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Innovative visualization technique for energy flow analysis: Waste heat recovery and energy savings opportunities
Applied Thermal Engineering 61 (2013) 143e148
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Innovative visualization technique for energy flow analysis: Waste heat recovery and energy savings opportunities Luciana Savulescu*, Zoé Périn-Levasseur, Marzouk Benali Natural Resources Canada, CanmetENERGY, Industrial Systems Optimisation 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, J3X 1S6 Quebec, Canada
h i g h l i g h t s Novel approach for energy flow analysis. Interactive visualization of energy and waste heat mapping. Rapid screening of steam use inefficiencies in existing industrial value chain. Thermal mapping led to up to 16 MW steam savings for the targeted pulping mill. Operating cost reduction of up to 3.5 M$ for the targeted pulping mill.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2012 Accepted 13 March 2013 Available online 28 March 2013
Improving the performance of existing industrial plants through new technologies and innovative ecoenergy solutions is raising a lot of challenges for researchers and practitioners. In the search for new sustainable designs, understanding the current energy distribution within a plant in regards to its energy requirements is essential for the decision-making process. The technique of Composite Curves is commonly employed to evaluate the scope for energy savings. However, it is seldom self-explanatory. In the present work, an alternative to the energy Composite Curves representation is proposed to complement the visualization of energy integration opportunities. A novel energy management assessment based on mapping of the overall process energy is introduced as a practical illustration to support the thermal energy integration. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Energy integration Waste heat Thermal mapping Energy flow analysis Energy savings
1. Introduction Industrial sites are continuously seeking and exploring new and innovative ways to improve their energy efficiency, and higher profitability in the current competitive market context. This challenging step requires a systematic strategy to account for the complex interactions between all sub-systems within a mill. The optimal integration of plant heat sources and demands while minimizing waste heat has been included as key priority in the energy loss reduction technology roadmap by the U.S. Department of Energy [1]. Most recently, the sustainable initiative addressing the development of pathways towards Clean Energy Systems [2] has emphasized the need to improve further the plant thermal energy management through heat recovery and integration of advanced heat upgrading technologies. Energy losses occur along
* Corresponding author. Tel.: þ1 450 652 0275; fax: þ1 450 652 5918. E-mail addresses: [email protected], zoe.perin-levasseur@ polymtl.ca (L. Savulescu).
the energy supply and distribution system as well as along the energy conversion systems where efficiencies are often thermally or mechanically limited. Recent editorial [3] pointed out that waste heat recovery and upgrading have the potential to reduce operating costs significantly by using energy more efficiently and lowering cooling requirements. Therefore, the optimal use of energy resources by revamping their overall integrated network distribution requires a global approach. This should include the identification and evaluation of all direct and indirect links between the process and utility through a site-wide system analysis [4]. Energy-efficient design and operation improvements are often achieved through the application of techniques such as energy audits, simulationbased analysis, process control, monitoring and targeting, and process integration. The audits are focused on equipment-based analysis, checking mostly the local performance while minimum considerations are given to the interactions within the energy system. On the other hand, an overall plant simulation accounts for all the links between the process and resources supporting systems as it incorporates a large amount of information albeit is limited in pointing out the thermodynamic inefficiencies. Evaluating savings
1359-4311/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.03.035
144
L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148
Nomenclature T Qc QH
temperature ( C) cold utility (kW) hot utility (kW)
Acronyms adt air dry ton BL black liquor CS cold stream GL green liquor HEN heat exchanger network HEX heat exchanger HS hot stream LP low pressure MP medium pressure M$ million-dollars PB power boiler PM paper machine RB recovery boiler
prior to design is a key feature that process integration is providing, being a global system approach. Also, this approach allows identifying energy bottlenecks such as steam usage for warm water production and waste heat losses. A combined approach based on these techniques is often considered. Researchers and engineers have been addressing the energy system design and management issues from different viewpoints and have developed a number of suitable methodologies. Linnhoff et al. [5], updated later by Kemp [6], introduced the key concepts of Pinch Analysis and its graphical representation through Composite Curves. Smith [7] provided practical guidelines for the design of optimized process unit operations and their efficient heat integration. Maréchal et al. [8] combined the concepts based on the thermodynamic analysis (e.g., Pinch Analysis) and the application of mathematical programming tools for energy analysis and synthesis of industrial processes. Several diagrams have been developed to visualize a plant energy distribution and characterize its steam use, waste heat sources, and non-isothermal stream mixing. Brown et al. [9] suggested a tool based on classical Pinch Analysis for early stages of process energy evaluation in which the minimum energy requirements (MER) of the process were calculated using a dual graphical representation that segregates the thermodynamic requirement of the process from its technological implementation. The MER are determined through: (a) the temperature profiles of the process streams that maximize the exergy supplied by the hot streams and minimize the exergy required by the cold streams; or (b) the analysis of the equipment used to convert utility streams into useful process heat. Further developments led Périn-Levasseur et al. [10] to set up a decision-making tool guiding an energy practitioner in establishing trade-offs between investment cost and energy penalty for all possible energy savings options. The visualization of the thermodynamic process insights was also explored by Wan Alwi et al. [11] through a novel superimposed curves concept that simultaneously illustrates the mass and energy aspects through cumulative mass load, temperature, and flowrate representation. This supports the assessment of the complex interaction and the identification of the impacts of mass and energy reduction. Pinch Analysis gained intensive interest towards advanced Composite Curves concept that Nordman and Berntsson [12] brought to the fore. The advanced curves represent the lower and upper boundaries for utility heaters and coolers in relation to the actual
