Studies on flue gas desulfurization using EDA/SiO₂–phosphoric acid solution as an absorbent

Abstract

Flue gas desulfurization by immobilized amines is a new desulfurization technique. EDA/SiO₂ and an H₃PO₄ additive were used as an absorbent for desulfurization. SO₂ was absorbed by the system and desorbed from the loaded solution. And the cycling operation was also analyzed in the experiments. Some technological conditions such as the amount of EDA/SiO₂, the temperature, concentration of SO₂ and pH value were experimentally researched. With the optimized process, the absorption efficiency of this system could reach 98% and the desorption efficiency was over 50%, showing good absorption/desorption capability and promising potential to be applied in desulfurization.

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1 Introduction

It is well known that the exhaust gas from burning coal is the main contributor to air pollution in the world. In China’s energy consumption structure, coal accounts for 76.12%.¹ Many pollutants are emitted during coal utilization, among which SO₂ is one of the most harmful emissions. SO₂ not only leads to acid rain, but also causes serious harm to the respiratory systems of human beings. Various desulfurization technologies have been developed to reduce pollutant emission.

Based on the morphology of desulfurization products, the methods of flue gas desulfurization can be divided into three types: dry, wet and semi-dry. Dry processes mainly include oxidative desulfurization (ODS), oxidation–extraction desulfurization (OEDS), the pulse plasma chemical process (PPCP) and adsorptive desulfurization (physical adsorption desulfurization, activated adsorption desulfurization).² Wet processes mainly include the limestone–gypsum method,^3,4 the seawater method, the citric acid–sodium citrate buffer solutions method,⁵ the sodium salt cycling method, the magnesia method and the alkali aluminum sulfate–gypsum method. Semi-dry desulfurization mainly includes the spray-drying process.⁶ However, one big issue with these techniques is that desulfur waste will re-pollute the environment. Therefore, it is necessary to develop a novel desulfurization technique with both good environmental and economic properties.

Ethylenediamine (EDA) has been intensively investigated recently as a wet desulfurization agent due to its high efficiency for desulfurization,⁷ low corrosivity and lack of re-pollution. However, some concerns about EDA must be overcome, such as its poor oxidation stability, low bubble temperature and the release of amine during the flue gas desulfurization (FGD) process.⁸ It is necessary to find a better desulfurizer to solve these problems.

In this paper, an EDA immobilized on SiO₂ material (EDA/SiO₂) was synthesized. As a desulfur absorber, it shows very promising properties and can achieve a good SO₂ absorption efficiency. This material has many advantages, such as low cost, high desulfurization efficiency, recycling of the absorbent, low amine loss and no re-pollution.

2 Desulfurization mechanism of EDA/SiO₂

SO₂ has a remarkable solubility in water, while on the other hand it can also dissociate with water. The equations of ionization in water are shown below:

SO₂(g) ⇋ SO₂(aq) (1)

SO₂(aq) + H₂ ⇋ OH⁺ + HSO₃⁻ (2)

HSO₃⁻ ⇋ H⁺ + SO₃²⁻ (3)

With increasing H₂SO₃ concentration in solution, reactions (1) and (2) move to the right and the [H⁺] increases. The reactions finally reach equilibrium and no more SO₂ can be dissolved in the solution. In order to increase the solubility of SO₂, the [H⁺] should be decreased, which can be realized by adding phosphoric acid to the solution.

The structure of EDA/SiO₂ is shown in Fig. 1.

Fig. 1 The structure of EDA/SiO₂.

EDA has a plurality of amine groups and is a kind of polyamine. The processes of adsorption and desorption are shown as follows.

