Prediction of SO₂ removal efficiency for ammonia-based wet flue gas desulfurization in a packed tower

Abstract

Wet flue gas desulfurization is widely used in power plants because of its high desulfurization efficiency. The reason why it is difficult to predict sulfur dioxide removal efficiency in flue gas is that it is related to many factors. In this paper, the SO₂ was removed by absorbing with ammonia solution in a packed tower. Experimental studies have shown that the operating conditions affecting the removal of sulfur dioxide include pH, liquid-to-gas, the concentration of SO₂ and inlet velocity. Based on the two-film theory, the mass transfer rate equation of SO₂ in the ammonia desulphurization process was studied, and the prediction of SO₂ removal efficiency based on certain assumptions was developed. In order to prove the responsibility of the model, significant factors such as pH, liquid-to-gas, the concentration of SO₂, and inlet velocity were studied in a packed tower. The experimental results show that it is possible to improve desulphurization efficiency by increasing the liquid-to-gas ratio and pH value, lowering the concentration of SO₂ and inlet velocity. Experimental results show that the experimental value is consistent with the predicted value.

1 Introduction

Sulfur dioxide (SO₂) discharged from thermal power plants is a significant pollutant in the atmosphere, which has a severe harm to human health and the environment [1,2,3]. Many countries have set strict emission standards to reduce sulfur dioxide emissions and various desulfurization methods came into being [4,5,6]. Flue gas desulfurization (FGD) is the most effective desulfurization method at present. According to whether water is added in the process and desulfurization behavior can be divided into wet method, semi-dry method and dry method. Among the different wet flue gas desulfurization processes, ammonia desulfurization technology has the advantages of simple equipment, low consumption, high flexibility of operation, and has the by-products of valuable ammonium sulfate which can be used as fertilizer [7, 8].

When ammonia solution contacts with SO₂ in flue gas and the following reactions occur [9, 10].

$$ 2{\text{NH}}_{3} + {\text{SO}}_{2} + {\text{H}}_{2} {\text{O}} \to \left( {{\text{NH}}_{4} } \right)_{2} {\text{SO}}_{3} $$

(1)

$$ {\text{NH}}_{3} + {\text{SO}}_{2} + {\text{H}}_{2} {\text{O}} \to {\text{NH}}_{4} {\text{HSO}}_{3} $$

(2)

$$ {\text{NH}}_{4} {\text{HSO}}_{3} + {\text{NH}}_{3} \to \left( {{\text{NH}}_{4} } \right)_{2} {\text{SO}}_{3} $$

(3)

In recent years, some studies have conducted experiments to describe the process of SO₂ absorption with ammonia solution. Johnstone revealed the mechanism of SO₂ absorption with the ammonia solution through the study of the gas–liquid equilibrium of the SO₂–NH₃–H₂O system [11].

Wu used ammonium sulfite as adsorbent and the relationships of affecting factors and desulphurization efficiency were analyzed, and suitable operating conditions were achieved as well [12]. Gao performed experiments with a stirred tank reactor to study the characteristics of gas–liquid reaction between ammonium sulfite solution and SO₂ [9].

The mass transfer and chemical reaction process of wet ammonia desulfurization process are complex and there are few studies on the prediction of wet ammonia desulfurization efficiency. Jia et al. have developed a mathematical model to simulate the absorption of SO₂ in a spray scrubber [13]. Liu et al. established a mathematical model of ammonia desulfurization in spray tower by analyzing the mass transfer rate equation of SO₂ in the process of ammonia desulfurization, and the experimental data agreed with the calculated values well [14]. In the process of deducing the mass transfer rate equation of ammonia desulfurization, most of the ammonia desulfurization processes are considered as a first-order irreversible reaction, and most of the absorption processes occur in a spray tower.

Sulfur dioxide emission standards are getting stricter in the world. How to improve desulfurization efficiency and reduce energy consumption is an important problem. In this study, the SO₂ removal with ammonia solution in packed power was tested under different operating conditions, including liquid–gas ratio, the pH, the concentration of SO₂, and inlet velocity. The goal is to estimate the model of mass-transfer in liquid sides with enhancement factors and sulfite concentration in the liquid phase based on the film theory to estimate the SO₂ removal efficiency.

2 Process fundamentals and prediction

2.1 Assumptions

The following assumptions were made for the ammonia desulfurization process to simplify the calculation:

• a. The flow rate of mixed gas and liquid flowing through the whole tower is considered as constant.

• b. Temperature is constant inside the absorption tower.

• c. The concentration and velocity distribution of flue gas and absorption liquid on the same section of the absorber are uniforms.

