Wen-Liang WANG, Xue-Yong REN, Yan-Zhe CHE, Jian-Min CHANG, Jin-Sheng GOU

Kinetics and FTIR characteristics of the pyrolysis process of poplar wood

Wen-Liang WANG, Xue-Yong REN, Yan-Zhe CHE, Jian-Min CHANG?, Jin-Sheng GOU

College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China

The pyrolysis characteristics of residues of poplar (Populussp.) wood were investigated using a thermogravimetric analyzer coupled with a Fourier transform infrared (TG-FTIR) spectrometer. The pyrolysis process was subdivided into four stages at a rate of 10°C·min?1, varying from 30 to 650°C. Below 180°C, a mass loss occurred for drying and preheating the sample and the onset temperature of pyrolysis ranged between 180–260°C. A signif i cant mass loss of 61.4 wt.% occurred between 260–380°C, followed by a slow and continuous mass change with lignin devolatilization.The analysis of kinetic reactions showed that the activation energy (78.29 kJ·mol?1) in the low-temperature section was much larger than that (6.40 kJ·mol?1) in the high-temperature section. The evolved gases formed by thermal degradation of poplar wood were simultaneously analyzed by FTIR. It was observed from the main peaks that the emissions evolved during poplar wood pyrolysis were acetic acid, carbon dioxide, carbon monoxide, methane, water, some volatile compounds of esters, alcohols and aldehydes. The emissions gradually increased with rising temperatures before a strong peak around 360°C and then decreased. Most gaseous products were emitted in the 320–380°C range, while CO2was continuously emitted in a wide range from 140–550°C.

poplar wood, pyrolysis, TG-FTIR, kinetics, characteristics

?Author for correspondence (Jian-Min CHANG)

E-mail: cjianmin@bjfu.edu.cn

Introduction

Woody biomass is considered a renewable and alternative resource for renewable production.Poplar (Populus) wood, an inexpensive sustainable source of carbon, is abundant in China and has great potential as a feedstock for the production of renewable fuels and commodity chemicals.Hence, eff i cient use of poplar wood, especially its residues, is of scientif i c importance and has application value.

Poplar wood residues can be directly burned for cooking or for power generation, but the effi ciency of direct burning is low. Therefore, many attempts have been made to utilize wood residues as raw material for the preparation of value-added products. Pyrolysis is one of the more promising thermochemical processes for transforming solid biomass mainly into liquid pyrolytic products, solid bio-char and combustible gases.

Some researchers have focused on the characteristics of pyrolysis of woody biomass. Lin and Jiang (2008) studied the characteristics of the pyrolysis process of poplar sawdust and its kinetics with thermogravimetric analysis (TG). Lee and Fasina (2009) studied the rate and kinetics of thermal decomposition of switchgrass (Panicum virgatum) and quantified the composition of gas evolved during the thermal pyrolysis process us-ing TG-FTIR (Fourier transform infrared). Ren et al. (2012) discussed the weight-loss characteristics and gas emission of larch (Larix gmelini) wood at different heating rates by TG-FTIR.

Poplar wood is expected to contribute significantly to renewable production. However, the pyrolysis of poplar wood is a complicated process involving numerous intermediate products and overlapping reactions, which depends on several factors such as decomposition temperature, heating rate, particle size and composition of poplar wood (Rao and Sharma, 1998). Therefore, it is necessary to investigate the characteristics and mechanism of pyrolysis in poplar wood. Thermogravimetric analysis is a technique giving characteristic information about the composition of the measured sample and quantifying thermal decomposition kinetics during pyrolysis (Vuthaluru, 2004).FTIR is most useful for identifying chemicals that are either organic or inorganic. It can be utilized to quantify the components and applied to the analysis of pyrolysis gas. A TG-FTIR combination yields a powerful analytical technique which combines the quantitative capabilities of TG and the qualitative capabilities of FTIR spectroscopy.

This study was aimed at determining the rate and kinetics of thermal decomposition of poplar wood and analyzing the composition and distribution of the pyrolysis emissions during the thermal pyrolysis process using TG-FTIR, which provides the scientif i c and theoretical basis for the characteristics of poplar wood.

