Techno-economic feasibility of high-temperature high-lift chemical heat pumps for upgrading industrial waste heat

Transcription

1 July 2002 ECN-RX Techno-economic feasibility of high-temperature high-lift chemical heat pumps for upgrading industrial waste heat S. Spoelstra W.G. Haije J.W. Dijkstra Accepted for publication in Applied Thermal Engineering A Date; final version B Made by: Approved by: Revisions S. Spoelstra Checked by: P.W. Bach Issued by: ECN Energy Efficiency in Industry W.G. Haije P.T. Alderliesten

2 Abstract This paper presents the results of a techno-economic feasibility study on two high-temperature high-lift chemical heat pumps for upgrading industrial waste heat. The study was set up in order to select the most promising heat pump concept for further development. First, a market study is performed to assess the amount of waste heat and the temperature dependence thereof. Based on the market potential, two heat pump concepts are selected for further detailed evaluation. These are an isopropanol heat pump and a salt/ammonia vapour heat pump. Both heat pump concepts are technically able to upgrade the waste heat to usable temperature levels. However, the salt/ammonia vapour heat pump has a much better technical performance. In addition, the economic analysis shows that this heat pump has a far better economic feasibility with a mean Internal Rate of Return of 14 %. Therefore, this heat pump is selected for further development. Keywords Industrial waste heat; Chemical heat pump; Isopropanol; Solid sorption; Feasibility study 2 ECN-RX

3 CONTENTS LIST OF TABLES 4 LIST OF FIGURES 4 1. INTRODUCTION 5 2. MARKET STUDY 7 3. CHEMICAL HEAT PUMPS 9 4. METHODOLOGY CONCEPTS STUDIED Isopropanol heat pump Salt/ammonia vapour heat pump EVALUATION/CONCLUSIONS ACKNOWLEDGEMENT REFERENCES 21 ECN-RX

4 LIST OF TABLES Table 5.1 Component results from ASPEN Plus for the isopropanol heat pump...14 Table 5.2 Total capital investment estimate for a 4 MW isopropanol heat pump...15 Table 5.3 Component results from the spreadsheet model for the salt/ammonia vapour heat pump...18 Table 5.4 Total capital investment estimate for a 5 MW salt/ammonia vapour heat pump...18 LIST OF FIGURES Figure 2.1 Cumulative waste heat in the refining and chemical industry in the Netherlands 7 Figure 5.1 Process flow diagram of the isopropanol/acetone heat pump 13 Figure 5.2 Vapour pressure as a function of temperature for Low (LTS) and High Temperature Salt (HTS). 16 Figure 5.3 Process flow diagram for a salt/vapour chemical heat pump 17 4 ECN-RX

5 1. INTRODUCTION In the 1990 s, the government policy in the Netherlands with respect to energy saving has been formulated with the objective of achieving an improvement of 33% in energy efficiency in the period from 1990 till The energy savings resulting from this should make an important contribution to the fulfilment of the Kyoto-protocol, as agreed upon in In order to reach this objective, an important contribution is expected from the energy intensive industry which is responsible for about one third of the total energy use in the Netherlands. Besides optimising and improving the industrial process itself, large quantities of energy can be saved by re-using waste heat that is now released to the atmosphere or surface water. Since the temperatures of this waste heat are rather low, conversion to electricity is not an attractive option. Moreover, 85% of the energy demand of industry consists of heat. In order for the waste heat to be reused, however, the temperature level has to be upgraded. This paper deals with a study on the technical and economical feasibility of using heat pumps based on reversible chemical reactions to upgrade industrial waste heat to reusable levels. Based on the outcome of this study, a technology development path is set up in order to achieve the implementation of the most feasible heat pump for this application. ECN-RX

