ARTICLE pubs.acs.org/EF
Reforming of Volatiles from the Biomass Pyrolysis over Charcoal in a Sequence of Coke Deposition and Steam Gasification of Coke Sou Hosokai,*,† Koyo Norinaga,‡ Tokuji Kimura,† Masaki Nakano,† Chun-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 § 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 ‡
ABSTRACT: Nascent volatiles from the pyrolysis of a type of woody biomass were reformed in a bed of charcoal at 750 850 °C. While the volatiles passed through the bed together with air at an air ratio of 0.115, the concentration of heavy tar (bp > 336 °C) decreased from 910 000 to 6 1020 mg/Nm3dry. This rapid and almost total decomposition of the tar can be ascribed to its deposition onto the charcoal surface, forming coke. The coke formation leads to the loss of the charcoal micropores that provide active sites. Therefore, simultaneous creation of micropores by gasification is necessary to maintain the charcoal activity. Steam played the role of gasifying agent, while O2 was consumed mainly by gas-phase oxidation that supplied the heat for the reaction.
1. INTRODUCTION Thermochemical decomposition of tar has been one of the most important subjects in the development and improvement of biomass gasification processes, because it can reduce or, in some cases, eliminate the cost of downstream processes of the gas purification and the associated loss of the energy efficiency.1,2 Tar decomposition by partial oxidation in the gas phase at elevated temperatures (e.g., >1000 °C) has been demonstrated, but it requires a substantial amount of air or even pure O2, inevitably resulting in a significant loss of chemical energy. Catalytic reforming has long been studied with the aim of rapid and total tar elimination at lower temperatures. Another feature of this process is less air or O2 consumption and maximum use of steam.3 5 NiBased steam reforming catalysts exhibiting high activities have been reported.6 14 However, a durable and economical catalytic system free from irreversible poisoning/deactivation has yet to be realized. Catalyst poisoning/deactivation can be caused by sulfur, chlorine, alkali/alkaline-earth metallic species, particulate matter, as well as coke deposition from tar. For durable catalytic reforming, heavy tar elimination is one of the necessary conditions. Caballero et al.15 demonstrated the sequential reforming of biomass tar using dolomite as a guard catalyst and Ni-based catalyst in series. They reported that the tar concentration below 2 g/Nm3dry was enough to avoid coke deposition, causing deactivation of the catalyst. Dolomite is a type of mineral having a porous structure for capturing tar. Several authors reported the tar-reforming activity of the dolomite in biomass reforming.16 18 Although dolomite is less expensive than synthesized catalysts, its deactivation is unavoidable. Carbonaceous solids, such as chars, from biomass or coal can provide active surfaces for the decomposition of hydrocarbons.19 26 The predominant mechanism of the decomposition is believed to occur by deposition of carbon (often termed soot or coke) from the hydrocarbons. Rapid decomposition of aromatic compounds has been achieved under suitable conditions.22,24 One of the advantages of employing char as an active material for the tar r 2011 American Chemical Society
decomposition is that the char is produced simultaneously with tar during the pyrolysis of biomass. In addition, deactivation of the char, should it occurred, is not a serious problem because the spent char can at least be used as a smokeless fuel.27,28 Hosokai et al.22 investigated the decomposition of mono- to tetra-aromatics over a biomass-derived char in the presence of steam. Naphthalene, phenol, and phenanthrene (initial concentration of 1.5 3 g/Nm3dry) were decomposed with a conversion >99.99% at 800 900 °C with a contact time of 0.2 s and a steam concentration of 15.5 vol %. They also described a simultaneous process of coke deposition and steam gasification of the char and/or the coke. In other words, the aromatics were reformed not directly to the gaseous products but in the following sequence: coke formation from the aromatics and subsequent gasification of the coke. They proposed that the coke deposition consumed micropores of the char, thus reducing its activity, whereas the gasification created micropores, thus maintaining its activity. It was also suggested that the rate of coke and/or char gasification needed to be faster than that of coke formation to maintain the char activity. Brandt et al.25 investigated a multi-stage gasification of a woody biomass, which consisted of a sequence of pyrolysis for producing char and volatiles, gas-phase partial combustion of the volatiles in air at 1050 1100 °C, and finally, decomposition of the residual tar within the 950 1000 °C bed of the char from the pyrolysis. The heavy tar content was reduced by the partial oxidation to levels of 0.08 0.13 wt % of dry biomass and further reduced to undetectable levels by the reforming process. The reforming also reduced the content of light tar (mainly polyaromatic hydrocarbons) by 98 99.7%. The results of the above-mentioned previous studies suggest the possibility of the direct reforming of tar from the pyrolysis Received: March 11, 2011 Revised: October 5, 2011 Published: October 06, 2011 5387
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Figure 1. Apparatus for the pyrolysis of biomass and reforming of pyrolytic gas.
