Energy Fuels 2010, 24, 2900–2909 Published on Web 05/04/2010
: DOI:10.1021/ef100161q
Characteristics of Gas-Phase Partial Oxidation of Nascent Tar from the Rapid Pyrolysis of Cedar Sawdust at 700-800 °C Sou Hosokai,*,† Kazuya Kishimoto,† Koyo Norinaga,‡ Chu-Zhu Li,§ and Jun-ichiro Hayashi‡ † Center for Advanced Research of Energy Conversion Materials, Hokkaido University, N13 W8, Kita-ku, Sapporo 080 8628, Japan, ‡Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816 8580, Japan, and §Curtin Center for Advanced Energy Science and Engineering, Curtin University of Technology, 1 Turner Avenue, Technology Park, WA 6102, GPO Box U1987, Perth, Western Australia 6845, Australia
Received February 9, 2010. Revised Manuscript Received April 17, 2010
Volatiles from the rapid pyrolysis of cedar sawdust were subjected in situ to gas-phase thermal cracking in the presence and absence of O2. It was found that O2 influenced the resulting product distribution in completely different manners at 700 and 800 °C. O2 was consumed at 700 °C mainly by the oxidation of tar and light oxygenates, forming CO, CO2, and H2O, unless the ratio of O2/carbon involved in the nascent volatiles (O/C) exceeded 0.7 mol of O/mol of C. On the other hand, the oxidation at 800 °C consumed H2 and lower hydrocarbons selectively, leaving the residual tar yield nearly unchanged. Such different behaviors of O2 were mainly due to the difference in characteristics of thermal cracking of the tar and light oxygenates between 700 and 800 °C. The thermal cracking at 700 °C left the tar with a molecular mass (MM) range up to >1000, which was decomposed in the presence of O2 but incompletely. At 800 °C, the tar was decomposed quickly even in the presence of O2, forming refractory aromatics with a MM range up to 400 together with soot, while light gases were oxidized. A portion of the tar from the cracking at 700 °C was deposited inside a ceramic-fiber filter downstream of the reactor, the temperature of which was 350-500 °C, and the deposit could not be removed mechanically. The entire part of the tar from the cracking at 800 °C was allowed to pass through the filter regardless of O/C. It was thus found that the gasphase temperature of 700-800 °C was critical to the tar property relevant to dust removal at 350-500 °C.
partial combustion, removal of inorganic dust (including alkali salts) from the fuel gas, and combustion of the fuel gas. In the case of dust removal with a filter made of ceramic fibers, the filter temperature must be lower than 500 °C, because otherwise the filter material undergoes irreversible chemical reactions with the alkali species.10 Accumulation of molten alkali salts leads to clogging of the filter. It is, on the other hand, better to maintain the filter temperature as high as possible for avoiding condensation of tarry material, which can cause clogging of the filter and also enhance it playing a role of binder of inorganic dust and soot.9,11 There is no need of either partial or complete elimination of tar in the gasification, but there is need of controlling the composition of tar, so that it can entirely pass through the filter without condensation. There is, thus, a range of temperatures and/or air ratios of the gasification in which minimization of the air ratio and no tar deposition in/on the filter are achieved simultaneously. At temperatures relevant to the gasification, it is believed that tar formed by the primary