In reaction (4), EDA has two amine groups, one of which can combine with H⁺ and generate a protonated amine. The protonated amine is relatively stable, and can be better desorbed after the absorption of SO₂. The characteristics of anion X⁻ can influence the desulfurization process. If the anion is Cl⁻, the causticity of the absorbent is increased. Also, SO₄²⁻ is not suitable because it influences the recovery of the absorbent. Part of SO₂ is oxidized into SO₄²⁻ in the FGD process, which should be removed in order to maintain the absorption capacity of the absorbent. Also, considering the excellent buffer effect and anti-oxidation of PO₄³⁻ in the sodium phosphate method, phosphoric acid is chosen as the best additive. Reaction (5) is the process of SO₂ being absorbed by the amine. Reaction (6) is the process of regeneration of EDA/SiO₂.

3 Experimental section

3.1 Materials

The simulated flue gas was prepared using air from an air compressor and SO₂ from a SO₂ generator, respectively. SO₂ was synthesized by decomposition of Na₂SO₃ with H₂SO₄. In each condition of the experiment, there is an equal amount of CO₂ in the flue gas. Since the solubility of SO₂ is greater than that of CO₂, the influence of CO₂ in the flue gas was not considered.

The absorption solution was prepared by EDA, SiO₂, γ-chloropropyltrichlorosiane (CPTCS) and 40 mL deionized water. Phosphoric acid was used to adjust the pH value of the solution.

EDA was purchased from Shanghai Jingchun Chemical Reagent Co. Phosphoric acid (purity ≥ 85%; the other component is water) was purchased from Tianjin Tianda Chemical Reagent Plant. SiO₂ was purchased from Tianjin Baishi Chemical Reagent Co. CPTCS was purchased from Zibo Qiquan Chemical Reagent Co.

3.2 The preparation of EDA/SiO₂

Activation of SiO₂: 30 g of SiO₂ was added into 1 mol L⁻¹ of nitric acid solution and refluxed at 383 K with stirring for 16 h. Then the product was washed to neutral pH with deionized water and carbinol, and dried in vacuum at a temperature of 383 K. The dried material was activated SiO₂.

Hydration of SiO₂ was performed by putting the activated SiO₂ into a constant temperature hydration device for 16 h at a constant temperature of 298 K.

Alkylation reaction: 50 mL of dehydrated n-hexane and 30 mL of CPTCS were mixed with the activated SiO₂, and the mixture was refluxed gently for 16 hours in a water bath at a temperature of 298 K. The product was washed to neutral pH by n-hexane, carbinol and deionized water, respectively. Then the product was dried in vacuum at a temperature of 363 K and CPTCS–SiO₂ was obtained.

The synthesis route of CPTCS–SiO₂ is shown as follows:

Preparation of EDA/SiO₂: 20 g of CPTCS–SiO₂ was dispersed into a mixture of 30 mL of carbinol and 40 mL of EDA, and the suspension was stirred for 16 hours at a temperature of 333 K. The product was washed to neutral pH by carbinol, deionized water and ammonia, respectively. Then it was dried in vacuum at a temperature of 363 K and EDA/SiO₂ was obtained.

The synthesis route of EDA/SiO₂ is as follows:

3.3 Experimental apparatus and methods

Fig. 2 shows a schematic map of the flue gas desulfurization process by the EDA/SiO₂–phosphoric acid system. Na₂SO₃ bottle and H₂SO₄ bottles were used to prepare SO₂. Air from an air pump and SO₂ flowed through the flowmeter, mixed at the manostat bottle and then flowed through the absorption tower. NaOH was used in a tail gas treatment tank to react with the SO₂ which was not absorbed in the absorption tower.

Fig. 2 Schematic map of the flue gas desulfurization process by the EDA/SiO₂–phosphoric acid system.

3.4 Count method of absorption and desorption efficiency

AE is defined as the desulfurization percentage to evaluate the absorption effects:where y_i is the inlet concentration of SO₂ in the gas phase and y_o is the outlet gas concentration of SO₂ after absorption.

DE is defined as the desorption percentage to evaluate the desorption effects:

where x_i denotes the inlet concentration of SO₂ in loaded absorption solutions and x_o denotes the outlet concentration of SO₂ in unloaded solutions after desorption. When absorption is used repeatedly, x_φ represents the initial SO₂ concentration before absorption. If a fresh absorption solution is prepared and used for first time, x_φ is equal to zero.

