Absorption cooling An analysis of the competition between industrial excess heat, waste incineration, biofuelled

Absorption cooling An analysis of the competition between industrial excess heat, waste incineration, biofuelled CHP and NGCC. Inger-Lise Svensson 1, Bahram Moshfegh 2 1,2 Energy Systems, Department of Management and Engineering, Linköping University, S-581 83 Linköping, Sweden 1 PhD student, Inger-Lise.Svensson@liu.se, +46(0)13-282767 (corresponding author) 2 Professor, Bahram.Moshfegh@liu.se, +46(0)13-281158 ABSTRACT: Utilization of excess heat from kraft pulp mills can improve the energy efficiency of a system, but the excess heat may be used either for internal purposes in the mill or externally, as district heating. Previous studies have shown that the trade-off between internal and external use of excess heat from kraft pulp mills depends mainly on energy prices and the demand for heat. The aim of this study is to investigate how the trade-off between the investment options can be altered when the option to produce district cooling through absorption cooling is introduced. The influence of absorption cooling on the trade-off between internal or external use of excess heat is investigated through modeling the pulp mill and the Energy Company (ECO) using the energy systems modeling tool MIND. To obtain a broader system perspective, the kraft pulp mill and ECO are modeled within the same system boundaries. The results of the optimizations show that absorption cooling mainly benefits investments in bio-fuelled CHP, but only influence the trade-off between internal or external use of excess heat when the conditions for CHP are unfavorable, for instance when the price of bio-fuels is high and the price of electricity certificates is low. Compared to the baseline scenarios the same investments were profitable, but the increased heat demand created a possibility for more electricity production which not only increases the system revenue but also decreases the CO 2 emissions of the system. Keywords: Absorption cooling, Industrial excess heat, CHP, Energy system optimization

1. INTRODUCTION With growing concern for global warming, energy efficiency has become increasingly important. The Swedish pulp and paper industry is a major Swedish energy user and has potential for investments in new, efficient technology that can make excess heat available [1-3]. Industrial excess heat from kraft pulp mills can be utilized both internally in e.g. processintegrated evaporation and drying of pulp [2], and externally, to produce district heating or cooling through an absorption cooling process. In an Energy Company (ECO) it is possible to make several new investments, including utilisation of industrial excess heat. Examples of other possible investments are bio-fuelled combined heat and plants (CHP), natural gas combined cycle plants (NGCC) or waste-fuelled CHP. Gebremedhin [4] has shown that by expanding the system boundaries to include not only the ECO s own utilities but also the industry, existing plants and new investments can be utilised more cost effectively. This approach has been adopted in a previous study by the authors [5, 6] where it was concluded that the trade-off between using excess heat from a kraft pulp mill either externally, as district heating, or for internal measures depends on multiple factors. Among the most important factors are: the composition of the already-existing capacity in the district heating system, the size of the district heating system, prices of fuels and electricity and a number of policy instruments, including the electricity certificate system 1. In the study optimizations using future energy market scenarios indicated that using excess heat externally was more profitable in smaller district heating systems. This is most likely due to the fact that in larger district heating 1 The electricity certificate system is a policy instrument to increase the electricity production from renewable resources for example solar, wind, water and bio-fuel, and was introduced in 2003. systems, utilisation of excess heat competes with large bio-fuelled CHP plants with high electric efficiency. When introducing absorption cooling in a system, the trade-off between internal or external use of excess heat may be altered due to increased heat demand, especially in the summer. Trygg et al. [7] argues that, in spite of the low coefficient of performance (COP) of absorption chillers, absorption technology is a competitive alternative to compression cooling when heat with low production costs is available. Shadow prices for heat are at the lowest level in the summer when the need for cooling is as highest, which favors the absorption cooling process. The objective of this study is to investigate how the introduction of absorption cooling affects the trade-off between utilizing industrial excess for internal measures in a kraft pulp mill and/or to use the excess heat to produce district heating and cooling. The research questions addressed in this study are: How does the introduction of absorption chillers affect the economic potential for external use of industrial excess heat? How does the choice of use for the excess heat influence the system s global CO 2 -emissions? 2. METHODOLOGY The study has been conducted through optimizations using the energy systems modelling tool MIND (Method for analysis of INDustrial energy systems). The MIND method uses mixed integer linear programming to minimise the system cost [8]. To investigate the influence of absorption chillers on the trade-off between internal or external use of excess heat, the model is optimized for different boundary conditions, in this study modelled as energy market scenarios. Using future energy market scenarios, different energy efficiency investments can be evaluated

