Effect of water-washing of wheat straw and hydrothermal temperature on its hydrochar evolution and combustion properties

In order to upgrade wheat straw, hydrothermal treatment at 160-240 °C was investigated. Meanwhile, the influence of temperature and leaching on the fuel's physicochemical and combustion properties were explored. A temperature of 180-220 °C was found to benefit the generation of hydrochar, with solid and energy yields of at least 57.3% and 69.9%, respectively. When temperature increased to 160 °C, hemicellulose was hydrolyzed and this led to the formation of carbon microspheres. The diameter of the carbon microspheres reached 0.05-0.7 μm. Hydrochars obtained at 160, 180, and 200 °C exhibited better combustion performance with higher comprehensive combustibility index value. While leaching heavily impacted the hydrochar derived at 160 °C and increased its activation energy (178 kJ/mol) above those obtained for treatment at 180 °C (164 kJ/mol) and 200 °C (169 kJ/mol). Overall, the recommended hydrothermal temperature for production of fuel from wheat straw is 180-200 °C.