Hybrid selective noncatalytic reduction (SNCR)/selective catalytic reduction (SCR) for NO_x removal using low-temperature SCR with Mn-V₂O₅/TiO₂ catalyst

Abstract

A hybrid selective noncatalytic reduction/selective catalytic reduction (SNCR/SCR) system that uses two types of technology, low-temperature SCR process and SNCR process, was designed to develop nitrogen oxide (NO_x) reduction technology. SCR was conducted with space velocity (SV) = 2400 hr⁻¹ and hybrid SNCR/SCR with SV = 6000 hr⁻¹, since the study focused on reducing the amount of catalyst and both achieved 98% NO_x reduction efficiency. Characteristics of NO_x reduction by NH₃ were studied for low-temperature SCR system at 150 °C using Mn-V₂O₅/TiO₂ catalyst. Mn-added V₂O₅/TiO₂ catalyst was produced, and selective catalyst reduction of NO_x by NH₃ was experimented. NO_x reduction rate according to added Mn content in Mn-V₂O₅/TiO₂ catalyst was studied with varying conditions of reaction temperature, normalized stoichiometric ratio (NSR), SV, and O₂ concentration. In the catalyst experiment according to V₂O₅ concentration, 1 wt.% V₂O₅ catalyst showed the highest NO_x reduction rate: 98% reduction at temperature window of 200~250 °C. As a promoter of the V₂O₅ catalyst, 5 wt.% Mn was added, and the catalyst showed 47~90% higher efficiency even with low temperatures, 100~200 °C. Mn-V₂O₅/TiO₂ catalyst, prepared by adding 5 wt.% Mn in V₂O₅/TiO₂ catalyst, showed increments of catalyst activation at 150 °C as well as NO_x reduction. Mn-V₂O₅/TiO₂ catalyst showed 8% higher rate for NO_x reduction compared with V₂O₅/TiO₂ catalyst in 150 °C SCR. Thus, (5 wt.%)Mn-(1 wt.%)V₂O₅/TiO₂ catalyst was applied in SCR of hybrid SNCR/SCR system of low temperature at 150 °C. Low-temperature SCR hybrid SNCR/SCR (150 °C) system and hybrid SNCR/SCR (350 °C) showed 91~95% total reduction rate with conditions of SV = 2400~6000 hr⁻¹ SCR and 850~1050 °C SNCR, NSR = 1.5~2.0, and 5% O₂. Hybrid SNCR/SCR (150 °C) system proved to be more effective than the hybrid SNCR/SCR (350 °C) system at low temperature.

Implications: NO_x control is very important, since they are the part of greenhouse gases as well as the cause of acid rain and ozone hole. A technology, so-called hybrid SNCR/SCR process, was tested using Mn-V₂O₅/TiO₂ monolithic catalyst for NO_x reduction, and the method is promising. The results of this study would provide some ideas to parties such as policy makers, environmental engineers, and so on.

Introduction

Various nitrogen oxide (NO_x) control methods are necessary because NO_x reduction uses different mechanisms according to the type of combustion engine. Reduction method is classified into two categories: combustion improvement method and aftertreatment method. The former reduces NO_x quantity by regulating combustion point and oxygen concentration, whereas the latter achieves NO_x reduction through postprocessing. Postprocessing is widely used for reduction of NO_x with high concentration. Aftertreatment of NO_x can be classified into SCR (selective catalytic reduction) process with low-temperature electric generation and SNCR (selective noncatalytic reduction) with high-temperature electric generation. The SCR process shows highly effective NO_x reduction rate at low temperature by using a catalyst (Bosch and Janssen, 1988), but economic feasibility should be considered for smaller industrial facilities due to increased operation cost and shortened life span of the catalyst caused by an expensive catalyst.

On the other hand, the SNCR not only promotes easy application in coal-fired power plants and industrial boilers by reverting NO to N₂ via direct injection of chemical additives in high temperature region of 900~1100 °C, but also promotes low-cost installation and operation compared with SCR (Lyon, 1975; Nam and Gibbs, 2000). The SNCR achieves approximately 30–60% NO_x reduction rate, and the reduction rate increases in laboratory-sized research (Miller and Bowman, 1989). On the other hand, SCR can reduce NO_x more than 90% depending on the catalyst volume and space velocity (Long and Yang, 2000; Nova et al., 2001). Both processes are affected by the temperature condition, degree of mixing, oxidation of reducing agent, and NH₃ slip.

