A Comparative Study on the Combustion Chemistry of Two Bio-hybrid Fuels: 1,3-Dioxane and 1,3-Dioxolane.

Bio-hybrid fuels are a promising solution to accomplish a carbon-neutral and low-emission future for the transportation sector. Two potential candidates are the heterocyclic acetals 1,3-dioxane (C4H8O2) and 1,3-dioxolane (C3H6O2), which can be produced from the combination of biobased feedstocks, carbon dioxide, and renewable electricity. In this work, comprehensive experimental and numerical investigations of 1,3-dioxane and 1,3-dioxolane were performed to support their application in internal combustion engines. Ignition delay times and laminar flame speeds were measured to reveal the combustion chemistry on the macroscale, while speciation measurements in a jet-stirred reactor and ethylene-based counterflow diffusion flames provided insights into combustion chemistry and pollutant formation on the microscale. Comparing the experimental and numerical data using either available or proposed kinetic models revealed that the combustion chemistry and pollutant formation differ substantially between 1,3-dioxane and 1,3-dioxolane, although their molecular structures are similar. For example, 1,3-dioxane showed higher reactivity in the low-temperature regime (500-800 K), while 1,3-dioxolane addition to ethylene increased polycyclic aromatic hydrocarbons and soot formation in high-temperature (>800 K) counterflow diffusion flames. Reaction pathway analyses were performed to examine and explain the differences between these two bio-hybrid fuels, which originate from the chemical bond dissociation energies in their molecular structures.

Theoretical Investigation of Key Properties of the Pyrolysis of Methyl, Ethyl, and Dimethyl Dioxolane Isomers.

Cyclic acetals are considered as carbon-neutral fuels that can be produced from biomass and renewable electricity. Recent investigations on 1,3-dioxolane, a five-membered cyclic acetal, revealed that unimolecular decomposition through H-atom migration within the ring governs thermal decomposition. For methyl- and ethyl-substituted dioxolane compounds, very limited information on thermodynamic, transport, bond dissociation, and thermal decomposition properties is available. The present study remedies this lack of information by providing these properties for methyl-, ethyl-, and dimethyl-substituted dioxolanes in a systematic manner. While adding substituents to the dioxolane ring has only a minor effect on the bond dissociation energies, the barrier heights for H-atom migration are clearly affected by the position of the substituents. Notably, the corresponding transition states are preferably in the boat ring configuration. However, two of the substituted dioxolanes do not allow for this configuration because of their respective bonding structures, resulting in larger barrier heights. The properties provided here will aid the detailed chemical kinetic modeling of substituted dioxolane combustion chemistry.