Sulfur Dioxide Removal Using Hydrogen Peroxide in Sodium- and

Article Cite This: Energy Fuels XXXX, XXX, XXX-XXX

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Sulfur Dioxide Removal Using Hydrogen Peroxide in Sodium- and Calcium-Based Absorbers Iman Bahrabadi-Jovein, Sadegh Seddighi,* and Javad Bashtani Department of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran ABSTRACT: This study presents the first results on comparing Na- and Ca-based absorbers for separating SO2 from flue gases in two-phase flows where the gas is modeled as a continuous phase and the absorber droplets are solved as a discrete phase. The reactions are solved as volumetric reactions where reactions are assumed to be in instantaneous equilibrium with finite rate. The results show that the Na-based absorbers have considerably higher SO2 removal efficiency compared to Ca-based absorbers. NaHCO3 is found to have the highest separation efficiency of around 96% while the lowest removal efficiency is for naturally abundant CaO. The physics behind the domination of Na-based over Ca-based absorbers is described in this work which shows that the lower activation energy and better dispersion of Na-based absorbers compared to Ca-based absorbers leads to high SO2 removal in Na-based absorbers. It is also found that the utilization of H2O2 increases the efficiency of SO2 removal for NaHCO3 to 99.8%, for CaO to 88%, and for CaCO3 to 98% due to the enhanced solubility in the presence of H2O2. In-furnace sulfur capture is mainly performed via fluidized bed combustion which enables the capture of sulfur emission in a furnace using lime stone.6,11,12 The utilization of FGD processes started in 1920 in the United Kingdom and has been in operation since then. Other European countries and Japan started using FGD processes in 1960. The United States Clean Air Act in 1970 boosted the FGD technology and lead to advanced FGD units for power plants.23 FGD from power plants has been widely studied in the literature as well. Scrubbers are well-known units in which an absorber is injected via a pneumatic method to a stream of flue gases. A new approach in small or medium industrial scales is using semidry methods in which the compressed air injects the absorber in a powdered format to the furnace, economizer, or downstream of the flue gases.24 Semidry desulfurization methods refer to scrubbers with dry waste products in contrast to wet scrubbers which have a slurry waste product. It should be noted that the functionality of the wet and semidry scrubbers has considerable similarity in which both may use similar wet absorbers. However, what makes the difference between wet and semidry scrubbers is the share of water in the absorber leading to the fact that the semidry scrubber waste products contain no water.25 Ma et al.26 experimentally investigated the effects of temperature and absorber concentration on the desulfurization extent in a semidry FGD process. They showed that the desulfurization increases with the decrease in absorber particles sizes and increase in the ratio of absorber to flue gas flow rate. The ratio of absorber to gas is known as the liquid to gas ratio (L/G). This ratio gives the available absorber for reaction with SO2. Gao et al.27 used CFD simulations to study the effect of absorber pH and gas sulfur concentration on desulfurization. Karatepe et al.28 studied the usage of different absorbers on the desulfurization process and categorized the absorbers based on economic

1. INTRODUCTION Power generation as a key element in our civilization is largely dependent on biofuel and fossil fuel power plants. In general the combustion of various types of fuels in furnaces for producing power and steam is associated with various emissions. However, there are tremendous industrial and academic efforts in preventing and removing the emissions from power generation.1−12 Hazardous sulfur emissions can be in the form of aerosols and particulate materials,5 sulfur dioxide (SO2), and sulfur trioxide (SO3) and are mainly produced due to sulfur-containing fuels. Sulfur contaminants are associated with serious damage to human health and the environment. Examples of diseases caused by sulfur emission are nausea, dyspnea, skin irritation, and cancer.13 Acid rain is another consequence of sulfur emission which has a dramatically negative impact on soil, aquatic plants, metals and buildings (due to corrosion), forest trees, and animal physiology.14 In addition to the lethal health and environmental consequences of sulfur emissions, SO2 and SO3 have a corrosive nature and sulfur particulate increases the erosive nature of flows which leads to damage and shorter lifetimes for pipelines.15−17 Around 70% of the global SO2 emission is produced by combustion of fuels.18 During the combustion of fuels, the fuel sulfur mainly produces SO2 according to the reaction below: S(fuel) + O2 → SO2

(1)

Understanding the dangers of sulfur emission lead to drastic measures for reducing sulfur emission. For example, the United States reduced its sulfur emission from 9 megatons in 1990 to 5 megatons in 2015 owing to power plant fuel change from coal to other fuels and various flue gas cleaning methods.19 There are various methods for avoiding or removing sulfur emissions such as utilization of low sulfur fuels, fuel desulfurization, in-furnace sulfur capture, and flue gas desulfurization (FGD).20 Fuel desulfurization as a preventive method of sulfur emission is based on producing low-sulfur fuels or cleaning the fuel from sulfurs prior to combustion.21 It is known that producing low-sulfur natural gas is easier and more practical than low-sulfur oil and heavy oil.22 © XXXX American Chemical Society

