Investigation of the Relationship between Droplet and Fine Particle

Apr 29, 2017 - Consequently, the optimization of operating parameters and desulfurization slurry is a feasible method to reduce the emission of drople...
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Investigation of the Relationship between Droplet and Fine Particle Emissions during the Limestone−Gypsum Wet Flue Gas Desulfurization Process Danping Pan, Hao Wu, and Linjun 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 ABSTRACT: Slurry droplets containing fine particles are entrained out of the scrubber during the wet flue gas desulfurization process, resulting in adverse impacts on the environment and human health. The characteristics of droplets and fine particles generated and entrained out of the scrubber were analyzed with an experimental system. The effects of desulfurization operating parameters and slurry properties on the droplet and fine particle emissions were investigated. The numerical calculation analysis indicated that the sprayed droplet size was slightly decreased after contact with high-temperature gas. The experimental results showed that the sizes of droplets and fine particles entrained out of the scrubber were closely related to the sprayed droplets in the scrubber. The sizes of droplets after desulfurization were mainly less than 30 μm. Meanwhile, the corresponding fine particles were mainly in the sub-micrometer range. As a result of the increases in the superficial gas velocity, gas−liquid ratio, and desulfurization slurry concentration and the decrease in the gypsum size distribution in the desulfurization slurry, the count percentages of micrometer droplets and number concentrations of corresponding fine particles increased. Consequently, the optimization of operating parameters and desulfurization slurry is a feasible method to reduce the emission of droplets and fine particles.

1. INTRODUCTION Limestone−gypsum wet flue gas desulfurization (WFGD) is the most effective and economic process to remove SO2 from stack gases of coal-fired power plants, constituting over 90% of flue gas desulfurization (FGD) capacity in China. After the elimination of gas−gas heaters (GGHs),1,2 the problem of gypsum rain was aggravated as a result of the lower flue gas temperature, which caused corrosion in equipment. Meanwhile, corresponding fine particles from desulfurization slurry droplets were emitted after desulfurization, resulting in reduced visibility of the flue gas and haze formation. These problems were discussed in previous studies,3−5 and they both led to adverse impacts on the surrounding environment and human health. As stricter standards were created for the control of pollutant emissions, the emission of desulfurization slurry droplets and fine particles became an urgent issue to be solved. Generally, a spray scrubber was adopted in the limestone− gypsum WFGD system, where the flue gas and desulfurization slurry maintained a countercurrent flow. During the desulfurization process, fly ash in the coal-fired flue gas was partly eliminated via the slurry scrubbing. Meanwhile, the desulfurization slurry droplets containing fine particles might be entrained out of the scrubber by the flue gas. Previous publications6−10 have found that unreacted CaCO3 and CaSO4·2H2O remained in the emitted particles after desulfurization, and the proportion could be over 50%. Therefore, in addition to the fly ash particles escaping from the scrubber, desulfurization slurry droplets were an important source of fine particle emissions. The process of particle removal via scrubbing was carefully analyzed by calculations and computer simulation, and the flow distribution and particle movement in the scrubber were demonstrated.11−14 However, these investigations were incom© XXXX American Chemical Society

plete as a result of the complicated conditions and the fact that the relationship between droplet and fine particle emissions in the scrubber was not addressed. In an attempt to reduce the emission of desulfurization slurry droplets and fine particles, different methods were proposed. Optimization of the demister and the operating conditions15−18 has been adopted in industry. The addition of steam, chemical agglomeration agents, and wetting agents during the desulfurization process19−22 has also been investigated, but several problems remain in their application. In addition, the applications of a wet electrostatic precipitator (WESP) downstream from the WFGD system were reported in some power plants, and high removal efficiencies for droplets and fine particles were achieved.23,24 However, this application was restricted by the high investment and operation costs. As a consequence, to control the emission of desulfurization slurry droplets and fine particles efficiently, the characteristics of droplets and fine particles during the desulfurization process require further investigation. In this work, the size change of a single droplet coming into contact with high-temperature flue gas was calculated. By use of an experimental system, the characteristics of droplets and fine particles generated and entrained out of the scrubber were analyzed. Furthermore, the effects of desulfurization operating parameters and slurry properties on the droplet and fine particle emissions were investigated. Received: February 11, 2017 Revised: April 7, 2017 Published: April 29, 2017 A

