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Integration of a cogeneration unit into a kraft pulping process
Applied Thermal Engineering 30 (2010) 2724e2729
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Integration of a cogeneration unit into a kraft pulping process Behnam Mostajeran Goortani*,1, Enrique Mateos-Espejel, Jean Paris Department of Chemical Engineering, Ecole Polytechnique de Montreal, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2010 Accepted 23 July 2010 Available online 3 August 2010
The integration of cogeneration with other measures that impact the power production capacity in a Canadian Kraft pulping mill is studied. Those measures are removal of pressure reduction valves, adjustment of the steam pressure level, biomass boiler capacity, and reduction in process energy demand. CADSIM Plus software is used to simulate the cogeneration plant. The dynamic behavior of the process during start-up and its effect on electricity generation are also considered. It is shown that by replacing the PRVs with turbines, 14.4 MW of power can be generated. Moreover, by implementing cogeneration units and process measures to recover 23% of internal energy, 44.5 MW of electricity can be generated in addition to shutting down the existing bunker oil boiler. Therefore, implementation of cogeneration in the pulp and paper industry is technically possible and it offers significant economic advantages. A cost analysis of the complete project gives a simple payback time of less than a year. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cogeneration Kraft process Condensing turbine Back pressure turbine Steam network
1. Introduction Cogeneration is the simultaneous production of heat and power. This concept has been used for a long time in various industries [1e3], and can be visualized by comparing the configuration of a typical steam power cycle (Fig. 1(a)) with that of a steam plant in a pulp and paper mill (Fig. 1(b)). The difference between the two configurations is that in Fig. 1(a) a significant amount of the exergy of the HP steam is changed into useful work, while in Fig. 1(b) this exergy is lost in the pressure reduction valves PRV (PRV1 and PRV2). Therefore, if a turbine is installed in the HP steam line to replace the PRVs this available energy can be efficiently converted into electricity and the remaining heating energy of the vapor can be used in the process as shown in Fig. 1(c). The pulp and paper (P&P) industry is the highest energy consumer in Canada (about 33% of the total consumption) and represents a high potential for cogeneration [4,5]. About 60% of the energy demand of this industry is provided by biomass. However, a significant portion is still obtained by the combustion of fossil fuels. The development of technologies with high energy performance is an attractive means to reduce the consumption of fossil fuels [6]. Power production and efficiency enhancements are interdependent approaches.
* Corresponding author. E-mail address: [email protected] (B.M. Goortani). 1 Present address: Waterloo University, Canada. 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.07.026
The objective of this study is to study the implementation of cogeneration in a Kraft process. Different scenarios of vapor production and consumption, the start-up and its effect on power generation are considered. The work is based on two previous studies on the optimization of cogeneration [7] and the implementation of energy measures for a mill. Cakembergh-Mas et al. have developed an optimization algorithm that considers the effect of energy reduction on cogeneration. The authors used the algorithm to study the impact of economic factors. Mateos et al. did a pinch analysis based on water and energy. They identified measures for recovery of internal energy and reutilization of water with the objective of increasing the power potential. The produced data in this work are new and original. To the knowledge of the authors, in the literature there is no parametric study about the effect of various process parameters on cogeneration. The new aspects of this work are the identification of the effect of the following parameters on cogeneration: removal of pressure reduction valves (PRVs), adjustment of the steam pressure level, reduction in process energy demand and variations in the electricity generation during plant start-ups. The final results on the electricity generation (44.5 MW for winter and 40.9 MW for summer) are more attractive than the results of the previous study of Cakembergh-Mas et al. To make the installation of cogeneration plant economically feasible, a boiler with a capacity of 116 t/h is shown to be sufficient. This capacity is more attractive and is lower than the capacity of 140 t/h which is envisaged in the previous study. The paper is organized as follows: Section 1 is the introduction. Section 2 presents the context of the work, previous studies done
B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729
Nomenclature BB BPT CT G HP I LP MB MCR MER MHR
Biomass boiler Back pressure turbine Condensing turbine Generator High pressure steam Investment Low pressure steam Bunker oil boiler Minimum cooling requirement Minimum energy requirement Minimum heating requirement
MP M$ NBB OC PBT P PRV T TS RB S W
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Medium pressure steam Millions US$ New biomass boiler Operating costs Payback time Annual profit Pressure reduction valve Turbine Paper machine turbine Recovery boiler Summer Winter
Fig. 1. (a) Steam power cycle. (b) Simplified flow diagram of the steam plant in a pulp and paper mill. (c) Cogeneration plant (combination of a and b).
