Phenolic hydrocarbons are produced in large quantities when lignocellulosic biomass is processed into fuels. Traditionally these phenols have been viewed as undesirable due to properties such as their acidity, so they are removed during subsequent fuel processing. However, they also have beneficial fuel properties such as high energy density and low sooting tendencies. Soot emissions are the second largest source of global warming and contribute to ambient fine particulates that cause millions of deaths annually. In this work quantitative sooting tendencies were measured for a wide range of phenolics, including benzenediols and benzenetriols, alkylphenols (e.g., n-propylphenol), methoxyphenols (e.g., guaiacol and syringol), allylphenols (e.g., eugenol), and phenolic aldehydes (e.g., vanillin). The sooting tendencies were determined by doping the test compounds into the fuel of a methane/air nonpremixed flame at low concentrations (typically 1000 ppm) and measuring the resulting soot concentration. The absolute soot concentrations were converted into yield sooting indices (YSIs) by rescaling them relative to n-heptane and toluene reference fuels. Many phenolics are solids at room temperature, so they were dissolved in ethanol and then the liquid solutions were injected into the flame. The results confirm that the oxygen in phenols reduces soot formation relative to the analogous regular hydrocarbons. For example, 1,3,5-trihydroxybenzene has a YSI of 9.7 versus 100.3 for benzene. The sooting tendencies depend strongly on molecular structure parameters such as the position of the hydroxyl group relative to other sidechains, the length of the other side-chains, branching in the other side-chains, etc. These results provide a basis for identifying specific phenols that can be produced as renewable fuels from biomass while combining low soot emissions with acceptable values of other fuel properties.

Sooting tendencies of phenolic hydrocarbons from lignin conversion and upgrading

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