Effect of torrefaction on pinewood pyrolysis kinetics and thermal behavior using thermogravimetric analysis
Introduction
Population growth and economic development have led to a growing demand for energy every year. Currently, the main source of energy comes from fossil fuels (Gong et al., 2017). The large-scale use of fossil fuels has caused serious environmental problems and resource depletion. So far the renewable energy has become of growing interest. Lignocellulosic biomass represents large amounts of renewable carbon, which has the potential to be converted into solid, liquid and gaseous products (Brunner et al., 2019, Mishra and Mohanty, 2018).
However, high water content, low energy density and difficulty in grinding for raw biomass limit the utilization. Due to these disadvantages, the pretreatment process of biomass has attracted extensive attention. Torrefaction pretreatment is a heat treatment between 200 °C and 300 °C, which has been proved to be a promising method for biomass upgrading (Chen et al., 2018a, He et al., 2018a). Besides, thermochemical conversion process is one of an effective way to utilize biomass, including pyrolysis, combustion and gasification. Among them, pyrolysis shows great potential to transform biomass into bio-fuels, absorbent bio-char, and various useful chemicals (Wang et al., 2016). Also, pyrolysis is considered as a transitional step in combustion and gasification (Li et al., 2015). In addition to the pyrolysis conditions, the literatures indicated that the torrefaction had a significant influence on the pyrolysis characteristics and kinetics (Bach et al., 2017b, Chen et al., 2015, Zhang et al., 2018). Therefore, it is essential to obtain a deep knowledge about the effect of torrefaction on biomass pyrolysis.
Studies on the wood pyrolysis have been reported in literatures. Mishra et al. studied the pyrolysis kinetics of three waste sawdust biomass using the iso-conversion method, in which the activation energy of pine sawdust was about 170 kJ/mol (Mishra and Mohanty, 2018). And Poletto et al. studied the influence of components on wood thermal decomposition (Poletto et al., 2012). As for pyrolysis of torrefied wood, Ru et al. studied the pyrolysis of torrefied polar wood, and found that the physicochemical characteristics and pyrolysis behaviors significantly changed after torrefaction, especially at 250 °C (Ru et al., 2015). Chen et al. reported the effect of torrefaction on the catalytic pyrolysis characteristics of pinewood using a fixed bed reactor (Chen et al., 2016). Neupane et al. investigated the effects of torrefaction temperature and residence time on the pinewood structure and pyrolysis product distribution (Neupane et al., 2015). Bach et al. pointed out that activation energy and pre-exponential factors increased after dry torrefaction (Bach et al., 2017a). However, these studies did not further analyze the pyrolysis mechanism and thermodynamic parameters. The literatures reported the effect of torrefaction on pyrolysis kinetics were not sufficient and the mechanisms were still unclear. It is necessary to conduct in-depth research about the effect of torrefaction on pyrolysis kinetics.
Thermogravimetric analysis (TGA) is the most commonly used method for pyrolysis kinetics. Due to the slow heating rate in TGA, the isothermal method will have a certain weight loss before reaching the set temperature, resulting in a deviation from the estimation of the kinetic parameters. Therefore, the non-isothermal method is more reliable and time-saving (Huang et al., 2016). Non-isothermal method includes model fitting method and model free (iso-conversion) method. The iso-conversion method can provides activation energy at progressive degrees of conversion without knowing the mechanism, including Kissinger-Akahira-Sunose (KAS), Ozawa-Flynn-Wall (OFW), Friedman, etc. (Ma et al., 2015, Mishra and Mohanty, 2018). Pyrolysis kinetics provides abundant information for process parameter optimization and new reactor design (Dhyani et al., 2017).
In this paper, pinewood, an important source of lignocellulosic biomass, was selected to study. The aim of this study is to perform a more detailed investigation about the effect of torrefaction on pinewood pyrolysis kinetics and thermodynamics. The structures of biomass were characterized by Fourier transform infrared spectrometer and X-ray diffraction. Kinetic parameters were calculated by iso-conversion methods. To establish kinetic-control, the master plots was utilized. And thermodynamic parameters were also determined. The differences of pyrolysis process were explained based on the variation of structure parameters. These careful comparisons help to understand the effect of torrefaction on pinewood pyrolysis and provide a reference for thermal conversion applications using pinewood as the raw material.
Section snippets
Sample preparation
Pinewood (PW) was selected as the raw material. The pinewood was crushed and sieved to 80 ∼ 120 mesh before use. The torrefaction was carried out in a fixed bed reactor as shown in Fig. 1. The torrefaction process was as follows: ∼10 g sample was placed in the hanging basket and stayed in the water cooling area; N2 was then fed into the reactor with a flow rate of 500 mL/min; after the set temperature was reached, the hanging basket was put into the constant temperature zone and stayed for 1 h.
Composition analysis
The ultimate analysis, proximate analysis and biochemical composition of PW and PW250 were shown in Table 2. It could be found that the volatiles decreased from 84.8% to 80.4% (wt.). The decrease of biomass volatiles was not obvious. And the H/C ratio of the biomass decreased from 0.13 to 0.12, the O/C ratio decreased from 0.90 to 0.83 after torrefaction. Specifically, the oxygen content was reduced from 43.5% to 41.9% (wt.), and the hydrogen content was decreased from 6.5% to 6.3% (wt.).
Conclusions
Effect of torrefaction on pinewood pyrolysis was investigated. After torrefaction at 250 °C, the cellulose content and the crystalline degree was increased, while the volatiles was reduced by only 4% (wt.). Devolatilization and heat transfer performances of torrefied pinewood were enhanecd. Pyrolysis activation energy were calculated by Frideman, OFW and KAS methods, and the results ranged from 160 to 200 kJ/mol. There was a little variation in thermodynamic parameters. However, the pyrolysis
Acknowledgements
This work was supported by National Key R&D Program of China (2017YFB0602601) and National Natural Science Foundation of China (21878093).
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