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Ethanol is partially oxidized in a continuous supercritical water reactor at temperatures from 500 to 530 °C, constant pressure of 25 MPa, initial ethanol concentration of 5 wt %, residence times of 3−8 s, and oxidant-to-fuel stoichiometric equivalence ratios of 5, 7.5, and 10%. The experimental conditions are selected to study the regime where ethanol oxidation happens rapidly but below the temperature necessary to initiate hydrolysis reactions. The reactions and interactions of intermediate species can be analyzed, leveraging previous experimental results and the existing body of literature on ethanol hydrolysis, pyrolysis, and oxidation. Higher oxidant concentration increases ethanol destruction and gasification efficiency, although significant coke/char buildup is qualitatively observed within the reactor. Product yields from the experiments are used to infer significant reaction mechanisms, and a pathway is postulated for the counterintuitive formation of char under the studied conditions.

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<p>A small-scale supercritical water oxidation reactor is designed and fabricated to study the destruction of hazardous wastes. The downward bulk flow is heated with the introduction of pilot fuel (ethanol/water mixture), and oxidant (H₂O₂/water mixture). Both streams are introduced coaxially. The fuel dilution is varied from 2 to 7 mol% ethanol/water, and the oxidant-to-fuel stoichiometric equivalence ratio (Φ_AF), is varied from 1.1 to 1.5. Higher ethanol concentrations in the pilot fuel stream and operation near-stoichiometric results in a more stratified temperature profile, i.e., highest local fluid temperatures near the top and the lowest temperatures at the bottom of the reactor. Steady operation at 603.5 °C is achieved with a nominal residence time of 25.3 s at 7 mol% fuel dilution and Φ_AF of 1.1. At the lowest pilot fuel dilution (2 mol%), the temperature profile is nearly uniform, approaching a distributed reaction regime.</p>

Supercritical water oxidation (SCWO) has been previously studied in the context of the destruction of organic compounds; however, information regarding the practical design and the details about the operation of these systems is not well presented in the literature. SCWO reaction rates have been studied in reactors with high surface-to-volume ratios and may not apply to practical reactors where volumetric reactions are prevalent. A modular small-scale supercritical water oxidation reactor is designed and fabricated to study kinetic rates, establish safe operating conditions, and test process control strategies. The reactor is heated by oxidation of pilot fuel (ethanol/water mixture) in an oxidant (H2O2/water mixture). Both streams are introduced coaxially in a downward direction stabilizing the oxidation by buoyancy. The fuel heating value was varied by adjusting the ethanol concentration in the 2–7 mol % ethanol/water range. The oxidant-to-fuel stoichiometric equivalence ratio (ΦAF) was varied from 1.1 to 1.5 by adjusting the oxidant mixture flow rate. Higher ethanol concentrations in the pilot fuel stream and operation near-stoichiometric result in a more stratified vertical temperature profile. The steady operation at a fluid temperature >600 °C is achieved with a nominal residence time of 25 s at a 7 mol % fuel dilution and ΦAF = 1.1. While the higher fluid temperature, the desired destruction of recalcitrant waste streams, was achieved, the wall temperature did not exceed material thresholds, establishing a safe operational envelope for the system. At the lowest pilot fuel dilution (2 mol %), the temperature profile is nearly uniform, approaching a distributed reaction regime with long residence times (>20 s) due to a buoyancy-driven stabilization scheme. The temperature distribution is used to validate the numerical modeling approach, presented in Part II of the study.