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Cleaner flue gas and energy recovery through pinch analysis
Journal of Cleaner Production 12 (2004) 165–170 www.elsevier.com/locate/jclepro
Cleaner flue gas and energy recovery through pinch analysis T.K. Zhelev a,∗, K.A. Semkov b b
a Chemical Engineering Department, University of Durban-Westville, Private Bag X54001, 4000 Durban, South Africa Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., Bl.103, 1113 Sofia, Bulgaria
Received 30 July 2002; accepted 5 December 2002
Abstract The paper addresses the problems of cleaner energy generation in industrial applications. Its aims further flue gas energy recovery accompanied with additional environmental impact. It shows how the widely accepted flue gas temperature of industrial coal fired boilers can be lowered substantially without increasing the risk of back-end condensation. This is possible through dehumidification and partial absorption of the flue gas in a packed-bed economiser system by circulating water. Different design contact economiser systems (CES) are presented and compared. Pinch principles are deployed for targeting, design and operation-guiding purposes, balancing the heat and mass transfer in a second generation CES of parallel type. The results are expected to make these systems more sophisticated and more suitable for industrial applications. They open the door for valuable boiler efficiency improvement and generalisation of methodology for simultaneous management of energy and water resources. 2003 Elsevier Ltd. All rights reserved. Keywords: Contact economiser; Pinch analysis; Flue gas
1. Introduction The implementation of pinch principles for analysis of energy resources in the mid-80s had a big impact on systematic energy management [1]. The success of pinch technology in hundreds of different industrial applications had provoked the implementation of the same principles for environmental protection. The water pinch [2,3] and the flue gas pinch were followed by hydrogen pinch and our modest contribution to wastewater management called oxygen pinch [4]. The affordable level of complication came closer to simultaneous heat and mass transfer considerations. The first communications to address this problem were announced recently in the EC Joule report [5] and by Savulescu and Smith [6]. Our experience in the field had followed the logical idea to consider the management of all heat and mass resources simultaneously, being fully convinced that they influence each other.
Corresponding author. Tel.: +27-31-204-4693; fax: +27-31-2044021. E-mail address: [email protected] (T.K. Zhelev). ∗
0959-6526/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0959-6526(02)00192-0
2. Thermal pinch and water pinch analogy
As thermal pinch analysis addresses heat exchange from a hot stream to a cold stream, so does water pinch addressing mass exchange from a rich stream to a lean stream. The initial methodology by El-Halwagi and Manousiouthakis [7] considered only a single contaminant. The targeting of minimum fresh water and wastewater using multiple contaminants was addressed by Wang and Smith [8]. The method again uses the composite curve concept for targeting and directly guided design. What is the situation when the processes of simultaneous heat and mass transfer are in place: chemical engineering theory and practice show a long history in simultaneous consideration of these processes. Pinch analysis has a little different approach to the process model than the classical unit operation modelling — it looks for firm constraints to get design guidelines out of them. Based on these constraints an idealised maximum of heat and mass transfer is to be set and accepted as the so-called target for future design. Taking this point of view back into the understanding of the classical process we may succeed in obtaining some additional bene-
166
T.K. Zhelev, K.A. Semkov / Journal of Cleaner Production 12 (2004) 165–170
fits. In our particular case a contact economiser system is used as an object for analysis.
3. Contact economiser system One of the problems of recent interest is the recovery of the low potential energy of flue gases from industrial boilers using contact economiser systems (CES). The operation of such systems involves direct heat transfer accompanied by mass transfer, dehumidification of the flue gas, controlled preheating and humidification of the combustion air, leading to improved efficiency and lower NOx emission. Efficient design and operation of CES involves also consideration of the system’s sensitivity, controllability, structural and parametrical optimisation. In our view the CES can be seen as a suitable set-up assigned to explore the more general methodology of simultaneous management of heat and water based on pinch analysis for combined heat and mass transfer. CES, if possible to be implemented, have several advantages: (a) improving the boiler’s efficiency (by returning back to the system a substantial part of wasted energy); (b) ash scrubbing and decreasing the concentration of harmful gases (like NOx) emitted to the atmosphere; (c) producing a valuable amount of water. The contact economiser is a classical packed-bed column ensuring high gas velocity and low pressure-drop. 3.1. Unit design The simplest contact economizer system, the so-called system of first generation, is shown in Fig. 1. For the particular case, the gas cooling is accompanied by water vapour condensation, well known as a dehumidification process. Assuming adiabatic conditions, the design equations are as follow: HV⫺HV1 ⫽ (LCL / V) / (TL⫺TL1), representing the operating line conditions, and
Fig. 1.
First generation CES.
