Soot is the second largest source of global warming after CO₂ and it contributes to ambient fine particulates that cause millions of deaths worldwide each year. Diesel fuels contributed to 41% of the total soot emission in the US in 2005 (Report to Congress on Black Carbon by EPA in 2012). Polyoxymethylene ethers (POMEs) are alternative diesel fuels that can be produced from biomass or synthesized from CO₂, and which have been reported to have low soot emissions from engines. The POMEs in these studies consisted of alternating oxygen and carbon atoms with oxymethylene backbones and methyl end groups. These methyl-POMEs suffer from high water solubility and low lower heating value (LHV). Replacing the methyl groups with larger alkyl groups can overcome these disadvantages. However, larger alkyl groups will lead to higher sooting tendencies. To provide a basis for optimizing the trade-off between soot production and other properties, we synthesized 13 individual POMEs and measured their sooting tendencies.

The sooting tendencies were characterized by the Yield Sooting Index (YSI), which is based on the maximum soot concentration measured in coflow diffusion flames whose fuel is CH₄ doped with a low concentration of each test compound. The figure shows YSI (proportional to soot/mole) and YSI/LHV (~ soot/energy). The sooting tendencies of the POMEs are roughly one order of magnitude lower than a certification diesel fuel (“CF diesel” in the figure). This result demonstrates that all the POMEs have soot emission benefits. The YSIs of the POMEs decrease with more oxymethylene units and increase with more carbon atoms in the end groups. Simplified POME decomposition pathways based on numerical simulations can explain these trends. POMEs with more oxymethylene units form larger numbers of formaldehyde molecules that do not contribute to soot and dilute soot formation from the other products. POMEs with more than one carbon atom in the end groups decompose to ethylene molecules and therefore form more soot.

Stephen Lucas

Graduate Research Assistant, Colorado State University

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