Primary Release of Alkali and Alkaline Earth Metallic Species during

Center for Advanced Research of Energy Conversion Materials, Hokkaido ..... Figure 2 YC, YT, and YLV from pyrolysis of PSD as a function of th at (a) ...
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Energy & Fuels 2005, 19, 2164-2171

Primary Release of Alkali and Alkaline Earth Metallic Species during the Pyrolysis of Pulverized Biomass Tsutomu Okuno,† Nozomu Sonoyama,† Jun-ichiro Hayashi,*,† Chun-Zhu Li,§ Chirag Sathe,† and Tadatoshi Chiba† 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, P.O. Box 36, Victoria 3800, Australia Received January 4, 2005. Revised Manuscript Received March 26, 2005

Release of alkali and alkaline earth metallic (AAEM) species was examined during pyrolysis of pulverized pine and sugarcane bagasse. The use of a wire-mesh reactor enabled the investigation of the primary release of AAEM species from pyrolyzing particles suppressing secondary interaction between them. Upon heating the pine at 1000 °C s-1 up to 800 °C, 15-20% of each AAEM species was released during the tar evolution and afterward. Further isothermal heating caused nearly complete release of alkalis within 150 s, while the release of alkaline earths terminated at levels of 20-40%. Heating the pine at 1 °C s-1 up to 800 °C brought about the release of AAEM species mainly after the tar evolution. Chlorides of AAEM species were found to be very minor volatiles over the range of conditions. Variations in K release with operating variables were reasonably explained by considering that elemental K volatilized from the charbonded AAEM species was a major volatile K species. None of AAEM species were significantly released when a fixed bed of the pine was heated at 1 °C s-1 up to 900 °C without forced gas flow through the bed. It was suggested that repeated desorption from and adsorption onto the char surface within the fixed bed inhibited the release of AAEM species from the fixed bed and resultantly allowed them to transform into thermally stable char-bonded ones and/or nonvolatiles such as silicates.

Introduction Woody and herbaceous biomass resources contain more or less alkali and alkaline earth metallic (AAEM) species that are associated with oxygen-containing functional groups of the organic substrate or are present inorganic salts in the cells. AAEM species are responsible for problems encountered in combustion and gasification.1-4 They are released from char and/or ash during pyrolysis and subsequent combustion or gasification,4-13 leading to slugging or defluidization in con* Author to whom correspondence should be addressed. Tel: +81 11 706 6850. FAX: +81 11 726 0731. E-mail: [email protected]. † Hokkaido University. § Monash University. (1) Jensen, P. A.; Stenholm M.; Hald P. Energy Fuels 1997, 9, 11048-1055. (2) Baxter, L. L.; Miles, T. R.; Miles, Jr. T. R.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (3) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-108. (4) Gabra, M.; Nordin, A.; O ¨ hman, M.; Kjellsto¨m, B. Biomass Bioenergy 2001, 21, 461-476. (5) Mojtahedi, W.; Backman, R. J. Inst. Energy 1989, 62, 189-96. (6) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 8, 855-865. (7) 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. (8) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385-1399. (9) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (10) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C.; Hald, P. Energy Fuels 1997, 11, 779-784.

ventional combustion processes and also erosion/corrosion of components in the power generation system. They also play catalytic roles in gasification and combustion of volatiles and char.14-17 Knowledge about the release of AAEM species during the primary pyrolysis as the common primary step of thermochemical conversion of biomass is therefore useful for developing combustion/gasification technologies in which the catalysis of AAEM species is maximized and their unpreferred influences are minimized. Dayton et al.6,7 investigated release of K from grasses, straws, and woods upon heating in atmospheres containing O2 in a fixed-bed reactor. They found that K was released mainly after devolatilization, i.e., during char combustion, and concluded that the predominant volatile K species was KCl while KOH was also involved in the volatiles under wet combustion conditions. Knud(11) Davidsson, K. O.; Korsgren, J. G.; Pettersson, J. B. C.; Ja¨glid, U. Fuel 2002, 81, 137-142. (12) Davidsson, K. O.; Stojkova, B. J.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1033-1039. (13) Keown, D.; Favas, G.; Hayashi, J.-i.; Li C.-Z. Bioresour. Technol., in press. (14) Raveendran, K.; Ganesh, A. Fuel 1998, 77, 769-781. (15) Zolin, A.; Jensen, A.; Jensen, P. A.; Frandsen, F.; DamJohansen, K. Energy Fuels 2001, 15, 1110-1122. (16) Elliott, D. C.; Hallen, T. R.; Sealock, L. J., Jr. J. Anal. Appl. Pyrol. 1984, 6, 299-316. (17) Ekstro¨m, C.; Lindman, N.; Pettersson, R. Catalytic conversion of tar, carbon black and methane from pyrolysis/gasification of biomass. In Fundamentals of thermochemical biomass conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; pp 601618.

