Thermal stability and decomposition of diesel fuel under subcritical and supercritical conditions

A novel concept of clean diesel combustion using supercritical fluids is proposed and being investigated to address some key challenges encountered in the fuel and transportation sector. The core of this concept is to inject diesel fuel (DF) in the supercritical state to achieve clean, high-efficiency combustion in diesel engines. Among other challenging issues that must be addressed for the implementation of this new concept is the thermal stability of DF and the potential decomposition and solid deposit formation under engine conditions. In this work, thermal stability of DF was experimentally evaluated under subcritical and supercritical conditions in both static (batch system) and dynamic (continuous flow system) thermal stressing systems. The effects of thermal stressing temperature (200-440 °C) and duration (10-600 min) and CO₂ concentration (∼10 wt%) were examined. DF decomposition is characterized by the average absolute deviation (AAD) of GC peak area percentages of all individual components. A temperature-time window (400-420°C, 0-60 min) where supercritical DF combustion in diesel engines may be possible was determined. CO₂ as a diluent could prevent or reduce accumulation of solid deposits inside fuel pipes mainly due to an increased solubilization capacity of DF. Finally, different structures and morphologies of solid deposits observed under different batch thermal stressing conditions were discussed.