Energy & Fuels 2006, 20, 1901-1905
1901
Enhancement of Ca(OH)2/Fly Ash Sorbent for the Dry-Desulfurization Process Mitsuo Yamamoto,*,† Satoshi Komaki,† Daichi Nakajima,† Norihiko Matsushima,† Dan Liu,‡ Masateru Nishioka,§ and Masayoshi Sadakata| Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department of Chemical Science and Engineering, Ariake National College of Technology, 150 Higashihagio-machi, Omuta, Fukuoka 836-8585, Japan, National Institute of AdVanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan, and Department of EnVironmental Chemical Engineering, Kogakuin UniVersity, 2665-1 Nakano-Machi, Hachioji-Shi, Tokyo 192-0015, Japan ReceiVed December 29, 2005. ReVised Manuscript ReceiVed July 1, 2006
Ca(OH)2/fly ash sorbent has been studied as an effective method for SO2 removal. The effect of iron and other species for enhancing the ability of Ca(OH)2/fly ash sorbent was investigated in this study. At first, Fe(NO3)3 was added in the preparation of the sorbent, and TG analysis was carried out. The Ca utilization rate over a period of 90 min was about 10% greater than that for Ca(OH)2/fly ash sorbent. However, it was found that iron is not effective for enhancing the ability of Ca(OH)2/fly ash sorbent but that NO3- was the most effective factor to enhance it. The mechanism of enhancing the Ca utilization rate was also investigated, and it was found that Ca(NO3)2 was produced in the sorbent and reacted with SO2, so that the reaction Ca(NO3)2 + SO2 f CaSO4 + 2NO + O2 proceeded.
Introduction SO2 emission from coal combustion in China causes serious environmental problems and negatively impacts human health. Although the ratio of coal as a primary energy source will drop to 67% in 2020, the total consumption of coal will increase to 2.3 billion tons per year in 2020 from 1.2 billion tons in 2000.1 There are three types of processes for desulfurization of flue gas; wet, semi-dry, and dry desulfurization. The wet desulfurization process is very effective for SO2 removal, and gypsum (CaSO4) can be obtained as a valuable byproduct in this process. However, large volumes of water are required, and the cost of facilities for purifying wastewater is high. Semi-dry processes are also effective processes and need less water than wet processes; however, since large volumes of water are still needed to achieve high desulfurization rates, it is difficult to introduce this process to regions in China with water shortages. Thus, wet and semi-dry processes are not applicable in China because of the large amount of water needed for SO2 removal. Therefore, a dry-desulfurization process that has low cost, needs no water, and produces CaSO4 as a valuable byproduct is desirable.2 A method to minimize sulfur dioxide emissions is to react SO2 with solid calcium oxide and produce CaSO4 as a byproduct, as with the following reaction:3 * Corresponding author. Tel: +81-3-5841-7299. Fax: +81-3-5841-7276. E-mail:
[email protected]. † The University of Tokyo. ‡ Ariake National College of Technology. § National Institute of Advanced Industrial Science and Technology. | Kogakuin University. (1) Xu, Z.; Chen, C.; Qi, H.; He R.; You, C.; Xiang, G. Fuel Process. Technol. 2000, 62, 153-160. (2) Matsushima, N.; Li, Y.; Nishioka. M.; Sadakata, M. EnViron, Sci. Thecnol. 2004, 38, 6867-6874 (3) Allen, D.; Hayhurst, A. N. J. Chem. Soc., Faraday Trans. 1996, 92 (7), 1227-1238.
