Novel Dry-Desulfurization Process Using Ca(OH)2 ... - ACS Publications

Nov 16, 2004 - A dry-desulfurization process using Ca(OH)2/fly ash sorbent and a circulating fluidized bed (CFB) was developed. Its aim was to achieve...
0 downloads 0 Views 512KB Size
Download PDF
Environ. Sci. Technol. 2004, 38, 6867-6874

Novel Dry-Desulfurization Process Using Ca(OH)2/Fly Ash Sorbent in a Circulating Fluidized Bed NORIHIKO MATSUSHIMA,* YAN LI,† MASATERU NISHIOKA,‡ AND MASAYOSHI SADAKATA Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan HAIYING QI AND XUCHANG XU Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

A dry-desulfurization process using Ca(OH)2/fly ash sorbent and a circulating fluidized bed (CFB) was developed. Its aim was to achieve high SO2 removal efficiency without humidification and production of CaSO4 as the main byproduct. The CaSO4 produced could be used to treat alkalized soil. An 83% SO2 removal rate was demonstrated, and a byproduct with a high CaSO4 content was produced through baghouse ash. These results indicated that this process could remove SO2 in flue gas with a high efficiency under dry conditions and simultaneously produce soil amendment. It was shown that NO and NO2 enhanced the SO2 removal rate markedly and that NO2 increased the amount of CaSO4 in the final product more than NO. These results confirmed that the significant effects of NO and NO2 on the SO2 removal rate were due to chain reactions that occurred under favorable conditions. The amount of baghouse ash produced increased as the reaction progressed, indicating that discharge of unreacted Ca(OH)2 from the reactor was suppressed. Hence, unreacted Ca(OH)2 had a long residence time in the CFB, resulting in a high SO2 removal rate. It was also found that 350 °C is the optimum reaction temperature for dry desulfurization in the range tested (320-380 °C).

Introduction Coal-fired power generation will continue to play a key role in power supply in the future because the estimated coal reserves are very large and the cost of coal is lower than that of other energy sources. However, SO2 emission from coal combustion causes many serious problems, such as respiratory disease in humans and acid rain (1). On the other hand, in northeastern China, the continued expansion of the desert area is becoming a very serious problem. One of the reasons for the progress of the desertification is the alkalization of soil. Alkali soil contains an excess of exchangeable sodium colloids and has soluble carbonates in the form of Na2CO3 and NaHCO3 (2, 3). Excessive Na in the exchangeable phase * Corresponding author e-mail: [email protected]; present address: Corporate Solar Energy Division, Kyocera Corporation. † Present address: Dept. of Chemical and Biological Engineering, The University of British Columbia, Canada. ‡ Present address: National Institute of Advanced Industrial Science and Technology, Japan. 10.1021/es035373p CCC: $27.50 Published on Web 11/16/2004

 2004 American Chemical Society

causes the collapse of soil particles, giving rise to soil with a dense structure and poor permeability. To reclaim alkali soils, the exchangeable Na+ needs to be replaced with Ca2+. CaSO4 is the most common supply of Ca2+ for alkali soil as it is nontoxic to plants, easy to handle, and moderately soluble. The chemical effect of CaSO4 on alkali soil is given by the following equation (4):

2NaX + CaSO4 f CaX2 + Na2SO4

(1)

where X represents the soil exchange phase. Exchange of Ca for Na in the soil complex results in flocculation of soil particles and restoration of the porous structure, causing high water permeability. According to previous research (5), addition of only 0.5 wt % CaSO4 to alkali soil is effective for alkali soil reclamation. Desulfurization byproducts containing CaSO4 and Ca(OH)2 were also shown to be effective, even though they include alkaline Ca(OH)2. The test field reclaimed from alkali soil by treatment with a desulfurization byproduct containing CaSO4 has been used for corn production for 5 yr. These results indicate that CaSO4 in desulfurization byproducts is effective for the reclamation of alkali soil. Wet and semi-dry desulfurization processes have been found to be very effective for SO2 removal. However, wet processes require large volumes of water and costly facilities for purifying wastewater, despite producing the valuable byproduct gypsum. Semi-dry processes need less water than wet processes. However, they still need large volumes of water to achieve high desulfurization rates. In many regions, water shortages are becoming a significant problem, and the use of large amounts of water for SO2 removal processes is not permitted. Therefore, a dry-desulfurization process that has a low cost, needs no water, and produces the valuable byproduct CaSO4 is desirable. If a dry process with such features could be developed, it could simultaneously eliminate both SO2 emission from power plants and the progress of desertification by using the byproduct of the desulfurization process. One of the most economical desulfurization processes is the direct limestone injection method. However, the SO2 removal efficiency of this method is very poor, because of the low calcium utilization rate and the short residence time of lime. Many studies have attempted to enhance the SO2 capturing capacity of lime (6-10). It was found that sorbent prepared by mixing fly ash with hydrated lime slurries can increase the calcium utilization rate. It is considered that the calcium silicate formed by the hydration reaction between Ca(OH)2 and fly ash leads to the enhancement of the SO2 capturing capacity. However, these methods are difficult to apply commercially because the hydration reaction generally requires a high temperature (about 100 °C) and a long reaction time (10-15 h). Li et al. (11) developed a new method for preparing low cost, highly active sorbent by mixing CaO and fly ash in water at ambient temperature with a short preparation time. The calcium utilization rate in the sorbent was found to be 2-3 times higher than that of the original CaO particles. It was also found that the mixing time of CaO and fly ash had no significant effect on the calcium utilization rate. Scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) results showed that the CaO particles were separated into small particles of Ca(OH)2. It is thought that the significant heat released by the reaction between CaO and water causes this separation and that the tiny particles of Ca(OH)2 cover the surface of the fly ash particles during the drying process. These results indicate that the significant VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6867