(1)
(HVi⫺HV) / (Ti⫺TL) ⫽ hla / KGaMBP,
(2)
showing the tie line conditions, where V is the volume of the flue gas, L is the liquid flowrate, H is the enthalpy, MB is molecular weight of the noncondensable component, hla and KGa are heat transfer and mass transfer coefficients, i is any point in the contact device, (LCL/V) gives the slope of the operating line. Fig. 2 represents a more complicated design where a combination of gas dehumidification and combustion air humidification is applied. The equilibrium curve and the operating line in their classical representation are shown in Fig. 3, where the closest approach (the pinch) between them in Fig. 3(a) is at the “top” of the exchange area, when in Fig. 3(b) it is at the “bottom” of the exchange area. A major factor in the design of the economiser is the circulating water flowrate. An increase in liquid loading leads to an increase of the slope of the operating line with corresponding implications to the design. The “pinched” operating line can also suggest a system structure change (for instance the introduction of a second tower, or a heat exchanger bringing the water temperature under the dew point of the flue gas). The favourable solutions are on the site of the pinch “at the top”, but the exact optimum is to be allocated upon implementation of the proper optimisation procedure. The approach justifies the need for combining pinch principles that have a conceptual character with the mathematical programming. The optimisation procedure can control the balance between the heat transfer (utilised heat’s potential) and the mass-transfer (condensation/evaporation) through the regime parameters. The choice of the objective function can give priority to the design parameters such as column height, or process parameters such as the liquid/gas ratio. 3.2. System design It is quite obvious that one important reason to prefer direct contact between the flue gas and the water is the
Fig. 2.
Second generation CES.
T.K. Zhelev, K.A. Semkov / Journal of Cleaner Production 12 (2004) 165–170
Fig. 3.
167
Temperature–enthalpy diagram of a dehumidification process.
benefit of sensible heat transfer boosted by latent heat transfer. In fact, the portion of heat transfer gained by condensation exceeds the sensible heat transfer by up to three times. One of the most important process parameters is the humidity (wet-bulb temperature) of the flue gas. This parameter plays a crucial role in the heat recovery process. The higher the wet-bulb thermometer reading is, the higher the temperature level of the utilised heat is. The wet-bulb temperature determines the dew point related to the partial water vapour pressure. The mass transfer is established solely by the difference between the dew point of the inlet gas and the partial pressure corresponding to the outlet water temperature. The optimisation of this parameter leads to changes in the economiser system structure as can be seen from Fig. 2. The so-called “second generation” contact economiser system proposed by Kolev et al. [9] has two stages of heat recovery: one utilises the heat of the flue gas and transfers it to corresponding users via a regenerative heat exchanger, usually through a water circuit, the second one uses part of the same heat source for preheating the combustion air. The humidification process in this case is arranged in a similar packed-bed column as in the first generation economiser systems, but the mass transfer is in the opposite direction, increasing the moisture content of the combustion air and hence suppressing the NOx formation in the burner and improving the humidity of the flue gas. This results in improvement of the recovered heat temperature by 10–15° C in comparison with the first generation case. Finally, the optimisation of integrated energy considering the possible changes of temperature and flowrate conditions as a result of topology change is another way to address the problem of heterogeneous process synthesis. Not forgetting the substantial environmental impact of this system expressed in terms of cleaner flue gas, characterised by lower ash content, indirect decrease of CO2 emissions, direct decrease of NOx, partial decrease of SOx and generating a substantial amount of water, condensed in the dehumidifier.