CaO + SO2 + 1/2O2 f CaSO4 Ca(OH)2/fly ash systems for SO2 removal have been investigated to increase desulfurization efficiency.4-6 Li et al.7 developed a new method for preparing a highly active sorbent for desulfurization at low cost. CaO particles were mixed with fly ash in water at ambient temperature, and the calcium utilization of prepared sorbent was achieved 2-3 times higher than that of just the original CaO particles. Observation of the surface structure of the sorbent by SEM (scanning electron microscopy) and EDX (energy-dispersive X-ray analysis) showed that the CaO particles were separated into small particles of Ca(OH)2 and that tiny Ca(OH)2 particles covered the surface of the fly ash particles. Li et al.8 also attempted to promote the calcium utilization rate of Ca(OH)2/fly ash sorbent and the formation of CaSO4 as a byproduct by investigating the effects of NOx, CO2, and the reaction temperature on the SOx removal process in a fluidized bed reactor. It was found that the presence of NOx enhanced the SO2 removal rate and that the negative effect of CO2 was reduced with the presence of NOx. Matsushima et al.2 utilized a circulating fluidized bed with Ca(OH)2/ fly ash sorbent to achieve a high SO2 removal rate without humidification and the production of mainly CaSO4; 83% SO2 removal efficiency was accomplished, and the byproducts produced had a high CaSO4 content. It was also found by testing in the range of 320-380 °C that the optimum reaction temperature for desulfurization is 350 °C. (4) Ho, C. S.; Shih, S. M. Ind. Eng. Chem. Res. 1992, 31, 1130-1135. (5) Davini, P. Fuel 1996, 75 (6), 713-716. (6) Sanders, J. F.; Keener, T. C. Ind. Eng. Chem. Res. 1995, 34, 302307. (7) Li, Y.; Nishioka, M.; Sadakata M. Energy Fuels 1999, 13, 10151020. (8) Li, Y.; Loh B. C.; Matsushima N.; Nishioka, M.; Sadakata, M. Energy Fuels 2002, 16, 155-160.
10.1021/ef050438q CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006
1902 Energy & Fuels, Vol. 20, No. 5, 2006
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Table 1. Chemical Composition of the Original Fly Ash [wt %] SiO2 CaO Fe2O3 Al2O3 MnO MgO TiO2 P2O5 SO3 K2O Na2O 53.9 7.34
5.62
24.0
0.06
1.62
1.24 0.53 0.31 2.74 1.13
However, the calcium utilization rate of Ca(OH)2/fly ash sorbent needs to be further improved because a higher SO2 removal rate is required for practical applications. One method for enhancing the ability of the Ca(OH)2/fly ash sorbent is to utilize the catalytic effect of metals. The effect of irons has been studied by many researchers.9-12 Yang et al.9 evaluated the catalytic effect of iron oxide (Fe2O3) on the sorption of SO2 by CaO in a chemical environment. They found that 4 wt % of Fe2O3 physically mixed with CaO approximately doubles the SO2 sorption rate at 850 °C while conversion of CaO to CaSO4 for the mixture was about 35%. Deker and Klabunde10 investigated the effect that a small amount of surface iron oxide can have on the effectiveness of nanocrystalline calcium oxide as an adsorbent for sulfur dioxide. They confirmed that iron oxide has a significant effect on the ability of CaO as an adsorbent for SO2. This study aimed to enhance the ability of Ca(OH)2/fly ash sorbent for desulfurization as developed by Li et al.7 At first, we investigated the effect of iron on the ability of Ca(OH)2/fly ash sorbent by adding Fe(NO3)2 in the preparation of the sorbent. Thermogravimetry analysis (TGA) and experimentation with a fixed bed reactor were examined to clarify the role of iron or other species in enhancing the desulfurization rate. The rates for calcium conversion from CaO to CaSO3 and CaSO4 were estimated under several conditions. Experimental Section Preparation of the Sorbent. The sorbents used in this study were prepared according to the method of Ca(OH)2/fly ash sorbent preparation developed by Li et al.7 We prepared and used five kinds of sorbents. One is a sorbent in which Fe(NO3)3 is added to the Ca(OH)2/fly ash sorbent; defined as sorbent 1 in this paper. The second is a sorbent in which HNO3 is added to the Ca(OH)2/fly ash sorbent; defined