Selective Mechanisms of WO₃ Catalyzing CO₂ Desorption and Inhibiting NH₃ Escape

Abstract

The high regeneration energy consumption and ammonia escape in the ammonia regeneration process are regarded as the main barriers for the commercial application of CO₂ capture technology based on ammonia solutions. Metal oxides can enhance the CO₂ desorption process and inhibit the ammonia escape at the same time, which can reduce the energy consumption of CO₂ capture systems. Both ammonium carbamate (NH₂COONH₄) and ammonium bicarbonate (NH₄HCO₃) are examined as the rich ammonia solution. The results show that when the concentration of tungsten trioxide (WO₃) was 0.1 mol/L, the CO₂ desorption efficiency could be promoted by 18.8% and the ammonia escape efficiency could be reduced about 14%. The mechanism by which WO₃ increased the CO₂ desorption process was clarified by XRD analysis as the production of ammonium tungstate. In addition, the other nine metal oxides exert no catalytic influence on the regeneration process.

1. Introduction

In the past several decades, climate change, which is caused by the excessive emission of greenhouse gases, has been established as one of many serious environmental issues. CO₂ capture technology based on ammonia solutions is regarded as the most effective method to achieve the emission peak and carbon neutrality [1,2]. Although ammonia solutions have the advantages of higher CO₂ loading capacity and lower regeneration energy compared with other absorbents, ammonia escape is also a main barrier to the commercial application of CO₂ capture technology based on ammonia. The temperature of the regeneration process is much higher than that of the absorbing process. Therefore, reducing the regeneration temperature is of significance for their application, which can contribute to reducing energy consumption and inhibiting ammonia escape.

It is widely accepted that a lower regeneration temperature can result in a lower energy consumption. Many investigations about the regeneration process were subsequently carried out. Cheng et al. [3] pointed out that Cu(II) could act as a catalyst to intensify regeneration by a complexation reaction. Li et al. [4] investigated the system with a metal ion (Ni,Cu,Zn)—NH₃-CO₂-H₂O and found that metal ions could promote the ammonia regeneration process. Voice [5] investigated the influences of metal ions (Ni,Cr,V,Mn) on the ammonia regeneration process and reported that Ni(II) had the best enhancing effect. Liu et al. [6] investigated three types of catalysts in various single and blended amine solvents and found that H-ZSM was the best catalyst. Bhatti et al. [7] found that metal oxides could catalyze the regeneration process of the aided amine solvent and reported the mechanism. Cheng et al. [3] reported that Cu(II) could reduce the regeneration energy consumption of MEA due to the complexation reaction. Li et al. [8] reported that the metal ion (Ni, Cu, Zn) could accelerate the CO₂ desorbing rate of the ammonia regeneration process in a thermodynamic equilibrium model. And Li et al. [8] reported that the additive of Ni²⁺ was the best metal ion additive for the ammonia reaction system. Pachitsas et al. [9] investigated the influence of K₂CO₃ on the ammonia reaction system, and found it could reduce the energy consumption of the regeneration process. However, it also enhanced ammonia escape. Lv et al. [10] added metal ions (Ni²⁺, Cu²⁺, Zn²⁺) to the ammonia reaction system and showed that the additives could promote the CO₂ desorption rate, which can reduce the energy consumption.

The temperature in the stripper is much higher than that in the absorber, which makes the ammonia volatilization much more critical than the absorbing process. Therefore, the ammonia escape-inhibiting technology is significant for the desorption process. The main technologies for inhibiting ammonia escape are ammonia washing and additives. Owing to the extra water consumption required for ammonia washing, the use of additives has been a focus [11]. Ma et al. [11] reported that Ni(II) and Co(II) can be used as additives to inhibit ammonia escape due to the complexation. Kim et al. [12] pointed out that the addition of Cu (II) can significantly reduce ammonia escape during the regeneration process. Mani et al. [13] carried out experiments on Zn(II) addition to the ammonia solution, and reported that Zn(II) could combine with NH₃ to produce the complex ion [Zn(NH₃)₄]²⁺, which can inhibit the evaporation of free ammonia as well as the absorption of CO₂. Ma et al. [14] concluded that the addition of Co(II) could reduce the free ammonia concentration in the liquid phase, which could move the desorption balance towards regeneration and increase the desorption efficiency of CO₂ by 2–5% [15]. Resnik et al. [16] reported that ammonia escape was influenced by the ammonia concentration and the desorption temperature. Ammonia escape could reach 43.1% when the ammonia concentration was 14%. Ma et al. [17] reported that polyethylene glycol dimethyl ether (NHD) could inhibit the ammonia escape process by forming hydrogen bonds with free NH₃.