4. Contact economiser systems for coal fired boilers South African coal is characterised by comparatively low sulphur content (in the range of 0.7%), which is a good prerequisite for flue gas energy recovery. The biggest coal consuming industry is power generation. The power station chosen as a case study consists of 6 units, 686 MW each. The station’s overall efficiency is 38%. Electrostatic precipitators let only 0.01% of the flyash into the atmosphere. The SO3 concentration in the flue gases is about 15 ppm with a temperature at the inlet of the stack of 120o C. Due to the dry cooling system, the raw water consumption of the plant is only 0.08 l/kWh, i.e. 91.44 l/s for the whole station. On this basis, the overall efficiency can be improved as follows. 4.1. First generation CES case Assuming the air excess of 1.05, humidity of the coal 7%, ash content in the coal 30%, the dry flue gas produced from burning 1 kg coal is 8.61 kg and the moisture generated 0.376 kg. If the combustion air is at ambient temperature of 20o C and relative humidity 80%, the moisture content in the flue gases will be 0.052 kg water vapour/kg dry gas. At such conditions the wet-bulb temperature of the flue gas will be 48.42o C. Considering the fact that the maximum temperature of the circulating water leaving the economiser can be 4.5 to 5.0o C below the wet-bulb temperature of the gas (allowing for a reasonable sized economiser and heat exchanger), the only way to return the recovered heat back to the process is through preheating the raw water to the station. Supposing the raw water’s temperature is 15o C, the heat load required to preheat it up to 43.5o C is Putl=4.186×91.44(43.5⫺15.0)=10909 kW. Temperature of the flue gas after the economiser system, 30.8o C; Humidity of the flue gas after the system, 0.0278 kg H2O vapour/kg dry gas; Economiser’s heat load, 322 kW/m2;
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Pressure drop, 13 mm Hg; Economiser’s cross-section, 33 m2; Heat exchanger surface, 1800 m2. The utilisation of such a heat load requires 67.8 kg/s dry flue gas. The actual emission of flue gases from one unit is 811 kg/s, thus the contact economiser system recovers only 8.36% of the heat wasted by one unit. As a result, the entire crude water used by the station can be preheated in one utilisation plant, attached to one of the units of the station. Since the flue gases from 3 units are collected in one stack, the calculated temperature of the gas at the inlet of the stack would be 117.5o C. Thus, practically there is no change of the temperature regime of the stack. The recovered energy is equivalent to 4.6 MW extra electrical power. Assuming 90% efficiency of the boiler, the amount of coal to be saved would be 2.35 t/h. At a coal cost of R40 a ton, the savings of fuel would be R752,000 p.a. The usual running costs of such a heat recovery system are about 1% of recovered energy. In this case the payback period of the system would be 1.5 years. There is an additional effect to be accounted for, the recovery of water from the flue gases. The recovery is due to the condensation accompanying the flue gas cooling process. For the given case the amount of recovered water would be 2.8 kg/s, which represents 10.76% of the water consumption of one unit, or 1.79% of the station’s water demand. The possible re-use of this water requires preliminary decarbonisation and relevant purification. 4.2. Second generation CES case and its contribution to cleaner flue gas Assuming a 10% increase of the combustion air humidity (the beneficial level established during our experiments), this will result in higher wet-bulb temperature of the flue gas raised to 58.5° C, boosting the exit water temperature to 57.5° C. The energy recovery from one unit in this case would be 130.5 MW when the cooling of the gases goes up to 30.8° C. If the recovered heat is returned to the system through preheating the boiler feed water (40° C), then the utilised energy will be 100 MW. This amount would be supplemented by another 11.8 MW raw water preheating power, which would yield 111.8 MW total heat recovery. The stack gas temperature would be 97.5° C. The coal saving would amount to 21.5 t/h, equivalent to R6.88×106 p.a., that corresponds to 42.16 MW extra electric power. Thus the overall effect, as it was calculated in the previous case, would reach up to R21.5×106 p.a. This would improve the efficiency of the block by 6% and, respectively, the entire station efficiency by 1%. The additional ecological effect would reflect in the following: (i) decreasing the CO2 emissions by 250,000 t/y; (ii)
decrease the NOx emissions by 118 t/y. Considering the fact that 3 blocks are served by one stack, the expected decrease of gas temperature would be minimal (from 120 to 97.5° C). This, as well as considering the insignificant concentration of SO3 in the stack gases (15 ppm) and the twice decreased moisture content, will exclude the danger of corrosion of the chimney. The amount of water recovered from the flue gases would be 15.03 kg/s. This represents 16.4% of the fresh water for the entire power station (330 m3/h) and is approximately two times more than the boilers’ make-up water for all 6 blocks. The utilisation of this water depends on the water treatment facilities on site.
5. Thermal pinch analysis The drawing in Fig. 4 shows how the simultaneous heat and mass transfer processes can be replaced by a pure heat transfer model in two stages, sensible heat recovery and latent heat recovery of vapour condensation (if a dehumidification process is concerned) or evaporation (if a humidification process takes place). The two-stream simultaneous heat and mass transfer in direct contact is replaced by three fictitious streams: (a) One of the flue gas entering the column with moisture content equal to the exit flue gas moisture content; (b) Second, the water stream leaving the packed column with the flowrate of the inlet conditions, and (c) The third, water vapour transferred from one stream to the second one having inlet temperature of the inlet gas and outlet temperature equal to the exit water temperature. Because of the phase change, this stream in the general case is to be split into three segments (as the pinch technology suggests). One segment will represent the cooling of vapour to the condensation temperature, the second will correspond to the actual phase change and the third segment will deal with the cooling of water to the exit water conditions. Note that the three new streams have constant flowrate as required by classical pinch analysis. The composite curve (Fig. 5) is drawn for the power station from our case study and corresponds to a second generation CES of parallel type. It shows the maximum possible heat recovery and the amount of heat to be wasted to the surroundings. The case can be specified as a typical threshold problem [1]. It also represents the heat load distribution in both packed bed columns (the humidifier and the dehumidifier). Note that the minimum permissible temperature approach is much smaller than in a typical heat exchanger network and can be around 3° C.
