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ModelingWetFlueGasDesulfurization

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Modeling Wet Flue Gas Desulfurization

Arif Arif1, Candice Stephen2, David Branken1,*, Raymond Everson1, Hein Neomagus1 & Stuart Piketh3

1EPPEI Emissions Control SC, School of Chemical and Minerals Engineering, North-West University, Private Bag X6001, Potchefstroom, 2520, South Africa

2Group Technology Boiler Engineering, ESKOM, SOC (Ltd), South Africa 3EPPEI Emissions Control SC, School of Geo- and Spatial Sciences, North-West University, Private Bag

X6001, Potchefstroom, 2520, South Africa

*dawie.branken@nwu.ac.za

Abstract Eskom’s new build power plants Kusile and Medupi will be fitted with wet flue gas desulfurization (WFGD)

plants to meet compliance levels for SO2 emissions. Although WFGD, as operated in Europe, is capable of achieving SO2 removal efficiencies between 95 – 98 %, operation with South African limestones are yet to be characterized.

An investigation was therefore undertaken to develop an integrated reaction rate model for wet desulfurization, which is used as input in detailed Computational Fluid Dynamics (CFD) models of WFGD absorption towers that includes detailed gas and slurry droplet flow dynamics. A semi-batch laboratory reactor was used with industrial type flue gases and commercial grade limestones, and the results have been modeled to identify the important mechanisms and the associated parameters. At this preliminary stage, it would seem that the dissolution of limestone, which is heavily influenced by the solution pH, is the rate determining step in the formation of gypsum, i.e. the final byproduct of the WFGD process. The results from these kinetic studies are to be used in conjunction with detailed CFD modeling, from which more simplified process models can be derived. It is therefore envisioned to use such process models to evaluate the use of limestones of varying quality and to characterize cost/process efficiency tradeoffs. Furthermore, the availability of accurate process models will enable efficient operation and control of Eskom WFGD plants. This paper therefore focuses on the construction of relevant CFD models, of which some preliminary results are showcased.

The water consumption for WFGD is approximately 30% more than for conventional semi-dry FGD processes. Medupi Power Station which is situated in the semi-arid region of Lephalale in the Limpopo Province is to be retrofitted with a FGD plant approximately six years after the commissioning of each boiler unit. Water optimization for WFGD remains an important aspect and options to reduce water losses are discussed in this paper.

1. Introduction Sulfur dioxide (SO2) is present in power plant flue gases as a result of burning fossil fuels during power

generation and SO2 originating from such point sources are one of the major contributors towards acid rain. Over the years, the allowed SO2 emission level has been lowered, thereby necessitating an increased efficiency of flue gas desulfurization plants. In South Africa, the national emission standards for SO2 emission from coal fired power plants are becoming increasingly stringent and by 2020 the minimum allowable emissions of SO2 will be limited to 500 mg/Nm3 at 10% O2 for both new and existing coal fired power stations as shown in Table 1 [1]. In this respect there is strong need to retrofit the existing coal fired power stations with desulfurization facilities. Several technologies are available for flue gas desulfurization of which wet flue gas desulfurization (WFGD) has been a popular choice internationally due to its high sulfur removal efficiencies. The WFGD treatment system is typically located after removal of particulate matter (PM) from the flue gas either by using bag-houses (fabric filters) or by electrostatic precipitators (ESPs). The WFGD process is also considered a commercially mature technology and is offered by a number of suppliers worldwide.

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Table 1: Minimum SO2 Emissions for South African Power Facilities [1].

