Production of High-Porosity Biochar from Rice Husk by the Microwave Pyrolysis Process

Abstract

This study focused on the highly efficient pyrolysis of rice husk (RH) for producing high-porosity biochar at above 450 °C under various microwave output powers (300–1000 W) and residence times (5–15 min). The findings showed that the maximal calorific value (i.e., 19.89 MJ/kg) can be obtained at the mildest microwave conditions of 300 W when holding for 5 min, giving a moderate enhancement factor (117.4%, or the ratio of 19.89 MJ/kg to 16.94 MJ/kg). However, the physical properties (i.e., surface area, pore volume, and pore size distribution) of the RH-based biochar products significantly increased as the microwave output power increased from 300 to 1000 W, but they declined at longer residence times of 5 min to 15 min when applying a microwave output power of 1000 W. In this work, it was concluded that the optimal microwave pyrolysis conditions for producing high-porosity biochar should be operated at 1000 W, holding for 5 min. The maximal pore properties (i.e., BET surface area of 172.04 m2/g and total pore volume of 0.1229 cm3/g) can be achieved in the resulting biochar products with both the microporous and the mesoporous features. On the other hand, the chemical characteristics of the RH-based biochar products were analyzed by using Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS), displaying some functional complexes containing carbon–oxygen (C–O), carbon–hydrogen (C–H), and silicon–oxygen (Si–O) bonds on the surface of the RH-based biochar.

Production of Porous Biochar from Cow Dung Using Microwave Process

“…On the other hand, the calorific values of CD-based biochar products were also determined to show their enhancement factors. The findings showed that higher calorific values were obtained under mild microwave conditions, which were consistent with those in a previous study [25]. For example, the The mass yield of CD-based biochar was weighed by the ratio of its residual mass to CD mass fed into the microwave pyrolysis system.…”

“…For example, the order of the mass yield is given below: 29.85 wt% (BC-DC-300W-10M), 24.37 wt% (BC-DC-440W-5M), 22.03 wt% (BC-DC-440W-10M), and 15.81 wt% (BC-DC-800W-5M). Obviously, the mass yields of CD-based biochar products were lower than those of biochar products from rice husk in a previous study [25], which could be attributed to the different ash contents between CD and rice husk. On the other hand, the calorific values of CD-based biochar products were also determined to show their enhancement factors.…”

“…The characterization analysis of the resulting biochar products was analyzed by the following methods: nitrogen adsorption-desorption isotherms for the pore properties (i.e., surface area, pore volume, and pore size distribution), scanning electron microscopy (SEM) for the textural structure on the biochar surface, Fourier-transform infrared spectroscopy (FTIR) for the surface oxygen-containing groups, and energy-dispersive X-ray spectroscopy (EDS) for the surface elemental compositions. The analytical procedures and conditions are described in a previous study [25]. However, the 2D-NLDFT-HS model was used in this work to depict the micropore (pores with diameters less than about 2 nm) size distribution, which was based on the slit-cylinder pore shape boundary fixed at 2 nm [26,27].…”

“…The pyrolysis experiments by microwave were similar to a previous study [25]. A modified microwave oven (Mei Lin Energy Technology Co., Ltd., Kaohsiung, Taiwan) with an oscillation frequency of 2450 MHz and a maximal power consumption of 1350 W was adopted to perform a series of biochar production experiments under the microwave conditions of output power (300-1000 W) and holding time (5-20 min).…”

“…As mentioned above, the thermochemical characteristics of the CD sample were analyzed to determine its relevance to be used as an available biomass for producing carbon materials. In this work, the adopted methods and analytical instruments may refer to previous studies [24,25]. The measurement of inorganic elemental contents was performed using the Agilent 725 ICP-OES instrument (Agilent Co., Santa Clara, CA, USA).…”