Biomass can be reformed into higher-value fuels using hydrothermal processes that employ high-temperature and high-pressure water as a reaction medium. Hydrothermal processing obviates feedstock drying and can achieve high energy efficiencies through heat integration. Hydrothermal liquefaction occurs under mild conditions (250–350 °C) in which biomass hydrolyzes rapidly and reacts to form a viscous bio-crude oil. At higher temperatures (350–500 °C), catalysts may be employed to promote the formation of CH4-rich gas in the process of catalytic hydrothermal gasification. Supercritical conditions (500–800 °C) may be used to achieve a H2-rich gas through supercritical water gasification (SCWG). The reaction chemistry underlying these hydrothermal processes is complex and not fully understood, but the influence of temperature, pressure, feedstock concentration, and the presence of catalysts on this chemistry has been extensively studied. In this chapter, we review hydrothermal processing of biomass, with a focus on the chemistry that describes biomass conversion under various hydrothermal conditions. Special attention is given to the relatively recent interest in processing aquatic feedstocks, such as algae, in a hydrothermal environment.
We processed phenol with supercritical water in a series of experiments, which systematically varied the temperature, water density, reactant concentration, and reaction time. Both the gas and liquid phases were analyzed post-reaction using gas chromatographic techniques, which identified and quantified the reaction intermediates and products, including H(2), CO, CH(4), and CO(2) in the gas phase and twenty different compounds--mainly polycyclic aromatic hydrocarbons--in the liquid phase. Many of these liquid phase compounds were identified for the first time and could pose environmental risks. Higher temperatures promoted gasification and resulted in a product gas rich in H(2) and CH(4) (33% and 29%, respectively, at 700 °C), but char yields increased as well. We implicated dibenzofuran and other identified phenolic dimers as precursor molecules for char formation pathways, which can be driven by free radical polymerization at high temperatures. Examination of the trends in conversion as a function of initial water and phenol concentrations revealed competing effects, and these informed the kinetic modeling of phenol disappearance. Two different reaction pathways emerged from the kinetic modeling: one in which rate ∝ [phenol](1.73)[water](-16.60) and the other in which rate ∝ [phenol](0.92)[water](1.39). These pathways may correspond to pyrolysis, which dominates when there is abundant phenol and little water, and hydrothermal reactions, which dominate in excess water. This result confirms that supercritical water gasification of phenol does not simply follow first-order kinetics, as previous efforts to model phenol disappearance had assumed.
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