Adhesion, Abrasion, and Desulfurization Characteristics of Rapidly

Oct 4, 2012 - first electrical field in the electrostatic precipitator (ESP) of a circulating fluidized bed (CFB) boiler, the fly ash from a chain boi...
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Adhesion, Abrasion, and Desulfurization Characteristics of Rapidly Hydrated Sorbent with Cheap and Easily Obtained Adhesive Carrier Particles Yuan Li and Changfu You* Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Adhesive carrier particles have important technical and economic influence on circulating fluidized bed flue gas desulfurization (CFB-FGD) systems that use rapidly hydrated sorbent. Several cheap and easily obtained materials with rough surfaces and porous structures were used as adhesive carrier particles to prepare rapidly hydrated sorbent: the fly ash from the first electrical field in the electrostatic precipitator (ESP) of a circulating fluidized bed (CFB) boiler, the fly ash from a chain boiler, and river sand. Scanning electron microscopy (SEM), particle size distribution (PSD) analysis, surface and pore analysis, abrasion tests, and thermal gravimetric analysis (TGA) tests were used to examine the influence of the adhesive carrier particles’ surface and pore characteristics on the adhesion, abrasion, and desulfurization characteristics of the rapidly hydrated sorbent. Experimental results showed that the CFBB ESP ash, chain boiler ash, and river sand all improved the abrasion and desulfurization characteristics of the rapidly hydrated sorbent compared to the coal fly ash. Specifically, the abrasion ratios of the experimental sorbents were all less than 25% compared to 45% for the coal fly ash sorbent, while the desulfurization abilities of the three experimental sorbents were more than 830 mg SO2/g lime compared to 725 mg SO2/g lime for the coal fly ash sorbent. The river sand sorbent had the greatest desulfurization ability and the lowest abrasion ratio, and it possesses the best industrial application potential of all the tested materials.

1. INTRODUCTION Dry and semidry flue gas desulfurization (FGD) technologies have excellent industrial application potential in regions where water is in short supply.1−3 Calcium-based sorbents modified by coal fly ash work quite effectively in dry/semidry FGD systems.4−9 Rapidly hydrated sorbent developed for dry/ semidry FGD systems is prepared by rapidly hydrating adhesive carrier particles and lime.10−14 In one experiment, coal fly ash sorbent provided 67−83% desulfurization efficiency for a circulating fluidized bed flue gas desulfurization (CFB-FGD) system, resulting in a Ca/S of 2.0 and a reaction temperature of 600−800 °C.15 These results are comparable (and sometimes superior) to the effects of sprayed water at 7.5 kg/h, which provided 58.7−88.4% desulfurization with a Ca/S of 1.2−3.0 and a reaction temperature of 65 °C.16 However, some of the fine calcium-containing particles that originally adhere to the coal fly ash surface can be abraded during the desulfurization process due to collisions in the CFB reactor. It can be difficult for the cyclone separator to collect these very active fine particles for recirculation, which greatly reduces the particle residence time and the Ca conversion ratio in the CFB reactor. Li et al.14,17 found that adhesive carrier particles with rough surfaces and porous structures can enhance adhesion intensity with fine calcium-containing particles, decrease the abrasion ratio of the sorbent, and improve the desulfurization ability of the sorbent. In another study, the circulation ash from a CFB boiler was used to prepare rapidly hydrated sorbent, and this circulation ash sorbent provided 90% desulfurization efficiency with a Ca/S of 1.0 and a reaction temperature of 750 °C; thus, the calcium conversion ratio of the sorbent reached 90% under this condition.18 However, the © 2012 American Chemical Society

application of circulation ash is limited because of insufficient sources, which significantly increases operating costs. Therefore, the development of cheap and easily obtained adhesive carrier particles is crucial for improving the economic feasibility of CFB-FGD systems that use rapidly hydrated sorbent. In this investigation, the fly ash from the first electrical field in the electrostatic precipitator of a circulating fluidized bed boiler (CFBB ESP ash), the fly ash from a chain boiler (Chain boiler ash), and river sand were used as adhesive carrier particles to prepare rapidly hydrated sorbent. The CFBB ESP ash, chain boiler ash, and river sand used in this study all have rough surfaces and porous structures, and they are all cheap and easily obtained. The adhesion, abrasion, and desulfurization characteristics of the rapidly hydrated sorbent were examined through scanning electron microscopy (SEM), particle size distribution (PSD) analysis, surface and pore analysis, abrasion tests, and thermal gravimetric analysis (TGA) tests.

