The modeling of thermal reaction furnaces of Sulfur Recovery Units (SRUs) is a rather complex problem since it involves different modeling scales such as the kinetic/molecular scale, the reactor scale, and the chemical process scale. This work introduces the multi-scale modeling approach to characterize the kinetic and reaction engineering scales for the thermal section of SRUs, involving the reactor furnace and the waste heat boiler. Specifically, also the waste heat boiler is modeled using detailed kinetics to characterize the recombination effects, which cannot be neglected any longer since they significantly influence the outlet compositions. The proposed models are validated on experimental and literature data for the kinetic scale. The reactor scale is validated on the industrial data coming from SRUs operating in Nanjing and Mumbai plants. (C) 2013 Elsevier Ltd. All rights reserved.