Scientists at Tomsk Polytechnic University conducted a comprehensive study and quantified how microdrops of water in biofuels affect the technological cycle of its use — spraying, evaporation, gorenje and emission indicators. The results of the study will help optimize the composition of biofuels in the future for more stable and environmentally friendly operation of diesel engines.
The research was supported by a grant from the Russian Ministry of Education and Science (No. 075-15-2025-572). The results of the scientists' work are published in the journal Biomass and Bioenergy (Q1, IF: 7.0).
Biofuels are considered a promising replacement for petroleum diesel fuel. However, different types of biodiesel behave differently in engines. The quality of their spraying and combustion is influenced by many factors: viscosity, surface tension, density, composition of fatty acids and impurities. In addition, biofuels contain water, which simultaneously reduces the amount of harmful emissions and changes the atomization, evaporation and stability of the gorenje flare. For the practical use of biofuels, it is necessary to understand which combination of "raw materials — water — heating mode" gives the best and stable result.
Tomsk Polytechnic University scientists conducted a comprehensive study to find out how water droplets affect the micro-explosive destruction of biofuels. The experiments were conducted with diesel fuel and bio-fuel obtained from rapeseed oil, distilled tall oil and methyl esters of tall oil fatty acids. Drops of this fuel were introduced into a convective heating chamber at a gas temperature from 450 to 750 K and a cylindrical combustion chamber. At the same time, scientists monitored changes in the physico-chemical properties of the fuel, droplet destruction modes, evaporation rate, and emissions composition.
When heated, the water in the fuel boils before the fuel itself, turning into steam and tearing the drop from the inside. Previously, scientists studied these processes separately: chemists analyzed the composition, thermophysicists analyzed droplet evaporation, and engineers analyzed emissions from the installation. We combined these three levels in one experimental scheme and were able to establish a quantitative relationship between the composition of biodiesel, the behavior of a single water-fuel droplet and emissions from its combustion.,
— notes Roman Fedorenko, one of the authors of the study, a research engineer at the TPU Heat and mass transfer laboratory.
The results of the study showed that the feedstock based on biodiesel affects the physico-chemical properties of the finished fuel: depending on the "base", the viscosity differed by more than 1.9 times, the interfacial tension by more than three times, the boiling point by no more than 3.5%, and the surface tension of the samples by 10%. According to scientists, these properties determine the dynamics of the decay of fuel droplets inside the combustion chambers.
Diesel fuel—based droplets disintegrate almost instantly (in less than 1 millisecond), while biofuel-based droplets disintegrate more slowly and form larger daughter droplets (droplets that separate from the "parent" during micro-explosive fragmentation - approx.) due to the growth of bubbles. At the same time, biodiesel from tall oil fatty acids proved to be more effective during secondary spraying, forming droplets on average 15-20% more than that of diesel fuel. However, compared to other types of biofuels, these drops were still 25-30% smaller. Even a small amount of biofuels from tall oil fatty acids in diesel fuel increased the decay delay time by 25-30% and increased the surface area by 8-10 %,
— adds Roman Fedorenko.
The biofuel and diesel emulsion samples also had different mechanisms of micro-explosive destruction. Thus, the delay time of diesel fuel-based emulsion droplets was 15-17% less than that of biodiesel-based emulsions. In addition, the increase in the surface area of diesel-based emulsion droplets was 40-45% less.
Separately, the polytechnics analyzed the effect of water in the fuel composition on the environment. The results showed that adding water reduced nitrogen oxide emissions by 10-12% for diesel fuel and by 18-20% for tall oil fatty acid biodiesel.
Scientists from the Tomsk Polytechnic University's School of Energy Engineering and the Research School of Physics of High-Energy Processes participated in the study.
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