Intensification of NOx absorption process by means of ozone injection into exhaust gas stream
Highlights
► Influence of nitric oxide initial concentration on the effectiveness of ozonation process. ► Influence of nitric oxide to nitrogen dioxide ratio on the effectiveness of ozonation process. ► Nitrogen oxides emission abatement with the use of ozonation and subsequent absorption of reaction gases.
Introduction
Nitrogen oxides are considered to be dangerous air pollutants. They contribute to the occurrence of several environmental problems, i.e. photochemical smog, acid rain, tropospheric ozone, ozone layer depletion and even greenhouse effect. NOx (NO and NO2) can lead to health problems in humans exposed to high doses of these gases. The wide range of NOx control technologies, e.g. techniques such as absorption, adsorption, selective catalytic reduction or electric discharge were developed. However, all these methods have its limitations and drawbacks.
Therefore, attempts to develop new technologies of reduction of NOx emission are carried out all around the world. In our previous work [1], devoted to trends in the NOx abatement methods from various emission sources, it was elucidated that vast majority of research is focused on NOx emission control from power plants and mobile vehicles, where the most popular are the catalytic methods. Fewer research are aimed to develop the NOx emission control technologies for chemical industry. The most common NOx emission reduction methods in chemical plants are absorption in alkali solutions and reduction with the use of hydrogen, methane or ammonia [2], [3]. It is well known that several reactions occur during absorption process of nitrogen oxides. The solubility of NO is very low, however it can be increased in the presence of NO2 as a result of formation of N2O3 (Eq. (1)), which is highly soluble.NO + NO2 → N2O3.
Absorption of nitrogen oxides like NO and NO2 leads to the formation of two acids, i.e. nitrous and nitric acid. Hence, the main drawback of the traditional absorption is disability to obtain flue gas without NOx. It is connected with nitrous acid decomposition in the presence of strong acid into nitric acid and nitric oxide according to [2], [3], [4]:3HNO2 → HNO3 + 2NO + H2O
Physical methods involving adsorption e.g. adsorption on molecular sieves have limited importance, since they just transmit nitrogen oxides into other phase. However, in the nitric acid industry the absorption technique used for NOx emission control can additionally increase the efficiency of nitric acid production without generation of waste products. As a way for intensification of absorption process Chacuk et al. proposed addition of an oxidizing agent (ozone) into absorption solution, resulting in the transformation of nitrous acid into nitric acid [2]. For this purpose also potassium permanganate, sodium chlorite, sodium hypochlorite, hydrogen peroxide and chlorine dioxide can be used [4]. According to Ellison ozone is the most expensive oxidizing agent, however it possesses many merits that make it the most useful oxidant for this application [5]. First of all ozonation process is very fast and does not cause the formation of waste products. Furthermore, residual ozone decomposes easily to benign product (i.e. oxygen). Another idea is to apply oxidant directly into the exhaust gas stream. Probably the best oxidizing agent for this solution is also ozone. Two approaches can be observed. First of all ozone can be applied into industrial exhaust gas stream in order to increase the amount of NO2 present. The second approach heads for oxidizing NOx into N2O5, since it has high solubility in water (around 500 g dm−3 [6]), which is significantly higher than solubility of nitric oxide or even nitrogen dioxide. The gases containing N2O5 then are passed through absorption column, where N2O5 reacts with water to produce HNO3. In this approach, higher amounts of ozone are needed to obtain N2O5. Many researchers have focused their studies on the first approach, i.e. homogeneous oxidation of NO into NO2 by means of ozone. The combination of this process with SCR or absorption was studied, i.e. by Jaroszyńska-Wolińska, Mok and Nam, Mok and Yoon, Wang et al., Dora et al. [6], [7], [8], [9], [10], [11]. In the literature less information can be found on the subject of NO ozonation into N2O5. In 2001 the process called Low-Temperature NOx absorption (LoTOx) won the Kirkpatrick Award. LoTOx enables to obtain 90% efficiency of NOx emission reduction [12].
The main objective of this paper was to confirm that the absorption of higher nitrogen oxides is more effective than absorption of NO2. The influence of NO initial concentration and NO/NO2 mole ratio in reaction gases on the nitrogen oxides ozonation process was also described. Presented results will be useful in establishing general recommendation for the intensification of NOx absorption process as a method for NOx emission abatement.
Section snippets
Materials and methods
Series of experiments were conducted for nitrogen oxides concentrations commonly occurring in flue gases from nitric acid plants. Various initial concentrations of nitric oxide (1.5–7.7 × 10−5 mol dm−3) were mixed with oxygen–ozone mixture under different mole ratios of O3/NO: 0.3–2.5. The ozonation process was conducted at the temperature of 298 K (25 °C) and the reaction gases flow rate equal to 1.5 dm3 min−1 was applied. An overview of the experimental set-up was described in the previous papers [13]
Influence of the NO initial concentration
Fig. 1 presents the concentrations of substrates and products after reaction of nitrogen monoxide (concentration 3.0 × 10−5 mol dm−3) with ozone at different molar ratios of O3/NO: 0.3–2.5. The data presented in the figure confirm the reaction sequence:NO + O3 → NO2 + O2NO2 + O3 → NO3 + O2NO2 + NO3 → N2O5N2O5 + H2O → 2HNO3
First of all nitrogen monoxide is oxidized by ozone into nitrogen dioxide (Eq. (3)). When the molar ratio of O3/NO exceeds 1, not only NO2 is formed but also NO3, and N2O5, which is in agreement with
Conclusions
It was confirmed that the ozonation of flue gases combined with the absorption process is a promising method for control of NOx emission from nitric acid production plants. The intensification of absorption process through the application of higher amounts of ozone reached up to 30%. The comparison of our results with those presented by Jakubiak and Kordylewski [20] as well as in our earlier work [21], proves that the residence time is extremely important for the effectiveness of NOx removal as
Acknowledgment
This work was supported through the grant PBZ – MEiN 3/2/2006 ‘Process engineering for the abatement of harmful and greenhouse gas emissions and their utilization’.
References (21)
- et al.
Trends in NOx abatement: a review
Science of the Total Environment
(2010) - et al.
Intensification of nitrous acid oxidation
Chemical Engineering Science
(2007) - et al.
The absorption–oxidation of NOx with hydrogen peroxide for the treatment of tail gases
Chemical Engineering Science
(1996) - et al.
Simultaneous removal of NOx, SO2 and Hg in nitrogen flow in a narrow reactor by ozone injection: experimental results
Fuel Processing Technology
(2007) - et al.
Kinetic model of NOx ozonation and its experimental verification
Chemical Engineering Science
(2011) - et al.
Removal of sulfur dioxide and nitrogen oxides by using ozone injection and absorption–reduction technique
Fuel Processing Technology
(2006) - et al.
Ozonation of nitrous acid in aqueous nitric acid solutions
Recent developments in the U.S.A. in the application of FGD-based technology for simultaneous SO2, NOx and mercury removal
Radiation Treatment of Gaseous and Liquid Effluents for Contaminant Removal. Proceedings of a Technical Meeting held in Sofia, Bulgaria, 7–10 September 2004
(2004)- et al.
Parametric studies of the effectiveness of oxidation of NO by ozone
Chemical and Process Engineering
(2009) Ozone application to a two-stage NO removal from waste gases
Polish Journal of Chemical Technology
(2002)
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