Simultaneous Removal of SO2 and NOx from Coal-fired Flue Gas

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Simultaneous Removal of SO and NOx from Coal-fired Flue Gas Using Steel Slag Slurry Ziheng Meng, Chenye Wang, Xingrui Wang, Yan Chen, and Huiquan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03385 • Publication Date (Web): 01 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Simultaneous Removal of SO2 and NOx from Coal-fired Flue Gas Using Steel Slag Slurry Ziheng Meng,†,‡ Chenye Wang,† Xingrui Wang,† Yan Chen,† and Huiquan Li*,†,‡ †

National Engineering Laboratory for Hydrometallurgical Cleaner Production

Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: coke oven flue gas, SO2 removal, NOx removal, steel slag, reaction mechanism ABSTRACT: A method for simultaneously removing sulfur dioxide (SO2) and nitrogen oxides (NOx) from coke oven flue gas using steel slag slurry was proposed. Due to the high removal efficiency of SO2, the effects of operation conditions on the removal of NOx were investigated emphatically. The results showed that the removal efficiency of NOx increased with an increase in steel slag slurry concentration, NOx oxidation ratio, steel slag slurry pH, and NOx and SO2 inlet concentrations, and a decrease in reaction temperature. The removal efficiencies of SO2 and NOx could reach 100% and 83.4%, respectively, by optimising the operation conditions. The mechanism of NOx removal using steel slag slurry was investigated. The results showed that (MgO)0.841·(MnO)0.159 (RO phase, a CaO–FeO–MnO–MgO solid solution) in steel slag was decomposed by H+ and produced Mg2+ and Mn2+. Furthermore, Mn2+ in steel slag slurry could promote the absorption of NO2 via redox reactions with the generation of solid Mn3O4 and MnO(OH).

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INTRODUCTION Fossil fuel combustion yields large volumes of flue gas with various types of air

pollutants, including nitrogen oxides (NOx) and sulfur dioxide (SO2), which produce acid rain, regional haze, and risk to human health.1, 2 Therefore, air pollutants should be urgently controlled in China. For example, the emissions of coke oven flue gas yielded by coke oven gas combustion were approximately 40,000 m3·h–1 per billion tons of coke, with flue gas concentrations of 50–600 mg·Nm–3 for SO2 and 450–1200 mg·Nm–3 for NOx.3, 4 Various technologies have been developed to treat air pollutants. Wet flue gas desulfurization (WFGD) is the dominant technology for SO2 reduction. Meanwhile, selective catalytic reduction (SCR) is a mature and efficient method for denitrification from coal-fired power plants with a flue gas temperature of over 300 °C.5 However, drawbacks, such as a narrow temperature range and ammonia escape, remain. For example, coke oven flue gas exhibits a low temperature (≤200 °C) after waste heat recovery,6 thereby making SCR difficult to apply. The main component of NOx in coke oven flue gas is NO.2 The wet scrubbing method is unsuitable for directly treating NOx in coke oven flue gas because of low solubility of NO. However, if NO is oxidized into more soluble NO2, then it can be easily and simultaneously removed with SO2 in WFGD equipment. Many chemical reagents, including oxidant (NaClO2, KMnO4, and H2O2),7–9 reductant (Na2SO3, urea, and Na2S),10–12 and chelating agents (FeSO4 and Fe(II)EDTA),13, 14 have been added into aqueous solutions to simultaneously removing SO2 and NOx. Recently, researchers15–22 have introduced a


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promising route for simultaneous desulfurization and denitrification by combining the gas-phase oxidation of NO and alkaline solution/slurry with or without additive absorption. Possible gas-phase oxidizers include ozone (O3),15–19 chlorine dioxide (ClO2),23 humidified/vaporized oxidant,24–27 and others.28, 29 The gas-phase oxidation of ozone is considered an appropriate method for NO oxidization because of its selectivity, high oxidization efficiency, and green reduction products.21, 30 However, ozone generation is costly, and NO cannot be completely oxidized into NO2. Thus, the simultaneous removal of NO and NO2 may be a feasible means to reduce the cost of NOx control. To date, many absorbents, such as alkaline solution/slurry,16, 17, 31, 32 and alkaline solution/slurry with reducing11, 15, 20, 33 or oxidizing agents,19, 21 have been investigated to remove NO/NO2 mixtures. Zhao et al.32 studied the simultaneous removal of SO2, NO2, and NO by using sodium humate (HA–Na). Under optimal reaction conditions, simultaneous removal efficiencies of 100%, 99.6%, and 65% for SO2, NO2, and NO, respectively, were achieved. Zhou et al.20 proposed a process of ozone oxidation and an alkaline countercurrent packed scrubber to treat NO, NOx, and SO2 in the exhaust gas of marine diesel engines. Simultaneous removal efficiencies of 90%, 93%, and 100%, respectively, were obtained under the following conditions: NaOH concentration (5 wt.%), pH (8), stoichiometric ratio of ozone to NO (0.6), and additive concentration of CO(NH2)2 (1 wt.%). The addition of reducing or oxidizing agents to enhance NOx absorption has a high cost and generates complex by-product. Therefore, an adaptable absorbent with high NOx and SO2 removal efficiencies and low cost should to be developed.