Research on hybrid SNCR/SCR process has been a trend to cut down the cost of business installation and operation (Roy et al., 2008; Wu et al., 2008; Chen et al., 2010; Koh and Park, 2013). Gullett et al. (1994) investigated on the optimal condition of combined process of SNCR and SCR, considering the change in space velocity (SV) during SCR, NH₃ injection quantity, and reaction temperature during SNCR process of pilot-scale combustor using natural gas. Urbas and Boyle (1998) discussed the application of SNCR/SCR hybrid process at GPU Genco Seward Station. Furthermore, Wendt et al. (2001) worked on the development of an accurate and applicable model of the hybrid process for measuring economic feasibility of SNCR/SCR hybrid process on site.

Hybrid process outperforms the independent SCR by cutting down the operation cost (Miller and Srivastava, 2000) and outperforms the independent SNCR by increasing the NO_x reduction efficiency and reducing the concentration of NH₃ slip. The state-of-the-art hybrid process uses V₂O₅/TiO₂ catalyst, which is common in SCR. This is because V₂O₅/TiO₂ achieves active reduction of NO generated from a fixed source by using NH₃ as the reducing agent. Also, V₂O₅/TiO₂ has superior endurance against SO₂ compared with other catalysts (Chen and Yang, 1992). For formulation of a practical V₂O₅/TiO₂ catalyst, WO₃ (Choo et al., 2003), MoO₃ (Lietti et al., 1999), SiO₂ (Sorrentino et al., 2001), or other chemicals is added or sulfated according to the processing condition. The catalyst promoter, greater than V₂O₅ in quantity, is selected and added according to the emission condition and plays an important role in boosting the performance of the V₂O₅/TiO₂ catalyst.

Although catalyst based on vanadium is highly active and durable against SO₂, it has several weaknesses. Activation only occurs within narrow temperature window of 300~400 °C, and SCR is applied within such temperature window to prevent pore blocking caused by the pigmentation of ammonium sulfates such as NH₄HSO₄ and (NH₄)₂S₂O₇ on the catalyst surface (Choo et al., 2003). For such reason, much attention is drawn on the development of a catalyst with high activity during low-temperature SCR.

The purpose of this research is to study Mn-V₂O₅/TiO₂ monolithic catalyst used in the SCR for developing a catalyst with high NO_x removal activity in the low temperature region below 200 °C. The SCR process with Mn-V₂O₅/TiO₂ monolithic catalyst only needs 150 °C for similar NO_x removal efficiency compared with typical SCR processes, which require the temperature of at least 250 °C. Also, the study evaluates denitrification efficiency of the hybrid SNCR/SCR system using the prepared catalyst in order to achieve power savings and avoid catalyst poisoning.

Experimental

Catalyst preparation

V₂O₅ catalyst was prepared via impregnation method using NH₄VO₃ (99%; Aldrich, Fisher Scientific, Tustin, CA, USA) with anatase TiO₂ (cell density: 36 CPI) as a supporting form. After being dried at 110 °C for 10–12 hr, it was burnt in a Maffle furnace at 500 °C for 5 hr. The catalyst content of V₂O₅ was set at 1 wt.%, and low-temperature catalyst was prepared by impregnating Mn(NO₃)₂3H₂O (99%; Junsei) each at 5 wt.% on the V₂O₅/TiO₂ monolithic standard catalyst.

Catalyst characterization

X-ray diffractometer (MP-XRD, USA) was used to perform qualitative analysis on the compound precipitate of the carrier and the catalyst. CuKα was used as an X-ray source to perform X-ray power diffraction in the range of 20~80°.

