Intensification of NO x Conversion over Activated Coke by Ozone Oxidation for Sintering Flue Gas at Low Temperatures.

Denitration (De-NO x ) over activated cokes (ACs) for sintering flue gas needs intensification. Gaseous reactions in a gas mixture containing NO, NO2, and NH3, with the effect of O2 concentration and moisture, were taken into consideration in the study of NO x conversion over ACs. Experimental studies on NO x conversion with and without NH3 over ACs were conducted using a fixed-bed reactor at 100 °C. The results demonstrated that moisture significantly affected NO x removal over ACs, especially the NO2 conversion. Under dry conditions, a disproportionation reaction of NO2 over ACs dominated NO x conversion with no NH3, whereas apparent fast selective catalytic reduction (SCR) over the ACs was observed in the presence of NH3. Regardless of the presence of absence of NH3 in wet mixtures, NO2 adsorption on ACs via the disproportionation route dominated the NO x conversion. Increasing the NO2/NO ratio in the simulated flue gas enhanced the NO x conversion rate over ACs. -C(ONO2) deposition on ACs generated by the disproportionation route inhibited NO x conversion with time. O3 oxidation was found to be efficient in increasing the NO2/NO ratio and intensifying the NO x conversion compared with commercially direct NH3-SCR over ACs. Increasing the temperature and decreasing the gas hourly space velocity can promote NO x conversion over ACs after O3 oxidation. NO oxidized with O3 coupled with NH3 spray and continuous regeneration of ACs is a potential method for removing NO x from sintering flue gas.

Introduction

SO2, NO, and particulate matter are the dominant flue gas pollutants generated by coal combustion, which remains the leading primary energy supply process in China. After the successful application of pollution control technologies in coal-fired power plants, emissions from other industrial processes have been increasingly attracting attention.[1−5] According to the latest China emission standard published in 2019, the emission limit of particulate matter, SO2, and NO for the sintering flue gases is 10, 35, and 50 mg/Nm3, respectively.[6] To meet the emission standard, activated cokes (ACs) have been widely recognized as a potential candidate for SO2 and NO removal from sintering flue gases[7] and the schematic diagram of the flue gas purification process using ACs is illustrated in Figure .

Figure 1

Recycling AC sintering flue gas pollutant controlling system.

In this process, SO2 can be captured and converted into sulfuric acid, which can be used in the steel manufacturing. The adsorption of SO2 on ACs as well as the regeneration of ACs have been widely studied and the emission standard of SO2 was achieved.[8−12] NO removal by using ACs was carried out after the desulfurization of flue gas, and NH3 has been commonly used as a reductant to react with NO to form gaseous nitrogen. ACs act as not only the adsorbent but also a catalyst in the process of NO removal. The adsorption properties of NO and NO2 on ACs have been studied in the absence of NH3.[13−17] Some researchers believed that NO was catalytically oxidized to NO2 on the surface of ACs, and NO2 was adsorbed and converted into HNO3 in the presence of O2 and moisture.[13,18,19] When NH3 was introduced into this process, ACs acted as a catalyst to convert NO into nitrogen via the following selective catalytic reduction (SCR) reaction.[20]

Because the temperature for SO2 adsorption at upstream must be controlled below 150 °C, the catalytic activity of ACs is limited for the NH3-SCR reaction.[21−23] Low-temperature NH3-SCR itself has been a research hotspot all over the world.[24] Development of a highly active catalyst at low temperatures in the presence of moisture and SO2 is of interest. Numerous studies have demonstrated that the doping of transition metals such as vanadium,[21,25,26] iron,[22,27,28] manganese,[29] and cerium[30,31] on ACs could effectively improve their catalytic activity at low temperatures. Although the NO removal efficiency could reach as high as 40%, the modified ACs still suffered from low-moisture resistance at low temperatures, which limits their application in NO removal. Besides, flue gas reheating by using a gas–gas heater fitted downstream of a flue-gas desulfurization (FGD) reactor has also been considered to improve the catalytic activity. In addition to improving the catalytic activity, previous works also demonstrated that the increase of the NO2/NO molar ratio could enhance the NO conversion using NH3 as a reductant (NH3-SCR). When the NO2/NO molar ratio = 1, the denitration (De-NO) rate was found to increase dramatically compared to the NO2/NO molar ratio of 0.[31−33] This fast SCR method consists of the following reaction