Received: September 12, 2017 Revised: October 31, 2017

A

DOI: 10.1021/acs.energyfuels.7b02722 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels aspects. They presented the effects of fuel type, unit thermal power, absorber availability, and price on flue gas desulfurization. Brown et al.29 used experimental and modeling tools to study the wet FGD. They found that the increased number of nozzles increases the rate of desulfurization. Wu et al.30 used the ionic liquid for desulfurization focusing on reactions between ionic liquid with SO2 at low temperatures and pressures achieving up to 90% desulfurization. Zhang and Gui31 studied the separation and reduction of SO2 in a magnetic scrubber and found that the enhanced magnetic field lead to increased particle separation efficiency. Gomez et al.32 used CaCO3 absorber for desulfurization focusing on the scrubber outgoing slurry and its potential applications. The parameters they changed were mass flow rate of recycled absorber, mass flow rate of used absorber, and mass flow rate of products. Olausson et al.33 investigated the effects of particle diameters, sulfur concentration, and fatty acid concentration on desulfurization. They found that the addition of organic acids to the absorber increases the desulfurization efficiency while increased particle size and sulfite concentration decrease the desulfurization efficiency. Zhou et al.34 experimentally studied the effect of H2O2 on the desulfurization process in a semidry FGD. They found that addition of 1%mass to 3%mass of H2O2 to Ca(OH)2 absorber adds 10% to the desulfurization at the same temperature and Ca/S ratio. They also showed that the increased Ca/S ratio leads to the enhanced desulfurization. Sun et al.35 experimentally compared H2O, NaOH, and HA-NA for desulfurization. They found that the H2O-based process has desulfurization efficiency compared to other two alternatives due to the fact that the H2O process is mixing controlled which is governed by molecular diffusion. Also they concluded that, in HA-Na desulfurization, the OH− radical rapidly reacts with the reduced SO2. Thus, the HA-NA has a better desulfurization efficiency compared to H2O and NaOH under similar conditions. They found that, while increased PH improves desulfurization, the addition of O2 up to 10%vol in the incoming flow considerably enhances the desulfurization. NO2 presence has also been shown to increase the emission reduction. Andreasen and Mayer36 used seawater for separation of sulfur from marine motors where they found that the desulfurization efficiency decreases with the decrease in water alkalinity and water salinity. They showed that desulfurization efficiency doubles for zero-salinity water. Zhang et al.37 used Ca(OH)2 for simultaneous separation of SO2 and nitrogen oxide (NO) from flue gases at low temperature. They found that KMnO4 leads to considerable increase in NO separation while it did not have an impact on SO2 separation. They also reported that steam can improve the separation of both NO and SO2. As for temperature, they found that the increase of temperature from 70 to 90 °C increases the SO2 separation from 40% to 80% and NO separation from 30% to 40%. Liu and Shih38 experimentally studied the effects of flue gas composition on desulfurization processes with Ca(OH)2 as the absorber. They found that the presence of CO2 and SO2 in the gaseous phase increases the sulfation of Ca(OH)2 only in the presence of NOx. They also found that the NOx and particularly NO enhances desulfurization much better in the presence of CO2 compared to when O2 is used. Jin et al.39 used chlorine dioxide (ClO2) for separation of SO2 and NO from flue gases. They used an experimental wet scrubber unit at the temperature of 45 °C achieving the maximum separation of 100% for SO2 and 66−72% for NO. They found that the increase in ClO2 from 0.7 to 2 mmol/min increases the NOx

reduction by 20% while it does not affect the SO2 reduction due to its 100% efficiency at even lower ClO2 flow rates. Hutson et al.40 experimentally evaluated the simultaneous removal of SO2, NOx and mercury from flue gases in a wet FGD unit using CaCO3 and other oxidizing agents. They found that among various oxidizing agents, NaClO2 had the highest emission reduction efficiency. For SO2, all sodium chlorite agents can reach 100% reduction. Glomba and Kordylewski41 used ozone gas as an oxidizer and NaOH as an absorber for the reduction of SO2 at the same time with CO, NOx, and Hg from flue gases. They reached full SO2 reduction at liquid to gas ratio of 7.5 dm3/m3 and achieved 95% NOx reduction at a O3/NO molar ratio of 2. Gavaskar and Abbasian42 used copper for desulfurization with Cu concentrations of 11.2% to 26.5% in the absorber. They found the optimum Cu concentration for SO2 removal to be around 14.1%. They also found that the desulfurization efficiency increases with the increase in temperature up to 450 °C. For a temperature higher than 500 °C, desulfurization efficiency decreases considerably which is due to sintering. Sulfur capture using Cu is performed via the following route: SO2 + CuO + 0.5O2 → CuSO4

(2)

43

Li et al. simulated the wet FGD using CaCO3 as the absorber. They studied various types of hollow conical and full nozzles reaching desulfurization efficiency of 90%. They also experimentally, with the help of a laser, investigated the effects of sizes of particles and droplets on desulfurization. They suggested the utilization of 16Cx and 20Cx nozzles for maximum desulfurization. Dahlan et al.44 studied sulfur separation from flue gases using absorbers based on Ca and silica in a dry FGD system. They used silica materials such as coal fly ash, oil palm ash, and rice husk ash (RHA) mixed with a Ca-based absorber using water hydration method reaching the reaction temperature of 100 °C. They found RHA to have the highest and coal fly ash to have the lowest absorption capacity among the three silica material alternatives. Ren et al.45 studied the effects of H2O on sulfur absorption from flue gases using ionic liquids of 1,1,3,3-tetramethylguanidinium lactate at various temperatures. H2O has generally a very strong interaction with ionic liquids, and H2O dissolves rapidly in ionic liquid which leads to a considerable density decrease for ionic liquid and consequently electrostatic interactions between ions. Decreasing the temperature lead to higher absorptivity of either SO2 or H2O in ionic liquid. Thomas et al.46 investigated the synthetic absorption of SO2 and converting it to a dense H2SO4 using H2O2. They studied the absorption rate for various SO2 partial pressures, various H2SO4 concentrations (between 0 and 40%mass), and varying H2O2 concentrations. They found SO2 absorption to enhance with increase in H2O2 concentrations. The SO2 separation capacity and the reaction rate was also found to decrease with the increase in H2SO4 concentration. They also studied SO2 separation using H2O, H2O2, and NaOH absorber. Among their alternatives, they found the strongest SO2 absorber to be NaOH and then H2O2 and H2O, respectively. They found that the increase in H2O2 rate from 0.05 to 0.2 molar leads to 5% increase in SO2 separation. In all, it can be concluded that desulfurization is affected by various parameters such as absorber type, flue gas temperature, absorber liquid to gas ratio, and particle sizes. The effect of particle sizes is already investigated given that larger particles give lower efficiency due to the limited mass transfer rate, as indicated for example in Ma et al.26 The impact of temperature on SO2 B


