Hydrothermal carbonization of millet stalk and dilute-acid-impregnated millet stalk: combustion behaviors of hydrochars by thermogravimetric analysis and a novel mixed-function fitting method

The combustion characteristics of hydrochars from millet stalk (MS) and dilute-acid-impregnated millet stalk (AMS) were investigated by thermogravimetric analysis. The Flynn-Wall-Ozawa (FWO) and Kissinger-AkahiraSunose (KAS) isoconversional approaches were used to determine the activation energies of the hydrochars. The pre-exponential factors were estimated by the compensation effect. A new mixed-function was proposed to fit the derivative thermogravimetric (DTG) curves to describe the combustion behaviors of the MS and AMS hydrochars. The results showed that compared with MS hydrochar, AMS hydrochar presented better fuel properties, including a higher yield, higher carbon content, lower ash content and lower nitrogen content. The fuel ratio and specific surface area of AMS hydrochar were also higher than those of MS hydrochar. The combustion kinetics indicated that the average activation energy of MS hydrochar first increased and then decreased with increasing HTC temperature. In the case of AMS hydrochar, the average activation energy decreased from 142.5 and 138.6 kJ/mol to 111.7 and 105.1 kJ/mol, as determined by the FWO and KAS approaches, respectively, as the HTC temperature increased from 210 degrees C to 300 degrees C. The average activation energy of AMS hydrochar was lower than that of MS hydrochar due to dilute-acid impregnation. The simulated DTG curves were in good agreement with the experimental data, indicating that the reaction functions for both the MS and AMS hydrochars can be accurately described by the mixed-function model. The proposed mixed-function fitting method, together with the obtained kinetics data for the MS/AMS hydrochars, can facilitate the design and optimization of combustion and hydrothermal carbonization systems.