T.K. Zhelev, K.A. Semkov / Journal of Cleaner Production 12 (2004) 165–170
169
FG FG W Fig. 4. Definition of streams in a contact economiser. Legend/nomenclature: FM = cFG in + cout; F =flowrate of the dry flue gas; F =flowrate of the circulating water; FM=flue gas moisture to be extracted by the water stream; TM=moisture temperature; TFG=flue temperature; cFG=flue gas moisture content.
Fig. 5.
Composite curves. Fig. 6. Grant composite curve.
Any shift of the curves decreasing the ⌬Tmin cannot affect the utility amount, but can affect the temperature level of utility application (threshold problem). In our case ⌬Tmin⬍⌬Tthreshold. The design in this case is suggested to start at the “no utility end” with the hottest “hot” stream matching the hottest “cold” stream. The number of possible matches after this suggested action is quite high. The Grant composite curve (GCC) is shown in Fig. 6. It gives an indication of the minimum cold utility, the temperature level (condensation, preheating) of the utility (cooling water) and the favourable condensation and evaporation shift throughout the tower’s height. The actual benefit can be found after including into consideration the eventual receptor of the recovered heat returned back to the system. For consistency, in our case a raw water stream is used. The maximum amount of heat recovered and transferred back to the system as well as the temperature level (potential) of this heat can be found directly from the GCC following one of the pinch rules: “Generate utilities at the highest possible level”. The
process can be controlled within constraints such as the minimum natural draft temperature of the stack gases and the acid dew point of the flue. The bigger the energy recovery the higher the exit water temperature, the higher the recovered heat potential. Raising energy recovery shifts the pinch touching point in the GCC presentation upwards. This shift is followed by the cooling utility, which role in this case is played by the recovered energy transporting fluid. The maximum temperature level of this heat corresponds to the first appearance of the hot utility requirement.
6. Conclusions The analysis of the energy utilisation integrated with the analysis of the mass transfer supported by the pinch concept can raise the opportunity for better energy and water management through process integration, better system design and bigger savings. The implementation
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of contact economiser systems in particular can lead to substantial improvement of power generation efficiency, accompanied by valuable water production and cleaner emissions. It opens the door for future investigations of combined energy and water resources management at a total site level. The results presented are based on the simplest CES flowsheet of the second-generation (parallel type). The optimal design of such systems will be a subject of a further pinch study presented separately. The case study presented here gives enough confidence about the potential of the approach and the impressive ecological outcome of its application. Acknowledgements We are grateful that our industrial partner ESKOM recognises the importance of this work under its TESP programme. The NRF additional state support under the THRIP programme is also acknowledged and much appreciated. References [1] Gundersen T, Naess L. Synthesis of cost optimal HEN. Comp. and Chem. Eng 1988;12(6):503.
[2] El-Halwagi MM, Manousiouthakis V. Synthesis of mass-exchange networks. AIChE Journal 1989;35(8):1233. [3] Wang Y-P, Smith R. Wastewater minimisation. Chem Eng Sci 1994;49:981. [4] Zhelev T, Ntlhakana L. Energy-environment closed loop through oxygen pinch. Comp. and Chem. Eng 1999;23:S79. [5] Klemesˇ J. (ed) Simultaneous energy and water minimisation, Contract JOE—3-CT950036, EC JOULE III, Publishable report, Brussels 1997. [6] Savulescu LE, Smith R. Simultaneous energy and water minimisation. AIChE Annual Meeting, 1998, Paper 13c. [7] El-Halwagi MM, Manousiouthakis V. Synthesis of mass exchange networks. AIChE J. 1989;8:1233. [8] Wang YP, Smith R. Wastewater mimimisation. Chem. Eng. Sci. 1994;13:981. [9] Kolev N, Darakchiev R, Semkov K. Systems containing contact economisers. In: Energy efficiency in process technology. London and New York: Elsevier; 1993. p. 68-3.
T.K. Zhelev holds an Associate Professor position at the University of Durban–Westville, Durban, South Africa. He spent between 1991 and 1994 at the Department of Process Integration, UMIST, UK. He is a Chartered Engineer, MIChemE and SAIChE.
K.A. Semkov has been professionally involved in research in heat and mass transfer of gas–liquid systems for more than 25 years. He is an Associate Professor of the Institute of Chemical Engineers, Bulgarian Academy of Sciences. He is co-developer of a number of industrial applications involving economizers and other practical equipment.
