Removal of SO2 from Flue Gas Using Basic Aluminum Sulfate

Jan 20, 2016 - This paper presents a novel wet flue gas desulfurization (FGD) technology based on a basic aluminum sulfate (BAS) desorption regenerati...
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Removal of SO2 from Flue Gas Using Basic Aluminum Sulfate Solution with the Byproduct Oxidation Inhibition by Ethylene Glycol Min Chen,* Xianhe Deng, and Feiqiang He Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China ABSTRACT: This paper presents a novel wet flue gas desulfurization (FGD) technology based on a basic aluminum sulfate (BAS) desorption regeneration process, in which ethylene glycol (EG) was first employed to inhibit the byproduct oxidation. The operating parameter effect on SO2 absorption efficiency and oxidation efficiency of sulfite was thoroughly examined in a labscale bubbling column. The results indicated that both the amount of aluminum and basicity play important roles on the desulfurization time with above 90% absorption efficiency. The BAS-based desulfurization process was more suitable for a low temperature and low gas flow rate. High inlet SO2 concentrations may contribute to the mass-transfer rate of SO2, and the SO2 absorption efficiency remained above 90% when the pH value was over 3.10. With the addition of 1% (v/v) EG in BAS solution, the oxidation efficiency dropped dramatically from 86 to below 10% (in 120 min). On the basis of the two-film theory, a model of the SO2 absorption process was developed and the mass-transfer characteristics were analyzed. The calculation results indicated that the SO2 absorption process for this system was decided by a combination of both the gas- and liquid-phase diffusion controls.

1. INTRODUCTION As a result of containing large amounts of sulfur dioxide (SO2) and other pollutants, flue gas emissions can cause serious atmospheric pollution and are strongly harmful toward human health, leading to asthma, respiratory diseases, etc.1−3 Therefore, removal of SO2 from flue gas is indispensable before discharge into the atmosphere. Currently, numerous desulfurization technologies have been developed; however, some can achieve end-use applications, but most still stay at the laboratory level.2 Wet flue gas desulfurization (FGD) is one of the most effective desulfurization technologies used to control SO2 emissions to the atmosphere. It can be classified as regenerative and non-regenerative. The majority of wet FGD processes used in industry is non-regenerative, such as the dual-alkali method, amine method, etc. Among them, the limestone−gypsum method is the most widely used for FGD because of its high SO2 removal efficiency, reliability, and low utility consumption.4 However, these methods suffer from some inherent drawbacks, such as a larger water consumption, waste liquid pollution, and secondary pollution of the byproduct.2 To address these limitations, many studies have been carried out for recycling the absorbents and acquiring the valuable side product SO2 after desorption by heating or reducing pressure. These processes have clear advantages of zero waste liquid emission, high value-added byproduct, low operating cost, etc.5,6 In terms of renewability, many successful outcomes to remove SO2 from flue gas have been presented, such as absorption with a buffer solution of sodium citrate and citric acid, chemical absorption using an ethylenediamine−phosphoric acid solution, magnesium-based sorbents, and absorption with sodium phosphate buffer solution.5,7−9 Nevertheless, the category of desulfurization processes faces a common problem that is the sulfite in solution easily oxidized to sulfate in operation, as a result of the loss of absorbents. © 2016 American Chemical Society

As a promising absorbent, basic aluminum sulfate (BAS) has been used in the industry to remove SO2 from flue gas. According to the byproduct, it can be divided into two desulfurization processes: BAS−gypsum method and BAS− desorption regeneration method.10 In 1988, the former was applied in a smelting plant in China because of simple operation, wide range of the sulfur dioxide concentration, difficulty plugging the pipeline and equipment, high desulfurization efficiency, etc.11 Wen et al.12 had studied the desulfurization performance of the BAS absorbent. Their studies revealed that both high basicity and a large amount of aluminum in BAS solution were more favorable for the removal of SO2 from flue gas. It should be noted that the basicity should be less than 40%; otherwise, it would reduce the stability of the BAS solution and lead to a flocculent precipitate. In 1943, Barwasser and Thumn13 developed a process of regeneration of an absorption liquid consisting of BAS for the recovery of sulfur dioxide. They demonstrated that the BAS−desorption regeneration process could achieve the regeneration of absorbents and recovery of SO2 successfully. However, a key issue is that the effective absorbents gradually decrease because of the byproduct oxidation during the desulfurization process. Thus, it is crucial to inhibit the oxidation of sulfite, to reduce the loss of absorbents based on the BAS−desorption regeneration process. Numerous works have studied the mechanism and effect of different inhibitors on the oxidation of sulfite, such as phenols, sodium thiosulfate, cellulose, glycerol, ethanol, butyl alcohols, and ascorbic acid.14−18 Despite these inhibitors suppressing the oxidation process of sulfite, an unsuitable desorption process could happen as a result of heating or reducing pressure, Received: October 14, 2015 Revised: January 14, 2016 Published: January 20, 2016 1183

