Hydrothermal Carbonization of Different Biomass Types

As a non-food renewable energy resource, lignocellulosic biomass is essential in augmenting limited fossil fuels to produce liquid fuels and chemicals. Hydrothermal carbonization (HTC) is a process that pretreats lignocellulosic biomass to produce sugars in its aqueous product and a solid hydrochar of increased heating value. Biomass is reacted in hot compressed water at temperatures between 200 °C and 260 °C at pressures required to maintain liquid water. HTC was performed on pelletized corn stover, rice hulls, Tahoe mix (Jeffrey pine and white fir), switch grass, and loblolly pine. These biomass sources cover a variety of types, including a primary agricultural residue, a secondary agricultural residue, a forest product removed to reduce wildfires, an energy crop, and a forest crop. HTC reaction temperature was found to be the process variable with the greatest effect. For the five biomass studied, mass yields of hydrochar were found to decrease with increasing reaction temperature, while higher heating values increased with increasing reaction temperature. Energy densification increased by up to 40%. Differences in these results among biomass sources were found to correlate with their original hemicellulose, cellulose, lignin, aqueous soluble, and ash contents. A reactive index was determined that could predict mass yield for a given biomass from its combined hemicellulose and aqueous soluble content divided by its combined lignin and ash content. These biomass components appeared to be observable in scanning electron microscope images of hydrochar.