Impact of HTC reaction conditions on the hydrochar properties and CO2 gasification properties of spent grains

- Increasing HTC reaction severity leads to formation of fixed carbon, which is conserved during pyrolysis.
- For high HTC reaction severity, the formation of fixed carbon significantly lowers char reaction rates with CO2.
- Mild HTC conditions are therefore favorable when considering HTC pretreatment for gasification applications.

The impact of residence time and temperature during hydrothermal carbonization (HTC) on hydrochar properties and CO2 gasification properties has been studied for brewers' spent grain (BSG), treated at temperatures from 180 °C to 280 °C and residence times from 0.5 to 12 hours. Lower heating values (LHV) of the hydrochars are found to increase to values of bituminous coal and anthracite as reaction severity increases. Temperature is found to have a greater influence on the LHV of the hydrochar than residence time. Mass and energy yields decrease with increasing reaction severity. With higher reaction severity decreased molar O/C and H/C ratios as well as decreased volatile contents and increased fixed carbon contents are observed. The influence of residence time is more pronounced for the formation of fixed carbon, main carbonization reactions occur for a reaction severity greater than 180 °C and 0.5 hours. Char reactivity is found to decrease with increasing carbonization reaction severity with a strong influence of both residence time and temperature due to the formation of fixed carbon in the hydrochar. Activation energies are decreased with increased carbonization temperature but only mildly affected by residence time. Hereby the catalytic influence of ash compounds has to be further determined.

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