There is a power plant with a capacity of 3 × 315 MW which has a tangential pulverized boiler type. This power plant uses an LRC and MRC Coal type for the fuel to produce steam which is used to rotate steam turbine that coupled with a generator to produce electric power. The Indonesian government is striving used biomass to decrease using coal fuel until 23%. Previous sub-critical power plant not designed to be a co-firing fuel, and the best way to implement that system is using the direct co-firing method. This power plant has been an experiment using biomass until 1%, and there is no significant effect for the equipment. For the effectiveness combustion still need to do research. In this research are using 3 types fuels, 97.5% coal and 2.5% biomass, 95% coal and 5% biomass, 92.5% coal and 7.5% biomass. And it is also using excess air variation of 15%, 17%, and 20%. Efficiency’s formula was carried out to find optimum variables with biomass and excess air at this power plant that the CFD numerical analysis was carried out to find an optimum model to analyze in more detail the effect to temperature contour, velocity particle combustion. In the end, we obtained that the efficiency biomass is still underusing coal but 97.5% biomass is the best for efficiency between the other variable.