over char even without the need for partial combustion at high temperatures between the pyrolysis and reforming steps. Such direct reforming, if successful, will minimize air consumption and, thereby, improve the fuel gas quality in terms of calories and energy efficiency of the gasification. In the present study, reforming of nascent tar from the pyrolysis of a woody biomass over a char was experimentally investigated with an emphasis on simultaneous progress of coke formation over the char and steam gasification of the coke and/or the char.
2. EXPERIMENTAL SECTION 2.1. Biomass and Charcoal Samples. Chipped Japanese cedar was used as the biomass feedstock. Its carbon, hydrogen, and oxygen contents and moisture content were 50.9, 6.4, and 43.2 wt % on a dry basis and 10 wt % on a wet basis, respectively. Commercially available silica sand (SS) and charcoal from another type of Japanese cedar (char A) were employed as bed materials for the reforming of the volatiles. SS and char A had particles sizes of 3.4 4.8 and 2.0 3.4 mm, respectively. 2.2. Pyrolysis/Reforming Experiments. A total of 10 pyrolysis/ reforming runs (R1 R10) were performed at atmospheric pressure in a two-stage reactor system, which is shown schematically in Figure 1. Cedar chips were fed into the screw-conveyer pyrolyzer at a constant feeding rate together with N2 gas (purity >99.9995 vol %) and heated to 550 °C at a heating rate of about 6 °C/s. The inner diameter of the pyrolyzer tube was 50 mm. Among the pyrolysates, the char was sent to a collector vessel, where it was quenched, while the volatiles and N2 were introduced into a reformer via the connecting tube with the temperature maintained at 550 °C. The reformer (inner diameter of 54 mm) contained a fixed bed of SS, char A, or char prepared by the pyrolysis of the feedstock as described above (char B). Char B was crushed to give particles 3.4 4.8 mm in size prior to being used as a bed material. The bed height in the reformer was 160 mm for each bed material, with a temperature distribution along with the bed axis. The temperature in an
effective height of 132 mm was distributed within a range of 750 800 or 800 850 °C. The reforming was performed in three different modes: thermal cracking (TC), partial oxidation (PO), and steam reforming (SR). In TC mode, the mixture of the volatiles and N2 was further diluted by N2 and then sent to the bed of SS, char A, or char B. For PO mode, the volatiles/ N2 mixture was instead diluted by the O2/N2 mixture, in which the overall O2/N2 ratio was adjusted to 21:79 (vol/vol). The rate of O2 feeding into the reformer was selected for the air ratio to the volatiles to be 0.115. The O2/N2 mixture was fed into the reformer by one of two methods: (a) the O2/N2 was mixed with the volatiles just upstream of the char bed (PO1 mode), or (b) the mixing of the gas and volatiles occurred within the bed (PO2 mode). In the SR mode, the feedstock was moistened to a moisture content of 0.46 kg/kg of dry cedar and also employed as the source of steam. The conditions for R1 R10 are summarized in Table 1. Only the pyrolyzer was used in R1. The reforming product was introduced into a train of solid/liquid collectors. An aerosol filter at 150 °C and three condensers (0, 30, and 70 °C) were connected in series. We first confirmed that even benzene vapor condensed mainly in the second and third condensers, with a total recovery >98%. At these temperatures, we also confirmed that almost no condensation of CO2 occurred. These facts ensured that the tar compounds with a higher boiling point successfully condensed in a condenser or on the filter. The first condenser was cooled with ice to condense the water. Although a portion of tar condensed in the first condenser, it could be separated from the water by the phase-separation method. The yield of water was determined by both weight and Karl Fischer titration. The second and third condensers were packed with glass beads, essential for the complete recovery of light liquids. The aerosol filter was indispensable for capturing aerosol particles of a portion of tar because complete deposition is not possible using only the condensers. Noncondensable gases were collected in gasbags and analyzed by gas chromatography (GC). The heaviest fractions of condensable compounds were deposited inside the tubes even upstream of the aerosol filter. Such deposits were recovered completely by washing 5388