DOI: 10.1021/acs.energyfuels.5b02411 Energy Fuels 2016, 30, 1183−1191

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Figure 1. Schematic diagram of the experimental apparatus: (1) air compressor, (2, 8, 10, and 16) needle valves, (3, 7, and 11) rotameter, (5) twostage stainless-steel regulators, (6 and 15) glass tee, (9) gas tank, (12) bubble column, (13) thermostatic bath, (14) sampling, (17) analysis bubbler, (18 and 19) glass stopcock, (20) water displacement column, (21) wet gas flow meter, (22) flat bottom, and (23) exhaust to the atmosphere.

resulting from volatility, flammability, instability, and toxicity. Ethylene glycol (EG) has favorable properties of low vapor pressure, nontoxicity, high chemical stability, and low melting point, which may remedy the above defects. In addition, no literature was found reporting on the oxidation inhibition of the byproduct in BAS solution by EG. In view of these considerations, we propose EG as the oxidation inhibitor and make a systematic study relevant to the BAS−desorption FGD process with the oxidation inhibition of the byproduct by EG.

HSO3−(aq) ↔ H+(aq) + SO32 −(aq)

In addition, a part of sulfite in the rich solution may be oxidized by O2 from flue gas, and the reaction formula is shown by eq 7: Al 2(SO4 )3 ·Al 2(SO3)3 (aq) + 1/2O2 (g) → 2Al 2(SO4 )3 (aq) (7)

In brief, SO2 from flue gas is absorbed into the BAS solution, after which the absorbent may be regenerated with the recovery of SO2 desorbed by heating or reducing pressure. Considering the oxidation of a part of sulfite in the absorption solution, it is necessary for high desulfurization performance to keep the basicity of absorption solution by the regeneration of BAS. 2.2. Mechanism of the Oxidation Inhibition of Sulfite by EG. The aerobic oxidation of sulfite in solution has long been considered as a radical chain reaction. With respect to the oxidation inhibition of sulfite, various alcohols (methanol, ethanol, and butyl alcohols) as a category of efficient inhibitors published by the literature17,20,21 were proven that they have different inhibitory actions and a similar inhibitory mechanism. Moreover, Wang et al.15 researched the inhibitory action of ethanol used as the oxidation inhibitor of sulfite and proposed a mechanism for oxidation inhibition of sulfite by ethanol. For reference, the oxidation process of sulfite in the presence of EG may be speculated as follows: First, the chain starts with the formation of the •SO3− radical, which usually occurs on the conditions of light or transition metal ion presented by eqs 8 and 9.14,22 Accordingly, the •SO3− radical can triggering a series of chain reactions.

2. THEORETICAL SECTION 2.1. Desulfurization Mechanism. BAS solution was prepared from aluminum sulfate solution by the neutralization reaction. Calcium carbonate was used as the neutralizing reagent, and the neutralization reaction (reaction 1) is as follows:10 Al 2(SO4 )3 (aq) + 3xCaCO3(s) + 6x H 2O → (1 − x)Al 2(SO4 )3 ·x Al 2O3(aq) + 3xCaSO4 ·2H 2O(s) + 3xCO2 (g)

(1)

The total amount of aluminum in a solution of BAS is represented by Al2O3 (g/L). Then, the amount of aluminum expressed as Al2O3 in formula (1 − x)Al2(SO2)3·xAl2O3 is termed as the “basic amount” (g/L). In reaction 1, x represents “basicity”, which can be calculated by the following equation (eq 2):19 basicity =

basic amount × 100% total amout of aluminum in solution

chain initiation

(2)

hν

The desulfurization mechanism in the BAS-based wet FGD process is according to the acid−base reaction. BAS can react with SO2 by the following neutralization reaction: Al 2(SO4 )3 ·Al 2O3(aq) + 3SO2 (g) ↔ Al 2(SO4 )3 ·Al 2(SO3)3 (aq)

(6)

SO32 − → •SO3− + e−

(8)

Me3 + + SO32 − → Me 2 + + •SO3−

(9)

chain propagation (3)

•

SO3− + O2 → •SO5−

Meanwhile, SO2 is initially dissolved into solution, and the dissociation reactions will accompany the absorption process by eqs 4−6: SO2 (g) + H 2O ↔ H 2SO3(aq)

