Environ. Sci. Technol. 2009, 43, 6101–6105
Energy Recovery Efficiency and Cost Analysis of VOC Thermal Oxidation Pollution Control Technology ARUNA S. K. WARAHENA AND YEW KHOY CHUAH* Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
Received March 30, 2009. Revised manuscript received May 8, 2009. Accepted June 11, 2009.
Thermal oxidation of VOC is extremely energy intensive, and necessitates high efficiency heat recovery from the exhaust heat. In this paper, two independent parameters heat recovery factor (HRF) and equipment cost factor (ECF) are introduced. HRF and ECF can be used to evaluate separately the merits of energy efficiency and cost effectiveness of VOC oxidation systems. Another parameter equipment cost against heat recovery (ECHR) which is a function of HRF and ECF is introduced to evaluate the merit of different systems for the thermal oxidation of VOC. Respective cost models were derived for recuperative thermal oxidizer (TO) and regenerative thermal oxidizer (RTO). Application examples are presented to show the use and the importance of these parameters. An application examples show that TO has a lower ECF while RTO has a higher HRF. However when analyzed using ECHR, RTO would be of advantage economically in longer periods of use. The analytical models presented can be applied in similar environmental protection systems.
1. Introduction Volatile organic compounds (VOC) emissions are important environmental problems recognized in recent years. VOC’s are widely used in semiconductor industries, chemical plants, paint booths, and printing industries. While thermal oxidation has been found to be effective in controlling VOC emissions, the use of large amounts of thermal energy is an environmental problem of concern. In a thermal oxidation process, VOC-laden air is thermally treated (oxidized/decomposed) at temperatures about 730-850 °C. This high temperature requirement results in intensive energy consumption, and therefore highest possible heat recovery efficiency would be needed in addition to the VOC destruction efficiency. This study considered both the cost of heat recovery equipment and the heat recovery efficiency in the analysis. There would be a point beyond which the marginal heat recovered may not payback the cost incurred for further increasing of heat recovery efficiency. The limit of energy saving often depends upon the payback time of the extra cost incurred for further efficiency improvements. Hence, the analysis of equipment cost against energy recovery is important in the thermal oxidation technology of VOC emissions. Murphy et al. (1) focused on the material and energy analysis in wafer fabrication in the semiconductor industry. * Corresponding author e-mail:
[email protected]. 10.1021/es900626e CCC: $40.75
Published on Web 06/24/2009
2009 American Chemical Society
Extension of such study to focus on the maximizing of energy efficiency associated with VOC thermal oxidation would benefit not only the semiconductor industry, but also many other industries. Maximizing energy efficiency in thermal processes was also emphasized in green engineering principles as mentioned by Murphy et al. (1). There are some studies on modeling, simulation, and evaluation, related to heat recovery associated with VOC thermal oxidation. Kays and London (2) presented extensive data on regenerative heat exchangers. Amelio and Morrone (3) worked on the simulation of energy performance of packed beds. Choi and Yi (4) worked on the simulation and optimization of regenerative thermal oxidizers. Abanto, Reggio, and Ouellet (5) used CFD in the design of RTO. Monte (6) studied the thermal response in the switching of fixedbed heat regenerators. Skiepo and Shah (7) studied the effect of leakages on heat transfer for fixed matrix regenerators. Warahena and Chuah (8) presented an analysis on the industrial application of regenerative heat recovery. However, the above works did not address the equipment cost concerns related to heat recovery efficiency. As an extension of RTO application, M. Bannai et al. (9) in their study of the development of efficiency enhanced cogeneration systems utilized high-temperature exhaust gas from RTO. A versatile mathematical model to design and evaluate RTO was presented by Warahena and Chuah (10). Most of the recent research works focused on VOC abatement and energy efficiency, but less consideration is given for the cost of the equipments. Warahena and Chuah (11) have also included the equipment costs into their study of TO and RTO heat recovery. This study is an extension of previous work by the authors. Parameters of merits that combine energy efficiency and cost factors in the practical aspect of value engineering are presented.