Cleaner flue gas and energy recovery through pinch analysis T.K. Zhelev a,∗, K.A. Semkov b b
a Chemical Engineering Department, University of Durban-Westville, Private Bag X54001, 4000 Durban, South Africa Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., Bl.103, 1113 Sofia, Bulgaria
Received 30 July 2002; accepted 5 December 2002
Abstract The paper addresses the problems of cleaner energy generation in industrial applications. Its aims further flue gas energy recovery accompanied with additional environmental impact. It shows how the widely accepted flue gas temperature of industrial coal fired boilers can be lowered substantially without increasing the risk of back-end condensation. This is possible through dehumidification and partial absorption of the flue gas in a packed-bed economiser system by circulating water. Different design contact economiser systems (CES) are presented and compared. Pinch principles are deployed for targeting, design and operation-guiding purposes, balancing the heat and mass transfer in a second generation CES of parallel type. The results are expected to make these systems more sophisticated and more suitable for industrial applications. They open the door for valuable boiler efficiency improvement and generalisation of methodology for simultaneous management of energy and water resources. 2003 Elsevier Ltd. All rights reserved. Keywords: Contact economiser; Pinch analysis; Flue gas
1. Introduction The implementation of pinch principles for analysis of energy resources in the mid-80s had a big impact on systematic energy management [1]. The success of pinch technology in hundreds of different industrial applications had provoked the implementation of the same principles for environmental protection. The water pinch [2,3] and the flue gas pinch were followed by hydrogen pinch and our modest contribution to wastewater management called oxygen pinch [4]. The affordable level of complication came closer to simultaneous heat and mass transfer considerations. The first communications to address this problem were announced recently in the EC Joule report [5] and by Savulescu and Smith [6]. Our experience in the field had followed the logical idea to consider the management of all heat and mass resources simultaneously, being fully convinced that they influence each other.
Corresponding author. Tel.: +27-31-204-4693; fax: +27-31-2044021. E-mail address: [email protected] (T.K. Zhelev). ∗
0959-6526/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0959-6526(02)00192-0
2. Thermal pinch and water pinch analogy
As thermal pinch analysis addresses heat exchange from a hot stream to a cold stream, so does water pinch addressing mass exchange from a rich stream to a lean stream. The initial methodology by El-Halwagi and Manousiouthakis [7] considered only a single contaminant. The targeting of minimum fresh water and wastewater using multiple contaminants was addressed by Wang and Smith [8]. The method again uses the composite curve concept for targeting and directly guided design. What is the situation when the processes of simultaneous heat and mass transfer are in place: chemical engineering theory and practice show a long history in simultaneous consideration of these processes. Pinch analysis has a little different approach to the process model than the classical unit operation modelling — it looks for firm constraints to get design guidelines out of them. Based on these constraints an idealised maximum of heat and mass transfer is to be set and accepted as the so-called target for future design. Taking this point of view back into the understanding of the classical process we may succeed in obtaining some additional bene-
166
T.K. Zhelev, K.A. Semkov / Journal of Cleaner Production 12 (2004) 165–170
fits. In our particular case a contact economiser system is used as an object for analysis.
3. Contact economiser system One of the problems of recent interest is the recovery of the low potential energy of flue gases from industrial boilers using contact economiser systems (CES). The operation of such systems involves direct heat transfer accompanied by mass transfer, dehumidification of the flue gas, controlled preheating and humidification of the combustion air, leading to improved efficiency and lower NOx emission. Efficient design and operation of CES involves also consideration of the system’s sensitivity, controllability, structural and parametrical optimisation. In our view the CES can be seen as a suitable set-up assigned to explore the more general methodology of simultaneous management of heat and water based on pinch analysis for combined heat and mass transfer. CES, if possible to be implemented, have several advantages: (a) improving the boiler’s efficiency (by returning back to the system a substantial part of wasted energy); (b) ash scrubbing and decreasing the concentration of harmful gases (like NOx) emitted to the atmosphere; (c) producing a valuable amount of water. The contact economiser is a classical packed-bed column ensuring high gas velocity and low pressure-drop. 3.1. Unit design The simplest contact economizer system, the so-called system of first generation, is shown in Fig. 1. For the particular case, the gas cooling is accompanied by water vapour condensation, well known as a dehumidification process. Assuming adiabatic conditions, the design equations are as follow: HV⫺HV1 ⫽ (LCL / V) / (TL⫺TL1), representing the operating line conditions, and
Fig. 1.
First generation CES.