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Researchers have used industrial solid wastes, such as fly ash,34 and red mud,35 as desulfurization absorbents because of their nearly free cost and solid waste utilization. However, these absorbents exhibit desulfurization efficiencies below 87% and cannot perform simultaneous removal of NOx. Steel slag is an industrial solid waste that is produced in the steel-making process;36 the amount of steel slag generated annually is approximately 100 million tons.37 However, its utilization is only approximately 22% in China.38 The accumulation and storage of steel slag lead to environmental pollution, occupation of farmlands, and waste of resources.38 Therefore, the comprehensive utilization of steel slag should be improved. Steel slag can react with water and generate gel and OH− because of its cementitious properties. In addition, free CaO and free MgO in steel slag also exhibit alkalinity.39 Therefore, steel slag can be used as an absorbent in desulfurization and denitrification. Furthermore, steel slag, which is an industrial waste, is nearly free of charge when used as an absorbent for removing NOx and SO2 compared with other absorbents. Researchers40–42 have investigated the desulfurization performance of steel slag in WFGD systems, and industrial-scale WFGD devices that use steel slag as an absorbent for flue gas from the sintering process have been operating smoothly. However, the simultaneous removal of SO2 and NOx using steel slag as an absorbent has not yet been reported. Thus, we developed a method for simultaneously removing SO2 and NOx based on existing desulfurization devices with steel slag slurry. In this study, a combined process of gas-phase oxidation and wet scrubbing was proposed for the simultaneous removal of SO2 and NOx using steel slag slurry. First, NO


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was partially oxidized into NO2 after gas-phase oxidation. The oxidation ratio of NOx (OR) was controlled by changing the concentrations of NO and NO2 in the simulated flue gas. Compared with NOx, SO2 can be easily removed. Hence, we emphasized investigating the effects of various factors, such as the concentration of steel slag slurry, OR, reaction temperature, pH of steel slag slurry, and inlet concentrations of NOx and SO2, on the removal of NOx. The optimal conditions for the simultaneous removal of SO2 and NOx were determined. The possible reaction mechanism of NOx removal using steel slag slurry was also proposed. 2.

EXPERIMENTAL SECTION 2.1. Raw Materials and Instruments. The steel slag samples used in this study were

collected from the basic oxygen furnace of a steel plant in Tangshan, China. The median size of the samples was 74µm. The samples were characterized by X-ray fluorescence (XRF) spectroscopy and X-ray diffraction (XRD) spectroscopy. The chemical composition was analyzed via XRF spectroscopy. The results, which are listed in Table 1, show that the main chemical components of raw steel slag are CaO, SiO2, Fe2O3, Al2O3, MgO, SO3, and MnO. Meanwhile, the XRD patterns, shown in Figure 1, indicate that the main crystallized mineral phases of raw steel slag are 2CaO·SiO2 (C2S), 3CaO·SiO2 (C3S), CaCO3, Ca(OH)2, (MgO)0.841·(MnO)0.159 (RO phase), free-MgO, SiO2, Fe2O3, Fe3O4, 2CaO·Fe2O3, and Ca3Mg(SiO4)2. All the chemicals used in the experiments were of analytical grade, and deionized water was used. Standard gases include N2 (99.999%), NO/N2 (4427 ppmv (parts per million by volume, ppmv)), NO2/N2 (4477 ppmv), SO2/N2


