Polychlorinated biphenyls (PCBs) are classified as persistent organic compounds and used in many industries because they are incombustible, heat resistant and electrical insulators. Owing to their strong toxicity and bioaccumulative nature, their transboundary movements are controlled by the Basel Convention and their levels are monitored by United Nations Environment Programme (UNEP). One of the most general methods for treating PCBs-containing waste is high-temperature decomposition; the removal efficiency of PCBs is 99.999% at over $1200^{circ}C$ for a retention time longer than 2 s. However, relatively low temperatures or low oxygen levels can produce highly toxic substances such as dioxins and dibenzofurans. Therefore, the reactor for the decomposition of PCBs should be very carefully designed. In this study, numerical simulations have been carried out to design a reactor for waste insulating oil containing PCBs. The flow patterns and temperature distributions of the gas in the reactor are examined. The path lines of the fluid particles are analyzed from the numerical results and the detention time of the PCBs-containing oil in the reactor is calculated. On the basis of this result, we try to scale up from a pilot plant to a commercial plant.