SO2 Emissions Limit Power Facilities Compliance Date

500 mg/Nm3 at 10 % O2 New Plants 2010

3500 mg/Nm3 at 10 % O2 Existing Plants 2015

500 mg/Nm3 at 10 % O2 Existing Plants 2020

Typical WFGD equipment consists of the absorption tower (absorber), the reaction tank and the associated recycling system. The flue gas is brought into contact with the lime or limestone slurry by dispersion of the liquid phase into droplets to remove SO2 from the flue gas. Limestone is the most widely used reagent because of its high SO2 removal efficiency, reliability, valuable by-product produced during the process (gypsum) and low utility consumption [2] The first and most important process taking place in the WFGD system is the absorption of SO2 in the aqueous phase of the dispersed slurry droplets and takes place in the spray absorption zone (absorption tower) where after it reacts with the dissolved limestone. Therefore, the absorption of SO2 in the absorber is the most important step taking place in the scrubber and is coupled with an intricate system of coupled chemical reactions. Generally speaking, there are four processes taking place in the absorber and reaction tank of a WFGD system as follows: Limestone dissolution, SO2 absorption, SO3

2− oxidation and gypsum crystallization. The simultaneous and interactive occurrences of these four processes make it difficult to accurately describe all the processes taking place in a WFGD system by simple mathematical models, especially in describing the transformations of all of components in the process of a WFGD system [3].

It is very important to evaluate the effect of different design parameters and process variables in the early stages of process design and development to reduce the development costs and the length of the development cycle. Therefore the use of numerical simulation is a key factor in this respect. Various studies have shown that CFD modeling is a valuable tool for evaluating the performance of WFGD systems and optimization of WFGD absorber as CFD provides an efficient platform to evaluate the effect of various process and design parameters on the performance [4, 5]. One of the objectives of the research currently being conducted, is to develop a comprehensive model for the design, optimization and operation of an industrial scale WFGD plant at Kusile, which will be Eskom’s first power station to be equipped with any desulfurization unit.

Another aspect that should be addressed such as to optimize WFGD operation is the water consumption. Water is of course lost through evaporation as the flue gas is cooled to the adiabatic saturation temperature while being contacted with the limestone slurry in the WFGD absorber, which must be compensated for through additional water make-up, and accounts for 85% of the total WFGD water consumption [6]. Flue gas typically enters the WFGD at a temperature of 120 - 150 °C which not only results in high WFGD water consumption but the sensible heat remaining in the flue gas is lost to the atmosphere and not beneficially utilized. Depending on the extent of flue gas cooling, a reduction of 30 - 80% in the total WFGD water requirement can be achieved, making the water consumption of the WFGD system comparable with that of conventional semi-dry FGD technologies.

Our current research on this aspect therefore focuses on a techno-economic evaluation (water savings, influences on WFGD plant performance, interfacing plant systems and overall power plant efficiency, and a life cycle cost assessment) of heat recovery options to reduce WFGD evaporative water losses for Medupi. Medupi power station will be the largest dry-cooled power station in the world which comes at a significantly high energy penalty and for this reason was chosen to be studied since additional energy penalties must be limited as far as possible. The options evaluated to reduce evaporative water loss include various system configurations for heat recovery through flue gas reheat, boiler feed water heating and combustion air preheating. While the cost of flue gas cooling systems may be significant due to the expensive heat exchanger materials, the enhanced flue gas heat recovery of the additional sensible heat could nonetheless be attractive due to the cumulative benefits of WFGD

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water savings and improved power plant performance, if the system can be practically integrated between the WFGD and the interfacing plant systems. It is therefore envisaged that concepts developed for Medupi Power Station will be evaluated based on a life cycle cost assessment and pre-defined evaluation criteria that address the technical feasibility of each option.

2. Modeling Methods

2.1 CFD Modeling of a WFGD Absorber

To model an industrial scale WFGD, slurry nozzles are represented by hollow cone injectors instead of modelling them geometrically. It is very important that the characteristic of the injector’s spray matches the behavior of actual nozzles. Most of the input data for modeling injectors is acquired from nozzle manufacturers where these nozzles have been extensively tested and key parameters affecting spraying performance are known. One of the most important user define input parameter for modeling the injector is number of parcel streams per injector. In reality hundreds of droplet streams are injected from a single nozzle cone and therefore it is necessary to provide the optimum number of parcel stream per injector to accurately represent the spray. By increasing the number of parcel streams, one increases the number of droplets to be tracked within the modeled volume, thereby increases the uniformity of the liquid slurry phase within the absorber, albeit at the expense of computational resources. This parameter has been addressed in detail in the current work along with other important parameters affecting the performance of WFGD. More in-depth detail on other aspects of model construction and validation is provided elsewhere [7].

