Effect of Internal Structure on Flue Gas Desulfurization with Rapidly

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Ind. Eng. Chem. Res. 2010, 49, 11464–11470

Effect of Internal Structure on Flue Gas Desulfurization with Rapidly Hydrated Sorbent in a Circulating Fluidized Bed at Moderate Temperatures Jie Zhang,†,‡ Changfu You,*,† and Changhe Chen† Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua UniVersity, Beijing 100084, China, and Centre for AdVanced Energy Science and Technology, Department of Chemical Engineering, Curtin UniVersity of Technology, Perth 6102, Australia

A moderate-temperature desulfurization process at 600-800 °C was studied in a pilot-scale circulating fluidized bed flue gas desulfurization (CFB-FGD) experimental facility with the addition of the internal structure. The rapidly hydrated sorbent and the desulfurization products were analyzed to clarify the influence mechanism of the internal structure on the moderate-temperature desulfurization process. The results show that the desulfurization efficiency with the internal structure at 600-800 °C was 74-93% for the calcium to sulfur (Ca/S) molar ratio of 2.0, which was higher than the desulfurization efficiency without the internal structure of 67-83%. As for the calcium-containing compositions, the desulfurization products included about 70% cyclone recirculation sample and about 30% bag filter sample. The cyclone recirculation sample was mainly composed of CaSO4 and CaO. The bag filter sample not only contained high contents of CaSO4 and CaO but also contained a considerable amount of CaCO3 and even some unreacted Ca(OH)2. With the addition of the internal structure, the calcium conversion rate of the cyclone recirculation sample greatly increased from 42.8 to 47.4% while that of the bag filter sample just increased from 29.4 to 30.7%. It demonstrated that the main contributor for the improved desulfurization efficiency was the cyclone recirculation sample due to the improved solids concentration distribution and the enhanced gas-solid contact efficiency. The calcium conversion rate for the bag filter sample depended on the solids concentration distribution as well as the particle residence time in the moderate temperature range, which indicated that prolonging the particle residence time for the fresh sorbent and the fall off calcium-containing particles was important to further improve the desulfurization performance. These results provided good guidance for realizing high desulfurization efficiency and low flow resistance in dry FGD processes. 1. Introduction All coals contain different amounts of sulfur in two major forms, i.e., inorganic and organic sulfur.1 The SO2 emissions in the flue gas from coal-fired utility boilers and industrial incinerators have caused significant environmental and human health effects. Various flue gas desulfurization (FGD) technologies have been developed to remove SO2 using low-cost Cabased sorbents. The moderate-temperature desulfurization process has the advantages of low capital expense, low operating cost, no water consumption, high desulfurization efficiency, and production of dry CaSO4 as the main byproduct, which is also promising to realize simultaneous removal of coal combustion pollutants.2 Thus, it is very attractive for coal combustion pollutant removal in developing countries, especially in very arid regions. The moderate temperature desulfurization process not only depends on the reaction temperature and the sorbent reactivity but also relies on the gas-solids flow structure, such as the contact efficiency and the contact residence time between the SO2 and the sorbent particles. Real flue gases contain not only SO2 but also much larger concentration of CO2. Zhang et al. reported that the 600-800 °C temperature range can effectively enhance the sulfate reaction rate and restrain the carbonate reaction.3 In addition, due to the gas-solid reaction mechanism in this moderate-temperature desulfurization process, the ultimate reaction extent depends on the SO2 diffusion into the sorbent particles. * To whom correspondence should be addressed. Tel.: +86-1062785669. Fax: +86-10-62770209. E-mail: [email protected]. † Tsinghua University. ‡ Curtin University of Technology.

