Research on plasma modified fly ash denitration

The effects of reactor parameters and process parameters on the denitration rate of modified fly ash in different gas atmospheres were studied by using a dielectric barrier plasma reactor and using orthogonal experiments. The characteristics of modified fly ash were analyzed using scanning electron microscope, specific surface area analyzer, X-ray diffraction, Boehm titration and Fourier transform infrared spectroscopy. The experimental data were processed by variance analysis and linear regression to induce the denitration mechanism. R2 of the linear regression analysis model is 0.789, which means that the adsorption pore size, acid groups and basic group can explain 78.9% of the change in denitration rate. The basic group will have a significant positive impact on the denitration rate, and the adsorption pore size and acidic group will have a significant negative impact on the denitration rate. Through variance analysis of the experimental data, it was found that the input power and discharge gap have a significant effect on the denitration rate, but the ionization time and discharge length have no significant effect. The input power affects the denitration rate by impacting the basic group, and the discharge gap affects the denitration rate by influencing the adsorption pore size. There are three denitration mechanisms on the surface of fly ash: physical adsorption, chemical adsorption and absorption process. Among them, chemical adsorption is the main mechanism of action, accounting for approximately 60.86%.


Materials and methods
Fly ash was used as the catalyst in this experiment.fly ash was obtained from Dalian Huaneng Power Plant, and its composition proportions are shown in Fig. 1.

Experimental procedure
(1) Modification Process The plasma modification system is shown in Fig. 2 and consists of a high-voltage pulse power supply, plasma reactor, gas supply system, and alkaline absorption solution.The gas supply system is composed of N 2 and O 2 gas cylinders, which control the airspeed of the reaction system through pressure reducing valves and flowmeters.The plasma reactor adopts a coaxial cylindrical double dielectric layer structure composed of a high-voltage electrode (copper rod), ground electrode (iron wire mesh), and discharge medium (quartz tube).During the experiment, the gas supply system controls the gas changes, and the input power and ionization time are adjusted by the high-voltage pulse power supply (CTP-2000).
The length of outer iron wire mesh is changed to control the discharge length, and the diameter of inner copper tube is changed to control the discharge gap.
Fly ash was washed with deionized water, dried, ground, and sieved through a 200-mesh screen.5 g of fly ash was evenly spread on the discharge area using a spoon at a time.The output power range of the high-voltage pulse power supply was between 1 and 80 W, and the output frequency was between 1 and 30 kHz.
The refractive index of fly ash is set to 1.54, with water as the dispersing medium having a refractive index of 1.333.The original particle size distribution of the ash is measured using a laser particle size analyzer as shown in Fig. 3.

Performance evaluation
As the core of the SCR system, the catalyst's function is to eliminate NO x from the exhaust gas, and the denitration rate directly reflects the effect of NO x removal.
where [NO x ] in is the inlet NO x concentration, and [NO x ] out is the outlet NO x concentration.

Characterization
SEM analysis was performed using a Korean company's EM-30 plus scanning electron microscope to observe the surface morphology of fly ash material.The specific surface area analysis was carried out using a Micromeritics ASAP 2010 rapid surface area and pore size analyzer, and calculated based on the BET equation.The surface functional group content of fly ash was determined using the Boehm method.The composition and elemental content changes of the original ash and modified fly ash were determined using X-ray fluorescence spectroscopy (XRF) analysis.For the catalyst crystal structure, testing was conducted using the X-ray diffractometer (XRD-6100).The testing range was from 20° to 80°, with a scanning speed of 16°/min, Cu-Kα radiation source, 40 kV tube voltage, and 40 mA tube current.Fourier transform infrared spectroscopy was used to determine the infrared spectra of the catalyst surface using the Spectrum 400.The spectral information was recorded in the wavelength range of 4000-500 cm −1 . (1)

Orthogonal experimental design
Plasma modified denitration is a reaction influenced by multiple factors.Adopting the orthogonal experimental method and variance analysis of experimental data can not only help in finding the best combination of factor levels with a small number of experiments but also in obtaining the main and secondary relationships and interaction effects between various factors.Studies [11][12][13] have shown that, among the reactor parameters, the discharge gap and discharge length have a significant effect, while among the process parameters, the input power and ionization time have a significant effect.In addition, different modified gases also have different effects on the removal of nitrogen oxides.Therefore, the modified gas type was determined to be O 2 and N 2 , and then 4 factors were selected under O 2 atmosphere, each with 3 different levels, randomly arranged, as shown in Table 1.
According to Table 1, 10 orthogonal experiments were conducted with denitration rate as the evaluation index.The results are shown in Table 2.
Variance analysis was performed using Multilevel Categoric analysis in Design-Expert13, and the results are shown in Table 3.
The order of the four factors affecting denitration rate is as follows: input power > discharge gap > discharge length > ionization time.The p-values indicate that input power and discharge gap have a significant impact on denitration rate, while the effects of ionization time and discharge length are not significant.The optimal parameter combination is an input power of 40 W, ionization time of 30 min, discharge length of 80 mm, and discharge gap of 2 mm.As this combination was not present in the experimental table, it was validated experimentally and yielded a denitration rate of 58.10%,which was higher than other combinations of factor levels.

