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The potential for energy savings when reducing the water consumption in a Kraft Pulp Mill
Applied Thermal Engineering 25 (2005) 1057–1066 www.elsevier.com/locate/apthermeng
The potential for energy savings when reducing the water consumption in a Kraft Pulp Mill Ulrika Wising a
a,b,*
, Thore Berntsson a, Paul Stuart
b
Department of Chemical Engineering and Environmental Science, Chalmers University of Technology, Go¨teborg, Sweden Department of Chemical Engineering, NSERC Chairholder in Process Integration for the Pulp and Paper Industry, E´cole Polytechnique, Montreal, Canada
b
Received 15 March 2004; accepted 20 July 2004 Available online 7 October 2004
Abstract An existing pulp and paper mill has been studied regarding the reduction of water consumption, and the resulting increased potential for energy integration. When the millÕs hot water consumption is decreased, the live steam demand for the mill also decreases. Also when decreasing the hot water consumption, the quantity and temperature of available excess heat increases. This excess heat can be used for evaporation, thereby reducing the live steam demand further by up to 1.5 GJ/t. A pinch analysis was performed and it was found that when removing pinch violations the hot water consumption is not an important factor any more. Removing all the pinch violations and using the remaining excess heat for evaporation yields a significantly larger energy savings for the mill (4.0 GJ/t). From an economic optimum perspective it is probably most profitable to do a combination of reducing water consumption, removing pinch violations, and use the remaining excess heat for evaporation. Ó 2004 Elsevier Ltd. All rights reserved.
*
Corresponding author. Address: Department of Chemical Engineering, NSERC Chairholder in Process Integration ´ cole Polytechnique, Montreal, Canada. Tel.: +1 514 340 4711; fax: +1 514 340 5150. for the Pulp and Paper Industry, E E-mail address: [email protected] (U. Wising). 1359-4311/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.07.023
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1. Introduction There is new motivation for implementing cost-efficient energy savings in the pulp and paper industry. One example is the adoption of new wastewater restrictions that increase the need for system closure and other system modifications, which can lead to decreased water consumption [1]. Another example relates to concerns regarding Kyoto Protocol requirements that might lead to increased CO2 taxes, CO2 trading and/or long-term agreements between government and industry concerning energy investments. Earlier work has shown that there is a potential for energy savings even in energy efficient, low water consuming mills [2]. Here the changes in the potential for energy savings when decreasing fresh water intake are discussed and visualized through a case study. While the importance of water consumption with regard to potential energy savings has been a topic of interest in the pulp and paper industry for years, there is no study in the literature that approaches the issue in a systematic fashion such as presented in this paper.
2. Process integration and excess heat Hot streams used today for the production of a surplus of warm and hot water can be made available if changes to the warm and hot water production system are made [6]. The hot streams that can be made available are referred to as excess heat. In particular, excess heat can be used in the evaporation plant to minimize live steam use if the evaporation plant is redesigned [7]. Thermal pinch analysis has been used in order to identify the amount and temperature of the excess heat that can be made available in a mill. Even when there are no pinch violations left in the mill, however, there is still a potential for energy savings from using excess heat. Several energyefficient model mills with minimal pinch violations have been evaluated in the literature, and all of them show that there is excess heat available above 80 °C of more than 0.5 GJ/air-dried metric tonne (GJ/t) [4–6]. It is not always the best option to first reduce the pinch violations and then use the remaining excess heat to reduce energy consumption. In many cases, reducing pinch violations may have practical constraints, and/or may not be economically attractive. It could be optimal from a cost reduction perspective to remove some pinch violations and using the remaining hot streams as excess heat. The new advanced pinch curves used in this paper give a better picture of the hot streams available in the mill compared to classical composite and grand composite curves [3,4].
3. The Eco-Cyclic Research Program The work presented here is a continuation of the work done within the Swedish National Research Program ‘‘The Eco-Cyclic Pulp Mill’’ [8]. In this research program, a reference model mill was developed, the ‘‘Reference Model Mill 2000’’. This is a state-of-the-art market kraft pulp mill, consisting of equipment built and run in the Scandinavian pulp and paper industry. It has low energy consumption (10.4 GJ/t) and low process water consumption (15.9 m3/t). Of that total
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water consumption, 7.8 m3/t is used as warm and hot water in the processes and the rest is cold water. Of the 7.8 m3/t of warm and hot water, 4.4 m3/t is used in the dryer.