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Energy & Fuels separation has several aspects: first, since the reactions are exothermic, augmented temperatures lead to reduced SO2 separation. On the other hand, the increase in temperature is associated with enhanced mass transfer rate and diffusivity. Finally, the increased temperature increases the entropy which leads to faster reactions.47 Practically, the scrubber inlet gas temperature lies in the range of 150 to 370 °C depending on the process type and reaction mechanism. It should be noted that in the dry and semidry FGD methods, the optimum temperature is 5−15 °C above the adiabatic saturation temperature.48 In dry and semidry methods, it is critical to take a temperature which ensures the outgoing flow does not contain liquid.49 This study focuses on wet and semidry FGD processes using Ca- and Na-based absorbers. The L/G ratio is assumed to be 15/1 as suggested by Gutiérrez Ortiz et al.50 The absorber is injected using rotary atomizers or two fluid nozzles leading to the formation of finer droplets. While the wet scrubbers give higher desulfurization efficiencies, this work investigates the semidry desulfurization due to its economic merits, easier operation, and reduced maintenance compared to wet methods.51 Figure 1

H 2SO3(aq) → HSO3−(aq) + H+(aq) −

2−

(5)

+

HSO3 (aq) → SO3 (aq) + H (aq)

(6)

1 O2 (aq) → SO4 2 −(aq) 2

(7)

SO32 −(aq) + HSO3−(aq) +

1 O2 (aq) → SO4 2 −(aq) + H+(aq) 2

(8)

where g refers to gas phase and aq refers to liquid phase. The reactions relevant to lime as absorber for FGD are given below where the focus is on the reactions between lime and sulfur oxides in wet FGD.

CaO(s) + H 2O → Ca(OH)2 (aq)

(9)

Ca(OH)2 (aq) → Ca 2 +(aq) + 2OH−(aq)

(10)

OH−(aq) + H+(aq) → H 2O

(11)

SO32 −(aq) + H+(aq) → H 2SO3−(aq)

(12)

Ca 2 +(aq) + SO32 −(aq) +

1 1 H 2O → CaSO3 · H 2O(s) 2 2

(13)

Ca 2 +(aq) + SO4 2 −(aq) + 2H 2O → CaSO4 · 2H 2O(s)

(14)

SOx + CaO + H 2O → CaSO3 + H 2O

(15)

where s refers to solids phase. The reaction in the dry FGD process are as below.53 CaO +

1 O2 + SO2 → CaSO4 2

(16)

The reactions in semidry FGD are given in the following two equations:

CaO(s) + H 2O → Ca(OH)2 (aq)

(17)

Ca(OH)2 + SO2 → CaCO3 + H 2O

(18)

The reactions for lime stone are given as below. Figure 1. Schematic model of a dry scrubber.

shows a schematic of the desulfurization from flue gases. As seen, the flue gases enter the reactor from the upper part of the rector and start reacting with absorbers. After the completion of the reactions, the clean gases leave the scrubber through an outgoing pipe in the side of the scrubber while the wastes leave the reactor from a duct located at the bottom of the scrubber. While it is possible to consider several inlet or outlets in the scrubber, in this work, one gas inlet and one gas outlet are assumed due to the relatively low volumetric flow rate of the flue gases compared to large-scale industrial applications. A major novelty of this work is that this work offers the first modeling work which compares the physics of the Na- and Ca-based absorbers for FGD. Also this work presents the first modeling results on the effects of H2O2 on FGD process. The aims of this work are (1) comparison of the efficiency of FGD processes using various absorbers and (2) understanding the physics behind the FGD processes for various absorbers.

SO2 (aq) + H 2O → H 2SO3(aq)

(4)

CaCO3(aq) → Ca 2 +(aq) + CO32 −(aq)

(20)

CO32 −(aq) + H+(aq) → HCO3−(aq)

(21)

SO32 −(aq) + H+(aq) → HSO3−(aq)

(22)

1 1 H 2O → CaSO3 · H 2O(s) 2 2

(23)

Ca 2 +(aq) + SO2 −4 (aq) + 2H 2O → CaSO4 · 2H 2O(s)

(24)

SO2 + CaCO3 + H 2O → CaSO3 + H 2O + CO2

(25)

Equation 24 shows the general reaction between SO2 and CaCO3 in a wet FGD process. In a dry process, reaction between SO2 and CaCO3 can be shown as below. CaCO3 + SO2 +

1 O2 → CaSO4 + CO2 2

(26)

Na-based absorbers are also used for desulfurization where examples of Na-based absorbers are sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and sodium bicarbonate (NaHCO3) with the main reaction given below: For NaOH:

Exact reactions in the FGD process depend on the type of process (like wet, dry, or semidry), absorber type, and operational conditions. Main reactions for desulfurization from flue gases are given below:52 (3)

(19)

Ca 2 +(aq) + SO32 −(aq) +

2. THEORY AND MODELING

SO2 (g) → SO2 (aq)

CaCO3(s) → CaCO3(aq)

SO2 + 2NaOH → Na 2SO3 + H 2O

(27)

For Na2CO3: Na 2CO3 + SO2 + C

1 O2 → Na 2SO4 + CO2 2

(28)


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Energy & Fuels

where u is the fluid phase velocity (m/s), up is the particle velocity (m/s), μ is the molecular viscosity of the fluid, ρ is the fluid density, ρp is the density of the particle, and dp is the particle diameter. Re is the Reynolds number, which is defined as

For NaHCO3: NaHCO3 + SO2 +

1 O2 → Na 2SO4 + 2CO2 + H 2O 2

(29)

This work focuses on FGD using sodium bicarbonate among Na-based absorbers. H2O2 is known to facilitate desulfurization using Na- and Ca-based absorbers. The nonequilibrium reaction between H2O2 and SO2 can be written as below:54 SO2 + H 2O2 → H+ + HSO−4 ( → 2H+ + SO−4 ) The general reaction also can be written as bellow.