Gas modification study
Due to the introduction of gas during the modification process, the mineral composition of fly ash before and after oxygen gas modification was analyzed using XRD, as shown in Fig. 5.  ).The characteristic peaks of modified fly ash are similar to those of original fly ash, with some characteristic peaks of quartz weakened, indicating that modification has some effect on the crystal structure of fly ash, but there is no evidence to suggest the introduction of new elements in the crystal structure.This may also be due to the fact that the amount of substances produced by plasma modification is insufficient for detection by XRD analyzer.

Level
This study utilized X-ray fluorescence (XRF) technology to determine the mineral composition and elemental content changes of fly ash in its original state and after oxygen modification, aiming to ascertain the effect of modification on the content of major components, and the results are shown in Tables 4 and 5. Table 4 presents a comparison of the elemental content changes of fly ash in its original state and after modification, while Table 5 compares the changes in the oxide content of fly ash in its original state and after modification.The oxide percentage content obtained in this experiment was derived from the elemental percentage content.
From Table 4, it can be seen that after plasma discharge modification, there is no significant change in the content of each element in fly ash compared to before modification.Looking at the composition content changes of fly ash before and after modification in Table 5, it can be observed that the proportion of SiO 2 and Al 2 O 3 is the highest, corresponding to the major components mullite and quartz in fly ash, and there was no significant change in the composition proportions before and after modification.Tables 4 and 5 indicate that the plasma discharge method did not overly alter the content of elements and components in fly ash.The slight variations in the content of each component may be due to inadvertent errors caused by the non-uniformity of the selected samples and differences in the test area during the testing process.
The effects of gas atmosphere changes on fly ash were characterized by SEM, Boehm titration, and FTIR, as shown in Figs. 6, 7 and 8, respectively.
In Fig. 6, the untreated fly ash was mostly spherical particles of varying sizes, with a relatively regular shape.The morphology of modified fly ash changed significantly, with more small pores on the surface compared to the original fly ash, and an increased number of small particles.This was due to the destruction of the original structure of fly ash, exposing many small particles.Comparing Fig. 6b, c, it could be seen that the shell of fly ash was etched more during oxygen gas modification, increasing the surface area and reducing the adsorption pore size, which further improved the denitration rate.
The catalyst surface has four types of functional groups: acidic groups, basic group, phenolic hydroxyl groups, and carboxyl groups.In Fig. 7, unmodified fly ash surfaces have many basic group.After modification, both  www.nature.com/scientificreports/acidic and basic group increased by about 2-3 times.Phenolic hydroxyl and carboxyl groups are present in small quantities and do not change significantly after modification.Comparing with the same functional groups, O 2 modification results in a more significant increase in basic group, while N 2 modification results in a more significant increase in acidic groups.Based on the experimental results, it can be concluded that the denitration effect of O 2 modification is better than that of N 2 , indicating that basic group have a more positive effect on improving denitration rate.In Fig. 8, the composition and quantity of functional groups on fly ash surface before and after modification have changed significantly, with peaks at 3450 cm −1 , 2360-2332 cm −1 , 1610 cm −1 , and 1100 cm −1 showing significant changes.The peak of 3450 cm −1 corresponds to the characteristic peak of O-H, 2360-2332 cm −1 corresponds to the characteristic peaks of C≡C and C≡N, 1610 cm −1 corresponds to the characteristic peaks of C=O, C=N, and N=O, and 1100 cm −1 corresponds to the characteristic peak of -O-.www.nature.com/scientificreports/Due to the drying process, the possibility of moisture content in fly ash is very low.The 3450 cm −1 peak corresponds to the O-H stretching vibration of fly ash surface, and its broadening and flattening indicates an increase in oxygen radicals.The 2360-2332 cm −1 peaks of modified fly ash are weakened or even disappeared, while the 1610 cm −1 peak becomes sharper, indicating that the original C≡C and C≡N have been oxidized to C=O and N=O.This is mainly generated by the loss on ignition of fly ash, i.e., the unburned carbon.The peak shape of unmodified fly ash near 1100 cm −1 is sharp because it contains a lot of SiO 2 , which is combined as Si-O-Si, and the peak shape becomes broader and flatter after modification, indicating that the crystallinity of SiO 2 is destroyed and becomes more dispersed.Overall, N 2 modification is weaker than O 2 modification in terms of increasing O-H and destroying SiO 2 crystallinity.
Combining Figs. 6 and 7, it can be inferred that O 2 -modified fly ash can produce high-energy charged particles in a plasma reactor, which can more effectively oxidize the functional groups on fly ash surface, generating more active sites of C=O and N=O and thus promoting catalytic reactions 14 .