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Numerical Analysis of Droplet Evaporation. When the sprayed droplets came in contact with high-temperature flue gas, the droplet sizes changed as a result of evaporation. To illustrate the droplet size variation during the desulfurization process, numerical calculation analyses for the single droplet25,26 were carried out. The physical situation was complicated during the desulfurization process; therefore, the following assumptions were proposed as follows: (1) the single droplet was a standard sphere, and the relative motion between the droplet and flue gas was ignored; (2) the radiation heat transfer between the droplet and flue gas was ignored; (3) the temperature inside the droplet was uniform; and (4) the flue gas was an ideal gas. The process of droplet evaporation depended upon the transfer of mass and heat between the droplet and surrounding environment. The mass diffusion equation of a symmetric spherical droplet can be expressed as follows:

2.1. Experimental System. The investigations were conducted in the experimental system outlined in Figure 1. Rated flue gas was

md = −Dc

P dYd + Ydmd RT dr

(1)

where md is the mass diffusion rate per unit area, Dc is the mass diffusion coefficient, P and T are the pressure and temperature, respectively, of the surrounding gas, Yd is the mass component of liquid, and r is the radial coordinate. According to the heat transfer mechanism of the boundary layer during the droplet evaporation, the heat transfer coefficient BT is

Figure 1. Schematic diagram of the experimental system.

generated from the air blower, and it then entered into the spray scrubber, where the flue gas and desulfurization slurry maintained a countercurrent flow. The scrubber was made up of polycarbonate pipes and plates with excellent heat resistance, and a demister was set at the top. The diameter and height of the scrubber were 120 and 1200 mm, respectively. Gypsum from a coal-fired power plant was used to prepare the desulfurization slurry that was circulated via the pump during the experiments. To measure the droplet size distributions, view ports of quartz glass were set at positions under the nozzle and after desulfurization. A measuring point was set for the size distributions of fine particles after desulfurization. 2.2. Measurement Technique. The size distributions of droplets were measured using a phase doppler analyzer (PDA, Dantec Dynamics A/S, Ltd., Denmark), and the measurement range of the size was from 0.5 to 2000 μm. A laser PDA is a non-contact real-time measurement technology that has been used extensively to measure droplet sizes. On the basis of the Lorenz−Mie scattering theory, as the droplets passed through the laser beam intersection region, the two fixed detectors received different phase shifts and the droplet size was analyzed according to the deviation. The concentrations and size distributions of particles with sizes ranging from 0.023 to 9.314 μm were measured in real time by means of an electrical low-pressure impactor (ELPI, Dekati, Ltd., Finland). The ELPI had 12 channels, and the sample gas stream from the main stream was routed through a cyclone to a dilutor before it went into the ELPI. The cyclone separated the particles with the aerodynamic diameters larger than 9.314 μm, and the dilutor diluted the gas with particle-free dry air (150 °C and dilution ratio of 67:1). To achieve an accurate measurement, the sample gas stream was heated to turn the water droplet into vapor before the measuring point. Although the PDA and ELPI relied on different principles, the measurements were online and precise, indicating the size change of droplets and fine particles, respectively, during the desulfurization process. In addition, the laser particle size analyzer (9300ST, Bettersize Instruments, Ltd., China) was used for the measurement of size distributions of particles in the desulfurization slurry, with the sizes ranging from 0.1 to 1000 μm.

BT =

c p, α(T∞ − Ts) L

(2)

where cp,α is the gas specific heat capacity, L is the latent heat of vaporization with the droplet surface temperature Ts, and T∞ is the gas temperature. According to the operating parameters during the desulfurization process in the industry, the initial temperatures of droplets and flue gas were assumed to be 50 and 100 °C, respectively. On the basis of the assumptions and the formulas mentioned above, the numerical calculation yielded the results shown in Figure 2, which demonstrated the droplet size variation by evaporation. According to the results, the size of the droplet with an initial size at 10 μm was decreased to 9.2 μm when the contact time approached 1 s. During the desulfurization process, the flue gas temperature was rapidly decreased by nearly 30 °C when the relative height reached

Figure 2. Droplet size variation by evaporation (initial temperature: Tdroplet = 50 °C, and Tgas = 100 °C). B