Re ≡

⎛ C − Cout ⎞ ηr = ⎜ in ⎟ × 100 C in ⎠ ⎝

(30)

(31)

(36)

(37)

where ηr is the efficiency, Cin is the SO2 concentration in the inlet, and Cout is the concentrations of SO2 at the outlet. Figure 2 shows the model geometry and a sample of meshing in a cross section. The unit has a gas inlet port at the bottom of

The mass transport is modeled using species equation: ∂ (ρYi ) + ∇·(ρνYi ) = −∇·Ji + R i + Si ∂t

μ

The efficiency of desulfurization is found as below:

46

SO2 + H 2O2 → H 2SO4

ρd p|u p − u|

(32)

where i shows the species, Ri is the source term, Si is dispersion rate of species i, Ri is the net rate of production of species i by chemical reaction, v is kinematic viscosity, Si is the rate of creation by addition from the dispersed phase plus any user-defined sources, and Ji is the flux of species i calculated as below for laminar flow: Ji = − ρDi , m∇Yi

(33)

where Di shows the diffusion flux. For turbulent flow Ji is calculated as below.

⎛ μ ⎞ Ji = − ⎜ρDi , m + t ⎟∇Yi Sct ⎠ ⎝

(34)

where μt is turbulent viscosity and Sct shows the Schmidt number which represents the ratio of momentum diffusivity (viscosity) to mass diffusivity.

3. MODELING The flow within the scrubber is solved in this work assuming a two-phase flow where the gas is modeled as the continuous phase and the absorber droplets are solved as the discrete phase. The reactions are solved as volumetric reactions where reactions are assumed to be in instantaneous equilibrium with finite rate. This work assumed the outgoing flow to be incompressible and particle free since almost all particles are assumed to be collected in the scrubber in a liquid or solid phase. The absorber particles and droplets are assumed to have a similar size. The scrubber walls are assumed to be adiabatic. The turbulent flow is solved using a two-equation realizable k−ϵ model which suits very well for solving turbulent flows from round jets.27 The boundary conditions for gas inlet and outlet are assumed to be velocity inlet and outflow, respectively. The boundary condition for absorber flow is assumed to be velocity inlet while the nozzle is assumed to be as a wall jet. The nozzle outlets are assumed to be as round jets.27 In this work, discrete phase models (DPM) are utilized for modeling the motion of particles using the Lagrangian approach. The gases are modeled as the continuous phase while the particles are modeled as the discrete phase. The simulations consider the effects of turbulence on the dispersion of particles in the flow. The interaction between the discrete and continuous phase is implemented assuming two-way coupling. In two way coupling both continuous and discrete phases affect each other.55 The particle force balance is calculated as below:56 du p dt

= FD(u − u p) +

g (ρp − ρ) ρp

Figure 2. Cross section mesh and the unit geometry used for the modeling in this work.

the unit, and the gas outlet is located at the top of the unit. Two nozzles for injection of absorber are located in the middle of the unit. The unit height is 1 m, the diameter is 0.08 m and the diameter of the gas inlet and outlet is 0.05 m.

4. GRID INDEPENDENCE STUDY Assuming the same geometry and conditions, four grid sizes are evaluated for grid independence. Table 1 shows the number of elements in addition to the corresponding grid sizes. As seen in Figure 3, the variations of the SO2 concentration when varying from 322 306 to 2 561 870 number of cells varies less than 1%. Thus, this work took the numerically optimized cell number of 634 281 corresponding to cell size of 5.3 × 10−4. Table 1 also shows the maximum error associated with the cell numbers. 5. VALIDATION The results in this work are validated with experimental data in Gao et al.27 with geometry and operational conditions including the absorber and inlet gas given in Tables 2 and 3, respectively. As seen in Figures 4 and 5, there is a good agreement between the

+ Fx (35) D


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Energy & Fuels Table 1. Number and Size of Grids no. of cells

size of cell [m]

322 306 634 281 1 349 605 2 561 870

0.0010 0.00053 0.00022 0.00011

Figure 4. Comparison of concentration of SO2 in modeling results in this work with experimental results from Gao et al.27 at a height of 0.225 m using high SO2 concentration at inlet.

Figure 3. Concentration of SO2 at the height of 0.225 m with various numbers of grid cells.

Table 2. Geometry of the Experimental Unit in Gao et al.27 Used for Validation in This Work size [m]

parameter

unit height unit diameter diameter of gas inlet and outlet distance between first nozzle and outlet distance between second nozzle and outlet nozzle diameter

1 0.08 0.05 0.325 0.725 0.003

Figure 5. Comparison of concentration of SO2 in modeling results in this work with experimental results from Gao et al.27 at a height of 0.225 m using low SO2 concentration at inlet.

Table 3. Operational Parameters of the Unit in Gao et al.27 Used for Validation in This Work

Table 4. Properties and Operational Conditions for FGD Using CaO as the Absorber

parameter

value

parameter

value

inlet gas density [kg/m3] CaCO3 absorber density [kg/m3] absorber density in nozzle [m/s] gas density [m/s] SO2 density in inlet [mol/m3] liquid to gas ratio (L/G)

1.34 1200 5.73 3 0.0015 15

density of inlet gas (kg/m3) density of absorber (kg/m3) velocity of absorber in nozzle (m/s) velocity of gas flow (m/s) CSO2in(mol/m3)

1.34 3320 5.73 3 0.0015

nozzle diameter (m) L/G

0.003 15

modeling in this work and the experimental results of Gao et al.27 The maximum error in the Figures 4 and 5 is less than 3%.

Table 5. Physical Properties of the Modeled Species viscosity

6. RESULTS AND DISCUSSION This section presents the results of the FGD from the flue gases using Na- and Ca-based absorbers. The simulated absorbers are CaO and NaHO3. 6.1. Results for CaO Absorber. The properties and operational conditions for CaO absorber are given in Table 4. In addition, the physical properties of the modeled gases contained in the scrubber are shown in Table 5. Also Figures 6 and 7 show the variations of SO2 concentrations at various heights of Z = 0.225, 0.425, 0.625, and 0.825 m. As seen, the efficiency of FGD with this absorber is 75% which is 15% less than the efficiency of CaCO3 that was 90% in ref 27.

species SO2 O2 H2O CO2 H2O2

−1

kg ms

−5

1.2 × 10 1.919 × 10−5 1.34 × 10−5 1.37 × 10−5 9.83 × 10−6

molecular weight −1

kg kmol

64.0648 31.9988 18.01534 44.00995 34.01474

Δh°f (298 K) −1

J mol

−297 269 0 −242 174 −394 088 −136 301

S° (298 K) J mol−1 K−1 248.468 205.310 188.995 213.984 232.965

Also Figures 6 and 7 show the SO2 is reducing with an increase in the height which is due to the presence of two nozzles. Since the nozzles are located at the center of the cross sections, the SO2 concentration is lowest at the center. E


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Figure 6. Concentration contours of sulfur dioxide gas at different heights for CaO.