To increase the NO2/NO molar ratio, lots of oxidizing regents have been utilized for oxidizing NO to NO2, such as ozone, hydrogen peroxide, chlorine hypochlorite, chlorine dioxide, etc.[34] As the typical oxidizing regent, ozone attracts lots of attention both in investigations and applications.[4,35−38] Among them, NO oxidized by ozone accompanied by a wet scrubber was the most popular technology. NO was oxidized to NO2 or N2O5, which was more soluble in water than NO, and can be removed using the scrubber. Generally, N2O5 was preferred when accompanying with a wet scrubber. NO oxidation efficiency reached higher than 90% with O3/NO ⇐ 1 at 100 °C.[35,36] No N2O5 formed when O3/NO ⇐ 1.[37] Increasing temperature also decreased N2O5 yield when O3/NO > 1, due to the decomposition of N2O5 to NO2.[37] NO2 was the mean oxidation product when O3/NO ⇐ 1 at low temperatures. NO2 is more susceptible to be adsorbed than NO. When NO oxidation was accompanied by adsorption, most studies focused on NO2 adsorption to the adsorbent.[13,17]

Studies on the fast SCR method have used TiO2 or AC-supported metal oxide as a catalyst. As for sintering flue gas De-NO over ACs, how the fast SCR reaction affects NO conversion was rarely studied. Besides, NO adsorption on ACs is always accompanied by oxidation of NO to NO2.[39] Furthermore, sintering flue gas is characterized by high O2 and moisture concentrations. How NO adsorption and NH3-SCR reaction affect NO conversion on ACs in a sintering flue gas atmosphere was unknown. Ozone (O3) was introduced into gas mixtures for increasing the NO2/NO molar ratio in the experiment to study its effect on NO removal.[35] Feasibility of O3 oxidation combined with NH3 spray for NO removal over ACs after FGD was also discussed.

Results

Gaseous Characteristics of NO and NH3 with the Effect of O2

Simulated gas flow used in the experiment is a mixture of gases from a cylinder. This would result in difference from the real flue gas. A clear understanding of the gaseous reaction among the gases is fundamental for following studies. Figure illustrates the effect of O2 and NH3 on the composition of a gas mixture containing NO and NO2. Experimental conditions for this study can be found in sets I and II of Table . As can be seen from Figure a, the NO2 concentration slightly increased with the increase of O2 concentration from 0 to 20%, while the NO concentration decreased continuously. It indicates that part of NO was oxidized to NO2 in the presence of O2. However, no apparent difference in the NO concentration between the gas mixture with and without the addition of NH3 was observed. Compared to wet conditions, both NO and NO2 concentrations in dry gas mixtures were lower. NO2 concentrations increased from 66.1 to 167.5 ppm in a wet gas mixture with the increase of the O2 concentration, while the NO2 concentration increased from 57.9 to 96.8 ppm under dry conditions. NH3 conversion rate increased from 5.8 to 11.8% under wet conditions, which ranged from 8.9 to 10.2% under dry conditions. The conversion of NH3 should include those being oxidized by NO or O2, and the oxidized products might include N2 and NO. According to the analysis of NH3 conversion and NO oxidation in dry and wet mixtures, more NO was reduced to N2 by NH3 under dry conditions than that under humid conditions.

Figure 2

Characteristics of NO mixtures with the effect of O2 and NH3 (a) NO mixtures; (b) NO2 mixtures.