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Table 1. Conditions for Pyrolysis or Pyrolysis/Reforming Runs run ID mode of reforming
R1
R2
none
TC
TC
TC
PO1
PO2
PO2
PO2
PO2
SR
SS
char B
char A
char A
char A
char A
char A
char A
char A
bed material
R3
R4
R5
R6
R7
R8
R9
R10
mass of bed material (g)
500
40
40
40
40
40
40
40
40
bed temperaturea (°C)
800
800
800
800
800
800
800
850
800
air ratio
0
0
0
0.115
0.115
0.115
0.115
0.115
0
S/B ratiob
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.46
biomass feeding rate (g dry/min)
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
4.3
2.8
biomass feeding period (min) O2 feeding rate (NL/min)
60 0
60 0
60 0
60 0
60 0.50
60 0.50
30 0.50
15 0.50
60 0.50
60 0
N2 feeding rate (NL/min)
2.25
gas residence time in reformer c (s)
2.25
2.23
2.23
1.88
1.88
1.88
1.88
1.88
1.55
0.40
0.38
0.38
0.38
0.38
0.37
0.35
0.34
0.40
a Peak temperature within the bed. b Mass ratio of moisture to dry biomass. c Calculated from the initial bed height of the effective section and the flow rate of the product gas at the reformer exit assuming the void fraction of bed = 0.50.
the tubes with acetone and tetrahydrofuran. Thus, in the present study, collection and quantification of the condensable materials were performed as precisely as possible. The condensable organic products were classified into three groups: BTX, light tar, and heavy tar. BTX consisted of benzene, toluene, styrene, xylenes, and trimethylbenzenes. Light tar comprised aromatics ranging from indene (bp = 180.8 °C) to phenanthrene (bp = 336 °C). Heavy tar consisted of aromatics with higher boiling point temperatures. The components of BTX and those of the light tar were quantified by GC, while the heavy tar was quantified gravimetrically. Carbonaceous deposits, hereafter termed coke, onto SS were quantified by means of a general combustion method. The amount of carbon was quantified by combustion of the coke and analysis of the combustion gas by GC. Spent char A and char B were subjected to elemental analysis as well as mass quantification. The net extent of the coke formation or that of char gasification was determined by weight and elemental analysis of the spent char. The yields of each product were calculated on the basis of the total amount of recovered products. Therefore, the yield is a time-average value. The total product recovery ranged from 99.2 to 101.4% on a feedstock carbon basis. The original and spent chars were also subjected to surface area and pore volume analysis by a N2 adsorption method.
Figure 2. Cumulative yield of BTX, light tar, and heavy tar on a biomass C basis.
3. RESULTS AND DISCUSSION 3.1. Product Distribution. Figures 2 4 show the product yields of the individual runs. The yields are indicated on the basis of either the total carbon content or the dry mass of the feedstock. As seen in Figure 4, the char yields from R1 R9 are about 36% C. In other words, the volatile products with yields of about 63% C were fed into the reformer in those runs. The data also demonstrate good reproducibility of the pyrolysis to form the char and volatiles. The char yield from R10 was greater than those from other runs. This can be rationalized by a much higher moisture content of the feedstock used in R10, which resulted in a peak temperature of 500 °C instead of 550 °C. A pyrolysis experiment was performed at 500 °C for the comparison of the volatile composition with that at 550 °C. Even at 500 °C, the yield of tar was 41%, which was almost equivalent to that at 550 °C in R1. The result from R10 was therefore comparable to other results. The coke and H2O yields are now discussed. The coke yield is given by the difference in the amount of carbon in the char bed before and after a run. A positive or negative value indicates net coke formation from the volatiles over the bed material or net
Figure 3. Heavy tar yield on the basis of the dry mass of biomass.