Figure 7. Concentration curves of sulfur dioxide gas at different heights for CaO.

6.2. NaHCO3 Absorber. Table 6 shows the properties and operational conditions of the FGD using the Na-based absorber of NaHCO3. Figures 8 and 9 show the SO2 concentrations during FGD with NaHCO3 as an absorber at various heights of

Z = 0.225, 0.425, 0.625, and 0.825 m. The SO2 concentration has its maximum close to the inlet and decreases with the increase in height as the flow approaches the outlet. As seen, when flow passes the second nozzle at the height of 0.725 m and becomes F


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reason for the better performance of the Na-based absorbers compared to Ca-based absorbers is not fully explained in the literature.57 Thus, we try in this work to explain the domination of Na-based absorbers over Ca-based absorbers using the physical nature and chemical behavior of absorption as below: 1. One reason for the domination of NaHCO3 over CaO in desulfurization is the lower activation energy for the reactions involving NaHCO3 compared to CaO leading to a higher reaction rate in the former absorber. The activation energy of NaHCO3 with SO2 is 2.8 times less than that of CaO58,59 leading to less energy required for the NaHCO3-based desulfurization compared to that for CaO. The higher sodium-based absorber reactivity compared to that of the calcium-based absorber is also reported in the literature.60−62 2. The more porous texture of sodium compared to that of the calcium based absorber makes sodium absorbers the better adsorbent (due to a better reactant diffusion) rather than calcium based absorbers, particularly in the early stages of the reaction. 3. The produced calcium sulfate causes pore mouth closure and increases the diffusion resistance leading to the reduced efficiency in Ca-based absorbers.63,64 4. The effectiveness and activation of sodium-based cations compared to calcium-based cations leads to a significant increase in the effectiveness of sodium bicarbonate over calcium oxide.65 It should be noted that generally the Ca-based absorbers are cheaper and more abundant materials compared to Na-based absorbers.66

Table 6. Properties and Operational Conditions for FGD Using NaHCO3 as the Absorber parameter

value

density of inlet gas (kg/m3) density of absorber (kg/m3) (NaHCO3) velocity of absorber in nozzle (m/s) velocity of gas flow (m/s) CSO2in (mol/m3)

1.34 1100 5.73 3 0.0015

nozzle diameter (m) L/G

0.003 15

closer the outlet, the SO2 concentration becomes more homogeneous in the cross section and approaches its minimal. The major reason for the inhomogeneity of SO2 concentration over cross sections is related the injection of the absorber at the center of the cross sections. Figures 10 and 11 show the SO2 concentration in the scrubber for sodium bicarbonate and calcium oxide absorbers, respectively. When comparing NaHCO3 to the Ca-based absorbers, NaHCO3 dominates the CaO in desulfurization which can be seen from the SO2 concentration in each cross section and also from the slope of the curves. Figure 10 shows more reduction in SO2 concentrations compared to Figure 11. Thus, it can be inferred that more SO2 is captured using NaHCO3 compared to CaO leading to lower outgoing SO2 in NaHCO3 compared to CaO. Also from Figures 10 and 11, it can be seen that the largest SO2 reductions occur in Z = 0.425 and 0.625m which are due to their proximity to the nozzles. NaHCO3 which has higher reactivity and smaller density compared to CaO becomes axisymmetric in some cross section such as in Z = 0.225 m. So far the

Figure 8. Concentration contours of sulfur dioxide gas at different heights for NaHCO3. G


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Figure 9. Concentration curves of sulfur dioxide gas at different heights for NaHCO3.

the cases where H2O2 is used. As seen in Figure 12, 3%mass of H2O2 leads to a considerable reduction of SO2 which increases the CaO desulfurization efficiency to 98% compared to 75% which was without H2O2. Hydrogen Peroxide enhances the rate of SO2 absorption while at the same time it increases the rate of SO2 dissolving34 leading to higher FGD efficiency when H2O2 is used. Figure 13 shows variations in the concentration of SO2 in the inlet and outlet of the scrubber in the presence of H2O2 for three sorbents of CaCO3, CaO, and NaHCO3. As seen NaHCO3 has the highest SO2 reduction compared to those of CaCO3 and CaO. This is consistent with the experimental results of ref 34 where H2O2 increased the CaO-based desulfurization by 15%. Also Thomas et al.46 evaluated the utilization of H2O and H2O2 in desulfurization which he found the desulfurization efficiency by 15% in H2O2 case. Table 7 also shows the efficiency of each absorber based on eq 37 for two cases with and without H2O2. As seen, the Na-based absorber dominates the Ca-based absorbers. In addition, the table shows the desulfurization efficiency of the NaHCO3 absorber in the presence of H2O2 approaches 100%. It also shows the effectiveness of H2O2 for all SO2 absorbers. In the case of the CaO absorber, the efficiency increases from 74% to 88%, which is a significant increase. The CaCO3 absorber has also reached 98% efficiency in the presence of H2O2, which can be used in various applications to reduce SO2 emissions. The reason behind the enhanced desulfurization in the presence of H2O2 can be described below:

Figure 10. Concentration of sulfur dioxide gas at different heights, for the NaHCO3 absorber.

Figure 11. Concentration of sulfur dioxide gas at different heights, for the CaO absorber.

1. In general, H2O2 increases the rate of the dissolution and absorption of SO2. 2. The reaction of H2O2 with the dissolved SO2 leads to formation of sulfuric acid which rapidly reacts with the absorber.