Experimental equipment

The hybrid SNCR/SCR reactor used in this study was designed as shown in Figure 1, and experimental conditions of SCR in hybrid SNCR/SCR are given in Table 1. Simulated gas of the hybrid SNCR/SCR reactor was composed of O₂ with N₂ balance (5%, 10%, 15%, 21%), 10% NO, and 81.5% NH₃. The simulated gas was controlled using the MFC (mass flow controller; Brooks 5850e series; Brooks Instrument, Tokyo, Japan), and the MFC regulated flow into the reactor by GMC1000 flow and pressure controller. Simulation gas was completely mixed in the mixing chamber and then guided into both reactors to investigate the characteristics of both SNCR and hybrid SNCR/SCR reactions. Reactor tubes were made of stainless metal.

Table 1. Experimental conditions of SCR in hybrid SNCR/SCR

		Conditions
Parameter		SNCR	SCR
Catalyst		Noncatalytic	(1 wt.%)V2O5/TiO2
			(5 wt.%)Mn-(1 wt.%)V2O5/TiO2
Temperature (°C)		800–1100	150–350 °C; (1 wt.%)V2O5/TiO2
			100–200 °C; (5 wt.%)Mn-(1 wt.%)V2O5/TiO2
Inlet gas concentration	NO (ppm)	500	Slip
	NSR	1–2.5	Slip
(N2 balance)	O2 (%)	5, 10, 15, 21	Slip
Space velocity (hr^−1)			2400–9000
Calcination temperature (°C)			500
Calcination time (hr)			5

Figure 1. A schematic diagram of the hybrid SNCR/SCR system.

Simulation gas was flown into each reactor for an hour to reach steady state, and initial concentration of NO_x was controlled at 500 ± 30 ppm for face-to-face comparison. Furthermore, NSR (normalized stoichiometric ratio; NH₃/NO_x) was altered to 1~2.5 based on NO_x concentration of 500 ppm. SNCR was driven in a high-temperature reactor in the temperature range of 800~1100 °C, and reactive internal materials were manufactured using quartz. The reduction rate of NO_x per reaction temperature and per NSR at 50 °C intervals was examined in the reactor (21 cm in length × 2.6 cm in diameter), and retention time was controlled at approximately 1.67 sec, with flow rate of 4 L/min. In hybrid process, SCR used a honeycomb quadrangle-shaped catalyst (cell density: 36 CPI) of sulfated V₂O₅-WO₃/TiO₂ commercialized for application at 350 °C, and the catalyst size was converted according to the SV (space velocity) from 2400 to 9000 hr⁻¹ for analyzing reduction efficiency under the same experimental conditions of the independent SNCR. Concentration of the postreaction gaseous substances NO, NO₂, and O₂ were analyzed using CombuCheck single-gas monitor (TSI, Shoreview, MN).

Results and Discussions

Comparison of SCR system response between the baseline catalyst and the Mn-added catalyst

The NO_x reduction efficiencies by reaction temperature and operating condition are compared between the baseline catalyst and the Mn-added catalyst. The catalyst used in this study is (5 wt.%)Mn-(1 wt.%)V₂O₅/TiO₂, containing 5 wt.% Mn in (1 wt.%)V₂O₅/TiO₂.

Figure presents the NO_x reduction efficiencies of the baseline catalyst and the Mn-added catalyst against the change in the reaction temperature, NSR, and O₂ concentration. As observed in Figure 2a and b, it is verified that although the best NO_x reduction efficiency of Mn-added catalyst is approximately 10% lower, it occurs in a low temperature region around 150 °C, which is 100 °C lower than for the baseline catalyst. Also, the NO_x reduction efficiency of the Mn-added catalyst is 8% higher than that of the baseline catalyst at 150 °C for 10% O₂, as shown in Figure 2a. This sets an important milestone for resolving the issue concerning high activity of NO_x released from a fixed source around 300 °C due to the V₂O₅/TiO₂ catalyst. The NO_x reduction efficiency decreases after the temperature for the best NO_x reduction efficiency, since concentration of NH₃ is getting lower after this temperature because of increasing NH₃ oxidation activities. As observed in Figure 2a, it is verified that NO_x reduction efficiencies of both baseline catalyst and Mn-added catalyst per reaction temperature decrease as O₂ concentration increases. This is because as O₂ concentration increases and temperature rises, reduction becomes less active whereas NH₃ oxidation becomes more active, reducing the NO_x reduction efficiency.