gasification of the char, respectively. The yield of H2O can be determined by subtracting the amount of moisture fed into the pyrolyzer from the total amount of water recovered in the condensers. Figure 4 demonstrates that the pyrolysis (i.e., R1) formed “pyrolytic” water with a yield of 23.5 mol of H2O/100 of mol C. A zero yield of H2O indicates the consumption of 23.5 mol of H2O/100 mol of C from the reforming. 5389
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Figure 4. Yields of char, coke, H2O, and gaseous products.
3.2. Reforming in TC Mode. Reforming in the bed of SS (R2) reduced the heavy and light tar yields from 25 to 3.0% C and from 15.4 to 4.3% C, respectively. Given the fact that SS possesses little or no catalytic activity and also considering the negligible steam consumption, it can be posited that the tars were decomposed in the SS bed mainly by thermal cracking to produce gaseous products and coke. Changing the bed material from SS to char B (R3) or char A (R4) further lowered the heavy tar yield to 0.46 0.48% C, which corresponded to a concentration of 3030 3320 mg/Nm3product gas (steam- and N2-free). R2 R4 gave similar yields of light tar and BTX. Reforming over the chars consumed steam. This was a result of the steam gasification of the char and/or coke and was also consistent with the slightly negative coke yields from R3 R4. As illustrated in Figure 5, the specific surface area and the micropore volume (pore width < 2 nm) of char A decreased from 654 to 394 m2/g and from 0.24 to 0.16 cm3/g, respectively. Thus, more than half of the initial micropore volume was lost during the reforming, despite a net progress of the char gasification. According to a previous study,18 tar compounds are converted to coke in the micropores of the char. The decrease in the micropore volume must be the result of coke deposition from the tars in the micropores. In other words, the activity of the tar reforming must be maintained by the generation of micorpores during the char gasification. 3.3. Reforming in PO and SR Modes. The introduction of air (i.e., O2) into the reformer could promote the tar reforming. O2 reacts not only with tar but also with fuel gas, such as H2, CO, and light hydrocarbons, to produce H2O and CO2. Combustion of
Figure 5. Specific Langmuir surface areas and micropore volumes of fresh and spent char A.
the fuel gas results in the decrease in the heating value of the gas. If the O2 react with char and/or coke selectively, it enhances the tar reforming, producing micropores. Furthermore, the decrease in the heating value of the gas could be avoided. In R5 R9, char A was exclusively employed because similar results were obtained when using either char A or char B in R3 and R4. The reforming was performed in R5 and R6 with an air ratio of 0.115 that corresponds to the feeding of 0.2 mol of O2/1 mol of C of the volatiles. R5, in which the PO1 mode was used, gave light and heavy tar yields similar to those obtained in TC mode (R4). In R5, O2 was introduced into the headspace of the reformer, and it was therefore suspected that O2 was not consumed in its 5390
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Figure 6. Changes in instantaneous yields of H2, COx, and CH4 with time in R6.
