Tar Constituents Analysis of Rice Straw via
Torrefaction
Yun-liang Zhang, Jian Deng, Wen-guang Wu, Yun Wang, Shan-hui Zhao, Pin Zhang and Yong-hao Luo
Institute of Thermal Engineering
Shanghai Jiao Tong University
Shanghai, P.R.China
zyl_1985@sjtu.edu.cn
Abstract—Torrefaction is a promising pretreatment technology
for biomass, with the merits of simplicity and economical high
efficiency. The pyrolysis products distribution of torrrefaction
with different temperatures (200℃-300℃) was obtained in a lab
scale hot-rod reactor. The tar constituents of various conditions
was analyzed by GC/MS. Studies indicated that the yield of the
tar was increasing when the temperature rising. There were
mainly three types of tar identified, including i) phenols; ii) esters
of phthalic acid; iii) acids, ketones, esters, alkanes, (not
containing benzene ring). The molecular weight distribution of
tar was investigated by GPC.
Keywords- torrefaction; tar constituents; GC/MS; GPC
I.
material. We investigated the effects of different pyrolysis
temperature(200 ℃ ,250 ℃ ,300 ℃ ) on the tar yield and
constituents. The torrefied char were also studied.
II.
MATERIALS AND METHODS
A. Materials
Rice straw (RS) used in our studies is from Jing Shan
Shanghai. The raw material was grinded into a particular size
range of 100-150 μm by a KER-1/100A sealed sample
preparation mill from Zhenjiang Kerui Zhiyang Shebei Co.
Ltd. Then the RS particles were dried in oven for 12 h at 30 ℃
before used.
INTRODUCTION
The primary energy is consuming increasingly every year,
therefore the renewable energy is getting more attentions.
Biomass is one of the renewable resources, which is promising
and in large amount. In China, about 630 million tons of crop
residues are produced every year, most of which are burned
and wasted in low efficiency [1]. Biomass thermo chemical
conversion technologies, including gasification and
combustion, have large potential to utilize biomass. However,
there are some demerits of biomass, such as large volume, low
energy density, high moisture content and high cost for
transport. Therefore it is very necessary to pre treat the biomass
to meet the demand of different utilization technologies.
B. Reactors and Procedures
K-type Thermocouple
MFC
N2
Heating electrodes
K-type Thermocouple
Controller & transformer
First stage
Temperature
controller
Copper gasket
Tar trap
MFC data
Computer
Torrefaction (low temperature pyrolysis), is one of the
pretreatment technologies for biomass. It provides a mild
pyrolysis temperature (200℃-300℃) to convert the biomass
into a solid fuel of which the grindability and energy density
are improved and properties are closer to the coal. many
research groups [2] have investigated the effects of
temperature, residence time, material types etc. on the
torrefaction process and the products distribution, including the
char and gaseous products. But few researchers studied the tars
produced during torrefaction process. The tar problems are the
key difficulties of biomass pyrolysis and gasification. How to
convert and remove the tar is a very important topic.
Torrefaction is a very mild pyrolysis process, of which the tars
are mainly the primary tars. If the pyrolysis temperature is
increased or the atmosphere is changed, the tars will further
convert to secondary and tertiary tars [3]. In our study, we
selected rice straw, the typical agricultural residuals as the raw
Liquid nitrogen
Figure 1. The schematic diagram of the first stage hot rod reactor
The reactor used in our studies is the first stage of the twostage hot rod reactor by which we mainly investigated
pyrolysis process of biomass, showed in Fig.1. The original
concept for the two-stage hot rod reactor is from the Kandiyoti
research group in Imperial College London who have done
tremendous work in biomass pyrolysis and gasification [4,5].
To cooperate with Professor Kandiyoti, our research group
constructed a two-stage reactor and did a lot of investigations
on biomass gasification and tar conversion, also including
some improvements for the reactor [6,7]. Figure 1 is the
schematic diagram of the first stage hot rod reactor.