4. The studied mill The mill where the case study has been performed is a pulp and paper mill in Quebec, Canada. This is a modern pulp and paper mill built in 1987 which produces 570,000 tonnes of fine paper per year. The processes in the mill are similar to the processes in the Reference Model Mill 2000. Only the pulp mill has been included for this study, in order to compare it to the Reference Model Mill 2000. There is some warm water produced in the pulp mill that is used in the paper producing part of the mill. In order to avoid sub-optimizing the mill, the warm water production for the paper producing part of the mill has been assumed constant. This situation also applies to the Reference Model Mill 2000 in the Eco-Cyclic Pulp Mill Research Program, where some warm and hot water is produced in the pulp mill for the pulp dryer.
5. Water use reduction The difference between cases 1 and 4, cases 2 and 5 and cases 3 and 6 respectively, is the way the water production has been reduced. In cases 4, 5 and 6 the fresh water has been replaced by condensate. In cases 1, 2 and 3 the water usage has been reduced, not just replaced. The approach to reducing the water consumption or increasing condensate reuse has been stepby-step. In Table 1, the extreme cases are shown. First there is the original warm and hot water production, referred to as the original mill. Then we have two cases (cases 1 and 4) where the warm water production is zero, just like in the Reference Model Mill 2000, but the hot water production is kept as it is originally in the mill. The hot water production is then reduced to the Reference Model Mill 2000 level and the warm water production is kept the same as originally in the mill (cases 2 and 5). Finally, both the warm and hot water production are reduced to the Reference Model Mill 2000 levels (cases 3 and 6).
Table 1 Water consumption for the different cases Original mill Reduced water consumption Case 1 Case 2 Case 3 Reused condensate Case 4 Case 5 Case 6 The Reference Model Mill
Warm water production, m3/t
Hot water production, m3/t
10.9
16.8
0 10.9 0
16.8 3.4 3.4
0 10.9 0 0
16.8 4.1 4.1 3.4
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The amount of warm water produced for the paper producing part of the mill is 10.1 m3/t. The corresponding amount for the Reference Model Mill 2000 is 1.9 m3/t of warm water and 2.5 m3/t of hot water. These values are not included in Table 1. When reducing the water consumption, the aforementioned measures are usually combined. Some equipment might be replaced with new, more water efficient equipment while at the same time some of the fresh water is replaced by condensate. Here, only one measure is taken at a time and no combinations are performed. The equipment changes required to achieve the water reduction have not been considered in this study, nor potential consequences due to water use reduction such as impacts on pulp quality or process operability.
6. Pinch analysis methodology Thermal pinch analysis has been used as the process integration tool in this study [9]. First, grand composite curves (GCCÕs) have been constructed for all the cases using a global temperature difference of 32 K, corresponding to the millsÕ actual live steam demand. In the GCCÕs with a 32 K global temperature difference the excess heat available without redesigning the heat exchanger network (HEN) can be seen in Fig. 1. With a global temperature difference of 32 K, the temperature difference throughout each heat exchanger is fairly large but varies between individual heat exchangers. When redesigning the HEN and installing new heat exchangers, a temperature difference of 10 K is most likely to be chosen for the new heat exchangers. If one keeps the same heat demand as for the case with a global temperature difference of 32 K while implying that no pinch violations are removed, but assumes that the temperature difference in the crucial heat exchangers around the pinch temperature is 10 K, then one obtains the true amount of excess heat available. The so-called hybrid curve (Fig. 2) is constructed by taking the GCC with a global temperature difference of 10 K and then 250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
Q (GJ/t)
Fig. 1. GCC for the original mill with a global temperature difference of 32 K.
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250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
11
Q (GJ/t)
Fig. 2. Hybrid curve for the original mill, showing the total amount of excess heat available.
adding the difference between that GCC and the GCC with a global temperature difference of 32 K as a straight line at the steam temperature. This is due to the fact that if pinch violations are removed, steam is made available. In this case the total amount of excess heat available is 3.5 GJ/t, and this excess heat can both be used to remove pinch violations as well as for other processes. If only the original GCC was used, the conclusion would be that there is no excess heat available for use (Fig. 1). Hence, the GCC cannot visualize the total amount of excess heat available. The hybrid curve is an example of the new advanced pinch curves [3,4].
7. Novel evaporator design The excess heat available can be used in the evaporation plant. The principles for excess heat supplied evaporation are shown in Fig. 3. Heat is introduced to the evaporation plant at two or more temperature levels and the temperature in the surface condenser is reduced. Earlier work has shown that using excess heat for evaporation is an effective way of reducing live steam demand in the mill [6,7]. On average, the saved live steam corresponds to the excess heat available (within certain limits), in GJ/t, plus 0.5 GJ/t [10]. In this work the saved live steam will be based on this average value, rather than being based on the actual design of an evaporation plant. The reason why the excess heat is worth more than live steam is that the temperature in the surface condenser is lowered and one or more effects can be placed below the original evaporation plant. In many mills all or part of the heat from the surface condenser is needed for the production of warm and hot water. Then, none or only part of that steam could be used for evaporation. Placing one or more effects below the original evaporation plant could be an option in a retrofit situation. This would lead to some steam savings at a lower investment cost compared to if the entire evaporation plant was redesigned, but with lower steam savings.