Table 1 Data for heat exchanger network example. Load Hot side Cold side (kW) Hot Tin ( C) Tout ( C) Cold stream stream Heater 1 Heater 2 Heater 3 HEX4 HEX5 Effluent
1000 2500 1500 2000 5000 3000
Steam Steam Steam HS4 HS5 Effluent
e e e 97 55 45
e e e 50 30 30
Tin ( C) Tout ( C)
CS1 10 CS2 35 CS3 100 CS4 30 CS5 5 Environment e
30 80 120 65 35 e
heating and cooling loads. Higher energy savings at lower operating cost could be obtained when the heaters and coolers are initially placed closer to the pinch. Such an innovative concept illustrates the impacts of the possible heat exchange enhancements on the utility system and the heat exchanger network [13]. An up-to-date overview of the integration and optimization methodologies for efficient use of resources is provided by Klemes et al. [14] as key strategies for the sustainable design development in process industries. Although the process integration (graphical and mathematical model-based) tools estimate energy targets and provide design guidelines, the data selection, representation, and interpretation of local inefficiencies are still challenging issues in the application of these methodologies. Specialized expertise is, therefore, required to screen and select the relevant data to be used in the retrofit analysis. The balance between simplified assumptions and practical elements has also to be captured to address the complexity of the process. From the perspective of energy sustainability, graphical methods capturing key energy system insights are proven to be essential. They deliver to engineers a global vision assisting them in prioritizing specific issues such as resources management. From the standpoint of industry operating with multidisciplinary teams, the application, interpretation, and benefits of Composite Curves might be diminished due to its intangible character. In this paper, a novel visualization technique is introduced to facilitate and increase the receptivity of the industrial engineers towards the need and benefits of a detailed process integration study. This technique aims at illustrating the current energy distribution of a targeted plant in a concrete and easy manner that reveals the allocation of energy sources and sinks within the whole process. It also highlights possible bottlenecks based on the specific repartition of energy as quality (temperature levels) and quantity
Fig. 1. Energy Composite Curves.
L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148 140 Zone 3
Temperature (°C)
120
0.35
1,500 kW 30%
0.33 M$/y
0.3
100
0.25
Zone 2
80
1,700 kW
0.22 M$/y
0.2
34%
0.18 M$/y
60
0.15
50 40
0.1
Economic penalty (M$/y)
0.4 0.36 M$/y
Zone 1
20
1,800 kW 36%
0
Economic penalty Zone 1 Zone 2 Zone 3
CS1-Heater 1
CS2-Heater 2
CS3-Heater 3
0.05 0
Steam consumption points
Fig. 2. Steam mapping for the example of the heat exchanger network.
(energy load) making it advantageous for quick and practical preliminary assessments by industrial engineers. Furthermore, the objective of such technique is to capture process know-how and ensure that appropriate operating details are included in userfriendly tools. 2. Thermal mapping The steam mapping diagram embeds the information on steam use from the process perspective. This plant-wide representation considers all the steam consumption points in relation with its corresponding process energy demands to reveal opportunities for reduction of steam demand. The diagram includes the details on the steam pressure levels, temperature levels, and energy loads. Waste heat sources are tracked and ranked through their energy load and temperature level. The end-users (PI experts and plant engineers) of the proposed visualization diagrams have to consider the following steps: Data collection Energy flow profiles representation
145
Process constraints identification Assessment through steam and waste heat mapping: preliminary targets and design insights Screening of heat recovery opportunities by matching steam users with waste heat sources to explore heat recovery opportunities with minimum changes in the existing HEN. The visualization diagrams illustrate a series of waste heat recovery opportunities, allowing the validation of the data extracted and matching constraints which facilitate defining the scope of detailed process integration study. A simple heat exchanger network example is considered in Table 1 to illustrate the thermal mapping concepts. It includes two heat exchangers (HEX4, HEX5), three steam heaters (Heaters 1e3) and one waste effluent stream (Effluent) discharged to sewer. The corresponding Composite Curves are depicted in Fig. 1. The Composite Curves provide the energy targeting for the minimum hot utility (QH) to be 2800 kW while the cooling represents (QC) 800 kW. The critical temperature level of pinch is found to be 50 C. The sources and sinks of energy as hot and cold streams are combined and globally illustrated as hot and cold Composite Curves. Although this is an overall illustration of the problem, the characteristics of the steam users across temperature intervals are lost in the representation. Therefore, a steam mapping has been considered to obtain a global profile of all steam users from the perspective of the cold streams as this will give indications on the inefficient use of steam. The steam mapping represents the process information as it indicates what are the streams receiving this heat and what is the temperature lift for each one, as well as the utility data (e.g., medium/low pressure levels and the amount of steam consumed with its corresponding process energy load of individual users). As illustrated in Fig. 2, the temperature range is divided by two temperature plateaus at 50 C and at 100 C into three zones: zone 1 e below 50 C; zone 2 e between 50 and 100 C; zone 3 e above 100 C. Such a temperature range is introduced as a reference for the assessment purpose prior to pinch study. The temperature level of 50 C corresponds to the pinch temperature and also can be
Fig. 3. Steam mapping for the case study.