as sorbent 2. The third is a sorbent in which FeCl3 is added to the Ca(OH)2/fly ash sorbent; defined as sorbent 3. The fourth is a sorbent in which HCl is added to the Ca(OH)2/ fly ash sorbent; defined as sorbent 4. Ca(OH)2/fly ash sorbent, the fifth, was prepared from CaO and fly ash. Fe(NO3)3 is used in sorbent 1 in order to confirm the effect of iron for desulfurization. However, as the effect of NO3- is also included in sorbent 1, the effect of it is estimated by comparing sorbent 1 with sorbent 2. The effect of Fe3+ can also be understood by comparing sorbent 1 with sorbent 3. CaO was generated by calcining reagent grade Ca(OH)2 (Wako Pure Chemical Industries) at 850 °C in an oven. Fly ash was secured from the Electric Power Development Co. Ltd, Isogo Power Station, with a mean diameter of about 23 µm as determined by a laser diffraction method. The chemical composition of the fly ash was analyzed by X-ray fluorescence, as shown in Table 1. The preparation method for sorbent 1 (Fe(NO3)3 + Ca(OH)2/fly ash) is as follows. Typically, 3 g of CaO, 16 g of fly ash, and Fe(NO3)3 solution at a time were used for preparation of the sorbent. Fly ash (16 g) was added to 30 mL of water at ambient temperature. CaO calcined from Ca(OH)2 was added to the fly ash slurry with stirring. Then, 10 mL of Fe(NO3)3 solution was added. Fe(NO3)3 solution was prepared by dissolving Fe(NO3)3 in 0.1 mol/L HNO3 solution in order that the concentration of Fe was (9) Yang, R. T.; Shen, M. S.; Steiberg, M. EnViron, Sci. Technol. 1978, 12 (8), 915-918. (10) Decker, S.; Klabunde K. J. J. Am. Chem. Soc. 1996, 118, 1246512466. (11) Ma, J.; Liu, Z.; Zhu, Z. Appl. Catal. B 2003, 45, 301-309. (12) Li, K. T.; Hung, Y. C.; Ko, T. T. Appl. Catal. B 2003, 46, 541549.
Figure 1. Dependence of the reaction time on the calcium utilization rates of sorbent 1 and Ca(OH)2/fly ash sorbent.
1000 ppm. The mixed slurry was dried at 85 °C in an oven for 12 h. The dried cake was crushed manually to regain the consistency of the original fine powder. The other sorbents were prepared by the same procedure. Apparatus and Procedure. Investigations of sorbent activity were carried out in TGA (Mac Science TGDTA2020S). In this paper, the Ca utilization rate is defined as the Ca conversion rate in the sorbent for the reaction with SO2 and was calculated by the equation that follows. The ratio of produced CaSO3 and CaSO4 is assumed to be 1:1 according to the experimental result by Li et al.7 They measured the IR absorption spectra of reaction products, such as CaSO3 and CaSO4, of SO2 + O2 with Ca(OH)2/fly ash sorbent at 300 °C: Ca utilization rate [mol%] ) amount of Ca reacted with SO2 [mol] amount of Ca(OH)2 included in sorbent [mol]
× 100 )
amount of CaSO3 and CaSO4 in sorbent after reaction [mol] × 100 amount of Ca(OH)2 in sorbent before reaction [mol] The reaction gas containing 1500 ppm of SO2 gas, 8% of O2, and nitrogen gas as a balance was fed into a thermogravimetric balance and reacted with prepared sorbent particles (7 mg), which were distributed over quartz wool in a bucket to avoid any diffusion effect among particles. The concentration of each reaction gas component in the experiment followed the actual exhausted gas of a boiler for coal thermal power generation. Other typical flue gas components, such as NOx and CO2, were not included in the experimental gas in order to investigate the ability of sorbents for desulfurization purely under the condition of O2 existence. The sorbent particles in TG were heated to the required reaction temperature (350 °C) in the presence of nitrogen gas, and finally the sorbent particles were reacted with the reaction gas, as mentioned above at 350 °C. The reasons of selecting 350 °C as a reaction temperature are as follows. One is that low reaction temperature is required in the case of desulfurization out of furnace. The other is that desulfurization by using Ca(OH)2/fly ash sorbent is most effective and moderate at 350 °C.2 The calcium utilization rate was determined as the average of three experimental results taken over a period of 90 min.
