HEAT RECOVERY AND SYSTEM CLOSURE IN A NEWSPRINT MILL

This study focussed on closure measures to reduce process water usage and on heat recovery measures to reduce process heating demand. The minimum process heating requirement was independent of process water use. Due to the large amount of refiner energy entering the process streams as heat, there was sufficient heat recovery potential to meet all process heating demands below the pinch temperature for current, moderate and advanced levels of closure. There was also excess heat available at all closure levels, some of which could be recovered for district heating. The maximum heat recovery was limited in winter by the temperature requirement for biological treatment. In some cases a cooling tower may be required to reduce effluent temperature in summer. A critical result is that the design of the heat exchanger network leading to minimum energy use will be a very strong function of water use. Reducing steam demand by recovering heat from a specific hot water stream depends on stream flows and temperatures; reducing water use may change the characteristics of that stream and thus the heat available for recovery from it. To ensure the best use of capital when installing heat recovery equipment, it is critical to consider potential water use reduction projects at the same time.