Effect of amino acid additives in ammonia solution on SO2 absorption and ammonia escape using bubbling reactor for membrane contactor applications

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Abstract

Controlling the SO2 emissions is crucial because they result in several environmental problems and affect human health. Several techniques have been proposed for controlling SO2 emissions. Limestone or lime-based absorbents are widely used for SO2 removal. However, their low solubility limits their applications. Ammonia solution has attracted attention owing to its higher SO2 removal efficiency. However, owing to its high volatility, it has the propensity to escape, which can cause secondary environmental pollution. Therefore, we investigated the effect of six amino acid additives to improve the SO2 absorption performance and inhibit the ammonia escape in aqueous ammonia solution. The surface tension and theoretical breakthrough pressure of the amino acid-containing ammonia solutions was investigated to evaluate their applicability to a membrane contactor process. L-histidine (His) exhibited the best performance. Therefore, the effect of His concentration, inlet SO2 concentration, and absorption temperature on SO2 absorption performance was explored. The 13C nuclear magnetic resonance (NMR) and 1H NMR analyses were used to study the SO2 absorption mechanism. This study provides a strategy for preparing an eco-friendly and highly efficient SO2 absorbent that can overcome the disadvantage of current ammonia solutions for the removal of low-concentration SO2 from industrial gas emissions for the membrane contactor process application.

Introduction

Owing to the increased use of fossil fuels due to rapid industrialization and population growth, 111 million tons of sulfur dioxide is emitted into the atmosphere annually (Zhong et al., 2020). The emitted SO2 results in serious environmental problems, such as acid rain and smog, and is converted into particulate matter (PM2.5) through a photochemical reaction in the atmosphere, causing respiratory diseases in humans (Huang et al., 2014; van Thriel et al., 2010). Furthermore, SO2 is included as a class 3 carcinogen in the list published by the International Agency for Research on Cancer of the World Health Organization (Li et al., 2022). Therefore, most countries strictly control the SO2 emissions from industrial processes with regulations becoming increasingly stringent. For example, in 2020, the standard for sulfur content in fuel of marine vessels was lowered from 3.5 % to less than 0.5 %, while the International Maritime Organization decreed that flue gas desulfurization (FGD) facilities should be installed on ships (Van et al., 2019).

The use of FGD technologies is a practical approach for controlling SO2 emissions. The FGD can be classified into three types: dry, semi-dry, and wet FGD (WFGD). In particular, WFGD has been extensively used owing to its high efficiency and reliability in industrial processes, such as coal-fired power plant (Zhao et al., 2021). Limestone or lime-based absorbents are the most used in WFGD for SO2 removal, with a market share of 70 % (Chu et al., 2014). However, the solubility of limestone in water is low, which can compromise its SO2 removal efficiency. Further, precipitates, such as gypsum, must be removed to avoid pipeline blockage, requiring additional processing. Limestone can also cause problems of dew-point corrosion (Chu et al., 2014).

Ammonia solution has attracted attention because it has a higher SO2 removal efficiency than typical alkaline absorbents, such as NaOH and NaHCO3. Furthermore, it is inexpensive, and the byproduct (ammonium sulfate) can be used as a fertilizer (Park et al., 2021, Yang et al., 2016). However, owing to the high volatility of ammonia, it has the propensity to escape, leading to a loss of the absorbent and reducing the desulfurization efficiency. This makes the use of ammonia solution less favorable. Further, ammonia released into the atmosphere can cause secondary environmental pollution. It forms a solid ammonia salt through a gas phase reaction with SO2, which can lead to fouling in the pipes and valves (Ma et al., 2013), requiring additional water and energy to remove ammonia via washing or condensation processes. Therefore, it is important to study the escape of ammonia in SO2 capture using ammonia solution.

Several methods to control the ammonia escape have been proposed. Alstom proposed a chilled ammonia CO2 capture method that effectively controlled the ammonia escape; however, its high cost and energy consumption hinder its commercialization (Peltier, 2008). Alternatively, organic additives have also been proposed. Ethylene glycol has been used as an additive, which reduced the inhibiting ammonia escape rate by 35 % in CO2 capture; however, it did not improve the CO2 absorption performance (Shuangchen et al., 2013). Although the use of ethanol or hindered amines as additives has a positive effect on the inhibition of ammonia escape, they are vulnerable to oxidative degradation (Shuangchen et al., 2015, Yang et al., 2016). Alternatively, some researchers have studied ammonia escape inhibition based on metal ions such as Zn(II), Cu(II), Ni(II). However, in addition to their high material cost, undesirable scaling products, such as ZnSO3, can interfere with the absorption performance (Li et al., 2014, Mani et al., 2008).