Therefore, the microstructure of the sorbent particle, including the surface area, the pore volume, and the pore size distribution, is very important to realize high calcium conversion ratios. Various high desulfurization activity sorbents have been developed by using a hydration mixture of lime and fly ash to replace commercial sorbents, such as the lime and limestone powder.4–13 The fly ash modified sorbent can mainly be classified into two types. One is to form some high activity materials at demanding preparation conditions, such as high hydration temperature and pressure, long hydration time, and long drying time. Another type is to form a coarse and porous composite structure with fine calcium-containing materials adhered on larger fly ash surfaces at moderate preparation conditions. The rapidly hydrated sorbent, belonging to the latter type, has been developed in which the calcium conversion rate increases rapidly with increasing temperature at moderate temperatures.9 The preparation principle relies on the rapid heat release of the lime hydration reaction to explode the lime lumps into tiny Ca(OH)2 particles, which are adsorbed onto the surface of the fly ash to form a cubic and porous sorbent surface structure. The calcium conversion rate at 750 °C reached 95.7% after 90 min of sulfate reaction in a thermogravimetric analyzer (TGA).12 Most research has focused on investigating the optimum preparation conditions to achieve high desulfurization performance with fixed bed reactors or TGA. However, whether the high-activity sorbent developed in fixed bed reactors or TGA will still realize high desulfurization performance in practical desulfurization processes, such as fluidized bed reactors, is much less reported. A pilot-scale circulating fluidized bed (CFB) reactor was used to realize the moderate temperature desulfurization process by providing intense gas-solid interactions and long sorbent residence time. Although the CFB reactor with the rapidly

10.1021/ie100988r  2010 American Chemical Society Published on Web 10/12/2010

Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010

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Figure 1. Pilot-scale CFB reactor experimental system diagram.

hydrated sorbent achieved high desulfurization efficiency above 95% at Ca/S ) 2.0, the flow resistance was very high due to the bed pressure drop of up to 200 mmH2O.2 Due to the engineering application background of the CFB-FGD process in coal-fired utility boilers and industrial incinerators, the capacity of induced draft fans was usually limited and the flue gas flow rate was always very high, which made the flow resistance and relevant operating costs crucial to the engineering application. The original pilot-scale CFB facility was retrofitted to reduce the flow resistance. The two-stage cyclone separator was changed into a one-stage cyclone separator, and the gas distributor was changed from a wind-cap type to a large-hole type. The experimental results showed that the retrofitted reactor structure reduced the flow resistance by over 50% but also reduced the desulfurization efficiency by 15%. In addition, the calcium conversion rate for this rapidly hydrated sorbent was much lower in CFB-FGD reactors than TGA results.3 The main reason was that reduced bed solids concentration and changed bed fluidized type unfavorably affected the particle distribution uniformity and gas-solids mixing.2,3 Various methods have been studied to improve the solids concentration distribution uniformity in CFB reactors, such as reactor structure modifications, particle characteristics designs, and elevated gas pressures.14,15 Optimizing the CFB reactor structure, such as the addition of internal structures, is one of the most practical solutions to realize high desulfurization efficiency and low flow resistance in CFB-FGD processes. The internal structures in CFB desulfurization reactors are mostly ring-shaped with the aim to improve the gas-solid contact efficiency through enhanced solids mixing and prolonged particle residence time.16 Wang et al. reported that the solids concentration distribution in a square CFB reactor was measured along the reactor height by optical fiber probes and electrical capacitance tomography (ECT) measurements at ambient temperature to determine the optimal CFB reactor structure.17,18 Furthermore, various internal structures were modeled in a 2D reactor to investigate the moderate temperature desulfurization process.3 However, these cold-state gas-solid measurements

and 2D numerical simulations were difficult to fully reflect the real moderate temperature CFB-FGD process with the rapidly hydrated sorbent, mainly due to the simplified assumptions of the sorbent particle characteristics and the gas-solid fluidization regime. Therefore, it is important to experimentally investigate the effect of internal structures on the moderate-temperature CFB-FGD process, which can include the complex interaction between the gas-solid flow and chemical reaction for the rapidly hydrated sorbent, for accurately evaluating the real desulfurization performance of the rapidly hydrated sorbent in CFB reactors. In this paper, the moderate temperature desulfurization process with the rapidly hydrated sorbent was studied in a pilot-scale CFB-FGD experimental facility with the addition of the internal structure at various temperatures and Ca/S ratios. In addition, the sorbent samples collected at various positions of the CFB-FGD facility were analyzed to reveal the influence mechanism of the internal structure on the moderate temperature CFB-FGD process. Furthermore, the particle abrasion and calcium utilization extent of the rapidly hydrated sorbent in the CFB reactor was quantitatively reported to explain why the high-activity sorbent tested in TGA achieved a much lower utilization rate in the pilotscale CFB facility. The results provide new insights into the flue gas desulfurization and high activity sorbent research fields to further improve the desulfurization performance. 2. Experimental System The pilot-scale CFB-FGD system is shown in Figure 1. The main subsystems were the sorbent preparation system, the flue gas generation system, and the CFB reactor. The CFB reactor included a main bed, a distributor, a cyclone separator, a sorbent recirculating facility, a sorbent feeder and drain, a bag filter, and a compressor. Detailed descriptions of the system were given elsewhere.2,3 The flue gas generated by the oil burner was mixed with a small amount of cool air to produce 600-800 °C simulated flue

