Soot has been identified as the second-largest source of climate change after CO2 and it contributes to ambient fine particulates that cause millions of deaths worldwide each year. Diesel engines contribute significantly to total soot emissions because diesel fuels have high sooting tendencies. Poly(oxymethylene) ethers (POMEs) are alternative diesel fuels that can be produced from waste CO2 with low soot emissions. The structures of the POMEs in these earlier studies are alternating oxygen and carbon atoms terminated with methyl groups on both ends. Unfortunately, these methyl-POMEs suffer from high water solubility and low energy density. Replacing the methyl groups with larger alkyl groups can overcome these disadvantages, but at the cost of higher sooting tendencies. To optimize this trade-off, the sooting tendencies of methyl-POMEs and alkyl-POMEs need to be quantified. In this work, a series of methyl-POMEs and alkyl-POMEs were synthesized and their sooting tendencies were quantified. The test compounds contained a wide range of terminating groups (methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl, iso-pentyl, and tetrahydrofurfuryl), cases where the terminating groups were identical at the two ends and where they differed, and from one to five oxymethylene units. 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 1000 ppm or 3000 ppm of each test compound. The YSIs of the POMEs vary significantly with fuel structure, but in all cases are at least one order of magnitude lower than a certification diesel fuel. We proposed some decomposition pathways that justified the difference among the YSIs. The lower heating value (LHV) was also measured to evaluate the energy penalty of the oxygen atoms. The calculated YSI/LHV of the POMEs are lower than conventional diesel fuels and their components.

Poly(oxymethylene) ethers: alternative diesel fuels with low sooting tendencies

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