10.1021/ef050002a CCC: $30.25 © 2005 American Chemical Society Published on Web 04/16/2005

Pyrolysis of Pulverized Biomass

sen8 studied release of K, Cl, and S species during fixedbed combustion of various biomass fuels with a variety of Cl/K molar ratios. For fuels with higher Cl/K ratios, it seemed that K was released as KCl at 600-800 °C, and as other species such as KOH and/or K2SO4 at higher temperatures. K release as KCl was not important for chars from fuels with a lower Cl/K ratio. Knudsen et al.8 also found that K was hardly released from the fixed bed of char when heated in N2 atmosphere up to 900 °C. K was thus released mainly from the char during combustion and/or the resultant ash. It cannot necessarily be said that the char combustion is essential for significant volatilization and release of K.9-13 Jensen et al.9 investigated releases of K and Cl during the pyrolysis of a fixed bed of wheat straw. K release took place at temperatures higher than 700 °C, resulting in about 25% loss of K up to 1050 °C. They concluded that the major volatile K species were KCl at 700-830 °C and KOH and/or elemental K at higher temperatures. Olsson et al.10 and Davidsson et al.11,12 monitored alkali (K and Na) release during the pyrolysis of biomass fuels by means of surface ionization techniques. They found that relative amounts of alkalis released at lower temperatures (200-500 °C) and at higher ones (above 700 °C) strongly depended on the nature of biomass. The alkali release occurred only at lower temperatures for the pyrolysis of wood waste11 but mainly at higher temperatures for the pyrolysis of straw and birch wood. In the reports by Olsson et al.10 and Davidsson et al.,11,12 however, the extents of K/Na releases were not indicated on the basis of their amounts in the original biomass samples. On the basis of the results from the previous studies4-12 as reviewed above, the composition of inorganic constituents is crucial to characteristics of K release during the pyrolysis of biomass and subsequent char combustion. It is believed that the release of K is facilitated by the presence of Cl6-10 while suppressed by that of Si, leading to formation of K-silicates6-9,18 that are much less volatile than KOH and KCl as well. It is also suggested that experimental conditions, including the configuration of reactor, have large impacts on the observed characteristics of K/Na release. Keown et al.13 studied pyrolysis of a sugarcane bagasse and trash in a fluidized-bed reactor. Pulverized biomass was continuously supplied into the hot fluidized-bed and pyrolyzed therein at a heating rate in the range of 103-104 °C s-1. The nascent char particles were carried away from the bed and then trapped underneath the frit above the bed, forming a fixed bed at the same temperature as the fluidized bed, and they were continuously exposed to flow of the carrier gas (N2) and nascent volatiles passing through the bed. The release of K took place above 500 °C after release of a major portion of Cl, and the extent of K release seemed to be more significant than those reported previously.6-12 Under the conditions used by Keown et al.,13 the fixed bed of char particles was exposed to gas flow that was forced through the bed. In most of the previous studies,6-12 biomass samples were heated slowly in a fixed bed where no or little forced gas flow was available and the volatiles had to escape (18) Hansen, L. A.; Nielsen, H. P.; Frandson, F. J.; Dam-Johansen, K.; Horlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189209.

