Microwave–assisted hydrothermal conversion of crop straw: Enhancing the properties of liquid product and hydrochar by varying temperature and medium

Abstract

Crop straw is gaining increasing attention as a green resource to be converted into biofuels, chemicals, and biomaterials through microwave-assisted hydrothermal conversion (MHTC). To reduce the energy consumption and improve the product quality, this research investigated the properties and formation mechanisms of products obtained from crop straws through MHTC by varying media and temperature. Increasing temperature, phosphoric acid, and magnesium acetate could promote the liquid yield, and increase the concentrations of organic matter and nutrient ions such as K+, Ca2+, and Mg2+ of liquid product. Magnesium acetate, whose liquid product exhibited the highest concentration of total organic carbon, further facilitated the conversion of sugars to acids, resulting in acidic liquid products with high electrical conductivity. Increasing temperature improved the crystallinity and pore structure of hydrochar, but reduced the hydrochar yield and surface active groups. Magnesium acetate was beneficial to generate ordered carbon structure, surface nitrogen- and oxygen-containing groups, and reduced the crystallinity of the hydrochar. The pH of liquid product in the phosphoric acid medium was the lowest, which improved the pore volume, the particle size of carbon microspheres, and the amount of carbon microspheres of hydrochar. Hydrochar exhibited good wettability and could be used as a precursor for high quality activated carbon. Compared with previous research, under the temperature of 180 °C in the magnesium acetate medium, nutrients were more preferentially accumulated in the liquid products and the hydrochar possessed more active groups. This work provides a theoretical reference and technical support for MHTC of crop straws and value-added utilization of the solid-liquid products.