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gas. SO2 was added to the flue gas before the CFB reactor. The flue gas passed through the CFB reactor and reacted with the sorbent and then through the cyclone separator and bag filter before being emitted from the stack. The sorbent particles collected in the cyclone separator and bag filter were fed back into the CFB reactor for further circulation or drained out of the system. The CFB reactor riser was 6 m high with a 0.305 m diameter and a flue gas flow rate of 300 N m3/h. The distributor was a large-hole distributor and the cyclone separator was a singlestage cyclone separator, which were described elsewhere.2 A ring type internal structure as shown in Figure 1 was added on the CFB reactor wall 3.25 m above the distributor. It was 1.75 m higher than the fresh sorbent screw feeder and 2.05 m higher than the sorbent recirculating inlet. The cross-section of the internal structure was a triangle shape with the projected length of 60 mm. The upper internal angle with the reactor wall was 45°, and the lower internal angle was 30°. The ratio between the throat diameter and the reactor diameter was 0.61. The flue gas temperatures and static pressures were measured automatically with thermocouples and pressure sensors at various positions along the riser height. The O2, CO2, and SO2 concentrations in the flue gas were measured online at the CFB reactor inlet and outlet. The desulfurization efficiency was directly calculated from the inlet and outlet SO2 concentrations. The rapidly hydrated sorbent was made from lime lumps and coal fly ashes at selected mass ratios, using hydration at an ambient temperature for about 2 h and drying at 150-300 °C for 0.5-1 h.12 The sorbent reactivity was 10 times higher than the original lime due to improved microscopic porous structure. The rapidly hydrated sorbent required a much shorter preparation time than the earlier sorbent made by hydration at 77 °C for 8 h and drying at 87 °C for 1 day.4 The sorbent preparation and sulfation characteristics were previously described in detail.12 Various measurements were used to characterize the solid samples, such as the fresh rapidly hydrated sorbent and the desulfurization products. A laser diffraction instrument (Malvern Mastersizer 2000) was used to evaluate the particle size distribution of the sorbent samples. A BRUKER D8 Advance X-ray diffractometer (XRD) was used to qualitatively study the sample compositions. The ULVAC-PHI type X-ray photoelectron spectroscopy (XPS) was used to determine the sulfur valence number in the desulfurization products. The specific surface area and the pore volume distribution were measured by nitrogen adsorption using an ASAP 2010 type BrunauerEmmett-Teller (BET) analyzer. The sulfation ability and the chemical compositions of the sorbent samples were studied with a TA Q500 type TGA. The sulfation ability of the samples (R) was defined as the mass of the absorption SO2 per mass sample. The calcium and sulfur contents in the sorbent samples were determined by the IRIS Advantage type inductively coupled plasma atomic emission spectrometry (ICP-AES). 3. Results and Discussion 3.1. Effect of the Internal Structure on the Desulfurization Efficiency. The effect of the internal structure on the desulfurization efficiency is shown in Figure 2 for the experimental conditions: Ca/S ) 1.5-2.0; the bed superficial velocity, U ) 2.5 m/s; the bed temperature, T ) 600-800 °C; the bed density, Fbed ) 16.7 kg/m3; the inlet SO2 concentration is 1500 ppm; the inlet CO2 concentration is 10%. The relationship between the bed temperature and the desulfurization efficiency was previously discussed in detail.2 For Ca/S ) 2.0, the desulfurization efficiency with the internal structure was 74-93% and the desulfurization efficiency without

Figure 2. Effect of the internal structure on the desulfurization efficiency at various temperatures and Ca/S ratios.

