Numerical evaluation of the effectiveness of NO2 and N2O5 generation during the NO ozonation process
Graphical abstract
Introduction
The most abundant gaseous pollutants emitted from power plants and industrial boilers are sulfur dioxide (SO2) and nitrogen oxides (NOx), which make a dramatic contribution to acid rain and smog formation (Price et al., 1997). For the recent severe hazy weather that invaded most areas of China more than once (Hou et al., 2011, Jiang et al., 2014, Liu et al., 2014), China has established more rigorous legislation to control the emission of SO2 and NOx from stationary sources (Huang et al., 2014). Generally, SO2 is controlled effectively by wet flue gas desulfurization methods (WFGD), and low NOx burners (Nishimura et al., 1997), selective catalytic reduction (SCR) (Topsøe, 1994) and selective non-catalytic reduction (SNCR) (Tayyeb Javed et al., 2007) are typically used for the removal of NOx. Since exhaust gas from power plants and industrial boilers contains multiple pollutants, combined technologies such as WFGD + SCR or WFGD + SNCR are commonly applied. However, these combined technologies require expensive investments and operating costs, resulting in an increasing amount of attention paid to simultaneous removal technologies with high efficiency, low investment and reasonable operating cost.
Up to now, numerous simultaneous removal technologies have been reported, such as activated carbon adsorption technology (Sumathi et al., 2010), the dielectric barrier discharge (DBD) plasma flue gas treatment (Obradović et al., 2011), and various types of wet scrubbing (Hu et al., 2010, Liu and Zhang, 2011, Raju et al., 2008, Wang et al., 2012, Zhao et al., 2011). For wet scrubbing technology, ozone has been a typical oxidant used for the oxidation of NO.
In 1997, Nelo et al. (1997) already began to study the use of ozone for the oxidation of nitrogen oxides. Mok and Lee (2006) reported a two-step process which combined rapid oxidation by ozone and aqueous absorption by sodium sulfide. The process achieved high removal efficiencies for both SO2 and NOx. Wang et al. (2007) also studied a similar two-step process that could remove NOx, SO2 and Hg effectively. As the higher oxidation states of NO (N2O3, N2O5) are much more soluble in water (500.0 g/dm3) than NO (0.032 g/dm3) and NO2 (213.0 g/dm3) (Dora et al., 2009), the wet-scrubbing removal efficiency of NOx relies highly on the resultant composition of NOx during the NO oxidation process; whereas, the oxidation products of NO ozonation vary as the reaction environment and running parameters change. Compared with NO2, N2O5 is easier to remove in scrubbing systems, but needs more O3 for its generation. Hence, detailed investigations on the reaction environment and running parameters for ozonation of NOx are required for the design and optimization of this ozone oxidation combined with WFGD method for the removal of NO.
For this complex reaction system, numerical simulation is needed because it can provide more detailed information about the reaction system. CHEMKIN is one of the most widely used and validated kinetic software programs (Zajemska et al., 2014). It is accurate for industrial burners, gas turbines, chemical processing and so on (Abián et al., 2011, Gui et al., 2014, Li et al., 2010, Yang et al., 2013). Therefore, CHEMKIN is suitable to study the effectiveness of NO2 and N2O5 generation during the NO ozonation process. Wang et al. (2006) studied an ozone-NOx reaction co-flow jet by direct numerical simulation. However, in that research, only the reactions between O3 and NOx in the turbulence time scale were observed. Because of the low reaction rate of O3–NO reactions, NO2 was the main product, with minor amounts of NO3 and N2O5. Furthermore, the concentration of N2O5 of the simulation was not verified by laboratory testing due to the difficulty of measuring of N2O5.
Recently, Skalska et al. (2011) have investigated the kinetic model of NOx ozonation and determined the rate constants of NOx (NO2, NO3, and N2O5) based on Fourier transform infrared (FT-IR) spectroscopy, although only at 25°C. Also, we studied the O3 oxidation processes of NO and SO2 on a qualitative scale, as well as their coexistence, with the help of an in-situ IR spectrometer (Sun et al., 2014). These studies suggested that in-situ FT-IR spectroscopy should be a suitable technology for the measurement of N2O5. However, these reported studies did not focus on the effect of running parameters, especially the residence time, on the production of NO2 or N2O5 for industrial application. Moreover, as far as we know, quantitative studies on the production of N2O5 have rarely been reported.
This article focused on simulating the reactions between O3 and NO under different reaction conditions and running parameters using a numerical simulation method. Meanwhile, the simulated concentration of N2O5 was verified by experimental studies with the help of in-situ FT-IR spectroscopy. More importantly, the suitable running parameters for the generation of NO2 and N2O5 including reaction temperature, residence time, and the molar ratio of O3/NO, were studied.
Section snippets
Experimental set-up
Fig. 1 illustrates the schematic diagram of the experimental system for NO oxidation by O3, consisting of a gas supply system, gas-phase mixer, reactor and flue gas analyzing system. The details of each apparatus and the experimental gas conditions were described in our previous research (Sun et al., 2014).
In Fig. 2, different IR spectra are shown. The bands at 743, 1246 and 1720 cm− 1 were assigned to N2O5 (Wängberg, 1993). The concentration of N2O5 was calculated according to the formula:
Kinetic mechanism of the reactions between O3 and NOx
The reactions between O3 and NOx are complicated, which include not only the decomposition of ozone and N2O5 but also numerous intermediate transient reactions. Mok and Lee (2006) came to the conclusion that 12 reactions were involved in the mechanism. However, several important species such as OH in the reactions between O3 and NOx were not considered. Wang et al. (2006) provided a more detailed kinetic mechanism. However, reactions related to NO3 were missed. NO3 is unstable due to its strong
Modeling
Computations of the NO ozonation process were all made using the CHEMKIN 4.1 software package (Reaction Design Co., Ltd, San Diego, USA) (Kee et al., 2006). The cylindrical shear-flow reactor model was used in the computations. The model took radial diffusion into account, neglecting axial diffusion. Moreover, the model only included gas chemistry. The reaction mechanism of the NO ozonation process has been described above. NO concentration in the inlet ranged from 200 to 400 ppm. NO2
Verification of the model
In Fig. 3 the simulation results and Mok's experiment results at 443 K and 2.94 sec, with 280 ppm NO and 20 ppm NO2, are compared (Mok and Lee, 2006). As shown in Fig. 3, the production of NO2 was determined by the total amount of added ozone, and the calculation results fitted quite well with those in Mok's experiment.
Both the results of the simulation and Mok's experiment showed that NO mainly reacted to form NO2 with the added O3 under this reaction condition. The mechanism indicated that
Conclusions
In this study, suitable process conditions of NO2 and N2O5 were investigated by numerical simulation. A new 20-species, 76-step detailed kinetic mechanism was proposed between O3 and NOx. The concentration of N2O5 was measured by an IR spectrometer. The calculation results showed good agreement with both the published experimental results and our experimental results. Due to the high oxidation rate of NO by ozone, the effect of temperature on the generation of NO2 can be neglected. The
Acknowledgments
This work was financially supported by the National High Tech Research and Development Program (863) of China (No. 2011AA060801) and the Program for Zhejiang Leading Team of S&T Innovation (No. 2013TD07).
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