- Email: [email protected]
A model for performance optimization of wet flue gas desulfurization systems of power plants
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
A model for performance optimization of wet flue gas desulfurization systems of power plants Yi Zhong, Xiang Gao⁎, Wang Huo, Zhong-yang Luo, Ming-jiang Ni, Ke-fa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
AR TIC LE I N FO
ABS TR ACT
Article history:
In this paper, a model of limestone/gypsum wet flue gas desulfurization (WFGD) system was
Received 20 June 2007
developed based on unsteady theory. The models of processes of absorption section and
Received in revised form
oxidation section were developed and incorporated into the WFGD integral model. The sub-
25 March 2008
model of motion of slurry drops, absorption of SO2, dissolution of limestone and
Accepted 2 April 2008
crystallization of gypsum were included in the model of absorption section, while the model of oxidation section was developed by the population balance theory. The calculation
Keywords:
results of the desulfurization system for 300 MW utility in China by this model was
Wet flue gas desulfurization
compared to that of corresponding measured results. The simulation results agreed with the
Unsteady theory
measurement results very well. The operation mode of boilers of power plant in China is
Spray scrubber
different to that of other countries since variable coal property and unstable loads of boilers.
Numerical calculation
The differences of operation mode lead to the variation of process parameters of WFGD
Optimization
system. The influences of liquid-to-gas ratio, SO2 concentration of inlet flue gas, and combination mode of different spray levels to the desulfurization efficiency were analyzed. Based on the analysis, some advices of performance optimization of flue gas desulfurization systems in China were suggested. © 2008 Published by Elsevier B.V.
1.
Introduction
It is well known that sulfur dioxide is not only harmful to health, but also contributes to acidification of soil and water. The emission of SO2 from coal-fired boilers is regulated strictly in many countries. There are many methods available for controlling the emission of SO2 of coal-fired boilers. Because of its high efficiency and reliability, limestone–gypsum wet flue gas desulfurization (WFGD) technology is the most commonly used technology for controlling the emission of SO2 in the world [1–3]. The typical scrubber of limestone/gypsum WFGD with forced oxidation system is shown in Fig. 1. The main task of WFGD system with forced oxidation is to remove sulfur dioxide from flue gas and to produce gypsum as a saleable product. The raw flue gas was boosted to the absorber through
flue gas inlet duct, which is located above the slurry tank. The raw flue gas flow upwards and contact slurry drops containing suspended limestone sprayed from the spray levels counter currently to remove the sulfur dioxide in flue gas after entering the scrubber. The slurry drops fall down to the slurry tank after reacting with the flue gas. The slurry is injected into the absorber over several spray levels each with a dedicated recirculation pump. Each spray bank is provided with numerous spray nozzles for a proper atomization of the slurry. The spray levels were named the 1st, 2nd, 3rd and 4th spray level from the bottom up, respectively. The clean flue gas leave the absorber through the outlet duct after removing the entrained water droplets. The water droplets in flue gas were removed by a mist eliminator (ME) to control the erosion of following equipments. The ME locates above the spray levels and consists of two stages. The first stage removes most of the
⁎ Corresponding author. Present address: Institute for Thermal Power Engineering of Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China. Tel.: +86 571 8795 1335; fax: +86 571 8795 1616. E-mail address: [email protected] (X. Gao). 0378-3820/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.fuproc.2008.04.004
1026
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Fig. 1 – Diagrammatic sketch of spray tower.
entrained liquid droplets; in the second stage also fine droplets are captured. The mist eliminators are frequently washed by process water to maintain a clear surface and a low pressure loss. The slurry tank is the lower end of the absorber which is designed with sufficient retention time for optimum utilization of limestone, oxidation and crystallization of gypsum. Oxidation air compressor is used to injected fresh air to the slurry tank by air spargers to oxidize the calcium sulfite in slurry to gypsum. Slurry agitators around the slurry tank are used to disperse the solid particle in the absorber slurry. The most important process in the WFGD system is the absorption of sulfur dioxide. Sulfur dioxide is absorbed in the spray absorption zone by reacting with the limestone slurry drops, so the chemical absorption in the absorber is the most complicated and important step in scrubber. Generally speaking, there are four processes in absorbers and slurry tanks as follows: limestone dissolution, SO2 absorption, SO2− 3 oxidation and gypsum crystallization. The simultaneous and interactive occurrences of these four processes make it difficult to describe all the processes in WFGD system acutely by mathematic models, especially in describing the transformations of all of components in the process of WFGD system. Many researchers have adopted different gas–liquid mass transfer theory to built different WFGD models. Gerbec et al. [4] introduced the unsteady-state theory to model of pilot scale WFGD system. Sub-models of the absorption of SO2 to the falling dispersed drops of liquid phase, chemical equilibriums of sulfite, sulfate, and carbonate, calcium ions in solution, oxidation of sulfite to sulfate, precipitation and dissolution of calcium sulfite and calcium sulfate, dissolution of limestone were built into the WFGD model. Based on various boundary conditions and input parameters, dynamic responses to chosen parameters can be calculated. The model can predict the SO2 removal efficiency and chemical component of slurry in each level. Based on penetration theory, Brogren et al. [5,6] built model of spray absorber of WFGD system and calculated the absorption rate of SO2 into a drop of limestone slurry. The model includes both instantaneous equilibrium reactions and chemical reactions with finite rate: dissolution of limestone,
oxidation of sulfite, gypsum crystallization and the hydrolysis reaction of CO2. The model was used to quantify the mass transfer within a spray scrubber and estimate the impact of the reactions with finite rate of SO2 mass transfer. Warych et al. [7,8] created a detailed model for the desulfurization process of utility boiler. The film theory was adopted in the model to simulate the absorption of SO2 and the dissolution of limestone. This model can predict the variation of SO2 removal efficiency influenced by ratio of volume of liquid to that of flue gas (Ra), pH, flow rate etc. The results agree with the records of experiment well. Kong [9] sets up the model of SO2 absorption in a pilot scale impinge stream scrubber with unsteady-state mass transfer theory. The calculation results fit the experimental results well. The absorption and mass transfer resistance in different zone of the impinge stream scrubber are analyzed based on the model. The effects of limestone dissolution, gypsum crystallization and sulfite oxidation on SO2 absorption are also analyzed. Xiang [10] developed a detailed model for slurry jet FGD pilot plant. The model describes the desulfurization processes in FGD tower. The calculation results can predict the desulfurization efficiency. Kiil [11] developed a detailed model for a wet flue gas desulfurization packed tower. Model predictions were compared to the experimental data such as gas-phase concentration profiles of SO2, slurry pH profiles, solid content of the slurry, liquid-phase concentrations, and residual limestone in the gypsum. The operation mode of utility boilers in China is different to that of other countries. Property of coal burned in utility boilers of China is unstable because the source of coal in power plant changes from time to time. Moreover, unstable electric power loads in China result in the low load operation of the utility boilers at night. The typical daily loads in different seasons are shown in Fig. 2. There is an obvious difference between loads of day and night in every season. The load of night is even about two the third of that of daylight in summer. The two differences of operation mode of boilers lead to some differences between Chinese WFGD systems of utility boilers and those of other countries. Moreover, there is no model available for performance and cost optimization of the spray scrubber of WFGD system of power plant in China now. It is meaningful to study the variation of removal efficiency because of the variable operation mode of boilers. Therefore,
Fig. 2 – Typical daily loads in different seasons.