reaction with char. According to our previous study,26 O2 introduction in the homogeneous gas phase results in the decomposition of functional groups and also gaseous species, such as CO and H2. The higher yields of CO2 in R5 must be the result of O2 consumption in the oxidation of CO. Tar, which does not possess functional groups, is more stable than functional-groupcontaining tar.26 Therefore, the tar passing through the char bed in R5 would be even less reactive than that in R6. On the other hand, the heavy tar yield in R6 was 0.23% C and was about half that in R4. In R6, which was performed in PO2 mode, O2 was directly injected into the char bed and mixed with the volatiles, resulting in a greater interaction of the char surface with O2. The effect of the introduction of air on the heavy tar yield was, however, not as significant as expected. Changes in the gas yield over time in R6 were investigated. As shown in Figure 6, both H2 and COx yields decreased with time, indicating a reduction in the rate of gasification (amount of gas formed by the gasification of char/coke per time). R7 and R8 were carried out in PO2 mode with a biomass-feeding period of 30 and 15 min, respectively. It was then found that the initial activity of char A was so high that the heavy and light tar yields in R8 were as low as 0.0017 and 0.035% C, respectively. The relatively large micropore volume of char A at a conclusion of R8 was consistent with its high activity during the initial 15 min feeding period (see Figure 5). The data shown in Figure 5 indicate that contact between the nascent volatiles and the char at 800 °C resulted in consumption of the micropores, even during co-feeding of O2. In Figure 4, it is seen that the yield of hydrcarbon gas in R6 is lower than in R4, while the yield of H2O and coke in R6 is higher than in R4. These results suggested that a direct reaction, if any, between O2 and the char/coke carbon, was not a major contributor to O2 consumption. Rather, reactions between O2 and the volatiles in the gas phase are the main source of the depletion in O2. Although not demonstrated, it was believed that further increasing the air ratio would not result in a dramatic increase in the gasification of the char/coke. Rather, it is believed that feeding air or O2 is important in a practical and adiabatic reformer, because exothermic O2 consumption causes an increase in the bed temperature and, thereby, faster progress of steam gasification of the char/ coke, suppressing the loss of the char activity. A simple calculation of the reforming process was performed, assuming that volatiles at 550 °C and dry air at 25 °C were fed into an adiabatic reformer with an air ratio of 0.115, while the product gas was exhausted from the reformer. The compositions of the volatiles from the pyrolyzer and product gas from the
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reformer were assumed to be the same as those in R1 and R8, respectively. We calculated the product gas temperature, satisfying the heat and mass balances around the reformer using the assumptions described in Table 2. The temperature of the product gas from the reformer is the only fitting parameter to satisfy the heat balance. As shown in Table 2, the simulation predicts a product gas temperature of 948 °C at the reformer exit. On the basis of this result, R9 was carried out in PO2 mode at 850 °C, the same air ratio as in R5 R8, and an identical biomassfeeding period to that in R5 and R6. The spent char A from R9 had a micropore volume of 0.16 cm3/g, almost equivalent to that of the spent char A from R8. The net progress of the char/coke gasification and the steam consumption in R9 were more significant than those in R8, despite the fact that the biomassfeeding period of R9 was 4 times longer than that of R8. The heavy tar yield from R9 was as low as 0.025% C and was about 1/9 of that reported for R6. The heavy tar yield from R9 was, however, 15 times that of R8. If the gasification of char/coke generates active sites, the yield of heavy tar in R9 could be lower than in R8. Therefore, the results impled that char A contained extremely active sites that could not necessarily be created by the gasification of char/coke. The heavy tar concentration in the product gas from R9 was 90 mg/Nm3dry. R10 was carried out in SR mode to examine the effect of enhanced steam gasification on the reforming in the absence of air. As seen in Figure 4, largely negative yields of H2O and coke showed significant progress of the steam gasification. The heavy tar yield from R10 was about 1/9 of that reported for R4, thus indicating the positive effect of the steam