The conventional two-step thermal processing of phosphoric acid wastes enormous amounts of energy, which causes serious environmental pollution in addition to bad economic performance. A two-step waste heat recovery method using a "phosphorus burning boiler" was developed to reduce energy loss. This paper presents an exergy analysis model based on two new indicators for the "process driving force" and the "energy-saving potential" for a thermal process. Based on the model, an evaluation method was established to indicate the energy utilization ratio, the economic performance and environmental effects of the thermal process. The method was used to analyze retrofitting for the waste heat recovery for thermal processing of phosphoric acid. The results demonstrate that the retrofit is feasible. Furthermore, the analysis was used to present two additional energy-saving retrofits. (c) 2005 Elsevier Ltd. All rights reserved.