1294
Energy & Fuels 2006, 20, 1294-1297
Interparticle Desorption and Re-adsorption of Alkali and Alkaline Earth Metallic Species within a Bed of Pyrolyzing Char from Pulverized Woody Biomass Nozomu Sonoyama,† Tsutomu Okuno,† Ondrˇej Masˇek,† Sou Hosokai,† Chun-Zhu Li,‡ and Jun-ichiro Hayashi*,† Center for AdVanced Research of Energy ConVersion Materials, Hokkaido UniVersity, N13-W8, Kita-ku, Sapporo, 060-8628, Japan, and Department of Chemical Engineering, Monash UniVersity, Post Office Box 36, Victoria 3800, Australia ReceiVed September 27, 2005. ReVised Manuscript ReceiVed March 26, 2006
Pulverized pine sawdust was pyrolyzed at 500 °C for a period long enough to complete tar evolution, and the resultant char was subjected to further heating at 700 °C in a fixed-bed reactor. Net release of alkali and alkaline earth metallic (AAEM) species from the char as a fixed bed was negligible unless the He flow was forced to pass through the bed. Even when the forced He flow was applied, the net release of each AAEM species was significantly influenced by the bed height even within a range up to 2.3 mm. These results showed that volatile AAEM species underwent repeated desorption from the char surface and adsorption onto it within the bed.
Introduction Alkali and alkaline earth metallic (AAEM) species contained in biomass are volatilized during pyrolysis, gasification, and combustion1-12 and therefore responsible for many problems encountered in processes for thermochemical biomass conversion.1,13-15 We recently investigated the release of AAEM species during pyrolysis of pulverized biomass, employing a particular type of pyrolyzer, a wire-mesh reactor (WMR).12 In WMR, biomass particles were dispersed in a form of monolayer * To whom correspondence should be addressed. Telephone: +81-11706-6850. Fax: +81-11-726-0731. E-mail:
[email protected]. † Hokkaido University. ‡ Monash University. (1) Gabra, M.; Nordin, A.; O ¨ hman, M.; Kjellsto¨m, B. Biomass Bioenergy 2001, 21, 461-476. (2) Mojtahedi, W.; Backman, R. J. Inst. Energy 1989, 62, 189-196. (3) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 8, 855-865. (4) 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. (5) Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860-870. (6) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385-1399. (7) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (8) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C.; Hald, P. Energy Fuels 1997, 11, 779-784. (9) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Ja¨glid, U. Fuel 2002, 81, 137-142. (10) Davidsson, K. O.; Stojkova, B. J.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1033-1039. (11) Keown, D.; Favas, G.; Hayashi, J.-i.; Li C.-Z. Bioresour. Technol. 2005, 96, 1570-1577. (12) Okuno, T.; Sonoyama, N.; Hayashi, J.-i.; Li; C.-Z.; Sathe, C.; Chiba, T. Energy Fuels 2005, 19, 2164-2171. (13) Jensen, P. A.; Stenholm M.; Hald P. Energy Fuels 1997, 9, 110481055. (14) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (15) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-108.