(1)
(HVi⫺HV) / (Ti⫺TL) ⫽ hla / KGaMBP,
(2)
showing the tie line conditions, where V is the volume of the flue gas, L is the liquid flowrate, H is the enthalpy, MB is molecular weight of the noncondensable component, hla and KGa are heat transfer and mass transfer coefficients, i is any point in the contact device, (LCL/V) gives the slope of the operating line. Fig. 2 represents a more complicated design where a combination of gas dehumidification and combustion air humidification is applied. The equilibrium curve and the operating line in their classical representation are shown in Fig. 3, where the closest approach (the pinch) between them in Fig. 3(a) is at the “top” of the exchange area, when in Fig. 3(b) it is at the “bottom” of the exchange area. A major factor in the design of the economiser is the circulating water flowrate. An increase in liquid loading leads to an increase of the slope of the operating line with corresponding implications to the design. The “pinched” operating line can also suggest a system structure change (for instance the introduction of a second tower, or a heat exchanger bringing the water temperature under the dew point of the flue gas). The favourable solutions are on the site of the pinch “at the top”, but the exact optimum is to be allocated upon implementation of the proper optimisation procedure. The approach justifies the need for combining pinch principles that have a conceptual character with the mathematical programming. The optimisation procedure can control the balance between the heat transfer (utilised heat’s potential) and the mass-transfer (condensation/evaporation) through the regime parameters. The choice of the objective function can give priority to the design parameters such as column height, or process parameters such as the liquid/gas ratio. 3.2. System design It is quite obvious that one important reason to prefer direct contact between the flue gas and the water is the
Fig. 2.
Second generation CES.
T.K. Zhelev, K.A. Semkov / Journal of Cleaner Production 12 (2004) 165–170
Fig. 3.
167
Temperature–enthalpy diagram of a dehumidification process.
benefit of sensible heat transfer boosted by latent heat transfer. In fact, the portion of heat transfer gained by condensation exceeds the sensible heat transfer by up to three times. One of the most important process parameters is the humidity (wet-bulb temperature) of the flue gas. This parameter plays a crucial role in the heat recovery process. The higher the wet-bulb thermometer reading is, the higher the temperature level of the utilised heat is. The wet-bulb temperature determines the dew point related to the partial water vapour pressure. The mass transfer is established solely by the difference between the dew point of the inlet gas and the partial pressure corresponding to the outlet water temperature. The optimisation of this parameter leads to changes in the economiser system structure as can be seen from Fig. 2. The so-called “second generation” contact economiser system proposed by Kolev et al. [9] has two stages of heat recovery: one utilises the heat of the flue gas and transfers it to corresponding users via a regenerative heat exchanger, usually through a water circuit, the second one uses part of the same heat source for preheating the combustion air. The humidification process in this case is arranged in a similar packed-bed column as in the first generation economiser systems, but the mass transfer is in the opposite direction, increasing the moisture content of the combustion air and hence suppressing the NOx formation in the burner and improving the humidity of the flue gas. This results in improvement of the recovered heat temperature by 10–15° C in comparison with the first generation case. Finally, the optimisation of integrated energy considering the possible changes of temperature and flowrate conditions as a result of topology change is another way to address the problem of heterogeneous process synthesis. Not forgetting the substantial environmental impact of this system expressed in terms of cleaner flue gas, characterised by lower ash content, indirect decrease of CO2 emissions, direct decrease of NOx, partial decrease of SOx and generating a substantial amount of water, condensed in the dehumidifier.