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
it is important to develop a model according to actual operation mode of WFGD systems to study the optimization of design and operation of WFGD in China. The variable coal rank and unstable loads of boilers will result in three kinds of variations of parameters. First, the inlet SO2 concentration varies great often, sometimes the concentration even increase as twice as that of design value; second, the flux of flue gas varies with the variation of load of boiler; third, the operation of spray tanks varies for the economic operation of WFGD system. Therefore, it is meaningful to study influences of Ra, SO2 concentration of inlet flue gas and combination mode of different spray levels to the desulfurization efficiency to keep the economic operation of WFGD in China. The model of limestone–gypsum WFGD system was developed by the unsteady theory and solved by the numerical method in this paper. The calculation results were compared with the measured results and they fit well. Based on the model, influences of Ra, SO2 concentration of inlet flue gas and the combination mode of different spray levels to the desulfurization efficiency were studied. According to the calculation results of model, the guideline of operation mode of WFGD system was set up to keep the pace of the boiler unit to keep efficiency and reliability.
2.
WFGD model
2.1.
Assumptions
The absorber that consists of absorption zone and slurry zone is the key component in WFGD system. The chemical processes in the absorber include lots of heat and mass transfer phenomenon and chemical reactions. In order to describe the processes in full-scale WFGD absorber better, absorber is separated to two zones: spray absorption zone and slurry oxidation zone. In spray zone, slurry drop contacts and reacts with the flue gas. The absorbent dissolution and oxidation and crystallization occur in the slurry zone. Size and its distribution of drops, velocity of drops, collisions and coalescence between drops, circulation inside drops, oscillation and distortion of drops and flow patterns in absorber are the complex factors in spray absorber [12]. It is difficult to describe all the processes in absorber acutely in mathematical theory. Therefore, some assumptions were made as below: a Flue gas is an ideal gas. In the falling process, the drops are considered to be the rigid ball in shape and the vertical line in trace. The interaction effects among slurry drops were described by the size and the penetration time of slurry drops. The variation of velocity of flue gas and drag coefficient caused by the evaporation, distortion and mass transfer of drops are neglected; b The charge balance, chemical equilibrium of slurry drops and the charge balance, ionic equilibrium in slurry tank are treated as instantaneous process. However, the limestone dissolution, SO2 absorption, liquid–gas mass transfer, gypsum crystallization and chemical reactions are considered as the finite rates process;
1027
c The oxidation of CaSO3, the dissolution and crystallization of CaSO4 in the absorption zone are neglected; d After combining with limestone slurry drops, SO2 and CO2 are totally considered as the hydration phase; CO2 desorption from slurry drops by the CO2 gas in flue gas is neglected; e The slurry drops in absorption zone and slurry zone were considered to be totally mixed. The radial concentration of SO2 is constant in absorber and axial concentration changed with the height of absorber. The content in slurry tank are the same everywhere and the absorption of SO2 gas in upward flue gas is neglected; f Only the O2, CO2, SO2 in gas phase and H2O, O2, CO2, SO2, H+, 2+ − 2− − 2− in liquid OH−, HCO−3, CO2− 3 , HSO3, SO3 , HSO4, SO4 , Ca phase were included in this model. The influences of Cl, and Mg2+ to the WFGD system are neglected.
2.2.
Model
2.2.1.
Model of absorption zone
2.2.1.1. Model of drop motion. The movement equation of falling drops in absorption zone can be determined as Eq. (1) by assumption a [13]: " # 2 qd qg dud 3 qg ud ug Cdrag ¼g : 4 dt qd qg d
ð1Þ
Moreover, the drag coefficient can be calculated by Eqs. (2) and (3) [13]:
Cdrag ¼
Red ¼
24 1 þ 0:125Re0:72 d Red
djud ug jqg : Ag
ð2Þ
ð3Þ
2.2.1.2. Model of SO2 absorption. The SO2 diffusion on the surface of drop and chemical absorption inside drop are the most important step in the process of WFGD. In classical chemical absorption process, 8 different cases can be proposed by the relative rate of mass transfer and chemical reaction. The Hatta number (MH) which is used to describe the contribution of each process to the overall absorption was calculated [14]. The chemical absorption inside the spray tower is fast reaction with low reagent concentration. In other words, the resistance of liquid bulk is negligible because chemical reaction is within the liquid film. Therefore, the overall chemical reaction rate, rSO2, can be described by Eq. (4) [14–16]. 1 HSO2 þ rSO2 ¼ pSO2 = kG bkL
ð4Þ
The partial pressure of SO2 in flue gas, pSO2, can be calculated by the ideal gas equation. The value of kG, kL and β is specified separately for calculate the general mass transfer coefficient. Mass transfer coefficient
1028
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
in gas phase, kG, is acquired by correlating Eqs. (5), (6), and (7) [9,10,17]. Sh ¼
Sc ¼
kG RTd 1=3 ¼ 2 þ 0:55Re0:5 d Sc Dg;SO2 p
ð5Þ
CaCO3) ACC 1 ¼ 2 ½j b2 DH2 CO3 jCH2 CO3 þ j b2 DHCO3 jCH2 CO3 b At þj b2 DCO2 jCCO2 þ rd;CaCO3 3
Ag qg Dg;SO2
DSO2 ;G
2− − 3 The variation of total C (including CO O 3 , HCO3, H2CO3,
ð11Þ
3
ð6Þ Boundary condition: DC jCC;b¼R Initial condition: CC = CC,0.
h i1=2 1 9:86 103 T1:75 M1a þ MSO 2 ¼ 2 1=3 1=3 p ma þ mSO2
ð7Þ
Mass transfer coefficient in liquid phase, kL, is achieved by Eq. (8) [15–18]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4DSO2 ;L : kL ¼ k tp
ð8Þ
The penetration time, t p, can be calculated by the characteristic of hydrodynamics inside slurry drops or sheets. In accordance with past works [6,19] and practicable operation conditions, the value of penetration time for slurry sheets and drops out of nozzles are set to different value in the simulation. Diffusion coefficient of SO2 in liquid phase, DSO2,L, is obtained from Ref. [19]. The enhancement factor for chemical reaction, β, Based on the experimental data in the lab and the real operation parameters, the value of β, can be fitted [7,10,19,20]. According to assumption c, there is no change for total S(VI) in the process of falling of drops. But the reciprocal transformation between HSO−4 and SO2− 4 because of the variation of pH value and the influence of this transformation to pH value are considered. Based on the spherical coordinate, the variations 2+ of the total S(IV) (SO2, HSO−3, SO2− 3 , CaSO3), total Ca(Ca , CaSO3, 2− − CaSO4, CaCO3), total C(CO3 , HCO3, H2CO3, CaCO3) in falling drops are modeled as below [5, 10]:
The nozzles (Pigtail type, STXP serious) used in the scrubber described in this paper are made by BETE fog nozzle Co. Ltd. The Sauter mean diameter is 1.96 mm for the nozzle. In this paper, the interaction effects among slurry drops were described by the size and penetration time. Take the interaction effects among slurry drops, 2.9 mm was taken as the diameter of the droplets.