Table 2

Experimental Conditions

sets	carrier gas (500 mL/min)	descriptions
set I	i: N2 + (0, 4, 8, 12, 16, 20%) O2 + 900 ppm NO + 900 ppm NH3;	gaseous reaction at 100 °C
	ii: N2 + (0, 4, 8, 12, 16, 20%) O2 + 8% H2O + 900 ppm NO + 900 ppm NH3;
	iii: N2 + (0, 4, 8, 12, 16, 20%) O2 + 8% H2O + 900 ppm NO
set II	i: N2 + 8% H2O + 700 ppm NO2; N2+ 8% H2O + 700 ppm NO2 + 700 ppm NH3;	gaseous reaction at 100 °C
	ii: N2 + 20% O2 + 8% H2O + 700 ppm NO2 + 700 ppm NH3;
	iii: N2 + 20% O2 + 8% H2O + 700 ppm NO2
set III	i: N2 + 20% O2 + 8% H2O + 900 ppm NO + 900 ppm (if present) NH3	NOx conversion over 1 g ACs at 100 °C
	ii: N2 + 20% O2 + 750 ppm NO + 150 ppm NO2 + 900 ppm (if present) NH3
set IV	i: N2 + 20% O2 + 450 ppm NO + 450 ppm NO2 + 900 ppm NH3	NOx conversion over 1 g ACs at 100 °C
	ii: N2 + 20% O2 + 700 ppm NO2 + 900 ppm NH3
set V	i: N2	regeneration of ACs (set IV-i)
set VI	i: N2 + 20% O2 + 8% H2O + 900 ppm NO + (0, 150, 300, 500, 700, 900, 1080, 1450 ppm) O3	gaseous reaction at 100 °C
	ii: N2 + 20% O2 + 8% H2O + 900 ppm NO + 900 ppm NH3 + (0, 150, 300, 500, 700, 900, 1080, 1450 ppm) O3
set VII	i: N2 + 20% O2 + 8% H2O + 900 ppm NO + 900 ppm NH3 + 500 ppm O3	NOx conversion over ACs at 100, 180, 250 °C
set VIII	i: N2 + 20% O2 +8% H2O + 900 ppm NO + (0, 900, 1800 ppm) NH3 + 500 ppm O3	NOx conversion over 4 g ACs at 100 °C

Figure 3

NO concentrations after ACs under dry and humid conditions.

Figure b illustrates the change in composition of wet NO2 balanced with N2 by the addition of O2 and/or NH3. It can be found that NO was detected in all experiments, indicating that NO2 was decomposed into NO and O2 at an experimental temperature of 100 °C. Compared with pure NO2 balanced with N2, O2 could inhibit the NO2 decomposition as the NO concentration decreased from 60.61 to 26.22 ppm by adding 20% O2 into the gas mixture. It can be seen from Figure b that the addition of NH3 can effectively convert NO2 into N2. Only 336.15 ppm NO2 and 39.36 ppm NO were detected at the outlet of the reactor, which means over 50% of the NO2 was converted. However, the conversion decreased to less than 32% in the presence of 20% O2, being indicative of the inhibition effect of O2 or the oxidized atmosphere on the gaseous reaction between NH3 and NO2.

NO Conversion over ACs under Dry and Humid Conditions

Water vapor is believed to have a significant impact on NO conversion over ACs. Sintering flue gas features high moisture. NO conversion over the commercial ACs under dry and humid conditions was conducted, and the results are shown in Figure . Comparable NO and NO2 concentrations were compounded in the humid or dry mixtures (set III from Table ). With increasing experimental time, the NO2 concentration at the outlet of the reactor decreased and leveled off at about 35 ppm under both humid and dry conditions. NO concentration from downstream of a fixed-bed reactor was 51 ppm lower than the NO concentration at the inlet under dry conditions, while 37 ppm higher than the inlet NO concentration in the presence of moisture. The drop of NO and NO2 concentrations in the dry mixture is derived from the fast SCR reaction and NO2 adsorption.

Figure 4

NO transient conversion over ACs with the effect of NH3 in dry mixtures.

According to the results reported in the literature,[15−17,40] there are two main routes for NO2 adsorption on ACs:

NO2 was adsorbed on −C(O) or −C(*) complexes as −C(ONO2) or −C(NO2).

This can be defined as the nondisproportionation route.

A pair of adsorbed NO2 (−C(NO2)) on one active site or two adjacent active sites reacts through the disproportionation route

When NO2 was adsorbed through the disproportionation route, the adsorption of 2 M NO2 will release 1 M NO and leave 1 mol of −C(ONO2) on the AC surface. Operation conditions, such as temperature, O2 concentration, and moisture, play an important role in determining the fraction of NO conversion via nondisproportionation and disproportionation routes. The increase of the NO concentration and the drop in the NO2 concentration under wet conditions implied that NO2 adsorption over ACs was dominated by the disproportionation route in the presence of H2O.

NO conversion over ACs with NH3 supposed to be derived from the complex interaction between NO2 adsorption through the two routes and the NH3-SCR reaction. Transient reaction analyses were conducted under wet and dry conditions, respectively, to investigate the effect of those processes on NO conversion.