1. Introduction A sequence of combustion, steam generation, and power generation with a steam turbine is the simplest way for power generation from biomass resources,1 and it is therefore more convenient than systems such as gasification combined cycles, particularly when the system is required to accept feedstock consisting of a variety of biomass fuels and municipal wastes, as well as unstable composition of the feedstock. However, even such a simple system will suffer from problems caused by alkali-containing species that are inevitably involved in the feedstock.2-9 The alkali species cause corrosion/erosion of heat-transfer tubes for generating steam or, otherwise, enhance accumulation of inorganic solid over them playing a role of binder.8,9 A reasonable way to avoid the above-mentioned events is to apply a sequence of gasification of the feedstock mainly by *To whom correspondence should be addressed. Telephone: þ81-11716-6849. Fax: þ81-11-726-0731. E-mail:
[email protected]. € Andries, J.; Hein, K. R. G.; Spliethoff, H. (1) de Jong, W.; Unal, O.; Biomass Bioenergy 2003, 25, 59–83. (2) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860–870. (3) De Bari, I.; Barisano, D.; Cardinale, M.; Matera, D.; Nanna, F.; Viggiano, D. Energy Fuels 2000, 14, 889–898. (4) Salo, K.; Mojtahedi, W. Biomass Bioenergy 1998, 15, 263–267. (5) Okuno, T.; Sonoyama, N.; Hayashi, J.-i.; Li, C.-Z.; Sathe, C.; Chiba, T. Energy Fuels 2005, 19, 2164–2171. (6) Wei, X.; Schnell, U.; Hein, K. R. G. Fuel 2005, 84, 841–848. (7) Keown, D. M.; Hayashi, J.-i.; Li, C.-Z. Fuel 2008, 87, 1187–1194. (8) Frandsen, F. J. Energy Fuels 2009, 23, 3343–3378. (9) Khana, A. A.; de Jonga, W.; Jansensb, P. J.; Spliethoff, H. Fuel Process. Technol. 2009, 90, 21–50. r 2010 American Chemical Society
€ (10) Brus, E.; Ohman, M.; Nordin, A. Energy Fuels 2005, 19, 825–832. (11) Brown, M. D.; Baker, E. G.; Mudge, L. K. Biomass 1986, 11, 255–270. (12) Morf, P.; Hasler, P.; Nussbaumer, T. Fuel 2002, 81, 843–853. (13) Narvaez, I.; Orı´ o, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110–2120. (14) Cao, Y.; Wang, Y.; Riley, J. T.; Pan, W.-P. Fuel Process. Technol. 2006, 87, 343–353. (15) Wang, Y.; Yoshikawa, K.; Namioka, T.; Hashimoto, Y. Fuel Process. Technol. 2007, 88, 243–250. (16) Padban, N.; Wang, W.; Ye, Z.; Bjerle, I.; Obenbrand, I. Energy Fuels 2000, 14, 603–611.
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Energy Fuels 2010, 24, 2900–2909
: DOI:10.1021/ef100161q
Hosokai et al.
Figure 1. Schematic diagram of TSR-1.
Figure 2. Conversion of O2 as a function of its feeding rate, which is normalized by that of volatiles for Tr = 700 °C.
Figure 3. Char yield for Tr = 700 °C as a function of the O/C ratio.
conversion, i.e., pyrolysis, undergoing thermal cracking and partial combustion simultaneously. There have been studies on the gas-phase conversion of tar from the biomass pyrolysis,12-16 but little is known about combined effects of the temperature and air ratio on the detailed composition and property of tar, in particular, those relevant to condensability. In the present study, nascent volatiles from the rapid pyrolysis of a type of biomass were subjected in situ to gasphase thermal cracking and oxidation at 700-800 °C and the investigation was made on the combined temperature/air ratio effects on the product distribution, composition of tar, and propensity of tar for condensation at 350-500 °C.
2. Experimental Section 2.1. Biomass Sample. As-received sawdust of a type of Japanese cedar was milled and sieved for collecting a fraction with sizes in the range of 350-590 μm. The fraction was further dried in air at 105 °C for 6 h and then used as the biomass fuel. The elemental compositions were C, 50.2 wt % [dry, ash free (daf)]; H, 6.3 wt % (daf); S, 0.6. The recovery for runs with Tr = 700 °C and O/C < 0.6 was lower because of the fact that light oxygenates, except for acetaldehyde, were not quantified. The yield of the light oxygenates was determined by difference.