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L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148
Fig. 4. Waste heat mapping for the case study.
considered as a low energy temperature level. In zone 1, steam use should be minimized and waste heat use should be maximized. The high temperature zone is bordered by the 100 C temperature level as above this value, steam is one of the common energy sources. This diagram provides the steam allocation for each zone and detailed information on the preliminary energy scope including its heat load and specific location. By analogy to the heat exchanger cross-pinch table, the steam mapping diagram visualizes the inefficiencies at the level of steam usage and can support the reallocation of energy within the network to reduce the overall steam consumption when coupled with the waste energy. This will provide a preliminary diagnostic for quick wins opportunities and minimum changes to the existing processeprocess heat exchangers. The repartition of the steam use for this example (Fig. 2) has been mapped and a preliminary target has been estimated at 36% steam savings (1800 kW), corresponding to 82% of the theoretical energy target (2200 kW) defined by the Composite Curves. The selection of an optimal solution requires an economic analysis and evaluation of system trade-offs. The operating cost is performed to distinguish between the options of converting the steam savings into fuel and/or additional power cogeneration. For detailed economic assessment, the capital cost of all energy saving options is required for the decision-making of the strategic investment planning. Therefore, the equipment cost, including piping and pumping, control system, manpower, and installation should be accurately evaluated. Assuming a 7 $/GJ steam cost, the economic projection of the steam usage has also been calculated and included in the diagram as an economic penalty (M$/y) from the perspective of lost opportunity for heat recovery. The economic values have been evaluated for each zone and then displayed for every steam users. These values are illustrated by triangle for zone 1, circle for zone 2 and square for zone 3. In consequence, the representation informs on the opportunity for steam reduction by 1800 kW and its equivalent steam cost reduction of 0.4 M$/y. 3. Case study A Canadian kraft pulp mill producing up to 400 adt of pulp per day is used as a case study to validate the effectiveness of this graphical technique as compared to the conventional representation of
Composite Curves. As a part of energy-based plant retrofit strategies, the thermal energy mapping diagrams were developed to complement the classical analysis and to facilitate the exchange with the industrial practitioners via a novel data/results visualization approach. The rules to evaluate the steam mapping diagram are very much based on the Pinch Analysis principles according to which steam should not be used below pinch to ensure minimum consumption in the mill. In typical long operated North American pulp and paper mills, the pinch point occurs commonly in the range of 50e100 C whereas it is about 120e140 C in modernized mills (most of Scandinavian and newly constructed Chinese mills). Pinch can occur at low temperature (50 C) when the mill has large water consumption or at high temperature level (100 C) when the mill is a low water consumer. Since steam mapping is performed prior to Pinch Analysis, the pinch point is unknown. Once the detailed pinch analysis is carried out, these temperature levels will delimit the heat recovery zone and will be replaced by the pinch temperature associated with the plant context.
Table 2 Load and steam costs for steam consumption points. Steam users
Pulp dryer Air preheating PM BL evaporation Water dearator Bleaching Whitewater PM1 RB air preheat Digester BL2 Chips preheating Whitewater PM2 Digester BL1 BL to digester GL recaustification Pulp preheating Washing water Total
Zone 1
Zone 2
Load (MW)
Steam cost (M$/y)
20.0
4.33
2.6
0.56
1.6
0.35
1.4
0.30
Zone 3
Load (MW)
Steam cost (M$/y)
Load (MW)
Steam cost (M$/y)
19.2 4 1.6 4.3 6.1 4.9 1.4
4.16 0.87 0.35 0.94 1.33 1.06 0.29
6.4
1.39
7.2 2.1
1.57 0.45
1.4 3.6
0.29 0.78
2 3.3
0.43 0.71
1.3 1.1 0.5 0.2 0.4 26 MW 5.66 M$/y 60 MW
2.2 0.47 0.29 0.6 0.13 0.23 0.05 0.08 10.1 M$/y 23 MW 5.07 M$/y
L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148
147
Fig. 5. Waste heat recovery opportunities for the case study.