(4)

H 2SO3(aq) ↔ H+(aq) + HSO3−(aq)

(5)

(10)

•

SO5− + HSO3− → HSO5− + •SO3−

(11)

product formation HSO5− + SO32 − → HSO4 − + SO4 2 − 1184

(12)

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±0.01, while the total sulfur(IV) concentration in the absorption solution was determined by a standard iodometric titration method within a small variation of ±1%. The absorption temperature was controlled with a constant temperature water bath within a small variation of ±0.1 K. The SO2 removal efficiency can be written as y − yi ηAE = 0 × 100% y0 (16)

chain termination •

SO5−

+

•

SO5−

→ inner product •

(13)

SO3−

Then, EG will act as the radical-trapping material and break the chain by reacting with the •SO3− radical (eq 14).14,15,20 Finally, EG may generate aldehyde or carboxylate as a result of an induced oxidation of EG. HOCH 2CH 2OH + •SO3− → HOCH 2CH 2OSO2− + •OH

where ηAE represents the SO2 absorption efficiency (%) and y0 and yi are the inlet and outlet concentrations of SO2 in the gas phase (ppm), respectively. The oxidation efficiency of total SO2 absorbed into BAS solution can be estimated by the following formulas:

(14) •

SO3− + •OH → SO4 2 − + H+

(15)

3. EXPERIMENTAL SECTION

SO2 absorbed for any time interval =

3.1. Materials. All chemicals and reagents used in the present investigation were of analytical reagent grade. Both aluminum sulfate [Al2(SO4)3·18H2O] and calcium carbonate (CaCO3) with more than 99% purity were purchased from Tianjin Kermel Chemicals Co., Ltd., China. EG (analytical reagent of >99%) was purchased from Shanghai Sinopharm Chemicals Co., Ltd., China. The simulated flue gas, containing 21% (v/v) O2, was prepared by mixing compressed air and pure sulfur dioxide (SO2) reagent. The SO2 cylinder (purity of ≥99%) was procured from Guangzhou Puyuan Gas Co., Ltd., China. 3.2. Preparation of BAS Solution. BAS solution was prepared as follows:10 Calcium carbonate powder was slowly added to aluminum sulfate solution and stirred for 24 h when pH was constant. After that, the slurry was filtered under reduced pressure, and the clear BAS solution was obtained. Water used in the experiments was deionized water. 3.3. Experimental Setup and Methods. All of the SO2 removal experiments were carried out in a laboratory-scale bubbling column. The schematic diagram of the experimental setup used was shown in Figure 1, which was similar to the system reported by Mondal.23 The system mainly includes three parts: a gas simulation system, a bubbling−absorption system, and a gas sampling−analysis system. The parameters of various main units were given in Table 1.

bubble column bubbling tube buffer tank bubbler for SO2 analysis bubbling tube water displacement column

height (mm)

internal diameter (mm)

material of construction

450 2200 300

32 2 300 23

borosilicate glass glass carbon steel borosilicate glass

600

1 45

22.4

(17)

Hence, total SO2 absorbed, N

N=

∫0

Q

ηAEy0 M 22.4

dq =

y0 M

∫ 22.4 0

Q

ηAE dq

(18)

Total sulfite in absorption solution can be obtained by

n = C TSVL

(19)

Finally

ηOE =

N−n × 100% N

(20)

where q stands for the volume of flue gas for a period of time (L), M is the molar mass of SO2 (kg/kmol), N stands for the amount of SO2 absorption (mol), n stands for the amount of sulfite in absorption solution (mol), VL stands for the volume of the absorption solution (L), CTS stands for the molar concentration of sulfite (mol/L), and ηOE stands for the oxidation efficiency of total SO2 absorbed (%). In addition, the loss of the absorption solution used in measuring the concentration of sulfite was taken into account for ηOE calculation.

4. RESULTS AND DISCUSSION 4.1. Effect of the Component of BAS Solution on the SO2 Absorption Efficiency. The component of the BAS solution is an important factor on desulfurization. Therefore, the effect of both the amount of aluminum and basicity in BAS solution on the SO2 absorption efficiency was investigated, respectively. It can be seen in Figure 2 that the amount of aluminum has little effect on the SO2 absorption efficiency but

