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Hydrothermal Carbonization of Lignocellulosic Biomass
AuthorReza, Mohammad T.
AdvisorCoronella, Charles J.
Chemical and Materials Engineering
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Hydrothermal Carbonization of Lignocellulosic BiomassAbstractHydrothermal carbonization (HTC) or wet torrefaction is a pretreatment process for lignocellulosic biomass where the biomass is treated with hot compressed water. The solid product of HTC is HTC biochar, which is friable, hydrophobic, and increased in mass and energy densification compared to the raw biomass. HTC biochar also is similar regardless of the type of biomass used. A liquid solution of five carbon and six carbon sugars, along with various organic acids and 5-HMF, is also produced from HTC of lignocellulosic biomass. The gaseous phase product consists mostly of CO<sub>2</sub>. Different types of lignocellulosic biomass were used for HTC under different conditions. Every type of biomass shows a significant decrease in mass yield with a significant increase in the energy densification ratio. Oxygen content decreases with increasing HTC reaction temperature. The oxygen carbon ratio decreases and as a result, HTC biomass has the same characteristics as a low rank coal. To optimize the reaction temperature and reaction time for HTC, reaction kinetics was studied for loblolly pine, which is a lignocellulosic biomass. A special two-chamber reactor was design and built to perform kinetic studies. It is found that hemicelluloses and cellulose degradation follow two parallel first order reactions. Hemicelluloses degrade much faster than cellulose, as the activation energy is lower for hemicelluloses than cellulose. Lignin behaves as an inert in the studied temperature range of 200-260 C and aqueous solubles are generated almost instantaneously in the reaction scheme. After a certain reaction time, the mass yield of HTC becomes steady, but it varies with the reaction temperature. Lignin in the HTC biochar shows glass transition behavior in the temperature range of 135-165 C. HTC biochar pellets were produced using a hydraulic press operating at 140 C temperature and 1000 MPa. As the lignin content becomes higher at elevated HTC temperatures, the HTC biochar pellets are more durable, abrasion resistant, and mass and energy denser than raw biomass pellets. The higher heating value (HHV) of the pellets is similar to the HHV of HTC biochar. However, the energy density is significantly higher, as the pellets have a higher mass density. HTC biochar pellets have a lower modulus of elasticity and a higher ultimate compressive strength relative to raw biomass. The trend of modulus of elasticity shows it lessens with increasing HTC treatment temperature. The HTC biochar pellets are more hydrophobic than raw biomass pellets. The equilibrium moisture content (EMC) of the HTC biochar pellets are in the same range as HTC biochar, but the pellets take 7-10 days more to reach equilibrium.