6.3. Effect of H2O2 on Desulfurization Efficiency. Figure 12 shows the effect of H2O2 on the performance of the CaO as an absorber for FGD process. 3%mass of H2O2 is used in H


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Energy & Fuels

Figure 12. Contour of SO2 concentration for CaO with and without H2O2, (a) at the inlet with H2O2, (b) at the inlet without H2O2, (c) at the outlet with H2O2, and (d) at the outlet without H2O2.

The Ca-based absorbers studied in this work are CaO and CaCO3 and the Na-based absorber used in this work is NaHCO3. The effects of H2O2 on the FGD performance is studied for both Na- and Ca-based absorbers. The key results of this work are summarized below. 1. The desulfurization efficiency is higher in Na-based absorbers compared to Ca-based absorbers. The reason for the domination of NaHCO3 over Ca-based absorbers is the lower activation energy and lower density of NaHCO3 compared to those for Ca-based absorbers leading to a boost in both kinetics and mass transport for Na-based absorbers. Also NaHCO3 is shown to have a higher reactivity with SO2 compared to those for CaCO3 and CaO. 2. Efficiency of desulfurization with CaCO3 is shown to be 96% which is higher than the 75% efficiency of CaO-based desulfurization. 3. We showed that H2O2 increases the efficiency of desulfurization for all CaO, CaCO3, and NaHCO3 absorbers due to a higher SO2 dissolving rate and higher SO2 absorption compared to cases without H2O2. In addition, H2O2 increases the desulfurization performance to 100% which is in part owing to a higher turbulence intensity achieved in the presence of H2O2 compared to the cases without H2O2.

Figure 13. Concentration of sulfur dioxide gas at the inlet and outlet of the scrubber for three absorber in the presence of H2O2.

Table 7. Efficiency of SO2 Removal for Different Absorbers absorbent calcium carbonate (CaCO3) calcium oxide (CaO) bicarbonate sodium (NaHCO3)

efficiency without the presence of H2O2

efficiency with the presence of H2O2

90%

98%

74% 96%

88% 99.8%

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3. H2O2 facilitates the dissolution of the sulfur dioxide in the liquid films around the particle surfaces leading to a faster desulfurization process.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sadegh Seddighi: 0000-0002-1148-1239

7. CONCLUSIONS This work develops and validates a model for the design of the SO2 reduction process using Na- and Ca-based absorbers.

Notes

The authors declare no competing financial interest. I


Article

Energy & Fuels

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(13) Kampa, M.; Castanas, E. Human health effects of air pollution. Environ. Pollut. 2008, 151 (2), 362−367. (14) Singh, A.; Agrawal, M. Acid rain and its ecological consequences. J. Environ. Biol. 2007, 29 (1), 15. (15) Zamani, M.; Seddighi, S.; Nazif, H. R. Erosion of natural gas elbows due to rotating particles in turbulent gas-solid flow. J. Nat. Gas Sci. Eng. 2017, 40, 91−113. (16) Stein-Brzozowska, G.; Norling, R.; Viklund, P.; Maier, J.; Scheffknecht, G. Fireside corrosion during oxyfuel combustion considering various SO 2 contents. Energy Procedia 2014, 51, 135−147. (17) Lu, J.-t.; Li, Y.; Yang, Z.; Huang, J.-y.; Zhu, M.; Gu, Y. Fireside Corrosion Behaviors of Inconel 740H Superalloy in Various SO2 Contents. Energy Materials 2017; Springer: Berlin, 2017; pp 193−201. (18) Smith, S. J.; Pitcher, H.; Wigley, T. M. Global and regional anthropogenic sulfur dioxide emissions. Global and planetary change 2001, 29 (1), 99−119. (19) Hood, M. M.; Groppo, J. G.; Johnston, M. N.; Hower, J. C.; Clack, H. L.; de Medeiros, D. S.; Taffarel, S. R.; Cutruneo, C. M.; Silva, L. F. Influence of Coal-Fired Power Plant Emissions Regulations and Consequent Engineering Controls and Coal-Supply Modifications on Fly Ash Chemistry and Petrology: Examples from Kentucky Power Plants 2016. (20) Nehlsen, J. P. Developing clean fuels: novel techniques for desulfurization; Princeton University: Princeton, NJ, 2006. (21) Javadli, R.; de Klerk, A. Desulfurization of heavy oil. Appl. Petrochem. Res. 2012, 1 (1−4), 3−19. (22) Speight, J. G. The desulfurization of heavy oils and residua; CRC Press: Boca Raton, FL, 1999. (23) Wang, L. K.; Williford, C.; Chen, W.-Y. Desulfurization and emissions control. Advanced Air and Noise Pollution Control; Springer: New York, 2005; pp 35−95. (24) Roy, P.; Sardar, A. SO2 Emission Control and Finding a Way Out to Produce Sulphuric Acid from Industrial SO2 Emission. J. Chem. Eng. Process Technol. 2015, 06, 10.4172/2157-7048.1000230 (25) Jafarinejad, S. Control and treatment of sulfur compounds specially sulfur oxides (SOx) emissions from the petroleum industry: a review. Chem. Int. 2016, 2 (4), 242−253. (26) Ma, X.; Kaneko, T.; Tashimo, T.; Yoshida, T.; Kato, K. Use of limestone for SO 2 removal from flue gas in the semidry FGD process with a powder-particle spouted bed. Chem. Eng. Sci. 2000, 55 (20), 4643−4652. (27) Gao, X.; Huo, W.; Luo, Z.-y.; Cen, K.-f. CFD simulation with enhancement factor of sulfur dioxide absorption in the spray scrubber. J. Zhejiang Univ., Sci., A 2008, 9 (11), 1601−1613. (28) Karatepe, N. A comparison of flue gas desulfurization processes. Energy Sources 2000, 22 (3), 197−206. (29) Brown, K.; Kalata, W.; Schick, R. Optimization of SO 2 Scrubber Using CFD Modeling. Procedia Eng. 2014, 83, 170−180. (30) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Desulfurization of flue gas: SO2 absorption by an ionic liquid. Angew. Chem., Int. Ed. 2004, 43 (18), 2415−2417. (31) Zhang, Q.; Gui, K. Effects of magnetic fields on the improving mass transfer in flue gas desulfurization with a fluidized bed. Heat Mass Transfer 2011, 52, 331. (32) Gómez, A.; Fueyo, N.; Tomás, A. Detailed modelling of a flue-gas desulfurisation plant. Comput. Chem. Eng. 2007, 31 (11), 1419−1431. (33) Olausson, S.; Wallin, M.; Bjerle, I. A model for the absorption of sulphur dioxide into a limestone slurry. Chemical Engineering Journal 1993, 51 (2), 99−108. (34) Zhou, Y.; Zhu, X.; Peng, J.; Liu, Y.; Zhang, D.; Zhang, M. The effect of hydrogen peroxide solution on SO 2 removal in the semidry flue gas desulfurization process. J. Hazard. Mater. 2009, 170 (1), 436−442. (35) Sun, Z.; Zhao, Y.; Gao, H.; Hu, G. Removal of SO2 from flue gas by sodium humate solution. Energy Fuels 2010, 24 (2), 1013−1019. (36) Andreasen, A.; Mayer, S. Use of seawater scrubbing for SO2 removal from marine engine exhaust gas. Energy Fuels 2007, 21 (6), 3274−3279. (37) Zhang, H.; Tong, H.; Wang, S.; Zhuo, Y.; Chen, C.; Xu, X. Simultaneous removal of SO2 and NO from flue gas with calcium-based