TABLE 1. Chemical Composition of the Original Fly Ash [wt %] SiO2 Al2O3 CaO Fe2O3 K2O3 MgO TiO2 Na2O P2O5

SO3

MnO

53.9 24.0 7.34 5.62 2.74 1.62 1.24 1.13 0.53 0.31 0.06

increase in the calcium utilization rate is mainly attributable to the tiny particles of Ca(OH)2 covering the surface of fly ash particles and that the hydration reactions were not significant in the sorbent preparation conditions at ambient temperature. In this research, the development of a novel drydesulfurization process using Ca(OH)2/fly ash sorbent prepared by Li et al.’s method (11) and a circulating fluidized bed (CFB) reactor was attempted. This was done in order to realize a high SO2 removal efficiency under dry conditions along with the production of CaSO4 for use as a soil amendment for alkalized soil. This would control both the pollution of air by SO2 and the progress of desertification.

Experimental Section Preparation of Sorbent. Ca(OH)2/fly ash sorbent was prepared from CaO and fly ash according to the method proposed by Li et al. (11). CaO was generated by calcining reagent grade Ca(OH)2 (Wako Pure Chemical Industries) at 850 °C in an oven. Fly ash was supplied by the Electric Power Development Co. Ltd, Isogo Power Station. The mean diameter of the original fly ash was 21 µm, as determined by laser diffraction. The chemical composition of the original fly ash was analyzed by X-ray fluorescence. The composition is shown in Table 1. This original fly ash was sieved. and fly ash with a mean particle diameter of about 80 µm was used to prepare the sorbent used in this study. Typically, 500 g of sorbent was prepared at a time. Fly ash (400 g) was added to 500 mL of water at ambient temperature. CaO calcined from 100 g of Ca(OH)2 was added to the fly ash slurry in small portions with stirring. The mixed slurry was dried at 85 °C in an oven for 12 h. The dried cake was crushed manually to regain the original fine powder. The sorbent size ranged from 2.6 to 150 µm, and the mean diameter was about 40 µm, as determined by laser diffraction. SEM photographs showed that the sorbent consists of coarse fly ash and fine Ca(OH)2 1-10 µm in diameter (11). Experimental Apparatus and Procedure. The dry-desulfurization experiments were carried out in the CFB reactor depicted schematically in Figure 1. The bed of the reactor was 30 mm in diameter and 1100 mm in height. A sintered Pyrex plate was used as the distributor. Two cyclones with diameters of 42 and 36 mm were designed according to a standard method (12). The bed was heated by electric furnaces. The temperature in the bed was measured automatically by three thermocouples and controlled to within (3 °C of the design temperature. A microfeeder (Hosokawa Micron GMD-60) was used to feed the sorbent. The typical gas composition was set at 1550 ppm SO2, 480 ppm NO or NO2, 8 vol % O2, and 12 vol % CO2, and the balance was N2. The gas concentration in the inlet and outlet flows was analyzed using a gas analyzer (HORIBA PG250). The amounts of particles collected by the bag filter were measured every 30 min. The particles collected by the bag filter are referred to as baghouse ash in this paper. The CFB was loaded with 80 g of fresh sorbent before the bed was heated. After stabilization of the temperature in a nitrogen atmosphere, the reactant gas was introduced into the CFB, and at the same time, the sorbent particles were fed into the CFB by the microfeeder. To fix the total amount of sorbent in the bed and establish steady-state operation, the sorbent in the bed was sometimes discharged through a drain located near the distributor as the pressure drop in the bed 6868