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

the contact area of sprayed droplets and flue gas was increased, resulting in the generation of more droplets. In addition, the kinetic energy of droplets was increased by the transformation of potential energy, and more fine droplets were generated by the collision and breakup of droplets. The sizes of sprayed droplets ranged mainly from 8 to 14 μm, which were easily entrained out of the scrubber by the flue gas. Because the sprayed droplets of the desulfurization slurry containing fine particles were partly entrained out of the scrubber by the flue gas after the demister, the size distributions of droplets and fine particles after desulfurization were measured online; the results are shown in Figure 4. Figure 4a

0.3.27 Generally, the retention time of flue gas in the scrubber is 2−3 s in the industry; thus, the contact duration between hightemperature gas and sprayed droplets was less than 1 s. As a consequence, the sprayed droplet size was slightly decreased as a result of contact with high-temperature gas during the desulfurization process. 3.2. Transformation of Droplets and Fine Particles during the Desulfurization Process. When the desulfurization slurry was sprayed into the scrubber, droplets with different sizes were generated. In addition, as a result of the countercurrent contact of the flue gas and the desulfurization slurry, fluid resistance was exerted on these droplets in the opposite direction of their velocity. As a consequence, parts of the sprayed slurry droplets containing fine particles were entrained out of the scrubber by the flue gas during the desulfurization process. The fluid resistance was closely related to the droplet sizes, and it was easier for small droplets to be entrained out of the scrubber. In an attempt to clarify the transformation of droplets and fine particles during the desulfurization process, investigations were carried out with the experimental system shown in Figure 1. With the application of a demister at the top of the scrubber, droplets emitted after desulfurization generally had sizes less than 50 μm.28 Thus, the droplet sizes ranged from 0.5 to 50 μm as measured by the PDA in the experiments. The experiments were conducted at a liquid−gas ratio of 15 L m−3, and the superficial gas velocity was 3.0 m/s. The size distributions of droplets generated from the sprayed desulfurization slurry are shown in Figure 3. With the increase of the radial direction

Figure 3. Size distributions of sprayed droplets (Qslurry = 1800 L h−1, and Pnozzle = 0.2 MPa): (a) droplets and (b) fine particles.

Figure 4. Size distributions of droplets and fine particles after desulfurization (L/G = 15 L m−3, and v = 3.0 m s−1).

shows that the sizes of droplets after desulfurization were mainly less than 30 μm, with a peak value of approximately 2 μm, and the proportion of droplets larger than 30 μm was less than 10%, which can be ignored. The mean diameter was 10.8 μm. Therefore, for droplets with sizes less than 50 μm, the droplets entrained out of the scrubber were closely related to the sprayed droplets in the scrubber. In an attempt to reduce the droplet entrainment during desulfurization, the sizes of sprayed droplets needed to be controlled. Meanwhile, as shown in Figure 4b, the number concentrations of corresponding fine particles emitted after desulfurization displayed a unimodal distribution mainly in the sub-micrometer range, with a peak value of approximately 0.1 μm. The ordinate in Figure 4b was formed by dividing the measured value of the number concentration in each stage by the logarithmic width of the stage, where Dp was the median diameter of each stage. The

distance, the mean diameters of droplets in the radial direction were decreased overall. Because inertial force was closely related with the mass, for the size distributions in the radial direction, droplets with larger sizes had greater inertial force and the horizontal fluctuating velocity was correspondingly lower. Thus, it was more difficult for these droplets to reach the edge. Meanwhile, the inertial force of smaller droplets was lower and the effect of the countercurrent gas on the breakup of the liquid sheet was strengthened, resulting in more small droplets generated. As a consequence, with an increase of the radial direction distance, the overall droplet sizes were decreased and more small droplets were spread at the external surface. Furthermore, when the distance between the measuring point and the nozzle was increased from 50 mm to 100 mm, the slurry droplet sizes were decreased and the size distribution was more uniform. With a decrease in the height, C

