Microwave-assisted hydrothermal carbonization of rice husk and rice straw for its applications as a solid fuel and a filler for polymer composites

Abstract

Biomass utilization has received much attention for production of high-density solid fuels. Utilization of cheap and naturally available precursors through environmentally friendly and effective processes is an attractive and emerging research area. Pyrolysis and hydrothermal carbonization (HTC) are well-known technologies available for production of solid biofuel using conventional or microwave heating. Microwave heating is a simpler and more efficient heating method than conventional heating. The process parameters of microwave hydrothermal carbonization (MHTC) have significant effect on yield of hydrochar. For the first time, this study discusses the effect of process parameters including reaction temperature, reaction time, particle size and biomass to water ratio on hydrochar yield produced from MHTC of rice husk and rice straw. Further, both the feed material and hydrochars have been characterized for energy, chemical, structural and thermal properties. Results revealed that, over the ranges tested, a lower temperature, lower reaction time, lower biomass to water ratio and higher particle size produce more hydrochar. The higher heating value (HHV) was improved significantly from 16.46 MJ/kg and 12.3 MJ/kg of rice husk and rice straw to 24.89 MJ/kg and 17.6MJ/kg of rice husk hydrochar and rice straw hydrochar, respectively. Elemental analysis results of rice husk and its hydrochar show that the carbon content increased, and oxygen content decreased in hydrochar from 25.9 to 44.9% and 68.5 to 47.0%. On the other hand, the carbon content and fixed carbon values were increased from 37.19% and 14.37% for rice straw to 48.8% and 35.4% for rice straw hydrochar. SEM analysis exhibited modification in structure of rice husk and improvement in porosity after MHTC, which was further confirmed from BET surface analysis. The BET surface area increased from 25.0656 m2/g (rice husk) to 92.6832 m2/g (hydrochar). Thermal stability of hydrochar was improved from 340 oC for rice husk to 370 oC for hydrochar. A polymer composite is fabricated by incorporating fillers into a polymer matrix at different concentrations. The addition of fillers helps to improve the physical, mechanical, chemical and thermal properties of the composite. This study utilizes hydrochar for the first time, synthesised by microwave-assisted hydrothermal carbonization of rice husk and rice straw, as filler for polylactide/hydrochar composites. The polylactide/hydrochar composites were fabricated by incorporating hydrochar in polylactide at 5%, 10%, 15% and 20 wt% by melt processing in a Haake rheomix at 170 ºC. Both the neat polylactide and polylactide/hydrochar composites were characterized for mechanical, structural, thermal and rheological properties. The tensile modulus of polylactide/hydrochar composites was improved from 2.63 GPa (neat polylactide) to 3.16 GPa, 3.33 GPa, 3.54 GPa, and 4.24 GPa after blending with hydrochar at 5%, 10%, 15%, and 20%, respectively. Further, the incorporation of hydrochar had little effect on storage modulus (G') and loss modulus (G"). The findings of this study suggest that addition of hydrochar improves the overall characteristics of polylactide composites confirming the suitability of hydrochar as filler for polymer/hydrochar composites