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004

FIGURE 1. Schematic diagram of the experimental setup. was kept at 1 or 2 kPa. The sorbent in the bed is referred to as bed sorbent in this paper. When the concentration of the exit gas was stable, it was considered that the steady-state had been achieved. Normally, it took 180 min until the concentration of the exit gas was stable. The SO2 removal rate was determined by eq 2 by using the concentration of SO2 at a steady state:

SO2 removal rate ) (CSO2,in - CSO2,out)/CSO2,in × 100 [%] (2) where CSO2,in is the concentration of SO2 at the inlet and CSO2,out is the concentration of SO2 at the outlet. The mole percent (mol %) of Ca species in baghouse ash became stable 240 min after the start of the experiment, as shown in Figure 2. Quantitative analyses of baghouse ash and bed sorbent were conducted using baghouse ash and bed sorbent sampled more than 240 min after the start of the experiment. The weight fractions of total sorbent in the bed and of Ca species such as Ca(OH)2, CaCO3, CaSO3, and CaSO4 were 0.12 and 0.03, respectively, in a typical experiment where the pressure drop was set at 1 kPa. The static bed height was 204 mm in a typical experiment with a 1 kPa pressure drop Analysis of the Reaction Products. Iodometric titration and thermogravimetric analysis (TGA) using both air and nitrogen atmospheres were used to quantify the chemical compositions of baghouse ash and bed sorbent after reaction. It was assumed that baghouse ash and bed sorbent consist of Ca(OH)2, CaCO3, CaSO3, CaSO4, and fly ash. Iodometric titration was performed by adding excess iodine solution and titrating back to the starch end point with sodium thiosulfate to determine the amount of CaSO3. TGA in an air atmosphere was used to determine the sum of the amounts of CaSO3 and CaSO4 and the amount of CaCO3. In the TGA, the temperature was kept at 500-530 °C for 30 min to oxidize CaSO3 to CaSO4. It was assumed that all CaSO3 was oxidized to CaSO4 during this 30 min. Then, the original CaSO4 and CaSO4 oxidized from CaSO3 were decomposed between 1000 and 1400 °C, and the sum of the amounts of CaSO3 and CaSO4 was calculated by using the mass loss between 1000 and 1400 °C in the TG curve (13). The amount of CaCO3 was also calculated by TGA in air by using the mass loss between 580 and 950 °C, on the assumption that all CaSO3 was oxidized to CaSO4 under 580 °C and that decomposition of CaSO3 did not occur between 580 and 950 °C. TGA in a nitrogen

FIGURE 2. Confirmation of a steady state in a typical experiment [350 °C, Ca/S 1.5, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, t 320 min, SO2 1540 ppm, NO 480 ppm, CO2 11.6%, O2 7.3%, balance N2; Ca(OH)2 (b), CaSO4 (×), CaSO3 (2)].

FIGURE 3. Effect of temperature on SO2 removal rate [Ca/S 1.7, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, SO2 1570 ppm, NO 470 ppm, CO2 11.7%, O2 7.4%, balance N2]. atmosphere was used to determine the amount of Ca(OH)2. The amount of Ca(OH)2 was calculated by using the mass loss between 350 and 550 °C. The weight fractions of Ca(OH)2, CaCO3, CaSO3, and CaSO4 in the samples could be determined from the analyses mentioned above. The remaining component was considered to be fly ash. Ca Collection Ratio by Bag Filter. The Ca collection ratio using a bag filter (CR) is defined by eq 3. This gives the ratio of Ca species collected in a bag filter to the total Ca species introduced:

CR ) sum of number of moles of Ca(OH)2, CaCO3, CaSO3, and CaSO4 in baghouse ash [mol/min] sum of number of moles of Ca(OH)2 and CaCO3 in fed sorbent [mol/min] (3) According to eq 3, when the calcium collection ratio is 1.0, all the calcium fed into the CFB is collected by the bag filter.

Results Effects of Temperature and Stoichiometric Ratio of Ca/S on SO2 Removal Rate. The effect of temperature on desulfurization was examined between 320 and 380 °C. The results presented in Figure 3 show that, in this temperature range, the SO2 removal rate was the highest at around 350 °C. The SO2 removal rate increased rapidly from 335 to 350 °C, decreased from 350 to 375 °C, and increased again with increasing temperature above 375 °C. In one experiment, the SO2 removal rate at 380 °C was almost the same as that