A full scale 3-D Numerical model of a commercial WFGD unit’s absorber installed in a high capacity coal fired power station has been developed accordingly, and simulated using the CFD framework as shown in Figure 1. Multiphase Euler-Lagrange (E/L) CFD model was applied for modelling multiphase flow while turbulence effects were introduced in the model using the realizable k-ε turbulence model [8]. The interaction between slurry droplets and flue gases was included in the model by two way coupling. Characteristics of the model and boundary conditions are listed in Table 2.

Figure 1: Geometric representation of the model showing simplified model geometry with cross section of absorber showing spray headers with nozzles. SH Refers to spray headers (banks of spray nozzles, and ME refers to the mist eliminator).

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Table 2: CFD Model characteristics of the WFGD absorber modelled in this study.

Parameters Modeling Method

Modeling approach Euler-Lagrange

Phase interaction Two way coupling

Turbulence model k-ε turbulence model

Nozzles Dual flow hollow cone point injectors

Drag force Liu dynamic drag coefficient model

Droplet distortion TAB distortion model

Mist eliminator Porous media with suitable pressure drop

Droplet-wall interaction Rebound and impingement model

Droplet size distribution Rosin Rammler particle size distribution model

Domain discretization Polyhedral and prism layer cells with surface remesher

Evaporation Quasi - steady state droplet evaporation model

2.2 Evaluation of options to minimize water consumption

The various water optimization options that have been identified are currently being evaluated by (i) establishing a WFGD process model to obtain the mass, energy and water balances for Medupi power station; (ii) integrating the water optimization options into the process model; (iii) evaluating the potential of the different water optimization options by taking into account the effects on the WFGD sulfur removal efficiency and interfacing plant systems; and (iv) determining the capital and operating cost implications associated with each evaluated aspect.

As reference, a proprietary WFGD design model developed by an Eskom-contracted engineering company, and has been used for comparison and validation of the newly obtained mass balances for the Medupi power station. In this paper, results obtained thus far regarding mainly point (i) above will be discussed. Some proprietary methods and results are therefore involved, and for this reason, the specifics of such methods and results cannot be disclosed.

3. Results and Discussion

3.1 CFD Modeling Results

As shown Figure 2, flue gas enters the absorber at c.a. 140 oC and exits the absorber at a uniform temperature of approximately 50 oC. Nonetheless, a relative humidity of 1.0 is predicted soon after the 1st spray header (SH) and these conditions prevail throughout the absorber. It was further observed that condensation did not have any significant effect on either the continuous or discrete phase since conditions under which condensation would occur is hardly reached inside the WFGD’s absorber.

As the flue gas flows inside through the WFGD absorber, cooling occurs due to evaporation of water from the slurry droplets which increases the moisture content of the flue gas. This is a result of counter current interaction of the flue gas with the slurry droplets, whereby heat and mass transfer occurs between the continuous and dispersed phases. Minimization of this evaporative water loss is further studied in a separate study as previously mentioned, and is discussed further in Sec. 3.2.

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The liquid-to-gas ratio (L/G ratio) which is considered an important parameter for the design and operation of absorbers was calculated, of which the results are shown in Figure 3. It can be seen that the slurry is distributed with an average L/G value of 11-13 which no doubt will influence the mass transfer of SO2 from the flue gas to the slurry droplets, such that the SO2 absorption rate will not be constant throughout the length of the column. The maximum L/G ratio was observed between the lowest spray headers (SH 1 and SH 2), followed by a decreases at the subsequent levels.

