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Pretreatment of Lignocellulosic Biomass Using Acetic Acid, Salt, and Ionic Liquids
AuthorLynam, Joan Goerss
Chemical and Materials Engineering
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As a renewable non-food resource, lignocellulosic biomass has great potential as an energy source or feedstock for further conversion. However, challenges exist in transportation, storage, and use of this perishable resource with relatively low fuel value compared to fossil fuels. Pretreatment of recalcitrant lignocellulosic biomass can facilitate its energy densification compared to the raw biomass. Pretreatment can also fractionate it into its components, which can then be used as platform chemicals for further conversion.Hydrothermal Carbonization (HTC), also known as wet torrefaction, is a pretreatment where biomass is immersed in liquid water under an inert atmosphere and heated to temperatures between 200 and 300 °C at pressures which ensure that the water remains liquid. HTC's products include a solid residue (biochar) of increased fuel value and a liquid solution of five carbon and six carbon sugars, along with acetic and other organic acids and furfurals. Additives to the basic process can enhance the higher heating value (HHV) of the biochar. Adding acetic acid and/or Li chloride to the standard HTC process, when applied to loblolly pine at 230 °C, increases the HHV of the biochar up to 30% and removes cellulose from the raw biomass. Adding Ca lactate and Ca chloride to the HTC pretreatment at 260 °C of loblolly pine also enhances the reactions occurring so that the resulting biochar has increased HHV compared to that with no salt added. Ca salts can reduce SOx and NOx emissions from coal thermal conversion processes. If pretreated biomass with a Ca salt adhering to it is co-fired with coal, emissions may be reduced while less coal is needed. Adding Ca lactate, Li chloride, and Ca chloride to HTC pretreatment also reduces reaction pressure, enhancing process safety.An alternative to HTC pretreatment is pretreatment of biomass with ionic liquids. Rice hulls, a particularly recalcitrant biomass, are available in vast quantities worldwide. Some ionic liquids,which have very low vapor pressures and are considered "green" solvents, can be used to dissolve biomass. The ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM Ac) is capable of removing lignin from rice hulls, while ethanol can precipitate lignin out of the used EMIM Ac. The remaining cellulose may be quite available for further conversion to bio-ethanol via enzymatic hydrolysis and fermentation. Additives to the HTC pretreatment process and the use of "green" ionic liquids at low temperatures and pressures show promise in replacing fossil fuel with renewable lignocellulosic biomass.