All the steam consumption points have been collected and plotted together (Fig. 3) while mapping of available waste heat is depicted in Fig. 4. Different bar widths were used to illustrate the steam consumption points, depending on the amount of consumed steam. As an example, the bar corresponding to the pulp dryer consuming 25 MW of steam is wider than the pulp preheating consuming 0.75 MW of steam. On the right side of Fig. 3, the steam cost has been calculated for each steam user and for each zone. These values are represented by triangle for zone 1, circle for zone 2, and square for zone 3. Table 2 summarized load and steam costs for all steam consumption points. For each zone, the potential of using waste heat instead of steam has been evaluated in terms of energy and steam cost avoided. For example, the water deaerator needs 9 MW to preheat water from 20 to 124 C. If the load of the zone 1 is heated by waste heat recovered from paper machine effluents PM1 and PM2, an equivalent of 2.6 MW and 0.56 M$/y can be saved (Table 2). For the same stream, as shown in Table 2, zone 2 corresponds to 4.3 MW and 0.94 M$/y while zone 3 is tied to 2.1 MW and 0.45 M$/y. Even though, thermodynamic opportunities exist for heat exchange between streams, their rather long distance position limits the scope of heat recovery. This practical consideration, that highly influences the project economics, could be added to the knowledge package delivered through the visualization. The matching should be done primarily with streams being relatively close to each other. Throughout the mapping assessment, seven key steam users have been selected and plotted together with seven relevant waste energy streams (Fig. 5), bringing them together such as in a heat exchanger network diagram for potential matching opportunities. The potential for heat recovery can be straightforwardly observed in the case of the waste energy recovery from the paper machine effluents to preheat make-up water supplying the deaerator (zone 1 in Fig. 5). For this opportunity, the detailed analysis indicates that 3.5 MW of steam energy can be substituted by waste heat, giving an opportunity for economic benefit of 0.76 M$/y based on the operating cost estimates. Other waste heat recovery opportunities could be screened as load and economic benefit for each zone. The waste heat mapping provides information that can support the user in the decision-making process for screening and selecting the most appropriate waste heat recovery opportunities and energy savings options (Fig. 5). The matching between steam
demand and waste heat sources is showing only the simple retrofit paths heaterecooler. The visualization can also be extended to the full heat exchanger network that will embed all the complex paths (e.g., heatereHEXecooler). The potential of increasing the waste heat temperature is not accounted in the current diagrams since this is related to the rearrangement of the heat exchanger network. Piacentino [15] is proposing a heat load plot (temperature versus heat capacity rate) representing the existing HEN that could complement this aspect. This is, however, recommended after the identification of energy paths via grid diagram for detailed design. These diagrams can be used as a short-cut analysis to identify opportunities to save steam through waste heat recovery. The detailed process integration study should be applied for evaluation of all opportunities, accounting for all the energy saving paths across the existing heat exchanger network for an improved retrofit solution. The mapping and Composite Curves diagrams of this case study have led to practical heat recovery opportunities in the same order of energy savings. Accounting for the process and operating constraints, the plant has selected projects leading to 16 MW steam savings, corresponding to 15% of energy savings and approximately 3.5 M$/y operating cost reduction.
4. Conclusions The presented diagrams provide new insightful visualization, and a systematic way of investigating the energy system of a process at the local level as well as the global level. It provides the global view of the steam users without losing its characteristics through merging streams as it is in the case of the Composite Curves. The representation should incorporate constraints on steam usage and temperature level for waste energy to allow practical evaluation of the energy scope. Moreover, the assembly of steam and waste mapping could provide preliminary design changes when placed at a minimum temperature difference relation. In the context of already highly integrated energy network, it is recommended to use the novel diagram to represent the existing processeprocess heat exchangers together with the heaters, coolers and waste energy streams as a visualization tool for the data and results of the detailed process integration.
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L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148
Mapping of steam and waste heat profiles through the thermal energy integration embeds information and process constraints to facilitate a quick screening of opportunities for impending energy recovery and upgrading. Thus, these graphs provide a different sitewide energy view for industry yet following the principles of process integration. Economic insights have been added to the profile to facilitate the ranking and selection of energy-efficient solutions. The application of the novel visualization technique of the energy flows to a Canadian kraft mill producing up to 400 adt of pulp per day led in rapid identification of steam savings and operating cost reduction of up to 16 MW and 5 M$/y.
Acknowledgements The authors acknowledge the financial support provided by the Program on Energy Research and Development of Natural Resources Canada.
References [1] U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies ProgramTechnology Roadmap Energy Loss Reduction and Recovery in Industrial Energy Systems (November 2004) [last access: February 2013], www1.eere.energy.gov/manufacturing/intensiveprocesses/ pdfs/reduction_roadmap.pdf. [2] Energy Technology Perspectives 2012: Pathways to a Clean Energy System, International Energy Agency, 2012, ISBN 978-92-64-17488-7, p. 690.