between two sheets of wire mesh and heated under He flow that was forced to pass through the particle layer. The forced He flow minimized the secondary interaction between the pyrolyzing biomass/char and AAEM species that had been released into the gas phase. The primary release of AAEM species was thus investigated successfully. Upon heating pine sawdust at a rate of 1000 °C s-1 up to 800 °C, 15-20% of each AAEM species was released. Further isothermal heating caused nearly the complete release of Na and K within 150 s, while the release of Mg and Ca terminated at levels of 2040%. Heating the sawdust at 1 °C s-1 up to 800 °C brought about the release of AAEM species mainly after the tar evolution. At both heating rates, chlorides of AAEM species were very minor volatiles. The pyrolysis was also performed in a general thermogravimetric analyzer (TGA), in which a fixed bed of the same sawdust as above (initial amount < 10 mg) was heated at 1 °C s-1 in the absence of forced gas flow through the fixed bed.12 It was then found that none of the AAEM species was released to a detectable degree. The difference in the behaviors of AAEM species between WMR and TGA suggests a chemical and/or physical interaction between AAEM species in the gas phase and the char surface, which was so strong as to prevent AAEM species from escaping from the char fixed bed in TGA. It was also suggested that volatile AAEM species underwent desorption from the char and adsorption onto it repeatedly within the bed. If such repeated desorption and adsorption of AAEM species were processes determining the net release, it would largely depend upon conditions of the char bed such as the bed height and gas velocity through the bed. However, the difference in the behavior of AAEM species between WMR and TGA could also have arisen from that in the pyrolysis conditions. As reported below, the char yield from the pyrolysis of the sawdust in WMR was clearly lower than that in TGA at temperatures over 400 °C. This result was due to the chemical interaction between nascent volatiles and the
10.1021/ef050316y CCC: $33.50 © 2006 American Chemical Society Published on Web 04/20/2006
AAEM Species within a Bed of Pyrolyzing Char
Figure 1. Schematic diagram of apparatus.
surface of pyrolyzing particles, which resulted in coking or charring of a portion of the volatiles.16 It was believed that the extent of coking/charring was influenced by the residence time of volatiles within the bed and therefore by the gas flow rate through the bed. Moreover, the interaction might have influenced the physical/chemical behavior of AAEM species as well as properties of the resultant char. Thus, to examine the repeated desorption and adsorption of AAEM species from char particles, it is necessary to investigate the release of AAEM species from char particles applying different conditions of the char bed such as bed height and gas velocity through the bed but employing char particles prepared under an identical pyrolysis condition. In continuation of our previous study,12 the release of AAEM species from biomass char particles was investigated with a purpose to examine repeated desorption/adsorption of AAEM species from/onto the char surface. This paper reports significant influences of the height of the fixed char bed on the net release of AAEM species from the bed, which is an indication of the repeated desorption and adsorption of AAEM species. Experimental Section The same pulverized pine sawdust as that used in our previous study12 was employed as the starting material. The sawdust particle had sizes in a range of 125-210 µm. Contents of AAEM species and Cl in the sawdust were as follows: Na, 0.006; K, 0.071; Ca, 0.054; Mg, 0.010; Cl, 0.005 wt % on a dry basis. The char sample was prepared by pyrolyzing the sawdust in a fixed-bed reactor. Figure 1 shows a schematic diagram of the reactor system. The reactor consisted of a concentric double tube made of transparent quartz glass. The inner and outer tubes had inner diameters of 8 and 16 mm, respectively. The lower end of the inner tube was closed by a sintered quartz plate (filter) that was used for gas-solid separation. A prescribed amount of the sawdust was placed over the filter and then heated to 500 °C with a heating rate and a holding time at 500 °C of 1 °C s-1 and 600 s, respectively, and then cooled to ambient temperature at a rate of about 10 °C s-1. Atmospheric He was continuously forced to pass through the fixed bed at a flow rate of 0.1 NL min-1 during the heating and cooling. The char yield from the pyrolysis was 19.8 wt % of the dry sawdust, and this was in good agreement with that for the pyrolysis in WMR with the (16) Antal, M. J., Jr.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703717.
Energy & Fuels, Vol. 20, No. 3, 2006 1295
Figure 2. Char yield from the pyrolysis of pine sawdust as a function of the temperature. (O and - - -) WMR. A monolayer of particles (mass ) 4-5 mg) sandwiched by SUS304-made wire meshes was heated at a rate of 1 °C s-1 up to a peak temperature of 300-700 °C and cooled to ambient temperature without an isothermal period. Atmospheric He was forced to pass through the particle layer at a velocity (at 25 °C) of 0.07 m s-1. The average cooling rate was within a range of 150-380 °C s-1, depending upon the peak temperature of 300-700 °C. Details of the procedure are reported in ref 12. (- - -) Drawn only for showing the trend. (s) TGA. A fixed bed of particles (mass ) 5 mg) was formed in a platinum cup (inner diameter ) 5 mm; depth ) 5 mm) and heated in atmospheric He flow at a rate of 1 °C s-1. Upon heating, the mass of the residual solid was measured continuously.