The first stage is made of 316-grade stainless steel with
diameters of 12 mm × 16mm and length of 300 mm. The
reactor is directly heated by electrical resistance and controlled
by K-type thermocouples. The system operates at atmospheric
pressure. The tar trap section is a U shape stainless steel tube
with diameters of 6 mm×8mm and length of 300 mm. The U
shape tube is connected with the first stage by a pair of flanges
and immerged into liquid nitrogen for tar trap.
4000000; column temperature: 35℃. Tetrahydrofuran (THF)
was used as the eluent solvent, flow rate is 1 mL/min. The
injection volume of the THF-diluted samples was 50μL.
A piece of a preweighed wire mesh plug was placed inside
the reactor tube and stuck at a fixed position at the bottom of
the isothermal area(about 40mm high) in the first stage. A total
mass of 1 ±0.01 g of rice straw was weighed and then dropped
into the reactor on top of the plug. N2 is the sweeping gas with
flow rate of 100 mL/min to provide a inert atmosphere and
reduce the secondary reactions. The temperature in the
isothermal area was set from the ambient to the set target (200
℃,250℃,300℃) by 20℃/min, then hold for 30 min. After the
pyrolysis process, the N2 was not turned off until the reactor
temperature cooled down below 50℃.
TABLE I.
C. Tar Sampling and Water Analysis
The tar trapped was washed by a mix solvent
(chloromethane and methanol) with a volume ratio of
CHCl3/CH3OH = 4:1. The U shape tube, the first stage reactor
and the char were all washed by the solvent. The char was
separated from the solution (including tar, moisture and
solvent) by a weighed filter paper. Then the char was dried and
weighed. The washed solution was rapidly tested for the
moisture by AKF-2010 Karl Fischer. Then the solution was put
into a RE3000A type rotary evaporator for 30 min to get rid of
most of the solvent. After this the tar was placed in a drying
oven with a temperature of 35℃ for 1 h in an inert atmosphere
for the final evaporation and then weighed as mass of the tar.
The dry tar was re-dissolved by the solvent and was tested by
GC/MS. This may caused a small mount of loss for the tar, but
it could keep consistent with the weighed tar.
D. Tar GC/MS and GPC Analysis
The tar constituents were tested by the Agilent 6890N GC
system and QP2010NC type gas chromatograph/mass
spectrometer (GC/MS) which was equipped with a splitter
injector. The split ratio is 10:1, and the oven temperature is 280
℃ . The mass spectrum was acquired using a quadrupole
instrument with an electron voltage of 70 eV. The GC column
was a HP-5MS column, 30 m long, 0.25 mm diameter, and
with a film thickness of 0.25 μm, with a flow rate of 2.4
mL/min. The column oven temperature was programmed from
45℃(held for 5 min) to 180 at 5 ℃/min, then increased from
180 to 300℃ at 20 ℃/min, and held for 10 min; each sample
volume is 1μL.
The molecular weight distribution of the tar constituents
were tested by a GPC（Gel Permeation Chromatography）
analyzer produced by Waters Inc., USA. The instrument
configuration and operating condition: Waters 1515 pump;
Waters 2414 differential refractive index detector; column
type: HR3, HR4, HR5(7.8×300 mm); detectable molecular
weight range: HR3:500-30000, HR4:5000-500000, HR5:2000-
III.
RESULTS AND DISCUSSION
A. Products Distribution via Torrefaction
PRODUCTS DISTRIBUTION OF RICE STRAW VIA TORREFACTION
T/℃
Tar /%
Char/%
H2O/%
Gas/%
200
2.42
85.83
9.32
2.43
250
5.87
68.55
14.30
11.28
300
13.47
46.93
16.72
22.88
The Table Ⅰ shows that, with the increasing of pyrolysis
temperature from 200℃ to 300℃, the char yield is decreasing
from 85.83% to 46.93%. The color of the char was getting
darken then be black. Deng [8] has investigated the
characteristics of the torrified char and found that the
grindability was greatly improved by torrefaction, most of the
alkali metal and about 60% of total energy were kept in the
char of 300 ℃ . The condensable product yield of tar and
moisture are increasing from 2.42%, 9.32% to 13.47%, 16.72%
respectively, with the rise of the temperature (200℃-300℃).