Results and Discussion Figure 1 shows the experimental results of the time dependence of the Ca utilization rate by TGA for sorbent 1 and Ca(OH)2/fly ash sorbent. The Ca utilization rate of sorbent 1 was 41.8%, while that of Ca(OH)2/fly ash sorbent was 32.9%. The Ca utilization rate of sorbent 1 over a 90-min period was about 10% higher than that of Ca(OH)2/fly ash sorbent. The reason for the improvement of the Ca utilization rate was thought to be the catalytic effect of Fe. Many researchers have found a positive effect of oxide iron for SO2 removal, as mentioned in the Introduction. However, there is a possibility that NO3- is effective for enhancing the Ca utilization rate. Thus, the effect
Ca(OH)2/Fly Ash Sorbent Enhancement
Energy & Fuels, Vol. 20, No. 5, 2006 1903
Figure 2. Dependence of the HNO3 concentration in the preparation of sorbent 2 on the calcium utilization rate. Table 2. Differences of Ca Utilization Rate among Sorbent 1, Sorbent 2, and Ca(OH)2/Fly Ash Sorbent sorbent
Ca utilization rate [mol %]
sorbent 1 sorbent 2 Ca(OH)2/fly ash
41.8 43.7 32.9
Table 3. Differences of Ca Utilization Rate among Sorbent 3, Sorbent 4, and Ca(OH)2/Fly Ash Sorbent sorbent
Ca utilization rate [mol %]
sorbent 3 sorbent 4 Ca(OH)2/fly ash
34.2 35.2 34.4
of NO3- was investigated using sorbent 2 (HNO3 + Ca(OH)2/ fly ash), which was prepared by adding only 0.1 M HNO3 solution, instead of Fe(NO3)3 solution to the Ca(OH)2/fly ash sorbent. The Ca utilization rate of sorbent 2 over a 90-min period is depicted in Table 2. It was higher than that of the Ca(OH)2/ fly ash sorbent; however, it was almost the same as that of sorbent 1. This result indicated that Fe3+ is not effective for enhancing desulfurization. There is, however, a possibility that NO3- is effective for enhancing desulfurization. So in order to evaluate the effect of Fe3+, Ca utilization rates of sorbent 3 (FeCl3 + Ca(OH)2/fly ash sorbent) and sorbent 4 (HCl + Ca(OH)2/fly ash sorbent) were also examined in TGA. The amount of Fe3+ in sorbent 3 was the same as that in sorbent 1 ([Fe3+] ) 0.018 mol/L), while the amount of Cl- in sorbent 4 was the same as that in sorbent 3 ([Cl-] ) 0.054 mol/L). Table 3 shows the differences in the Ca utilization rates for 90 min among three samples, sorbent 3, sorbent 4, and the Ca(OH)2/fly ash sorbent. The Ca utilization rate of each sorbent was shown to be almost the same, about 35 mol %. It was found that not only was Cl- not effective for the improvement of Ca utilization rate but also neither was Fe3+. We ascertained that Fe, especially Fe3+, is not effective for improving the Ca utilization rate for desulfurization at 350 °C (as depicted in Table 3). The temperatures of previous works that have investigated the effect of Fe were over 500 °C, while the reaction temperature in the experimental condition of this study is relatively low (350 °C). It is considered that Fe is not effective for SO2 removal at near 350 °C. Furthermore, it was confirmed that there was no effect of low pH on desulfurization because the Ca utilization rate of sorbent 4 was the same as that of Ca(OH)2/fly ash sorbent. On the other hand, NO3- concentration in Fe(NO3)3 solution in the preparation of sorbent 1 was not the same as NO3- concentration in 0.1 M HNO3 solution in the preparation of sorbent 2. NO3concentration in Fe(NO3)3 solution in the preparation of sorbent 1 is 0.15 mol/L, while NO3- concentration in HNO3 solution in the preparation of sorbent 2 is 0.1 mol/L. Figure 2 shows that the dependence of HNO3 concentration in the preparation
Figure 3. TG/MS data of sorbent 2 (HNO3 concentration: 1.67 mol/ L, rising rate of temperature: 5 °C/min, He balance).