the internal structure was 67-83% at 600-800 °C. For Ca/S ) 1.5, the desulfurization efficiency with the internal structure was 63-81% and the desulfurization efficiency without the internal structure was 59-71% at 600-800 °C. Therefore, the desulfurization efficiency with the internal structure was 5-10% higher compared to the desulfurization efficiency without the internal structure. The relative difference of the desulfurization efficiency was calculated to be mostly above 10%, which clearly showed the effectiveness of the internal structure on the desulfurization efficiency. It was mainly due to improved solids concentration distribution and enhanced gas-solid contact efficiency between the sorbent and the SO2, which was consistent with previous numerical simulation results.3 The fresh sorbent was fed into the CFB reactor by a screw feeder, which led to very low sorbent injection velocity. In addition, the heights from the screw feeder and the sorbent recirculating inlet to the distributor were 1.5 and 2.0 m, respectively. They were both located above the dense-phase regime in the CFB reactor and led to low horizontal diffusion of the sorbent particles.2 For the CFB reactor without internal structures, most of the sorbent particles flowed in the near-wall regions and the annular-core particle flow structure also made the sorbent particles concentrate in the near-wall regions, which resulted in decreased gas-solid contact efficiency. The solids concentration distribution was improved after installing the internal structure in the CFB reactor. It was mainly due to the acceleration effect through the reducer section and the deceleration effect through the diffuser section when the gas-solid flow passed through the internal structure. The upper and lower wall surfaces of the internal structure caused blockage, separation, and guidance effects on the particle movement, which improved the particle horizontal mixing and the destruction of the near-wall particle flow layer.17 In addition, the inertia guidance effect of the internal structure led to different improvement performances of the particle diffusion characteristics for various particle sizes. As for the rapidly hydrated sorbent, the Ca(OH)2 particles were mainly adhered on the fly ash surface. The average diameter of the fly ash particles was about 83 µm, and the Ca(OH)2 particles were mostly below 10 µm.2 The internal structure had greater inertial guidance effect on the


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Figure 4. Sulfur 2p XPS spectrum for the desulfurization products.

Figure 3. XRD results for the desulfurization products (s, CaSO4; t, CaCO3; 1, CaO; m, 3Al2O3 · 2SiO2; q, SiO2).

rapidly hydrated sorbent because the sorbent particles were larger than the fine Ca(OH)2 particles. It improved the horizontal diffusion ability of the Ca(OH)2 particles adhered on the sorbent particles and then enhanced the gas-solid contact efficiency between the Ca(OH)2 particles and the SO2. Compared with previous reported experimental results in the same CFB-FGD facility using the wind cap distributor and the double-stage cyclone separator,2 the improved CFB reactor structure with the large hole distributor, the internal structure, and the single-stage cyclone separator greatly reduced the bed solids concentration from 33.3 to 16.7 kg/m3 while maintaining the desulfurization efficiency above 90%. It provided good guidance to achieve high desulfurization efficiency and low flow resistance in the moderate temperature CFB-FGD process. 3.2. Physical and Chemical Analysis of the Desulfurization Products. Various physical and chemical analysis was conducted for the desulfurization products collected from the cyclone recirculation inlet (named as the cyclone recirculation sample) and the bag filter hopper (named as the bag filter sample). The main calcium-containing compositions in the desulfurization products were qualitatively characterized by the XRD and XPS methods. The mass ratios of the main compositions were quantitatively analyzed by the ICP-AES and TGA measurements to determine the falloff extent of the calciumcontaining fine particles. Due to the composition similarity of the desulfurization products from the CFB reactor with and without the internal structure, only the results for the CFB reactor with the internal structure were presented in this section. 3.2.1. Main Composition Determination of the Desulfurization Products. Figure 3 shows the XRD results of the cyclone recirculation sample and the bag filter sample. The calciumcontaining compositions in the cyclone recirculation sample were mainly CaSO4, and the main compositions for the bag filter sample were CaSO4, CaCO3, and CaO. The XRD method was just suitable for crystalline-phase identification and was also difficult to detect compositions with the content below 5%. Therefore, it was not appropriate to claim no CaSO3 composition