2.2.1.3. Model of limestone dissolution and sulfite crystallization. The dissolution of limestone occurred in drop was described by Eqs. (12) and (13) [9,10]. rd;CaCO3 ¼ kd;CaCO3 ðRSCaCO3 1Þ RSCaCO3 ¼
1 The variation of total S(IV) (including SO , O 2
cCa2þ cCO2 3
KSP;CaCO3
ð12Þ ð13Þ
According to assumption c, the crystallization of CaSO3 and CaSO4 inside the drops was described by Eqs. (14), (15), and (16) [9,10]. 5153 A ðRSCaSO3 1Þm rc;CaSO3 ¼ 1:62 104 exp T RSCaSO4 ð14Þ RSCaSO3 ¼
RSCaSO4 ¼ HSO−3,
R3 rC;b¼R ¼ 0
SO2− 3 ,
cCa2þ cSO2 3
KSP; CaSO3 cCa2þ cSO2 4
KSP; CaSO4
ð15Þ
ð16Þ
CaSO3) ACSðIVÞ 1 ¼ 2 ½j b2 DSO2 ;L jCSO2 þ j b2 DHSO3 jCHSO3 b At þj b2 DSO2 jCSO2 þ j b2 DCaSO3 jCCaSO3 3
2.2.2. ð9Þ
3
rc;CaSO3
Boundary condition: kG pg; SO2 pi; SO2 ¼ DS jCS;b¼R R3 rS;b¼R Initial condition: CS(IV) = CS(IV),0 2+ 2 The variation of total Ca (including Ca , CaSO3, CaSO4, CaCO3) O ACCa 1 ¼ 2 ½j b2 DCa2þ jCCa2þ þ j b2 DCaSO2 jCCaSO3 b At þj b2 DCaSO4 CCaSO4 þ j b2 DCaCO3 jCaCO3
X dck;out ¼ Ck þ Ck;rea dt
Ck ¼ ð10Þ
þrd;CaCO3 rc;CaSO3 Boundary condition: DCa jCCa;b¼R Initial condition: CCa =CCa,0
Model of oxidation zone
According to assumptions a, c, d and theory of population balance, all the contents including S(IV), S(VI) and Ca, all the ions and molecules are in balance conditions when the system operates in normal condition. It can be described by Eqs. (17) and (18) [10].
1 F c þ Ffal ck;fal Frec ck;rec Fout ck;out Vtk in k;in
2.2.3.
R3 rCa;b¼R ¼ 0
ð17Þ
ð18Þ
Numerical method and calculation process
All the species concentrations are calculated from the total S (IV), total Ca, total C and charge. A drop is a calculation unit for the calculation of SO2 absorption. The spatial discretization
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
1029
tration of China) and The Determination of Particulates and Sampling Methods of Gaseous Pollutants Emitted Gas of Stationary Source (Chinese Standard (recommended) GB/T 16157-1996). The protocol for determining measurement dot in the duct is The Determination of Particulates and Sampling Methods of Gaseous Pollutants Emitted Gas of Stationary Source (Industry Standard DL/T 467-2004). The SO2 concentration of flue gas is measured by Rosemount gas analyzer (NGA 2000).
4.
Fig. 3 – Simulated SO2 removal efficiency versus measured removal efficiency.
and the temporal discretization are carried out by divide radius and falling time of slurry drop, respectively. The partial differential equations are solved by finite differences method in a every orthogonal collocation. In order to simplify the calculation, the initial pH value of slurry in slurry tank was set as known value at the beginning of the calculation. The calculation results of concentrations of all the icons were input to the model of absorption zone as known value directly. The two steps shown as below were added to the calculation of scrubber on the base of the SO2 absorption model. All the droplets of the four spray levels take part in the calculation by the two steps: 1) The falling height was taken as the criterion to determine if the drops had reached the next spray level; 2) When the droplets reached the next spray level, the new slurry droplets were added into calculation. The original droplets keep calculating according the divided levels. The coherent data for the model computation were cited or calculated with the data and methods introduced in Refs. [9,21,22].
3.
Results and discussion
The input parameters of model include physical dimension of absorber, the number of nozzles of every spray level, the physical properties of flue gas, pH value of slurry in the slurry zone and so on. Based on the model, SO2 removal efficiency at different operation conditions can be gotten. To verify the reliability of the model, comparisons of different operation conditions between calculation results and measurement results were carried out. Comparisons were made in different operation conditions: pH value, 5.2–5.8; mass concentration of SO2 in flue gas, 2000–4500 mg/N m3; gas flux, 9 × 105–1.25 × 106 N m3/h. Fig. 3 shows the simulated values of desulfurization efficiency versus the measured values of a FGD plant. The average difference between modeled results and measurement results equals 0.87%, with a standard deviation equals to 1.14%. The modeled results agree with the measurement results. The comparison reveals that the model can describe the processes in this absorber well. As shown in Fig. 4, when the flue gas flux is of the design value, desulfurization efficiency increases exponentially with the increase of Ra at three different SO2 concentrations in flue gas. Gerbec [4], Warych [7,8] and Kong [9] obtained same conclusion by their models. The desulfurization efficiency at the high value of Ra increases faster than that at low Ra value. For the operation condition in design value of flue gas flux, the Chinese emission standard of SO2 can be met with the Ra value of 12 and 15 when the SO2 concentrations in flue gas is 3000 mg/N m3 and 5000 mg/N m3. In order to meet the Chinese emission standard of SO2 of flue gas of boiler in power plant at the some severe change of operation, the Ra value is set at 15 at the normal operation condition in this WFGD system.
Experimental
All the experimental data in this paper were taken from a wet limestone with forced oxidation and a gypsum byproduct FGD system. This WFGD system is applied in a power plant in North China and consists of two separate scrubbers. Each scrubber was installed for each boiler of 300 MW unit. The two boilers were sub-critical natural circulation boiler. The maximum continuous rating of the boiler is 1025 t/h. The sulfur content of design coal rank is 1.34% and the corresponding SO2 volume concentration in flue gas is 1200 ppm. The flow of flue gas is 995,000 N m3/h (wet, 6% O2) when burning the design coal. The design values of operation temperature, capacity of slurry tank and retention time of flue gas for scrubber are 50–60 °C, 1900 m3 and 4 s, respectively. The design value of desulfurization efficiency of the WFGD system is more than 95%. The structure of scrubber is the same to the structure shown in Fig. 1. Each absorber consists four recirculation pumps for the four spray levels. The analytical protocols of SO2 concentration of scrubber used in the experiments are Analytical Method for Ambient Air and Exhaust Gas (issued by State Environmental Protection Adminis-
Fig. 4 – Influence of L/G on desulfurization efficiency.
1030
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Fig. 6 – Influence of concentration of SO2 on desulfurization efficiency for design load of the boiler. Fig. 5 – Influence of concentration of SO2 on desulfurization efficiency.
The variation of desulfurization efficiency with the concentration of SO2 in flue gas in different flue gas flux was shown in Fig. 5. The desulfurization efficiency decreases with the increase of concentration of SO2 in flue gas. When the four spray levels are in operation, Chinese emission standard of SO2 of flue gas of boiler in power plant can be met even when the flue gas flux increased to 1, 200,000 N m3/h and the concentration of SO2 in flue gas increase to 5000 mg/N m3. For optimization of performance and cost, the combination of different spray levels are calculated in different operation conditions. The combinations of different spray levels are shown in Table 1. The calculation results of desulfurization efficiency with different combination of three spray levels in the flue gas flux of design value were shown in Fig. 6. Chinese emission standard of SO2 can be met with any combination scheme with three spray levels. The desulfurization efficiency of scheme 2 and scheme 4 is higher than that of scheme 1 and scheme 3. This phenomenon reveals that the higher spray level influence the desulfurization efficiency more than lower spray level. The slurry drops from higher spray level travels longer than those of low spray level in absorber. The corresponding contact time of flue gas and slurry drops are longer. Therefore, chemical absorption in the absorber can proceed sufficiently. The generation unit often operate in part load mode in the night. The flue gas flux changes with the load of boiler. The typical value of flue gas flux is 800,000 N m3/h with the part load. The influence of concentration of SO2 on desulfurization
efficiency in part load of boiler is shown in Fig. 7. In this operation mode, any schemes can be used to satisfy the SO2 emission standard when the concentration of SO2 in flue gas is below the design value. When the concentration of SO2 in flue gas is in 5000 mg/N m3, only scheme 8, scheme 9 and scheme 10 can be used. At the same time, the effect of higher spray level is the same to the condition of three spray levels. Based on the calculation results of combination of spray levels, it can conclude that combinations of higher spray levels are more effective in cleaning the flue gas. When the variation of concentration of SO2 is great, the scheme of combinations of higher spray levels is effective. But the energy consumption of higher spray levels is higher than low spray levels. For the optimization of both performance and cost, the scheme with rational desulfurization efficiency and energy consumption is preferred. For this WFGD system, scheme 2 is recommended for the design operation mode. While in its typical part load, scheme 8 and scheme 9 are recommended. If the load of boiler is lower to half load, which happened in many utility boilers in China, the scheme 1 will be a preferable choice for optimization of operation cost.