[3] A.P. Roskilly Tony, Special issue: sustainable thermal energy management in the process industries international conference (SusTEM2011), Appl. Therm. Eng. (2012). htpp://dx.doi.org/10.1016/j.applthermaleng.2012.09.010. [4] P.Y. Liew, S.R. Wan Alwi, P.S. Varbanov, Z.A. Manan, J.J. Klemes, A numerical technique for total site sensitivity analysis, Appl. Therm. Eng. 40 (2012) 397e408. [5] B. Linnhoff, D.W. Townsend, D. Boland, G.F. Hewitt, B.E. Thomas, A User Guide on Process Integration for the Efficient Use of Energy, last ed., Rugby, U.K, 1994. [6] I.C. Kemp, Pinch Analysis and Process Integration: A User Guide on Process Integration for Efficient Use of Energy, Elsevier, Amsterdam, The Netherlands, 2007. [7] R. Smith, Chemical Process Design and Integration, John Wiley, Chichester, U.K, 2005. [8] F. Maréchal, B. Kalitventzeff, Energy integration of industrial sites: tools, methodology and application, Appl. Therm. Eng. 18 (1998) 921e933. [9] D. Brown, F. Maréchal, J. Paris, A dual representation for targeting process retrofit, application to a pulp and paper process, Appl. Therm. Eng. 25 (2005) 1067e1082. [10] Z. Périn-Levasseur, Energy efficiency and conversion in complex integrated industrial sites: application to a sulphite pulping facility, Ph.D. Thesis No. 4433, École Polytechnique Fédérale de Lausanne (Switzerland) 2009. [11] S.R. Wan Alwi, A. Ismail, Z.A. Manan, Z.B. Handani, A new graphical approach for simultaneous mass and energy minimisation, Appl. Therm. Eng. 31 (2011) 1021e1030. [12] R. Nordman, T. Berntsson, Use of advanced Composite Curves for assessing cost-effective HEN retrofit: theory and concepts, Appl. Therm. Eng. 29 (2009) 275e281. [13] R. Nordman, T. Berntsson, Use of advanced Composite Curves for assessing cost-effective HEN retrofit: case studies, Appl. Therm. Eng. 29 (2009) 282e289. [14] J. Klemes, F. Friedler, I. Bulatov, P. Varbanov, Sustainability in the Process Industries. Integration and Optimisation, McGraw-Hill, New York, USA, 2011. [15] A. Piancentino, Thermal analysis and new insights to support decision making in retrofit and relaxation of heat exchanger networks, Appl. Therm. Eng. 31 (2011) 3479e3499.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Innovative visualization technique for energy flow analysis: Waste heat recovery and energy savings opportunities Luciana Savulescu*, Zoé Périn-Levasseur, Marzouk Benali Natural Resources Canada, CanmetENERGY, Industrial Systems Optimisation 1615 Lionel-Boulet Blvd., P.O. Box 4800, Varennes, J3X 1S6 Quebec, Canada
h i g h l i g h t s Novel approach for energy flow analysis. Interactive visualization of energy and waste heat mapping. Rapid screening of steam use inefficiencies in existing industrial value chain. Thermal mapping led to up to 16 MW steam savings for the targeted pulping mill. Operating cost reduction of up to 3.5 M$ for the targeted pulping mill.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2012 Accepted 13 March 2013 Available online 28 March 2013
Improving the performance of existing industrial plants through new technologies and innovative ecoenergy solutions is raising a lot of challenges for researchers and practitioners. In the search for new sustainable designs, understanding the current energy distribution within a plant in regards to its energy requirements is essential for the decision-making process. The technique of Composite Curves is commonly employed to evaluate the scope for energy savings. However, it is seldom self-explanatory. In the present work, an alternative to the energy Composite Curves representation is proposed to complement the visualization of energy integration opportunities. A novel energy management assessment based on mapping of the overall process energy is introduced as a practical illustration to support the thermal energy integration. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Energy integration Waste heat Thermal mapping Energy flow analysis Energy savings
1. Introduction Industrial sites are continuously seeking and exploring new and innovative ways to improve their energy efficiency, and higher profitability in the current competitive market context. This challenging step requires a systematic strategy to account for the complex interactions between all sub-systems within a mill. The optimal integration of plant heat sources and demands while minimizing waste heat has been included as key priority in the energy loss reduction technology roadmap by the U.S. Department of Energy [1]. Most recently, the sustainable initiative addressing the development of pathways towards Clean Energy Systems [2] has emphasized the need to improve further the plant thermal energy management through heat recovery and integration of advanced heat upgrading technologies. Energy losses occur along
* Corresponding author. Tel.: þ1 450 652 0275; fax: þ1 450 652 5918. E-mail addresses: [email protected], zoe.perin-levasseur@ polymtl.ca (L. Savulescu).
