Experimental Study on In Situ Preparation of Supported Sorbent for

Jan 30, 2018 - The heated flue gas was mixed with pure SO2 to a given SO2 concentration. Next, flue gas passed through an air cooler in which the flue...
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Experimental Study on In Situ Preparation of Supported Sorbent for Moderate Temperature CFB-FGD Dong Xie,† Changfu You,*,† and Qingxia Liu‡ †

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing, China, 100084 ‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 1H9, Canada ABSTRACT: Supported sorbent for flue gas desulfurization was prepared by a process of hydrating and drying a mixture of carrier particles and lumps of CaO in previous studies. Supported sorbent achieves high desulfurization efficiency in a Circulating Fluidized Bed (CFB) reactor at moderate temperature. However, the preparation process is complicated, which makes industrial application difficult. In this study, supported sorbent is prepared in situ in a CFB reactor by a coating process, in combination with SO2 removal. The experiments are conducted in a pilot-scale CFB experimental facility at 700−750 °C. The influence of operation parameters on supported sorbent formation and desulfurization efficiency is tested, including bed inventory, sprayer location, and spray method. The results show that the present process achieves 85.3% desulfurization efficiency with a Ca/S molar ratio of 2.01 when the bed inventory is 25 kg. This result is approximately 10 times higher than that of a zero bed inventory. The scanning electron microscope and energy dispersive spectrometer results show that the supported sorbent forms when fine Ca(OH)2 particles in the lime slurry adhere to the bed material surface. The residence time of the supported sorbent in the CFB reactor is significantly longer than that of the fine Ca(OH)2 particles. The adhesion efficiency is influenced by the spray location and spray method. The desulfurization performance of the supported sorbent prepared by the original hydration and drying method is also tested. The desulfurization efficiency is 25−35% higher than the present process because there is better adhesion in the original method than the in situ method. But the process complexity is significantly reduced when using the present process. The present process is a substantial attempt toward the application of supported sorbent.

1. INTRODUCTION Sulfur dioxide (SO2) is one of the main gaseous pollutants that is emitted by a coal-fired power plant.1 SO2 is the cause of many environmental problems, such as acid rain and adverse effects to human health. SO2 is also considered to be an important precursor for secondary PM2.5 formation in the atmosphere.2 There are hundreds of Flue Gas Desulfurization (FGD) processes available for the removal of SO2.3 The limestone wet FGD process is the most widely used, mature process. This process achieves up to 90% desulfurization efficiency at a relatively low Ca/S molar ratio.4 In addition, the operation stability along with fuel and load adaptability of the wet FGD are better than that of other FGD processes. For wet FGD, capital costs and operation expenses are high and water consumption is substantial, which is unacceptable for small capacity units and areas where there is a shortage of water. Moreover, the wet FGD process produces large amounts of sludge byproduct, which causes secondary pollution.5 Dry FGD processes have the advantages of less capital costs and significantly less water consumption.6−9 The reaction between the absorbent and SO2 is conducted on the particle surface and particle pores, with no water participating in the process. The overall reaction rate is significantly lower than the wet FGD, which leads to low absorbent utilization. The gas− solid reaction between the absorbent and SO2 is strongly influenced by the microstructure of the absorbent. High specific surface area and rich porosity of the sorbent results in a high desulfurization performance due to the reduction in diffusion resistance of SO2 to the absorbent surface.10 A supported © XXXX American Chemical Society

sorbent is prepared using a process of hydrating and drying a mixture of lumps of CaO and coarse carrier particles (e.g., fly ash and circulating ash).11,12 During the hydration process, lumps of lime break apart to form ultrafine Ca(OH)2 particles ranging from 1 to 10 μm. These ultrafine Ca(OH)2 particles adhere to the surface of the carrier particles in the drying process. The specific surface area of the supported sorbent is much higher than that of the common sorbent. The sorbent utilization is measured by a Thermogravimetric Analysis (TGA) and reaches 95.7% at 750 °C.13 In a CFB reactor, supported sorbent can be captured by the cyclone separator. Hence, Ca(OH)2 particles circulate in the CFB reactor before detaching from the carrier particles. The supported sorbent has a long residence time. The experimental results show that circulation of the supported sorbent is necessary to achieve high desulfurization efficiency.14 In a pilot-scale CFB reactor, the desulfurization efficiency of the supported sorbent reaches 95% at 750 °C when the Ca/S molar ratio is 2.0.12 Although the desulfurization performance of the supported sorbent is high, industrial application of the supported sorbent is difficult due to the complex nature of the preparation process. The hydration process is approximately 2 h; upon completion, the slurry is filtered to remove most of the water. The residue is dried at 150 °C for approximately 1 h to reduce the water content below 10%. Next, the cake is crushed to a specific granularity.15 Here, Received: December 10, 2017 Revised: January 29, 2018 Published: January 30, 2018 A

DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX

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process lasted for 2 h and the mixture was continuously stirred. Next, the mixture was filtered to remove most of the water. The residue was then placed in an oven to further reduce the water content below 10%. The oven was set to 150 °C and the drying time was 2 h. Finally, the cake was crushed to prepare it for usage. 2.2. Experimental Facility. Figure 1 shows the pilot-scale CFB reactor, which comprises a flue gas system and a CFB reactor system. Figure 1 also shows the main components of each system. The opening degree of the fan was adjusted to control the air quantity, which ranged from 200 to 1000 N m3/h. An oil burner heated the air to 400−800 °C. The nozzle was adjusted to modify oil flow rates, thereby adjusting the temperature. The heated flue gas was mixed with pure SO2 to a given SO2 concentration. Next, flue gas passed through an air cooler in which the flue gas temperature was adjusted by the exchange heat with the cool air. Flue gas entered the CFB riser through a distributor. The height between the distributor and the outlet of the riser was approximately 6 m. The diameter of the riser was 0.305 m. Along the riser there were eight temperature sensors that measured the temperature of the flue gas and seven pressure sensors that measured the pressure of flue gas. A two-phase nozzle was placed at the centerline of the riser for the lime slurry spray. SO2 in the flue gas reacted with Ca(OH)2 particles in the lime slurry droplets and on the bed material surface. A PS3400 gas analyzer was used to measure the SO2 concentrations in the flue gas. The SO2 concentration range was 0−5000 ppm and the accuracy was ±1%. There were measurement locations at the entrance of the riser and the outlet of the cyclone separator. The desulfurization efficiency was directly calculated from the SO2 concentrations at these two locations. Flue gas passed through the cyclone separator and bag filter and was then emitted into the atmosphere. Coarse particles, such as bed material and bed material coated with Ca(OH)2 particles, were collected by the cyclone separator and sent back through the return valve. Fine particles (i.e., mainly Ca(OH)2 particles) were captured by the bag filter. For experiments of supported sorbent prepared by the hydration and drying method, the supported sorbent was fed into the CFB reactor by a screw feeder. The feed rate was controlled by the precalibrated motor frequency. The air velocity in the inlet pipe was measured by a speedometer. The superficial velocity in the riser was calculated from the bed average temperature and inlet air velocity. The solid concentration around the slurry sprayer location was measured by an optical fiber (PV-6D, Institute of Process Engineering, Chinese Academy of Sciences). The solid concentration measurements were conducted at ambient temperature with the bed inventory and superficial velocity in the same way as the experimental conditions. The surface of the original silica sand and postreaction silica sand was observed by a scanning electron microscope (SEM) to confirm the adhesion of Ca(OH)2 particles onto the sand. The chemical element content on the surface was measured by an energy dispersive spectrometer (EDS).

the processing time is lengthy and the drying process requires numerous pieces of equipment as well as consuming a substantial amount of energy. Therefore, a new preparation process is required for supported sorbent to create industrial application feasibility. The structure of the supported sorbent is like the microencapsulation structure that is widely used in pharmaceutical and food industries.16−18 The general preparation process for microencapsulation is a fluidized bed coating from solutions and suspensions. A fluidized bed is widely used in the microencapsulation preparation process because the mass and heat transfer is much stronger than other reactors. In this process, the coating material dissolves in water or another solvent. Next, the solution is continuously sprayed into the hot fluidized bed. The solvent is dried by hot flue gas and bed material. Coating material in the solution is coated onto the bed material surface. This process is like the drying process of the supported sorbent preparation and is under continuous operation. Previous experimental results showed that high desulfurization efficiency can be achieved in a CFB reactor.12 Hence, using the microencapsulation production process to prepare the supported sorbent in situ combined with SO2 removal in a CFB reactor is a feasible alternative for industry application. This work aims to determine the desulfurization performance of the present desulfurization process. In this process, bed material is fluidized in a CFB reactor by hot flue gas with a specific SO2 concentration at approximately 700−750 °C and the lime slurry is continuously sprayed into the CFB reactor. The adhesion of Ca(OH)2 particles onto the bed material surface is investigated. The effects of operation parameters on the desulfurization efficiency is tested. Under the same experimental conditions, the supported sorbent prepared by the hydration and drying method is also tested. The desulfurization performances of the two processes are compared.

