Sooting Tendencies of Phenolic Hydrocarbons

Lignin is an abundant renewable energy resource that constitutes about one-third of all biomass. Its chemical structure is a complex network of monolignol subunits that contain hydroxy-substituted benzenoid rings; thus, fuel processing of lignin produces phenols. The objectives of this study were to measure the sooting tendencies of lignin-derived phenols, compare them to the aromatic hydrocarbons that are found in petroleum-derived fuels, and identify the relevant chemical kinetic pathways with numerical simulations. Sooting tendency was quantified in terms of Yield Sooting Index (YSI), which is based on the maximum soot concentration when a small concentration (1000 ppm) of the test compound is doped into the fuel of a methane/air flame. In this methodology the test compound is usually injected as a liquid into the gas-phase fuel; however, many phenols are solids at room temperature. Therefore, a new procedure was developed wherein the phenols were dissolved at a 1:4 molar ratio in ethanol, then the solution was atomized into the methane. Validation experiments demonstrated that the ethanol has negligible influence on soot formation from the test compounds. The measured sooting tendencies show that the addition of a hydroxyl functional group to a benzenoid ring significantly reduces soot formation. Reactive molecule dynamics simulations with ReaxFF force fields indicated that the phenols form phenoxy radicals, which then eject CO molecules; this sequesters some of the carbon in a chemical form that does not contribute to soot. Overall, this study shows that lignin- derived fuels offer the benefit of low particulate emissions despite containing aromatic hydrocarbons.