in the desulfurization products according to no CaSO3 peaks appeared in the XRD results. XPS method could analyze the valence number of the elements regardless of the sample crystallization condition. Therefore, XPS method was suitable for analyzing the sulfur valence number to confirm whether the CaSO3 composition existed in the desulfurization products. Figure 4 shows the XPS spectra for the desulfurization products. The single peak of the S 2p spectrum for the desulfurization products was concentrated at the binding energy of 168 eV. It showed just the sulfate type of sulfur existence, which confirmed that no CaSO3 exist in the desulfurization products. The reason may lie in that CaSO3 could not stably exist in the desulfurization products at the moderate temperature range of 600-800 °C. It was consistent with previous results that the desulfurization products produced by CaO at the TGA reaction conditions above 740 °C with the existence of oxygen were all CaSO4 without CaSO3.19 3.2.2. Main Composition Mass Ratios of the Desulfurization Products. ICP-AES and TGA were used to further determine the main composition contents of the desulfurization products. The total Ca and S contents in the desulfurization products were analyzed by ICP-AES method. The CaSO4 content was calculated by the S content due to no CaSO3 existence in the desulfurization products. The contents of Ca(OH)2 and CaCO3 were measured by TGA in a nitrogen atmosphere using different decomposition temperature ranges. The CaO content was calculated by subtracting the Ca contents of Ca(OH)2, CaCO3, and CaSO4 in the desulfurization products from the total Ca content. Garea et al.5 reported that the starting decomposition temperatures of Ca(OH)2 and CaCO3 were about 360 and 600 °C, respectively. Therefore, the sorbent samples were heated in TGA by a constant heating rate of 10 °C/min and then consecutively kept at 150, 450, and 850 °C for 30 min to completely decompose the corresponding chemical compositions. The weight loss below 150 °C was correlated to the water content due to the sorbent moisture absorption. The amount of the Ca(OH)2 was calculated by using the weight loss between 150 and 450 °C. The remaining component in the solid sample was considered to be fly ash. To explain the main compositions of the desulfurization products, the particle size distributions of the fresh sorbent and the desulfurization products are shown in Figure 5. The fresh sorbent had an average particle size of 88.7 µm. However, the average particle size of the cyclone recirculation sample increased slightly to 116 µm, while that of the bag filter sample decreased greatly to 8.9 µm, which demonstrated that the

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Figure 5. Particle size distributions of the fresh sorbent and the desulfurization products.

Figure 6. Main composition mass ratios of the desulfurization products (wt %).

cyclone separator had low collection efficiency for the fine particles less than 10 µm. Figure 6 shows the main composition mass ratios in the fresh rapidly hydrated sorbent and the desulfurization products obtained at the bed temperature of 730 °C for Ca/S ) 2.0 condition. The cyclone recirculation sample mainly contained CaSO4 and CaO due to the relatively large particle sizes. The large particles guaranteed high collection and recirculation efficiency in the cyclone separator and provided enough particle residence time in the bed temperature range of 700-800 °C to decompose Ca(OH)2 and CaCO3. However, the bag filter sample not only contained a high content of CaSO4 and CaO but also contained a considerable amount of CaCO3 and even some unreacted Ca(OH)2. It was mainly due to the small particle sizes for the bag filter sample. After falling off the rapidly hydrated sorbent surfaces, some fine Ca(OH)2 particles could not be effectively collected by the cyclone separator and then quickly entered into the downstream bag filter. The particle residence time of these Ca(OH)2 particles in the bed was too short to guarantee complete decomposition even though the bed temperature was 700-800 °C. Furthermore, the flue gas temperature gradually decreased due to external heat exchange along the gas flow direction. The CaO and Ca(OH)2 in these fine particles escaped from the cyclone separator and reacted with the high concentration CO2 in the flue gas, which contributed to the relatively high CaCO3 content in the bag filter sample. Therefore, it is important to prolong the particle residence time and reduce the carbonization reaction extent to reach higher calcium utilization rate for these calcium-containing fine particles. The cumulative specific surface area and pore volume distributions for the rapidly hydrated sorbent and the desulfurization products are shown in Figure 7. The pore structure

Figure 7. Pore structure characteristics for the fresh sorbent and desulfurization products (a) cumulative specific surface area; (b) cumulative pore volume.

characteristics of the rapidly hydrated sorbent were much better than the desulfurization products. The specific area and pore volume of the bag filter sample were 5.9 and 3.6 times higher than the cyclone recirculation sample. The specific area and pore volume of the cyclone recirculation sample was slightly higher than those of the original fly ash. The worsened pore structure characteristics in the CFB reactor further supported the particle abrasion of the rapidly hydrated sorbent into the larger particles of the cyclone recirculation samples and the smaller particles of the bag filter sample. The sulfation ability of the rapidly hydrated sorbent and the desulfurization products are shown in Figure 8. About 10 mg solid samples were preheated in the TGA to 750 °C. After the weight became stable, the reaction gas was introduced into the TGA with the gas flow rate of 100 mL/min for 80 min. The reaction gas consisted of SO2 (2000 ppm), O2 (5%), and N2 (balance gas). The sulfation ability of the rapidly hydrated sorbent was the highest (174.1 mg/g). The sulfation ability of the cyclone recirculation sample was just 14.7 mg/g, which was slightly higher than that of the original fly ash (2.23 mg/g). It indicated that the sulfation ability of the cyclone recirculation sample greatly decreased due to the loss of the calcium-containing fine particles and the worsened pore structure characteristics in the CFB reactor. The sulfation ability of the bag filter sample was 150.9 mg/g, which was much higher than the cyclone recirculation sample. It indicated that the reaction extent of the bag filter sample was much less than the cyclone recirculation sample, which still had good pore structure and sulfation ability. The main reason was that the particle residence time of the bag filter sample was greatly reduced without enough contact with SO2