Figure 7. Yields of CO, CO2, and H2O as a function of O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
Tr = 700 °C or 0.33 s at Tr = 800 °C, respectively. The main purpose of using this reactor was to know the product distribution with a minimized extent of cracking of the nascent volatiles in the absence of O2. It was confirmed in preliminary experiments that both TSR-1 and TSR-2 gave char yields equivalent to each other. 2.4. Product Analyses. The tar collected with the thimble filter is hereafter referred to as “heavy tar”. It was dissolved in tetrahydrofuran (THF) and weighed after rotary evaporation of THF and vacuum drying of the evaporation residue at ambient temperature. The heavy tar was redissolved in THF and analyzed by means of liquid chromatography and mass spectrometry. The liquid chromatography was performed in a gel permeation mode at 20 °C using two columns (Shodex KF801 and KF802.5, Showa Denko K.K.) in series and THF as the mobile phase. The mass spectra of the heavy tar were obtained with a time-of-flight (TOF) mass spectrometer (Voyager DE, Applied Biosystems, Inc.). The tar, which was condensed in the cold trap, was dissolved in methanol and analyzed with a flame ionization detector (FID)-equipped gas chromatograph (GC) for quantifying the aromatic compounds ranging from mono- to triaromatics, which are defined as the components of “light tar”. After the GC analysis, the methanol solution was subjected to rotary
3. Results and Discussion 3.1. Thermal Cracking. Figure 4 shows the combined effects of τ and Tr on the product distribution from the cracking at O/C = 0. The liquid yield means the sum of the heavy tar, light tar, and light oxygenate. It is seen that employing higher temperatures and longer τ decreases the tar yield from 58 to 20% while increasing the gas yield. It is also noted that as much as 20% of the tar remains even after the cracking at 800 °C with τ = 4.0 s. Figure 5 compares mass spectra of the heavy tars from the cracking under different combinations of τ and Tr. The 700 °C cracking seems to be ineffective for decomposition of high mass components of the heavy tar (molecular mass > 500), which is, on the other hand, eliminated by the cracking at 800 °C. The heavy tar yields at 700 and 800 °C were 15 and 12%, respectively, at τ = 4.0 s. The cracking conditions thus 2903
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: DOI:10.1021/ef100161q
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Figure 8. Yields of H2 and C1-C2 hydrocarbons as a function of O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
Figure 9. Yields of C3-C4 hydrocarbons and acetaldehyde as a function of O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
influenced the molecular mass range of the heavy tar more significantly than its yield. 3.2. Oxidation at Tr = 700 °C. Figure 6 shows the yields of heavy tar, light tar, and light oxygenates in a cumulative manner as a function of O/C at Tr = 700 °C. The yields of heavy tar and light oxygenates decrease linearly with O/C up to 0.6-0.7. Decomposition of the light oxygenates is
completed within this range of O/C. On the other hand, the light tar yield remains around 7%. Changes in the yields of gaseous products are exhibited in Figures 7-9. Increasing O/C up to 0.6 causes net increases in CO, CO2, and H2O yields, while those of H2, CH4, and C2H4 remain steady. It is also seen in Figure 9 that the yields of C3-C4 hydrocarbons and acetaldehyde decrease considerably at O/C = 0-0.6. 2904