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MP steam to digester and bleaching
Temperature, °C 200 LP –steam to evaporation and drying
150
Live steam saved
100
50 Surplus heat to cooling system and atmosphere
0 0
5
10
15 Heat flow, GJ/t
Fig. 3. Evaporation design that uses excess heat.
8. System consequences The warm and hot water production system needs to be redesigned to make the excess heat available for evaporation. It can be reduced in size since less warm and hot water is produced. In order to use the excess heat in the evaporation plant, it needs to be in the form of steam; therefore a steam reformer is most likely necessary as well as heat exchangers. When saving live steam, the need for cooling is reduced in the plant because less heat is transferred through the system. Also, the temperature of the heat sources needing cooling is reduced, which can lower the size of cooling towers.
9. Results and discussion Theoretically, if the outgoing water streams in the mill were exactly the same flow as the incoming water, the incoming water could be heated to the temperature of the effluent minus the temperature difference in the heat exchanger. The rest of the heating to the target temperature must be satisfied with steam. Therefore, if the global temperature difference is 32 K, more steam is needed to satisfy the heating of the water to the target temperature compared to if the global temperature difference is 10 K. In this work, the outgoing water streams are not of the same flow as the incoming water streams, but the overall principle is the same. Therefore, when the amount of water being heated is reduced, the steam needed to bring the water temperature up to the target temperature will be reduced as well (when the global temperature difference is kept constant). Thus, when the global temperature difference is large and the amount of water being heated is reduced, the live
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steam demand is reduced to a greater extent compared to when the global temperature difference is smaller. When comparing the original mill with case 3 for example, the live steam demand is reduced by 1.3 GJ/t, which is a 12% reduction of the total steam demand due to water reduction (Table 2). When the global temperature difference is 10 K, i.e. there are no pinch violations; the live steam demand is not dependent on the warm and hot water production. The reduction of hot water production influences the amount and temperature of the excess heat more than the reduction of warm water production. In cases 1 and 4 the warm water is reduced to the same level as the Reference Model Mill 2000, but the hot water is kept the same as in the original mill. In these cases the live steam demand decreases only slightly, and there is no excess heat available above 80 °C. If the hot water is reduced to the Reference Model Mill 2000 level and the warm water is kept the same as in the original mill (cases 2 and 5), then the live steam demand is decreased considerably and there is between 0.5 and 0.6 GJ/t of excess heat available. When comparing cases 2 and 3 or cases 5 and 6 this becomes more evident. The reason why the hot water production has a larger influence on the pinch temperature as well as the amount of excess heat available is the same as discussed earlier. Heating warm water can be performed to a larger extent by internal heat sources at a fairly low temperature; no steam is needed since the heat sources that (today) need to be cooled in a cooling tower can be used. The hot water heating is on the other hand to a larger extent satisfied by steam, and therefore the steam demand is decreased when the hot water production is decreased. When the pinch violations have been removed (global temperature difference of 10 K), the amount of water produced does not affect the heat consumption. All of the cases, including the original mill, have a live steam demand of 8.0 GJ/t, pinch temperature of 95 °C and excess heat available that varies marginally around 0.8 GJ/t after the pinch violations have been removed. The specific amount of excess heat for the cases with a global temperature difference of 10 K can be seen in Table 2 in the last column titled ‘‘Excess heat with no pinch violations’’. When reducing the water production, the pinch temperature is increased and the temperature of the excess heat below the pinch temperature is increased as well. Comparing Fig. 1 with Figs. 4 or 5, it is clear that the pinch temperature has increased. When less heat is needed to heat hot water,
Table 2 Results of the pinch analysis for the different cases
Original Mill Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Reference Model Mill 2000 a
Live steam demand, GJ/t
Global temp. difference, K
Pinch temp., °C
Pinch violations, GJ/ta
Excess heat above 80 °C, GJ/t
Excess heat with no pinch violations, GJ/t
10.8 10.5 9.5 9.5 10.4 9.5 9.5 7.5
32 32 32 32 32 32 32 10
50 50 86 86 50 86 86 122
2.8 2.5 1.5 1.5 2.5 1.5 1.5 0
0 0 0.7 0.8 0 0.6 1.0 0.5
0.7 0.7 0.8 0.8 0.7 0.8 0.8 0.5
With the defined stream data used in this study.
