Hydrothermal Treatment for Biofuels: Lignocellulosic Biomass to Bioethanol, Biocrude, and Biochar
Type of Degreedissertation
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Biofuels are viewed as an alternative, renewable fuel and an important component of energy security for the future. The dissertation work is directly aimed at supporting the commercial production of biofuels from lignocellulosic biomass. The prime objective is to apply chemical engineering fundamentals for capitalizing on the extraordinary solvent properties of water at elevated temperature and support green and sustainable chemistry. Hydrothermal treatment refers to processing of biomass in sub- and super-critical water medium. Microcrystalline cellulose (MCC) was pretreated (Chapter 2) using subcritical water in a continuous flow reactor for enhancing its enzymatic digestibility. The degree of polymerization of cellulose steadily decreased with an increase in the pretreatment temperature, with a rapid drop occurring above 300°C. The partial transformation of cellulose I to II polymorph was noticed in the MCC treated at ≥ 300 °C in subcritical water. A nearly three-fold increase in the initial enzymatic reactivity was observed for the sample treated at 315°C. The hydrothermal pretreatment of switchgrass (Chapter 3) was conducted in a flow through reactor to enhance and optimize the enzymatic digestibility of pretreated biomass. More than 80% of glucan digestibility could be achieved at 190°C in hydrothermal medium. Addition of K2CO3 (0.45-0.9 wt%) in water helped in enhancing the enzymatic activity of biomass. An alternate pathway for the utilization of liquid hydrolyzate was developed and sugar loss in the liquid fraction during pretreatment was recovered as high heating value solids. Liquefaction of cellulose in sub- and super-critical water was studied (Chapter 4) in a continuous flow reactor. The focus of this study was to maximize the yield of sugar products (oligomers and monomers) from cellulose hydrolysis. About 65% of cellulose converted to the oligomers and monomers at 335°C in 4.8 s and also at 354°C in 3.5 s. In the supercritical region, the produced oligomers and monomers partially degraded to degradation products. Liquefaction of switchgrass (Chapter 5) in subcritical water was studied to produce biocrude. The effects of reaction temperature and catalysis by K2CO3 were examined. Potassium carbonate significantly enhanced the hydrolysis of switchgrass components into water soluble products. More than 50 wt% of organic carbon available in switchgrass was converted to biocrude at 235°C in the presence of 0.15 wt% of K2CO3. Biocrude contained oxygenated hydrocarbons. The subcritical water treatment caused the complete breakdown of lignocellulosic structure of switchgrass. Hydrothemal carbonization of switchgrass (Chapter 6) was studied to produce the high energy density (coal-like) biochar. The effects of temperature, residence time, and pressure on the yield and heating value of biochar were examined. Besides the solid fuel application, biochar was also studied for its sorption properties for removing heavy metal contaminants from ground water. The batch adsorption results (Chapter 7) with uranium [U(VI)] as a solute showed that biochar can be a potential low cost adsorbent for such application. Future work (Chapter 8) involves the upgrade of biocrude to liquid / gaseous fuel. Also pyrolysis / gasification properties of biochar may be investigated. Biochar adsorption properties may be further tested for removing other metal contaminants.