Figure shows the change in concentrations of NO and NO2 along with the reaction time under dry conditions. When experiment was conducted without the addition of NH3, the NO2 concentration sharply dropped to nearly zero at the initial state and then slowly increased to the initial NO2 concentration in 294 min. A reverse trend was observed for the NO concentration, which rapidly increased to about 624 ppm that was higher than the initial NO concentration and then slowly dropped to the level equal to the initial concentration. In comparison, the decrease in the concentration of NO2 is about twice the increase in the concentration of NO, indicating that the increase of the NO concentration is mainly attributable to the NO2 adsorption on ACs via the disproportionation route. When NH3 was introduced to the mixture at 294 min, the NO concentration sharply dropped within 20 min and remained stable at about 36 ppm, which was much lower than the initial NO concentration. Meanwhile, the NO2 concentration rapidly dropped to near zero and then slowly increased, which was still much lower than the initial NO2 concentration after 356 min. It can be found from Figure that the NO2 conversion in the presence of NH3 could be divided into two parts: the reduction of NO2 and NO to N2 via fast NH3-SCR (blue area) and the adsorption of NO2 through a nondisproportionation route (gray area). There was no significant increase of the NO concentration being found when NH3 was introduced, which meant that the adsorption of NO2 on ACs via a disproportionation route was limited in the presence of NH3. However, as aforementioned, without the addition of NH3, NO2 adsorption on ACs via a disproportionation route played the dominant role in NO2 removal.

Figure 5

NO transient conversion over ACs with the effect of NH3 under wet conditions.

Figure shows the effect of NH3 on NO conversion over ACs under wet conditions. When NH3 flow was turned off, no significant change in the NO2 concentration was observed, whereas the NO concentration slightly increased. When NH3 flow was turned on again, the NO concentration decreased by 12 ppm, while it was still higher than the initial NO concentration, which means that the addition of NH3 partially inhibited the NO2 conversion via a disproportionation route under wet conditions. However, the NO2 concentration remained unchanged, indicating that the addition of NH3 had no contribution to NO2 removal under wet conditions. Hence, it can be concluded that increasing the NO2/NO ratio supposed to enhance the NO conversion ratio.

Figure 6

NO conversion over ACs with the effect of the NO2/NO ratio.

Figure illustrates the NO conversion rate along with reaction time with different NO2/NO ratios. NO conversion was only about 12% and remained stable during the experiment period when the NO2/NO ratio was 0.25. When the NO2/NO ratio increased to 1.21, the NO conversion rate was about 30% at the initial stage and decreased linearly to about 3% after 300 min. With a further increase of the NO2/NO ratio to 24, the NO conversion ratio reached 46% at the initial stage and then linearly decreased to about 20% and maintained at about 20% after 250 min. The above results confirmed that increasing the NO2/NO ratio could effectively improve the NO conversion rate, while further investigation is required to explain why the NO conversion rate decreased along with reaction time at a NO2/NO ratio higher than 0.25.

Figure 7

Breakthrough of NO2 over ACs in wet mixtures.

Figure shows the breakthrough curves of NO2 conversion over ACs at a NO2/NO ratio of 1.21 and 24 under wet conditions. A significant decrease of the NO2 concentration and an increase of the NO concentration compared with initial concentration were observed, indicating that the disproportionation route of NO2 adsorption is dominant in NO conversion at NO2/NO = 24 and 1.21. Different from the stable NO and NO2 outletconcentrations in the ratio NO2/NO = 0.25, the NO2 concentration increased slightly after reaching nearly zero and the NO concentration decreased slightly after cresting. The adsorbed NO2 in the form of −C(ONO2) occupied the active sites on the AC surface. The active sites were reduced with the increase of the adsorbed NO2 molecules, which resulted in the breakthrough of NO2. The initial NO2 concentration of the gas mixture with a NO2/NO ratio of 0.25 is too low, and the adsorption time was not long enough to achieve the breakthrough of NO2.

Figure 8

NO2 adsorption and reduction over ACs.