the energy supply and distribution system as well as along the energy conversion systems where efficiencies are often thermally or mechanically limited. Recent editorial [3] pointed out that waste heat recovery and upgrading have the potential to reduce operating costs significantly by using energy more efficiently and lowering cooling requirements. Therefore, the optimal use of energy resources by revamping their overall integrated network distribution requires a global approach. This should include the identification and evaluation of all direct and indirect links between the process and utility through a site-wide system analysis [4]. Energy-efficient design and operation improvements are often achieved through the application of techniques such as energy audits, simulationbased analysis, process control, monitoring and targeting, and process integration. The audits are focused on equipment-based analysis, checking mostly the local performance while minimum considerations are given to the interactions within the energy system. On the other hand, an overall plant simulation accounts for all the links between the process and resources supporting systems as it incorporates a large amount of information albeit is limited in pointing out the thermodynamic inefficiencies. Evaluating savings
1359-4311/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.03.035
144
L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148
Nomenclature T Qc QH
temperature ( C) cold utility (kW) hot utility (kW)
Acronyms adt air dry ton BL black liquor CS cold stream GL green liquor HEN heat exchanger network HEX heat exchanger HS hot stream LP low pressure MP medium pressure M$ million-dollars PB power boiler PM paper machine RB recovery boiler
prior to design is a key feature that process integration is providing, being a global system approach. Also, this approach allows identifying energy bottlenecks such as steam usage for warm water production and waste heat losses. A combined approach based on these techniques is often considered. Researchers and engineers have been addressing the energy system design and management issues from different viewpoints and have developed a number of suitable methodologies. Linnhoff et al. [5], updated later by Kemp [6], introduced the key concepts of Pinch Analysis and its graphical representation through Composite Curves. Smith [7] provided practical guidelines for the design of optimized process unit operations and their efficient heat integration. Maréchal et al. [8] combined the concepts based on the thermodynamic analysis (e.g., Pinch Analysis) and the application of mathematical programming tools for energy analysis and synthesis of industrial processes. Several diagrams have been developed to visualize a plant energy distribution and characterize its steam use, waste heat sources, and non-isothermal stream mixing. Brown et al. [9] suggested a tool based on classical Pinch Analysis for early stages of process energy evaluation in which the minimum energy requirements (MER) of the process were calculated using a dual graphical representation that segregates the thermodynamic requirement of the process from its technological implementation. The MER are determined through: (a) the temperature profiles of the process streams that maximize the exergy supplied by the hot streams and minimize the exergy required by the cold streams; or (b) the analysis of the equipment used to convert utility streams into useful process heat. Further developments led Périn-Levasseur et al. [10] to set up a decision-making tool guiding an energy practitioner in establishing trade-offs between investment cost and energy penalty for all possible energy savings options. The visualization of the thermodynamic process insights was also explored by Wan Alwi et al. [11] through a novel superimposed curves concept that simultaneously illustrates the mass and energy aspects through cumulative mass load, temperature, and flowrate representation. This supports the assessment of the complex interaction and the identification of the impacts of mass and energy reduction. Pinch Analysis gained intensive interest towards advanced Composite Curves concept that Nordman and Berntsson [12] brought to the fore. The advanced curves represent the lower and upper boundaries for utility heaters and coolers in relation to the actual
Table 1 Data for heat exchanger network example. Load Hot side Cold side (kW) Hot Tin ( C) Tout ( C) Cold stream stream Heater 1 Heater 2 Heater 3 HEX4 HEX5 Effluent
1000 2500 1500 2000 5000 3000
Steam Steam Steam HS4 HS5 Effluent
e e e 97 55 45
e e e 50 30 30
Tin ( C) Tout ( C)
CS1 10 CS2 35 CS3 100 CS4 30 CS5 5 Environment e
30 80 120 65 35 e
heating and cooling loads. Higher energy savings at lower operating cost could be obtained when the heaters and coolers are initially placed closer to the pinch. Such an innovative concept illustrates the impacts of the possible heat exchange enhancements on the utility system and the heat exchanger network [13]. An up-to-date overview of the integration and optimization methodologies for efficient use of resources is provided by Klemes et al. [14] as key strategies for the sustainable design development in process industries. Although the process integration (graphical and mathematical model-based) tools estimate energy targets and provide design guidelines, the data selection, representation, and interpretation of local inefficiencies are still challenging issues in the application of these methodologies. Specialized expertise is, therefore, required to screen and select the relevant data to be used in the retrofit analysis. The balance between simplified assumptions and practical elements has also to be captured to address the complexity of the process. From the perspective of energy sustainability, graphical methods capturing key energy system insights are proven to be essential. They deliver to engineers a global vision assisting them in prioritizing specific issues such as resources management. From the standpoint of industry operating with multidisciplinary teams, the application, interpretation, and benefits of Composite Curves might be diminished due to its intangible character. In this paper, a novel visualization technique is introduced to facilitate and increase the receptivity of the industrial engineers towards the need and benefits of a detailed process integration study. This technique aims at illustrating the current energy distribution of a targeted plant in a concrete and easy manner that reveals the allocation of energy sources and sinks within the whole process. It also highlights possible bottlenecks based on the specific repartition of energy as quality (temperature levels) and quantity
Fig. 1. Energy Composite Curves.
L. Savulescu et al. / Applied Thermal Engineering 61 (2013) 143e148 140 Zone 3
Temperature (°C)
120
0.35
1,500 kW 30%
0.33 M$/y
0.3
100
0.25
Zone 2
80
1,700 kW
0.22 M$/y
0.2
34%
0.18 M$/y
60
0.15
50 40
0.1
Economic penalty (M$/y)
0.4 0.36 M$/y
Zone 1
20
1,800 kW 36%
0
Economic penalty Zone 1 Zone 2 Zone 3
CS1-Heater 1
CS2-Heater 2
CS3-Heater 3
0.05 0
Steam consumption points
Fig. 2. Steam mapping for the example of the heat exchanger network.