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(4978 ppmv), and O2 (99.999%) span gas (Haipu Beifen Gas Industrial Co.). Note: 1 ppmv equal to 1.2479 mg·m–3 for NO, 1.9135 mg·m–3 for NO2, and 2.6622 mg·m–3 for SO2 at ambient temperature and pressure (20 °C and 101.325 kPa). The chemical compositions and mineral phases of the solid samples were analyzed using an XRF spectrometer (AXIOS, PANalytical Instruments) and an XRD spectrometer (Empyrean, PANalytical Instruments), respectively. The metal contents of the liquid samples were determined using an inductively coupled plasma-optical emission spectroscopy (ICP-OES) spectrometer (iCAP 6300, Thermo Scientific Inc.) The concentrations of NOଶି and NOି ଷ in the liquid samples were characterized with an ion chromatography (IC) (Dionex ICS-5000+, Thermo Fisher Scientific Inc.). The pH of the solution or slurry was determined with a pH meter (FiveEasy FE20, Shanghai Toledo Instrument Co., Ltd.). The NO, NO2, and SO2 concentrations in the simulated flue gas were measured using a Fourier transform infrared (FTIR) spectrometer (18) (Bruker Tensor 27, Bruker Optik Inc.) with a 2.4 m gas cell. The detection wavenumber range of the FTIR spectrometer was from 600 cm−1 to 4000 cm−1 at a resolution of 4 cm−1. 2.2. Preparation of Absorbent. A certain concentration of steel slag slurry was first stirred for 10 h, and the obtained slurry was alkaline. However, the operational pH of absorbents in the industrial scale is weak acid.43 To approximate an actual absorbent in industrial-scale desulfurization processes with steel slag slurry in terms of absorbent properties, such as pH, main compositions, solid contents, and viscosity, the pH of steel slag slurry was adjusted to a certain value by adding 27 wt.% H2SO4 dropwise into raw


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steel slag slurry.11, 44 Afterward, steel slag slurry was obtained at a certain pH. The acid steel slag slurry (pH = 5.5−6.0, the range of pH varies within 30 min) was filtered and dried at 105 °C for 12 h, and then acidized steel slag was obtained. The chemical compositions and mineral phases of the acidized steel slag are provided in Table 1 and Figure 1, respectively. The main chemical components of the acidized steel slag are CaO, SiO2, Fe2O3, Al2O3, MgO, SO3, and MnO. The mineral phases of the acidized steel slag are CaSO4·0.5H2O, SiO2, and Fe3O4. The analyses of the involved reactions during the adjustment process of the pH of steel slag slurry are presented in the Supporting Information. The forms of Ca, Mg, Mn, and Si in the liquid phase are Ca2+, Mg2+, Mn2+, and H4SiO4, respectively. The concentrations of Ca2+, Mg2+, Mn2+, and Si in the acid steel slag slurry are provided in Table 2. 2.3. Experimental Apparatus. The experimental system used for simultaneously removing NOx and SO2 is shown in Figure 2. This system consists of a simulated flue gas generation system, a bubbler reactor, and a simulated flue gas analysis system. The simulated flue gas was supplied by five standard gas cylinders (1–5), metered by mass flow controllers (7) (D07-7K, Beijing Sevenstar Electronics Co., Ltd), and mixed in a gas mixing tank (8), in which all gases were diluted by N2 to the desired concentrations. Then, the simulated flue gas came in contact with 300 mL steel slag slurry through an aerator in the bubbler reactor. The total gas flow rate of simulated flue gas was 1000 mL·min–1. Moreover, the concentration ranges of NO, NO2, SO2, and O2 in the simulated flue gas were 20–150 ppmv, 80–392 ppmv, 0–300 ppmv, and 0–5 vol.%, respectively.


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The bubbler reactor (14) is made of glass and has a height of 100 mm and an inner diameter of 80 mm. Two syringes (15) on the bubbler reactor were used to sample or add dilute CaCO3 slurry and dilute H2SO4 solution to the absorbent. A hot plate magnetic stirrer (10) (C-MAG HS-7, IKA Works Inc.) was used to control the temperature of steel slag slurry. The pH of steel slag slurry was detected with a pH meter (13). The pH levels of various absorbents during the absorption process were controlled by the dilute CaCO3 slurry and the dilute H2SO4 solution. The operating conditions for all the experiments are listed in the Supporting Information. The NO, NO2, and SO2 concentrations in the simulated flue gas at the inlet and in the exhaust gas from the bubbler reactor, which removed water vapor by using a condenser (16) and a drying tube (17) filled with anhydrone particle (Mg(ClO4)2), were measured via FTIR spectroscopy. Then, the off-gas was absorbed by a KMnO4/H2SO4 solution. The concentrations of NOଶି and NOି ଷ in steel slag slurry were characterized via IC. The metal contents (Ca, Mg, Mn, Al, and Si) of the liquid samples and steel slag were determined via ICP-OES and XRF spectroscopy, respectively. The phase compositions of the samples were examined through XRD spectroscopy. The oxidation ratio of NOx was defined as eq 1: OR =

CNO2

(1)

CNO + CNO2

where CNO and C NO

2

are NO and NO2 inlet concentrations (ppmv), respectively.

The removal efficiencies of the air pollutants were obtained at different conditions


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after 20 min at steady state, which were calculated by eq 2: η=

Cin − C out × 100 % Cin

(2)

where η is the removal efficiency (%) of the target air pollutant (NO, NO2, or SO2), and Cin and Cout are the inlet and outlet concentrations of the target air pollutants (ppmv), respectively. 3.

