Factors controlling the formation of persistent free radicals in hydrochar during hydrothermal conversion of rice straw

In the environment, persistent free radicals (PFR) have adverse effects on human health. PFR are generated by thermal conversion of biomass, such as hydrothermal carbonization to produce hydrochar. So far, the mechanism and control factors of PFR production in hydrochar are poorly known. Therefore, we investigated here the impacts of hydrothermal temperature, residence time and solid load on the formation of PFR in hydrochar from hydrothermal carbonization of rice straw, by electron paramagnetic resonance (EPR) coupled with Fourier transform infrared spectrometers. Results show that the EPR signal intensity increased with increasing hydrothermal temperature from 180 to 240 degrees C and then decreased at 260 degrees C. A shorter residence time and a higher solid load led to formation of more PFR in hydrochar. The types of PFR also depended on hydrothermal temperature, residence time and solid load. This is the first report on the formation of PFR and relevant influencing factors during hydrothermal conversion of biomass. Based on these results, hydrochar from hydrothermal conversion of biomass at relatively higher temperature, i.e., 260 degrees C, longer residence time, i.e., 4h, and lower solid load, i.e., 1:10, issuggested for safer application.