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those hot streams used for that purpose can be made available for other uses. As can be seen in Table 2 for cases 2, 3, 5 or 6, the excess heat above 80 °C is increased from zero to between 0.6 and 1.0 GJ/t when the hot water production is decreased, without removing any pinch violations. This excess heat can be seen for cases 3 and 6 in Figs. 4 and 5.
250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
11
12
9
10
11
12
Q (GJ/t) Fig. 4. Grand composite curve for case 3.
250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
Q (GJ/t) Fig. 5. Grand composite curve for case 6.
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Table 3 Saved live steam for different approaches
Original Mill Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Reference Model Mill 2000
Removing pinch violations, GJ/t
Removing pinch violations and using remaining excess heat, GJ/t
Using excess heat, GJ/t
Using all heat sources as excess heat, GJ/ta
2.8 2.5 1.5 1.5 2.5 1.5 1.5 0
4.0 3.7 2.8 2.8 3.7 2.8 2.8 1.0
0 0 1.2 1.3 0 1.1 1.5 1.0
3.3 3.0 2.7 2.8 3.0 2.6 3.0 1.0
a
Using all heat sources including the heat sources intended to remove pinch violations. If the total amount of excess heat is large, using it all for evaporation might not be possible.
The live steam savings that can be achieved by using the total amount of excess heat available, either for evaporation and/or to remove pinch violations, can be seen in Table 3. When using the available excess heat for evaporation without removing pinch violations, the mill can save up to 1.5 GJ/t as for case 6. If some or all of the pinch violations are removed, up to 2.8 GJ/t of live steam can be saved (Original Mill). If all of the pinch violations are removed while using the remainder excess heat for evaporation, then up to 4.0 GJ/t can be saved for the Original Mill. If instead the global temperature difference is reduced to 10 K, using all the heat sources made available for evaporation and retaining all of the pinch violations, the maximum live steam reduction for the studied cases is 3.3 GJ/t.
10. Conclusions When reducing the warm and hot water produced in a mill, the pinch temperature is increased and there is excess heat available that can be used for other purposes. The live steam demand is also reduced when reducing the warm and hot water production. Reducing the hot water need is more important than reducing the warm water need in order for the pinch temperature to increase. For this case study, the live steam demand at the mill can be reduced by up to 4.0 GJ/t if pinch violations are removed and the remaining excess heat is used for evaporation. The total amount of excess heat available, including the excess heat that can be used to remove pinch violations, can be used for evaporation and considerably lower the live steam demand for that process. Usually a combination of both removing pinch violations and using excess heat for evaporation is the most optimal solution from a cost perspective. If the mill approaches a situation where the energy system in the mill is very effective, i.e. there are few pinch violations, then the amount of warm and hot water produced is of less importance compared to when the energy system is not as effective. If all the pinch violations are removed, there is still excess heat available that can be used for evaporation.
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Acknowledgments We would like to thank the personnel at the studied mill for their invaluable help. Financial support from the Swedish National Energy Administration and the mill is gratefully acknowledged.
References [1] L. Ledung et al., Overall energy balances for bleached kraft pulp mills with a high degree of system closure, in: 6th International conference on new available technologies, SPCI, 1999. [2] U. Wising, The potential for generating a surplus of biomass fuel in the pulp and paper industry, Licentiate Thesis, Department of Heat and Power Technology, Chalmers University of Technology, Sweden, 2001. [3] C. Bengtsson, et al., Co-ordination of pinch technology and the MIND method—applied to a Swedish board mill, Applied Thermal Engineering (2001) 133–144. [4] C. Bengtsson, R. Nordman, T. Berntsson, Utilization of excess heat in the pulp and paper industry—a case study of technical and economical opportunities, Applied Thermal Engineering 22 (9) (2002) 1069–1081. [5] U. Wising, T. Berntsson, A. Asblad, Usable excess heat in future kraft pulp mills, TAPPI Journal (November) (2002) 27–29. [6] U. Wising, T. Berntsson, A. Asblad, Energy consequences in minimum effluent market kraft pulp mills, TAPPI Journal (Nov.) (2002) 30–32. [7] J. Algehed, Energy efficient evaporation in future kraft pulp mills, PhD-Thesis, Department of Heat and power technology, Chalmers University of Technology, Sweden, 2002. [8] KAM, Eco-Cyclic Pulp Mill. Final report, 1996–2002, Report A100, STFI, Stockholm, 2003. [9] B. Linnhoff, D.W. Townsend, D. Boland, G.F. Hewitt, B.E.A. Thomas, A.R. Guy, Marshland R.H.A User Guide on Process Integration for the Efficient Use of Energy. Rugby: Institution of Chemical Engineers, 1982, last edition 1994. [10] J. Algehed., U. Wising, T. Berntsson, Energy efficient evaporation in future pulp and paper mills, in: 7th International conference on new available technologies, Stockholm, Sweden, 2002, pp. 112–116.
