Reduction of SO<sub>2</sub> to Elemental Sulfur in Flue Gas Using Copper-Alumina Catalysts

Mousavi, Seyyed Ebrahim; Pahlavanzadeh, Hassan; Khalighi, Reza; Khani, Masoud; Ebrahim, Habib Ale; Abbasizadeh, Saeed; Mozaffari, Abbas

doi:https://doi.org/10.1155/2023/3723612

Journal of Nanotechnology

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Abstract Introduction Results and Discussion Conclusion Data Availability Conflicts of Interest References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 3723612 | https://doi.org/10.1155/2023/3723612

Reduction of SO₂ to Elemental Sulfur in Flue Gas Using Copper-Alumina Catalysts

Seyyed Ebrahim Mousavi,¹Hassan Pahlavanzadeh,¹Reza Khalighi,²Masoud Khani,³Habib Ale Ebrahim,³Saeed Abbasizadeh,¹and Abbas Mozaffari⁴

Academic Editor: Mozhgan Afshari

Received15 Aug 2023

Revised02 Nov 2023

Accepted28 Nov 2023

Published23 Dec 2023

Abstract

This study aims to propose an advanced catalyst for the selective catalytic reduction of SO₂, as a sustainable process to mitigate the emission of this toxic gas, which is a significant environmental concern. The conversion of SO₂ through catalytic reduction with CH₄ to elemental sulfur was investigated using Al₂O₃-Cu catalysts. The reaction was conducted under atmospheric pressure and at a temperature range of 550–800°C. A remarkable 99.9% SO₂ conversion rate and 99.5% sulfur selectivity were achieved using the Al₂O₃-Cu (10%) catalyst at 750°C. The highest conversion rates of SO₂ to elemental sulfur, with minimal production of undesirable by-products such as H₂S and COS, were obtained when the SO₂/CH₄ molar feed ratio was set at 2, which is the stoichiometric ratio. Furthermore, the optimal catalyst exhibited excellent long-term stability for SO₂ reduction with methane.

1. Introduction

Today, SO₂ remains a critical airborne pollutant [1, 2]. Alongside nitrogen oxides, it remains a primary contributor to acid rain formation. The impact of sulfur dioxide on human well-being is substantial, causing detrimental health effects, diminishing agricultural output, leading to fish fatality by lowering river pH, and instigating numerous other hazardous consequences. Both stationary and mobile sources contribute to SO₂ emissions. Vehicles running on high-sulfur-content gasoline and diesel represent the mobile sources, while stationary sources encompass metallurgical facilities (such as copper, zinc, and lead roasting units), coal-fired power plants, oil and gas refineries, and petrochemical industries. Given the detrimental repercussions, the development of effective strategies to manage these emissions is imperative.

In general, flue gas desulfurization (FGD) techniques are categorized into two main groups: disposability and regenerability [1, 2]. The primary disposability methods involve lime sorption, offering a major pathway for curbing atmospheric SO₂ emissions. Nonetheless, these techniques are better suited for treating modest quantities of SO₂ within flue gas, proving inadequate for large flows, which generate a substantial volume of nonuseable by-products, a challenge exacerbated by landfill disposal.

On the contrary, regenerative techniques are designed for handling higher SO₂ volumes, such as those found in copper converting and zinc roasting plants. Notably, the catalytic conversion of SO₂ into sulfuric acid and elemental sulfur is the cornerstone of regenerative methods. When a robust demand exists for sulfuric acid, it becomes justifiable to reduce SO₂ emissions while producing this substance. However, the corrosive nature of sulfuric acid poses significant challenges in terms of storage and transportation. In comparison, the selective catalytic reduction of SO₂ to sulfur, facilitating simpler transport and storage of solid sulfur, holds greater appeal and promise [3]. The reduction of sulfur dioxide to sulfur has been explored using both dry-based and wet-based catalysts. Dry-based catalysts are of greater interest, primarily because of their applicability in industrial processes. [4]. Various reducing agents have been explored for this process, including CO [5], CH₄, and H₂ [6] as the primary contenders, while syngas (CO + H₂) [7, 8] and carbon [9, 10] have also been tested.

Various catalysts, such as CoMo/γ-Al₂O₃ [5], SnO₂ [11], Ir/CeO₂ [12], and Ce-Al-O_x composite oxide catalyst [13] for the reduction of SO₂ with carbon monoxide and NiO/γ-Al₂O₃ [6], FeS₂/γ-Al₂O₃ [14], and Al₂O₃/MoS₂ [15] for the reduction of SO₂ with hydrogen, have been investigated. The use of plasma for the catalytic reduction of sulfur dioxide has also been investigated [16, 17].