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7 2. MARKET STUDY To assess the amount of industrial waste heat, a study is conducted in co-operation with an engineering and contracting firm active in the process industry to quantify the amount of waste heat and the temperature thereof. Figure 2.1 shows the results of this study for the chemical and refining industry in the Netherlands. The picture shows the cumulative waste heat released to the environment in 1999 as a function of the temperature of this waste heat. Clearly, there is a huge potential for energy savings. 120 Cumulative waste heat (PJ) Temperature (ºC) Figure 2.1 Cumulative waste heat in the refining and chemical industry in the Netherlands If the waste heat can be upgraded to about 230ºC, this upgraded heat can be easily supplied to the MP-steam grid. This provides a flexible way of incorporating a heat pump without interfering with the main process. The challenge is then to find a way of upgrading the waste heat from the temperature levels shown in Figure 2.1 to 230ºC. Conventional (compression, absorption) heat pumps are not able to function at these temperature levels, nor can they provide the temperature lifts necessary. Therefore, other heat pumps concepts should be considered. The requirements, which should be met by new heat pump concepts, are defined as: input temperature ºC, temperature lift ºC, cooling at ambient temperature with cooling water or air, unit size ~ 1 MWth. There are very few heat pump concepts that are able to fulfil these requirements. Chemical heat pumps, however, based on reversible chemical reactions, have the possibility to do so. ECN-RX

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9 3. CHEMICAL HEAT PUMPS A large variety of chemical heat pumps exists. A recent paper [1] gives a broad overview and classification of the different types. In order to select the most promising chemical heat pump for further development, the different concepts were assessed with help of the list of criteria like temperature, pressure, reaction enthalpy, and so forth. A qualitative judgement is made with respect to two classes of chemical heat pumps, being the organic and the inorganic systems. Organic systems can in general be operated continuously while inorganic systems are usually operated batch-wise. A total number of 33 candidates have been assessed against these criteria. No clear winners were identified. Organic systems show a bad score on selectivity and conversion compared to inorganic systems. On the other hand, inorganic systems perform worse on criteria like mass and heat transfer. The advantage of organic systems is that they can be implemented as continuous systems, which provides easier integration. From both the organic and inorganic types one promising representative chemical heat pump type is selected and assessed in more detail. The first is a concept that has been widely studied and works with the dehydrogenation of isopropanol at low temperature and the reverse reaction at high temperature. The second one is based on the desorption of ammonia vapour from a low temperature salt, followed by the absorption of this vapour in a high temperature salt. Before going to the detailed analyses of these heat pumps, first the general methodology of the technoeconomic evaluation will be described. ECN-RX

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11 4. METHODOLOGY The technical assessment is performed by modelling the system with help of the ASPEN Plus flow-sheet simulation program for the isopropanol heat pump and with a spreadsheet model for the salt/ammonia vapour heat pump. The outcomes of the calculations indicate the technical performance of the system. This performance is characterised by three parameters: Enthalpic efficiency: high-temperature heat divided by waste heat supply; Exergetic efficiency: high-temperature heat exergy divided by waste heat exergy plus electrical power exergy; Coefficient of Performance: high-temperature heat divided by electrical power. The economic feasibility is based on the calculation of the Internal Rate of Return, see [2]. It is assumed that all investments made are taken from the company s own funds. The depreciation period is taken equal to the project lifetime, being 10 years. Other assumptions are: Yearly O&M costs, excluding electricity costs, amount to 5% of the direct investment (process equipment and additional direct costs). Yearly insurance payments equal 1% of the investment. The cash flow is calculated by determining the profit minus 35% tax payments. Inflation and escalation rates are assumed to be zero. Availability of the installation is 95%, resulting in 8320 running hours per year. The total capital investment is estimated by first taking the costs associated with the main equipment. Thereafter, factors taken from the literature [2, 3] are used to account for additional direct costs for installation, instrumentation, piping, and electrical equipment. Costs associated with buildings, service facilities, and land have not been taken into account since these systems are installed on an existing site. The indirect costs relate to engineering & supervision, construction & supply, and contingencies. These are calculated as a percentage of the direct costs. Finally, start-up costs and working capital are added to complete the total capital investment. With respect to the variable costs and the price of the product (MP-steam) some uncertainties exist. In order to account for these uncertainties, a probabilistic uncertainty analysis is performed for the economic analysis. More information on this type of analysis can be found in [4]. For the uncertain parameters, a distribution function is defined within a certain range. The ultimate result forms a distribution function of the desired output parameter, in this case the Internal Rate of Return. The following parameter have been varied: The cost of electricity varies between 14 and 45 /MWh, with a most likely value of 27 /MWh. The costs for cooling water varies between /GJ, based on [5] These costs are used to quantify the savings on cooling water. The price of MP-steam varies between 3.6 and 8.2 /GJ, with a most likely value of 4.8 /GJ. The total capital investment is varied between the first estimate and 60% of this value, with 80% as the most likely value. Cost reduction is achieved by installing multiple units at the same site and by serial production of these installations. ECN-RX