Table 1 – Combinations of different spray levels Scheme
1
2
3
4
5
6
7
8
9
10
1st spray level 2nd spray level 3rd spray level 4th spray level
+ + + −
+ − + +
+ + − +
− + + +
+ + − −
+ − + −
+ − − +
− + + −
− + − +
− − + +
Note: + represents the spray levels are in operation, − represents the spray levels are out of operation.
Fig. 7 – Influence of concentration of SO2 on desulfurization efficiency for partial load of the boiler.
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
5.
Conclusions
A model describing the processes in the absorber was developed by unsteady theory. The results of model are in good agreement with the measured results from a WFGD unit of a 300 MW utility boiler. The model could be used to analyze influence of process parameters such as Ra, SO2 concentration in flue gas and the different combination mode of spray levels on desulfurization efficiency. Most of boilers of power plants in China are facing the problem of variable coal and unstable loads. According to the calculation results of model, the guideline of operation mode of the WFGD system for a 300 MW unit in China was determined.
Acknowledgements The authors wish to thank the National Key Technologies R&D Program in the 11th Five-Year Period (No. 2006BAA01B04) and New Century Excellent Talents Program (NCET-06-0513) for their financial support in the course of our technical study of simultaneous removal of NO and SO2 in flue gas in wet flue gas desulfurization system.
REFERENCES
1031
[14] O. Levenspiel, Chemical Reaction Engineering, 3rd ed.Chemical industry press, Beijing, 2002. [15] W.J. Beek, K.M.K. Muttzall, J.W. van Heuven, Transport Phenomena, Chemical industry press, Beijing, 2003. [16] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, 2nd ed., Chemical industry press, Beijing, 2002. [17] B.M. Pamm, Gas Absorption, 2nd ed., Chemical industry press, Beijing, 1985. [18] P.V. Danckwerts, Gas–Liquid Reaction, McGraw-Hill press, New York, 1970. [19] K.Y. Norman, Liquid Phase Mass Transfer in Spray Contactors, The University of Texas, Austin, 2002, pp. 37–94. [20] E.R. Altwicker, E. Chapman, Mass transfer of sulfur dioxide into falling drops: a comparison of experimental data with absorption models, Atmos. Environ. 15 (1981) 299233. [21] R.H. Perry, D.W. Green, J.O. Maloney, Perry's Chemical Engineers' Handbook, McGraw-Hill press, New York, 1984. [22] D.S. Henzel, B.A. Laseke, E.O. Smith, D.O. Swenson, Handbook for Flue Gas Desulfurization Scrubbing with Limestone, Noyes Data Corporation, New Jersey, 1982.
Glossary A: BET surface area of unit mass of calcium sulfite (m2/g) b: distance from center of the drop (m) c: concentration in liquid phase (mol/m3) C: concentration variation (mol/m3) Cdrag: drag coefficient of single droplet motion in flue gas, dimensionless number
[1] P.S. Nolan, K.E. Redinger, G.T. Amrhein, G.A. Kudlac, Demonstration of additive use for enhanced mercury emissions control in wet FGD systems, Fuel Process. Technol. 85 (2004) 587–600. [2] H.G. Nygaard, S. Kiil, J.E. Johnsson, et al., Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber, Fuel 83 (2004) 1151–1164. [3] D.S. Jin, B.R. Deshwal, Y.S. Park, H.Y. Lee, Simultaneous removal of SO2 and NO by wet scrubbing using aqueous chlorine dioxide solution, J. Hazard. Mater. 135 (2006) 412–417. [4] M. Gerbec, A. Stergarsek, R. Kocjancic, Simulation model of wet flue gas desulphurization plant, Comput. Chem. Eng. 19 (1995) s283–s286. [5] C. Brogren, Models for Wet Scrubbing of SO2 and NOx, Ph. D. Lund University, Lund, 1997. [6] C. Brogren, H.T. Karlsson, Modelling the absorption of SO2 in a spray scrubber using the penetration theory, Chem. Eng. Sci. 50 (1997) 3085–3099. [7] J. Warych, M. Szymanowski, Model of the wet limestone flue gas desulfurization process for cost optimization, Ind. Eng. Chem. Res. 40 (2001) 2597–2605. [8] J. Warych, M. Szymanowski, Optimum values of process parameters of the “wet limestone flue gas desulfurization system”, Chem. Eng. Technol. 25 (2002) 427–432. [9] H. Kong, Experimental and Theoretical Study on Limestone Wet Flue Gas Desulfurization, Zhejiang University, Hangzhou, 2001. [10] G.M. Xiang, Studies of Slurry Jet Flue Gas Desulfurization, Tsinghua University, Beijing, 2002. [11] S. Kiil, M.L. Michelsen, K. Dam-Johansen, Experimental and theoretical investigations of wet flue gas desulphurisation pilot plant, Ind. Eng. Chem. Res. 37 (1998) 2792–2806. [12] K.C. Mthta, M.M. Sharma, Mass transfer in spray columns, Br. Chem. Eng. 15 (1970) 1140–1444. [13] J.A. Michalski, Aerodynamic characteristics of flue gas desulfurization spray towers-polydispersity consideration, Ind. Eng. Chem. Res. 39 (2000) 3314–3324.
d: diameter of drop (m) D: diffusion coefficient (m2/s) F: liquid phase flow rate in sump (m3/s) g: acceleration of gravity (9.8 N/kg) H: Henry's constant (Pa m3/mol) kd,CaCO3: the crystallization constant of limestone (mol/(m3 s)) kG: mass transfer coefficient of gas phase, kmol/(m2 s kPa) kL: mass transfer coefficient of gas phase, m/s KSP: solubility product ((mol/m3)2) m: constant, 3 in the crystallization of WFGD M: molar weight (g/mol) MH: Hatta number, dimensionless number n: amount of substance (mol) p: pressure (Pa) Q: flow rate of flue gas (N m3/h) r: chemical absorption rate (mol/(m2 s)) rc: crystalline rate (mol/(m3 s)) rd: solution rate of component (mol/(m3 s)) rreact: the molar concentration variation by reaction (mol/m3) R: drop radius (m) Ra: liquid-to-gas ratio Re: Renault number, dimensionless number RS: degree of supersaturation, dimensionless number Sc: Schmidt number, dimensionless number Sh: Sherwood number, dimensionless number t: movement duration of drops (s) tp: penetration time (s) T: temperature in absorber (K) u: velocity (m/s) V: volume of slurry tank (m3) Greek letter β: enhancement factor for chemical reaction, dimensionless number
ρ: density (kg/m3) μ: viscidity, Pa•s ν: molecular volume, cm3/mol
1032
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Subscripts a: air c: carbon CaCO3: calcium carbonate CaSO3: calcium sulfite CaSO4: calcium sulfate d: slurry drop fal: falling drops in absorption zone g: flue gas
k: component k in slurry in: fresh slurry fed into slurry tank out: slurry discharged from slurry tank rea: reaction slurry in WFGD rec: recycled slurry in WFGD SO2: sulfur dioxide tk: slurry tank
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
A model for performance optimization of wet flue gas desulfurization systems of power plants Yi Zhong, Xiang Gao⁎, Wang Huo, Zhong-yang Luo, Ming-jiang Ni, Ke-fa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, PR China
AR TIC LE I N FO
ABS TR ACT
Article history:
In this paper, a model of limestone/gypsum wet flue gas desulfurization (WFGD) system was
Received 20 June 2007
developed based on unsteady theory. The models of processes of absorption section and
Received in revised form
oxidation section were developed and incorporated into the WFGD integral model. The sub-
25 March 2008
model of motion of slurry drops, absorption of SO2, dissolution of limestone and
Accepted 2 April 2008
crystallization of gypsum were included in the model of absorption section, while the model of oxidation section was developed by the population balance theory. The calculation
Keywords:
results of the desulfurization system for 300 MW utility in China by this model was
Wet flue gas desulfurization
compared to that of corresponding measured results. The simulation results agreed with the
Unsteady theory
measurement results very well. The operation mode of boilers of power plant in China is
Spray scrubber
different to that of other countries since variable coal property and unstable loads of boilers.