The potential for energy savings when reducing the water consumption in a Kraft Pulp Mill Ulrika Wising a
a,b,*
, Thore Berntsson a, Paul Stuart
b
Department of Chemical Engineering and Environmental Science, Chalmers University of Technology, Go¨teborg, Sweden Department of Chemical Engineering, NSERC Chairholder in Process Integration for the Pulp and Paper Industry, E´cole Polytechnique, Montreal, Canada
b
Received 15 March 2004; accepted 20 July 2004 Available online 7 October 2004
Abstract An existing pulp and paper mill has been studied regarding the reduction of water consumption, and the resulting increased potential for energy integration. When the millÕs hot water consumption is decreased, the live steam demand for the mill also decreases. Also when decreasing the hot water consumption, the quantity and temperature of available excess heat increases. This excess heat can be used for evaporation, thereby reducing the live steam demand further by up to 1.5 GJ/t. A pinch analysis was performed and it was found that when removing pinch violations the hot water consumption is not an important factor any more. Removing all the pinch violations and using the remaining excess heat for evaporation yields a significantly larger energy savings for the mill (4.0 GJ/t). From an economic optimum perspective it is probably most profitable to do a combination of reducing water consumption, removing pinch violations, and use the remaining excess heat for evaporation. Ó 2004 Elsevier Ltd. All rights reserved.
*
Corresponding author. Address: Department of Chemical Engineering, NSERC Chairholder in Process Integration ´ cole Polytechnique, Montreal, Canada. Tel.: +1 514 340 4711; fax: +1 514 340 5150. for the Pulp and Paper Industry, E E-mail address: [email protected] (U. Wising). 1359-4311/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.07.023
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1. Introduction There is new motivation for implementing cost-efficient energy savings in the pulp and paper industry. One example is the adoption of new wastewater restrictions that increase the need for system closure and other system modifications, which can lead to decreased water consumption [1]. Another example relates to concerns regarding Kyoto Protocol requirements that might lead to increased CO2 taxes, CO2 trading and/or long-term agreements between government and industry concerning energy investments. Earlier work has shown that there is a potential for energy savings even in energy efficient, low water consuming mills [2]. Here the changes in the potential for energy savings when decreasing fresh water intake are discussed and visualized through a case study. While the importance of water consumption with regard to potential energy savings has been a topic of interest in the pulp and paper industry for years, there is no study in the literature that approaches the issue in a systematic fashion such as presented in this paper.
2. Process integration and excess heat Hot streams used today for the production of a surplus of warm and hot water can be made available if changes to the warm and hot water production system are made [6]. The hot streams that can be made available are referred to as excess heat. In particular, excess heat can be used in the evaporation plant to minimize live steam use if the evaporation plant is redesigned [7]. Thermal pinch analysis has been used in order to identify the amount and temperature of the excess heat that can be made available in a mill. Even when there are no pinch violations left in the mill, however, there is still a potential for energy savings from using excess heat. Several energyefficient model mills with minimal pinch violations have been evaluated in the literature, and all of them show that there is excess heat available above 80 °C of more than 0.5 GJ/air-dried metric tonne (GJ/t) [4–6]. It is not always the best option to first reduce the pinch violations and then use the remaining excess heat to reduce energy consumption. In many cases, reducing pinch violations may have practical constraints, and/or may not be economically attractive. It could be optimal from a cost reduction perspective to remove some pinch violations and using the remaining hot streams as excess heat. The new advanced pinch curves used in this paper give a better picture of the hot streams available in the mill compared to classical composite and grand composite curves [3,4].
3. The Eco-Cyclic Research Program The work presented here is a continuation of the work done within the Swedish National Research Program ‘‘The Eco-Cyclic Pulp Mill’’ [8]. In this research program, a reference model mill was developed, the ‘‘Reference Model Mill 2000’’. This is a state-of-the-art market kraft pulp mill, consisting of equipment built and run in the Scandinavian pulp and paper industry. It has low energy consumption (10.4 GJ/t) and low process water consumption (15.9 m3/t). Of that total
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water consumption, 7.8 m3/t is used as warm and hot water in the processes and the rest is cold water. Of the 7.8 m3/t of warm and hot water, 4.4 m3/t is used in the dryer.
4. The studied mill The mill where the case study has been performed is a pulp and paper mill in Quebec, Canada. This is a modern pulp and paper mill built in 1987 which produces 570,000 tonnes of fine paper per year. The processes in the mill are similar to the processes in the Reference Model Mill 2000. Only the pulp mill has been included for this study, in order to compare it to the Reference Model Mill 2000. There is some warm water produced in the pulp mill that is used in the paper producing part of the mill. In order to avoid sub-optimizing the mill, the warm water production for the paper producing part of the mill has been assumed constant. This situation also applies to the Reference Model Mill 2000 in the Eco-Cyclic Pulp Mill Research Program, where some warm and hot water is produced in the pulp mill for the pulp dryer.
