Energy & Fuels 2008, 22, 1041–1045
1041
Flue Gas Desulfurization with an Electrostatic Spraying Absorber Binlin Dou, Young-Cheol Byun, and Jungho Hwang* Department of Mechanical Engineering, Yonsei UniVersity, Seoul, Korea ReceiVed October 31, 2007. ReVised Manuscript ReceiVed January 22, 2008
A flue gas desulfurization device and process were developed and tested in this study. The process used an electrostatic spraying absorber (ESA) as the reactor, where SO2 was absorbed into an aqueous slurry of reactive Ca(OH)2. The absorption process was analyzed by using the two-film theory of mass-transfer. Both the liquid and gas side resistances were important, and the absorption rate was controlled by a combination of both gas-film and liquid-film diffusion controls. The ESA characteristics were investigated between applied voltages from -10 to 10 kV at various slurry flow rates. The SO2 removal efficiency was independent of the polarity of the applied voltage. A slightly higher efficiency was obtained with the conduction charging configuration than with the induction charging configuration. A model of external mass-transfer with a chemical enhancement factor was proposed for estimation of the absorption efficiency; the theoretical SO2 removal efficiency obtained was compared with the experimental data.
1. Introduction The removal of SO2 from various industrial sources has received considerable attention over the years.1–3 Recently, various semidry flue gas desulfurization (FGD) processes have been developed to avoid the disadvantages of wet and dry FGD techniques.3,4 Flue gas is typically brought into contact with the lime or limestone slurry by dispersion of the liquid/particle phase into droplets to increase the area of the gas–liquid interface. The mass-transfer is particularly important when the chemical reaction between the dissolved reactants can be considered instantaneous and irreversible. Zhou et al.3 developed a FGD process with a multifluid alkaline spray generator to improve the SO2 removal efficiency and investigated the effect of temperature on their system. Wang et al.5 found that the spray water flow rate and the droplet’s size had an obvious influence on the SO2 removal efficiency. Akbar and Ghiaasiaan6 studied the gas absorption in a spray scrubber by using dissolved reactive particles. Scala and D’Ascenzo7 developed a model for gas absorption via an * To whom correspondence should be addressed. Telephone: 82-2-21232821. E-mail:
[email protected]. (1) Gómez, A.; Fueyo, N.; Tomás, A. Detailed modelling of a flue-gas desulfurisation plant. Comput. Chem. Eng. 2007, 31 (11), 1419–1431. (2) Jannelli, E.; Minutillo, M. Simulation of the flue gas cleaning system of an RDF incineration power plant. Waste Manage. 2007, 27 (5), 684– 690. (3) Zhou, Y.; Zhang, M.; Wang, D.; Wang, L. Study on a novel semidry flue gas desulfurization with multifluid alkaline spray generator. Ind. Eng. Chem. Res. 2005, 44 (23), 8830–8836. (4) Xu, G.; Guo, Q.; Kaneko, T.; Kato, K. A new semi-dry desulfurization process using a powder-particle spouted bed. AdV. EnViron. Res. 2000, 4 (1), 9–18. (5) Wang, L.; Song, Y. B.; Zhang, M. C.; Fan, H. J.; Zhou, Y. G.; Fan, W. D.; Wu, J. Modeling study on the impaction and humidification process in desulfurization activation reactor. Chem. Eng. Sci. 2005, 60 (4), 951– 962. (6) Akbar, M. K.; Ghiaasiaan, S. M. Modeling the gas absorption in a spray scrubber with dissolving reactive particles. Chem. Eng. Sci. 2004, 59 (5), 967–976. (7) Scala, F.; D’Ascenzo, M. Absorption with instantaneous reaction in a droplet with sparingly soluble fines. AIChE J. 2002, 48 (8), 1719–1726.