In contrast, there are only a few studies using amino acid as additives to prevent the ammonia escape. Amino acids exhibit low volatility, resistance to oxidative degradation, good reactivity with SO2, and are environmentally friendly, making them viable candidates over conventional additives to prevent the ammonia escape and improve the SO2 absorption performance. He et al. reported a new high-quality bio-organic fertilizer containing amino acids, which improved soil composition and quality, and promoted plant growth (He et al., 2019). Therefore, the use of the byproducts generated in the process of using ammonia solution-containing amino acids as an absorbent as fertilizers can provide better value over conventional fertilizers. Further, the high surface tension of the amino acids is suitable for acid absorption in membrane contactor applications. The wetting phenomenon is a disadvantage for membrane contactors, the effect of which can be reduced owing to the high surface tension of the amino acids, allowing for long-term operation.

In this study, we evaluated the effect of six amino acid additives on the SO2 absorption performance and ammonia escape in aqueous ammonia solution. Glycine, alanine, β-alanine, serine, histidine, and arginine have been mainly used in the capture of acid gases (CO2 or SO2) because of their good reactivity (Deng et al., 2012); therefore, they were selected for the current study and their performances were compared in terms of duration time of high efficiency and absorption loadings. The absorption experiments were conducted at 298.15 K with 2000 ppmv SO2 and 14 % CO2 (N2 balance) gases, and equilibrium data regarding the reactivity of SO2 with the ammonia solution-containing amino acids were collected. Furthermore, the effect of the amino acids on inhibiting the ammonia escape was evaluated using an ammonia analyzer. In addition, the surface tension and theoretical breakthrough pressure of the ammonia solution-containing amino acids was investigated to determine a suitable additive for reducing the wetting phenomenon of a membrane contactor. Based on the results obtained, SO2 performance of the ammonia solution with amino acid was investigated by varying the operating conditions. Thus, we intend to develop an eco-friendly and highly efficient SO2 absorbent that can overcome the disadvantages of current ammonia solutions for the removal of low-concentration SO2 from industrial gas emissions for membrane contactor process applications.

Section snippets

Materials

Ammonia solution (NH4OH, 28 % NH3 in H2O) was provided by FUJIFILM Wako Pure Chemical Co., (FUJIFILM Wako, Japan). Glycine (Gly, 99 %), L-alanine (Ala, 99 %), β-alanine (β-Ala, 99 %), dl-serine (Ser, 98 %), l-histidine (His, 99 %), and l-(+)-arginine (Arg, 99 %) were all purchased from Tokyo Chemical Industry (TCI) Co., Ltd. (Tokyo, Japan). Ultrapure distilled water (DI water) was prepared using a purification system (Human Science Co., Ltd., Korea). The N2 (99.99 %), SO2 (1 %, N2 balance), and

Effect of amino acids additives on the SO2 absorption performance

The additives that inhibit ammonia escape should be able to improve the SO2 absorption performance in order to respond to the strengthened SO2 emission regulation. Therefore, it is important to compare the SO2 absorption performances according to the use of amino acids additives in aqueous ammonia solution. In this study, the SO2 absorption performances were compared using the duration time of high efficiency (SO2 removal efficiency above 95 %) and SO2 absorption loading on the amino

Conclusions

In this study, six amino acids were investigated as additives in aqueous ammonia solution to enhance the SO2 absorption performance and inhibit the ammonia escape. Although all the additives improved the SO2 absorption performance (duration time of high efficiency and SO2 absorption loading), His and Arg showed superior performances compared to amino acids with a single amino group. In addition, the rate of ammonia escape inhibition in the presence of the amino acids was at least 44 % or

CRediT authorship contribution statement

Kwanghwi Kim: Investigation, Data curation, Formal analysis, Writing – original draft. Hyunji Lim: Conceptualization. Hyun Sic Park: Methodology. Jo Hong Kang: Format analysis. Jinwon Park: Conceptualization. Hojun Song: Writing – review & editing, Supervision, Funding acquisition.

Funding sources

This study was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Korean government (MOTIE) (Project No. 20005884).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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