The advantage of CO and H₂ lies in their lower operating temperatures, although their production is notably expensive.

The use of CH₄ as a reducing agent has a drawback in its higher operating temperatures compared to H₂ and CO. However, CH₄’s affordability and accessibility make it an attractive option, particularly for nations endowed with substantial natural gas reserves, such as Iran, Russia, and others.

Catalysts for the reduction of SO₂ to elemental sulfur using CH₄ have included bauxite [18], alumina [19–21], metal oxides, and sulfides supported on alumina and activated carbon [22–26], along with transition metal sulfides [27], ferromanganese nodules [28], and cobalt oxide on different supports [29]. Cerium oxide has also demonstrated significant catalytic activity in this context [30–35]. The application of plasma for catalytic reduction of sulfur dioxide using FeS/Al2O3 has also been subject to investigation [17]. Although copper on ceria-based catalysts has exhibited reliable performance in CH₄-assisted SO₂ reduction, the industrialization of cerium is hindered by its high cost.

Therefore, alumina was selected as the catalyst support in this study, offering a well-established substrate. Moreover, its greater surface area, compared to ceria, is a pivotal factor in solid-gas reactions, rendering it a superior choice. Furthermore, our prior research has assessed the catalytic efficiency of nickel oxide, molybdenum oxide, and cobalt oxide catalysts supported on alumina [36–38].

In this investigation, Cu-Al₂O₃ catalysts with varying concentrations were synthesized and characterized for CH₄-assisted SO₂ reduction. Subsequent reactor tests were performed and compared under stoichiometric molar feed ratios and within a temperature range of 550–800°C to identify the most effective catalyst.

Finally, the study delved into the impact of feed ratio, space velocity, and long-term stability (crucial for industrial applications) on Cu-Al₂O₃ (10%), identified as the optimal catalyst.

2. Experimental Section

2.1. Catalyst Preparation

This study employed the wet impregnation technique for catalyst preparation [39, 40]. An aqueous solution of copper nitrate trihydrate (Cu(NO₃)₂.3H₂O, sourced from Merck) was employed as the copper precursor for impregnating onto a commercial γ-Al₂O₃ support with a particle size ranging from 2.5 to 3 mm. The well-impregnated catalyst was then allowed to stand for 1 hour, subjected to overnight drying at 120°C within an oven, and ultimately calcined at 550°C for a duration of 4 hours. The copper-alumina catalysts employed in this research were characterized by the loading of copper oxide onto γ-alumina, yielding copper weight percentages of 5%, 10%, and 15%. These catalysts are, respectively, denoted as Al₂O₃-Cu (5%), Al₂O₃-Cu (10%), and Al₂O₃-Cu (15%).

2.2. Catalyst Characterization

The BET-specific surface area, pore size distributions, and adsorption isotherms of the catalysts were assessed using the nitrogen adsorption method via the Autosorb-1-MP apparatus from Quantachrome, operating at 77 K. To determine phase composition, XRD patterns of the catalysts were captured using a diffractometer (PHILIPS-PW1730) operating at 40 kV and 30 mA, utilizing Cu Kα radiation (λ = 1.5406 A˚). The catalysts’ morphology was examined using field-emission scanning electron microscopy (FE-SEM) with a TE-SCAN instrument from the Czech Republic.

The acidic properties of the catalysts were gauged through temperature-programmed ammonia desorption (NH₃-TPD) analysis. To examine the catalyst’s characteristics in the form of a TPO (temperature-programmed oxidation) test, thermogravimetric analysis was performed using a Rheometric Scientific instrument, as explicitly mentioned in the relevant section.

2.3. Catalyst Performance Tests

The experiments were conducted in a fixed-bed stainless steel tubular reactor with loading 1 g of the catalyst in each reactor test. The flow diagram of the system is shown in Figure 1.

At first, the reactor is purged by an inert gas stream (gas 1). Then, the system is heated to reach the desired temperatures under a mixture of reaction gases. The reacting gas (gas 2) is a combination of CH₄, SO₂, and inert (argon) streams with predefined concentrations.

SO₂, CH₄, and argon inlet concentrations in the mixture were adjusted by three mass flow controllers. The reaction outlet was analyzed online by a mass spectrometer (MS) from Leda Mass.

After converting base peak heights to partial pressures, it is possible to plot mole fractions of up to 12 different gases versus time with ppm sensitivity by the mass spectrometer [41].

3. Results and Discussion

3.1. Catalyst Characterization

XRD patterns of the alumina and Al₂O₃-copper catalysts are shown in Figure 2.