3.5 Analytical methods

The concentration of SO₂ in the flue gas and in the aqueous solution was analyzed by the iodine method, as was recommended in the literature.^9,10

4 Results and discussion

4.1 Physical properties of EDA/SiO₂

EDA/SiO₂ is a kind of milky particle which is semi-transparent and spherical. Its particle size is 150–250 μm, aperture is 40–70 Å, pore volume is 0.60–0.85 mL g⁻¹ and specific surface area is 400–600 m² g⁻¹. The surface morphology of EDA/SiO₂ is shown in Fig. 3. It can be seen that the overall structure of the material is loose, which is favorable for adsorption and desorption.

Fig. 3 SEM image of EDA/SiO₂.

4.2 Effect of the EDA/SiO₂ amount in an absorption solution

Fig. 4 shows the effects of the amount of EDA/SiO₂ in the absorption solution on absorption efficiency. 0.2 g, 0.4 g, 0.6 g, 0.8 g and 1.0 g of EDA/SiO₂ were put into 40 mL of water and phosphoric acid was used to control the pH of the solution. A sloped curve was observed when less than 0.4g of EDA/SiO₂ was present and then a maximum like value was reached as the amount of EAD/SiO₂ increased over 0.4 g. This phenomenon was related to the concentration of effective amine groups in the solution.

Fig. 4 Effects of the EDA/SiO₂ amount on absorption (experimental conditions: flow rate of gas G = 400 mL min⁻¹; preheat temperature t = 60 °C; pH = 6.0–6.5; the concentration of SO₂ C = 3000 mg m⁻³; adsorption time is 60 minutes).

At small EDA/SiO₂ amounts, the effective amine groups increase. Therefore the absorption efficiency increased. But if the amount of EDA/SiO₂ is too large, it will lead to the combination of SO₂ with EDA/SiO₂ and it is difficult to desorb the absorbed SO₂, which is not good for the cyclic utilization of absorption material. So, the suitable amount of EDA/SiO₂ is set as 0.4 g in 40 mL water.

The amount of EDA/SiO₂ in solution also affects the desorption efficiency, as shown in Fig. 5. When the amount of EDA/SiO₂ was increased, the desorption efficiency decreased, which was associated with the inhibition of alkali (the alkali was from the reaction of EDA/SiO₂ with water). The higher the alkalinity of the loaded solution, the less ideal the desorption efficiency.

Fig. 5 Effects of the EDA/SiO₂ amount on desorption (experimental conditions: initial concentration of SO₂ in the loaded solution w = 1.8 g L⁻¹; preheating temperature t = 70 °C; initial pH of the loaded solution pH = 5; desorption time is 60 min).

With increased amounts of EDA/SiO₂, desorption becomes difficult. The reason for this phenomenon is that EDA has two NH₂ groups and one of them is more active in reacting with SO₂ to form the corresponding salt, which is difficult to decompose in the desorption process by heating.¹¹

Since SO₂ is an acidic gas, higher amounts of EDA/SiO₂ with strong basicity are favored for absorption but are harmful for desorption. Based on previous results, the optimized amount of EDA/SiO₂ was fixed at 0.4 g in 40 mL water. In this condition, the corresponding absorption and desorption efficiencies were more than 98% and 50%, respectively.

4.3 Effect of temperature

Fig. 6 shows the effects of temperature on absorption efficiency, where the desulfurization efficiency decreased with the increase of temperature. When the temperature was over 60 °C, absorption efficiency decreased quickly.

Fig. 6 Effects of temperature on absorption (experimental conditions: flow rate of gas G = 400 mL min⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; pH = 6.0–6.5; the concentration of SO₂ C = 3000 mg m⁻³; adsorption time is 60 minutes).

