Combustion control techniques have become a legal requirement to minimize pollution in municipal solid waste incinerators. In-furnace destruction of pollutants is achieved when 2-second gas residence time at 850 C and 6% O2 are guaranteed. Incinerator performance is analyzed numerically to validate good combustion performance. Computational Fluid Dynamics (CFD) modeling of gas flow inside the furnace chamber provides three-dimensional temperature, concentration, and velocity vectors. General flow patterns and the presence of recirculation pockets are traditionally observed. Local temperature and oxygen concentration can also be checked. The CFD results are analyzed further in terms of residence time, mixing, and thermal decomposition of potential pollutants. The residence time needs to be carefully determined based on the gas inlet position. The statistical variation requires evaluation of the average and minimum (or shortest) residence time. Mixing is quantified by defining a local mixedness parameter, a, with which the effects of secondary air jets are interpreted. Thermal decomposition parameter B integrates the temperature and oxygen availability over the residence time, which may be directly related to good combustion.