NOMENCLATURE A,B = experimental constant C = concentration (mol m−3) Di = diffusion flux (m2 s−1) J = flux of a specie (mol m−2 s−1) Re = Reynolds number Ri = source term Sct = turbulent Schmidt number T = temperature (K) u = velocity (m s−1) ν = kinematic viscosity (m2 s−1) Y = mass fraction

Greek letters

μt turbulent dynamic viscosity (kg m−1 s−1) ρ density (kg m−3) ηr efficiency Subscripts

aq g in out P R S

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aqueous gas inlet outlet product reactant solid

REFERENCES

(1) Qi, Z.; Li, J.; Wu, D.; Xie, W.; Li, X.; Liu, C. Particulate Matter Emission Characteristics and Removal Efficiencies of a Low-Low Temperature Electrostatic Precipitator. Energy Fuels 2017, 31 (2), 1741−1746. (2) Wu, J.; Wang, X.-B.; Zeng, R. J. Reactivity enhancement of iron sulfide nanoparticles stabilized by sodium alginate: Taking Cr (VI) removal as an example. J. Hazard. Mater. 2017, 333, 275−284. (3) Seddighi, S. CO2-based reconfigurable enhanced oil recovery using the oxy-fuel-fired boilers: Thermal and flow properties. J. Cleaner Prod. 2017, 162, 1346−1355. (4) López, M. E.; Rene, E. R.; Boger, Z.; Veiga, M. C.; Kennes, C. Modelling the removal of volatile pollutants under transient conditions in a two-stage bioreactor using artificial neural networks. J. Hazard. Mater. 2017, 324, 100−109. (5) Seddighi, S. Design of large scale oxy-fuel fluidized bed boilers: Constant thermal power and constant furnace size scenarios. Energy 2017, 118, 1286−1294. (6) Seddighi, K. S.; Pallarès, D.; Normann, F.; Johnsson, F. Progress of Combustion in an Oxy-fuel Circulating Fluidized-Bed Furnace: Measurements and Modeling in a 4 MWth Boiler. Energy Fuels 2013, 27 (10), 6222−6230. (7) Zhao, Y.; Hao, R.; Xue, F.; Feng, Y. Simultaneous removal of multipollutants from flue gas by a vaporized composite absorbent. J. Hazard. Mater. 2017, 321, 500−508. (8) Seddighi Khavidak, S.; Pallarès, D.; Normann, F.; Johnsson, F.; YläOutinen, V. Heat transfer in a 4−MWth circulating fluidized bed furnace operated under oxy-fired and air-fired conditions: Modeling and measurements. Int. J. Greenhouse Gas Control 2015, 37, 264−273. (9) Seddighi, K. S.; Pallarès, D.; Normann, F.; Johnsson, F. Carbon Monoxide Formation during Oxy-fuel-Fired Fluidized-Bed Combustion. Energy Fuels 2013, 27 (4), 2275−2282. (10) Shen, S.; He, J.; Pan, M.; Zhou, Z.; Feng, C.; Liang, G. Effective removal of sulfur from high-sulfur coal prior to use by dry chlorination at low temperature. J. Hazard. Mater. 2012, 217, 116−122. (11) Seddighi, S.; Pallarès, D.; Normann, F.; Johnsson, F. Heat extraction from a utility-scale oxy-fuel-fired CFB boiler. Chem. Eng. Sci. 2015, 130, 144−150. (12) Seddighi, S.; Pallarès, D.; Normann, F.; Johnsson, F. Carbon Monoxide Formation during Oxy-fuel-Fired Fluidized-Bed Combustion. Energy Fuels 2013, 27 (4), 2275−2282. J