2. VOC Emission Treatments In semiconductor fabrication or in manufacturing of liquid crystal displays, emissions of VOC are strictly regulated in many countries, such as in Taiwan. The emission volume is enormous and therefore would consume great amounts of energy in the treatment of VOC. Usually millions of cubic meters of natural gas would be required each year for a single semiconductor plant, unless effective heat recovery is achieved. Energy saving consideration in the VOC emission treatment would involve both concentration and thermal oxidation of VOC laden air. The state of the art concentrator as shown in Figure 1 is used to reduce the amount of air (to 1/10 or below) carrying VOC. The concentration of VOC laden air
FIGURE 1. Schematic of VOC thermal oxidation process including heat recovery and VOC concentration. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The definition of Ntu for a heat exchanger is, Ntu )
US US ) ˙ Fcp m ˙ cp V
(3)
Where S, heat transfer surface area, m2; U, overall heat transfer ˙ , volume flow rate, m3/s; F, air density, coefficient, W/m2°C; V 3 kg/m ; cp, air specific heat capacity, J/kg°C. Therefore from eqs 2 and 3, η)f FIGURE 2. Schematics of TO with the heat recovery heat exchanger for VOC concentration.
( ) US ˙ Fcp V
(4)
The overall heat transfer coefficient U in eq 4 would stay unchanged for the same equipment design. For given design conditions and operation ranges, variation of F and cp are less significant. Therefore, the efficiency η is a function of heat transfer surface area and volume flow rate as in eq 5. ˙) η ) f (S, V
(5)
Equation 5 can be rewritten as eq 6, ˙) S ) f (η, V
(6)
By rearranging eq 1, and by substituting for Ntu from eq 3,
( )
1 US ) exp ˙ Fcp 1-η V
(7)
Maximum efficiency (η ) 1) corresponds to infinitely large heat transfer surface area. Therefore, the cost of a thermal oxidizer is related to the heat recovery efficiency η. Then a cost function CTO can be introduced as eq 8, FIGURE 3. Schematics of RTO with the heat recovery heat exchanger for VOC concentration. involves the adsorption of VOC in a concentrator, and subsequent desorption of the concentrator using a much smaller air volume which is heated to temperatures of above 200 °C. As shown in Figure 1, the reduced volume of concentrated VOC laden air is preheated in the heat recovery device, before undergoing VOC thermal oxidation.
3. Heat Recovery Mechanisms Both regenerative and recuperative thermal oxidizers have been proved to be effective for VOC thermal oxidation. Figures 2 and 3 are the schematics that show the designs of the TO and RTO systems. Both systems have heat recovery for thermal oxidation and for VOC concentration.
4. Equipment Cost Model Costs of TO and RTO systems can be represented by some key parameters that are related to energy efficiency. This study used application data provided by a major supplier of TO and RTO for industrial air-pollution control. The cost models were derived from the efficiency models and the equipment cost data. Cost Model for TO. Thermal oxidizers (TO) can be considered as a counter flow heat exchanger. In heat exchanger analysis for gas, efficiency η is related to number of transfer units (Ntu), according Kays and London (2), as, η ) 1 - exp(-Ntu)
(1)
η ) f(Ntu)
(2)
Therefore,
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CTO ) f
( 1 -1 η , R) ) f [exp( V˙USFc ), R]
(8)
p
In eq 8, S can be seen as the key cost sensitive parameter of TO. Therefore the cost function can be written as, CTO ) f [exp f1(S), R]
(9)
Where R is a currency parameter for economic analysis. Then eq 9 can be written for the cost model of thermal oxidizer CTO, for actual application as eqs 10-12. CTO ) 103.75Rexp(0.0025S)
(10)
˙ ) × expb.η S ) A(V
(11)
˙ ) ) 1.1958 × exp0.0002V˙ A(V
(12)
In the above equations, R is the currency parameter set as 1000 (relative to U.S. dollars), b ) 6.4. The applicable range ˙ from 1000 of the above equations is for volume flow rate V to 10000 m3/h. The efficiency of the heat recovery preheater, η, usually ranges from 55 to 70%. The above empirical eqs 10-12 were obtained from application data provided by a supplier. Similar models can be formed with any given supplier’s data. Costing Model for RTO. The efficiency of a regenerative thermal oxidizer (RTO) approaches the efficiency of a counter flow heat exchanger, since the heat capacity of RTO heat media is much larger than that of the airflow through RTO, according to Kays and London (2). Therefore a regenerative thermal oxidizer (RTO) can be considered as counter flow heat exchanger. The cost model for RTO can be drawn from the cost model of TO as in the previous section.