from an economic as well as an environmental point of view. In the text following, data concerning the model is displayed. For a more detailed description of the model see previous work by the authors [6]. 2.1. Model The studied energy system consists of two parts; a model of an average Scandinavian kraft pulp mill and a model of an ECO. The general structure of the model is displayed in Figure 1. Broken lines indicate investment options and solid lines already existing capacity in the system. Figure 1. The general structure of the model. The model of the kraft pulp mill includes the present energy balance along with technically possible investments in new, efficient technology. The model of the ECO consists of the existing heat, and cooling capacity in the system and possibilities for investments in new capacity, including the option to invest in an absorption cooling plant and to buy excess heat from the mill. The two parts are integrated and optimized as one system. Through optimizing the mill and the ECO within the same system boundary, the optimal solution from a common investment perspective is obtained. In the optimization the system cost for the total system consisting of both the kraft pulp mill and the ECO is minimized. The objective function includes the costs of investments, fuel costs and income from sales of electricity and received electricity certificates. 2.2. The mill The kraft pulp mill in the model is based on data from the national Swedish research programme Future Resource Adapted Pulp Mill (FRAM) and is a model of an average Scandinavian kraft pulp mill [9]. Using data from the FRAM mill, several possibilities for energy efficiency within the modelled mill have been identified [2, 10]. The suggested new investments are, e.g. process-integrated evaporation, a new conventional evaporation plant, an increase of the dry solid content, a retrofit of the hot and warm water system, steam savings at the wood yard and a new three-stage flash. Some of the new investments decrease the steam demand and therefore make more steam available. In this study the steam is assumed to be used to increase electricity production. To be able to increase electricity production, further investments in new turbines are needed. Some of the new investments generate excess heat of temperatures that are suitable for district heating and others, like process-integrated evaporation, use excess heat [2, 9]. The mill has the option of selling the excess heat to the ECO, but can also use it for internal purposes; hence there is a trade-off between using the excess heat internally or externally. In addition to the excess heat recovered through the previously mentioned measures it is possible to use heat pumps to increase the temperature of excess heat of lower temperatures and use this heat as district heating. Another possibility is flue gas heat recovery, the heat

recovered from the flue gases can be used either for internal processes or as district heating. 2.3. The ECO The modelled ECO is based on data from existing Swedish district heating systems located near kraft pulp mills [6]. The total heat demand and the different heat production plants for the studied district heating system are displayed in Table 1. Table 1. Existing heat production in the district heating system. Heating Top load (MW) * 483 * Heat demand (GWh) * 1651 * Waste CHP (GWh heat ) 616 Bio-fuel CHP (GWh heat ) 591 boiler (GWh) 115 Top load, oil boiler (GWh) 329 Cooling Cooling demand (GWh) 34 Compression cooling 34 * Information about the heat load was received from the local energy company Tekniska Verken in Linköping District cooling is not as common as district heating in Sweden, but there is a rising demand for cooling in, e.g. office buildings and industrial processes. The cooling demand in the model is based on a study made by Trygg et al. [7]. The cooling demand is assumed to originally be covered by compression chillers in the baseline scenarios. For the ECO there are several possible investment options, including new combined heat and plants, buying excess heat from the kraft pulp mill and absorption chillers. The investment options are described in Table 2. The capacities of the possible investments are variable, so that an optimal size of the investment may be chosen. Table 2. Investment options for new district heating and cooling capacity Heat production Excess heat 100 C (MW) * 0-13 Excess heat 40/60 C (MW) * 0-43 FGHR (MW] * 0-4 Bio-fuel CHP (MW heat ) ** 18.5-235 NGCC (MW heat ) ** 18.5-126 Cooling production Absorption cooling (MW cooling ) 0-5 * In accordance with [2]. COP of the heat pumps is 6.05 for 60 C and 3.63 for 40 C. ** In accordance with [11] Absorption chillers use excess heat to produce cooling. The COP of absorption chillers is low (about 0.7) compared to compression chillers (1-3) but the fact that they utilize heat instead of electricity makes them competitive with compression chillers when heat with low production cost is available. In this study the absorption chillers are assumed to have a COP of 0.7 and the compression chillers a COP of 3. Absorption chillers use a small amount of electricity for pumping (in the present study it is assumed to be negligible) and compared to compressions chillers the amount of electricity used can be reduced substantially [7]. A schematic overview of an absorption chiller s role in a district heating and cooling system is shown in Figure 2. Figure 2. An absorption chiller in an integrated district heating and cooling system. 2.4. Economic conditions The investment costs of the possible measures in the model are described in more detail in a previous article by the author [6].