Numerical calculation
The differences of operation mode lead to the variation of process parameters of WFGD
Optimization
system. The influences of liquid-to-gas ratio, SO2 concentration of inlet flue gas, and combination mode of different spray levels to the desulfurization efficiency were analyzed. Based on the analysis, some advices of performance optimization of flue gas desulfurization systems in China were suggested. © 2008 Published by Elsevier B.V.
1.
Introduction
It is well known that sulfur dioxide is not only harmful to health, but also contributes to acidification of soil and water. The emission of SO2 from coal-fired boilers is regulated strictly in many countries. There are many methods available for controlling the emission of SO2 of coal-fired boilers. Because of its high efficiency and reliability, limestone–gypsum wet flue gas desulfurization (WFGD) technology is the most commonly used technology for controlling the emission of SO2 in the world [1–3]. The typical scrubber of limestone/gypsum WFGD with forced oxidation system is shown in Fig. 1. The main task of WFGD system with forced oxidation is to remove sulfur dioxide from flue gas and to produce gypsum as a saleable product. The raw flue gas was boosted to the absorber through
flue gas inlet duct, which is located above the slurry tank. The raw flue gas flow upwards and contact slurry drops containing suspended limestone sprayed from the spray levels counter currently to remove the sulfur dioxide in flue gas after entering the scrubber. The slurry drops fall down to the slurry tank after reacting with the flue gas. The slurry is injected into the absorber over several spray levels each with a dedicated recirculation pump. Each spray bank is provided with numerous spray nozzles for a proper atomization of the slurry. The spray levels were named the 1st, 2nd, 3rd and 4th spray level from the bottom up, respectively. The clean flue gas leave the absorber through the outlet duct after removing the entrained water droplets. The water droplets in flue gas were removed by a mist eliminator (ME) to control the erosion of following equipments. The ME locates above the spray levels and consists of two stages. The first stage removes most of the
⁎ Corresponding author. Present address: Institute for Thermal Power Engineering of Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China. Tel.: +86 571 8795 1335; fax: +86 571 8795 1616. E-mail address: [email protected] (X. Gao). 0378-3820/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.fuproc.2008.04.004
1026
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Fig. 1 – Diagrammatic sketch of spray tower.
entrained liquid droplets; in the second stage also fine droplets are captured. The mist eliminators are frequently washed by process water to maintain a clear surface and a low pressure loss. The slurry tank is the lower end of the absorber which is designed with sufficient retention time for optimum utilization of limestone, oxidation and crystallization of gypsum. Oxidation air compressor is used to injected fresh air to the slurry tank by air spargers to oxidize the calcium sulfite in slurry to gypsum. Slurry agitators around the slurry tank are used to disperse the solid particle in the absorber slurry. The most important process in the WFGD system is the absorption of sulfur dioxide. Sulfur dioxide is absorbed in the spray absorption zone by reacting with the limestone slurry drops, so the chemical absorption in the absorber is the most complicated and important step in scrubber. Generally speaking, there are four processes in absorbers and slurry tanks as follows: limestone dissolution, SO2 absorption, SO2− 3 oxidation and gypsum crystallization. The simultaneous and interactive occurrences of these four processes make it difficult to describe all the processes in WFGD system acutely by mathematic models, especially in describing the transformations of all of components in the process of WFGD system. Many researchers have adopted different gas–liquid mass transfer theory to built different WFGD models. Gerbec et al. [4] introduced the unsteady-state theory to model of pilot scale WFGD system. Sub-models of the absorption of SO2 to the falling dispersed drops of liquid phase, chemical equilibriums of sulfite, sulfate, and carbonate, calcium ions in solution, oxidation of sulfite to sulfate, precipitation and dissolution of calcium sulfite and calcium sulfate, dissolution of limestone were built into the WFGD model. Based on various boundary conditions and input parameters, dynamic responses to chosen parameters can be calculated. The model can predict the SO2 removal efficiency and chemical component of slurry in each level. Based on penetration theory, Brogren et al. [5,6] built model of spray absorber of WFGD system and calculated the absorption rate of SO2 into a drop of limestone slurry. The model includes both instantaneous equilibrium reactions and chemical reactions with finite rate: dissolution of limestone,
oxidation of sulfite, gypsum crystallization and the hydrolysis reaction of CO2. The model was used to quantify the mass transfer within a spray scrubber and estimate the impact of the reactions with finite rate of SO2 mass transfer. Warych et al. [7,8] created a detailed model for the desulfurization process of utility boiler. The film theory was adopted in the model to simulate the absorption of SO2 and the dissolution of limestone. This model can predict the variation of SO2 removal efficiency influenced by ratio of volume of liquid to that of flue gas (Ra), pH, flow rate etc. The results agree with the records of experiment well. Kong [9] sets up the model of SO2 absorption in a pilot scale impinge stream scrubber with unsteady-state mass transfer theory. The calculation results fit the experimental results well. The absorption and mass transfer resistance in different zone of the impinge stream scrubber are analyzed based on the model. The effects of limestone dissolution, gypsum crystallization and sulfite oxidation on SO2 absorption are also analyzed. Xiang [10] developed a detailed model for slurry jet FGD pilot plant. The model describes the desulfurization processes in FGD tower. The calculation results can predict the desulfurization efficiency. Kiil [11] developed a detailed model for a wet flue gas desulfurization packed tower. Model predictions were compared to the experimental data such as gas-phase concentration profiles of SO2, slurry pH profiles, solid content of the slurry, liquid-phase concentrations, and residual limestone in the gypsum. The operation mode of utility boilers in China is different to that of other countries. Property of coal burned in utility boilers of China is unstable because the source of coal in power plant changes from time to time. Moreover, unstable electric power loads in China result in the low load operation of the utility boilers at night. The typical daily loads in different seasons are shown in Fig. 2. There is an obvious difference between loads of day and night in every season. The load of night is even about two the third of that of daylight in summer. The two differences of operation mode of boilers lead to some differences between Chinese WFGD systems of utility boilers and those of other countries. Moreover, there is no model available for performance and cost optimization of the spray scrubber of WFGD system of power plant in China now. It is meaningful to study the variation of removal efficiency because of the variable operation mode of boilers. Therefore,
Fig. 2 – Typical daily loads in different seasons.
