Fine Particle Transformation during the Limestone Gypsum

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Fine Particle Transformation during the Limestone Gypsum Desulfurization Process Danping Pan, Hao Wu, and Linjun Yang* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ABSTRACT: Special attention is required on the fine particle control after desulfurization because a large quantity of fine particles are emitted into the atmosphere from the coal-fired power plants. In an attempt to figure out the transformation of fine particles during the limestone gypsum desulfurization, the physical properties of fine particles before and after the desulfurization system were measured in a power plant. Moreover, the investigations on the removal and formation characteristics of fine particles during the desulfurization process were carried out in a pilot plant. The results showed that the particle characteristics changed after desulfurization and new particles of CaSO4 may be generated. The scrubbing of the desulfurization slurry generated large numbers of fine particles, which were mainly located in the sub-micrometer range, and the inlet SO2 concentration had little influence on the fine particle formation. Besides, the desulfurization slurry characteristics were closely related to the concentration and size distribution of fine particles after desulfurization. With the lower proportion of fine particles and the decrease of the solid mass concentration in the desulfurization slurry, the generated fine particles were reduced. The fine particle emission can be controlled by the adjustment of operation parameters. With the decrease of the gas flow velocity and the liquid/ gas ratio, fewer droplets of the desulfurization slurry were entrained out of the scrubber, resulting in the corresponding decrease of fine particle concentrations after desulfurization. desulfurization.11−14 The methods of injecting steam, agglomeration solution, or wetting agent onto the WFGD system were beneficial to improve the removal efficiency of fine particles, whereas for the industrial application, many problems still needed to be solved.15−18 Recently, the wet electrostatic precipitator (WESP) installed after the desulfurization system was demonstrated to have high efficiency for the fine particle removal,19,20 but the required high investment and operation expense greatly restricted its widespread application. In comparison to these reported works, they mainly focused on the reduction of fine particles before and after the desulfurization and the effect of the WFGD system on the fine particle emission, while the fine particle transformation during the desulfurization process was barely investigated, which was also very essential to promote the removal of the WFGD system on the fine particles. In this paper, fine particle properties at the inlet and outlet of the WFGD system in a 300 MWth coal-fired power plant were measured. The removal and formation characteristics of fine particles during the desulfurization process were then investigated with a pilot plant. With respect to the current investigations centered on the effect of the WFGD system on the fine particle emission, the aim is to figure out the fine particle transformation during the limestone gypsum desulfurization system, which is the foundation of the optimization of the desulfurization system for fine particle reduction.

1. INTRODUCTION Emissions of fine particles have been one of the most important problems in the atmospheric environment, especially for some toxic heavy metals, which can be easily absorbed on the surface as a result of their relatively high specific surface area, leading to adverse effects on the environment and human health.1,2 For these fine particles, they mainly come from the coal-fired power plant through the processes of coal combustion3,4 and selective catalytic reduction (SCR),5,6 but they could not be removed efficiently by the normally used electrostatic precipitator (ESP).7,8 Meanwhile, to address the issue of SO2 removal in the flue gas, most of the coal-fired power plants at present have installed the wet flue gas desulfurization (WFGD) system after the ESP and at least 80% of them have adopted the technology of limestone gypsum desulfurization. Because the desulfurization system is generally the last treatment process of the coalfired flue gas, it is essential to focus on the fine particle transformation during the process. During the desulfurization process, the flue gas is scrubbed by the desulfurization slurry and, thus, the characteristics of particles may be changed. For instance, Nielsen et al.9 made spot measurements and found that the particle removal efficiency after the limestone gypsum desulfurization was 50− 80%, while the sub-micrometer particle concentration was increased by 20−100%. Wang et al.10 investigated the particle emissions from coal-fired power plants, and the results showed that the particle sizes shifted to the smaller end after desulfurization as the mass ratio of PM2.5 and PM10 changed from 0.434 to 0.764. Hence, the total particle concentration revealed a decreasing tendency after desulfurization, while the fine particle concentration was increased. Meanwhile, the particle compositions changed correspondingly, and particles of unreacted CaCO3 and CaSO4 ·2H2 O were found after © XXXX American Chemical Society

Received: May 11, 2016 Revised: September 21, 2016

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DOI: 10.1021/acs.energyfuels.6b01133 Energy Fuels XXXX, XXX, XXX−XXX

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has 13 stages for the particle collection. Before entering into the ELPI, the sample gas stream was diluted with free-particle, hot dry air (150 °C and dilution ratio of 8.18:1) to avoid the condensation of water vapor on the wall of sampling pipelines and on the impact plate of the ELPI. The SO2 concentration of the flue gas was measured by the flue gas analyzer (J2KN Pro, Ecom, Ltd., Germany). Liquid droplets after desulfurization were collected by the droplet catcher (Duohatu, Ltd., China) designed by the national standard. Particles were collected by the aluminum film on the impact plate of the PM2.5/10 impactor (Dekati, Ltd., Finland) for morphology and composition analyses, which were carried out using Zeiss Ultra Plus scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) detector system and D8 Advance X-ray diffraction (XRD).

