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Cost-benefit analysis of electrocyclone and cyclone
Resources Conservation and Recycling 31 (2001) 285–292
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Cost-benefit analysis of electrocyclone and cyclone Chi-Jen Chen a,b, Leonard F.S. Wang b,c,* a
Department of En6ironmental Engineering and Health, Tajen Institute of Technology, En-Pu, Ping-Tung 907, Taiwan, ROC b Department of Business Management, National Sun Yat-sen Uni6ersity, Kaoshiung 804, Taiwan, ROC c Department of Applied Economics, National Uni6ersity of Kaoshiung, Kaohsiung 811, Taiwan, ROC Received 6 December 1999; accepted 12 September 2000
Abstract We propose a new method of cost-benefit analysis to investigate whether the new air pollution control equipment-electrocyclone vis-a-vis cyclone has the potential for practical use. When the flow rate of waste gas is 1000 m3/min, the cost of cyclone is then compared with that of electrocyclone provided that the benefit side is being fixed. The results show that the capital cost of electrocyclone is higher than that of cyclone, but the operating cost of electrocyclone is much lower than that of cyclone. Straight-line depreciation method is used to calculate the depreciation of capital cost per year. The total cost of electrocyclone is NT$ 160 290 per year which is cheaper than that of cyclone NT$ 225 356. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cost-benefit analysis; Electrocyclone; Cyclone; Straight-line depreciation
1. Introduction Cyclones are simple, mechanical collectors which use centrifugal forces to separate particles from the gas stream. They are relatively inexpensive to fabricate, economical to operate, and adaptable to a wide range of operating conditions. Cyclones can typically achieve moderate to high efficiencies for particles larger than about 5 mm in diameter and can operate at very high dust loading. Cyclones are * Corresponding author. Tel.: +886-7-5252000/4647; fax: + 886-7-5254648. E-mail address: [email protected] (L.F.S. Wang). 0921-3449/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-3449(00)00086-0
286
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
used for the removal of large particles for both air pollution control and process use. Application in air pollution includes the control of grain dust, sawdust, and rock dust. They are also used as a precleaner upstream of a high-efficiency particulate control device. Process applications presented by Overcamp and Mantha (1998) include catalyst recovery in petrochemical plants and product recovery downstream of spray dryers. Much research has been done on the improvement of collection efficiency of the cyclone by adjusting the geometry and operation conditions. Leith and Mehta (1973) used an optimization procedure to develop a cyclone design method and found it theoretically to have better efficiency compared with a standard high efficiency cyclone under identical operating conditions. Biffin and Syred (1983) introduced a novel design of cyclone separator and presented some results obtained in optimizing the design. The new design, which effectively combined two stages in one unit, could increase collection efficiency. Kim and Lee (1990) discussed the effect of the geometry on collection efficiency and reported that as the exit tube size was reduced, the collection efficiency increased. Hoffmann et al. (1992) investigated the influence of dust loading and gas inlet and outlet dimensions on the performance of cyclones. They found that increasing the solid loading increased the collection efficiency. Boericke et al. (1983) used a electrostatically augmented cyclone to remove the coal ash from combustion gas at high temperature and pressure and concluded that the improvement of total efficiency was significant when applying an external high voltage. Plucinski et al. (1989) analyzed the centrifugal force and electrostatic force acting on a particle. The results showed that an electric field strength of 8 kv/cm had a significant effect on its collection efficiency. The effect of the electric field was particularly important for smaller particles. Although much research has been done on the improvement of cyclone collection efficiency and have had significant progress technically, little work has been done on the cost-benefit analysis. By fixing the benefits of cyclone and electrocyclone devices, the cost of the electrocyclone could be compared with that of the cyclone. The purpose of this paper is to analyze the cost of cyclone and electrocyclone devices and provide the probability of application of the electrocyclone.