Figure shows the reduction percentage of NO2, calculated based on the production of NO (gray shadow in Figure ), and the adsorption percentage of NO2, calculated by subtracting the reduced NO2 from the total NO2 conversion (red shadow in Figure ), along with reaction time, within the first 150 min, the NO2 reduction rate was almost equal to the NO2 adsorption rate. This was consistent with the disproportionation route. After 150 min, NO2 reduction was getting higher than NO2 adsorption and the difference increased along with reaction time. The decrease of NO2 reduction and adsorption was due to the reducing active sites on ACs for the formation of −C(ONO2) along with reaction time. The production of NO did not follow the production of −C(ONO2). This means that the direct reduction of NO2 to NO occurs, which might not make any contribution to final NO conversion.

Figure 9

Gaseous oxidation of NO by ozone.

According to Gao[15] and Jeguirim,[40] the adsorbed NO2 would release NO and leave −C(O) on the surface of ACs. Generally, −C(O) would further react with NO2 to form −C(ONO2), which follows the disproportionation route. With the increase of the adsorption time, not every −C(O) would react with NO2, which resulted in a higher NO2 reduction rate than the adsorption rate as shown in Figure .

NO Conversion over ACs with Ozone Oxidation

As discussed above, a higher NO2/NO ratio could improve the NO conversion. Ozone oxidation has been considered as a common method for NO removal. The effect of O3 on NO oxidation, especially in the presence of NH3, was studied before the mixture went through the fixed-bed reactor containing ACs, and the results are shown in Figure .

Figure 10

IR spectra of the produced crystalline phase in experiment at O3/NO = 1.5.

It can be seen that with the increase of the O3 concentration from 0 to 900 ppm (O3/NO ⇐ 1), the NO concentration sharply dropped, whereas the NO2 concentration increased rapidly whether with or without NH3. With a further increase of the O3 concentration to higher than 900 ppm (O3/NO > 1), the NO concentration dropped to zero for both with and without NH3. The NO2 concentration stabilized at about 650 ppm in the absence of NH3 in the gas mixture. As for gas mixtures with NH3, the NO2 concentration sharply dropped to about 166 ppm at a O3/NO ratio of 1.5. The NH3 conversion rate was 10–20% at O3/NO ⇐ 1, and sharply increased to 67% at O3/NO = 1.5. Furthermore, white crystals were found on the inside wall of the tubes after NH3 addition. The produced crystalline solids were collected and characterized by infrared spectroscopy (IR) using a Fourier-transform infrared spectroscopy (FTIR) spectrometer (Thermo Scientific Nicolet 6700). The sample was mixed with KBr at a weight ratio of 1:200 and milled before being flaked. The IR spectra of the white crystals are shown in Figure . According to literature,[41,42] the solid is identified as ammonium nitrate (NH4NO3). The utilization rate of O3 was around 80% at O3/NO ⇐ 1, which dropped upon further increasing the O3/NO ratio. When O3/NO > 1, N2O5 was generatedin the gas mixture,[37] which resulted in the decrease of the O3 utilization rate. N2O5 further reacts with NH3 and produced NH4NO3,[43] which caused the decrease of the NO2 concentration and the increase of the NH3 conversion rate.

Figure 11

NO transient conversion over ACs after oxidized with O3 with the effect of temperature.

Transient experiments were conducted over ACs in a wet mixture consisting of 900 ppm NO, 500 ppm O3, and the results are in Figure . At 100 °C, NO conversion after O3 oxidation exhibits a similar trend compared to directly mixing of NO and NO2 at a ratio of 1.21. The produced NO decreased along with NO2 breakthrough. This indicates that the existence of O3 in the gas mixture exhibits hardly any impact on NO conversion over AC when O3/NO ⇐ 1. When the experimental temperature increased to 180 °C, the NO concentration dropped to 413 ppm following a crest. The NO2 concentration stabilized at about 28 ppm after the crest. With the increase of experimental temperature to 250 °C, the NO concentration further decreased to 385 ppm after a crest. The NO2 concentration dropped to nearly zero. The decrease of NO and NO2 concentrations with increasing temperature was attributed to the weakening NO2 adsorption via a disproportionation route and the strengthening fast SCR reaction. The crests of NO and NO2 concentrations at 180 °C and the NO concentration at 250 °C were resulted from the decomposition of deposited NH4NO3 in the ACs. The results can prove that higher operation temperature can promote total NO conversion.

Figure 12

NO transient conversion over ACs after oxidized with O3 at a low GHSV.