As mentioned before, the ammonia regeneration process has rarely been investigated, and the catalysis of the metal oxide on the regeneration process was never reported. In this work, ten kinds of metal oxide catalysts were studied: ZnO, WO₃, MnO₂, TiO₂, CuO, MoO₃, Nb₂O₅, Cr₂O₃, ZrO₂, and Al₂O₃. The effects of the metal oxides on the desorption of CO₂ and the ammonia escape were clarified, and the catalytic mechanism was determined by XRD analysis, which can contribute to the industrial application of CO₂ capture using ammonia.

2. Reactions and Experiment

2.1. Reactions

In the absorption process, the main reactions occurred in the liquid phase, and are the following:

$${\mathrm{CO}}_2{\left(\mathrm{g}\right)+\mathrm{NH}}_3\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{\mathrm{NH}}_2\mathrm{COOH}\left(\mathrm{aq}\right)$$

(1)

$${\mathrm{NH}}_2{\mathrm{COOH}\left(\mathrm{aq}\right)+\mathrm{NH}}_3\left(\mathrm{aq}\right)\stackrel{}{\to }{\mathrm{NH}}_2{\mathrm{COONH}}_4\left(\mathrm{aq}\right)$$

(2)

$${\mathrm{NH}}_2{\mathrm{COONH}}_4{\left(\mathrm{aq}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{}{\leftrightarrow }{\mathrm{NH}}_4{\mathrm{HCO}}_3{\left(\mathrm{aq}\right)+\mathrm{NH}}_3\left(\mathrm{aq}\right)$$

(3)

$${\mathrm{NH}}_4{\mathrm{HCO}}_3{\left(\mathrm{aq}\right)+\mathrm{NH}}_3\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{{(\mathrm{NH}}_4)}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)$$

(4)

$${\mathrm{NH}}_3(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{aq})\stackrel{}{\leftrightarrow }{\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}(\mathrm{aq})$$

(5)

$${\mathrm{NH}}_2{\mathrm{COONH}}_4{\left(\mathrm{aq}\right)+\mathrm{CO}}_2{\left(\mathrm{g}\right)+2\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{}{\leftrightarrow }{2\mathrm{NH}}_4{\mathrm{HCO}}_3\left(\mathrm{aq}\right)$$

(6)

$${\mathrm{NH}}_4{\mathrm{HCO}}_3\left(\mathrm{aq}\right)+{\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{{(\mathrm{NH}}_4)}_2{\mathrm{CO}}_3{\left(\mathrm{aq}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)$$

(7)

The main species in the rich solution were NH₂COONH₄ and NH₄HCO₃ [18,19].

2.2. Materials

Ammonia solution (25–28%), zinc oxide (ZnO, 99%), tungsten oxide (WO₃, 99%), manganese dioxide (MnO₂, 85%), titanium dioxide (TiO₂, 99%), copper oxide (CuO, 99%), molybdenum trioxide (MoO₃, 99.5%), niobium pentoxide (Nb₂O₅, 99.9%), chromium trioxide (Cr₂O₃, 99%), zirconia (ZrO₂, 99%), aluminum trioxide (Al₂O₃, 99.9%), and ammonium tungstate ((NH₄)₁₀H₂(W₂O₇)₆, 98.5%) were purchased from Shanghai Macklin Biochemical Co. Ltd., Shanghai, China. Nitrogen (N₂, 99.9%) was obtained from Tianjin FuChen Chemical Reagent Co., Ltd. (Tianjin, China).

2.3. Apparatus and Methods

Figure 1a shows the schematics of the experimental devices, which were mainly composed of an ammonia regeneration reactor, oil bath, cooling and drying system, and an on-line monitoring system. The oil bath provided heat for regeneration of the rich solution, which was monitored by thermocouple. The main regeneration reactions of the ammonia took place in the three-necked, round-bottomed flask (Yancheng Pritch Experimental Instrument Co., Ltd., Yancheng, China). A thermostatic magnetic stirrer (DF-101S, Shanghai Lichen-bx Instrument Technology Co., Ltd., Shanghai, China) was used to control the temperature around 363K during the regeneration reactions, and 480 r/min was selected every time to remove the influence of the rotating speed on the regeneration process, which would ensure the reliability of the experimental data. The unit mass flow controllers (CS200, Beijing Sevenstar Microelectronics Co., Ltd.; Beijing, China) were used to control the flow rates and pressure of the gas. N₂ was used to carry the desorbed CO₂ and escaped NH₃ out. The flow rate of N₂ was set at 0.1 L/min and the inlet N₂ pressure was 0.35 MPa. In experiments, the flow of N₂ can dilute ammonia concentration in the gas phase to within the detection range, and the volume fraction was recorded every 0.5 min. The ammonia escape was calculated as the average concentration within 1 min after air intake. A glass airway with an outer diameter of 6 mm and a silicone tube with an inner diameter of 6 mm and an outer diameter of 9 mm were used to connect the apparatus.

