) systems in recent years, many research studies have been performed to enhance the performance of WFGD systems for the removal of fine particles by heterogeneous vapor condensation. Fan et al. studied the establishment of supersaturation in a desulfurization scrubber by numerical simulation and found that the essential supersaturated degree for the nucleation of fine particles can be achieved by increasing the inlet flue gas humidity.11 Wu et al. experimentally studied the removal behavior of WFGD systems for fine particles by adjusting the humiture of the flue gas before © 2017 American Chemical Society
the desulfurization scrubber. The necessary supersaturated degree can also be achieved in the scrubber.12 The fine particles can be enlarged by heterogeneous vapor condensation and then efficiently removed by the desulfurization process. Although the performance of the fine particle removal in WFGD can be enhanced by these processes, the fine particle emission in the final exhausted flue gas is still too high. The main factor restricting the improvement of the removal performance is the fine particle concentration at the WFGD inlet.13 Higher concentrations of fine particles work against the enlargement of fine particle sizes in the same supersaturation, which results in a reduction in the improvement of the removal efficiency.7 To further reduce the fine particle emissions in coal-fired flue gas, low-low temperature electrostatic precipitator (LLT-ESP) technology has become an effective measure.14,15 In comparison to the conventional electrostatic precipitators (ESPs), the inlet flue gas temperature of a LLT-ESP is lowered to approximately 90−110 °C, which is below the acid dew point. The lower temperature of the flue gas at the LLT-ESP inlet will lead to a lower specific resistance of fine particles;15 moreover, when the flue gas temperature is lower than the acid dew point, SO3 will condense on the fine particle surfaces and the specific resistance of these particles will decrease further.16 According to previous studies, a new process to reduce fine particle emissions from coal-fired flue gas via condensation of SO3 and water vapor has been proposed. The increase in the flue gas humidity and decrease in the flue gas temperature were Received: November 18, 2016 Revised: January 26, 2017 Published: January 31, 2017 3219
DOI: 10.1021/acs.energyfuels.6b03069 Energy Fuels 2017, 31, 3219−3226
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Figure 1. Schematic diagram of the experimental facility. To emulate the actual conditions, the operating voltage of the ESP system was set to 50 kV and the residence time of flue gas was 1 s. In the WFGD system, the limestone−gypsum method was simulated. The liquid−gas ratio was set to 15 L Nm−3, and the desulfurization slurry temperature was 50 °C. 2.2. Measurement Techniques. The particle concentration and size distribution were measured in real time by an electrical lowpressure impactor (ELPI, Finland) at different points. The measuring range of the ELPI was from 0.023 to 9.314 μm. Because the flue gas humidity was high, a diluter (DI-1000, Finland) was used prior to the ELPI to decrease the measuring errors. The concentration of SO3 in the flue gas was measured via the controlled condensation method (CCM).17,18 The sample gas was first heated to 180 °C and then cooled until completely condensed on the surface of a spiral condenser. Water was used as the cooling media in the spiral condenser, and the temperature of the water was set to 60 °C. An ion chromatograph analyzer (ICS-2100, Sunnyvale, CA, U.S.A.) was used to determine the concentrations of SO42− in the collected condensate. Finally, the SO3 concentration was obtained. To measure the humiture of the flue gas at different points, a humiture transmitter (HMT337, Finland) was used.
realized by spraying hot atomized water before the ESP, and the further decrease of flue gas temperature was obtained by a fluoroplastic heat exchanger located between the ESP and WFGD. In this new process, the flue gas humidity would be increased and the flue gas temperature would be decreased to an appropriate value, leading to the condensation of SO3 on fine particle surfaces and a reduction in the fine particle specific resistance before the ESP. Then, the removal efficiency of the ESP would be enhanced. The fine particle concentration of the WFGD inlet flue gas would also decrease. The application of the fluoroplastic heat exchanger located between the ESP and WFGD would lead to a further flue gas temperature reduction at high humidity, which could satisfy the requirement of the establishment of supersaturation in the desulfurization process. The fine particles were enlarged in the desulfurization scrubber, which would enhance the fine particle removal performance of the desulfurization process. In this paper, the feasibility analysis of this new process was carried out. The removal performance of this new process was also presented under typical operating conditions. Furthermore, the influence of SO3 concentrations of the original flue gas, the temperature drop of the flue gas before the ESP, and the flue gas temperature at the WFGD inlet were investigated.