• d. The oxidation of sulfites is negligible

2.2 Mass transfer rate equations

In the process of wet ammonia desulfurization, the absorption rate is determined as follows:

$$ N_{{{\text{SO}}_{ 2} }} = {\text{K}}_{G} \left( {{\text{p}}_{{{\text{SO}}_{ 2} }} - {\text{p}}_{{{\text{SO}}_{ 2} }}^{ *} } \right) = {\text{k}}_{{\rm G}} ( {\text{p}}_{{{{\rm SO}}_{ 2} }} - {\text{p}}_{{{{\rm SO}}_{ 2} ,i}} ) = {\text{Ek}}_{{\rm L}} {\text{c}}_{{{{\rm SO}}_{2} ,i}} $$

(4)

Sulfur dioxide is easily soluble in water, and the liquid film resistance is very small. So K_G = k_G.

The equilibrium partial pressure at the gas–liquid interface directly affects the mass transfer force with the expression of mass transfer rate. The partial pressure of SO₂ in the gas–liquid interface [11] can be obtained by:

$$ p_{{SO_{2,i} }} \approx p_{{_{{\text{SO}_{2} }} }}^{ *} = 0.1333 \times \frac{{MN(2S/N - 1)^{2} }}{1 - S/N} $$

(5)

where lgM = 5.865–2369/T.

It can be seen from the above correlation formula that there are various types of ions in the ammonia desulfurization process. The concentration of the main elements of N and S can be used to characterize the characteristics of the solution. The model assumes that the absorption solution does not contain sulfate ions.

$$ S = \frac{X + Y}{Z} \times 100 $$

(6)

$$ N = \frac{2X + Y}{Z} \times 100 $$

(7)

$$ N = \frac{{1000D_{r} - 99Y - 116X - 132N^{\prime}}}{18} $$

(8)

$$ pH = 8.88 - 4\frac{S}{N} $$

(9)

The total reaction of ammonia flue gas desulfurization could be expressed as follows:

$$ {\text{SO}}_{2} + \left( {{\text{NH}}_{4} } \right)_{2} {\text{SO}}_{3} + {\text{H}}_{2} O = 2{\text{NH}}_{4} {\text{HSO}}_{3} $$

(10)

The study of Chen [15] shows the reaction of the ammonia flue gas desulfurization is an irreversible second-order reaction. For a second-order irreversible reaction A + bB → Q, The enhancement factor is determined by the following equation:

$$ E = {{Ha\sqrt {\frac{{E_{\infty } - E}}{{E_{\infty } - 1}}} } \mathord{\left/ {\vphantom {{Ha\sqrt {\frac{{E_{\infty } - E}}{{E_{\infty } - 1}}} } {th\left( {Ha\sqrt {\frac{{E_{\infty } - E}}{{E_{\infty } - 1}}} } \right)}}} \right. \kern-0pt} {th\left( {Ha\sqrt {\frac{{E_{\infty } - E}}{{E_{\infty } - 1}}} } \right)}} $$

(11)

where E∞ is the instantaneous irreversible reaction enhancement factor, which could be expressed as follows:

$$ E_{\infty } = 1 + \frac{{D_{{\left( {NH_{4} } \right)_{2} SO_{3} }} c_{{\left( {NH_{4} } \right)_{2} SO_{3} }} }}{{bD_{{SO_{2} }} c_{{SO_{2} ,i}} }} $$

(12)

When the reaction rate constant and absorption liquid component concentration is large, the reaction at this time could be considered as an instantaneous reaction. The enhancement factor E of the second-order reaction is equal to the instantaneous reaction enhancement factor E∞. The above equation can be written approximately as following:

$$ E \approx E_{\infty } = 1 + \frac{{D_{{\left( {NH_{4} } \right)_{2} SO_{3} }} c_{{\left( {NH_{4} } \right)_{2} SO_{3} }} }}{{bD_{{SO_{2} }} c_{{SO_{2} ,i}} }} $$

(13)

Diffusion coefficients at different temperatures can be calculated by the Stokes–Einstein equation [16]:

$$ \frac{{D_{{SO_{2} }} \mu }}{T} = constant $$

(14)

According to Henry’s law, the concentration of sulfur dioxide at the interface can be expressed by the partial pressure of sulfur dioxide at the interface, as showed in Eq. (15).

$$ c_{{SO_{2,i} }} = p_{{SO_{2,i} }} H $$

(15)

The correlation formula proposed by Setchenow [17] is adopted to estimate the solubility constant of the absorbed solution.

$$ \log [H_{w} /H] = - k_{s} \cdot I $$

(16)

Since the reaction is rapid, SO₂ reacts completely on the membrane surface. The concentration in the bulk of the liquid phase is zero, c_so2 = 0. Therefore, the mass transfer rate equation could be expressed as:

$$ N_{{SO_{2} }} \approx \left( {1 + \frac{{D_{{\left( {NH_{4} } \right)_{2} SO_{3} }} c_{{\left( {NH_{4} } \right)_{2} SO_{3} }} }}{{bD_{{SO_{2} }} p_{{SO_{2} ,i}} H}}} \right)k_{L} p_{{SO_{2} ,i}} H $$

