**1. Introduction**

The most common biomass feedstocks for the production of energy and carbon materials are plant, wood, agricultural waste or crop residues, which are mainly composed of water, lignocellulosic components (lignin, hemicellulose and cellulose), extractives and ash [1]. Notably, the energy-containing biomass is derived from the sun by converting atmospheric carbon dioxide (CO2) and water into carbohydrates (or lignocelluloses) through photosynthesis, thus mitigating greenhouse gas (GHG) emissions by displacing fossil fuel use. In this regard, the valorization of biomass for fuels and chemicals was motivated mainly by the benefits of renewable resources, global warming (environmental protection) and social economy [1]. For example, biomass has been considered as a carbon-neutral feedstock or fuel from the viewpoint of the carbon cycle principle regarding the environment. Furthermore, biomass can be converted by biochemical and thermochemical methods into useful products [2]. Pyrolysis, one of the commonly used thermochemical conversion processes, involves decomposition of biomass in the absence of air or oxygen at an elevated temperature [3]. The resulting biochar can be further used as solid fuel, carbon material, soil amendment, environmental adsorbent (biosorbent), functional catalyst or feedstock for chemicals, depending on its final applications [4–9].

In Asia, rice is the most important crop, suggesting that rice husk is an important crop residue because it accounts for around 20% of grain weight. Approximately 150 million metric tons of rice husk are produced annually based on the world production of paddy rice (i.e., 750 million metric tons) [10]. According to the agricultural statistics [11], it was thus

**Citation:** Tsai, W.-T.; Lin, Y.-Q.; Huang, H.-J. Valorization of Rice Husk for the Production of Porous Biochar Materials. *Fermentation* **2021**, *7*, 70. https://doi.org/10.3390/ fermentation7020070

Academic Editor: Alessia Tropea

Received: 31 March 2021 Accepted: 27 April 2021 Published: 30 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

estimated that around 0.4 million metric tons of rice husk are generated annually in Taiwan. Due to its richness in silica and lignocellulosic constituents [12], the biomass is currently used for bioenergy (or solid fuel) in rice milling plants, as a paving/bedding material in poultry farms, animal feed and as a soil amendment in different forms in agricultural lands to increase soil fertility and crop productivity [13–15]. Rice husk is directly reused without converting it into useful materials by thermochemical or biochemical processes. As compared to uncontrolled burning on fields, these direct reuse approaches do not valorize the energy content of the material and may generate toxic emissions without leading to valuable applications such as porous carbon materials.

In order to increase the pore properties, mediate environmental pollution and also mitigate the carbon dioxide release as GHG forms by valorizing rice husk, the potential to enhance the porous structure of resulting biochar products at limited pyrolysis conditions has been widely investigated [16–26]. Vassileva et al. pyrolyzed rice husk at 250, 350, 480 and 700 ◦C at a heating rate of 4 ◦C/min and subsequently maintained this temperature for 4 h [16]. Jindo et al. charred rice husk for 10 h at different temperatures (400–800 ◦C) at a heating rate of 10 ◦C/min [17]. Phuong et al. investigated the effects of pyrolysis temperature (350, 450 and 550 ◦C) and heating rate (10 and 50 ◦C/min) on the yield and properties of the resulting biochar derived from rice husk [18]. Ahiduzzaman and Sadrul Islam produced rice husk biochar at 650 ◦C for 60 min, which was further activated to produce activated carbon [19]. Wei et al. prepared rice husk biochars at 300, 500 and 750 ◦C, which were used as adsorbents for comparing the adsorption performance of herbicide metolachlor with their physicochemical characteristics [20]. Zhang et al. reported the physicochemical properties of rice husk biochars prepared under different temperatures (200–800 ◦C) at a fixed heating rate of 10 ◦C/min and then kept for 60 min [21]. Dissanayake et al. conducted pyrolysis experiments on rice husk at 350, 500 and 650 ◦C at a heating rate of 10 ◦C/min [22]. In this case, the experiment at 350 ◦C took around 2 h to complete pyrolysis, while the experiments at 500 ◦C and 650 ◦C completed the process in around 25 min after reaching the pyrolysis temperature. Jia et al. produced rice husk biochar under 300, 400, 500, 600 and 700 ◦C (heating rate of 15 ◦C/min) for 3 h [23]. Shi et al. investigated adsorption interactions between lead ion and biochars produced at 300, 500 and 700 ◦C [24]. Singh et al. used rice husk biochars, prepared at 300, 450 and 600 ◦C, as adsorbents of nutrient nitrogen (i.e., urea), showing the huge sorption potential of the biochar due to high functionality and porosity [25]. Saeed et al. performed a pyrolysis experiment at a constant temperature of 500 ◦C for 60 min [26]. It is clear that the pore properties of rice husk biochar will be more developed at higher pyrolysis temperatures because of the greater formation of turbostratic crystallites [27]. Regarding the applications of rice husk biochar, it has been used as an effective adsorbent for the removal of trichloroethylene [28], a good medium for the growth of soursop (*Annona muricata L.*) seedlings [29] and a soil amendment for increasing nutrient retention [30,31].

In Taiwan, rice husk biochar has been extensively applied to agricultural soils for enhancing soil fertility and crop yields in recent years due to the promotion of organic farming [30]. However, these biochars showed poor pore properties [32]. For example, the values of the Brunauer–Emmett–Teller (BET) surface area were lower than 5 m2/g. In addition, few studies on the production of rice husk biochar at higher temperatures (e.g., 900 ◦C) for different holding times have been reported in the literature, as mentioned above. Therefore, this work focused on investigating the variations in the yields and pore properties of rice husk biochar in the pyrolysis process as a function of temperature (400, 500, 600, 700, 800 and 900 ◦C) and residence time (0, 30, 60 and 90 min) at a commonly used heating rate (10 ◦C/min). The instrumental analyses, including nitrogen adsorption–desorption isotherms, true density (gas pycnometry using helium displacement principle), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) and Fourier-transform infrared spectroscopy (FTIR), were performed to determine the physicochemical properties of the resulting rice husk biochar.