5. Water use reduction The difference between cases 1 and 4, cases 2 and 5 and cases 3 and 6 respectively, is the way the water production has been reduced. In cases 4, 5 and 6 the fresh water has been replaced by condensate. In cases 1, 2 and 3 the water usage has been reduced, not just replaced. The approach to reducing the water consumption or increasing condensate reuse has been stepby-step. In Table 1, the extreme cases are shown. First there is the original warm and hot water production, referred to as the original mill. Then we have two cases (cases 1 and 4) where the warm water production is zero, just like in the Reference Model Mill 2000, but the hot water production is kept as it is originally in the mill. The hot water production is then reduced to the Reference Model Mill 2000 level and the warm water production is kept the same as originally in the mill (cases 2 and 5). Finally, both the warm and hot water production are reduced to the Reference Model Mill 2000 levels (cases 3 and 6).
Table 1 Water consumption for the different cases Original mill Reduced water consumption Case 1 Case 2 Case 3 Reused condensate Case 4 Case 5 Case 6 The Reference Model Mill
Warm water production, m3/t
Hot water production, m3/t
10.9
16.8
0 10.9 0
16.8 3.4 3.4
0 10.9 0 0
16.8 4.1 4.1 3.4
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The amount of warm water produced for the paper producing part of the mill is 10.1 m3/t. The corresponding amount for the Reference Model Mill 2000 is 1.9 m3/t of warm water and 2.5 m3/t of hot water. These values are not included in Table 1. When reducing the water consumption, the aforementioned measures are usually combined. Some equipment might be replaced with new, more water efficient equipment while at the same time some of the fresh water is replaced by condensate. Here, only one measure is taken at a time and no combinations are performed. The equipment changes required to achieve the water reduction have not been considered in this study, nor potential consequences due to water use reduction such as impacts on pulp quality or process operability.
6. Pinch analysis methodology Thermal pinch analysis has been used as the process integration tool in this study [9]. First, grand composite curves (GCCÕs) have been constructed for all the cases using a global temperature difference of 32 K, corresponding to the millsÕ actual live steam demand. In the GCCÕs with a 32 K global temperature difference the excess heat available without redesigning the heat exchanger network (HEN) can be seen in Fig. 1. With a global temperature difference of 32 K, the temperature difference throughout each heat exchanger is fairly large but varies between individual heat exchangers. When redesigning the HEN and installing new heat exchangers, a temperature difference of 10 K is most likely to be chosen for the new heat exchangers. If one keeps the same heat demand as for the case with a global temperature difference of 32 K while implying that no pinch violations are removed, but assumes that the temperature difference in the crucial heat exchangers around the pinch temperature is 10 K, then one obtains the true amount of excess heat available. The so-called hybrid curve (Fig. 2) is constructed by taking the GCC with a global temperature difference of 10 K and then 250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
Q (GJ/t)
Fig. 1. GCC for the original mill with a global temperature difference of 32 K.
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250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
11
Q (GJ/t)
Fig. 2. Hybrid curve for the original mill, showing the total amount of excess heat available.
adding the difference between that GCC and the GCC with a global temperature difference of 32 K as a straight line at the steam temperature. This is due to the fact that if pinch violations are removed, steam is made available. In this case the total amount of excess heat available is 3.5 GJ/t, and this excess heat can both be used to remove pinch violations as well as for other processes. If only the original GCC was used, the conclusion would be that there is no excess heat available for use (Fig. 1). Hence, the GCC cannot visualize the total amount of excess heat available. The hybrid curve is an example of the new advanced pinch curves [3,4].
7. Novel evaporator design The excess heat available can be used in the evaporation plant. The principles for excess heat supplied evaporation are shown in Fig. 3. Heat is introduced to the evaporation plant at two or more temperature levels and the temperature in the surface condenser is reduced. Earlier work has shown that using excess heat for evaporation is an effective way of reducing live steam demand in the mill [6,7]. On average, the saved live steam corresponds to the excess heat available (within certain limits), in GJ/t, plus 0.5 GJ/t [10]. In this work the saved live steam will be based on this average value, rather than being based on the actual design of an evaporation plant. The reason why the excess heat is worth more than live steam is that the temperature in the surface condenser is lowered and one or more effects can be placed below the original evaporation plant. In many mills all or part of the heat from the surface condenser is needed for the production of warm and hot water. Then, none or only part of that steam could be used for evaporation. Placing one or more effects below the original evaporation plant could be an option in a retrofit situation. This would lead to some steam savings at a lower investment cost compared to if the entire evaporation plant was redesigned, but with lower steam savings.