4. Contact economiser systems for coal fired boilers South African coal is characterised by comparatively low sulphur content (in the range of 0.7%), which is a good prerequisite for flue gas energy recovery. The biggest coal consuming industry is power generation. The power station chosen as a case study consists of 6 units, 686 MW each. The station’s overall efficiency is 38%. Electrostatic precipitators let only 0.01% of the flyash into the atmosphere. The SO3 concentration in the flue gases is about 15 ppm with a temperature at the inlet of the stack of 120o C. Due to the dry cooling system, the raw water consumption of the plant is only 0.08 l/kWh, i.e. 91.44 l/s for the whole station. On this basis, the overall efficiency can be improved as follows. 4.1. First generation CES case Assuming the air excess of 1.05, humidity of the coal 7%, ash content in the coal 30%, the dry flue gas produced from burning 1 kg coal is 8.61 kg and the moisture generated 0.376 kg. If the combustion air is at ambient temperature of 20o C and relative humidity 80%, the moisture content in the flue gases will be 0.052 kg water vapour/kg dry gas. At such conditions the wet-bulb temperature of the flue gas will be 48.42o C. Considering the fact that the maximum temperature of the circulating water leaving the economiser can be 4.5 to 5.0o C below the wet-bulb temperature of the gas (allowing for a reasonable sized economiser and heat exchanger), the only way to return the recovered heat back to the process is through preheating the raw water to the station. Supposing the raw water’s temperature is 15o C, the heat load required to preheat it up to 43.5o C is Putl=4.186×91.44(43.5⫺15.0)=10909 kW. Temperature of the flue gas after the economiser system, 30.8o C; Humidity of the flue gas after the system, 0.0278 kg H2O vapour/kg dry gas; Economiser’s heat load, 322 kW/m2;
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Pressure drop, 13 mm Hg; Economiser’s cross-section, 33 m2; Heat exchanger surface, 1800 m2. The utilisation of such a heat load requires 67.8 kg/s dry flue gas. The actual emission of flue gases from one unit is 811 kg/s, thus the contact economiser system recovers only 8.36% of the heat wasted by one unit. As a result, the entire crude water used by the station can be preheated in one utilisation plant, attached to one of the units of the station. Since the flue gases from 3 units are collected in one stack, the calculated temperature of the gas at the inlet of the stack would be 117.5o C. Thus, practically there is no change of the temperature regime of the stack. The recovered energy is equivalent to 4.6 MW extra electrical power. Assuming 90% efficiency of the boiler, the amount of coal to be saved would be 2.35 t/h. At a coal cost of R40 a ton, the savings of fuel would be R752,000 p.a. The usual running costs of such a heat recovery system are about 1% of recovered energy. In this case the payback period of the system would be 1.5 years. There is an additional effect to be accounted for, the recovery of water from the flue gases. The recovery is due to the condensation accompanying the flue gas cooling process. For the given case the amount of recovered water would be 2.8 kg/s, which represents 10.76% of the water consumption of one unit, or 1.79% of the station’s water demand. The possible re-use of this water requires preliminary decarbonisation and relevant purification. 4.2. Second generation CES case and its contribution to cleaner flue gas Assuming a 10% increase of the combustion air humidity (the beneficial level established during our experiments), this will result in higher wet-bulb temperature of the flue gas raised to 58.5° C, boosting the exit water temperature to 57.5° C. The energy recovery from one unit in this case would be 130.5 MW when the cooling of the gases goes up to 30.8° C. If the recovered heat is returned to the system through preheating the boiler feed water (40° C), then the utilised energy will be 100 MW. This amount would be supplemented by another 11.8 MW raw water preheating power, which would yield 111.8 MW total heat recovery. The stack gas temperature would be 97.5° C. The coal saving would amount to 21.5 t/h, equivalent to R6.88×106 p.a., that corresponds to 42.16 MW extra electric power. Thus the overall effect, as it was calculated in the previous case, would reach up to R21.5×106 p.a. This would improve the efficiency of the block by 6% and, respectively, the entire station efficiency by 1%. The additional ecological effect would reflect in the following: (i) decreasing the CO2 emissions by 250,000 t/y; (ii)
decrease the NOx emissions by 118 t/y. Considering the fact that 3 blocks are served by one stack, the expected decrease of gas temperature would be minimal (from 120 to 97.5° C). This, as well as considering the insignificant concentration of SO3 in the stack gases (15 ppm) and the twice decreased moisture content, will exclude the danger of corrosion of the chimney. The amount of water recovered from the flue gases would be 15.03 kg/s. This represents 16.4% of the fresh water for the entire power station (330 m3/h) and is approximately two times more than the boilers’ make-up water for all 6 blocks. The utilisation of this water depends on the water treatment facilities on site.
5. Thermal pinch analysis The drawing in Fig. 4 shows how the simultaneous heat and mass transfer processes can be replaced by a pure heat transfer model in two stages, sensible heat recovery and latent heat recovery of vapour condensation (if a dehumidification process is concerned) or evaporation (if a humidification process takes place). The two-stream simultaneous heat and mass transfer in direct contact is replaced by three fictitious streams: (a) One of the flue gas entering the column with moisture content equal to the exit flue gas moisture content; (b) Second, the water stream leaving the packed column with the flowrate of the inlet conditions, and (c) The third, water vapour transferred from one stream to the second one having inlet temperature of the inlet gas and outlet temperature equal to the exit water temperature. Because of the phase change, this stream in the general case is to be split into three segments (as the pinch technology suggests). One segment will represent the cooling of vapour to the condensation temperature, the second will correspond to the actual phase change and the third segment will deal with the cooling of water to the exit water conditions. Note that the three new streams have constant flowrate as required by classical pinch analysis. The composite curve (Fig. 5) is drawn for the power station from our case study and corresponds to a second generation CES of parallel type. It shows the maximum possible heat recovery and the amount of heat to be wasted to the surroundings. The case can be specified as a typical threshold problem [1]. It also represents the heat load distribution in both packed bed columns (the humidifier and the dehumidifier). Note that the minimum permissible temperature approach is much smaller than in a typical heat exchanger network and can be around 3° C.