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
it is important to develop a model according to actual operation mode of WFGD systems to study the optimization of design and operation of WFGD in China. The variable coal rank and unstable loads of boilers will result in three kinds of variations of parameters. First, the inlet SO2 concentration varies great often, sometimes the concentration even increase as twice as that of design value; second, the flux of flue gas varies with the variation of load of boiler; third, the operation of spray tanks varies for the economic operation of WFGD system. Therefore, it is meaningful to study influences of Ra, SO2 concentration of inlet flue gas and combination mode of different spray levels to the desulfurization efficiency to keep the economic operation of WFGD in China. The model of limestone–gypsum WFGD system was developed by the unsteady theory and solved by the numerical method in this paper. The calculation results were compared with the measured results and they fit well. Based on the model, influences of Ra, SO2 concentration of inlet flue gas and the combination mode of different spray levels to the desulfurization efficiency were studied. According to the calculation results of model, the guideline of operation mode of WFGD system was set up to keep the pace of the boiler unit to keep efficiency and reliability.
2.
WFGD model
2.1.
Assumptions
The absorber that consists of absorption zone and slurry zone is the key component in WFGD system. The chemical processes in the absorber include lots of heat and mass transfer phenomenon and chemical reactions. In order to describe the processes in full-scale WFGD absorber better, absorber is separated to two zones: spray absorption zone and slurry oxidation zone. In spray zone, slurry drop contacts and reacts with the flue gas. The absorbent dissolution and oxidation and crystallization occur in the slurry zone. Size and its distribution of drops, velocity of drops, collisions and coalescence between drops, circulation inside drops, oscillation and distortion of drops and flow patterns in absorber are the complex factors in spray absorber [12]. It is difficult to describe all the processes in absorber acutely in mathematical theory. Therefore, some assumptions were made as below: a Flue gas is an ideal gas. In the falling process, the drops are considered to be the rigid ball in shape and the vertical line in trace. The interaction effects among slurry drops were described by the size and the penetration time of slurry drops. The variation of velocity of flue gas and drag coefficient caused by the evaporation, distortion and mass transfer of drops are neglected; b The charge balance, chemical equilibrium of slurry drops and the charge balance, ionic equilibrium in slurry tank are treated as instantaneous process. However, the limestone dissolution, SO2 absorption, liquid–gas mass transfer, gypsum crystallization and chemical reactions are considered as the finite rates process;
1027
c The oxidation of CaSO3, the dissolution and crystallization of CaSO4 in the absorption zone are neglected; d After combining with limestone slurry drops, SO2 and CO2 are totally considered as the hydration phase; CO2 desorption from slurry drops by the CO2 gas in flue gas is neglected; e The slurry drops in absorption zone and slurry zone were considered to be totally mixed. The radial concentration of SO2 is constant in absorber and axial concentration changed with the height of absorber. The content in slurry tank are the same everywhere and the absorption of SO2 gas in upward flue gas is neglected; f Only the O2, CO2, SO2 in gas phase and H2O, O2, CO2, SO2, H+, 2+ − 2− − 2− in liquid OH−, HCO−3, CO2− 3 , HSO3, SO3 , HSO4, SO4 , Ca phase were included in this model. The influences of Cl, and Mg2+ to the WFGD system are neglected.
2.2.
Model
2.2.1.
Model of absorption zone
2.2.1.1. Model of drop motion. The movement equation of falling drops in absorption zone can be determined as Eq. (1) by assumption a [13]: " # 2 qd qg dud 3 qg ud ug Cdrag ¼g : 4 dt qd qg d
ð1Þ
Moreover, the drag coefficient can be calculated by Eqs. (2) and (3) [13]:
Cdrag ¼
Red ¼
24 1 þ 0:125Re0:72 d Red
djud ug jqg : Ag
ð2Þ
ð3Þ
2.2.1.2. Model of SO2 absorption. The SO2 diffusion on the surface of drop and chemical absorption inside drop are the most important step in the process of WFGD. In classical chemical absorption process, 8 different cases can be proposed by the relative rate of mass transfer and chemical reaction. The Hatta number (MH) which is used to describe the contribution of each process to the overall absorption was calculated [14]. The chemical absorption inside the spray tower is fast reaction with low reagent concentration. In other words, the resistance of liquid bulk is negligible because chemical reaction is within the liquid film. Therefore, the overall chemical reaction rate, rSO2, can be described by Eq. (4) [14–16]. 1 HSO2 þ rSO2 ¼ pSO2 = kG bkL
ð4Þ
The partial pressure of SO2 in flue gas, pSO2, can be calculated by the ideal gas equation. The value of kG, kL and β is specified separately for calculate the general mass transfer coefficient. Mass transfer coefficient
1028
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
in gas phase, kG, is acquired by correlating Eqs. (5), (6), and (7) [9,10,17]. Sh ¼
Sc ¼
kG RTd 1=3 ¼ 2 þ 0:55Re0:5 d Sc Dg;SO2 p
ð5Þ
CaCO3) ACC 1 ¼ 2 ½j b2 DH2 CO3 jCH2 CO3 þ j b2 DHCO3 jCH2 CO3 b At þj b2 DCO2 jCCO2 þ rd;CaCO3 3
Ag qg Dg;SO2
DSO2 ;G
2− − 3 The variation of total C (including CO O 3 , HCO3, H2CO3,
ð11Þ
3
ð6Þ Boundary condition: DC jCC;b¼R Initial condition: CC = CC,0.
h i1=2 1 9:86 103 T1:75 M1a þ MSO 2 ¼ 2 1=3 1=3 p ma þ mSO2
ð7Þ
Mass transfer coefficient in liquid phase, kL, is achieved by Eq. (8) [15–18]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4DSO2 ;L : kL ¼ k tp
ð8Þ
The penetration time, t p, can be calculated by the characteristic of hydrodynamics inside slurry drops or sheets. In accordance with past works [6,19] and practicable operation conditions, the value of penetration time for slurry sheets and drops out of nozzles are set to different value in the simulation. Diffusion coefficient of SO2 in liquid phase, DSO2,L, is obtained from Ref. [19]. The enhancement factor for chemical reaction, β, Based on the experimental data in the lab and the real operation parameters, the value of β, can be fitted [7,10,19,20]. According to assumption c, there is no change for total S(VI) in the process of falling of drops. But the reciprocal transformation between HSO−4 and SO2− 4 because of the variation of pH value and the influence of this transformation to pH value are considered. Based on the spherical coordinate, the variations 2+ of the total S(IV) (SO2, HSO−3, SO2− 3 , CaSO3), total Ca(Ca , CaSO3, 2− − CaSO4, CaCO3), total C(CO3 , HCO3, H2CO3, CaCO3) in falling drops are modeled as below [5, 10]:
The nozzles (Pigtail type, STXP serious) used in the scrubber described in this paper are made by BETE fog nozzle Co. Ltd. The Sauter mean diameter is 1.96 mm for the nozzle. In this paper, the interaction effects among slurry drops were described by the size and penetration time. Take the interaction effects among slurry drops, 2.9 mm was taken as the diameter of the droplets.