instantaneous reaction in a droplet. Muginstein et al.8 developed a model relevant to spray towers for gas absorption into a large slurry droplet, with the results indicating that internal circulation enhanced the mass-transfer with respect to a stagnant droplet for large droplets. Some studies9–11 have been carried out by applying electric fields to enhance the gas–liquid mass-transfer. Yang and Carleson9 studied the effects of electric fields on the dispersed liquid phase flow behavior and mass-transfer efficiency of several liquid–liquid systems by using a single/multiple nozzle extraction column. Wang et al.10 investigated the atomization characteristics of an electrohydrodynamics limestone water spray and observed the changes in the droplet diameter and the specific charge in relation to various applied voltages. Carleson and Berg11 experimentally examined the effect of an electric field on the absorption of SO2 by pure water droplets. In this paper, a device and method for enhancing the removal of SO2 from flue gas by producing Ca(OH)2 slurry droplets have been introduced, which increase the contact area between the droplets and SO2 via electrostatic spraying. A model of the external mass-transfer on both the gas and liquid sides, using a chemical enhancement factor to estimate the slurry absorption efficiency, has also been proposed. 2. Modeling Studies on the absorption of SO2 accompanied by a fast chemical reaction in slurry were initially reported by Ramachandran and Sharma using the film theory of mass-transfer.12 (8) Muginstein, A.; Fichman, M.; Gutfinger, C. Gas absorption in a moving drop containing suspended solids. Int. J. Multiphase Flow 2001, 27, 1079–1094. (9) Yang, W.; Carleson, T. E. Several effects of electric fields on liquid extraction. IEEE Trans. Ind. Appl. 1990, 26 (2), 366–373. (10) Wang, S. H.; Chang, J. S.; Berezin, A. A. Atomization characteristics of electrohydrodynamics limestone-water slurry spray. J. Electrostatics 1993, 30, 235–246. (11) Carleson, T. E.; Berg, J. C. The effect of electric fields on the absorption of pure sulfur dioxide by water drops. Chem. Eng. Sci. 1983, 38 (6), 871–876. (12) Ramachandran, P. A.; Sharma, M. M. Absorption with fast reaction in a slurry containing sprayingly soluble fine particles. Chem. Eng. Sci. 1969, 24, 1681–1686.
10.1021/ef700646c CCC: $40.75 2008 American Chemical Society Published on Web 02/20/2008
1042 Energy & Fuels, Vol. 22, No. 2, 2008
Dou et al.
The following reaction scheme has been represented for the removal of SO2 by using a Ca(OH)2 slurry: Ca(OH)2 + SO2 + H2O ) CaSO3 · 1⁄2H2O + 3⁄2H2O
(1)
H ) exp[32143.3/T + 198.14 ln T - 0.3384T - 1135.62] (8) Then, the absorption rate of SO2 can be represented by combining eqs 3, 5, and 7: NA ) KGaPA
investigations,4,8,12,13
According to some the rate of the above chemical reaction is rapid relative to the diffusion. Considering the use of a dilute reactive suspension, the desulfurization process is also not limited by the dissolution rate of Ca(OH)2. The heats of reaction and dissolution can be neglected, and the system is assumed as isothermal. The rate of SO2 absorption in the reactor can be given as14 NA )
( )
QG -dPA QLRT dt
NA ) kGa(PA - PAi)
(3)
where kG is the gas side mass-transfer coefficient (kmol/ m2 · s · atm), PAi is the partial pressure of SO2 in the interface (atm), and a is the gas–liquid interface area per unit volume of liquid (m2/m3), defined as15 a)
6εG dp
NA ) EkLa(ci - 0)
(5)
where kL is the liquid-side mass-transfer coefficient (m/s), ci is the interfacial concentration of SO2 in the liquid phase (kmol/ m3), and E is the mass-transfer enhancement factor following the two-film model for instantaneous chemical reaction, which can be given as13
(
)
(6)
where Da is the diffusion coefficient of SO2 in water (m2/s), Db is the diffusion coefficient of Ca(OH)2 in water (m2/s), cb is the concentration of Ca(OH)2 in water (kmol/m3), and ξ is the mole ratio of SO2 to Ca(OH)2 reagent (mol/mol). The interfacial concentration of SO2 is obtained from the following relationship: ci ) HPAi
1 1 1 ) + KG EHkL kG
(7)
where H is the thermodynamic equilibrium constant (kmol/ m3 · atm), and the value can be obtained by13 (13) Bandyopadhyay, A.; Biswas, M. N. Prediction of the removal efficiency of a novel two-stage hybrid scrubber for flue gas desulfurization. Chem. Eng. Technol. 2006, 29 (1), 130–145. (14) Dagaonkar, M. V.; Beenackers, A. A. C. M.; Pangarkar, V. G. Absorption of sulfur dioxide into aqueous reactive slurries of calcium and magnesium hydroxide in a stirred cell. Chem. Eng. Sci. 2001, 56 (3), 1095– 1101. (15) Liu, S.; Xiao, W. Modeling and simulation of a bubbling SO2 absorber with granular limestone slurry and an organic acid additive. Chem. Eng. Technol. 2006, 29 (10), 1167–1173.
