Investigation on the formic acid evaporation and ignition of formic acid/octanol blend at elevated temperature and pressure

Abstract

With the growing advancements in new energy sources, formic acid has become a subject of interest as a hydrogen carrier. Apart from being a hydrogen carrier source, to explore the possibility of formic acid being used as a potential fuel in engines, the combustion of formic acid is yet to be explored fully. Few studies have been conducted to determine its laminar burning velocity. The study of droplet combustion and evaporation is also critical if formic acid is to be deployed as a potential fuel. Therefore, the evaporation of formic acid with a droplet diameter of ∼1.00 mm is investigated in a constant volume combustion chamber for pressures ranging from 5 bar to 20 bar at a wide temperature range (150–300 °C). A model was developed to get a fundamental understanding of the experimental results. The D2 plot from experimental data exhibited the presence of two zones for evaporation. Zone 1 showed a slower evaporation rate compared to zone 2. Due to a thin layer of formic acid remaining on the thermocouple, the surface area of the droplet increased in zone 2, resulting in faster evaporation of the droplet. The steeper decline in droplet evaporation occurred at the point of complete loss of sphericity of the droplet. Even so, the trend for total evaporation time from the model and experiments was consistent. Formic acid is a low reactive fuel, and it did not ignite at the ambient temperature and pressure of 300 °C and 20 bar, respectively. So, to determine the auto-ignition behavior of formic acid, a separate experimental setup that could exceed the ambient temperature higher than the auto-ignition temperature of formic acid was used. When pure formic acid did not ignite, it was mixed with octanol at different concentrations and further investigated. The mixture droplet’s ignition probability slightly decreased to 65 vol% of formic acid addition in the octanol. Further addition of formic acid in octanol significantly decreased the ignition probability of the mixture droplet. Lastly, a polynomial equation is proposed to extrapolate the auto-ignition time of pure formic acid droplets with a droplet diameter of ∼0.7 mm.