and the methodology. Section 3 presents the results and discussion and the Section 4 is the conclusion. 2. Context of the work and methodology 2.1. Mill specifications The mill under study produces about 700 adt/y of bleached Kraft pulp. A simplified diagram of the process is shown in Fig. 2. Wood chips are first sent to a digester, where the lignin (the binding material of the cellulose fibers) is dissolved [13]. From the resulting solution, black liquor, a by-product, is separated and the resulting pulp is bleached dried and finally formed into pulp sheets. The black liquor, is first concentrated in a series of multiple effect evaporators and then sent to recovery boilers, where it is burned to produce heat. The oxidized chemicals are regenerated in recaustisizers and a lime kiln. The regenerated chemicals are finally returned to the digester. The steam production and distribution system of the process is shown in Fig. 3. High pressure steam (HP: 3100 kPa, 371 C) is produced by two recovery boilers (RBI and RBII), one biomass boiler, and one bunker oil boiler (MB). The process uses 32 t/h of HP directly and the remaining steam is depressurized via pressure reduction valves to obtain medium pressure steam, MP (965 kPa, 179 C) and low pressure steam, LP (345 kPa, 143.5 C). A turbine (TS) fed by HP supplies the power requirements of the drying and sheet forming equipment. The total steam production consists of 65% from recovery boilers, 25% from biomass, and the remaining 10% from the bunker oil. The amount of HP production and the consumptions of the three steam levels are listed in Table 1. 2.2. Previous work 2.2.1. Cogeneration Cakembergh-Mas et al. [8] developed an optimization algorithm (MILP) to analyze the profitability of the power house. Several scenarios were studied, including the reduction of steam consumption to 15%, district heating, and type and arrangements of turbines (back pressure BPT and condensing CST turbines). The
decision variables considered are fuel flow, amount of steam produced by each boiler, and sale or purchase of electricity. The constraints are the production capacity of the boilers and the available fuel, various factors influencing process steam demand and electricity, and mass and energy balance on the turbines and boilers. The main simulation parameters and the results are given in Table 2. The additional considerations for that study are the replacement of the bark boilers (BB) and bunker oil boilers (MB) by a new biomass boiler with a production capacity of 140 t/h. This new boiler was selected based on the fact that the present bark boiler efficiency is very low (43%) compared to the efficiency of new industrial bark boilers. The integration of this boiler makes the mill independent from the present bunker oil boiler and allows implementation of a cogeneration unit. The results show that the most effective economic results are obtained by installing a BPT and a CT and implementing energy measures and district heating. In their configuration, PRVs from HP to LP are removed; however, the configuration includes PRVs for the production of LP steam from MP steam. The average and maximum generated power are 28 MW and 36.3 MW, respectively. The payback time (PBT) is 0.9 year. The index costs are for 2006. A sensitivity analysis shows that the PBT depends strongly on the selling price of electricity and the cost of biomass. 2.2.2. Energy efficiency measures The proposed energy efficiency measures are water closure and internal energy recovery.2 The objective is to optimize the amount of steam consumption, to close the bunker oil boiler, and to use the excess steam for power generation. A new methodology is used to analyze the impacts of water and steam reductions [9,10]. Water pinch analysis yields the potential economic savings [14]. This analysis is a balance between the water effluents of the process that can be reused in the process (sources) and process water demand (sinks). The objective of the analysis,
2 Water closure is the reutilization of used water in the process to decrease fresh water consumption while internal energy recovery is the reutilization of internal energy in the process to decrease the steam consumption.
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B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729 Table 1 Mass balance on the steam network. Production (t/h)
Winter
RBI RBII BB MB
Fig. 2. Simplified diagram of a Kraft process.
which takes into account the dissolved solid, is to maximize the reutilization of water in the process (Fig. 4). The composite curves of the sources and sinks are included at the pinch point, for this study at 0 ppm. The minimum water requirement is 1000 m3/h and the minimum production of effluents is 875 m3/h. The proposed measures are the reutilization of condensates from the evaporators, bleaching effluents, water from drying section and vacuum pumps sealing water. The consumption of water and steam is reduced to 540 m3/h and 14 MW, respectively. Water closure affects the process energy balance and consequently that of internal energy recovery. Thermal pinch analysis [15] is done to characterize the thermal profiles of the process. This analysis is constructed in the form of a diagram of temperature vs. enthalpy. This diagram is composed of cold and hot composite curves. The results of the analysis are the minimum energy requirement (MER) and the pinch point. The minimum heat requirement (MHR) is 95.8 MW and minimum cooling requirement (MCH) is 22.1 MW with a pinch point at 57 C corresponding to a minimum temperature difference of 10 C [16]. The proposed economically feasible measures are recovery of the extra heat from the stack gases of the boilers and from the bleaching effluents. The recovered heat replaces the steam used to heat black liquor, boiler feed water, and mill fresh water [17]. Steam consumption is reduced to 20.9 MW. They have also proposed installation of a new heat pump in the ClO2 department to reduce MHR, resulting in an extra 3.6 MW reduction in process steam [11]. Globally, there is a 40.8 MW reduction in steam consumption, equivalent to 23% of present consumption. Therefore, the bunker oil boiler can be permanently closed and 15.8 MW of the produced vapor can be used to generate electricity to district heating.
Summer
88.3 56.4 63 30
86.1 56.7 54.7 11
Total HP
237.7
208.5
Utilization (t/h)
Winter
Summer
HP to MP Water to PRV2 Total MP HP to LP HP to TS Water to PRV1 Total LP HP process
70.9 16 86.9 110.7 24 21 155.7 32
65 13 78 88 24 19 130 32
2.3. Methodology The study by Cakembergh-Mas et al. has been done before the detailed analysis of Mateos-Espejel et al. In the present work additional assumptions are made, namely on the steam economy and the available steam for cogeneration and export and also removal of PRVs between MP and LP lines. Moreover, the plan to change the bunker oil and biomass boilers has evolved. The principal conclusions of utilizing a condensing turbine and a back pressure turbine remain the same. Therefore, this work presents A broader study by combining the results of the two previous works. CADSIM Plus is used to model the cogeneration plant. The model consists of two recovery boilers, a new biomass boiler, two turbines (condensing CST and back pressure BPST) and deaerator, as shown in Fig. 5. The feed water for the boilers (138 C) is prepared in the deaerator (not shown) where make up water, returned condensates of CST and LP steam are mixed together. The recovery boilers RB1 and RB2 produce the HP steam for the process and the excess steam is fed to the BPST to generate electricity, MP steam and some LP steam. The steam from the NBB is sent directly to CST to generate electricity and the remaining LP steam. The CADSIM model results for the generated electricity were validated by comparing the results with the data of an existing cogeneration plant in a mill in Western Canada. The specification of the turbines (as isentropic and mechanical efficiencies) and that of boilers are set according to Ref. [20].