Table 1. Parameters of Various Main Units unit

ηAEqy0 M

glass glass

The simulated flue gas with a fixed concentration SO2, which was prepared by mixing air from an air compressor and SO2 from a cylinder in a buffer tank, flowed through a bubbling column containing 200 mL of BAS solution. The flow rate of inlet gas stream was regulated and measured with a calibrated rotameter. Then, the cumulative volume of the outlet gas stream was recorded by the wet gas flow meter for each 10 min interval. Finally, surplus SO2 in outlet gas was removed through a flat-bottomed flask filled in tap water, and after that, the gas was emitted to the atmosphere. The SO2 concentration in both inlet and outlet gases was measured for every 10 min by the following method:7,23,24 A fixed volume of gas was fed into the analysis bubbler containing a fixed volume (100 mL) of mixed solution (0.096 M ammonium sulfamate and 0.053 M ammonium sulfate at pH 5.2). Meanwhile, the volume of gas was measured by the water displacement column and also summed up with the wet gas flow meter reading to obtain the total volume of gas passed through the bubbling column. For each 20 min interval, the pH of liquid was measured by the pH meter within a small variation of

Figure 2. Effect of the amount of aluminum on the SO2 absorption efficiency. Gas flow, 0.8 L/min; SO2, 2600 ppm; basicity, 25%; EG, 1% (v/v); and T, 303 K. 1185

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4.2. Effect of the Inlet SO2 Concentration. Figure 4a shows that the inlet SO2 concentration has no significant influence on the SO2 absorption efficiency, but the desulfurization time for absorption efficiency above 90% gradually declines with the increase of the inlet SO2 concentration. This can be well-interpreted as follows: according to the two-film theory of mass transfer, the SO2 partial pressure in flue gas increases when the inlet SO2 concentration increases; meanwhile, the driving force of SO2 mass transfer from the gas phase to the liquid phase has been enhanced, causing faster removal of SO2 from flue gas. Total SO2 absorbed increases at the same time with the increase of the inlet SO2 concentration; therefore, the consumption of Al2O3 in BAS solution will be larger. The results indicate that the BAS-based wet FGD process is more suitable for the removal of high SO2 concentration flue gas. Figure 4b shows that the oxidation efficiency of sulfite in absorption solution slightly increases as the inlet SO 2 concentration increases, and the curves have a similar tendency that the oxidation efficiency tardily increases with time. The reason can be illustrated as follows: with the increase of the inlet SO2 concentration, the sulfite concentration in solution increases faster. However, a higher sulfite concentration may increase the oxidation rate of sulfite in solution.25 4.3. Effect of the Temperature. Figure 5a shows that the absorption temperature has a significant influence on the SO2 absorption efficiency. The SO2 absorption efficiency dramatically decreases from 99 to 94% at the initial absorption time (20 min) as the absorption temperature ascends from 303 to 313 K. In addition, the desulfurization time for absorption efficiency above 90% markedly reduces from 150 to 80 min with the rise of the absorption temperature from 303 to 313 K. The reasons can be given as follows: the SO2 solubility in absorption solution drops with increasing the absorption temperature, which results in the escape of more SO2 from absorption solution. In accordance with reaction 3, it is an exothermic reaction, so that increasing the temperature promotes it left to react, leading to less SO2 absorbed into BAS solution. Hence, a low temperature is a benefit of the desulfurization performance of BAS. Then, considering the desulfurization cost, room temperature should be suitable to remove SO2 from flue gas. Figure 5b shows that the absorption temperature has a considerable effect on the oxidation efficiency of sulfite in absorption solution. It is obvious that the oxidation efficiency

influences the desulfurization time with above 90% absorption efficiency. The desulfurization time with above 90% absorption efficiency significantly increases with an increase in the amount of basicity. It can be explained that the basic amount in BAS solution increased with the rise of the amount of aluminum according to eq 1. Hence, it can extend the efficient desulfurization time by appropriately increasing the amount of aluminum in industrial application, which should be 15−20 g/L because the excessive amount of aluminum caused the loss of aluminum precipitated in gypsum.12 On the other hand, the effect of basicity in BAS solution on the SO2 absorption efficiency is shown in Figure 3. It

Figure 3. Effect of the basicity on the SO2 absorption efficiency. Gas flow, 0.8 L/min; SO2, 2600 ppm; amount of aluminum, 15 g/L; EG, 1% (v/v); and T, 303 K.

demonstrates that the desulfurization time with above 90% absorption efficiency increases as the basicity increases from 20 to 30% and the SO2 absorption efficiency has a little rise. The reasons can be given as follows: first, the basic amount rises with the rise of the basicity, causing better absorption performance. In addition, the initial pH value slightly increases from 3.38 to 3.45 with increasing the basicity from 20 to 30%, which may make better SO2 absorption efficiency. Thus, a relatively high basicity is more favorable to remove SO2 from flue gas, while it should be below 40%, over which may result in aluminum hydroxide precipitate.10

