Fast pyrolysis of biomass in free-fall reactor for hydrogen-rich gas

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Fuel Processing Technology 85 (2004) 1201–1211

Fast pyrolysis of biomass in free-fall reactor for

hydrogen-rich gas

Shiguang Li, Shaoping Xu*, Shuqin Liu, Chen Yang, Qinghua Lu

Institute of Coal Chemical Engineering, Chemical School of Dalian University of Technology,

158 Zhongshan Road, PO Box 33, Dalian, LiaoNing, 116012, China

Received 28 April 2003; received in revised form 10 October 2003; accepted 1 November 2003

Abstract

Fast pyrolysis of two typical biomasses (legume straw and apricot stone) in a free-fall reactor was

studied. The pyrolysis behavior of the biomass is strongly relevant to its chemical composition.

Under fast heating and pyrolysis conditions as in the free-fall reactor, an interaction of the biomass

pyrolysis with the in situ steam reforming of the pyrolysis intermediate products occurs, which leads

to more hydrogen-rich gas yield. The total ratio of CO and H2 in the gas product reaches 65.4 mol%

for legume straw and 55.7 mol% for apricot stone, respectively. Effects of sample particle size and

pyrolysis temperature on the fast pyrolysis of the biomass were also studied.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Biomass; Fast pyrolysis; Free-fall reactor; Hydrogen-rich gas

1. Introduction

Biomass (which includes wood, agricultural residues, and so on) is widely available. It

is the fourth largest source of energy in the world, supplying about 14% of the primary

energy. In China the yield of agricultural residues is up to 0.81 billion tons, as much as

0.39 billion tons of standard coal, supplying about 32.1% of the primary energy in 1998

[1]. Biomass is a renewable, zero CO2 net emission energy resource. Due to these

characters, it attracts many efforts to develop various processes to make full use of

biomass.

0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.fuproc.2003.11.043

* Corresponding author. Fax: +86-411-3646633.

E-mail address: [email protected] (S. Xu).

S. Li et al. / Fuel Processing Technology 85 (2004) 1201–12111202

There are several thermochemical conversion methods of biomass, such as direct

combustion, liquefaction, gasification, pyrolysis, and so on. Among those, more interest

was put on pyrolysis, especially fast pyrolysis. And hydrogen-rich gas production from

biomass fast pyrolysis attracts much attention recently [2,3].

Many pyrolysis conditions such as particle size, temperature, heating rate, residence

time, S/B (steam/biomass) ratio, catalyst, and so on, strongly affect the yield and properties

of products. It needs a better understanding of these effects on both the primary pyrolysis

and the secondary reactions of fast pyrolysis in order to develop a model for biomass fast

pyrolysis that produces hydrogen-rich gas.

The present paper deals with fast pyrolysis of two typical biomasses in the temperature

range 500–800 jC in a free-fall reactor. Interest is focused on the yield of products,

especially that of hydrogen-rich gas.

2. Experimental

2.1. Raw material

Two typical biomasses were selected: legume straw (LS) and apricot stone (AS). The

approximate analysis and ultimate analysis of the biomass samples used are given in Table

1. The air-dried biomass was milled, sieved and classified to obtain fractions of uniform

particle size. For TG analysis, both the sizes of the two biomasses are 0.076–0.150 mm.

For fast pyrolysis, the size of the LS is 0.45–0.90 mm; and the sizes of the AS are 0.20–

0.30, 0.30–0.45, 0.45–0.90 and 0.90–2.00 mm, respectively.

Table 1

Proximate analysis, ultimate analysis and component analysis of the biomass

Parameter Legume straw Apricot stone

Proximate analysis (wt.%, ad. basis)

Moisture 9.80 8.52

Ash 1.62 0.17

Volatile matter 73.74 75.14

Fix carbon 14.84 16.17

Ultimate analysis (wt.%, daf. basis)

Carbon 43.30 44.39

Hydrogen 5.62 5.74

Oxygen (by difference) 50.35 49.45

Nitrogen 0.61 0.37

Sulfur 0.12 0.05

Component analysis (wt.%, daf. basis)

Extractives 1.96 5.20

Hemicellulose 34.08 20.83

Lignin 34.03 51.43

Cellulose 28.13 22.36

2.2. Component analysis of the biomass samples

The data of the component analysis of the biomass samples used are given in Table 1.