Table 2 Simulation parameters and results of the previous of Cakembergh-Mas et al. Fuel
Cost (US$/t)
Caloric value (kJ/kg)
BL Biomass Bunker oil
0 18 333
14,700 20,000 42,000
Boilers
Efficiency (%)
MB RBI RBII NBB Cost analysis
86 53 53 70 I (M$)
Steam (t/h) max.
0.5 10 10 20
35 99 65 140
Power (MW) Min.
Fig. 3. Steam network.
Best configuration (BPT þ CT)
15.9
Fuel (t/h) Min.
Min.
28
0.5 10 10 20 PBT
Max. 36.3
0.91
B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729
Fig. 6. CPU analysis for the process.
Fig. 4. Water composite curves [9].
3. Results and discussion The new subjects that are covered in this study are: Consideration of the final and complete economic aspects of energy measures Adjustment of pressure levels of the process steam Optimal selection of the new biomass boiler Sale of steam for district heating to a city close to the mill Dynamics of the cogeneration unit Implementation of an adsorption heat pump (trigeneration) Complete removal of PRVs for the production of LP The cost analysis is also revised based on these new conditions. The following sections discuss the results of the above subjects.
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3.2. Optimization of steam network pressure The power generated by a turbine mainly results from the pressure difference between inlet and discharge. The inlet pressure is the pressure of the new biomass boiler. The pressure drop can be increased by decreasing the pressure levels in the steam network, which is presently 345 kPa. To determine the minimum level of pressure that is compatible with the steam consuming equipments, a combined analysis of utility systems (CPU) is used by Mateos et al. [12] and Marinova et al. [18]. This analysis takes into account the process temperatures and process pressure levels and the objective to obtain the minimum pressure level at which the process operates well by maximizing the power generation. The results of this analysis are illustrated in Fig. 6, which shows a new LP steam level of 220 kPa and the utilization of MP steam instead of HP in the pulp dryer. The corresponding increase in the power generation is represented by the surface area between the two graphs.
3.1. Updating the reduction on the process steam demand Reducing the steam demand to 40.8 MW allows the bunker oil boiler to be shut down. However, the process needs 16.2 MW (during summer) and 31.3 MW (during winter), more steam. This can be provided by a new biomass boiler. The higher capacity of this boiler of very high pressure (8825 kPa, 482 C) allows two cogeneration units, as recommended for maximum sale of electricity to the grid.
3.3. New biomass boiler Implementation of the new biomass boiler (NBB) with the specifications proposed by Cakembergh-Mas et al. was revised in the context of a 23% reduction in the process energy demand. Therefore, it is possible to produce a maximum of 44.5 MW of electricity. It should be noted that about 69% of the vapor production capacity of the NBB is used to generate electricity. An interesting result of this study is the opportunity to install a boiler of lower capacity. If 35 MW was used for power generation as the goal in the CADSIM model, a corresponding capacity of 116 t/ h for the boiler is required, in other words, a boiler of lower capacity compared to the previous results (140 t/h, Cakembergh-Mas et al.) is sufficient to generate 35 MW of electricity based on the new energy saving measures.
Table 3 Possible options in power generation.
Fig. 5. Proposed configuration of the cogeneration plant modeled on CADSIM Plus.
Configuration
Power, MW
1. 2. 3. 4. 5.
14.4 30.7 36.3 35 44.5
Present configuration NBB (140 t/h) NBB (140 t/h), configuration a NBB (116 t/h), configuration b NBB (140 t/h), configuration b
Configuration a, 15% reduction in steam demand, configuration; b, 23% reduction in steam demand, adjustment of pressure level, and implementation of absorption heat pump.
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B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729
Fig. 7. Variations of electricity generation during start-ups.
3.4. Factors contributing to electricity generation Many factors contribute to the generation of electricity, namely removal of pressure reduction valves (PRV), decrease in turbine discharge pressure, steam production capacity of the NBB, and reductions in energy demand. Table 3 illustrates the weight for each factor. The table shows that replacing the PRVs with a turbine generates considerable electricity, which is highly profitable (option 1). Option 2 illustrates the effect of increasing the available steam. The other three options show the effect of different reductions of steam demand and optimization of pressure level in the steam distribution system. For each option, there is room to maximize the effect of these factors to increase profit. 3.5. Dynamic study during start-ups During start-ups, a considerable amount of steam is needed to heat the piping and equipment and to reach the steady state operation. Fig. 7 shows the generation of electricity as the turbines are put into service during start-ups. The procedure is based on the following hypothesis: at time zero, only one part of the condensing turbine is used because HP is needed for the process. At 50% startup time, HP and MP are available and at 70%, consumption of LP begins. 3.6. Cost analysis A cost analysis for the case of maximum production of 44.5 MW of electricity is performed. The simple payback time PBT method is used. All the costs are indexed in 2006. Payback time is defined by:
PBT ¼ I=½ðP þ SÞ OC where I is investment, P is profits, S is savings, and OC is operating costs per year. Operating costs include the cost of biomass and electricity. Electricity cost is based on the average cost listed in the plant bills for 2006. Electricity is sold at a price of 90 US$/MWh, which is greater than that assumed in Cakembergh-Mas et al. The steam cost for the biomass boiler is 3.3 US$/t and is an average value provided by the mill. The cost of installed turbines is based on the methodology of Seider et al. [19]. The cost of a new biomass boiler is not Table 4 Cost analysis for cogeneration alone (option A) and trigeneration (option B).