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13 5. CONCEPTS STUDIED 5.1 Isopropanol heat pump The possibility of using the dehydrogenation of isopropanol in a heat-driven chemical heat pump has been the subject of many papers since the early nineties [6-14], the greater part being devoted to the catalytic aspects of both the dehydrogenation and the hydrogenation reactions. This heat pump is based on the endothermic dehydrogenation of isopropanol into acetone at the low temperature (waste heat) level and the reverse reaction at a high temperature level. The reaction is given by: C 3 H 7 OH! C 3 H 6 O + H 2 ΔH = kj/mol Figure 5.1 shows a process flow diagram of this heat pump. The system is designed to use waste heat of 80 C (Q L ). The reaction is catalysed by a Ru-Pt or CuCr catalyst present in the reboiler of the distillation column that separates the two fluids acetone and isopropanol. The reaction products acetone and hydrogen are directed to the hydrogenation reactor via a blower and a heat exchanger to preheat the mixture. The exothermic hydrogenation reaction is catalysed by Ni and produces heat at about 200 C. The isopropanol formed is then returned back to the distillation column to close the cycle. Blower Q C W Condenser 4 Column HE Q H Q L Reboiler/ Reactor Reactor Figure 5.1 Process flow diagram of the isopropanol/acetone heat pump Technical assessment The following assumptions are made with respect to the ASPEN Plus calculations: The input of waste heat amounts to 4 MW. The rigorous equilibrium distillation model of ASPEN is used. The reaction is specified only to take place in the reboiler. The equilibrium conversion at 80 C is 7%. This case will be referred to as the equilibrium conversion case. ECN-RX

14 ASPEN Plus does not account for the shift in equilibrium that occurs when the reaction proceeds in the liquid phase and the hydrogen escapes from the liquid. To account for this effect, a separate calculation is made with an optimistic fixed conversion of 30%. This case is referred to as the 30% conversion case. The top product contains 1% isopropanol. This requirement sets the reflux ratio. The distillation column runs at atmospheric pressure. The pressure drop within the system is set at 0.5 bar. The blower has an isentropic efficiency of 72%. The hydrogenation reactor is modelled as a thermodynamic equilibrium reactor. Kinetics has not been taken into account: heat transfer is taken as the critical process (thermal design). All equipment is sized according to well-known design methods [15]. The main component, the reactive distillation column, is about 12 meters high and has a diameter of 2.5 meters. The hydrogenation reactor is a relatively small vessel with a diameter of 1.4 meter and a height of 2.4 meters. The total mass flow circulating through the system amounts to 2904 kg/h. When the conversion of isopropanol is fixed at 30% this mass flow increases to kg/h. The composition of the streams is almost identical to the equilibrium conversion case. The energy balances for the two cases analysed are listed in Table 5.1. Table 5.1 Component results from ASPEN Plus for the isopropanol heat pump Component Equilibrium conversion 30% conversion Power (kw) Exergy (kw) Power (kw) Exergy (kw) Dehydrogenation column reboiler duty condenser duty Blower Heat exchanger duty Hydrogenation reactor Enthalpic efficiency (%) Exergetic efficiency (%) COP Clearly, there is a large difference between the two cases. Since the COP is sufficiently high, the enthalpic and exergetic efficiency are the dominant criteria. The performance of this heat pump is very sensitive to the conversion of isopropanol in the reactive distillation column. Attempts have been made to optimise the system by changing column and hydrogenation reactor pressures, undercooling the condensate and changing the column top-product composition. It is also possible to replace the expensive distillation column by liquid-phase and vapour-phase dehydrogenation reactors in series. None of the alternatives considered proved to have a profound benefit on the thermodynamic performance of the system. Economic evaluation Based on the thermal sizing of the equipment, an estimate has been made of the corresponding costs [16]. Table 5.2 presents these estimates. The reactive distillation column is by far the most important component with respect to costs. It represents more than 50% of the equipment costs. 14 ECN-RX