FIGURE 4. Effect of Ca/S on SO2 removal rate [350 °C, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, t 191-331 min, SO2 1550 ppm, NO 480 ppm, CO2 11.7%, O2 7.4%, balance N2]. at 350 °C. However, 350 °C is the optimum temperature in the range 320-380 °C because a lower reaction temperature is desirable for practical applications. Therefore, all further experiments were conducted at 350 °C. The relationship between the SO2 removal rate and the Ca/S ratio was examined at 350 °C with a superficial gas velocity of 0.75 m/s. As shown in Figure 4, the SO2 removal rate increased with an increasing Ca/S ratio, and an 83% SO2 removal rate was achieved without humidification under the optimum conditions (Ca/S 2.2, 350 °C, U0 0.75 m/s, SO2 1550 ppm, NO 480 ppm, CO2 11.7%, O2 7.4%, balance N2). Byproduct for Amendment of Alkali Soil. To determine whether CaSO4 was the main byproduct of this process, the chemical compositions of bed sorbent and baghouse ash at a steady state were investigated. The results of the quantitative analyses of (column a) fresh sorbent, (column b) bed sorbent, (column c) baghouse ash, and (column d) alkali soil amendment are shown in Figure 5. Fresh sorbent (column a) is the sorbent before the experiment. Alkali soil amendment (column d) is the byproduct from desulfurization facilities, which has already been proven effective in the treatment of alkalized soil in a field test (5). Bed sorbent and baghouse ash were sampled at the same time. From the results in Figure 5, it is evident that bed sorbent contained as much fly ash as 80.8 wt % because the fresh sorbent originally contained 78.3 wt % fly ash. However, baghouse ash only had a 31.2% fly ash content because fly ash more than 10 µm in diameter was separated by the cyclones. This separation of fly ash resulted in the increase in the ratio of CaSO4 in baghouse ash. Hence, baghouse ash contained as much as 23.9 wt % of CaSO4, as is evident from Figure 5. On the other hand, alkali soil amendment contained 20.0 wt % CaSO4. Comparison of the composition of baghouse ash with that of alkali soil amendment showed that the process reported here VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6869

constant, it alternately increased and decreased. On the other hand, in the experiment with NO2, the Ca collection ratio increased for first the 150 min and was then constant at about 0.26 after 180 min. In the experiment with only N2, the Ca collection ratio reached a constant value of about 0.2 after 180 min. These results indicate that the amount of baghouse ash increased as the reaction progressed. Clear differences were observed, particularly during the first 180 min.

FIGURE 5. Chemical compositions of fresh sorbent, bed sorbent, baghouse ash, and alkali soil amendment [350 °C, Ca/S 1.5, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, t 320 min, SO2 1540 ppm, NO 480 ppm, CO2 11.6%, O2 7.3%, balance N2]. Fresh sorbent (a), bed sorbent (b), baghouse ash (c), and alkali soil amendment (d). produced baghouse ash that could be used as a soil amendment for the treatment of alkalized soil. Increase in the Ca Collection Ratio on Reaction. The profiles of the Ca collection ratios from the start of each experiment are shown in Figure 6. The experiments were carried out with three kinds of gas compositions to examine the effect of gas composition on the Ca collection ratio. The first experiment, with only N2, was conducted in a N2 atmosphere to exclude the effects of any reactions between the sorbent and reactive gases such as SO2, CO2, NO, NO2, or O2. The second experiment, with NO, was conducted using the typical gas composition including NO. In the third experiment, with NO2, the reactant gas contained 340 ppm NO2 in place of NO in the typical gas composition. In the last two experiments, desulfurization and carbonation reactions occurred. From Figure 6, it is evident that the Ca collection ratio ranged from 0.2 to 0.8 in the experiment using the typical gas composition containing NO. This result indicated that 2080 mol % of the calcium fed into the CFB was collected as baghouse ash. However, the Ca collection ratio was not

Comparison of the Chemical Compositions of Ca Species in Baghouse Ash and Bed Sorbent at a Steady State. Ca species in baghouse ash and bed sorbent were analyzed to compare their chemical compositions at the steady state. The results obtained are shown in Figure 7. In this analysis, we focused only on Ca species such as Ca(OH)2, CaCO3, CaSO3, and CaSO4 and disregarded the amount of fly ash to compare the ratios of Ca species in the products. Therefore, the results of the quantitative analysis are given as mole percent of Ca species in Figure 7, not as weight percent as in Figure 5. It is evident from Figure 7 that baghouse ash contained less Ca(OH)2 and more CaCO3 and CaSO3 than bed sorbent. Although baghouse ash contained slightly less CaSO4 than bed sorbent, overall, it can be said that baghouse ash contained more reaction products and less unreacted Ca(OH)2 than bed sorbent. Effects of NO and NO2. The effects of NO and NO2 on the SO2 removal rate were investigated, and the results obtained are shown in Figure 8. Addition of NO or NO2 increased the SO2 removal rate by 20%. This tendency agrees well with our previous results (14). The effects of NO and NO2 on the reaction product were also investigated by a semibatch operation in the CFB. Figure 9 shows FTIR spectra of baghouse ash in the experiments with NO and NO2. The difference in the CaSO4/CaSO3 ratio of baghouse ash is evident when the result in Figure 9a is compared with that in Figure 9b. With NO, although the dominant product was CaSO4, some CaSO3 was also observed. On the other hand, with NO2, a negligible amount of CaSO3 was found in the reaction product. This indicates that more CaSO4 is obtained when NO2, rather than NO, is used in the reactant gas. In addition, the effects of NO and NO2 concentrations on the SO2 removal rate were investigated. The results shown in Figure 10 indicate that the presence of only 10 ppm of NO or NO2 can enhance the SO2 removal rate. It was also found that SO2 removal rates increased with increasing NO and NO2 concentrations in the range 0-60 ppm and that both removal rates were almost stable in the range 60-500 ppm. It was also confirmed that the SO2 removal rate was slightly