RESULTS AND DISCUSSION 3.1. Effects of Various Factors on NOx Removal Efficiency. 3.1.1. Effect of Reaction

Time. Figure 3 shows the effect of reaction time on NOx removal efficiency. The removal efficiencies of NO, NO2, and NOx maintained a steady state after 20 min. The removal efficiencies of NO, NO2, and NOx are 77.5%, 82.7%, and 81.6%, respectively. Therefore, a reaction time of 20 min was selected for the follow-up experiments. 3.1.2. Effect of Reaction Temperature. Figure 4 shows the effect of reaction temperature on NOx removal efficiency. The removal efficiency of NOx decreased gradually from 86.7% to 57.9% with increasing reaction temperature. Meanwhile, NO removal efficiency dropped sharply when reaction temperature increased from 30 °C to 70 °C. A possible explanation for this result is the double effect of reaction temperature on NOx removal. During the absorption of NO and NO2, an increase in reaction temperature can accelerate the chemical reaction rate and the mass transfer rate of the reactants or products in the slurry. Simultaneously, however, when the reaction temperature increases, NO and NO2 solubilities decrease. Furthermore, the release of NO through the decomposition of absorption products (HNO2) (eqs 3−8) is enhanced in this process. Overall, the removal


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efficiency of NO decreased rapidly when the reaction temperature increased from 30 °C to 70 °C, thereby indicating that the latter factor plays a leading role in absorption reaction. Meanwhile, the removal efficiency of NO2 remained stable when the reaction temperature increased from 30 °C to 70 °C, thereby indicating that the aforementioned two factors play nearly the same role in absorption reaction. Notably, the removal efficiency of NO exhibited a negative value when the reaction temperature exceeded 50 °C. Furthermore, NO concentration at the outlet was higher than that at the inlet. This finding is attributed to the acceleration of the decomposition rate of HNO2 (eq 8) as temperature increases. Hence, an optimal reaction temperature of 40 °C was selected for the other experiments. NO + NOଶ ↔ Nଶ Oଷ

(3)

ା Nଶ Oଷ + Hଶ O → 2NOି ଶ + 2H

(4)

2NOଶ ↔ Nଶ Oସ

(5)

ି ା 2NOଶ + Hଶ O → NOି ଶ + NOଷ + 2H

(6)

ା Nଶ Oସ + Hଶ O → NOଶି + NOି ଷ + 2H

(7)

3HNOଶ ↔ 2NO + Hଶ O + NOଷି + H ା

(8)

3.1.3. Effect of Steel Slag Slurry Concentration. The effect of steel slag slurry concentration on NOx removal efficiency was investigated, and the results are shown in Figure 5. The removal efficiency of NOx increased from 40.3% to 81.9% when the concentration of steel slag increased from 0% to 15%. Meanwhile, a sharp increase from −39.5% to 78.4% for NO and a slight increase from 53.9% to 73.2% for NO2 were observed when steel slag concentration increased from 0% to 6%. Subsequently, NO


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removal efficiency remained stable, whereas NO2 removal efficiency increased gradually as steel slag concentration increased from 6% to 15%. As steel slag concentration increases, the viscosity of steel slag slurry also increases gradually. However, a high viscosity of slurry leads to poor fluidity and mixing performance of steel slag slurry. Therefore, the optimal steel slag concentration for NOx removal was determined as 15%. The removal efficiency of NO was negative when the concentration of steel slag was 0, thereby indicating that the addition of steel slag to the absorbent exerted a crucial effect on NO removal. When the concentration of steel slag increases, the content of suspended fine particles, which can provide a reactive surface, also increases, thereby improving NOx removal efficiency.45, 46 3.1.4. Effect of OR. The effect of OR on NOx removal efficiency is shown in Figure 6. NOx concentration at the inlet was maintained at 300 ppmv and OR was adjusted by changing the concentrations of NO2 and NO. Figure 6 indicates that NOx removal efficiency increased from 68.6% to 83.4% when OR increased from 0.5 to 1.0. Removal efficiencies increased sharply from 47.5% to 77.7% for NO but decreased gradually from 89.7% to 82.9% for NO2, as OR increased from 0.5 to 0.8. When OR was higher than 0.8, the removal efficiency of NOx remained stable. When OR was 1.0, which indicates that only NO2 is present in the simulated flue gas, NO2 removal efficiency was 84.6%. When OR is below 0.5, N2O3 is the main removal material, and an increase in OR can accelerate the removal of NO in the form of N2O3, as shown in eqs 3−4.31 Therefore, the removal efficiency of NO increased sharply when OR increased from 0.5 to 0.9. Then


