Due to the growing increase in fuel price, a significant part of costs of power plants is expended on obtaining high-quality fuels. The change of fuel type from the natural gas to the liquid in the winter due to the increase in natural gas consumption is the main reason for this research. The goal is to select the appropriate fuel type from the Energy, Exergy, Exergy-economic, and Environmental (4E) aspects. The topic of this paper is an irreversible comparison of combined cycle power plant (CCPP) components by changing fuel type from natural gas to liquid fuels with different APIs. The costs resulting from the pollution of normalized air with fuel change were compared. The results were validated by the data from the largest CCPP in Iran. Multi-objective optimization of the CCPP was implemented by objective functions of the price of the power produced, exergy efficiency and normalized production amount of CO2. The results showed that decrease in APt led to proper exergy efficiency and cost but exposed very high environmental impacts compared with gas fuel. Results of changing objective functions demonstrated that propagation rate of CO2 in gas fuel was much lower than that of liquid fuel. (C) 2018 Elsevier Ltd. All rights reserved.