In some complex energy conversion process system related to hydrogen, as like as an acetylene production process, fuel cell and a H(2)/O(2) power generation cycle, hydrogen can be considered as a process key component. The influence of hydrogen conversion and utilization on the characteristics of energy conversion is very important. This paper presents a new graphic method for thermodynamic analysis and energy integration used in hydrogen systems. The proposed tool, known as the flowrate-exergy diagram (FED), visually and concisely describe the conversion rates of hydrogen and hydrogenous chemicals associating with the exergy loss, based on the second law analysis and the exergy concept. The principles and a series of revelatory criteria for using the FED are also introduced. Based on the analysis of an acetylene production process and a H(2)/O(2) cycle system, a novel acetylene and power polygeneration system was proposed and the validity of the new method was proved. (C) 2008 Elsevier Ltd. All rights reserved.