FIGURE 6. Profile of Ca collection ratios in experiments with three different gas compositions [350 °C, U0 0.5 m/s, ∆P 2 kPa, feed rate of the sorbent 0.5 g/min]. With NO (2): SO2 1560 ppm, NO 540 ppm, CO2 11.9%, O2 7.7%, balance N2. With NO2 (×): SO2 1440 ppm, NO 100 ppm, NO2 330 ppm, CO2 12.5%, O2 7.5%, balance N2. N2 only (O): N2 atmosphere in the absence of SO2, NO2, NO, CO2, and O2. 6870

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004

FIGURE 7. Difference between the compositions of bed sorbent and baghouse ash at a steady state [350 °C, Ca/S 1.5, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, t 320 min, SO2 1540 ppm, NO 480 ppm, CO2 11.6%, O2 7.3%, balance N2].

FIGURE 9. FTIR spectra of baghouse ash after 150 min of desulfurization in the CFB [350 °C, SO2 1500 ppm, NO 1000 ppm or NO2 800 ppm, CO2 12%, O2 8%, balance N2]. With NO (a); with NO2 (b). Experiments were conducted in semibatch operation in the CFB.

FIGURE 8. Effects of NO and NO2 on SO2 removal rate [U0 0.75 m/s, 350 °C, τ 1.5 s, ∆P 1 kPa, SO2 1500 ppm, NO 480 ppm or NO2 480 ppm, or NO and NO2 0 ppm, CO2 12%, O2 7.5%, balance N2]. With NO (O), with NO2 ([), without NO and NO2 (2). higher with NO2 than with NO when the concentrations of NO and NO2 were less than 300 ppm and that it was almost identical for NO and NO2 when their concentrations were about 480 ppm.

Discussion High SO2 Removal Rate without Humidification and Production of Amendment for Alkali Soil. As shown in Figure 4, the dry-desulfurization process proposed in this study achieved an 83% SO2 removal rate without any humidification under optimum conditions (Ca/S 2.2, 350 °C, U0 0.75 m/s, SO2 1550 ppm, NO 480 ppm, CO2 11.7%, O2 7.4%, balance N2). It can also be seen from Figure 5 that baghouse ash (column c) included more than 20 wt % CaSO4. The byproduct with 20 wt % CaSO4 that was obtained from the desulfurization facility (alkali soil amendment (column d)) had previously been shown to be effective for the treatment of alkalized soil (5). Therefore, it was concluded that the process presented here also produced a soil amendment that could be used to treat alkalized soil, in the form of baghouse ash. Thus, this novel dry-desulfurization process realizes both high SO2 removal efficiency under dry conditions and production of a byproduct with a high CaSO4 content for use as an amendment to treat alkalized soil. The high SO2 removal rate is considered to occur for the following reasons: First, use of a highly active sorbent. Second, a long residence time for the sorbent was obtained by using a CFB. Third, reactive fine Ca(OH)2 particles 1-10 µm in diameter could be fluidized smoothly. Last, these fine Ca(OH)2 particles have a long residence time in the CFB.