2. MATERIAL AND METHODS 2.1. Material and Sorbent Preparation. High grade silica sand was used as bed material in the experiments due to the high SiO2 content, i.e., approximately 98%. Circulating ash was not used due to the large amount of unreacted CaO, which could affect the desulfurization efficiency. Further, the surface of the silica sand was smooth, which was favorable for observation of Ca(OH)2 particles on the bed material surface. XRF was used to measure the compositions of the silica sand and lime, as listed in Table 1.

3. RESULTS AND DISCUSSION 3.1. Desulfurization Performance of Present Process. 3.1.1. Bed Inventory Effect. The bed inventory is an important parameter for the CFB reactor because it affects gas−solid interaction and pressure drop.19 Two experiments are conducted under identical conditions to investigate the influence of the bed inventory. One experiment uses a 25 kg bed inventory and the other experiment uses a zero bed inventory. The average bed temperature is maintained at Tbed = 700−750 °C. The bed superficial velocity in the riser is U = 3 m/s. The flow rate of pure SO2 is controlled by a mass flowmeter. The SO 2 concentration in the flue gas is approximately CSO2 = 900−1100 ppm. The slurry sprayer is located at h = 0.1 m above the distributor. The mass ratio of water to Ca(OH)2 in the lime slurry is approximately w = 6.0− 7.6. Figure 2 shows the results of the experiments.

Table 1. Main Material Composition (%) lime silica sand

SiO2

Al2O3

CaO

MgO

Fe2O3

K2O

0.48 98.24

0.17 0.44

92.21 0.12

0.64 0.07

0.05 0.11

0.02 0.02

The lumps of lime were first crushed down to a size of 1−5 mm. Next, they were dissolved in water at a mass ratio of 1:8. The mixture was stirred for approximately 1 h to ensure a full reaction between the lime and water. Large particles were removed from the slurry by filtering to avoid pipe blocking. Finally, the obtained slurry was sprayed into the CFB reactor. Some of the slurry was sampled and dried at 150 °C for 3 h. The Ca(OH)2 content in the slurry was calculated from the mass change. The supported sorbent was prepared by the hydration and drying method. During the hydration process, lumps of lime were first dissolved in water and then mixed with the silica sand. The mass ratio of water, silica sand, and lime was approximately 6:2:1. The hydration B

DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Pilot-scale CFB reactor: 1. Fan, 2. Oil burner, 3. SO2 mixing chamber, 4. Air cooler, 5. CFB reactor, 6. Sorbent feeder, 7. Slurry sprayer, 8. Bag filter.

The results show that, for the zero bed inventory, the desulfurization efficiency is only 10.08% even when the Ca/S molar ratio is 2.45. The calcium conversion rate is approximately 4%. Lumps of lime fragment into ultrafine Ca(OH)2 particles when they react with water. The reaction releases a substantial amount of heat in a short period of time. The particle size distribution in the lime slurry was measured by Zhang et al.15 The results show that the average diameters of the Ca(OH)2 particles are approximately 7 μm. When the lime slurry is sprayed into the CFB reactor, the water is evaporated by the hot flue gas and the remaining Ca(OH)2 particles are carried by the flue gas. In this process, SO2 in the flue gas reacts with Ca(OH)2. The Ca(OH)2 particles are not captured as they exit the riser and enter the cyclone separator. This is because the cyclone separator is not efficient with fine particles (even though it has a high capture efficiency for large particles). Hence, the Ca(OH)2 particles pass through the riser only once. The residence time of Ca(OH)2 particles is approximately 2 s, which is the same as the flue gas. Therefore, the short residence time of Ca(OH)2 particles results in low desulfurization

Figure 2. Effect of bed inventory on desulfurization efficiency (Tbed = 700−750 °C; U = 3 m/s; CSO2 = 900−1100 ppm; h = 0.1 m; w = 6.0− 7.6).