Although increasing the operation temperature is an effective method to enhance NO conversion, it is difficult to be achieved in practical applications. Increasing the gas hourly space velocity (GHSV) and the NH3/NO ratio is also believed to be beneficial for NO conversion. The NO conversion of the gas mixture with different NH3/NO ratios at a low GHSV of only 4500 was studied and the results are shown in Figure . Compared with the breakthrough of NO2 in Figures and 12, the NO2 concentration stabilized at near zero for 350 min after putting more ACs in the reactor. This means lower GHSV can contribute to stable NO conversion for a longer reaction time. Transient change of NH3 in mixtures effected the NO concentration at the outlet of the reactor. When the NH3/NO ratio was 1 in the gas mixture, the NO concentration stabilized at about 500 ppm, which was approximately 170 ppm higher than the initial NO concentration. Taking NH3 away from mixtures resulted in the increasing of the NO concentration. The NO concentration resumed after NH3 reloaded as NH3/NO = 1. A further increase of the NH3/NO ratio to 2 led to a slight decrease of the NO concentration through the ACs. The NO concentration stabilized at around 490 ppm (10 ppm lower than NH3/NO = 1). The amounts of NO production and NO2 reduction approximately meet the disproportionation route molar ratio. This has demonstrated the domination of the disproportionation route in the process.

Figure 13

NO conversion mechanisms in the gaseous phase and over ACs in a highly oxidizing atmosphere.

Discussion

Reaction Mechanism over ACs

NO conversion mechanisms are summarized in Figure . Oxidation of NO by O2 or O3 under both wet and dry conditions as well as in the presence of NH3 was studied before the gas mixture was sent to the fixed bed containing ACs. O2 can oxidize NO with low efficiency, while O3 is more efficient. A slight reduction of NO and NO2 by NH3 was observed. If the O3 concentration was higher than the NO concentration, NH4NO3 crystals were formed in the gas mixture containing NH3. Moisture plays an important role in determining the conversion routes of NO over ACs. Under dry conditions, NO conversion was dominated by the disproportionation route if there was no NH3 in the mixture. Obvious fast SCR reaction as well as direct adsorption of NO2 to ACs were found after NH3 was added to gas mixtures. As for wet mixtures, the fast SCR reaction was too weak to be observed under most operating conditions, especially for the gas mixture containing a higher NO2 concentration at 100 °C. The disproportionation reaction dominated under most operation conditions and under wet conditions. Because of the deposition of −(ONO2) on the ACs, the disproportionation reaction was inhibited, which has resulted in the increase of the outlet NO2 concentration and the decrease of NO removal with time. With the breakthrough of NO2, the NO2 conversion gradually shifted from adsorption via the disproportionation route to direct reduction to NO. Increasing operation temperature can strengthen the fast SCR reaction over ACs, which has become quite important at 250 °C under wet conditions.

Figure 14

Regeneration curves of ACs after the reaction under the conditions of set IV-i.

Method of NO Removal with ACs in a Highly Oxidizing Atmosphere

According to the analysis discussed in section , increasing the NO2/NO molar ratio by O3 oxidation can promote NO conversion over ACs at 100 °C with moisture in flue gas. The NO conversion cannot exceed 50% (CNOx = CNO2) due to the dominant disproportionation reaction of NO2 over ACs with one-time oxidation. Increasing operation temperature is an efficient method to break the conversion limit, while energy consumption created difficulties. NH3 is essential in NO conversion. Lack of NH3 will resulted in the increase of direct reduction of NO2 to NO over ACs, which is negative for total NO conversion. Further study is required to optimize the amount of NH3 added into the flue gas in order to prevent NH3 escape. The O3/NO ratio should be lower than 1 to prevent the formation of ammonium nitrate in the reactor with NH3.

Considering–C(ONO2) deposition on ACs via the disproportionation reaction, ACs need to be continuously regenerated to ensure stable NO removal efficiency. In the design of reactors, GHSV should be coordinated with AC replacement. AC regeneration can be coupled with SO2-saturated ACs in the regenerator in Figure . Products of AC regeneration after NO conversion are shown in Figure . When temperature increased to 150 °C, NO, NO2, N2O, and NH3 were produced. Upon further increase of temperature, only NO was produced. The released NO and NH3 are expected to be further converted into N2 or acid liquor. The produced mixtures by regeneration are characterized by high temperature, low moisture, and low flux, which could be easily converted to N2 with the SCR reaction.