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NO2 (or N2O4) becomes the main material in the absorption process (eqs 5−7) with an increase in OR. The absorption rate of NO2 is lower than that of N2O3, thereby leading to a decrease in NO2 removal efficiency44. The appropriate OR was selected as 0.8 considering the high cost of NO oxidation. This amount can reduce the consumption of gas-phase oxidation agent, thereby decreasing the cost of denitrification processes. 3.1.5. Effect of Steel Slag Slurry pH. Slurry pH can affect the removal of NOx and the stability of HNO2 in slurry. Therefore, the effect of steel slag slurry pH on NOx removal efficiency was investigated. Figure 7 shows that an elevated pH significantly promotes NO and NO2 removal. Efficiencies were significantly increased from 35.5% to 85.4% for NO within the pH range of 4.7−6 and from 72.3% to 82.7% for NO2 when slurry pH increased from 4.7 to 5.5. Afterward, both values remained constant. When the pH of steel slag slurry was lower than 5.0, a sharp decrease in NO removal occurred because the absorption products (HNO2) decomposed rapidly into NO from steel slag slurry in a strong acid liquid phase. Similarly results were reported by Sun et al.22 When pH increased from 4.7 to 5.5, the removal efficiency of NO2 also increased. The increase in pH of steel slag slurry will accelerate NO2 hydrolysis reaction (eq 6) rate and increase the NO2 removal efficiency. Hence, the optimal pH of steel slag slurry for the removal of NOx can be considered 5.5, which is consistent with the actual slurry pH condition of a typical Ca-based WFGD system. 3.1.6. Effect of Gas Flow Rate. As an important operating parameter of gas flow rate, the effect of gas flow rate on the removal of NOx was investigated. As shown in Figure 8,


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the removal efficiency of NOx decreased gradually from 88.0% to 66.9% with increasing gas flow rate. Meanwhile, a sharp decrease from 88.9% to 50.6% for NO and a slight decrease from 89.0% to 69.3% for NO2 were observed when gas flow rate increased from 600 mL·min–1 to 1600 mL·min–1. The results indicated that an increase in gas flow rate will result in low removal efficiencies of NO, NO2, and NOx because of a decrease in contact time of simulated flue gas and slurry. 3.1.7. Effect of NOx Inlet Concentration. As shown in Figure 9, the removal efficiency of NOx increased from 64.8% to 83.7% when NOx concentration increased from 100 ppmv to 490 ppmv. The removal efficiencies of NO and NO2 exhibited the same tendency when NOx concentration increased from 100 ppmv to 490 ppmv. This increase is attributed to the fact that the hydrolysis reaction of NO2 is pseudo-second with respect to NO2.47 Meanwhile, as NOx concentration increases, the removal of NO and NO2 can be accelerated because of an increase in N2O3 concentrate, in which the hydrolysis reaction of N2O3 is pseudo-first order with respect to N2O3.48 Overall, as NOx concentration increases, the removal efficiency of NOx also increases; this result is consistent with the findings of other literature.18, 49 The method proposed in this study exhibits a good removal effect for NOx within a wide NOx concentration range. 3.1.8. Effect of SO2 Inlet Concentration on NOx and SO2 Removal Efficiencies. Figure 10 shows the effects of SO2 concentration on the simultaneous removal efficiencies of NOx and SO2. The removal of SO2 is not affected by the variation in SO2 concentration and efficiency is constant at 100% within the SO2 concentration range of 100–300 ppmv. In


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addition, when SO2 concentration increased from 0 to 300 ppmv, NOx removal efficiency increased from 81.6% to 88.0%, thereby indicating that an increase in SO2 concentration promotes the removal efficiency of NOx. When SO2 concentration increased from 0 to 300 ppm, the removal efficiency of NO2 increased sharply from 82.7% to 95.4%. Nevertheless, when SO2 concentration increased from 100 ppmv to 300 ppmv, the removal efficiency of NO decreased from 77.5% to 58.7%. When SO2 is absorbed into steel slag slurry, SO2 will react with water and produce ଶି HSOି ଷ and a small amount of SOଷ (eqs 9−11) when the pH of slurry is controlled at

approximately 5.5. In addition, when SO2 is present in the simulated flue gas, Mg2+ in steel slag slurry can increase the amount of dissolved sulfite species in the form of neutral MgSO30 ion pairs, which can promote NO2 absorption (eqs 12−13).50 Furthermore, dissolved SO2 can react with NO2 (eqs 14−15), thereby accelerating NO2 absorption.51−53 Therefore, an increase in SO2 concentration will result in high NO2 removal efficiency. However, the removal efficiency of NO decreases as the concentration of SO2 increases because of the competitive reduction of NO2 by dissolving NO and SO2 (eqs 3−4, 14−15). The rate of reduction reaction between S(IV) and NO2 was faster than that of the hydrolysis reaction of N2O3. Thus, an increase in SO2 concentration will decrease the removal of NO.54 Overall, as SO2 concentration increases, the removal of NOx is enhanced. In summary, a high removal efficiency of NOx that exceeds 80% by steel slag slurry is obtained within a wide SO2 concentration range. SOଶ + Hଶ O ↔ Hଶ SOଷ