Therefore, similar to eq 9, cost function of RTO becomes, CRTO ) f [exp f1(S), R]
(13)
Since the heat transfer area S of a RTO is proportional to the heat media volume V, similarly using eq 6 and eq 13, the following equations can be derived: ˙ , η) V ) f (V
(14)
CRTO ) f [exp f1(V), R]
(15)
By considering eqs 14 and 15, the following expressions can be obtained for the costing of RTO: CRTO ) 134.42R(exp0.1288V )
(16)
˙ )expaη V ) A(V
(17)
˙ ) ) (0.005V ˙ + 0.14).(10-5) A(V
(18)
˙ is the In the above equations, CRTO is the cost of RTO, V process airflow rate through RTO, V is RTO heat media volume, R is a currency parameter (set as 1000 for USD), a ) 12.0317, and η is RTO heat recovery efficiency. Similarly ˙ is from 1000 the applicable range of the process air volume V to 10000 m3/h. The heat recovery efficiency of a RTO, η, usually ranges from 80 to 95%. The above empirical eqs 16-18 were obtained from application data provided by a supplier. Similar models can be plotted with any given supplier’s data.
5. Equipment Heat Recovery Model Let the total heat required for a thermal oxidation process ˙ T, then beQ ˙ R + ∆Q ˙ ˙T ) Q Q
(19)
˙ Ris the recovered heat and ∆Q ˙ is the heat energy Where, Q ˙ added. Higher heat recovery efficiency would minimize ∆Q and hence save energy. Energy recovery efficiency is the ratio of recovered heat to the total heat and can be expressed as ˙R Q η) ˙T Q
C ˙ tβ V
(22)
Then for TO and from eqs 10 and 22, ECFTO )
103.75Rexp(0.0025S) C ) ˙ tβ ˙ tβ V V
(23)
Then for RTO and from eqs 16 and 22, ECFRTO )
134.42R(exp0.1288V ) C ) ˙ tβ ˙ tβ V V
(24)
Heat recovery factor (HRF) reflects the cumulative heat recovery against the cumulative amount of airflow, during ˙ , t, β), and ˙ R,V the time period of t, is defined as HRF ) f (Q can be written as HRF )
˙ Rtβ ˙R Q Q ) ˙ ˙ Vtβ V
(25)
Therefore from eqs 20 and 25, HRF )
˙T ˙R ηQ Q ) ) ηq˙ ˙ ˙ V V
(26)
Where q˙, specific heat rate (watt/m3/h) to raise the VOC ˙, laden air temperature to thermal oxidation temperature; V process airflow rate in m3/h; t, equipment nominal age in hours; β, usage time factor (ratio of actual operated hours to equipment age in hours), and C, cost of equipment. The above derivation shows that HRF and ECF can be used to separately evaluate the merits of cost effectiveness and energy efficiency of VOC oxidation systems. Equipment cost against heat recovery (ECHR) reflects the cost of equipment against the heat recovery payback. ECHR is then an index used to evaluate and compare the cost effectiveness of heat recovery systems, and is defined as, ECHR )
ECF C ) ˙ tβ HRF ηq˙V
(27)
From eqs 10 and 27 for TO, (20)
Then from eqs 19 and 20, the heat energy added is ˙ ) (1 - η)Q ˙T ∆Q
ECF )
ECHRTO )
103.75Rexp(0.0025S) C ) ˙ ˙ tβ ηq˙Vtβ ηq˙V
(28)
Then from eqs 16 and 27 for RTO, (21)
˙ T, the heat According to eq 21 and for a given total heatQ ˙ is lower when the recovery efficiency η energy addition ∆Q is higher. Higher η would result in lesser fuel or electricity consumption and hence lower energy cost and less environmental impact. To achieve higher heat recovery efficiency, larger heat recovery components (heat exchangers in TO, and ceramic beds in RTO) would be needed. Ever larger heat exchange components meant for higher heat recovery would reach ever less margin of benefits. Therefore, simply increasing the recovery efficiency by over sizing the equipment cannot achieve optimal cost-effective operation.