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from the bed mainly by diffusion. Knudsen et al.8 implied that volatile K species could experience repeated desorption and adsorption from/onto char particles within the fixed bed, being allowed to chemically transform into less volatile or nonvolatile species. Davidsson et al.12 found a significant negative influence of increasing particle size on the extent of K release for the pyrolysis of the birch wood, and they ascribed the influence to transformation of K species in its way out of the particle. This finding suggests that the secondary interaction between the volatile K species and char can suppress the net release from the fixed bed in the absence of forced gas convection through the bed. It is also suggested from results of Keown et al.13 that exposure of char particles to vapor of radical-laden volatiles, i.e., tar and light gases, at above 600 °C enhances volatilization of K. It has been demonstrated for the rapid pyrolysis of brown coal19-22 that hydrogen and other radicals supplied by the volatiles can dissociate bonds between Na and char matrix, thereby volatilizing Na. Thus, the characteristics of K/Na and also Ca/ Mg may largely depend on not only their abundances relative to Cl and Si but also on their secondary transformations both inside and over char particles. In view of the above, it is worthwhile to investigate the release of K and the other AAEM species during the pyrolysis of biomass under conditions with no or minimized secondary reactions/interactions of volatiles (including volatile inorganic species) with char because characteristics of the primary release of AAEM species are essential to understand and also quantify the impacts of the secondary transformations of the metallic species on their fates in practical gasifiers and combustors. The present authors have been studying the pyrolysis of pulverized biomass in a particular type of reactor, wire-mesh reactor, which enables the minimization of interactions between char particles and reactive species in the gas phase, providing a simple and well-definable atmosphere in the particles’ vicinities. The extents of volatilization of AAEM species were measured as a function of the heating rate, peak temperature, and holding time at the peak temperature and were compared with those for pyrolysis in a conventional fixed-bed reactor. The primary purpose of this work was to clarify the characteristics of the primary release of AAEM species during the pyrolysis of pulverized biomass and then examine effects of the secondary interactions between volatile AAEM species and char particles. Attempts were made to estimate main mechanisms of volatilization and release of AAEM species with the aid of knowledge in volatilization and release of AAEM species from biomass 4-13 and brown coal 19-26 in the literature. (19) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033-1039. (20) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143-149. (21) Quyn, D. M.; Wu, H.; Bhattacharya, S. P.; Li, C.-Z. Fuel 2002, 81, 151-158. (22) Li, X.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2004, 83, 12731279. (23) Manzoori, A. R.; Agarwal, P. K. Fuel 1992, 71, 513-522. (24) Li, C.-Z.; Riley, K. W.; Kelly, M. D.; Nelson, P. F. In Coal Science and Technology 24; Pajares, J. A., Tasco´n, J. M. D., Eds.; Elsevier: London, 1995; pp 683-686. (25) Li, C.-Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fuel 2000, 79, 427-438. (26) Sathe, C.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Fuel 2003, 82, 1491-1497.

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Okuno et al.

Table 1. Elemental Compositions and Moisture Contents of Samplesa PSD 50.1 6.0 0.1 10 s at 500 °C and th > 0 s at 800 °C are due to the release AAEM species after the tar evolution. At 500 °C, the K, Ca, and Mg retentions decrease to 70-75%, 90-95%, and 90-95%, respectively, and then become steady. Upon the isothermal heating at 800 °C, the K retention decreases to less than 1% at th ) 100-150 s. The release of Mg seems to terminate at the retention of 60-65%, whereas Ca is hardly released. The nearly complete release of K at 800 °C shows that the formation of nonvolatile K species such as K-silicates is unlikely upon the fast heating of PSD up to 800 °C and the subsequent isothermal heating, although PSD contains

Figure 6. Retention of K, Ca, and Mg as a function of holding time, th, for fast pyrolysis of PSD in WMR at (a) 500 and (b) 800 °C.

Figure 7. Retention of K, Ca, and Mg as a function of peak temperature for fast pyrolysis of SCB in WMR.

Si with a sufficient amount to fix K in a stoichiometric sense (see Table 1). Release of AAEM Species during Fast Pyrolysis of SCB. In Figure 7 are plotted K, Ca, and Mg retentions against the peak temperature for the fast pyrolysis of SCB. The retentions change with trends clearly different from those for the fast pyrolysis of PSD. K and Mg are released mainly at 300-600 °C in overlap with the evolution of tar and lighter volatiles. In contrast, Ca is hardly volatilized over the temperature range examined. The retentions of K, Ca, and Mg were also measured as a function of th in a range from 0 to 120 s upon the isothermal heating at 800 °C. The retention of K decreased only by 10% and those of Ca and Mg remained unchanged. Thus, the release of AAEM species during the isothermal heating at 800 °C was less extensive compared with that from PSD char (see Figure 6). The Cl retention in the 400 °C char was 17%, and this corresponded to Cl/K and Cl/AAEM ratios of 0.057 and 0.012, respectively. This indicates that K and Mg were released at above 400 °C mainly as species other than chlorides. Mechanism of Volatilization and Release of AAEM Species. As reviewed in the Introduction, it is believed that K is released mainly as KCl and KOH during pyrolysis of biomass and subsequent char com-