2.2.1.3. Model of limestone dissolution and sulfite crystallization. The dissolution of limestone occurred in drop was described by Eqs. (12) and (13) [9,10]. rd;CaCO3 ¼ kd;CaCO3 ðRSCaCO3 1Þ RSCaCO3 ¼
1 The variation of total S(IV) (including SO , O 2
cCa2þ cCO2 3
KSP;CaCO3
ð12Þ ð13Þ
According to assumption c, the crystallization of CaSO3 and CaSO4 inside the drops was described by Eqs. (14), (15), and (16) [9,10]. 5153 A ðRSCaSO3 1Þm rc;CaSO3 ¼ 1:62 104 exp T RSCaSO4 ð14Þ RSCaSO3 ¼
RSCaSO4 ¼ HSO−3,
R3 rC;b¼R ¼ 0
SO2− 3 ,
cCa2þ cSO2 3
KSP; CaSO3 cCa2þ cSO2 4
KSP; CaSO4
ð15Þ
ð16Þ
CaSO3) ACSðIVÞ 1 ¼ 2 ½j b2 DSO2 ;L jCSO2 þ j b2 DHSO3 jCHSO3 b At þj b2 DSO2 jCSO2 þ j b2 DCaSO3 jCCaSO3 3
2.2.2. ð9Þ
3
rc;CaSO3
Boundary condition: kG pg; SO2 pi; SO2 ¼ DS jCS;b¼R R3 rS;b¼R Initial condition: CS(IV) = CS(IV),0 2+ 2 The variation of total Ca (including Ca , CaSO3, CaSO4, CaCO3) O ACCa 1 ¼ 2 ½j b2 DCa2þ jCCa2þ þ j b2 DCaSO2 jCCaSO3 b At þj b2 DCaSO4 CCaSO4 þ j b2 DCaCO3 jCaCO3
X dck;out ¼ Ck þ Ck;rea dt
Ck ¼ ð10Þ
þrd;CaCO3 rc;CaSO3 Boundary condition: DCa jCCa;b¼R Initial condition: CCa =CCa,0
Model of oxidation zone
According to assumptions a, c, d and theory of population balance, all the contents including S(IV), S(VI) and Ca, all the ions and molecules are in balance conditions when the system operates in normal condition. It can be described by Eqs. (17) and (18) [10].
1 F c þ Ffal ck;fal Frec ck;rec Fout ck;out Vtk in k;in
2.2.3.
R3 rCa;b¼R ¼ 0
ð17Þ
ð18Þ
Numerical method and calculation process
All the species concentrations are calculated from the total S (IV), total Ca, total C and charge. A drop is a calculation unit for the calculation of SO2 absorption. The spatial discretization
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
1029
tration of China) and The Determination of Particulates and Sampling Methods of Gaseous Pollutants Emitted Gas of Stationary Source (Chinese Standard (recommended) GB/T 16157-1996). The protocol for determining measurement dot in the duct is The Determination of Particulates and Sampling Methods of Gaseous Pollutants Emitted Gas of Stationary Source (Industry Standard DL/T 467-2004). The SO2 concentration of flue gas is measured by Rosemount gas analyzer (NGA 2000).
4.
Fig. 3 – Simulated SO2 removal efficiency versus measured removal efficiency.
and the temporal discretization are carried out by divide radius and falling time of slurry drop, respectively. The partial differential equations are solved by finite differences method in a every orthogonal collocation. In order to simplify the calculation, the initial pH value of slurry in slurry tank was set as known value at the beginning of the calculation. The calculation results of concentrations of all the icons were input to the model of absorption zone as known value directly. The two steps shown as below were added to the calculation of scrubber on the base of the SO2 absorption model. All the droplets of the four spray levels take part in the calculation by the two steps: 1) The falling height was taken as the criterion to determine if the drops had reached the next spray level; 2) When the droplets reached the next spray level, the new slurry droplets were added into calculation. The original droplets keep calculating according the divided levels. The coherent data for the model computation were cited or calculated with the data and methods introduced in Refs. [9,21,22].
3.
Results and discussion
The input parameters of model include physical dimension of absorber, the number of nozzles of every spray level, the physical properties of flue gas, pH value of slurry in the slurry zone and so on. Based on the model, SO2 removal efficiency at different operation conditions can be gotten. To verify the reliability of the model, comparisons of different operation conditions between calculation results and measurement results were carried out. Comparisons were made in different operation conditions: pH value, 5.2–5.8; mass concentration of SO2 in flue gas, 2000–4500 mg/N m3; gas flux, 9 × 105–1.25 × 106 N m3/h. Fig. 3 shows the simulated values of desulfurization efficiency versus the measured values of a FGD plant. The average difference between modeled results and measurement results equals 0.87%, with a standard deviation equals to 1.14%. The modeled results agree with the measurement results. The comparison reveals that the model can describe the processes in this absorber well. As shown in Fig. 4, when the flue gas flux is of the design value, desulfurization efficiency increases exponentially with the increase of Ra at three different SO2 concentrations in flue gas. Gerbec [4], Warych [7,8] and Kong [9] obtained same conclusion by their models. The desulfurization efficiency at the high value of Ra increases faster than that at low Ra value. For the operation condition in design value of flue gas flux, the Chinese emission standard of SO2 can be met with the Ra value of 12 and 15 when the SO2 concentrations in flue gas is 3000 mg/N m3 and 5000 mg/N m3. In order to meet the Chinese emission standard of SO2 of flue gas of boiler in power plant at the some severe change of operation, the Ra value is set at 15 at the normal operation condition in this WFGD system.
Experimental
All the experimental data in this paper were taken from a wet limestone with forced oxidation and a gypsum byproduct FGD system. This WFGD system is applied in a power plant in North China and consists of two separate scrubbers. Each scrubber was installed for each boiler of 300 MW unit. The two boilers were sub-critical natural circulation boiler. The maximum continuous rating of the boiler is 1025 t/h. The sulfur content of design coal rank is 1.34% and the corresponding SO2 volume concentration in flue gas is 1200 ppm. The flow of flue gas is 995,000 N m3/h (wet, 6% O2) when burning the design coal. The design values of operation temperature, capacity of slurry tank and retention time of flue gas for scrubber are 50–60 °C, 1900 m3 and 4 s, respectively. The design value of desulfurization efficiency of the WFGD system is more than 95%. The structure of scrubber is the same to the structure shown in Fig. 1. Each absorber consists four recirculation pumps for the four spray levels. The analytical protocols of SO2 concentration of scrubber used in the experiments are Analytical Method for Ambient Air and Exhaust Gas (issued by State Environmental Protection Adminis-
Fig. 4 – Influence of L/G on desulfurization efficiency.
1030
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Fig. 6 – Influence of concentration of SO2 on desulfurization efficiency for design load of the boiler. Fig. 5 – Influence of concentration of SO2 on desulfurization efficiency.
The variation of desulfurization efficiency with the concentration of SO2 in flue gas in different flue gas flux was shown in Fig. 5. The desulfurization efficiency decreases with the increase of concentration of SO2 in flue gas. When the four spray levels are in operation, Chinese emission standard of SO2 of flue gas of boiler in power plant can be met even when the flue gas flux increased to 1, 200,000 N m3/h and the concentration of SO2 in flue gas increase to 5000 mg/N m3. For optimization of performance and cost, the combination of different spray levels are calculated in different operation conditions. The combinations of different spray levels are shown in Table 1. The calculation results of desulfurization efficiency with different combination of three spray levels in the flue gas flux of design value were shown in Fig. 6. Chinese emission standard of SO2 can be met with any combination scheme with three spray levels. The desulfurization efficiency of scheme 2 and scheme 4 is higher than that of scheme 1 and scheme 3. This phenomenon reveals that the higher spray level influence the desulfurization efficiency more than lower spray level. The slurry drops from higher spray level travels longer than those of low spray level in absorber. The corresponding contact time of flue gas and slurry drops are longer. Therefore, chemical absorption in the absorber can proceed sufficiently. The generation unit often operate in part load mode in the night. The flue gas flux changes with the load of boiler. The typical value of flue gas flux is 800,000 N m3/h with the part load. The influence of concentration of SO2 on desulfurization
efficiency in part load of boiler is shown in Fig. 7. In this operation mode, any schemes can be used to satisfy the SO2 emission standard when the concentration of SO2 in flue gas is below the design value. When the concentration of SO2 in flue gas is in 5000 mg/N m3, only scheme 8, scheme 9 and scheme 10 can be used. At the same time, the effect of higher spray level is the same to the condition of three spray levels. Based on the calculation results of combination of spray levels, it can conclude that combinations of higher spray levels are more effective in cleaning the flue gas. When the variation of concentration of SO2 is great, the scheme of combinations of higher spray levels is effective. But the energy consumption of higher spray levels is higher than low spray levels. For the optimization of both performance and cost, the scheme with rational desulfurization efficiency and energy consumption is preferred. For this WFGD system, scheme 2 is recommended for the design operation mode. While in its typical part load, scheme 8 and scheme 9 are recommended. If the load of boiler is lower to half load, which happened in many utility boilers in China, the scheme 1 will be a preferable choice for optimization of operation cost.