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels emitted fine particles were generated during the desulfurization process and could hardly be removed from the flue gas, resulting in the increase of sub-micrometer particle concentrations. Because the fine particles were from the droplets entrained out of the scrubber, the droplets and fine particles entrained out of the scrubber were both closely related to the sprayed droplets in the scrubber. 3.3. Influences of Desulfurization Operating Parameters on Droplet and Fine Particle Emissions. 3.3.1. Superficial Gas Velocity. Because the superficial gas velocity played an important role in the process of gas entrainment, the effects of superficial gas velocity on droplet and fine particle emissions after desulfurization are shown in Figure 5. When the superficial

there were slight increases of count percentages when the peak was approximately 30 μm. However, the count percentages of droplets with sizes from 5 to 30 μm decreased. With an increased superficial gas velocity, the relative motion between the droplets and flue gas was enhanced; thus, more droplets were entrained out of the scrubber. The gas entrainment was related to the droplet size, and the smaller sized particles were much more easily entrained out of the scrubber. As a consequence, the count percentages of small droplets were increased, leading to the decrease of the overall droplet size. As seen in Figure 5b, the corresponding number concentrations of fine particles after desulfurization were increased and the variation rates revealed a unimodal distribution with a peak at 0.05 μm in the measurement range of the ELPI. The concentrations of fine particles with sizes less than 0.1 μm showed a significant increase, as did those larger than 1 μm. The variation rate of particles ranging from 0.1 to 1 μm showed a gentle curve with a value of 5%. Therefore, with the increase of the superficial gas velocity, the concentrations of droplets and fine particles were increased, especially for droplets with sizes less than 5 μm and sub-micrometer particles. 3.3.2. Gas−Liquid Ratio. The gas−liquid ratio was an important operating parameter in the desulfurization system; thus, the effect of the gas−liquid ratio on the droplet and fine particle emissions after desulfurization was investigated. When the gas−liquid ratio was increased from 10 to 15 L m−3, the droplet count percentage and fine particle number concentrations increased, as shown in Figure 6. Figure 6a shows the increase of the droplet count percentage after desulfurization. The count percentages of micrometer droplets were obviously increased with the increase of the gas−liquid ratio. In

Figure 5. Effect of the superficial gas velocity on droplet and fine particle emissions (L/G = 15 L m−3).

gas velocity was increased from 3.0 to 3.6 m s−1, the size distributions of droplets and fine particles were measured and the increase in droplet count percentage and the increase rate of the fine particle number concentrations were analyzed. The increase rate of fine particle number concentrations was calculated via the following formula: c − c1 η= 2 × 100% c1 where η is the increase rate of fine particle number concentration, c1 is the initial number concentration, and c2 is the number concentration when the operating condition changed. Figure 5a illustrates the increase of the droplet count percentage after desulfurization, which occurred in a bimodal distribution with peaks at 1 and 30 μm. The count percentages of droplets with sizes less than 5 μm had obvious increases. For droplets with sizes of 1 μm, the increase of the droplet count percentage was approximately 5%. Meanwhile,

Figure 6. Effect of the gas−liquid ratio on droplet and fine particle emissions (v = 3.0 m s−1). D

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels comparison to the results in Figure 5a, the curve was similar, while the peak value was lower. As a result of the increase of the gas−liquid ratio, the flow of sprayed desulfurization slurry was increased. Accordingly, the momentum rate between the flue gas and desulfurization slurry was increased, resulting in the generation of more small droplets. Figure 6b shows the increase rate of fine particle number concentrations after desulfurization. With the increase of the gas−liquid ratio, the corresponding number concentrations of sub-micrometer particles were significantly increased and that of particles with sizes of approximately 0.1 μm was increased by 32%. Meanwhile, the increase of micrometer particle number concentrations was relatively gentle, with value at approximately 20%. 3.4. Influences of Slurry Properties on Droplet and Fine Particle Emissions. 3.4.1. Gypsum Size Distribution. Because fine particles emitted after desulfurization were from the entrained droplets, the size distribution was closely related to the particle size distribution in the desulfurization slurry, which was mainly gypsum. Gypsum of different sizes was obtained by sieving, and its size distributions are shown in Figure 7, which reflected a significant difference. The mean

Figure 8. Effect of the gypsum size distribution on droplet and fine particle emissions (L/G = 15 L m−3, and v = 3.0 m s−1).

Figure 7. Gypsum size distributions in the desulfurization slurry.