Figure 2. Effect of Mn addition to V₂O₅/TiO₂ by O₂ concentration and NSR.

NH₃ shows more active self-oxidation with respect to the elevation of the reaction temperature in SCR system and reaches a maximum value as NO_x conversion rate decreases. It can be verified by measuring NO_x concentration at the reactor exit while providing only NH₃ to the reactor. Figure 2b plots NO_x reduction efficiency against reaction temperature and NSR for the baseline catalyst and Mn-added catalyst. NO_x reduction efficiency increases or stays nearly the same when NSR is greater than 1, and it decreases when NSR is smaller than 1. Such result is due to the stoichiometric requirement of NH₃ during the catalyst reaction of NO_x in SCR system. According to Muzio et al. (1977), NO_x reduction efficiency increased to 22% (NSR = 0.3), 75% (NSR = 1.1), and 90% (NSR = 1.6) when discharge condition of combustion gas was set to 300 ppm NO_x and 4% O₂. Other experiment results also showed similar tendency.

SV, which is closely linked to catalyst performance, is generally defined by the catalyst quantity regarding the gas flow cleared through reaction or as the proportion of the reactor volume occupied by catalyst. In case of a catalyst with good performance, only small quantity of catalyst is required to process simulation gas, enabling high SV operation. Figure 3a and b show decline in NO_x reduction efficiency with increased SV in reaction temperature range of optimum efficiency when the operation conditions were set to 500 ppm NO_x, NSR = 1.2, and 10% O₂. Increase in SV forces more simulation gas passing through the catalyst layer per time unit, and NO_x activation decreases as catalyst surface contact is shortened. Furthermore, it can be seen that the NO_x reduction efficiency is higher in Mn-added catalyst than in the baseline catalyst at SV = 3000 hr⁻¹. This is due to the support of Mn even in equal temperature window and increase of the activation point for reducing NO_x in the low temperature region when Mn is added.

Figure 3. NO_x removal versus increasing SV in the temperature window 100~350 °C.

Although NO_x reduction efficiency against the change in reaction temperature, O₂ concentration, and NSR drops by 10% in the Mn-added catalyst, the reaction temperature of the Mn-added catalyst shifts to low temperature region of 150 °C, which is 100 °C lower than the baseline catalyst. Also, NO_x reduction efficiency is 8% higher with the Mn-added catalyst than the baseline catalyst. Furthermore, it is shown that NO_x reduction efficiency differs between Mn-added catalyst and the baseline catalyst according to the reaction temperature, even in temperature window of optimum efficiency with equal SV. In our study, the role of Mn is not increasing the efficiency of V₂O₅/TiO₂ catalyst, but decreasing the temperature for NO_x oxidation, since catalytic activity energy for NOx oxidations with adding Mn is lower than that with adding other components.

Although V₂O₅/TiO₂ catalyst, the current commercial catalyst, displays optimum activation in the high temperature region, it also faces technical and economic challenges such as decreased NO_x reduction efficiency due to NH₃ oxidation, shortened life of the catalyst, increased overhead, and additional installation cost from heating equipment. This study can resolve such challenges by dropping the gas discharge temperature by 100 °C during SCR application of the catalyst with high activation in low temperature region. Power cost will be reduced, and the catalyst life will be extended. Therefore, hybrid system with low-temperature SCR using Mn-added catalyst seems to be more suitable than the hybrid system using the baseline catalyst.

XRD analysis

To identify the crystal structure form of V₂O₅/TiO₂ monolithic standard catalyst applied in low-temperature SCR system, XRD patterns were analyzed and are shown in Figure 4. When 1 wt.% V₂O₅ is contained in anatase TiO₂ carrier, a peak due to V₂O₅ is absent, and this signifies that V₂O₅ is evenly dispersed. However, V₂O₅ peak appears at 2θ = 21.787° (110), 25.637° (210), 26.221° (101), 31.138° (400), 32.470° (011), 33.419° (111), and 34.406° (301) when V₂O₅ content is increased to 10 wt.%. This reflects the work by Reddy et al. (1999), which illustrates that excess V₂O₅ in forming a single layer of the carrier causes crystallization of V₂O₅. The result is also similar to Serwicka’s (2000) research, which verified thatthe peak at 2θ = 15~32° signified V₂O₅ crystal content.