15 Table 5.2 Total capital investment estimate for a 4 MW isopropanol heat pump Costs (k ) Purchased equipment costs 612 Additional direct costs 530 Total direct investment 1142 Total indirect investment 183 Total process investment 1325 Start-up costs 66 Working capital 133 Total capital investment 1524 The economic evaluation is carried out in a probabilistic way, as explained in section 4. The results show that both cases have a negative Internal Rate of Return, even for the most optimistic set of parameters. Therefore, it can be concluded that this type of heat pump has no economic prospect for an application in Western Europe. The most important factor determining the IRR is the selling price of the MP-steam. An important remark with respect to this type of heat pump should be made concerning byproducts. In the ideal situation, chemical reactions proceed with 100% selectivity. However, in organic systems by-products will always be formed. This will have the following implications: Lowering the coefficient of performance. By-products form a vapour stream that has to be pumped around without contributing to the heat pump process. A lower selectivity leads to a higher circulating mass flow to obtain the same capacity. By-products may poison the catalysts in the system. By-products may affect the performance of the distillation column (higher reboiler temperature, less efficient separation). By-products may affect the performance of the heat exchangers. From this list, it is clear that the formation of by-products may seriously hamper the functionality and the capacity of the system. Purging, compensated by make-up, may provide a solution to the by-product problem at the expense of increasing the complexity and the operating costs of the system. 5.2 Salt/ammonia vapour heat pump The second case analysed concerns a solid sorption heat pump. Such a heat pump does not make use of an expensive distillation column. The system consists of two solids (salts) which are able to absorb/desorb ammonia vapour. The operating principle is illustrated in Figure 5.2. This picture shows the ammonia vapour pressure as a function of temperature for two different salts, called a low temperature salt (LTS) and a high temperature salt (HTS). The desorption reaction is an endothermic reaction. The heat supplied at T M to the LTS leads to the release of ammonia vapour at high pressure. When this ammonia vapour flows to the HTS, the ammonia vapour will be absorbed leading to the release of heat at T H. Cooling the LTS with coolant at T L and heating the HTS with heat supplied at T M regenerates the system so the cycle can start all over again. For industrial waste-heat applications, T M represents the waste-heat temperature in the range of C. The useful heat is generated at T H = 230 C, while cooling at T L is done at ambient temperature. ECN-RX

16 LTS Ln P HTS T L T M T H Temperature Figure 5.2 Vapour pressure as a function of temperature for Low (LTS) and High Temperature Salt (HTS). The pair of salts that was selected for this application is the couple MnSO 4 /NH 3 (LTS) and NiCl 2 /NH 3 (HTS). The operation of the heat pump is based on the following reactions: MnSO 4 6NH 3 + Q waste MnSO 4 2NH 3 + 4NH 3 ΔH = kj/mol NH 3 NiCl 2 2NH 3 + 4NH 3 NiCl 2 6NH 3 + Q MPsteam ΔH = kj/mol NH 3 In order to evaluate this system, reliable thermodynamic data are required. A search for these data in the available literature did not prove very successful. The data are very sparse and often inconsistent between different experiments [17]. For the time being, best guesses have been made for the thermodynamic data. In order to allow continuous operation of this heat pump, two batch units are foreseen operating in an alternating way. Waste heat at 140 C is used as the heat source at T M. Cooling water at 30 C is used as T L. The high-temperature heat becomes available at 240 C. This temperature is sufficient to generate medium-pressure steam. The heat supply/removal to/from the salts has to be switched, depending on the operating state. An intermediate circuit is required to avoid contamination of streams. This circuit uses a water/steam system to transfer the heat from and to the reactor vessels containing the HTS and LTS. Figure 5.3 shows the process scheme as evaluated. As may be understood from Figure 5.3, the configuration of streams will be changed after one cycle. The configuration of streams is changed in such a way that the vessels at the top will take over the function of the vessels at the bottom of the figure, and vice versa. 16 ECN-RX

