Progress on Separation and Hydrothermal Carbonization of Rice Husk Toward Environmental Applications

Abstract

Owing to the increasing global demand for carbon resources, pressure on finite materials, including petroleum and inorganic resources, is expected to increase in the future. Efficient utilization of waste resources has become crucial for sustainable resource acquisition for creating the next generation of industries. Rice husks, which are abundant worldwide as agricultural waste, are a rich carbon source with a high silica content and have the potential to be an effective raw material for energy-related and environmental purification materials such as battery, catalyst, and adsorbent. Converting these into valuable resources often requires separation and carbonization; however, these processes incur significant energy losses, which may offset the benefits of using biomass resources in the process steps. This review summarizes and discusses the high value of RHs, which are abundant as agricultural waste. Technologies for separating and converting RHs into valuable resources by hydrothermal carbonization are summarized based on the energy efficiency of the process.

Keywords: biomass; hydrothermal carbonization; rice husk.

Figure 1

Value‐added rice husk (RH) via separation and hydrothermal carbonization (HTC) processes.

Figure 2

a) Images of silica inside the RH. The image was adapted and modified from ref. [27] with permission from the Royal Society of Chemistry, copyright 2010. b) Optical microscope image of an RH shell and silicon mapping (inset). c) Scanning electron microscopy images of silica derived from the RH. Images (b and c) were adapted and modified from ref. [28] with permission from the National Academy of Science, Copyright 2013.

Figure 3

a) Photograph of a hydrothermal vessel. b) Diagram of the hydrothermal vessel. c and d) Photographs of RH before and after hydrothermal treatment (HTC), respectively.

Figure 4

Hydrolysis and carbonization of cellulose with HTC.

Figure 5

a) Van‐Krevelen diagram from previous studies. b) Energy yields and hydrochar yields under different conditions. The data values of A, B, C, D, and E are reproduced from ref.[60, 64, 65, 66, 69] respectively.

Figure 6

a and b) Graphical scheme of the synthesis SiOx/C from RH and the electrochemical performance of a Li‐ion battery (LIB), respectively. The images were adapted from ref. [106] with permission from the American Chemical Society, Copyright 2019. c and d) Scanning electron microscope (SEM) images of a carbon‐coated ZnO particle and electrochemical performance of the LIB, respectively. The images were adapted from ref. [115] with permission from Elsevier, Copyright 2020.

Figure 7

a and b) Graphical scheme of synthesis of hierarchical carbon from RHs and electrochemical performance as supercapacitors, respectively. The images were adapted from ref. [122] with permission from Elsevier, Copyright 2020. c and d) Graphical scheme of synthesizingiron‐doped hierarchical carbon from RHs and the electrochemical performance as a supercapacitor, respectively. The images were adapted from ref. [128] with permission from Elsevier, Copyright 2022.

Figure 8

a and b) Graphical scheme of synthesis of heteroatom‐doped RH‐derived carbon and electrochemical performance as an electrocatalyst for the oxygen reduction reaction, respectively. The images were adapted and modified from ref. [142] with permission from Elsevier Copyright 2022. c) Electrochemical performance of RH‐derived carbon with metal oxides as electrocatalysts for the oxygen reduction reaction (ORR). The images were adapted from ref. [143] with permission from MDPI, Copyright 2020.