Figure 4. XRD patterns of V₂O₅/TiO₂ catalysts and Mn-V₂O₅/TiO₂ catalysts.

Figure 4 shows the XRD patterns when Mn, a multivalent base metal oxide, is added to the baseline catalyst. XRD peak does not appear when Mn concentration is low; this indicates even dispersion on V₂O₅/TiO₂ monolithic catalyst surface. However, crystal structure of Mn is present in the V₂O₅/TiO₂ monolithic catalyst as Mn concentration increases to 15 wt.%, and XRD peak of manganese oxide (Mn₂O₃) appears at 2θ = 32.978° (222).

NO_x removal characteristics of hybrid SNCR/SCR system with applied low-temperature SCR system

NO_x reduction characteristics of hybrid system using low-temperature SCR considering economic aspect of power cost and catalyst life span are evaluated. Experimental conditions are all equivalent to former hybrid systems, with the only exception that reaction temperature of the SCR system using 5 wt.% Mn in (1 wt.%)V₂O₅/TiO₂ was set to 150 °C.

Figure 5a plots NO_x reduction characteristics of hybrid system against SNCR system reaction temperature and NSR change under conditions of 5% O₂ and low-temperature SCR system with SV = 2400 hr⁻¹. Reaction temperature of SNCR is within the range of 850~1050 °C, and NO_x reduction efficiency of hybrid system is above 97% when NSR is greater than or equal to 1.0.

Figure 5. NO_x reduction characteristics as a function of temperature and NSR in hybrid SNCR-SCR using low-temperature SCR.

When operation temperature of SNCR system is low at 850 °C, NO_x concentration increases in low-temperature SCR system, and when operation temperature of SNCR system rises to 950 °C, NO_x amount to be reduced decreases in low-temperature SCR system. This achieves extension of the catalyst life span used in the hybrid system.

Furthermore, NO_x reduction efficiency in SNCR system increases as NSR increases, so NO_x amount to be reduced in low-temperature SCR system decreases. When SNCR system reaction temperature is 1100 °C and low-temperature SCR system reaction temperature is 350 °C, NO_x reduction efficiency of the hybrid system with NSR = 1.0~2.5 drops by 10~12% when compared with 950 °C SNCR system and 150 °C SCR hybrid system. This is due to decrease in total NO_x reduction efficiency as NH₃ is used up through oxidation in the high temperature region above 1100 °C and lacks the capability as a reducing agent in the low-temperature SCR system.

To evaluate the effect of O₂ concentration on the hybrid system, NO_x reduction efficiency at 15% O₂ is plotted in Figure 5b against the efficiency at 5% O₂ in Figure 5a, which is the optimum condition for NO_x reduction. All variables except O₂ concentration are equivalent between Figure 5a and b. Although reduction characteristics between Figure 5a and b seem similar, NO_x reduction efficiency shows 13~16% drop in optimum temperature region of SNCR system as O₂ concentration increases from 5% to 15%, increasing NO_x amount to be reduced in the low-temperature SCR system. Such variation in reduction efficiency according to the O₂ concentration is suspected to have an influence on the life span of the low-temperature SCR catalyst.

For evaluating effect of low-temperature SCR system SV on the hybrid system, NO_x reduction efficiencies with SV from 2400 to 6000 hr⁻¹ are plotted in Figure 5c and d. Figure 5c presents NO_x reduction characteristics of hybrid system against the SNCR system reaction temperature and NSR change at 5% O₂. NO_x reduction efficiency of hybrid system at 850 °C SNCR system is shown as 87% when NSR = 1.0, 91% when NSR = 1.5, and 94~95% when NSR = 2.0~2.5. When SNCR system is in optimum operating temperature, NO_x reduction efficiency of hybrid system is shown as 88% when NSR = 1.0 and above 96% when NSR = 1.5~2.5. So, total NO_x reduction efficiency with NSR greater than 1.5 does not vary much even when low-temperature SCR system SV is increased from 2400 to 6000 hr⁻¹.