Figure 3: Profile of L/G ratio (dm3 of slurry / m3 of flue gas) along the absorber’s height. Due to the fact that these results are likely to be published in an international, peer reviewed scientific journal, axis values cannot be shown.

Velocity profiles for the droplets and gas inside the absorption tower are shown in Figure 4 (a). A high flue gas velocity exists near the absorber’s wall at the opposite side of the gas entrance duct with high velocity propagating along the wall throughout the column. It is obvious that the high velocity zone could

Figure 2: Scalar profile of (a) gas temperature, and (b) relative humidity. Due to the fact that these results are likely to be published in an international, peer reviewed scientific journal, axis values cannot be shown.

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be eliminated by placing more slurry nozzles near the absorber wall as compared to the middle section of the absorber.

It was further found that the entrance velocity to the absorber has a great influence on the gas velocity profile inside the absorber as shown in Figure 4(b). Flue gas entrance velocities ranging from 10 m/s to 20 m/s were simulated and it was found that for entrance velocity of less than 15.5 m/s that the carryover of droplets could be eliminated by the mist eliminator, which is an important operating parameter.

Figure 4: A CFD representation (a), showing the flow of slurry droplets (grey) from the spray banks and flue gas (coloured) counter-currently. The relationship between inlet gas velocity and the axial gas velocity at various heights within the column (b) as calculated using the present CFD model. Due to the fact that these results are likely to be published in an international, peer reviewed scientific journal, axis values for Figure (a) cannot be shown.

Three parameters, i.e. the maximum face velocity upstream of the ME, the flue gas velocity CV-value upstream of the ME, and the droplet size distribution upstream of the ME were noted as critical performance indicators for the ME, and various sets of simulations have thus been performed to further study the dependence of the ME performance on these parameters. The flue gas velocity upstream of the mist eliminator was found to be very important for the operation and durability of the ME. A smooth upstream velocity profile is thus essential for optimum performance of the ME. In addition, the maximum velocity at the mist eliminator face gives an indication of the risks of fouling on the mist eliminator surface since the lower the maximum velocity, the lower the amount of droplets colliding with the mist eliminator surface.

The velocity profile upstream of the ME is illustrated in Figure 5 (a). The profile was found to be uniform across the cross-section of the absorber as shown by the lower value of CV for velocity and the maximum face velocity was also found to be relatively low which is an indication of good performance and durability of the mist eliminator. The numerical value of the CV for velocity, and the maximum face velocity may be used in the design and optimization stage of ME equipment. Different droplet sizes were introduced using the RR droplet size distribution model and it can be seen from Figure 5 (b) that smaller droplets can be expected to be found in upper part of the absorber. This information may be helpful in the design of the ME to avoid fouling, and may also help in the design of an efficient ME washing system. It was observed that only the Lagrangian phase is sensitive to slurry droplet distortion and breakup, since the CV-value for the L/G ratio when introducing droplet distortion and breakup in the model because smaller droplets (child parcels) are formed. This nonetheless further improves slurry distribution and results in the CFD model having a closer resemblance to the real situation.

(a) (b)

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3.2 Medupi Process Model and Mass Balances

In the WFGD process as discussed above, the SO2 contained in the flue gas is absorbed by the liquid slurry droplets, after which further reaction with the limestone reactant takes place in the reaction tank that is located below the absorber. Sulfite ions are converted to sulfate ions in a forced oxidation process, and reaction with dissolved calcium produces gypsum slurry as the main by-product. The gypsum slurry is then passed through a gypsum dewatering system which transforms the gypsum slurry into gypsum that conforms to certain specifications [9].

Figure 6: WFGD Process Flow Diagram for Medupi Power Station.

Figure 5: Profile of (a) velocity upstream of ME (b) droplet size distribution. Due to the fact that these results are likely to be published in an international, peer reviewed scientific journal, axis values cannot be shown.

