An exergy-based approach to determine production cost and CO2 allocation in refineries

In view of the continuous yet finite exergy supply to Earth, the determination exergy cost together with CO2 emission for fuel production is essential to the environmental evaluation of most processes. A petroleum refinery comprising atmospheric and vacuum distillation, delayed coking, fluidized catalytic cracking, hydrotreating, hydrogen generation, as well as residue/waste treatment units, such as sulphur recovery and sour water treatment, was analysed. Although high indexes of exergy conservation were observed for process units (>97%), the total exergy destroyed in the refinery was almost 800 MW. The calculation of the unit exergy cost and unit CO2 cost for produced fuels were performed by solving the set of linear equations used to describe the exergy cost formation for these fuels. The unit exergy costs found for diesel, gasoil, gasoline and hydrotreated diesel were 1.026 MJ/MJ, 1.028 MJ/MJ, 1.049 MJ/MJ and 1.10 MJ/MJ, respectively, while the unit CO2 costs for these fuels were 1.49 gCO2/MJ, 1.20 gCO2/MJ, 4.86 gCO2/MJ, 6.34 gCO2/MJ, respectively, reflecting the processing level and its efficiency as well as the C/H ratio of the burnt fuels.