(17)

2.3 Modeling of the packed tower

The packed column is divided into several micro elements from top to bottom, as showed in Fig. 1. For each micro element, the amount of concentration reduction of SO₂ is equal to the amount absorbed by the absorbing solution, and the following equations are obtained [18]:

Fig. 1

Gas–liquid absorption diagram of packed column

$$ \frac{G}{ART}\frac{dp}{dh} = N_{{SO_{2} }} a $$

(18)

$$ \frac{L}{A}\frac{dc}{dh} = N_{{SO_{2} }} a $$

(19)

In the absorption process of the packed column, the absorption process accompanied by a chemical reaction, the specific surface area of effective mass transfer can be calculated according to the following equation [19]:

$$ \frac{a}{{a_{t} }} = 1 - \exp \left\{ { - 1.45\left( {\frac{{\sigma_{C} }}{{\sigma_{L} }}} \right)^{0.75} \left( {\frac{{L_{G} }}{{a_{t} \mu_{L} }}} \right)^{0.1} \left( {\frac{{L_{G}^{2} a_{t} }}{{\rho_{L}^{2} g}}} \right)^{ - 0.05} \left( {\frac{{L_{G}^{2} }}{{\rho_{L} \sigma_{L} a_{t} }}} \right)^{0.2} } \right\} $$

(20)

The mass transfer coefficient of gas–liquid absorption is calculated by the correlation Eq. (21) [20]:

$$ k_{L} = 0.0095\left( {\frac{{L_{G} }}{{a\mu_{L} }}} \right)^{{\frac{2}{3}}} \left( {\frac{{\mu_{L} }}{{\rho_{L} D_{L} }}} \right)^{{\frac{ - 1}{2}}} \left( {\frac{{\mu_{L} g}}{{\rho_{L} }}} \right)^{{\frac{1}{3}}} \varphi^{0.4} $$

(21)

The expression of desulfurization rate can be obtained by sorting out the above formula:

$$ \eta = \frac{{\int {\frac{{M_{{SO_{2} }} \times 10^{6} }}{G}N_{{SO_{2} }} adh} }}{{c_{in} }} \times 100{\text{\% }} $$

(22)

3 Experimental

A schematic of the experimental setup is shown in Fig. 2. The experimental equipment consists of three parts: smoke simulation system, absorption system, and tail gas detection device. The desired amount of sulfur dioxide at inlet gas is prepared by mixing SO₂ with air. The simulated smoke enters from the bottom of the packed tower, and countercurrent contacted with the descending liquid. The solution of ammonia (10%) added into the bottom of the tower depends on the pH of the solution. Gas flow rates are metered by rotameters and gas samples at the inlet and outlet of power are measured by a gas analyzer. The inner diameter of the tower is 0.05 m. The height of the packing layer is 0.3 m. The following equation calculates desulfurization efficiency:

Fig. 2

Schematic diagram of experimental system. 1—flue gas analyzer; 2—air compressor; 3—SO₂ cylinder; 4—mixing box; 5—packed tower; 6—pump; 7—heater; 8—ammonia tank; 9—flowmeter; 10—value

$$ \eta_{{}} = \frac{{c_{in} - c_{out} }}{{c_{in} }} \times 100\% $$

(23)

pH was measured by PHS-25 digital pH meter. Concentrations of SO₂ inlet and outlet were measured with a flue gas analyzer (KM940). The concentration of each component in the absorption solution was determined by chemical analysis. Experimental conditions are shown in Table 1.

Table 1 Experimental conditions

The diagram of the calculation process is present in Fig. 3.

Fig. 3

Schematic diagram of calculating

4 Results and discussion

4.1 The effect of pH

When gas flow was 1 m³/h, the liquid–gas ratio was 3 L/m³, the absorption temperature was 25 °C, and the SO₂ inlet concentration was 1500 mg/m³, the effect of desulfurization efficiency with pH is shown in Fig. 4. The pH of the absorbent is an important factor affecting the desulfurization efficiency. As can be seen in Fig. 4, at a lower pH, there is a big difference between the removal rate calculated by the experiment and the model, and after the pH is higher than 6.4, the difference between the experimental value and the calculated value of the model is decreasing. This is because the absorption process of SO₂ is complex, and there are many ions in the solution. Many influencing factors are not considered in the model, resulting in the difference between the calculated value of the model and the experimental value. On the whole, the trend of model calculated value and the experimental value is generally consistent.