2. EXPERIMENTAL SECTION 2.1. Experimental Conditions. The fine particles at the inlet and outlet of the WFGD system in a 300 MWth coal-fired power plant were collected, and the physical properties were analyzed. The power plant installed a SCR, an ESP, and a WFGD system to purify the coal-fired flue gas. During each test period, the fuel, boiler testing load, and operation conditions of the equipment were kept the same. The WFGD system using two spray scrubbers adopted limestone gypsum desulfurization technology, in which the flue gas and desulfurization slurry maintained a countercurrent flow and a pre-scrubber was installed before the absorption scrubber. The main operation parameters were listed in Table 1.

Table 1. Main Parameters of the WFGD System

3. RESULTS AND DISCUSSION 3.1. Fine Particle Characteristics before and after Desulfurization in the Power Plant. 3.1.1. Fine Particle Concentrations and Size Distributions. Fine particle concentrations of the coal-fired flue gas at the inlet and outlet of the desulfurization scrubber in the power plant were shown in Figure 2a. The removal efficiency of the WFGD system on the

parameter gas flow rate (Nm3/h) inlet gas temperature (°C) outlet gas temperature (°C) inlet concentration of SO2 (mg/m3) gas/liquid ratio (L/Nm3) flue gas speed (m/s) pH value of desulfurization slurry SO2 removal efficiency (%)

(130−140) × 104 150−155 50−60 2000−2500 15 3.7 5.5−6.5 99

The experimental investigations were carried out in a pilot plant, which was outlined in Figure 1. Flue gas with a volume flux up to 300

Figure 2. Characteristics of fine particles before and after desulfurization.

Figure 1. Schematic diagram of the pilot plant. Nm3/h was generated by a coal-fired boiler, and the particle concentration was ensured to be uniform by a buffer tank installed downstream. SO2 was added to the buffer tank to adjust the SO2 concentration of the flue gas. After the ESP, large particles were removed efficiently from the coal-fired flue gas. Then, the flue gas entered a spray scrubber for desulfurization with a diameter of 200 mm and a height of 6000 mm. The temperature of the flue gas at the inlet of the scrubber was from 120 to 125 °C; the scrubber had three levels; and a demister was set at the top. Gypsum from a certain coal-fired power plant was used to prepare the desulfurization slurry. The temperature of the desulfurization slurry was maintained to be 50 °C by the heater in the desulfurization slurry tank, and limestone slurry was added to the desulfurization slurry to ensure a constant pH value at 5.5 ± 0.1. 2.2. Measurement Technique. The concentration and size distribution of the fine particles were measured in real time by the electrical low-pressure impactor (ELPI, Dekati, Ltd., Finland), which

fine particle number concentration was calculated to be 53.3% when the number concentration of fine particles decreased significantly from 4.5 × 106 to 2.1 × 106 cm−3 after desulfurization. Figure 2b showed the size distributions of fine particle number concentrations before and after desulfurization in the power plant. It is noteworthy that both of them reveal bimodal distributions in the measurement range of the ELPI, and the particle sizes were mainly located in the submicrometer range. Before desulfurization, the bimodal distribution was more apparent, whose peak values were presented at the size of about 0.2 and 1 μm. In contrast, the proportion of smaller particles increased significantly after desulfurization, with its peak values shifted to the smaller end at about 0.07 and 0.3 μm. Although the total number concentration of fine B

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Figure 3. SEM micrographs of particles collected in the power plant.

