Biomass utilization to generate electricity via combustion simply can be classified into firing and cofiring. Biomass cofiring into the pulverized coal boilers has some advantages compared to dedicated biomass firing in terms of capital cost and combustion efficiency. To understand the cofiring behavior of biomass and coal comprehensively, computational fluid dynamics (CFD) method can be used to analyze and solve problems involving fluid flows inside a combustor. A CFD modeling is significantly more effective from the perspectives of time and cost and safety and ease of scaling up; hence, it is usually performed before conducting a physical investigation through experiment. Moreover, the current state-of-the-art CFD modeling-based study is capable of solving the complexity of the interdependent processes such as turbulence, heat transfer via radiation, produced gas, and reactions in both the particle and gas phases during combustion. This chapter focuses on the study of cofiring of biomass, which is palm mill wastes, into the existing coal-fired power plant. Two palm mill wastes are evaluated: palm kernel shell and hydrothermally treated empty fruit bunch. Distributions of temperature and the produced are simulated to find the most optimum and applicable cofiring conditions.