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CO, CO2, and H2O but not incorporated into the organics. From the formation of H2O, which is 0.49 mol of H2O/mol of O consumed, and in addition, from the total conversion of the heavy tar, light oxygenates, and lower hydrocarbons, which is 0.48 mol of C/mol of O consumed, it is estimated that the overall H/C atomic ratio of the volatiles decomposed by the oxidation was about 2:1. This ratio was clearly higher than that of the volatiles at O/C = 0, i.e., 1.56. Thus, the oxidation at O/C = 0-0.6 converted hydrogen-rich components of the heavy tar and light oxygenates selectively to hydrogen-poor components. As shown in Figure 6, increasing the O/C ratio beyond 0.7 decreases not only the heavy tar but also light tar yields but with much less efficiencies compared to that for the heavy tar at lower O/C. As seen in Figure 8, the yields of H2 and C1-C2 hydrocarbons decrease at O/C > 0.6. The heavy and light tars remaining after the oxidation with O/C of 0.6-0.7 were as reactive with O2 as H2 and C1-C2 hydrocarbons or even less reactive. Figure 10 presents the mass spectra of the tars from the oxidation at Tr = 700 °C with different O/C. It was found that high mass components with a molecular mass of >500 were hardly eliminated by the oxidation with O/C lower than 0.6, which is represented by the spectrum of the tar with O/C = 0.46. It seemed that such high mass components finally became less abundant by applying O/C higher than 0.7. The difficulty in elimination of the high mass components of the heavy tar was supported by the results from the liquid chromatography. Figure 11 shows chromatograms of the heavy tars at different O/C for two different wavelengths of UV light at 300 and 400 nm. The chromatograms are indicated on the basis of the same unit, i.e., absorbance per fuel mass. PS4000, PS800, and PAH indicate polystyrenes with average molecular masses of 4000 and 800 and polyaromatic hydrocarbons without oxygen-containing substituents, respectively. The chromatography was performed under a general size-exclusion mode, and it was therefore believed that earlier elution corresponded to compounds with higher molecular mass and/or polarity because
According to the data shown in Figures 7-9, it is reasonable to assume that the changes in the individual product yields are linear with O/C at 0-0.6. The effects of increasing O/C on the net changes in the individual yields are quantified, as listed in Table 1. This table indicates that consumption of 1 mol of O (=0.5 mol of O2) results in a decrease of the heavy tar by 0.27 mol of C. The total reduction of carboncontaining volatiles, except CO and CO2, 0.48 mol of C/mol of O, agrees well with the total increase in the CO and CO2 yields, 0.47 mol of C/mol of O. The fate of O2 can be quantified on the basis of the information given in Table 1. As seen in Table 2, consumption of 1 mol of O at O/C = 0-0.6 results in increases in CO, CO2, and H2O by 0.32, 0.30, and 0.49 mol of O (total = 1.10 mol of O), respectively, while converting 0.10 mol of O of the heavy tar and light oxygenates. Thus, O2 was converted into Table 1 product
increase or decrease in yield (mol of C/mol of consumed O)
CO CO2 subtotal C3H6 C3H8 C4 CH3CHO light liquid heavy tar subtotal
0.323 0.151 0.474 -0.021 -0.0004 -0.0041 -0.001 -0.137 -0.267 -0.479
Table 2 reactant
mol of O/mol of O consumed
O2 (consumed by oxidation) heavy tar and light oxygenates
1.00 0.11
product
mol of O/mol of O consumed
CO2 CO H2O
0.32 0.30 0.49
Figure 10. Mass spectra of the tars from the oxidation with different O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
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Figure 11. Liquid chromatograms of the heavy tars with different O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
Figure 13. Effect of the setting of the CF filter on the liquid chromatogram of heavy tar at Tr = 700 °C and O/C = 0.
Figure 12. Changes in the yields of soot-f and soot-r with O/C. A ceramic fiber filter was set at the reactor bottom in every run.
of oxygen-containing functionalities, except for PAHs of that elution controlled by adsorption rather than the size exclusion.17,18 It is seen that the oxidation with O/C < 0.6
Figure 14. Yield of soot-f normalized by that of heavy tar as a function of O/C at Tr = 700 °C. A ceramic fiber filter was set at the reactor bottom in every run.
(17) Janos, P.; Tokarova, V. Fuel 2002, 81, 1025–1031. (18) Herod, A. A.; Zhuo, Y.; Kandiyoti, R. J. Biochem. Biophys. Methods 2003, 56, 335–361.