(9)


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Hଶ SOଷ ↔ H ା + HSOି ଷ

(10)

ଶି ା HSOି ଷ ↔ H + SOଷ

(11)

଴ Mg ଶା + SOଶି ଷ → MgSOଷ

(12)

ଶି ଶା MgSO଴ଷ + 2NOଶ + Hଶ O → 2NOି + 2H ା ଶ + SOସ + Mg

(13)

ି ଶି ା 2NOଶ + HSOି ଷ + Hଶ O → 2NOଶ + SOସ + 3H

(14)

ି ଶି ା 2NOଶ + SOଶି ଷ + Hଶ O → 2NOଶ + SOସ + 2H

(15)

3.2. Optimal Reaction Conditions. The aforementioned experimental results indicate that the optimal reaction conditions for NOx removal were established, in which the concentration of steel slag slurry was 15%, reaction temperature was 40 °C, the pH of steel slag slurry was 5.5, gas flow rate was 1000 mL·min−1, and OR was 0.8. To confirm the effectiveness of the simultaneous removal of SO2 and NOx, the simultaneous removal of SO2 and NOx by steel slag slurry under optimal reaction conditions was investigated. Figure 11 shows that the average removal efficiencies of SO2 and NOx are 100% and 83.4%, respectively. 3.3. Reaction Mechanism of NOx Removal by Steel Slag Slurry. To analyze the role of steel slag in the removal of NOx, the effects of various absorbents on NOx removal efficiency were investigated, as shown in Figure 12. There absorbents include the acid steel slag slurry (pH = 5.5−6.0, as mentioned in Section 2.2.), filtrate, and residue slurry. The filtrate was obtained after the filtration of acid steel slag slurry. Then, the remaining washed filter cake was mixed with a certain amount of deionized water to obtain 300 mL residue slurry.


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Figure 12 shows that the removal efficiency of the residue slurry is only 39.1%, which is lower compared with the NOx removal efficiencies of the acid steel slag slurry and the filtrate, which are 81.6% and 82.3%, respectively. The acid steel slag slurry and the filtrate play leading roles in the removal of NOx. Thus, the analyses of the acid steel slag slurry and the filtrate are discussed further in this article. As shown in Figure 13, the distribution percentages of nitrite and nitrate, which are the ି absorption products of NOx removal, are 12.96% for NOି ଷ , 87.04% for NOଶ by the acid

steel slag slurry, and 12.59% for NOଷି , 87.41% for NOଶି by the filtrate. The results showed that the acid steel slag slurry and the filtrate achieved similar distribution percentages of nitrite and nitrate. As shown in Figure 14, the XRD pattern of acidized steel slag changed slightly after the denitration experiments with 12 h absorption time. In addition, the ion concentrations in acid steel slag slurry and its filtrate before/after denitration experiments are shown in Table 3. The pH of the filtrate decreased gradually during the NOx absorption process. To maintain stability in pH, dilute CaCO3 slurry was added. Thus, Ca2+ concentration in the filtrate increased because of the added CaCO3 dissolved in the filtrate. Ca2+, Mg2+, Mn2+, and Si concentrations increased slightly for the acid steel slag slurry, and Mg2+, Mn2+, and Si concentrations decreased slightly for the filtrate after an absorption time of 12 h. The results showed that Ca, Mg, Mn, and Si were leached from steel slag (the reactions related to the leaching process are listed in the Supporting Information) during the NOx removal process. The role of steel slag in steel slag slurry for NOx removal may merely be as a buffer material to avoid decreases in


