An investigation on NO removal by wet scrubbing using NaClO2 seawater solution
© The Author(s) 2016
Received: 9 April 2016
Accepted: 7 June 2016
Published: 17 June 2016
The experiments were conducted to investigate the NO removal by wet scrubbing using NaClO2 seawater solution in a cyclic scrubbing mode. Results show that, when the concentration of NaClO2 in scrubbing solution is higher than 10 mM, a complete removal of NO can be achieved during the cyclic scrubbing process. The breakthrough time for seawater with 15 mM NaClO2 is enhanced by 34.3 % compared with that for NaClO2 freshwater. The extension of the breakthrough time for NaClO2 seawater is mainly ascribed to the improved utilization of NaClO2 in the solution. The good buffering ability of seawater could suppress the acidic decomposition of NaClO2 into ClO2 effectively. The analysis of reaction products indicates that the main anions in the spent liquor are chloride ions and nitrate ions. The calculation of NaClO2 utilization according to the ion chromatography also agrees well with the experimental results of breakthrough times.
Air pollution is currently one of the major problems worldwide, resulting that more rigorous environmental laws are introduced. The main source of air pollution is the combustion process of fossil fuels used in power plants, vehicles, ships and other incineration processes (Skalska et al. 2010). Sulfur oxides (principally SO2), nitrogen oxides (NOx), particulate matter (PM) are the key combustion-generated air contaminants. Since NOx have been implicated in a variety of environmental effects such as acid rain, photochemical smog, tropospheric ozone layer depletion and even global warming, they are considered as the primary pollutants of the atmosphere (Wei et al. 2009a).
At present, the implementation of stringent regulations of NOx emission requires the development of new technologies and improvement of currently used methods for NOx removal from exhaust gas. Effective reduction of NOx emissions from both stationary and mobile sources poses a major challenge. Technologies for the NOx removal can be divided into combustion control and post-combustion treatment. Combustion control aims at reducing the formation of NOx during the combustion of fossil fuel (Adewuyi et al. 1999). Post-combustion control methods include a variety of techniques such as selective catalytic reduction (SCR) (Zhang et al. 2015; Jiang et al. 2015), selective non-catalytic reduction (SNCR) (Lee et al. 2008), thermal DeNOx and scrubbing. SCR can remove NOx with an efficiency of 80–95 % and it has been applied in power plants extensively. Up to date, SCR is also considered as one of the most promising techniques for ocean-going ships. But it requires additional space, high investment and operating cost. Another major concern of SCR is the deleterious effect of SO2-laden flue gas on the life of catalyst which is an important factor of this technology’s cost. SNCR approach requires high reaction temperature (900–1000 °C) with an elaborate temperature control to avoid ammonia slip and to achieve effective NOx removal. Scrubbing method is one of the advanced air pollution control technologies. It has the ability of removing other acid gases and particulates simultaneously (Makansi 1990).
Nitric oxide (NO) is a major component of NOx emitted from incineration processes and is of low solubility in aqueous solution, so it can not be removed easily from the flue gas by scrubbing. But a feasible method is to oxidize NO to other soluble NOx species (NO2, N2O3, N2O4, N2O5, etc.). For this purpose, various oxidants such as hydrogen peroxide (H2O2) (Liu and Zhang 2011), potassium permanganate (KMnO4) (Chu et al. 2001a), peracids (Littlejohn and Chang 1990), ferrous-chelating agents (Wang et al. 2007; Yan et al. 2015), sodium hypochlorite (NaClO) (Chen et al. 2005) and sodium chlorite (NaClO2) (Deshwal et al. 2008; Wei et al. 2009b) have been investigated to enhance the NO removal efficiency of the scrubbing solution. Among these oxidants, NaClO2 has been found one of the most promising chemicals for NO oxidation. In the late 1970s, Sada et al. performed early studies on the absorption of NOx in a NaClO2 solution and proposed the overall reactions of NO and NO2 with NaClO2 in alkaline solutions (Sada et al. 1978, 1979). Brogren et al. studied the kinetics of the absorption of NO in an alkaline solution of NaClO2 and found that the chlorite ion oxidized NO mainly to NO2 or NO 2 − and NO2 to NO 3 − (Brogren et al. 1998). Yang and co-workers conducted experiments with acid solution of NaClO2 and determined the overall reaction of NO with NaClO2 in acidic conditions (Yang and Shaw 1998). Apart from the considerable studies on kinetics of NO absorption with NaClO2 (Hsu et al. 1998; Chu et al. 2001b), more and more research on simultaneously removal of NOx and other pollutants (such as SO2 and Hg) by scrubbing NaClO2 solution has been conducted throughout the world (Jin et al. 2006; Huston et al. 2008; Zhao et al. 2010; Park et al. 2015). However, all of the work above-mentioned is focused on experimentations using NaClO2 freshwater solution. Since seawater has some unique properties such as the natural alkalinity (pH 7.5–8.3) and good buffering ability, it has already been used to remove SO2 from the flue gas of on-going ships and onshore power plants (Andreasen et al. 2007; Vidal et al. 2007; Lan et al. 2012). To the best of our knowledge, research on NO removal by using NaClO2 seawater has not been reported yet. The purpose of this work is to use seawater to take place of fresh water as solvent and to study the effect of seawater on the NO removal of NaClO2 solution. The experimental results showed that NaClO2 seawater can remove NO from the simulated flue gas with a high efficiency. Besides, the breakthrough time for NO absorption with NaClO2 seawater is obviously enhanced due to the buffering ability of seawater compared with that for NaClO2 freshwater and the possible reaction mechanism has also been discussed.