of sorbent 2 on the Ca utilization rate. Ca utilization rate in the case of 0.15 mol/L HNO3 concentration is expected to be the same as Ca utilization rate by using sorbent 1. Figure 2 also shows that the Ca utilization rate of the sorbent with 0.1 mol/L HNO3 added was the highest and its utilization rate was about 43%, although we expected that the Ca utilization rate would increase as HNO3 concentration increased. From the above results, it was found that NO3- is effective for enhancing the ability of Ca(OH)2/fly ash sorbent. We consider the mechanism for enhancing the ability of Ca(OH)2/ fly ash sorbent by utilizing NO3- in the next step. It was assumed that the constituent composition of Ca(OH)2/ fly ash sorbent consisted of 16 wt % Ca(OH)2, 4 wt % CaCO3, and 80 wt % fly ash. However, the composition may be altered because Ca(NO3)2 can be produced as HNO3 is added. The catalytic effect of NO for removal of SO2 has been investigated previously.5,13,14 Li et al.7 found that NO has positively enhances the Ca utilization rate of Ca(OH)2/fly ash sorbent, clarified the mechanism of the effects of NOx on the SO2 removal process, and proposed the following reactions:
NO + 1/2O2 f NO2
(1)
Ca(OH)2 + 3NO2 f Ca(NO3)2 + NO + H2O
(2)
CaCO3 + 3NO2 f Ca(NO3)2 + NO + CO2
(3)
Ca(NO3)2 + SO2 f CaSO4 + 2NO + O2
(4)
The characteristic of this mechanism is that NO in the gas phase plays an important role as a medium of the chain reaction of the desulfurization process. If Ca(NO3)2 was produced by adding HNO3 in the preparation of sorbent 2, reaction 4 occurs and SO2 could be reduced by the chain reaction (reactions 1-4). TG/MS analysis of sorbent 2 was examined to confirm the production of Ca(NO3)2 and to determine the constituents of the sorbent. Figure 3 shows the results of TG/MS analysis (Mac Science TGDTA2020S, Thermo ONIX Pro Lab) of sorbent 2. The experimental conditions were as follows. Temperature was increased until 1200 °C at 5 °C/min under He conditions. The concentration of added HNO3 was 1.67 mol/L in the preparation of sorbent 2. Three large gravity changes of TG were observed from 320 to 710 °C. When the change of gravity occurred at from 320 to 380 °C, a peak of mass 18 appeared, which was thought to be H2O produced from the thermal decomposition (13) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Ind. Eng. Chem. Res. 1995, 34, 1404-1411 (14) Tsuchiai, H.; Ishizuka, T.; Nakamura, H.; Ueno, T.; Hattori, H. Ind. Eng. Chem. Res. 1996, 35, 851-855.
1904 Energy & Fuels, Vol. 20, No. 5, 2006
Figure 4. Experimental fixed bed reactor apparatus.
of Ca(OH)2, although Ca(OH)2 was predicted to be decomposed at higher temperature. The reasons of Ca(OH)2 decomposition at lower temperature were thought to be the influence of other components and the differences between crystal structure of Ca(OH)2 in sorbent 2 and that of pure Ca(OH)2. A peak of mass 30 was detected at about 400 °C as the second large gravity change; this was thought to be NO. This result indicated that Ca(NO3)2 was produced by the addition of HNO3 in the preparation of sorbent 2 and that NO was emitted by the thermal decomposition of Ca(NO3)2. The gravity change at 500 °C was predicted to be due to the thermal decomposition of CaCO3 and that the peak of mass 44 was CO2. As a result, it was found that sorbent 2 consists of Ca(OH)2, Ca(NO3)2, CaCO3, and fly ash. Although the dependence of the concentration of HNO3 in the preparation of sorbent 2 was evaluated in Figure 2, two reasons can be considered why the Ca utilization rate in the condition of 0.1 mol/L HNO3 was the highest. One is that the Ca utilization rate in Figure 2 was calculated on the assumption that Ca(NO3)2 was not included in sorbent 2, and only Ca(OH)2 can be reacted with SO2. As sorbent 2 contained Ca(NO3)2, there is a possibility that the profile in Figure 2 is changed to some extent. However, the other important reason that can be considered is that as Ca(NO3)2 reacted with SO2, according to reaction 4, it can be predicted that more SO2 is removed as the ratio of Ca(NO3)2 is increased in sorbent 2. On the other hand, reactions 2 and 3 are difficult to proceed if a large amount of Ca(NO3)2 is contained in sorbent 2 because of the chemical equilibrium of the reactions. Hence, it was considered that the Ca utilization rate at 0.1 mol/L was the highest in Figure 2. Nevertheless, the reason for enhancing the Ca utilization rate in desulfurization is that the chain reaction noted above proceeds and that added HNO3 plays the same role as NO gases. However, further experiments are needed to confirm that this assumption is correct. We then carried out the analysis of NO in a gas phase with a plug flow reactor (PFR). Experiments by Using a PFR. As the next step, a PFR was used to detect production in gas phase from the reaction of SO2 gas with sorbent 2. Figure 4 shows the experimental PFR apparatus. Pyrex glass with i.d. 20.5 mm was used as a reactor, and 0.65 g of sample was mixed with glass beads (φ 0.5 mm). The reaction zone was 50 mm, and quartz wool was used to fix the sample. The experimental conditions were as follows: 1 L/min of reaction gas, 1500 ppm of SO2, and 8% O2 (N2 balance) at a reaction temperature of 350 °C was flowed into the reactor to achieve 1 s of residence time. The reaction and production gas were detected by a gas analyzer (HORIBA, PG250). The sample used in this experiment was sorbent 2 (1.67 M HNO3 + Ca(OH)2/fly ash). Figure 5 indicates the Ca
Yamamoto et al.
