Catalytic hydrothermal carbonization of invasive macrophyte Hornwort (<Emphasis Type="Italic">Ceratophyllum demersum</Emphasis>) for production of hydrochar: a potential biofuel

Biofuels are environment friendly and economical. First-generation biofuels are made from sugar, starch and vegetable oils, and have an impact on food security, while second generation biofuels are generated from plant or algal material and are greener and sustainable. The present work focused on lab-scale application of hydrothermal carbonization to convert aquatic weed Ceratophyllum demersum into a second-generation biofuel—hydrochar. Hydrothermal carbonization has been carried out with and without catalysts in a high-pressure reactor under subcritical temperatures 240–320 °C, residence times 10–40 min and biomass to water dilution ratios varying from 1:4 to 1:12. Under noncatalytic conditions higher heating value and energy density were highest, 24.11 MJ/kg and 1.64 respectively at temperature of 300 °C and residence time of 30 min. The calorific value of noncatalytic hydrochar was enhanced further, respectively, to 29.0, 27.5 and 26.48 MJ/kg by catalysts KOH, Na₂CO₃ and acetic acid. Catalytic hydrochar had H/C and O/C atomic ratios of same order as that of high-rank bituminous coal. Van Krevelen diagram indicated KOH to be better catalyst followed by Na₂CO₃ and acetic acid. Hydrochar yield of 38% obtained in noncatalytic hydrothermal carbonization decreased with catalysts in following order; acetic acid > Na₂CO₃ > KOH, however, energy density was enhanced in order KOH > Na₂CO₃ > acetic acid. The research can prove a way forward in the direction of meeting part of global energy demand. At the same time, the problem of secondary pollution caused by piling up and decay of aquatic weeds will also be addressed to.