2. Cyclone and electrocyclone The cyclone used in this study is one of the most often used types in the industry, and is generally named as a reverse-flow cyclone. The 0.22-cm diameter cyclone shown in Fig. 1 has been designed to measure the collection efficiency of fly ash at various flow rates and feeding rates. The system consisted of a dust feeder, a cyclone, and a fan. Air was drawn through the system by a fan located near the outlet. A screw feeder was used to introduce the flyash at feed rates in the range 7.2 – 21.2 g/min and the flow rates in the range 15.8 –44.3 m3/min. The source of the particles came from the flyash of a coal-fired power plant. The density of the flyash was 2.2 g/cm3 and the particle size distribution upstream of the cyclone ranged
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
287
Fig. 1. Schematic diagram of electrocyclone.
from 0.5 to 30 mm. The results have been published by Chen et al. (1999) and listed in Table 1. The electrocyclone concept is a synthesis of cyclone and electrostatic precipitator. By applying the high voltage electric field in the cyclone, the electrostatic force is incorporated into the centrifugal force so that the efficiency of operation can be enhanced. A principal advantage of the electrocyclone is that a large diameter unit can be built to give the same efficiency as a small conventional cyclone. Another principal advantage of the electrocyclone is its cost. Cost estimates indicate a potential advantage in plant cost of $40 –180 per kw using the electrocyclone as Table 1 Collection efficiency of cyclone at various flow rates and feeding rates
288
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
Table 2 Collection efficiency of electrocyclone at various flow rates and feeding rates
compared to low face velocity approache designs, which are necessarily much larger. This cost advantage is the key to early commercialization of the PFBC system. Other advantages of the electrocyclone are apparent in turndown operation and in its self-cleaning capability. During part-load operation of a PFBC power plant, the gas turbine inlet temperature and volumetric flow are reduced, resulting in a lower inlet velocity in the electrocyclone. While this velocity reduction lowers the inertial force, the electrostatic force is unaffected and the longer gas residence time at part load results in an overall improvement in efficiency (Boericke et al., 1983). The schematic diagram of the experimental system is shown in Fig. 1. The electrocyclone which used a high voltage corona electrode to charge the particles and improved the separation efficiency was the combination of the traditional cyclone and electrostatic precipitator. The results have been also published by Chen et al. (1999) and listed in Table 2. Based on the results of Tables 1 and 2, the feeding rate multiplied by collection efficiency could obtain the mass accumulation rate of fly ash in the hopper. The results are listed in Tables 3 and 4. Traditionally, most researchers seek after the improvement of cyclone collection efficiency. Little work has been done on the cost-benefit analysis. In order to have practical application, the cost-benefit analysis is necessary especially for new air pollution equipment. In this paper, the costbenefit analysis will be done using a new concept. The mass rate of collected fly ash in the hopper will be utilized to be the benefit of both cyclone and electrocyclone because the air quality improved by the increasing efficiency of air pollution control equipment is very hard to determine. By fixing on this benefit, the cost of the electrocyclone could be compared with that of the cyclone. If the cost of electrocyclone is less than that of the cyclone, we could say that the new equipment has the potential to be practical.
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
289
Table 3 Fly ash mass rates of cyclone in the hopper
3. Cost-benefit analysis Traditionally, the cost-benefit analysis evaluates the monetary value of the benefit, divided by the cost and compares them with each other. But, because the benefits (air pollution prevention) of the cyclone and electrocyclone are difficult to determine, the methodology used in this study, therefore, is to fix the benefit of both cyclone and electrocyclone, mainly comparing the cost of the two units; the cheaper one having higher potential for being chosen for practical use. The first step of cost-benefit analysis is to fix the benefit (the mass rate of fly ash in the Table 4 Fly ash mass rates of electrocyclone in the hopper
290
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Table 5 Selected operating conditions of cyclone and electrocyclone
Cyclone Electrocyclone
Flow rate (m3/min)
Feeding rate (g/min)
Flyash mass rate (g/min)
Concentration (g/m3)
44.3 23.7
13.9 7.2
10.84 6.88
0.3 0.3