Figure 5. Differences in the Ca utilization rates between TGA and a fixed bed reactor.
Figure 6. NO and NOx produced by the reaction of sorbent 2with SO2.
utilization rates of each sample. The Ca utilization rate in the PFR experiment was defined by the following equation:
Ca utilization rate [mol %] ) amount of Ca calculated by amount of decreased SO2 concentration [mol] amount of Ca(OH)2 in sample [mol]
× 100
Figure 5 shows that the Ca utilization rate of Ca(OH)2/fly ash sorbent with PFR over a 60-min period was the same as the Ca utilization rate of Ca(OH)2/fly ash sorbent with TG over 60 min. The Ca conversion rate of sorbent 2 over a 60-min period in Figure 5 was 40%; being larger than that of Ca(OH)2/ fly ash sorbent, which was the same as the result with TG. We attempted to confirm the production of NOx in the gas phase from the reaction of SO2 with Ca(NO3)2 by detecting NO and NO2. The experimental conditions were that the composition of the reaction gas was 900 ppm of SO2 and 8% O2 in N2. The dependence of the reaction time on NO and NOx concentration is shown in Figure 6. NO and NO2 gas could be detected under this condition, and we ascertained that 70% of NOx was NO. Since Ca(NO3)2 could not be decomposed thermally at 350 °C, it was predicted that the following reaction (reaction 4) occurred under this condition:
Ca(NO3)2 + SO2 f CaSO4 + 2NO + O2 After this reaction, reaction 1 proceeds, and the chain reaction (reactions 1-4) occurs. But the next reaction may proceed instead of reaction 2 because thermal decomposition of Ca(OH)2 occurs at 320-380 °C as shown in Figure 3:
Ca(OH)2/Fly Ash Sorbent Enhancement
CaO + 3NO2 f Ca(NO3)2 + NO
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(2′)
In conclusion, it was expected that the chain reaction proceeds by adding HNO3 to Ca(OH)2/fly ash sorbent and that NO3- is considered to enhance the ability of the Ca(OH)3/fly ash sorbent. Conclusions We investigated further the method developed by Li et al.7 for enhancing the ability of Ca(OH)2/fly ash sorbent for desulfurization. Fe(NO3)3 was added in the preparation of the sorbent (sorbent 1) to evaluate the effect of iron, and TG analysis
was carried out. The Ca utilization rate of sorbent 1 over a 90min period was about 10% higher than that of Ca(OH)2/fly ash sorbent. Two reasons for the ability to enhance desulfurization were considered. One is Fe3+; the other is NO3-. Experiments in TGA showed that NO3- was the most effective factor for enhancing the desulfurization ability of Ca(OH)2/fly ash sorbent. Then, TGA and experiments with a fixed bed reactor were examined to clarify the role of NO3- for enhancing the Ca utilization rate. It was concluded that Ca(NO3)2 is produced in sorbent 2 because added NO3- reacts with Ca(OH)2 in the preparation of sorbent 2 and that a chain reaction for enhancing desulfurization proceeds. EF050438Q