2. EXPERIMENTAL SECTION 2.1. Materials and Sorbent Preparation. The lime used in this study was from Hebei Province, the CFBB ESP ash was from a CFB boiler, the chain boiler ash was from the heating boiler in Tsinghua University, and the river sand was from a construction site. Their main components were measured using the ARL Advant’XP+ X-ray fluorescence spectrometer. Table 1 shows that the main component of the lime is CaO; it also Received: Revised: Accepted: Published: 13833

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Table 1. Main Components of Materials (%) lime CFBB ESP ash chain boiler ash river sand

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

0.82 46.12 41.64 62.70

0.33 33.53 14.08 14.19

0.15 3.52 14.71 4.44

93.49 9.60 9.66 9.03

0.92 1.13 3.00 2.73

0.08 0.39 2.43 3.26

0.09 0.06 2.13 2.36

MnO

P2O5

SO3

0.02

0.19 1.38 0.14

2.71 8.36 0.06

0.08

minutes later, the fan was stopped, and the particles in the cyclone separator and the bag filter were measured. An abrasion ratio, Ra, was defined to reflect the mass change when the sorbent flowed in the fluidized abrasion test bed:

shows that the three adhesive carrier particles have similar components, containing relatively large amounts of Si, Al, Fe, and Ca. The three adhesive carrier particles were mixed with the lime individually to prepare three different rapidly hydrated sorbents. First, the lime, the adhesive carrier particles, and water were mixed in a hydration mixer and stirred at room temperature to produce a sorbent slurry. The water/solid mass ratio was 1:1, the adhesive carrier particles/lime ratio was 2:1, and the hydration time was 2 h. Then, the sorbent slurry was dried in an infrared desiccator for 1.5 h at a temperature of 150 °C until its water content, which was tested by TGA in N2 atmosphere at a temperature of 150 °C, was less than 10%. 2.2. Abrasion Experiments. The intense collisions between different sorbent particles, as well as those between sorbent particles and the inner wall of a CFB reactor, can lead to sorbent abrasion and detachment of the fine particles from adhesive carrier particle surfaces. A fluidized particle abrasion test bed was developed to simulate the actual abrasion of the sorbent in a CFB. A schematic of the test bed is shown in Figure 1. The sorbents used for the abrasion experiments were

Ra =

mc + mb × 100% m0

(1)

where mc represents the mass of particles collected by the cyclone separator, mb represents the mass of particles collected by the bag filter, and m0 is the initial mass of precalcined sorbent. Lv et al.19 found that the particle abrasion ratio became almost constant after about 15 min, with the average residence time of particles in a CFB reactor being about 2 h.20 Therefore, the total abrasion time should be between 15 min and 2 h. A time of 60 min was used here. 2.3. Desulfurization Experiments. To quantitatively evaluate the variance in the desulfurization abilities of the sorbents, the desulfurization ability of the precalcined sorbent before the abrasion experiment and that of the sorbent remaining in the test bed after the abrasion experiment were measured using TGA tests, which used about 10 mg of sorbent preheated to 750 °C. After the weight became stable, the reaction gas was input into the TGA unit at a gas flow rate of 100 mL/min for 80 min. The reaction gas consisted of SO2 (2000 ppm), O2 (5%), and N2 (balance gas). CO2 was not added to the reaction gas because CO2 does not significantly affect the desulfurization ability of sorbent at 750 °C.10 The desulfurization ability of the sorbent, Rd (mg/g), is represented by the mass of SO2 absorbed per gram of sorbent. The TGA test10 showed that almost all of the desulfurization reaction product was CaSO4 for this condition. Thus, the sulfur reaction can be summarized as: CaO (solid) + SO2 (gas) + 1/2O2 (gas) → CaSO4 (solid); Rd can be calculated by eq 2: Rd =

(m i − m0) × 64 × 1000 80 × m0

(2)

where mi represents the sorbent mass at a given time (mg) and m0 represents the initial mass of precalcined sorbent (mg). A Malvern Mastersizer 2000 laser diffraction particle size analyzer was used to evaluate the particle size distributions of the samples. A KYKY-2800 scanning electron microscope was used to observe the surface micrographs of the adhesive carrier particles and the sorbents. Specific surface areas and specific pore volume distributions were measured by nitrogen adsorption using a Micromeritics ASAP 2010 analyzer according to the Brunauer−Emmett−Teller (BET) method.