hopper) of cyclone and electrocyclone. The operation conditions of the cyclone and electrocyclone selected are listed in Table 5 based on Tables 3 and 4. If two electrocyclones are used to collect the fly ash, the fly ash mass rate in the hopper will be a little more than one cyclone. It means that the benefit from two electrocyclones will be a little higher than one cyclone. If we could prove that the cost of these two electrocyclones would be cheaper than one cyclone, the potential of the electrocyclone for application will be verified. In order to be closer to the real situation, we will take a flow rate of 1000 m3/min for the example. According to Table 5, the number of cyclones is 23 and electrocyclones is 43 to treat the same 1000 m3/min flow rate. The second step is to evaluate the capital cost of both cyclones and electrocyclones. There are three main parts including cyclone bodies, fan and motor that have to be considered in the capital cost calculation of a cyclone. The prices of the fan and motor of the electrocyclone are NT$ 150 000 ($1=NT$31) and NT$ 200 000, which are much more expansive than those of the cyclone (NT$ 30 000 and NT$ 20 000) because the pressure drop across the cyclone is 2.3 in Aq, which is much higher than that of the electrocyclone (0.5 in Aq). In addition to the three main parts of cyclone, the electrocyclones have extra electric devices. As listed in Table 6, the total capital cost of cyclone is NT$ 650 000 and electrocyclone is NT$ 1 080 000. The third step is to calculate the operating costs of cyclone and electrocyclone. The operating cost of cyclones is entirely from the energy consumption of the fan and motor. The power consumption is calculated by the following equation: W =0.00415 ×Q × DP Table 6 Capital cost of cyclone and electrocyclone Cyclone Number Cyclone bodies Fan Motor Electric devices Total capital cost
23 300 000 150 000 200 000 0 650 000
Electrocyclone 43 600 000 30 000 20 000 430 000 1 080 000
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291
Table 7 Operating cost of cyclone and electrocyclone per year Cyclone Flow rate (m3/min) DP (in Aq) W (kw) Electric devices (kw) Energy consumption (kw-h per year) Unit price of electric power Operating cost
1000 2.3 9.545 0 80 178 2 160 356
Electrocyclone 1000 0.5 2.075 1.0375 26 145 2 52 290
where W is power consumption (kw), Q is flow rate (m3/min) and DP is pressure drop (in Aq). Table 7 shows that the value of energy consumption is 9.545 kw for cyclone and 2.075 kw for electrocyclone. In addition to the energy consumption of fan and motor, the electrocyclone has extra electric power consumption from the high voltage electrode inside the electrocyclone. The applied voltage within the electrocyclone is 25 kv, which is about half of the applied voltage in the general electrostatic precipitator. The energy consumed by the electric devices is about half of the electrostatic precipitator assuming that the corona currents of electrocyclone and the general electrostatic precipitator are the same. To treat the same flow rate of waste gas, the energy consumption of the general electrostatic precipitator is almost the same as that of the cyclone (Chen, 1994). Thus, the energy consumption of the electric devices is half of the electrocyclone. As listed in Table 7, the energy consumption is 1.0375 kw. The total energy consumption is 80 178 kw-h per year for cyclone and 26145 kw-h per year for electrocyclone. For a plant running 350 days per year and the price of 1 kw-h is NT$ 2, the operating cost of cyclones and electrocyclones are listed in Table 7. The results show that the operating cost of cyclones per year is NT$ 160 356, which is much higher than that of electrocyclone NT$ 52 290. This means that the energy consumption of electrocyclone is much lower than that of cyclone. The last step is to calculate the total cost. Due to the maintenance of the cyclone is negligible (Doerschlag and Miczek, 1977) the maintenance cost is negligible. The total cost is the sum of the operating cost per year plus the depreciation of the devices per year. We use the most popular method, straight-line depreciation (Zerbe and Dively, 1994), to calculate and assume that the useful lives of both cyclones and electrocyclones are ten years. The depreciation of cyclones and electrocyclones are calculated by the following equation: K −S D(t) = N where D(t) is the depreciation per year, K is capital cost, S is the scrap at the end of the useful life and N is the useful life. The results of total cost calculated by the previous equation based on Tables 6 and 7 are listed in Table 8. The results show that the total cost of electrocyclone per year is NT$ 160 290 which is cheaper than that of cyclone NT$ 225 356.
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Table 8 Total cost of cyclone and electrocyclone
Operating cost Depreciation Total cost
Cyclone
Electrocyclone
160 356 65 000 225 356
52 290 108 000 160 290
4. Conclusion This paper has presented a new concept for cost-benefit analysis of cyclone and electrocyclone. By fixing the benefit (mass flow rates of collected flyash in the hopper), the cost of electrocyclone could be compared with that of cyclone. The results indicate that the capital cost of the cyclone is NT$ 650 000 and electrocyclone is NT$ 1 080 000. The operating cost of the cyclone is NT$ 160 356 per year which is much higher than that of the electrocyclone NT$ 52 290 per year. It means that the energy consumption of the electrocyclone is much lower than that of the cyclone. Straight-line depreciation method is used to calculate the depreciation of the capital cost. The total cost of the electrocyclone is NT$ 160 290 per year which is cheaper than that of the cyclone NT$ 225 356. On the basis of these results, we can conclude that the new air pollution control equipment-electrocyclone has lower totals cost by saving energy consumption than that of the traditional cyclone. The new equipment has the potential to be practical.