particles decreased after desulfurization, the number concentration of part particles was increased, which was mainly in the sub-micrometer range, indicating that new particles might be formed during the desulfurization process. 3.1.2. Morphology and Compositions. To investigate the characteristics of fine particles formed during the desulfurization process, the morphology of particles collected from the desulfurization slurry and at the inlet and outlet of the desulfurization system was measured, as shown in Figure 3. According to Figure 3a, particles in the desulfurization slurry were complex with various shapes, e.g., clintheriform, prismatic, and spherical shapes. Before desulfurization, as indicated in Figure 3b, fine particles with similar spherical appearance were different in size and some tiny particles were absorbed on their surfaces. The morphology of fine particles after desulfurization (Figure 3c), however, was changed significantly, where the particles with spherical appearance were analogous to those before desulfurization but their sizes were more uniform (mostly less than 1 μm). Moreover, some particles with clintheriform and prismatic shapes in different sizes were also observed, which were similar to those in the desulfurization slurry (Figure 3a), and the particle diameters with prismatic shape were less than 1 μm, whereas those with clintheriform shape were larger, ranging from 1 to 4 μm with the widths of about 0.5 μm. Furthermore, the XRD analysis of fine particles before and after desulfurization was also carried out to verify the particle compositions, as illustrated in Figure 4. Before desulfurization, Al6Si2O13, SiO2, and CaO could be clearly observed, corresponding to the fly ash with spherical structures. In contrast, after desulfurization, the main compositions were turned into CaSO4·0.5H2O, CaSO4·2H2O, Al6Si2O13, and SiO2. Therefore, on the basis of the above results, it could be concluded that the physical properties of fine particles obviously changed after desulfurization and it was related to

Figure 4. XRD spectrum of particles collected in the power plant.

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shown. It is worthwhile to note that the inertial and diffusion forces imposed on the particles as they were removed from the flue gas in the scrubber took vital important roles in their particle sizes. For instance, when the particle size was larger than 1 μm, the inertial force was the primary mechanism to separate them from the flue gas. With the further decrease of the particle size, however, the effect of the inertial force weakened gradually and the diffusion force became dominant, particularly for the particles with their sizes less than 0.1 μm. Therefore, a decrease of the removal efficiency occurred as the particle size ranged from 0.1 to 1 μm, which was difficult to separate from the flue gas by the inertial or diffusion force. Furthermore, the results in Figure 5b displayed an obvious difference when the particle sizes were less than 0.1 μm. With the decrease of the diameter, the grade removal efficiency increased for water scrubbing, while it decreased for slurry scrubbing. For particles with sizes less than 0.1 μm in the flue gas, the capture efficiency depended upon the diffusion force, which was strengthened when the particle sizes decreased. Thus, the grade removal efficiency increased for water scrubbing. As for the scrubbing of the desulfurization slurry, the sprayed droplets contained particles that had some correlations with the crystallization in the desulfurization slurry. The large droplets fell into the bottom of the scrubber by gravity, while the small droplets were entrained out of the scrubber by the flue gas, whose gravity was lower than their resistance. With the decrease of their sizes, the entrainment was correspondingly easier. By comparison, the grade removal efficiency of particles with their diameters less than 1 μm was clearly lower, especially for the particles with size less than 0.2 μm, resulting in the different results in Figure 5b. Therefore, more particles were generated by scrubbing the desulfurization slurry, and they were mainly located in the sub-micrometer range. 3.3. Formation Characteristics of Fine Particles during the Desulfurization Process. 3.3.1. Effect of the Inlet SO2 Concentration. Because the fine particles formed during the desulfurization process were related to the desulfurization product, the effect of the SO 2 concentration before desulfurization was investigated in a pilot plant, as shown in Figure 6, where the liquid/gas ratio was 15 L/m3 and the amount of the limestone slurry added to the desulfurization slurry was kept the same during the test. With the decrease of the SO2 concentration from 2500 to 500 mg m−3, the number concentration of fine particles after desulfurization decreased slightly from 8 × 106 to 7 × 106 cm−3. Furthermore, the

the crystal characteristics in the desulfurization slurry. During the desulfurization process, new particles were generated during the desulfurization process, where CaSO4 became dominant. 3.2. Removal Characteristics of Fine Particles by the WFGD System. The emission of fine particles after desulfurization was related to the particles in the inlet flue gas and the desulfurization slurry; thus, the removal characteristics of fine particles by the water scrubbing and scrubbing of the desulfurization slurry were analyzed in a pilot plant. The experiments were conducted at a liquid/gas ratio of 15 L/m3. The total removal efficiency of the desulfurization scrubber on the fine particles and their grade removal efficiency of the number concentrations were illustrated in Figure 5. As seen

Figure 5. Fine particle removal efficiency of the desufurization scrubber.

from Figure 5a, the removal efficiency of the mass concentration was higher than that of the number concentration because the large particles were removed more easily through scrubbing, leading to a significant influence on the mass concentration. In comparison to the water scrubbing, removal efficiencies of the number concentration and mass concentration were 24 and 28% lower, respectively, as a result of the scrubbing of the desulfurization slurry, indicating that new particles were generated during the desulfurization process. Moreover, Figure 5b depicted the grade removal efficiency of the number concentrations, where the relationship between the removal efficiency and the size of the separated particles was