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slurry pH. Therefore, the main influencing factor on the removal of NOx is the ions in the liquid phase that are leached from steel slag rather than steel slag in the acid steel slag slurry. The filtrate can replace the acid steel slag slurry during the mechanism investigation of NOx absorption. To ensure the role of ions in the liquid during the denitration process, the long-period denitration experiments by the filtrate were investigated. Figures S2 and S3 indicate that the concentrations of the ions in the filtrate increase sharply for Ca2+ and then become constant because of the dissolution of CaCO3, decrease slightly for Mg2+ and Si, and decrease gradually with time for Mn2+. In particular, the effects of Mn2+ concentration in the filtrate on NOx removal efficiency are emphasized. Figure 15 shows the changing curve of denitration efficiency, Mn2+ concentration, and the ratio of Mn2+ concentration to denitration efficiency using the filtrate during the long-period denitration experiment. As time increases, NOx removal efficiency and Mn2+ concentration in the filtrate decrease simultaneously, but the ratio of Mn2+ concentration in the filtrate to the removal efficiency of NOx remains nearly stable at approximately 2.2. Thus, the removal efficiency for NOx may be affected by Mn2+ concentration in the filtrate. As shown in Figure 3, when the inlet concentration of NOx was 300 ppmv, OR was 0.8, steel slag slurry concentration was 15%, reaction temperature was 40 °C, pH was 5.5±0.1, and absorption time was 1 h, the removal efficiencies of NO, NO2, and NOx were 77.5%, 82.7%, and 81.6%, respectively. Theoretically, the distribution percentages of NOx absorption products in the acid steel slag slurry should be 69.0% for NOଶି and


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31.0% for NOି ଷ when the removal efficiencies were 77.5% for NO and 82.7% for NO2 based on eqs 3−7. However, the results in Figure 13 show that the mole percentage of nitrite in the acid steel slag slurry is approximately 87.04%, which is higher than the theoretical calculation. This result indicated that more nitrite was produced, which was probably attributed to the reduction of parts of NO2 by Mn2+ in the liquid phase. During the long-period denitration experiment process, in which the filtrate was used as an absorbent, a light brown solid was generated in the solution, which was obtained after filtration, washed with deionized water, and then dried at 105 °C for 12 h. Figure 16 shows the XRD patterns of the precipitation. The mineral phase of the precipitation was represented by Mn3O4 and MnO(OH), thereby confirming that NO2 could oxidize Mn2+ in the liquid phase given the generation of solid Mn3O4 and MnO(OH). Therefore, Mn2+ in the acid steel slag slurry or its filtrate can promote the absorption of NO2 via redox reactions as shown in eqs 16−17. In summary, the mechanism of NOx removal by steel slag slurry is a combination of acid-base reaction (eqs 3−7) and redox reaction (eqs 16−17). ା 2NOଶ + 3Mnଶା + 4Hଶ O → 2NOି ଶ + Mnଷ Oସ + 8H

(16)

ା NOଶ + Mnଶା + 2Hଶ O → NOି ଶ + MnO(OH) + 3H

(17)

4.

CONCLUSIONS A method for simultaneously removing of SO2 and NOx from coke oven flue gas using

steel slag slurry was proposed in this study. It has some superiorities, e.g. nearly free cost of absorbent, less consumption of gas-phase oxidation agent, and the consistency of


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operation steel slag slurry pH and actual slurry pH condition of a typical Ca-based WFGD system. Nearly 100% SO2 removal efficiency and moderate 83.4% NOx removal efficiency were obtained under optimal operation conditions, in which the concentration of steel slag slurry was 15%, reaction temperature was 40 °C, steel slag slurry pH was 5.5, gas flow rate was 1000 mL·min−1, and OR was 0.8. The mechanism of NOx removal by steel slag slurry was also proposed. The existence of Mn2+ in steel slag slurry could accelerate the removal of NO2 via redox reactions. This study indicates that gas-phase oxidation (such as ozone oxidation) coupled with existing wet flue gas desulfurization devices using steel slag as an absorbent may solve the pollution problem of SO2 and NOx emissions from coke oven flue gas.

FIGURES.

Figure 1. XRD patterns of raw and acidized steel slag.


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Figure 2. Schematic diagram of the experimental apparatus. 1−5, N2, NO/N2, NO2/N2, SO2/N2, O2 cylinders; 7, mass flow controllers; 8, gas mixing tank; 9, triple valve; 10, hotplate magnetic stirrer; 11, aerator; 12, magnetic stirrer; 13, pH meter; 14, bubbler reactor; 15, syringe; 16, condenser; 17, drying tube; 18, FTIR spectrometer; 19, KMnO4/H2SO4 solution; 20, computer.

Figure 3. Effect of reaction time on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1.


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Figure 4. Effect of reaction temperature on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; pH, 5.5±0.1.

Figure 5. Effect of steel slag slurry concentration on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; reaction temperature, 40 °C; pH, 5.5±0.1. Note: NO removal efficiency exhibits a negative value, indicating that the NO concentration at the outlet is higher than that at the inlet because of the decomposition of HNO2.


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Figure 6. Effect of OR on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1.