decreases the heavy tar yield but allowing survival of highmass and/or highly polar components, which are finally 2906
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Figure 15. Continued
microscope, there was no evidence that soot-f was in the form of nano-sized particles. It was rather believed that a portion of the heavy tar was condensed over ceramic fibers coating them. Although not shown in the figure, the soot-f yield was a function of Tf. In the cases of Tr = 700 °C and O/C = 0, the soot-f yield changed gradually from 0.54 to 0.91% on the fuel carbon basis as Tf decreased from 484 to 360 °C. Thus, the soot-f formation for Tf = 415 °C, as shown in Figure 12, was not a particular case. The behavior of soot-f at Tr = 800 °C will be discussed later. Figure 13 compares liquid chromatograms of two different heavy tars at Tr = 700 °C and O/C = 0. One of them was collected downstream of the CF filter, while no CF filter was set in the run for collection of the other. The difference between the two chromatograms is clear, showing that a portion of the heavy tar with relatively high molecular mass and/or high polarity, in other words, lower saturated vapor pressure, was deposited, forming soot-f. Figure 14 plots sootf yields for the oxidation at Tr = 700 °C, which has been normalized by the heavy tar yield, against O/C. This figure
eliminated with O/C as high as 1.0. Thus, the oxidation at Tr = 700 °C was not effective on the elimination of heavier components of the heavy tar, unless a sufficiently high O/C was applied. Excess O2 loading, however, resulted in the substantial loss of important components of fuel gas, such as H2, CH4, and C2H4. Carbon deposit in/on the CF filter was quantified at different O/C ratios. The results for Tf = 415 °C are illustrated in Figure 12. Soot-f and soot-r are carbon deposits in/on the CF filter and on the inner wall of the tubular reactor, respectively. The amount of soot-f is more important than that of soot-r in consideration of the possibility of filter clogging during operation of the gasification. At Tr = 700 °C, the soot-f yield decreases as O/C increases, but O/C as high as 1.0 is required for reducing soot-f to a negligible level. It was found from the observation of the filter crosssection that soot-f was deposited mainly at inner layers of the filter. It was also confirmed that soot-f was not dissolved at all in any organic solvents, such as THF, chloroform, and pyridine, even under ultrasonic irradiation. Without saying, soot-f was not removed mechanically. In microscopic observations with a field-emission-type scanning electron 2907
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Figure 15. Changes in the yields of selected aromatic compounds with O/C at Tr = 700 °C. No CF filter was set at the reactor bottom.
oxidation of hydrogen-rich components of the heavy tar to hydrogen-poor ones. 3.3. Oxidation at Tr = 800 °C. The oxidation at Tr = 800 °C influenced the product distribution in a completely different manner from that at Tr = 700 °C. Effects of O/C on the individual product yields are shown in Figure 16. It is obvious that the oxidation consumes H2 and lower hydrocarbon gases (C1-C4) while hardly consuming the heavy tar. O2 was thus consumed by oxidation of the light gases selectively. This was probably due to the fast progress of the thermal cracking of the heavy tar into refractory aromatics, which were involved in both the heavy and light tars. Light oxygenates, which were abundant at Tr = 700 °C, were decomposed almost completely, even at O/C = 0. As mentioned previously, high mass components with a molecular mass greater than 500 were eliminated at Tr = 800 °C even in the absence of O2 (see Figure 5). Figure 17 compares a liquid chromatogram of the heavy tar at Tr = 800 °C and O/C = 0 to those at Tr = 700 °C. The individual chromatograms are shown in different arbitrary units, so that
thus indicates a change in the propensity of the heavy tar to soot-f formation. The normalized soot-f yield starts to diminish at O/C > 0.6, and this is in good agreement with the results shown in Figures 10 and 11. In other words, the oxidation at Tr = 700 °C and O/C < 0.6 was effective for decreasing the yields of the heavy tar and soot-f but not for diminishing the soot-f-forming propensity of the heavy tar. Figure 15 shows changes with O/C of the yields of selected aromatic compounds that were involved in the light tar. The light tar yield remained nearly unchanged at O/C = 0-0.6, as mentioned previously. However, those of the individual compounds changed to more or less extents. It is seen that the yields of the non-substituted aromatics and phenol remain steady or increase slightly, while those of the alkyl-substituted aromatics tend to decrease. The apparent steady yield of the light tar at O/C < 0.6 thus resulted from a balance between the formation and the oxidative decomposition. Higher reactivity of alkyl-substituted aromatics with O2 than non-substituted ones was consistent with the preferential 2908
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Figure 17. Liquid chromatograms of heavy tars at Tr = 700 °C (O/C = 0, 1.02) and Tr = 800 °C (O/C = 0). Wavelength for detection = 300 nm. No CF filter was set at the reactor bottom.