Figure 1. Schematic of the fluidized abrasion test bed: 1 = fan; 2 = rotameter; 3 = fluidized bed; 4 = cyclone separator; 5 = bag filter.

calcined for 30 min at 750 °C to simulate moderatetemperature conditions.19 A 250-W fan with an inner diameter of 0.09 m and a height of 1.10 m pushed air into the fluidized bed to fluidize the precalcined rapidly hydrated sorbents. The large particles remained inside the bed while the fine particles were entrained in the cyclone separator (d99 = 40 μm). Some of the fine particles from the fluidized bed were collected by the cyclone separator, and others were collected by the bag filter (efficiency = 99%). The test began with 200 g of sorbent being put into the fluidized bed at an air flow rate of 5 m3/h. A few

3. RESULTS AND DISCUSSION 3.1. Physical Characteristics of the Particles. Figure 2 shows the SEM results of the CFBB ESP ash, chain boiler ash, river sand, and their sorbents. Figure 2 panels a−c show that the CFBB ESP ash has a smaller particle diameter than both the chain boiler ash and the river sand, but all three adhesive carrier 13834

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Figure 2. SEM images of the three adhesive carrier particles and their sorbents.

with an average diameter of 398 μm and no particles smaller than 10 μm. 3.2. Adhesion Characteristics of the Adhesive Carrier Particles. VPM10 represents the volume ratio of particles smaller than 10 μm to the total volume of particles, which was obtained from the particle size distributions. ΔVPM10 represents the increase of VPM10 of the sorbent compared to the adhesive carrier particles. A larger ΔVPM10 means fewer lime particles smaller than 10 μm adhered to the adhesive carrier particles. Therefore, the lower the ΔVPM10 is, the greater is the adhesion characteristics of the adhesive carrier particles. Table 3 shows the calculated ΔVPM10 for the three adhesive carrier particles compared with that of coal fly ash.14 Table 3 shows that the ΔVPM10 values of the three adhesive carrier particles with rough surfaces and porous structures are lower than that of the coal fly ash, proving that adhesive carrier particles with rough surfaces and porous structures have superior adhesion intensity with fine calcium-containing particles. Because the porous structures of the CFBB ESP ash and the chain boiler ash are loose and friable, the particles of these sorbents break up during the hydrating and milling processes, so the fine calcium-containing particles fall off from their adhesive carrier particles’ surfaces. This increases the VPM10 of the CFBB ESP ash and chain boiler ash sorbents, explaining why the adhesion characteristics of the CFBB ESP ash and chain boiler ash are not as good as those of the river sand. 3.3. Abrasion Characteristics of the Sorbents. Figure 4 shows the abrasion ratios of the three experimental sorbents over 60 min compared with that of the coal fly ash sorbent.14 It is clear that the abrasion ratio of the coal fly ash sorbent is larger than the ratios of all three experimental sorbents, proving that adhesive carrier particles with rough surfaces and porous structures can improve the abrasion characteristics of rapidly hydrated sorbent. The particle sizes of the CFBB ESP ash are smaller than those of the chain boiler ash and the river sand, and because the surface structures of the CFBB ESP ash are

particles have rough surfaces and porous structures. Figure 2 panels d−f show that a large amount of fine calcium-containing particles adhered to the three adhesive carrier particles; for the CFBB ESP ash sorbent, there were some unadhered fine calcium-containing particles. Table 2 shows the specific surface areas and pore volumes of the three adhesive carrier particles and their sorbents. The Table 2. The Specific Surface Areas and Pore Volumes of the Adhesive Carrier Particles and Their Sorbents CFBB ESP ash specific surface area (m2/g) specific pore volume (cm3/g)

4.39

29.62

0.0140 CFBB ESP ash sorbent

specific surface area (m2/g) specific pore volume (cm3/g)

chain boiler ash

8.30 0.0411

0.0341 chain boiler ash sorbent 25.82 0.0591

river sand

coal fly ash

2.06

0.306

0.0037

0.0017

river sand sorbent

coal fly ash sorbent

8.04

5.60

0.0514

0.0391

CFBB ESP ash, chain boiler ash, and river sand all have larger specific surface areas and pore volumes compared to the coal fly ash because of their rough surfaces and porous structures. Accordingly, the specific surface areas and pore volumes of the experimental sorbents are larger than those of the coal fly ash sorbent. Figure 3 shows the particle size distributions of the lime, the three carrier particles, and their sorbents. The average particle diameter of the lime is 3.7 μm, and most of the lime particles are less than 10 μm in size. The CFBB ESP ash has a slightly larger particle size, with an average diameter of 36.7 μm and a relatively larger proportion of particles smaller than 10 μm. The chain boiler ash has medium-sized particles, with an average diameter of 301.9 μm and a lower percentage of particles smaller than 10 μm. The river sand has the largest particle size, 13835

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Figure 3. Particle size distributions.