same heating rate and peak temperature.12 The char sample was stored in a dry N2 atmosphere prior to use. Analyses of the char confirmed negligible release of AAEM species during the pyrolysis at 500 °C, while 88% of Cl in the sawdust was released probably as HCl. Then, Cl/K and Cl/total AAEM species ratios of the char were 0.008 and 0.003 on a molar basis, respectively. The pyrolysis of the sawdust was also performed in a TGA with the same heating rate as above. Details of the procedure were reported previously.12 Figure 2 compares the change in the char yield with the temperature between WMR12 and TGA. It is clearly seen that coking/charring of nascent volatiles was suppressed by applying the forced gas flow through the fixed bed and/or reducing the bed height, and as a result, the char yield was decreased. The char formed in TGA was not further employed for investigation of the release of AAEM species. The char sample was heated in the same fixed-bed reactor as mentioned above. A fixed bed of char with a mass of 3, 9.5, or 23 mg was formed over the filter, and the bed height was measured. The masses of 9.5 and 23.0 mg corresponded to bed heights of 1.0 and 2.3 mm, respectively. The other mass, 3 mg, was chosen to form a monolayer of char particles over the filter. The char sample was heated to a peak temperature of 700 °C, further heated at this temperature for a period up to 3600 s, and then cooled to ambient temperature. It was found in our previous study12 that 42, 13, and 8% of K, Mg, and Ca, respectively, in the original sawdust was released upon heating it up to 700 °C at 1 °C s-1 in WMR, while no release was detected in TGA. This temperature was thus high enough to examine the effects of the char bed conditions on the net release of AAEM species from the sawdust char. During the heating and the subsequent cooling, atmospheric He was continuously forced to pass through the fixed bed at a flow rate of 0.036 or 1.0 NL min-1, which corresponded to linear gas velocities through the empty inner tube of 0.039 and 1.08 m s-1, respectively, at 700 °C. The height of the fixed bed changed to an insignificant degree (well below 10%) during the heating. The resultant char was recovered from the inner tube and then analyzed to determine the amounts of AAEM species remaining within/on the char particles. Details of the analyses were reported previously.12,17 In brief, the char sample was burnt completely without ignition and volatilization of AAEM species by means of a temperature-
1296 Energy & Fuels, Vol. 20, No. 3, 2006
Figure 3. Effects of the height of the char bed on retentions of K, Ca, and Mg. (O) Fixed-bed reactor, He flow rate ) 1.0 NL min-1. (4) Fixed-bed reactor, He flow rate ) 0.036 NL min-1. (b) TGA.
programmed combustion technique. The ash was washed with HFHNO3 mixed acids, dissolved into an aqueous solution of CH3SO3H, and analyzed by ion chromatography. The char was also heated in TGA. A prescribed mass, ca. 3 mg, of the char sample was charged into a platinum cup with an inner diameter and depth of 4.9 and 5.0 mm, respectively, and then heated at a rate of 1 °C s-1 up to 700 °C under atmospheric flow of He, while no forced gas flow through the char fixed bed was available. The retentions of AAEM species in/on the char after the heating were measured in the same way as above. Reproducibility of the retentions of AAEM species in the char after heating at 700 °C was examined choosing a set of conditions: He flow rate, 1.0 L min-1; height of char bed, 2.3 mm; holding time at 700 °C, 0 s. The results from five independent runs of char heating and char analysis showed that errors in determining K, Ca, and Mg retentions were within (2, (3, and (3% of the respective retentions in the char before the pyrolysis. For each of the other sets of conditions, two or more independent runs were performed, and errors of the AAEM retentions were within the same ranges as above. The Na retention was poorly reproduced in comparison to the other AAEM species, but the Na retention changed with the conditions in a manner very similar to that of K.