17 LTS HTS MP steam Process Stream Unloading HTS LTS Loading Figure 5.3 Process flow diagram for a salt/vapour chemical heat pump Cooling Water Technical assessment The following assumptions have been made in order to calculate the technical performance of this heat pump with help of a spreadsheet model. The process stream is a condensing hydrocarbon stream of 140 C. The waste-heat power equals 5 MW. T M = 120 C, taking into account a 20 C temperature loss for the heat exchangers and intermediate circuit. T H = 259 C. The temperature of the MP steam equals 239 C, which corresponds to 33 bara, taking into account a 20 C temperature difference across the heat exchanger. Cooling water is available at 30 C, resulting in T L = 40 C, taking a 10 C temperature difference across the heat exchanger. No kinetics or hysteresis are modelled for the chemical reactions. Heat transfer is the dominating mechanism during the reactions. Therefore, mass transfer limitations and reaction kinetics between ammonia vapour and the salt have not been taken into account. The reactors are shell-and-tube like vessels filled with finned tubes. The salt is present between the fins of the heat exchanger. The salt has a uniform axial temperature distribution. Sizing of all heat exchangers done on the basis of well-known design rules [15], with fixed heattransfer coefficients. An important assumption relates to the heat transfer between the intermediate streams in Figure 5.3 and both the LTS and HTS salts. Since the salts are very bad thermal conductors this imposes a restriction on the amount of heat that can be transferred. The assumption is that an overall heat transfer coefficient of 1000 W/m 2 K is applicable. ECN-RX

18 To give an impression of the size of the installation, each reactor vessel has a height of 6 meters and a diameter of 2 to 3 meters. The total weight of one vessel is about 50 t, including the salt and the heat exchanger tubes. The calculations show that the overall enthalpic efficiency amounts to 34.8% (including sensible heat losses). The pure thermodynamic efficiency equals 41%. The cycle time amounts to a little more than 1 h. Table 5.3 presents the energy balance. Table 5.3 Component results from the spreadsheet model for the salt/ammonia vapour heat pump Component Power (kw) Exergy (kw) Waste heat at T M = 140 C Low-temperature heat at T L = 30 C High-temperature heat at T H = 240 C High-temperature heat including heat-up/cool-down losses Pumps Enthalpic efficiency (%) 34.8 Exergetic efficiency (%) 51.7 COP 97 Clearly, the technical performance of this system is far better than the isopropanol heat pump. The reason for a very high COP lies in the fact that electrical power is only required to pump around the liquid streams for the intermediate loops. Economic evaluation The investment costs are estimated based on the assumption that all equipment is built of carbon steel, except for the heat exchangers and finned tubes which are made of stainless steel. The procedure for the additional costs is identical to the one used for the isopropanol heat pump, that is fixed percentages of the process equipment costs are taken. Table 5.4 shows the build-up of the estimate for the capital needed. The reactor vessels make up about 70% of the process equipment costs. Table 5.4 Total capital investment estimate for a 5 MW salt/ammonia vapour heat pump Costs (k ) Purchase equipment costs 507 Additional direct costs 445 Total direct investment 952 Total indirect investment 153 Total process investment 1105 Start-up costs 55 Working capital 111 Total capital investment 1271 The economic analysis is then carried out in a probabilistic way, resulting in a cumulative distribution function for the IRR. The mean value of the IRR equals 14%. The 5% and 95% probability range number are 7% and 22%, respectively. This means there is a large probability of a good return on investment. The most important parameter is again the selling price of the MP-steam. 18 ECN-RX