3.4.2. Desulfurization Slurry Concentration. To investigate the effect of desulfurization slurry concentrations on the droplet and fine particle emissions after desulfurization, the change of size distributions of droplets and fine particles after desulfurization is shown in Figure 9 when the desulfurization slurry concentration was increased from 10 to 15%. As shown in Figure 9a, the increase of the droplet count percentage displayed a bimodal distribution with peaks at approximately 2 and 30 μm. The count percentages of micrometer droplets were significantly increased, while those approximately 10 μm in size decreased. The desulfurization slurry concentration was related to the viscosity and surface tension of the slurry, which played an important role in the character of the spray from the nozzle. Accordingly, the droplets sprayed into the scrubber had corresponding changes. With the increase of desulfurization slurry concentrations, the viscosity and surface tension were increased; thus, higher energy was required for the generation of sprayed droplets. As a consequence, the sprayed droplet sizes were larger29 and were more difficult to be entrained out of the scrubber. This resulted in the decrease of count percentages of droplets with sizes of approximately 10 μm. Correspondingly, the count percentages of micrometer droplets increased. Furthermore, the results in Figure 9b showed that the number concentrations of fine particles after desulfurization increased. With the higher concentration of the desulfurization slurry, the number concentrations of fine particles in the slurry increased, leading to the higher concentration of fine particles after desulfurization. The increase rates of fine particles were relatively close, ranging mainly from 20 to 30%.

diameters were 14.2 and 23.8 μm, and the proportion of fine particles with a mean diameter of 23.8 μm was obviously decreased from 9.1 to 3.1% compared to the slurry with a mean diameter of 14.2 μm. Panels a and b of Figure 8 depict the change of size distributions of droplets and fine particles after desulfurization with the decrease of the particle size in the desulfurization slurry. When the particle size in the desulfurization slurry was decreased, the count percentages of micrometer droplets were increased, which could be attributed to the change of desulfurization slurry properties. The viscosity and surface tension of the desulfurization slurry could change, with great influence on the character of the spray from the nozzle. Furthermore, with the higher proportion of fine particles in the desulfurization slurry, more fine particles were emitted out of the desulfurization scrubber, resulting in an increase in number concentrations. The increase rate displayed an obvious bimodal distribution with peaks at 0.2 and 7 μm. The peak values were 40 and 10%, respectively. Because the concentration and size distribution of fine particles in the entrained droplets were related to those in the desulfurization slurry, the fine particle concentrations in the droplets from the scrubber were correspondingly increased when the proportion of fine particles in the desulfurization slurry was higher, resulting in more fine particles emitted after desulfurization. As a consequence, a feasible method to reduce the droplet and fine particle emissions after desulfurization was the optimization of gypsum crystallization to inhibit the generation of fine particles in the desulfurization slurry. E

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21276049), the National Basic Research Program of China (973 Program, 2013CB228505), and the Jiangsu Science and Technology Support Program (BE2014856) for their financial support.

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Figure 9. Effect of the slurry concentration on droplet and fine particle emissions (L/G = 15 L m−3, and v = 3.0 m s−1).

4. CONCLUSION In this work, the size change of certain droplets that came in contact with high-temperature flue gas was calculated. The characteristics of droplets and fine particles generated and entrained out of the scrubber were analyzed. The effects of desulfurization operating parameters and slurry properties on the droplet and fine particle emissions were investigated. The following conclusions were drawn: (1) The numerical calculation analysis indicated that the sprayed droplet size was slightly decreased after contact with high-temperature gas. (2) The droplets and fine particles entrained out of the scrubber were closely related to the sprayed droplets in the scrubber. The sizes of droplets after desulfurization were mainly less than 30 μm, and the mean diameter was 10.8 μm. Meanwhile, the corresponding fine particles were mainly in the sub-micrometer range. (3) Upon the increases in the superficial gas velocity, gas−liquid ratio, and desulfurization slurry concentration and the decrease in the gypsum size distributions in the desulfurization slurry, the count percentages of micrometer droplets and number concentrations of corresponding fine particles increased. As a consequence, the optimization of operating parameters and desulfurization slurry proved to be a feasible method to reduce the emissions of droplets and fine particles.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-2583795824. E-mail: [email protected]. ORCID

Linjun Yang: 0000-0002-6208-0582 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.energyfuels.7b00423 Energy Fuels XXXX, XXX, XXX−XXX