The reagents used in this investigation were: N2 pure gas, NO span gas (10 %) with N2 as the balance gas, NaClO2, sodium chloride (NaCl), magnesium chloride hexahydrate (MgCl2∙6H2O), sodium sulfate (Na2SO4), anhydrous calcium chloride (CaCl2), potassium chloride (KCl), sodium bicarbonate (NaHCO3), sodium hydroxide (NaOH) and hydrochloric acid (HCl). All chemicals were analytical reagent (AR) degree without further purification. The deionized water was prepared in a two-stage ELGA PURELAB Option R15 purification system and had a resistivity of 15 MΩ∙cm.
The substitute ocean water, which is prepared according to an American national standard (ASTM 2008), is used as seawater solvent in the experiments. For simplicity, minor elements, occurring naturally in concentration below 0.2 g/L, are not included. The process for preparing the substitute ocean water is as follows: firstly, stock solution No. 1 is prepared by dissolving 11.11 g MgCl2∙6H2O and 1.16 g CaCl2 in 50 mL deionized water. Secondly, stock solution No. 2 is prepared by dissolving 0.695 g KCl and 0.201 g NaHCO3 in 50 mL deionized water. Thirdly, dissolve 24.53 g NaCl and 4.09 g Na2SO4 in 900 mL deionized water and then add stock solution No. 1 and No. 2 slowly with vigorous stirring. Finally, adjust the pH value of the synthetic seawater to 8.2 with 0.2 M NaOH solution.
The initial pH values of the scrubbing solution of various NaClO2 concentrations
NaClO2 concentration (mM)
The pH value of freshwater solution
The pH value of seawater solution
The scrubber gaseous effluent is analyzed with a Madur GA-21 gas analyzer which is a multi-functional flue gas analyzer. Electrochemical sensors are used for the measurement of gas concentration. NO and NO2 are measured directly using the electrochemical cells while the NOx concentration is calculated by the analyzer as a simple sum of measured NO and NO2 concentration. In order to protect the flue gas analyzer, the simulated flue gas is dried by anhydrous CaCl2 particles after being scrubbed. A pH meter is used to on-line monitoring the pH value and temperature of the scrubbing solution. After scrubbing, sample solution is withdrawn from the spent solution in the beaker for ion chromatographic analysis (Dionex ICS-1500).
Results and discussion
When the concentration of NaClO2 in the scrubbing solution is 5 mM, a complete removal of NO from the flue gas can not be achieved during the whole scrubbing process, so the breakthrough time is zero for 5 mM NaClO2 solution. From Fig. 3, one can see that, the breakthrough times for NaClO2 seawater solution have been enhanced obviously compared with those for freshwater solution of the same concentration of NaClO2 oxidant. When the concentration of NaClO2 is 15 mM, the breakthrough time for seawater can be improved by 34.3 % compared with that for freshwater. Since the breakthrough time for NO absorption during the cyclic scrubbing process represents the ability of NO absorption for the scrubbing solution, NaClO2 seawater exhibits much higher NO absorption potential than NaClO2 freshwater.