References Biffin M, Syred N. A Novel Design of Cyclone Dust Separator, Filtration and Separation, 1983:189– 91. Boericke RR, Giles WG, Dietz PW, Kallio GA, Kuo JT. Electrocyclone for high-pressur dust removal. J Energ 1983;7(1):43–9. Chen CJ. Study on the Collection Efficiency of Fly Ash by Electrocyclone, Master Thesis. Taichung, Taiwan: National Chung-Hsing University, 1994. Chen CJ, Wang LFS, Chang MT. Enhanced total collection efficiency of fly ash by combining cyclone with electrostatic precipitator. Industr Pollut Preven Control 1999;18(1):40– 60. Doerschlag C, Miczek G. How to choose a cyclone dust collector. Chem Eng 1977;14:64– 72. Hoffmann AC, van Santen A, Allen RWK, Clift R. Effects of geometry and solid loading on the performance of gas cyclones. Powder Technol 1992;70:83– 91. Kim JC, Lee KW. Experimental study of particle collection by small cyclones. Aerosol Sci Technol 1990;12:1003–15. Leith D, Mehta D. Cyclone performance and design. Atmosph Environ 1973;7:527– 49. Overcamp TJ, Mantha SV. A simple method for estimating cyclone efficiency. Environ Progr 1998;17(2):77–9. Plucinski J, Gradon L, Nowicki J. Collection of Aerosol particles in a cyclone with an external electric field. J Aerosol Sci 1989;20(5):695–700. Zerbe RO, Dively DD. Benefit-Cost Analysis: In Theory and Practice. New York, NY: The Lehigh Press, 1994. .
www.elsevier.com/locate/resconrec
Cost-benefit analysis of electrocyclone and cyclone Chi-Jen Chen a,b, Leonard F.S. Wang b,c,* a
Department of En6ironmental Engineering and Health, Tajen Institute of Technology, En-Pu, Ping-Tung 907, Taiwan, ROC b Department of Business Management, National Sun Yat-sen Uni6ersity, Kaoshiung 804, Taiwan, ROC c Department of Applied Economics, National Uni6ersity of Kaoshiung, Kaohsiung 811, Taiwan, ROC Received 6 December 1999; accepted 12 September 2000
Abstract We propose a new method of cost-benefit analysis to investigate whether the new air pollution control equipment-electrocyclone vis-a-vis cyclone has the potential for practical use. When the flow rate of waste gas is 1000 m3/min, the cost of cyclone is then compared with that of electrocyclone provided that the benefit side is being fixed. The results show that the capital cost of electrocyclone is higher than that of cyclone, but the operating cost of electrocyclone is much lower than that of cyclone. Straight-line depreciation method is used to calculate the depreciation of capital cost per year. The total cost of electrocyclone is NT$ 160 290 per year which is cheaper than that of cyclone NT$ 225 356. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cost-benefit analysis; Electrocyclone; Cyclone; Straight-line depreciation
1. Introduction Cyclones are simple, mechanical collectors which use centrifugal forces to separate particles from the gas stream. They are relatively inexpensive to fabricate, economical to operate, and adaptable to a wide range of operating conditions. Cyclones can typically achieve moderate to high efficiencies for particles larger than about 5 mm in diameter and can operate at very high dust loading. Cyclones are * Corresponding author. Tel.: +886-7-5252000/4647; fax: + 886-7-5254648. E-mail address: [email protected] (L.F.S. Wang). 0921-3449/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-3449(00)00086-0
286
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
used for the removal of large particles for both air pollution control and process use. Application in air pollution includes the control of grain dust, sawdust, and rock dust. They are also used as a precleaner upstream of a high-efficiency particulate control device. Process applications presented by Overcamp and Mantha (1998) include catalyst recovery in petrochemical plants and product recovery downstream of spray dryers. Much research has been done on the improvement of collection efficiency of the cyclone by adjusting the geometry and operation conditions. Leith and Mehta (1973) used an optimization procedure to develop a cyclone design method and found it theoretically to have better efficiency compared with a standard high efficiency cyclone under identical operating conditions. Biffin and Syred (1983) introduced a novel design of cyclone separator and presented some results obtained in optimizing the design. The new design, which effectively combined two stages in one unit, could increase collection efficiency. Kim and Lee (1990) discussed the effect of the geometry on collection efficiency and reported that as the exit tube size was reduced, the collection efficiency increased. Hoffmann et al. (1992) investigated the influence of dust loading and gas inlet and outlet dimensions on the performance of cyclones. They found that increasing the solid loading increased the collection efficiency. Boericke et al. (1983) used