(energy load) making it advantageous for quick and practical preliminary assessments by industrial engineers. Furthermore, the objective of such technique is to capture process know-how and ensure that appropriate operating details are included in userfriendly tools. 2. Thermal mapping The steam mapping diagram embeds the information on steam use from the process perspective. This plant-wide representation considers all the steam consumption points in relation with its corresponding process energy demands to reveal opportunities for reduction of steam demand. The diagram includes the details on the steam pressure levels, temperature levels, and energy loads. Waste heat sources are tracked and ranked through their energy load and temperature level. The end-users (PI experts and plant engineers) of the proposed visualization diagrams have to consider the following steps: Data collection Energy flow profiles representation
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Process constraints identification Assessment through steam and waste heat mapping: preliminary targets and design insights Screening of heat recovery opportunities by matching steam users with waste heat sources to explore heat recovery opportunities with minimum changes in the existing HEN. The visualization diagrams illustrate a series of waste heat recovery opportunities, allowing the validation of the data extracted and matching constraints which facilitate defining the scope of detailed process integration study. A simple heat exchanger network example is considered in Table 1 to illustrate the thermal mapping concepts. It includes two heat exchangers (HEX4, HEX5), three steam heaters (Heaters 1e3) and one waste effluent stream (Effluent) discharged to sewer. The corresponding Composite Curves are depicted in Fig. 1. The Composite Curves provide the energy targeting for the minimum hot utility (QH) to be 2800 kW while the cooling represents (QC) 800 kW. The critical temperature level of pinch is found to be 50 C. The sources and sinks of energy as hot and cold streams are combined and globally illustrated as hot and cold Composite Curves. Although this is an overall illustration of the problem, the characteristics of the steam users across temperature intervals are lost in the representation. Therefore, a steam mapping has been considered to obtain a global profile of all steam users from the perspective of the cold streams as this will give indications on the inefficient use of steam. The steam mapping represents the process information as it indicates what are the streams receiving this heat and what is the temperature lift for each one, as well as the utility data (e.g., medium/low pressure levels and the amount of steam consumed with its corresponding process energy load of individual users). As illustrated in Fig. 2, the temperature range is divided by two temperature plateaus at 50 C and at 100 C into three zones: zone 1 e below 50 C; zone 2 e between 50 and 100 C; zone 3 e above 100 C. Such a temperature range is introduced as a reference for the assessment purpose prior to pinch study. The temperature level of 50 C corresponds to the pinch temperature and also can be
Fig. 3. Steam mapping for the case study.
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Fig. 4. Waste heat mapping for the case study.
considered as a low energy temperature level. In zone 1, steam use should be minimized and waste heat use should be maximized. The high temperature zone is bordered by the 100 C temperature level as above this value, steam is one of the common energy sources. This diagram provides the steam allocation for each zone and detailed information on the preliminary energy scope including its heat load and specific location. By analogy to the heat exchanger cross-pinch table, the steam mapping diagram visualizes the inefficiencies at the level of steam usage and can support the reallocation of energy within the network to reduce the overall steam consumption when coupled with the waste energy. This will provide a preliminary diagnostic for quick wins opportunities and minimum changes to the existing processeprocess heat exchangers. The repartition of the steam use for this example (Fig. 2) has been mapped and a preliminary target has been estimated at 36% steam savings (1800 kW), corresponding to 82% of the theoretical energy target (2200 kW) defined by the Composite Curves. The selection of an optimal solution requires an economic analysis and evaluation of system trade-offs. The operating cost is performed to distinguish between the options of converting the steam savings into fuel and/or additional power cogeneration. For detailed economic assessment, the capital cost of all energy saving options is required for the decision-making of the strategic investment planning. Therefore, the equipment cost, including piping and pumping, control system, manpower, and installation should be accurately evaluated. Assuming a 7 $/GJ steam cost, the economic projection of the steam usage has also been calculated and included in the diagram as an economic penalty (M$/y) from the perspective of lost opportunity for heat recovery. The economic values have been evaluated for each zone and then displayed for every steam users. These values are illustrated by triangle for zone 1, circle for zone 2 and square for zone 3. In consequence, the representation informs on the opportunity for steam reduction by 1800 kW and its equivalent steam cost reduction of 0.4 M$/y. 3. Case study A Canadian kraft pulp mill producing up to 400 adt of pulp per day is used as a case study to validate the effectiveness of this graphical technique as compared to the conventional representation of
Composite Curves. As a part of energy-based plant retrofit strategies, the thermal energy mapping diagrams were developed to complement the classical analysis and to facilitate the exchange with the industrial practitioners via a novel data/results visualization approach. The rules to evaluate the steam mapping diagram are very much based on the Pinch Analysis principles according to which steam should not be used below pinch to ensure minimum consumption in the mill. In typical long operated North American pulp and paper mills, the pinch point occurs commonly in the range of 50e100 C whereas it is about 120e140 C in modernized mills (most of Scandinavian and newly constructed Chinese mills). Pinch can occur at low temperature (50 C) when the mill has large water consumption or at high temperature level (100 C) when the mill is a low water consumer. Since steam mapping is performed prior to Pinch Analysis, the pinch point is unknown. Once the detailed pinch analysis is carried out, these temperature levels will delimit the heat recovery zone and will be replaced by the pinch temperature associated with the plant context.
