Intensification of NO_x absorption process by means of ozone injection into exhaust gas stream

Abstract

The growing emissions of nitrogen oxides into atmosphere endanger the human health and degrade our environment. In this work the application of ozone for the oxidation of nitrogen oxides present in exhaust gases was described. The main concept of the process is to oxidize nitrogen monoxide (NO) and nitrogen dioxide (NO₂) into dinitrogen pentoxide (N₂O₅) which reacts with water giving nitric acid. This way nitrogen oxides (NO_x) can be absorbed in water with good effectiveness. In presented studies we investigated the influence of the NO initial concentration and the ratio of NO/NO₂ in reaction gases on the NO conversion and selectivities into products. Furthermore, the two stage process composed of NO_x ozonation and the subsequent absorption of reaction gases in the 0.01 M solution of NaOH was performed. This experiments proved that due to ozonation the efficiency of NO_x absorption process is much higher than without ozone injection.

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. NO_x (NO and NO₂) can lead to health problems in humans exposed to high doses of these gases. The wide range of NO_x 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 NO_x emission are carried out all around the world. In our previous work [1], devoted to trends in the NO_x abatement methods from various emission sources, it was elucidated that vast majority of research is focused on NO_x emission control from power plants and mobile vehicles, where the most popular are the catalytic methods. Fewer research are aimed to develop the NO_x emission control technologies for chemical industry. The most common NO_x 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 NO_x. 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 NO_x 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 NO_x 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 NO_x absorption (LoTOx) won the Kirkpatrick Award. LoTOx enables to obtain 90% efficiency of NO_x 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 NO_x absorption process as a method for NO_x emission abatement.