An infrared CO₂ gas analyzer (PGA 620, PhyMetrix Inc.; Medford, NY, USA) was used to measure the volume fraction of CO₂ at the outlet, and the measurement range was 0–100%. Silica gel was selected as the desiccant, because it cannot react with the desorbed CO₂. By recording the volume fraction of CO₂ and flow rate of gas, the CO₂ desorption efficiency was calculated successfully. An infrared NH₃ gas analyzer (PTM 600, Shenzhen Eranntex Co., Ltd.; Shenzhen, China) was used to measure the volume fraction of NH₃ at the outlet, and the range was 0–20%. Alkali lime was selected as the desiccant, because it would not react with the products. Through recording the ammonia volume fraction in gas phase and flow rate, ammonia escape summary and ammonia escape reduction can be obtained.

The volume of the regeneration solution was 150 mL. Referring to the process of CO₂ absorption by the ammonia solution, the solution was prepared by using the ammonia solution and two kinds of intermediate products, NH₂COONH₄ and NH₄HCO₃. In the two kinds of desorption solution, the concentrations of ammonia were 2 wt%. The concentrations of ammonium carbamate and ammonium bicarbonate were 0.5 mol/L.

2.4. Order Independence Validation of Addition

As NH₂COONH₄ can react with the water and the ammonia, the influences of the addition order (water and ammonia) on the rich solution components should be investigated before the experiments. Therefore, experiments with different orders of addition were carried out and Figure 1b presents the variation in CO₂ volume fraction under different addition orders, which illustrated that the regeneration process was not influenced by the addition order. Therefore, the ammonia solution was added to the reactor first and then the water was added, for both the NH₂COONH₄ and NH₄HCO₃ experiments.

3. Results and Discussion

3.1. Effects of Catalyst on CO₂ Desorption

The energy consumption of the regeneration was mainly influenced by the CO₂ desorption process, so it was of benefit to clarify the influences of the catalyst on the CO₂ desorption process for the application of CO₂ capture technology. The experiments were carried out both for NH₂COONH₄ and NH₄HCO₃ solutions.

3.1.1. Ten Kinds of Metal Oxides

The regeneration experiments were carried out with 2 wt% ammonia solution and 0.5 mol/L NH₂COONH₄ at 363 K. The variations in CO₂ volume fraction with time under different metal oxides are shown in Figure 2a, and the concentration of each metal oxide was 0.1 mol/L. Compared with the blank series, only WO₃ can significantly promote the CO₂ desorbing process. The variations in CO₂ desorption amount with different catalysts are shown in Figure 2b, which illustrated that WO₃ could enhance the CO₂ desorption amount by 18.8%. Although MnO₂, TiO₂, and MoO₃ could also promote the CO₂ desorption amount by 3.6%, 2.2%, and 0.1%, the enhancement of the three metal oxides was too small to be investigated. Combined with Figure 2a,b, this showed that CuO, ZnO, Al₂O₃, Cr₂O₃, ZrO₂, and Nb₂O₅ inhibited the desorption of CO₂ from the rich solution. Compared with the blank group, Nb₂O₅ had the strongest influence on the desorption of CO₂, inhibiting CO₂ desorption by 21%. The same influences of metal oxides on the CO₂ desorption from the NH₄HCO₃ rich solution were discovered; the detailed results are not shown here.

3.1.2. WO₃ Concentration Effects

The influences of WO₃ on NH₂COONH₄ and NH₄HCO₃ solutions were the same. Therefore, the NH₄HCO₃ solutions were introduced in this part. The rich solution was under the condition of 2 wt% ammonia solution and 0.5 mol/L NH₄HCO₃ at 363 K. The variations in CO₂ volume fraction under different WO₃ concentration (0.05, 0.1, 0.15, 0.175 mol/L) are shown in Figure 3a, which illustrated that 0.175 mol/L WO₃ concentration caused the best promotion. The variations in CO₂ desorption amount with different WO₃ concentration are presented in Figure 3b, showing that the improvement in CO₂ desorption increased as the WO₃ concentration increased. The improvement caused by 0.175 mol/L was 19.4% compared with the blank series. Figure 3c shows the variations in CO₂ volume fraction under different conditions, which indicated that the addition of WO₃ could improve the CO₂ desorption below 90 °C. The CO₂ volume fraction under 90 °C and WO₃ addition was clearly higher than that under 95 °C and in the blank group. Therefore, WO₃ can reduce the regeneration temperature significantly, which can lead to the reduction in energy consumption. The addition of WO₃ was beneficial to the ammonia regeneration process, which can save energy.

