Due to the depletion of non-renewable natural resources, the increase in the emission of pollutants and the growing global energy demand, it is increasingly important to implement strategies that lead to a sustainable use of available resources and to improve the efficiency of the processes currently employed; A tool widely used for this purpose is the exergy analysis. Currently, refining processes use refinery gases as the main source of energy, which consist of mixtures of natural gas and residual gases; these are obtained as by-products in different processing units, so their composition varies according to the process they come from, highlighting their content of gases such as ethylene, hydrogen, propane and propylene. In this order of ideas, in this work we evaluated the effect of the variation of the proportion of residual gas (propane) with respect to natural gas, on the thermodynamic behaviour of combustion in a refinery furnace; through the methodology of exergy analysis. The process was simulated using the software Aspen-Plus®-v.8.8, obtaining thermodynamic properties of the flows involved in the combustion for each of the propane-natural gas mixtures analysed. Then, the exergy analysis was performed, obtaining profiles of irreversibility and exergetic efficiency, finding that the variation of the residual gas at the entrance of the furnace does not greatly affect the exergetic efficiency (variations less than 1 %); however, the irreversibilities of the process increase up to 70 % as the proportion of residual gas increases.