Deploying sustainable hydrochars from food waste as solid fuels: The limits of thermogravimetric analysis to describe true combustion behavior

This work compares fuel science and combustion properties for solid biofuel made via the hydrothermal carbonization (HTC) of food waste (FW). Because HTC of FW produces a hydrochar (HC) that comprises two phases – a solid primary char and an amorphous secondary char that limits fluidization – the HCs are first solvent-extracted to separate the char phases. We investigate the combustion properties of the remaining primary char by coupling TGA with experiments in a Hencken burner instrumented with CH∗ chemiluminescence imaging, particle image velocimetry, and two-color pyrometry. TGA data confirms reduction of hydrochar reactivity by secondary char removal; primary char behaves similarly to Illinois #6 coal under slow oxidation conditions. However, ignition delay time measurements at realistic combustion conditions indicate primary char ignites 4–4.5 times faster than coal, despite their similar ignition and combustion modes. Comparing the results of a TGA-informed combustion model with combustion data highlights the limitations of using thermogravimetry as the sole classifier for suitable biofuels and the necessity of real combustion analysis in assessing the suitability of biofuels combustion or co-combustion in burners that are designed for coals.