Power generated (MW) Investment (CSTþBPST) (M$) Operating costs (M$) PBT (year)
Option A
Option B
43.9 14.2 15.9 0.8
44.5 17 16 0.9
Fig. 8. Change of PBT with the selling price of electricity and with the cost of biomass steam (Cbio).
taken into account because the mill is obligated to buy it. The results for two options, namely cogeneration alone (option A) and trigeneration unit (option B) are summarized in Table 4. PBT of the current study, 0.8e0.9 years, is in the same range as those of Cakembergh-Mas et al.; however, this PBT is much less than 2e6 years, reported in the literature [21,22]. One principal reason for this difference is that the cost of the boiler (about 69 M$) is not considered in the analysis because of the special situation of the mill under study. The mill envisages to replace the biomass boiler with a new high efficiency one irrespective of the installation of the cogeneration plant. If this term was considered, for example, for option B of Table 4, the new PBT would be PBTnew ¼ Inew/ [(P þ S) OC] ¼ (Inew/I) PBT ¼ (69 þ 17)/17 PBT ¼ 86/17 0.9 ¼ 4.5 years, which is in the same range as reported in the literature. A sensitivity analysis is done to determine the effect of the cost factors on PBT. This analysis is based on a 50% change in the selling price of electricity (from 60US$/MWh to 130US$/MWh) and a 50% to þ100% in the cost of biomass steam (from 2.1US$/t to 6.3US$/t). According to Fig. 8 when the selling price of electricity increases from 60US$ to 130US$ there is a sharp decrease in PBT from 2.3 to 0.5 years. The figure further shows that PBT is not affected significantly by the cost of biomass steam. Therefore, the implementation of cogeneration is very attractive with a PBT of 0.8e0.9 year. 4. Conclusion Implementation of cogeneration in the pulp and paper industry is technically and economically favorable. The optimization of the steam network and sensitivity analysis of the variables affecting profit is essential to determine the favorable conditions of cogeneration. Various energy saving measures and optimization of pressure levels increase power generation. For the case presented, the capacity of the new biomass boiler (140 t/h) is more than the actual need; however, the required investment is compensated by the sale of more electricity to the grid. The results indicate maximum generation of 44.5 MW of electricity. The implementation of a double cogeneration unit and reductions in steam demand permit permanent shut down of the bunker oil boiler to be closed. Acknowledgements This study was supported by the collaborative R & D grant of the Natural Sciences and Engineering Research Council of Canada. The authors would like to thank Aurel Systems Inc. for providing the
B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729
license for CADSIM Plus simulation software. Mateos-Espejel acknowledges the Mexican Science and technology council (CONACYT) for his PhD scholarship. The comments of Dr. Louis Fradette during the development of the model on CADSIM Plus, were helpful. Thanks to the mill personnel, who provided the data, and to the other industrial partners of CRD project. References [1] R. Budin, M.A. Bogdanic, E. Vujasinovic, Cogeneration and heat recovery in the industrial process, Chemistry in Industry 56 (2007) 551e555. [2] Yin H, Qui R, and Luo X. International congress on energy and environment: Shangai, China; 2003. [3] W.K. Tien, R.H. Yeh, J.M. Hong, Theoretical analysis of cogeneration systems for ships, Energy Conversion and Management 48 (2007) 1965e1974. [4] CIPEC, 2007 Annual report, Canadian Industry Program for Energy Conservation. Natural Resources Canada, Ottawa, 2007. [5] C. Strickland, J. Nyboer, A Review of Existing Cogeneration Facilities in Canada. Canadian Industrial Energy End-Use Data and Analysis Center. British Columbia, Vancouver, 2004. [6] T.C. Browne, in: Réduction des coûts énergétiques dans l’indurstrie des pâtes et papiers, first ed. PAPRICAN, Montreal, Canada, 1999. [7] Cakembergh-Mas A. Modélisation Stratégique de la Gestion Énergétique d’une usine de Pâte Kraft. M.Sc. thesis, École Polytechnique de Montréal; 2007. [8] A. Cakembergh-Mas, M. Trépanier, J. Paris, Strategic simulation of energy management in a Kraft mill, Energy Conversion and Management 51 (2009) 988e997. [9] Mateos-Espejel E, Marinova M, Bararpour S, Paris J. Energy implications of water reduction strategies in Kraft process. Part I: methodology. In: PAPTAC 94th Ann. Mtg., Montreal, QC; 2008. [10] Mateos-Espejel E, Marinova M, Bararpour S, Paris J. Energy implications of water reduction strategies in Kraft process. Part II: results. In: PAPTAC 94th Ann. Mtg. Montreal, Qc; 2008.