a electrostatically augmented cyclone to remove the coal ash from combustion gas at high temperature and pressure and concluded that the improvement of total efficiency was significant when applying an external high voltage. Plucinski et al. (1989) analyzed the centrifugal force and electrostatic force acting on a particle. The results showed that an electric field strength of 8 kv/cm had a significant effect on its collection efficiency. The effect of the electric field was particularly important for smaller particles. Although much research has been done on the improvement of cyclone collection efficiency and have had significant progress technically, little work has been done on the cost-benefit analysis. By fixing the benefits of cyclone and electrocyclone devices, the cost of the electrocyclone could be compared with that of the cyclone. The purpose of this paper is to analyze the cost of cyclone and electrocyclone devices and provide the probability of application of the electrocyclone.
2. Cyclone and electrocyclone The cyclone used in this study is one of the most often used types in the industry, and is generally named as a reverse-flow cyclone. The 0.22-cm diameter cyclone shown in Fig. 1 has been designed to measure the collection efficiency of fly ash at various flow rates and feeding rates. The system consisted of a dust feeder, a cyclone, and a fan. Air was drawn through the system by a fan located near the outlet. A screw feeder was used to introduce the flyash at feed rates in the range 7.2 – 21.2 g/min and the flow rates in the range 15.8 –44.3 m3/min. The source of the particles came from the flyash of a coal-fired power plant. The density of the flyash was 2.2 g/cm3 and the particle size distribution upstream of the cyclone ranged
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
287
Fig. 1. Schematic diagram of electrocyclone.
from 0.5 to 30 mm. The results have been published by Chen et al. (1999) and listed in Table 1. The electrocyclone concept is a synthesis of cyclone and electrostatic precipitator. By applying the high voltage electric field in the cyclone, the electrostatic force is incorporated into the centrifugal force so that the efficiency of operation can be enhanced. A principal advantage of the electrocyclone is that a large diameter unit can be built to give the same efficiency as a small conventional cyclone. Another principal advantage of the electrocyclone is its cost. Cost estimates indicate a potential advantage in plant cost of $40 –180 per kw using the electrocyclone as Table 1 Collection efficiency of cyclone at various flow rates and feeding rates
288
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
Table 2 Collection efficiency of electrocyclone at various flow rates and feeding rates
compared to low face velocity approache designs, which are necessarily much larger. This cost advantage is the key to early commercialization of the PFBC system. Other advantages of the electrocyclone are apparent in turndown operation and in its self-cleaning capability. During part-load operation of a PFBC power plant, the gas turbine inlet temperature and volumetric flow are reduced, resulting in a lower inlet velocity in the electrocyclone. While this velocity reduction lowers the inertial force, the electrostatic force is unaffected and the longer gas residence time at part load results in an overall improvement in efficiency (Boericke et al., 1983). The schematic diagram of the experimental system is shown in Fig. 1. The electrocyclone which used a high voltage corona electrode to charge the particles and improved the separation efficiency was the combination of the traditional cyclone and electrostatic precipitator. The results have been also published by Chen et al. (1999) and listed in Table 2. Based on the results of Tables 1 and 2, the feeding rate multiplied by collection efficiency could obtain the mass accumulation rate of fly ash in the hopper. The results are listed in Tables 3 and 4. Traditionally, most researchers seek after the improvement of cyclone collection efficiency. Little work has been done on the cost-benefit analysis. In order to have practical application, the cost-benefit analysis is necessary especially for new air pollution equipment. In this paper, the costbenefit analysis will be done using a new concept. The mass rate of collected fly ash in the hopper will be utilized to be the benefit of both cyclone and electrocyclone because the air quality improved by the increasing efficiency of air pollution control equipment is very hard to determine. By fixing on this benefit, the cost of the electrocyclone could be compared with that of the cyclone. If the cost of electrocyclone is less than that of the cyclone, we could say that the new equipment has the potential to be practical.
