ARTICLE pubs.acs.org/EF
Thermogravimetric Pyrolysis and Gasification of Lignocellulosic Biomass and Kinetic Summative Law for Parallel Reactions with Cellulose, Xylan, and Lignin Hyung Chul Yoon,† Peter Pozivil,‡ and Aldo Steinfeld*,‡,§ †
Clean Fuels Research Center, Korea Institute of Energy Research (KIER), 305-343 Taejon, South Korea Department of Mechanical and Process Engineering, Eidgen€ossische Technische Hochschule Z€urich (ETH Z€urich), 8092 Z€urich, Switzerland § Solar Technology Laboratory, Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland ‡
ABSTRACT: Pyrolysis and gasification of lignocellulosic (woody) biomass and its main components, cellulose, xylan, and lignin, were studied using a combined thermogravimetry and gas chromatography experimental setup at a heating rate of 10 °C min1 in the temperature range of 140900 °C using air and/or steam gasifying agents (100% air, 50% air/50% steam, 25% air/75% steam, and 70% steam/30% argon). Simulated biomass composed of a mixture of cellulose, xylan, and lignin at 50:25:25 wt % was also investigated. A three-parallel-reaction kinetic model was formulated on the basis of the weighted sum of reaction rates for the individual components and experimentally validated in terms of reaction rates, carbon conversions, and product gas yields for lignocellulosic and simulated biomass.
1. INTRODUCTION The pyrolysis and gasification behavior of lignocellulosic biomass (LB) depends upon its main components, namely, cellulose, xylan (hemicelluloses), and lignin.13 Hardwoods have a higher proportion of cellulose and xylan than softwoods.4 Forest biomass contains a high weight percentage of cellulose and lignin, whereas agricultural biomass contains a high weight percentage of cellulose and xylan.5 The thermochemical conversion of different types of biomass and its components has been examined,2,3,613 and several kinetic models have been proposed to describe the complex reaction pathways.1425 Of interest is the modeling approach of the summative law, which assumes no interactions among the main components.2628 This study presents a kinetic investigation of LB and its components during pyrolysis and gasification under inert, steam, and air/steam atmospheres. Weight loss and gas evolution profiles were measured using a combined thermogravimetry and gas chromatography (TGGC) experimental setup. A kinetic model based on the weighted sum of three parallel Arrhenius-type nthorder reaction rates for cellulose, xylan, and lignin is formulated, and the corresponding kinetic parameters are experimentally determined. The selection of different reacting gases elucidates the effects of the various oxidizing agents on the kinetics of pyrolysis and gasification of biomass and its main individual components cellulose, xylan, and lignin.
Figure 1. Thermogravimetric experimental setup. lignin in LB provided by the supplier is 50, 25, and 25%, respectively (Table 1). SB was prepared on the basis of the structural components of conifers: a mixture of cellulose (catalog number S5504, Sigma-Aldrich, CAS 9004-34-6), xylan from beech wood (catalog number X4252, Sigma-Aldrich, CAS 9014-63-5), and commercial kraft pine lignin powder (Indulin AT, MeadWestvaco) at a 50:25:25 wt % ratio. Note
2. MATERIALS AND METHODS 2.1. Materials. Ultimate and proximate analyses of cellulose, xylan, lignin, the (real) LB from selected conifers, and simulated biomass (SB) are shown in Table 1. Xylan has been previously used as a model of hemicellulose,2,6,12 and alkali lignin has been used for pyrolysis and gasification studies.2932 The weight percentage of cellulose, xylan, and r 2011 American Chemical Society
Received: August 24, 2011 Revised: November 1, 2011 Published: December 06, 2011 357
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Table 1. Ultimate and Proximate Analyses of Cellulose, Xylan, Lignin, LB, and SB (Dry Sample) sample
cellulose
xylan
lignin
supplier
Sigma-Aldrich
Sigma-Aldrich
MeadWestvaco (Indulin AT)
ultimate analysis (daf) (wt %)
proximate analysis (wt %)
(real) LB
SB
J. Rettenmaier and Sohne GmbH and Co. KG, Germany
C
43.58
43.77
64.36
48.63
H
6.09
5.91
5.88
6.5
5.99
O
50.27
50.26
27.65
44.16
44.61
N
0.05
0.05
0.65
0.18
0.20
S
0.01
0.02
1.46
0.03
0.38
91.64 8.54
74.11 21.94
58.17 39.88
84.76 14.67
78.89 19.73
3.95
1.95
0.57
1.48
volatile compound fixed carbon ash
0
48.82
Figure 3. Pyrolysis/gasification of LB and its components in 50% air/ 50% steam: (a) cellulose, (b) xylan, (c) lignin, and (d) LB. The left and right panels represent gas evolution and conversion as well as gas yield, respectively. Legend: (—) H2, (- - -) CO2, (- 3 3 -) CO, and ( 3 3 3 ) CH4.