Table 2 Load and steam costs for steam consumption points. Steam users
Pulp dryer Air preheating PM BL evaporation Water dearator Bleaching Whitewater PM1 RB air preheat Digester BL2 Chips preheating Whitewater PM2 Digester BL1 BL to digester GL recaustification Pulp preheating Washing water Total
Zone 1
Zone 2
Load (MW)
Steam cost (M$/y)
20.0
4.33
2.6
0.56
1.6
0.35
1.4
0.30
Zone 3
Load (MW)
Steam cost (M$/y)
Load (MW)
Steam cost (M$/y)
19.2 4 1.6 4.3 6.1 4.9 1.4
4.16 0.87 0.35 0.94 1.33 1.06 0.29
6.4
1.39
7.2 2.1
1.57 0.45
1.4 3.6
0.29 0.78
2 3.3
0.43 0.71
1.3 1.1 0.5 0.2 0.4 26 MW 5.66 M$/y 60 MW
2.2 0.47 0.29 0.6 0.13 0.23 0.05 0.08 10.1 M$/y 23 MW 5.07 M$/y
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Fig. 5. Waste heat recovery opportunities for the case study.
All the steam consumption points have been collected and plotted together (Fig. 3) while mapping of available waste heat is depicted in Fig. 4. Different bar widths were used to illustrate the steam consumption points, depending on the amount of consumed steam. As an example, the bar corresponding to the pulp dryer consuming 25 MW of steam is wider than the pulp preheating consuming 0.75 MW of steam. On the right side of Fig. 3, the steam cost has been calculated for each steam user and for each zone. These values are represented by triangle for zone 1, circle for zone 2, and square for zone 3. Table 2 summarized load and steam costs for all steam consumption points. For each zone, the potential of using waste heat instead of steam has been evaluated in terms of energy and steam cost avoided. For example, the water deaerator needs 9 MW to preheat water from 20 to 124 C. If the load of the zone 1 is heated by waste heat recovered from paper machine effluents PM1 and PM2, an equivalent of 2.6 MW and 0.56 M$/y can be saved (Table 2). For the same stream, as shown in Table 2, zone 2 corresponds to 4.3 MW and 0.94 M$/y while zone 3 is tied to 2.1 MW and 0.45 M$/y. Even though, thermodynamic opportunities exist for heat exchange between streams, their rather long distance position limits the scope of heat recovery. This practical consideration, that highly influences the project economics, could be added to the knowledge package delivered through the visualization. The matching should be done primarily with streams being relatively close to each other. Throughout the mapping assessment, seven key steam users have been selected and plotted together with seven relevant waste energy streams (Fig. 5), bringing them together such as in a heat exchanger network diagram for potential matching opportunities. The potential for heat recovery can be straightforwardly observed in the case of the waste energy recovery from the paper machine effluents to preheat make-up water supplying the deaerator (zone 1 in Fig. 5). For this opportunity, the detailed analysis indicates that 3.5 MW of steam energy can be substituted by waste heat, giving an opportunity for economic benefit of 0.76 M$/y based on the operating cost estimates. Other waste heat recovery opportunities could be screened as load and economic benefit for each zone. The waste heat mapping provides information that can support the user in the decision-making process for screening and selecting the most appropriate waste heat recovery opportunities and energy savings options (Fig. 5). The matching between steam
demand and waste heat sources is showing only the simple retrofit paths heaterecooler. The visualization can also be extended to the full heat exchanger network that will embed all the complex paths (e.g., heatereHEXecooler). The potential of increasing the waste heat temperature is not accounted in the current diagrams since this is related to the rearrangement of the heat exchanger network. Piacentino [15] is proposing a heat load plot (temperature versus heat capacity rate) representing the existing HEN that could complement this aspect. This is, however, recommended after the identification of energy paths via grid diagram for detailed design. These diagrams can be used as a short-cut analysis to identify opportunities to save steam through waste heat recovery. The detailed process integration study should be applied for evaluation of all opportunities, accounting for all the energy saving paths across the existing heat exchanger network for an improved retrofit solution. The mapping and Composite Curves diagrams of this case study have led to practical heat recovery opportunities in the same order of energy savings. Accounting for the process and operating constraints, the plant has selected projects leading to 16 MW steam savings, corresponding to 15% of energy savings and approximately 3.5 M$/y operating cost reduction.
4. Conclusions The presented diagrams provide new insightful visualization, and a systematic way of investigating the energy system of a process at the local level as well as the global level. It provides the global view of the steam users without losing its characteristics through merging streams as it is in the case of the Composite Curves. The representation should incorporate constraints on steam usage and temperature level for waste energy to allow practical evaluation of the energy scope. Moreover, the assembly of steam and waste mapping could provide preliminary design changes when placed at a minimum temperature difference relation. In the context of already highly integrated energy network, it is recommended to use the novel diagram to represent the existing processeprocess heat exchangers together with the heaters, coolers and waste energy streams as a visualization tool for the data and results of the detailed process integration.
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Mapping of steam and waste heat profiles through the thermal energy integration embeds information and process constraints to facilitate a quick screening of opportunities for impending energy recovery and upgrading. Thus, these graphs provide a different sitewide energy view for industry yet following the principles of process integration. Economic insights have been added to the profile to facilitate the ranking and selection of energy-efficient solutions. The application of the novel visualization technique of the energy flows to a Canadian kraft mill producing up to 400 adt of pulp per day led in rapid identification of steam savings and operating cost reduction of up to 16 MW and 5 M$/y.
Acknowledgements The authors acknowledge the financial support provided by the Program on Energy Research and Development of Natural Resources Canada.
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