Figure 6. Effect of the SO2 concentration before desulfurization on the fine particle emission. D

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Energy & Fuels particles from the coal-fired flue gas nearly did not have chemical reactions during the desulfurization process, and then they could be partly removed through the physical mechanisms, which would not be affected by the change of the SO2 concentration in the inlet flue gas. Thus, the decrease of the number concentration for the fine particles was only related to the chemical reactions in the scrubber. During the desulfurization process, SO2 was removed by the reactions with CaCO3 in the desulfurization slurry, which were listed as follows: SO2 + H 2O → H 2SO3 → H+ + HSO3−

(1)

H+ + HSO3− + 1/2O2 → SO4 2 − + 2H+

(2)

CaCO3 + 2H+ + H 2O → Ca 2 + + 2H 2O + CO2 ↑

(3)

Ca 2 + + SO4 2 − + 2H 2O → CaSO4 ·2H 2O

(4)

Figure 7. Particle size distributions in the desulfurization slurries.

and the value was obviously lower with the desulfurization slurry from power plant 1. With larger particles in the desulfurization slurry, the droplets would be harder to be entrained by the flue gas. Meanwhile, the demister achieved higher removal efficiency of larger particles. Thus, the solid content percentage with the slurry from power plant 1 was lower. Furthermore, the fine particle concentration and size distribution after desulfurization with different desulfurization slurries were measured by ELPI, as shown in panels b and c of Figure 8. With the higher proportion of fine particles in the desulfurization slurry, the fine particle number concentration was increased from 1.18 × 107 to 1.25 × 107 cm−3 and the peak value of the fine particle size distribution was shifted to the smaller end, leading to more smaller particles emitted out of the desulfurization scrubber. The concentration and size distribution of fine particles in the entrained droplets were closely related to those in the desulfurization slurry. Correspondingly, the fine particle concentration in the droplets out of the scrubber was increased with a higher proportion of fine particles in the desulfurization slurry. With the droplet drying in the following process, fine particles were separated out. As a consequence, more fine particles were emitted after desulfurization when the desulfurization slurry with a higher fine particle proportion was used. Therefore, the fine particle characteristics after desulfurization were related to the particle size distribution in the desulfurization slurry. With the lower proportion of fine particles in the desulfurization slurry, it was feasible to reduce the fine particle emission. The profile of the fine particle number concentration varying with the solid mass concentration in the desulfurization slurry in a pilot plant was shown in Figure 9. The increase of the solid mass concentration in the desulfurization slurry from 20 to 60 g L−1 led to an increase in the number concentration of fine particles from 2.7 × 106 to 3.8 × 106 cm−3. With a higher solid mass concentration in the desulfurization slurry, the sprayed droplets containing more fine particles were entrained out of the scrubber, resulting in the increase of fine particle number concentration after desulfurization. Therefore, the reduction of the solid mass concentration in the desulfurization slurry was worth exploring to control the fine particle emission after desulfurization. 3.4. Effect of Operation Parameters of the WFGD System on the Fine Particle Emission. 3.4.1. Effect of the Gas Flow Velocity. Gas flow velocity in the scrubber (3−4 m/s in the industry) also exhibited an important effect on the relative movement between the flue gas and the desulfurization slurry. Accordingly, the fine particle number concentrations at different gas flow velocities in a pilot plant were analyzed, as

When the concentrations of Ca 2+ and SO 4 2− in the desulfurization slurry were supersaturated, new particles consisting of CaSO4 were formed during the desulfurization process. Because the flue gas and the desulfurization slurry were maintained countercurrent and the slurry was circulatory, the slurry droplets could partly be entrained out of the scrubber by the flue gas, whose gravity was lower than the resistance, instead of directly entering into the circulation system of the desulfurization slurry. Correspondingly, these new particles in the droplets were entrained out of the scrubber. In addition, the size of these particles was small because the residence time was short in the scrubber and there was not enough time for the particle growth. The decrease of the SO2 concentration reduced the chemical reactions, and then fewer fine particles were generated, resulting in the decease of the number concentration after desulfurization correspondingly. In the desulfurization slurry, however, it did not exert great effect on the number concentration of fine particles because the amount of particles formed by the reactions between CaCO3 and SO2 was much less than the initial particle amount in the desulfurization slurry. Therefore, the decrease of the number concentration was slight, indicating that the inlet SO2 concentration had little influence on the fine particle formation. 3.3.2. Effect of Desulfurization Slurry Characteristics. The limestone gypsum desulfurization slurries from two different power plants were used to investigate the effect of the particle size distribution in the desulfurization slurry on the fine particle emission after desulfurization in a pilot plant. The solid matter in the slurry was mainly gypsum crystals. The experiments were conducted at the SO2 concentration of 1800 ± 20 mg m−3, with the results shown in Figures 7 and 8. The particle size distributions of the desulfurization slurries (Figure 7) reflected a significant difference, and smaller particles were generated in power plant 2. The proportion of fine particles in the desulfurization slurry from power plant 2 was 4.8%, almost 65% higher than that of the fine particles from power plant 1, which was 2.9%. As a consequence, the corresponding median diameters of the particles were 26.85 and 38.22 μm, respectively. As mentioned above, the fine particles generated during the desulfurization process were mainly from the entrainment of the sprayed droplets. To illustrate the relationship of the particle concentration between the droplets and the desulfurization slurry, the solid content percentages of entrained droplets and the slurry with desulfurization slurries from different power plants were investigated, as shown in Figure 8a. The solid content percentages were from 30 to 40%, E