Figure 2. Pyrolysis/gasification of LB and its components in air: (a) cellulose, (b) xylan, (c) lignin, and (d) LB. The left and right panels represent gas evolution and conversion as well as gas yield, respectively. Legend: (—) H2, (- - -) CO2, (- 3 3 -) CO, and ( 3 3 3 ) CH4.
70% steam/30% argon. Mass flow rates were electronically controlled (Bronkhorst Hi-Tec) and maintained at a total of 100 mLN min1 (mLN means milliliters under normal conditions at 273 K and 1 atm). A sample of 20 mg was used for each experiment. The steam generator unit (Bronkhorst Hitec CEM) was connected to the thermogravimetric analysis (TGA) furnace via a transfer line heated to 200 °C to avoid condensation. Dynamic TG runs were conducted in the temperature
that the SB contains a higher amount of ash and fixed carbon than the woody biomass sample. All samples were dried for 4 h and sieved for particles with a mean size of 5080 μm to minimize heat-/mass-transport limitations. 2.2. Experimental Setup. A schematic of the thermogravimetric experimental setup (TG, Netzsch 409STA) is shown in Figure 1. Reacting gases were air, 50% air/50% steam, 25% air/75% steam, and 358
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Table 2. Kinetic Parameters of the Air-Based Gasification of Cellulose, Xylan, Lignin, and LB kinetic parameters
cellulose
k0 (s1)
xylan
1.98 1030 1
Ea (kJ mol )
260360 °C
n k0 (s1) 1
Ea (kJ mol ) n
455522 °C
357.51
lignin 1.46 101
6.26 107 180350 °C
LB
140342 °C
104.58
23.55
1.22 107 205355 °C
103.27
1.91
1.36
0
1.02
9.38 106
1.38 101
2.65 108
4.59 105
133.30
353523 °C
0.83
367490 °C
49.36
142.13
0.79
k0 (s1)
357450 °C
1.06
106.37 0.48
638.42
Ea (kJ mol1) n
526812 °C
k0 (s1)
105.71 0.64 2.53 105
1
Ea (kJ mol )
812900 °C
n
174.75 0.15
correct for the effects of buoyancy and thermal expansion. All TG runs were repeated to ensure reproducibility, and the root mean square (rms) of the differences was less than 0.01. 2.3. Kinetic Model. The Arrhenius-type rate law is formulated as dα Ea ¼ k0 exp ð1Þ ð1 αÞn dt RT where α is the reaction extent, α = (mi m)/(mi mf). Kinetic parameters were determined by employing the least-squares method. The integral form of eq 1 was adapted to elucidate devolatilization, intermediates, and char gasification. The carbon conversion (Xc) is defined as
∑
j ¼ CO, CO2 , CH4
Xc ¼
nj ð2Þ
nc, sample
where nj and nc are the moles of C-1 gaseous products and carbon in the sample, respectively. The yield of char and gas is defined as the sum of the moles of each product divided by the moles of carbon in the initial sample. The reactivity R0 is defined as R0 ðs1 Þ ¼
1 dm ðm mf Þ dt
ð3Þ
Assuming no interactions between the biomass components, the gasification/pyrolysis of LB in the presence of air and/or steam can be described by the summative law for the parallel gasification/pyrolysis of each biomass component (i.e., a three-parallel-reaction model), formulated as dα ¼ dt
∑
i ¼ celluose, xylan, lignin
γi ðk0, i ðEa, i =RTÞð1 αi Þni Þ
ð4Þ
where α is the reaction extent of LB, αi is the reaction extent of its components (i = cellulose, xylan, and lignin), and γi is the initial mass fraction of the component. The predicted gas yield is then Figure 4. Pyrolysis/steam gasification of LB and its components in 70% steam/30% argon. The left and right panels represent gas evolution and conversion as well as gas yield, respectively. Legend: (—) H2, (- - -) CO2, (- 3 3 -) CO, and ( 3 3 3 ) CH4.