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was conducted on the basis of the following equations. The detailed chemical reaction equations and descriptions were illustrated by Fan et al.20 The mass ratios of the cyclone recirculation sample and the bag filter sample was determined from FSBCa -

∑ X PiCa/(100 - M i

(1)

i,up)

PiCa ) PiCaCaSO4 + PiCaCaSO3 + PiCaCaO + PiCaCa(OH)2 (2) Mi,up ) (62PiCaCaSO4 + 26PiCaCaSO3 - 18PiCaCaO)/40

(3)

Figure 8. Sulfation ability for the fresh sorbent and desulfurization products.

Figure 9. Total calcium content of the desulfurization products at Ca/S ) 2.0 (wt %).

in the flue gas due to the low collection efficiency of the cyclone separator for these fine particles. 3.2.3. Calcium Balance Analysis for the Desulfurization Products. Since the bag filter sample contained a high content of effective calcium-containing compositions, such as Ca(OH)2, CaO, and CaCO3, the calcium balance analysis was adopted to determine the mass ratios of the cyclone recirculation sample and the bag filter sample in the desulfurization products. It is helpful to quantitatively analyze the fall off extent of the calcium-containing fine particles in the rapidly hydrated sorbent during the moderate-temperature CFB-FGD process. The calcium balance analysis was based on the total mass conservation principle of the calcium element in the fresh sorbent and the desulfurization products during the desulfurization process.20 The calcium content in the desulfurization products was composed of the calcium contents from the produced CaSO4, CaCO3, CaO, and the unreacted Ca(OH)2. Compared with Ca(OH)2 in the fresh sorbent, there were weight increases for the desulfurization products by chemical reactions during the moderate-temperature CFB-FGD process, which was considered in the calcium balance analysis. The calcium balance analysis

where FSBCa is the calcium mass content in the fresh rapidly hydrated sorbent (%); Xi is the mass ratios of the corresponding desulfurization products (i ) 1 represents the cyclone recirculation sample, and i ) 2 represents the bag filter sample; X1 + X2 ) 100) (%); PiCa is the sum of the calcium mass content for various compositions in the corresponding desulfurization products (%); and Mi,up is the total weight increase due to various compositions formed in the desulfurization process in the corresponding desulfurization products, such as CaSO4, CaCO3, and CaO (%). Figure 9 shows the total calcium contents (PiCa) of in the fresh sorbent and the desulfurization products, which were the sum of the calcium contents directly calculated from the corresponding main composition mass ratios in Figure 6. The calcium mass content in the fresh sorbent was 12.86%. The calcium content of the cyclone recirculation sample decreased to 5.79% and that of the bag filter sample increased to 26.27%. It verified that some calcium-containing fine particles fell off the rapidly hydrated sorbent surfaces, escaped from the cyclone separator and were finally collected in the bag filter hopper. After the calcium balance analysis, the mass ratio of the cyclone recirculation sample and the bag filter sample was obtained from eq 1 to be 71.0 and 29.0%, respectively. 3.3. Influence Mechanism of the Internal Structure on the Desulfurization Process. The main composition contents and the mass ratios of the bag filter sample in the desulfurization products were also calculated for the same experimental conditions without the internal structure. Compared with the CFB reactor without the internal structure, the total calcium content of the bag filter sample with the addition of the internal structure just decreased from 27.93 to 26.27% and the corresponding mass ratio of the bag filter sample just increased from 27.6 to 29.0%. It indicated that the addition of the internal structure did not improve the fall off extent of the calciumcontaining fine particles. In addition, the system calcium utilization rate and the sorbent calcium conversion rate are compared in Table 1 to reveal the sorbent utilization at various positions in the CFB reactor and clarify the influence mechanism of the structure retrofit on the desulfurization process. The calcium utilization rate was calculated by dividing the desulfurization efficiency (ηSO2) with the Ca/S ratio. The sorbent calcium conversion rate was calculated by dividing the total calcium content with the sulfur content, which was equal to the calcium content in CaSO4 due