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
289
Table 3 Fly ash mass rates of cyclone in the hopper
3. Cost-benefit analysis Traditionally, the cost-benefit analysis evaluates the monetary value of the benefit, divided by the cost and compares them with each other. But, because the benefits (air pollution prevention) of the cyclone and electrocyclone are difficult to determine, the methodology used in this study, therefore, is to fix the benefit of both cyclone and electrocyclone, mainly comparing the cost of the two units; the cheaper one having higher potential for being chosen for practical use. The first step of cost-benefit analysis is to fix the benefit (the mass rate of fly ash in the Table 4 Fly ash mass rates of electrocyclone in the hopper
290
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Table 5 Selected operating conditions of cyclone and electrocyclone
Cyclone Electrocyclone
Flow rate (m3/min)
Feeding rate (g/min)
Flyash mass rate (g/min)
Concentration (g/m3)
44.3 23.7
13.9 7.2
10.84 6.88
0.3 0.3
hopper) of cyclone and electrocyclone. The operation conditions of the cyclone and electrocyclone selected are listed in Table 5 based on Tables 3 and 4. If two electrocyclones are used to collect the fly ash, the fly ash mass rate in the hopper will be a little more than one cyclone. It means that the benefit from two electrocyclones will be a little higher than one cyclone. If we could prove that the cost of these two electrocyclones would be cheaper than one cyclone, the potential of the electrocyclone for application will be verified. In order to be closer to the real situation, we will take a flow rate of 1000 m3/min for the example. According to Table 5, the number of cyclones is 23 and electrocyclones is 43 to treat the same 1000 m3/min flow rate. The second step is to evaluate the capital cost of both cyclones and electrocyclones. There are three main parts including cyclone bodies, fan and motor that have to be considered in the capital cost calculation of a cyclone. The prices of the fan and motor of the electrocyclone are NT$ 150 000 ($1=NT$31) and NT$ 200 000, which are much more expansive than those of the cyclone (NT$ 30 000 and NT$ 20 000) because the pressure drop across the cyclone is 2.3 in Aq, which is much higher than that of the electrocyclone (0.5 in Aq). In addition to the three main parts of cyclone, the electrocyclones have extra electric devices. As listed in Table 6, the total capital cost of cyclone is NT$ 650 000 and electrocyclone is NT$ 1 080 000. The third step is to calculate the operating costs of cyclone and electrocyclone. The operating cost of cyclones is entirely from the energy consumption of the fan and motor. The power consumption is calculated by the following equation: W =0.00415 ×Q × DP Table 6 Capital cost of cyclone and electrocyclone Cyclone Number Cyclone bodies Fan Motor Electric devices Total capital cost
23 300 000 150 000 200 000 0 650 000
Electrocyclone 43 600 000 30 000 20 000 430 000 1 080 000
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291
Table 7 Operating cost of cyclone and electrocyclone per year Cyclone Flow rate (m3/min) DP (in Aq) W (kw) Electric devices (kw) Energy consumption (kw-h per year) Unit price of electric power Operating cost
1000 2.3 9.545 0 80 178 2 160 356
Electrocyclone 1000 0.5 2.075 1.0375 26 145 2 52 290
where W is power consumption (kw), Q is flow rate (m3/min) and DP is pressure drop (in Aq). Table 7 shows that the value of energy consumption is 9.545 kw for cyclone and 2.075 kw for electrocyclone. In addition to the energy consumption of fan and motor, the electrocyclone has extra electric power consumption from the high voltage electrode inside the electrocyclone. The applied voltage within the electrocyclone is 25 kv, which is about half of the applied voltage in the general electrostatic precipitator. The energy consumed by the electric devices is about half of the electrostatic precipitator assuming that the corona currents of electrocyclone and the general electrostatic precipitator are the same. To treat the same flow rate of waste gas, the energy consumption of the general electrostatic precipitator is almost the same as that of the