3. RESULTS AND DISCUSSION 3.1. Feasibility Analyses. 3.1.1. Acid Dew Point of Flue Gas. Because the main purpose of the flue gas temperature reduction before the ESP was to make SO3 condense onto the fine particle surfaces, the acid dew point was very important. It is reported that the acid dew point is mainly influenced by the flue gas humidity and the SO3 concentration,19 as shown in Figure 2. In this new process, when the hot atomized water was sprayed before the ESP, the flue gas humidity was increased,
2. EXPERIMENTAL SECTION 2.1. Experimental Facility. Figure 1 shows the schematic diagram of the experimental facility. The experimental facility consists of a coalfired boiler, an evaporation chamber, an ESP system, a fluoroplastic heat exchanger, and a WFGD system. The coal-fired boiler used in the experimental setup is a self-sustained stokerfeed boiler (CZML-0.12, China), which burns coal briquettes with an average size of about 20 mm. The flue gas was produced by the boiler at approximately 350 Nm3 h−1, and the particle concentration and size distribution in the flue gas were kept stable. Before entering the evaporation chamber, the SO3 concentration and humidity of flue gas could be adjusted by the SO3 and steam generators to simulate different conditions of flue gas. After mixing with SO3 and water vapor, the flue gas went through the evaporation chamber. In the evaporation chamber, hot atomized water was sprayed into flue gas by a two-fluid atomizing nozzle, and the average droplet size was about 20 μm. With the evaporation of hot atomized droplets, the flue gas temperature decreased and the humidity increased. After flowing through the ESP, the temperature of flue gas was further decreased to appropriate values at high humidity by a fluoroplastic heat exchanger. Then, the flue gas with high humidity and low temperature entered the desulfurization scrubber. In the desulfurization scrubber, many entrained droplets, inactivated fine particles, and condensation-grown droplets in the flue gas would be efficiently separated.
Figure 2. Relationship among the sulfuric acid dew point, SO3 concentration, and vapor concentration. 3220
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Energy & Fuels leading to a change in the acid dew point. The acid dew point can be calculated using an equation derived by Okkes,19 and the equation is shown as follows: tsld = 10.8809 + 27.61 log pH O + 10.83 log pSO 2
+ 1.06(log pSO + 2.9943)2.19 3
dqg = αg(Tg − Tf )A f dz
(2)
dqw = αw(Tf − Tw )A f dz
(3)
where qg and qw were the quantities of heat convection on the gas side and fluid side, respectively, αg and αw were the heat transfer coefficients on the gas side and liquid side, respectively, Tg and Tw were the bulk temperatures of the gas phase and liquid phase, respectively, Tf was the temperature of the interface, Af was the total surface area of the droplets per unit height, and dz was the height of an infinitesimal section. The spraying droplet motion model was also established, and the equation was as follows:11
3
(1)
where tsld is the temperature of the acid dew point, pH2O is the partial pressure of water vapor, and pSO3 is the partial pressure of SO3 vapor. The dew points of the acid calculated from the equation of Okkes are shown in Figure 3. The acid dew point was directly
md
dVd = Gd − Ffd − Fvd dt
(4)
where md was the mass of a single droplet, Vd was the velocity of a droplet, t was the time, Gd was the gravity of the droplet, Ffd was the buoyancy force, and Fvd was the resistance force. The calculation method and determination of the parameters were based on various publications.10,11,20 Figure 4 shows the distribution of the saturation in the desulfurization scrubber at different flue gas humidity values.
Figure 3. Sulfuric acid dew point at different humidities and SO3 concentrations of flue gas.
proportional to the flue gas humidity and SO3 concentration. With flue gas humidity increasing from 40 to 160 g Nm−3 at a SO3 concentration of 20 mg Nm−3, the acid dew point increased from approximately 119 to 130 °C. As the SO3 concentration increased to 80 mg Nm−3, the acid dew point increased from approximately 130 to 149 °C, with the flue gas humidity increasing from 40 to 160 g Nm−3. Therefore, if the flue gas temperature is reduced below 120 °C at a relatively high humidity, most SO3 condenses onto the particle surfaces before the ESP to form larger particles with relatively low specific resistance,15,16 which can be easily collected in the ESP. 3.1.2. Saturation Established in the Scrubber. The supersaturation obtained in the scrubber depended upon the humiture of flue gas at the WFGD inlet.11,12 For the purpose of illustrating the distribution of saturation established in the desulfurization scrubber at different humitures of the WFGD inlet flue gases, a numerical calculation was performed. Because the physical processes in the desulfurization scrubber were too complex, many assumptions were performed: (1) the flue gas was an ideal gas; (2) the desulfurization scrubber was heatinsulated; (3) the flue gas humiture and the temperature of droplets were a continuous distribution; (4) the desulfurization slurry was regarded as pure water, and the change of the vapor equilibrium pressure caused by the desulfurization slurry was ignored; and (5) the droplet diameter of the desulfurization slurry was assumed to be 2 mm. According to the principle of mass conservation, the principle of energy conservation, and the double-film theory, the heat transfer equation of the gas phase and the interface and the heat transfer equation of the interface and the liquid phase could be written as11
Figure 4. Distribution of saturation in the scrubber at different flue gas temperatures at the WFGD inlet.