FIGURE 10. Effects of concentrations of NO and NO2 on SO2 removal [Ca/S 1.8, 350 °C, U0 0.75 m/s, τ 1.5 s, ∆P 1 kPa, SO2 1570 ppm, CO2 12.5%, O2 7.5%, balance N2]. With NO (O); with NO2 (b). The desulfurization activity can be increased by using smaller Ca(OH)2 particles. However, normal fluidization is extremely difficult for fine particles classified as group C powders according to Geldart’s classification (15). The prepared sorbent consists of coarse fly ash with a mean diameter of 80 µm and fine Ca(OH)2 1-10 µm in diameter. These fine Ca(OH)2 particles are classified as group C powders. Hence, they cannot be fluidized by themselves despite their high reactivity. However, the total sorbent is classified as a group A powder according to Geldart’s classification and exhibits good fluidity because the fine Ca(OH)2 particles are dispersed on the surface of the coarse fly ash used as a support. Thus, the prepared sorbent has two useful characteristics; the high desulfurization activity of fine particles and good fluidity. In addition, these fine Ca(OH)2 particles, which have small terminal velocities and cannot in theory be separated by cyclones, are not able to be discharged from the CFB before reaction. As is evident in Figure 6, the amount of baghouse ash increased as the reaction progressed. Moreover, it is clear from the result presented in Figure 7 that the amount of Ca(OH)2 in baghouse ash was less than that in bed sorbent and that more reaction products such as CaCO3 and CaSO3 were present in baghouse ash than in bed sorbent. These two results indicate that the sorbent is discharged more VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6871

FIGURE 11. Possible models for the increase in baghouse ash on reaction. easily after reaction. Namely, unreacted Ca(OH)2 remained in the CFB and reaction products were discharged more easily from the CFB. This characteristic promotes the long residence time of unreacted Ca(OH)2 in the CFB, resulting in the high SO2 removal rate. The production of a byproduct high in CaSO4 results from the separation of the fly ash from the sorbent by the cyclones. As mentioned above, the sorbent was prepared using fly ash with a mean diameter of 80 µm. Cyclones can remove particles more than 10 µm in diameter from exhaust gas. Therefore, in this study, about 60 wt % of fly ash in the sorbent could be separated by the cyclones and returned to the bed, which increased the ratio of CaSO4 in the baghouse ash. Increase in the Amount of Baghouse Ash on Reaction. The results presented in Figures 6 and 7 show that the amount of baghouse ash is increased on reaction. Two possible models, as illustrated in Figure 11, are proposed to explain the increase in the amount of baghouse ash on reaction. In model A, the surface of the fly ash is covered by the very fine Ca(OH)2 particles in the sorbent. In the CFB, attrition of the fine particles on the surface of the fly ash is assumed to occur due to constant collision and abrasion between particles. Reaction products such as CaSO3, CaSO4, and CaCO3 detach more rapidly than Ca(OH)2 because of the expansion of their molar volume on reaction. These fine, reacted particles detach from the coarse fly ash and can be discharged from the CFB due to their small terminal velocity. In model B, before any reaction takes place, a large amount of the sorbent in the CFB exists as agglomerates of single sorbent particles (11). Agglomerates are formed by bridge-building and by weak forces between sorbent particles. Because CaSO3, CaSO4, and CaCO3 have larger molar volumes than Ca(OH)2, reaction of Ca(OH)2 expands the bridges built between sorbent particles and makes the agglomerates friable, which encourages agglomerates to split into single sorbent particles. When the agglomerates are split, some tiny sorbent particles included in the agglomerates are released into the exhaust gas, resulting in an increased amount of baghouse ash. Effect of Temperature on SO2 Removal Rate. As shown in Figure 3, the SO2 removal rate increased rapidly from 335 to 350 °C, decreased from 350 to 375 °C, and increased again with increasing temperature above 375 °C. This tendency 6872

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004

can be explained as follows. Generally, the rate of Ca(OH)2 conversion in the reaction between Ca(OH)2 and SO2increases with increasing temperature. Hence, the SO2 removal rate increased with temperature in the range 320-350 °C. Moreover, according to previous research, the rate of Ca(OH)2 conversion in the reaction between Ca(OH)2 and SO2 increases dramatically at around the dehydration temperature because the dehydration reaction produces a more porous structure inside the Ca(OH)2 particles (16). Dehydration of Ca(OH)2 started at around 350 °C under the dry conditions used in this experiment. Therefore, the SO2 removal rate increased rapidly from 335 to 350 °C. In the range 350-375 °C, the SO2 removal rate decreased. This decrease is considered to be a result of the main absorbent being converted from Ca(OH)2 to CaO by the dehydration of Ca(OH)2. Previous studies have reported that the rate of the reaction between CaO and SO2 is lower than that between Ca(OH)2 and SO2 (17, 18). Hence, the SO2 removal rate decreased in the range 350-375 °C due to the conversion of Ca(OH)2 to CaO. Above 375 °C, the SO2 removal rate increased again as the rate of the reaction between CaO and SO2 increases with increasing temperature. The mechanism presented above is one possible mechanism. Previous studies on the reactivity of CaO with SO2 have reached differing conclusions. Fernandez et al. (17) and Bortz et al. (18) reported that the rate of reaction of CaO with SO2 was lower than that of Ca(OH)2 with SO2. In contrast, Khang et al. (16) reported that CaO formed by the dehydration of Ca(OH)2 was as active as Ca(OH)2. More research is needed to clarify the reason for this temperature dependence of the SO2 removal rate. Effects of NO, NO2, and CO2 on the Dry-Desulfurization Reaction of Ca(OH)2. The reaction route of the sorbent was investigated in our previous research (14). It was found that CO2 had a negative effect on the desulfurization activity of the sorbent and that the presence of NO or NO2 reduced this negative effect. According to the previous research, these reactions can be outlined as follows: k1