Table 3. ΔVPM10 of Various Particles

VPM10 (%) VPM10 (%) ΔVPM10

CFBB ESP ash sorbent

chain boiler ash sorbent

river sand sorbent

coal fly ash sorbent

48.45

13.85

4.04

26.90

CFBB ESP ash

chain boiler ash

river sand

coal fly ash

35.63 12.82

3.81 10.04

0 4.04

8.33 18.57

Figure 5. TGA-tested desulfurization abilities of various particles. T = 750 °C; CSO2 = 2000 ppm; CO2 = 5%.

Figure 4. Sorbent abrasion ratios.

loose and friable, the CFBB ESP ash sorbent has a higher abrasion ratio. Looking at the ΔVPM10 results in Table 3, it is clear that lower sorbent abrasion ratios correspond to smaller ΔVPM10 values, indicating that adhesive carrier particles with better adhesion characteristics provide better abrasion characteristics for rapidly hydrated sorbent. 3.4. Desulfurization Abilities of the Sorbents. Figure 5 shows the TGA-tested desulfurization abilities of the CFBB ESP ash, chain boiler ash, river sand, and their sorbents. The three experimental sorbents have similar desulfurization abilities after 80 min (300 mg/g). TGA-tested lime desulfurization ability, which was obtained by separating the desulfurization ability of the adhesive carrier particle from that of the sorbent, is represented by the mass of SO2 absorbed per gram of lime (mg SO2/g lime). Figure 6 illustrates the values of TGA-tested lime desulfurization abilities for the various sorbents. It can be seen that the lime desulfurization abilities of the first three

Figure 6. TGA-tested lime desulfurization abilities of various sorbents.

sorbents are similar, and are clearly greater than that of the coal fly ash sorbent.14 The main reason is that the specific surface areas and pore volumes of the three sorbents are larger than those of the coal fly ash sorbent, especially for medium pores (2−50 nm) which have obvious promotion for sulfur reaction21 shown in Figure 7. Among the three sorbents that use cheap and easily obtained adhesive carrier particles, the river sand sorbent has the best desulfurization ability with the lowest abrasion ratio. Consid13836

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Special Funds for Major State Basic Research Projects (No. 2006CB200305).

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Figure 7. Pore diameter distributions of various sorbents.

ering that river sand is commonly found in nature, it has better application potential used as the adhesive carrier particles than either CFBB ESP ash or chain boiler ash. On the other hand, river sand has a greater density than CFBB ESP ash, chain boiler ash, and coal fly ash, which may have a negative effect on CFB-FGD systems. Thus, the technical and economic feasibility of using river sand as an adhesive carrier particle will require further investigation on actual CFB-FGD system.

4. CONCLUSIONS CFBB ESP ash, chain boiler ash, and river sand, which are cheap and easy to obtain, were used as adhesive carrier particles to prepare rapidly hydrated sorbent. The adhesion, abrasion, and desulfurization characteristics of the sorbents were examined through an analysis of the physical characteristics of the particles, abrasion tests, and TGA desulfurization tests. The conclusions can be summarized as follows: (1) The CFBB ESP ash, chain boiler ash, and river sand all have rough surfaces and porous structures, which improved their adhesion characteristics as adhesive carrier particles. The rapidly hydrated sorbents prepared using the three adhesive carrier particles have large specific surface areas and pore volumes, which also improved their abrasion and desulfurization characteristics. (2) The particle sizes of the CFBB ESP ash are relatively small compared to those of the chain boiler ash and the river sand, and the surface structures of the CFBB ESP ash are loose and friable, which led to a larger abrasion ratio for the CFBB ESP ash sorbent. The three experimental sorbents had similar TGA-tested desulfurization abilities (830−878 mg SO2/g lime) that were much greater than that of the coal fly ash sorbent (725 mg SO2/g lime) due to their large specific surface areas and pore volumes. (3) The abrasion tests and the TGA desulfurization tests indicated that the CFBB ESP ash, chain boiler ash, and river sand are all suitable for using as adhesive carrier particles for rapidly hydrated solvent, with river sand having the best application potential. The application of these types of sorbents in actual CFB-FGD systems must be investigated in future research in order to verify the preliminary findings of this study.

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*E-mail: [email protected]. 13837

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