Effects of temperature and catalytic methods on the physicochemical properties of microwave-assisted hydrothermal products of crop residues

We studied the effects of temperature (180-260 degrees C) and catalytic methods (ultrafine grinding and phosphoric acid treatment) on the microwave-assisted hydrothermal carbonization of crop residues by taking rice straw, corn stover and rape stalk as the research materials. The physicochemical and structural properties of hydrochars and liquid products generated under different conditions were characterized. The results showed that with increasing temperature, the hydrochar yield of crop residues decreased from 62.97-83.09% to 42.73-57.38%; the liquid yield rose from 15.03-33.38% to 37.94-47.58%; and the gas yield increased from 1.88-4.92% to 4.68-10.90%. The liquid products were all acidic (pH = 2.53-4.40), and the NH4+-N and PO43-P concentrations respectively showed an upward trend and downward trend with increasing temperature. Higher temperature led to enhanced carbonization degree, causing decreases in H/C and O/C atomic ratios but increases in higher heating value (HHV). Hydrochars prepared at 240 degrees C had the most intensified oxygenated functional groups, the largest specific surface area (5.5652-15.5058 m(2)/g), total pore volume (0.0403-0.1116 cm(3)/g) and mesopore volume (0.0395-0.1101 cm(3)/g), and the most carbon microspheres. However, temperature had a rather limited effect on the micropore volume of hydrochars derived from the three types of crop residues. Both ultrafine grinding and phosphoric acid catalysis could enhance hydrochar properties, with the latter being the more low-cost and efficient method. Hydrochars from ultrafine grinding crop residues had the highest peak strength of oxygenated functional groups, while those from phosphoric acid catalysis had higher fixed carbon (23.58-35.91%), C (45.03-65.50%) and HHV (17.45-27.18 MJ/kg), and lower ash content (4.21-15.67%), as well as the largest specific surface area (6.9140-15.5058 m(2)/g), total pore volume (0.0638-0.1116 cm(3)/g), micropore volume (0.0007-0.0015 cm(3)/g) and mesopore volume (0.0631-0.1101 cm(3)/g). Besides, crop residue type was also found to be an inherent factor determining hydrochar property. (c) 2020 Elsevier Ltd. All rights reserved.