Physicochemical and Energy Characteristics of Biochar and Hydrochar Derived from Cotton Stalks: A Comparative Study

Abstract

Burning agricultural biomass in the field significantly contributes to air pollution, particularly in the Indian context, where numerous cities have consistently ranked among the world's most polluted over the past few decades. The investigation endeavors to examine the potential utilization of cotton stalks as an environmentally friendly and sustainable energy source. During the investigation, biochar was generated through pyrolysis at temperatures of 300, 500, and 700 ˚C for 4 h, while hydrochars were produced via hydrothermal carbonization (HTC) at 180, 210, and 240 ˚C for the same duration. The findings revealed that hydrochar exhibited higher mass and energy yields, with mass yields of 60 ± 7% compared to 41 ± 10% for biochar, and energy yields of 87 ± 1% compared to 63 ± 5% for biochar. Elemental analysis results indicated an increase in carbon percentage with rising process temperatures, with carbon content increasing from 59% at 300 ˚C to 78% at 700 ˚C for pyrolysis, and from 49% at 180 ˚C to 63% at 240 ˚C for HTC. The biochar synthesized at 700 ˚C demonstrated the highest measured high heating value (HHV_m) of 29.83 MJ/kg, whereas for HTC, the HHV_m of 25.88 MJ/kg was reported for hydrochar synthesized at 240 ˚C. From the computed thermal kinetic parameters, it is evident that the biochars are more thermally stable than hydrochars and raw cotton. According to the Van Krevelen diagram, both biochar and hydrochar products exhibited improved fuel properties. The cumulative evidence suggests a ground-breaking potential for utilizing these char products as sustainable solid fuel alternatives.

Graphical Abstract