gasification on the reforming. The pore structure showed a different trend in SR mode from that in PO mode. As seen in Figure 5, the specific surface area significantly decreased in R10, while the pore volume slightly decreased. This phenomenon can be explained by linkage or aggregation of micropores to become larger pores. Such enlargement of the pores can enhance the diffusion of the tar into the char particle. This can be one of the reasons for the maintenance of char activity. On the other hand, the heavy tar yield from R10 was more than twice that from R9. This is explained partly by a temperature effect on the rate of coke deposition and also by a significant consumption (∼50%) of char A during R10; this reduced the residence time of the volatiles within the bed. Even under these conditions, steam reforming in R10 maintains the char activity longer than the partial oxidation in R8. However, partial oxidation generates a temperature increase in a practical reformer, which will result in an enhancement of the tar reforming in the char bed, as was simulated in R9. Therefore, steam reforming of char and partial oxidation cooperatively improve the tar-reforming activity of the char and its durability, respectively. With regard to the product gas quality, the partial oxidation mode gives a lower heating value as a result of the oxidation of the product gas, as indicated in R5 R9 (Table 3). To obtain a high calorific value, steam introduction is preferable. In the steam gasification mode, care needs to be taken when considering the heat supply because steam reforming is largely endothermic. Figure 7 illustrates cumulative yields of BTX and light tar as a function of the boiling point temperature. Naphthalene was the most abundant component of the light tars from R6 R10. The naphthalene yields from R8, R9, and R10 were 0.0017, 0.19, and 0.18 wt %, respectively, corresponding to the concentrations in the product gas of 12, 1320, and 1400 mg/Nm3dry (on a N2-free basis for R10), as seen in Table 3. The naphthalene concentrations 5391
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Table 2. Results of the Calculation for Predicting the Syngas Temperature at the Reformer by Assuming Its Adiabatic Nature volatiles and air at reformer inlet
product O2a
yield (mol of C/
temperature
enthalpy
yield (mol of C/
temperature
enthalpy
100 mol of C)
(°C)
(MJ/100 mol of C)
100 mol of C)
(°C)
(MJ/100 mol of C)
0
0
0.0 100
46.0
1.3 101
18.0
3.7 10
12.2
N2a
25
46.0 a
H2O (steam)
syngas at reformer exit
0 8.1 10
1
36.5
1
0.647
9.9 10
41.8
1.1 10
9.68
9.2 100
33.3
2.7 10
1
5.75
2.1 10
25.5
8.9 10
1
CH4 C2H4
2.90 0.885
1.4 10 3.9 10
C2H6
0.521
H2
a
CO CO2
C3H6 C3H8 C4H8 CH3OH (vapor) CH3CHO (vapor) liquid (crude, vapor) total C (mol of C/100 mol of C)
2
1 0 1
6.45 3.78
1
1.2 10 2.4 100
0
948
1.0 10
1
0.00
0.685
1.7 10
1
0.00
0.0 100
0.153
2.1 10
2
0.00
0.0 100
0.382
6.9 10
2
0.00
0.0 100
0.030
5.1 10
2
0.00
0.0 100
0.220
1.4 10
1
0.00
0.0 100
1.4 10
1
0.50 69.5d
2.0 10
550
b
41.5
62.7
total H (mol of H/100 mol of C)
151
total O (mol of O/100 mol of C)
102 1.27 102
a
1c
153
99.8
total enthalpye
0.0 100
1.27 102
b
Unit of yield: mol of compound/100 mol of C. Liquid = heavy tar + light tar + BTX. Overall H/C and O/C atomic ratios of the liquid were 1.42 and 0.313, respectively. Enthalpy of the liquid product (as vapor) was given by an equation developed by the present authors, which has not been published. c Enthalpy was approximated by that of benzene, because 93.3 and 6.3% of the liquid were explained by benzene and naphthalene, respectively, on a carbon basis. d The total C-based yield of the syngas was greater than that of the volatiles by 6.8 mol of C/100 mol of C, and this was the result of the net progress of steam gasification of char in the reformer. e Enthalpies of the products except for the liquid were calculated using thermodynamic data available on the National Institute of Standards and Technology (NIST) Chemistry WebBook (http://webbook.nist.gov/chemistry/).
Table 3. Properties of Product Gasa run
heavy tar
naphthalene
LHV of syngas
ID
(mg/Nm3dry syngas)
(mg/Nm3dry syngas)
(MJ/Nm3dry)
R2
2.7 104
5.8 103
20.2
R3 R4
3
3.0 10 3.3 103
4.6 103 5.3 103
16.4 17.1
R5
2.1 103
2.4 103
9.2
R6
3
1.0 10
2.6 103
8.1
R7
1.6 102
2.3 103
8.2
R8
6.0 100
1.2 101
7.9
R9
9.0 101
1.3 103
8.6
R10
2.3 102
1.4 103
12.0
a R2 R4 and R10, N2-free basis; R5 R9, N2 is involved with a volume of 3.76 times that of O2 fed into the reformer.
for R9 and R10 were lower than that at 40 °C saturation (2560 mg/Nm3 dry ) but higher than that at 25 °C saturation (620 mg/Nm 3 dry). 29,30 Milne et al.1 reported that the tar concentration of