On the one hand, the temperature affects mass transfer. The equation of convective mass transfer rate can be defined from the following relationship:

N_A = k_G(p_A − p_Ai) (11)

where N_A denotes the convective mass transfer rate of solute A, k_G denotes the gas film mass transfer coefficient, p_A denotes the partial pressure of solute A in gas phase and p_Ai denotes the partial pressure of solute A at the interface.

It is well known that when other parameters are stable, the value of gas film mass transfer coefficient k_G increases as temperature decreases. Therefore, low temperature favors the absorption of SO₂.

In addition, since the SO₂ absorption is an exothermal reaction, low temperature is also helpful for the desulfurization process. However controlling low temperature consumes much more energy. In order to balance the cost and absorption efficiency, the optimum temperature was set at 60 °C.

Fig. 7 shows the effects of the preheat temperature on desorption efficiency. It can be seen that the desorption efficiency increased as the temperature rose. However, when the temperature was over 70 °C, the desorption efficiency became stable and the system reached its maximum capacity.

Fig. 7 Effects of temperature on desorption (experimental conditions: initial concentration of SO₂ in the loaded solution w = 1.8 g L⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; initial pH of the loaded solution pH = 5; desorption time is 60 min).

Desorption is the inverse process of absorption because the mass transfer direction of desorption is directly opposite to absorption. Increased temperature is not conducive to absorption, but is conducive to desorption. But in actual industrial operation, much higher temperatures may cause higher energy consumption for heating, so 70 °C is considered to be a suitable temperature.

4.4 Effect of SO₂ concentration

The effect of the concentration of SO₂ on absorption efficiency is shown in Fig. 8. The data indicate that the desulfurization efficiency increases with a decreasing concentration of SO₂ in flue gas.

Fig. 8 Effects of the initial SO₂ concentration on absorption (experimental conditions: flow rate of gas G = 400 mL min⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; pH = 6.0–6.5; preheat temperature t = 60 °C; adsorption time is 60 minutes).

Increased concentrations of SO₂ in flue gas will promote SO₂ diffusion to the internal liquid through the solution surface and then accelerate the speed of reaction. But higher SO₂ concentrations will lead to part of the SO₂ not being absorbed by the absorber. Therefore, higher concentrations of SO₂ will make the absorption efficiency decrease.

Fig. 9 shows the effects of the SO₂ concentration in solution on desorption. From the aspect of chemical equilibrium, increased SO₂ concentration will lead to the movement of chemical equilibrium in the direction of desorption. But if the SO₂ concentration is too high, it will influence the recycling of absorbent. So in this investigation, the initial SO₂ concentration is specified as 1.8 g L⁻¹.

Fig. 9 Effects of the SO₂ concentration on desorption (experimental conditions: amount of EDA/SiO₂ A = 0.4 g in 40 mL water; initial pH of the loaded solution pH = 5; preheating temperature t = 70 °C; desorption time is 60 min).

4.5 Effect of pH

Fig. 10 shows the effects of pH on absorption. The pH value of the absorption agent is adjusted by tuning the ratio of H₃PO₄ and EDA/SiO₂ in the system. The results indicated that the desulfurization efficiency for AE increases with increasing pH value.

Fig. 10 Effects of pH on absorption (experimental conditions: flow rate of gas G = 400 mL min⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; preheat temperature t = 60 °C; concentration of SO₂ C = 3000 mg m⁻³; adsorption time is 60 minutes).

With the increase of the pH value, the free active amines in solution increase, and the buffer capacity increases. So it will easy for the absorption liquid to combine with SO₂ and dthe esulfurization efficiency increases. From the aspect of chemical equilibrium, the increase of pH value inhibits the [H⁺], and the chemical equilibrium moves to the right, so the desulfurization efficiency increases. However, at high pH, the selectivity of EDA/SiO₂ between CO₂ and SO₂ would decrease. Therefore, the pH range was optimized from 6.0 to 6.5.

