Gasification, Liquefaction and Deoxy-Liquefaction of Switchgrass using Sub- and Supercritical Water

Abstract

Increased demand in transportation fuels, environmental concerns and depletion of fossil fuel require development of efficient conversion technologies for second-generation biofuels. In this dissertation, sub- and supercritical water have been used as the medium to transform biomass (here, switchgrass) into biochar, syngas, and biocrude through carbonization, gasification, liquefaction and deoxy-liquefaction. The challenges with thermal gasification of biomass to produce syngas include high transportation and drying costs of biomass, the need for high temperature (700-1000 oC) gasifiers and conditioning of the produced gases. Some of these challenges can be addressed by first converting biomass into high-energy-density biochar before transportation, and then hydrothermal gasification of biochar at the site of Fischer-Tropsch plant for liquid fuels synthesis. In the first part of this work (Chapter 2), a high-energy density (> 27 MJ/kg) biochar is first produced via hydrothermal carbonization of switchgrass at 300 oC, and then the biochar is gasified in hydrothermal medium at 400-650 °C. The carbon gasification efficiency in hydrothermal medium is much better than that in the thermal medium. For example, at 550 °C, only 5.9% carbon gasification is achieved in the thermal medium as compared to 23.8% in hydrothermal medium. The addition of 25 wt% K2CO3 catalyst enhances the hydrothermal gasification to 43.8%. The gasification can be further enhanced if the biochar is passivated with a small amount of Ca(OH)2 when producing from biomass. The use of Ca(OH)2 passivation during hydrothermal carbonization coupled with the use of K2CO3 catalysis during hydrothermal gasification with short reaction times of 5 and 30 minutes, respectively result in a high carbon gasification efficiency of 75% at 600 °C. The heating value of the syngas obtained increased with passivation (due to enhanced gasification) and was comparable to that obtained from low-grade coal gasification. Apart from producing syngas, biomass can also be liquefied into energy-dense biocrude. In Chapter 3, a novel two-step process is proposed in which acidic subcritical-water followed by alkaline supercritical-water media are utilized for the liquefaction. The concept is tested with switchgrass. The first step is carried out at 200 °C in acidic subcritical water to liquefy hemicelluloses to biocrude. In the second step, the remaining un-liquefied biomass (biomass-H) is subjected to supercritical water at 380 °C with Ca(OH)2 as catalyst for minimizing the formation of char, enhancing lignin solubilization and therefore increasing liquefaction of the remaining polysaccharides toward biocrude. The extraction of biocrude from the first step before subjecting the unliquefied biomass to the second step is very crucial. It avoids repolymerisation reactions between the hemicelluloses degradation products and lignin degradation products which would have resulted in char. The proposed two-step liquefaction produces significantly higher amount of biocrude as compared to the traditional one-step process. The yield of biocrude from the proposed process is 40% on mass basis and 67% on energy basis of the feedstock biomass. The biocrude obtained from hydrothermal liquefaction has to be upgraded to be able to be used as a “drop-in” fuel. In the last part of this work, switchgrass was liquefied in supercritical water (SCW) using calcium formate as an in-situ source of hydrogen to enhance its deoxygenation and hence improve the quality of the biocrude obtained. In supercritical water, calcium formate produces hydrogen via decomposition and hydrolysis reactions, and simultaneously switchgrass hydrolyzes to form oxygenated hydrocarbon compounds from cellulose, hemicelluloses, and lignin. Because of the close proximity of the newly-formed hydrogen and active oxygenated hydrocarbons, hydrodeoxygenation occurs whereby some of the oxygenated compounds are upgraded by the removal of oxygen in the form of water. The analysis of the biocrude, obtained in the presence of calcium formate, shows the presence of benzene, polyaromatic hydrocarbons, and alkyl phenolics as opposed to alkoxy phenolic compounds (obtained without formate addition). The benzene formation is attributed to the hydrogenation of phenols (from lignin decomposition) but also due to the Diels-Alder alkene addition (alkenes formed by liquefaction of unsaturated triglycerides), followed by dehydrogenation. The addition of calcium formate doubles the yield of oily biocrude to about 10 wt% and increases the heating value from 28 to 34 kJ/g. The content of formic acid increases in the aqueous biocrude (biocrude-W) due to the enhanced decomposition of xylose and glucose. This study has therefore used a low-value catalyst to increase the energy content of the upgraded biocrude by 20% in a simple one-pot reaction.