Biomass supply is complicated by diversity of feedstocks, seasonal availability, and widely distributed feedstock. The costs of biomass supply and logistics hinder commercialization for advanced fuel and power production. Hydrothermal carbonization (HTC, or wet torrefaction) is a pretreatment process for making a homogenized, carbon rich, and energy dense solid fuel, called HTC biochar, from lignocellulosic biomass. Compared to raw biomass, HTC biochar is both more hydrophobic for better storage and more friable for better processing. In this pretreatment method, the biomass is treated with hot compressed water in an inert environment in the temperature range of 200-260°C. The range of mass yield of biochar is 60-90% and its energy yield is 75-95%, indicating that the process causes an energy densification of 6-25%. Higher reaction temperature decreases mass yield and increases energy densification.
Kinetic data show that hemicelluloses and water extractives react very quickly compared to cellulose and that lignin is relatively inert, resulting in a biochar with increased lignin content. Lignin extracted from raw wood by Van-Soest method exhibits a glass transition temperature at about 135-165 ˚C. Similarly, lignin extracted from HTC biochar exhibits a glass transition temperature in the same range. Lignin acts as a natural binder for pelletization of HTC biochar, if pelletizing is done in the temperature range of glass transition. A hydraulic press with a controllable heated die was used for producing uniform pellets. The materials used to make pellets were raw loblolly pine and pine pretreated at 200°C, 230°C, and 260°C. The sample was compressed into a 13 mm die to make cylindrical pellets 8-10 mm long. The pressure applied was 1000 MPa while a temperature of 140 ˚C was maintained for 30 seconds to allow the lignin showing the glass transition behavior.
Pellets made from HTC biochar exhibit favorable properties resulting from high levels of lignin, including increased abrasion resistance, increased durability, increased modulus of elasticity, increased energy density, and increased mass density. On the other hand, ultimate breaking strength of pellets is decreased with increasing pretreatment temperature. Pellets produced from biomass pretreated at 260°C, have volumetric fuel value 70% greater than pellets produced from untreated pine and 142% greater than the raw biomass. Equilibrium moisture content (EMC) of pellets of HTC biochar pretreated at 260 ˚C is about one fourth that of pellets produced from raw biomass. Pellets produced from HTC biochar maintain structural integrity significantly longer than do pellets produced from raw woody biomass.
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