Study on the influence of parameter change
In the orthogonal experiments of 10 groups, BET and Boehm titration analyses were performed, and the results are shown in Table 6.
Using specific surface area, adsorption pore size, total pore volume, acidic groups, basic group, phenolic hydroxyl groups, and carboxyl groups as independent variables and denitration rate as the dependent variable, linear regression analysis was performed.The VIF value in the obtained model is greater than 10, which means that there is a collinearity problem, and there are closely related independent variables in the model.
Generally, the mean pore size is proportional to the specific pore volume and inversely proportional to the specific surface area.Combining the p-value significance of adsorption pore size, the specific surface area,and total pore volume were removed as independent variables.Acidic groups include phenolic hydroxyl and carboxyl groups, carboxyl groups decompose at temperatures ranging from 200 to 300 °C, but the content is too low.Phenolic hydroxyl groups have a high content but usually decompose at temperatures higher than 600 °C15 .www.nature.com/scientificreports/Therefore, hydroxyl and carboxyl groups were removed as independent variables.By refitting the model, Table 7 can be obtained.The model's R 2 value of 0.789 indicates that the adsorption pore size, acidic groups, and basic group can explain 78.9% change of the variation in denitration rate.The model passed the F-test, and all the VIF values were less than 5, indicating that there is no collinearity problem.The DW value is around 2, indicating that the model does not exhibit autocorrelation and that there is no correlation between the sample data, indicating a good model.
The model shows that the basic group have a significant positive effect on denitration rate, while the adsorption pore size and acidic groups have a significant negative effect on denitration rate.This suggests that the more basic group generated by plasma modification, the more denitration rate can be increased, while an increase in acidic groups will actually reduce denitration rate.Meanwhile, the smaller the adsorption pore size, the easier it is to improve the denitration rate.
The variance analysis of the orthogonal experimental results shows that the input power and discharge gap have a significant effect on denitration rate.Examining the effects of these two factors on adsorption pore size and basic group, as shown in Tables 8 and 9.
In Table 8, the model is significant at the 0.10 level.From the p-value observation, the discharge gap has a significant effect on adsorption pore size.Therefore, reducing the discharge gap can simultaneously reduce the adsorption pore size.In Table 9, the model is significant at the 0.05 level.From the p-value observation, the input power has a significant effect on the basic group.Therefore, increasing the input power can simultaneously increase the number of basic groups.

Discuss
The functional group changes of fly ash before and after denitration were characterized by Boehm titration and FTIR analysis, as shown in Fig. 9. www.nature.com/scientificreports/In Fig. 9a, the number of surface functional groups of the modified fly ash all decreased after reaction, and the reduction in basic functional groups was greater than that of acidic group.
In Fig. 9b, both the composition and content of surface functional groups of fly ash changed significantly after denitration.The peak at 3440 cm −1 was attributed to the vibration of O-H, and it was inferred that crystalline water was physically adsorbed on the surface after the loss of oxygen free radicals, based on the actual engine operating conditions.The peaks at 2924 cm −1 and 2858 cm −1 were both related to the vibration of CH 2 , indicating that residual hydrocarbons in the engine exhaust are adsorbed on the surface of fly ash catalyst.The continuous stretching vibration near 1492 cm −1 was due to the unburned alkane in diesel fuel.The peak at 1606 cm −1 was attributed to the characteristic peaks of C=O, C=N, and N=O, and its decrease after denitration indicated the involvement of the corresponding functional groups in chemical reactions.Therefore, C=O, C=N, and N=O were found to have a positive impact on the catalytic effect of the SCR reaction.
It is speculated that the possible denitration mechanism of plasma-modified fly ash includes physical adsorption, chemical adsorption, and absorption processes, as shown in Fig. 10.
Due to its porous structure and large surface area, NO and NO 2 molecules are enriched on the surface through physical adsorption 16 .And the smaller the surface pore size, the more favorable it is for adsorption.

Figure 1 .
Figure 1.Composition and content of oxide in fly ash.

Figure 2 .
Figure 2. Schematic diagram of the plasma-modified system.

Figure 3 .
Figure 3. Particle size distribution of original fly ash.

Figure 4 .
Figure 4. Schematic diagram of the denitration evaluation device.

Figure 5 .
Figure 5. XRD of fly ash before and after modification with different gases.

Table 4 .
Comparison of element content changes.

Table 5 .
Comparison of composition changes.

Table 6 .
Bet and Boehm titration results of fly ash.