The tar will analyze in next part. The moisture is increasing not
only because of the vaporization of external moisture contained
in RS, but also the breaking of chemical bond to release of OH, -H radicals and form moisture. The non condensable gas is
greatly increasing when the temperature rising. Most of the
detected gas are CO2, CO and small amount of CH4 [8].
B. The Tar GC-MS Analysis
Fig.2 shows the GC/MS chromatogram of rice straw tar
samples with different pyrolysis temperature (200℃,250℃,300
℃ ). To analyze the tar in macro ways, the tar yield is
increasing from 2.42% to 13.47% with the temperature rising.
And the numbers of constituents of tar is obviously increasing
from 200℃ to 300℃ as can be seen in Fig.2. The constituents
of tar before residence time 15 min were not detected. It is
probably because of the loss of some easily-volatilized small
molecular weight tars due to the rotary evaporator process
(mentioned above). However, the yield of the light tars lost is
small which could be explained by the tar GPC analysis.
To analyze the tar in micro way, Table Ⅱ shows the
identified tar constituents under different pyrolysis conditions
(200℃,250℃,300℃). The identified tar constituents in Table 2
was selected by principles which was that the quality value of
each constituent is larger than 60 (for the MS chromatogram
analyze). That made the tar identified relatively more precise
and believable. The constituents detected with quality value
lower than 60 and large molecular weight (> 400g/mol) were
all out of considerations.
There are mainly three types of tar constituents, that is i)
phenols , which are the phenol derivates with forked chain
including methoxy, hydroxyl, aldehydes, alkanes, olefins
group. ii) esters of phthalic acid. iii) acids, ketones, esters,
alkanes, which are not containing benzene ring. Except for the
three main types of tar, the furans (furan derivates) and sugars
(monomer) are also detected. Some of the tar constituents we
Abundanc e
T IC: 20110413-R S200-Z YL(2).D\ da ta.ms
38.146
38.756
55000
Ab u n d an c e
Ri ce st r aw, 200¡æ£¬30mi n
50000
75 0 0 0
39.438
T IC: 2 01 1 0 4 15 -R S2 5 0 -Z Y L.D \ da ta .ms
3 8.1 4 6
Ri ce st r aw, 250¡æ£¬30mi n
70 0 0 0
45000
65 0 0 0
3 4 .8 61
40000
60 0 0 0
40 .236
38.580
35000
55 0 0 0
50 0 0 0
38.390
38.492
38.421
37.544
30000
45 0 0 0
37.996
36.872
38.065
25000
38 .3 8 9
40 0 0 0
35 0 0 0
34.046
36.077
20000
30 0 02
0.5 35
25 0 0 0
35.050
15000
2.947
10000
1 8.6 6 0
2 4 .28 5
3 7 .29 4
3 4 .0 4 9
20 0 0 0
36.402
15 .4 0 8
1 9 .30 1
15 0 0 0
2 7 .59 1
10 0 0 0
5000
50 0 0
0
0
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
5.0 0
1 0.0 0
1 5 .0 0
2 0 .00
2 5 .0 0
30 .0 0
3 5.0 0
4 0 .00
4 5 .0 0
T ime -->
T ime-->
Ab u n d an c e
T IC: 2 0 1 1 04 1 8 -R S3 0 0-Z YL .D \ d a ta .ms
3 8 .1 45
Ri ce st r aw, 300¡æ£¬30mi n
22 0 0 0 0
3 8 .7 57
20 0 0 0 0
37 .5 4 1
3 9 .44 1
18 0 0 0 0
1 8.6 8 7