Y ¼
∑
i ¼ celluose, xylan, lignin
γi Yi
ð5Þ
The error between the measured and predicted conversions for N data points is given by the rms
range of 140900 °C at a 10 °C min1 heating rate and ambient pressure. Ar was pumped into the balance system to protect the scale. The composition of the dried gas products (i.e., H2, CO, CO2, and CH4) was analyzed by gas chromatography (GC, Varian cp4900 equipped with Molsieve-5A/Poraplot-U columns). Blank runs were performed to
rms ¼ 359
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u ðαi, measured αi, predicted Þ2 t
∑i
N
ð6Þ
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Table 3. Kinetic Parameters of the Gasification of Cellulose, Xylan, Lignin, and LB in 50% Air/50% Steam kinetic parameters
cellulose
k0 (s1)
xylan
2.49 1028 1
Ea (kJ mol )
260360 °C
150385 °C
81.19
LB 9.10 103
2.82 105 190350 °C
337.11
lignin
1.59 105 182347 °C
12.0
84.78
n
1.82
1.15
0
0.622
k0 (s1)
4.60 100
4.99 1021
4.33 106
4.11 1010
Ea (kJ mol1)
422577 °C
n
575645 °C
49.29
390510 °C
408.35
0.62
k0 (s1)
405480 °C
127.27
0.74
172.64
0.77
0.81
3.30 105
Ea (kJ mol1) n
750860 °C
164.14 0.76
Table 4. Kinetic Parameters of the Gasification of Cellulose, Xylan, Lignin, and LB in 25% Air/75% Steam kinetic parameters
cellulose
k0 (s1)
xylan
2.13 1025 1
265367 °C
lignin 2.48 102
8.08 107
207382 °C
45.92
1.62
1.17
0
0.478
k0 (s1) Ea (kJ mol1)
2.11 101 32.53
6.55 1012 258.63
2.09 105 112.66
640670 °C
1.73 1014 314.17
424580 °C
n
304.57
546648 °C
0.27
155429 °C
2.25 101
n
Ea (kJ mol )
197335 °C
LB
106.16
18.20
431540 °C
0.61
k0 (s1)
0.72
0.811
2.88 103
Ea (kJ mol1)
667800 °C
118.75
n
0.48
Table 5. Kinetic Parameters of the Gasification of Cellulose, Xylan, Lignin, and LB in 70% Steam/30% Argon kinetic parameters
cellulose
1
k0 (s ) Ea (kJ mol1)
275375 °C
n
3.92 10 301.97
xylan 24
197340 °C
1.62
k0 (s1) Ea (kJ mol1)
lignin 6.83 10 106.51
600695 °C
n
480690 °C
2.46 10 7.60
205397 °C
3.53 104 80.55
1.14
0
0.89
3.10 1012
3.05 1024
4.20 104
264.31 0.69
k0 (s1)
LB 2
7
697770 °C
521.92 0.66
650870 °C
150.75 0.58
3.30 1011 1
Ea (kJ mol )
700810 °C
n
272.79 1.15
3. RESULTS AND DISCUSSION
during the smoldering pyrolysis of intermediates and char in the range of 390760 °C, as observed previously,38 while CH4 evolved during the intermediate reactions in the range of 400600 °C. For xylan, Xc = 0.84, indicating less tar compared to cellulose. The devolatilization of lignin (Figure 2c) began at 135 °C and occurred concurrently with char gasification in the range of 300500 °C. H2 evolution was also observed during smoldering char pyrolysis in the range of 390450 °C. For lignin, Xc = 0.99. The devolatilization of the LB (Figure 2d) occurred at above 170 °C and yielded CO and CO2 during the gasification of ligninderived char. H2 and CH4 evolutions were observed during devolatilization and smoldering char gasification. The kinetic parameters are shown in Table 2. The calculated curves are also indicated in panels ad of Figure 2 and agreed well with those measured by TG. Previous analysis of the pyrolysis of cellulose and xylan was based on three first-order consecutive reactions.39 In our study, the pyrolysis/gasification of xylan in air was observed to follow four consecutive reactions, including
3.1. Air-Based Gasification. The gasification of LB and its components cellulose, xylan, and lignin in the presence of air is shown in panels ad of Figure 2. The progressive depolymerization of cellulose (Figure 2a) started at above 250 °C and was followed by its thermal decomposition,33,34 resulting in CO2, CO, and char. The peaks of the CO2 and CO evolutions were attributed to the primary reactions, including depolymerization/ devolatilization,35 intermediate/secondary reactions,36 and char gasification. Because the amount of CnHn produced was below the GC detection limit, the relative tar yield can be assumed to be the difference between the moles of gaseous products and the thermogravimetric data.34,37 For cellulose, Xc = 0.50, indicating substantial tar production during devolatilization, consistent with a previous observation,34 because tar barely oxidizes at