Results and Discussion Figure 3 shows combined effects of the height of the fixed bed and He flow rate on the retentions of K, Ca, and Mg after heating at 700 °C with a holding period of 0 s. The retention is defined as the amount of AAEM species remaining in the char after heating relative to that before heating. In this figure are also shown the retentions for the heating in TGA with the same heating rate, peak temperature, and holding time as in the fixedbed reactor. It is seen that the retention is affected by the bed height significantly, while the increasing gas velocity from 0.039 to 1.08 m s-1 causes very minor changes in the retentions. It is also seen that none of the AAEM species was released from (17) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427438.
Sonoyama et al.
the char without forced gas flow through the char bed. These results are explained qualitatively by considering that the char played a role of strong adsorbent of AAEM species, thereby inducing desorption and adsorption of volatile AAEM species within the char bed. In the absence of gas flow through the char bed, the transport of volatile AAEM species through the char bed would be driven mainly by their diffusion in the interstitial gas phase, the rate of which was not so high as to cause their net release from the char bed. On the other hand, in the presence of the gas flow through the char bed, the gas convection in the interstitial gas phase helped the transport of AAEM species, causing the net release. The gas flow through the char bed was thus essential for the net release of AAEM species. The insignificant effect of the gas velocity over the range from 0.039 to 1.09 m s-1 can be explained as follows: Within the char bed, residence time of AAEM species in the interstitial gas phase was much shorter than that on the char surface. Then, the former had very little contribution to the overall residence time. An experimental proof of this will be shown later. The results shown in Figure 3 show the importance of defining the height of the char bed and the presence of gas convection through the bed in investigating volatilization/release of AAEM species in fixed-bed reactors. Without the definition of these parameters, it seems to be difficult to generalize the experimental results. In our previous study,12 volatilization characteristics of AAEM species during the pyrolysis of the sawdust and sugarcane bagasse were reasonably explained by the fact that AAEM species were volatilized from the char surface mainly as elemental species, rather than hydroxides and carbonates. It is agreed that AAEM species are volatilized mainly from those bonded to the carbonaceous part of the char in a form of C-OM+ or C-O-M2+-O-C (M+, Na or K; M2+, Mg or Ca), and these bonds undergo dissociation forming elemental AAEM species.11,12,18-22 Thus, repeated dissociation and formation of C-O-M+ and C-O-M2+-O-C bonds corresponded to repeated volatilization (desorption) and adsorption (chemisorption) of AAEM species, respectively. It is reasonable that AAEM species experienced repeated desorption and adsorption not only between the gas phase and the char surface but also within the pore systems of char particles. In this sense, well-known negative effects of increasing particles size on the release of AAEM species10 may be explained partly in the way as above. However, it is difficult to simply compare the impact of increasing particles size on the release of AAEM species with that of increasing bed height because of the following reasons: Dissociation of C-O-M+ and C-O-M2+-O-C bonds is greatly enhanced by the presence of radicals such as hydrogen radicals,20-22 and the enhancement is often so strong that alkali metallic species are released nearly completely.21 The char undergoes thermal cracking, generating hydrogen radicals even after completion of the main part of volatile evolution. As suggested by Sathe et al.,23 the concentration of hydrogen radicals in the pore system of char is an important factor for the release of AAEM species. It is at present difficult to compare the concentration of hydrogen radicals on the surface of char with that in its matrix. (18) Wigmans, T.; Elfring, R.; Moulijn, J. A. Carbon 1983, 21, 1-12. (19) Hashimoto, K.; Miura, K.; Xu, J.-J.; Watanabe, A.; Masukami, H. Fuel 1986, 65, 489-494.38. (20) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033-1039. (21) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143-149. (22) Quyn, D. M.; Wu, H.; Bhattacharya, S. P.; Li, C.-Z. Fuel 2002, 81, 151-158. (23) Sathe, C.; Hayashi, J.-i.; Li, C.-Z.; Chiba, T. Fuel 2003, 82, 14911497.