cyclone (Chen, 1994). Thus, the energy consumption of the electric devices is half of the electrocyclone. As listed in Table 7, the energy consumption is 1.0375 kw. The total energy consumption is 80 178 kw-h per year for cyclone and 26145 kw-h per year for electrocyclone. For a plant running 350 days per year and the price of 1 kw-h is NT$ 2, the operating cost of cyclones and electrocyclones are listed in Table 7. The results show that the operating cost of cyclones per year is NT$ 160 356, which is much higher than that of electrocyclone NT$ 52 290. This means that the energy consumption of electrocyclone is much lower than that of cyclone. The last step is to calculate the total cost. Due to the maintenance of the cyclone is negligible (Doerschlag and Miczek, 1977) the maintenance cost is negligible. The total cost is the sum of the operating cost per year plus the depreciation of the devices per year. We use the most popular method, straight-line depreciation (Zerbe and Dively, 1994), to calculate and assume that the useful lives of both cyclones and electrocyclones are ten years. The depreciation of cyclones and electrocyclones are calculated by the following equation: K −S D(t) = N where D(t) is the depreciation per year, K is capital cost, S is the scrap at the end of the useful life and N is the useful life. The results of total cost calculated by the previous equation based on Tables 6 and 7 are listed in Table 8. The results show that the total cost of electrocyclone per year is NT$ 160 290 which is cheaper than that of cyclone NT$ 225 356.
292
C.-J. Chen, L.F.S. Wang / Resources, Conser6ation and Recycling 31 (2001) 285–292
Table 8 Total cost of cyclone and electrocyclone
Operating cost Depreciation Total cost
Cyclone
Electrocyclone
160 356 65 000 225 356
52 290 108 000 160 290
4. Conclusion This paper has presented a new concept for cost-benefit analysis of cyclone and electrocyclone. By fixing the benefit (mass flow rates of collected flyash in the hopper), the cost of electrocyclone could be compared with that of cyclone. The results indicate that the capital cost of the cyclone is NT$ 650 000 and electrocyclone is NT$ 1 080 000. The operating cost of the cyclone is NT$ 160 356 per year which is much higher than that of the electrocyclone NT$ 52 290 per year. It means that the energy consumption of the electrocyclone is much lower than that of the cyclone. Straight-line depreciation method is used to calculate the depreciation of the capital cost. The total cost of the electrocyclone is NT$ 160 290 per year which is cheaper than that of the cyclone NT$ 225 356. On the basis of these results, we can conclude that the new air pollution control equipment-electrocyclone has lower totals cost by saving energy consumption than that of the traditional cyclone. The new equipment has the potential to be practical.
References Biffin M, Syred N. A Novel Design of Cyclone Dust Separator, Filtration and Separation, 1983:189– 91. Boericke RR, Giles WG, Dietz PW, Kallio GA, Kuo JT. Electrocyclone for high-pressur dust removal. J Energ 1983;7(1):43–9. Chen CJ. Study on the Collection Efficiency of Fly Ash by Electrocyclone, Master Thesis. Taichung, Taiwan: National Chung-Hsing University, 1994. Chen CJ, Wang LFS, Chang MT. Enhanced total collection efficiency of fly ash by combining cyclone with electrostatic precipitator. Industr Pollut Preven Control 1999;18(1):40– 60. Doerschlag C, Miczek G. How to choose a cyclone dust collector. Chem Eng 1977;14:64– 72. Hoffmann AC, van Santen A, Allen RWK, Clift R. Effects of geometry and solid loading on the performance of gas cyclones. Powder Technol 1992;70:83– 91. Kim JC, Lee KW. Experimental study of particle collection by small cyclones. Aerosol Sci Technol 1990;12:1003–15. Leith D, Mehta D. Cyclone performance and design. Atmosph Environ 1973;7:527– 49. Overcamp TJ, Mantha SV. A simple method for estimating cyclone efficiency. Environ Progr 1998;17(2):77–9. Plucinski J, Gradon L, Nowicki J. Collection of Aerosol particles in a cyclone with an external electric field. J Aerosol Sci 1989;20(5):695–700. Zerbe RO, Dively DD. Benefit-Cost Analysis: In Theory and Practice. New York, NY: The Lehigh Press, 1994. .