(a) (b)

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The process flow diagram for the Medupi WFGD plant is shown in Figure 6, where this configuration includes the recycle and reclaim of process streams to optimize water consumption. The overall mass balance and water balances for the plant are also illustrated schematically in Figure 7 (a) and (b).

The water required for fresh limestone slurry preparation and gypsum washing represent internal uses of water in the system. The major consumers of water in the WFGD system for the Medupi Power station are summarised in Figure 8, based on results obtained from the WFGD process model developed as part of this research. Figure 8 also highlights that water lost through evaporative cooling of the flue gas to the adiabatic saturation temperature is the largest consumer of water and represents the greatest opportunity for WFGD water reduction. The amount of water lost through evaporative cooling of the flue gas depends on the relative humidity of the raw flue gas, as well as on the temperature difference between the raw flue gas and the saturation temperature. Therefore the higher the temperature of the raw flue gas the greater the water lost through evaporative cooling. Reducing the temperature of the raw flue gas closer to saturation before entering the absorber is therefore desired to reduce the amount of water lost through evaporation [10].

The influence of raw flue gas temperature on WFGD water consumption for Medupi Power Station was therefore calculated, and the results are shown in Figure 9. From the results presented in Figure 9, it is clear that the amount of water that can be saved through upstream cooling of the raw flue gas is limited by the water required for

Figure 7: Overall mass balance (a), and overall water balance for the Medupi WFGD process.

(a) (b)

Figure 8: WFGD Water consumers for Medupi Power Station.

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mist eliminator washing, limestone preparation and to compensate for water lost through the wastewater bleed and the final gypsum product. Make-up water is usually fed to the absorber through the mist eliminator washing stage, and therefore represents the minimum water requirement for the absorption process, and can therefore be used to determine the desired raw flue gas temperature to achieve the desired reduction in evaporative water losses.

In principle the flue gas entering the WFGD can be cooled using an external cooling medium (eg. water/air) or through recovering the heat present in the flue gas by utilising an existing source from the power plant. This research focuses on the recovery and reuse of the remaining sensible heat from the flue gas for flue gas reheat or for heat integration with other process streams in the power plant. A simplified schematic representation of the Medupi Power Plant in which the three options thus identified for heat recovery such as to lower the WFGD raw flue gas temperature is given in Figure 10.

The three different options shown in Figure 10 include:

• Option 1: Flue gas reheat: Recovery of sensible heat from the raw flue gas to reheat the cleaned flue gas. Benefits include improved plume buoyancy and dispersion of residual pollutants [10].

• Option 2: Feedwater (FW) heating: In this configuration the sensible heat recovered from the raw flue gas is used for FW heating which reduces the requirements for steam extraction for the FW heaters. Benefits include improved overall power plant efficiency.

• Option 3: Combustion air pre-heating: In this configuration the sensible heat recovered from the raw flue gas can be used to pre-heat the combustion air before it enters the main air preheaters. Conventionally, steam extracted from the turbines is used to pre-heat the combustion air before entering the air heaters to prevent corrosion on the cold end. Benefits include improved overall power plant efficiency.

The remainder of the research focuses on the integration of these options and associated impacts, taking into consideration the influence of fuel quality, load flexibility and ambient conditions.

Figure 9: Calculated influence of raw flue gas temperature on WFGD water consumption for Medupi Power Station.

Wat

er R

equi

rem

ents

(m3 /h

r) 1000

800

600

400

200

0

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4. Conclusions A numerical model was developed and used for CFD simulation using the commercially available software

STAR-CCM+ to study the effect of hydro/aerodynamics, and other design parameters on the performance of a WFGD absorber. A sufficiently large number of droplets (158000) were tracked to understand the complex multiphase flow inside the absorber. The detailed modeling enabled accurate estimations of the gas and droplet behaviour with particular reference to the velocity fields of the droplets and gas and the L/G ratio. It was further found that the flue gas entrance velocity to the absorber has a great influence on the velocity profile inside the absorber. Flue gas entrance velocities ranging from 10 m/s to 20 m/s were simulated and with an entrance velocity of less than 15.5 m/s almost no carryover of droplets were observed. Therefore, it is recommended that proper duct work is essential for feeding the flue gases to the absorber to reduce the flue gas entrance velocity. The developed numerical model was also shown to be capable of accounting for complex droplet phenomena such as droplet distortion, droplet breakup, drag force acting on the droplets, droplet size distribution and droplet-wall interactions.