Fig. 4

Effect of pH on desulfurization efficiency

4.2 The effect of liquid–gas ratio

The liquid–gas ratio is vital in the absorption process. When the gas flow was 1 m³/h, SO₂ concentration in the flue gas was 2500 mg/m³. The pH of the absorption solution was 8. The relationship between the liquid–gas ratio and desulfurization efficiency is shown in Fig. 5. It can be seen that the calculated values of the model are consistent with the experimental values. When the liquid–gas ratio is higher than 3 L/m³, the experimental value is close to the theoretical value. When the liquid–gas ratio is lower than 3 L/m³, the experimental value is higher than the theoretical value. The reason may be with the increase of the liquid–gas ratio, the content of (NH₄)₂SO₃ in the spray solution increases, and the mass transfer impetus increases, which is good for the absorption process. The flow of flue gas enhances the surface fluctuation of the liquid film and it increases the actual active contact area, the desulfurization rate measured by the experiment is higher than the theoretical value in the process of the experiment.

Fig. 5

Effect of liquid–gas ratio on desulfurization efficiency

4.3 The effect of inlet concentrations of SO₂

Experimental conditions: temperature was 25 °C, the pH was 8, the liquid–gas ratio was 3 L/m³, the gas flow rate was 1 m³/h.

Since SO₂ concentration in industrial coal combustion flue gas varies greatly with the sulfur content of coal burning, the range can be from l000 to 3500 mg/m³, that is, SO₂ concentration in flue gas is not a constant value, so it is of great significance to explore the influence of SO₂ concentration on desulfurization rate. The experimental results are shown in the Fig. 6. The variation trend of the fitted curve is basically consistent with the experimental results. Desulfurization efficiency gradually decreases with the increase of the initial concentration of SO₂. When the inlet concentration of SO₂ is less than 2000 mg/m³, calculation value and experimental value fitted better. The reason may be the concentration of sulfite in the absorption solution exceeds the required for gas phase reactant when the inlet concentration is less than 2000 mg/m³. When the inlet concentration is greater than 2000 mg/m³, the calculated value of the model is higher than the experimental value. The reason may be the volatilization of ammonia in the experimental operation, which reduces the concentration of ammonium sulfite in the absorption solution and reduces the absorption rate.

Fig. 6

Effect of the concentration of SO₂ on a desulfurization efficiency

4.4 The effect of flue gas velocities

Experimental conditions: absorption temperature was 25 °C, pH was 8, liquid–gas ratio was 3L/m³, the concentration of SO₂ was 2000 mg/m³.

It can be seen from Fig. 7 that the variation trend of the fitted curve is basically consistent with the experimental results, and the desulfurization rate decreases with the increase of inlet speed. The absorption rate obtained by the simulation calculation is lower than the experimental value. The reason may be the absorption reaction is carried out under the countercurrent condition, and the reduction of absorption liquid will appear back mixing phenomenon which strengthens the mass transfer in the system. This also shows that the ratio of liquid to gas is not completely proportional to the desulfurization reaction rate. When the ratio of liquid to gas reaches a certain value, the enhancement effect of the ratio of liquid to gas on the desulfurization rate decreases accordingly.

Fig. 7

Effect of the velocities of flue gas rate on desulfurization efficiency

4.5 Effect of temperature on desulfurization efficiency

Experimental conditions: pH was 8, liquid–gas ratio was 3 L/m³, the concentration of SO₂ was 2000 mg/m³. The effect of temperature on absorption of SO₂ is observed in Fig. 8. An examination of Fig. 8 shows that when the inlet temperature increases from 20 to 100 °C, the desulfurization efficiency decreases slightly. The change of absorption reaction temperature has an important effect on the equilibrium partial pressure of SO₂ besides the basic data of diffusion coefficient, density, viscosity and surface tension in the mass transfer rate formula. With the increase of temperature, the equilibrium partial pressure of SO₂ also increases, and the driving force of chemical absorption decreases, thus reducing the desulfurization rate. Therefore, in the actual wet desulfurization device, the high temperature original flue gas is first cooled down and then entered into the absorption tower, which is conducive to SO₂ absorption.

Fig. 8

Effect of temperature on desulphurization efficiency

5 Conclusions

In this paper, experiments for SO₂ absorbed by the ammonia solution in the packed power were carried out to get the removal efficiency of SO₂. The results show that the reasonable values of the process parameters: pH value is 7–8, inlet concentration of SO₂ is 1500–2000 mg/m³, gas–liquid ratio is 3–4 L/m³. Mass transfer equation of SO₂ in flue gas removal by the ammonia method was developed for the prediction of the SO₂ removal efficiency using the two-film theory. The results show that the desulfurization efficiency increased by increasing the liquid–gas ratio and pH of the absorbed liquid, and decreased by increasing the inlet concentration of flue gas flow. The desulfurization model is calculated by using the operating parameters of a laboratory-scale packed column, and the results show that the theoretical calculated values are basically consistent with the experimental data.