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Figure 8. Fine particle emissions after desulfurization with different desulfurization slurries.

shown in Figure 10, where the desulfurization liquid/gas ratio was 15 L m−3 and the SO2 concentration was 1800 ± 20 mg m−3. If the gas flow velocity increased from 2.6 to 3.8 m s−1, the number concentration of fine particles would also increase from 1.8 × 106 to 2.4 × 106 cm−3 correspondingly. Moreover, the gas flow distribution around droplets and particle motion trail were affected with the increase of the gas flow velocity, resulting in the increase of the Reynolds number of the gas flow and the inertial force parameter St, which were beneficial for the particle capture by inertial force. Besides the effect of gas flow velocity on particles from the flue gas, particles generated from the entrainment of the desulfurization slurry were also related to the gas flow velocity. With a higher gas flow velocity, more droplets were entrained during the desulfurization process as their resistance against the gravity increased, resulting in more fine particles generated. Therefore, in comparison to the

Figure 9. Effect of the solid mass concentration in the desulfurization slurry on fine particle emission.

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of fine particles emitted out of the scrubber displayed a distinct difference and new particles of CaSO4 may be generated during the desulfurization process. (2) The fine particles in the flue gas before desulfurization were partly removed by the scrubbing, while the scrubbing of the desulfurization slurry generated large numbers of fine particles, which were mainly located in the submicrometer range. (3) The inlet SO2 concentration of the flue gas had little influence on the fine particle emission, while the desulfurization slurry characteristics were closely related to the fine particle characteristics after desulfurization. With the lower proportion of fine particles and the decrease of the solid mass concentration in the desulfurization slurry, the generated fine particles were reduced. (4) The fine particle emission can be controlled by the adjustment of operation parameters. With the decrease of the gas flow velocity and the liquid/gas ratio, fewer droplets of the desulfurization slurry were entrained out of the scrubber, resulting in the corresponding decrease of fine particle concentrations after desulfurization.

Figure 10. Effect of the gas flow velocity on fine particle emission.

removal of fine particles via inertial force, the fine particle generation through the entrainment of the desulfurization slurry took greater effect on the fine particle emission and the number concentrations of fine particles increased correspondingly with the increase of the gas flow velocity. 3.4.2. Effect of the Liquid/Gas Ratio. Figure 11 illustrated the effect of the liquid/gas ratio on the number concentration

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-025-83795824. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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Figure 11. Effect of the gas/liquid ratio on fine particle emission.

of fine particles after desulfurization in a pilot plant, where the SO2 concentration was 1800 ± 20 mg m−3 in the experiments. When the liquid/gas ratio was increased from 16 to 23 L m−3, the fine particle concentration was also increased from 3.2 × 106 to 3.6 × 106 cm−3. With a higher liquid/gas ratio, more desulfurization slurry was sprayed into the desulfurization tower, resulting in more droplets generated. The increase of the droplet amount gave rise to the higher collision probability between particles and droplets, which was beneficial for particle capture, while more droplets containing fine particles were entrained out of the scrubber at the same time. In addition, the size distribution of sprayed droplets was related to the flux of the desulfurization slurry. A higher volume flux, which corresponded to a higher nozzle pressure, resulted in a higher proportion of smaller droplets generated.21 These smaller droplets with lower gravity were entrained out of the scrubber more easily. Owing to the entrainment of the desulfurization slurry, the number concentration of fine particles was increased with the increase of the liquid/gas ratio.

4. CONCLUSION To investigate the fine particle transformation during the limestone gypsum desulfurization system, the experiments were carried out in a power plant and a pilot plant. The conclusions can be drawn from the results as follows: (1) In comparison to the fine particles before desulfurization, the physical properties G

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