The exergy analysis of low carbon or carbon free fuels: Methane, methanol, and hydrogen under engine like conditions

Abstract

The carbon dioxide (CO2) emitted from the combustion of fossil fuels is considered the main contributor to the warming of the climate system. Low carbon or carbon free fuels like methane, methanol, and hydrogen have gained a lot of attention on their ability of CO2 reduction in Internal Combustion (IC) engines. In this context, improving thermal efficiency at reduced levels of emissions by adopting low carbon fuels is meaningful in the current situation. However, the exergy loss of unconstrained combustion degrades the exergy work transform in IC engines. The present study focuses on the exergy loss and the thermomechanical exergy of these low carbon or carbon free dual fuels blended with high reactive n-heptane as the diesel surrogate under engine like conditions. Results reveal that these fuels lower the exergy loss by reducing the rate of n-heptane dehydrogenation and isomerization reaction. In addition, hydrogen in blends shows more influence on exergy loss reduction than methanol and methane. Using low carbon or carbon free fuels in blends improves the thermomechanical exergy distribution at 1000–2000 K combustion range. This is due to the fact that the reactions concerning radicals are more active than a traditional isooctane/n-heptane mixture, especially under low initial temperature and rich mixture conditions. Methanol in blend promotes the thermomechanical exergy at 1500–2000 K temperature range due to its highly intense H2O2-OH, CH2O-CO oxidation reaction, especially for the rich mixture. Furthermore, with regard to these low carbon fuels, one unit increment of thermomechanical exergy leads to approximately 0.65–1.2% improvement of second law efficiency. In conclusion, the optimization of low carbon fuels on exergy is mainly reflected in the low temperature and rich mixture combustion conditions, thus, the use of low carbon fuels does improve exergy use in combustion processes where the mixture is not uniformly distributed.