Fig. 11 shows the effect of pH on desorption. The results indicate that desorption efficiency decreases with increasing pH value. The stronger the loaded solution acid is, the higher the desorption efficiency. While the initial pH value of the loaded solution is largely determined by the absorption amount of SO₂ and the initial pH value of the absorption solution.

Fig. 11 Effects of pH on desorption (experimental conditions: initial concentration of SO₂ in the loaded solution w = 1.8 g L⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; preheating temperature t = 70 °C; desorption time is 60 min).

This is because a decrease of [H⁺] with an increase of the pH value strengthens the combined ability of the absorption agent with SO₂. But after absorbing SO₂; the pH value of the loaded solution becomes lower. A strong acidity of the loaded solution will not only have an influence on the absorption, but also increase corrosion to equipment. So the suitable pH value is specified as 5.0.

4.6 Circulating experiment

In order to investigate the desulfurization capability of the absorption reagent after regeneration, the absorption–desorption cycles were performed several times. The experimental results are shown in Fig. 12 and 13.

Fig. 12 Effect of the number of cycles on absorption (experimental conditions: flow rate of gas G = 400 mL min⁻¹; amount of EDA/SiO₂ 0.4 g in 40 mL water; absorption temperature t = 60 °C; pH = 6.0–6.5; concentration of SO₂ C = 3000 mg m⁻³).

Fig. 13 Effect of the number of cycles on desorption (experimental conditions: initial concentration of SO₂ in the loaded solution w = 1.8 g L⁻¹; amount of EDA/SiO₂ A = 0.4 g in 40 mL water; preheating temperature t = 70 °C; initial pH of the loaded solution pH = 5).

As shown in Fig. 12, the absorption efficiency of SO₂ in EDA/SiO₂–phosphoric acid system decreased as the number of cycles increased. After nine cycles, absorption efficiency is maintained at about 91%. The fading of the absorption efficiency of the solution in the initial stages (less than 9 cycles) was related to the oxidation of SO₂ to SO₃ and SO₃ is difficult to release from the solution.

As shown in Fig. 13, the desorption efficiency of SO₂ increases as the number of cycles increases. The reason is that SO₂ accumulated in the loaded solution during the absorption cycle experiment, and the initial SO₂ concentration increases with increasing numbers of cycles.

5 Conclusions

This paper presented a novel FGD process in which an EDA/SiO₂–phosphoric acid system was used to absorb sulfur dioxide in flue gas.

(1) EDA/SiO₂ was prepared. With the addition of phosphoric acid, an EDA/SiO₂–phosphoric acid system was established which can be used to remove SO₂ from flue gas.

(2) Using the EDA/SiO₂–phosphoric acid solution as the absorbent, technical conditions for absorption and desorption were experimentally researched. The optimum conditions for absorption were as follows: the amount of EDA/SiO₂ was 0.4 g in 40 mL water, the preheat temperature was 60 °C, the flow rate of gas was 400 mL min⁻¹, the concentration of SO₂ was 3000 mg m⁻³ in flue gas, the adsorption time was 60 minutes and the pH was between 6.0 and 6.5. Under the optimum conditions, the absorption efficiency for SO₂ was more than 98%.

(3) The optimal conditions for desorption were as follows: temperature was 70 °C, the original pH value of the loaded solution was 5, the amount of EDA/SiO₂ was 0.4 g in 40 mL water and the desorption time was 60 min. Under these conditions, the desorption efficiency of the loaded solution was more than 50%.

(4) The prepared EDA/SiO₂ as a desulfurization agent had many advantages. The mechanical properties of silicone are good, grafted EDA makes the specific surface area bigger and adsorption performance better. It was solid, so it solved the problem of traditional desulfurization agent loss. So EDA/SiO₂ material should be better reused in flue gas desulfurization.

Acknowledgements

This work was supported by Hebei Iron and Steel Joint Fund and by Hebei Provincial Education Department (Z2009431).

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