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bustion.6,7 Thermodynamic calculations indicate that major volatile K species are KCl4,6,9,31 and/or K2Cl24,9 at temperatures from 500 to 1200 °C and that KOH,6,9 K2SO4,6 and elemental K9 can be present in the gas phase above 800 or 900 °C but at lower concentrations than chlorides. It is also predicted that K2SiO3 is a major K species in the solid phase over the temperature range. The volatilization of K, particularly that from the ash phase, may be predicted well on the basis of chemical equilibrium models. However, the thermodynamic calculation does not necessarily consider the presence of K directly bonded to carbonaceous part of the char as an intermediate species, as well as release of Cl preferential to K. Jensen et al.9 and Knudsen et al.8 suggested that not only KOH but also elemental K can be formed from K2CO3 and/or char-bonded K and then released from char. Under the present experimental conditions, K and the other AAEM species were released mainly as volatiles other than chlorides. It is thus inferred that K associated with the carbonaceous part of char is a major precursor of volatile K. It is well known that K and Na are excellent catalysts for steam gasification of coal char and carbon.32-34 Wigmans et al.35 and Hashimoto et al.36 proposed the following reaction scheme:

M + H2O ) M(O) + H2

(R1)

M(O) + C ) C(O) + M

(R2)

C(O) ) CO

(R3)

C(O) + M(O) ) M + CO2

(R4)

M represents reduced alkali that may be weakly associated with carbon.35 Wigmans et al.35 suggested that oxygen is trapped by potassium on the carbon surface in a form of K-O-C. Hashimoto et al.36 confirmed that K(O) and Na(O) have chemical forms of K-O-C and Na-O-C, respectively. The reactions R2 and R4 indicate that M(O), i.e., char-bonded alkali, can be decomposed into M and an oxygen-containing functional group, C(O), or M and gaseous CO or CO2, even when the char is not exposed to steam. The abundance of M depends on the thermal stability of M(O) and also on the abundance of oxygen that is reactive with M to form M-O-C. M undergoes desorption from and adsorption onto the carbon surface, and it is finally released into the gas phase. Hashimoto et al.36 also found that K species are associated with the char/carbon surface in the form of K-O-C regardless of the chemical forms of initial salts such as KOH, K2SO4, and KNO3 loaded to the carbon substrate. This intimates that KOH, if it is presented initially or formed during the pyrolysis, is highly reactive with carbon and readily converted into K(O) or K species unless isolated from (31) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Frandsen, F. J.; DamJohansen, K. Energy Fuels 2004, 18, 810-819. (32) Mckee, D. W. Fuel 1983, 62, 170-175. (33) Kapteijn, F.; Abbel, G.; Moulijn, J. A. Fuel 1984, 63, 10361042. (34) Moulijn, J. A.; Cerfontain, M. B.; Kapteijn, F. Fuel 1984, 63, 1043-1047. (35) Wigmans, T.; Elfring, R.; Moulijn, J. A. Carbon 1983, 21, 1-12. (36) Hashimoto, K.; Miura, K.; Xu, J.-J.; Watanabe, A.; Masukami, H. Fuel 1986, 65, 489-494.

the carbon substrate. With the same reasoning, it is unlikely that K2CO3 is an important volatile K species. Bazardorj et al.37 performed steam gasification of nascent char from the pyrolysis of a brown coal at 800900 °C in a reactor where interactions between char and volatiles were minimized. They found that exposure of the char to steam caused not only char gasification that was catalyzed by inherent Na, Ca, and/or Mg species but also rapid release of these metallic species from the gasifying char. Analyses of char samples revealed that AAEM species were highly dispersed in the carbonaceous matrix of the char without forming detectable inorganic phases. Thus, AAEM species were volatilized and released directly from the carbonaceous matrix. The above-mentioned mechanism, R1-R4, seems to explain extensive release of Na during the gasification. Elemental Na and K are in fact much more volatile than hydroxides and carbonates at temperatures of interest.38 It is also implied that even Ca and Mg can be volatilized as elemental species and then released from the char, although the formation rate of elemental Ca/Mg may be slower than that of elemental K/Na due to divalent nature of the formers. Volatilization of char-bonded K to elemental K is also consistent with results of recent studies on Na volatilization during pyrolysis of Victorian brown coal.19-21,24 Quyn et al.21 observed nearly complete release of Na during the fast pyrolysis of a NaCl-loaded Loy Yang brown coal at 800-900 °C in a fluidized-bed reactor that had a configuration similar to that used by Keown et al.13 A major portion, up to 80%, of Cl in the coal was released preferentially to Na.20 Na release was much more extensive than that during the pyrolysis in a thermogravimetric analyzer and even in a wire-mesh reactor. In the fluidized-bed reactors used by Quyn et al.20,21 and Keown et al.,13 the nascent char particles were continuously exposed to vapor of nascent volatiles. Following the studies by Quyn et al.,20,21 Wu et al.19 demonstrated that the interaction between volatiles and char induces the release of Na. The effects of the volatiles-char interaction on Na release have been explained by the following sequence: (i) thermal cracking of volatiles, particularly that of tar, over the char surface and the resultant formation of radicals such as H radical, (ii) migration of radicals into the char matrix, and (iii) their reactions with char-bonded Na to volatilize Na. The reactions can be expressed as

