The pyrolysis of willow samples from various plant positions was analysed using thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR). The results indicate that pyrolysis can be divided into four stages. The first stage from 30 to 120°C involves free evaporation of moisture, with a mass loss of 5%. The second stage from 120 to 200°C involves the pyrolysis of hemicellulose and unstable cellulose, with a mass loss of 4%. The third stage is from 200 to 400°C, with a weight loss of 60%, in which the chemical components of wood thermally decompose and emit heat, carbon dioxide, and so on. In the final stage, which occurs above 400°C, the pyrolysis of lignin and charring of cellulose occur, with a mass loss of 10%. Moreover, in FTIR, the samples exhibit the highest absorbance during the main pyrolysis phase, from which wood vinegar ingredients mainly arise, including CO2, H2O, CO, and small amounts of hydrocarbons, alcohols, phenols, acids, esters, and aromatic compounds. Additionally, leaves are decomposed more thoroughly before the main pyrolysis phase, whereas decomposition of branches occurs fullest during this phase. Finally, we put forward some suggestions to support further research on conversion of willow into wood vinegar products.
The willow tree has some excellent characteristics, such as fast growth, strong transpiration, and high biomass content. Willow not only provides a purification effect to the surrounding environment but also plays an important role as a natural landscape with good ecological effects. The willow wastewater facility (see Supplementary Material available online at
Beside willow, there are a number of other sources of wood vinegar with varied performance characteristics. The main components of wood vinegar, with the exception of water with a content of 80%–90%, are organic acids, phenols, alcohols, and ketones. Acids are the most typical components of wood vinegar and often account for more than 50% of the organic matter. The composition and component content of wood vinegar can be affected by different preparations, pyrolysis temperatures, and the type of raw materials [
Pyrolysis is a key step in the forest biomass conversion process and in the production of wood vinegar. However, this method lacks reliability when used alone [
The formation mechanisms of different types of vinegar have been examined and their compositions and applications in agriculture have been reported in some previous studies [
However, there has been little research on the pyrolysis of willow specimens from different parts of the plant. In this study, we have examined the pyrolysis of three types of willow samples collected from different parts of the plant, namely, leaves, stems, and branches, using TG-FTIR [
Selected leaves, stems, and branches from three parts of the willow plant in the zero-discharge wastewater system were used as samples for the experiments. The willow samples were air-dried at 90°C for 24 h. Then, the samples were ground into particles with a 0.1 mm grinder to obtain particle sizes of ≤0.1 mm.
The biomass of willow cultivars is known to have good thermophysical compositions and contain cellulose and hemicelluloses, with only small amounts of undesirable components, such as ash, sulfur, and chlorine [
The willow samples were first analysed using a PerkinElmer STA6000 TGA instrument to obtain baseline data. To reduce the impact of heat and mass transfer, as well as temperature gradients inside the material during the pyrolysis process, about 10 mg of the sample was placed in a crucible made of alumina. Prior to collecting the baseline data, the sample in the crucible was subjected to high temperature calcination at about 1300°C to reduce the impact of other impurity components on the TGA curves. Nitrogen (99.99%) at a flow rate of 30 mL min−1 was passed through the furnace to ensure an inert atmosphere. The initial temperature of the furnace was set at 30°C. Once the furnace stabilised at 30°C, the sample was heated from 30 to 600°C at a heating rate of 20°C min−1.
The decomposition gases generated during pyrolysis were analysed by IR spectroscopy using a PerkinElmer Spectrum 100 FTIR spectrometer. The line connecting the TGA unit with the IR spectrometer was insulated to ensure that all the gases produced during pyrolysis entered the IR spectrometer. The compositions of the pyrolysis gases were analysed in real time in the IR frequency range of 4500–600 cm−1.
The willow samples underwent a series of complex chemical reactions during the pyrolysis process. Figures
TGA curves for the three types of willow samples.
Derivative TGA curves for the three types of willow samples.
Based on Figure
It is evident from Figure
The differential scanning calorimetry (DSC) curves for the three types of willow samples are shown in Figure
DSC curves for the three types of willow samples.
A large absorbance value implies that the gas being analysed has a high concentration. From Figures
FTIR spectra of willow leaves at different times. (a) Three-dimensional IR spectrum of willow leaves and IR spectra obtained at (b) 200 s, (c) 525 s, (d) 830 s, (e) 1100 s, and (f) 1420 s.
FTIR spectra of willow stems at different times. (a) Three-dimensional IR spectrum and IR spectra obtained at (b) 200 s, (c) 525 s, (d) 830 s, (e) 1100 s, and (f) 1420 s.
FTIR spectra of willow branches at different times. (a) Three-dimensional IR spectrum and IR spectra obtained at (b) 200 s, (c) 525 s, (d) 830 s, (e) 1100 s, and (f) 1420 s.
Figure
As evidenced from Figures
Above all, it can be seen that monitoring the pyrolysis process via IR spectroscopy enables verification of the steps that occur during the process and the products generated. Moreover, the pyrolysis products mainly consist of CO2, H2O, and CO, as well as a small amount of hydrocarbons, alcohols, phenols, acids, esters, and aromatic compounds. Based on the thermal IR spectra, the decomposition of the willow particles can be divided into four stages, namely, water loss, degradation of hemicellulose and unstable cellulose, decomposition of cellulose and lignin, and charring. Wood vinegar is formed from the main pyrolysis stage until the charring stage. The analysis results are consistent with those of Biagini et al. and Pétrissans et al. [
From the DTG and DSC plots, it can be concluded that the primary pyrolysis stage for the three types of willow samples occurs in the 200–400°C range. However, the peak in the TG curve for the willow branches is more pronounced than those for the willow stems and leaves, indicating that the willow branches decompose more evenly and thoroughly compared with the other two samples. Therefore, the actual rate of heating in large particles is below the values set in the heating rate experiments [
Thermal decomposition of the willow particles occurs in four stages, namely, water loss, hemicellulose and unstable cellulose decomposition, decomposition of cellulose and lignin, and charring. These stages reflect the formation of wood vinegar. From the three-dimensional IR spectra, willow particles are found to exhibit higher absorbance during thermal decomposition at 200, 525, 830, 1100, and 1420 s. The thermal IR spectroscopy results suggest that the formation of wood vinegar, including CO2, H2O, CO, and small amounts of hydrocarbons, alcohols, phenols, acids, esters, and aromatic compounds, occurs mainly from the main pyrolysis stage until the charring stage.
According to the qualitative results of these experiments, we put forward some suggestions to improve conversion of willow into wood vinegar products. For willow materials with the same particle size, when the collecting temperature is below 200°C, we can acquire more active ingredient from willow leaves, whereas when the temperature is above 200°C, branches should be chosen. With an increase of temperature, more types of ingredients arise. It is suggested to collect willow wood vinegar over different temperature ranges to obtain targeted components for various purposes more efficiently and effectively. In the preparation of rough wood vinegar, the amount of organic content obtained directly through pyrolysis is small. Therefore, it is necessary to optimize equipment and improve purification during the refining process.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the Fundamental Research Funds for the Central Universities (no. YX2013-09) and the Forestry Scientific and Technological Achievements of the National Forestry Bureau (no. 2012-39). The authors would like to thank Editage [