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MP steam to digester and bleaching
Temperature, °C 200 LP –steam to evaporation and drying
150
Live steam saved
100
50 Surplus heat to cooling system and atmosphere
0 0
5
10
15 Heat flow, GJ/t
Fig. 3. Evaporation design that uses excess heat.
8. System consequences The warm and hot water production system needs to be redesigned to make the excess heat available for evaporation. It can be reduced in size since less warm and hot water is produced. In order to use the excess heat in the evaporation plant, it needs to be in the form of steam; therefore a steam reformer is most likely necessary as well as heat exchangers. When saving live steam, the need for cooling is reduced in the plant because less heat is transferred through the system. Also, the temperature of the heat sources needing cooling is reduced, which can lower the size of cooling towers.
9. Results and discussion Theoretically, if the outgoing water streams in the mill were exactly the same flow as the incoming water, the incoming water could be heated to the temperature of the effluent minus the temperature difference in the heat exchanger. The rest of the heating to the target temperature must be satisfied with steam. Therefore, if the global temperature difference is 32 K, more steam is needed to satisfy the heating of the water to the target temperature compared to if the global temperature difference is 10 K. In this work, the outgoing water streams are not of the same flow as the incoming water streams, but the overall principle is the same. Therefore, when the amount of water being heated is reduced, the steam needed to bring the water temperature up to the target temperature will be reduced as well (when the global temperature difference is kept constant). Thus, when the global temperature difference is large and the amount of water being heated is reduced, the live
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steam demand is reduced to a greater extent compared to when the global temperature difference is smaller. When comparing the original mill with case 3 for example, the live steam demand is reduced by 1.3 GJ/t, which is a 12% reduction of the total steam demand due to water reduction (Table 2). When the global temperature difference is 10 K, i.e. there are no pinch violations; the live steam demand is not dependent on the warm and hot water production. The reduction of hot water production influences the amount and temperature of the excess heat more than the reduction of warm water production. In cases 1 and 4 the warm water is reduced to the same level as the Reference Model Mill 2000, but the hot water is kept the same as in the original mill. In these cases the live steam demand decreases only slightly, and there is no excess heat available above 80 °C. If the hot water is reduced to the Reference Model Mill 2000 level and the warm water is kept the same as in the original mill (cases 2 and 5), then the live steam demand is decreased considerably and there is between 0.5 and 0.6 GJ/t of excess heat available. When comparing cases 2 and 3 or cases 5 and 6 this becomes more evident. The reason why the hot water production has a larger influence on the pinch temperature as well as the amount of excess heat available is the same as discussed earlier. Heating warm water can be performed to a larger extent by internal heat sources at a fairly low temperature; no steam is needed since the heat sources that (today) need to be cooled in a cooling tower can be used. The hot water heating is on the other hand to a larger extent satisfied by steam, and therefore the steam demand is decreased when the hot water production is decreased. When the pinch violations have been removed (global temperature difference of 10 K), the amount of water produced does not affect the heat consumption. All of the cases, including the original mill, have a live steam demand of 8.0 GJ/t, pinch temperature of 95 °C and excess heat available that varies marginally around 0.8 GJ/t after the pinch violations have been removed. The specific amount of excess heat for the cases with a global temperature difference of 10 K can be seen in Table 2 in the last column titled ‘‘Excess heat with no pinch violations’’. When reducing the water production, the pinch temperature is increased and the temperature of the excess heat below the pinch temperature is increased as well. Comparing Fig. 1 with Figs. 4 or 5, it is clear that the pinch temperature has increased. When less heat is needed to heat hot water,
Table 2 Results of the pinch analysis for the different cases
Original Mill Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Reference Model Mill 2000 a
Live steam demand, GJ/t
Global temp. difference, K
Pinch temp., °C
Pinch violations, GJ/ta
Excess heat above 80 °C, GJ/t
Excess heat with no pinch violations, GJ/t
10.8 10.5 9.5 9.5 10.4 9.5 9.5 7.5
32 32 32 32 32 32 32 10
50 50 86 86 50 86 86 122
2.8 2.5 1.5 1.5 2.5 1.5 1.5 0
0 0 0.7 0.8 0 0.6 1.0 0.5
0.7 0.7 0.8 0.8 0.7 0.8 0.8 0.5
With the defined stream data used in this study.
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those hot streams used for that purpose can be made available for other uses. As can be seen in Table 2 for cases 2, 3, 5 or 6, the excess heat above 80 °C is increased from zero to between 0.6 and 1.0 GJ/t when the hot water production is decreased, without removing any pinch violations. This excess heat can be seen for cases 3 and 6 in Figs. 4 and 5.