16 0 0 0 0
3 6.8
36
.0 76
39
4 0 .2 43
14 0 0 0 0
12 0 0 0 0
41 .2 2 0
3 4.8 6 2
3 5 .0 4 5
3 8 .5 81
373.9
93
9.2
342
8 .4 4 4
10 0 0 0 0
1 9.2 9 4
23 .0 2 4
80 0 0 0
1 8 .9 82 2 4.2 7 5
18 .7 5
22
2 .0 1 8
60 0 0 0
40 0 020.5 4 9
2 5 .71
1 85
27 .5
01
11
45
.6.4
06
33
7.3
797
.8
980
2
36
.3
3
7 .2 72
34
.0
3
29
6
34
2.71
0
3.9
5
.9
2 57
3
33
3
.5
.74
86
90
4
3
3.4
.7
66
33
4 3 .9 90
20 0 0 0
0
5 .0 0
10 .0 0
1 5.0 0
2 0 .0 0
2 5 .00
3 0 .0 0
3 5 .0 0
40 .0 0
4 5.0 0
T ime-->
Figure 2. The GC/MS chromatogram of rice straw tar samples with different pyrolysis temperature
(a)
(b)
(c)
Figure 3. The GPC chromatogram of rice straw tar samples with different pyrolysis temperature (a-200℃,b-250℃,c-300℃)
TABLE II.
IDENTIFIED TAR CONSTITUENTS UNDER DIFFERENT PYROLYSIS CONDITIONS
Reletive Concentration /%
Residence Time
(min)
Formula
MW
(g/mol)
Library/ID
15.6805
C15H32
212
Dodecane, 2,6,10-trimethyl-
15.6862
C15H28O4
272
Oxalic acid, isobutyl nonyl ester
17.7576
C8H10O
122
Phenol, 3-ethyl-
18.3985
C5H8O3
116
18.6617
C9H18O2
158
(S)-(+)-2',3'- Dideoxyribonolactone
Propanoic acid, 2-methyl-,
3-methylbutyl ester
18.6846
C5H8O3
116
2-Oxo-n-valeric acid
18.7532
C6H6O2
110
9.19
18.9821
C6H8O4
144
1,2-Benzenediol
1,4:3,6-Dianhydro-.alpha.
-d-glucopyranose
19.2968
C8H8O
120
Benzofuran, 2,3-dihydro-
5.34
RS200
RS250
RS300
0.46
1.74
2.06
2.50
2.68
9.83
16.60
5.61
6.63
6.75
22.0205
C9H10O2
150
2-Methoxy-4-vinylphenol
3.91
4.32
23.0218
C8H10O3
154
Phenol, 2,6-dimethoxy-
3.82
6.19
23.2793
C7H6O2
122
Benzaldehyde, 4-hydroxy-
24.2749
C8H8O3
152
Vanillin
25.7111
C10H10O4
194
Dimethyl phthalate
25.7168
C9H9NO2
163
27.4678
C9H14N2S
182
Benzonitrile, 3,4-dimethoxy4,4-Dimethyl-5-methylene-2allylamino-2-thiazoline
27.5822
C10H14O2
166
Phenol, 2-methoxy-4-propyl-
27.5879
C9H12O3
168
Homovanillyl alcohol
32.1712
C10H12O4
196
Benzaldehyde, 3,4,5-trimethoxy-
33.7677
C13H26O
198
33.9107
C16H22O4
278
2-Undecanone, 6,10-dimethyl1,2-Benzenedicarboxylic acid,
bis(2-methylpropyl) ester
33.9679
C27H44O4
432
Phthalic acid, hexadecyl propyl ester
1.81
34.0309
C26H42O4
418
4.01
34.048
C21H28O4
344
34.048
C16H22O4
278
34.5688
C13H26O2
214
Phthalic acid, isobutyl propyl ester
Phthalic acid, isobutyl
non-5-yn-3-yl ester
1,2-Benzenedicarboxylic acid,
butyl 2-methylpropyl ester
Undecanoic acid, 10-methyl-,methyl
ester
34.8606
C16H32O2
256
35.0437
C19H23NO
281
n-Hexadecanoic acid
N-Benzyl-N-ethyl-pisopropylbenzamide
36.9663
C20H42
282
Eicosane
3.40
37.4412
C17H36
240
2.75
38.145
C19H26O4
318
38.391
C13H12O5S
280
Heptadecane
Phthalic acid, cyclohexyl
2-pentyl ester
3,5,6-Trimethyl-p-quinone,2-(2,5dioxotetrahydrofuran-3-yl)thio-
2.08
9.64
4.73
1.93
1.46
0.72
3.13
4.21
1.94
4.02
3.04
4.09
100
1.09
14.88
4.51
7.70
25.37
5.87
detected are similar to the results concluded by Elliott [9], but
their conditions with fast pyrolysis and higher temperatures
were different from ours. There was only one identified tar
