(323e) Thermal Pretreatment Options for Thermochemical Conversion of Lignocellulosic Biomass

Thermochemical conversion of lignocellulosic biomass is a promising technology for production of renewable power, fuels, and chemicals. Both pyrolysis and gasification of biomass have significant technical barriers that must be eliminated for successful widespread commercialization. Feedstock handling is complex due to the diverse nature of important feedstocks, including commercial timber, slash removed due to fire hazard reduction, switchgrass, and agricultural residues as diverse as rice hulls, corn stover and wheat chaff. Seasonal availability and low forest density make feedstock logistics complex and expensive. A process to homogenize the feedstocks that will simultaneously produce a stable energy-dense fuel is needed. Two such processes are considered here: dry torrefaction, and wet torrefaction.

Dry torrefaction is a low-temperature pyrolysis process in which the biomass is heated in a chemically inert environment at temperatures ranging between 200 and 300 ˚C. The reaction produces two products, a solid and a gas. The solid has about 70% - 80% of the mass, and approximately 90% of the fuel value of the original biomass.

Biomass is treated in hot compressed water in the case of wet torrefaction, also known as hydrothermal pretreatment. This process results in three products, including gases, aqueous chemicals, and a solid fuel. The temperature is in the range of 225˚ - 275 ˚C, and the pressures are up to 60 bar. The solid product contains about 50 ? 70% of the mass and 70 ? 90% of the fuel value of the original feedstock. The gas product is about 10%, and the aqueous chemicals make up the balance.

We have characterized experimentally the solid fuels resulting from both processes, which indicate significant similarity. Solid product from both processes have an enhanced energy density relative to the starting feedstock, and are is friable (and might be easily pelletized.) In both cases, an ultimate analysis of the solid fuels reveals decreased oxygen content and increased carbon content. In both cases, a fiber analysis of the fuel demonstrates that a significant fraction of the original hemicellulose component has been extracted. Proximate analyses reveal fuels suitable for gasification, and possibly for pyrolysis. The solid fuel in both cases exhibits hydrophobic qualities, as demonstrated by equilibrium moisture.