Figure 15

Schematic diagram of the fixed-bed experiment system.

Conclusions

Moisture and the oxidizing atmosphere contributed to a significant difference in NO conversion both with the gaseous reaction and adsorption/reduction over ACs, especially for NO2 conversion. The disproportionation reaction of NO2 over ACs dominated NO conversion under dry conditions with no NH3. Apparent fast SCR was observed under dry conditions with NH3 over the ACs. Under wet conditions, the disproportionation reaction dominated NO conversion over ACs both with or without NH3 in the gas mixtures. Increasing the NO2/NO ratio in the gas mixture enhanced the NO conversion rate over ACs. −C(ONO2) deposition on ACs generated by the NO2 disproportionation route resulted in the decrease of the NO conversion rate along with the reaction time. O3 oxidation was efficient in increasing the NO2/NO ratio, whereas NH3 is necessary for NO conversion under wet conditions. Increasing temperature and decreasing GHSV can promote NO conversion over ACs after O3 oxidation. NO oxidation by O3 coupled with NH3 and continuous regeneration of ACs is a potential method for NO removal from sintering flue gas.

Materials and Methods

Materials and Characterization

The commercial coal-based AC specialized for desulfurization and denitration was utilized in the study. The ACs received are columnar with a diameter of 9 mm, and were crushed and sieved into particles in the mesh range of 80–150 for this study.

The chemical composition of the sample was determined using an elemental analyzer (Vario EL). The textural properties of the sample were characterized by an automatic surface analyzer (Quantachrome Autosorb 1C) as N2 adsorption/desorption isotherms at 77 K. The specific surface area was calculated by the Brunauer–Emmett–Teller method using the N2 adsorption isotherm. The single-point adsorption method was employed to calculate the total pore volume of the sample. The micropore volume was calculated using the t-plot method. The chemical composition and textural properties of the sample are shown in Table .

Table 1

Chemical Composition and Porous Texture of the Commercial Activated Coke

elemental analysis (wt %)a	C	H	O	N	S
	83.76	1.32	0.89	0.76	0.3
porous texture	SBET (m2/g)	Vtotal (cm3/g)	Vmic (cm3/g)	Vmeso–macro (cm3/g)	D (nm)
	192.26	0.109	0.054	0.055	2.28

Air-dry; SBET: specific surface area; Vtotal: total pore volume; Vmic: micropore volume; Vmeso–macro: mesopore and macropore volume; and D: average pore size.

NO Adsorption, Reduction, and Desorption Tests

The NO conversion (adsorption and reduction) and desorption tests were carried out using a 500 mm long quartz fixed-bed tube reactor (17 mm i.d.), as shown in Figure . All flue gas components except ozone (O3) and water vapor were supplied in cylinders and were mixed in a gas mixer to simulate the flue gas. The flow rate was precisely controlled using mass flow controllers. O3 was made of pure O2 using an ozonator, and water vapor was generated using a heated water bubbler. All the tubes, valves, and joints in contact with SO2 were constructed from either quartz or polytetrafluoroethylene. Moreover, electric-heating tape (Thermolyne) embedded with temperature controllers was used to heat the transport line both upstream and downstream of the fixed-bed reactor to preheat the simulated flue gas and prevent any possible condensation before analysis. The NO, NO2, and NH3 concentrations were monitored and recorded continuously every 5 s using an on-line FTIR spectroscopy gas analyzer (Dx4000, Gasmet Company, Finland). The O3 concentration was analyzed using an ozone monitor (GF-Z-3-50, Shenzhen).

The experimental conditions are summarized in Table . In each typical conversion experiment operation, ACs (if required) were put into the glass reactor. Before each experiment, the gas mixtures compositions were measured by FTIR spectroscopy through the bypass of the reactor. When the desired value was reached and stabilized, the gas flow was switched to the glass reactor to start the NO conversion and desorption experiments.

NO conversion, NO2 adsorption, NO2 reduction, NH3 conversion, and O3 utilization were calculated according to the following equationswhere, CNH3,inlet and CNH represent the NH3 concentration (ppm) in the gas mixture at the inlet and outlet of the reactor, respectively, while CNO,inlet and CNO,outlet represent the NO concentration (ppm) in the gas mixture at the inlet and outlet of the reactor, respectively. CO is the O3 concentration (ppm) at the inlet of the reactor.