Table 1. Comparison between the Sorbent Calcium Conversion Rate and the Calcium Utilization Rate (%) exptl condition

sample name

sample Ca conversion rate

sample mass ratio

without internal structure

cyclone recirculation sample bag filter sample cyclone recirculation sample bag filter sample

42.8 29.4 47.4 30.7

72.4 27.6 71.0 29.0

with internal structure

sorbent Ca conversion rate

Ca/S ratio

ηSO2

Ca utilization rate

39.1

2.0

79.2

39.6

42.6

2.0

85.6

42.8

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to the stoichiometric ratio of 1:1 between CaO and SO2. The comparison showed good agreement which demonstrated that the effect of the internal structure on the desulfurization efficiency described in section 3.1 was reliable. With the addition of the internal structure, the calcium conversion rate for the cyclone recirculation sample increased from 42.8 to 47.4%, which clarified that the effect of the internal structure on the improved desulfurization performance mainly resulted from the increased calcium conversion rate of the cyclone recirculation sample and also demonstrated that the solids concentration distribution of the sorbent particles was important for the moderate-temperature CFB-FGD process. However, the calcium conversion rate for the bag filter sample just slightly increased from 29.4 to 30.7%, which indicated that the desulfurization performance could be further improved by increasing the calcium conversion rate of the bag filter sample taking account of the considerable mass ratio (around 30%) of the bag filter sample in the desulfurization products. The calcium conversion rate for the bag filter sample not only depended on the solids concentration distribution but also relied on the particle residence time. One feasible method was to further optimize the reactor structure, such as the bed structure and the cyclone structure, to prolong the particle residence time for the fresh sorbent and the falloff calcium-containing fine particles in the moderate temperature window. Another feasible method was to develop new sorbent preparation method to reduce the fall off extent of the calcium-containing fine particles from the rapidly hydrated sorbent in CFB reactors. 4. Conclusions The moderate temperature desulfurization process was studied in a pilot-scale CFB-FGD experimental facility with the addition of the internal structure. The main conclusions were as follows. (1) The desulfurization efficiency with the internal structure at 600-800 °C was 74-93% for Ca/S ) 2.0, which were higher than the desulfurization efficiency without the internal structure of 67-83%. It was mainly due to the improved solids concentration distribution and the enhanced gas-solid contact efficiency between the sorbent and the SO2, which provided good guidance to realize high desulfurization efficiency and low flow resistance in the moderate-temperature CFB-FGD process. (2) The cyclone recirculation sample was mainly CaSO4 and CaO due to the larger particle sizes providing good cyclone separator efficiency and enough particle residence time in the moderate temperature range. The bag filter sample not only contained high contents of CaSO4 and CaO but also contained considerable amount of CaCO3 and even some unreacted Ca(OH)2. It mainly resulted from the smaller particle sizes of the bag filter sample, which was difficult to be effectively collected and decomposed in the moderate temperature range. (3) With the addition of the internal structure, the calcium conversion rate for the cyclone recirculation sample greatly increased from 42.8 to 47.4% while the calcium conversion rate for the bag filter sample just increased from 29.4 to 30.7%. It demonstrated that the main contributor for the improved desulfurization efficiency was the increased calcium conversion rate of the cyclone recirculation sample, and the solids concentration distribution of the sorbent particles was important for the moderate-temperature CFB-FGD process. (4) The calcium conversion rate for the bag filter sample not only depended on the solids concentration distribution but also relied on the particle residence time in the moderate temperature range, which indicated that prolonging the particle residence