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[11] Mateos-Espejel E, Marinova M, Mostajeran-Goortani B, Paris J. Eco-industrial cluster centered on a Kraft pulp mill. In: World Congress of Chemical Engineering (WCCE08), Montreal, QC; 2009. [12] Mateos-Espejel E, Marinova M, Maréchal F, Paris J. Analyse énergétique d’évaporateurs para la méthode de la double représentation. in VIIIéme Colloque Interuniversitaire Franco-Québecois sur la Thermique des Systèmes. Montréal, QC, Canada; 2007. [13] G.A. Smook, in: Handbook for pulp & paper technologists, 3rd ed. Angus Wild Publication Inc., Vancouver, Canada, 2002. [14] V.R. Dhole, in: U.S. Patent (Ed.), Fluid efficiency, Linnhoff March Limited, United States, 1998. [15] B. Linnhoff, D.W. Townsend, G.F.H.D. Boland, B.E.A. Thomas, A.R. Guy, R.H. Marsland, in: A user guide on process integration for the efficient use of energy, 2nd ed. Institution of Chemical Engineers, Rugby, UK, 1994. [16] L.E. Savulescu, A. Alva-Argaez, Direct heat transfer considerations for improving energy efficiency in pulp and paper Kraft mills, Energy 33 (2008) 1562e1571. [17] E. Mateos-Espejel, M. Marinova, D. Diamantis, L. Fradette, J. Paris, Strategy for converting a conventional Kraft pulp mill into a sustainable “Green” mill. in: A. Sayigh (Ed.), World Renewable Energy Congress (WRECX). Elsevier, Glasgow, UK, 2008, pp. 176e180. [18] Marinova M, Mateos E, Bakhtiari B, Paris J. A new methodology for the implementation of trigeneration in industry: application to the Kraft process. In: First European conference on polygeneration: technologies and application, Tarragona, Spain; 2007. [19] W. Seider, J. Seader, D. Lewin, Product and process design principles. John Wiley and Sons, New York, 1999. [20] P.B. Meherwan, in: Handbook for cogeneration and combined cycle power plants, 2nd ed. ASME, 2002. [21] A. Adahl, S. Harvey, Energy efficiency investments in Kraft pulp mills given uncertain climate policy, International Journal of Energy Research 31 (2007) 486e505. [22] D.K. Jain, J.C. Mora, Cogeneration potential in the Indian pulp and paper industries, Energy 17 (12) (1992) 1249e1254.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Integration of a cogeneration unit into a kraft pulping process Behnam Mostajeran Goortani*,1, Enrique Mateos-Espejel, Jean Paris Department of Chemical Engineering, Ecole Polytechnique de Montreal, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2010 Accepted 23 July 2010 Available online 3 August 2010
The integration of cogeneration with other measures that impact the power production capacity in a Canadian Kraft pulping mill is studied. Those measures are removal of pressure reduction valves, adjustment of the steam pressure level, biomass boiler capacity, and reduction in process energy demand. CADSIM Plus software is used to simulate the cogeneration plant. The dynamic behavior of the process during start-up and its effect on electricity generation are also considered. It is shown that by replacing the PRVs with turbines, 14.4 MW of power can be generated. Moreover, by implementing cogeneration units and process measures to recover 23% of internal energy, 44.5 MW of electricity can be generated in addition to shutting down the existing bunker oil boiler. Therefore, implementation of cogeneration in the pulp and paper industry is technically possible and it offers significant economic advantages. A cost analysis of the complete project gives a simple payback time of less than a year. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cogeneration Kraft process Condensing turbine Back pressure turbine Steam network
1. Introduction Cogeneration is the simultaneous production of heat and power. This concept has been used for a long time in various industries [1e3], and can be visualized by comparing the configuration of a typical steam power cycle (Fig. 1(a)) with that of a steam plant in a pulp and paper mill (Fig. 1(b)). The difference between the two configurations is that in Fig. 1(a) a significant amount of the exergy of the HP steam is changed into useful work, while in Fig. 1(b) this exergy is lost in the pressure reduction valves PRV (PRV1 and PRV2). Therefore, if a turbine is installed in the HP steam line to replace the PRVs this available energy can be efficiently converted into electricity and the remaining heating energy of the vapor can be used in the process as shown in Fig. 1(c). The pulp and paper (P&P) industry is the highest energy consumer in Canada (about 33% of the total consumption) and represents a high potential for cogeneration [4,5]. About 60% of the energy demand of this industry is provided by biomass. However, a significant portion is still obtained by the combustion of fossil fuels. The development of technologies with high energy performance is an attractive means to reduce the consumption of fossil fuels [6]. Power production and efficiency enhancements are interdependent approaches.
* Corresponding author. E-mail address: [email protected] (B.M. Goortani). 1 Present address: Waterloo University, Canada. 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.07.026
The objective of this study is to study the implementation of cogeneration in a Kraft process. Different scenarios of vapor production and consumption, the start-up and its effect on power generation are considered. The work is based on two previous studies on the optimization of cogeneration [7] and the implementation of energy measures for a mill. Cakembergh-Mas et al. have developed an optimization algorithm that considers the effect of energy reduction on cogeneration. The authors used the algorithm to study the impact of economic factors. Mateos et al. did a pinch analysis based on water and energy. They identified measures for recovery of internal energy and reutilization of water with the objective of increasing the power potential. The produced data in this work are new and original. To the knowledge of the authors, in the literature there is no parametric study about the effect of various process parameters on cogeneration. The new aspects of this work are the identification of the effect of the following parameters on cogeneration: removal of pressure reduction valves (PRVs), adjustment of the steam pressure level, reduction in process energy demand and variations in the electricity generation during plant start-ups. The final results on the electricity generation (44.5 MW for winter and 40.9 MW for summer) are more attractive than the results of the previous study of Cakembergh-Mas et al. To make the installation of cogeneration plant economically feasible, a boiler with a capacity of 116 t/h is shown to be sufficient. This capacity is more attractive and is lower than the capacity of 140 t/h which is envisaged in the previous study. The paper is organized as follows: Section 1 is the introduction. Section 2 presents the context of the work, previous studies done
B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729
Nomenclature BB BPT CT G HP I LP MB MCR MER MHR
Biomass boiler Back pressure turbine Condensing turbine Generator High pressure steam Investment Low pressure steam Bunker oil boiler Minimum cooling requirement Minimum energy requirement Minimum heating requirement
MP M$ NBB OC PBT P PRV T TS RB S W
2725
Medium pressure steam Millions US$ New biomass boiler Operating costs Payback time Annual profit Pressure reduction valve Turbine Paper machine turbine Recovery boiler Summer Winter
Fig. 1. (a) Steam power cycle. (b) Simplified flow diagram of the steam plant in a pulp and paper mill. (c) Cogeneration plant (combination of a and b).