The WFGD inlet flue gas was assumed to be 80 °C, and the superficial gas velocity was set at 3.5 m s−1 in the calculation process. With an increase in the relative height, the degree of saturation increased. When flue gas humidity increased from 60 to 120 g Nm−3, the maximum degree of saturation in the outlet of the desulfurization scrubber increased from 0.80 to 1.26. When the hot flue gas entered the desulfurization scrubber from the bottom to counter-current contact with the cold desulfurization slurry, the mass and heat transfer occurred simultaneously. The flue gas temperature would decrease, and relative humidity would increase. Meanwhile, some desulfurization slurry droplets would also evaporate and enter the gas phase. As a result, the flue gas relative humidity could be greatly raised, causing the degree of saturation to increase along the height of the scrubber. Although the degree of saturation would increase, supersaturation could not be achieved in the desulfurization process for the original flue gas.21 However, as the humidity of the WFGD inlet flue gas increased from 60 to 120 g Nm−3, the situation was different. The degree of saturation would be further increased, and the supersaturation could be established in the scrubber. As reported, the supersaturation degree of 1.15−1.20 is a suitable value for 3221
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above 150 °C, and the humidity should be above 80 g Nm−3. Therefore, the new process proposed in this paper was only feasible for the original flue gas with a high SO3 concentration, high temperature, and high humidity. 3.2. Fine Particle Characteristics in the Original Flue Gas. Figure 7 shows the particle concentration and size
fine particle nucleation;22 thus, the humidity of the flue gas after humidification by hot atomized water should be 100 g Nm−3 or above. 3.1.3. Humiture of Flue Gas after Water Spray Humidification. Because the flue gas humiture was mainly dependent upon the original flue gas humidity and the effect of the hot atomized water spray humidification, the humiture of the flue gas after the hot atomized water spraying was discussed. Figure 5 depicts the evaporation capacity as a function of the flue gas temperature drop before the ESP. Because the enthalpy
Figure 5. Evaporation capacity as a function of the flue gas temperature drop.
of the flue gas was not the same at different temperatures, the same temperature drop might lead to different energy releases. However, according to the thermodynamic calculation, the deviation of the released energy for the same flue gas temperature drop was less than 0.05; thus, the difference in the enthalpy of the initial flue gas temperature was ignored. In Figure 5, the variations of evaporation capacity of the hot atomized water presented as an approximately linear relationship with the changes of the flue gas temperature drop. When the temperature drop increased from 10 to 80 °C, the evaporation capacity increased from approximately 6 to 47 g Nm−3. Figure 6 gives the final humidity of the flue gas after the different flue gas temperature drops before the ESP. In combination with the previous calculation results (see Figures 3 and 4), it can be inferred that, in actual coal-fired power plants, the appropriate temperature drop should be between 30 and 60 °C to establish supersaturation in the desulfurization scrubber. That is, the original flue gas temperature should be
Figure 7. Concentration and size distribution of fine particles in original flue gas: (a) concentration of fine particles, (b) size distribution of fine particles, and (c) cumulative size distribution of fine particles.
distribution in the original flue gas. As shown in Figure 7a, the number concentration was approximately 1.4 × 107 cm−3 and the mass concentration was approximately 350 mg Nm−3. It can also be observed from panels b and c of Figure 7 that, for the mass size distribution, the fine particles were mostly in the micrometer range; for the number size distribution, the clear majority of fine particles was concentrated at the submicrometer scale. In this case, it can be noted that controlling number concentration of fine particles would make more sense. Therefore, in this paper, more attention was focused on the number concentration of fine particles.