The analytical methods for extractives, hemicellulose, lignin and cellulose are as follows.

2.2.1. Analysis of extractives

The dried biomass sample (G0, g) is leached with mixture of benzene/ethanol (2:1 in

volume) at a constant temperature for 3 h. After air-drying, the residue is dried in an oven

at 105–110 jC to a constant weight. Then the residue is cooled to room temperature in a

desiccator and then weighted (G1, g). The extractive wt.% is calculated as

W1ðwt:%; dÞ ¼ G0 � G1

G0

� 100% ð1Þ

2.2.2. Analysis of hemicellulose

Put the residue G1 from the extractive analysis above in a flask and then add it into 150

ml NaOH solution (20 g/l). Boil the mixture for 3.5 h with recycled distilled water. Filter

and wash the residue till no more Na+, and dry it to a constant weight. The residue is then

cooled to room temperature in a desiccator and weighted (G2, g). The hemicellulose wt.%

is calculated as

W2ðwt:%; dÞ ¼ G1 � G2

G0

� 100% ð2Þ

2.2.3. Analysis of lignin

Put about 1 g of residue after extractives analysis as above into a weighed flask and dry

it to a constant weight. The sample is then cooled in a desiccator and weighed (G3, g).

Slowly pour 30 ml of sulphuric acid (72%) into the sample. Keep the mixture at 8–15 jCfor 24 h. Then transfer it into a flask and dilute it with 300 ml of distilled water. Boil the

sample for 1 h with recycled distilled water. After cooling and filtration the residue is

washed until there is no more sulfate ion in the filtrate (detected by 10% barium chloride

solution). The residue is then dried to a constant weight, cooled to room temperature in a

desiccator and weighted (G4, g). The hemicellulose wt.% is calculated as

W3ðwt:%; dÞ ¼ G4ð1�W1ÞG3

� 100% ð3Þ

2.2.4. Analysis of cellulose

The hemicellulose wt.% is calculated as

W4ðwt:%; dÞ ¼ 100� ðAd þW1 þW2 þW3Þ ð4Þ

2.3. Equipment and procedure

The fast pyrolysis of the biomass was conducted in a free-fall reactor shown in Fig. 1.

The reactor tube has a length of 1.8 m and an i.d. of 20 mm and is heated by three

independent electrical heaters. Three thermocouples are inserted in the middle of the three

S. Li et al. / Fuel Processing Technology 85 (2004) 1201–1211 1203

Fig. 1. Schematic diagram of free-fall reactor.


heaters outside the wall of the reactor tube, respectively. A screw feeder is situated on the

top of the reactor to give the pyrolysis sample. The carrier gas is N2.

For every experiment run, the reactor tube was heated to a setting temperature. The fast

pyrolysis took place when the biomass particles passed through the heated zone of the

reactor. The reaction pressure in the reactor was controlled around atmospheric by a

vacuum pump. The char was collected in a char receiver. The volatile passed through a

metallic filter to remove solid particles and was further cooled in four ice-cooled

condensers in series, where the condensable components were separated. The remaining

aerosol was removed in a filter filled with glass wool. The gaseous product was washed

with water (saturated with CaCl2) before it was pumped out by vacuum pump and

collected. At the end of the run the collected gas mixture was analyzed by gas

chromatograph (column: packed GDX104 and 5A molecular sieve columns; detector:


TCD; carrier gas: Ar, flow rate: 30 ml/min for 5A column and 25 ml/min for GDX104

column; column temperature: 35 jC ). Temperature program heating (from 35 to 150 jC)was used in GC for GDX104 column. The analysis data were recorded by a NC-2000

chromatograph data workstation.

2.4. TG analysis

TG analysis was carried out in a PCT-1 TGA. The experimental setup is as follows: a

small sample (about 30 mg) of starting material is weighted and spread evenly in a sample

cup. The cup is then placed on the balance sample holder. The startup protocol is initiated.

And finally, the sample is heated at a constant preset heating rate to a desired temperature

(800 jC) using N2 (99.99 vol.%) as the carrier gas at a constant flow rate of 40 ml min� 1.