constituent at 200 ℃ , which belonged to the ii) type. It is
probably because that most of tar with 200℃ are large MW tar
which could not be detected by GC/MS and the small MW tar
was of poor quality value. Under 250℃ pyrolysis condition,
the relative concentration of the i), ii), iii) types of tar, are
32.23%, 29.46% and 30.74% respectively. At 300 ℃ , the
values are 30.06%, 10.79% and 28.81% respectively. Furans
were detected both at 250℃ and 300℃. The long chain alkanes
and sugars were only found at 300℃.
secondary reaction with the i) and iii) type tar. There was no i)
type tar detected at 200 ℃, it is consistent to the decomposition
temperature range of 280-500 ℃[12].
To analyze the tar constituents in terms of lignocellulose
level, the content of the cellulose, hemicellulose and ligin for
rice straw are 34%,27.2% and 14.2% respectively [10]. The
rest of the rice straw are inorganic minerals and organic
extractives. The i) type tar are oriented from the thermal
decomposition of lignin, which was concluded by T.A. Milne
etc.[3].The iii) type tar (especially long chains) may be from
the pyrolysis of organic extractives(including fats and waxes).
The furals and sugars are from decomposition of cellulose and
hemicellulose [3,11]. The ii) type tar may be from the
Fig.3 shows the GPC chromatogram of rice straw tar
samples with different pyrolysis temperature (200℃, 250℃,
300℃). The peaks between the residence time 20-35min are
for the tar. According to the specification curve, the residence
time 31.803 min refer to the molecular weight of 500 g/mol. To
integral the peak area before and after the selected residence
time, the molecular weight distribution of tar will be obtained.
Table Ⅲ is the results after calculation. It indicated that, with
the growing pyrolysis temperature, the amount of smaller MW
C. The Tar GPC Analysis
TABLE III.
MOLECULAR WEIGHT DISTRIBUTIONIN OF THE BIOMASS TAR
SAMPLES (WT%)
T /℃
200
250
300
MW<500
34.93
53.77
65.54
MW>500
65.07
46.23
34.46
tar constituents(<500g/mol) increased and the amount of lager
MW tar (>500 g/mol)decreased.
IV.
CONCLUSION
The effect of the temperature on rice straw torrefaction
products distribution was investigated in a lab scale hot rod
reactor. Tar constituents under different conditions were
analyzed by GC/MS. The molecular weight distribution of tar
was obtained by GPC analysis. There were mainly three types
of tar, i) phenols; ii) esters of phthalic acid; iii) acids, ketones,
esters, alkanes, which are not containing benzene ring. The i)
tar type oriented from the decomposition of lignin. The yield of
small molecular weight (<500g/mol) of tar was increasing
when the temperature was rising. The mechanism of the low
temperature pyrolysis and production of the primary tar needs
to be studied in future.
ACKNOWLEDGMENT
The authors are grateful to the guiding of Professor Yonghao Luo.
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[1]