Investigation on NO Oxidation during Ozone Oxidation Denitration of Sintering Flue Gas

With the intensified requirements on ambient air quality, controlling NOx emission from sintering flue gas has become the most important priority in steel industry. Ozone oxidation denitration process has become the most closely watched sintering flue gas denitration technology. In order to investigate the influence of different operating conditions on NO oxidation in practical engineering application of ozone oxidation denitration process, an ozone oxidation system was built, and sintering flue gas was extracted from the pipeline between the induced draft fan and the desulfurization tower. The influence of discharge power and O2 flow rate of ozone generator on O3 concentration, O3 yield and NO oxidation efficiency were investigated. The results indicated that NO oxidation efficiency increased with the discharge power. And the increase of O3 dosage imposed a significant influence on NO oxidation efficiency. However, little effect of O2 flow rate on NO oxidation efficiency was observed.


Introduction
With the intensified requirements on ambient air quality, controlling NO x emission from sintering flue gas has become the most important priority in steel industry.However, the performance of flue gas denitration technology implemented in steel industry is not satisfactory compared with other air pollution control technologies such as desulfurization.
It is widely acknowledged that NO x generated by sintering process of steel industry is mainly comprised of NO (95% vol.) [1] which is difficult to be removed from flue gas due to its stability especially the limited solubility.The solubility can be improved by oxidation where NO is transformed into high-valence nitrogen oxides (such as NO 2 , NO 3 , N 2 O 5 , etc.), thus the removal could be readily performed by wet scrubber.As a reactive oxidant, ozone has drawn great attention in oxidation denitration since it exhibits many advantages which are low economic costs, high selectivity to NO, and resistance to temperature fluctuations [2].In the research conducted by Mok [3], after the exhaust gas passed through the ozonizing chamber and the absorber sequentially, NOx removal efficiency of about 95% and SO 2 removal efficiency of 100% were obtained.Similar results were observed by Wang [4] who found that 97% of NO and nearly 100% of SO 2 can be removed simultaneously by alkaline washing tower after the injection of ozone.
According to the research results of Wang [5], the oxidation reactions between O 3 and NO could be expressed by equation ( 1) to (3) where NO was oxidized into NO 2 which further transforms into N 2 O 5 in the presence of excessive O 3 .
As a result, the O 3 concentration would not only dominate the performance of NO oxidation effect, but also play a significant role in the removal of NO in the whole process of oxidation denitration.In order to investigate the influence of different operating conditions on NO oxidation in practical engineering application of ozone oxidation denitration process, an ozone oxidation system was built, and sintering flue gas was extracted from the pipeline between the induced draft fan and the desulfurization tower.The influence of discharge power and O2 flow rate of ozone generator on O 3 concentration, O 3 yield and NO oxidation efficiency were investigated.

Experiment part 2.1 Process description
The on-site operation of ozone oxidation experiments were conducted at a steel-making plant in Tangshan, Hebei province.An ozone oxidation system was built, and sintering flue gas was extracted from the pipeline between the induced draft fan and the desulfurization tower at 30000 m 3 /h.The temperature of flue gas was about 140℃.The NO concentration was 70 mg/m 3 .As shown in figure 1, O 3 generated by an ozone generator (GUOLIN, CF-G-2-7kg) was injected into the flue gas by ozone feeding distributor at the inlet of the mixing chamber where O 3 and flue gas were mixed thoroughly.The mixed flue gas resided in the system for 0.6~1.0s.Finally, the oxidized flue gas entered into the desulfurization tower to remove the SO 2 and NO x from the flue gas simultaneously.

Generation of ozone
The ozone generator generated O 3 through dielectric barrier discharge (DBD) with an adjustable discharge power ranging from 13 to 45 kW.O 2 gas (99.5% vol.) was used as the gas source of generator, and its rated consumption was 70 kg/h (50m 3 /h).The energy consumption of generator to generate 1kg O 3 was 7.5 kWh.The rated O 3 concentration and O 3 yield of generator were 148 mg/L (50m 3 /h) and 7 kg/h (with an adjustable range from 10 to 100%) respectively.

Evaluation of NO oxidation efficiency
The O 3 concentration at the outlet of ozone generator was monitored online by ozone concentration detector, and O 3 yield was calculated by the control system of generator.The NO concentration at the inlet and outlet of ozone oxidation system was measured by an exhaust gas analyser (Testo 350).Based on the monitoring results, the NO oxidation efficiency can be estimated by equation (4).
In equation ( 4), [NO] in and [NO] out represent the NO concentration at the inlet and outlet of system respectively.