3. Results and discussion

3.1. TG analysis of biomass

Typical TG and DTG curves for LS and AS are shown in Figs. 2 and 3. Around 100 jCboth samples have a DTG peak of dehydration. In the temperature range 200–500 jCevident difference in weight loss behavior for the two samples is shown: LS sample has

one DTG peak (Fig. 2), while AS sample has two peaks (Fig. 3). This is relevant to the

composition of the biomass and the characteristic pyrolysis property of the components.

Biomass is typically composed of cellulose, hemicellulose and lignin. Under ordinary

heating rate the hemicellulose pyrolysis is completely under 350 jC, cellulose pyrolysis

Fig. 2. TG and DTG curves of LS.


between 250 jC and 500 jC, and lignin pyrolysis slowly spreading almost all the pyrolysis

temperature range even over 500 jC and no sharp weight loss peak appears. The lignin is

relatively more thermally stable than hemicellulose and cellulose. With regard to the

components of the two samples, LS is relatively more hemicellulose and cellulose and

decomposes rapidly under relatively lower temperature. Due to its relatively more

hemicellulose and cellulose composition, the corresponding weight loss peaks become

wider and fully overlap one another and as a result show a single DTG peak (10 jC/min)

in Fig. 2. AS, on the other hand, is relatively more lignin and less hemicellulose and

cellulose, and only partial overlap of the peaks happened and there are two peaks (10 jC/min) in Fig. 3. From the TG curves (10 jC/min) in Figs. 2 and 3, we can also see that the

higher lignin composition of the apricot stone leads also to higher char yield as compared

with that of legume straw.

The influences of heating rate on the pyrolysis of the two biomasses are also shown in

Figs. 2 and 3. With the increase of the heating rate (10–40 jC/min) the DTG peaks move

to a higher temperature range and the peak of dehydration becomes wider. It could be

expected that under very fast heating and pyrolysis condition the so-called dehydration and

pyrolysis processes could happen simultaneously. This provides an opportunity for

producing more hydrogen and carbon monoxide through an interaction of the fast

pyrolysis of the biomass with the in situ steam gasification or steam reforming of the

pyrolysis intermediate products, especially in the occasion of the free-fall reactor pyrolysis

system.

3.2. Effect of sample particle size on fast pyrolysis of AS

The effect of sample particle size on the yield of products from AS fast pyrolysis is

shown in Fig. 4. With the decrease of particle size (from 0.90–2.00 to 0.20–0.30 mm),

Fig. 3. TG and DTG curves of AS.

Fig. 4. Effect of sample particle size on yield of products from fast pyrolysis of AS (at 800 jC).


both the yield of the solid product (char) and that of the liquid product (bio-oil) are

decreased and that of the gas product is increased: from 30.7 to 3.2 wt.% for char, 48.3 to

17.8 wt.% for bio-oil and 16.3 to 71.3 wt.% for gas product. The smaller particle size

favors the yield of gas product.

The effect of sample particle size on the composition of gas product from AS fast

pyrolysis is shown in Fig. 5. With the decrease of particle size from 0.90–2.00 to 0.20–

Fig. 5. Effect of sample particle size on composition of gas product from fast pyrolysis of AS (at 800 jC).

Table 2

Effect of sample particle size on H2 +CO and CO/H2 in gaseous product from fast pyrolysis of AS (at 800 jC)

Particle size

(mm)

CO+H2

(mol%)

H2/CO

(mol/mol)

0.90–2.00 40.8 0.09

0.45–0.90 55.6 0.47

0.30–0.45 60.0 0.47

0.20–0.30 60.6 0.57


0.30 mm, H2 concentration is increased from 3 to 22 mol%. It is shown in Table 2 that the

total concentration of CO+H2 (component of hydrogen-rich gas) reaches more than 60

mol% when particle size is 0.20–0.30 mm. The H2/CO ratio is also greatly increased with

the decrease of the particle size. The smaller particle size favors the yield of hydrogen-rich

gas.

Comparing the char yields shown in Figs. 4 and 2, it can be seen that the yield of char

from fast pyrolysis at 800 jC is lower than that from slow pyrolysis. The lower yields of

char and liquid and, at the same time, the higher yields of hydrogen-rich gas from fast

pyrolysis could be attributed partly to the in situ gasification or steam reforming of the

pyrolysis intermediate products; this could be seen from the decrease in CO2 concentration

and the increase in CO and H2.