6. Model for the Evaluation of Equipment Cost and Heat Recovery Equipment cost factor (ECF) reflects the equipment cost (C) against the cumulative amount of airflow handled during ˙ , t, β), and can the time period of t, is defined as ECF ) f (C,V be written as
ECHRRTO )
134.42R(exp0.1288V ) C ) ˙ tβ ˙ tβ ηq˙V ηq˙V
(29)
General comparison of ECHR between two thermal oxidizer systems can be performed as follows, ECHR for equipment 1, ECHR1 )
ECF (HRF ) ) ( ηq˙CV˙tβ )
ECHR for equipment 2, ECHR2 )
ECF (HRF ) ) ( ηq˙CV˙tβ )
Then take the ratio
1
ECHR2 ) ECHR1 η2 η1
1
2
C2 /C1 ˙ 2 t2 q˙2 V ˙ 1 t1 q˙1 V
2
( )( )( )( )( ) β2 β1
If(ECHR2)/(ECHR1) > 1, then comparatively equipment 2 costs more against cumulative heat energy recovered. Therefore equipment 1 can be considered better than equipment 2 in terms of cost effectiveness. In such evaluations “t” can be VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. ECF, HRF, and ECHR with Time Periods of Use for TO time period of use hr (years)
ECF
HRF
ECHR
8640 (1 year) 17280 (2 year) 25920 (3 year) 34560 (4 year) 43200 (5 year)
0.002949 0.001147 0.000983 0.000737 0.000590
0.3 0.3 0.3 0.3 0.3
0.009829 0.004915 0.003276 0.002457 0.001965
considered as the expected lifetime or time period of use of the equipment. Equipments 1 and 2 should not necessarily to be of the same type. One can be a recuperative thermal oxidizer (TO) and the other can be regenerative thermal oxidizer (RTO). Such analysis can be applied in general to industrial heat recovery systems. According to eqs 22 and 27, ECF and ECHR improve (reduce) with the expected age of the thermal oxidizer, and with a higher usage-time-factor β. Also according to eqs 26 and 27, higher η and q˙ values improve both HRF (higher) and ECHR (lower). If fixed values are considered for q˙ and η, then the value of HRF remains unchanged.
7. Application Example for Thermal Oxidizer (TO) A typical VOC abatement system (as shown and described with Figures 1-3) in semiconductor industry is used to treat VOC laden airflow rates of tens of thousands of m3/h. VOC abatement is the largest heat energy item for a typical semiconductor fabrication plant in Taiwan. A typical application case of airflow rate of 6280 m3/h is demonstrated below. ECF, HRF, and ECHR are obtained for varying periods of use for q˙ ) 0.5 kW/(m3/h), η ) 60%, and β ) 100%. The analysis for a TO unit is referred to eqs 23, 26, and 28, and the results are shown in Table 1 and Figure 4. Since q˙ and η are considered as fixed values, HRF is a fixed value of 0.3. However, the values of ECF and ECHR vary with different periods of use.
TABLE 2. Variation of ECF, HRF, and ECHR with Airflow Capacity for TO Aairflow capacity (m3/h)
ECF
HRF
ECHR
1570 3140 4710 6280 7850
0.001784 0.001025 0.000713 0.000589 0.000498
0.3 0.3 0.3 0.3 0.3
0.005947 0.003417 0.002375 0.001963 0.001660
8. Application Example for Regenerative Thermal Oxidizer (RTO) As similar to thermal oxidizer (TO) application shown above, ECF, HRF, and ECHR are obtained for varying periods of use for airflow rate of 6280 m3/h, q˙ ) 0.5 kW/(m3/h), η ) 60%, and for β ) 100%, for a RTO. The results are shown in Table 3 and Figure 6 (by referring to eqs 24, 26, and 29). Since q˙ and η are considered as fixed values, HRF is a fixed value of 0.425. However, the values of ECF and ECHR vary with different periods of use. Values of ECF, HRF, and ECHR for regenerative thermal oxidizer (RTO) units with different rated airflow capacities, for time period t of 5 years (43 200 h), q˙ ) 0.5 kW/ (m3/h), η ) 60%, β ) 100%, are obtained. The results are shown in Table 4 and Figure 7 (by referring to eqs 24, 26, and 29). Since q˙ and η are considered as fixed values, HRF is a fixed value of 0.425. Tables 1 and 3 show the trend of the improvement of ECF and ECHR with longer time of usage for thermal oxidizer (TO) and regenerative thermal oxidizer (RTO), respectively. According to the results in Tables 1 and 3, ECHR of RTO is comparatively lower and hence better than that of TO, while ECF has shown to be the reverse. Tables 2 and 4 show the variations of ECF and ECHR for different equipment capacities for TO and RTO, respectively. Both ECF and ECHR are better when the rated-airflowcapacity of the units is higher. ECHR values of regenerative
Values of ECF, HRF, and ECHR calculated for TO units with different rated airflow capacities, for time period t of 5 years (43 200 h), q˙ ) 0.5 kW/ (m3/h), η ) 60%, β ) 100%, are shown in Table 2 and Figure 5. The analysis referred to eqs 23, 26, and 28. Since q˙ and η are fixed values, HRF is a fixed value of 0.3.
FIGURE 5. ECF and ECHR for TO units with different rated airflow capacities.
TABLE 3. ECF, HRF, and ECHR with Time Period of Use for RTO
FIGURE 4. ECF and ECHR for a TO for varying period of use. 6104
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Ttime period of use hr (years)
ECF
HRF
ECHR
8640 (1 year) 17280 (2 years) 25920 (3 years) 34560 (4years) 43200 (5 years)
0.003741 0.001871 0.001247 0.000935 0.000748
0.425 0.425 0.425 0.425 0.425
0.008802 0.004401 0.002934 0.002201 0.001760
FIGURE 6. ECF and ECHR for a RTO for varying period of use.