The alumina peaks (number 1) and copper oxide peaks (number 2) are distinguished in Figure 2. According to Figure 2(a), the major peaks of alumina are obvious while, by adding copper oxide (Figure 2(b)–2(d)) to the catalyst, the height of alumina peaks drops and copper oxide peaks arises which is fair clear for Al₂O₃-Cu (15%) catalyst in Figure 2(d).

Table 1 shows the results of BET (Brunauer, Emmett, and Teller) surface area, total pore volume, and average pore diameter of γ-Al₂O₃ support and Al₂O₃-Cu catalysts. It was expected that the impregnation of CuO (as a promoter) on Al₂O₃ would decrease the surface area of the catalyst concurrent with its total pore volume due to the blockage of support pores.

Although the presence of CuO as an active species on the support pores would boost desired catalytic performances of the catalyst, N₂ adsorption isotherms of alumina and Al₂O₃-Cu synthesized catalysts are given in Figure 3. With a similar trend to Table 1, alumina support shows the highest N₂ adsorption while, with increasing the amount of CuO on the support, the adsorbed N₂ by the catalysts decreases.

Specifically, through CuO impregnation of the Al₂O₃ support, copper precursor molecules penetrate into the support pores reducing the surface area, pore volume, and consequently N₂ adsorption ability. It should be noticed that smaller pores would be filled first which causes an ascending trend along with increasing CuO weight percent for the average pore diameter of the prepared catalysts according to Figure 4 and data given in Table 1. BJH pore size distributions of various catalysts are shown in Figure 4.

3.2. Catalysts Activity Tests

The principal reaction for SO₂ reduction by CH₄ can be represented as follows:

The main side reaction that may occur between SO₂ and CH₄ is as follows:

While the first reaction produces a suitable sulfur product, the second reaction produces toxic H₂S and CO gases. SO₂ conversion was calculated from inlet and outlet SO₂ volume fractions through the following equation: and are the volumetric velocity of SO₂ at the reactor inlet and outlet, respectively, while sulfur selectivity was estimated by the molar difference of the sum of all sulfur-containing products (including H₂S, COS, CS₂, and unreacted SO₂) from the reacted share of SO₂. Figure 5 shows SO₂ conversion plots for the prepared catalysts against operating temperatures.

At 550°C, the SO₂ conversation rate is very low. When the temperature increases, the SO₂ conversion rate extremely increases for all catalysts. This proves a strong dependency of the reaction to the temperature.

At temperatures higher than 550°C, the increasing rate of SO₂ conversion for Al₂O₃-Cu (5%) and Al₂O₃ is lower than Al₂O₃-Cu (10%) and Al₂O₃-Cu (15%). At 600 and 650°C, Al₂O₃-Cu (10%) performance is slightly better, but at higher temperatures, both Al₂O₃-Cu (10%) and Al₂O₃-Cu (15%) catalysts show a similar performance.

Therefore, adding more than 10 percent of copper not only has no beneficial effect but also could increase the operating costs.

It is worth noting that all the Al₂O₃-Cu catalysts presented a much better performance than alumina support in the temperature range of 550–800°C indicating CuO performance as a suitable active species for this process.

The partial pressure curves of H₂S, produced from reactions for different catalysts, are compared in Figure 6.

For all the catalysts, by increasing the temperature, the amount of produced H₂S decreases which is in contrast to the SO₂ conversion ascending trend. Seemingly, at lower temperatures, the conversion is incomplete, and there is a large share of unreacted CH₄ and SO₂. Subsequently, this unreacted CH₄ can be decomposed according to the following reaction:

Thereafter, it is likely that produced H₂ could react catalytically with unreacted SO₂ to form H₂S and water via the following reaction:

Given that no significant amount of hydrogen is produced and that H₂S decreases with increasing conversion rate, this possibility is confirmed [24]. It is noteworthy that, even at 550°C with maximum produced H₂S, its share is less than 0.35% of the outlet flow.

For temperatures over 700°C, the H₂S produced for Al₂O₃-Cu (5%) is higher than that of alumina, while the amount of unreacted SO₂ is more significant for alumina. This can be due to the more active catalytic behavior of Al₂O₃-Cu (5%) than alumina for methane decomposition (reaction (4)), and the reaction of produced H₂ with SO₂ (reaction (5)) forming more H₂S.

COS partial pressure profiles from the reaction versus temperature are illustrated in Figure 7 for different catalysts.

The amount of produced COS for Al₂O₃-Cu catalysts at temperatures lower than 700°C is less than that of alumina while at higher temperatures is in contrast. This may be due to the fact that the amount of CS₂ production increases [33] with increasing temperature and its further reaction with CO₂ to produce COS, indicated in reaction (6). No CS₂ was detected at the reactor outlet during the experiments.