(10)
The gas side mass-transfer coefficient can be calculated from the Frossling correlation:16 Sh )
kGdpRT ) 2 + 0.6Re1/2Sc1/3 DG
(11)
Re )
FGdpuG µG
(12)
Sc )
FG µGDG
(13)
where
where Sh, Re, and Sc are the Sherwood number, Reynolds number, and Schmidt number, respectively, DG is the diffusion coefficient of SO2 in the gas phase (m2/s), FG is the gas density (kg/m3), µG is the gas viscosity (kg/m · s), and uG is the gas velocity (m/s). The liquid-side mass-transfer coefficient is estimated by16
(4)
where G is the gas fraction and dp is the average size of the slurry droplets (m). The rate of SO2 mass-transfer through a liquid film can be given as4
Db cb E) 1+ξ Da ci
where KG is the overall gas side mass-transfer coefficient, given as
(2)
where NA is the absorption rate (kmol/m3 · s), PA is the partial pressure of SO2 in the gas phase (atm), R is the universal gas constant (atm · m3/kmol · K), T is the temperature (K), QG and QL are the gas and liquid flow rates (m3/s), respectively, and t is the time (s). The rate of SO2 mass-transfer through a gas film can be given as
(9)
kL ) 0.88√fDa
(14)
where f)
8σ 3πmp
(15)
where σ is the surface tension of the liquid (N/m) and mp is the mass of a droplet (kg). The residence time of the gas in the reactor is obtained by τ)
Vd QG
(16)
where τ is the residence time (s) and Vd is the reactor volume (m3). The absorption efficiency of SO2 is defined as ηA ) 1 -
PA2 PA1
(17)
where ηA is the absorption efficiency of SO2, PA1 is the partial pressure of SO2 in the gas phase at the inlet (atm), and PA2 is the outlet partial pressure of SO2 (atm). After combining eqs 2 and 9, the efficiency can be written as ηA ) 1 - exp(-KGaRTQLVd /QG2)
(18)
3. Experimental Section A schematic of the experimental setup is shown in Figure 1. The system consisted of a gas delivery system, an electrostatic spraying absorber (ESA), a slurry system, a power supply, and a gas sampling and analysis system. Slaked lime slurry, prepared by (16) Warych, J.; Szymanowski, M. Model of the wet limestone flue gas desulfurization process for cost optimization. Ind. Eng. Chem. Res. 2001, 40, 2597–2605.
Flue Gas Desulfurization with an ESA
Energy & Fuels, Vol. 22, No. 2, 2008 1043 Experiments were carried out in two configurations: conduction charging, where the mesh was grounded and a high voltage was applied to the nozzle, and induction charging, where the nozzle was grounded and a high voltage was applied to the mesh. The applied voltages ranged between -10 and 10 kV. For a given slurry flow rate, current flowing, voltage distribution, and SO2 concentration at the exit were measured at various applied voltages. A digital amperemeter and a 1000:1 high voltage probe were used for measurements of the current and voltage, respectively. A portable SO2 analyzer was used for measurements of the SO2 concentrations, with its probe positioned in the gas exhaust pipeline. All of the measurements were performed at STP conditions.
Figure 1. Schematic diagram of the experimental setup: (1) water; (2) CaO feeder; (3) Ca(OH)2 slurry storage; (4) pump; (5) rotameter; (6) power supply; (7) ESA reactor; (8) products tank; (9) gas sampling and analysis; (10) gas outlet; (11) gas flow meter; (12) mixing chamber; (13) compressed air; (14) SO2 cylinder; (15) N2 cylinder.
Figure 2. Schematic diagram of the ESA.
mixing CaO (2 wt %) with tap water, was stored in a cylindrical acrylic container. Temol NN 8906 was added to enhance the dissolution of CaO. The slurry concentration was kept uniform by rotating a metal propeller attached to an ac motor. A microdiaphragm pump was used to deliver the dilute suspension to an internally mixing twin-fluid type nozzle. The nozzle was located in the ESA reactor and used as an atomizer. Because the pumping flow rate (3 L/min) was much higher than the required slurry flow, a bypass was formed to provide an appropriate flow rate, with the rest of the slurry returned to the container. Thus, the slurry and pressurized air delivered to the nozzle through different paths were mixed inside the nozzle to form a finely atomized delivery. Commercially available SO2 with N2 as the carrier gas was used to maintain a uniform atomizing pressure. The schematic of the ESA reactor is shown in Figure 2. The body of the reactor (30 cm in diameter and 60 cm in height) was made of acryl to enable visualization, with a high voltage power supply (maximum voltage ) (10 kV, maximum dc current ) 5 mA) via a voltage applied to enhance atomization of the slurry. The applied voltages (up to (10 kV) and the nature of the electrodes precluded the formation of any corona discharge with the associated ion formation. A stainless steel mesh was located 3 cm below the nozzle tip. When the distance between the nozzle tip and the mesh was shorter than 3 cm, electrical breakdown occurred. A hopper was installed at the bottom of the reactor to drain the sulfate reaction products and unreacted slurry. A PVC gas pipeline was connected to the ventilating exhaust hood. Various combinations of inlet flow rates of SO2, N2, compressed air, and slurry were tested, with the flow rates finally selected as those that yielded a noticeable electric field effect; SO2 was mixed with the carrier gas, N2, whose flow rate was 2 L/min. The flow rate of compressed air was 3 L/min. The slurry flow rate was selected to be in the range of 15–35 mL/min. The mole ratio of liquid to gas was about 2.0-6.0.