Figure 3. Surface morphology. C

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Energy & Fuels efficiency. Similar results were obtained by Akira when fine CaCO3 particles were used for desulfurization in a pulverized coal combustion process.20 When 25 kg of bed material is added to the CFB reactor, the desulfurization efficiency is much higher, reaching 85.30% at a Ca/S molar ratio of 2.01 with a calcium conversion rate of approximately 40%. This is approximately 10 times higher than that of the zero bed inventory. The surface morphology of the original silica sand and postreaction silica sand is observed by an SEM. Figure 3 shows the micrographs. The surface morphologies of the original silica sand and postreaction silica sand are different: the former is smooth, but the latter is full of tiny particles. The diameter of the tiny particles is less than 10 μm. An EDS combined with the SEM is used to measure the chemical element contents on the surface. Table 2 lists the results. There is no Ca and S in the

time, which is approximately 45 min. The overall calcium conversion rate is only approximately 40%, which is significantly lower than the previous value of 74%. This is due to the many Ca(OH)2 particles that leave the CFB reactor during the experiment process. First, some slurry droplets fully dry before adhering to the silica sand surface. Ca(OH)2 particles in these droplets directly exit the CFB reactor. Second, when droplets collide with the silica sand, the kinetic energy of the droplet and wettability of the silica sand determine whether the droplets bounce off or adhere to the sand surface.22 As such, not all collisions lead to adhesion of Ca(OH)2 particles on the silica sand surface. There is a substantial number of Ca(OH)2 particles that do not adhere to the silica sand surface; as such, they directly exit the CFB reactor. The reaction time of these Ca(OH)2 particles with SO2 is only approximately 2 s. For the supported Ca(OH)2 particles, some detach from the silica sand surface due to the intense collision in the CFB reactor, thereby reducing the reaction time. According to the results, high bed inventory leads to high desulfurization efficiency. But there are two disadvantages when the bed inventory is high. The first one is that high desulfurization efficiency corresponds to high pressure drop. The second one is that high solid concentration may lead to large agglomerates around the sprayer. When choosing bed inventory in the practical application, both of these two factors should be taken into account. 3.1.2. Effect of Spray Location. The spray location is an important parameter for industrial application. It primarily influences the solid and gas flow conditions around the slurry sprayer. Figure 4 shows the desulfurization efficiency that is

Table 2. EDS Results for Chemical Element Contents (%) original silica sand postreaction silica sand

C

O

Si

S

Ca

9.52 6.86

52.90 39.97

37.58 15.30

15.06

22.80

original silica sand. After the desulfurization test, S contents on the silica sand surface significantly increase to 15.06% and Ca contents on the silica sand surface significantly increase to 22.80%. This shows that the fine particles on the silica sand surface are CaSO4 as well as unreacted CaO. The above analysis shows that the improved desulfurization efficiency (with a 25 kg bed inventory) is mainly due to the adhesion of Ca(OH)2 particles on the silica sand surface. When lime slurry is sprayed into the CFB reactor, some slurry droplets collide with the silica sand. The liquid bridge force allows the silica sand particles to trap the slurry droplets. As the liquid water is evaporated by the hot flue gas, the liquid bridge becomes a solid bridge. Next, the Ca(OH)2 particles in the slurry droplets adhere to the silica sand surface.21 Given that the silica sand is captured by the cyclone separator and then flows back to the riser through the return valve, the supported Ca(OH)2 pass through the riser more than once. Here, the supported Ca(OH)2 circulate in the CFB reactor before being separated from the silica sand. The separation could be due to the intense collision between the particles in the CFB reactor. Therefore, the reaction time between the SO2 and the supported Ca(OH)2 particles is much longer than that of the SO2 with the single-pass Ca(OH)2 particles. This is the reason for the improved desulfurization performance with the 25 kg bed material. XRF is used to measure the chemical compositions of postreaction silica sand. Table 3 lists the results. The main

Figure 4. Effect of spray location on desulfurization efficiency (Iinv = 25 kg; Tbed = 700−750 °C; U = 3 m/s; CSO2 = 900−1100 ppm; w = 6.0−7.6).

Table 3. Composition of Postreaction Silica Sand content (%)

SiO2

CaO

SO3

MgO

Al2O3

P2O5

Fe2O3

79.37

9.36

9.83

0.29

0.10

0.01

0.06

achieved under different Ca/S molar ratios when spraying lime slurry at two different heights above the distributor. The bed inventory is maintained at Iinv = 25 kg. When spraying lime slurry at h = 0.1 m, the desulfurization efficiency increases with the Ca/S molar ratio. The average calcium conversion rate is calculated as approximately 43.24%. When the spray location increases to h = 0.9 m, the average calcium conversion rate decreases to approximately 33.18%. This result is 10.06% lower than the result for h = 0.1 m. The solid concentration decreases along the height in the CFB riser due to the interaction of particles and the gas flow.23 The solid volume fraction at the sprayer location is 0.08 for h = 0.9 m and 0.18 for h = 0.1 m. If

composition of postreaction silica sand is still SiO2. The Ca and S contents significantly increase. Here, the S and Ca molar ratio is calculated as 0.74, which shows that approximately 74% of Ca(OH)2 reacts with SO2 in the bed material. The Ca(OH)2 particles are supported Ca(OH)2 particles that have not separated from the silica sand in the experimental process. The residence time of the Ca(OH)2 particles is the experiment D

DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX

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3.2. Desulfurization Performance of Supported Sorbent. 3.2.1. Physical Properties. The supported sorbent is prepared by the hydration and drying method. Figure 6

the solid concentration around the sprayer is high, then the collision frequency between the slurry droplet and bed material is larger. More Ca(OH)2 particles adhere to the silica sand surface before the water is fully evaporated, which corresponds to the numerical result for the fluidized bed coating.24 Therefore, there are more Ca(OH)2 particles circulating in the CFB riser when spraying at h = 0.1 m. This is the reason that the calcium conversion rate improves for h = 0.1 m. 3.1.3. Effect of Spray Method. In industry applications, absorbent particles easily deposit onto the reactor wall when using an improper spray method.25,26 This decreases the operation stability of the FGD system. Two different droplet− gas contact patterns (i.e., co-current and countercurrent) are tested to investigate the influence of the spray method on desulfurization efficiency and wall deposition. Figure 5 shows

Figure 6. Particle size distribution of raw silica sand and supported sorbent.

shows the particle size distribution of raw silica sand and supported sorbent. The peak of the particle size distribution moves to the larger diameter because of the supported Ca(OH)2 particles on the silica sand surface. There is a distribution peak of the supported sorbent located in the range of 1−10 μm. This peak is not observed in the raw silica sand. These particles are Ca(OH)2 particles that are not supported on the silica sand surface. The volume fraction of these particles is approximately 10%. Figure 7 shows the micrographs of the raw silica sand and the supported sorbent observed under the SEM. The surface of the raw silica sand is angular and relatively smooth. After hydration and drying, the surface becomes rough. Many fine particles adhere to the silica sand surface. In addition to the large particles, there are also some smaller particles that comprise nonadhering Ca(OH)2 particles and their aggregate. 3.2.2. TGA Test. The desulfurization capacity of the supported sorbent is tested in a TGA. Approximately 10 mg of sorbent is used for the TGA test. The TGA is preheated to 750 °C, and the reaction gas is input into the TGA for a reaction time of 70 min. The reaction gas comprises 2000 ppm of SO2 with 5% O2 and N2 as a balance gas. The desulfurization capacity of the supported sorbent is represented by mole of SO2 absorbed per mole of Ca in the supported sorbent, which is also the maximum sorbent utilization of the supported sorbent under the given fixed bed conditions. The desulfurization reaction product is almost all CaSO4 under the test conditions.30 The desulfurization capacity is calculated from the sample weight change. The results show that the desulfurization capacity of the supported sorbent is approximately 0.98 mol of SO2 per mole of Ca in the supported sorbent. This means that the maximum sorbent utilization of the supported sorbent can achieve 98%. 3.2.3. Desulfurization Performance in CFB Reactor. The desulfurization performance of the supported sorbent is tested under the same conditions as the present process. Desulfurization efficiency that is calculated from the inlet and outlet SO2 concentrations changes with time, as shown in Figure 8. The desulfurization efficiency of the supported sorbent increases with time due to the accumulation of Ca(OH)2 particles in the riser. As the particle composition in the riser stabilizes, the

Figure 5. Effect of spray method on desulfurization efficiency (Iinv = 25 kg; Tbed = 700−750 °C; U = 3 m/s; CSO2 = 900−1100 ppm; h = 0.1 m; w = 6.0−7.6).

the experimental results. The desulfurization efficiency of the countercurrent spray is higher than that of the co-current spray. The average calcium conversion rate is calculated as 43.24% for the countercurrent spray compared with 33.52% for the cocurrent spray. The improved calcium conversion rate is due to the improved contact efficiency between the droplets and the bed material particles when using the countercurrent spray. Droplets move downward after leaving the nozzle when using the countercurrent spray. The solid concentration in the lower area is higher than that in the upper area. Therefore, the collision frequency between droplets and bed material is higher when using the countercurrent spray.27 This leads to a higher fraction of Ca(OH)2 particles adhering to the bed material surface. Hence, the average reaction time of SO2 with Ca(OH)2 increases when using the countercurrent spray. This is the reason for the improved desulfurization performance. In the experimental process for the two spray methods, the deposition on the reactor wall is not observed. The FGD system has a stable operation because the usual operation temperature of the spray-dry is below 200 °C.28,29 Deposition occurs because the wet particles collide with the reactor wall. The temperature in the CFB reactor is 700−750 °C, which is much higher than that of the usual spray-dry process. The evaporation of liquid water is significantly accelerated; therefore, less wet particles impact on the wall. In addition, the sprayer is surrounded by high concentration bed material, which also reduces the deposition of wet particles on the reactor wall. E

DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 7. Micrographs of raw silica sand and supported sorbent.

droplets that do not contact with the bed material before the water is fully evaporated directly exit the CFB reactor. The adhering efficiency is significantly influenced by the contact efficiency between the slurry droplets and the bed material. Comparatively speaking, the Ca(OH)2 particles and silica sand fully contact in the mixing slurry when using the hydration and drying method. Therefore, the adhering efficiency is higher for the supported sorbent than that for the present process. This is the reason that there is a higher calcium conversion ratio for the supported sorbent. Table 4 lists the chemical composition of the postreaction supported sorbent. The S and Ca molar ratio is calculated as Table 4. Composition of Postreaction Supported Sorbent Figure 8. Desulfurization efficiency tendency with experiment time.

content (%)

desulfurization efficiency tends to be steady after 45 min. Figure 9 shows the stable desulfurization efficiencies at different Ca/S

SiO2

SO3

CaO

MgO

Fe2O3

K2O

61.57

20.39

15.61

0.32

0.17

0.03

0.91, which means that the calcium conversion rate of Ca(OH)2 particles that remain in the CFB reactor is approximately 91%. This result is close to the sorbent utilization achieved in the TGA test. The overall calcium conversion rate is less than 75% because there are 10% unsupported Ca(OH)2 particles in the preparing process and Ca(OH)2 particles could detach from the silica sand surface due to the intense collisions in the CFB reactor.

4. CONCLUSION This paper presents a moderate temperature CFB-FGD process with in situ preparation of a supported sorbent. A series of experiments are conducted in a pilot-scale CFB reactor to study the influence of operation parameters on desulfurization efficiency. The results show that the bed inventory significantly influences the desulfurization efficiency. Desulfurization efficiency is 85.3% at a Ca/S molar ratio of 2.01 when the bed inventory is 25 kg. The calcium conversion rate is approximately 10 times higher than that of the zero bed inventory. The improved calcium conversion rate is due to the adhesion of Ca(OH)2 on the bed material surface. The reaction time between the SO2 and Ca(OH)2 is significantly extended. The position of spray location should be informed by the distribution of the solid concentration. A higher solid concentration near the sprayer usually results in a higher desulfurization. A countercurrent spray is preferred over a cocurrent spray for this process because it has better contact efficiency between the droplets and bed material. The desulfurization performance of the supported sorbent prepared by the original hydration and drying method is also tested in

Figure 9. Desulfurization efficiency of supported sorbent (Tbed = 700− 750 °C; U = 3 m/s; CSO2 = 900−1100 ppm).

molar ratios. The desulfurization efficiency increases with the Ca/S molar ratio. However, the calcium conversion rate decreases with the Ca/S molar ratio. Here, it is 74.37% for a Ca/S molar ratio of 0.83 and 64.83% for a Ca/S molar ratio of 1.34. The calcium conversion rate is approximately 25−35% higher than that of the present process because the adhering efficiency of the Ca(OH)2 particles on the silica sand surface is much higher for the supported sorbent than for the present process. In the present desulfurization process, the slurry F

DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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the experimental facility. The calcium conversion rate of the supported sorbent is approximately 25−35% higher than that of the present process because the adhesion efficiency is higher for the hydration method. Most of the Ca(OH)2 is retained in the CFB reactor throughout the experimental process. Although the desulfurization performance is better with the supported sorbent prepared by the hydration and drying method, the preparation method is not feasible for industry application due to its complexity. The present process is a substantial attempt toward the application of supported sorbent.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-62785669. E-mail: [email protected]. ORCID

Changfu You: 0000-0003-4174-2177 Qingxia Liu: 0000-0002-4587-2086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the National Key Research and Development Program (2016YFC0204100) and National Natural Science Foundation of China (No. 51476089) and Key Science and Technology Project for Coal of Shanxi Province (MD2014-03).

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DOI: 10.1021/acs.energyfuels.7b03887 Energy Fuels XXXX, XXX, XXX−XXX