Ca(OH)2 + SO2 98 CaSO3 + H2O k2

Ca(OH)2 + CO2 98 CaCO3 + H2O

(4) (5)

k3

CaCO3 + SO2 98 CaSO3 + CO2

(6)

NO + 1/2O2 98 NO2

(7)

Ca(OH)2 + 3NO2 98 Ca(NO3)2 + NO + H2O

(8)

CaCO3 + 3NO2 98 Ca(NO3)2 + NO + CO2

(9)

Ca(NO3)2 + SO2 98 CaSO4 + 2NO + O2

(10)

When the reactant gas contains both SO2 and CO2, reactions 4 and 5 occur simultaneously. The rate of the reaction between CaCO3 and SO2 is smaller than that of the reaction between Ca(OH)2 and SO2 (14). Therefore, the presence of CO2 results in partial conversion of Ca(OH)2 into the less reactive CaCO3 and causes a decrease in the overall desulfurization activity. However, when NO or NO2 are included in the reactant gas, the CaCO3 formed can react with NO or NO2 to form Ca(NO3)2 as shown in reaction 9, and then Ca(NO3)2 reacts with SO2 to form CaSO4 as shown in reaction 10. This explains why the negative effect of CO2 on SO2 removal was reduced when NO or NO2 were present. In the present study, we also confirmed the strong effects of NO and NO2 on the SO2 removal rate, as shown in Figure 8. The above reaction scheme indicates that the chain reactions caused by NO and NO2 occur through reactions 7-10. NO in the reactant gas reacts with O2 to form NO2 (reaction 7). The NO2 formed then reacts with Ca(OH)2 or CaCO3 to form Ca(NO3)2 (reactions 8 and 9), and this Ca(NO3)2 then reacts with SO2 to form CaSO4 (reaction 10). In reactions 8-10, NO is released. Released NO can react again with O2 to form NO2 according to reaction 7, and then reactions 8-10 are repeated to form even more NO. By this reaction route, NO and NO2 can function cyclically as gaseous catalysts. When the SO2 removal rate is enhanced according to this reaction route, the stoichiometric ratio of NO or NO2 to absorbed SO2 should be less than 1. The results shown in Figure 10 indicate that just 10 ppm of NO or NO2 was able to enhance the absorption of SO2 by about 300 ppm. Here, the stoichiometric ratio of NO or NO2 to absorbed SO2 is 1/30. This means that one molecule of NO or NO2 functioned cyclically as a gaseous catalyst 30 times. This result confirms the existence of the chain reactions involving NO and NO2. We have already reported the effects of NO and NO2 on the SO2 removal rate and the selectivity of CaSO4 in the final product (14). However, we did not previously compare enhancement of the SO2 removal rate and selectivity of CaSO4 in the final product between NO and NO2. Enhancement of these two factors by NO2 was examined in this study. Although the SO2 removal rate was slightly higher with NO2 than with NO when the concentration was less than 300 ppm, not much difference could be observed in the SO2 removal rate between NO and NO2 in Figure 10. However, a clear difference was found in the chemical compositions of the byproducts. According to the results presented in Figure 9, a greater amount of the valuable byproduct CaSO4 can be produced when the reactant gas contains NO2 rather than NO. It seems that oxidization of CaSO3 to CaSO4 is encouraged by the presence of NO2. This indicates that more CaSO4 can be produced when NO in the reactant gas is converted to NO2. According to previous studies, a solid surface such as fly ash, Al2O3, or zeolite helps convert NO to NO2. When those methods are combined with this process, greater amounts of valuable byproducts can be produced. Periodic Behavior of the Amount of Baghouse Ash. As shown in Figure 6, the amount of baghouse ash obtained when the reactant gas contained NO was not constant. Instead, it periodically increased and decreased. Although the mechanism for this behavior is not clear, the following

explanation is suggested. When NO is included in the reactant gas, NO is absorbed onto the surface of Ca(OH)2 or CaCO3 before NO is converted to NO2 by reaction 7. The concentration of NO on the solid surface increases with time due to absorption. When the concentration of NO on the surface reaches a certain level, reactions 7-10 suddenly occur, because the local concentration of NO on the surface is high. Therefore, formation of CaSO4 occurs rapidly as shown in reactions 7-10, along with rapid expansion of the sorbent, resulting in a sudden increase in baghouse ash. After the sudden increase in baghouse ash, its production decreases until the local concentration of NO on the surface of Ca(OH)2 or CaCO3 once again reaches a high concentration. Consequently, the amount of baghouse ash produced periodically increased and decreased when the reactant gas contained NO. On the other hand, when the reactant gas contained NO2, reactions 8-10 could occur without reaction 7. Hence, the amount of baghouse ash produced was constant after 180 min.