and the methodology. Section 3 presents the results and discussion and the Section 4 is the conclusion. 2. Context of the work and methodology 2.1. Mill specifications The mill under study produces about 700 adt/y of bleached Kraft pulp. A simplified diagram of the process is shown in Fig. 2. Wood chips are first sent to a digester, where the lignin (the binding material of the cellulose fibers) is dissolved [13]. From the resulting solution, black liquor, a by-product, is separated and the resulting pulp is bleached dried and finally formed into pulp sheets. The black liquor, is first concentrated in a series of multiple effect evaporators and then sent to recovery boilers, where it is burned to produce heat. The oxidized chemicals are regenerated in recaustisizers and a lime kiln. The regenerated chemicals are finally returned to the digester. The steam production and distribution system of the process is shown in Fig. 3. High pressure steam (HP: 3100 kPa, 371 C) is produced by two recovery boilers (RBI and RBII), one biomass boiler, and one bunker oil boiler (MB). The process uses 32 t/h of HP directly and the remaining steam is depressurized via pressure reduction valves to obtain medium pressure steam, MP (965 kPa, 179 C) and low pressure steam, LP (345 kPa, 143.5 C). A turbine (TS) fed by HP supplies the power requirements of the drying and sheet forming equipment. The total steam production consists of 65% from recovery boilers, 25% from biomass, and the remaining 10% from the bunker oil. The amount of HP production and the consumptions of the three steam levels are listed in Table 1. 2.2. Previous work 2.2.1. Cogeneration Cakembergh-Mas et al. [8] developed an optimization algorithm (MILP) to analyze the profitability of the power house. Several scenarios were studied, including the reduction of steam consumption to 15%, district heating, and type and arrangements of turbines (back pressure BPT and condensing CST turbines). The
decision variables considered are fuel flow, amount of steam produced by each boiler, and sale or purchase of electricity. The constraints are the production capacity of the boilers and the available fuel, various factors influencing process steam demand and electricity, and mass and energy balance on the turbines and boilers. The main simulation parameters and the results are given in Table 2. The additional considerations for that study are the replacement of the bark boilers (BB) and bunker oil boilers (MB) by a new biomass boiler with a production capacity of 140 t/h. This new boiler was selected based on the fact that the present bark boiler efficiency is very low (43%) compared to the efficiency of new industrial bark boilers. The integration of this boiler makes the mill independent from the present bunker oil boiler and allows implementation of a cogeneration unit. The results show that the most effective economic results are obtained by installing a BPT and a CT and implementing energy measures and district heating. In their configuration, PRVs from HP to LP are removed; however, the configuration includes PRVs for the production of LP steam from MP steam. The average and maximum generated power are 28 MW and 36.3 MW, respectively. The payback time (PBT) is 0.9 year. The index costs are for 2006. A sensitivity analysis shows that the PBT depends strongly on the selling price of electricity and the cost of biomass. 2.2.2. Energy efficiency measures The proposed energy efficiency measures are water closure and internal energy recovery.2 The objective is to optimize the amount of steam consumption, to close the bunker oil boiler, and to use the excess steam for power generation. A new methodology is used to analyze the impacts of water and steam reductions [9,10]. Water pinch analysis yields the potential economic savings [14]. This analysis is a balance between the water effluents of the process that can be reused in the process (sources) and process water demand (sinks). The objective of the analysis,
2 Water closure is the reutilization of used water in the process to decrease fresh water consumption while internal energy recovery is the reutilization of internal energy in the process to decrease the steam consumption.
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B.M. Goortani et al. / Applied Thermal Engineering 30 (2010) 2724e2729 Table 1 Mass balance on the steam network. Production (t/h)
Winter
RBI RBII BB MB
Fig. 2. Simplified diagram of a Kraft process.
which takes into account the dissolved solid, is to maximize the reutilization of water in the process (Fig. 4). The composite curves of the sources and sinks are included at the pinch point, for this study at 0 ppm. The minimum water requirement is 1000 m3/h and the minimum production of effluents is 875 m3/h. The proposed measures are the reutilization of condensates from the evaporators, bleaching effluents, water from drying section and vacuum pumps sealing water. The consumption of water and steam is reduced to 540 m3/h and 14 MW, respectively. Water closure affects the process energy balance and consequently that of internal energy recovery. Thermal pinch analysis [15] is done to characterize the thermal profiles of the process. This analysis is constructed in the form of a diagram of temperature vs. enthalpy. This diagram is composed of cold and hot composite curves. The results of the analysis are the minimum energy requirement (MER) and the pinch point. The minimum heat requirement (MHR) is 95.8 MW and minimum cooling requirement (MCH) is 22.1 MW with a pinch point at 57 C corresponding to a minimum temperature difference of 10 C [16]. The proposed economically feasible measures are recovery of the extra heat from the stack gases of the boilers and from the bleaching effluents. The recovered heat replaces the steam used to heat black liquor, boiler feed water, and mill fresh water [17]. Steam consumption is reduced to 20.9 MW. They have also proposed installation of a new heat pump in the ClO2 department to reduce MHR, resulting in an extra 3.6 MW reduction in process steam [11]. Globally, there is a 40.8 MW reduction in steam consumption, equivalent to 23% of present consumption. Therefore, the bunker oil boiler can be permanently closed and 15.8 MW of the produced vapor can be used to generate electricity to district heating.