Figure 4. (a) Effect of the inlet SO2 concentration on the SO2 absorption efficiency and (b) oxidation efficiency of sulfite in absorption solution. Gas flow, 0.8 L/min; basicity, 25%; amount of aluminum, 15 g/L; EG, 1% (v/v); and T, 303 K. 1186

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Figure 5. (a) Effect of the temperature on the SO2 absorption efficiency and (b) oxidation efficiency of sulfite in absorption solution. Gas flow, 0.8 L/min; SO2, 2600 ppm; basicity, 25%; amount of aluminum, 15 g/L; and EG, 1% (v/v).

Figure 6. (a) Effect of the gas flow rate on the SO2 absorption efficiency and (b) oxidation efficiency of sulfite in absorption solution. SO2, 2600 ppm; basicity, 25%; amount of aluminum, 15 g/L; EG, 1% (v/v); and T, 303 K.

Figure 7. (a) Effect of EG on the SO2 absorption efficiency and (b) oxidation efficiency of sulfite in absorption solution. Gas flow rate, 0.8 L/min; SO2, 2600 ppm; basicity, 25%; amount of aluminum, 15 g/L; and T, 303 K.

reaction, increases as the temperature increases, causing the rise of the oxidation rate.26 In addition, a high absorption temperature may undermine the inhibitory action of EG in BAS solution. 4.4. Effect of the Gas Flow Rate. The effect of the gas flow rate on the SO2 absorption efficiency is shown in Figure 6a. Obviously, it indicates that the SO2 absorption efficiency

significantly increases from 9.6 to 14.8% when the temperature increases from 303 to 313 K at 120 min. This can be explained as follows: A high temperature can speed the ionic interaction in aqueous solution, which accelerates the oxidation reaction (reaction 7). According to the Arrhenius formula, ln k = ln k0 − (Ea/R)(1/T), the reaction rate constant k between SO32− and O2 in solution, which quantifies the speed of a chemical 1187

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Energy & Fuels reduces slightly as the gas flow rate increases and the desulfurization time for absorption efficiency above 90% declines dramatically as the gas flow rate increases. The reasons are as follows: The gas−liquid contact time of SO2 with BAS solution may reduce with the increasing of the gas flow rate, causing more SO2 to be unreacted passing through the bubble column. Furthermore, more absorbent will be consumed during the same reaction time. Hence, the gas flow rate should be below 1.2 L/min to keep a high SO2 absorption efficiency for a longer time. The effect of the gas flow rate on the oxidation efficiency of sulfite in absorption solution is shown in Figure 6b. The results show that the oxidation efficiency of sulfite increases quickly as the flow rate increases from 0.4 to 0.8 L/min, above which it increases slowly. The reasons can be explained as follows: The gas−liquid contact area increases with the rise of the gas flow rate, which not only speeds the reaction rate between SO32− and O2 but also reduces the influence of EG on the oxidation inhibition of sulfite. When the gas flow rate is below 0.8 L/min, the bubbled flue gas disperses uniformly, resulting in a quiet bubbling area and a little stirring effect on the absorption solution. However, when the flow rate increases to 1.2 L/min, some air collide and agglomerate, leading to a turbulent bubbling area and little change of the gas−liquid contact area.27 4.5. Effect of the EG Volume Concentration. The effect of the EG volume concentration on the SO2 absorption efficiency has also been conducted. Figure 7a shows that the oxidation inhibitor EG has little effect on the SO2 absorption efficiency and slightly influences the desulfurization time for absorption efficiency above 90%. Without the addition of EG in absorption solution, the SO2 absorption efficiency can reach above 96% in a nearly level section (160 min). The absorption curve with an EG volume concentration of 1% in absorption solution is basically consistent with that of 1.5%, while the absorption efficiency has a little reduction as the EG concentration ascends from 0 to 1% (v/v). This can be explained as follows: In the presence of EG, which can inhibit the oxidation of sulfite in BAS solution, sulfite in solution is rarely oxidized. Meanwhile, less sulfite is oxidized as the EG concentration increased. On the basis of eq 3, a higher concentration of sulfite inhibits the reaction right to react and weakens the SO2 absorption performance. Thus, the experimental results indicate that the effect of the sulfite concentration in BAS solution should be fully considered for the desulfurization performance based on the BAS−desorption regeneration process in industrial running. Figure 7b shows that the EG concentration has a critical effect on the oxidation efficiency of sulfite in BAS solution. It is clear to see that the oxidation efficiency can maintain extremely high values in the range of 72.4−86.8% (120 min) in the absence of EG. However, the oxidation efficiency of sulfite reduces obviously as the EG concentration increases from 0 to 1% (v/v). When the EG concentration is above 1% (v/v), the oxidation efficiency of sulfite has little change and remains below 10%. In view of the considerations, EG has an excellent antioxidation performance and its volume concentration in BAS solution should be above 1% (v/v), which is conducive to strengthen the recycling of absorbents and recover more sulfur dioxide for the BAS−desorption regeneration process. 4.6. Effect of the pH. Figure 8 shows that the SO2 absorption efficiency is closely related to pH of the absorption solution. The change of the pH value could be divided into two stages. In the first stage (80 min), the pH value remains at 3.40