Materials and methods

Plant materials and experiments

Poplar (Populussp.) wood samples, collected from the Xiao Hinggan mountains of Heilongjiang Province, northeastern China, were crushed into particles with an average diameter of 150–250 μm after being dried in an oven at 90°C for 4 h.

Methods

An on-line TG system was applied. The pyrolysis of poplar wood samples was carried out using a thermogravimetric analyzer, the STA 449C(NETZSCH, Selb, Germany). Our experiment with the samples was conducted at a rate of 10°C·min?1with a heating temperature range of 30–650°C.High purif i ed nitrogen was used as carrier gas at a fl ow rate of 50 mL·min?1. A FTIR spectrometer(TENSOR 27, Bruker, Billerica, USA) was used to quantify the gases evolved during pyrolysis in the TG. A transfer line was used to connect the FTIR to the TG. Resolution in FTIR was set at 4 cm?1and the spectral region at 4500–600 cm?1.

The Coats-Redfern method (Coats and Redfern,1964) was used to analyze the kinetic characteristics of pyrolysis of poplar wood. The kinetics of pyrolysis are largely described by the fi rst-order Arrhenius law (White et al., 2011), which was divided into two sections: 1) a low temperature section ranging from 260 to 380°C, during which the primary decomposition reactions occurred and 2) a high temperature section ranging from 380 to 480°C, during which nearly all the pyrolysis reactions took place. The mass loss fraction (α) is def i ned as follows:

whereMois the initial mass,Msthe mass during pyrolysis andM∞the fi nal mass.

The Coats–Redfern formula used to calculate the kinetic parameters of fi rst-order reactions is as follows:

whereEis the activation energy (kJ·mol?1),Athe pre-exponential factor (min?1),βthe heating rate(°C·mol?1), R the universal gas constant (8.314 J·K?1·mol?1) andTthe temperature (K).

Results

TG analysis

The result of component analysis is shown in Table 1. The TG and derivative TG (DTG) curves for poplar wood samples are shown in Fig. 1. The onset temperature of pyrolysis was in the rangeof 180–260°C. The main mass loss ended at 380°C for poplar wood samples, followed by a slow and continuous mass change with lignin devolatilization. The pyrolysis process was subdivided into four stages based on the DTG prof i le.

Table 1 Component analysis of poplar wood samples

Fig. 1 TG and DTG curves for pyrolysis of poplar wood

The first stage was characterized by a mass loss for drying and preheating the sample below 180°C. The mass decreased by about 0.6 wt.% due to the release of surface moisture and inherent moisture. In this stage, the mass loss was small and the rate of mass loss low because of the low moisture content of the samples. Desorption of moisture and melting of wax composition also occurred for the poplar wood.

In the second stage the temperature ranged between 180–260°C and the mass loss was about 3.3 wt.% of the original weight, due to a small amount of depolymerization and recombination of internal material in wood and a slow changing process with glass transition.

Thermal decomposition was fast in the third stage for poplar wood between 260–380°C. As can be seen from Fig. 1, a significant loss of sample mass (about 61.4 wt.% of the original weight) occurred at this stage. The mass decreased rapidly due to decomposition of large amounts of cellulose and hemicellulose into small molecular gases and macromolecular condensed volatiles.The DTG curves showed that poplar wood exhibited two overlapping peaks: a single peak (at a temperature of 361.8°C and mass loss rate of–10.96 wt.%·min?1) on the right and a shoulder peak (at a temperature of 316.0°C and mass loss rate of –5.36 wt.%·min?1) on the left. According to previous studies on biomass feedstocks (Tsamba et al., 2006; Fu et al., 2009), the thermal decomposition of cellulose and hemicellulose in a lower heating rate led to the separation of DTG peaks;the shoulder peak on the left side corresponds to hemicellulose decomposition while the higher temperature peak represents the degradation of cellulose.

The last stage in poplar wood pyrolysis was characterized by a further cracking process of the residues in a wide temperature section from 380°C to 650°C at the end of this experiment.About 12.2 wt.% of the total mass loss occurred at a lower rate during this stage. The slow rate of mass loss might be due to lignin decomposition.In general, lignin is harder to decompose than cellulose because part of the lignin consists of benzene rings (Gani and Naruse, 2007).