19 6. EVALUATION/CONCLUSIONS This paper presents the results of a techno-economical feasibility study on two chemical heat pumps. From an energetic point of view, both heat pump concepts are feasible. Waste heat is upgraded to usable heat in both concepts. The technical performance of the salt/ammonia vapour heat pump, however, is much better than the isopropanol heat pump, since the former has a higher enthalpic efficiency, a higher COP, and does not suffer from by-product formation. From an economic standpoint, the differences become even greater. The isopropanol heat pump is not economically feasible, while the salt/ammonia vapour heat pump shows good IRR results. Based on the results of this study, development work is going on with respect to the salt/ammonia vapour heat pump. In the first phase, more precise thermodynamic data are required for a better evaluation. In the next phase, a 1 kw test system will be designed and constructed. 7. ACKNOWLEDGEMENT The work described in this paper has received financial support from both the Netherlands Agency for Energy and the Environment and the Ministry of Economic Affairs. The authors would like to thank E. van der Werf (University of Twente, Faculty of Chemical Engineering) for his contribution to the work on the isopropanol heat pump. ECN-RX

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21 8. REFERENCES [1] Wongsuwan, W., et al., A review of chemical heat pump technology and applications. Applied Thermal Engineering, : p [2] Brennan, D., Industry Economics, An International Perspective. 1998: Institute of Chemical Engineers. [3] Peters and Timmerhaus, Plant design and economics for Chemical Engineers, in Chemical Engineering Series, 4th edition [4] Vose, D., Risk analysis, A quantitative guide. Second edition. 2000: John Wiley & Sons. [5] European Commision, Integrated Pollution Prevention (IPPC), Reference document on the application of Best Available Techniques to Industrial Cooling Systems, 2000, European Commission. [6] Anikeev, V.I., A.V. Ooudkov, and A.S. Bobrin, Study of catalytic heat pumps of upgrading low-level thermal energy, in New Energy Systems and Conversions. 1993, Universal Academy Press Inc. p [7] Cunningham, J., J.N. Hickey, and Z. Wang, Activities and selectivities of copper/metaloxide catalysts at temperatures relevant to chemical heat pumps based on isopropanol/acetone interconversions. International Journal of Energy Research, : p [8] Gandia, L.M. and M. Montes, Effect of the design variables on the energy performance and size parameters of a heat transformer based on the system acetone/h2/2-propanol. International Journal of Energy Research, : p [9] Gandia, L.M. and M. Montes, Effect of the reduction of temperature on the selectivity of the high temperature reaction of acetone and hydrogen over alumina and titania supported nickel and cobalt catalysts. Journal of Molecular Catalysts, : p [10] Ito, E., et al., A composite Ru-Pt catalyst for 2-propanol dehydrogenation adaptable to the chemical heat pump system. Chemistry Letters from the Chemical Society of Japan, 1991: p [11] Saito, Y., et al., Hydrogen production from 2-propanol and cyclohexanes with noble metal catalysts aiming at chemical conversion of low quality heat, in 10th World Hydrogen Energy Conference Cocoa Beach, Florida, USA. [12] Taneda, D., et al., Studies of 2-propanol/acetone/hydrogen energy conversion system, in 10th World Hydrogen Energy Conference Cocoa Beach, Florida, USA. [13] Yamashita, M. and Y. Saito. Catalyst study on liquid phase dehydrogenation of 2- propanol for newly-proposed chemical heat pump, in International Symposium on CO2- fixation & efficient utilisation of energy Tokyo, Japan. [14] Taneda, D., et al., Pilot plant proving tests of solar chemical heat pump system, in Solar Engineering. 1995, ASME. p [15] Sinnot, R.K., Coulson & Richardson's Chemical Engineering, 1993, Pergamon Press. [16] DACE, Webci prijzen boekje (in Dutch). 1997: Dutch Assocatioan of Cost Engineers. [17] Touzain, P. Thermodynamic values of ammonia-salts reactions for chemical sorption heat pumps, in International sorption heat pump conference Munich, Germany. ECN-RX