Figure 8. Relationship between the pH value and SO2 absorption efficiency. Gas flow rate, 0.8 L/min; SO2, 2600 ppm; basicity, 25%; amount of aluminum, 15 g/L; EG, 1% (v/v); and T, 303 K.

± 0.02 and the absorption efficiency can be maintained above 96% with little reduction. The reason may be that BAS has a certain pH buffer effect. In the second stage, pH decreases slowly from 3.39 to 2.82. When the pH value is higher than 3.10, the SO2 absorption efficiency tardily reduces from 99 to 90% during 150 min. After that, the SO2 absorption efficiency decreases rapidly with the reduction of the pH value. Hence, the pH value in BAS solution should remain above 3.10 for high SO2 absorption efficiency. 4.7. Mass-Transfer Analysis. 4.7.1. Process Modeling for SO2 Absorption. In the present study, a bubble column was employed for SO2 absorption. The SO2 absorption process in BAS solution can be divided into two steps: SO2 transfer from the gas phase to the liquid phase and reaction with BAS. On the basis of the two-film theory of mass transfer, all of the mass transfer resistances are only contained in both gas film and liquid film and the gas−liquid interface is always in equilibrium with the following relationship:4,28

cA* = HPA*

(21)

where cA* is the interface concentration of SO2 (kmol/m3), PA* is the partial pressure of SO2 in the interface (atm), and H is the thermodynamic equilibrium constant, which has a value of 1.04 kmol m−2 atm−1 at 303 K. The absorption rate of SO2 in the bubbling reactor was calculated by the following various formulas: gas-phase mass transfer

NA = k Ga(PA − PA*)

(22) −3

−1

where NA is the absorption rate of SO2 (kmol m s ), kG is the gas-phase mass-transfer coefficient (kmol m−2 s−1 atm−1), PA is the partial pressure of SO2 in the gas phase (atm), and a is the gas−liquid interfacial area per unit volume of liquid (m2/ m3). The specific gas−liquid interfacial area (a) can be expressed by23,29 a=

6εG dVS

(23)

with 1188

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⎛ gD 2ρ ⎞−0.5⎛ gD 3ρ 2 ⎞−0.12 ⎛ u R L ⎟⎟ ⎜⎜ R 2 L ⎟⎟ ⎜⎜ G = 26DR ⎜⎜ σ ⎝ gDR ⎝ ⎠ ⎝ μL ⎠ L

⎞−0.12 ⎟⎟ ⎠

where VL and VG are the volume of the liquid and the gas bubbling in liquid (m3), respectively, and R is the universal gas constant, which has a value of 8.314 atm m3 kmol−1 K−1. The residence time of the gas t0 in the bubbling reactor is given by23

(24)

where εG is the gas holdup, dVS is the volume−surface mean bubble diameter (m), DR is the diameter of the bubbling column (m), ρL is the density of solution (kg/m3), σL is the surface tension of solution (N/m), μL is the viscosity of solution (Pa s), μG is the superficial velocity (m/s), and g is gravitational constant (m/s2). However, the gas holdup εG in the bubble column reactor was written as29,30 7/24 ⎛ uGμL ⎞⎛ ρL σL 3 ⎞ ⎟⎟ = 0.27⎜ ⎟⎜⎜ (1 − εG)4 ⎝ σL ⎠⎝ gμL 4 ⎠

t0 =

VG = εGVL

(25)

∫P

PA i

A0

(26)

ηAE = 1 − (27)

(28)

(29)

(30)

(31)

where the total gas-phase mass transfer KG can be expressed by 1 1 1 = + KG kG EHkL

VG ⎛ −dPA ⎞ ⎜ ⎟ VLRT ⎝ dt ⎠

PA i PA 0

(37)

(38)

parameter

value

DA DG DB ρL T σL μL E H VL DR

1.83 × 10−9 1.44 × 10−5 1.62 × 10−9 1050 303 7.2 × 10−2 1.012 × 10−3 1.09 1.04 0.2 × 10−3 3.2 × 10−2

unit m2/s m2/s m2/s kg/m3 K N/m N m−2 s−1 kmol m−3 atm−1 m3 m

It is clearly seen that the gas−liquid interfacial area (a) significantly increases with the rise of the gas flow rate; therefore, it is likely to explain why the SO2 absorption efficiency still retained high values as the gas flow rate increased. The values of both KG,cal and KG,exp were obtained, and the relative error was below 20%, indicating that the masstransfer model presented in this work can be considered to be reasonable for this system with various conditions.