Table 1 – Combinations of different spray levels Scheme
1
2
3
4
5
6
7
8
9
10
1st spray level 2nd spray level 3rd spray level 4th spray level
+ + + −
+ − + +
+ + − +
− + + +
+ + − −
+ − + −
+ − − +
− + + −
− + − +
− − + +
Note: + represents the spray levels are in operation, − represents the spray levels are out of operation.
Fig. 7 – Influence of concentration of SO2 on desulfurization efficiency for partial load of the boiler.
FUE L PR O CE SS I N G TE CH N O LO G Y 89 ( 20 0 8 ) 1 0 2 5–1 0 3 2
5.
Conclusions
A model describing the processes in the absorber was developed by unsteady theory. The results of model are in good agreement with the measured results from a WFGD unit of a 300 MW utility boiler. The model could be used to analyze influence of process parameters such as Ra, SO2 concentration in flue gas and the different combination mode of spray levels on desulfurization efficiency. Most of boilers of power plants in China are facing the problem of variable coal and unstable loads. According to the calculation results of model, the guideline of operation mode of the WFGD system for a 300 MW unit in China was determined.
Acknowledgements The authors wish to thank the National Key Technologies R&D Program in the 11th Five-Year Period (No. 2006BAA01B04) and New Century Excellent Talents Program (NCET-06-0513) for their financial support in the course of our technical study of simultaneous removal of NO and SO2 in flue gas in wet flue gas desulfurization system.
REFERENCES
1031
[14] O. Levenspiel, Chemical Reaction Engineering, 3rd ed.Chemical industry press, Beijing, 2002. [15] W.J. Beek, K.M.K. Muttzall, J.W. van Heuven, Transport Phenomena, Chemical industry press, Beijing, 2003. [16] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, 2nd ed., Chemical industry press, Beijing, 2002. [17] B.M. Pamm, Gas Absorption, 2nd ed., Chemical industry press, Beijing, 1985. [18] P.V. Danckwerts, Gas–Liquid Reaction, McGraw-Hill press, New York, 1970. [19] K.Y. Norman, Liquid Phase Mass Transfer in Spray Contactors, The University of Texas, Austin, 2002, pp. 37–94. [20] E.R. Altwicker, E. Chapman, Mass transfer of sulfur dioxide into falling drops: a comparison of experimental data with absorption models, Atmos. Environ. 15 (1981) 299233. [21] R.H. Perry, D.W. Green, J.O. Maloney, Perry's Chemical Engineers' Handbook, McGraw-Hill press, New York, 1984. [22] D.S. Henzel, B.A. Laseke, E.O. Smith, D.O. Swenson, Handbook for Flue Gas Desulfurization Scrubbing with Limestone, Noyes Data Corporation, New Jersey, 1982.
Glossary A: BET surface area of unit mass of calcium sulfite (m2/g) b: distance from center of the drop (m) c: concentration in liquid phase (mol/m3) C: concentration variation (mol/m3) Cdrag: drag coefficient of single droplet motion in flue gas, dimensionless number
[1] P.S. Nolan, K.E. Redinger, G.T. Amrhein, G.A. Kudlac, Demonstration of additive use for enhanced mercury emissions control in wet FGD systems, Fuel Process. Technol. 85 (2004) 587–600. [2] H.G. Nygaard, S. Kiil, J.E. Johnsson, et al., Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber, Fuel 83 (2004) 1151–1164. [3] D.S. Jin, B.R. Deshwal, Y.S. Park, H.Y. Lee, Simultaneous removal of SO2 and NO by wet scrubbing using aqueous chlorine dioxide solution, J. Hazard. Mater. 135 (2006) 412–417. [4] M. Gerbec, A. Stergarsek, R. Kocjancic, Simulation model of wet flue gas desulphurization plant, Comput. Chem. Eng. 19 (1995) s283–s286. [5] C. Brogren, Models for Wet Scrubbing of SO2 and NOx, Ph. D. Lund University, Lund, 1997. [6] C. Brogren, H.T. Karlsson, Modelling the absorption of SO2 in a spray scrubber using the penetration theory, Chem. Eng. Sci. 50 (1997) 3085–3099. [7] J. Warych, M. Szymanowski, Model of the wet limestone flue gas desulfurization process for cost optimization, Ind. Eng. Chem. Res. 40 (2001) 2597–2605. [8] J. Warych, M. Szymanowski, Optimum values of process parameters of the “wet limestone flue gas desulfurization system”, Chem. Eng. Technol. 25 (2002) 427–432. [9] H. Kong, Experimental and Theoretical Study on Limestone Wet Flue Gas Desulfurization, Zhejiang University, Hangzhou, 2001. [10] G.M. Xiang, Studies of Slurry Jet Flue Gas Desulfurization, Tsinghua University, Beijing, 2002. [11] S. Kiil, M.L. Michelsen, K. Dam-Johansen, Experimental and theoretical investigations of wet flue gas desulphurisation pilot plant, Ind. Eng. Chem. Res. 37 (1998) 2792–2806. [12] K.C. Mthta, M.M. Sharma, Mass transfer in spray columns, Br. Chem. Eng. 15 (1970) 1140–1444. [13] J.A. Michalski, Aerodynamic characteristics of flue gas desulfurization spray towers-polydispersity consideration, Ind. Eng. Chem. Res. 39 (2000) 3314–3324.
d: diameter of drop (m) D: diffusion coefficient (m2/s) F: liquid phase flow rate in sump (m3/s) g: acceleration of gravity (9.8 N/kg) H: Henry's constant (Pa m3/mol) kd,CaCO3: the crystallization constant of limestone (mol/(m3 s)) kG: mass transfer coefficient of gas phase, kmol/(m2 s kPa) kL: mass transfer coefficient of gas phase, m/s KSP: solubility product ((mol/m3)2) m: constant, 3 in the crystallization of WFGD M: molar weight (g/mol) MH: Hatta number, dimensionless number n: amount of substance (mol) p: pressure (Pa) Q: flow rate of flue gas (N m3/h) r: chemical absorption rate (mol/(m2 s)) rc: crystalline rate (mol/(m3 s)) rd: solution rate of component (mol/(m3 s)) rreact: the molar concentration variation by reaction (mol/m3) R: drop radius (m) Ra: liquid-to-gas ratio Re: Renault number, dimensionless number RS: degree of supersaturation, dimensionless number Sc: Schmidt number, dimensionless number Sh: Sherwood number, dimensionless number t: movement duration of drops (s) tp: penetration time (s) T: temperature in absorber (K) u: velocity (m/s) V: volume of slurry tank (m3) Greek letter β: enhancement factor for chemical reaction, dimensionless number
ρ: density (kg/m3) μ: viscidity, Pa•s ν: molecular volume, cm3/mol
1032
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 0 25 –1 0 3 2
Subscripts a: air c: carbon CaCO3: calcium carbonate CaSO3: calcium sulfite CaSO4: calcium sulfate d: slurry drop fal: falling drops in absorption zone g: flue gas
k: component k in slurry in: fresh slurry fed into slurry tank out: slurry discharged from slurry tank rea: reaction slurry in WFGD rec: recycled slurry in WFGD SO2: sulfur dioxide tk: slurry tank