3.3. Effect of pyrolysis temperature on products from fast pyrolysis of biomass

The effects of pyrolysis temperature on the yield of products from fast pyrolysis of LS

and AS are shown in Figs. 6 and 7. For LS (in Fig. 6), with a temperature increase from

Fig. 6. Effect of pyrolysis temperature on yield of products from fast pyrolysis of LS (sample particle size:

0.45–0.90 mm).

Fig. 7. Effect of pyrolysis temperature on yield of products from fast pyrolysis of AS (sample particle size:

0.45–0.90 mm).


500 to 700 jC, the yield of gas product increases and both liquid and solid decrease. At

higher temperatures (700–800 jC), the yields change more slowly or remain constant. For

AS (in Fig. 7), in the temperature range 500–800 jC, the gas product yield increases and

the solid yield decreases with the temperature increase, while the liquid product yield

Fig. 8. Effect of pyrolysis temperature on composition of gas product from fast pyrolysis of LS (sample particle

size: 0.45–0.90 mm).

Fig. 9. Effect of pyrolysis temperature on composition of gas product from fast pyrolysis of AS (sample particle

size: 0.45–0.90 mm).


reaches a maximum (about 66 wt.%) around 600 jC. Compared with AS, LS produces

more gaseous products at the same pyrolysis temperature. In other words, the pyrolysis of

cellulose and hemicellulose produces more gaseous products than that of lignin at the same

temperature.

The effects of pyrolysis temperature on the composition of gas product from fast

pyrolysis of LS and AS are shown in Figs. 8 and 9. Higher temperature is in favor of

hydrogen-rich gas product. With a temperature increase from 500 to 800 jC, the H2

concentration in product gas increases from 3.2 to 28.2 mol% for LS, and from 2.4 to 17.8

mol% for AS, respectively. And it is shown in Table 3 that the total concentration of

CO+H2 (syngas) reaches more than 65 mol% for LS and 55 mol% for AS at 800 jC,respectively. The H2/CO ratio is also increased with the increase of the pyrolysis

temperature for the two biomasses. At the same pyrolysis temperature LS produces more

hydrogen-rich gas than AS. In other words, biomass composed of more cellulose and

Table 3

Effect of pyrolysis temperature on H2 +CO and CO/H2 in gaseous product from fast pyrolysis of LS and AS

(sample particle size: 0.45–0.90 mm)

Temperature (jC) LS AS

H2 +CO

(mol%)

H2/CO

(mol/mol)

H2 +CO

(mol%)

H2/CO

(mol/mol)

500 32.5 0.11 27.3 0.10

600 51.9 0.51 34.6 0.24

700 61.5 0.49 35.4 0.34

800 65.4 0.76 55.6 0.47


hemicellulose is a better source for hydrogen-rich gaseous production than that composed

of lignin.

4. Conclusion

The results of this study show that the fast pyrolysis of biomass produces more volatile

than the slow pyrolysis at the same temperature. The yield and composition of the

hydrogen-rich gas product from fast pyrolysis are relevant to the composition of the

biomass. Cellulose and hemicellulose produce more hydrogen-rich gas than lignin.

The results also show that smaller particle size of biomass and higher fast pyrolysis

temperature are in favor of hydrogen-rich gas production. With the decrease of the particle

size and increase of the fast pyrolysis temperature, the heat shock on biomass pyrolysis

becomes stronger. This results in the increase of the smaller molecular product (such as

bio-oil and gas, especially syngas) and the decrease of the heavier molecular product (such

as char). The in situ gasification of the char and/or the steam reforming of the bio-oil

intermediate might be one of the reasons that lead to more hydrogen-rich gas.

References

[1] S.P. Xu, et al. Investigation of the technique for straw conversion and utilization in northeast of China. Report

to NEDO, March 2000, pp. 1–7.

[2] D. Ferdous, A.K. Dalai, S.K. Bej, R.W. Thring, N.N. Bakhshi, Production of H2 and medium Btu gas via

pyrolysis of lignins in a fixed-bed reactor, Fuel Process. Technol. 70 (1) 2001, pp. 9–26.

[3] A. Demirbas, Yields of hydrogen-rich gaseous products via pyrolysis from selected biomass samples, Fuel 80

(13) (2001) 1885–1891.

Fast pyrolysis of biomass in free-fall reactor for hydrogen-rich gas

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