TABLE 4. Variation of ECF, HRF, and ECHR with Airflow Capacity for RTO airflow capacity (m3/h)
ECF
HRF
ECHR
1570 3140 4710 6280 7850
0.002137 0.001245 0.000889 0.000748 0.000643
0.425 0.425 0.425 0.425 0.425
0.005028 0.002929 0.002091 0.001760 0.001512
thermal oxidizer units are shown to be lower and hence better than that of thermal oxidizer in the case study. Whereas ECF values of thermal oxidizer units are lower and better than that of regenerative thermal oxidizer. The above study has shown that heat recovery factor (HRF) and equipment cost factor (ECF) are two independent parameters that can separately evaluate the merits of VOC treatment equipment or systems in terms of energy efficiency and cost factors. Then this study introduced another parameter, equipment cost against heat recovery (ECHR),
FIGURE 7. ECF, and ECHR for RTO units with different rated airflow capacities.
which is a ratio of ECF and HRF can be used as a single parameter to evaluate the cost effectiveness of a VOC abatement system. It has been shown with two types of VOC abatement heat recovery that, the parameters introduced could provide a value-engineering tool in the design of VOC abatement systems. The cost model included in this paper acts as a bridge between technology and economics, for cost and efficiency comparison of thermal oxidation equipments associated with heat recovery. TO and RTO are two types of VOC abatement systems operating at elevated temperature, and hence make heat recovery of great importance. Therefore, the methods used in this study can be advantageously applied to other high temperature waste gas treatment technologies. The parameters introduced in this study could be used generally in the engineering analysis that considers both the equipment cost and energy analysis.
Acknowledgments This work was partly supported by DTECH, Inc. with data used in the study, and the Bureau of Energy of Taiwan.
Literature Cited (1) Murphy, C. F.; Kenig, G. A.; Allen, D. T.; Laurent, J.-P.; Dyer, D. E. Development of parametric material, energy, and emission inventories for wafer fabrication in the semiconductor industry. Environ. Sci. Technol. 2003, 37, 5373–5382. (2) Kays, W. M.; London, A. L. Compact Heat Exchangers; McGrawHill International: New York, 1995. (3) Amelio, M.; Morrone, P. Numerical evaluation of the energetic performances of structured and random packed beds in regenerative thermal oxidizers. Appl. Therm. Eng. 2007, 27, 762– 770. (4) Choi, B.-S.; Yi, J. Simulation and optimization on the regenerative thermal oxidization of volatile organic compounds. Chem. Eng. J. 2000, 76, 103–114. (5) Abanto, J.; Reggio, M.; Painchud-Ouellet, S. On the design of RTO unit using CFD. Appl. Therm. Eng. 2006, 26, 2327–2335. (6) de Monte, F. Cyclic steady thermal response of rapidly switched fixed-bed heat regenerators in counter flow. Int. J. Heat Mass Transfer 1999, 42, 2591–2604. (7) Skiepo, T.; Shah, R. K. Modeling and effect of leakages on heat transfer performance of fixed matrix regenerators. Int. J. Heat Mass Transfer 2005, 48, 1608–1632. (8) Warahena A. S. K.; Chuah Y. K. Regenerative Heat RecoverysIts Industrial Applications, IEEE conference proceedings of International Conference on Information and Automation for Sustainability (ICIAfS), Melbourne, 2007. (9) Bannai, M.; Houkabe, A.; Furukawa, M.; Kashiwagi, T.; Akisawa, A.; Yoshida, T.; Yamada, H. Development of efficiency-enhanced cogeneration system utilizing high-temperature exhaust-gas from a regenerative thermal oxidizer for waste volatile-organiccompound gases. Appl. Energy 2006, 83, 929–942. (10) Warahena A. S. K.; Chuah Y. K. Analytical Model for Transient and Stabilized Conditions of RTO Operation, IEEE conference proceedings of International Conference on Information and Automation for Sustainability (ICIAfS), Melbourne, 2007. (11) Warahena A. S. K.; Chuah Y. K. the Economics of Equipment Cost and Energy Recovery in VOC Thermal Oxidation in Industrial Air Pollution Control Applications, Monash University Conference of International Conference on Business, Environment, International Competitiveness and Sustainable Development of Asia Pacific Economies, Kuala Lumpur, Malaysia, 2007.
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