(32)

On the basis of the mass balance for SO2 absorbed into BAS solution, the absorption rate of SO2 in the bubbling reactor can also be written as32 NA =

(36) 4

Table 2. Parameters for This System

Combining eqs 21, 22, and 30 can deduce the total mass transfer rate and gives NA = K GaPA

K GaVLRT dt VG

4.9.2. Calculations for Mass-Transfer Coefficients and Interfacial Area. The mass-transfer characteristics (kG, kL, and a) for the bubbling reactor have important influences on the removal of SO2. For these calculations, Table 2 shows the parameters for the desulfurization process.5,33,34 The calculation results for mass-transfer coefficients and interfacial area under various conditions are given in Table 3.

According to the present experimental results, the SO2 absorption in BAS solution was assumed to be a rapid chemical reaction. Thus, the SO2 concentration in the liquid phase cA can be negligible with respect to the SO2 concentration in the gas− liquid interface, and eq 27 is simplified as NA = EkLacA*

t0

⎡ ⎛ K aRTV ⎞⎤ L ⎟⎥ ηAE = ⎢1 − exp⎜⎜ − G ⎟⎥ ⎢⎣ q ⎝ ⎠⎦ G

where DA and DB are the diffusion coefficients of SO2 and BAS in water (m2/s), respectively, ξ is the mole ratio of SO2 to BAS absorbent, and cB is the concentration of BAS in water (kmol/ m3). Then, the liquid-phase mass-transfer coefficient can be obtained by29 kL = 0.5gDA1/2ρL 3/8 σL−3/8dVS1/2

∫0

Finally, solving the differential eq 36 and substituting eq 37 gives

where kL is the liquid-phase mass-transfer coefficient (m/s) and cA is the concentration of SO2 in the liquid phase (kmol/m3). The mass-transfer enhancement factor E can be given by4,5,31 D Bc B DA cA*

1 dPA = − PA

The SO2 absorption efficiency can also be written as

liquid-phase mass transfer

E=1+ξ

(35)

where t0 is the residence time of the gas through the bubbling column (s) and qG is the gas flow rate (m3/s). In association of eqs 31 and 33, a differential equation from initial time t = 0 and pressure p = PA0 to time t0 and pressure PAi can be obtained by

The gas-phase mass-transfer coefficient can be estimated by the correlation (eq 26)30

NA = EkLa(cA* − cA )

(34)

with

εG

k Ga = 0.00051uG 0.73

VG qG

(33) 1189

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Energy & Fuels Table 3. Calculations for the Mass-Transfer Coefficients and Interface Area number parameter

1

2

3

4

5

y0 (ppm) qG (L/min) ηAE (%) a (m2/m3) kG (×104, kmol m−2 atm−1 s−1) kL (×104, m/s) KG,cal (×104, kmol m−3 s−1) KG,exp (×104, kmol m−3 s−1) relative error of KG,cal and KG,exp (%) (1/kG)/(1/KG,cal)

2600 0.4 99.5 37.1 3.41 5.67 2.23 1.82 18.4 0.65

1500 0.8 98.9 70.0 2.92 5.67 2.01 1.70 15.4 0.69

2600 1.2 96.9 101.3 2.78 5.67 1.95 1.60 17.9 0.70

3800 0.8 97.8 70.0 2.92 5.67 2.01 1.70 15.4 0.69

2600 0.8 98.4 70.0 2.92 5.67 2.01 1.70 15.4 0.69

equation

eq eq eq eq eq

23 26 29 32 38

desorption regeneration process and provides a benign alternative for the oxidation inhibition of sulfite.