Figure 6. Final humidity as a function of the flue gas temperature drop. 3222
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Energy & Fuels 3.3. Removal of Fine Particles via the New Process. Figure 8 presents the removal performance of fine particles with
ESP, the application of this new process would lead the number concentration of fine particles at the ESP outlet to decrease from 6.2 × 107 to 4.3 × 107 cm−3. Simultaneously, the mass concentration of fine particles would decrease from 59 to 47 mg Nm−3. Figure 8c shows that the number removal efficiency of the ESP increased from 55 to 67%, while the mass removal efficiency only increased from 82 to 85%. When the flue gas temperature decreased from 165 to 115 °C, the humidity of the flue gas increased from 85 to 115 g Nm−3. Hence, the flue gas temperature would be below the acid dew point of 140 °C before the ESP (see Figure 3). In this case, SO3 in the flue gas would condense on the particle surfaces, leading to larger particles with low specific resistances to be formed; moreover, the temperature drop of the flue gas could also increase the flue gas residence time in ESP and decrease the specific resistance of fine particles. Therefore, more fine particles would be collected by the ESP. However, because many of the removed fine particles were lightweight sub-micrometer particles,23 the downward trend in the number concentration of fine particle emission was more obvious than that of the mass concentration. For the removal performance of the WFGD, it can be seen in panels a and b of Figure 8 that the number concentration of fine particles at the WFGD outlet decreased from 5.1 × 107 to 2.4 × 107 cm−3, while the mass concentration decreased from 27.6 to 16.6 mg Nm−3. It can also be noted that the mass and number removal efficiencies of the WFGD increased from 17.4 to 44.7% and from 53.4 to 63.4%, respectively. It can be explained by the supersaturation achieved in the desulfurization scrubber. When the new process was applied in the experiment, the flue gas temperature decreased to 80 °C and the humidity increased to 115 g Nm−3 ahead of the WFGD. As calculated (see Figure 4), the supersaturation could be established in the scrubber and the maximum supersaturated degree would exceed 1.20. In this case, many fine particles would be activated and become larger droplets with the fine particle acting as nuclei of condensation. The enlarged fine particles were easily removed by the scrubbing of the slurry and intercepting of the demister. In addition, numerous studies have shown that fine particles will be generated by the desulfurization process, and the evaporation of the slurry droplets entrained by the flue gas is a major cause of this phenomenon.24,25 The establishment of supersaturation in the scrubber was also beneficial for the inhibition of slurry droplet evaporation, which would reduce the fine particles generated by the desulfurization process.12 In comparison to the concentration of fine particles in the final exhausted flue gas without the application of this new process, the application of this new process leads the number concentration and mass concentration of fine particles in final exhausted flue gas to be reduced by 52.9 and 39.9%, respectively. It is noteworthy that, in the application of this new process, the reduction rate of fine particles based on the mass concentration was much lower than that based on the number concentration. As shown in Figure 7b, with a decreasing particle size, the mass concentration of the particles significantly decreased, while the number concentration of the particles significantly increased. The number concentration of fine particles with a small mass was much higher than that of the fine particles with a large mass, as shown in Figure 7c. Hence, when many fine particles with a small mass were removed, the variation in the total number concentration was significant, while the variation in the total mass concentration was not obvious.
Figure 8. Removal performance of fine particles [mean ± 2 standard deviation (SD)] by this new process: (a) variation of the number concentration of fine particles (mean ± 2SD) by this new process, (b) variation of the mass concentration of fine particles (mean ± 2SD) by this new process, and (c) removal efficiency of fine particles (mean ± 2SD) by this new process.
and without this new process. In experiments, the original flue gas humiture was 165 °C and 85 g Nm−3 and the concentration of SO3 was 60 mg Nm−3. In the evaporation chamber, the temperature drop of the flue gas after hot atomized water spraying was 50 °C and, thus, the humidification amount was 30 g Nm−3. The temperature of the flue gas was adjusted to 80 °C before WFGD. Panels a and b of Figure 8 display the concentration variation of fine particles at different points with and without this new process. When this new process was applied, the fine particle concentration decreased. For the removal performance of the 3223
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Energy & Fuels 3.4. Influence of the Main Parameters on the New Technique. 3.4.1. Influence of the SO3 Concentration in the Original Flue Gas. The enhancement of the removal efficiency of the ESP by this new process was mainly caused by the flue gas temperature drop and the condensation of SO3. Thus, the concentration of SO3 would have a great influence on this new process. Figure 9 gives the removal efficiency of the ESP and
WFGD was very limited. In Figure 9, when the SO 3 concentration increased from 20 to 80 mg Nm−3, the removal efficiency of the WFGD only increased from 43.5 to 45.6%. 3.4.2. Influence of the Flue Gas Temperature Drop before the ESP. The flue gas temperature drop before the ESP was due to the spraying of hot atomized water. Thus, different flue gas temperature drops meant different evaporation capacities, which had a significant influence on the flue gas humiture. The number removal efficiency of the ESP and WFGD as a function of flue gas temperature drop before the ESP is shown in Figure 10. In the experiments, the original flue gas humiture
Figure 9. Number removal efficiency of ESP and WFGD (mean ± 2SD) as a function of the SO3 concentration.