4. Results and Discussion Initial experiments were carried out with no voltage applied to the ESA reactor. When the concentration of SO2 at the inlet of the reactor was fixed at 1100 ppm, the concentrations of SO2 at the exit of the reactor were 700, 500, and 400 ppm for slurry flow rates of 15, 25, and 35 mL/min, respectively. Therefore, the SO2 removal efficiencies were 36, 54, and 63% for the slurry flow rates of 15, 25, and 35 mL/min, respectively. Experiments were then conducted to investigate the enhancement of the SO2 removal efficiency when a high voltage was applied to the ESA reactor by using electrostatic spraying as well as to determine the effect of the electrical configuration on the removal of SO2. Electrostatic spraying implies that individual drops comprising a spray are electrostatically charged at the spray nozzle. Droplets are charged by either induction or conduction and an electrostatic field; therefore, they interact with the liquid during the dispersion process. Figure 3 shows the results of the visualization experiments performed with a conventional camera system. The droplet sizes became smaller with an applied voltage. The effect of a charge on the droplet sizes can be explained by using the following equation:17,18 dp3/2 ) 6(8σγ)1/2/FLqd
(19)
where FL is the density of the liquid (kg/m3) and γ is the permittivity of a vacuum (F/m). The specific charge of a droplet (the ratio between the charge of a droplet and the mass of a droplet), qd, becomes qd )
I FLQL
(20)
where I is the current (A). Consequently, if the current and the flow rate of liquid into the nozzle are measured, the specific charge can be calculated from eq 20. Then, the droplet size can be calculated from eq 19. The electrical currents and SO2 concentrations at the exit of the ESA reactor were measured for various slurry flow rates and applied voltages. All data points were averages of three or more repeated measurements. As shown in Figure 4, the electrical current flowing between the electrodes was symmetrical with respect to the applied voltage. The current for the conduction charging configuration was slightly higher than that for the induction charging configuration. Figure 5 shows the experimental results of the SO2 removal efficiencies at various liquid flow rates. From this figure, the effect of the liquid flow rate on the SO2 removal efficiency can be seen to be very strong. With an increase in the liquid flow rate, the SO2 removal efficiency gradually increased. In all cases, (17) Roth, D. G.; Kelly, A. J. Analysis of the disruption of evaporating charged droplets. IEEE Trans. Ind. Appl. 1983, IA-19 (5), 771–775. (18) Cloupeau, M.; Prunet-Foch, B. Electrohydrodynamic spraying functioning modes: a critical review. J. Aerosol Sci. 1994, 25, 1021–1036.
1044 Energy & Fuels, Vol. 22, No. 2, 2008
Dou et al.
Figure 3. Spray visualization with a slurry flow rate of 15 mL/min: (a) without an applied voltage and (b) with an applied voltage of –10 kV. Table 1. Parameters for the System parameter
value
Da Db DG G σ µG H E
1.8 × 10-9 1.6 × 10-9 1.4 × 10-5 0.5 7.20 × 10-2 17.81 1.25 1.028
unit m2/s m2/s m2/s N/m kg/m · s kmol/m3 · atm
Table 2. Mass-Transfer Coefficients dp (µm) kG (kmol/m2 · s · atm) kL (m/s) KG (kmol/m2 · s · atm)
Figure 4. Relationship between current and applied voltage for different slurry flow rates.