AAEM Species within a Bed of Pyrolyzing Char
Figure 4. Effects of the holding period at 700 °C on retentions of K, Ca, and Mg in char. He flow rate ) 1.0 NL min-1. Bed height ) 2.3 mm.
Figure 4 illustrates changes in the K, Ca, and Mg retentions with the holding period at 700 °C for the 2.3 mm high fixed bed. It is seen that the isothermal heating of the char at 700 °C for 60 min decreases the K retention from 93 to 85%. At a gas velocity of 1.08 m s-1, the gas residence time within the char bed was shorter than 2 ms, and it contributed little to the overall residence time of K within the char bed. It is also noted that the K retention of 85% is higher than that for the monolayered char, as low as 60%, even without the holding time (see Figure 3). Moreover, the Ca and Mg retentions seem to remain almost unchanged over the holding period. Thus, negligible release of Ca and Mg, compared with their release from the monolayered char particles, reveals considerable negative effects of increasing bed height on the release of Ca and Mg from the bed. The difference in the net release between K and Ca/Mg was more significant for a thicker char bed. As shown in Figure 3, about 30% of Mg and about 20% of Ca were released from the monolayered char particles, and these extents were in the same order of magnitude as that of K release, ca. 40%. On the other hand, no or negligible amounts of Mg/ Ca were allowed to escape from the 2.3 mm high char bed, while ca. 15% of K was released (see Figure 4). The results for the monolayered char particles and 2.3 mm high char bed were
Energy & Fuels, Vol. 20, No. 3, 2006 1297
thus in contrast to each other, which could not be explained simply by the difference in the desorption/adsorption kinetics between K and Mg/Ca. Although not evidenced yet, consideration of chemical transformation of AAEM species on the surface of char and within its matrix may be needed to explain the appreciable difference in the bed height effect between the net release of K and that of Ca/Mg. Along with repeated desorption and adsorption of AAEM species, in other words, repeated formation and dissociation of C-O-M+ or C-OM2+-O-C, these bonds could become more stable thermally and therefore less volatile.24 It is also possible that AAEM species reacted with Si species to form nonvolatile silicates on their way to the gas-phase outside the char bed.6,25 In fact, Si was contained in the sawdust with Si/K and Si/AAEM ratios of 11 and 5.0 on a molar basis, respectively. In a thermodynamic sense, the formation of Ca/Mg silicates is much more favorable than that of K silicates.12 It is thus speculated that Ca and Mg staying in the char bed transformed to silicates more extensively than K and also that, for every metallic species, the transformation became more considerable as the bed height increased. Conclusions Volatilization of AAEM species from the pine-sawdust char were investigated at 700 °C, focusing on factors influencing the net release of AAEM species from the fixed bed of char particles. Within the conditions employed, the following conclusions are drawn: (1) The height of the fixed bed of char particles is a crucial factor for the net release of AAEM species from the bed upon heating. Increasing the bed height suppressed the release of Ca and Mg much more intensively than that of K and Na. (2) The presence of the gas flow through the fixed char bed is essential for the net release of AAEM species from the bed. (3) The combined effects of the bed height and the gas flow rate are qualitatively explained on the basis of the fact that volatile AAEM species undergo repeated desorption from and re-adsorption onto the char surface. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the New Energy and Industrial Technology Development Organization (NEDO), Japan. EF050316Y (24) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Energy Fuels 2000, 79, 427-438. (25) Rines, H.; Fjellerup, J.; Henriksen, U.; Moilanen, A.; Norby, P.; Papadakis, K.; Posselt, D.; Sørenen, L. H. Fuel 2003, 82, 641-651.