CM-M + H ) CM-H + M (CM, char matrix) (R5) The radical-induced volatilization and release of Na reasonably explain combined effects of temperature, heating rate, and pressure on the release of Na during the pyrolysis of Loy Yang coal in He flow in a wire-mesh reactor, as reported by Sathe et al.24 Their results strongly suggest that increasing concentration of radicals in the char matrix enhances volatilization and the volatilization rather than intraparticle diffusion of volatile AAEM species controls the overall rate of their release. Hydrogen-radical-induced volatilization would also be important in steam gasification of char, which (37) Bazardorj, B.; Hayashi, J.-i.; Shimada, T.; Sathe, C.; Li, C.-Z.; Chiba, T. Fuel, in press. (38) Handbook of Chemistry, 6th Edition; Chemical Society of Japan: Tokyo, 2003.

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produces hydrogen radical as an intermediate. Rapid volatilization of AAEM species during the steam gasification of char from the brown coal36 may be contributed by R5 and similar reactions. The above-described mechanisms support that charbonded alkalis are converted into elemental ones by reactions such as R2 and R5 and are released from PSD char. It is reasonable that PSD char undergoes thermal cracking even after its yield becomes nearly constant. As shown in Figure 5, the release of Ca and Mg is as extensive as that of K upon the fast heating of PSD. This cannot be explained if the release of AAEM species was controlled by their intraparticle diffusion because Ca and Mg species, regardless of their chemical forms, are much less volatile than elemental K. As shown in Figures 3, 5, and 6, a minor part of release of AAEM species is not associated with evolution of tar and lighter volatiles. This may be explained by formation of lower carboxylates of AAEM such as acetates and formates21,25 and/or heavier organic salts.24-26 In the present work, water-soluble products were recovered in the fast pyrolyses of PSD and SCB at 500 °C and analyzed by ion chromatography. The analysis detected acetate, formate, glycolate, and oxalate ions, of which total yields was 0.5-1.0 wt% of mass of each sample. This result, though not a direct evidence, is consistent with the volatilization of AAEM species as lower carboxylates. The volatilization of AAEM species may be caused by reactions such as

CM-CH3-COO-M + H ) CM-H + CH3COOM (in the case of acetate formation) (R6) CM-M + R ) CM + R-M (R, radical) (R7) Elemental AAEM species may also be important volatile species during the tar evolution. In the course of the pyrolysis, intraparticle concentration of hydrogen radical is expected to be highest during the formation of fragment radicals as precursors of tar and lighter volatiles. AAEM species can therefore be involved in reactions such as R5. Effects of Heating Rate on the Release of AAEM Species. Here are assessed the characteristics of volatilization and release of AAEM species during the pyrolysis of PSD and SCB on the basis of the abovedescribed volatilization mechanisms. Upon the slow heating of PSD, AAEM species are released mainly after the tar evolution. Char-bonded K and Na are volatilized through reactions such as R2, R4, and R5 and are then released from the char at above 500 °C. Ca and Mg are bonded to the carbonaceous matrix of the char via oxygen and then volatilized in similar ways to those of K and Na but much more slowly due to their divalent natures. Upon the fast heating of PSD up to 800 °C, release of AAEM species during the tar evolution takes place as extensively as after the tar evolution. As explained in the previous section, intraparticle residence time of volatile precursors involving organic salts of AAEM species is shortened to a considerable degree by increasing the heating rate from 1 to 1000 °C s-1. This suppresses intraparticle secondary reactions of volatiles and also transformation of the organic salts into charbonded ones. Increasing the heating rate also raises

