Effects of nitric acid modification on hydrochar's combustion, fuel and thermal properties are dependent on feedstock type

Elevated metal (e.g., alkali metals) and ash contents can negatively impact the use of biomass-derived solid fuels, including hydrochars, in clean energy generation. The effects of nitric acid modification on those and other properties (combustion, fuel and thermal) were studied for hydrochars produced at three temperatures from four feedstocks. Through side-chain oxidation and surface protonation, nitric acid significantly leached metals from pristine hydrochars to a maximum of five order and increased their burnout temperature by 9–41%, but its effect on ash content, gross calorific values and ignition temperature depended on feedstock type and carbonization temperature. Ignition temperature increased by > 2 times for modified manure pellet hydrochar produced at 300 °C. The combustion characteristics index for the hydrochars was above the minimum benchmark (2 × 10^-7 %² min⁻² °C⁻³) for a typical solid fuel. Therefore, nitric acid can effectively improve hydrochar's combustion and fuel properties and reduce slagging in industrial boilers.