Summer
88.3 56.4 63 30
86.1 56.7 54.7 11
Total HP
237.7
208.5
Utilization (t/h)
Winter
Summer
HP to MP Water to PRV2 Total MP HP to LP HP to TS Water to PRV1 Total LP HP process
70.9 16 86.9 110.7 24 21 155.7 32
65 13 78 88 24 19 130 32
2.3. Methodology The study by Cakembergh-Mas et al. has been done before the detailed analysis of Mateos-Espejel et al. In the present work additional assumptions are made, namely on the steam economy and the available steam for cogeneration and export and also removal of PRVs between MP and LP lines. Moreover, the plan to change the bunker oil and biomass boilers has evolved. The principal conclusions of utilizing a condensing turbine and a back pressure turbine remain the same. Therefore, this work presents A broader study by combining the results of the two previous works. CADSIM Plus is used to model the cogeneration plant. The model consists of two recovery boilers, a new biomass boiler, two turbines (condensing CST and back pressure BPST) and deaerator, as shown in Fig. 5. The feed water for the boilers (138 C) is prepared in the deaerator (not shown) where make up water, returned condensates of CST and LP steam are mixed together. The recovery boilers RB1 and RB2 produce the HP steam for the process and the excess steam is fed to the BPST to generate electricity, MP steam and some LP steam. The steam from the NBB is sent directly to CST to generate electricity and the remaining LP steam. The CADSIM model results for the generated electricity were validated by comparing the results with the data of an existing cogeneration plant in a mill in Western Canada. The specification of the turbines (as isentropic and mechanical efficiencies) and that of boilers are set according to Ref. [20].
Table 2 Simulation parameters and results of the previous of Cakembergh-Mas et al. Fuel
Cost (US$/t)
Caloric value (kJ/kg)
BL Biomass Bunker oil
0 18 333
14,700 20,000 42,000
Boilers
Efficiency (%)
MB RBI RBII NBB Cost analysis
86 53 53 70 I (M$)
Steam (t/h) max.
0.5 10 10 20
35 99 65 140
Power (MW) Min.
Fig. 3. Steam network.
Best configuration (BPT þ CT)
15.9
Fuel (t/h) Min.
Min.
28
0.5 10 10 20 PBT
Max. 36.3
0.91
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Fig. 6. CPU analysis for the process.
Fig. 4. Water composite curves [9].
3. Results and discussion The new subjects that are covered in this study are: Consideration of the final and complete economic aspects of energy measures Adjustment of pressure levels of the process steam Optimal selection of the new biomass boiler Sale of steam for district heating to a city close to the mill Dynamics of the cogeneration unit Implementation of an adsorption heat pump (trigeneration) Complete removal of PRVs for the production of LP The cost analysis is also revised based on these new conditions. The following sections discuss the results of the above subjects.
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3.2. Optimization of steam network pressure The power generated by a turbine mainly results from the pressure difference between inlet and discharge. The inlet pressure is the pressure of the new biomass boiler. The pressure drop can be increased by decreasing the pressure levels in the steam network, which is presently 345 kPa. To determine the minimum level of pressure that is compatible with the steam consuming equipments, a combined analysis of utility systems (CPU) is used by Mateos et al. [12] and Marinova et al. [18]. This analysis takes into account the process temperatures and process pressure levels and the objective to obtain the minimum pressure level at which the process operates well by maximizing the power generation. The results of this analysis are illustrated in Fig. 6, which shows a new LP steam level of 220 kPa and the utilization of MP steam instead of HP in the pulp dryer. The corresponding increase in the power generation is represented by the surface area between the two graphs.
3.1. Updating the reduction on the process steam demand Reducing the steam demand to 40.8 MW allows the bunker oil boiler to be shut down. However, the process needs 16.2 MW (during summer) and 31.3 MW (during winter), more steam. This can be provided by a new biomass boiler. The higher capacity of this boiler of very high pressure (8825 kPa, 482 C) allows two cogeneration units, as recommended for maximum sale of electricity to the grid.
3.3. New biomass boiler Implementation of the new biomass boiler (NBB) with the specifications proposed by Cakembergh-Mas et al. was revised in the context of a 23% reduction in the process energy demand. Therefore, it is possible to produce a maximum of 44.5 MW of electricity. It should be noted that about 69% of the vapor production capacity of the NBB is used to generate electricity. An interesting result of this study is the opportunity to install a boiler of lower capacity. If 35 MW was used for power generation as the goal in the CADSIM model, a corresponding capacity of 116 t/ h for the boiler is required, in other words, a boiler of lower capacity compared to the previous results (140 t/h, Cakembergh-Mas et al.) is sufficient to generate 35 MW of electricity based on the new energy saving measures.
Table 3 Possible options in power generation.
Fig. 5. Proposed configuration of the cogeneration plant modeled on CADSIM Plus.
Configuration
Power, MW
1. 2. 3. 4. 5.
14.4 30.7 36.3 35 44.5
Present configuration NBB (140 t/h) NBB (140 t/h), configuration a NBB (116 t/h), configuration b NBB (140 t/h), configuration b
Configuration a, 15% reduction in steam demand, configuration; b, 23% reduction in steam demand, adjustment of pressure level, and implementation of absorption heat pump.
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Fig. 7. Variations of electricity generation during start-ups.