250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10
11
12
9
10
11
12
Q (GJ/t) Fig. 4. Grand composite curve for case 3.
250
200
T (°C)
150
100
50
0 0
1
2
3
4
5
6
7
8
Q (GJ/t) Fig. 5. Grand composite curve for case 6.
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Table 3 Saved live steam for different approaches
Original Mill Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Reference Model Mill 2000
Removing pinch violations, GJ/t
Removing pinch violations and using remaining excess heat, GJ/t
Using excess heat, GJ/t
Using all heat sources as excess heat, GJ/ta
2.8 2.5 1.5 1.5 2.5 1.5 1.5 0
4.0 3.7 2.8 2.8 3.7 2.8 2.8 1.0
0 0 1.2 1.3 0 1.1 1.5 1.0
3.3 3.0 2.7 2.8 3.0 2.6 3.0 1.0
a
Using all heat sources including the heat sources intended to remove pinch violations. If the total amount of excess heat is large, using it all for evaporation might not be possible.
The live steam savings that can be achieved by using the total amount of excess heat available, either for evaporation and/or to remove pinch violations, can be seen in Table 3. When using the available excess heat for evaporation without removing pinch violations, the mill can save up to 1.5 GJ/t as for case 6. If some or all of the pinch violations are removed, up to 2.8 GJ/t of live steam can be saved (Original Mill). If all of the pinch violations are removed while using the remainder excess heat for evaporation, then up to 4.0 GJ/t can be saved for the Original Mill. If instead the global temperature difference is reduced to 10 K, using all the heat sources made available for evaporation and retaining all of the pinch violations, the maximum live steam reduction for the studied cases is 3.3 GJ/t.
10. Conclusions When reducing the warm and hot water produced in a mill, the pinch temperature is increased and there is excess heat available that can be used for other purposes. The live steam demand is also reduced when reducing the warm and hot water production. Reducing the hot water need is more important than reducing the warm water need in order for the pinch temperature to increase. For this case study, the live steam demand at the mill can be reduced by up to 4.0 GJ/t if pinch violations are removed and the remaining excess heat is used for evaporation. The total amount of excess heat available, including the excess heat that can be used to remove pinch violations, can be used for evaporation and considerably lower the live steam demand for that process. Usually a combination of both removing pinch violations and using excess heat for evaporation is the most optimal solution from a cost perspective. If the mill approaches a situation where the energy system in the mill is very effective, i.e. there are few pinch violations, then the amount of warm and hot water produced is of less importance compared to when the energy system is not as effective. If all the pinch violations are removed, there is still excess heat available that can be used for evaporation.
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Acknowledgments We would like to thank the personnel at the studied mill for their invaluable help. Financial support from the Swedish National Energy Administration and the mill is gratefully acknowledged.
References [1] L. Ledung et al., Overall energy balances for bleached kraft pulp mills with a high degree of system closure, in: 6th International conference on new available technologies, SPCI, 1999. [2] U. Wising, The potential for generating a surplus of biomass fuel in the pulp and paper industry, Licentiate Thesis, Department of Heat and Power Technology, Chalmers University of Technology, Sweden, 2001. [3] C. Bengtsson, et al., Co-ordination of pinch technology and the MIND method—applied to a Swedish board mill, Applied Thermal Engineering (2001) 133–144. [4] C. Bengtsson, R. Nordman, T. Berntsson, Utilization of excess heat in the pulp and paper industry—a case study of technical and economical opportunities, Applied Thermal Engineering 22 (9) (2002) 1069–1081. [5] U. Wising, T. Berntsson, A. Asblad, Usable excess heat in future kraft pulp mills, TAPPI Journal (November) (2002) 27–29. [6] U. Wising, T. Berntsson, A. Asblad, Energy consequences in minimum effluent market kraft pulp mills, TAPPI Journal (Nov.) (2002) 30–32. [7] J. Algehed, Energy efficient evaporation in future kraft pulp mills, PhD-Thesis, Department of Heat and power technology, Chalmers University of Technology, Sweden, 2002. [8] KAM, Eco-Cyclic Pulp Mill. Final report, 1996–2002, Report A100, STFI, Stockholm, 2003. [9] B. Linnhoff, D.W. Townsend, D. Boland, G.F. Hewitt, B.E.A. Thomas, A.R. Guy, Marshland R.H.A User Guide on Process Integration for the Efficient Use of Energy. Rugby: Institution of Chemical Engineers, 1982, last edition 1994. [10] J. Algehed., U. Wising, T. Berntsson, Energy efficient evaporation in future pulp and paper mills, in: 7th International conference on new available technologies, Stockholm, Sweden, 2002, pp. 112–116.