time for the fresh sorbent and the fall off calcium-containing particles was important to further improve the desulfurization performance. Acknowledgment This research was supported by the Special Funds for Major State Basic Research Projects (Grant No. 2006CB200305). Literature Cited (1) Yani, S.; Zhang, D. K. Transformation of organic and inorganic sulphur in a lignite during pyrolysis: Influence of inherent and added inorganic matter. Proc. Combust. Inst. 2009, 32, 2083–2089. (2) Zhang, J.; You, C. F.; Qi, H. Y.; Hou, B.; Chen, C. H.; Xu, X. C. Effect of operating parameters and reactor structure on moderate temperature dry desulfurization. EnViron. Sci. Technol. 2006, 40, 4300–4305. (3) Zhang, J.; You, C. F.; Qi, H. Y.; Chen, C. H.; Xu, X. C. Effect of solids concentration distribution on the flue gas desulfurization process. EnViron. Sci. Technol. 2006, 40, 4010–4015. (4) AI-Shawabkeh, A.; Matsuda, H.; Hasatani, M. Comparative reactivity of treated FBC- and PCC-fly ash for SO2 removal. Can. J. Chem. Eng. 1995, 73 (5), 678–685. (5) Garea, A.; Ortiz, M. I.; Viguri, J. R.; Renedo, M. J.; Fernandez, J.; Irabien, J. A. Desulfurization yield of calcium hydroxide fly-ash mixtures. Thermogravimetric determination. Thermochim. Acta 1996, 286 (1), 173–185. (6) Ishizuka, T.; Tsuchiai, H.; Murayama, T.; Tanaka, T.; Hattori, H. Preparation of active absorbent for dry-type flue gas desulfurization from calcium oxide, coal fly ash, and gypsum. Ind. Eng. Chem. Res. 2000, 39, 1390–1396. (7) Renedo, M. J.; Fernandez, J. Preparation, characterization, and calcium utilization of fly ash/Ca(OH)2 sorbents for dry desulfurization at low temperature. Ind. Eng. Chem. Res. 2002, 41, 2412–2417. (8) Lin, R. B.; Shih, S. M.; Liu, C. F. Structural properties and reactivities of Ca(OH)2/fly ash sorbents for flue gas desulfurization. Ind. Eng. Chem. Res. 2003, 42, 1350–1356. (9) Hou, B.; Qi, H. Y.; You, C. F.; Zhang, Q. New calcium-based sorbent used for medium temperature FGD process. J. Eng. Thermophys. 2004, 25 (1), 159–162. (10) Shi, L. M.; Xu, X. C. Partially sulfated lime-fly ash sorbents activated by water or steam for SO2 removal at a medium temperature. Energy Fuels 2005, 19, 2335–2339. (11) Fernandez, J.; Rico, J. L.; Garcia, H.; Renedo, M. J. Development of sorbents for SO2 capture prepared by hydration of fly ash and hydrated lime in seawater. Ind. Eng. Chem. Res. 2006, 45, 856–862. (12) Zhang, J.; Zhao, S. W.; You, C. F.; Qi, H. Y.; Chen, C. H. Rapid hydration preparation of calcium-based sorbent made from lime and fly ash. Ind. Eng. Chem. Res. 2007, 46, 5340–5345. (13) Li, Y.; Song, C. X.; You, C. F. Experimental study on abrasion characteristics of rapidly hydrated sorbent for moderate temperature dry flue gas desulfurization. Energy Fuels 2010, 24, 1682–1686. (14) Basu, P.; Scott, A. F. Circulating Fluidized Bed Boilers: Design and Operations; Butterworth-Heinemann: Stoneham, MA, 1991. (15) Li, H. Z.; Guo, M. S. Particulatization of Gas-Solids Fluidization; Chemical Industry Press: Beijing, 2002. (16) Zhu, J. X.; Salah, M.; Zhou, Y. Radial and axial voidage distributions in circulating fluidized bed with ring-type internals. Jap. J. Chem. Eng. 1997, 30 (5), 928–937. (17) Wang, C. P.; Yang, Y.; Li, D. K. Experiment of optimizing gassolids flow regime in the square-section circulating suspension bed. Chin. J. Process Eng. 2003, 3, 200–205. (18) Wang, C. P.; Lv, Z. A.; Li, D. K. Experimental study on gas-solids flows in a circulating fluidised bed using electrical capacitance tomography. Powder Technol. 2008, 185 (2), 144–151. (19) Marsh, A. W.; Ulrichson, D. L. Rate and diffusional study of the reaction of calcium oxide with sulphur dioxide. Chem. Eng. Sci. 1985, 40 (3), 423–433. (20) Fan, B. G.; Xiao, Y. H.; Tian, W. D.; Qi, H. Y. An analytic method featuring calcium balance during a gas-solid calcium-based desulfurization reaction. Journal of Engineering for Thermal Energy and Power (in Chin.) 2007, 22 (1), 101–105.

ReceiVed for reView April 30, 2010 ReVised manuscript receiVed September 27, 2010 Accepted October 1, 2010 IE100988R

Effect of Internal Structure on Flue Gas Desulfurization with Rapidly

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