800 1.43 × 10-3
1000 1.15 × 10-3
1200 9.55 × 10-4
1500 7.64 × 10-4
7.75 × 10-4 5.87 × 10-4
6.56 × 10-4 4.83 × 10-4
5.72 × 10-4 4.15 × 10-4
4.84 × 10-4 3.43 × 10-4
transfer resistances, defined as 1/kG and 1/EHkL, can be calculated. The ratios of the gas side resistance to the overall gas side resistance were 0.41, 0.42, 0.43, and 0.45 for droplet sizes of 800, 1000, 1200, and 1500 µm, respectively, indicating that both liquid and gas side resistances were important, with the absorption rate likely to be controlled by a combination of both gas-film and liquid-film diffusion controls. The reduction in both the gas and liquid side resistances resulted in an increase of the SO2 removal. Figure 6 shows the predicted and experimental results of SO2 removal efficiencies for various slurry flow rates and droplet sizes. The SO2 removal efficiency increased with decreasing droplet size or increasing slurry flow rate. The calculated results were closely related to the experimental data. From an economic point of view, the liquid to gas flow rate ratio has been found to be one of the most important criteria
Figure 5. ηA vs applied voltage at various slurry flow rates.
the efficiency slightly increased with increasing strength of the applied voltage and was relatively independent of the polarity of the applied voltage for a given charging configuration. Theoretical calculations of the removal of SO2 were carried out by using eq 18. For these calculations, the droplet sizes were assumed to be 800–1500 µm, which were obtained from eq 19. Table 1 shows the parameter values for the calculations. Table 2 shows the effect of droplet size on the mass-transfer coefficients, with both the gas and liquid side mass-transfer coefficients gradually decreasing with increasing droplet size. From the coefficients, the gas and liquid side mass-
Figure 6. Relationship between ηA and QL for different droplet sizes.
Flue Gas Desulfurization with an ESA
for reporting the reactor performance. Since this novel ESA combines some regimes of electrostatic spraying, multiphase dispersions, and chemical reactions, the removal of SO2 can be significantly controlled by using electrostatic spraying, even with very low liquid to gas flow mole ratios. The true gas-side and liquid-side mass-transfer coefficients of the present ESA have been calculated and found to be almost equivalent in importance for a typical droplet size. Thus, by decreasing the external resistance to mass-transfer on both the gas and liquid sides, it would be expected that the removal of SO2 by electrostatic spraying should be a competitive FGD process with lower cost and higher efficiency. 5. Conclusions A flue gas desulfurization method using an electrostatic spraying absorber (ESA) was developed. The SO2 absorption process was analyzed by using the two-film theory of gas–liquid mass-transfer, which indicated that the absorption rate was controlled by a combination of both gas-film and liquid-film diffusions. The ESA characteristics were investigated between applied voltages of -10 to 10 kV at various slurry flow rates. The SO2 removal efficiency was found to be independent of the polarity of the applied voltage. A slightly higher efficiency was obtained with the conduction charging configuration than with the induction charging configuration. The theoretical SO2 removal efficiency was compared with the experimental data. Acknowledgment. This work was supported by the Development Institute Project of Seoul (Grant No. 20060124-2-1-001) and the Science and Technology Commission of Shanghai Municipality (07DZ12013).
Nomenclature a ) gas-liquid interface area per unit volume of liquid (m2/m3) cb ) concentration of Ca(OH)2 in water (kmol/m3) ci ) interfacial concentration of SO2 in liquid phase (kmol/m3)
Energy & Fuels, Vol. 22, No. 2, 2008 1045 dp ) average droplet size (m) Da ) diffusion coefficient of SO2 in water (m2/s) Db ) diffusion coefficient of Ca(OH)2 in water (m2/s) DG ) diffusion coefficient of SO2 in gas phase (m2/s) E ) enhancement factor for instantaneous chemical reaction H ) thermodynamic equilibrium constant (kmol/m3 · atm) I ) current (A) kG ) gas-side mass-transfer coefficient (kmol/m2 · s · atm) kL ) liquid-side mass-transfer coefficient (m/s) KG ) overall gas-side mass-transfer coefficient (kmol/m2 · s · atm) mp ) mass of a droplet (kg) NA ) absorption rate (kmol/m3 · s) PA ) partial pressure of SO2 in the gas phase (atm) PAi ) partial pressure of SO2 in the interface (atm) PA1 ) partial pressure of SO2 in the gas phase at the inlet (atm) PA2 ) partial pressure of SO2 in the gas phase at the outlet (atm) QG ) gas flow rate (m3/s) QL ) liquid flow rate (m3/s) qd ) specific charge of a droplet R ) universal gas constant (atm · m3/kmol · K) T ) temperature (K) t ) time (s) uG ) gas velocity (m/s) Vd ) reactor volume (m3) G ) gas fraction γ ) permittivity of vacuum (F/m) FG ) density of gas (kg/m3) FL ) density of liquid (kg/m3) τ ) residence time (s) µG ) gas viscosity (kg/m · s) σ ) surface tension of liquid (N/m) ξ ) mole ratio of SO2 and Ca(OH)2 reagent (mol/mol) ηA ) absorption efficiency of SO2 Sh ) Sherwood number Re ) Reynolds number Sc ) Schmidt number EF700646C