Both N2O3 and N2O4 are very easy to react with H2O, so most of them will be removed during the wet scrubbing process. To some extent, N2O3 and N2O4 can be considered as intermediate products. The NOx in exit flue gas are mainly NO and/or NO2. The reaction between NO and NO2, as well as the hydrolysis of N2O3 and N2O4, will not affect the analysis result of NOx absorption obviously.
Figure 4 also shows that, for 20 mM NaClO2 seawater solution, the concentration of NO2 in exit flue gas is more stable than that of freshwater solution, suggesting that seawater is better for balancing the NO absorption during the cyclic scrubbing process. Besides, for 20 mM NaClO2 seawater solution, the total removal efficiency of NOx (here it is the sum of NO and NO2) can be approximately calculated as 70 % during the cyclic scrubbing process.
As shown in Fig. 5, the initial pH value of NaClO2 freshwater is about 10.5 and it drops sharply with the addition of HCl. An addition of 6.6×10−2 mmol HCl can make the pH value of 0.2 L NaClO2 freshwater drop below 7. Whereas the initial pH value of NaClO2 seawater is 8.6, which is just a little higher than that of seawater alone. With the addition of HCl, the pH value of NaClO2 seawater drops slowly. It requires 1.14×10−1 mmol HCl to make the solution pH drop to 7. The result shows that, for NaClO2 solution, seawater has a much better buffering ability in maintaining the solution pH than freshwater. It is well known that the buffering ability of seawater is mainly related with the bicarbonate salt which is only 0.48 mmol in 0.2 L seawater. Though the amount of HCl addition, that can make pH value of NaClO2 seawater drop to below 7, seems to be much less compared with the molar quantities of bicarbonate salt and NaClO2 (5 mmol) in the solution, the buffering ability of seawater might be high enough to enhance the breakthrough time for NaClO2 seawater during the cyclic scrubbing experiments.
Figure 7 indicates that Cl − and NO 3 − are the major anion products in the spent liquor. There is no ClO 2 − in the spent liquor because NaClO2 in the solution has been consumed completely during the cyclic scrubbing process. For 20 mM NaClO2 freshwater, there is certain amount of ClO 3 − in the spent solution, which may be formed by the acidic decomposition of ClO 2 − in Eq. (9).
As shown in Fig. 7, the concentration of Cl − in freshwater represents the actual value of Cl − which exist in the spent liquor and comes from the NaClO2 oxidant. While the concentration of Cl − in seawater represents the value of Cl − left in the sample solution after being filtered by Ag cartridge.
Note that the treatment of the spent liquor does not affect the concentrations of other ions, such as NO 3 − and ClO 3 − . As shown in Fig. 7, the concentration of NO 3 − in spent liquor of NaClO2 seawater is a little higher than that in freshwater. It means that, with the same concentration of NaClO2, the seawater solution has absorbed more NO during the cyclic scrubbing process than freshwater solution. It also demonstrates that, seawater is helpful to improve the utilization of NaClO2 compared with freshwater.
In this work, the effect of seawater on NO removal of NaClO2 solution has been investigated in a cyclic scrubbing mode. Compared with NaClO2 freshwater solution, NaClO2 seawater solution exhibits much longer breakthrough time for NO absorption. The result of blank experiment and acidic titration demonstrates that the extension of breakthrough time for NaClO2 seawater might be ascribed to the buffering ability of seawater, which could suppress the acidic decomposition of NaClO2 into ClO2. The reduction of ClO2 escape may be the main reason for the enhancement of NO removal for NaClO2 seawater during the cyclic scrubbing process. The analysis of liquid reaction products indicated that NaClO2 has been consumed completely during the cyclic scrubbing process and turned into Cl − in the spent liquor. The calculation result from the ion chromatography demonstrates the enhancement of NaClO2 utilization in seawater, which agrees well with the breakthrough times for NO removal. This work suggests that NaClO2 seawater solution is a promising choice for NOx removal, especially for the applications in onshore power plants and ocean-going ships.
ZH participated in the design of the study, performed the statistical analysis and paper writing. DZ carried out the NO removal experiment. SY participated in the NO removal experiment. ZY and XP took part in the guidance of the study and the paper preparation. All authors read and approved the final manuscript.
This study has been financially supported by the Natural Science Foundation of China (Grant no. 51402033), the Scientific Research Fund of Liaoning Provincial Education Department of China (No. L2014198), the Science and Technology Plan Project of China’s Ministry of Transport (Grant no. 2015328225150) and the Fundamental Research Funds for the Central Universities (Grant no. 3132016018).
The authors declare that they have no competing interests.
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