WFGD at different SO3 concentrations in the original flue gas. During the experiment, the original flue gas temperature was 165 °C and the humidity was 85 g Nm−3. The temperature drop of the flue gas ahead of the ESP was set at 50 °C; thus, the final humidity of the flue gas was approximately 115 g Nm−3. The temperature of the flue gas at the WFGD inlet was still set to 80 °C. As shown in Figure 9, the removal performance of the ESP and WFGD improved as the concentration of SO3 increased. However, the increased removal efficiency of the ESP was more significant than that of the WFGD. Before ESP, the flue gas temperature was decreased from 165 to 115 °C. According to the calculated results (see Figure 3), even if the SO 3 concentration increased from 20 to 80 mg Nm−3, the temperature of flue gas was always below the acid dew point. In this case, the higher SO3 concentration meant that more SO3 would condense on the particle surfaces. More SO3 condensing on the particle surfaces would lead to further reductions in the specific resistance of fine particles;26 moreover, it would also lead the size of fine particles to slightly increase.27 Hence, when the SO3 concentration increased from 20 to 80 mg Nm−3, the removal performance of the ESP for fine particles increased from approximately 60.3 to 68.3%. Although many fine particles with SO3 condensing onto them would be efficiently removed by the ESP, some of them still escape the ESP. Because the surfaces of these escaped fine particles are covered with sulfuric acid, according to the classic heterogeneous nucleation theory, the critical supersaturated degree of these particles is lower than the original fine particles of the same size.10 That is, these particles will more easily become condensation-grown droplets in the WFGD system, which will lead to an improvement in the removal efficiency of the WFGD. However, the maximum supersaturated degree of the obtained supersaturation was unchanged, at approximately 1.20 during the experiment; therefore, many of the escaped fine particles with SO3 condensed onto their surfaces could still not be activated. Thus, the actual improvement of the removal efficiency of the
Figure 10. Number removal efficiency of ESP and WFGD (mean ± 2SD) as a function of the flue gas temperature drop before ESP.
was 165 °C and 85 g Nm−3 and the SO3 concentration was 60 mg Nm−3. Then, the flue gas temperature was further decreased to 80 °C before the desulfurization scrubber. The number removal efficiency of the ESP and WFGD increased as the flue gas temperature drop was increased, as shown in Figure 10. For the removal performance of the ESP, when the flue gas temperature drop changed from 10 to 20 °C, the change in the removal efficiency was not obvious. However, when the temperature drop continuously increased from 20 to 30 °C, the change in the removal efficiency was obvious. Afterward, when the temperature drop increased, the removal performance of the ESP slightly increased. This trend was ascribed to changes in the acid dew point. When the temperature drop was below 20 °C, the calculated temperature of flue gas was higher than 145 °C and the acid dew point was lower than 139 °C. That is, the temperature of flue gas was higher than the acid dew point. On this occasion, SO3 would not condense onto the fine particle surfaces. The slight improvement of the removal efficiency was mainly because of the reduction of the fine particle specific resistance and the increase of the flue gas residence time in the ESP, which was caused by the decrease in the flue gas temperature. As the temperature drop continued to increase, the situation was very different. When the flue gas temperature drop was above 30 °C, the flue gas temperature was lower than 135 °C and the acid dew point was higher than 142 °C. It is obvious that the flue gas temperature was below the acid dew point. Hence, SO3 would condense onto the fine particle surfaces, causing larger particles with low specific resistance to be formed. As a result, the removal efficiency significantly increased as the flue gas temperature drop increased from 20 to 30 °C. With the further increases in the flue gas temperature drop, the removal efficiency of the ESP continued to grow. However, the upward trend was slight. This was mainly linked to the reduction of the 3224
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inlet flue gas temperature also led to a decrease in the condensable vapor; therefore, the condensation of the vapor decreased, causing the final condensation-grown droplets to become smaller, which were more difficult for the WFGD to separate.28 Because of the interactions of the above processes, the degree of obtained supersaturation in the scrubber would decrease with the increasing temperature of inlet flue gas, leading to a decrease in the number of activated fine particles.
