The release of the Ministerial of Energy and Mineral Resources of Indonesia Decree number 14.K/TL.04/MEM.L/2023 about coal-fired power plant (CFPP) greenhouse gas (GHG) emission cap, PLN as the only Indonesia- owned electricity company try to reconsider optimizing the GHG production of its CFPP’s at a particular value through several decarbonization programs which are conducting biomass co-firing on its existing CFPP as the first step action. PLN has various CFPP types and classes, from two megawatts (MW) to 660 MW. In this study, the basis used for CFPP is 60 MW class, PC boiler type derived from Asam-asam CFPP unit 1–2. The CFPP emits 1.428 kgCO2/kWh, which is higher than the GHG cap mentioned by the Ministerial Decree of 1.297 kgCO2/kWh. The CFPP uses sawdust as the biomass to be co-fired to reduce its carbon intensity. The gap between the actual GHG production and the GHG cap becomes a value that PLN should pay as a carbon tax, which another regulation mentioned the value. This program is expected to boost the use of green energy and prevent the CFPP from paying the carbon tax. However, the implementation of biomass co-firing is not always going smoothly. Based on the study, a specific ratio (%) becomes the best choice to be implemented because it produces the least cost. Based on the study, it can be concluded that the effort to avoid carbon tax must not be profitable. When the carbon cap value becomes a mandatory factor that shall be obeyed, it shall conduct the biomass co-firing with a ratio of about 10.5% at the minimum. However, the action recommended to the CFPP is to conduct the biomass cofiring in the 5 – 9% ratio in terms of economics.