Key issues and solutions in a district heating system using low-grade industrial waste heat

Industrial waste heat is increasingly being recognized as an important source of heat for DH (district heating) systems in cold regions to fill shortfalls in heating requirements, while consuming less fossil energy than with conventional heating sources. Most existing cases of industrial waste heat utilization for heating merely focus on heat recovery from a single waste heat source for heating either in the factory in which the heat is generated or other buildings in the vicinity. The purpose of this paper is to discuss the key issues related to a DH system using two or more kinds of low-grade industrial waste heat, at a temperature between 20 degrees C and 90 degrees C, for users a long distance away from the heat sources, including the collection and integration of multiple-grade waste heat sources, long-distance delivery of waste heat, and peak shaving of the system. Solutions to these three issues are proposed to increase the efficiency of the system and for further and better promotion of such a system: 1) "Tangency technology" is designed and applied to find the optimal method of collecting heat from multiple waste heat sources. 2) Lowering the temperature of the return water on the primary side has been proven to be crucial to the collection and long-distance delivery of industrial waste heat. 3) Systems integrating both industrial waste heat and fossil-fuel heat are depicted. Industrial waste heat always provides the base load for the DH system, while the fossil-fuel heat acts as the peak shaver. Lastly, a case study undertaken in Chifeng in northern China demonstrates how these solutions act in a first-ever DH demonstration project using multiple sources of waste heat from a copper smelter. In all, 390,000 GJ of waste heat was recovered, 35,000 t of CO2 emission was reduced, and over 150,000 t of water was saved. (C) 2015 Elsevier Ltd. All rights reserved.