In order to simulate and evaluate decisions for future investments, future energy market prices have been estimated using energy market scenarios. The energy market scenarios reflect four different combinations of level of fuel prices and CO 2 charge as can be seen in Table 3. Table 3. Input data for the energy market scenarios used, based on Axelsson et al. [12]. Future energy market scenario: 2006 1 2 3 4 Policy instruments CO 2 charge ( /tonne) 27 27 44 27 44 Electricity cert. ( /MWh el ) 22 16 5 16 5 Transport cert. ( /MWh fuel ) 0 0 0 0 39 Electricity price ( /MWh el ) 40 55 60 59 63 Fuel prices including CO 2 charge ( /MWh fuel ) Oil, EO5 40 29 34 41 46 Natural gas 36 26 30 34 38 15 15 20 16 21 Bio-fuel, chips 15 17 24 18 34 Waste a -21.8-21.8-21.8-21.8-21.8 CO 2 emissions b Bio-fuel (tonne/gwh fuel ) 329 329 329 329 159 Electricity (tonne/gwh el ) 779 374 136 723 136 Marginal technology for production NGCC with CCS with CCS Marginal technology for bio-fuel use a The price of waste is negative due to the fact that waste incineration plants are often paid to take care of waste from several other municipalities [11] b Well-to-gate from marginal use DME prod. The future energy market scenarios and prices from 2006 are applied to the model of the kraft pulp mill and the ECO. Each scenario has a corresponding baseline scenario where the cooling demand is covered by compression chillers. 3. RESULTS In all of the studied scenarios, all possible investments in energy efficiency in the kraft pulp mill that have no need for excess heat are economically profitable. These measures do not compete directly with the option of using the excess heat externally, but an external use of excess heat will lead to a decreased electricity production within the mill. The measures that use excess heat of the same quality as district heating and absorption chillers are influenced by the increased heat demand in the ECO, but since the heating demand for the absorption chillers is relatively small compared to the total heating demand, the trade-off between using excess heat internally or externally is not altered compared to the baseline scenarios except when the price of electricity and electricity certificates is too low to make increased CHP production profitable. 3.1. Use of excess heat The trade-off between using excess heat either internally or externally is in all cases but one not altered when introducing absorption cooling, compared to the baseline scenarios where only compression cooling was available. The only scenario where the increased heat demand is covered by excess heat is scenario 2, where the price of bio-fuel is high combined with a moderate electricity price and a low electricity certificate price, which makes biofuelled CHP less profitable than in the other scenarios. The fact that excess heat is used externally as district heating influences the investments in the mill that are dependent on access to excess heat, and less steam will be made available for electricity production. In scenarios 2006, 1, 3 and 4 the increased heat demand is mainly covered by a small increase of heat production in the waste CHP and the biofuelled CHP which does not affect the possible investments in new technology within the mill. The distribution of the utilities used to cover the heat demand in each scenario and the corresponding baseline scenarios are displayed in Figure 3.

