The yellow smoke problem is the oxidation product NO₂ has not been effectively reduced or absorbed. Because NO₂ is a red-brown gas, obvious yellow color could appear when the emission concentration of NO₂ in the flue gas exceeds 30 mg/m³. This problem occurs in imperfect oxidation and denitrification engineering, which is for supervision and regulation [24] . Generally, the oxidation method of denitrification with yellow smoke phenomenon cannot achieve ultra-low emission levels, and it is also the “false denitrification” project that has been questioned the most, especially in the case of wet desulfurization, it is advisable to use peroxide method to oxidize NO to N₂O₅, which could solve the problem of yellow smoke and actual emission exceeding the standard due to low absorption efficiency of NO₂ in conventional desulfurization system. It can also be solved by using the special denitrification agent absorption tower to greatly improve the absorption efficiency, and the oxidation denitrification method combined with semi-dry desulfurization has a low initial concentration of nitrogen oxides (<150 mg/m³), the route of initial oxidation to NO₂ could be adopted, under which conditions cheaper oxidants (such as chlorine dioxide [25] , sodium chlorite [26] [27] , etc.) might be considered to reduce construction and operating costs.

Nitrate has a positive effect on the application of wet desulfurization gypsum and semi-dry desulfurization ash in building materials’ industry, improving the thermal insulation capacity of the material, and applied to other occasions to determine whether there is secondary pollution, nitric acid and nitrite in wet desulfurization wastewater if improper disposal will pollute the water body, the desulfurization wastewater of sintering machine using such denitrification technology should not be used for slag washing. It can be added to a mixture for consumption and utilization. From the perspective of laboratory irrigation mechanism analysis and practical engineering application, most of the decomposition products of nitric acid and nitrite carried in wastewater in the sintering machine production process are nitrogen, which does not cause the initial concentration of nitrogen oxides to be enriched and increased, and the secondary pollution problem can be completely avoided and solved.

The greatest risk of ozone escape lies in the ozone generator workshop rather than the flue gas, and the ozone that does not participate in the reaction will quickly decompose into oxygen in the high humidity reducing atmosphere in the desulfurization tower, which theoretically does not produce secondary pollution, unless there is serious flue gas bias in the desulfurization tower design. Therefore, under the condition that excessive ozone injection does not cause secondary pollution, the problem of large fluctuations in flue gas parameters of the sintering machine can be effectively solved. After measuring several peroxide demonstration projects, ozone cannot be detected by ppm level instruments in the chimney position, while a very small amount of ozone can be detected by ppb level instruments, almost within the error range of 0 points. Ozone has been applied in China’s water treatment industry for many years, the equipment is all domestic, and there are also perfect preventive measures for workshop escape.

In the process of denitrification by oxidation method, NO in the flue gas is first oxidized to NO₂ or N₂O₅, and all kinds of oxidants can achieve oxidation efficiency of more than 96%. There are two reactions of reduction and absorption of oxidation products in the desulfurization system, mainly reduction reaction. The difficulty of analysis of nitrogen element balance is that the part reduced to nitrogen cannot be measured. Therefore, in the actual nitrogen balance analysis process, the nitrogen element outlet is lower than the nitrogen element inlet, and this part of the difference is the part that is converted into nitrogen. The same problem exists in other denitrification technical routes. The proportion of nitrogen converted by different projects is also different, which is affected by the concentration of sulfur dioxide in the flue gas, the oxygen content, the temperature and the water content of the flue gas.

The process of ozone oxidation combined with semi-dry absorption reduction denitrification is to quickly and evenly mix ozone after the dust collector, oxidize the vast majority of NO in the flue gas to NO₂, and then reduce and absorb NO₂ in the semi-dry desulfurization tower, using sulfurized and absorption accelerator to produce N₂ and calcium nitrite, so as to achieve the purpose of denitrification. In the actual production process, the initial concentration of nitrogen oxides (calculated as NO₂) in a single sintering machine is 220 mg/m³ as an example. The smoke volume is 900,000 Nm³/h, and the desulfurization agent is about 1.8 t/h, and the desulfurization ash is about 2.5 t/h, and the denitrification agent is about 60 kg/h. The molar ratio of ozone to nitrogen oxides is 1.25, and under this condition, the main oxidation product is NO₂, which cannot form higher nitrogen oxides.

