Elsevier

Bioresource Technology

Volume 312, September 2020, 123614
Bioresource Technology

A critical review of the production and advanced utilization of biochar via selective pyrolysis of lignocellulosic biomass

https://doi.org/10.1016/j.biortech.2020.123614Get rights and content

Highlights

  • In-situ pyrolysis regulation and post-modification of biochar were summarized.

  • The relationship between physicochemical properties and applications of biochar was analyzed.

  • The future research requirements for biochar preparation and applications were proposed.

Abstract

Biochar is a carbon-rich product obtained from the thermo-chemical conversion of biomass. Studying the evolution properties of biochar by in-situ modification or post-modification is of great significance for improving the utilisation value of lignocellulosic biomass. In this paper, the production methods of biochar are reviewed. The effects of the biomass feedstock characteristics, production processes, reaction conditions (temperature, heating rate, etc.) as well as in-situ activation, heteroatomic doping, and functional group modification on the physical and chemical properties of biochar are compared. Based on its unique physicochemical properties, recent research advances with respect to the use of biochar in pollutant adsorbents, catalysts, and energy storage are reviewed. The relationship between biochar structure and its application are also revealed. It is suggested that a more effective control of biochar structure and its corresponding properties should be further investigated to develop a variety of biochar for targeted applications.

Introduction

In the past few years, biomass as an organic solid waste and renewable resource has attracted an increasing amount of attention. Biomass refers to biological materials from living organisms or related biological organisms. Lignocellulosic biomass is composed of carbohydrate polymers (hemicellulose and cellulose) and aromatic polymers (lignin) (Dai et al., 2019a, Dai et al., 2019b), and it can be converted into liquid, gas, and solid products via thermo-chemical conversion (Dai et al., 2019a, Dai et al., 2019b). The resultant solid biochar is a carbonaceous material produced through the thermochemical transformation of biomass in an anaerobic or anoxic environment (Ma et al., 2017a, Ma et al., 2017b). In addition to the main element “carbon”, there are many other elements in biochar that affect the corresponding action and function of materials. Biochar has a porous structure with abundant functional groups (i.e. it is rich in surface free radicals and surface charges) and a high surface area, and also contains minerals and trace metals (Wang & Wang, 2019). Biochar is a reservoir of electron acceptors and donors with a pH buffering capacity and cation exchange capacity (Leng et al., 2020). These properties lend a high reactivity to biochar, and are mainly affected by the composition of the raw materials and production methods (Fig. 1). Production methods include slow or fast pyrolysis (Wang et al., 2016), gasification (Dissanayake et al., 2020), hydrothermal carbonisation (Afolabi et al., 2020), torrefaction (Ma et al., 2019a, Ma et al., 2019b), and flash carbonisation (Kumar et al., 2020a, Kumar et al., 2020b) as well as the regulation of the pyrolysis process and subsequent modification (Leng & Huang, 2018).

Due to its unique physical and chemical characteristics, biochar is often used in the area of removal of water pollutants (Li et al., 2019a, Li et al., 2019b, Li et al., 2019c, Luo et al., 2019), catalysis (Chen et al., 2018a, Chen et al., 2018b, Liu et al., 2018, Wang et al., 2019a, Wang et al., 2019b), composting (Guo et al., 2020), fermentation detoxication (Sun et al., 2020) as well as electrochemical energy storage (Liu et al., 2019a, Liu et al., 2019b, Liu et al., 2019c, Liu et al., 2019d). Biomass raw materials and various process parameters have important influences on the physicochemical properties of biochar, and thus directly determine its use (Ma et al., 2017a, Ma et al., 2017b). The yield, physicochemical properties, and quality of biochar are determined by the composition of biomass raw materials and process conditions under the thermo-chemical conversion platform. The physical and chemical properties of biochar can be improved by its post-treatment (activation and modification) (Yan et al., 2020). In recent years, there has been a great interest in optimising the pyrolysis conditions to improve the yield and quality of biochar; however, there is still a lack of research into the design of biochar and the structure-application relationship between the physicochemical properties and application of biochar.

The purpose of this study was to review and summarize the recent advances on biochar production via selective pyrolysis, biochar design and application. Based on the understanding of the properties of biochar, the development of the preparation system design and application of biochar was reviewed from the perspective of the relationship between physicochemical properties and applications.

Section snippets

Biomass pyrolysis for biochar production

The production process parameters (temperature, residence time, heating rate, and pressure) of biochar have an important impact on its yield, properties (amorphous or porous), and quality (shape, size and chemical composition) (Tripathi et al., 2016). In addition, the composition, structure, and intrinsic binding of the original biomass also influence the physicochemical properties of biochar (Ruan et al., 2019). In recent years, a considerable amount of research has been conducted into the

Pyrolysis regulation for biochar production

As mentioned in Section 2, the properties of biochar are not only closely related to the biomass raw materials, but also to the production conditions, including the temperature, heating rate, and pyrolysis time. However, in addition to altering these pyrolysis parameters, the pyrolysis process of biomass can also be regulated by changing the pyrolysis atmosphere and in-situ activation/doping as a means of obtaining excellent biochar.

Biochar modification

The characteristics of biochar can be further enhanced by activation (physical or chemical) or modification (Wang & Wang, 2019). The latter can be achieved by creating new functional groups on the surface of biochar to prepare biochar matrix composites, or by biological modification (Fig. 1). Some examples of biochar production through methods of activation and modification are provided in Table 3. These methods include the treatment of steam, bases, acids, carbonaceous materials, metal oxides,

Challenge and future research directions

Biochar can be regarded as a renewable and eco-friendly carbon based material. As a new and exciting research field, many gaps and uncertainties still exist in the field of biochar production and valorisation, requiring further investigation.

  • (1)

    In-depth understanding of biochar structure at multi-scale level. Extensive work is needed to reveal the correlation between the biochar structure features and its properties. The introduction of some free radicals and functional groups and the formation of

Conclusions

As the main pyrolysis product, biochar is influenced greatly by the biomass feedstock and pyrolysis parameters. Biomass with high lignin content contributes the formation of biochar. Among many operation parameters, reaction temperature has a dominant role in the production of high-quality biochar. In addition to improving the quality of biochar by adjusting operating parameters, in-situ activation and doping with heteroatom could alter the properties of biochar obviously, especially the

CRediT authorship contribution statement

Yunchao Li: Writing - original draft. Bo Xing: Formal analysis. Yan Ding: . Xinhong Han: . Shurong Wang: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support from the National Science Fund for Distinguished Young Scholars (51725603), the Project funded by China Postdoctoral Science Foundation (2019M652080), the Innovative Research Groups of the National Natural Science Foundation of China (51621005) and the Open Project of State Key Laboratory of Clean Energy Utilization, Zhejiang University (ZJUCEU2018007).

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