3.4. Factors contributing to electricity generation Many factors contribute to the generation of electricity, namely removal of pressure reduction valves (PRV), decrease in turbine discharge pressure, steam production capacity of the NBB, and reductions in energy demand. Table 3 illustrates the weight for each factor. The table shows that replacing the PRVs with a turbine generates considerable electricity, which is highly profitable (option 1). Option 2 illustrates the effect of increasing the available steam. The other three options show the effect of different reductions of steam demand and optimization of pressure level in the steam distribution system. For each option, there is room to maximize the effect of these factors to increase profit. 3.5. Dynamic study during start-ups During start-ups, a considerable amount of steam is needed to heat the piping and equipment and to reach the steady state operation. Fig. 7 shows the generation of electricity as the turbines are put into service during start-ups. The procedure is based on the following hypothesis: at time zero, only one part of the condensing turbine is used because HP is needed for the process. At 50% startup time, HP and MP are available and at 70%, consumption of LP begins. 3.6. Cost analysis A cost analysis for the case of maximum production of 44.5 MW of electricity is performed. The simple payback time PBT method is used. All the costs are indexed in 2006. Payback time is defined by:
PBT ¼ I=½ðP þ SÞ OC where I is investment, P is profits, S is savings, and OC is operating costs per year. Operating costs include the cost of biomass and electricity. Electricity cost is based on the average cost listed in the plant bills for 2006. Electricity is sold at a price of 90 US$/MWh, which is greater than that assumed in Cakembergh-Mas et al. The steam cost for the biomass boiler is 3.3 US$/t and is an average value provided by the mill. The cost of installed turbines is based on the methodology of Seider et al. [19]. The cost of a new biomass boiler is not Table 4 Cost analysis for cogeneration alone (option A) and trigeneration (option B).
Power generated (MW) Investment (CSTþBPST) (M$) Operating costs (M$) PBT (year)
Option A
Option B
43.9 14.2 15.9 0.8
44.5 17 16 0.9
Fig. 8. Change of PBT with the selling price of electricity and with the cost of biomass steam (Cbio).
taken into account because the mill is obligated to buy it. The results for two options, namely cogeneration alone (option A) and trigeneration unit (option B) are summarized in Table 4. PBT of the current study, 0.8e0.9 years, is in the same range as those of Cakembergh-Mas et al.; however, this PBT is much less than 2e6 years, reported in the literature [21,22]. One principal reason for this difference is that the cost of the boiler (about 69 M$) is not considered in the analysis because of the special situation of the mill under study. The mill envisages to replace the biomass boiler with a new high efficiency one irrespective of the installation of the cogeneration plant. If this term was considered, for example, for option B of Table 4, the new PBT would be PBTnew ¼ Inew/ [(P þ S) OC] ¼ (Inew/I) PBT ¼ (69 þ 17)/17 PBT ¼ 86/17 0.9 ¼ 4.5 years, which is in the same range as reported in the literature. A sensitivity analysis is done to determine the effect of the cost factors on PBT. This analysis is based on a 50% change in the selling price of electricity (from 60US$/MWh to 130US$/MWh) and a 50% to þ100% in the cost of biomass steam (from 2.1US$/t to 6.3US$/t). According to Fig. 8 when the selling price of electricity increases from 60US$ to 130US$ there is a sharp decrease in PBT from 2.3 to 0.5 years. The figure further shows that PBT is not affected significantly by the cost of biomass steam. Therefore, the implementation of cogeneration is very attractive with a PBT of 0.8e0.9 year. 4. Conclusion Implementation of cogeneration in the pulp and paper industry is technically and economically favorable. The optimization of the steam network and sensitivity analysis of the variables affecting profit is essential to determine the favorable conditions of cogeneration. Various energy saving measures and optimization of pressure levels increase power generation. For the case presented, the capacity of the new biomass boiler (140 t/h) is more than the actual need; however, the required investment is compensated by the sale of more electricity to the grid. The results indicate maximum generation of 44.5 MW of electricity. The implementation of a double cogeneration unit and reductions in steam demand permit permanent shut down of the bunker oil boiler to be closed. Acknowledgements This study was supported by the collaborative R & D grant of the Natural Sciences and Engineering Research Council of Canada. The authors would like to thank Aurel Systems Inc. for providing the
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license for CADSIM Plus simulation software. Mateos-Espejel acknowledges the Mexican Science and technology council (CONACYT) for his PhD scholarship. The comments of Dr. Louis Fradette during the development of the model on CADSIM Plus, were helpful. Thanks to the mill personnel, who provided the data, and to the other industrial partners of CRD project. References [1] R. Budin, M.A. Bogdanic, E. Vujasinovic, Cogeneration and heat recovery in the industrial process, Chemistry in Industry 56 (2007) 551e555. [2] Yin H, Qui R, and Luo X. International congress on energy and environment: Shangai, China; 2003. [3] W.K. Tien, R.H. Yeh, J.M. Hong, Theoretical analysis of cogeneration systems for ships, Energy Conversion and Management 48 (2007) 1965e1974. [4] CIPEC, 2007 Annual report, Canadian Industry Program for Energy Conservation. Natural Resources Canada, Ottawa, 2007. [5] C. Strickland, J. Nyboer, A Review of Existing Cogeneration Facilities in Canada. Canadian Industrial Energy End-Use Data and Analysis Center. British Columbia, Vancouver, 2004. [6] T.C. Browne, in: Réduction des coûts énergétiques dans l’indurstrie des pâtes et papiers, first ed. PAPRICAN, Montreal, Canada, 1999. [7] Cakembergh-Mas A. Modélisation Stratégique de la Gestion Énergétique d’une usine de Pâte Kraft. M.Sc. thesis, École Polytechnique de Montréal; 2007. [8] A. Cakembergh-Mas, M. Trépanier, J. Paris, Strategic simulation of energy management in a Kraft mill, Energy Conversion and Management 51 (2009) 988e997. [9] Mateos-Espejel E, Marinova M, Bararpour S, Paris J. Energy implications of water reduction strategies in Kraft process. Part I: methodology. In: PAPTAC 94th Ann. Mtg., Montreal, QC; 2008. [10] Mateos-Espejel E, Marinova M, Bararpour S, Paris J. Energy implications of water reduction strategies in Kraft process. Part II: results. In: PAPTAC 94th Ann. Mtg. Montreal, Qc; 2008.
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