OPTIMISATION OF STEAM AND CONDENSATE SYSTEMS OF PAPER MACHINES

Transcription

1 Parallel session Innovative energy efficiency examples of different industrial sectors Energy efficiency in pulp & paper and sugar industry OPTIMISATION OF STEAM AND CONDENSATE SYSTEMS OF PAPER MACHINES Gerald Bachmann Allplan GmbH Schwindgasse 10, 1040 Vienna, Austria

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3 OPTIMISATION OF STEAM AND CONDENSATE SYSTEMS OF PAPER MACHINES 1. Introduction Gerald Bachmann, Allplan GmbH In paper mills steam is used for drying wet paper. Therefore extremely complex steam and condensate systems are utilized. These systems consist of the following main parts with the following functions: 1. Different heating groups with heating cylinders for heating and drying the paper. 2. Separators at the end of every heating group are installed for the separation of vapour and condensate of each heating group. 3. Vapour heat exchanger in the hood air heating system are installed for the condensation of the vapour with the lowest pressure at the last separator. 4. Auxiliary condensers for total condensation of none condensated vapour. Figure 1: Steam and condensate system of a paper mill Each different heating group requires a different necessary pressure for optimal drying of the paper. Due to the fact that it is always tried to increase the velocity of the paper machine there are always changes necessary in the steam and condensate system (putting an heating cylinder from one heating group to an other, increase or decrease the pressure in different heating groups, ). Due to these changes it is difficult to find the optimum mode of operation. There are two main possibilities to identify better operating conditions for the steam and condensate system of paper machines. These two possibilities are: 1. Simulation 2. Experiments without influences on the production With a simulation model the paper machine could be analysed and optimized. The result meets the reality quite closely. With special experiments during normal production an optimization potential can be given. 3

4 2. Simulation Creating good quality simulation models is quite difficult because of the complexity of the steam and condensate systems. The first step is creating the model and the second to feed the model with data (first existing parameters and then choosing the last unknown parameters). 2.1 Creating the Simulation Model There are different programs like Aspen or Ipse which could be used to create the model. In the model creating process all relevant equipment and processes have to be built up in the model. The following picture gives the main part of a certain simulation model of an existing paper mill, which was optimized using the simulation. Figure 2: Main part of the steam and condensate system model of a paper mill in Austria The model in the figure above consists of steam input (STI 1), separator 2 (S 2), cylinder heating section 1 (CHS), separator 1 (S 1), condensate output 1 (CO 1), hood air heating section (HAHS), steam input 2 (STI 2), condenser (C), cooling water input (CWI) and cooling water output (CWO). 2.2 Feeding the Model with Data After creating the simulation model the data must be entered. Therefore the values mass flow in kg/s pressure in bar and temperature in C have to be entered. The last value enthalpie in kj/kg could be calculated by the simulation model program. 4

5 Some data is easy to fill in because these parameters are well known. In this certain simulation model this is all data of the cooling water input and output, the condensate output 2 and the hood air heating section. The unknown data has to be assumed. After assuming the unknown data the model has to be checked by experiments. With these experiments different production situation could be simulated and the results of the simulations could be compared to the reality. In the following two figures parameters of different production situation are given. paper output [t/h] coolant temperature [ C] coolant flow [l/s] water temperature [ C] HS 3 [bar] Figure 3: Parameter 1 steamconsumption [t/h] HS 5 [bar] HS 4 [bar] HS 3 [bar] HS 6 [bar] HS 2 [bar] HS 1 [bar] Figure 4: Parameter 2 After entering in these parameters into the simulation model, a technical conflict should not occur. If there is a conflict, the model has to be corrected. The following figure shows the correct simulation model. 5

6 Figure 5: Correct simulation model 2.3 Simulation Calculation After crating a correct simulation model, different scenarios could be calculated. For example the partly separating of condensate after separator 2 which results in a dramatic reduction of flow of cooling water. The following figure shows, that partly separating 20.45kg/s of condensate after separator 2 the cooling water flow will be reduced from to kg/s (marked red). Figure 6: Simulation of taking off condensate 6

7 Due to the fact, that the energy is not heating the cooling water it is possible to use this energy at a level of 133 C. The energy saving is about 3MW and with 8000 operating hours per year the savings are 24GWh/year and more than /year. 3. Experiments without influences on the production Beside the simulation experiments without an influence on the production could be a possibility to reduce energy demand. Therefore with a high level of knowledge of steam and condensate systems different experiments could be selected and should be carried out very carefully. The most important aspect in carrying out these experiments is that the production is operating during the experiment and the quality of the product and the capacity of the process are not decreasing. While carrying out these experiments it is very useful, if a process control system is installed where all relevant parameters are shown. These relevant parameters have to be observed during the entire experiments. Changing defined values such as the position of a valve results in changing the conditions and could lead to energy savings. For example the simulation case above that could mean the following: without any simulation it is assumed, that the energy of the condensate from separator 2 is not necessary for the vapour production in separator 1 therefore an outlet valve at the bottom of separator 2 is slowly opened and a valve at the connecting tube between separator 2 and 1 is slowly closed The result of this experiment is the same as in the simulation, the cooling water flow decreases. 4. Summary There are two main possibilities to identify better operating conditions for the steam and condensate system of paper machines. These two possibilities are Simulation Experiments without influences on the production An example is given for these two possibilities. The optimization of the steam and condensate system in this case leads to 3MW savings which means 24GWh/year respectively more than Euros per year. Allplan carried out such energy optimization several times in Austria as well as in other countries like Switzerland, Germany, Netherlands or Slovenia and found savings each time from several 100kW up to several MW. 7