Comparing the SV = 2400 hr⁻¹ hybrid system of low-temperature SCR system with identical conditions, the total NO_x reduction efficiency shows clear drop of 3~18% as NSR changes from 1.0 to 2.5 at SNCR system reaction temperature of 1000 °C. The efficiency clearly drops by 9~22% at 1050 °C SNCR system and 5~18% at 1100 °C SNCR system. This is speculated to be caused by NH₃ oxidation occurring in high temperature region above 1000 °C, which does not occur under 950 °C for SNCR system.

Figure 5b and d compare NO_x reduction efficiency according to the low-temperature SCR system SV with O₂ concentration of 15%. As NSR changes from 1.0 to 2.5, it can be seen that the total efficiency drops by 7~13% in hybrid system at 850 °C, 11~21% in 900 °C SNCR system, 17~28% in 950 °C SNCR, 20~29% in 1000 °C SNCR, 17~28% in 1050 °C SNCR, and 13~20% in 1100 °C SNCR, showing distinct differences throughout the entire temperature window of SNCR system, unlike in Figure 5c and a. This is because there are both the effect of SV increase and more active NH₃ oxidation due to increased O₂ concentration.

Conclusions

Denitrification characteristics of the hybrid SNCR/SCR system using Mn-V₂O₅/TiO₂ monolithic catalyst in SCR process are as follows.

First, although Mn-added catalyst is approximately 10% lower in NO_x reduction efficiency compared with V₂O₅/TiO₂ monolithic standard catalyst, the reaction temperature window shifts to low temperature region of 150 °C, which is approximately 100 °C lower than the standard catalyst. Also, NO_x reduction efficiency is 8% higher in the Mn-added catalyst at 150 °C with 10% O₂.

Second, the highest NO_x reduction efficiency of Mn-added monolithic low-temperature catalyst is found in (5 wt.%)Mn-(1 wt.%)V₂O₅/TiO₂ monolithic catalyst at 81%, which is 14% less than the standard catalyst, but the reaction temperature window shifts to low temperature region of 150 °C.

Third, after examining XRD patterns of V₂O₅/TiO₂ monolithic standard catalyst including a multivalent base metal oxide of Mn, even dispersion on the catalyst surface is verified from absence of XRD peak in case of small amount of Mn; in case of 15 wt.% Mn, XRD peak of manganese oxide (Mn₂O₃) appears at 2θ = 32.978° (222) as Mn crystal structure in V₂O₅/TiO₂.

Fourth, (1 wt.%)V₂O₅/TiO₂ monolithic standard catalyst presents two reduction peaks at 570 and 657 °C. A major reduction peak appears at 595 °C in case of 5 wt.% Mn-added catalyst, but it still includes the reduction peaks at 570 and 657 °C present in the monolithic standard catalyst.

Fifth, NO_x reduction characteristics of the hybrid system using low-temperature SCR is evaluated considering economic aspects of power cost and catalyst life span. At 5% O₂ and SV = 2400 hr⁻¹ of the low-temperature SCR system (150 °C), reaction temperature of SNCR is within the range of 850~1050 °C, and total NO_x reduction efficiency of the hybrid system is over 97% when NSR is above 1.0. NO_x reduction efficiency drops by 13~16% at optimum temperature in SNCR system as O₂ concentration increases from 5% to 15%, increasing NO_x amount to be reduced in the low-temperature SCR system. To evaluate the effect of low-temperature SCR system SV on the hybrid system, NO_x reduction efficiency is measured when SV increases from 2400 to 6000 hr⁻¹. The NO_x reduction efficiency is 91~95% even when low-temperature SCR system SV is increased from 2400 to 6000 hr⁻¹ when NSR is above 1.5.

Additional information

Notes on contributors

Sung-Woo Choi

Sung-Woo Choi is a Professor and Sang-Ki Choi is a lecturer at the Department of Environmental Science at KMU.

Sang-Ki Choi

Sung-Woo Choi is a Professor and Sang-Ki Choi is a lecturer at the Department of Environmental Science at KMU.

Hun-Kyun Bae

Hun-Kyun Bae is a Professor at the Department of Global Environment at KMU.