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FG FG W Fig. 4. Definition of streams in a contact economiser. Legend/nomenclature: FM = cFG in + cout; F =flowrate of the dry flue gas; F =flowrate of the circulating water; FM=flue gas moisture to be extracted by the water stream; TM=moisture temperature; TFG=flue temperature; cFG=flue gas moisture content.
Fig. 5.
Composite curves. Fig. 6. Grant composite curve.
Any shift of the curves decreasing the ⌬Tmin cannot affect the utility amount, but can affect the temperature level of utility application (threshold problem). In our case ⌬Tmin⬍⌬Tthreshold. The design in this case is suggested to start at the “no utility end” with the hottest “hot” stream matching the hottest “cold” stream. The number of possible matches after this suggested action is quite high. The Grant composite curve (GCC) is shown in Fig. 6. It gives an indication of the minimum cold utility, the temperature level (condensation, preheating) of the utility (cooling water) and the favourable condensation and evaporation shift throughout the tower’s height. The actual benefit can be found after including into consideration the eventual receptor of the recovered heat returned back to the system. For consistency, in our case a raw water stream is used. The maximum amount of heat recovered and transferred back to the system as well as the temperature level (potential) of this heat can be found directly from the GCC following one of the pinch rules: “Generate utilities at the highest possible level”. The
process can be controlled within constraints such as the minimum natural draft temperature of the stack gases and the acid dew point of the flue. The bigger the energy recovery the higher the exit water temperature, the higher the recovered heat potential. Raising energy recovery shifts the pinch touching point in the GCC presentation upwards. This shift is followed by the cooling utility, which role in this case is played by the recovered energy transporting fluid. The maximum temperature level of this heat corresponds to the first appearance of the hot utility requirement.
6. Conclusions The analysis of the energy utilisation integrated with the analysis of the mass transfer supported by the pinch concept can raise the opportunity for better energy and water management through process integration, better system design and bigger savings. The implementation
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of contact economiser systems in particular can lead to substantial improvement of power generation efficiency, accompanied by valuable water production and cleaner emissions. It opens the door for future investigations of combined energy and water resources management at a total site level. The results presented are based on the simplest CES flowsheet of the second-generation (parallel type). The optimal design of such systems will be a subject of a further pinch study presented separately. The case study presented here gives enough confidence about the potential of the approach and the impressive ecological outcome of its application. Acknowledgements We are grateful that our industrial partner ESKOM recognises the importance of this work under its TESP programme. The NRF additional state support under the THRIP programme is also acknowledged and much appreciated. References [1] Gundersen T, Naess L. Synthesis of cost optimal HEN. Comp. and Chem. Eng 1988;12(6):503.
[2] El-Halwagi MM, Manousiouthakis V. Synthesis of mass-exchange networks. AIChE Journal 1989;35(8):1233. [3] Wang Y-P, Smith R. Wastewater minimisation. Chem Eng Sci 1994;49:981. [4] Zhelev T, Ntlhakana L. Energy-environment closed loop through oxygen pinch. Comp. and Chem. Eng 1999;23:S79. [5] Klemesˇ J. (ed) Simultaneous energy and water minimisation, Contract JOE—3-CT950036, EC JOULE III, Publishable report, Brussels 1997. [6] Savulescu LE, Smith R. Simultaneous energy and water minimisation. AIChE Annual Meeting, 1998, Paper 13c. [7] El-Halwagi MM, Manousiouthakis V. Synthesis of mass exchange networks. AIChE J. 1989;8:1233. [8] Wang YP, Smith R. Wastewater mimimisation. Chem. Eng. Sci. 1994;13:981. [9] Kolev N, Darakchiev R, Semkov K. Systems containing contact economisers. In: Energy efficiency in process technology. London and New York: Elsevier; 1993. p. 68-3.
T.K. Zhelev holds an Associate Professor position at the University of Durban–Westville, Durban, South Africa. He spent between 1991 and 1994 at the Department of Process Integration, UMIST, UK. He is a Chartered Engineer, MIChemE and SAIChE.
K.A. Semkov has been professionally involved in research in heat and mass transfer of gas–liquid systems for more than 25 years. He is an Associate Professor of the Institute of Chemical Engineers, Bulgarian Academy of Sciences. He is co-developer of a number of industrial applications involving economizers and other practical equipment.