To judge the controlling stems of mass transfer, it is based on comparing the mass-transfer resistance in the gas phase to that in the liquid phase. According to the coefficients, the gas- and liquid-phase mass-transfer resistances, defined as 1/kG and 1/ EHkL, respectively, can be calculated. If 1/kG is nearly equal to 1/KG, the transfer is controlled by the gas phase and a ratio of KG/kG ≤ 0.1 is the criterion for liquid-phase control.4,28 The ratios of the gas-phase resistance to the overall gas-phase resistance under various gas flow rates and inlet SO 2 concentrations are shown in Table 3. The results show that the value of KG/kG was in the range of 0.65−0.70 for numbers 1−5, meaning that the gas-phase resistance to mass transfer is the dominating resistance but the liquid-phase resistance cannot be neglected. Thus, the SO2 absorption rate in BAS solution for this system is likely to be decided by a combination of both gas- and liquid-phase diffusion controls.5

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-020-87111814. E-mail: c.m03@mail. scut.edu.cn. Notes

The authors declare no competing financial interest.

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5. CONCLUSION The wet FGD performance for the BAS−desorption regeneration process has been examined comprehensively in a bubbling column under various conditions. The paper first used EG as an efficient and novel oxidation inhibitor of sulfite in BAS solution. The experimental results reveal that the SO2 absorption efficiency could reach a high level above 90% (in 150 min) and the oxidation efficiency of sulfite in BAS solution can be controlled below 10% (in 120 min) with the addition of above 1% (v/v) EG on the conditions of the gas flow rate of 0.8 L/ min, inlet SO2 concentration of 2600 ppm, temperature of 303 K, amount of aluminum of 15 g/L, and basicity of 25%. Both the amount of aluminum and basicity significantly influence the desulfurization time with above 90% SO2 absorption efficiency, which should keep 15−20 g/L and 25−30%, respectively. The desulfurization performance is related to the absorption pH, which should remain above 3.10. In addition, the absorption temperature has a considerable effect on the removal of SO2, while the BAS-based FGD process is suitable for the removal of a high SO2 concentration. Furthermore, a model for the SO2 absorption process using the bubbling column was established by the two-film theory to mass transfer, and it is reasonable to a certain extent through comparison betweenKG,cal and KG,exp. The calculation results indicate that the SO2 absorption process for this system is decided by a combination of both gas- and liquid-phase diffusion controls. Then, a further study is quite necessary for an in-depth understanding of the mechanism of oxidation inhibition of sulfite by EG. The present study is useful for the design and industrial application relevant to the BAS− 1190

NOMENCLATURE a = specific gas−liquid interface area (m2/m3) cA = SO2 concentration in the liquid (kmol/m3) cA* = SO2 concentration in the interface (kmol/m3) cB = concentration of BAS (kmol/m3) CTS = total concentration of sulfite (mol/L) dVS = volume−surface mean bubble diameter (m) DA = diffusion coefficient of SO2 in water (m2/s) DB = diffusion coefficient of BAS in water (m2/s) DG = diffusion coefficient of SO2 in the gas phase (m2/s) DR = diameter of the bubbling column (m) E = mass-transfer enhancement factor g = gravitational constant (m/s2) H = thermodynamic equilibrium constant (kmol m−3 atm−1) kG = gas-phase mass-transfer coefficient (kmol m−2 s−1 atm−1) kL = liquid-phase mass-transfer coefficient (m/s) KG = overall gas-phase mass-transfer coefficient (kmol m−2 s−1 atm−1) M = molar mass of SO2 (kg/kmol) n = amount of sulfite in solution (mol) N = amount of SO2 absorption (mol) T = temperature (K) t = time (s) NA = absorption rate (kmol m−3 s−1) PA = partial pressure of SO2 in the gas phase (atm) P*A = partial pressure of SO2 in the interface (atm) q = volume of flue gas for a period of time (L) qG = gas flow rate (m3/s) Q = total volume of flue gas (L) R = gas constant (atm m3 kmol−1 K−1) t0 = residue time of gas in liquid (s) uG = superficial gas velocity (m/s) VL = volume of the absorption solution (L) VG = volume of gas bubbling in liquid y0 = SO2 concentration in the gas phase at the inlet (ppm) yi = SO2 concentration in the gas phase at the outlet (ppm) DOI: 10.1021/acs.energyfuels.5b02411 Energy Fuels 2016, 30, 1183−1191

Article

Energy & Fuels Greek Symbols

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εG = gas holdup (%) ρL = density of solution (kg/m3) μL = viscosity of solution (Pa s) σL = surface tension of solution (N/m) ξ = mole ratio of SO2 and absorbent reagent (mol/mol) ηAE = SO2 absorption efficiency (%) ηOE = oxidation efficiency of SO2 absorbed (%)

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DOI: 10.1021/acs.energyfuels.5b02411 Energy Fuels 2016, 30, 1183−1191