Additionally, according to Figure 7, it could be seen that despite larger values of produced COS for Al₂O₃-Cu (5%) than the others, Al₂O₃-Cu (10%) and Al₂O₃-Cu (15%) catalysts present similar performances with much lower toxic COS. Generally, the important thing is that the total amount of undesirable H₂S and COS is negligible resulting in an outstanding catalyst selectivity of more than 99.5% for Al₂O₃-Cu (10%) which was chosen as the best catalyst. The temperature-programmed desorption plots of ammonia (NH₃-TPD) for Al₂O₃ and Al₂O₃-Cu (10%) are illustrated in Figure 8 with related results of weak, moderate, and strong acid sites as given in Table 2.

Accordingly, the density of moderate and weak acid sites was increased significantly by adding CuO to alumina support, and alumina’s weak acid sites were converted into very weak acid sites (first peak) and weak acid sites (second peak) [42–44].

The density of strong acid sites is decreased because of replacing CuO with Brønsted strong acid sites. In other words, CuO has weak and moderate acidity leading to an increase in weak and moderate acid sites after support modification with CuO [42–44] to achieve a more active catalyst for selective catalytic reduction of SO₂ with CH₄.

The FE-SEM images shown in Figure 9 illustrate the placement of copper nanoparticles on the surface and pores of Al₂O₃-Cu (10%) catalysts fairly well. This causes the reduction of pores and the specific surface area of the Al₂O₃-Cu (10%) than the Al₂O₃ (Table 1). However, these copper nanoparticles create very active sites for reaction.

(a)

(b)

3.3. Effects of Feed Gas Composition

The effect of changing the SO₂/CH₄ molar ratio on SO₂ conversion and H₂S-COS production is shown in Figure 10.

It can be seen that high conversion values were attained for SO₂/CH₄ molar ratios below 2 when CH₄ is in excess and larger than the stoichiometric ratio. Substantially, entering SO₂ more than the stoichiometric ratio required for reaction (1) leads to a conversion drop and a remarkable unreacted SO₂, same as the case with SO₂/CH₄ ratio of 3 with a reported SO₂ conversion of 71%.

However, when the SO₂/CH₄ ratio is less than the stoichiometric ratio (excess methane), H₂S and COS production greatly increase as a result of reacting SO₂ with CH₄ through reaction (2) which is now more favorable than reaction (1). Moreover, the excess methane could be decomposed through reaction (4) to produce further H₂ with its share via reaction (2). Total produced H₂ could react with SO₂ to form H₂S according to reaction (5).

In addition, CH₄ can react with S₂ to produce CS₂ through reaction (8). Then, the produced CS₂ reacts with H₂O and produces H₂S and COS according to reaction (9). In the next step, COS reacts with water (product of reaction (1)) in reaction (10) and produces more H₂S. The drastic increase in H₂ at low ratios of SO₂/CH₄ confirms the possibility.

For COS, except the mentioned mechanism, the generated CS₂ according to reaction (8) could produce COS through reaction (6). Hence, the amount of COS greatly increases with decreasing SO₂/CH₄ ratio.

3.4. Effects of Space Velocity

The effect of space velocity is shown in Figure 11.

When space velocity increased from 3000 to 12000 ml/h, SO₂ conversion decreased from 99.8% to 70%. H₂S and COS production showed no significant change, and finally, selectivity in the same value remained high. The sharp decline in conversion can be due to reducing contact time between the reactants and the catalyst while the dependency of sulfur selectivity is independent of the space velocity increment.

3.5. Stability of Catalyst

The stability of Al₂O₃-Cu (10%) as the best catalyst was tested at 750°C for 20 hours. As illustrated in Figure 12, in the first 9 hours, the conversion rate was constant. In this initial period, the amount of H₂S by-product was almost constant. Afterward, the conversion rate started to diminish.

The SO₂ conversion rate experienced only a slight 3% decrease between 9 and 12 hours, while, for durations exceeding 13 hours, the conversion rate remained relatively constant.

In this regard, SO₂ conversion reduction is consistent with H₂S production ascending trend, as a result of increasing unreacted CH₄ which could be decomposed to H₂ with further reactions according to the scenario discussed earlier. Therefore, Al₂O₃-Cu (10%) showed a good stability for SO₂ reduction with CH₄.

The main threat at high temperatures for catalyst deactivation is coke production which can be produced by CH₄ decomposition according to reaction (4). However, in the main reaction per each consumed mole of SO2, two moles of water vapor are produced that could prevent coke deposition through reaction (10) as an additional advantage.