specific resistance, which was caused by the reduction of the flue gas temperature. For the removal performance of WFGD, when the flue gas temperature drop increased to 20 °C, the removal efficiency of the WFGD changed very little. As shown in Figures 4−6, the humidity of the flue gas was always below 97 g Nm−3 and the maximum supersaturated degree was lower than 1.15. On the basis of the previous studies of the supersaturated degree of fine particles in coal-fired flue gas,9,10 it can be found that, although the supersaturation was already achieved in the desulfurization process, only some micrometersized fine particles would be enlarged to larger droplets that could be separated by WFGD. That is, most sub-micrometer fine particles could still not be activated. Because the peak of the distribution of the number concentration of fine particles was in the sub-micrometer range (see Figure 7), the variation in the removal efficiency was not obvious. However, when the flue gas temperature drop continued to increase, the situation was different. The higher flue gas temperature drop meant higher flue gas humidity at the WFGD inlet. That is, the higher maximum supersaturated degree would be achieved in the scrubber, which would cause more fine particles to be enlarged and separated in the desulfurization scrubber. 3.4.3. Influence of the Flue Gas Temperature before WFGD. The WFGD inlet flue gas temperature is another critical parameter that determined the removal performance of the WFGD for fine particles. In the experiments, the original flue gas humiture was 165 °C and 85 g Nm−3 and the SO3 concentration was 60 mg Nm−3. The flue gas temperature drop ahead of the ESP was kept at 50 °C, and the final humidity of flue gas was about 115 g Nm−3. Figure 11 shows the number removal efficiencies of the WFGD at different inlet flue gas temperatures. As shown in
4. CONCLUSION A new process was proposed to reduce the fine particle emissions in coal-fired flue gas by the condensation of SO3 and water vapor. The condensation of SO3 was achieved ahead of the ESP by flue gas temperature reduction via spraying of hot atomized water before the ESP. The condensation of water vapor was obtained in the desulfurization scrubber by adjusting the flue gas humiture before the ESP via spraying of hot atomized water and the application of a fluoroplastic heat exchanger located between the ESP and WFGD. The feasibility analysis indicated that the new process was feasible for the original flue gas with high SO3 concentrations, a high temperature, and a high humidity. The experimental results showed that the SO3 concentration of the original flue gas, the temperature drop of the flue gas before the ESP, and the flue gas temperature at the WFGD inlet had large influences on the effectiveness of this new process. Higher SO3 concentrations of the original flue gas and a higher temperature drop of the flue gas before the ESP were beneficial to reduce the fine particle emissions in the final exhausted flue gas. However, a higher flue gas temperature at the WFGD inlet worked against this goal. The new process proposed in this paper was feasible, and the emission of fine particles in the final exhausted flue gas would be reduced by 50−70%.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Linjun Yang: 0000-0002-6208-0582 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was implemented under financial support from the Jiangsu Science and Technology Support Program (BE2014856), the National Basic Research Program of China (973 Program, 2013CB228505), and the National Natural Science Foundation of China (21276049).
Figure 11. Number removal efficiency of WFGD (mean ± 2SD) as a function of the WFGD inlet flue gas temperature.
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Figure 11, when other parameters remained constant and the inlet flue gas temperature increased, the removal efficiency decreased. The higher inlet flue gas temperature would make a larger temperature difference between the flue gas and desulfurization slurry, causing the heat and mass transfer to intensify. Hence, more desulfurization slurry would evaporate and enter the gas phase, leading an increase in the flue gas humidity. However, when the inlet flue gas temperature increased, the flue gas temperature in the desulfurization process would also increase. According to the definition of supersaturation,9 higher flue gas temperatures will increase the saturation vapor partial pressure, which is disadvantageous to obtain a higher supersaturated degree. Furthermore, the higher
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DOI: 10.1021/acs.energyfuels.6b03069 Energy Fuels 2017, 31, 3219−3226
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DOI: 10.1021/acs.energyfuels.6b03069 Energy Fuels 2017, 31, 3219−3226