Different droplet sizes were introduced using the Rosin Rammler droplet size distribution model and it was observed that smaller droplets were concentrated in the upper part of the absorber as compared to the lower part and this information may also be helpful in the design of the mist eliminator It is observed that droplet distortion and breakup positively affect the discrete (dispersed) phase and its uniform distribution in the absorber as smaller droplets are formed due to distortion and breakup, while it has no noticeable effect on the continuous phase. By inclusion of evaporation it was found that the flue gas is relatively rapidly cooled to a temperature of 50 °C and that a relative humidity of 1.0 was reached and these conditions prevailed throughout the absorber column. It is also noted from the results that condensation did not have any significant effect on either continuous or discrete phase because conditions for condensation hardly occur inside the WFGD’s absorber. It can therefore be deduced from the simulation results that a large number of parameters play an important role in the optimization of WFGD absorber and CFD is proving to be very helpful in this respect.

Parallel to the CFD modelling, mass balance principles were used to identify the major water consumers for the envisaged Medupi WFGD plant. Evaporative water loss was confirmed to be the most significant factor, which corresponds with the CFD results that showed that a relative humidity of 1.0 is rapidly achieved in the absorber. Consequently, it was shown that in principle significant water savings can be realised by implementing raw flue gas cooling and three different options were identified thus far which is the subject of on-going study.

Figure 10: Simplified schematic of Medupi Power Station indicating options for heat recovery to reduce WFGD raw flue gas temperature.

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6. Acknowledgement The authors thank Eskom for supporting this research within the Eskom Power Plant Engineering Institute

(EPPEI) framework, and providing relevant plant data. We are also thankful to ALSTOM and Steinmüller Engineering GmbH for their advice and technical assistance.

7. References [1] National Gazette: National Environmental Management, Air Quality Act [Act 39/2004], 2010, Notice 248.

Pretoria: South African Government.

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[3] Y. Zhong, X. Gao, W. Huo, Z.Y. Luo, M.J. Ni, & K.F. Cen, A model for performance optimization of wet flue gas desulfurization systems of power plants. Fuel Process. Technol., 89 (2008) 1025-1032.

[4] C. Chen, F. Li, & H.Y. Qi, Modeling of the flue gas desulfurization in a CFB riser using the Eulerian approach with heterogeneous drag coefficient. Chem. Eng. Sci., 69 (2012) 659-668.

[5] T. Neveux, & Y. le Moullec, Wet industrial flue gas desulfurization unit: model development and validation on industrial data, Ind. & Eng. Chem. Res., 50 (2011) 7579-7592.

[6] A.M. Carpenter, Low water FGD technologies, IEA Clean Coal Centre, CCC/210 (2012) ISBN: 978-92-9029-530-3.

[7] A. Arif, The simulation of an industrial wet flue gas desulfurization absorber, P.hD. Thesis, North-West University, Potchefstroom, South Africa. Currently in preparation, expected date of completion: Sep 2016.

[8] L. Marocco, Modeling of the fluid dynamics and SO2 absorption in a gas-liquid reactor, Chem. Eng. J., 162, (2010) 217 – 226.

[9] Electric Power Research Institute, FGD Chemistry and Analytical Methods Handbook, Vol. 1, (2007), California: EPRI.

[10] X. Haiping, D. Lin, H. Gaoyan, N. Xiang, Influence of gas-gas heater on wet flue gas desulfurization, Adv. Mater. Res., 986-987, (2014) 92 – 69.

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