## 3 Study on the Mechanism of Yellow Plume
### Mechanism Analysis  
Figure 6 shows that, as the temperature increases, the NO conversion rate first increases and then decreases. In the interval of 650-700 \({}^{\circ}\)C, the NO conversion rate reaches the highest, which is about 60%. When the temperature is lower than 450 \({}^{\circ}\)C or higher than 850 \({}^{\circ}\)C, the conversion rate of NO to NO\({}_{2}\) is extremely low. Figure 6 shows that, when there is no CO in the inlet gas, the conversion of NO is very low. The conversion of NO significantly increases when the CO is introduced into the resistance furnace. The concentration of CO in flue gas is 50 ppm. It is indicated that CO has the effect of promoting the conversion of NO to NO\({}_{2}\). Figure 4: Test system: A, N\({}_{2}\) gas tank; B, O\({}_{2}\) gas tank; C, CO gas tank; D, NO gas tank; E, flow controller; F, peristaltic pump; G, heating cable; H, resistance furnace; I, smoke analysis instrument. Figure 5: Effect of temperature on the conversion. Figure 7 shows that, as the volume fraction of the inlet CO increases, the conversion rate of NO increases, when the reaction temperature is 550-650 \({}^{\circ}\)C. It takes a certain amount of time for the gas to be heated to the corresponding temperature during the experiment, but the gas stays in the resistance furnace for a short time. Thus, due to the limitation of the retention time, the experimental was not fully carried out under the ideal condition. As a result, the conversion rate of the simulation calculation is higher than that of the experiment. Figure 8 shows that, in the experiment, the O\({}_{2}\) concentration change has little effect on the conversion of NO to NO\({}_{2}\). When the oxygen concentration reaches 12% or more and the reaction temperature is around 650 \({}^{\circ}\)C, the conversion rate of NO in the experiment is basically maintained at 40-50%. However, if there is no oxygen, the conversion rate will be very low. Figure 9 shows that the NO to NO\({}_{2}\) conversion rate decreases with the increase of the inlet NO volume fraction when the reaction temperature is around 650 \({}^{\circ}\)C. Because NO and NO\({}_{2}\) are  
Figure 8: Effect of O\({}_{2}\) concentration on the conversion. Figure 6: Effect of temperature and CO on the conversion. Figure 7: Effect of CO concentration on the conversion. contaminants, it is not recommended to reduce the conversion rate from NO to NO\({}_{2}\) by increasing the concentration of NO. Figure 10 shows that, in the absence of water vapor, the NO conversion ratio is lower. The NO conversion ratio in the experiment is slightly higher than the calculation results, but it is still below 10%. When water is added, the volume fraction of water vapor reaches 9%, and the conversion rate is greatly improved. Therefore, water vapor is an important condition for the conversion of NO to NO\({}_{2}\). In the experiment on water vapor, the residual gas was discharged using N\({}_{2}\) at the end of the experiment, and the sudden expansion and shrinkage section in the heater caused a small portion of the gas residue, which may have resulted in a small portion of the gas residue in the heating furnace. Therefore, the conversion rate without water addition in experiment is higher than the conversion rate of simulated calculation. During the start-up period of the gas turbine, the main factors affecting the generation of yellow plume are the CO content and temperature. CO, which is produced by incomplete combustion, is the main influencing factor of NO conversion to NO\({}_{2}\). At the temperature range of 600-700 \({}^{\circ}\)C, the concentration ratio of NO to NO\({}_{2}\) is the highest. Simultaneously, the oxygen content and water vapor content in the flue gas are important influencing factors of the generation of yellow plume. The presence of oxygen and water vapor has a significant impact on conversion rates, but, if oxygen and water vapor are present and reach a certain concentration, then the effect of increased concentration on the conversion rate is less pronounced.