When the gas-phase temperature is, on the other hand, as low as 700 °C, there would be an accumulation of the carbonaceous deposit from the heavy tar inside the dustremoval filter, unless the oxidation of the volatiles is extremely extensive. The deposit would accumulate in inner layers, leading to clogging of the filter, and/or help with the formation of a layer of dust over the outer surface of the filter playing a role of a binder. Keeping the gas-phase temperature higher than 700 °C may thus be needed for avoiding troubles in the dust removal process.
Figure 16. Effects of O/C on the product yields at Tr = 800 °C. No CF filter was set at the reactor bottom.
the highest absorbances agree with one another. The thermal cracking at Tr = 800 °C, even in the absence of O2, seems to be as effective for eliminating high-mass components of the heavy tar as the oxidation at Tr = 700 °C with O/C = 1.0. Peaks around an elution time of 22 min are attributed to PAHs. The soot-f and soot-r yields at Tr = 800 °C are shown in Figure 12. The soot-f yield remains around 0.2% over the range of O/C up to 0.61. A negligible effect of O/C on the soot-f yield strongly suggests the formation of the soot-f inside the tubular reactor and that O2 was minimally involved in its oxidation, as well as that of the refractory aromatics. One of the most important findings on the sootf property was that it was deposited over the outer surface of the CF filter and easily removed mechanically. This is proof that the soot-f was formed in the tubular reactor and then deposited over the filter. 3.4. Consideration of the Operating Condition of Gasification. In practical gasification of biomass with air or O2, its loading, i.e., air or O2 ratio, is a factor most crucial for the operating temperature, while the moisture content of the fuel is another important factor. The results from the present study show a definite importance of the temperature history of the volatiles for avoiding clogging of the filter for dust removal and minimizing the loss of chemical energy of the gas. The volatiles, if they undergo the thermal cracking at temperatures as high as 800 °C, would entirely pass through the dust-removal filter regardless of the extent of its oxidation. More or less amount of soot would be produced during the thermal cracking, deposited over the filer, but easily removed together with inorganic dust (ash and alkali salts) by applying a general method, such as pulse-jet cleaning.19,20 Thus, maintenance of the gas-phase temperature around 800 °C is a condition necessary for successful operation of the dust removal process.
4. Conclusions The gas-phase thermal cracking and oxidation of nascent volatiles from the rapid pyrolysis of the cedar sawdust were investigated under independent control of O2 consumption and temperature. O2 was consumed at 700 °C with the heavy tar and light oxygenates at high selectivities. The oxidation of the heavy tar, however, was ineffective for elimination of high-mass components of the heavy tar, a portion that was condensed at inner layers of the CF filter at 350-500 °C. The resulting carbonaceous deposit could not be removed from the filter unless burned off. It was also difficult to completely avoid such a deposit even by the oxidation at O/C as high as 1.0. At 800 °C, the thermal cracking rapidly eliminated highmass components of the heavy tar, forming refractory aromatics as well as light gases. The entire part of the resulting aromatics passed through the filter, forming no carbonaceous deposit, such as above, regardless of O/C. O2 was consumed mainly by oxidation of light gases, while the heavy and light tar yields hardly changed. Soot was formed from the tar by the thermal cracking and then deposited over the CF filter. The soot was easily removed from the filter mechanically. On the basis of the above-mentioned findings, it is concluded that the temperature of 700-800 °C is critical for the tar property relevant to dust removal by filtration downstream of the gasifier. Acknowledgment. The authors are grateful to the JFE Engineering Corporation and the New Energy and Industrial Technology Development Organization (NEDO) for financial support of this work.
(19) Smith, D. H.; Powell, V.; Ahmadi, G. Powder Technol. 1998, 97, 139–145. (20) Kim, J.-H.; Liang, Y.; Sakong, K.-M.; Choi, J.-H.; Bak, Y.-C. Powder Technol. 2008, 181, 67–73.
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