Article

Energy & Fuels sorbent at low temperature. Ind. Eng. Chem. Res. 2006, 45 (18), 6099− 6103. (38) Liu, C.-F.; Shih, S.-M. Effects of flue gas components on the reaction of Ca (OH) 2 with SO2. Ind. Eng. Chem. Res. 2006, 45 (26), 8765−8769. (39) Jin, D.-S.; Deshwal, B.-R.; Park, Y.-S.; Lee, H.-K. Simultaneous removal of SO 2 and NO by wet scrubbing using aqueous chlorine dioxide solution. J. Hazard. Mater. 2006, 135 (1), 412−417. (40) Hutson, N. D.; Krzyzynska, R.; Srivastava, R. K. Simultaneous removal of SO2, NOx, and Hg from coal flue gas using a NaClO2enhanced wet scrubber. Ind. Eng. Chem. Res. 2008, 47 (16), 5825−5831. (41) Głomba, M.; Kordylewski, W., Simultaneous removal of NOx, SO2, CO and Hg from flue gas by ozonation. Pilot plant studies. Environ. Protect. Eng. 2014, 40, (3). (42) Gavaskar, V. S.; Abbasian, J. Dry regenerable metal oxide sorbents for SO2 removal from flue gases. 1. Development and evaluation of copper oxide sorbents. Ind. Eng. Chem. Res. 2006, 45 (17), 5859−5869. (43) Li, F.; Brown, K.; Kalata, W.; Schick, R. Analysis and Optimization of Spray Tower in WFGD 2014. (44) Dahlan, I.; Mohamed, A. R.; Kamaruddin, A. H.; Lee, K. T. Dry SO2 Removal Process Using Calcium/Siliceous-Based Sorbents: Deactivation Kinetics Based on Breakthrough Curves. Chem. Eng. Technol. 2007, 30 (5), 663−666. (45) Ren, S.; Hou, Y.; Wu, W.; Chen, X.; Fan, J.; Zhang, J. Effect of H2O on the desulfurization of simulated flue gas by an ionic liquid. Ind. Eng. Chem. Res. 2009, 48 (10), 4928−4932. (46) Thomas, D.; Colle, S.; Vanderschuren, J. Kinetics of SO 2 absorption into fairly concentrated sulphuric acid solutions containing hydrogen peroxide. Chem. Eng. Process. 2003, 42 (6), 487−494. (47) Koech, L.; Everson, R.; Neomagus, H.; Rutto, H. Dissolution kinetics of sorbents and effect of additives in wet flue gas desulfurization. Rev. Chem. Eng. 2014, 30 (6), 553−565. (48) Mussatti, D. Air pollution control technology fact sheet; EPA-CICA Fact Sheet: Washington, DC, 2003. (49) Yelverton, T. L.; Nash, D. G.; Brown, J. E.; Singer, C. F.; Ryan, J. V.; Kariher, P. H. Dry sorbent injection of trona to control acid gases from a pilot-scale coal-fired combustion facility. AIMS ENVIRONMENTAL SCIENCE 2016, 3 (1), 45−57. (50) Gutiérrez Ortiz, F.; Vidal, F.; Ollero, P.; Salvador, L.; Cortés, V.; Gimenez, A. Pilot-plant technical assessment of wet flue gas desulfurization using limestone. Ind. Eng. Chem. Res. 2006, 45 (4), 1466−1477. (51) Poullikkas, A. Review of Design, Operating, and Financial Considerations in Flue Gas Desulfurization Systems. Energy Technology & Policy 2015, 2 (1), 92−103. (52) Scala, F.; D’Ascenzo, M.; Lancia, A. Modeling flue gas desulfurization by spray-dry absorption. Sep. Purif. Technol. 2004, 34 (1), 143−153. (53) Krawczyk, E.; Zajemska, M.; Wyleciał, T. The chemical mechanism of SOx formation and elimination in coal combustion process. CHEMIK nauka-technika-rynek 2013, 1 (67), 856−862. (54) Colle, S.; Vanderschuren, J.; Thomas, D. Simulation of SO 2 absorption into sulfuric acid solutions containing hydrogen peroxide in the fast and moderately fast kinetic regimes. Chem. Eng. Sci. 2005, 60 (22), 6472−6479. (55) Flows, S. I. A. f. M.; Sommerfeld, M. Best practice guidelines for computational fluid dynamics of dispersed multi-phase flows; European Research Community on Flow, Turbulence and Combustion (ERCOFTAC), 2008. (56) Delnoij, E.; Lammers, F.; Kuipers, J.; van Swaaij, W. P. M. Dynamic simulation of dispersed gas-liquid two-phase flow using a discrete bubble model. Chem. Eng. Sci. 1997, 52 (9), 1429−1458. (57) Yassin, L.; Lettieri, P.; Simons, S. J.; Germanà, A. Study of the process design and flue gas treatment of an industrial-scale energy-fromwaste combustion plant. Ind. Eng. Chem. Res. 2007, 46 (8), 2648−2656. (58) KEENER, S. U.; Khang, S.; Keener, T. A calcination and sulfation reaction model for calcium carbonate with sulfur dioxide. Adv. Environ. Res. 1998, 2 (3), 251−268.

(59) Keener, T. C.; Khang, S.-J. Kinetics of the sodium bicarbonate sulfur dioxide reaction. Chem. Eng. Sci. 1993, 48 (16), 2859−2865. (60) Carpenter, A. M. Low water FGD technologies; IEA Clean Coal Centre: London 2012; CCC/210. (61) Ben-Said, L. Reaction kinetics and mechanisms of low temperature SO2 removal by dry calcium-based sorbents; Ohio University: Athens, OH, 1993. (62) Majeed, J.; Korda, B.; Bekassy-Molnar, E. Comparison of the efficiencies of sulfur dioxide absorption using calcium carbonate slurry and sodium hydroxide solution in an ALT reactor. Gas Sep. Purif. 1995, 9 (2), 111−120. (63) Davini, P.; DeMichele, G.; Ghetti, P. An investigation of the influence of sodium chloride on the desulphurization properties of limestone. Fuel 1992, 71 (7), 831−834. (64) Doğu, G.; Doğu, T. Kinetics of capture of sulfur dioxide and applications to flue gas desulfurization. Chemical reactor technology for environmentally safe reactors and products; Springer: Berlin, 1992; pp 467−498. (65) Verdone, N.; De Filippis, P. Thermodynamic behaviour of sodium and calcium based sorbents in the emission control of waste incinerators. Chemosphere 2004, 54 (7), 975−985. (66) Wang, L. K.; Pereira, N. C.; Hung, Y.-T. Air pollution control engineering; Springer: Berlin, 2004; Vol. 1.

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Sulfur Dioxide Removal Using Hydrogen Peroxide in Sodium- and

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