The whole process involves a reaction pathway as shown in Figure 2 below, where oxygen generates ozone with oxidizing properties through an ionization reaction in the ozone generator, and ozone generates NO₂ with NO through a gas-phase reaction in the flue gas, the whole reaction being a stepwise reaction. The process was illustrated by the following Equations.

${\text{O}}_3+\text{NO}\to {\text{NO}}_2+{\text{O}}_2$(1)

$2{\text{NO}}_2+{\text{H}}_2\text{O}\to 2{\text{H}}^{+}+{\text{NO}}_2^{-}+{\text{NO}}_3^{-}$(2)

${\text{H}}_2\text{O}+2{\text{NO}}_3^{-}+5{\text{SO}}_3^{2-}+\text{accelerator}\to 5{\text{SO}}_4^{2-}+{\text{N}}_2+2{\text{OH}}^{-}$(3)

The oxidized gas enters the semi-dry fluidizer bed desulfurization and denigration undergoes the fluidizer lime trapping and high pressure spray humidification and efficiency enhancement reaction. During this process, excess O₃ is reduced to O₂ on the surface of porous solid powder, NO₂ is finally formed into N₂ under the action of OH⁻, stabilizer and accelerator, and ${\text{SO}}_3^{2-}$ in the liquid phase plays the main role of reaction. NO₂ in the fluidizer lime and gentrifying agent to participate in the subsequent chemical reaction, and ultimately most of the NO₂ is reduced to N₂, generating a very small amount of nitrate and nitrite. This mechanism approach has been verified by the mechanism test of Professor Tianle Zhu’s [28] research group (chromatographic verification as shown in Figure 3 below) and engineering practice (as shown in Table 1 , Table 2 , Table 3 ). As shown in Figure 3(a) , the peak time of N₂ under the corresponding column was determined by testing, and as shown in Figure 3(b) , 20 - 40 min was the optimal time for denitrification, and with the increase of time, the denitrification capacity gradually decreased and less N₂ was produced. Comprehensive detection of high-temperature catalytic reduction NOx was carried out on the flue gas at the tail of the sintering system, and the total amount of NOx was always maintained below 50 mg/nm³, and the desulfurization gypsum ion detection met the requirements of ultra-low emission NOx emission.

In the process of oxidation combined with semi-dry absorption and denitrification, the input of nitrogen element comes from NO in the flue gas after the dust collector, and the total input nitrogen element (nitrogen is not calculated) is: (equations number)

$\text{N}=\text{V}*\text{c}*14/46=48.7\text{kg}/\text{h}$(4)

----V Smoke flow rate

----c Nitrogen oxide concentration

The nitrogen outlet consists of three parts, which are the residual emission of nitrogen oxides in the flue gas, nitrogen generated by the reaction of NO₂ and denitrification agent, and calcium nitrite (calcium nitrate) in desulfurization ash. Due to the high concentration of nitrogen in the flue gas itself, the nitrogen part cannot be detected and can only be analyzed through balance.

According to the field measurement, the average concentration of nitrogen oxides in the final emission of flue gas calculated as NO₂ is 15 mg/m³, so the nitrogen element of this part is calculated as:

$\text{N}=\text{V}*\text{c}*14/46=3.65\text{kg}/\text{h}$(5)

The initial total nitrogen ratio was a1 = 3.65/48.7*100% = 7.5%.

The calcium nitrate content of the desulfurization ash is less than 2%, and at 2%, an average of 2 tons of desulfurization ash is produced per hour, so this fraction of nitrogen is calculated as:

$\text{N}=\text{G}*\text{c}*28/132=8.5\text{kg}/\text{h}$(6)

The initial total nitrogen ratio is a2 = 8.5/48.7*100% = 17.5%.

According to the decrement method, the nitrogen element of this part is:

$\text{a}3=1-7.5\%-17.5\%=75\%$