Acknowledgments This research was supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology (JST) Corporation. The authors appreciate their Grant-in-Aid in support of this research. The authors thank Mr. Nakajima, Mr. Seta, and Mr. Komaki for their help with the experimental work.

Nomenclature CSO2,in

concentration of SO2 at the CFB inlet [ppm]

CSO2,out concentration of SO2 at the CFB outlet [ppm] Ca/S

stoichiometric ratio of Ca to S

U0

superficial gas velocity [m/s]

τ

(height of reactor [m]/superficial gas velocity[m/ s]); mean residence time of gas [s]

t

(total amount of sorbent in the CFB[g]/feed rate of sorbent[g/min]); mean residence time of solid [min]

∆P

pressure drop in bed [kPa]

CR

collection ratio of Ca by a bag filter according to eq 3

Literature Cited (1) Xu, X.; Chen, C.; Qi, H.; He, R.; You, C.; Xiang, G. Development of coal combustion pollution control for SO2 and NOx in China. Fuel Process. Technol. 2000, 62, 153. (2) Suarez, D. L.; Rhoades, J. D.; Lavado, R.; Griee, C. M. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J. 1984, 48, 50. (3) Gupta, R. J.; Dick, W. A. Salt-affected soils: their reclamation and management for crop production. Adv. Soil Sci. 1990, 11, 223. (4) U.S. Salinity Laboratory Staff. Diagnosis and Improvement of Saline and Alkali Soils; U.S. Department of Agriculture Handbook 60; U.S. Gobvernment Printing Office: Washington, DC, 1954. (5) Sakai, Y.; Matsumoto, S.; Nitta, Y.; Sadakata, M. Alkali soil reclamation in China using gypsum produced in flue gas desulfurization process; a case study. J. Global Environ. Eng. 2002, 8, 55. (6) Jozewicz, W.; Chang, C. S. J.; Brna, G. T.; Sedman, B. C. Reactivation of solids from furnace injection of limestone for SO2 control. Environ. Sci. Technol. 1987, 21, 664. (7) Ho, C. S.; Shih, S. M. Ca(OH)2/fly ash sorbents for SO2 removal. Ind. Eng. Chem. Res. 1992, 31, 1130. VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

6873

(8) Sanders, J. F.; Keener, T. C.; Wang, J. Heated fly ash/hydrated lime slurries for SO2 removal in spray dryer absorbers. Ind. Eng. Chem. Res. 1995, 34, 302. (9) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H. Kita, H. Highly active absorbent for SO2 removal prepared from coal fly ash. Ind. Eng. Chem. Res. 1995, 34, 1404. (10) Davini, P. Investigation of the SO2 adsorption properties of Ca(OH)2-fly ash systems. Fuel 1996, 75, 713. (11) Li, Y.; Nishioka, M.; Sadakata, M. High calcium utilization and gypsum formation for dry desulfurization process. Energy Fuels 1999, 13, 1015. (12) The Society of Chemical Engineers, Japan. Kagaku Kougaku Binran, 6th ed.; Maruzen: Tokyo, 1999 (Japanese). (13) 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, 173. (14) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Chain reaction mechanism by NOx in SO2 removal process. Energy Fuels 2002, 16, 155.

6874

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004

(15) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Stoneham, MA, 1991. (16) Khang, S. J., Keener, T. C., Wang, A., Wang, Z., Toole-O’Neil, B., Eds. Dry Scrubbing Technologies for Flue Gas Desulfurization; Kluwer Academic Publishers: Boston, 1998; p 529. (17) Fernandez, I.; Garea, A.; Irabien, A. SO2 reaction with Ca(OH)2 at medium temperatures (300-450 °C): Kinetic behavior. Chem. Eng. Sci. 1998, 10, 1869. (18) Bortz, S. J.; Roman, V.; Offen, G. R. Hydrate and process parameters controlling SO2 removal during hydroxide injection near 1000°F. Proceedings First Combined FGD and Dry SO2 Control Symposium; EPA/EPRI: St. Louis, MO, October 25-28, 1988.

Received for review December 9, 2003. Revised manuscript received September 11, 2004. Accepted September 14, 2004. ES035373P