Elsevier

Bioresource Technology

Volume 218, October 2016, Pages 1303-1306
Bioresource Technology

Short Communication
Effect of pyrolysis temperature on the chemical oxidation stability of bamboo biochar

https://doi.org/10.1016/j.biortech.2016.07.112Get rights and content

Highlights

  • Carbon yield of biochar decreased from 71.72% to 38.48% as the temperature increased.

  • The chemical oxidation stability of biochar was determined using the K2Cr2O7 method.

  • The aromatization degree of biochar increased with increasing pyrolysis temperature.

  • Carbon loss proportion decreased from 16.52% to 6.69% with increasing temperature.

  • Biochar showed better stability at a higher temperature in the range 300–700 °C.

Abstract

Biochar produced by biomass pyrolysis has the advantage of carbon sequestration. However, some of the carbon atoms in biochar are not very stable. In this study, the effect of pyrolysis temperature on the chemical oxidation stability of bamboo biochar was investigated using the atomic ratios of H/C and O/C, Fourier transform infrared spectroscopy, and potassium dichromate (K2Cr2O7) oxidation spectrophotometric method. The results show that the carbon yield and ratios of H/C and O/C decreased from 71.72%, 0.71, and 0.32 to 38.48%, 0.22, and 0.06, respectively, as the temperature was increased from 300 °C to 700 °C. Moreover, the main oxygen-containing functional groups gradually decreased, while the degree of aromatization increased accordingly. The biochar showed a better stability at a higher pyrolysis temperature. The proportion of carbon loss, i.e., the amount of oxidized carbon with respect to the total carbon of the biochar, decreased from 16.52% to 6.69% with increasing temperature.

Introduction

Biochar is generally produced by biomass pyrolysis. Biochar contains abundant amounts of carbon and some plant nutrients; it is characterized by a rich pore structure and large specific surface area. Biochar has many advantages such as carbon sequestration, high stability, and it can improve soil fertility, promote crop production, and reduce heavy-metal pollution (Chen et al., 2016a, Ippolito et al., 2012, Lehmann, 2007, Sohi, 2012). Therefore, biochar can be used as a multifunctional material. However, some of the carbon atoms in biochar are not very stable (Masek et al., 2013). Some carbon atoms are released to the atmosphere in diverse ways (Ameloot et al., 2013, Lehmann, 2007). Thus, it is very important to determine the chemical oxidation stability of a biochar, because the stability of biochar not only determines the beneficial effect on soil fertility and plant growth, but also determines the carbon sequestration potential of biochar (Li et al., 2014, Spokas, 2010, Woolf et al., 2010).

Pyrolysis is one of the most important and common methods to prepare biochar. Previous studies showed that pyrolysis conditions, particularly the pyrolysis temperature significantly affects the elemental composition, chemical structure, and stability of biochar (Angin, 2013, Chen et al., 2016b, Crombie et al., 2013, Lee et al., 2013, Park et al., 2014, Tripathi et al., 2016). Atomic ratios (H/C and O/C) and Fourier transform infrared spectroscopy (FTIR) are widely used to evaluate the stability of biochars because of their simplicity, reliability, convenience, low cost, and short turnaround time (Chen and Chen, 2009, Crombie et al., 2013, Li et al., 2014). Currently, studies related to the stability of a biochar are mainly focused on the elemental analysis of the biochar and the effects of soil characteristics on the stability of biochar (Ameloot et al., 2013, Spokas, 2010). However, the effects of pyrolysis temperature on carbon retention and the chemical oxidation stability of a biochar have been rarely studied.

On the other hand, atomic ratios (H/C and O/C) and FTIR spectroscopic analysis cannot provide sufficient information on the stability of a biochar, particularly the chemical oxidation stability of a biochar. The chemical oxidation stability of biochars has been rarely reported in the literature, because only a few methods are available for the quantitative determination of oxidation carbon in a biochar. Biochar is widespread in soil and is the main source of soil organic matter or soil organic carbon. The potassium dichromate (K2Cr2O7) oxidation method has been widely used for decades to determine the amount of soil organic carbon or oxidation carbon (Chan et al., 2001, Meersmans et al., 2009, Nobrega et al., 2015). In this study, the K2Cr2O7 oxidation method was used to evaluate the chemical oxidation stability of biochar, thus providing a promising indicator of biochar stability.

Bamboo has the advantages of fast growth and high strength-to-weight ratio. The bamboo forest area of China was as high as 6.72 million hectares in 2011 (Chen et al., 2015). The production of bamboo biochar was also very high. In this study, bamboo was selected to investigate the effect of pyrolysis temperature on the carbon retention and chemical oxidation stability of the biochar. It is expected that this study would provide useful data to better understand the carbon sequestration ability of biochar.

Section snippets

Feedstock materials

Moso bamboo (Phyllostachys edulis) was obtained from the Yuyao city of China. The bamboo was first ground using a multifunctional disintegrator and then dried at 105 °C for 6 h. The bamboo particles with a size of 0.38 mm to 0.25 mm were used.

Preparation of biochar

The pyrolysis of bamboo was conducted in a fixed bed furnace, as shown in our previous study (Chen et al., 2014). Nitrogen gas was fed from the top at a flow rate of 500 mL/min. After the temperature reached the four selected gradient temperatures (300 °C, 400 

Mass yields of pyrolysis products

The biochar yield is shown in Table 1. It depends on the pyrolysis temperature. The yield gradually decreased with the increase in the final pyrolysis temperature. At 300–400 °C, the biochar yield sharply decreased from 53.62% to 31.87%, mainly because of the mass loss of cellulose and hemicellulose. With further increase in temperature, the biochar yield decreased at a relatively slower rate, as the main pyrolysis reaction above 400 °C involved the pyrolysis of lignin. Lignin is a highly

Conclusions

Pyrolysis temperature directly affects the carbon sequestration efficiency of biochar by affecting the mass yield, amount of carbon retention, and chemical oxidation stability of the biochar. With increasing pyrolysis temperature, the carbon content of the biochar and aromatization degree gradually increased, whereas the content of oxygen-containing functional groups constantly decreased. The K2Cr2O7 oxidation spectrophotometric method is a promising and alternative method for determining the

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51406089, 31570552), Natural Science Foundation of Jiangsu Province (No. BK20151521), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), Students Practice Innovation Training Program of Nanjing Forestry University (NO. 2016NFUSPITP067), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors also acknowledge the Advanced Analysis &

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