PNNL

Abstract

Electrocatalytic oxidation is an attractive process for valorizing the carbon and removal of nitrogen present in the aqueous waste streams at ambient conditions. Here, we evaluated the electrocatalytic oxidation as a function of applied potential over a commercial Pt electrode of aqueous waste stream generated via hydrothermal liquefaction. We validated the conversion of quantifiable carbon (e.g., carboxylic acids, alpha hydroxyacids, alcohols, ketones, amides) and unquantifiable carbon as well as nitrogen removal. The unquantifiable carbon was valorized to quantifiable short chain organic molecules (e.g., acetic). Model studies showed that carboxylic acids, alcohol and ketones converted following (non-) Kolbe chemistry. Amides and alpha hydroxyacids formed carboxylic acids that subsequently converted via (non-) Kolbe chemistry. Ammonia was oxidized into N2. The reaction rate varied for each molecule family with carboxylic acids and ammonia having the highest reaction rates. The main reaction products from the electrocatalytic oxidation at the anode were volatile, short chain hydrocarbons (i.e., olefins, paraffins) and CO generated via Kolbe chemistry, while H2 was generated at the cathode. The parallel denitrification, valorization of carbon, and H2 generation of the aqueous waste can replace and simplify the current unit operations used in a commercial hydrothermal liquefaction process. The high operation potential required to drive the electrocatalytic reaction is responsible for high operation costs; however, they can be either partially subsidized with sale of excess H2 at the current DOE goal of $2/kg H2. Thus, the present work shows how electrocatalytic oxidation can be used to valorize aqueous waste streams into volatile hydrocarbons and H2.