To investigate Al₂O₃-Cu (10%) changes during the reactor test, the catalyst after 40 hours of reactor lifetime test was analyzed by thermogravimetric (TG) analysis in an air environment by connecting its outflow to the mass spectrometer (MS) device for a better understanding of the phenomena.

The changes in the catalyst weight and SO₂ MS peaks obtained from TG-MS analysis are indicated in Figure 13. For this test, the temperature was increased to 1000°C with a constant rate and then for a few minutes was maintained isothermally at 1000°C.

According to Figure 13, it can be seen at first some weight loss appeared due to the loss of the adsorbed water by the catalyst. At temperatures about 300°C, the catalyst weight increased rapidly while at the temperature range of 400–700°C decreased slowly, and finally at temperatures more than 700°C decreased again sharply.

It could be deduced that during the reactor lifetime test, CuO is sulfided and converted into copper sulfide [45, 46]. This copper sulfide would be sulfated in temperatures more than 300°C forming copper sulfate and causing the catalyst weight gain at these temperatures. Thereafter, at temperatures more than 700°C, the copper sulfate is decomposed to copper oxide and the catalyst weight decreases [45, 46].

The sharp increase for SO₂ in output at 700°C was fully confirmed by copper sulfate decomposition. It should be noted that a small increase in SO₂ and a small decrease in the catalyst weight at a temperature range of 400–700°C could be due to the oxidization of sulfur product trapped in the catalyst cavities.

The TG-MS analysis shows the sulfidation of CuO changes the catalytic performance of Al₂O₃-Cu (10%) over time which results in decreasing SO₂ conversion and increasing improper by-product (H₂S). These results also show that CuO is more suitable than copper sulfide for the selective catalytic reduction of SO₂ with CH₄.

In general, Al₂O₃-Cu (10%) showed a reliable performance for this process and with further modifications against sulfidation could be more applicable in terms of industrialization.

3.6. Determining Activation Energies

Activation energies are determined by assuming Arrhenius’s dependency on the temperature for the reaction constants [47].

While k = 8.6173324 × 10⁵ eV/K is the Boltzmann constant, T is the absolute temperature and A_i is preexponential factor. Arrhenius plot obtained from SO₂ conversion values for Al₂O₃-Cu (10%) is given in Figure 14.

According to Figure 14, Al₂O₃-Cu (10%) activation energy equals 0.2690 eV in the temperature range of 550–800°C, while it was calculated 0.33 eV for Al₂O₃-Mo (%10) catalyst [37] and 0.228 eV for Al₂O₃-Co (%15) [38] catalyst in a same operating condition. However, with respect to the work by Guiance et al. [47], the activation energy for the Al2O3-Cr2O3 catalyst is calculated at 0.43 eV at temperatures between 25°C and 45°C.

4. Conclusion

The current investigation focused on the selective catalytic reduction of SO₂ using CH₄ over CuO alumina-based catalysts. The catalysts were prepared using the wet impregnation technique. Four distinct catalysts were synthesized, featuring copper loadings of 0%, 5%, 10%, and 15% by weight on γ-Al₂O₃ support. These catalysts were subjected to testing within a fixed-bed pilot reactor. Notably, all CuO-containing catalysts outperformed the pure alumina base. Among the four catalysts, Al₂O₃-Cu (10%) demonstrated the most favorable outcomes in terms of catalytic activity and sulfur selectivity. Interestingly, the inclusion of additional CuO did not impart any discernible advantageous effects.

The optimal catalyst showcased remarkable performance, achieving a remarkable SO₂ conversion of 99.9% coupled with a sulfur selectivity exceeding 99.5% at a temperature of 750°C. Furthermore, a meticulous assessment of the influence of the SO₂/CH₄ molar feed ratio revealed that the peak catalytic efficiency was achieved at the stoichiometric ratio pertinent to the primary reaction (reaction (1)). The minimal presence of undesirable by-products such as H₂S and COS indicated the prominence of reaction (1) as the predominant reaction pathway. However, over the course of the lifetime test, a reduction in catalytic activity by approximately 30% was observed due to the conversion of CuO into copper sulfide species, a phenomenon unveiled through TG-MS analysis. Ultimately, the activation energy for Al₂O₃-Cu (10%) was computed to be 0.1355 eV within the temperature spectrum spanning 550–800°C.

Data Availability

All the data and results have been presented in the manuscript and supplementary documents, and they can be published as a public report.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Copyright © 2023 Seyyed Ebrahim Mousavi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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