Influence of discharge power on ozone concentration and yield
As shown in figure 2, when the O 2 flow rate was 30 Nm 3 /h, the discharge power of ozone generator in the preheating stage was 12.9 kW, while the concentration of generated O 3 was 65 mg/L which was equivalent to 2.52 kg/h.With the increase of discharge power, O 3 concentration and yield increased.

Influence of ozone dosage on NO oxidation
To investigate the influence of O 3 /NO molar ratio on NO oxidation efficiency, O 3 concentration at the outlet of ozone generator was converted into concentration in the flue gas.As shown in figure 4, there was a significantly positive correlation between NO oxidation efficiency and O 3 /NO molar ratio, and the increase of O 3 dosage imposed a significant influence on NO oxidation efficiency.When the O 3 /NO molar ratio increased from 0.84 to 1.37, NO oxidation efficiency increased correspondingly from 70.8% to 87.7%.
It was observed that NO oxidation efficiency was 75% under the O 3 /NO molar ratio of 1 which was lower than the theoretical value indicated by equation ( 1).This could be ascribed to that O 3 oxidized generated NO 2 into high-valence nitrogen oxides (such as NO 3 , N 2 O 5 , etc.) or reacted with other components in flue gas, which made some of O 3 not be used for NO oxidation.

Influence of oxygen flow rate on NO oxidation
In order to investigate the influence of O 2 flow rate on NO oxidation efficiency, O 2 flow rate was increased from 34.9 Nm

Assessment of NO oxidation energy consumption
When inlet NO concentration was 70 mg/m 3 and O 3 /NO molar ratio was 1, the NO oxidation efficiency was 75%.In this case, the energy consumption of ozone generator to generate 1kg O 3 was 7.5 kWh which was equivalent to 62.5 g/kWh.According to these data, the average oxidation energy consumption of each NO molecule was 0.05 eV which was much lower than that of 30 eV in discharge plasma oxidation.It could be concluded that ozone oxidation denitration process was feasible and economical.

Conclusions
In this investigation, the influence of discharge power and O 2 flow rate of ozone generator on O 3 concentration, O 3 yield and NO oxidation were investigated.The results indicated that O 3 concentration, O 3 yield and NO oxidation efficiency increased along with the discharge power under the same O 2 flow rate and inlet NO concentration.There was a significantly positive correlation between NO oxidation efficiency and O 3 /NO molar ratio, and the increase of O 3 dosage imposed a significant influence on NO oxidation efficiency.These test results showed that applying ozonation denitration process to dealing with sintering flue gas could achieve a NO oxidation efficiency higher than 90%.
Besides, under the same discharge power and inlet NO concentration, with the increase of O 2 flow rate, O 3 concentration decreased gradually while O 3 yield increased gradually.However, little effect of O 2 flow rate on NO oxidation efficiency was observed.In the application of ozone oxidation denitration engineering, when the O 2 was supplied in a certain amount, we should improve NO oxidation efficiency by adjusting the discharge power of ozone generator.But considering some of O 3 may not be used for NO oxidation in practical engineering application, O 3 dosage should be moderately higher than the theoretical value under the reasonable economic consideration.

Fig. 2 Fig. 3
Fig. 2 Influence of discharge power on ozone concentration and yield

Fig. 4
Fig.4 Influence of ozone dosage on NO oxidation efficiency (The O2 flow rate is 30 Nm 3 /h.)

Figure 5
Figure 5 indicated a significantly negative correlation between O 3 concentration and O 2 flow rate, while O 3 yield was positively associated with O 2 flow rate.Within a range of O 2 flow rate from 28.6 Nm 3 /h to 45.2 Nm 3 /h, the O 3 generation rate increased with O 2 flow rate, leading to a higher O 3 yield.However, the generated O 3 gas was diluted by the increased feed of O 2 gas leading to a reduced observed concentration.

Fig. 5 Fig. 6
Fig.5 Influence of oxygen flow rate on ozone concentration and yield (The discharge power is 30 kW.)As shown in figure6, O 3 yield increased with discharge power which was consistent with the above conclusion.Besides, O 3 yield of different discharge power increased gently with O 2 flow rate.

Fig. 7
Fig. 7Influence of oxygen flow rate on ozone concentration and NO oxidation efficiency (The discharge power is 30 kW.)