Initially, the pyrolysis behavior of poplar wood was investigated up to 650°C. However,the results generated by TG curves showed that between 30 and 450°C, the mass loss varied from 0 to 71.0 wt.% and reached 77.3 wt.% at the fi nal temperature. Hence, we show that the temperature of 450°C is enough for thermal decomposition and pyrolysis of poplar wood.

Analysis of kinetic parameters of pyrolysis

We used the Coats–Redfern method (Coats and Redfern, 1964) to analyze the kinetic characteristics of pyrolysis of poplar wood. The calculated kinetic parameters are listed in Table 2.The activation energy (78.29 kJ·mol?1) in the low temperature section was much larger than in the high temperature section (6.40 kJ·mol?1), suggesting that the pyrolysis reaction of poplar wood is prone to occur in the higher temperature section.Macromolecules in poplar wood were completely broken and reactions of dehydration and depolymerization occurred which, more easily, resulted in further condensation of intermediates. Hence,in the high temperature section, condensation occurred more easily and manifested as a smaller amount of activation energy on the kinetics of reactions.

FTIR analysis of gaseous products

The gaseous products from TG analysis were detected on-line using FTIR. Figure 2 shows the FTIR spectra of the gases released from poplar wood during the different stages of pyrolysis treatment. The main peaks observed are listed in Table 3. FTIR analysis of poplar wood showed that prominent peaks were in the region of 4500–600 cm?1and the main volatile compounds emitted at typical stages were identif i ed. It was found that the emission of gases gradually increased with rising temperatures before a strong peak around 360°C and then decreased till the end of the experiment. The maximum intensities of gases in FTIR spectra agree well with the maximum mass loss rate of the DTG curve, which occurred at 361.8°C. This is exactly the same temperature of the maximum mass loss rate during pyrolysis.CO2and some volatile organic compounds, such as carboxylic acid and aldehyde, were identif i ed as the main pyrolysis gaseous products. Small amounts of NH3and CO were also detected. Most gaseous products were emitted in the 320–380°C range, while CO2was continuously emitted from a wide temperature range of 140–550°C.

At the stage of maximum absorption intensities in FTIR (361.8°C), the ratio of the highest absorbance values in these regions (1846–1624 cm?1)indicates the relative tendencies of carboxylic acid and aldehyde (C=O bonds). The absorptions of O―H bonds found in the range of 3686–3484 cm?1are well known to be due to water. The absorptions of C―H bonds found between 3044–2760 cm?1are normally attributed to hydrocarbons, often specif i cally to methane, but it is not appropriate and specif i c to regard this region as a methaneabsorption band. The formation of CO2and CO is due to the presence of stretching vibration of C=O and C―O bonds in the range of 2388–2278 and 2241–2141 cm?1, respectively. C―H bonds show important absorptions between 1561–1487 cm?1, indicating the formation of some organic compounds related with volatile hydrocarbons.An important and strong peak between 1240–993 cm?1was seen as an important characteristics of the carboxyl group, suggesting the compounds of esters and alcohols. C―C bonds could be found in the range of 707–652 cm?1.

Table 2 Kinetic parameters from the pyrolysis of poplar wood

Fig. 2 FTIR spectrogram for gaseous products during poplar pyrolysis at different temperatures

Table 3 Analysis of poplar IR functional groups

Conclusions

1) The pyrolysis process of poplar wood was subdivided into four stages. The onset temperature of pyrolysis was in the range of 180–260°C. A significant mass loss occurred between 260–380°C,followed by a slow and continuous mass change with lignin devolatilization.

2) The analysis of kinetic reactions showed that the pyrolysis process could be described by a simple fi rst-order reaction and was divided into two sections: low and high temperature sections. The activation energy in the low temperature section of 260–380°C was 78.29 kJ·mol?1and 6.40 kJ·mol?1in the high temperature section of 380–480°C.

3) FTIR analysis of poplar wood showed prominent peaks in the region of 4500–600 cm?1.The emission of gases gradually increased with the increasing temperature before a strong peak around 360°C and then decreased. The major gases evolved during poplar wood pyrolysis were acetic acid, carbon dioxide, carbon monoxide,methane, water, some volatile compounds of esters, alcohols and aldehydes.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 30972309)and the Doctoral Fund of the Ministry of Education of China (No. 20090014110015).

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14 June 2012; accepted 27 August 2012