Design of pulp mill hot and warm water systems-A new method that maximizes excess heat
Examensarbete för masterexamen
In this master thesis the hot and warm water system in pulp mills has been studied. Many hot and warm water systems of today are overdimensioned and produce more hot and warm water than demanded. This overproduction could instead be used for other applications, if heat could be released at high enough temperatures. The aim of the thesis has been to release as much excess heat of as high temperature as possible. In order to maximize the amount of excess heat a new method has been developed. The basic idea of the new method is to decrease the ∆Tmin in the system. In practise using pinch technology does this. The cold composite curve is moved towards the hot curve in the composite curve diagram until ∆Tmin for the system is reached. The hot composite curve contains all hot streams available, while the cold curve contains only the process streams. In order to investigate how to design a hot and warm water system a third curve has been introduced. This curve is called the “tank curve” represents the actual streams to the different tank temperature levels in the hot and warm water system. To investigate the number of tank levels needed and the temperatures of these levels a program has been developed in Visual Basic. This program checks all possible combinations of temperature levels for the water tanks for a given number of tanks, in order to find the combination of temperatures that maximizes the excess heat. Two different ways to use the excess heat has been investigated, to use the excess heat for production of district heating or to replace live steam in the evaporation plant. If the excess heat is used in the evaporators the demand of live steam will decrease and thereby the fuel consumption also will decease. Today oil or bark is used on the margin to produce live steam. The economic consequences will differ depending on how much the fuel consumption is lowered. Stream data for a Swedish kraft pulp mill has been used with the new method. The method shows that 6-7 MW excess heat with a temperature over 90 °C can be released if the system is rebuilt. The study also shows how such heat exchanger networks should be designed. Finally an economic analysis has been done, showing that it is economically favourable to rebuild the hot and warm water system to release the excess heat. Most favourable is to decrease the use of oil, then to use it for production of district heating. Least profitable is it to use the excess heat to lower the consumption of bark, but still this option is profitable.
Kemiteknik , Chemical Engineering