GWh 1800 GWh 1200 1600 Oil boiler 1000 Baseline 1400 1200 boiler 800 600 1000 800 600 400 200 0 2006 2006 (b) 1 1 (b) 2 n 2 (b) Scenario 3 3 (b) 4 4(b) Excess heat Bio CHP New Bio CHP Waste CHP Figure 3. Heat supply in the district heating system. Scenarios marked with (b) distinguish the baseline scenarios. 3.2. Amount of electricity produced and used The amount of electricity produced in the system is increased in scenarios 2006, 1, 3 and 4 compared to the baseline scenarios. The increase is due to the larger heat demand which is covered by CHP plants in the ECO. In scenario 2 the increased heat demand is covered by excess heat which also replaces some of the heat produced in CHP plants in the baseline scenario, leading to an overall reduction of electricity production. This is due to the high price of bio-fuel and a high CO 2 charge which makes an increased use of excess heat as district heating more profitable. Some of the reduction in electricity production in scenario 2 is also related to the reduced electricity production in the mill. The production of electricity is displayed in Figure 4. 400 200 0 2006 1 2 3 4 Scenario With absorption cooling Figure 4. Electricity production in the system. The amount of electricity used in heat pumps and chillers is reduced in all cases except scenario 2 (see Figure 5), where the increased heat demand for absorption chillers is covered by excess heat. The excess heat is to some extent heat pumped to increase the temperature to a suitable level for district heating which increases the total use of electricity. 70 60 50 40 30 20 10 0 GWh 2006 1 2 3 4 Scenario Baseline With absorption cooling Figure 5. Use of electricity in heat pumps and chillers. 3.3. System revenue and CO 2 emissions The total system revenue increases or remains unchanged in all scenarios. The increased revenue is due to increased electricity production in the ECO, see Table 4. The magnitude of the system revenue increase depends on the given energy market scenario. For almost all of the studied scenarios the global CO 2 -emissions will decrease when implementing absorption cooling due to the fact that

absorption cooling creates a larger heat demand. The increased heat demand makes it possible to produce more electricity with CHP that can replace marginal electricity production, as is shown in Table 4. The exception in scenario 1 is due to the fact that bio-fuel is used to cover the increased heat demand. In scenario 1 bio-fuel is strained with rather large CO 2 emissions since the marginal users of bio-fuel are coal condensing plants [12]. Table 4. System revenue and CO 2 emissions. System revenue (M ) Baseline EE 2006 24.6 25.0 1 46.8 47.7 2 33.8 34.2 3 48.0 49.1 4 25.6 26.8 CO 2 emissions (ktonnes) Baseline EE L:2006-296.6-304.3 L:1 128.7 133.3 L:2 408.1 355.9 L:3-305.1-772.5 L:4 77.4 55.4 4. CONCLUDING DISCUSSION The results of the optimizations shows that implementation of absorption cooling is a profitable investment independent of which scenario is used to analyze the system. The increase in system revenue is related to the increase in heat demand caused by the absorption chillers. The larger heat demand makes it possible to produce more electricity with CHP, but the increase in electricity production is more favourable when the price of electricity is high and thus the greatest rise in electricity production is seen in scenarios 1, 3 and 4. The increased production benefits not only the system revenue but also the CO 2 emissions of the system. The CO 2 emissions are reduced compared to the baseline scenarios due to the reduced use of electricity in the compression chillers, but also as a result of the increased electricity production in the ECO which can replace marginal electricity production. The use of excess heat is influenced only to a smaller extent by the introduction of absorption cooling. The cooling demand is relatively small compared to the heating demand and in only one of the studied scenarios (scenario 2) is the use of excess heat altered compared to the baseline scenario. As long as the prices of electricity and electricity certificates are rather high, an increase of the CHP production is more beneficial than to cover the increased heat demand caused by the absorption chillers with excess heat. Introduction of absorption cooling in a district heating and cooling system benefits the CHP production in the system. Presently the cooling demand in Sweden is relatively small compared to the heating demand but with a rising demand for cooling in industries and public buildings, absorption cooling can help to reduce the CO 2 emissions. Acknowledgements The work has been carried out under the auspices of The Energy Systems Programme, which is primarily financed by the Swedish Energy Agency. REFERENCES [1] IEA. Process Integration in the Pulp and Paper Industry, Final report from Annex XIII. IEA Pulp and Paper, 2004. [2] Axelsson, E., M.R. Olsson, and T. Berntsson, Heat Integration Opportunities in Avarage Scandinavian Kraft Pulp Mills: Pinch Analysis of Model Mills. Nordic Pulp Paper Res. J. 2006: 4(21): 466-474. [3] Wising, U., Process Integration in Model Kraft Pulp Mills: Technical, Economical and Environmental Implications. Department of Energy

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