3.2. Effect of Inhibiting Ammonia Escape

Besides energy consumption, ammonia escape is another barrier for the commercial application of the CO₂ capture technology based on ammonia. It was therefore of significance to clarify the influence of WO₃ on the ammonia escape process. Both the NH₂COONH₄ rich solution and NH₄HCO₃ rich solution were investigated, and the variations in NH₃ volume fraction in the gas phase over time were shown in Figure 4, in which (a) is for NH₂COONH₄ rich solution and (b) is for NH₄HCO₃. It can be seen that WO₃ can inhibit the ammonia escape process both for the NH₂COONH₄ rich solution and NH₄HCO₃ rich solution. For the peaks of ammonia escape, the WO₃ group in the NH₂COONH₄ desorption solution was 11% lower than the blank, and 14% for the NH₄HCO₃ desorption solution. Lv et al. [10] also reported that the addition of divalent metal ions (Ni, Cu, and Zn) can inhibit the ammonia escape during the regeneration by about 26.2%, 9.7%, and 9.6%, respectively. However, the addition of divalent metal ions may be complex with NH₃, which might influence the regeneration process.

In conclusion, WO₃ not only can promote CO₂ desorption but also can inhibit NH₃ escape, both of which can reduce the energy consumption significantly.

3.3. Analysis and Mechanism

The mechanism of WO₃ was studied by various means. As shown in Equation (3), adequate NH₂COONH₄ would become NH₄HCO₃ in the solution. The desorption mechanisms of NH₂COONH₄ and NH₄HCO₃ were similar. Therefore, the rich solution of NH₄HCO₃ was mainly investigated. A UV–VIS-NIR spectrophotometer (UV-3600, Shimadzu Scientific Instruments, Inc.; Kyoto, Japan) was used and the scanning spectra of rich solutions are shown in Figure 5a; these were similar without obvious peaks. Therefore, there was no complex being formed.

For further investigations, XRD analysis was used. The comparison of rich solution with WO₃ and the ammonium tungstate was shown in Figure 5b, which illustrated that the rich solution with WO₃ was 90% similar to the fingerprint of pure ammonium tungstate. Therefore, the addition of WO₃ can generate ammonium tungstate to inhibit ammonia escape and enhance the CO₂ desorption process, as shown in Equation (8) [20] and Equation (9) [20].

According to the two-film theory, the main resistance to mass transfer between liquid and gas phase exists in the liquid film, and the ammonia escape rate is influenced by the concentration of free ammonia in the liquid phase [13,21]. The balance of the ammonia in the liquid phase is shown in Equation (10). The addition of WO₃ can reduce the concentration of free ammonia as shown in Equation (11) and Figure 6. The reduction in free ammonia in the liquid phase can also enhance the regeneration reaction, which can promote CO₂ desorption.

$${\mathrm{WO}}_3{\left(\mathrm{aq}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{\mathrm{H}}_2{\mathrm{WO}}_4\left(\mathrm{aq}\right)$$

(8)

$${\mathrm{H}}_2{\mathrm{WO}}_4{\left(\mathrm{aq}\right)+2\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{{(\mathrm{NH}}_4)}_2{\mathrm{WO}}_4{\left(\mathrm{aq}\right)+2\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)$$

(9)

$${\mathrm{NH}}_4^{+}(\mathrm{aq})+{\mathrm{OH}}^{-}(\mathrm{aq})\stackrel{}{\leftrightarrow }{\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{\mathrm{NH}}_3(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{aq})$$

(10)

$${2\mathrm{NH}}_3·{\mathrm{H}}_2{\mathrm{O}\left(\mathrm{aq}\right)+\mathrm{WO}}_3\left(\mathrm{aq}\right)\stackrel{}{\leftrightarrow }{2\mathrm{NH}}_4^{+}{\left(\mathrm{aq}\right)+\mathrm{WO}}_4^{2-}{\left(\mathrm{aq}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)$$

(11)

4. Conclusions

In this work, the influences of ten metal oxides catalysts on the regeneration process were studied. The results showed that WO₃ can enhance the desorption of CO₂ and significantly inhibit ammonia escape. Compared with the blank group, WO₃ can promote the CO₂ desorption by 18.8% and inhibit ammonia escape by 14%. The catalytic reaction mechanism of WO₃ was examined by XRD analysis, which illustrated that ammonium tungstate was generated during the reactions. The addition of WO₃ can reduce the regeneration temperature, which can then reduce the energy consumption of regeneration and is beneficial for the application of CO₂ capture technology.