Okuno et al. Table 2. Gibb’s Free Energy Change (∆G°) for Formation of Silicates ∆G°, kJ/mol reaction

500 °C

800 °C

K2O + SiO2 ) K2SiO3 MgO + SiO2 ) MgSiO3 2MgO + SiO2 ) Mg2SiO4 CaO + SiO2 ) CaSiO3 (R) 2CaO + SiO2 ) Ca2SiO4

-0.3 -35.4 -62.7 -88.7 -130.2

-0.3 -33.1 -60.9 -88.6 -131.7

concentration of hydrogen radical during the tar evolution, thereby encouraging reactions such as R5 and R7. Volatilization of AAEM species continues after the tar evolution being governed by the same mechanism as that upon the slow heating. Alkaline earths are released as fast as alkalis upon the fast heating while much more slowly upon the slow heating and isothermal heating. The char undergoes thermal cracking, forming hydrogen radical upon the fast heating at a much higher rate than upon the slow heating, as well as isothermal heating, and this results in much higher concentration of hydrogen radical within the char matrix. Thus, a higher concentration of hydrogen radical accelerates R5 volatilization of alkaline earths as extensive as that of alkalis, though the mechanism causing this trend is unknown. Upon the isothermal heating of PSD char at 500 °C (see Figure 6), the release of AAEM species terminates at th ) 100 s at retentions of 70-90%. This is primarily due to the fact that only a portion of functional groups bonded to AAEM species undergoes reactions such as R2 and R4 releasing elemental metallic species. The rate of thermal cracking of char decreases with an increase in the holding time, while hydrogen radical concentration decreases toward termination of reactions such as R5. During the isothermal heating at 800 °C, decomposition of functional groups bonded to AAEM species and formation of hydrogen radical are so significant as to cause nearly complete volatilization of K. In contrast to K, Ca is hardly released from PSD char during the isothermal heating at 800 °C while Mg is released but at a lower rate than K. It is also seen in Figure 7 that Ca is not released at all from SCB char regardless of the pyrolysis condition. This could be explained if bonds between Ca and the char matrix (i.e., C-O-Ca-O-C bonds) were more stable than those between Mg and the char matrix (C-O-Mg-O-C bonds). This is, however, inconsistent with comparable releases of Ca and Mg from PSD char after the tar evolution. Another explanation is based on difference in reactivity with Si species between AAEM species. As shown in Table 1, the Si content in SCB is about four times that in PSD. Table 2 shows changes in Gibb’s free energy due to formation of K-, Ca-, or Mg-silicate. Clearly, the formation of the Ca-silicates is more favored than that of the Mg-silicates, as well as the K-silicate, in a thermodynamic sense. Though the thermodynamic states of charbonded AAEM species are different from those of corresponding oxides, the data shown in Table 2 strongly suggest that char-bonded Ca has a stronger propensity to react with Si species than does char-bonded Mg. For both the fast and slow pyrolyses of PSD, the release of Ca was as significant as that of Mg. This is reasonably explained if transformation of AAEM species

Pyrolysis of Pulverized Biomass

into silicates is much less important in the PSD char than in the SCB char. This hypothesis is consistent with the Si content in PSD, which is only one-fourth that in SCB. Release of K and Mg from SCB char is insignificant upon heating of the SCB char at temperatures from 600 to 800 °C (after volatile evolution) and also upon the subsequent isothermal heating at 800 °C. This result is in contrast with the release of K and Mg from PSD char after the volatile evolution (see Figures 5 and 6). It is implausible that oxygen in the SCB, which is essential for binding AAEM species to the char matrix, is much more abundant than in the PSD char. More frequent formation of K-silicates and Mg-silicate is therefore a more plausible explanation of difference in K/Mg release between PSD and SCB chars. Conclusions The release of AAEM species during the pyrolysis of pulverized PSD and SCB has been studied employing a wire-mesh reactor, which enables investigation of the

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primary release of AAEM species from char surface minimizing secondary interactions between them. The following conclusions are drawn within the present experimental conditions. The secondary interaction between volatile AAEM species and char surface even inhibits their release out of the fixed bed. Regardless of the heating rate, AAEM species were released mainly as species other than chlorides. The observed characteristics of release of AAEM species are reasonably explained by considering that elemental species are major volatiles. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors are also grateful to Drs. T. F. Dixon and P. Hobson at The Sugar Research Institute (SRI), Australia, for supplying SCB and also providing useful information on SCB. EF050002A