Figure 7. Effect of steel slag slurry pH on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C.


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Figure 8. Effect of gas flow rate on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1.

Figure 9. Effect of NOx inlet concentration on NOx removal efficiency. Experimental conditions: gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1.


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Figure 10. Effects of SO2 inlet concentration on the simultaneous removal efficiencies of NOx and SO2. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C, pH, 5.5±0.1.

Figure 11. Simultaneous removal efficiencies of NOx and SO2 with steel slag slurry under optimal condition. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; SO2 inlet concentration, 200 ppmv; O2 inlet concentration, 5 vol.%; steel slag slurry concentration, 15%; reaction temperature, 40 °C;


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pH, 5.5±0.1.

Figure 12. Effects of various absorbents on NOx removal efficiency. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1; reaction time, 1 h.

Figure 13. Effects of various absorbents on the distribution percentages of nitrite and nitrate. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1; reaction time, 1 h.


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Figure 14. XRD patterns of acidized steel slag and slag after denitration.

Figure 15. The changing curve of denitration efficiency, Mn2+ concentration, and the ratio of Mn2+ concentration to denitration efficiency using the filtrate during the long-period denitration experiment. Experimental conditions: NOx inlet concentration, 300 ppmv; gas flow rate, 1000 mL·min‒1; OR, 0.8; steel slag slurry concentration, 15%; reaction temperature, 40 °C; pH, 5.5±0.1.


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Figure 16. XRD pattern of the precipitation.

TABLES. Table 1. Chemical compositions of raw and acidized steel slag (wt.%) Samples

CaO

SiO2

Fe2O3

Al2O3

MgO

SO3

MnO

Raw steel slag

41.38

17.63

17.67

8.16

6.50

1.17

1.13

Acidized steel slag

27.57

10.22

6.81

5.25

1.17

47.48

0.45

Table 2. Concentrations of ions in the acid steel slag slurry (mg·L–1) Samples

Ca2+

Mg2+

Mn2+

Si*

Acid steel slag slurry

429.63

4890.82

170.59

32.22

*The form of Si is H4SiO4.

Table 3 The ion concentrations in acid steel slag slurry and its filtrate before/after


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denitration experiments. Samples (mg·L–1)

Ca2+

Mg2+

Mn2+

Si*

Acid steel slag slurry

429.63

4890.82

170.59

32.22

Acid steel slag slurry after denitration

450.83

4909.53

189.81

48.83

Filtrate

429.63

4890.82

170.59

32.22

Filtrate after denitration

689.16

4625.48

158.06

33.00

*The form of Si is H4SiO4; no detection of the Fe, Cr, Ti, P，Pb, Cu, and Zn in liquid phase. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge.

The analyses of the involved reactions during the adjustment process of the pH of steel slag slurry; operation parameters of this study; the changing curve of ion concentrations (Ca2+, Mg2+, Mn2+, and Si) in the filtrate during the long-period of denitration experiment (pdf)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone: +86-10-82544825. Fax: +86-10-62621355. ORCID


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Huiquan Li: 0000-0002-3702-3164 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the Science and Technology Service Network Initiative Project of the Chinese Academy of Sciences (KFJ-SW-STS-178) and the Young Scientists Fund of the National Natural Science Foundation of China (21706264). REFERENCES (1) Razzaq, R.; Li, C. S.; Zhang, S. J. Fuel 2013, 113, 287−299. (2) Weiss, C.; Rieger, J.; Rummer, B. Environ. Eng. Sci. 2012, 29, 555−562. (3) Hou, X.; Zhang, H. Coal Conversion 2007, 30, 39−42, 48 (in Chinese). (4) Saikia, J.; Saikia, P.; Boruah, R.; Saikia, B. K. Sci. Total Environ. 2015, 530–531, 304−313. (5) Zhao, K.; Han, W.; Lu, G.; Lu, J.; Tang, Z.; Zhen, X. Appl. Surf. Sci. 2016, 379, 316−322. (6) Bisio, G.; Rubatto, G. Energy 2000, 25, 247−265. (7) Zhao, Y.; Guo, T. X.; Chen, Z. Y.; Du, Y. R. Chem. Eng. J. 2010, 160, 42−47. (8) Chu, H.; Chien, T. W.; Li, S. Y. Sci. Total Environ. 2001, 275, 127−135. (9) Liu, Y.; Wang, Q.; Yin, Y.; Pan, J.; Zhang, J. Chem. Eng. Res. Des. 2014, 92, 1907−1914. (10) Chen, L.; Lin, J. W.; Yang, C. L. Environ. Prog. 2002, 21, 225−230.


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Simultaneous Removal of SO2 and NOx from Coal-fired Flue Gas

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